Fluid-solid-thermal Physical Field Coupling Analysis And Structure Optimization Of Ventilated Disc Brake

During the working condition of vehicle ventilated disc brake,the friction heat generated by the ventilated disc and brake pads will cause thermal stress and non-uniform temperature field,along with heat conduction,thermal radiation and other physical phenomena.At the same time,due to the ventilation slot structure,there is an obvious forced convection heat transfer between the surface and the air flow field,the whole process is a typical fluid-solid-thermal coupling process.The current thermo mechanical coupling and fluid solid coupling research program are calculated based on two-phase coupled physical field while the actual boundary conditions have a certain deviation.Therefore,according to the variation of convective heat transfer coefficient in time and space,the integrated use of theoretical analysis,numerical simulation and experimental verification are applied in the paper for friction thermodynamics research,which can achieve direct fluid-solid-thermal coupling calculation,and heat transfer performance is improved by reasonable optimization method.The friction load of disc brake is analyzed according to the working principle and stuucture characteristics.The ventilated disc brake transition condition of elastic-plastic contact state and convection heat transfer principle is analyzed by using micro convex theory and coupling control equations.According to Hooke's law,the two-dimensional temperature-displacement coupling equation of the ventilated disc is derived,and the solution conditions are given.Considering the relationship of thermal conductivity,specific heat and temperature,the ratio between excess temperature and thermal conductivity is obtained by linear fitting,and the excess temperature of steady-state distribution characteristics in fin thickness direction is calculated.In order to research the heat dissipation characteristics of ventilated disc,the numerical solution of transient heat conduction is obtained base on explicit difference method.Through the analysis of separated variables derived from the transient heat transfer equation,the solution is extended to muti-dimensional heat transfer with product method.According to the characteristics of air flow and heat transfer in the boundary layer,the fluid governing equations(momentum equation and energy conservation equation)are simplified,and the solving method of the local convection heat transfer coefficient is obtained.Considering the rotation factor of the disk,the local and mean Nusselt numbers are concluded and used to express the dynamic(changes in radial direction)heat transfer characteristics of different structure(the end face and the fin)in laminar and turbulent conditions.Under the condition of continuous uniform braking,the two-dimensional heat transfer mathematical model is establishned,of which boundary conditions are determined by the local Nusselt number under different working conditions,and it is more accurate than the experience method.Based on the transient heat transfer equation under different conditions,the transient temperature field of the disc is obtained based on PDE model of MATLAB.The fluid-solid-thermal coupling process is simulated by Link3900 NVH test platform based on the bench test method,of which test and evaluation parameters are brake pressure,friction coefficient,spindle speed and temperature.Friction stability coefficient is introduced in the test process,and the changes at different braking speed,braking pressure and temperature conditions are obtained,along with the average friction coefficient,of which results show that only by increasing the braking pressure to increase the braking effect is not desirable.In the single test period,temperature changes in different brake pressure and brake speed conditions are obtained through the temperature sensor(thermocouple),which can verify the result of numerical simulation.The calculation solver of ABAQUS(solid model,thermo mechanical coupling)and FLUENT(fluid model,fluid solid coupling)are intergrated by the platform MPCCI for data exchange,which can realize full fluid-sol id-thermal coupling calculation.Among them,the coupling of the temperature field and stress field is proceeded by tight coupling algorithm,and the air convection effect and temperature field is proceeded by loose coupling algorithm.During the pretreatment of fluid model,sliding grid technology is applied to deal with the dynamic region.The turbulence model is chosen as RNG k-? model,and the near wall treatment is chosen as non equilibrium wall function.In order to make sure that the fluid mesh is reasonable,refine and adjust mesh demension until the y+of first layer nodes are near 30?60.Ensure that the boundary conditions of experiment and simulation calculation are the same,and the analysis results show that the simulation node value is corresponding to the temperature,which shows a good match with the test results,and the average deviation of each peak temperature is 4.2%,it can be concluded that the numerical simulation scheme has good accuracy and feasibility.According to the structure characteristics of disc and fin,different paths are set for comparison,and the changes of path convection heat transfer coefficient,temperature and stress along the paths are acquired.The parameterized finite element model of ventilated disc brake is established based on ANSYS/Workbench for the solution of transient heat transfer process.The brake pad model is omitted and the heat bearing surface of the model is finished by freezing and cutting.With the using of central composite design method,the optimization variables(total disc thickness,ventilation slot height,fin lateral angle and fin number)and optimization(disc quality and peak temperature)are calculated as sample test design according to the decision space of 145 design points.Through multivariate two regression model technology,these design points are fitting as response surface function,of which correction is judged by fitting coefficient,correction coefficient and root mean square deviation,metting the requirements of the approximation model.The multi objective optimization problem is transformed into one single objective optimization problem,and the optimization mathematical model is established.The Pareto optimal solution are obtained by the sequence of sequential quadratic programming algorithm.The normalized weighted method(min-max standard)is used for price evaluation,and the multi-objective decision can be merged into a single target,which can meet the requirements of different types of locomotive brake.