The overall heat transfer by combined Modes is usually expressed on terms of an overall conductance or overall heat transfer coefficient ‘U’. A simple method for determining an overall heat transfer coefficient that is useful to find the heat transfer between simple elements such as walls in buildings or across modes of heat transfer pdf exchangers is shown below.
Note that this method only accounts for conduction within materials, it does not take into account heat transfer through methods such as radiation. In the walls of buildings the above formula can be used to derive the formula commonly used to calculate the heat through building components. Although convective heat transfer can be derived analytically through dimensional analysis, exact analysis of the boundary layer, approximate integral analysis of the boundary layer and analogies between energy and momentum transfer, these analytic approaches may not offer practical solutions to all problems when there are no mathematical models applicable. Therefore, many correlations were developed by various authors to estimate the convective heat transfer coefficient in various cases including natural convection, forced convection for internal flow and forced convection for external flow. These empirical correlations are presented for their particular geometry and flow conditions.
Recommendations by Churchill and Chu provide the following correlation for natural convection adjacent to a vertical plane, both for laminar and turbulent flow. For laminar flows, the following correlation is slightly more accurate. For cylinders with their axes vertical, the expressions for plane surfaces can be used provided the curvature effect is not too significant. The induced buoyancy will be different depending upon whether the hot surface is facing up or down. The characteristic length is the ratio of the plate surface area to perimeter.