Design And Performance Analysis Of Variable Diffuser Centrifugal Compressor

The turbocharger can effectively improve the power density of the diesel engine and ameliorate the diesel engine emissions,but due to the limitation of turbocharger performance,the turbocharged diesel engine can not maintain good performance under the full working condition,especially at low load,the power is reduced,the temperature before turbine is increased and the emission performance is deteriorated.Therefore,for turbocharged diesel engines,especially marine turbocharged diesel engines,improving their performance under low load has become an important research field.This paper mainly completed the design and performance analysis of the variable diffuser centrifugal compressor,including the analysis of the flow field of the variable geometry compressor,the design of the variable geometry compressor and the performance test of the variable geometry compressor,including the steady state performance and transient state performance,and steady and transient performance modeling,and simulation studying the effects of variable geometry compressors on the performance of turbocharged diesel engines.The main contents of this paper are as follows:(1)The influence of the structural parameters on the performance and flow field of the variable geometry compressor were analyzed by using the fluid calculation software.Based on the CFX fluid calculation software,the fluid simulation model of the variable geometry compressor was established.The influence of different diffuser blade inlet angle and different radius ratio on the performance and flow field of the compressor was analyzed.The results show that as the angle of the diffuser blade increases,the surge boundary of the compressor moves to the left.The flow range of the compressor extends from 1.4kg/s?2.8kg/s to 0.2kg/s?2.8kg/s,and the flow range is expanded by 85.7 %.The maximum efficiency of the compressor will decrease as the angle of the diffuser vanes increases.It was found that at the low-speed surge boundary,the trailing edge of the diffuser blades will appear to detach the vortex,and as the angle of the diffuser vane increases,the vortex will gradually develop upstream and eventually spread throughout the impeller flow field.As deeper analysis,it was found that the development of this vortex in the impeller was caused by the fluid return of the impeller outlet at a large angle of diffuser blades.At the same time,the effects of different inlet radius ratios on the flow field and performance of the compressor were analyzed.It was found that with the increase of the radius ratio,the pressure of the vane-less diffuser section increases,the speed of flow decreases,and the high pressure zone and the low velocity zone are formed,which easily leads to the generation of eddy currents and affects the uniform distribution of the flow field,and the efficiency of the compressor is reduced.By comparing the static pressure recovery coefficient and the total pressure loss coefficient of different radius ratios in low speed,medium speed and high speed,the effective static pressure recovery coefficient and effective total pressure loss coefficient based on the speed weight are proposed,and the inlets of different diffuser blades are compared.The effective static pressure recovery coefficient and the effective total pressure loss coefficient of the angle,the effective static pressure recovery coefficient with the inlet radius ratio of 1.12 is the highest,and the effective total pressure loss coefficient is the smallest,so the diffuser inlet radius ratio of the modified variable geometry compressor is set to 1.12.(2)The variable geometry compressor was designed and modified,and the structural strength was checked.The experimental bench of the variable geometry compressor was established,and the steady state performance and transient performance of the variable geometry compressor were obtained on the experimental bench.According to the layout and structure of the original compressor,the variable diffuser vanes is selected as the realization of the variable geometry compressor,and the rocker rotating disc mechanism is creatively selected to realize the synchronous rotation of the diffuser vanes,and the gear pair is used to realize the precise control of the angle of the blade.mounting the rocker rotating disc on the back of the diffuser chassis avoids the influence on the flow field of the diffuser;the stepping motor and the gear pair are selected according to the external installation space to realize the function of deceleration and torque transmission.According to the structural strength check and deformation analysis of the designed structure,the minimum safety factor of the structure was 1.48 and the maximum deformation was 0.35 mm,meeting the strength requirement.Using a large compressor as a power source,a plurality of electromagnetic control valves control the inlet and outlet conditions of the variable geometry compressor,and install a variety of steady state and transient data recording devices,and establish a variable geometry compressor test bench.The steady-state performance and transient performance test points were selected according to the requirements of modeling,and the steady-state performance experiments and transient performance experiments of the variable geometry compressor were completed on the experimental bench.(3)The steady-state performance and transient performance model of the variable geometry compressor are established.The steady-state performance of the variable geometry compressor was modeled by elliptic equation and partial least squares method.In the first step,the elliptic equation is used to convert the compressor characteristic lines of different speeds at different diffuser angles into unrelated independent parameters.The second step uses partial least squares method to establish the model of independent parameters and compressor speed and diffuser blade angle.The results show that the maximum fitting error of the pressure ratio-mass flow rate characteristic diagram and the efficiency-mass flow rate characteristic diagram is 2%,the maximum prediction error of the pressure ratio is 1.6% at relative speed equals 0.625,and the efficiency is 4.4%;at a fixed angle,the pressure ratio and the efficiency prediction ability is almost same,and the maximum prediction errors of the two are 3% and 2.5%,respectively.In general,the established steady-state performance model shows good prediction ability,and its prediction accuracy is far better than conventional look-up tables and artificial neural networks.The transient performance curves obtained by the experiment are analyzed to extract the performance parameters that determine the transient performance,and the transient performance curves of the variable geometry compressor are expressed by the mathematical expressions of the performance parameter combination;then the performance parameters are related to the control parameters.The performance is analyzed,the mathematical expressions of performance parameters and control parameters are established,and the transient performance model of the variable geometry compressor is established by using polynomial and partial least squares method.Finally,the fitting performance and prediction performance of the model are tested and demonstrated.The results show that the established model can not only effectively fit the transient performance of the known variable geometry compressor,but also accurately predict the transient performance of the variable geometry compressor,showing good qualitative prediction and quantitative prediction.And the model can meet the accuracy requirements of mathematical modeling.(4)A combination simulation model of variable diffuser centrifugal compressor and diesel engine is established and the influence of variable geometry compressor on the performance of diesel engine is studied.The joint simulation platform of variable geometry compressor and diesel engine was established by using simulink platform.The influence of variable geometry compressor on diesel engine under propulsion characteristics,load characteristics and high back pressure conditions were studied.The results show that the diesel engine's propulsion characteristics and the load characteristics are improved with the variable geometry compressor,the fuel consumption is reduced,the maximum reduction is 4.58%,and the diesel engine power is also recovered effectively under high back pressure condition,power recovery factor increased by up to 96%.Under the propulsion characteristics,as the load decreases,the optimum diffuser blade inlet angle increases;at high back pressure,as the back pressure increases,the optimum diffuser blade inlet angle increases.