With the advantages of high efficiency and compact structure,supercritical carbon dioxide(sC02)Brayton cycles have bright prospects for development in energy conversion field.As one of the core components of the power...With the advantages of high efficiency and compact structure,supercritical carbon dioxide(sC02)Brayton cycles have bright prospects for development in energy conversion field.As one of the core components of the power cycle,the centrifugal compressor tends to operate near the critical point(304.13 K,7.3773 MPa).Normally,the compressor efficiency increases as the inlet temperature decreases.When the inlet temperature is close to the critical point,the density increases sharply as the temperature decreases,which results in quickly decreasing of volume flow rate and efficiency reducing.The flow loss mechanism of the sCO_(2) compressor operating at low flow rate is studied in this paper.Computational fluid dynamics(CFD) simulations for sCO_(2)compressor were carried out at various inlet temperatures and various mass flow rates.When the sCO_(2)compressor operates at low volume flow rate,the flow loss is generated mainly on the suction side near the trailing edge of the blade.The flow loss is related to the counterclockwise vortexes generated on the suction side of the main blade.The vortexes are caused by the flow separation in the downstream region of the impeller passage,which is different from air compressors operating at low flow rates.The reason for this flow separation is that the effect of Coriolis force is especially severe for the sCO_(2) fluid,compared to the viscous force and inertial force.At lower flow rates,with the stronger effect of Coriolis force,the direction of relative flow velocity deviates from the direction of radius,resulting in its lower radial component.The lower radial relative flow velocity leads to severe flow separation on the suction side near the trailing edge of the main blade.展开更多
Supercritical carbon dioxide(SCO_(2))centrifugal compressor is a key component of a closed Brayton cycle system based on SCO_(2).A comprehensive understanding of the loss mechanism within the compressor is vital for i...Supercritical carbon dioxide(SCO_(2))centrifugal compressor is a key component of a closed Brayton cycle system based on SCO_(2).A comprehensive understanding of the loss mechanism within the compressor is vital for its optimized design.However,the physical properties of SCO_(2) are highly nonlinear near the critical point,and the internal flow of the compressor is closely related to its properties,which inevitably influences the generation of aerodynamic losses within the compressor.This paper presents a comprehensive investigation of the compressor's loss mechanism with an experimentally validated numerical method.The real gas model of CO_(2) embodied in the Reynolds-Averaged Navier-Stokes(RANS)model was used for the study.Firstly,the numerical simulation method was validated against the experimental results of Sandia SCO_(2) compressor.Secondly,performance and loss distribution of the compressor were compared among three fluids including SCO_(2),ideal CO_(2)(ICO_(2))and ideal air(IAir).The results showed that the performance of SCO_(2) was comparable to IAir under low flow coefficient,however markedly inferior to the other two fluids at near choke condition.Loss distribution among the three fluids was distinctive.In the impeller,SCO_(2) was the most inefficient,followed by ICO_(2) and IAir.The discrepancies were magnified as the flow coefficient increased.This is due to a stronger Blade-to-Blade pressure gradient that intensifies boundary layer accumulation on walls of the shroud/hub.Furthermore,owing to the reduced sonic speed of SCO_(2),a shock wave appears earlier at the throat region and SCO_(2) encounters more intenseboundarylayerseparation.展开更多
Flow instability in the centrifugal compressor should be detected and avoided for stable and safe operation.Due to the popularity of electric centrifugal compressors,instability detection could be achieved by measurin...Flow instability in the centrifugal compressor should be detected and avoided for stable and safe operation.Due to the popularity of electric centrifugal compressors,instability detection could be achieved by measuring motor signals instead of traditional aerodynamic signals.In this paper,the feasibility of instability detection by motor signals(i.e.rotating speed and phase current)was studied experimentally.The physical structure and control method of the electric centrifugal compressor were discussed to reveal the