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Review

Application and Challenge of High-Speed Pumps with Low-Temperature Thermosensitive Fluids

by
Beile Zhang
1,
Ben Niu
1,
Ze Zhang
1,2,
Shuangtao Chen
1,2,
Rong Xue
1,2 and
Yu Hou
1,2,*
1
School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
2
MOE Key Laboratory of Cryogenic Technology and Equipment, Xi’an Jiaotong University, Xi’an 710049, China
*
Author to whom correspondence should be addressed.
Energies 2024, 17(15), 3732; https://doi.org/10.3390/en17153732
Submission received: 24 June 2024 / Revised: 20 July 2024 / Accepted: 26 July 2024 / Published: 29 July 2024
(This article belongs to the Special Issue Intelligent Phase Change Control and Thermal Management for Energy Applications)
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Abstract

:
The rapid development of industrial and information technology is driving the demand to improve the applicability and hydraulic performance of centrifugal pumps in various applications. Enhancing the rotational speed of pumps can simultaneously increase the head and reduce the impeller diameter, thereby reducing the pump size and weight and also improving pump efficiency. This paper reviews the current application status of high-speed pumps using low-temperature thermosensitive fluids, which have been applied in fields such as novel energy-saving cooling technologies, aerospace, chemical industries, and cryogenic engineering. Due to operational constraints and thermal effects, there are inherent challenges that still need to be addressed for high-speed pumps. Based on numerical simulation and experimental research for different working fluids, the results regarding cavitation within the inducer have been categorized and summarized. Improvements to cavitation models, the mechanism of unsteady cavity shedding, vortex generation and cavitation suppression, and the impact of cavitation on pump performance were examined. Subsequently, the thermal properties and cavitation thermal effects of low-temperature thermosensitive fluids were analyzed. In response to the application requirements of pump-driven two-phase cooling systems in data centers, a high-speed refrigerant pump employing hydrodynamic bearings has been proposed. Experimental results indicate that the prototype achieves a head of 56.5 m and an efficiency of 36.1% at design conditions (n = 7000 rpm, Q = 1.5 m3/h). The prototype features a variable frequency motor, allowing for a wider operational range, and has successfully passed both on/off and continuous operation tests. These findings provide valuable insights for improving the performance of high-speed refrigerant pumps in relevant applications.

1. Introduction

Centrifugal pumps are the most prevalent type of pump due to their stable output, compact structure, and high reliability. These characteristics make them widely used in many applications such as industrial, agricultural, military, and domestic fields [1,2]. To achieve the demands of complex operating environments and conditions, there is a growing need for centrifugal pumps to develop in the direction of miniaturization, having a high head and high efficiency, and being lightweight. This is particularly critical in military applications [3] and aerospace systems [4], where equipment must reliably operate under all or specific operating conditions.
By increasing the pump rotational speed under identical inlet conditions, it is possible to achieve a higher head and simultaneously reduce the impeller diameter, overall dimension, weight, and manufacturing costs. The specific speed, n q = n Q 0.5 / H 0.75 , is a dimensionless parameter that evaluates a pump’s performance across different design conditions, which depends on the flow rate Q, head H, and rotational speed n. When the flow rate and head are constant, increasing the rotational speed enhances the specific speed n q . For low-specific-speed centrifugal pumps, a higher specific speed number usually leads to better impeller efficiency. In multistage centrifugal pumps, increased speed can augment the pressure-boosting capacity of each stage, thereby minimizing interstage losses and enhancing pump efficiency [5].
As fluid machinery used for liquid transport or pressurization, centrifugal pumps are applicable not only for water and aqueous solutions [6,7,8,9,10,11,12] but also for low-temperature thermosensitive fluids such as refrigerants and cryogens. In fields such as new energy-saving cooling technologies, aeronautics and astronautics industries, chemical industries, and cryogenic engineering, centrifugal pumps designed for thermosensitive fluids play an essential role. Typical engineering application situations are shown in Figure 1. In most of these applications, there are high standards for pump performance and reliability. Increasing the pump rotational speed emerges as an effective strategy to fulfill these stringent requirements.
However, several engineering challenges must be addressed before high-speed pumps can be widely applied, especially as rotational speeds increase. One significant issue is cavitation, which occurs when the liquid pressure drops below the local saturation pressure. The rapid expansion of vaporized working fluid blocks the flow passages, significantly reducing the hydraulic performance of the equipment. The shock waves generated by collapsing vapor bubbles damage the impeller surface material and ultimately lead to pump failure or malfunction [13]. Additionally, flow-induced vibration and noise from cavitation exacerbate recirculation and vortices within the impeller, causing operational instability and increased energy losses [14]. Under the combined effects of these external working environments and operating conditions, the dynamic characteristics and energy efficiency of the hydraulic system are significantly impacted [15].
The cavitation phenomena and their impacts on pumps differ with various fluid media [16,17]. For instance, heat transfer during cavity growth in room-temperature water is negligible; thus, it is considered as an isothermal fluid. In contrast, the physical properties of thermosensitive fluids such as high-temperature water, Freon refrigerants, and cryogens can change greatly depending on temperature. The temperature drop in the fluid phase transition region leads to a decrease in the saturation pressure, which suppresses the phase transition process. This characteristic is known as the thermal effect [18]. When pumping low-temperature thermosensitive fluids, the inlet pressure of pumps is often near the saturation pressure. In practical applications, an inducer is often used to improve the flow state within the impeller, thus enhancing the pump performance.
Although high-speed centrifugal pumps using water as a medium have been widely applied, with extensive and in-depth studies on water cavitation, the application of high-speed centrifugal pumps for low-temperature thermosensitive fluids still requires further technical breakthroughs due to differences in thermal properties and pump operating conditions. Research on cavitating flow for refrigerants is also limited. Thus, establishing a deep and comprehensive understanding of the cavitation characteristics and mechanisms of low-temperature thermosensitive fluids is crucial. It is necessary to combine experimental research and numerical simulations to establish a deep and comprehensive understanding of the cavitation characteristics and mechanisms of low-temperature thermosensitive fluids.
This paper reviews the current applications and challenges of high-speed pumps using low-temperature thermosensitive fluids. It proposes a high-speed refrigerant pump solution with hydrodynamic bearings for data center cooling systems, providing guidance for the further development and application of high-speed centrifugal pumps.

