Convective Heat and Mass Transfer
This chapter on convective heat and mass transfer starts out from Prandtl’s original chapter Heat Transfer in Flowing Liquids. We will treat free convection flows, caused by the density changes in the fluid due to temperature and concentration gradients. These cause a lift in the gravitational field, which in turn causes convection flows. Examples of free convection flows at heated cylinders and plates were shown in the introductory chapter in Figure 1.6. The Rayleigh–Bénard convection of Figure 1.5 and diffusion convection are also examples of free convection flows.
Forced convection flows occur when an external force, such as a pressure gradient, also acts on the flow. Forced convection flows occur, for example, in heated or cooled pipes such as those used in heat exchangers.
Heat and mass exchange processes are found in the ocean and in many different processes in chemical technology, such as absorption, adsorption, extraction, and distillation. When water evaporates on the surface of the oceans, a high salt concentration remains, and an unstable density layer with diffusion instabilities occurs. The expansion of substances in solvents and the separation of substances in centrifuges are further examples. Examples of biological mass exchange processes are the supply of oxygen to the blood and absorption of food in the body.
KeywordsNusselt Number Prandtl Number Rayleigh Number Free Convection Local Nusselt Number
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