Atomization characteristics of impinging Liquid-Gas jets

  • Yakang Xia

Student thesis: Master's Thesis

Abstract

Atomization by impinging liquid jets is a widely used process in propulsion and chemical engineering system, especially in liquid propellant rocket engines due to the advantages of simple fabrication, low costs and satisfactory mixing and atomization performance besides the advantage of controlling the drop size and spatial distribution. The atomization characteristics are of great significance to these applications and it is necessary to get a good understanding of the spray characteristics, especially velocity and size of the droplets in order to improve the efficiency of various related applications. In this study three configurations of turbulent impinging jet atomizers are manufactured and studied. They are Liquid-Liquid (L-L), Liquid-Air-Liquid (L-A-L) and Liquid-Air (L-A) impinging jets. Thesis work included design and manufacturing of an experimental test rig. The study consists of flow visualization using high speed photography to investigate the topological structure of the generated spray for a range of liquid and air flow rates. The study then extends to use an advanced electro-optical technique, Phase Doppler Anemometry (PDA) to obtain the droplet size and velocity distribution characteristics of the spray. The Reynolds number of the water jet based on the velocity and the water pipe diameter ranges from 1540 to 5400 (2.25 m/s - 7.89 m/s). The Reynolds number of the air jet for the size and velocity measurement is maintained around 18300. In the L-L configuration, five spray patterns are identified and confirmed the previously identified regimes. These are: pre-length, closed rim, ruffled sheet, open rim and turbulent regime. The breakup length of water sheet is found to increase with increasing Weber number in the laminar regime, and in the turbulent case, it is found to increase and then remain almost constant. The breakup length also decreases with increasing impinging angle. Spray angle in the L-A-L configuration increases with the water jet velocity and air flow rate when the water jet is totally disintegrated. It also increases with the impingement angle. In the L-A configuration, the spray turns out to be a half cone, and there are two edges containing relatively large droplets. PDA results indicate that in the L-A-L impinging jets configuration, droplets size is the smallest in the spray center, with minimum values of D10 (arithmetic mean diameter) and D32 (Sauter mean diameter SMD) of 20 μm and 50 μm, respectively, when the air flow rate is 11.4 L/min. The size of droplets increases towards the spray outer region gradually to about 80 μm and 120 μm for D10 and D32 , respectively 40 mm away from the spray axis. The mean vertical droplet velocity component W is the highest in the spray center and decreases gradually with increasing distance from the spray center. The radial component V is quite small due to the fact that it is mainly generated from the expansion of air jet and the liquid jet impact. At higher air flow rates, SMD in the spray center is smaller than that at a relatively small air flow rates, and the difference decreases towards the spray edge. The vertical velocity of the droplets at a higher air flow rate is larger than that at a lower air flow rate. The difference between the vertical velocities at different air flow rates is the largest in the spray axis and decreases with the increasing distance away from the spray axis. In the L-A configuration where the spray adopts a half cone shape, the spray center is not located below the air jet orifice but a little off-axis, and the droplets are smaller in the spray center and increase towards the outer region, which is similar to that of L-A-L configuration. The maximum SMD in the L-A configuration is about 40 μm, which is smaller than the (SMD)min in L-A-L of 50 μm at the same water and air flow rates. The PDA measurements in the air-assisted impinging jets flow are obtained for the first time. The study sheds light on the performance of the L-A-L and L-A atomizers providing useful information for future CFD simulation works based on Eulerian-Eulerian interpenetrating models or those using population balance modeling.
Date of AwardDec 2016
Original languageAmerican English
SupervisorLyes Khezzar (Supervisor)

Keywords

  • Applied sciences
  • Atomization
  • Droplet size and velocity
  • Impinging jets
  • Phase doppler anemometry (pda)
  • Mechanical engineering
  • 0548:Mechanical engineering

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