Effective NTU of a counterflow heat exchanger with unbalanced flow and longitudinal heat conduction through fluid separating and outer walls

Abstract A closed form expression for effective NTU of a heat exchanger is very useful for performance prediction and design optimization of the heat exchanger with less computational efforts. However, in literature it is available only for some limited cases such as infinite design NTU, balanced flow etc. In this paper, we present effective NTU of heat exchangers with finite design NTUs and unbalanced flows. Three separate cases, (i) longitudinal conduction only through the fluid separating wall, (ii) longitudinal conduction only through the outer wall and (iii) longitudinal conduction through both fluid separating and outer walls, have been considered. Accuracy of the proposed expressions is verified with the numerical solution of the corresponding governing equations. It is proved that an existing expression for balanced flow can be used for unbalanced flow by using a modified axial conduction parameter. This principle is applied on perforated plate type matrix heat exchangers.