| As more and more high-rise buildings appear in the city, the fire-fighting tasks are also increasing. As a result, the performance of fire-fighting trucks is largely improved, such as the growing power of fire engine. However, the temperature of transmission gear is higher and higher. Power takeoff is a power transmission tool which is to transmit the engine power of fire-fighting truck to fire extinguishing pump so as to carry out the watering operations. Its temperature changes a lot because of its own structure and position in the fire engine. Though power takeoff is an essential part in special cars, it is not designed and made by the professional manufacturer. When the fire engine carries out a parking work with heavy load for a long time, the temperature of power takeoff increases rapidly; as a result, the output shaft bearing temperature would exceed the upper limit of national standard temperature. This thesis has firstly got the steady-state temperature distribution of box when power takeoff is working with heavy load for a long time. Then, this thesis has designed to install a set of power takeoff oil cooling equipment to the existing fire engine, in order to solve the problem that power takeoff would stop working because of the high temperature.At first, the author set up a three-dimensional geometric model of power takeoff. After a necessary simplification and based on the theory of heat transfer and heat conduction, the author established a grid model and computed the respective heat flux density value on the gear surface and in the bearing inner and outer rings. Meantime, the corresponding convective heat transfer coefficient on the inner and outer surface of power takeoff box is also computed. Afterwards, the author made a thermal analysis on the power takeoff box based on finite element method, by which a steady-state temperature distribution of box is obtained. The result has shown that after four hours of continuous full load working of fire engine, the power takeoff output shaft bearing's temperature exceeds that of the national standard which is not exceeding 100℃.Based on a comprehensive consideration of the fire engine's spatial parameters, and in order to guarantee an effective heat exchange and low cost of investment and delivery, this thesis has adopted an ordinary single-arched shell and tube heat exchanger as the power takeoff oil heat exchanger. Besides, this thesis set up three-dimensional geometric model and grid model of heat exchanger, and simulated the shell side fluid convection and heat transfer process of six baffle heat exchangers, by using the standard K-e model and the standard wall function. The result shows that this type of heat exchanger can decrease the oil temperature effectively, which provided a better heat dissipation for power takeoff.The heat exchanger is easy to form scale because that when the fire engine is carrying out fire-fighting operations, it fetches water nearby which often contains a lot impurities, and that there is a water back-flow phenomenon inside the heat exchanger since inside the single-arched shell and tube heat exchanger, the baffle would change the fluid direction. By adjusting the heat exchanger baffle spacing and cutting height, this thesis has compared four baffles with different spacing values and that with different cutting heights. Then, this thesis analyzed their respective temperature, pressure and velocity fields, and compared their different influences on the shell side flow, both of which have provided theoretical basis for setting suitable baffle spacing value and cutting height, and reducing pressure loss on heat exchanger shell side, as well as improving the heat transfer coefficient and getting rid of internal scaling of heat exchanger. The experiment and analysis provide a theoretical basis for applying this type of heat exchanger to fire-fighting truck in practice. |
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