Body surface cooling

High heat of vapourization Helps the evaporation process and cooling of body surfaces. 

It is well known that exposure of the human body to cryogenic fluids or to surfaces cooled by cryogenic fluids can result in severe cold burns since damage to the skin or tissue is similar to that caused by an ordinary burn. The severity of the burn depends on the contact area and the contact time prolonged contact results in deeper burns. Severe burns are seldom sustained if rapid withdrawal is possible. 

The three bodies — plate, very long cylinder and sphere — shall have a constant initial temperature d0 at time t = 0. For t > 0 the surface of the body is brought into contact with a fluid whose temperature ds d0 is constant with time. Heat is then transferred between the body and the fluid. If s < i90, the body is cooled and if i9s > -i90 it is heated. This transient heat conduction process runs until the body assumes the temperature i9s of the fluid. This is the steady end-state. The heat transfer coefficient a is assumed to be equal on both sides of the plate, and for the cylinder or sphere it is constant over the whole of the surface in contact with the fluid. It is independent of time for all three cases. If only half of the plate is considered, the heat conduction problem corresponds to the unidirectional heating or cooling of a plate whose other surface is insulated (adiabatic). 

The pure fluidized bed crystallizers described later in Example 7-6, while designed for a particular purpose, have many similarities with the Oslo commercial surface-cooled crystallizer shown in Fig. 7-10. A fluidized magma in the crystallizer body (E) carries out the crystallization. Feed enters the clear overflow stream (at G), is cooled within the metastable super-saturation region in the cooler (H), and then enters as a fluidizing stream at the bottom of (E). 

The conditions (1 l-7a) and (1 l-7c) correspond to the assumption that the surface of the body is heated (or cooled) to a constant temperature T0 beyond a certain position denoted as x. Upstream of x, the body is not heated. Hence, at steady state in the present boundary layer limit, both the body surface and the fluid remain at the ambient temperature T,x, for x < x. If the body is heated over its whole surface, then x = 0. In this case, the leading edges of the thermal and momentum boundary layers are coincident. 

N. R. DesRuisseaux and R. D. Zerkle, Temperature in Semi-Infinite and Cylindrical Bodies Subjected to Moving Heat Sources and Surface Cooling, J. Heal Transfer, 92, pp. 456-464,1970. 

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