tailieunhanh - Heat Transfer Handbook part 71

Heat Transfer Handbook part 71. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | 694 BOILING the wall is too hot to be rewetted by the liquid and a continuous stable if chaotic vapor film is formed between the wall and the continuous liquid core. For horizontal and inclined tubes dryout typically initiates on the upper perimeter of the tube while the lowerperimeterremains wet and may also occuronly on one side of a vertical tube heated nonuniformly by say a radiant heat source. Post-dryout heat transfer is characterized by the following modes Wall-to-vapor heat transfer turbulent or laminar convection to the continuous vaporphase Wall-to-droplet heat transfer evaporation of droplets that impinge on the hot wall Vapor-to-droplet heat transfer convection from the bulk superheated vapor to the saturated liquid in the droplets including any droplets passing through the thermal boundary layer on the wall that do not actually contact the wall Radiation heat transfer from wall-to-droplets vapor upstream wall net radiation flux dependent on the view factor emissive properties transparency of the vapor and respective temperatures. Inverted Annular Flow Heat Transfer This regime is also referred to as forced-convective film boiling. From observations of film boiling inside a vertical tube Dougall and Rohsenow 1963 observed that the flow consisted of a central liquid core surrounded by a thin annular film of vapor on the heated wall when occurring at low vapor quality and low flow rates. The interface was not smooth but wavy. Because of the density difference between the two phases the vaporwas assumed to be traveling at a much highervelocity than the liquid core. Depending on the conditions imposed on vertical upward flow the liquid core was observed to flow upward remain more or less stationary or even flow downward. Entrained vapor bubbles were also observed in the liquid core. The simplest inverted annular flow to analyze is heat transfer through a laminar vapor film. For a verticfl flat isrn fect this is simile to the Nusselt solution for .

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