Advances on Deflagration to Detonation Transition Methods in Pulse Detonation Engines

Abstract

Pulse detonation engines (PDEs) have become a transformative technology in the field of aerospace propulsion due to the high thermal efficiency of detonation combustion. However, initiating detonation waves within a limited space and time is key to their engineering application. Direct initiation, though theoretically feasible, requires very high critical energy, making it almost impossible to achieve in engineering applications. Therefore, indirect initiation methods are more practical for triggering detonation waves that produce a deflagration wave through a low-energy ignition source and realizing deflagration to detonation transition (DDT) through flame acceleration and the interaction between flames and shock waves. This review systematically summarizes recent advancements in DDT methods in pulse detonation engines, focusing on the basic principles, influencing factors, technical bottlenecks, and optimization paths of the following: hot jet ignition initiation, obstacle-induced detonation, shock wave focusing initiation, and plasma ignition initiation. The results indicate that hot jet ignition enhances turbulent mixing and energy deposition by injecting energy through high-energy jets using high temperature and high pressure; this can reduce the DDT distance of hydrocarbon fuels by 30–50%. However, this approach faces challenges such as significant jet energy dissipation, flow field instability, and the complexity of the energy supply system. Solid obstacle-induced detonation passively generates turbulence and shock wave reflection through geometric structures to accelerate flame propagation, which has the advantages of having a simple structure and high reliability. However, the problem of large pressure loss and thermal fatigue restricts its long-term application. Fluidic obstacle-induced detonation enhances mixing uniformity through dynamic disturbance to reduce pressure loss. However, its engineering application is constrained by high energy consumption requirements and jet–mainstream coupling instability. Shock wave focusing utilizes concave cavities or annular structures to concentrate shock wave energy, which directly triggers detonation under high ignition efficiency and controllability. However, it is extremely sensitive to geometric parameters and incident shock wave conditions, and the structural thermal load issue is prominent. Plasma ignition generates active particles and instantaneous high temperatures through high-energy discharge, which chemically activates fuel and precisely controls the initiation sequence, especially for low-reactivity fuels. However, critical challenges, such as high energy consumption, electrode ablation, and decreased discharge efficiency under high-pressure environments, need to be addressed urgently. In order to overcome the bottlenecks in energy efficiency, thermal management, and dynamic stability, future research should focus on multi-modal synergistic initiation strategies, the development of high-temperature-resistant materials, and intelligent dynamic control technologies. Additionally, establishing a standardized testing system to quantify DDT distance, energy thresholds, and dynamic stability indicators is essential to promote its transition to engineering applications. Furthermore, exploring the DDT mechanisms of low-carbon fuels is imperative to advance carbon neutrality goals. By summarizing the existing DDT methods and technical bottlenecks, this paper provides theoretical support for the engineering design and application of PDEs, contributing to breakthroughs in the fields of hypersonic propulsion, airspace shuttle systems, and other fields.

1. Introduction

As a novel thermodynamic cycle propulsion system, pulse detonation engines have attracted considerable attention in the aerospace propulsion field in recent years due to their potential advantages such as high cycle thermal efficiency, self-pressurization, a wide working range, simple structure, low fuel consumption rate, and high thrust-to-weight ratio [1,2,3,4]. The engine can not only be independently used as a propulsion system for transonic aircraft and cruise missiles but also be combined with turbofan or turbojet engines to provide power for subsonic or supersonic aircraft. The instantaneous energy release characteristics of detonation combustion necessitate the reliable initiation and stable propagation of detonation waves within a limited space and time. The rapid initiation and stable maintenance of the detonation wave are key to realizing efficient energy release in PDEs. As the key mechanism that enables this process, DDT is not only the basis of efficient operation of PDEs but also the key scientific challenge constraining their engineering application.

Traditional detonation wave initiation methods are divided into direct initiation and indirect initiation. Direct initiation generates a strong shock wave instantaneously in the combustible mixture through high-energy ignition to directly trigger detonation. However, this method demands extremely high energy input and exhibits high sensitivity to fuel reactivity, mixture homogeneity, and ambient conditions, making practical application extremely difficult. In contrast, indirect initiation is a complex dynamic process in which deflagration waves are generated by low-energy ignition, and then deflagration waves are transformed into detonation waves through turbulent acceleration, shock wave compression, and chemical reaction coupling, and finally, self-sustaining detonation waves are formed. It has been proven to be a feasible detonation initiation method [5].

In recent years, in order to improve the efficiency of DDT, researchers have put forward various schemes, such as enhancing the turbulence of flow and the effect of shock wave reflection through geometric structure design to accelerate the coupling between flame fronts and compression waves and elevating the chemical reaction rate by adjusting fuel activity to shorten DDT time [6,7]. Although significant research progress has been achieved, there are still many challenges in understanding the detonation initiation mechanism. For example, the multi-scale flow and the coupling mechanism of chemical reaction and turbulence interaction in the DDT process are still not fully elucidated. Additionally, the regulatory effects of complex geometric boundaries on shock wave evolution and flame acceleration, as well as the initiation mechanism of detonation waves under different incoming flow conditions, are still unclear [8,9]. Furthermore, indirect initiation usually requires a longer DDT time and distance and may be accompanied by total pressure loss and a decline in propulsion performance, which can adversely impact the overall performance of pulse detonation engines [10]. Therefore, low-resistance and short-distance initiation technology has consistently been a key focus and major challenge in detonation research.

After years of extensive research, a variety of initiation methods have been developed, including hot jet ignition initiation, obstacle-induced detonation, shock wave focusing initiation, and plasma ignition initiation [11,12,13,14]. This review focuses on the research progress of DDT methods in pulse detonation engines and systematically examines the physical mechanisms, influencing factors, and optimization strategies during the DDT process. Based on the research results of numerical simulation and experimental studies, the bottleneck in detonation initiation technology is summarized, and its future application prospects are discussed in order to provide theoretical support and a technical path for the practical application of detonation propulsion technology.

2. Hot Jet Ignition Initiation

Hot jet ignition initiation is a kind of technology in which a hot jet tube is installed at an appropriate position in the main detonation chamber, and the combustible mixture in the tube is ignited by low-energy ignition methods [15]. The high-energy jet of high temperature and high pressure is ejected from the hot jet tube into the main detonation chamber, and the combustible mixture in the chamber is successfully detonated within a certain distance. Hot jet ignition is essentially a process of energy amplification, and the initial ignition energy required is low.

