Analysis On The Aerodynamic Characteristic Of The Interior Flow Field Of A Truck

With the development of the modem society, We have more requirement for the high speed, light weight and comfort while driving of an automobile. The comfort performance of the air-condition is more and more important in the automobile design, gradually it decides the success of automobiles in the furious market competition.Previously most study was focused on the comfort performance of the internal environment of the saloon, especially the limousine. Nowadays the research region is extending to the truck. For the truck of line-haul, as the driver will stay for a long time in the carriage, we have more and more requirement for the comfort performance of the internal environment, so it's meaningful to study the internal flow field in the cab of the truck.In the carriage in which the air-condition is turned on, the air which is disposed by the air-condition enters the carriage though the inlet. And after it exchanges the heat with the air in the carriage, the air vents though the outlet. So the air flow distribution in the cab is the important factor which influences the passenger's comfort performance. And it also influences the uniformity of the temperature distribution in the cab. Whether the air flow distribution is reasonable is related with the factors of the dimension and the position of the inlet and the outlet, the supply air, and the seal capability of the carriage. So it's necessary to study these aspects to improve the comfort performance in the carriage.The simulation method of CFD is used to discuss the influence of the dimension and the position of the 4 inlets on the instrument panel and the supply air and the mode of the circulation of the air-condition to the interior flow field. By the simulation, the influence of the factors hereinbefore to the interior flow field varies from each other. The computation and the analysis offer the reference to improve the comfort performance of the passengers in the interior flow field. It also lays the foundation for the forward study of the temperature distribution in the cab and the complicated instance of the different inlets while working together.In this paper, a real 3-dimentional model of a truck is used, and it is simplified. Some parts such as the seat and steering wheel which influence the interior flow field sharply are reserved, while the less one is ignored. The door inner panel, roof panel, floor and instrument panel should be smoothed properly. The human bodies of the driver and assistant driver are established in order to ensure the accuracy of the simulation. The human bodies are simplified from the SAE 50th percentile male model. Otherwise the nature convection, heat exchange, thermal radiation and thermal-physical model of the human body are not involved. Here follows the details of the projects: The study of the difference of the supply air is defined as the first group, which is composed of the model 1, model 2 and model 3. The study of the difference of the circulation is defined as the second group, which is composed of the model 1 and model 4. The study of the difference of the inlets' shape is defined as the third group, which is composed of the model 1, model 5 and model 6. The study of the difference of the inlets' area is defined as the fourth group, which is composed of the model 7 and model 8.In this simulation, the cells we used are the tetrahedral cells, which are generated by a special software. This kind of mesh is simple, and it can compute the vortex which is in a complicated position more accurately. So it is very suitable to this study. The standard k -εmodel is used as the turbulence model in the simulation. The first order difference scheme is used to do the computation. The inlets are the velocity inlets, and the outlet is the pressure outlet.At last, the FLUENT software is used to do the numerical computation for each model, and the computational results are analyzed. Here follows the conclusion:1. The supply air of the four inlets on the instrument panel influences the characteristic of the interior flow field prodigiously. When the supply air of inlet2 and inlet3 occupies 30% while inletl and inlet4 are 20% respectively, the comfort performance of the passengers on the seats is good, which means the velocity on the face is uniform, the range of the velocity varies from 0.2 to 0.35m/s. By reducing the supply air and changing the air direction, the velocity on the passengers' faces can reach the range of 0.2 to 0.25m/s which is the most comfortable range. When the supply air of the 4 inlets is separately 25%, both passengers on the seats and on the bed feel comfortable. As the supply air transfers more to the inletl and inlet4, the comfort of the passenger decreases. To avoid losing energy in the air duct of the air-condition, most air will blow off from the inlet which is closer to the main air duct, which means the air is blown off from the two inlets in the middle. This also matches the best project which meets the needs of the comfort performance of the passenger.2. The different circulations of the air-condition have influence on the characteristic of the interior flow field. When in the refresh, as the outlet is on the symmetry face of the body rear wall, the characteristic of the flow around the driver and the assistant driver is similar to each other, and it can meet the needs of the comfort performance of the passengers. When in the encircle, as the outlet is in front of the assistant driver's legs, the characteristic of the flow around the driver and the assistant driver is different from each other. The velocity of the flow around the assistant driver's body is very slow. That's very terrible for the heat exchanging. As the encircle contains the virtue that the refresh can't replace, which means the high efficiency of cooling and warming, both circulations are used in a real automobile. We can adjust the ratio of the encircle and the refresh to meet the different needs.3. The shape of the inlets has some influence on the interior flow field. In the cab with the circular inlets, the comfort performance is the best while the quadrate one is the worst. However by rounding the edge of the quadrate inlets can improve the comfort performance.4. The dimension of the inlets has a great influence on the interior flow field. Decreasing the dimension of the inlets will lead the high velocity of the blowing flow. The velocity of the whole interior flow field becomes no more uniform, it worsens the comfort performance of the passengers. And the two inlets in the middle contribute more comfort performance than the other two inlets on the side. So when in the practical design, the dimension of the inlet should be as large as possible especially the two inlets in the middle under the precondition of meeting the needs of the styling and the dispose. This is propitious to increase the comfort performance of the passengers.