Study On The Internal Flow And Loss Mechanism Of Turboexpander In The Compressed Air Energy Storage System

The Compressed Air Energy Storage(CAES)system is widely applied to the power system with renewable energy.The turboexpander is applied as an important device in the power generation process of the CAES system.It converts the intrinsic energy of compressed air into the mechanical energy,which could drive generators to output power.The CAES turbine operates at a lower inlet temperature and a higher expansion ratio under off-designed conditions.Its operating conditions are much different from that of gas/steam turbine in the power plant,high/low pressure turbine in the aeroengine,However,the lack of an in-depth investigation of internal flow and loss mechanisms for CAES turbines restricts the optimization of aerodynamic design and efficient operation,especially under variable expansion ratio or variable rotating speed conditions.For the CAES axial turbine and shrouded radial turbine,their internal flow and loss mechanisms were analyzed in the current work.The transportation of secondary vortices in the mainstream,the leakage characteristics and losses in the secondary path were studied numerically.Based on the numerical analyses of shrouded radial turbine,the experiment was also conducted to measure detailed aerodynamic parameters in the collector,shroud cavity and outflow pipe,respectively.The internal flow characteristics were also discussed under off-designed conditions.The results revealed the development of internal flow field,explained the loss mechanism of leakage flow,and proposed some loss evaluation,optimal design and efficient operation methods.As the main leakage region of the axial turbine,the impact of the rotor tip clearance on the performance of C AES turbine is apparently lesser known.Thus,the distribution of flow patterns at rotor tip clearance and leakage loss mechanisms were analyzed numerically.A typical secondary flow structure in the clearance is the separation bubble,whose reattachment occurs when the static pressure of tip is equal to that of endwall.The high-loss region is located at the suction side of tip clearance rather than the separation bubble.To evaluate the tip leakage loss of axial turbine in the CAES system,correlations proposed by Denton and Yaras and Sjolander give satisfactory results,whose deviations are within 5%.The shrouded radial turbine is applied to the high expansion ratio and small mass flow rate conditions in the CAES system.However,the secondary flow structures in the shrouded impeller and leakage loss in the shroud cavity under off-designed conditions are still unknown.The numerical investigation of distributions of secondary vortices and their losses were conducted in the shrouded radial turbine.The high dissipation near the suction-side endwall,which is caused by the cross flow in the shrouded impeller,accounts for over one third of the total loss.The leakage vortex of the shroud cavity exists at the outflow pipe.It not only causes high energy loss or entropy generation,but also reduces flow uniformity on the cross-sectional planes.To improve the stability of the numerical simulation,it is recommended the outflow pipe should be designed longer than the rotor axial chord length to reduce the impact of cavity leakage.To further study the complex flow field in turbines,the analytical method of physical quantity synergy was proposed.Based on the synergy principle,the leakage and loss mechanisms were further researched,and several optimizations of CAES turbines were also suggested.In the analyses of internal leakage by velocity-pressure gradient synergy,the larger the synergy angle is,the higher flow resistance would be.According to the synergy angle between the velocity and pressure gradient,a novel model of leakage mass flow rate for the axial turbine was proposed,and the dimensionless seal clearance of the shrouded radial turbine should be smaller than 1.5%to reduce the leakage efficiently.In the loss analyses by velocity-pressure gradient synergy,a relatively low synergy angle could indicate the high-loss region.An apparent negative correlation of the loss coefficient versus synergy angle has been perceived.It is hoped that this correlation could further explore the understanding of losses in turbines.A lack of clarity in the experiments of CAES shrouded radial turbine prevents a detailed approach to the turbine performance under off-designed conditions.To acquire the off-designed performance and verify numerical results of the shrouded radial turbine,experiments were designed and conducted to measure aerodynamic parameters in the collector,shroud cavity and outflow pipe,respectively.The experimental analyses of shrouded radial turbine were first reported.The results show that there exists a wide range of circumferential non-uniformity in the collector.High static pressure regions,especially from 90° to 135° and from 225° to 270° in the clockwise direction,show relatively low velocity.The leakage of shroud cavity,whose numerical results agree well with experiments,decreases when the rotating speed rises.The total pressure loss and non-uniformity of outflow angle along spanwise direction reach the minimum at the designed condition.The cavity leakage not only increases the loss,but also reduces the outflow angle at the upper region.The present study conducted numerical and experimental investigations about the internal flow and losses of CAES turboexpanders in detail.The results revealed the distribution of secondary vortices,leakage characteristics and loss mechanisms.Several novel leakage models,sealing structures and loss evaluation methods were put forward based on physical quantity synergy.It is hoped the present work could provide a guiding effect on the optimal design of CAES turboexpanders.