Cfd Analysis of Impinging Axisymmetric Turbulent Fountains

When a positively buoyant vertical fluid jet directed downwards from its source impinges on a horizontal flat surface, the resulting flow is termed an “impinging fountain”. These flows arise in a number of practical situations, for example, in gas metal arc welding (GMAW). Developing a description of this flow presents the researcher with a number of challenges and forms the motivation for this work. In this paper, CFD simulations of impinging fountain flows in a brine environment are presented, and the predictions compared with earlier experimental work carried out by two of the authors (Cooper and Hunt, 2004). Close agreement is achieved between experiment and simulation. In particular, the CFD accurately predicts the lateral spread of the impinging fountain along the horizontal surface. This spread determines the initial size of the source of the buoyant plume that is subsequently formed and is, thus, a crucial parameter in predicting the spread of pollutants, such as welding fume, via the plume. The results presented herein are a precursor to theoretical and experimental investigations of GMAW-induced flow fields by the authors. NOMENCLATURE B Buoyancy Flux (m/s) D Diameter (m) Fr Froude Number g Gravitational Acceleration (m/s) H Standoff Distance (m) L Length Scale (m) M Momentum Flux (m/s) m Mass Fraction of Salt p Pressure (Pa) Q Volume Flow Rate (m/s) R Radial Distance (m) u Velocity Vector (m/s) V Average Velocity (m/s) v,w Velocity Components along y, z axes (m/s) ρ Density (kg/m) φ General Conserved Variable μ Dynamic Viscosity (Pa-s) INTRODUCTION Buoyancy-driven convective flows abound in the natural and artificial environments. An understanding of the structure of these flows is of interest from both theoretical and practical points of view. In plumes, buoyancy and momentum fluxes at the source act in the same direction, as in the case of thermal plumes arising from an upward injection of warm air in a cooler environment. Flows that develop from sources of buoyancy and momentum fluxes that act in opposite directions are known as “fountains” (e.g. Turner, 1966). These flows result from a combination of forced and natural convective effects, and are encountered in a number of industrial and natural settings. In industrial workplaces, for example, warm (positively buoyant) air curtains formed by jets directed downward from the ceiling give rise to turbulent fountains. Flows induced by the gas metal arc welding (GMAW) process bear a close resemblance to impinging fountain flows, as established by Norrish, et al. (2005). On a different scale and in a different setting, the jets formed by V-STOL aircraft engines are an example of similar flows. Common to these examples is a positively buoyant jet directed downward from a source in the vicinity of a horizontal surface (the ground in the case of air curtains and V-STOL aircraft; the surface of the workpiece in the case of GMAW-induced flows). In the absence of density differences, the impinging jet flow (as opposed to fountain) is relevant to the cooling of microelectronic components (e.g. Chiriac and Ortega, 2002). The descending jet-like flow from the fountain source initially impinges on the horizontal surface and is forced to travel radially outward. After having travelled a certain distance along the surface, the buoyancy force becomes dominant, causing the flow to detach from the surface and rise up, forming the source of a thermal plume. Of concern in the case of GMAW-induced flows are the welding fume and other gaseous/particulate contaminants transported by the plume into the breathing zone of operators. If the spread of contaminants via impinging fountain flows is to be controlled effectively, it is necessary to understand and predict the structure of these flows.