Boiling Heat Transfer Source: • Vishwas V. Wadekar, HTFS, Aspen Technology • J.P. Holman
Jan 03, 2016
Boiling Heat Transfer
Source: • Vishwas V. Wadekar, HTFS, Aspen Technology• J.P. Holman
• 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
Boiling Heat Transfer
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
• Tx increases, bubbles form more rapidly and rise to surface of liquid and dissipated.
Nucleate boiling
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.
Film boiling region: this region is transition region (from nucleate to film boiling)
The film is unstable.
transition boiling
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
Bubbles
• 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
pv , Tv
pl , Tl
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 boilingq/a = 242.5 kW/m2
Temp excess = 37C
Transition boilingq/a = 217.6 kW/m2
Temp excess = 62C
Film boilingq/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 boilingoccurred 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 atmp 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.