Visualization Study On Thermo-physical Characteristics And Mechanism Of Convective Boiling Of Liquid Nitrogen In Mini/Micro-Channels

Flow boiling heat transfer in mini/micro-channels has attracted a great deal of attention in the past a few years due to the various applications in electronics, astronautics, medical treatment, etc. With the aid of high-speed photography, the present study aims to uncover the physical mechanism of convective boiling of liquid nitrogen in mini/micro-channels. Moreover, numerical simulation of the bubble dynamics is developed to further understand the phase change phenomenon in mini/micro-channels. The main conclusions are shown below:Successfully solving the two difficulties in cryogenic visualization, i.e., illumination and magnification, the present study set up the visualization system for micro-scale two-phase flow in cryogenic temperature. It was found from the experimental results that the flow patterns were mainly bubbly flow, slug flow, churn flow and annular flow. And the confined bubbles and mist flow were also observed in micro-tubes of 1.042 mm and 0.531 mm in inner diameters in the experiments. Compared with flow regime maps for gas-water flow in tubes with similar hydraulic diameters, the region of slug flow in the present study reduces significantly. Correspondingly, the transition boundary from the bubbly flow to slug flow shifts to higher superficial gas velocity, and the transition line of churn to annular flow moves to lower superficial gas velocity.To explore the mechanism of flow boiling of liquid nitrogen in mini/micro-channels, it employed a segment of upward vertical quartz glass tube with the inner diameter range of 1.3-1.5 mm, which was coated with a layer of transparent ITO film （Indium Tin Oxide） as the heater on the outer surface. The bubble growth, departure and the following flow pattern evolution in the micro-tube were visualized and quantitatively investigated, along with the simultaneous measurement of local heat transfer coefficient around a specified nucleation site. The bubble departure diameter and bubble period were investigated and satisfy （ D_d ）^1.46·（1/τ）=constant, which showed that the tube size of the micro-tube had no notable effect on the bubble departure and the trend of the bubble departure was similar to that in macro-tubes. Whereas the following flow pattern evolution was apparently confined due to the size effect, which presented the acceleration of flow pattern transition and desirable heat transfer performance in micro-tubes. The heat transfer coefficients for different flow patterns along the micro-tube were obtained in terms of bubbly, slug, annular flow and the flow regimes of flow reversal and post-dryout. It was found that the dominant heat transfer mechanism was the liquid film evaporation which offered desirable heat transfer capability. The heat transfer performance would be deteriorated in the post-dryout regime, while flow reversal could somewhat enhance the heat transfer upstream of the nucleation site. Flow pattern evolution beyond the boiling crisis was also investigated. Post-dryout regimes such as inverted bubbly, inverted slug and inverted annular flow were observed in the micro-tube. Flow reversal and liquid entrainment, which were relevant to flow instability in the flow pattern evolution, was demonstrated clearly. Other than macro-channels, the liquid entrainment usually occours in slug flow in mini/micro-channels.Flow pattern visualization is essential for understanding the mechanism of two-phase flow in the micro-scale passages like micro-tubes and micro-channels, etc. However, the front view of the two-phase flow is the only information obtained in the most flow pattern visualization researches in micro-scale passages, so far. The two-dimensional images can only provide partial information and sometimes important information such as the bubble nucleation sites, bubble shapes and spatial distribution of the bubbles, which could provide an in-depth understanding of two-phase flow, are missed or not accurately obtained. Due to the limitation of the working distance of the conventional visualization system, it is very difficult for applying the conventional three-dimentional photography method for the visualization of the two-phase flow in mini/micro-channels. The present study proposed a simple but effective method to visualize the two-phase flow in mini/micro-channels three-dimensionally, which was validated by the difficult experiment in the cryogenic temperature and could be extended to the flow condition in room temperature. An isosceles right-angle prism combined with a mirror located 45°bevel to the prism was employed to obtain synchronously the front and side views of the flow patterns with a single camera, where the locations of the prism and the micro-tube for clearly imaging should satisfy a fixed relationship which was specified in the present study. The image deformation due to refraction and chromatic aberration due to the prism were clearly specified and corrected.Numerical simulation was conducted to clarify the flow boiling process in micro-tubes by using Volume-of-Fluid （VOF） method which was modified to include the effect of phase change. A specially treated micro-layer model was used, in which the evaporative heat flux through the micro-layer was approximated in the simulation. The effect of heat flux, mass flux, surface tension force, contact angle and channel size on the bubble growth and heat transfer were analyzed. It was found that the bubble growth rate displayed linear trend in relatively high flow rate, while the bubble growth curve shaped parabolic under low flow rate. The effect of heat flux on the bubble dynamics was specified and it showed that the controlled mechanism of bubble growth during flow boiling in micro-tubes was thermally controlled mechanism. The thermal properties such as surface tension, contact angle and density ratio played significant role in the bubble growth and the following pattern evolution. For the working fluid with small surface contact angle and surface tension, the vapor bubble departed from the heating wall easily in the nucleate boiling. Bubble growed fast as the liquid-vapor density ratio increased. The confined bubble occurred and the corresponding heat transfer performance was enhanced as the channels size reduces. The influential region of the confined bubble was specified, which covered more than two-fold area of the confined bubble and could heavily influence the region upstream of the confined bubble.The experimental study was performed on the convective boiling of liquid nitrogen through stainless steel heat sink. Heat transfer characteristics and associated flow patterns were quantified. The appearing typical flow patterns in the micro-channel heat sink were bubbly, slug and annular flow, as well as flow reversal. The greatest heat capability was up to 21.35 W/cm² in the flow rate range of 50.1-880.5 kg/m²s. The maldistribution of mass flux in each channels of the heat sink was investigated and it was found the maximum difference was around 18% in the present experimental range. Moreover, the maldistribution of mass flux in each channel became apparent as the flow rate increases. The pressure drop characteristics of different channels for different flow patterns were experimentally investigated. It was found that the pressure drop curves almost overlapped for single liquid phase. Whereas the curves diversified as the flow enters into two phase, even out of phase with each other in some cases.