CFD MODELLING OF LIQUID JET AND CASCADE BREAKUP IN CROSSFLOWS

Formation and breakup of liquid droplets play key roles in a wide range of chemical processes, most of which are concerned with atomising the liquid into fine droplet sprays in a co-flow environment where the liquid jet is sandwiched between high-velocity air streams to produce a droplet cloud. The present work deals with droplet breakup in a cross-flow environment where the air stream directly impacts onto a liquid jet or column. A CFD model has been developed for the breakup of this liquid jet and subsequent formation of droplets. The model has been tested for liquid jets in a crossflow that are documented in the published literature. Both the shape of the liquid column and dispersion pattern of the droplets as predicted by the model agree well with the published data. NOMENCLATURE C model constant CD drag coefficient d droplet diameter F momentum source k turbulence kinetic energy M Mach number n droplet number density On Ohnesorge number P pressure q liquid/air momentum flux ratio, 2 2 / a a f l v v ρ ρ r volume fraction T characteristic timescale for droplet breakage U mean velocity vector u′ fluctuating velocity vector Vslip slip velocity w cascade width We Weber number α phase ε turbulence dissipation rate ρ density σ Prandtl number μ dynamic molecular viscosity INTRODUCTION Formation and breakup of liquid droplets play key roles in a wide range of chemical processes, including fuel spray in combustors, spray coating, powder formation, and gas scrubbing. Most of these processes are concerned with atomising the liquid into fine droplet sprays in a co-flow environment in which the liquid jet is sandwiched between high-velocity air streams to produce a droplet cloud. There is a large body of scientific knowledge addressing this topic (Varga et. al., 2003; Inamura and Daikoku, 2002; Park et. al, 2002) in the published literature. By comparison, liquid droplets used in some industrial processes, such as spray cooling towers, need to be introduced in such a way that facilitates maximum contact between a large body of gas and the liquid. In cases where the gas temperature is considerably less than boiling point of the liquid, liquid droplets can be sprayed directly onto the gas to strip the solids off the particleladen gas stream. However, for processes where the liquid also plays a significant role in cooling the gas through vaporisation, it is not appropriate to spray fine liquid droplets onto the hot gas streams. Liquid droplets thus need to be generated using an opposingor crossflow arrangement with respect to the gas stream. Wu et. al. (1995, 1997, 1998) have performed a series of experiments to examine the atomisation of an upflowing liquid jet injected into a strong crossflow. Parameters investigated include liquid injection velocity, gas velocity and liquid surface tension. Their flow visualisation study has revealed a series of events taking place during the liquid jet breakup process. Breakup first starts at the liquid jet surface from which some liquid fragments are stripped and droplets form. This causes instability to the jet surface in the form of acceleration waves which deform and flatten the liquid column. The liquid column eventually disintegrates into ligaments and droplets. The present paper outlines the development of a droplet breakup model that is suitable for Eulerian-Eulerian twophase flow applications where the particle phase is solved as a continuum fluid. The model is based on a populationbalance approach with one governing equation representing the transport of droplet number density. Droplet breakup is evaluated based on local droplet Weber number and reflected as a source term to the populationbalance equation. The model is validated using experimental data provided in Wu et. al. (1997) and Lim et. al. (2006). Model development and results of model validation are presented in this paper. MODEL DESCRIPTION Gas and liquid flow models were developed using a commercial CFD code ANSYS CFX-11 and are based on a two-fluid approach. The flow field is assumed to be in a steady-state throughout the flow domain. Two-Fluid Model Gas and liquid flow properties are calculated by numerically solving the following set of Reynoldsaveraged Navier-Stokes equations for each phase α 0 ) ( = ⋅ ∇ α α α ρ U r (1)