Numerical Study of Thermal Plume Characteristics and Entrainment in an Enclosure with a Point Heat Source

Abstract

The structure of a buoyant plume above a point heat source in a ventilated enclosure has been investigated using large-eddy simulation (LES). The aim of the work is to assess the performance and the accuracy of LES for modelling buoyancy-driven displacement ventilation of an enclosure and investigate the role of coherent structures in the plume entrainment mechanism which is important in these flow types because, for example, entrainment determines the ventilation flow rate. The Smagorinsky subgrid-scale model is used for the unresolved small-scale turbulence. The Rayleigh number Ra is chosen in the range where spatial transition from laminar to turbulent flow takes place (Ra=1.5×109). The stratification height and temperature of the stratified layer deduced from the mean field of the LES data is in good agreement with the theory of Linden, Lane-Serff and Smeed (1990). The plume entrainment coefficient is in good agreement with experimental values determined by Morton, Taylor and Turner (1956), Rouse, Yih and Humphreys (1952), and Baines and Turner (1969). Instantaneously, the plume develops through expansion and contraction phases, where the expansion phase is associated with the existence of coherent large-scale structures leading to an outward stretching of the plume and the contraction occurs as a result of partial breakdown and/or loss of coherence of these structures. As a result, the instantaneous entrained mass (and thus the entrainment coefficient) at different heights below the mean interface height were found to fluctuate about a mean value. Visualization of the computed flow showed that the stretching mechanism of the large-scale structures, which governs the expansion-contraction behaviour of the plume, occurs in such a way that the coherent structure dominating the flow below the interface height takes a spiral shape.