2. Applications Status of High-Speed Pumps

2.1. Cryogenic Liquid

Cryogenic pumps often have stringent requirements for weight and volume, and in certain conditions, the flow rate is also very low. Although positive displacement pumps have the advantage of providing a high head at low flow rates, they are complex in structure and less suitable for cryogenic applications. High-speed centrifugal pumps can better adapt to cryogenic working conditions.
Currently, cryogenic pump products are available from only limited global commercial companies, such as Nikkiso (Tokyo, Japan), Cryostar (Hésingue, France), EBARA (Tokyo, Japan), Barber-Nichols (Arvada, CO, USA), etc. These products are primarily designed for large-scale cryogenic systems and come at a high cost [19]. In the realm of basic physics research, cryogenic pumps are commonly employed to transport various cryogenic fluids, including liquid helium, liquid argon, and liquid xenon, as Figure 2 shows. The pumping of these liquid cryogens reduces the temperature to the required experimental cryogenic environment, facilitating the cooling of superconducting coils for applications such as particle detection and control research [20,21,22,23,24,25]. The applications of cryogenic pumps include the use of liquid helium pumps in CERN’s ATLAS Detector, pumping liquid xenon in a gamma-ray detector utilizing liquid Xe as a scintillation material and cooling neutrons using cryogenic fluid circulation systems in the Spallation Neutron Source laboratory, etc.
In industrial production and energy development, the use of liquid cryogens is integral to processes such as air liquefaction and separation, oil well development, and liquefied natural gas (LNG) storage and transportation [26,27,28,29,30,31]. To facilitate equipment installation, maintenance, and the extraction of cryogens, the pumps employed in these fields are typically submersible pumps, as shown in Figure 3. Moreover, during the design and selection process of these pumps, it is essential to consider factors such as efficiency, leakage, and reliability to ensure economic and safe operation.
Furthermore, cryogenic pumps play a pivotal role in wind-tunnel testing for aerospace applications. Wind tunnels facilitate various aerodynamic experiments based on the principles of relativity and similarity. To increase the Reynolds number within wind tunnels, strategies such as enlarging tunnel dimensions, elevating operating pressure, and reducing the airflow temperature can be employed. Consequently, liquid nitrogen (LN2) and cryogenic nitrogen are used as simulated flow media in cryogenic wind tunnels, requiring LN2 pumps that meet high-flow-rate pumping demands [32,33].
The development and promotion of hydrogen energy also necessitate the use of cryogenic pumps for the long-distance transportation and storage of liquid hydrogen (LH2). These pumps must ensure the continuous and stable delivery of the fluid and handle long-distance, low-flow-rate transportation [34,35]. Another significant application of high-speed cryogenic pumps is within the aerospace industry [36,37,38,39]. Modern liquid rocket engines are advancing toward a high specific impulse and environmental sustainability, typically utilizing cryogenic propellants such as LH2 and liquid oxygen (LOX). Turbopumps are employed to supply these cryogenic propellants to combustion chambers operating under extremely high pressure. Unlike closed-loop cryogenic systems, turbopumps are characterized by high flow rates and very high rotational speeds, as illustrated in Figure 4. The design of the impeller profile, internal flow-field distribution, rotor stress distribution, and pump performance are critical factors in these applications.

2.2. Liquid Refrigerant

The procurement and storage of liquid cryogenic fluids are challenging and expensive, making the setup and operation of cryogenic test rigs costly and experimentally demanding. As illustrated in Table 1, the physical properties of refrigerants closely resemble those of cryogens. There are substantial differences between the physical properties of room-temperature water and cryogens, particularly concerning the saturation pressure change with the temperature increase d p v / d T , liquid/vapor density ratio ρ l / ρ v , latent heat of vaporization c p , thermal conductivity k , and viscosity ν . Consequently, some researchers use R114 refrigerant as a surrogate for cryogens, applying similarity laws to investigate the internal flow and hydraulic performance of refrigerants in centrifugal pumps [18].
With the rapid advancement of electronic information technology and increased focus on energy conservation, devices are becoming smaller and more powerful, leading to higher heat fluxes [41]. Pump-driven two-phase flow systems have been increasingly utilized for cooling high-heat-flux electronic devices and data centers due to their superior heat transfer capabilities and energy efficiency. The primary working fluids in these systems include fluorocarbons such as R142b, R22, R32, and R410A [42,43,44,45]. Our research team has previously developed a high-speed pump utilizing R134a refrigerant [46]. This single-stage pipeline pump is suitable for high-heat-flux electronic thermal management systems, which is shown in Figure 5. During operation, the refrigerant upstream of the pump is in a low-temperature and low-pressure state. After being pressurized by the refrigerant pump, it enters the cooling equipment. The net positive suction head (NPSH) is a critical factor influencing the pump performance.