Early research on hot jet initiation was mainly based on prototype experiments [16,17]. The US Naval Research Laboratory and the French Directorate General of Armament designed the pre-detonation tube structure and successfully achieved stable multi-cycle operation. Subsequently, the feasibility of hot jet ignition initiation was verified experimentally by comparing other ignition methods [18]. Lieberman et al. [19] conducted an experimental study on hot jet initiation, demonstrating that high-speed flame jets could trigger detonation through DDT within a certain distance. Shimada et al. [18] conducted experimental studies on the ignition effect of flame jet ignition. Compared with traditional spark ignition, flame jet ignition significantly reduced the time and distance of DDT, especially DDT time, which was shortened by about 3.9 times.

The key to hot jet initiation lies in generating an intense turbulent flame in the detonation tube to accelerate flame propagation and enhance the DDT process. In order to analyze the effects of pre-detonation tube structural parameters and ignition parameters on hot jet ignition initiation, the French Ministry of Armed Forces [20] conducted an experiment by filling the detonation chamber and pre-detonation tube with a mixture of air and hydrogen. The pre-detonation tube generated detonation jets, thereby successfully realizing the multi-cycle stable operation of secondary detonation in a valveless air-breathing pulse detonation engine. Wang et al. [21] studied the influence of the intensity of flame jets on the initiation performance. The intensity of the flame jet was controlled by changing the length of the hot jet tube. Zheng et al. [22] studied the influence of ignition position on the ignition and initiation process of hot jets. It was shown that the distance and time of the detonation wave were the shortest because of the highest jet intensity occurring when the ignition source was 30 mm away from the top of the hot jet tube. Figure 1 shows the DDT time and distance at different ignition positions.

The previous studies indicate that the initiation performance of detonation jets is usually superior to that of high-speed flame jets. However, it is difficult to trigger detonation in the hot jet tube in advance. In this regard, filling the hot jet tube with reactants that are easy to initiate is a quick method to obtain a detonation jet. Brophy et al. [23,24] used ethylene-oxygen as the reactant in a hot jet tube to initiate a JP10-air mixture in the main detonation chamber. It was found that the diffraction phenomenon occurred after the detonation wave spread out of the hot jet tube, and the shock wave intensity decreased, which led to the decoupling of the detonation wave. However, the diffracted shock wave would reflect and focus on the wall of the main detonation chamber, forming a local hot spot to realize the secondary initiation. He et al. [25] found that it was more conducive to the initiation of the main detonation chamber when the detonation jet was in a state of overdrive before it was about to enter the main detonation chamber. Wang et al. [26] adopted the initiation method of a pre-detonation tube in their experiments and obtained the shortest initiation time of 2–3 ms and a DDT distance of 700 mm.

With the maturity of high-resolution experimental observation methods and high-precision numerical simulation techniques, researchers have conducted detailed studies on the DDT process induced by a hot jet and analyzed the complex wave structure and the interaction between the wall and shock waves in the detonation tube. Cai et al. [27,28] studied the effect of the uniformity of premixed gas and the intensity of a hot jet on the detonation initiation process of supersonic airflow based on the open-source program AMROC. Figure 2 shows the schlieren of hot jet initiation. Chen et al. [29,30] studied the initiation process of hot jets and the propagation mechanism of detonation waves in a circular tube; they analyzed the complex wave system structure and the interaction between the wall and shock waves in the detonation tube. Figure 3 presents the contours of temperature, showing different results of jet initiation using different free stream Mach numbers. Wang et al. [31] carried out a numerical study on a rapid DDT process in high-frequency pulse detonation in rocket engines and found a new mechanism of vortex–flame interaction to accelerate the flame.

To improve the acceleration effect of a jet on a flame, Wang et al. [32] designed a segmented acceleration structure in the detonation tube, and the jet was ejected from the transverse jet hole in the acceleration section. The results showed that a shorter DDT distance and time were realized by the structure than transverse jets when jet intensity was high. Subsequently, Wang et al. [33] put forward a new initiation method for internal jet tubes and compared the traditional transverse jet initiation process with the internal jet tube initiation process, which verified the feasibility and high efficiency of the method. Figure 4 illustrates the evolution of temperature, shock wave, and vortex under an internal jet tube.

In contrast to single-point jet ignition initiation, multi-point jet ignition initiation adopts the sequential triggering of multiple ignition sources to continuously strengthen the combustion front and finally trigger the detonation wave, which plays an important role in improving ignition energy. Wang et al. [34,35] numerically studied the detonation initiation process of double jets and analyzed the effects of the position parameters of the two jet tubes and the interval time of jets on the detonation initiation characteristics. Dai et al. [36,37] conducted a study on the initiation and propagation mechanism by colliding jets and staggering opposed jets in a supersonic airflow; they analyzed the effect of jet strength and jet spacing on the flow field and the initiation process induced by the interaction of bow shock waves induced by double jets. Figure 5 shows the density contours of detonation initiation under the action of a continuous injection of a symmetric jet.

The current research indicates that hot jet ignition initiation technology provides a highly controllable and adaptable solution for detonation wave initiation through efficient energy injection and flow control. However, its engineering application still faces multiple challenges, including energy loss, structural reliability, and multi-field coupling mechanisms. Future studies should emphasize the integration of interdisciplinary technology and multi-scale optimization design to accelerate the practical application of this technology in the field of hypersonic propulsion.

3. Obstacle-Induced Detonation

3.1. Solid Obstacle-Induced Detonation

Early studies on DDT were all carried out in smooth tubes. However, a long axial distance was usually required to trigger detonation in smooth tubes when using DDT, and detonation cannot even be initiated smoothly [38]. Consequently, solid obstacle-induced detonation promotion technology emerged, which promoted flame acceleration and effectively shortened the DDT process through the interaction of turbulence, shock waves, and flames.

Solid-obstacle structures have evolved into various types through years of exploration and research, among which the most typical is the Shchelkin spiral structure. Shchelkin [39] proposed placing a spiral wire in the detonation tube to promote the DDT process. The initial deflagration wave was accelerated by the spiral wire and, ultimately, achieved a successful transition into a detonation wave; hence, the configuration was named the Shchelkin spiral. Meyer et al. [12] conducted experimental studies to investigate the detailed mechanisms of detonation promotion by the Shchelkin spiral. The results revealed that the initially generated local detonation propagated along the spiral and continuously generated detonation centers along the spiral; ultimately, it successfully triggered detonation.

In addition to the Shchelkin spiral, current mainstream solid obstacles also include semi-circular protrusions, blockage plates, orifice plates, and inclined wedges. The detonation promotion mechanism of these structures is similar to that of the Shchelkin spiral, which promotes flame entrainment and stretching by enhancing turbulence intensity, thereby improving heat and mass transfer efficiency, as well as increasing the heat release rate, ultimately leading to local detonation that further develops into self-sustained detonation waves.

