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Research On The Parameter Optimization Method Of Automotive Hydraulic Retarder Based On Unsteady Flow Field Analysis

Posted on:2011-10-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:X S LiFull Text:PDF
GTID:1102360305453400Subject:Vehicle Engineering
Abstract/Summary:PDF Full Text Request
At the present time, with constant development of China's economy, the conditions of the road transport are improved year by year, both the trucks and the buses are developed towards the direction of high-power and high-load. Much severer requirements of the vechicle's drving safty have been proposed because of the vechicle's heavy-duty and high-speed, which means a severer requirement has been proposed to improve the vechicle's braking perfermance. For the longtime frequent braking, the conventional drum brake or disc brake couldn't meet the referred need any more. In this context, an additional brake assist system is a must for the automobile. The hydraulic retarder, a brake assist parts, is well worth to be used widely, for it has the advantages of providing braking force durably, more effective, much safer and energy-conserving and environment-protective. However, there are not enough studies on the hydraulic retarder in our country at present. And there is no formed quantification theory for designing the hydraulic retarder either. For these reasons, there won't be any breakthrough development on the technology of retarding and braking in our country. Especially for the three dimensional flow field theory which has been developed lately in the outer world, relatively there are hardly any studies and products in our country. Hencely, associating with the high-tech research project"The study of the key technology of vechicle used hydraulic retarder", the enterprise project"The development of vechicle used electric controlled hydraulic retarder", three-dimentional unsteady numerical caculations on the internal flow of the hydraulic retarder based on the theory of Computational Fluid Dynamics(CFD) and three-dimensional multiphase flow have been done. Based on the deep study on how the single parameter affects the retarder's braking performances, the correlation among the structural parameters is analyzed. Finally, the optimal selection for structural parameters of the hydraulic retarder is acquired, and at the same the optimal design method for structural parameters of the hydraulic retarder is obtained, which will have important reference value on further studying and improving the hydraulic retarder's performances. 1. Numerical Simulation of Unsteady Flow in Hydraulic RetarderDepending on the variation of the amount of charging oil in working cavity, hydraulic retarder achieves the retarding capacity of different working conditions, which has internally complex three-dimensional unsteady viscous flow. For the working characteristics of the hydraulic retarder, the internal flow field using three-dimensional numerical simulation of transient flow when the oil charging rate of the internal flow field is 25%, 50%, 75%, 100% are analyzed. The specific methods of the calculation are as follows: the SIMPLEM algorithm is used in velocity-pressure coupling algorithm; the second-order upwind scheme is used in space discrete format; the sliding-mesh method is used in interface between the stator and rotor, the blade's surface of the flow pass and the external ring surface are set as the wall conditions of the internal grid in relative boundary with non-sliding.The extracted entire flow pass is used to calculate the flow field, so that obtains its transient characteristics. The calculation methods and processes of three-dimensional flow field are based on the reference prototype product that produced in Shaanxi Fast Auto Drive Group, including the basic assumptions, geometric model, grid model, boundary conditions, calculation steps, the convergence criteria, and so on.When doing numerical calculations of partial charging condition, based on the full understand of the basic control equation , the computing model , the commonly used turbulence models and turbulent numerical method of gas-liquid two-phase flow , the three multi-phase flow model methods in CFD are studied deeply, including VOF model, Mixture model and Eulerian model. For the flow characteristics of hydraulic retarder, Mixture model is selected to compute the retarder transient gas-liquid two-phase flow.2. The Analysis of the Unsteady Flow Field of RetarderBy defining spatial geometry of the working flow pass of the hydraulic retarder, the velocity and pressure field of the inner chord surface, wing surface and section surface of the working flow pass are analyzed. The flow field characteristics of the hydraulic retarder's rotor and stator are deeply studied, and the comprehensive discussion of the cause of formation is conducted. At the same time, combining with the loss analysis method, the flow field analysis is more in-depth, and many valuable conclusions are obtained.Through the pressure distribution on the chord surface, the maximum pressure appears near the angle between the outer ring and the pressure surface. After contrasting the velocity distribution, it is found that the fluid mainly impact here. So when designing the retarder's structure, round corners should be properly added to let the transition be evenly. The pressure on the front edge of the blade rises as the rotate speed increases. This phenomenon will threaten the strength of the blade edge. Consequently, proper blade wedge angle is important when conduct the structure design.From the distribution of relative velocity of the wing surface, it is not difficult to find that the main factor that influences the velocity vector's direction is the blade's lean angle. The smoother the velocity distributes, the steadier the brake torque outputs. So when designing the stator and the rotor, it can be based on velocity vector distribution of the flow field to select the proper blade inclination.From the velocity field and pressure field of the grid-interface, it is not difficult to find that the liquid inflows and outflows at the surface at the same time, which makes the interchange between the entrance and the exit where the liquid flows circularly produces a large number of vortex. The more the vortex increased and strengthed, the bigger.the brake torque becomes.By the analysis of the internal flow field of the hydraulic retarder, a better understanding of its working mechanism could be aquired. The analysis of the loss of hydraulic can give a direct viewing of the relationship between the internal characteristics and the external characteristics, and all of these can lay an important foundation for the hydraulic retarder's optimization design.3. The Characteristics Prediction and Experimental Verification of Hydraulic RetarderBased on the numerical solution of the three-dimensional flow field, the external characteristic of the hydraulic retarder is calculated. At the same time, the performance test of the prototype retarder is performed on hydraulic transmission test bench. The errors between simulation and test are within the allowable engineering range, which proves the validity of the simulation method used in this paper, simultaneously proves the feasibility of calculating the performance of hydraulic retarder by using CFD numerical simulation method.4. Structural Parameters Optimization of Automotive Hydraulic Retarder Based on Unsteady Flow Field AnalysisThe main structure parameters of hydraulic retarder are the shape of circulation circle, blade number, blade front rake angle and blade wedge angle, etc. In this paper, based on the original prototype, the loss analysis is used to obtain how each of the structural parameters influence the braking performance of hydraulic retarder, then the correlation among the structural parameters is analyzed and the model is further optimized. At the same time, the flow field analysis results are combined with finite element analysis to check the blade strength. The three-dimensional model of the blade is imported into the ANSYS—a finite element analysis software, and the pressure datas from the fluid numerical simulation results are also imported into the ANSYS and loaded on the blades by the method of coordinate transformation and multiple linear regression. The working blade strength in the condition of high-speed and fully liquid-filled are checked, then the optimization design is obtained. The simulation results between the original prototype and optimized model show that the optimized model not only increases braking torque but also ensures a reliable work of the retarder. Based on the optimized model, a charging and draining oil mathematic model is established and the oil charging and draining time is calculated, by which the calculation method using numeirical simulation results for oil charging and draining time is also studied.The heavy truck braking performance simulation model is established to analyze the influence of the optimized hydraulic retarder on the whole vehicle braking performance. After experimental verification for the whole vehicle braking model, a long time downhill braking under low speed and emergency braking in high speed is analyzed. Both of the simulation results show that the optimized hydraulic retarder matches the vehicle more reasonable than before.The parametric design method, study content and results of the hydraulic retarder based on the unsteady flow field analysis will have some theoretical and technical reference value on the optimization design and performance prediction for the hydraulic retarder's braking performances.
Keywords/Search Tags:Hydraulic retarder, Unsteady flow field, Two-phase flow, Parameter Optimization, Retarder Braking
PDF Full Text Request
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