3. Current Challenges

3.1. Cavitation in Inducer

Cavitation generally occurs at the inlet of centrifugal pumps and is more likely to occur at high rotational speeds [47]. Cavitation not only damages the blade material, thereby reducing the operational lifespan of pumps, but also induces pressure pulsations that lead to vibrations. These vibrations can jeopardize the safe and stable operation of aerospace engines or petrochemical systems or cryogenic systems. Additionally, high-frequency noise (above 500 Hz) and a reduction in pump capacity are observed. In rocket engines, high-speed turbopumps are subject to size constraints, with impellers rotating at extremely high speeds, resulting in severe cavitation of the propellant. Numerous rocket launch failures have been attributed to the instability caused by turbopump cavitation. In industrial systems, cavitation in cryogenic fluids not only reduces production line efficiency but also impacts the resonance of transport pipelines.
For high-speed centrifugal pumps, radial impellers offer significant pressure-boosting capabilities but are highly sensitive to cavitation, making them susceptible to breakdown. Presetting an axial-flow inducer is an effective solution to the cavitation problem. While the pressure-boosting capability of an inducer is lower than that of a radial impeller, it provides superior cavitation resistance. Extensive studies have been conducted on the cavitation behavior of low-temperature thermosensitive fluids within inducers, primarily using safe and readily available LN2.
Ito et al. [48] introduced a bubble size distribution model into numerical simulations to investigate the cavitation characteristics of LN2, focusing on bubble development and thermal effects. Numerical results indicated that the rotational speed of the backflow vortex is consistently lower than that of the impeller and is concentrated near the inducer axis, with bubbles of different sizes exhibiting relatively fixed positional distributions. Chen et al. [49] examined the mechanism of tip leakage vortex cavitation and the associated cavity structure in inducers. Their numerical results demonstrated that the periodic coupling of cavity development, flow rate, and local incidence angle variations led to the instability of tip leakage vortex cavitation. The axial vorticity distribution of the tip leakage vortex and the cavity structure are shown in Figure 6. Zhang et al. [50] evaluated the prediction accuracy of four cavitation models—full cavitation, Kunz, Schnerr–Sauer, and Zwart–Gerber–Belamri models—using Ito et al.’s [51] experimentally captured images of LN2 cavitation as a reference. The full cavitation model provided the most accurate numerical predictions without modifying computational parameters, whereas the other models achieved satisfactory predictions through empirical parameter adjustments. This demonstrates that by improving computational models and validating them against experimental results, CFD numerical simulations can achieve high computational accuracy, providing reliable guidance for analyzing hydrodynamic phenomena in hydraulic devices [52].
In addition to LN2, cavitation in cryogenic propellants such as LH2 and LOX used in liquid rockets has also received considerable attention and research. In an experimental study, Lettieri et al. [40] conducted visual studies on the cavitation instability of the inducer in rocket engine turbopumps. Optical experiments demonstrated that the generation of rotating cavitation is related to the interaction between cavities at the leading edges of adjacent blades. Changes in the incidence angle exacerbate cavitation, leading to the apparent supersynchronous rotation of the cavities around the annulus. Shao and Zhao [34,53] developed a LH2 centrifugal pump to meet the demand for low-flow-rate and high-head applications. They conducted hydraulic performance tests on a prototype designed using LN2 as the working fluid, employing similarity laws. The pump performance met the design expectations, with simulation results closely matching experimental data. Based on this, they optimized the impeller design using genetic algorithms. Cavitation characteristics can be effectively enhanced by decreasing the inlet angle, enlarging the impeller diameter, or incorporating an inducer. Additionally, slightly reducing the impeller’s outlet diameter can decrease hydraulic losses, thereby increasing the head. The performance characteristics are shown in Figure 7. Kim et al. [54,55] used water at 310–323 K and LOX as working fluids to study cavitation instability by measuring the vibration of the pump casing. Under low-flow-rate conditions, both fluids exhibited supersynchronous rotating cavitation. In the LOX experiments, asymmetric cavitation was predominant, and its cavitation instability at high-flow-rate conditions was lower than that of hot water. Shimura et al. [56] conducted experimental studies on the vibrations caused by cavitation in the inducer of a turbopump and modified the inducer blades to suppress rotating cavitation and mitigate vibration effects.
Tani et al. [11] studied the relationship between rotating cavitation and the flow coefficient in the oxidizer inducer of a turbopump. Numerical simulations discussed the relationship between tip vortices and inducer blades, while unsteady simulations revealed that tip vortices were not the primary factor driving cavitation. Instead, negative flow divergence caused by bubble collapse affected the flow angle, leading to tip gap recirculation and exacerbating cavitation. Kimura et al. [57] investigated the tip leakage vortex structure and the rotating cavitation it induced. Their study found that the development of the vortex structure is strongly influenced by the geometry of the inlet casing and flow rate. Adding a gutter to the inlet casing effectively suppressed cavitation and recirculation, as shown in Figure 8.
Research on cavitation in inducers for LNG has also been extensive. Son et al. [28] developed a pump for LNG transportation systems, investigating its hydraulic performance and NPSH and addressing cavitation and surge issues. Their study revealed that the pressure loss generated within the inducer is not recovered in the impeller and recirculation channels, and this pressure loss further increases. Vortices generated at the trailing edge of the blades negatively affect the operation of the impeller and the hydraulic performance of the pump. Li et al. [27] performed both numerical and experimental studies on the cavitation behavior and pressure pulsation of the impeller in a two-stage LNG submersible pump. Their research found that as the flow rate increases, cavitation becomes more severe and periodic, with a greater impact on the first-stage impeller. The low-frequency pressure pulsation frequency of the impeller gradually becomes dominant. Karakas et al. [58] also performed numerical and experimental studies on the effects of inducer tip clearance on the cavitation characteristics and hydraulic performance of an LNG submersible pump. The results indicated that wider inducer tip clearances lead to back leakage and more severe vortex recirculation, resulting in local pressure drops and cavitation. Moreover, the impact of tip clearance on cavitation is more pronounced in variable pitch inducers.