Ciccarelli et al. [40] experimentally studied the detonation propagation process in a cylindrical tube equipped with orifice plates, using a self-luminous high-speed camera to visualize the detonation phenomenon of different-sized orifice plates. Rakotoarison et al. [41] revealed the mechanism of flame acceleration and DDT in a tube containing circular obstacles with a mixture of propane-oxygen. Shi et al. [42] investigated the quenched detonation re-initiation processes behind obstacles of different shapes. It was demonstrated that detonation re-initiation could be realized under cylindrical and triangular obstacles, while square obstacles and inverted triangular obstacles suppressed the re-ignition of detonations. Wang et al. [43] conducted a numerical study on the effects of different shaped obstacles on flame acceleration and the detonation initiation process. The results indicated that triangular obstacles possessed the optimal flame acceleration effect and detonation-assisting capability. Saeid et al. [44] analyzed the effect of obstacle shapes on DDT mechanisms in an inhomogeneous mixture. It was found that rectangular obstacles could more effectively accelerate flame propagation, while semi-circular obstacles promoted a more controlled DDT process.

The solid obstacles significantly increased the flow resistance of the airflow, leading to an increase in total pressure loss. Therefore, the blockage ratio of obstacles has become a research hotspot. Ciccarelli et al. [45] investigated the influence of blockage ratio on the flame acceleration process in a square channel with obstacles and visualized the flow field of unburned gas in front of the flame and the interaction between shock waves and flames based on schlieren techniques. Figure 6 displays a schlieren diagram of the flame acceleration process in obstacles. Sun et al. [46,47] experimentally investigated the effects of induced perturbations on detonation propagation under different blockage ratios. The experimental results showed that it was easier to initiate at the fuel-lean side when the blockage ratio was in the range of 0.802 to 0.96. In addition, the critical condition limit for detonation propagation was quantified. Ni et al. [48] studied the influence of the blocking ratio of arc obstacles on the flame acceleration process and detonation characteristics. A comparative study of detonation characteristics in a tube with rectangular obstacles was also conducted.

Goodwin et al. [49] numerically simulated the DDT process of an ethylene-oxygen mixture in obstructed tubes under different blockage ratios and found that several different DDT mechanisms were observed with the decrease in blockage ratio. Figure 7 exhibits the flame acceleration and initiation process at a blockage ratio of 0.8. Ago et al. [50] utilized a multi-step reaction model to simulate the DDT process under different blockage ratios in a two-dimensional channel with obstacles. It was found that the initiation of DDT had a different dependence on the height of obstacles. Ahumada et al. [51] conducted an experimental investigation on the initiation process of hydrogen-oxygen mixtures with different blockage ratios. The important impact of the blockage ratio on the DDT process was also observed.

Furthermore, the arrangement and spacing of obstacles and the initial parameters of the mixed gas also have a significant influence on the DDT process. Zhu et al. [52,53] numerically studied the effect of the distribution of obstacles and the concentration gradient of premixed gas on flame acceleration and the DDT process. It was demonstrated that DDT occurred earliest when obstacles were symmetrically distributed in a uniform concentration field. When the concentration was not uniform, the earliest initiation occurred with the negative lateral concentration gradient and the interlaced distributed obstacles. Figure 8 shows the evolution of the distorted flame and reflected shock waves in the shock-accelerated flow. Xiao et al. [54] performed numerical simulations to study the effect of obstacle arrangement on detonation initiation in fuel-rich hydrogen-air mixtures. The results indicate that unilateral obstacle arrangement was more conducive to flame acceleration and DDT than bilateral obstacle arrangement, and the placement of unilateral obstacles on the upper or lower wall did not have significant effects on DDT distance. Debnath et al. [55] analyzed the effect of obstacle spacing on the detonation wave propagation of a hydrogen-air mixture in a pulse detonation chamber. It was shown that the larger obstacle spacing played a significant role in the stable propagation of detonation waves.

Obara and Maeda et al. [56,57] experimentally observed the effects of obstacle spacing and initial pressure on the detonation initiation process of a hydrogen-oxygen mixture, analyzed the flame acceleration mechanism, and obtained the optimal condition for the DDT process. Saeid et al. [58] found that the concentration of hydrogen had a significant effect on DDT and the detonation diffraction mechanisms of hydrogen-air mixtures. Wang et al. [59] conducted an experimental investigation on the effects of initial pressure, slit width, slit spacing, and obstacle spacing on the detonation propagation process. It was shown that re-initiation could be generated in various ways. Song et al. [60] numerically investigated the effects of initial pressure, hydrogen concentration, and obstacle distribution on detonation propagation, as well as the changes in the shape and size of detonation cells. It was demonstrated that the position of detonation generation with obstacles distributed on one side was further away than with obstacles distributed on both sides, but there was no significant change in the size of the detonation cell. As the initial pressure increased, the distance of DDT shortened, and the size of the detonation cell also decreased.

In recent years, research on obstacle-induced detonation has gradually shifted from research on DDT distance and time to the study of DDT mechanisms. Valiev et al. [61] revealed the physical mechanism of flame acceleration in a channel with obstacles, which was essentially different from the classical Shchelkin spiral structure. Li et al. [62] experimentally studied the flame propagation process of stoichiometric hydrogen-oxygen mixtures in a narrow channel using schlieren and smoked foil techniques; they found that the blockage ratio had a significant influence on the detonation limit. Figure 9 illustrates Mach stem reflection leading to chevron formation in stoichiometric H₂-O₂ for 33% BR obstacles. Zhong et al. [63] studied the potential mechanism of obstacles influencing flame propagation and detonation through flame shock waves and the interaction between flames and vortices, analyzing the mechanism of different detonation modes caused by different obstacle positions. The results indicated that the detonation combustion mode was determined by the compression and preheating temperature of the unburned mixture. Xiao et al. [64,65] studied the flame acceleration and DDT process of hydrogen-air mixtures in narrow channels using high-speed schlieren photography and pressure transducers combined with high-precision numerical simulations, revealing the essence of flame acceleration and DDT mechanisms. Zhao et al. [66] carried out numerical simulations to investigate flame acceleration, DDT, and detonation propagation in a combustion chamber filled with a subsonic or supersonic mixture. Two DDT mechanisms were discovered in the combustion chamber of a mixture with different initial velocities.

Obstacle-induced detonation structures inevitably cause thrust loss. Consequently, research efforts are simultaneously focused on evaluating performance loss caused by these structures and exploring low-loss structures. Brophy et al. [67] investigated the initiation performance of various swept-ramp structures. It was found that compared with wall spiral structures with the same initiation performance, the total pressure loss of swept-ramp structures could be reduced by at least 50%. Li et al. [68] conducted an experimental investigation into the detonation promotion performance of spiral grooves, and the results revealed that spiral grooves had lower flow loss and better propulsion performance than Shchelkin spirals.