potential of instability detection by motor signals.Dynamic pressure signals and motor signals measured during unsteady experiments were analyzed in the time domain and frequency domain.Characteristics of these signals were then compared under different operating conditions to indicate the feasibility of instability detection by motor signals.Finally,the ability of Short-Time Fourier Transform(STFT)of rotating speed signals in real-time instability detection was discussed.Results showed that the rotating speed signal is a good alternate for instability detection in spite of signal distortion,while the phase current signal can only detect surge due to the low resolution of the controller.Based on the variations of the amplitude and frequency of rotating speed signals,the real-time instability can be captured accurately by STFT with a window size of 0.5 s.Besides,the interference caused by the controller can be removed by STFT.展开更多
温度是影响锂离子动力电池性能的关键因素,本文采用平板热管作为电池热管理的传热部件,实验研究了平板热管在不同电池产热功率条件下的传热性能和均温性,理论计算了平板热管扩散热阻及导热系数.研究表明,在25 W产热条件下,平板热管扩散...温度是影响锂离子动力电池性能的关键因素,本文采用平板热管作为电池热管理的传热部件,实验研究了平板热管在不同电池产热功率条件下的传热性能和均温性,理论计算了平板热管扩散热阻及导热系数.研究表明,在25 W产热条件下,平板热管扩散热阻为0.044℃W-1,等效导热系数650 W K-1,随着电池产热功率的增大,平板热管的扩散热阻降低,等效导热系数显著增大.在多热源条件下,平板热管表面最大温差低于4℃,表明其较好的均温性,在电池热管理系统中具有较好的应用前景.展开更多
Recovery of heat energy from internal combustion engine exhaust will achieve significant road transportation CO2 reduction. Turbocharging and turbogenerating are most commonly used technologies to recover engine exhau...Recovery of heat energy from internal combustion engine exhaust will achieve significant road transportation CO2 reduction. Turbocharging and turbogenerating are most commonly used technologies to recover engine exhaust heat energy.Engine exhaust pulse flow can significantly affect the turbine performance of turbocharging and turbogenerating systems,and it is necessary to consider the pulse flow effects in turbine design and performance analysis.An investigation was carried out by numerical simulation on the mixed flow turbine pulse flow performance and flow fields.Results showed that the variations of the turbine efficiency and flowfiled under pulsating flow conditions demonstrate significant unsteady effects.The effect of blade leading edge sweep on turbine pulse flow performance was studied.It is shown that increasing of the leading edge sweep angle can improve the turbine average instantaneous efficiency by about 2 percent under pulsating flow conditions.展开更多
The effects of Reynolds number on the performance of a high pressure-ratio turbocharger compressor were investigated by both experiments and numerical simulation. The experimental results show that the pressure ratio ...The effects of Reynolds number on the performance of a high pressure-ratio turbocharger compressor were investigated by both experiments and numerical simulation. The experimental results show that the pressure ratio and the efficiency of the compressor respectively decrease by 7.9% and 6.9% when Reynolds number drops from 9.86×10 5 to 2.96×10 5 . The numerical simulation predicts a similar trend as the experimental results although it underestimates the deterioration of the performance under low Reynolds number conditions. According to simulation results, the boundary layer thickness increases at the inducer, which decreases the throat area and leads to smaller choke mass flow rate. The increments of the boundary thickness are relatively small at the rear part of the impeller. The boundary layer separation flow is severe. The interaction between boundary layer separation flows and leakage flows causes the high loss region at the rear part of the impeller passage under low Reynolds number condition.展开更多
The present study focuses on the influence of the swirling flows on flow behaviors and performance of a radial-flow turbocharger turbine under pulsating inflow condition.To characterize the effects of swirling flow,th...The present study focuses on the influence of the swirling flows on flow behaviors and performance of a radial-flow turbocharger turbine under pulsating inflow condition.To characterize the effects of swirling flow,three sets of simulations of the turbine were carried out,which are an unsteady simulation under pulsating swirling inflow,an unsteady simulation under equivalent pulsating uniform inflow,and quasi-steady simulations under uniform inflow.Results proved that swirling flow has a considerable negative influence on turbine instantaneous performance and lead to 2.5%cycle-averaged efficiency reduction under pulsating flow condition.Swirling inflow