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The Mystery of Contact Thermal Resistance: Why does heat lose part of it when it flows between two objects?
Thermal contact conductance is a key concept when we discuss thermal conduction, which involves the transfer of heat between two objects. In this process, heat does not always pass through completely and is often lost on the contact surface of the two objects. The reason behind this is thermal contact resistance. This article will delve into the roots and factors that influence contact thermal resistance and explain why this phenomenon is critical in many practical applications.
What is thermal contact resistance?
Thermal contact resistance refers to the resistance to heat flow between two contacting objects due to the unevenness of the contact surface and the presence of gas or liquid. Therefore, when heat flows from a hot object to a cold object, a certain temperature drop will occur on the contact interface.
When two objects (such as metals A and B) are in contact, heat flows from A (the hotter object) to B (the cooler object). Ideally, heat flow is continuous, but in reality, at the contact point between two objects, thermal contact resistance is formed due to surface unevenness and tiny gaps. This phenomenon causes part of the energy of heat flow to disappear during the transfer process, and is converted into other forms or dissipated.
Factors affecting contact thermal resistance
Contact thermal resistance is affected by many factors, the following are some of the most critical factors:
Contact pressure
Contact pressure is one of the most important factors affecting heat conduction. When two objects are pressed together, the increase in contact pressure increases the true contact area, thereby reducing the thermal contact resistance.
The relationship between contact pressure and thermal contact conductivity has been explored in depth in many studies, showing that as contact pressure increases, thermal contact conductivity also increases.
Interface Materials
There can't be a completely smooth surface between two objects, which greatly reduces the actual contact area. In addition, the gas or fluid filled in these tiny gaps will affect the heat conduction.
The thermal conductivity and pressure of these filler materials have a significant impact on the overall contact thermal conductivity. In a vacuum the contact resistance increases significantly, since the heat flow is mainly conducted through the contact points.
Surface roughness and waviness
Surface characteristics, such as roughness and waviness, affect how well heat is transferred. These factors typically narrow the path for heat flow, further increasing the thermal contact resistance.
In practical applications, the degree of metal surface finishing will directly affect its contact thermal conductivity with other materials.
Surface cleanliness
Impurities such as dust and oil will affect contact thermal conductivity. Keeping the contact surfaces clean helps reduce thermal contact resistance.
Measurement of thermal contact conductivity
Due to the difficulty in measuring the contact area, the calculation of thermal contact conductivity often needs to be obtained through experiments. Many research institutions and companies are working hard to establish databases in this area so that they can be applied more accurately in design and calculation.
SummaryThe study of thermal contact resistance not only helps us understand the process of heat transfer between objects, but also helps us improve thermal management performance in different industrial applications. Good thermal contact is critical in many fields such as electronics, construction, and aerospace. However, can we pay more attention to the impact of these physical phenomena in our daily lives?
