Abstract

Abstract This paper deals with effectiveness-NTU relations for crossflow heat exchangers. The literature contains algebraic relations for two limiting cases: “unmixed” and “mixed” flows. These are simply thermodynamically defined limiting cases. The authors argue that many practical flow situations may not satisfy either of the above cases. Rather, conditions of “partial mixedness” may exist, which will yield performance between the above limiting cases. Algebraic relations are provided to calculate the effectiveness-NTU relationship for arbitrary values of partial mixedness. Graphs are provided for 25, 50, and 75 % mixedness. It is argued that inadequate knowledge of the actual flow structure exists to define the actual mixedness that will occur in various actual heat exchanger-surface geometries. Definition of the actual mixedness would require measurement of the spatial temperature distribution leaving the heat exchanger. The problem addressed is of particular importance for high effectiveness designs. The assumption of the “unmixed” condition would lead to over prediction of the performance of a heat exchanger having partial mixedness.