Flow distribution and boiling heat transfer of R-1336mzz(Z) in falling film evaporators with various distributor geometries under high-temperature conditions

초록

Falling film evaporators (FFEs) have been widely used in high-temperature heat pumps because of their high heat transfer performance and reduced refrigerant charge. However, studies on the combined effects of distributor design and operating conditions in FFEs using R-1336mzz(Z) remain limited. This study experimentally investigates the effects of distributor hole diameter (D), pitch (P), and aspect ratio on the flow distribution and evaporation heat transfer of R-1336mzz(Z) in FFEs at various vapor qualities (x). Consequently, a larger dimensionless design index of the distributors stabilizes the liquid column and promotes uniform wetting. In contrast, a higher two-phase Weber number at an elevated x induces transitions to the jet and mist modes. Circular holes provide superior heat transfer coefficients compared to elliptical holes, balancing axial and circumferential film spreading. Although decreasing D promotes liquid atomization at high x, it degrades performance at low x owing to crest stagnation. Furthermore, the pressure drop is more sensitive to P at small values of D. Finally, based on the present database, an effective Nusselt number (Nueff) is developed to evaluate the total heat transfer performance. Based on the developed correlation, the proposed optimal distributor design can enhance Nueff by 44% compared to the lowest-performing design. Enhancing film renewal on the first tube does not necessarily maximize the overall heat transfer performance because excessive liquid breakup and vapor momentum can reduce the liquid supplied to the second tube. These findings provide a practical design guideline for distributor geometry to balance heat transfer enhancement and liquid-supply preservation in FFEs under twophase inlet conditions.

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