In summary, solid obstacles effectively accelerate the DDT process through turbulence enhancement and shock wave modulation, which provides a cost-effective and highly reliable solution for the rapid initiation of detonation waves. However, its engineering application is limited due to flow resistance and energy loss; thermal stress and structural reliability; and the uncontrollability of the DDT process. Future advancements require breakthroughs in material innovations, multi-field collaborative optimization, and intelligent design methodologies.

3.2. Fluidic Obstacle-Induced Detonation

Fluidic obstacle-induced detonation (transverse jet) is proposed as a feasible alternative in order to balance initiation performance and propulsion efficiency in pulse detonation engines. In this method, gas is injected through small holes or slits on the wall of the detonation tube to form transverse jets, which induce turbulence, promote flame acceleration, and finally realize low resistance and short-distance rapid detonation initiation.

The comparative experimental study on the influence of transverse jets and orifice plates on the DDT process was first carried out by Knox [69,70,71]. The results revealed that transverse jets exhibit greater advantages in inducing turbulence. The jet with a high momentum ratio propagated upstream with turbulence, reduced ignition time, and, consequently, shortened DDT time. In addition, it was also found that the transverse jet demonstrated higher stability and reliability regarding initiation at a lower initial pressure.

As a novel technology for enhancing the DDT process, fluid obstacles have been the focus of extensive research in recent years to elucidate their advantages over traditional solid obstacles. Knox et al. [69,70] conducted experiments to study the influence of fluid obstacles on the DDT process. It was found that fluidic obstacles provided active control over flame speed, increased turbulence intensity, and shortened DDT times compared to the passive control of physical obstacles. Peng et al. [72,73] conducted comparative experiments on detonation initiation processes by using solid obstacles and transverse jets, respectively. The experimental observations confirmed the significant potential of transverse jets for detonation promotion. Furthermore, the study investigated the effect of the number of jets on the detonation initiation process of methane-oxygen mixtures, as well as the influence of initial jet parameters (mixture composition, stagnation temperature, pressure, and mass flow rate) on flame propagation and DDT in hydrogen-oxygen-argon mixtures. The results demonstrated that dual-jet injection was more conducive to flame acceleration compared to single-jet injection. DDT distance and time decreased with the increase in initial injection temperature, pressure, and mass flow rate. Figure 10 illustrates the density contours and temperature contours of the interaction of the flame and jet. Tarrant et al. [74] explored the flame acceleration effects of flames interacting with solid obstacles and fluidic obstacles. The main mechanisms of interaction between flames and fluid obstacles to facilitate the acceleration of the DDT process were discovered and confirmed.

In order to explore the mechanism of interaction between turbulence and flames, McGarry et al. [75] conducted experimental investigations into the interaction between laminar flame and transverse jets utilizing schlieren imaging and particle image velocimetry techniques; they analyzed the effect of equivalent ratio and jet momentum ratio on flame acceleration. Chambers et al. [76,77] found that the acceleration effect of transverse jets on laminar flames was different from that of turbulent flames, and the flame acceleration of turbulent flames was higher under the action of transverse jets. McGarry et al. [78] conducted an experimental comparison and analysis of the interaction between physical plates, transverse jets, and flames, respectively. The results revealed that a transverse jet could induce turbulence more effectively and promote flame acceleration compared with the physical plate. Figure 11 shows the flame acceleration process under different working conditions. Huang et al. [79] experimentally investigated the effects of jet delay time, jet position, jet quantity, and jet distribution on the DDT process. It was found that transverse jets significantly enhanced the acceleration of the initial flame. The DDT time was shorter when the jet injection position was close to the ignition position, and there was an optimal jet delay time to make the DDT time shortest.

The previous research primarily focused on flame acceleration prior to detonation triggering. It was not until recent years that the key process of successful detonation triggering by transverse jets was deeply studied. Wang et al. [80,81] conducted an in-depth analysis of the process of flame acceleration and detonation initiation promoted by transverse jets. The study also investigated the effects of initial pressure, initial temperature, and equivalence ratio on the initiation process. It was demonstrated that the initiation distance and time initially decreased and then increased with higher initial pressure and equivalence ratio; they then decreased monotonically with increases in initial temperature. The effect of jets on a flame at the same moment in the turbulence–-flame interaction stage under different initial pressures is depicted in Figure 12.

Zhao et al. [82] conducted numerical simulations to study the mixing, flame acceleration, and DDT triggered by transverse jet obstacles. The results demonstrated that multiple transverse jet obstacles significantly enhanced mixture uniformity and promoted the DDT process, while the occurrence of DDT was extremely difficult in non-uniform supersonic mixtures. Wang et al. [83,84] conducted a detailed simulation to investigate the effects of multiple groups of fluidic obstacles on flame acceleration and the DDT process under different initial velocities and gas types. It was demonstrated that increasing the initial jet velocity or using reactive jet gases could shorten the detonation initiation time and distance. Figure 13 displays the sequence of flame propagation images. In addition, the position and the delayed injection time of fluidic obstacles on flame acceleration and the DDT process were studied. An optimal position of a fluidic obstacle was obtained that could improve initial flame acceleration and accommodate DDT. The effectiveness of the delayed injection strategy was constrained by the position of fluidic obstacles.

The type of jet injection medium is also a critical factor influencing the detonation performance of transverse jets. Wang et al. [85] conducted numerical simulations to investigate the impact of fluidic obstacles on flame acceleration and the DDT process. The computational results indicated that fluid obstacles could reduce DDT time by 37.5% compared with smooth tubes. Compared to physical obstacles, fluid obstacles could significantly decrease the total pressure loss and have the potential for application in propulsion systems. Zhao et al. [86] conducted a study on the state of an injection medium and found that using a combustible mixture as the injection medium could effectively promote flame acceleration. However, when air was used as the injection medium, the effect on flame acceleration was not significant. Cheng et al. [87,88,89] experimentally investigated the process of promoting detonation using different types of non-combustible gases as an injection medium; they analyzed in detail the combined effects of turbulence intensity and dilution induced by non-combustible gas injection on the DDT process. Figure 14 depicts Ar distribution and vortex evolution for the case with a jet. Wang et al. [90,91] numerically studied the influence of the gas type of fluid obstacle on flame acceleration and the DDT process. It was found that reactive gas jets had better flame acceleration and DDT performance compared with inert gas jets.

Fluid obstacles have demonstrated significant performance advantages in promoting flame acceleration and detonation initiation. Compared to solid obstacles, using fluid obstacles to accelerate the DDT process can effectively reduce total pressure loss. The characteristics of active control and high adaptability provide a flexible technical path for detonation wave initiation. However, its engineering application is limited by various challenges, such as high energy consumption, control complexity, and unclear multi-field coupling mechanisms, indicating that significant work remains to overcome these barriers.