would lead to significant losses in both the volute and the rotor,while the pulsating inflow leads to higher losses in the rotor and shows little influence on the losses in the volute.The instantaneous efficiency reduction of the turbine could be correlated with the time-varying inlet swirl strength.Under the influence of unsteady inlet swirls,the volute flow field is highly distorted and the free vortex relation is no longer valid.The swirling flow has strong interactions with the wake flow of the volute tongue,leading to additional losses.Relative flow angle at rotor inlet is remarkably reduced and its distribution is significantly distorted.Strong separation flows and passage vortices would appear in the rotor because of the swirling inflow,leading to inferior rotor performance.展开更多
Heightened interests have been laid at the preliminary design and optimization of the centrifugal compressor for the fuel cell vehicle.The centrifugal compressor for fuel cell vehicle is driven by a high-speed motor;h...Heightened interests have been laid at the preliminary design and optimization of the centrifugal compressor for the fuel cell vehicle.The centrifugal compressor for fuel cell vehicle is driven by a high-speed motor;however,the limit of the motor speed makes the flow passage of the impeller long and narrow,which leads to a serious tip leakage loss.Serious tip leakage loss deteriorates the compressor performance.In this paper,3-D numerical simulations were carried out with the aim of investigating the tip leakage loss in a prototype centrifugal compressor for a 100 kW fuel cell stack.The results revealed that the mixing loss caused by the interaction between the tip leakage vortex and the downstream tip leakage flow contributed to the major part of the tip leakage loss.The path of the tip leakage vortex almost followed the streamwise direction,while the downstream tip leakage flow exhibited strong circumferential momentum,which referred to the fact that they were nearly orthogonal.Therefore,a flow control approach,which was realized by enhancing the blade loading around the leading edge of blade tips in this paper,was proposed to decrease the interaction angle between the tip leakage vortex and the downstream tip leakage flow and then mitigate mixing loss by changing the flow direction of the tip leakage vortex.The results showed a smaller interaction angle was achieved in the optimized impeller compared with the baseline one.Meanwhile,the efficiency was also improved by 1.30%at design condition and the maximum efficiency improvement could be up to 10%at large mass flow condition of 92000 r/min.Being manufactured and tested,the optimized compressor was proved to achieve an isentropic efficiency of 75.84%at design condition.展开更多
基金supported by the National Key Research and Development Program of China (No. 2018YFB1501004)。
文摘With the advantages of high efficiency and compact structure,supercritical carbon dioxide(sC02)Brayton cycles have bright prospects for development in energy conversion field.As one of the core components of the power cycle,the centrifugal compressor tends to operate near the critical point(304.13 K,7.3773 MPa).Normally,the compressor efficiency increases as the inlet temperature decreases.When the inlet temperature is close to the critical point,the density increases sharply as the temperature decreases,which results in quickly decreasing of volume flow rate and efficiency reducing.The flow loss mechanism of the sCO_(2) compressor operating at low flow rate is studied in this paper.Computational fluid dynamics(CFD) simulations for sCO_(2)compressor were carried out at various inlet temperatures and various mass flow rates.When the sCO_(2)compressor operates at low volume flow rate,the flow loss is generated mainly on the suction side near the trailing edge of the blade.The flow loss is related to the counterclockwise vortexes generated on the suction side of the main blade.The vortexes are caused by the flow separation in the downstream region of the impeller passage,which is different from air compressors operating at low flow rates.The reason for this flow separation is that the effect of Coriolis force is especially severe for the sCO_(2) fluid,compared to the viscous force and inertial force.At lower flow rates,with the stronger effect of Coriolis force,the direction of relative flow velocity deviates from the direction of radius,resulting in its lower radial component.The lower radial relative flow velocity leads to severe flow separation on the suction side near the trailing edge of the main blade.
