Experimental Investigation On Vapor-Liquid Phase Change Heat Transfer In Vapor Chambers

Vapor chambers have been regarded as an effective heat transfer component to undertake the high heat flux removal in confined space. It has a promising application in aerospace thermal control system and microelectronics cooling due to its simple structure, low fabrication cost, good heat transfer performance, strong adaptability, etc. The space between evaporation section and condensation section is generally as thin as a few millimeters long, which indicates that evaporation and condensation may be strongly inter-related and exert significant interactions with each other in this small and confined space, and thus, the complex vapor-liquid two-phase flow as well as phase change heat and mass transfer phenomena are involved in its operation. Therefore, the studies on vapor-liquid phase change heat transfer inside the vapor chambers not only have great engineering value but also possesses scientific significance in the exploration of vapor-liquid two-phase flow and heat and mass transfer with evaporation-condensation co-existing phase change process in small confined spaces.So far, the mechanism and inherent law of vapor-liquid phase change heat transfer in vapor chambers are still not completely known, especially the underlying relationship between the vapor-liquid two-phase fluid dynamic behavior and phase change heat transfer is less understood. In addition, the influence of the coupling effect of evaporation and condensation on heat transfer performance in small confined space is still waiting to be developed. For this reason, the visualization of vapor-liquid two-phase flow pattern in vapor chambers and experimental studies on heat transfer performance of vapor champers are conducted to observe the phase change heat transfer characteristics and explore the mechanism of vapor-liquid two-phase flow and heat transfer in small confined space in this context. The result and conclusions are summarized as follows:(1) The visualization experiment of vapor-liquid phase change heat transfer on vapor chamber is performed to capture the vapor-liquid two-phase operation and flow patterns as well as the change of wall temperature. Four quasi-steady operation modes are defined based on experimental observation in vapor chamber and the fluctuation feature of the wall temperature. The results indicate that four quasi-steady operation modes, including continuous large pulsation operation, alternate operation of large pulsation and transient slight pulsation, alternate operation of slight pulsation and transient large pulsation, and continuous slight pulsation operation, will evolve orderly with the increase of heat flux. The fluctuation characteristics of corresponding wall temperature can be used to judge the operation mode of vapor-liquid inside vapor chamber. In addition, the vapor-liquid two-phase dynamic behaviors include the formation, growth, movement, coalescence, breakage and congeal of bubbles and the fluctuation of liquid, which plays a key role in the stable operation and high efficient heat transfer of vapor chambers.(2) An experimental study on the heat transfer performance of the vapor chamber is conducted to analyze the influence of operating parameters, such as heat flux, filling ratio and so on, on heat transfer performance of vapor chamber, and to correlate the diverse fluid dynamic behaviors and heat transfer property. The result indicates that increase in the heat flux can prompt more bubbles in the heating surface and thus the vapor chamber conducts a better thermal performance. There exists an optimum water filling ratio (approximately 49%) at which the thermal resistance acquires its minimum value, which results from the fact that the evaporation and condensation heat transfer are strongly inter-related in this filling ratio. Furthermore, the visible dynamic behaviors of two-phase vapor-liquid are introduced to analyze the influence of filling ratio. That is, in lower filling ratio, the bubbles as well as boiling water has fewer opportunities to contact with the condensing surface, which signifies the interaction of evaporation and condensation is particularly weak. Instead, when the filling ratio is quite high, the behaviors, such as decrease of bubbles'departure frequency, most of the condensing surface is covered by turbulent water and the liquid bridge phenomenon, will degrade the thermal performance of vapor chamber. While the filling ratio is in a mid-range level (approximately 49%), the height as well as the space between the water level and condensing surface is in an optimum condition and the evaporation and condensation heat transfer will promote with each other. When the chamber space decreases from 20mm to 5mm, the thermal resistance of vapor chamber will decrease gradually.(3) An experimental study of evaporation and condensation coupled phase change heat transfer in a new type vapor chamber is conducted to acquire the boiling and condensation heat transfer coefficient in various filling ratio. The result indicates that there respectively exists an optimum water filling amount at which the boiling and condensation heat transfer coefficient acquire their maximum value, and the optimal filling ratio for the maximum boiling and condensation coefficient are correspondingly 33% and 52%. Under these filling ratio, the liquid height as well as the space between liquid level and condensing surface is in an appropriate condition at which the boiling and condensation processes are inter-related and have significant coupling effects over each other, which promotes the phase change heat transfer to the largest extent.(4) An experimental study of the influence of mesh wick on the thermal performance of vapor chamber is conducted to compare the working performance of different mesh wick and capture the vapor-liquid two-phase flow patterns. The result indicates that mesh wick can induce more nucleation sites which generally come into being from the sides or corners area of the mesh wick. The vapor chamber embedded with array-type mesh wick has a better heat transfer performance than those with cross-shaped mesh wick. The vapor chamber embedded with roll-type mesh wick operates normally at the filling ratio range of 20%-70% in gravity, and the optimum water filling ratio is approximately 50% at which the thermal resistance acquires its minimum value. The vapor-liquid two-phase operation in such vapor chambers that embedded with mesh wick becomes more stable and the boiling process shows no intermittent characteristics which exist in those without mesh wick.(5) An experimental investigation on the heat transfer performance of vapor chamber in anti-gravity is conducted to analyze the influence of filling ratio, space height and the structures of both heating surface and mesh wick on the thermal performance of vapor chamber. The result indicates that with the increase of the filling ratio, the thermal resistance of the vapor chamber which is full of roll-type mesh wick in anti-gravity increases at first and then decreases, and the optimum filling ratio is 60%. The vapor chamber conducts more excellent heat transfer performance with lower chamber space under anti-gravity condition. The fractal groove heating surface associating with combined type mesh wick improves the circulation of vapor and liquid, and thus the heat transfer performance of vapor chambers.