A flat plate is an ideal heat transfer geometry. Truck radiators are used for heat exchange between two fluids, one fluid is air and the other fluid is coolant. Part of the heat generated when the engine's fuel is burned is transferred to the coolant through the coolant boundary layer (liquid film for short) close to the inner wall of the water jacket, the metal wall of the truck radiator (including water pipes and heat sinks), and the The cooling air boundary layer (referred to as gas film) on the outer surface of the metal wall is transmitted to the atmosphere.
Heat transmission path of truck radiator:
Coolant→Liquid film→Metal wall of truck radiator (including water pipe and heat sink)→Air film→Atmosphere
The total thermal resistance of a truck radiator is equal to the sum of the thermal resistance of the boundary layer of the coolant on the inner wall of the water pipe, the thermal resistance of the metal material itself and the thermal resistance of the boundary layer of the cooling air outside the water pipe, and the reciprocal of the thermal resistance is the heat dissipation coefficient.
It can be expressed by the following formula: total thermal resistance = liquid side thermal resistance (liquid film thermal resistance) + pipe wall material thermal resistance + gas side thermal resistance (gas film thermal resistance + heat sink material thermal resistance)
According to the test, if the total thermal resistance is 100%, the distribution of each thermal resistance is approximately: liquid film thermal resistance accounts for 10%, tube wall material thermal resistance accounts for 1%, gas film thermal resistance accounts for 80.1%, and heat sink material accounts for 8.9%. Therefore, in the entire process of heat transfer from the coolant to the atmosphere, the main hindrance to heat transfer is the thermal resistance of the boundary layer of the cooling air on the surface of the truck radiator, followed by the thermal resistance of the boundary layer of the coolant on the inner wall of the water pipe. While the absolute value of the thermal resistance of the metal material is very small, it has little effect on the heat dissipation coefficient K value, therefore it is usually omitted in the calculation.