การศึกษาลักษณะการถ่ายเทความร้อนของเจ็ทพุ่งชนด้วยน้ำผสมฟองอากาศ

Abstract

Thermal energy management has importance for electronic devices. These have continuously produced overheated while it is operated. In the present, the application of liquid jet impingement is widely utilized. This technique has a high cooling rate in the impingement region and can eliminate heat rapidly. Therefore, this research has an idea using two-phase flow which is air mixed with water jet for heat transfer enhancement for cooling the surface. This objective of the research is to study the flow and heat transfer characteristics of impinging two-phase jet flow which has mixed between water and air. This research can be divided into two parts; the first part focused on flow characteristics in the pipe nozzle and jet flow, which affected to the heat transfer on the impingement surface. This study used the pipe nozzle which has an inner diameter (D) of about 9.5 mm. Reynolds number of water (Rew) was fixed at 24,000. The effect of volumetric fraction was investigated at B = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7, respectively. Furthermore, the nozzle to impingement plate distances was studied at L = 2D, 4D, 6D, 8D, and 10D. The second part focused on the effect of bubble size on heat transfer characteristics of impinging jet for water jet mixing with air microbubble. The air microbubble was generated by compressing air and dissolving in the water pressure tank and then decompressed suddenly. The Reynolds number of water (Rew) was fixed at 16,000, and the gage pressure in the pressure tank was varied at 3, 4, 5, and 6 bars. The nozzle to impingement surface distance was investigated at L = 1D, 2D, 4D, and 8D For the study of flow characteristics of the air-water jet, the high-speed camera was applied to record the flow in the pipe nozzle and the jet flow before impingement. For the study the heat transfer characteristics on the impingement surface, the thermal infrared camera was used to record the temperature distribution, and the Nusselt number on the impingement surface was then calculated. The microbubble was recorded with a digital microscope camera, and image processing method was then used to evaluate the bubble size. For two-phase flow characteristics in the pipe nozzle with increase the flow rate of air, it was found that the water flow rate at Reynolds number of water between Rew =8,400 to 21,400 showed two types of flow patterns: the air bubble like a bullet shape or slug flow, and the group of air bubbles characterized with turbulence or churn flow. Moreover, the case of water flow rate between Rew = 24,000 and 26,700 showed three types of flow patterns: water flows with small bubbles or bubbly flow, slug flow and churn flow when increasing air flow rate. These results led to selection for one case of water flow rate at Rew = 24,000, which covering all types of flow patterns for studying the effect of air flow rate on heat transfer characteristics of an impinging jet. For the flow characteristic of the impinging jet at volumetric fraction B =0.1, it found that the air bubbles that were impinging on the surface were similar to bubbly flow in the pipe nozzle. For volumetric fraction at B=0.2 and 0.3, it found that the behavior of the bubble that near the exit nozzle was slug flow, and when this bubble was interacted by shearing with the surrounding water and break down to some small bubbles. For volumetric fraction at B =0.4 and 0.5, it found that the behavior of the bubbles that were near the exit nozzle was a cloud of the bubble or churn flow, and then it was sheared by the surrounding water and broken down to some small bubbles. For volumetric fraction at ß =0.6 and 0.7, it found that the small bubbles appeared near the exit nozzle and extended to the impingement surface. This resulted in the impingement surface mostly covered by the gas phase. However, the behavior does not observe for the case of L=10D. For the study of heat transfer of impinging jet at all jet impingement distance, the air bubbles in impinging jet had affected significantly on heat transfer enhancement when compared to the case of a water jet. This is due to the bubbles increasing the turbulent intensity in water jet and disturbing the thermal boundary layer overall the impingement surface. Particularly, the case of volumetric fraction B =0.3 and impingement distance L=4D gives the highest average Nusselt number on the surface. It can enhance heat transfer about 47% when compared case of impinging water jet. However, when volumetric fraction increased, it may also decrease the heat transfer of the impinging jet when compared with impinging water jet. Because the quantity of air increased in the jet flow will generate the air film on the impingement surface.