Turbulent plumes in confined spaces

Abstract

Studies pertaining to turbulent plumes in confined spaces are of utmost interest due to its relevance in practical flows that are associated with, but not restricted to, the propagation of smoke and hot gases generated by fires in buildings, road and railway tunnels, etc. In this dissertation, direct numerical simulations (DNS) of the governing equations are carried out to analyze such flows with the focus on (i) free turbulent line plumes, and (ii) wall attached turbulent line plumes, in confined spaces. In all cases, the computation domain is rectangular with no-slip and adiabatic boundary conditions at the top, bottom, and lateral side walls. In free turbulent line plume simulations, the plume originates from a line heat source of length, L, located at the centre of the bottom wall and rises until it impinges on the top wall and eventually spreading out laterally thereby producing a buoyant fluid layer at the top wall. Since the region is confined, the continuous heat source forces the top layer to move downwards, until it reaches the bottom wall, when the flow is said to be at the asymptotic state (Baines and Turner 1969). DNS data at three Reynolds numbers (ReH), 1800, 3600 and 7200, based on box height H and the buoyant velocity scale, F_1/3 0 , where F_0 is buoyancy flux per unit length, are presented for plume lengths, L/H = 1, 2 and 4 and box aspect ratio, R/H = 1. Here, R is the box half-width. Following the initial transient dynamics, a flapping motion of the plume is observed, where the plume oscillates around the centre plane of the box. The DNS results reveal that the long-term behavior of the flow consists of a meandering, flapping plume with a counter-rotating vortex pair on either side of the plume. Additionally, the plume volume, momentum, and buoyancy fluxes obtained from the simulations are compared to the theoretical models proposed by Baines and Turner (1969) and Barnett (1991). Further, simulations of turbulent line plumes are carried out at increased box aspect ratios R/H = 1, 2, 4, 8 and 16, to study the horizontal outflow of the buoyant fluid layer after the plume impinges on the top wall. Following the axisymmetric plume model of Kaye and Hunt (2007), a theoretical model to compute the horizontal outflow properties is developed for turbulent line plumes. In the case of wall attached thermal plumes, the plume originates from a local line heat source placed at the bottom left corner of the box. The plume develops along the vertical side wall while remaining attached to it before spreading across the top wall forming a buoyant fluid layer and eventually moving downwards and filling the whole box. The simulations are carried out at ReH = 14530 and L/H = 0.5, and a parametric study is conducted for boxes of aspect ratios R/H = 1 and 2. Furthermore, the original filling box model of Baines and Turner Baines and Turner (1969) is modified to incorporate the wall shear stress and are compared against the results obtained from the DNS. A reasonable agreement is observed for the volume and momentum fluxes in the quiescent uniform environment and for the time-dependent buoyancy profiles calculated further away from the plume. Finally, the entrainment processes in both free and wall attached line plumes are assessed, using the DNS data. Both cases show similar contributions to entrainment due to net buoyancy. However, a deficit in the entrainment coefficient is observed for wall plumes due to the effect of the wall, which in turn suppressed the turbulent kinetic energy production.