Study of external heat transfer mechanisms in single-row fin and tube heat exchangers.

摘要

This is the study of some phenomena which are important for the performance of fin and tube heat exchangers, especially those related to external heat transfer. Analytical, numerical and experimental techniques are used to improve our understanding of plate fin and tube heat exchangers performance.The problem of tube-to-tube conduction through the fins is first studied. A mathematical model to quantify the effect of heat transfer between adjacent fins is developed this formulation is used to determine the conditions under which tube-to-tube conduction through the fins may or may not be neglected.A mathematical model helpful for the understanding of the vortex induced convection is developed next. An analytical solution is made possible by a perturbation technique in which the nonadmission strength of the vortex is the small parameter. Three different configurations of vortex are analyzed and the role of the secondary flow is found to be different for each one.The three-dimensional hydrodynamics and heat transfer in the plate-fin and tube geometry is also studied numerically. The effect of flow and geometrical parameters on the hydrodynamics is determined. The stabilizing effect of the fin distance is shown. The hydrodynamics in turn affects the convective heat transfer to the fins. The overall heat transfer per unit length of the tube per unit pressure drop across the heat exchanger, which is a relevant parameter for design purposes, shows a maximum value between the extremes of high pressure drop for small fin spacing and small heat transfer area for large fin distances.Experimental visualization of the flow between fins is also performed. The experiments are found in qualitative agreement with the steady state numerical solutions if the wake is trapped inside the heat exchanger fins. Otherwise the behavior is different since the wake becomes unsteady as the experimental results show.