3.2. Thermal Effect

The temperature reduction induced by thermal effects is a characteristic of cavitation in thermosensitive fluids, which complicates the cavitation behavior and has been a significant area of research in low-temperature thermosensitive fluid cavitation studies. Thermal cavitation not only suppresses the formation and growth of cavities but also impacts the cavitation dynamics of thermosensitive fluids [59,60].
Researchers have extensively studied the cavitation dynamics in simple cavity-generating devices such as hydrofoils, Venturi tubes, and ogives [61,62,63]. Chen et al. [64] developed a hydrofoil cavitating flow model using fluoroketone as the working fluid and discovered that the thermodynamic cavitation effects of room-temperature fluoroketone resemble those of LN2 at cryogenic temperatures. Their findings identified a transition temperature at which the dominant factor influencing cavitation shifts from the liquid/vapor density ratio to thermal effects. Zhang et al. [65] carried out experimental studies on the cavitation characteristics of R134a refrigerant in a Venturi tube and observed that the formation, shedding, and collapse of cavities become more frequent and complex under the combined influence of reentrant jet flow and thermal effects.
For cryogens, Chen et al. [66] examined the influence of thermal effects on the cavitating flow of LN2 in a hydrofoil, discussing how thermal effects specifically impact the mass transfer processes and cavity structures in cavitating flows. They found that the phase transition of cryogenic liquids causes changes in fluid properties, which in turn alter the reference free-stream conditions equivalently. Long et al. [67] performed numerical studies on the shedding process of LH2 cavitation on an ogive surface under thermal effects, clarifying the interaction between cavitation and vortices in unsteady thermal cavitating flows. Chen et al. [68,69] investigated the thermal transition process and transition temperature in the evolution of unsteady cavitating flows of LN2, identifying two modes of thermal cavitation dynamics, termed the inertial mode and thermal mode, as shown in Figure 9. As the temperature rises, the shedding frequency and the number of simultaneously shedding cavities increase consistently, while the characteristic frequency of individual shedding processes and quasi-periodic features first decreases and then increases. Zhu et al. [70,71] visualized the cavitating flow of LN2 in a Venturi tube and established a one-dimensional theoretical equation considering thermal effects to estimate the speed of the condensation front, applying a two-dimensional thermal effect parameter to quantify the intensity of cavitation thermal effects. Murakami and Harada [72] utilized Particle Image Velocimetry to investigate the cavitating flow of superfluid helium, focusing on the effect of the void fraction in the Venturi tube on cavitation thermal effects. Niiyama et al. [73] studied the influence of turbulence around cavities on cavitation thermal effects based on LN2 cavitation experiments in an orifice, finding that heat transfer is enhanced during cavitation. Liang et al. [74] demonstrated that the thermal cavitation mode transition of LH2 is influenced by both thermal effects and Reynolds number, with the entropy production rate increasing with temperature according to the entropy transport equation.
As a complex three-dimensional hydraulic structure, the study of cavitation behavior within an inducer holds significant practical value. Extensive research on the thermal effects of cavitation within inducers has been conducted through a combination of experimental and simulation approaches. Yoshida et al. [75,76,77] studied the thermal effects of LN2 on synchronous rotating cavitation within an inducer, examining characteristics such as the cavity length at the blade tip, temperature, and fluid forces. They found that, as the cavity length increased, the thermal effects intensified. The growth of cavity non-uniformity due to synchronous rotating cavitation was identified as a primary cause of shaft vibration. Ito et al. [51,78] established the first visualization experimental setup suitable for studying an inducer with both LN2 and water, comparing the unsteady cavitation characteristics of LN2 to those of water, as shown in Figure 10. They elucidated the cavitation developments of backflow vortices and tip vortices as well as the thermal effects of cryogen. The experimental results indicated that backflow vortices in LN2 cavitation are more intense, and tip vortices are smaller compared to those in water.
