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Second law optimization of heat exchangers in waste heat recovery

This paper numerically investigates the first and second law efficiencies of transient crossflow over tube banks. A significant number of simulations were performed using a commercial computational fluid dynamics (CFD) tool, ANSYS Fluent 18.2, with varying inlet temperature, Reynolds number, pitch ratio (PR), number of inline cylinders, and heat transfer coefficient defining a heat leakage on the system. Numerical results compare favorably with published data in terms of Nusselt number and pressure drop. Five hundred seventy‐six simulations were performed, and energy efficiencies ranged from 72% to 99%, while exergy efficiencies ranged from 26% to 70%. No discernable, universal trends were observed across all data sets, but the staggering amount of results spans a wide range of operational characteristics for tube bundle waste heat recovery. To illustrate the insight such data can provide, we consider four sample cases for analysis: well‐insulated low‐grade heat recovery, poorly insulated low‐grade heat recovery, well‐insulated high‐grade heat recovery, and poorly insulated high‐grade heat recovery. Low‐ and high‐grade heat recovery cases were simulated with differing inlet heat transfer fluid (HTF) values, and heat leakage was introduced as a source term in the 2D energy equation. Detailed descriptions of the efficiencies of all cases are described, with references to the immense body of simulation data included in the appendix. Overall, the highest efficiencies of the first and second laws were obtained in domains that have three cylinders and inlet temperature of 500 K and eight cylinders and an inlet temperature of 400 K, respectively. The main findings of this study could be beneficial in designing a highly efficient waste heat recovery system, especially when source temperatures vary.