Effect of liquid jet diameter on performance of coaxial two-fluid airblast atomizers

Abstract

In this paper, the effect of liquid jet diameter on performance of coaxial two-fluid airblast atomizer has been investigated with water–air system in a wide region of liquid/gas mass flux ratio (0.137 < m < 15.6). The Sauter mean diameters (SMD) were measured using Malvern Laser Particle Sizer. The experimental results showed that the liquid jet diameter has striking effect on SMD for large m, but has no effect for small m. For a fixed large m, a decrease of SMD with liquid jet diameter was observed first, followed by an increase. A similar non-monotonic trend was also observed between SMD and liquid jet velocity, We, Re, and M (gas/liquid momentum flux ratio) at the same m, respectively. The minimum SMD is obtained when the velocity of liquid jet flow is about 1.5–4.0 m/s. The following correlation for SMD is developed from the measured data:

$$\text{SMD}=685.8{(u_{\text{g}}-3.297u_{\text{l}})}^{-0.4813}{m}^{0.3665}+0.1824d_{\text{l}}m.$$

Introduction

Atomization of liquids constitutes a technology presently used in almost all industrial operations. It covers a broad range of applications, such as evaporative cooling, combustion, gasification, fire suppression, agriculture, and spray drying [1], [2]. Numerous devices have been developed, and they are generally designated as atomizers or nozzles. Airblast atomizers are ideally suited for atomizing liquid fuels in continuous-flow combustion systems and entrained flow gasifiers. Droplet size in sprays is a crucial parameter of atomization process that is needed for the fundamental analysis of the transport of mass, momentum, and heat in engineering systems. Owing to the complicated and random nature of the atomization process, most practical atomizers do not produce sprays of uniform droplet size at any given operating conditions. Instead, the spray can be regarded as a spectrum of droplet sizes distributed about some defined mean droplet size. At present, the most widely used mean diameter may be the Sauter mean diameter ($\text{SMD}=\sum n_i{d}_i^3/\sum n_i{d}_i^2$, where n_i is the number of droplets per unit volume in size class i and d_i is the droplet diameter) for airblast atomization process [1].

Coaxial two-fluid airblast atomizer is often used in liquid propellant rocket engines, gas turbines, and other industrial applications [1]. The first major work of coaxial atomization was an experimental study by Nukiyama and Tanasawa [3], who obtained an expression for the SMD as a function of the injection parameters. Since these early experiments, many other investigations [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] have been carried out. The results showed that the SMD of this type of atomizer depends on its geometry and size, the physical properties of the liquid and gaseous medium, gas velocity u_g, and liquid/gas mass flux ratio m. Recently, Lasheras et al. [8] and Varga et al. [11] using high-speed digital video camera and phase-doppler particle analyzer, and Marmottant and Villermaux [14] and Villermaux et al. [15] using high-speed digital video camera, investigated the atomization mechanism (according to Kelvin–Helmholtz instability and Rayleigh–Taylor instability) of this type of atomizer, respectively. Now the influences of physical properties, gas velocity, and liquid/gas mass flux ratio are comparatively clear, which have been well reviewed by Lefebvre [1] and Lasheras and Hopfinger [9], respectively. Lasheras and Hopfinger [9] also proposed the breakup regime diagram of this type of atomizer.

However, owing to the complexity of the underlying physical processes involved in the breakup of a liquid jet by a high-speed turbulent gas jet, some influencing factors of this type of atomization, especially the effect of liquid jet diameter d_l, are still poorly understood. Lefebvre [1] pointed out that there are appreciable differences in regard to the effect of liquid jet diameter on SMD. Lorenzetto and Lefebvre [4] observed, in agreement with Nukiyama and Tanasawa [3], that liquid jet diameter appears to have little influence on SMD for liquid of low viscosity. Rizk and Lefebvre [5] found that $\text{SMD}\propto d_{\text{l}}^{0.6}$ for liquid of low viscosity. But Varga et al. [11] observed that SMD is larger for the case of the smaller liquid nozzle diameter that was reduced by a factor of approximately three, which is actually opposite to intuition. Therefore, it is necessary to investigate the dependence of SMD on liquid jet diameter, which is one of the foundations to design atomizers with reliable atomization performance.

In the above studies, the liquid/gas mass flux ratio is usually less than 1.0 [1], [3], [4], [5], [11]. But in chemical industry, people often need to deal with much larger liquid/gas mass flux ratio because of the strict process requirements, such as the case in the entrained flow gasifiers of residual oil [16] or coal water slurry [17] based on the partial oxidation with pure oxygen. In the present work, the effect of liquid jet diameter on SMD was investigated experimentally using a Malvern Laser Particle Sizer with water–air systems in a very wide region of liquid/gas mass flux ratio.