Effects of Large Aspect ratio on Atomization Performance of a Micro-nozzle

• Main Authors: Yi-Feng Chen, 陳義豐
• Other Authors: Muh-Rong Wang
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/9sz78z

碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 90 === ABSTRACT This research is to investigate the effects of large aspect ratio of the nozzle on atomization performance and the droplet evolution in the spray jet of a micro-nozzle. All experiments are measured under room temperature and atmosphere pressure. Working pressure is 5atm to 11atm. The orifice hydraulic diameter is 66.67μm and the aspect ratio is 1:5. Flow visualization with stroboscope shows that the liquid film is formed near the nozzle. Results also show that the ratios of mass flow rate and exit-velocity are almost constant at different aspect ratios when pressure is increased. It also shows that the mass flow rate as low as 0.013g/sec-atm is achieved with this nozzle. It can be used in the micro-engine or micro-flow applications. The energy conversion rate is decreased when the aspect ratio is increased i.e., only 30% conversion in high pressure range. The pressure drop is almost constant under high working pressure. Hence the energy conversion rate increases in the high-pressure range. Measurements show that the proportion of droplets-probability with diameter more than 100μm tends to decrease in the downstream. Measurements by PDPA further show that the number density and volume flux in the core region is 2.3 times more than that in the shear layer region. Hence, it can be considered as a single stream spray jet. The mean drop size tends to increase and the reduction of the axial velocity of the droplets in the down stream is more significant when the aspect ratio is 1:2. On the other hand, the mean drop size is almost constant and the axial velocity of the droplets decreases slowly in the down stream when the aspect ratio is 1:5. The radial velocity distribution of droplets concentrates in the vicinity of core region due to the single stream effects. It is also found that the velocities of droplets from 4μm to 176μm are all the same value in the core region when operating at Pt=7atm. A shear layer is measured at Z=5cm, resulting in an extreme high velocity gradient（ΔU/ΔR）in this particular flow field . It is believed that the momentum transport is very high in this case because its velocity gradient is two orders of magnitude higher than the conventional spray flow. Data measured by PDPA also indicates that droplet coalescence takes place in the down stream because the droplet velocity decreases in the down stream due to the momentum transfer in the two phase flow.