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| {{expert-subject|Physics|ex2=Chemical and Bio Engineering|date=February 2009}}
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| '''Nucleate boiling''' is a type of [[boiling]] that takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the [[heat flux]] is below the [[critical heat flux]]. For water, as shown in the graph below, nucleate boiling occurs when the surface temperature is higher than the [[saturation temperature]] (T<sub>S</sub>) by between {{convert|4|C-change}} to {{convert|30|C-change}}. The critical heat flux is the peak on the curve between nucleate boiling and transition boiling.
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| ==Mechanism==
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| [[File:Heat transfer leading to Leidenfrost effect for water at 1 atm.png|thumb|450px|border|right|Behavior of water on a hot plate. Graph shows heat transfer (flux) v. temperature (in degrees Celsius) above T<sub>S</sub>, the [[saturation temperature]] of water, {{convert|100|C}}.]]
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| Two different regimes may be distinguished in the nucleate boiling range. When the temperature difference is between approximately {{convert|4|C-change}} to {{convert|10|C-change}} above T<sub>S</sub>, isolated bubbles form at [[nucleation]] sites and separate from the surface. This separation induces considerable fluid mixing near the surface, substantially increasing the convective [[heat transfer coefficient]] and the heat flux. In this regime, most of the heat transfer is through direct transfer from the surface to the liquid in motion at the surface and not through the [[vapor]] bubbles rising from the surface.
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| Between {{convert|10|C-change}} and {{convert|30|C-change}} above T<sub>S</sub>, a second flow regime may be observed. As more nucleation sites become active, increased bubble formation causes [[Liquid bubble|bubble]] interference and coalescence. In this region the vapor escapes as jets or columns which subsequently merge into slugs of vapor.
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| Interference between the densely populated bubbles inhibits the motion of liquid near the surface. This is observed on the graph as a change in the direction of the gradient of the curve or an inflection in the boiling curve. After this point, the heat transfer coefficient starts to reduce as the surface temperature is further increased although the product of the heat transfer coefficient and the temperature difference is still increasing.
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| When the relative increase in the temperature difference is balanced by the relative reduction in the heat transfer coefficient, a maximum heat flux is achieved as observed by the peak in the graph. This is the critical heat flux. At this point in the maximum, considerable vapor is being formed, making it difficult for the liquid to continuously wet the surface to receive heat from the surface. This causes the heat flux to reduce after this point. At extremes, film boiling commonly known as the [[Leidenfrost effect]] is observed.
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| [[File:Boiling Curve.jpg|thumb|450px|border|right|Boiling Curve for water at 1atm]]
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| The process of forming [[steam]] [[Liquid bubble|bubbles]] within [[liquid]] in micro cavities adjacent to the wall if the wall temperature at the [[heat transfer]] surface rises above the [[saturation temperature]] while the bulk of the liquid ([[heat exchanger]]) is [[subcooled]]. The bubbles grow until they reach some critical size, at which point they separate from the wall and are carried into the main [[fluid]] stream. There the bubbles collapse because the temperature of bulk fluid is not as high as at the heat transfer surface, where the bubbles were created. This collapsing is also responsible for the sound a water kettle produces during heat up but before the temperature at which bulk boiling is reached.
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| [[Heat transfer]] and [[mass transfer]] during nucleate boiling has a significant effect on the heat transfer rate. This heat transfer process helps quickly and efficiently to carry away the [[energy]] created at the heat transfer surface and is therefore sometimes desirable — for example in [[nuclear power plant]]s, where liquid is used as a [[coolant]].
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| The effects of nucleate boiling take place at two locations:
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| * the liquid-wall interface
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| * the bubble-liquid interface
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| The nucleate boiling process has a complex nature. A limited number of experimental studies provided valuable insights into the boiling phenomena, however these studies provided often contradictory data due to internal recalculation (state of [[Chaos theory|chaos]] in the fluid not applying to classical [[thermodynamic]] methods of calculation, therefore giving wrong return values) and have not provided conclusive findings yet to develop models and correlations. Nucleate boiling phenomenon still requires more understanding.<ref>[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050195867_2005195125.pdf "Nucleate Boiling Heat Transfer Studied Under Reduced-Gravity Conditions"], Dr. David F. Chao and Dr. Mohammad M. Hasan, Office of Life and Microgravity Sciences and Applications, [[NASA]].</ref>
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| ==Boiling Heat transfer correlations==
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| The nucleate boiling regime is important to engineers because of the high heat fluxes possible with moderate temperature differences. The data can be correlated by equation of the form,<ref>{{cite journal|title=Fundamentals of Heat and Mass Transfer 6th Edition by Incropera }}</ref>
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| <math>N{{u}_{b}}={{C}_{fc}}\left( R{{e}_{b}},P{{r}_{L}} \right)</math>
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| The Nusselt number is defined as,
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| <math>N{{u}_{b}}=\frac{\left( \frac{q}{A} \right){{D}_{b}}}{\left( {{T}_{s}}-{{T}_{sat}} \right){{k}_{L}}}</math>
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| where q/A is the total heat flux, <math>D_b</math> is the maximum bubble diameter as it leaves the surface, <math>{{T}_{s}}-{{T}_{sat}}</math> is the excess temperature, <math>k_L</math> is the [[thermal conductivity]] of the liquid and <math>Pr_L</math> is the [[Prandtl number]] of the liquid. The bubble [[Reynolds number]], <math>Re_b</math> is defined as,
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| <math>R{{e}_{b}}=\frac{{{D}_{b}}{{G}_{b}}}{{{\mu }_{L}}}</math>
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| Where <math>G_b</math> is the average mass velocity of the vapor leaving the surface and <math>{{\mu }_{L}}</math> is the liquid [[viscosity]].
