GAS-LIQUID-PHASE HEAT TRANSFER ON VERTICAL-TUBE BAFFLES IN MECHANICALLY-AGITATED VESSELS

An experimental investigation of heat transfer coefficients has been carried out in mechanically agitated air-water dispersions to and from vertical tubes acting as baffles in addition to heat exchange surface in a vertical cylindrical tank. Air was dispersed into water, which served as a two-phase fluid. Agitation was provided by a dual, 4-blade turbine impeller. A modified Reynolds number has been introduced into the heat-transfer equation which contains the sum of the mixer Reynolds number and the dispersed-gas Reynolds number.It was learned that, in general, the film coefficients of heat transfer in mechanically agitated air-water mixtures were higher than single-phase liquid agitated in vessels. The exponent on the modified Reynolds number (0.73) from this investigation was larger than the value 0.64 reported by Rao and Murti. This was believed to be as a result of more gas-bubble contacts with the vertical-tube heat-transfer surfaces used in this study compared to the helical-coil heat-transfer surface used by Rao and Murti. The location of the gas disperser right below the heat-transfer surface gave a maximum concentration of gas bubbles near the heat-transfer surface in this investigation. This in turn, allowed the air bubbles to hit the surface of the tube more frequently resulting in the more effective erosion of the laminar sub-layer.The heat-transfer results showed that vertical-tube heat exchangers with dispersers located below the tubes gave higher film coefficient of heat transfer than those of helical-tube heat exchanger with air disperser located in the center of tank that was used by other investigators.It was concluded that the increase in film coefficient in air-water-mixture heat transfer units is mainly attributable to an increase in turbulence near the heat-transfer surface. This turbulence enhancement is caused by movement of gas bubbles in the liquid. A second factor of importance is the decrease in the effective thickness of the laminar sub-layer adjacent to the heat-transfer tubes. The enhancement of film coefficient over air velocity range of 2.0 to 10.0 ft/min was approximately 48%. The influence of the gas velocity on the heat-transfer rate has been shown to be more pronounced at lower impeller speeds than at higher speeds. This is believed to be caused by the marginal increase in turbulence created by the impeller at high agitation speed compared with the relatively large increase in turbulence caused by the impeller at lower agitation speed.