4. Shock Wave Focusing Initiation

Shock wave focusing initiation refers to the process in which energy converges in a small region within a medium through the interaction of shock waves, generating a high-temperature and high-pressure center that ignites and initiates the detonation of the combustible mixture in the main detonation chamber. It is primarily realized through planar shock wave axial incidence, cylindrical shock wave focusing collision, annular shock wave focusing collision, and annular jet-induced shock wave focusing [92].

The research on shock wave focusing initiation can be traced back to the 1970s. It was discovered that shock wave focusing could create a hot spot with extremely high energy density and directly initiate a detonation wave. Chan [13] first studied the ignition of combustible mixtures through shock wave reflection, diffraction, and focusing. The detonation waves were successfully initiated when the incident shock wave was sufficiently strong. RWTH Aachen University and the Russian Institute of Chemical Physics [93,94] conducted experimental studies on shock wave focusing ignition using planar shock waves and concave cavities and compared the ignition characteristics of four types of concave cavities. They observed three combustion modes: deflagration, direct initiation, and DDT, and found that there were three locations for the ignition points when using planar shock wave focusing to initiate detonation. In addition to the method of planar shock wave axial incidence focusing to initiate detonation waves, researchers have proposed initiating detonation waves through cylindrical shock wave diffraction focusing generated by annular shock tubes. Teng et al. [95] numerically studied the focusing process of toroidal shock waves in a cylindrical chamber. The study revealed the mechanism of shock wave focusing and demonstrated shock reflection styles and focusing characteristics. Subsequent research revealed that initiating detonation waves through cylindrical shock wave diffraction focusing required an extremely high incident shock wave intensity, which was difficult to achieve under actual flight conditions. Therefore, this method is considered infeasible for practical detonation wave initiation applications.

In subsequent research, scholars have proposed initiating detonation waves through annular shock waves or detonation wave implosive collision. Murray [96] and Jackson et al. [97,98,99,100] experimentally confirmed that the efficiency of detonation initiation in the main chamber using a pre-detonation tube could be improved through the implosive collision of detonation waves. The study revealed that the maximum diameter of the main detonation chamber that could be successfully initiated by the pre-detonation tube was 3.2 times larger than that of conventional pre-detonation tubes. Jackson et al. [101] successfully initiated a combustible mixture in the main detonation tube by utilizing the implosive collisions of the detonation wave generated by the pre-detonation tube along the central axis of the detonation tube, combined with reflection and focusing induced by the upstream planar reflector.

Although these shock wave focusing methods—including planar shock wave axial incidence, cylindrical shock wave focusing collision, and annular shock wave focusing collision—have been theoretically and experimentally proven to induce detonation wave initiation, it is difficult to continuously generate high-intensity and unsteady incident shock waves under actual flight conditions; shock wave focusing initiation usually requires an independent shock generating device, indirectly leading to significant challenges in practical engineering applications.

Levin et al. [102] proposed a two-stage PDE scheme based on the principle of thermoacoustic coupling and shock wave focusing. This shock wave focusing initiation method utilizes the shock wave formed by the supersonic jet from the annular channel to reflect and focus in the cavity to trigger detonation. This approach demonstrated greater potential for practical applications due to its lower energy requirements and simplified system design. Subsequently, the United States and Japan have also carried out related research. Leyva et al. [103] conducted a series of experimental and numerical studies to investigate the collision phenomena of non-reactive jets in a binary concave cavity and the resulting aero-acoustic oscillations. In order to verify the feasibility of annular supersonic jet focusing collision to initiate a detonation wave, Jackson and Shepherd et al. [101] utilized the focusing collision of supersonic jets to generate shock waves on the central axis of a reflection cavity. The mixture of ethylene-oxygen-nitrogen and propane-oxygen-nitrogen was successfully initiated under the action of a plane reflector, and three initiation modes were identified.

McManus et al. [104] conducted experiments on a two-stage PDE and observed two different combustion modes. In the first mode, the combustion within the concave cavity was incomplete, with a turbulent flame visible in the downstream region, and no pressure pulsations were observed at the nozzle exit, indicating the absence of detonation waves. In the second mode, a short blue flame was observed at the exit, accompanied by pressure oscillation frequencies as high as 1200 Hz, initially interpreted as high-frequency detonation. However, further analysis using high-speed photography and pressure measurements confirmed that no detonation occurred, and the measured thrust was comparable to that of deflagration, with no significant enhancement.

In order to further advance the engineering application of two-stage PDEs, researchers have focused on the effects of types of focusing reflectors (wedge, semi-spherical, three-wall, conical, annular, etc.) and mixtures under different initial conditions on detonation waves initiated by shock wave focusing. In the study of the wedge reflector, Smirnov et al. [105,106] conducted numerical simulations and experimental studies on the shock wave reflection and focusing processes of hydrogen-air mixtures in wedge-shaped cavities under different scenarios. The developed three-dimensional transient mathematical model for chemical reaction gas mixing flow was well suited to hydrogen-air mixtures, and improving the kinetic scheme for hydrogen-air mixtures under high pressure was highly feasible. Allah et al. [107] presented an experimental investigation into the effect of methane addition to methane-hydrogen-air mixtures on the critical conditions for transition to detonation in a 90-degree wedge. The results showed three ignition modes, namely, flame ignition, strong ignition, and weak ignition. De La Hoz et al. [108] conducted a numerical study on the initiation of detonation by shock wave reflection and focusing on a 90-degree wedge in hydrogen-air mixtures. The results unveiled three potential outcomes: deflagrative ignition in the corner, deflagrative ignition with intermediate transient phases, and ignition with an immediate transition to detonation.

Zhang et al. [109,110,111] combined transient over-pressure recording technology with a high-speed schlieren photography system to study the ignition modes of methane-based mixtures. Three ignition modes in the 90-degree and 60-degree wedge reflectors with shock wave focusing were observed, namely peak local ignition mode, boundary ignition mode, and strong ignition mode. Subsequently, they utilized two reflectors, namely the 60-degree and 90-degree wedge reflectors, and four incident shock wave velocities as variable parameters to investigate the effects on the ignition modes and detonation propagation caused by shock wave focusing. The results showed that as the intensity of the incident shock wave increased, three ignition modes emerged. The formation of new hot spots and transverse waves was crucial for ignition and maintaining detonation waves. Moreover, the energy accumulation capabilities of seven different reflectors—including wedge reflectors, parabolic reflectors, and semi-cylindrical reflectors—were compared using the internal energy density near the reflector vertex area as an evaluation metric. The results indicated that the energy accumulation capabilities of the seven reflectors were at least 2.5 times that of a flat reflector, demonstrating the significant effect of shock wave focusing. Figure 15 displays typical detonation initiation and propagation in a wedge reflector.

