| With the increase of China's high-speed railway operating mileage,the national economy has developed rapidly and people's livelihood has been greatly improved.The primary goal of high-speed rail construction and operation is to ensure safety and reliability,and the key equipment to achieve this goal is the foundation brake device.Braking technology has become one of the important factors affecting the development of high-speed railway technology.With the improvement of train running speed,the heat generated by braking also increases significantly.How to ensure the safety of train deceleration has become an urgent problem to be studied and solved.At present,At present,disc braking is the main mode of high-speed train,and resistance or regenerative braking is the auxiliary mode.During the braking process,when the electrical braking suddenly fails,the friction braking system will bear a huge impact of thermal load,which requires the brake disc to have higher braking performance.The performance of disc brake system mainly depends on the ability of brake disc and brake pad to withstand heat capacity,wear and heat dissipation.Heat capacity and wear are mainly affected by friction surface temperature and stress.Good heat dissipation performance of brake disc helps to reduce the temperature and thermal stress on the surface of brake disc,and ensures the safe parking or deceleration of high-speed train.This paper mainly studies the heat dissipation performance of brake disc,the distribution and variation of temperature field and thermal stress field of brake disc under emergency braking conditions.It provides a basis for optimizing the structure of brake disc,improving the heat transfer performance of brake disc and realizing the safe operation of high-speed train.The main research contents and conclusions are as follows:(1)The convection heat transfer coefficient on the surface of the brake disc during emergency braking is analyzed by numerical simulation.Fluid-solid coupling heat transfer model is established in software ANSYS FLUENT,which is solved by partition and boundary coupling method,and the rotation and translation of brake disc are considered in the calculation model.The distribution of convection heat transfer coefficient on each surface of the brake disc and its change rule with braking time are solved by transient analysis.The results show that during the braking process,the air around the brake disc forms a complex vortex flow,which is helpful for convection and heat dissipation.The average convection heat transfer coefficient on each surface of the brake disc decreases with the increase of braking time,and the average convection heat transfer coefficient on each surface is close to the natural convection heat transfer coefficient at the end of braking.Except the moments close to the end of braking,at the same braking moment,the convective heat transfer coefficient in the windward area of the outer diameter edge of the brake disc is higher than that in the leeward area,while the convective heat transfer coefficient in the inner diameter edge of the brake disc is opposite to that in the outer diameter edge.The average convection heat transfer coefficient on the outer surface of the brake disc is higher than that on the inner surface of the inner passage,and the average convection heat transfer coefficient on the outer surface changing with braking time is more stable than that on the inner passage.The numerical results obtained by grid independence verification are accurate.(2)Applying ANSYS Transient Thermal and ANSYS Transient Structural modules integrated by ANSYS Workbench platform,the thermal-mechanical coupling symmetrical structure model of the braking system was established respectively.The calculated boundary conditions are heat flow input boundary and convection heat dissipation boundary.The transient thermal analysis was used to analyze the distribution and variation rules of the temperature field and thermal stress field of the brake disc in the emergency braking condition of 250km/h high-speed train.The influence of different load methods and different materials on the temperature field and thermal stress field of the brake disc was also considered.The results show after calculation analysis: use input the average convective heat transfer coefficient of the various surfaces of the brake disc method loading convection load to calculate the instantaneous maximum temperature of the brake disc and the instantaneous maximum thermal stress in the process of braking,which along with the growth of the braking time first increase then decrease.The convection load is loaded by the direct coupling method,the calculated instantaneous maximum temperature of the brake disc increases first,then decreases,and finally increases gradually with the increase of the braking time,the instantaneous maximum thermal stress is along with the growth of the braking time first increases then decreases,this solution method helps to reflect the actual thermal load when the brake disc is braked.The results of the two methods were compared and analyzed,and it was found that in the whole braking process,the thermal stress peak of the brake disc appeared several seconds earlier than the temperature peak,indicating that the thermal stress of the brake disc was not only related to the temperature field,but also related to the structure of the brake disc;Using direct coupling convection load to get the brake disc temperature peak load and thermal stress peak are better than in the average convective heat transfer coefficient of load flow load to solve the two peak time in advance,and the former method of peak temperature and thermal stress peak value are lower than the latter,which indicates that the data obtained by loading the convective load by the average convective heat transfer coefficient is more conservative.The comparative analysis of the influence of different materials on the thermal load of the brake disc shows that the material HT300 and 25Cr2 MoV do not meet the performance requirements of brake disc of 250km/h high-speed train.Therefore,it is suggested to consider materials with large tensile strength and specific heat capacity but small thermal expansion coefficient and elastic modulus when selecting brake disc materials. |
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