基金supported by the National Natural Science Foundation of China(Grant No.52076130)。
文摘Supercritical carbon dioxide(SCO_(2))centrifugal compressor is a key component of a closed Brayton cycle system based on SCO_(2).A comprehensive understanding of the loss mechanism within the compressor is vital for its optimized design.However,the physical properties of SCO_(2) are highly nonlinear near the critical point,and the internal flow of the compressor is closely related to its properties,which inevitably influences the generation of aerodynamic losses within the compressor.This paper presents a comprehensive investigation of the compressor's loss mechanism with an experimentally validated numerical method.The real gas model of CO_(2) embodied in the Reynolds-Averaged Navier-Stokes(RANS)model was used for the study.Firstly,the numerical simulation method was validated against the experimental results of Sandia SCO_(2) compressor.Secondly,performance and loss distribution of the compressor were compared among three fluids including SCO_(2),ideal CO_(2)(ICO_(2))and ideal air(IAir).The results showed that the performance of SCO_(2) was comparable to IAir under low flow coefficient,however markedly inferior to the other two fluids at near choke condition.Loss distribution among the three fluids was distinctive.In the impeller,SCO_(2) was the most inefficient,followed by ICO_(2) and IAir.The discrepancies were magnified as the flow coefficient increased.This is due to a stronger Blade-to-Blade pressure gradient that intensifies boundary layer accumulation on walls of the shroud/hub.Furthermore,owing to the reduced sonic speed of SCO_(2),a shock wave appears earlier at the throat region and SCO_(2) encounters more intenseboundarylayerseparation.
基金supported by the National Key Research and Development Program of China(No.2018YFB1501004)。
文摘Flow instability in the centrifugal compressor should be detected and avoided for stable and safe operation.Due to the popularity of electric centrifugal compressors,instability detection could be achieved by measuring motor signals instead of traditional aerodynamic signals.In this paper,the feasibility of instability detection by motor signals(i.e.rotating speed and phase current)was studied experimentally.The physical structure and control method of the electric centrifugal compressor were discussed to reveal the potential of instability detection by motor signals.Dynamic pressure signals and motor signals measured during unsteady experiments were analyzed in the time domain and frequency domain.Characteristics of these signals were then compared under different operating conditions to indicate the feasibility of instability detection by motor signals.Finally,the ability of Short-Time Fourier Transform(STFT)of rotating speed signals in real-time instability detection was discussed.Results showed that the rotating speed signal is a good alternate for instability detection in spite of signal distortion,while the phase current signal can only detect surge due to the low resolution of the controller.Based on the variations of the amplitude and frequency of rotating speed signals,the real-time instability can be captured accurately by STFT with a window size of 0.5 s.Besides,the interference caused by the controller can be removed by STFT.
文摘温度是影响锂离子动力电池性能的关键因素,本文采用平板热管作为电池热管理的传热部件,实验研究了平板热管在不同电池产热功率条件下的传热性能和均温性,理论计算了平板热管扩散热阻及导热系数.研究表明,在25 W产热条件下,平板热管扩散热阻为0.044℃W-1,等效导热系数650 W K-1,随着电池产热功率的增大,平板热管的扩散热阻降低,等效导热系数显著增大.在多热源条件下,平板热管表面最大温差低于4℃,表明其较好的均温性,在电池热管理系统中具有较好的应用前景.
基金supported by the National Basic Research Program of China("973"Program)(Grant No.2011CB707204)the National Natural Science Foundation of China(Grant No.50706020)
文摘Recovery of heat energy from internal combustion engine exhaust will achieve significant road transportation CO2 reduction. Turbocharging and turbogenerating are most commonly used technologies to recover engine exhaust heat energy.Engine exhaust pulse flow can significantly affect the turbine performance of turbocharging and turbogenerating systems,and it is necessary to consider the pulse flow effects in turbine design and performance analysis.An investigation was carried out by numerical simulation on the mixed flow turbine pulse flow performance and flow fields.Results showed that the variations of the turbine efficiency and flowfiled under pulsating flow conditions demonstrate significant unsteady effects.The effect of blade leading edge sweep on turbine pulse flow performance was studied.It is shown that increasing of the leading edge sweep angle can improve the turbine average instantaneous efficiency by about 2 percent under pulsating flow conditions.