Building on this visualization experiment, Fan et al. [79] set a cavitation model to predict the performance of the LN2 inducer. Their numerical study revealed that, in the cavitation region, as temperature increased, the temperature drop within the cavities became significant and the cavity volume decreased, contributing to a delay in the head variation. Additionally, an increased rotational speed affected the cavity volume, exacerbating cavitation. They also analyzed sub-synchronous rotating cavitation phenomena in an inducer in terms of a temperature drop, thermal effects, and bubble volume changes. Chen et al. [80] utilized an enhanced cavitation model that includes thermal effects to investigate how cavitation and tip leakage vortices influence hydraulic losses in an LN2 inducer. They found that the tip leakage flow exhibited a helical distribution, with its shape negatively correlated with the nitrogen temperature. Thermal effects partially suppressed the development of tip leakage vortices. Enstrophy analysis revealed energy losses within tip leakage vortices, identifying these vortices as key parameters influencing thermal cavitation. Wei et al. [81] investigated cavitating flow characteristics in a LN2 submersible pump. They improved the cavitation model by incorporating corrections for rotation and thermal effects and validated their numerical framework by comparing it with transient cavitation images captured by Ito et al. [51]. Their study indicated that cavitation within the inducer was primarily caused by vortices at the tip clearance, with the evolution of cavities shown in Figure 11. Using vortex identification methods and the vorticity transport equation, they identified vortex structures, analyzed the interaction between cavitation and vortices, and assessed the impact of thermal effects on cavitation.
Blumenthal and Kelecy [82] incorporated the thermal properties of LH2 into a cavitation model and utilized CFD methods to predict the cavitating flow state of the NASA TM X-1360 inducer with LH2. The revised numerical model provided the most accurate predictions at 23.3 K. Goncalves et al. [83] conducted numerical investigations into the cavitating flow characteristics of LH2 and LOX within the inducer of a NASA rocket engine turbopump. Their computational model accounted for the impact of the thermal properties of cryogens on the flow characteristics, revealing that, at high rotational speeds, the wall viscosity effect and vaporization, respectively, led to heating and cooling effects within the pump.
Xiang et al. [37,84] analyzed the internal cavitating flow characteristics of an inducer using LOX as the working fluid based on a transport-based cryogen cavitation model. Their findings indicated that thermal effects significantly reduced the cavitation region and bubble formation, thereby delaying the head breakdown, as illustrated in Figure 12. Higher temperatures exhibited stronger thermal effects and improved cavitation performance. Shi et al. [85] explored the thermal cavitation characteristics of LOX, discovering that, as the temperature was raised, the thermal effects became more pronounced, effectively suppressing cavitation phenomena in steady-state cavitating flows within the inducer.
As more commonly used thermosensitive fluids, refrigerants have practical significance for engineering applications in the study of their cavitation thermal properties. Holl et al. [86] developed a correlation for temperature depression based on temperature and pressure from cavitation experiments with water and Freon R113 to describe cavitation intensity. Franc et al. [18,87] performed visualization experiments on the cavitating flow of R114 refrigerant within an inducer, exploring how temperature affects cavitation development, as shown in Figure 13. They estimated the temperature drop around the cavitation region based on the cavity length, finding that the temperature drop increased with an increasing cavity length. Additionally, they established a model for analyzing the thermal effects of cavitation in inducers. Fang et al. [46] employed an improved Sauer–Schnerr cavitation model to investigate the impact of variables such as NPSH, temperature, and flow rate on the internal flow characteristics and cavitating flow properties of a centrifugal pump with an inducer.
The cavitating flow of low-temperature thermosensitive fluids within the inducer is highly unsteady, with its evolution process accompanied by dramatic changes in physical quantities such as temperature, pressure, and velocity. Due to the suppression of thermal effects, the interaction mechanism between the cavitating flow and turbulence, as well as vortex structures in the flow field, becomes more complex. Therefore, conducting experimental studies on cavitation in the inducer with low-temperature thermosensitive fluids is valuable for revealing the mechanisms of cavitating flow.
The importance of synchronously collecting multiple physical parameters, such as pressure, temperature, and frequency, and using high-speed photography and Particle Image Velocimetry (PIV) technology to study cavitating flow is increasing. However, when the working fluid temperature is below room temperature, especially for cryogenic fluids, setting up cavitation visualization testing equipment is highly challenging due to limitations in adiabatic conditions, frosting on visual windows, material brittleness, and high-speed rotating mechanical processing techniques.
Numerical simulation has become an indispensable research method for solving complex multiphase cavitating flow problems. Extensive and in-depth research on the calibration and improvement of cavitation models and turbulence models can accurately present and predict the cavitating flow of low-temperature thermosensitive fluids within the inducer. Additionally, the suppression of cavity evolution by thermal effects has also received attention. Furthermore, the use of data-driven methods to perform the modal decomposition of cavitating flow and obtain coherent structures of cavitating flow is also worthy of attention.