For the semi-spherical reflector, Hatanaka et al. [112] numerically investigated the effect of boundary layer and flow field inhomogeneities caused by detonation waves on the stability of inward shock waves in a hemispherical combustion chamber. It was found that the influence of the boundary layer was more significant than that of the inhomogeneities caused by detonation. Xue et al. [113] studied the shock focusing process of kerosene-air mixtures under different temperatures and Mach numbers in a semi-cylindrical reflector and analyzed the interaction between the flame and the shock wave. Two modes of detonation initiation were discovered, namely the direct initiation mode and the reflected shock wave collision initiation mode. Zhang et al. [114] investigated the ignition behavior of methane-oxygen-argon mixtures in a hemispherical reflector through numerical and experimental research. The results revealed that the ignition delay time and the maximum pressure after ignition in the hemispherical reflector were completely opposite for the strong ignition mode compared to the weak ignition mode.

During the study of the three-wall reflector, Rudy [115,116] presented an experimental investigation on the DDT process due to shock wave focusing in a 90-degree corner and three-wall 90-degree corner in hydrogen-air mixtures. The results showed three ignition modes, namely weak ignition, deflagration, and detonation. In addition, the detonation ability range of hydrogen-air in a three-wall 90-degree reflector was higher than that in a 90-degree wedge reflector under the same initial conditions. For the conical reflector, Zhang et al. [114,117] experimentally investigated the effect of incident shock Mach on the ignition delay time for methane-oxygen-argon mixtures with flat and conical reflectors. The results indicated that the conical reflector significantly reduced the ignition delay time, and strong deflagration waves and quasi-detonation waves were observed in the conical reflector, while only a weak deflagration wave was formed in the flat reflector. Furthermore, the ignition behavior of methane-oxygen-argon mixtures in 60 and 90-degree conical reflectors was studied, revealing two ignition modes when the incident shock wave velocity exceeded a certain critical value; the boundary between weak ignition and strong ignition depended on incident shock wave velocity.

In the research on annular reflectors, He et al. [118,119,120] studied the method of annular jet-induced shock wave focusing detonation initiation in a two-stage PDE. The research focused on the effects of nozzle geometry, cavity structure, flow exit profile, annular jet injection parameters, the initial state of the combustible mixture, and the deflection angle and depth of the guide ring on the performance of shock focusing initiation; they analyzed the mechanisms of detonation initiation. Zheng et al. [121,122] proposed a compact annular shock wave focusing initiation structure and revealed the physical mechanisms of detonation wave triggering within this structure and the initiation characteristics under different initial conditions. The influence of ignition and structural factors on the detonation initiation process was obtained. The results demonstrated that this novel initiator could obviously shorten DDT time and distance, and detonation initiation time increased with the decrease in ignition energy. Moreover, both DDT time and distance increased with the increase in energy release time. Xu et al. [123] numerically studied the influence mechanism of parabolic cavity geometry on the detonation wave formation process in a shock wave focusing engine. The results indicated that an increase in the inlet jet temperature facilitated the coupling of the hot spot with the reflected shock wave at the cavity base, thereby successfully forming detonation waves. Figure 16 demonstrates the process of shock wave focused initiation with a jet temperature of 500 K.

With the improvement of experimental observation techniques and high-precision numerical simulation methods, researchers have conducted detailed studies on the process of initiation induced by shock wave focusing. Xiao et al. [124] numerically studied the interaction between focused shock waves and flame fronts during detonation wave induction. It was found that shock wave focusing played a significant role in the DDT process. Figure 17 shows the temperature fields and the corresponding schlieren fields of the DDT process. Maclucas et al. [125] experimentally investigated the development of the reflection mode of incident shock waves on the profile wall and the detailed process of gas dynamic focusing, analyzing flow characteristics under different reflector geometries. Yang et al. [126] conducted numerical studies on detonation initiation through shock wave focusing under different hydrogen concentrations. The results showed that higher hydrogen concentrations significantly accelerated the attenuation of overdriven detonation waves, which was beneficial in terms of facilitating the rapid and stable formation of detonation waves.

In other words, shock wave focusing initiation provides a highly efficient and controllable initiation method for pulse detonation waves through aerodynamic energy amplification. This technology effectively addresses the challenges of the initiation of low-reactivity fuels, reducing dependence on turbulence and minimizing energy loss. However, shock wave focusing and its induced detonation process—which involves coupled physical and chemical processes such as interaction between shock waves, chemical reactions, rapid energy conversion, and detonation initiation—are extremely complex. These processes are characterized by high temperature, high pressure, and high transience. Future research should focus on systematic and in-depth investigations into material innovations, intelligent optimization algorithms, and multi-modal synergy technologies to overcome the existing technical barriers.

5. Plasma Ignition Initiation

Plasma ignition refers to the process of utilizing electrical discharge to form localized high-temperature regions and excite a significant number of active particles, thereby achieving the rapid ignition of combustible gas mixtures and enhancing combustion. Compared to traditional spark ignition, plasma ignition has the advantages of a larger ignition zone, higher energy utilization efficiency, shorter ignition delay time, broader ignition boundaries, and greater ignition reliability [127].

In order to address the challenges of long DDT distances and times in straight tube pulse detonation engines, Starikovskiy et al. [128,129] proposed the use of plasma ignition technology to initiate pulse detonation engines to address various issues hindering the application of pulse detonation engines. Wang et al. [130] conducted a comparative study on the effects of transient plasma ignition and spark ignition on ignition delay time; they found that transient plasma ignition effectively shortened the ignition delay time. Sinibaldi et al. [131] conducted an experimental study on the ignition and detonation initiation processes of ethylene-air mixtures. The results revealed that transient plasma ignition significantly shortened DDT distance and time compared with spark ignition, and they successfully achieved 40 Hz high-frequency initiation. Naples et al. [132] conducted comparative studies on the detonation initiation processes of spark plug ignition and plasma ignition using aviation gasoline, ethylene, and hydrogen as fuels. It was illustrated that the DDT distance and time could be shortened by 50% and 30% using plasma ignition, respectively.

Starikovskiy et al. [133] summarized the research achievements of plasma ignition in the initiation of pulse detonation engines and highlighted its promising application prospects in the fields of high-speed combustion, lean combustion control, and detonation engines. Subsequent researchers have conducted in-depth research on the detailed mechanism of detonation waves initiated by plasma ignition, as well as the effects of the performance parameters of plasma igniters and reactant parameters on the initiation process. Gray et al. [134,135] confirmed that plasma discharge could accelerate the propagation of turbulent flames and significantly reduce the initiation distance. The results indicated that low-energy active devices might be suitable for replacing passive devices. Furthermore, an effective alternative method was proposed that allowed for the application of plasma discharge near the wall while successfully maintaining a promising success rate for detonation. Figure 18 displays the schematic representation of the experimental test, including a photograph detailing the electrode configuration.