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| Rohsenow has developed the first and most widely used correlation for nucleate boiling,<ref>James R. Welty; Charles E. Wicks; Robert E. Wilson; Gregory L. Rorrer., "Fundamentals of Momentum, Heat and Mass transfer" 5th edition, John Wiley and Sons</ref>
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| <math>\frac{q}{A}={{\mu }_{L}}{{h}_{fg}}{{\left[ \frac{g\left( {{\rho }_{L}}-{{\rho }_{v}} \right)}{\sigma } \right]}^{{}^{1}\!\!\diagup\!\!{}_{2}\;}}{{\left[ \frac{{{C}_{pL}}\left( {{T}_{s}}-{{T}_{sat}} \right)}{{{C}_{sf}}{{h}_{fg}}Pr_{L}^{1.7}} \right]}^{3}}</math>
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| Where <math>C_{pL}</math> is the heat capacity of the liquid. <math>C_{sf}</math> is the surface fluid combination and vary for various combinations of fluid and surface. For example, water and nickel have a <math>C_{sf}</math> of 0.006.
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| {| class="wikitable" border="2"
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| |+ Values of <math>C_{sf}</math> for various surface fluid combinations<ref>James R. Welty; Charles E. Wicks; Robert E. Wilson; Gregory L. Rorrer., "Fundamentals of Momentum, Heat and Mass transfer" 5th edition, John Wiley and Sons</ref>
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| ! Surface fluid combinations!! <math>C_{sf}</math>
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| |-
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| | Water/copper|| 0.013
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| |-
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| | Water/nickel|| 0.006
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| |-
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| | Water/platinum|| 0.013
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| |-
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| | Water/brass|| 0.006
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| |-
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| | Water/Stainless steel, mechanically polished|| 0.0132
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| |-
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| | Water/Stainless steel, Chemically etched|| 0.0133
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| |-
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| | Water/Stainless steel, Ground and polished|| 0.0080
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| |-
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| | <math>CCl_4</math>/copper|| 0.013
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| |-
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| | Benzene/Chromium|| 0.0101
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| |-
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| | n-Pentane/Chromium|| 0.015
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| |-
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| | Ethyl alcohol/Chromium|| 0.0027
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| |-
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| | Isopropyl alcohol/copper|| 0.0025
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| |-
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| | n-Butyl alcohol/copper|| 0.003
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| |}
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| ==Departure from nucleate boiling==
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| If the [[heat flux]] of a boiling system is higher than the [[critical heat flux]] (CHF) of the system, the bulk fluid may boil, or in some cases, ''regions'' of the bulk fluid may boil where the fluid travels in small channels. Thus large bubbles form, sometimes blocking the passage of the fluid. This results in a '''departure from nucleate boiling''' ('''DNB''') in which steam bubbles no longer break away from the solid surface of the channel, bubbles dominate the channel or surface, and the heat flux dramatically decreases. Vapor essentially insulates the bulk liquid from the hot surface.
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| During DNB, the surface temperature must therefore increase substantially above the bulk fluid temperature in order to maintain a high heat flux. Avoiding the CHF is an [[process engineering|engineering problem]] in heat transfer applications, such as [[nuclear reactor]]s, where fuel plates must not be allowed to overheat. DNB may be avoided in practice by increasing the [[pressure]] of the fluid, increasing its [[flow rate]], or by utilizing a lower temperature bulk fluid which has a higher CHF. If the bulk fluid temperature is too low or the pressure of the fluid is too high, nucleate boiling is however not possible.
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| DNB is also known as '''Transition boiling''', '''unstable film boiling''', and '''partial film boiling'''. For water boiling as shown on the graph, transition boiling occurs when the temperature difference between the surface and the boiling water is approximately {{convert|30|C-change}} to {{convert|120|C-change}} above the T<sub>S</sub>. This corresponds to the high peak and the low peak on the boiling curve. The low point between transition boiling and film boiling is the [[Leidenfrost effect|Leidenfrost point]].
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| During transition boiling of water, the bubble formation is so rapid that a vapor film or blanket begins to form at the surface. However, at any point on the surface, the conditions may oscillate between film and nucleate boiling, but the fraction of the total surface covered by the film increases with increasing temperature difference. As the [[thermal conductivity]] of the vapor is much less than that of the liquid, the convective [[heat transfer coefficient]] and the heat flux reduces with increasing temperature difference.
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| ==See also==
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| *[[Heat transfer]]
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| *[[Fluid physics]]
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| *[[Cavitation]]
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| *[[Sonoluminescence]]
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| *[[Leidenfrost effect]]
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| *[[Boiling]]
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| *[[Chemical engineering]]
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| ==References==
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| {{reflist}}
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| [[Category:Thermodynamic entropy]]
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| [[Category:Nuclear technology]]
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| [[Category:Cooling technology]]
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| [[Category:Heat transfer]]
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| [[Category:Transport phenomena]]
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Hi there, I am Andrew Berryhill. For years he's been residing in Mississippi and he doesn't plan on changing it. Invoicing is what I do. The preferred pastime for him and his kids is style and he'll be starting some thing else alongside with it.
Feel free to visit my web page :: psychic solutions by lynne - Our Web Site -