Effectiveness-NTU relations for parallel flow heat exchangers: The effect of kinetic energy variation and heat leak from outside

The effectiveness-NTU relations are valuable information for heat exchangers with fluids having significant variation in velocities in the presence of heat leak. In this paper, the closed-form relations for parallel-flow heat exchangers, when the heat leak is either on the cold or hot side of the heat exchanger in the presence of kinetic energy variation, are presented. The effectiveness was found to depend on NTU and fluid capacity ratio along with six other quantities that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are presented in a graphical form demonstrating the variation of effectiveness of the heat exchanger with the relevant parameters. It was demonstrated that in the limiting case, the solution reduces to the classical effectiveness-NTU relations for parallel-flow heat exchangers.