Study On The Heat Transfer Characteristics Of The Internal Channel Of The Brake Disc Considering Ground Effect

In the process of braking,the friction between the brake disc and the brake disc slows down the moving vehicle until it stops moving.In the braking process,the kinetic energy of the vehicle is transformed into heat energy at the friction point,which makes the temperature of the brake disc rise rapidly.If the brake disc can not dissipate heat effectively,the uneven distribution of temperature in the brake disc will increase,resulting in great thermal stress.On the one hand,the increase of the temperature of the brake disc decreases the friction coefficient and the braking ability.On the other hand,the brake disc appears hot spots,hot cracks and thermal fatigue,reducing the braking effect and the service life of the brake disc,affecting the safe operation of the vehicle.Therefore,it is of great significance to study the heat transfer characteristics of brake disc channel and to understand and strengthen its heat transfer capacity for reducing vehicle running costs and improving vehicle running safety.At present,the equivalent models used by scholars in the study of brake discs can be divided into three types.The first is the fixed-axis rotation model,the second is the equivalent brake disc rolling process through the fixed-axis rotation model plus air flow,and the third is the rolling model.In the first part of this paper,the pin-fin brake disc specimen used in the experiment is taken as the object,and the simulation model of the brake disc specimen rolling on the circular wall is obtained by simplifying the experimental device.Then,the dynamic mesh method is used in Fluent to simulate the rolling process of the pin-fin brake disc specimen on the wall.The distribution of the flow field around the brake disc during rolling and the heat transfer characteristics of the internal channel of the brake disc are analyzed.In the second part of the paper,the straight rib brake disc is taken as the research object,and the brake disc model is equivalent to the rotating plus air flow model.The simulation model is established and the grid is divided.The heat transfer characteristics and ventilation characteristics of the internal channel of the straight rib brake disc are analyzed by using the sliding grid method considering the influence of the ground.The results show that for the pin-fin brake disc specimen rolling on the circular rolling wall:（1）The air velocity in the internal channel is relatively large in the leeward area,and the air velocity near the rolling wall is small,and there is obvious vortex structure in the air wake area.（2）The convective heat transfer coefficient of the internal channel disk surface on the windward side is larger,and the heat transfer coefficient on the leeward side is smaller.The high convective heat transfer area around the rib pressure side is very obvious U-shaped.The convective heat transfer coefficient of the heat dissipation rib on the windward side is larger than that on the leeward side,and the convective heat transfer coefficient on the pressure side of the heat dissipation rib is larger than that on the back pressure side.（3）The relationship between the average convective heat transfer coefficient and the angular velocity of the internal disk surface and the heat dissipation rib of the brake disc specimen is h=0.8255ω^0.8093and h=0.6667ω^0.9185.For the straight rib brake disc equivalent to the rotating plus incoming flow model:（1）On the windward side,a stagnation point and a stagnation area are formed on the outer edge of the heat sink due to the incoming flow.For the upper side of the windward area,there is backflow and vortex at the inlet,and a relatively smooth flow is formed on the pressure side of the heat sink,and the flow velocity on the back pressure side is relatively low.Vortex is generated at the inner edge exit.For the upper side of the leeward side,a stagnation point is formed on the inner edge of the heat dissipation rib due to the impact effect in the upper area of the leeward side.The air in the channel flows from the inner edge side to the outer edge side,and the air velocity on the pressure side of the heat sink is high,and the air velocity on the back pressure side is low.（2）For the internal surface,the maximum convective heat transfer coefficient is on the windward side.On the windward side,the convective heat transfer coefficient on the outer edge of the inner disk is larger than that on the inner circle side,and a U-shaped high convective heat transfer area is formed on the outer side of the heat sink.The heat transfer coefficient of the pressure side near the fin is greater than that of the back pressure side.The local heat transfer coefficient of the pressure side of the heat sink is larger than that of the non-pressure side.（3）The relationship between the average convective heat transfer coefficient of the internal surface of the brake disc and the average convective heat transfer coefficient of the cooling rib and the angular velocity is h=3.236ω^0.8089and h=3.516ω^0.8128.（4）The mass flow rate of the internal channel of the brake disc is linear with the speed.When the velocity increases,the mass flow through the channel at the same position increases.The proportion of fluid flowing from the inner edge to the outer edge in the rotating channel of the brake disc is larger than that from the outer edge to the inner edge.（5）P=0.00481v³-0.00676v²+0.914129v-7.2227 can be obtained by fitting the pump air loss at different speeds of the straight rib brake disc used in the calculation.