Abstract
In this paper, the influence of wettability properties on the start-up characteristics of two-phase closed thermosyphons (TPCTs) is investigated. Chemical coating and etching techniques are performed to prepare the surfaces with different wettabilities that is quantified in the form of the contact angle (CA). The 12 TPCTs are processed including the same CA and a different CA combination on the inner surfaces inside both the evaporator and the condenser sections. For TPCTs with the same wettability properties, the introduction of hydrophilic properties inside the evaporator section not only significantly reduces the start-up time but also decreases the start-up temperature. For example, the start-up time of a TPCT with CA = 28° at 40 W, 60 W and 80 W is 46%, 50% and 55% shorter than that of a TPCT with a smooth surface and the wall superheat degrees is 55%, 39% and 28% lower, respectively. For TPCTs with combined hydrophilic and hydrophobic properties, the start-up time spent on the evaporator section with hydrophilic properties is shorter than that of the hydrophobic evaporator section and the smaller CA on the condenser section shows better results. The start-up time of a TPCT with CA = 28° on the evaporator section and CA = 105° on the condenser section has the best start-up process at 40 W, 60 W and 80 W which is 14%, 22% and 26% shorter than that of a TPCT with smooth surface. Thus, the hydrophilic and hydrophobic modifications play a significant role in promoting the start-up process of a TPCT.
Highlights
As a two-phase passive device, the thermosyphon has a wide-range of various industrial applications, for instance, electronic equipment [1], heat-recovery systems [2], solar water heater systems [3] and space applications [4] due to the simple structure, reliability, high efficiency and low cost
The start-up performance is a crucial indicator of the thermosyphon operation
In order to evaluate the influence of wettability on the start-up performance of the thermosyphon, the start-up times and two typical temperatures, i.e., the average temperatures of evaporator Te.ave and condenser Tc.ave, are selected and measured by thermocouples
Summary
As a two-phase passive device, the thermosyphon has a wide-range of various industrial applications, for instance, electronic equipment [1], heat-recovery systems [2], solar water heater systems [3] and space applications [4] due to the simple structure, reliability, high efficiency and low cost. A thermosyphon is a gravity-assisted heat pipe without wicks that depends on phase-change heat transfer in both the evaporator and condenser sections to transfer large amounts of heat with relatively small temperature difference [6] and low thermal resistance [7]. A TPCT is composed of evaporator, adiabatic and condenser sections. The heating converts the saturated liquid into vapor that rises to the condenser section. Afterwards, the vapor condenses into liquid, which flows back down to the evaporator section by gravity, in the process transferring heat to the heat sink, such as cold water [8]
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