Mathematical Modeling and Experimental Assessment of Heat Transfer in a Field-Tube Evaporator of a Closed-Loop Two-Phase Thermosyphon

This paper presents an experimental and mathematical evaluation of heat transfer in a Field tube heat exchanger integrated into the evaporator section of a closed-loop two-phase thermosyphon. Propylene glycol flowed through 19 parallel Field tubes and transferred heat to distilled water boiling on their outer surfaces. A lumped thermal-resistance model included convective heat transfer in the annular channel, heat conduction through the outer tube wall, nucleate boiling, and secondary heat redistribution between the annulus and the inner return tube. The Hausen, Bergman et al., Gnielinski, and Sieder–Tate formulations were compared for heat transfer in the annulus. Experiments were conducted at propylene glycol volumetric flow rates from 0.0746 to 0.1729 m3·h−1. The experimental cooling capacity increased from 203.58 to 485.92 W, with a combined relative standard uncertainty of 6.6–8.1%. All models captured the increasing cooling-capacity trend but differed in their absolute predictions. The mean absolute relative deviations were 6.32% for Sieder–Tate, 7.49% for Hausen, 13.89% for Bergman et al., and 17.73% for Gnielinski. The Sieder–Tate correlation therefore provided the lowest deviation for the investigated geometry and operating conditions.