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Abstract
An external forced, free and mixed convection heat transfer, with opposing buoyant and inertial forces from heated vertical cylinders, was experimentally and numerically investigated. A Mach-Zehnder interferometer was used for the experiment, and a discretization method with Patankar's algorithm "SIMPLER" was used for the numerical analysis.
The mixed convection behavior which appears during the transition from downward forced flow to free convection was simulated by quasi-unsteady state conditions. Forced downward flow is commonly used in gas-cooled nuclear reactors. The results of this study have a bearing on gas-cooled reactor safety.
The transition was characterized by five distinctive heat and flow regimes. They are free-, suppressed-, vortex-, unsteady-, and forced-convection regimes. Local and average heat transfer coefficients were determined at forced flow rates from 30 to 130 cm/sec., over a temperature difference of 85 to 130 K with heater diameters of 12.7, 19.05, and 25.4 mm. The effect of the spacing-to-diameter ratio, S/D, was also investigated. The average heat transfer coefficients for both single and multiple heaters revealed a strong dependence on free stream velocities and curvature of the heater. For both cases the ranges of experimental parameters were 1.27 * 10('4) (LESSTHEQ) Gr(,D) (LESSTHEQ) 1.17 * 10('5); .0517 (LESSTHEQ) GR(,z)/Re(,z)('2) (LESSTHEQ) 1.3937.
For a numerical study, the conservation equations for mixed convection from a vertical cylindrical heater were derived using the Boussinesq assumptions. The boundary layer simplifications were not used. The two dimensional elliptic governing equations were discretized and solved using Patankar's algorithm "SIMPLER". The computed profiles of the temperature and heat transfer coefficient showed good agreement with those of the experiments. The range of the numerical solution was limited to the forced through vortex regimes.
