Boiling Heat Transfer Source: • Vishwas V. Wadekar, HTFS, Aspen Technology • J.P. Holman Boiling Heat Transfer • Definitions/Terminology – T surface > Tsat of liquid boiling may occur and heat flux depends on T – Pool boiling process heated surface is submerged below a free surface of liquid. – Subcooled or local boiling Tof liquid < Tsat – Saturated or bulk boiling Tof liquid = Tsat Two Modes of Heating Region I = Single phase • No bubbles, wall superheat too low • Motion of fluid near surface = free convection currents • Liquid near heated surface = superheated slightly, when it rises to liquid surface, it evaporates. • Calculation uses free convection relations. Region II • Bubbles begin to form on surface of a wire and dissipated in liquid after breaking away from surface. • This region indicates the beginning of nucleation boiling Nucleate boiling Coefficient increases with Temp excess Region III Nucleate boiling • Tx increases, bubbles form more rapidly and rise to surface of liquid and dissipated. Coefficient increases with Temp excess Region IV • Bubbles form so rapidly and they blanket the heating surface and prevent the inflow and of fresh liquid from taking their place. • Bubbles coalesce and form vapor film (cover the surface) • Film cause thermal resistance due to reduction in heat flux. transition boiling Film boiling region: this region is transition region (from nucleate to film boiling) The film is unstable. Region V • Vapor film at wall • Stable film boiling • Surface temperature is high to maintain stable film boiling. film boiling Region VI • Heat loss from surface is the result of thermal radiation. • • • Point a wire is unstable, small increase in T Critical heat flux Point b this temp. is higher than melting Temp. of wire (cause of burnout results) If maintain at point a partial nucleate boiling and unstable film region p = pv-pl pl , Tl Bubbles pv , Tv • If Tv = Tsat and Tl < Tsat heat conducted out of bubble and vapor condense bubble collapse • If Tl > Tv a metasatable condition bubble growth after leaving the surface Copper rod heated and immersed in isopropanol free convection boiling nucleate boiling film boiling • Boiling of methanol on a horizontal steam-heated copper tube Nucleate boiling q/a = 242.5 kW/m2 Temp excess = 37C Transition boiling q/a = 217.6 kW/m2 Temp excess = 62C Film boiling q/a = 40.9 kW/m2 Temp excess = 82C Calculation of boiling heat transfer • Nucleate pool boiling : Rohsenow • This eqn. can use for geometries other than horizontal wire. • Geometry is not a strong factor in determining heat flux for pool boiling. Vapor-liquid surface tension for water Heat flux data for water boiling on a platinum wire (numbers in parentheses are pressure in MN/m2) Example • A heated brass plate is submerged in a container of water at atmospheric pressure. The plate temperature is 242F. Calculate the heat transfer per unit area of plate. Forces convection boiling occurred when surface Temp > Tsat of liquid This equation is applicable to forced convection where the bulk liquid temp. is subcooled (local forced convection boiling) For fully developed nucleate boiling independent of flow velocity or forced convection effects For low pressure boiling water For high pressure boiling water Peak heat flux for nucleate pool boiling • Zuber equation: Simplified relations for boiling heat transfer with water For forced convection local boiling inside vertical tubes: Valid for 5-170 atm p is pressure in Mpa Example: Water at 5 atm flows inside a tube of 2.54 cm diameter under local boiling conditions where the tube wall temperature is 10C above the saturation temperature. Estimate the heat transfer in a 1.0 m length of tube.