Effect of vessel dimensional ratio on heat conveyance capabilities of gravity-assisted heat pipes: Theoretical and experimental approach

This study conducts an experimental performance assessment of different gravity-assisted heat pipes, with length-to-diameter ratios ranging from 50 to 200. Process modelling is based on a mechanistic approach and complemented by a comprehensive analysis of internal entropy generation rates and convective phenomena. Findings show that increasing the dimensional ratio extends the range of the device, from 200 to 800 W to 1000-2800 W. Similarly, the temperature difference of the cooling fluid increases, achieving 5.3 degrees C and 15.9 degrees C. Nonetheless, these increment rates remain constant for all treatments, between similar to 1.6 and 1.8 degrees C. Thermosyphons with L/D < 50 display a linear behaviour, with an inverse Stanton-Reynolds dispersion index of 0.0089, and no dominant internal mechanism, with ranges of 0.99-4.27 kW/K for the vapour core; 0.52-4.94 kW/K for condensation and; 0.82-3.68 kW/K for evaporation, hence their separate characterization. Contrarily, for L/D > 100, entropy generation rates at evaporation process is between 25 and 45% higher than the other thermally-driven processes, hence identified as the dominant mechanism inside these vessels. For all instances, conden-sation is the most susceptible process to external stimuli, with dispersion indexes 1.98 to 5.2 times higher than the rest. As heat flow supply rises, vapour core contribution to total entropy generation is reduced by 10-15%, entailing that external heat transfer becomes more intense than internal heat conveyance. Finally, statistical correlations were constructed to estimate inverse Stanton number and condensation vapour core temperature. For L/D < 50 this estimation achieved a maximum relative error of 0.03%. For the remaining treatments, es-timations achieved a maximum relative error of 14.5%.