1 Proceedings of the 37 th International & 4 th National Conference on Fluid Mechanics and Fluid Power FMFP 2010 December 16-18, 2010, IIT Madras, Chennai, India FMFP 2010 ______________ MERGING OF SHEET PLUMES IN TURBULENT CONVECTION G. S. Gunasegarane & Baburaj A. Puthenveettil Department of Applied Mechanics, IIT Madras, Chennai, Tamil Nadu, India. e-mail: apbraj@iitm.ac.in ABSTRACT We present results from a visualization experiment in turbulent free convection in water over a horizontal heated surface at moderately high Rayleigh number (Ra=10 5 to 10 9 ). We observe the random formation of sheet plumes very close to the heated surface. The rising sheet plumes also merge laterally, presumably due to the entrainment flow into the plumes. We study the merging dynamics of two parallel sheet plumes by measuring the lateral velocities at different times of their merging, at different heat fluxes and aspect ratios. We observe that the merging rate increase with the increase in heat flux. The effect of aspect ratio on merging rate is found to be negligible for constant heat flux. Due to the local shear created by the large scale flow the merging rates decreased. Key words: turbulent convection, sheet plumes, merging. INTRODUCTION Natural convection over horizontal surfaces has been studied in simple geometries to understand the nature of buoyancy induced turbulence. These flows abound in nature and have major technological applications. Effectiveness of many engineering processes such as electronics cooling and chemical vapour deposition is decided by the natural convection dynamics near the horizontal heated surfaces. Due to the strong density gradients created over the heated surface, convection with continuous rising columns of buoyant fluid called “plumes” occur. Various theories of turbulent natural convection approximate the near-wall region as turbulent shear boundary layer [Castaing et al, 1989], mixing zone [Siggia, 1990], Balsius boundary layer [Grossmann & Lohse, 2000]. The Rayleigh number (Ra = gβΔTD 3 /αν), a ratio of buoyancy to dissipative effects and the Prandtl number (Pr= ν/α), a fluid property defined as the ratio of momentum to thermal diffusivity characterize such flow regimes. Here g = the acceleration due to gravity, β = the thermal expansion coefficient, ΔT = the temperature difference between the walls, D = the height of the fluid layer, α = the thermal diffusivity and ν = the kinematic viscosity. However, visualization results presented [Puthenveettil and Arakeri, 2005] on high Rayleigh number (Ra) unsteady turbulent free convection in the high Prandtl number (Pr) regime showed that, the near wall coherent structures are sheet plumes.