With the development of electronic equipment industry, the systems and components are pursuing high-performance and high-efficiency, and the size of system is gradually downsizing as well. Furthermore, the development of technology that can efficiently eliminate the heat generated inside a miniature system is demanded. Thermal and fluidic characteristics in micro-channels used by heat sink have been proved by many experimental studies and can be applied to various areas.
This study analyzed the characteristics on heat transfer and pressure drop of FC-72 in parallel micro-channels through an experimental study. Parallel micro-channel is 450μm in width, 200μm in depth, 60mm in length, and has 15 channels.
Before the experiment, the shape of the manifold, length of the channel and the flow field based on inlet condition are analyzed using CFD. The parallel micro-channel to be used in the experiment was designed based on the analysis results. The experiment is conducted at the mass flux of 100.3-408.5 kg/m2s and heat flux of 3.2-49.0 kW/m2 and the vapor quality at this time is within the range of 0.00-0.96.
The total pressure drop in the channel is expressed as the sum of frictional pressure drop and the accelerational pressure drop. Since the frictional pressure drop is considerably larger than accelerational pressure drop in most cases, it is essential to precisely calculate the frictional pressure drop in order to predict the total pressure drop. Frictional pressure drop is usually calculated by using the homogeneous model and the separated flow model, the frictional pressure drop obtained from the experiment is compared to those calculated by the homogeneous model and the separated flow model. In order to accurately predict the frictional pressure drop, this study expresses the two-phase friction multiplier as the function for Reynolds number, Weber number, and Martinelli parameter to develop a new correlation that considers the inertial force, viscous force, and surface tension of fluid. The new correlation predicts well the frictional pressure drop obtained in this study within 5.5% MAE.
The flow boiling heat transfer in the channel is known to occur by nucleate boiling and forced convective boiling. In the region where nucleate boiling dominates the flow, heat transfer usually occurs from the creation and breakaway of bubbles, and the flow pattern in such a region indicates mainly bubbly and slug flow. Also, the heat transfer coefficient normally depends on the heat flux, but is not affected too much by mass flux and vapor quality. In contrast, in the region where the forced convective boiling dominates, most of the flow pattern is annular flow, and the heat transfer usually occurs through the evaporation of the thin liquid film around the heating surface. The heat transfer coefficient usually depends on the mass flux and vapor quality in this region, and is nearly unrelated to the heat flux. The heat transfer mechanism in micro-channel is being reported differently by researchers.
According to the result of this experiment, in the region of lower vapor quality, nucleate boiling plays a role as the main mechanism of heat transfer. The influence of nucleate boiling gradually decreases as the vapor quality increases. As the effects of forced convective boiling gradually increases, nucleate boiling and forced convective boiling occur compositively. Based on such a mechanism of heat transfer, a new correlation to predict heat transfer coefficient is developed. A new correlation obtained in this study considers the effects of both nucleate boiling and forced convective boiling, and predicts the result within 4.6% MAE.