4.17 External wall heat flux
The boundary condition for temperature (or energy) dictates the heat transfer across a boundary. At a boundary that represents a solid wall, simple conditions can sometimes be applied. However, specialised boundary conditions are often required that control the heat flux across the boundary.
The fixed temperature is the simplest condition, setting a fixed value . This condition provides an approximation for cases for a solid with high thermal mass, due to a large mass of material and high conductivity , which helps to maintain constant .
Otherwise the boundary condition sets the heat flux normal to the boundary, derived from Eq. (2.54 ) by
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(4.28) |
Otherwise a fixed heat flux condition specifies an inward heat flux as a fixed gradient type with a reference gradient by
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(4.29) |
Fixed heat transfer coefficient
Another way to specify the heat transfer at an external wall is by Newton’s law of cooling.7 This general law states the rate of heat loss of a body is directly proportional to the difference between the body temperature and a surrounding, ambient temperature .
Applied as a boundary condition, is the fluid temperature at the boundary, and a temperature some distance beyond the solid boundary. A heat transfer coefficient , with SI units , provides the constant of proportionality such that
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(4.30) |
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(4.31) |