4. Prospects and Solutions

4.1. Development of a Low-Specific-Speed Centrifugal Pump with an Inducer

In large systems, high-flow-rate cryogenic pumps have been successfully implemented. However, for small systems, due to factors such as the flow resistance of the pipeline and components, pumps need to have characteristics that can handle low flow rates and high pumping pressures, which is indicative of a lower specific speed. Based on the summary of current applications and challenges, the development of low-specific-speed pumps with high performance metrics will be the future trend and direction for centrifugal pumps utilizing thermosensitive fluids.
The development of low-specific-speed pumps will focus on the following directions:
  • Enhancing cavitation resistance: Incorporating an inducer at the pump inlet to enhance the NPSH, thereby mitigating the effects of high rotational speeds and complex operating conditions on pump performance. The design of the inducer should account for the impacts of cavitation dynamics and thermal effects.
  • Optimizing impeller design: optimizing impeller profile design methods to improve hydraulic performance, addressing issues of high energy loss and low efficiency at ultra-low specific speeds.
  • Flow-induced noise and vibrations: attention should be given to flow-induced noise and bearing vibrations during high-speed operation, as the performance of high-speed bearings directly influences the operational lifespan of the pump.
  • Compact and maintenance-friendly design: the pump design should be more compact and easier to maintain, with efforts to reduce the weight and volume while also lowering manufacturing and operational maintenance costs.

4.2. Application of a Centrifugal Pump with Hydrodynamic Bearings

4.2.1. The Structural Design of the Prototype

Building on the previous applications review, naturally cooled pump-driven two-phase flow systems have significant development potential for data center cooling. Considering the actual working conditions of vapor-compression refrigeration systems, we propose a high-speed centrifugal pump with hydrodynamic bearings, utilizing R410A refrigerant as the working fluid. The design operating conditions are a flow rate of 1.5 m3/h and a designed rotational speed of 7000 rpm. To ensure hydraulic efficiency, a two-stage impeller configuration is employed, with specific speeds of n q 1 = 15 and n q 2 = 11 for each stage, respectively. The primary components of the high-speed centrifugal pump, as shown in Figure 14, include the inducer, primary impeller and vaned diffuser, secondary impeller and vaned diffuser, discharge volute, hydrodynamic bearing, shaft, motor, and casing. Considering the weight and strength of the prototype, the pump casing, vaned diffuser, and discharge volute are made of aluminum alloy, while the shaft, impeller, hydrodynamic bearings, and bearing housing are made of stainless steel.
To achieve the high-speed rotation of the pump, herringbone-grooved radial hydrodynamic bearings and spiral-grooved thrust hydrodynamic bearings are employed as support components, using R410A refrigerant as the lubricating medium. The hydrodynamic effect of the liquid maintains the high-speed operation of the motor. Since the bearing rotating surface does not come into direct contact with the bearing housing, the friction loss and wear of the hydrodynamic bearings are minimal, resulting in smooth operation, high reliability, and low noise [88].
To further integrate the pump structure, the motor is housed within the casing, eliminating the need for couplings and other transmission components. The high-speed centrifugal pump is designed for a pipeline configuration. The working fluid is initially drawn in from the inlet to the inducer for pre-pressurization; then it enters the primary impeller and vaned diffuser, flows through the motor casing to the secondary impeller and vaned diffuser, and is finally discharged through the volute. The design parameters of the hydraulic components are listed in Table 2.

4.2.2. Research Methods

As previously reviewed, CFD numerical calculations can shorten the design time and reduce manufacturing costs for the development of new pump models. The development process of the centrifugal pump with a hydrodynamic bearing combined with CFD numerical performance predictions and experimental testing for validation. First, the profiles of the main hydraulic components, such as the impeller, were designed. Suitable numerical models were selected to predict the pump performance. Subsequently, tests were conducted on a refrigerant pump external characteristic test rig, with experimental results guiding the pump design and numerical simulation outcomes.
Pressure and temperature measurement points were placed at both the inlet and outlet of the prototype. A flow meter and flow valve were installed downstream of the pump, while the inflow pressure at the pump upstream could be adjusted by changing the height of the refrigerant storage tank. The refrigerant pump test rig allowed the determination of key parameters such as the head and power under different flow rates, speeds, and NPSH conditions. These parameters were recorded using a data acquisition system to infer the operating status of the pump. Additionally, an automatic control system enabled start/stop testing and long-term operational testing of the prototype.

