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Introduction
The thermal boundary layer is a region whereby the temperature gradient (dT/dy) is at 90 degrees or in a direction perpendicular to a flow of a free stream. The free stream usually approaches with a temperature- T to a different temperature plate of Ts , so that T not equal to Ts , then the generation of the thermal boundary layer is said to have happened. Therefore, only when the temperature of the surface of such a plate is not equal to the free stream’s temperature, thermal boundary layer can exist.Thermal Boundary Layer Definition
To explain the concept one needs to look into this example. Suppose a fluid of a certain temperature is passing through a surface of a different temperature, the temperature gradient causes a boundary between the fluid passing with the different temperature and the surface having a different temperature. This will be clearer if we consider touching a very hot plate with a flowing fluid of different temperatures. The hotness of the plate will transfer to the fluid only to a certain extent after which the fluid will remain at the same temperature as it was. The boundary in this case is created by only that much fluid to which the temperature of the plate was transferred. This boundary separates the temperature of the hot plate from the fluid away from the plate. It has to be understood that such an occurrence is possible only when the fluid is in a moving state or when it is a free stream. This is why when a hot object touches a free stream, the side immediately to the object gets heated to some extent but the other side of the free stream, away from the object, remains of the same temperature as it was. This is because of a thermal boundary layer that has been created by the immediate part of the stream to which the temperature from the object has transferred or flown. It is as if the immediate part of the stream touching the object sticks to the object and makes its movement slow. This is also known as the hydrodynamic boundary layer.Velocity and Viscosity of the Free Stream
The free stream in the vicinity of the surface with different temperatures will create the boundary which is usually a very thin layer. The velocity gradient is normal towards the direction of the flow of this thin layer. The gradient is formed because the immediate contact slows it down. The gradient of velocity can be large. Even if the viscosity of the flowing fluid is small, in this thin region or the boundary created it exerts shearing stress as well as substantial influence, which can have large values. The thickness of the thermal boundary layer is quite less. Moreover, the less viscous the fluid is, the thinner boundary layer is created. However, other than this region of boundary, the rest of the stream remains unaffected due to this boundary created between the surface and the rest of the stream of fluid. Therefore, no velocity gradient exists in the rest of the stream, flow is frictionless and the viscosity of the fluid does not matter because this region or part of the fluid flow remains unaffected.Essential Characteristics of the Boundary Layer
- The leading edge of any plate touching which a free stream is passing does not have any boundary layer. Therefore, the leading edge of such a plate has zero boundary layer thickness.
- The trailing edge of such a plate has the maximum boundary layer thickness.
- The thickness of the boundary layer is inversely proportional to the free stream velocity. This means when the stream has more velocity less is the thickness of the boundary layer.
- The thickness of the boundary layer is directly proportional to the fluid viscosity. This means that the more viscous the fluid of the free stream is the more thick the boundary layer will be.
