| Aerodynamic and heat transfer performance of the rear-mounted engine cabin have important influence on security of engine and stability of vehicle traveling. And they are also closely related to the structural design of engine cabin. With the increasing functional demands of modern automobile and expansion of passenger space, the structure of engine cabin becomes more and more compact. These above will deteriorate the performance of ventilation and heat transfer, especially for its overheating problem of rear-mounted engine cabin. Therefore, the research and optimization on cabin heat dissipation play a key role in the rational design of engine cabin.The cooling performance of engine cabin in the past researches mainly depended on empirical coefficients and experimental test datum. Due to the limited amount of collected information, it was difficult to sort out the law of engine cabin heat transfer. Based on the known thermal boundary conditions,3D model can calculate flow resistance characteristics and boundary heat dissipations of engine cabin by simulating the spatial distributions of flow and temperature field. By setting parameters such as drag coefficient, heat transfer coefficient and ambient temperature,1D model of engine cooling system can simulate the variations of engine heat dissipation with different conditions.This paper, through conducting field trials on the rear-mounted engine cabin, compiles temperature distribution of cabin interior component surface under different working conditions. A3D model of wind tunnel and a1D model of engine cooling system have been respectively established by STAR-CCM and GT-COOL software. Otherwise, a1D/3D manual coupled model is built by exchanging parameters of flow and heat transfer performance. Based on thermal equilibrium theory of engine cooling system, these exchanging parameters models are determined by excluding secondary factors. Contrast to direct coupled model, the manual coupled model is more suitable to study the characteristics of heat transfer in this paper. In order to ensure the rationality of coupled simulation, coupled model is calibrated and validated by using field test datum in the coupled modeling process. In1D model, fitting polynomial of fan performance under different speed is derived according to the performance under the rated speed. Then the cooling fan performance is matched to engine speed so as to reflect actual working condition of engine cabin. This paper calculated cooling performance of engine cabin in different ambient temperatures, road grades and vehicle speeds. Drag coefficient, average heat transfer coefficient and bulk temperature of engine cabin are analyzed and fitted into associated formulas. The analytical results show that the generations of air reflow are easier and drag coefficient is larger at low vehicle speed. It indicates that the drag coefficient can be reduced apparently with the increasing vehicle speed but below20km/h. Average heat transfer coefficient of engine cabin increases in type of power function with the growing vehicle speed, but decreases with the growing drag coefficient. It indicates that cooling performance of engine cabin is affected by flow field of engine cabin. Engine cabin bulk temperature increases with growing engine load, but decreases with growing vehicle speed. This shows that bulk temperature is affected by vehicle speed under a certain engine load.Drag coefficient, average heat transfer coefficient and bulk temperature of engine cabin, which reflect the influence of vehicle speed and cabin structure on dragging and cooling performance, provide a more rational method and basis for the evaluation of cabin cooling performance. Based on average heat transfer coefficient and bulk temperature associated formulas of engine cabin,1D model of engine cooling system can work independently divorcing from coupling environment. It can provide more effective and efficient method for simulating cooling performance and optimizing equipment in engine cooling system. |
