A numerical study on the internal heat leak mechanisms in a variable property three-stream heat exchanger

Heat leakage mechanisms need to be addressed in the thermal analysis of multi-stream heat exchangers due to their effects on the intended heat transfer between the streams. In this paper, multi-dimensional heat transfers between various fluid streams and also between the fluid and solid parts of a three-stream plate-fin heat exchanger is numerically modeled considering the variation of thermo-physical properties of both solid and fluid parts. All internal heat leak mechanisms, i.e. longitudinal heat conduction, transverse bypass through fins, and heat transfer reversal in a stream are taken into consideration. The distribution of longitudinal heat conduction along the stream’s separating plates (plates) is also explored. It is shown that the longitudinal heat conduction depends strongly on the variation of properties in some flow arrangements. For such cases, the plates experience areas with relatively low temperature, and a new longitudinal heat conduction, mainly induced by property variation, is identified and presented. This induced longitudinal conduction is close to 1% of the maximum heat exchange between the streams in this study. Another interesting result is that the longitudinal temperature distribution in the plates does not necessarily follow the temperature distribution along the nearby streams due to the entrance effects and unbalanced heat capacity rates. Numerical results show that property variations affect all of the thermal leakage phenomena and, therefore, need to be considered in the modeling and thermal analysis of multi-stream heat exchangers.