Cherif et al. [136,137] experimentally investigated the effect of volumetric nanosecond discharge on the size of detonation cells. The analysis revealed that the size of the detonation cells was decreased by 1.5 to 3 times when passing through the discharge region. Moreover, the impact of initial pressure on the deposited energy and homogeneity was also studied. The research confirmed the relationship between plasma parameters and detonation cell widths and demonstrated the possibility of controlling detonability. Figure 19 shows ICCD images of the discharge and the detonation wave front positions at the moment of the discharge pulses P1 (−47 kV) and P2 (−34 kV). Starikovskaia et al. [138] summarized recent results on plasma-assisted ignition and combustion and presented applications of plasma-assisted combustion and detonation activated by various types of discharges. Lafaurie et al. [139] developed a novel nanosecond non-equilibrium discharge configuration capable of producing a gradient of atomic oxygen through discharge between variable gaps in a planar configuration.

Tropina et al. [140] numerically investigated the effect of equilibrium and non-equilibrium plasma on DDT and detonation cell size for hydrogen-air mixtures in a two-dimensional channel with obstacles. The results suggested that non-equilibrium plasma could generate higher velocities and maintain higher pressures and velocities for longer periods of time, showing potential for DDT control in scramjets engines. Vorenkamp et al. [141,142] experimentally investigated the influence of plasma discharge on the initiation time and distance of DDT. It was shown that a small number of plasma discharge pulses prior to ignition could reduce DDT initiation time and distance by 60% and 40%, respectively. Furthermore, the enhancement of low-temperature chemistry in a premixed dimethyl ether-oxygen-argon mixture through nanosecond dielectric barrier discharge plasma for DDT was also explored. The results indicated that non-equilibrium plasma generated active species and kinetically accelerated low-temperature chemistry of dimethyl ether and DDT. An appropriate number of nanosecond dielectric barrier discharges enhanced the low-temperature chemistry of dimethyl ether and accelerated the DDT process. Shi et al. [143] numerically studied plasma-assisted DDT in a microscale channel. It was demonstrated that there was no monotonic dependency between the initiation time of DDT and the number of discharge pulses. In addition, two different DDT regimes were observed: via acoustic choking of the burned gas and plasma-enhanced reactivity gradient without acoustic choking. Figure 20 summarizes the plasma-assisted DDT mechanism.

Zheng et al. [144,145] innovatively used alternating current (AC) low-temperature dielectric barrier discharge plasma to successfully achieve ignition in pulse detonation engines. They systematically investigated the effects of discharge zone length, low-voltage electrode holes, and combustion kernel pressure on the ignition and initiation process. The results indicated that plasma ignition significantly shortened DDT time and broadened the equivalent ratio range of successfully triggering detonation; they realized multi-cycle detonation initiation using plasma ignition in pulse detonation engines.

Zhou et al. [146,147] investigated the effects of alternating current dielectric barrier discharge non-equilibrium plasma on the detonation initiation process of hydrogen-oxygen mixtures and the plasma-assisted initiation process under lean conditions. The results revealed that plasma ignition shortened the initiation time and distance under various conditions, with the degree of reduction decreasing at lower equivalence ratios. Additionally, a dual-zone quasi-direct current discharge plasma ignition scheme was proposed for a numerical study. It was demonstrated that dual-zone plasma ignition could shorten DDT time by 17.9% and DDT distance by 14.2%. Yang et al. [148] experimentally investigated the cracking of propane using sliding arc plasma and the detonation characteristics of propane-air mixtures under the action of plasma. It was found that adding a small amount of propane cracking gas to the propane/air detonation tube not only significantly improved ignition stability but also enhanced propane/air detonation performance.

The above research demonstrates that plasma ignition, with its advantages of high energy density, enhanced chemical activity, and precise control, plays an obvious role in improving the initiation performance of pulse detonation engines compared with traditional spark plug ignition. However, the detailed working mechanisms of plasma ignition are still unclear, and its engineering application is limited by challenges such as the structural design of plasma igniters, high energy consumption, electrode lifespan, and the miniaturization of plasma power supplies. In the future, it will be necessary to promote its application process through power supply innovation, material breakthroughs, and multi-modal synergistic design.

6. Other DDT Methods

In addition to hot jet ignition initiation, obstacle-induced detonation, shock wave focusing initiation, and plasma ignition initiation, scholars have proposed some other methods based on the above four to promote detonation initiation, including side wall cavities, converging obstacles, stratified concentration gradient, multi-point ignition, laser ignition, and a combination of solid and transverse jet obstacles.

Cai et al. [149] numerically investigated a detonation combustion process induced by hot jets in a cavity filled with supersonic combustible mixtures. The results suggested that the cavity could realize detonation initiation in the combustible mixtures using hot jets and played a significant role in the propagation of detonation in supersonic combustible mixtures. Ma et al. [150] analyzed the effects of perturbations on the DDT process by utilizing a cavity. It was found that there were three modes of cavity perturbation on deflagration acceleration: direct transition to detonation, secondary acceleration to detonation after a period of stable propagation or slight decrease in velocity, and failure to transition to detonation.

Bengoechea et al. [151,152,153] studied the detonation initiation process of a hydrogen-oxygen enriched air mixture in a pipe containing a convergent-divergent nozzle, revealing the essence of detonation wave initiation and optimizing the geometric structure through an adjoint approach. Li et al. [154] proposed a novel design concept that utilized the geometric effects of channels with varying diameters to enhance flame acceleration and detonation in micro-channels; they discovered that the converging spiral channels significantly promoted flame acceleration.

Grune et al. [155,156] studied the DDT process of stratified hydrogen-air mixtures in semi-confined flat layers. The results indicated that mixtures with a linear concentration gradient could significantly accelerate flame propagation, thereby quickly reaching a combustion state close to detonation; they determined the critical conditions for the maximum hydrogen concentration at the top of the channel. Saeid et al. [157] investigated the effect of fuel diffusion time on flame acceleration and ignition location in both homogeneous and stratified hydrogen-air mixtures by varying the duration of hydrogen injection in closed channels to generate different vertical concentration gradients. It was found that reducing the diffusion time and increasing the mixture’s inhomogeneity could accelerate flame propagation and shorten the DDT distance in the lean mixture.

