Micro-scale Nucleate Pool Boiling Heat And Mass Transfer Characteristics And Mechanisms

As the size of heating components decreases and power level increases, the heat and mass transfer mechanisms often differ from traditional heat transfer phenomena and traditional theory. Micro-scale boiling phenomena are especially important due to their higher heat transfer rates and simple structure with no moving parts. However, due to the extremely complex phenomena and mechanisms, there is still no dominate model confirmed by all studies, especially for micro-scale geometries. This paper analyzes the microscale nucleate boiling heat transfer characteristics and mechanisms.A micro-scale 96 heater array is used in the experiment and each heater(approximately 0.1 mm × 0.1 mm) is controlled by an individual feedback circuit at a constant temperature. The bubble dynamics were observed from both side and under the bubble by a high-speed camera. Heat fluxes in different ebullition cycles were recorded synchronously by a high-speed data acquisition system. Each individual region was first heated to generate separate bubbles and bubble departure frequencies, departure diameters and heat fluxes were analyzed. The heat fluxes remained low during bubble growth period and increased sharply at bubble departure. As the subcooling increases, the bubble departure frequency decreases. The energy requied for bubble growth during saturated boiling partly comes from superheated liquid layer and partly comes from microlayer evaporation. Two separate regions with a distance in between were heated so two bubbles can grow to a certain size and coalesce and form into a big bubble and depart. Bubble coalescence process is associated with a combination of bubble stretching, oscillation and sliding, which greatly enhanced the heat transfer and bubble departure frequencies. The heat fluxes for each heater under different positions of bubbles are different. Transient conduction is found to be the main heat transfer mechanism in bubble departure and coalescence processes.The effects of nucleation site spacing, arrangement and subcooling on bubble coalescence and heat transfer were also investigated. The bubble coalescence type, bubble departure diameter, bubble departure frequencies, and heat flux distributions under bubbles are different at various nucleation site spacings and arrangements. For very small dimensionless spacing, a single bubble formed on all the heaters. Moderate spacing resulted in frequent sequential coalescence events. Horizontal coalescence with immediate departure always occurs when dS D is large. The average heat flux at three nucleation sites is larger than the heat flux at two nucleation sites. The coalescence dynamics differed for the three different subcoolings. At saturated conditions, two bubbles grew very fast and coalesced soon. At a subcooling of 21?C, sequential coalesce occurred when the coalesced bubble stayed on the surface and coalesced with a small bubble. At a subcooling of 13?C, the coalesced bubble deforms on the surface for relatively long time. The bubble departure frequency at saturated conditions was much faster than that at subcooled conditions.The paper also theoretically analyzes the heat transfer mechanisms during boiling process. Transient conduction model predictions were very close to experimental measurements of high heat flues in bubble departure events. The 3-D numerical simulation of bubble coalescence shows that liquid rewetting is the main reason of the high heat fluxes at edge heaters.