Evaluation of Cavitation in a Liquid-Liquid Ejector

This paper summarizes the numerical analysis of a liquid-liquid ejector pump with focus on the accuracy of the employed cavitation model and in view of its potential application as a dualor multi-liquid mixing and injection system. While the use of gas-phase or gas/liquid-phase ejection systems has found wide application in gas/hybrid burners and scrubbers for air pollution control; the use of liquid-phase ejectors for the mixing and preparation of different liquids and additives prior to atomization or other processing steps is not common place. As a metered-delivery and mixing device, the ejector system has to be properly designed in order to omit onset of cavitation within the employed working fluids and across the system operating range. Cavitation would not only impact ejector performance, it could also affect steady-state flow conditions and result in poor mixing of the liquid phases which can affect the quality of the subsequent processing steps. In order to utilize CFD analysis as part of the design process for the prescribed system, it is important that the employed cavitation model accurately predicts onset and extend of cavitation within the system. As a first step, the present work focuses on the numerical analysis of a liquid fuel ejector pump over a range of operating conditions and including onset of cavitation. The analysis is carried out by employing ANSYS/CFX v13.0 with its implementation of the Rayleigh-Plesset cavitation model. Comparison with empirical data shows that, the numerical analysis accurately tracks ejector performance and the employed cavitation model accurately predicts the onset of cavitation within the ejector. The relevance of dissolved gases and viscous stresses on the cavitating ejector flow is discussed. Corresponding author: [email protected] ILASS Americas, 24 Annual Conference on Liquid Atomization and Spray Systems, San Antonio, TX, May 2012 Introduction Ejector or jet pumps are commonly used to pump fuel within an aircraft fuel system. One application includes scavenging fuel from remote corners or the bottom of fuel tanks and discharge that fuel at the inlet to the main fuel feed pump(s). See Figs. 1 and 2. In an ejector or jet pump, a driving fluid is expanded through a nozzle (motive flow), converting its pressure energy into kinetic energy thereby reducing its static pressure according to Bernoulli’s Principle. The low pressure high-speed fluid zone downstream of the motive nozzle draws in and entrains the surrounding suction fluid fed from a separate inlet. Motive and suction fluids mix as the static pressure of the mixture increases further downstream when kinetic head is transformed back into pressure head. The suction fluid can be of the same type as the motive flow or it can be a different fluid. The fact that an ejector pump does not have any moving parts allows the pumping of suction fluids that cannot be delivered by other pumps, e.g., due to impurities such as particle loading, for example. Figure 1. Schematic of aircraft fuel system including ejector pump for fuel scavenging [1]. Figure 2. Two typical ejector pumps out of the Parker product line. In the fuel scavenging application described above, the fuel scavenged from the bottom of fuel tanks is often contaminated with considerable amounts of water (originally dissolved within the fuel or entering the fuel tank via condensation through vent lines); consequently, an important function of ejector pumps in this application is to disperse any water present within the scavenged liquid volume (i.e., suction fluid) into fine drops, so that the fuel with its finely dispersed water droplets can be safely consumed by the engine without any performance impact. Since traditional ejector pumps do not have any moving parts, they are highly reliable if used within their operating range, latter being limited by the onset of cavitation. Here, two types of cavitation are to be distinguished; i.e., gaseous cavitation and vapor cavitation both of which are very different in nature. The formation and ‘collapse’ of gaseous cavitation bubbles is a gas diffusion process taking place across the bubble interface. This is in contrast to vapor cavitation, where bubble formation and collapse is the result of a phase change of the liquid from liquid-phase to gas-phase, i.e., a very rapid process taking place in microseconds. This rapid phase-change causes the release of considerable amounts of energy when vapor bubbles collapse, which can result in cavitation damage if the bubble collapse occurs close to walls. Gaseous cavitation, on the other hand, will not cause any material damage; however, just like vapor cavitation, it can be the performance limiting factor in ejector pumps. The present paper investigates a fuel ejector pump whose performance limit is determined by the onset of vapor cavitation. Accordingly, comparison of the subsequently presented CFD results with actual test or empirical data provides a means to evaluate the performance of the cavitation sub-model employed within the analysis. Problem Set-Up and Operating Conditions Figure 3 shows a cut-away of the fluid inverse generated from the ejector pump geometry considered for this study. The suction flow is fed from a side inlet into a plenum chamber where the suction fluid interacts with the motive flow which enters the cylindrical plenum axially. Geometric dimensions of motive nozzle, plenum, mixing section and diffuser section are fixed. Various operating points of the ejector pump are analyzed governed by motive and suction flow static inlet pressures and mixture flow static pressure at the diffuser outlet. Figure 3. Fluid inverse of ejector pump including inlet and exit flow boundaries. Table 1 summarizes the analyzed operating points, identified by flow ratio Φ = Wi/ Wm with Wi and Wm representing the induced/suction flow and motive flow, respectively; and the non-dimensional total pressure ratio formed by the differences of total downstream and suction pressures (Pt,d Pt,s), and total motive and downstream pressures (Pt,m Pt,d), respectively.