Miniature Planar Nozzles

• Main Authors: Wu Chao Ching, 吳招慶
• Other Authors: Rong Fung Huang
• Format: Others
• Language: zh-TW
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/54821449522882723093

碩士 === 國立臺灣科技大學 === 機械工程系 === 88 === Presented in this thesis is a preliminary study of miniature planar nozzles - a major component of micron-sized heat engines to be fabricated by the photo lithography technique. Experiments were carried out to assess the feasibility and the pros and cons of these small nozzles. Deviations of the nozzle performance and behaviors from those of large, axial symmetrical nozzles were also investigated. The planar nozzles were fabricated in acrylic using a high-precision CNC milling machine. The thickness and throat width of the nozzles were approximately 1 mm. Maximum nozzle length and exit width were 20 and 10 mm, respectively. The planar nozzles and their lids were glued together using epoxy. In the experiments the back pressure was maintained at the atmospheric pressure while the stagnation pressure at the nozzle inlet varied from 1 to 10 atm. The designed exit Mach numbers were between 2.5 and 4. The thrusts produced by the small nozzles were only a few grams and the mass flow rates were in the neighborhood of 1 gram/s. Consequently, equipment and devices capable of measuring extremely small thrusts and flow rates, such as high-precision balances and Venturi, were developed and applied to the measurement of nozzle performance. Two nozzle contours were tested. The first nozzle contour was designed based on the isentropic flow theory of one-dimensional ideal gas flows. With the area - Mach number relationship determined from the isentropic flow theory, the length of the divergent section was then varied to maximize the nozzle thrust. Except for very low operating pressures, the thrusts and mass flow rates of the first nozzle design were measured to be approximately 70 and 50 % of those of isentropic flows. The ratio of thrust to mass flow efficiency of these nozzles was about 1.4 After the optimal length of the first nozzle design was determined, a simple method which required zero slope of nozzle contour at the nozzle throat and exit was then employed to improve the nozzle contour for better performance. The second nozzle design has a funnel-shaped divergent section. Test results revealed that, the thrust and mass flow rate of the improved nozzle contour could reach 83 and 66 % of the theoretical values, but the ratio of thrust to mass flow efficiency dropped to 1.2. The schlieren technique was employed for the diagnosis of flow field at the nozzle exit. When the flow leaving the nozzle became supersonic, the diamond-shaped expansion / compression wave pattern appeared and repeated itself several times in the flow direction. The jet boundary was also clearly visible in the schlieren photos. The dimensions of the waves increased slightly as the jet expanded. Among the various nozzles tested in the present study, those of low efficiency showed a jet cross section less than the nozzle exit area, indicating the gas flow in this case was unable to follow the nozzle contour in the divergent section. Flow visualization therefore proved to be a handy tool for a quick diagnosis of the design of miniature planar nozzles.