Abstract:

This work addresses the challenge of accurately modeling heat transfer in two-phase heat exchangers using zeotropic refrigerants, where conventional methods often fail due to the omission of pressure drop and temperature glide effects. To overcome these limitations, we present modified formulations of the Log Mean Temperature Difference (LMTD) and effectiveness–NTU methods, introducing dimensionless correction parameters that quantify glide intensity and pressure-induced saturation temperature variation. Closed-form solutions are developed for parallel-flow, counterflow, and cross-flow configurations. Comparative analysis shows that the classical LMTD method can overestimate heat transfer by more than 10% in counter flow and underestimate it by a similar margin in parallel flow under strong glide and pressure drop. The modified effectiveness–NTU approach offers even greater improvements, with corrections up to 40%, depending on glide magnitude and heat capacity ratio. In cross-flow systems, the combined influence of glide and pressure drop causes non-monotonic deviations reaching 30% under high-glide, high-pressuredrop conditions. A curvature-based evaluation of temperature profiles offers additional insight into the thermodynamic asymmetries that distort classical predictions. The proposed framework applies to both single- and two-phase regimes, providing a unified, accurate, and analytically tractable tool for heat exchanger design and thermal performance evaluation under realistic conditions.