An investigation in the effects of recycles on laminar heat transfer enhancement of parallel-flow heat exchangers

It was demonstrated that fluid recycling could effectively enhance heat transfer rates of heat exchangers, however, related investigations were limited. In the current work, parallel-flow heat exchangers with basic recycles or revised recycles are investigated in the laminar regime. Theoretical models of thermo-hydraulic performances are established. The effects of reflux ratio, capacitance rate ratio, heat transfer area, and recycle length are investigated. The results demonstrate that the dimensionless heat transfer rate rises with the increase of reflux ratio or capacitance rate ratio, or with the decrease of heat transfer area, and the maximum values reach up to 127% and 121% for basic internal and external recycles, respectively. Basic internal recycles generate larger dimensionless heat transfer rates under larger reflux ratios, while basic external recycles perform more reliably over the whole reflux ratio range. Compared with basic recycles, revised recycles (i.e., partial-length recycles) require smaller pumping powers. Thus, partial-length recycles can improve the dimensionless overall performance of full-length recycle heat exchangers, e.g., half-length recycles increase the dimensionless overall performance by 65%. Fluid recycling does not need to change geometrical structures and fluid flow rates, thus it is a competitive approach of thermal augmentation in heat exchangers.