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Effects of Pool Subcooling on Coalescence Heat Transfer and Bubble Dynamics

DOI: 10.1615/IHTC15.pbl.009320
pages 6205-6216

Jingliang Bi
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University

David Christopher
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University

Xipeng Lin
Institute of Engineering Thermophysics, Chinese Academy of Sciences

Xuefang Li
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University


KEY WORDS: Boiling and evaporation, Microscopic measurement, Coalescence, bubble dynamics

Abstract

Boiling is an effective way to dissipate large amounts of heat from small areas. However, even though boiling has been investigated for many years, the heat transfer mechanisms are still not fully understood. Coalescence phenomena were investigated experimentally in this study at three subcooling conditions on a microheater array. Two groups of heaters each containing 12 micro-scale heaters were used as two nucleation sites. Each heater was approximately 0.1 mm × 0.1 mm with the heater temperature automatically controlled by the control system while the heat transfer rate from each microheater was measured. Bubble images were taken from the bottom at a rate of 2000 frames per second, while a high speed data acquisition system recorded the voltages across the heaters at 2000 readings per second per channel. The coalescence dynamics differed for the three different subcoolings. At saturated conditions, two bubbles grew very fast and quickly coalesced, with the two bubbles sometimes even departing individually before they coalesced. 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 experienced a long deformation time. The bubble departure frequency at saturated conditions was much faster than that at subcooled conditions. The heat flux plots were very different for the three different coalescence types with heat flux spikes occurring much more often at saturated conditions and greater heat transfer enhancement at the lowest subcooling than for the other two conditions.

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