Characteristic Analysis And Experimental Study Of Flow Boiling In Rectangular Minichannel

Recently, study on the phenomenon of flow boiling in mini/micro-channel has become a new research focus in the field of heat transfer. The main purpose of the present study is to clarify the heat transfer mechanism and new features of flow boiling under mini/micro-scale condition. With emphasis on flow patterns transition, heat transfer and pressure drop characteristics during flow boiling in rectangular minichannel, studies including theoretical analysis, experimental observation and numerical simulation are performed in this thesis work.Based on the bubble dynamics analysis of a single bubble during flow boiling in mini/micro-channel, an equation to calculate the bubble departure diameter Dd is theoretically derived. By the comparison between channel width S and bubble departure diameter Dd, it is pointed out that when S is smaller than Dd, the flow pattern of "confined bubbly flow" would occur, and the flow features of such flow pattern is further analyzed. Study of the confined bubbly flow as an individual flow pattern, can not only enlighten new classification method of mini-and micro-channel, but also provide reasonable explanation to some unusual phenomina in existing experiments.Experimental study of water flow boiling in vertical rectangular minichannel is conducted using self-designed and established experiment rig. A method combining the CCD imaging technology and artificial neural networks is employed to enable a precise and objective identification of different flow patterns. Flow patterns such as isolated bubbly flow, slug flow, annular flow and partial tryout area, and confined bubbly flow are identified in experiment, which also verify the result of theoretical analysis. The artificial neural networks show relatively high reliability and accuracy on flow patterns identification, and can be used as important supplements to traditional flow pattern identification technology.Based on the theoretical analysis and experimental observation results, the traditional flow patterns are further subdivided taking into account the presence of confined bubbly flow. Flow patterns transition criteria from bubble growth to confined bubbly flow, from isolated/confined bubbly flow to slug flow, and from slug flow to annular flow are theoretically derived. The prediction of the theoretical model agrees well with experimental data. By carrying out comparative study, it is indicated that there is an apparent postponement of flow patterns transition of flow boiling in mini/micro-channel than that under adiabatic conditions. Moreover, for flow boiling under the same heat flux and mass flux conditions, the flow patterns transition may occur earlier with the decrease of channel size.Some modifications are made with new features of confined bubbly flow to the existing three zone model, so that a new theoretical model is established to calculate the heat transfer coefficient of confined bubbly flow region. With the Mean Absolute Error being9.72%, the new model provides a good prediction to the experimental results. The influence of heat flux, mass flux and vapor quality on heat transfer coefficient is also analyzed. It is revealed that as a transitional flow pattern from the early stage of boiling to the stable slug and annular flow, the change of heat transfer coefficient in this area shows characteristics of both nucleate boiling and foced convection heat transfer.The various components and their corresponding calculation methods of the pressure drop of flow boiling in vertical rectangular minichannels are described in this thesis. It can be seen from the analysis about pressure drop characteristics in experiment that two-phase frictional pressure drop increases with the drop of the mass flux, the decline of channel width and the rise of heat flux. Especially, the pressure drop in0.5mm wide rectangular minichannel is significantly higher than that of other channel widths. Taking into account the impact of the channel size, mass flux and heat flux on pressure drop, a new experimental correlation of two-phase frictional pressure drop is fitted in this thesis in the form of Lockhart-Martinelli expression. The predicted result of the new correlation is in good agreement with the experimental results. Especially for0.5mm wide channel, the predicted result is much bettern than that of other existing correlations.A numerical simulation corresponding to the experiment is carried out in this thesis. Key processes of the flow boiling are controlled by writing supplementary program in combination with general CFD software. Flow patterns such as the isolated bubbly flow in unconfined channels and the confined bubbly flow in confined channels are obtained in the simulation, which further reveals the influence of channel size on the flow patterns and the related heat transfer and pressure drop characteristics. Moreover, with the flexible control of time steps, the process from growth to confinement of a single bubble in rectangular microchannel is precisely captured in numerical simulation. The velocity vector and temperature distribution diagrams are obtained and analyzed. In addition, by the comparison of bubble length curves under the conditions with and without gracity, it is indicated that the influence of gravity on bubble flow properties is negligible under the simulated channel scale.