Special Issue: “Turbomachinery: Theory, Design and Application”

Turbomachinery is an essential part of the industrial field, and it is usually used for transporting fluids, extracting energy from flows, and cooling heated surfaces in many industrial sites and inside machines. In order to design and manufacture efficient turbomachines that are suitable for many industrial applications, many researchers so far have tried to develop theories, experimental techniques, and numerical methods for understanding the internal flow within turbomachines. This Special Issue of Applied Sciences, “Turbomachinery: Theory, Design and Application”, deals with all topics related to turbomachinery such as fans, compressors, pumps, turbines and wind turbines. In particular, thirteen papers published in this Special Issue focus on advancements in detailed internal flow analysis and methodologies for designing various fluid machines and devices, and they are summarized as follows.

Firstly, the authors of five papers tried to design efficient compressors and to understand the internal flow inside compressors. Yu et al. [1] applied the design of experiments (DOE) and numerical simulations to predict the aerodynamic performance of an axial compressor rotor blade profile by changing the profile locally at two chord positions. They finally established the surrogate model to predict the coupled influence of multiple deviations in the blade profile on the performance. In Borovkov et al.’s research [2], they designed a centrifugal compressor for an internal combustion engine turbocharger and compared the experimental data with the calculated performance of the use of both a one-dimensional algebraic model with several empirical coefficients and CFD simulations. They concluded that CFD simulations are not yet a reliable means of calculating all the characteristics of compressors in design and off-design operating conditions. Yang et al. [3] improved the throughflow calculation by applying Newton’s method to achieve robust and fast simulation results. It is well-known that throughflow calculations are very fast compared to full 3D CFD simulations and have often been used in the design process of turbomachinery. A method of enforcing the characteristic boundary condition based on Newton’s iteration method suggested by the authors improved the robustness of the throughflow calculation significantly for two centrifugal compressors with different rotating speeds. Han et al. [4] tried to improve the internal flow conditions and to achieve better performance of the radial inlet chamber for an industrial centrifugal compressor stage using numerical simulations. However, they found that the increases in the performance and overall flow uniformity of the radial inlet chamber do not mean a performance improvement in the downstream centrifugal compressor. Zamiri et al. [5] applied an inclined leading edge to the diffuser and analyzed its effects on the performance of and the noise induced by a transonic centrifugal compressor. Their numerical results showed that the hub-to-shroud inclined leading edge with a proper inclination angle effectively improves the compressor performance at the design point, increasing the stall margin and reducing the tonal blade passing frequency (BPF) noise.

Many researchers are still interested in several types of pumps due to their importance in industry. Zhou et al. [6] investigated the evolution behaviors of separated vortices and near-wall flow in an impeller of a centrifugal pump, based on unsteady flow simulations. They showed that both adverse pressure gradient and wall shear stress are two major causes for the boundary layer separation on the impeller surface, and the separated vortices reduce the performance of the impeller. Shamsuddeen et al. [7] designed a five-stage centrifugal pump for low-viscous, highly volatile and flammable chemicals and performed numerical simulations to analyze the performance of the pump at design and off-design operating points. Wang et al. [8] optimized an impeller of a mixed flow pump twice with and without considering the spanwise distribution of impeller exit circulation. They found that considering the spanwise distribution of impeller exit circulation can achieve higher performance in the final optimization results. Na et al. [9] analyzed the effects of process parameters, including the slurry particle diameter, rotation speed of the impeller, and gap between the impeller and the throat bush on the erosion wear of a throat bush of a slurry pump, caused by the friction between the solid and liquid particles. Based on their model, they suggested the optimized parameters to reduce the erosion rate density of the throat bush.

Beyond the scope of basic fluid machines, researchers have also submitted many papers on gas turbines, wind turbines, and torque converters. Baek and Ahn [10] investigated the effects of bulk flow pulsations on film cooling in gas turbine blades using both large eddy simulation (LES) and Reynolds-averaged Navier–Stokes simulation (RANS). Based on the comparison of the simulation results to the experimental data, they concluded that LES predicts the flow and cooling effectiveness in a film-cooling problem more accurately than RANS. Cican et al. [11] investigated the effects of a chevron nozzle on the acoustics and performances of a micro turbojet engine based on their experiments. They found based on their measurement in a micro turbojet engine test bench that the chevron nozzle is effective in an overall noise reduction at a high-speed regime but not effective at a low-speed regime, at the cost of the small penalty to engine performance. Hao et al. [12] established a multiple-degree-of-freedom finite element model of a wind turbine and analyzed the effect of the flexible tower vibration on the aerodynamic load of the wind turbine. Based on their model, they found that the aeroelastic load of the wind turbine increased gradually as the flexibility of the tower system increased. Ran et al. [13] analyzed the interaction between cavitation and vortex formation in a hydrodynamic torque converter using unsteady numerical simulations. They showed that the re-entrant jet driven by the reverse pressure gradient on the stator is a main factor in periodically inducing shedding cavitation.

As the guest editors of this Special Issue, we would like to sincerely express our gratitude to all the authors who submitted their valuable research results. The guest editors have no doubt that all the papers will be helpful to other researchers in designing efficient turbomachinery or understanding the underlying mechanisms related to the flow and structure of turbomachinery. Finally, we would especially like to sincerely express our gratitude to the Section Managing Editor, Ms. Michelle Wu, for organizing and helping the Special Issue of Applied Sciences. We are also thankful to all the reviewers for their valuable comments for improving the quality of papers published in this Special Issue. This valuable Special Issue is available at https://www.mdpi.com/journal/applsci/special_issues/Turbomachinery_Theory_Design_and_Application.