Jianting Sun

A high-pressure centrifugal compressor with a shroud cavity is a key piece of energy equipment found in a compressed air energy storage (CAES) system. The leakage flow of the shroud cavity is an important factor affecting the efficiency of the system. Using systematic numerical studies, the influence of leakage flow on high-pressure centrifugal compressors under different leakage rates has been comprehensively elucidated. The study reveals that the leakage flow significantly increases the circumferential swirl angle of the flow at the impeller inlet, thereby exerting a significant influence on the flow characteristics within the impeller. As the leakage rate increases, the stable operating range of the compressor is significantly reduced, with the total pressure ratio decreasing by up to 2% and the isentropic efficiency dropping by up to 4 percentage points.

The oblique flow compressor is one of the important components in the compressed air energy storage (CAES) system. The structural shape of the radial inlet chamber (RIC) directly affects the compressor performance, and a reasonable RIC design should achieve the smallest total pressure loss and outlet distortion as much as possible to meet the structural design. To study the influence of splitter blades, 4 RICs equipped with different numbers of splitter blades are designed, and the performance of 4 RICs and the overall performance of the compressor is calculated. The results show that with the increase in the number of splitter blades, the stall margin increases from 6.3% to 13.94%. At the design point, the isentropic efficiency is highest for the RIC with 17 splitter blades, and the pressure ratio is highest for the RIC with 11 splitter blades. Compared with the direct axial intake mode, the uniformity of the relative leakage distribution and the attack angle distribution of the impeller leading edge under 4 radial intake modes are poor. However, with an increase in the number of splitter blades, the uniformity of the relative tip leakage and the attack angle distribution gradually increase. The flow loss of RIC will increase simultaneously, though the uniformity of the outlet aerodynamic parameters distribution improves, and the influence on the downstream component performance gradually weakens. There is an optimal number of splitter blades in RIC, which balances the total pressure loss and distortion coefficient.

The internal flow field and loss distributions are quite complicated in the high-pressure compressor with shroud and backside cavity applied in the compressed air energy storage (CAES) system. It’s necessary to develop physical understandings on the internal flow and losses inside the impeller, shroud cavity and backside cavity by the physical synergy relationship between some key quantities, which is innovatively applied to the internal flow of a high-pressure compressor used in CAES. First, the author successfully built a high-pressure centrifugal compressor test rig for CAES, and carried out the high pressure performance experiment for the first time. The aerodynamic performance of the high-pressure centrifugal compressor is compared by experiment and calculation. Then, the main flow characteristics and loss generation inside the impeller has been studied by analyzing the synergy between velocity and temperature gradient, and some other important quantities. And, the leakage flow characteristics inside the shroud cavity and backside cavity has been discussed by analyzing the synergy between velocity and pressure gradient, and some other key quantities. It is found that the regions where the high energy losses and entropy generation locate correspond to the relatively high synergy angle. At last, the interaction between leakage flow and main flow has been researched through clarifying the loss of temperature and radial velocity. It is found that there exit two boundary lines for leakage flow injecting into main flow.

Relying on a closed test rig of a high-power intercooling centrifugal compressor for compressed air energy storage (CAES), this study measured the static pressure and static temperature at different radii on the static wall of the impeller backside cavity (IBC) under variable rotating speeds. Simultaneously, the coupled computations of all mainstream domains with IBC or not were used for comparative analysis of the aerodynamic performances of the compressor and the internal flow field in IBC. The results show that IBC has a significant impact on coupling characteristics including pressure ratio, efficiency, torque, shaft power, and axial thrust of the centrifugal compressor. The gradients of radial static pressure and static temperature in IBC both increase with the decrease of mainstream flow or the increase of rotating speed, whose distributions are different under variable rotating speeds due to the change of the aerodynamic parameters of mainstream.

The impeller backside cavity (IBC) is a unique structure of centrifugal compressor in compressed air energy storage (CAES) systems, which affects the aerodynamic performance of centrifugal compressor, and the angle change of the downstream coupled adjustable vaned diffusers (AVDs) will affect the flow field inside the cavity and compressor performance. This paper relies on the closed test facility of the high-power intercooling compressor to measure static pressure and static temperature at different radii on the static wall of the IBC. The coupling relationship between the IBC and compressor under variable operating conditions is analyzed, and the influence of AVDs on the internal flow in IBC is studied. The results show that static pressure and static temperature rise along the direction of increasing radius, but static temperature drops near the coupling between the impeller outlet and the cavity inlet. Under AVDs’ design angle, static pressure and static temperature at each point, static pressure loss and static temperature loss in the direction of decreasing radius all increase as the flow decreases. Under variable AVDs’ angles, static pressure and static temperature will change differently, and respective loss will also be different.

Wet compression is widely used to reduce compression work and improve efficiency in gas turbines. However, wet compression has not been industrially applied in the compressed air energy storage system (only few studies on isothermal compressed air energy storage system exist), which has an urgent demand to reduce the compression work. The high-pressure section of the compressed air energy storage system usually contains supercritical air, and the influence of supercritical wet compression is not yet clear. Thus, in this study, supercritical wet compression was numerically investigated in a centrifugal compressor used for the compressed air energy storage system, and its effects on performance and internal flow were also studied. The results show that the ideal maximum evaporation of water droplets in supercritical air is very low, which limits the maximum available water injection ratio; however, wet compression still has some benefits, such as increasing the total pressure ratio and isentropic efficiency and reducing the compression specific work. Moreover, wet compression reduces the diffuser loss but raises the wake loss in the impeller at the near-stall point. For this compressor, the optimum water injection ratio is 0.3%, which reduces the specific work by 0.65% at the designed pressure ratio.

There is an urgent demand to reduce compression power consumption in Compressed Air Energy Storage (CAES) systems. Wet compression has been widely used in gas turbines to reduce compressor power consumption and improve thermal efficiency, but this technology has not been applied yet in the CAES field. In this paper, a centrifugal compressor for CAES was numerically studied to investigate the effect of wet compression on compressor and droplet motion. The results showed that wet compression makes the performance curve shift to a high-pressure ratio/efficiency. Meanwhile, wet compression lowers the stall margin and narrows the stable operation range, and the effect is enhanced with the increase of water injection ratio or the decrease of average droplet diameter. Wet compression can effectively save compressor power consumption during energy storage, and at the designed pressure ratio, the power consumption can be reduced by 1.47% with a water injection ratio of 3% and an average droplet diameter of 5 μm. Influenced by the inertia and secondary flow, the droplets migrate to the impeller pressure and shroud side, thus causing brake loss by impacting on blades. The migration of droplets strengthens with the increase in the average droplet diameter and flow coefficient.