Natural convection heat transfer in hemisphere

Natural convection in a hemisphere was studied with bottom surface heated and hemispherical surface cooled. Heat transfer and flow visualization experiments were performed. The range of Rayleigh number and Prandtl number were 9 x 106 < Ra < 7 x 109 and 6 < Pr < 860 for heat transfer experiment and 2 x 105 < Ra < 3 x 107 and 6 < Pr < 860 for flow visualization experiment. Heat transfer characteristic was empirically correlated by the following relation as a function of Rayleigh number: Nu = 0.2352 Ra0.242. The value of the exponent 0.242 is close to 0.25 which corresponds to the case of the laminar natural convection in vertical parallel plates. The measurement of the fluid temperature in vertical direction showed that hot thermal plumes reached near the top of the hemisphere for low Rayleigh number (∼Ra < 1 x 109) and the hot thermal plumes played an important role on the liquid temperature reversal observed for low Rayleigh number. For high Rayleigh number (∼Ra > 1 x 109), the effect of the hot thermal plumes was confined in the vicinity of the hot surface and the temperature reversal disappeared. Flow visualization experiment revealed the existence of downward flow along the inner hemispherical surface. The existence of the downward flow suppresses the generation of the cold thermal plumes. (author)