Computational Fluid Dynamics is used to assess the thermal (heat transfer) performances of an automobile engine considering different grille opening and closing degrees. For this purpose the entire vehicle is modelled and three fundamental aspects are examined, namely, the open area of the air intake grille, the position of the upper and lower grilles and their shape. The results show that the opening area and position of the grille have some influence also on the aerodynamic characteristics of the automobile. With an increase in the opening angle of the grille, the C_{D} (Drag Coefficient) value of the whole vehicle becomes higher. When the air intake grille of the car is fully open or closed, the C_{D} value is 0.35434 or 0.31777, respectively, that is, the flow resistance in the engine compartment accounts for 10.32% of the C_{D} value for the whole automobile.

The automobile has entered the ordinary people’s family as a common means of transportation [

Different opening areas, positions and forms of grille influence the wind resistance and engine compartment heat dissipation [

The method of computer numerical simulation technology can visualize the velocity field and temperature field of the whole vehicle flow field, and calculate the C_{D} (Drag Coefficient) value of the whole vehicle under different grille structures, which facilitates the data reading and analysis. In a word, there are still multiple deficiencies in the research of heat transfer function of automotive engine, so the further theoretical and practical research of active grille air intake is still needed.

Based on the professional commercial CFD (computational fluid dynamics) analysis software Fluent, the method of numerical simulation technology [

CFD is the product of the combination of modern fluid mechanics, numerical mathematics and computer science, which is a cross science with strong vitality. It approximately expresses the integral and differential terms in the governing equations of fluid mechanics as discrete algebraic forms, making them become algebraic equations. Then, it solves these discrete algebraic equations by computer to obtain the numerical solution at discrete time/space points [

Mass-conservation equation

Mass conservation means that the increase of unit mass is the same as the outflow in unit time, and the mass is constant in large watershed, so it is also called continuity equation.

^{3};

The research object of

The energy conservation equation is as follows:

_{P} suggests the specific heat capacity, _{T} represents the viscous dissipation term, and

Based on the above conservation equations, it can be found that there are six variables to be solved, namely,

For ideal gas, the equation of state is:

R is the molar gas constant [

Numerical simulation parameter setting: the air intake grille of the basic vehicle simulation model is in the form of a horizontal bar [^{2} and 0.05 m^{2}, respectively. In the four simulation models, when the opening area of the air inlet grille is 0, it is completely closed. Only the upper grille is open at 0.078 m^{2}, only the lower grille is open at 0.05 m^{2}, and all the grilles are open at 0.13 m^{2}.

Heat transfer parameter setting: the main function of the opening and closing of the air intake grille is to dissipate heat [

Therefore, the effective area of the active grille when it is fully open is set to 0.11825 m^{2}.

Parameter setting of combustion switch closing angle: the cooling water temperature shall not be higher than 110°C. When it reaches about 95°C, the engine is in a more suitable working state [

The opening area and opening and closing situation of the upper and lower grilles suggest that: (1) the trend of air intake increases when the opening area increases. The trend of the two is the same, showing a positive correlation, when the front structure of the car first contacts with the air along the surface, the opening area becomes larger, and the direct blocking effect decreases. (2) when the air intake of the grille increases, the C_{D} value is not positively correlated with it. That is because the opening position of the grille is different when it is only opened up and only opened down, affecting the flow field distribution in the engine compartment, so the position of the grille will also affect the C_{D} value. (3) The C_{D} value is 0.35434 when the grille is fully open and 0.31777 when the grille is fully closed. Hence, the flow resistance coefficient of the engine compartment is 0.03657, accounting for 10.32% of the total resistance coefficient of the car. (4) The cooling capacity of the cooling module is not fully utilized when the fan speed is high and the vehicle is driving at high speed on the straight road, and the cooling performance is surplus.

The crucial results concerned are counted after the completion of numerical calculation.

_{D} value and intake volume of the whole vehicle also increase. The opening area increases by 33.86% from area scheme 1 to area scheme 4, resulting in the increase of grille air intake by 41.38%, the increase of radiator air flow by 58.28%, the increase of vehicle C_{D} value by 5.68%, and the decrease of temperature by 7.21%. The goodness of fit R^{2 }=^{ }0.9941.

