Research On Heat-flow Field Simulation Of Hydrodynamic Retarder With SBES And Muti-objective Optimization Of Its Plate-fin Heat Exchanger

With the rapid development of economy in our country, the demand for highway freight and passenger transportation are increasing, commercial vehicle also incessant transitions to the direction of high power and heavy load. Hydrodynamic retarder, as one kind of auxiliary brake, has obtained extensive application and commend with its unique advantages in the field of vehicle auxiliary brake. Despite all this, during the process of braking, if cooling system can’t provide sufficient heat transfer rate and the heat generated by the braking can’t be released timely, the temperature of working oil will also be increasing continually, which will trigger a series of thermal issues.In this paper, aiming at solving some thermal issues induced by the braking during the working process of hydrodynamic retarder, YH293 is selected as research object to in-depth investigate the variable viscosity numerical computation method of retarder and the optimization of heat exchanger structure from the change of working oil viscosity caused by the heat and its heat transfer. The major research works and related conclusions listed as follows.1. SBES numerical computation method of hydrodynamic retarder In order to simulate transient heat-flow coupling condition with gas-liquid two phase within hydrodynamic retarder, the most advanced hybrid RANS/LES turbulence model at present, SBES(Stress-Blended Eddy Simulation), is introduced into establish heat-flow coupling mathematical model combined with gas-liquid two phase for retarder itself. Furthermore, flow around cylinder as classic case is employed to verify the effectiveness of SBES by comparing with Dynamic Large Eddy Simulation and Delayed Detached Eddy Simulation from the capability of computing and flow structure capture. The results show that SBES can capture the more abundant flow structures with less computation time.2. Heat-flow field numerical calculation and analysis of hydrodynamic retarder based on variable viscosity One variable viscosity numerical calculation method is presented by selecting YH293 hydrodynamic retarder as research object. The predicted values of braking torque for different filling factors and rotational speeds are acquired under the conditions of both constant and variable viscosity. Through comparing the values of experiment and CFD involving constant and variable viscosity conditions, it is indicated that the predicted value after considering the changes in viscosity with temperature during the braking process is more precise and the prediction deviation is decreased from 6% ~ 10% to 4% ~ 7%. At the same time, the reasons for variable viscosity method can improve the predictive accuracy of braking torque value are analyzed by comparing heat-flow field structures. In addition, the needed heat transfer rate for retarder under 2000 rpm condition is calculated.3. Structure design and multi-objective optimization of the plate-fin heat exchanger The reliability of the CFD method for predicting the performance of serrated platefin heat exchanger is verified based on the existing experiment date for 95JC1803 heat exchanger with steam-air for heat exchange way. Transforming heat exchange medium into oil-water, the fin height, length, spacing and thickness are selected as the optimization variables, variable constraints are determined by combining with design experience, the maximum heat transfer factor j and minimum friction factor f are defined as multi-goal optimizing function, Opt LHD is employed to set sample point, Kriging is applied to constitute agent model, NSGA-II is selected to optimize fin structure. Through comparing the response value of optimal Pareto solution and CFD calculated value, it is found that the deviation between response and calculated value is small, thus the reliability of optimal solution is verified. Additionally, the heat transfer performance both for before and after optimization under the different Re are compared, it turned out that the heat transfer factor increased by 12% and the friction factor decreased by 25% for the optimized. The heat transfer rate both for before and after optimization are compared, the result show that the optimized model satisfies the heat transfer requirement of hydrodynamic retarder.4. Optimized analysis of the serrated plate-fin heat exchanger and its field synergy number comparison The inferences of each design variable on heat transfer performance are analyzed, the results show that fin height should be smaller for decreasing thermal resistance, fin length should be smaller for increasing the disturbance to the heat transfer medium and strengthening heat transfer capacity, fin spacing should be larger for adding first heat exchange area and decreasing heat transfer medium velocity, fin thickness should be smaller for decreasing the resistance of the flow passage, then the heat transfer factor j increased and the friction factor f decreased. Meanwhile, the optimization process of each design variable are analyzed. Furthermore, each physical field structure and corresponding field synergy number are employed to evaluate the heat transfer performance of heat exchanger.