What is thermal resistance?
Thermal resistance refers to the thermal resistance encountered when heat is transferred by heat conduction inside an object. For the cross-sectional area through which the heat flow passes unchanged
Schematic diagram of thermal resistance
Flat plate with thermal conductivity of L/(k*A). Where l is the thickness of the plate, a is the cross-sectional area of the plate perpendicular to the heat flow direction, and k is the thermal conductivity of the plate material.
In the process of convective heat transfer, the thermal resistance between solid wall and fluid is called convective heat transfer thermal resistance, 1/(hA). Where h is the convective heat transfer coefficient and a is the heat transfer area. The thermal resistance when two objects with different temperatures exchange heat with each other is called radiant thermal resistance. If both objects are blackbody (see blackbody and graybody) and the absorption of heat by the gas between the two objects is ignored, the radiant thermal resistance is 1/(A1F1-2) or 1/(A2F2-1). Where A1 and A2 are the surface areas of two objects radiating from each other, F1-2 and F2-1 are the radiation angle coefficients.
When heat flows through the interface of two contacting solids, the interface itself presents obvious thermal resistance to heat flow, which is called contact thermal resistance. The main reason for thermal contact resistance is that the actual area of direct contact between any two objects that appear to be in good contact on the outside is only a part of the interface (see figure), and the rest is a gap. Heat is transferred by heat conduction and heat radiation of the gas in the gap, but their heat transfer capability is far lower than that of ordinary solid materials. Contact thermal resistance causes a sudden drop in temperature T along the direction of heat flow when heat flow flows through the interface, which is a phenomenon to be avoided as much as possible in engineering applications. The measures to reduce thermal contact resistance are as follows: ① increase the pressure on the contact surface of two objects to deform the protruding part on the interface of the objects, thus reducing the gap and increasing the contact surface. (2) Coat the interface of the two objects with a gel-like object with higher thermal conductivity-thermal grease.