4.2.3. Test Results of Prototype

Tests were conducted on the constructed refrigerant pump external characteristic test rig. The prototype had an inlet temperature of 10 °C and an NPSH of 3 m. The hydraulic performance test results of the high-speed centrifugal pump at the rated speed are shown in Figure 15. Under the design conditions, the pump achieves a head of 56.5 m with an efficiency of 36.1%. It can be observed that, when the prototype operates at the rated speed, the head gradually decreases with the increasing flow rate, without exhibiting a hump. The power increases at a relatively gentle rate with the increasing flow rate. The flow rate–efficiency curve reaches a peak and then decreases as the flow rate increases, indicating an optimal flow rate at which the prototype achieves maximum efficiency. The maximum efficiency point (BEP) of the centrifugal pump at the rated speed is 37.9%, with a corresponding flow rate of 2.1 m3/h and a head of 47.5 m. The trend of the hydraulic characteristics of the prototype obtained from the experiments was consistent with the hydraulic characteristics of pumps with similar specific speeds studied in Ref. [89]. The process of pumping liquid refrigerant was continuous and stable, meeting design expectations. Compared to an earlier single-stage high-speed centrifugal pump developed by the research team that used water as the working fluid, the prototype achieved higher efficiency at a lower designed specific speed [12].
The designed high-speed pump can adapt to a broader range of applications through variable frequency motor control. According to similarity laws, pumps operating under approximately similar conditions exhibit nearly equal efficiencies. Using the BEP at the design speed as a baseline, predictions for similar conditions at speeds ranging from 4000 to 6000 rpm were made and compared with measured data, as shown in Figure 16. The experimentally measured efficiencies at 4000 rpm, 5000 rpm, and 6000 rpm were 36.7%, 37.8%, and 37.4%, respectively. It can be seen that the deviation between the theoretical predictions and the experimental results for the head and power increases as the rotational speed decreases, with a maximum deviation of 2.4%. This demonstrates that similarity laws provide accurate predictions of the hydraulic performance of centrifugal pumps under various operating conditions.
Additionally, the reliability and service life of the hydrodynamic bearings in the centrifugal pump were empirically evaluated. The pump successfully underwent over 100,000 on/off cycles and more than 150 days of continuous overload operation testing within a data center cooling system. After the tests, the degradation in the external characteristics of the high-speed centrifugal pump was less than 5%. This prototype demonstrates substantial application potential in data center cooling systems and novel electronic device cooling technologies.

5. Conclusions

This paper summarizes the current applications of high-speed pumps using low-temperature thermosensitive fluids and reviews the challenges encountered in their application. These challenges include the mechanisms of cavitation inception, development, and collapse within the inducer as well as the impact of fluid thermal properties and thermal effects on cavitating flow. Understanding these aspects is crucial for studying the performance of high-speed pumps and guiding their design. Based on industry development trends, a high-speed centrifugal refrigerant pump incorporating hydrodynamic bearings is proposed, with an overview of its performance and potential applications. The key conclusions drawn from this review and discussion are as follows:
(1)
Cavitation is the most common issue encountered in the pumping process of high-speed pumps. An inducer can enhance the cavitation resistance of pumps, significantly improving their hydraulic performance and stability. The accurate prediction of the cavitation intensity in an inducer using improved cavitation models is crucial for the effective design and performance optimization of high-speed pumps.
(2)
Compared to cavitation studies with room-temperature water, cavitation experiments with low-temperature thermosensitive fluids are relatively few due to the complexity and difficulty of setting up the testing systems. Capturing cavity flow behavior through high-speed photography has become one of the important research methods for exploring cavitation characteristics and mechanisms. However, limited by current technological conditions, conducting cryogenic cavitation flow experiments in inducers is extremely challenging.
(3)
Thermal effects have become the focus of research on cavitation dynamics of thermosensitive fluids. The local temperature drop caused by thermal effects suppresses the growth and collapse of cavities. Affected by the thermal effects, the cavitating flow characteristics in hydraulic components such as hydrofoils, Venturi tubes, and inducers have been extensively studied and demonstrated.
(4)
A two-stage high-speed refrigerant pump with an inducer and hydrodynamic bearings is proposed. Under design conditions (n = 7000 rpm, Q = 1.5 m3/h), the pump achieves a head of 56.5 m with an efficiency of 36.1%. The stability and reliability of the prototype have been thoroughly validated, indicating significant application potential in data center cooling systems and novel electronic device cooling technologies.