基金supported by the National Natural Science Foundation of China (Grant No. 51176087)
文摘The effects of Reynolds number on the performance of a high pressure-ratio turbocharger compressor were investigated by both experiments and numerical simulation. The experimental results show that the pressure ratio and the efficiency of the compressor respectively decrease by 7.9% and 6.9% when Reynolds number drops from 9.86×10 5 to 2.96×10 5 . The numerical simulation predicts a similar trend as the experimental results although it underestimates the deterioration of the performance under low Reynolds number conditions. According to simulation results, the boundary layer thickness increases at the inducer, which decreases the throat area and leads to smaller choke mass flow rate. The increments of the boundary thickness are relatively small at the rear part of the impeller. The boundary layer separation flow is severe. The interaction between boundary layer separation flows and leakage flows causes the high loss region at the rear part of the impeller passage under low Reynolds number condition.
基金the foundation of Science and Technology on Diesel Engine Turbocharging Laboratory(No.6142212190101)Young Elite Scientists Sponsorship Program by CAST(2021QNRC001)for the supports。
文摘The present study focuses on the influence of the swirling flows on flow behaviors and performance of a radial-flow turbocharger turbine under pulsating inflow condition.To characterize the effects of swirling flow,three sets of simulations of the turbine were carried out,which are an unsteady simulation under pulsating swirling inflow,an unsteady simulation under equivalent pulsating uniform inflow,and quasi-steady simulations under uniform inflow.Results proved that swirling flow has a considerable negative influence on turbine instantaneous performance and lead to 2.5%cycle-averaged efficiency reduction under pulsating flow condition.Swirling inflow would lead to significant losses in both the volute and the rotor,while the pulsating inflow leads to higher losses in the rotor and shows little influence on the losses in the volute.The instantaneous efficiency reduction of the turbine could be correlated with the time-varying inlet swirl strength.Under the influence of unsteady inlet swirls,the volute flow field is highly distorted and the free vortex relation is no longer valid.The swirling flow has strong interactions with the wake flow of the volute tongue,leading to additional losses.Relative flow angle at rotor inlet is remarkably reduced and its distribution is significantly distorted.Strong separation flows and passage vortices would appear in the rotor because of the swirling inflow,leading to inferior rotor performance.
基金the National Key R&D Program of China(Grant No.2018YFB0106502)Open Fund of Science and Technology on Thermal Energy and Power Laboratory(No.TPL2017AB008).
文摘Heightened interests have been laid at the preliminary design and optimization of the centrifugal compressor for the fuel cell vehicle.The centrifugal compressor for fuel cell vehicle is driven by a high-speed motor;however,the limit of the motor speed makes the flow passage of the impeller long and narrow,which leads to a serious tip leakage loss.Serious tip leakage loss deteriorates the compressor performance.In this paper,3-D numerical simulations were carried out with the aim of investigating the tip leakage loss in a prototype centrifugal compressor for a 100 kW fuel cell stack.The results revealed that the mixing loss caused by the interaction between the tip leakage vortex and the downstream tip leakage flow contributed to the major part of the tip leakage loss.The path of the tip leakage vortex almost followed the streamwise direction,while the downstream tip leakage flow exhibited strong circumferential momentum,which referred to the fact that they were nearly orthogonal.Therefore,a flow control approach,which was realized by enhancing the blade loading around the leading edge of blade tips in this paper,was proposed to decrease the interaction angle between the tip leakage vortex and the downstream tip leakage flow and then mitigate mixing loss by changing the flow direction of the tip leakage vortex.The results showed a smaller interaction angle was achieved in the optimized impeller compared with the baseline one.Meanwhile,the efficiency was also improved by 1.30%at design condition and the maximum efficiency improvement could be up to 10%at large mass flow condition of 92000 r/min.Being manufactured and tested,the optimized compressor was proved to achieve an isentropic efficiency of 75.84%at design condition.