The idea of using multiple ignition sources to trigger detonation and reduce the ignition energy was first proposed by Zel’dovich and Kompaneetz [158]. Its essence is to simulate the strong coupling between shock waves and chemical energy release, igniting and releasing the energy behind weak shock waves to enhance the shock waves and form detonation waves. Ciccarelli et al. [159] enhanced the acceleration of flames by using multi-point ignition to achieve the transition to detonation. The results suggested that the time and distance required for the flame to accelerate to the speed of sound were reduced by approximately 10%. Schild et al. [160] experimentally studied the initiation process of ethylene-air mixtures in a detonation tube with a cross-section of 50 square millimeters using multi-point ignition. It was demonstrated that the DDT time was reduced by 40% using four spark plugs for ignition compared to a single spark plug. Frolov et al. [161,162,163,164] established multiple sets of gas-phase and spray test systems to conduct extensive research on the number of igniters, fuels, detonation tube structures, and energy utilization efficiency during the multi-point ignition detonation process. The experiments confirmed that the multi-point ignition method was feasible in both gas-phase and spray two-phase systems. Wang et al. [11,165] conducted a numerical study on the mechanism and process of detonation wave initiation with two point ignitions. The study indicated that when a single ignition source could not initiate detonation waves, adding another ignition source with the same energy at a certain distance downstream and sequentially igniting at an appropriate ignition interval could initiate detonation waves. As the ignition energy decreased, the range of ignition time intervals capable of successfully triggering detonation waves narrowed while the time and distance for detonation increased.

Sato et al. [166,167] proposed a combination of laser-induced spark ignition and shock focusing to generate detonation waves in a shorter distance. It was discovered that the initiation occurred at the center line where the two combustion waves and incident shock wave collided through the schlieren visualization of the flow field within the elliptical and rectangular cavities.

Zhao et al. [168,169] investigated the mechanisms of flame acceleration and DDT triggered by a combination of solid and transverse jet obstacles; they analyzed the effects of jet position, multiple jets, jet start time, and jet stagnation pressure on the DDT process. The results showed that detonation flames were obtained by the transverse jet, although at a lower blockage ratio. Compared to solid obstacles, the combined obstacles improved the DDT time by 22.26% and reduced the DDT distance by 33.36%. Zhu et al. [170] numerically studied the effect of combining fluid and solid obstacles on flame acceleration and DDT in non-uniform concentration fields. The results revealed that the combination of fluids and solid obstacles could effectively reduce the initiation distance and time for DDT. Moreover, a more uniform distribution of hydrogen concentration led to quicker detonation initiation.

7. Conclusions

The transition from deflagration to detonation in pulse detonation engines is the core process for achieving efficient detonation combustion. Advances in detonation initiation methods have directly impacted the engineering progress of PDEs. This paper reviews DDT methods, including hot jet ignition initiation, obstacle-induced detonation, shock wave focusing detonation, and plasma ignition initiation, and summarizes their technical characteristics, research bottlenecks, and future prospects. Important conclusions from the present review are summarized as follows:

Hot jet ignition initiation: The research on detonation waves initiated by hot jet ignition mainly focuses on the optimization of jet parameters, combustion chamber configuration design, and new jet generation technologies. Its advantages lie in efficiently injecting energy through high-temperature and high-speed airflows, accurately controlling the spatiotemporal position of ignition, and enhancing turbulent mixing, thereby significantly shortening DDT distance. It exhibits strong adaptability to non-uniform mixing and dynamic incoming flow conditions. However, this method faces various challenges, such as the high energy transmission loss of jets, the ease of instability of flow fields due to coupling with the mainstream, the complex structure of high-temperature resistant jet sources, and difficulty in modeling the coupling of multiple physical fields. It is urgent to achieve engineering breakthroughs through low-power jet technologies, intelligent control strategies, and cross-scale simulation methods.

Obstacle-induced detonation: Obstacle-induced detonation is mainly divided into solid obstacle-induced detonation and fluidic obstacle-induced detonation. Among them, solid obstacle-induced detonation focuses on the optimization of geometric structures and the study of the initiation mechanism. It has the advantages of a simple structure and high reliability, and it can shorten the DDT distance by using passive turbulence enhancement and shock wave reflection. However, it faces many challenges, such as large flow resistance and energy loss, short structural life due to the accumulation of thermal stress, etc.

Fluidic obstacle-induced detonation mainly focuses on the study of jet parameters, the jet medium, and the initiation mechanism. This technology enhances mixing uniformity through active perturbation while effectively reducing pressure loss and exhibiting strong adaptability to working conditions. However, its high energy consumption requirements, jet-mainstream coupling instability, and the complexity of dynamic control remain the main bottlenecks for engineering applications. Future research needs to combine the advantages of both approaches and develop a strategy for solid-fluid cooperative obstacles, supplemented by precise dynamic jet control for the formation of shock waves in order to balance energy efficiency and controllability.

Shock wave focusing detonation: The study on shock wave focusing detonation primarily focuses on the optimization of the initiation structure, as well as initiation methods and mechanisms. Its advantage lies in triggering detonation waves through aerodynamic energy concentration, significantly shortening DDT distance and reducing flow resistance while possessing high precision controllability over the position and time of initiation. However, this method faces challenges such as structural thermal fatigue and ablation under high-frequency detonation, the instability of the shock path in unsteady incoming flows, strong sensitivity to geometric parameters and incident shock conditions, and high computational cost regarding multi-physical field coupling modeling. It is urgent to achieve a breakthrough in reliability and applicability through the innovation of thermal shock-resistant materials, real-time shock wave feedback control, and cross-scale experiment-simulation integration technology.

Plasma ignition initiation: The research of plasma ignition initiation mainly focuses on high-energy discharge technology, chemically active species generation mechanisms, electrode material, structure optimization, etc. It can directly trigger detonation through the instantaneous release of high energy density, significantly shorten DDT time, and accurately control initiation position and time, especially when adapting to the requirements of low-reactivity fuel and high-frequency initiation. However, it faces many challenges, such as high energy consumption, electrode ablation and lifespan limitations, drastic discharge efficiency reduction under high-pressure environments, and difficulties in modeling plasma-turbulence-combustion multi-physics coupling. In the future, it will be necessary to achieve breakthroughs in low-power pulse circuits, bio-inspired anti-erosion electrode materials, and cross-scale intelligent control technologies for engineering applications.

Inspirations and prospects: The research on the DDT method for pulse detonation engines has advanced from single-mechanism exploration to the phase of multi-physics collaborative optimization. Although there are differences in energy efficiency, controllability, and adaptability among the various initiation methods, their core objective remains to achieve the rapid triggering and stable propagation of detonation waves using the lowest energy consumption and the highest reliability. Future advancements require breakthroughs in material and structural innovation, intelligent optimization and cross-scale modeling, low-power and high-reliability technologies, interdisciplinary technological innovations, and engineering validation; these would be beneficial in promoting the practical application of detonation propulsion technology in the aerospace field and providing revolutionary power solutions for hypersonic vehicles and aerospace shuttle systems.