Author Contributions

Writing—original draft preparation, B.Z.; investigation, B.Z. and B.N.; data curation, B.N.; validation, Z.Z.; formal analysis, B.Z. and Z.Z.; writing—review and editing, S.C. and Y.H.; resources, S.C.; funding acquisition, R.X.; supervision, Y.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Project No. 52276017) and the Youth Innovation Team of Shaanxi Universities.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Typical applications of centrifugal pumps with low-temperature thermosensitive fluids.
Figure 1. Typical applications of centrifugal pumps with low-temperature thermosensitive fluids.
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Figure 2. Centrifugal pump with liquid xenon [20].
Figure 2. Centrifugal pump with liquid xenon [20].
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Figure 3. Schematic diagram of LNG submersible pump [26].
Figure 3. Schematic diagram of LNG submersible pump [26].
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Figure 4. Liquid rocket engine turbopump with inducer [40].
Figure 4. Liquid rocket engine turbopump with inducer [40].
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Figure 5. Pump-driven two-phase cooling loop using high-speed R134a refrigerant pump.
Figure 5. Pump-driven two-phase cooling loop using high-speed R134a refrigerant pump.
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Figure 6. Axial vorticity distribution and cavity evolution of tip leakage vortex in inducer [49].
Figure 6. Axial vorticity distribution and cavity evolution of tip leakage vortex in inducer [49].
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Figure 7. Effect of the inducer on the hydraulic and cavitation performance of the LH2 pump: (a) frequency–head curve, (b) flow rate–head curve, (c) flow rate–efficiency curve, and (d) NPSH–head curve [34].
Figure 7. Effect of the inducer on the hydraulic and cavitation performance of the LH2 pump: (a) frequency–head curve, (b) flow rate–head curve, (c) flow rate–efficiency curve, and (d) NPSH–head curve [34].
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Figure 8. Suppression of backflow cavitation in the inducer by gutter at the inlet casing [57]. The distribution of the axial velocity component is plotted on the plane, which includes the rotation axis by a gray-scale contour map. The brightest area represents the backflow region. Red lines and the colored inducer indicate the vortex cores and pressure, respectively.
Figure 8. Suppression of backflow cavitation in the inducer by gutter at the inlet casing [57]. The distribution of the axial velocity component is plotted on the plane, which includes the rotation axis by a gray-scale contour map. The brightest area represents the backflow region. Red lines and the colored inducer indicate the vortex cores and pressure, respectively.
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Figure 9. Cavity shedding of LN2 cavitation under different thermal cavitation modes. Within the gray-shaded cavities, the curves with blue and red arrows represent the direction of cavity rotation. The green straight line with arrows represents the movement of the detached cavity. The numbers indicate the cavity shedding process [69].
Figure 9. Cavity shedding of LN2 cavitation under different thermal cavitation modes. Within the gray-shaded cavities, the curves with blue and red arrows represent the direction of cavity rotation. The green straight line with arrows represents the movement of the detached cavity. The numbers indicate the cavity shedding process [69].
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Figure 10. Tip vortex and backflow vortex cavitation occurring on the inducer with thermosensitive fluid LN2 vs. room-temperature water. The arrows indicate the locations where tip vortex and backflow vortex occur [48].
Figure 10. Tip vortex and backflow vortex cavitation occurring on the inducer with thermosensitive fluid LN2 vs. room-temperature water. The arrows indicate the locations where tip vortex and backflow vortex occur [48].
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Figure 11. The cavity evolution of LN2 cavitation, considering thermal effects [81].
Figure 11. The cavity evolution of LN2 cavitation, considering thermal effects [81].
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Figure 12. The impact of the thermal effect on the hydraulic performance of pump [84].
Figure 12. The impact of the thermal effect on the hydraulic performance of pump [84].
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Figure 13. The impact of temperature on the R114 refrigerant cavitation [18].
Figure 13. The impact of temperature on the R114 refrigerant cavitation [18].
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Figure 14. Major components of two-stage centrifugal refrigerant pump. The arrows indicate the direction of fluid flow.
Figure 14. Major components of two-stage centrifugal refrigerant pump. The arrows indicate the direction of fluid flow.
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Figure 15. Experimentally obtained external characteristic curve of the prototype.
Figure 15. Experimentally obtained external characteristic curve of the prototype.
Energies 17 03732 g015
Energies 17 03732 g016
Figure 16. The theoretical and experimental values of the BEP at different rotational speeds.
Figure 16. The theoretical and experimental values of the BEP at different rotational speeds.
Energies 17 03732 g016
Table 1. Thermal properties of thermosensitive fluids vs. room-temperature water.
Table 1. Thermal properties of thermosensitive fluids vs. room-temperature water.
ParametersWaterLN2LH2R114R134aR410A
T/K2837723283283283
d p v / d T / Pa ⋅ K − 1 8412,10853,690470013,98032,500
ρ l / ρ v 107,286.1182.026.7154.062.727.5
c p / k j ⋅ k g − 1 2477.5199.6431.2133.7190.9/
k / m W ⋅ m − 1 ⋅ K − 1 578.4145.5103.464.387.797.5
ν / μ P a ⋅ s 1311.7162.911.0325.8235.3143.0
Table 2. Main parameters of hydraulic components.
Table 2. Main parameters of hydraulic components.
Design ParameterValue
InducerNumber of blades3
Inducer diameter/mm21.4
Hub diameter/mm12
Tip clearance/mm0.3
Primary impellerNumber of blades6
Inlet diameter/mm22
Outlet blade angle/°28
Outlet width/mm3
Outlet diameter/mm52
Primary vaned diffuserNumber of blades8
Outlet blade angle/°12
Outlet diameter/mm68
Secondary impellerNumber of blades6
Inlet diameter/mm19
Outlet blade angle/°30
Outlet width/mm3
Outlet diameter/mm68
Secondary vaned diffuserNumber of blades8
Outlet blade angle/°12
Outlet diameter/mm88
Discharge voluteInlet width/mm3.6
Discharge diameter/mm18
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MDPI and ACS Style

Zhang, B.; Niu, B.; Zhang, Z.; Chen, S.; Xue, R.; Hou, Y. Application and Challenge of High-Speed Pumps with Low-Temperature Thermosensitive Fluids. Energies 2024, 17, 3732. https://doi.org/10.3390/en17153732

AMA Style

Zhang B, Niu B, Zhang Z, Chen S, Xue R, Hou Y. Application and Challenge of High-Speed Pumps with Low-Temperature Thermosensitive Fluids. Energies. 2024; 17(15):3732. https://doi.org/10.3390/en17153732

Chicago/Turabian Style

Zhang, Beile, Ben Niu, Ze Zhang, Shuangtao Chen, Rong Xue, and Yu Hou. 2024. "Application and Challenge of High-Speed Pumps with Low-Temperature Thermosensitive Fluids" Energies 17, no. 15: 3732. https://doi.org/10.3390/en17153732

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