Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.
-
Upload
janel-mcgee -
Category
Documents
-
view
300 -
download
7
Transcript of Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.
![Page 1: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/1.jpg)
Boiling Heat Transfer
Source: • Vishwas V. Wadekar, HTFS, Aspen Technology• J.P. Holman
![Page 2: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/2.jpg)
• 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
![Page 3: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/3.jpg)
Two Modes of Heating
![Page 4: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/4.jpg)
![Page 5: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/5.jpg)
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.
![Page 6: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/6.jpg)
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
![Page 7: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/7.jpg)
Region III
• Tx increases, bubbles form more rapidly and rise to surface of liquid and dissipated.
Nucleate boiling
Coefficient increases with Temp excess
![Page 8: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/8.jpg)
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
![Page 9: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/9.jpg)
Region V
• Vapor film at wall• Stable film boiling• Surface temperature
is high to maintain stable film boiling.
film boiling
![Page 10: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/10.jpg)
Region VI
• Heat loss from surface is the result of thermal radiation.
![Page 11: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/11.jpg)
• 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
![Page 12: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/12.jpg)
p = pv-pl
![Page 13: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/13.jpg)
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
![Page 14: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/14.jpg)
Copper rod heated and immersed in isopropanol
free convection boiling nucleate boiling film boiling
![Page 15: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/15.jpg)
• 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
![Page 16: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/16.jpg)
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.
![Page 17: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/17.jpg)
![Page 18: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/18.jpg)
Vapor-liquid surface tension for water
![Page 19: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/19.jpg)
Heat flux data for water boiling on a platinum wire(numbers in parentheses are pressure in MN/m2)
![Page 20: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/20.jpg)
![Page 21: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/21.jpg)
![Page 22: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/22.jpg)
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.
![Page 23: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/23.jpg)
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)
![Page 24: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/24.jpg)
For fully developed nucleate boiling independent of flow velocity or forced convection effects
For low pressure boiling water
For high pressure boiling water
![Page 25: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/25.jpg)
Peak heat flux for nucleate pool boiling
• Zuber equation:
![Page 26: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/26.jpg)
Simplified relations for boiling heat transfer with water
![Page 27: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/27.jpg)
![Page 28: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/28.jpg)
For forced convection local boiling inside vertical tubes:
Valid for 5-170 atmp is pressure in Mpa
![Page 29: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/29.jpg)
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.
![Page 30: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/30.jpg)
![Page 31: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/31.jpg)
![Page 32: Boiling Heat Transfer Source: Vishwas V. Wadekar, HTFS, Aspen Technology J.P. Holman.](https://reader033.fdocuments.in/reader033/viewer/2022061414/56649ef65503460f94c09af3/html5/thumbnails/32.jpg)