Experimental Study And Simulation About Flow Boiling And CHF In Narrow Rectangular Channel

Recently, with the increasing application of MEMS and micro machining technology, equipments become more compact and modular. Highly integrated of it, equipments show dramatic higher heat flux during operation, so the measures to achieve effective cooling and keep devices in appropriate temperature under high heat flux have been the key issues of research. The way that flow boiling in micro-channel could solve the above problems effectively, however, the mechanism of which has not been clear until now. Flow boiling and CHF characteristics of water in rectangular channel was investigated through experimental study and numerical simulation in present work to provide theoretical description for micro-cooling channel designment and optimization, which has great practical significance.Firstly, a set of experimental loop suitable for flow boiling in narrow rectangular channel was established. Then, the experimental system, experimental equipments, data measurement and data processing methods as well as uncertainty analysis were described in this paper. A new heat transfer correlation was obtained based on experimental results.Secondly, VOF multiphase model coupled with surface tension model was employed to investigate flow boiling characteristics in narrow rectangular channel using FLUENT. The phase changing process was realized with UDF and the physical properties changing of water due to temperature increasing had also been taken into consideration. The results indicated as follows:(1) The characteristics of flow pattern in rectangular channel were obtained, meanwhile flow pattern maps of different form was drawn, and the influence of heat flux, mass flux,saturation temperature, channel size and vapor quality on flow pattern were analyzed.(2) The bubble growth characteristics were obtained assisted with phase distribution map and the growth history of the bubbles was summarized.(3) The influence of heat flux, mass flux, saturation temperature, channel size and vapor quality on heat transfer coefficient were analyzed. The heat transfer mechanism was divided into nucleate boiling and convective boiling, based to which the flow boiling could be classified into four regions.(4) The effects of heat flux, mass flow rate and channel size on the pressure drop were summarized, and the results were compared with the experimental facts to prove its reliability.Finally, the critical heat flux was studied by means of numerical simulation, and its determination method was introduced emphatically. The factors influencing CHF were analyzed subsequently, the results showed that: CHF increases monotonically with increasing mass velocity while decrease with larger /eL D; it is independent of the inlet sub-cooling. CHF of up-flow is slightly larger than that of down-flow under the same channel dimensions and inlet condition. The CHF decreases as the pressure increases, however, when the channel is narrow enough, it may show an unique trend of increasing at the very beginning and then dropping.