Heat Transfer And Flow Visualization Of Silicon-based Micro Pulsating Heat Pipe

In this thesis, four Micro Flat Pulsating Heat Pipes (MPHP) havebeen designed and fabricated by Micro-Electrical Mechanical Systems(MEMS), where chemical wet-etching was used on single-crystallinesilicon wafers. Thus the cross sections of all channels are trapezoidal andthe sloped angles of the trapezoid are samely at54.7°, which isdetermined by silicon’s crystal structure. The former three MPHPs#1,#2and#3have uniform cross sections with the hydraulic diameters of305.4,407.2and260.5μm respectively, while the last one#4(d varies from342.7to439.6μm) has non-uniform cross section. Series of experimentson these MPHPs have been carried out to observe flow and heat transfercharacteristics, and detailed discussion on various parameters such as tiltangle, hydraulic diameter, filling ratio, input power and cross section ofchannels are provided in the thesis. Based on experimental results, twomodified designs have been proposed and tested, and test results showedthey did achieve better performances. The following are the majorconclusions of this study:In MPHPs with sharp angled trapezoidal cross section channels, liquid slug/vapor plug alternation distribution was not observed at thebeginning even with Bo numbers far smaller than2. Thus, there are notenough bubbles to play the role of ‘pump’, which goes against triggeringtypical self-sustained violent oscillation. Besides, with highly smoothedinner walls and super pure working fluid, it needs a great degree of superheat to start to boil, and the average nucleate temperature of the testedMPHPs is140℃. Once a nucleation site appears, super-heated liquidstarts bulk boiling.Two operation modes have been observed in MPHP#1and#2:atypical oscillation and intermittent washing. Flow pattern of atypicaloscillation mainly includes plug flow and stratified flow. Typical andatypical oscillations have similar frequency, but former has largeramplitudes than latter; besides, for atypical oscillation a high frequency(amplitude~1mm,9Hz) is superimposed on a low frequency (amplitude~9mm,0.7Hz). Under this operation mode, working fluid flows back tothe evaporating section with the help of gravity. Hence heat transferperformance in horizontal direction is relatively poor when gravitycomponent is zero and oscillation is suppressed. However, after reachinga certain tilt angle of30°(#2) or60°(#1), influence of tilt angle (alsogravity component) is no longer obvious. Increasing inner diameterwithin a little range also helps in heat transfer enhancement. In addition,filling ratio doesn’t affect much on this mode’s heat transfer performance. Intermittent washing usually happens in horizontal direction with acertain input power. The intermittence means there is a big time gapbetween every two washing behaviors. Under this intermittent washing,three conditions periodically repeat: stop-over, oscillation with highfrequency and small amplitude (frequency at~27.5Hz, amplitude at~2mm), washing the evaporator. The cycle periods of heat pipe#2undermy tested working conditions are respectively10.6s,15s, and24s. Thefrequency decreases with an increasing input power while thetemperature fluctuation increases with higher input power. As one of thethree repeating conditions, behavior of washing the evaporator takes avery short time at around0.3s, but it is very violent, and the flow patternis mainly annular flow and wavy flow.Through the experiment on MPHP#3, it shows for channels withsharp angles, although decreasing inner diameter still cannot produce aninitial liquid slug/vapor plug alternation distribution, anyway during theoperation plug flow occurs more easily and frequently. The heat transferresistance is5~7K/W, which is lowest among all tested MPHPs. Besides,#3is able to operate under any tilt angle, but it doesn’t mean heat pipeoperates100%for sure even with enough input power, in other words,even under the same working condition, it may or may not oscillate, andthe oscillation occurring probability is different for different tilt angles.Result shows that when tilt angle is beyond15°, oscillation occurring probabilities are basically beyond60%.With the non-uniform cross section design, MPHP#4successfullyaccomplished circulation flow in clock wise direction. Like#1and#2,gravity has a big influence on the heat transfer performance, so inhorizontal direction the heat pipe cannot work very well. Thenon-uniform cross section design helps to increase capillary force, whichis more obvious under high input power, and delays the dry-out limit.Under advantageous working conditions, the heat transfer enhancementratio of#4is around61%, which is the best of all tested heat pipes, with athermal conductivity of222W/(mK) in average.