The air intake is positively correlated with the opening area, while the maximum temperature of the cooling module is negatively correlated with the opening area. The whole trend can be summarized as follows. The air flow into the engine compartment increases with the increase of the effective opening area, and the heat carried away increases correspondingly; while the temperature of the radiator and cabin decreases. The analysis suggests that the maximum temperature of the cooling module in the opening area scheme 1 is 107.8°C. Too high temperature will cause serious consequences to the service life and operation of the engine compartment cooling module. The opening area of the car should be larger than it.

The first is the relationship between the drag coefficient and the opening. _{D} value and the opening of the whole vehicle.

The relationship in _{D} value of the whole vehicle is. As long as the opening angle of the air intake grille is not 0, a part of the air flow along the surface will enter the engine compartment through the air intake grille in the front of the vehicle, and this part of the air flow in the engine compartment will produce internal flow resistance.

The second is the analysis of the velocity field of the active air intake grille. The speed change at the grille is that the closer it is to the outside of the vehicle, the greater the speed is, and the greater the mass flow of the intake air is. This is because when the car is driving to the left, part of the air will flow to both sides of the car. Due to the interaction, the closer it is to the outside, the more the air volume and kinetic energy increase, the greater the speed and amount of air intake of the external grille. Furthermore, the flow velocity near the grille accelerates and the air intake volume increases with the increase of the opening. This is because the effective air intake area increases after the opening is large, so the air intake volume increases. The air flow basically follows a straight line when the opening angle is 75° and 90°. There is obvious direction change when the opening angle is 60°. Next, the direction change increases a lot with the decrease of the opening angle; the turbine is formed in the upper right corner when the opening angle is 30°; even when the opening angle is 15°, the air flow has backflow phenomenon, greatly affecting the air intake efficiency and heat dissipation. Hence, the smaller the opening of the air intake grille is, the smaller the air intake of the grille is, the smaller the capacity of the radiator is, and the smaller the C_{D} value of the whole vehicle is.

The initial position of each state is determined by the combination of three factors and seven levels orthogonal experimental design and simulation results. Obviously, the speed of fan and the vehicle increases gradually with the increase of the grille opening, which will reach the peak when the grille opening reaches 90°.

The upper area of each fan speed curve in

To sum up, the proposed numerical simulation method based on CFD analysis software Fluent can simulate the grid parameters of the actual traffic volume. The flow field simulation analysis is carried out through the establishment of different inlet grille opening and closing angle models, and the influence law of grille opening on vehicle C_{D} value is studied, so as to optimize the wind resistance of vehicle and the heat dissipation performance of engine compartment. In the study of the fluid mechanics performance characteristics of small mobile vehicles, Sun et al. [

The computer numerical simulation technology is adopted to analyze and summarize the influence of the structural parameters of the air intake grille on the heat transfer function of the whole vehicle. Then, the calibration of the active air intake grille is studied. The flow field simulation analysis is conducted through the establishment of different opening and closing angle models of the air intake grille, and the influence law of the grille opening on the vehicle C_{D} value is evaluated. The C_{D} value of the whole vehicle increases with the increase of the opening angle of the grille. Besides, the simulation results of different grille forms are analyzed by the control variable method. The influence of the grille form on the heat transfer function of the whole vehicle is interpreted from different angles. Although the influence of different opening and closing degrees of the grille on the heat transfer function of a vehicle has been explored, there are still multiple deficiencies. First, the ventilation in the passenger compartment, the brake cooling ventilation and the flow of engine intake and exhaust are ignored. Then, the numerical simulation analysis of the flow field in the engine compartment needs to be put into the wind tunnel calculation domain together with the vehicle flow field. In the future, these two aspects will be further explored to continuously improve the model results.

This article has received a lot of help from the outside world from the topic of research to the final writing of the manuscript. Here, I would like to express my heartfelt thanks to the units and individuals who have provided help and support for this research.