THE WAKE BEHAVIOR BEHIND A HEATED CYLINDER IN FORCED AND MIXED CONVECTION REGIMES

The effect of buoyancy on the wake instability of a heated circular cylinder in a contra flow arrangement is investigated experimentally in the present study. A novel optical diagnostic technique, Molecular Tagging Velocimetry and Thermometry (MTV&T), was used for qualitative flow visualization and quantitative simultaneous measurements of velocity and temperature fields in the wake of the heated cylinder. The experiment was conducted in a water channel with the temperature and Reynolds number of the approaching flow being held constant at C T o 24 = ∞ and 130 / Re = = ∞ ∞ ∞ μ ρ D U . The temperature of the heated cylinder varied between C o 24 (unheated cylinder) and C o 85 , corresponding to a Richardson number (Ri) varying between zero (unheated) and unity. The heat transfer process around the heated cylinder changes from the forced convection regime to the mixed convection regime. With increasing Richardson number, significant modifications of the wake instability were revealed from both qualitative flow visualization images and quantitative simultaneous velocity and temperature fields. The effect of buoyancy on the wake instability of the heated cylinder was discussed in the terms of vortex shedding pattern, vortex shedding frequency, turbulent heat flux distribution, the wake closure length and averaged Nusselt number of the heated cylinder. INTRODUCTION An understanding of the flow around a bluff body is of great importance due to its fundamental nature as well as many related engineering applications. A circular cylinder is a most commonly studied bluff body. Despite of its simple shape, a circular cylinder generates a wake that is dynamically complex. By varying the Reynolds number, a variety of flow patterns and vortex shedding characteristics in the wake of a circular cylinder have already been observed and discussed in literature. The wake behavior behind a heated circular cylinder is physically more complicated due to the buoyancy effect added to the viscous phenomena. The heat transfer from the surface of a heated cylinder to surrounding fluids can be either pure forced convection, free convection or mixed convection depending on the Richardson number ( 2 Re / Gr i R = ) of the case. In the forced convection (Ri<<1) regime, where the effect of buoyancy is neglected, the heat transfer is a function of Reynolds (Re) number and the Prandtl (Pr) number. In free convection (Ri>>1) regime, where the forced convection is negligible, the heat transfer is a function of the Grashof (Gr) number and the Prandtl ( a / Pr ν = ) number. In the mixed convection regime, both forced convention and free convection are important, and the heat transfer is a function of the Grashof number ( 2 3 / ) ( ν β D T T g r G w ∞ − = ), the Reynolds Number (Re), the Prandtl number and the forced flow direction. Despite the fact of that mixed convection around bluff bodies is of great importance for various engineering applications (for example, electronics cooling, compact heat exchangers, etc. ), the thermal effect (buoyancy effect) on the wake stability of a bluff body in mixed convection regime has received little attention compared to that in forced convection regime. When a heated cylinder operates in mixed convection regime, buoyancy force plays an important role to the flow pattern in the wake of a heated cylinder. As shown in Fig. 1, for a horizontal heated cylinder, the forced flow approaching to the heated cylinder can be either horizontal, parallel (vertically upward) or contra (vertically down) depending on the angle between the buoyancy force acting on the fluid around the heated cylinder and the approaching flow direction.