Investigating the Bubble Behavior in Pool Boiling in Microgravity Conditions

摘要

In space, objects float without falling down. The floating effect happens because of the absence of gravity which is "No-Gravity", also known as Zero-gravity. Even though it is mentioned as zero-gravity, there is small portion of gravity level available; it is in micro level (10{sup}(-6)). Therefore it is known as Microgravity. It is important to study and conduct microgravity research in order to understand the outer space environmental behaviors. Liquids (fluids) behave differently in microgravity level than the earth's gravity. Experimental research on "Boiling" was done in microgravity conditions to study the "Kelvin force effect in microgravity". Experimental Research was mainly funded by NASA-WV Space Grant Consortium. The bubble behavior, which is deformation (change in shape) and the change in radius of a bubble, and the velocity of a bubble in microgravity conditions, was studied. Parabolic path of an aircraft (going approximately 45° nose up and down) can create microgravity conditions inside an aircraft within a period of 20-30 seconds. That method was used to create microgravity conditions in a short period of time to carry out the experiment. Several parabolas were done to get the required microgravity periods to conduct the research. Boiling was done in two cylindrical tanks with fixed boiling pads in the bottom of the tank. Thermo couples were used to determine the temperature while running the experiment and temperature data was stored in the computer. While the experiment was running, video cameras were used to capture the boiling behavior which includes the bubble behavior. After conducting 40 parabolas of microgravity levels, most of the successful microgravity level parabolas were examined by reviewing the video clips. Frame by frame analysis of half a second time-frame captures of the clip were used to determine the bubble position. Center of the bubble, shape of the bubble (maximum radius and minimum radius in vertical and horizontal axis), rate of change of radius, vertical and horizontal displacement of the center, rate of change of the position of the bubble coordinates and the vertical component of the bubble velocity were determined by manually analyzing the data. Bubble displacement and the coordinates were measured in pixels. A couple of softwares such as Power DVD and Photoshop image processing softwares were used to examine the characteristics of the bubble frames. In one particular parabolic path (a parabolic path with a most clear and detectable bubble visibility) one single bubble was selected in microgravity boiling period from the bottom of the tank and until it reached the top of the tank to determine the data. After producing the Excel spread sheets of data, results were gathered. As an outcome of the results, bubble radius in vertical and horizontal direction was calculated with the average radius. Under the microgravity conditions bubble's vertical displacement was in a spiral path. It could be expressed graphically in 2-D coordinate system.