Phase-change heat transfer of bare surface evaporator with phase-separating wick in downward facing orientation
Egbo, Munonyedi Kelvin
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Egbo, M., & Hwang, G. (2021). Phase-change heat transfer of bare surface evaporator with phase-separating wick in downward facing orientation. International Journal of Heat and Mass Transfer, 173 doi:10.1016/j.ijheatmasstransfer.2021.121206
Liquid-vapor, phase-change heat transfer using wicks can provide reliable and high heat flux cooling capability, especially in microgravity applications. However, the maximum heat removal capacity, also known as Critical Heat Flux (CHF), is related to the capillary-driven liquid supply limit and/or vapor removal limit. A key is to develop a novel wick structure, offering efficient liquid supply as well as vapor removal pathways. In this study, the Bare Surface Evaporator with Phase-Separating Wick (BEPSW) is examined to understand the liquid supply and vapor removal limits in a downward facing orientation for microgravity environment. The BEPSW consists of a bare surface evaporator for the efficient evaporation, and the distributed, sintered-particle post wicks with the phase-separating wick for enhanced liquid supply and vapor removal. The bare surface is fabricated from a copper disk 19.1 mm in diameter, while the post and phase-separating wicks are manufactured using 10 and 3 layers of sintered copper particles, respectively. The heat flux is measured as a function of the surface superheat for different post-post pitch distances ($L_p$ = 2.5, 3.5, 4.5, and 7 mm) and average particle sizes (<$d_p$> = 350 and 550 μm), using distilled water as a working fluid. The results show that the CHF increases as the pitch distance decreases from $L_p$ = 7 to 3.5 mm in both particle sizes due to the increased liquid supply through the post wicks, while it decreases below $L_p$ = 2.5 mm in both particle sizes due to the liquid entrainment limit, i.e., the maximum CHF is observed at $L_p$ = 2.5 to 3.5 mm. The measured maximum CHF and Heat Transfer Coefficient (HTC) with 350 μm particle size is 184 W/$cm^2$ (24.9°C superheat) and 7.7 W/$cm^2$-K (144 W/cm2 heat flux), respectively, and 207 W/$cm^2$ (44.3°C superheat) and 5.4 W/$cm^2$-K (118 W/$cm^2$ heat flux) for the 550 μm particle size, both at $L_p$ = 3.5 mm. The CHF models, both the capillary-viscous and entrainment limits, predict the optimal particle size, showing that it increases with increasing pitch distance of the post wicks at given post wick geometries. This is related to the type of limitation that controls the CHF, i.e., either capillary-viscous limit or the entrainment limit. Also, the results show that the CHF using the 550 μm wick exceeds that of 350 μm at given pitch distance, due to the large liquid permeability.
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