Thermal radiation and view factor effects on a U-shaped heat pipe heat sink with circular fins under partial vacuum conditions for avionics applications

This study experimentally investigates the thermal performance of a U-shaped heat pipe heat sink (HPHS) integrated with circular fins for avionics thermal management under partial vacuum conditions. The investigation focuses on the influence of thermal radiation, radiation view factor, apparent natural convection, fin performance, and optimisation of the evaporator-to-condenser length ratio. Experiments were conducted under both partial vacuum and atmospheric conditions with heating powers ranging from 3 to 22 W. Three thermal models were evaluated, including radiation analysis with radiation view factor, radiation analysis without view factor, and convection-only analysis. The results show that the total thermal resistance decreases with increasing heating power due to enhanced phase-change heat transport within the heat pipe. Neglecting the radiation view factor substantially overestimates the radiation heat transfer by approximately 198 %, leading to unrealistic prediction of the radiation-to-convection heat transfer ratio and underestimation of the apparent natural convection coefficient. Despite the reduced-pressure environment, the apparent natural convection remains the dominant heat transfer mechanism throughout the investigated operating range. The fin efficiency and fin effectiveness decrease with increasing heating power, while the optimisation analysis indicates that the optimal evaporator-to-condenser length ratio under partial vacuum conditions remains relatively small, ranging from approximately 0.015 to 0.038, indicating the necessity of a substantially longer condenser section for effective heat dissipation. The findings provide important design guidelines for compact passive cooling systems in avionics and aerospace electronic applications.