10 Convection Part 1

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L.D.D Advanced Building Physics - Convection, Thermal circuits, Thermal bridges 1 - HEAT TRANSFER BY CONVECTION AND NEWTON'S POSTULATE The natural convection is a phenomenon tha occurs as a result of the fact that if two points of a fluid are at different temperatures, they also have different density and this influences the forces they are subjected to in the presence of a gravitational field. Example: if we heat a fluid from below, the less dense part of fluid (higher T), subjected to hydrostatic pressure for Archimedes' law, will move to other areas at lower temperature carrying with it its own temperature. The phenomenon described is called natural or free convection. If the movement of the fluid is forced by a fan, pump or so, the phenomenon is called forced convection. In both cases, the energy transport called convection is related to motion of macroscopic portions of matter (carrying internal energy U) but also to transport phenomena due to molecular collisions (always present wherever there's matter). Comsider a surface S of area A that separates a solid from a liquid. Consider the temperature profile of the liquid. Near the wall it is equal to the wall surface. In the immediate vicinity of the wall the fluid has an high thermal gradient that diminishes moving away from the wall, till reaching Tinfinite, not influenced by the wall temperature. Also, near the wall the fluid is stationary (no macr. velocity ) due to friction (--> heat is transferred by conduction in this small layer) while away from the wall macroscopic portions of fluid are in motion and thus the transport of energy occurs via convection. All the complexities involved in the analysis of molecular and turbolent fluid flow can be merged in a single parameterby introducing the law or Newton's…