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Nucleate boiling is a type of
boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Th ...
that takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the
heat flux Heat flux or thermal flux, sometimes also referred to as ''heat flux density'', heat-flow density or ''heat flow rate intensity'' is a flow of energy per unit area per unit time. In SI its units are watts per square metre (W/m2). It has both a ...
is below the
critical heat flux Critical heat flux (CHF) describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the efficiency of heat transfer, thus causing ...
. For water, as shown in the graph below, nucleate boiling occurs when the surface temperature is higher than the
saturation temperature The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
(TS) by between . The critical heat flux is the peak on the curve between nucleate boiling and transition boiling. The heat transfer from surface to liquid is greater than that in
film boiling The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this re ...
. Nucleate boiling is common in electric kettles and is responsible for the noise that occurs before boiling occurs. It also occurs in water boilers where water is rapidly heated.


Mechanism

Two different regimes may be distinguished in the nucleate boiling range. When the temperature difference is between approximately above TS, isolated bubbles form at
nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
sites and separate from the surface. This separation induces considerable fluid mixing near the surface, substantially increasing the convective
heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ). ...
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 In physics, a vapor (American English) or vapour (British English and Canadian English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critic ...
bubbles rising from the surface. Between above TS, a second flow regime may be observed. As more nucleation sites become active, increased bubble formation causes
bubble Bubble, Bubbles or The Bubble may refer to: Common uses * Bubble (physics), a globule of one substance in another, usually gas in a liquid ** Soap bubble * Economic bubble, a situation where asset prices are much higher than underlying funda ...
interference and coalescence. In this region the vapor escapes as jets or columns which subsequently merge into slugs of vapor. 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 (the heat flux) is still increasing. 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 The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this re ...
is observed. The process of forming
steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...
bubbles within
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...
in micro cavities adjacent to the wall if the wall temperature at the
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
surface rises above the
saturation temperature The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...
while the bulk of the liquid (
heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...
) is
subcooled The term subcooling (also called undercooling) refers to a liquid existing at a temperature below its normal boiling point. For example, water boils at 373 K; at room temperature (293 K) the water is termed "subcooled". A subcooled liqu ...
. The bubbles grow until they reach some critical size, at which point they separate from the wall and are carried into the main
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
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.
Heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
and
mass transfer Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, ...
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 In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
created at the heat transfer surface and is therefore sometimes desirable—for example in
nuclear power plant A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a electric generator, generato ...
s, where liquid is used as a
coolant A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosio ...
. The effects of nucleate boiling take place at two locations: * the liquid-wall interface * the bubble-liquid interface 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 Chaos or CHAOS may refer to: Arts, entertainment and media Fictional elements * Chaos (''Kinnikuman'') * Chaos (''Sailor Moon'') * Chaos (''Sesame Park'') * Chaos (''Warhammer'') * Chaos, in ''Fabula Nova Crystallis Final Fantasy'' * Cha ...
in the fluid not applying to classical
thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of ther ...
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.


Boiling heat transfer correlations

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, N=\left( R,P \right) The Nusselt number is defined as, N=\frac where q/A is the total heat flux, D_b is the maximum bubble diameter as it leaves the surface, - is the excess temperature, k_L is the
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
of the liquid and Pr_L is the
Prandtl number The Prandtl number (Pr) or Prandtl group is a dimensionless number, named after the German physicist Ludwig Prandtl, defined as the ratio of momentum diffusivity to thermal diffusivity. The Prandtl number is given as: : \mathrm = \frac = \frac ...
of the liquid. The bubble
Reynolds number In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be domi ...
, Re_b is defined as, R=\frac Where G_b is the average mass velocity of the vapor leaving the surface and is the liquid
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
. Rohsenow has developed the first and most widely used correlation for nucleate boiling,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 \frac= Where c_ is the specific heat of the liquid. C_ is the surface fluid combination and vary for various combinations of fluid and surface. \sigma is the surface tension of the liquid-vapor interface. The variable n depends on the surface fluid combination and typically has a value of 1.0 or 1.7. For example, water and nickel have a C_ of 0.006 and n of 1.0.


Departure from nucleate boiling

If the
heat flux Heat flux or thermal flux, sometimes also referred to as ''heat flux density'', heat-flow density or ''heat flow rate intensity'' is a flow of energy per unit area per unit time. In SI its units are watts per square metre (W/m2). It has both a ...
of a boiling system is higher than the
critical heat flux Critical heat flux (CHF) describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the efficiency of heat transfer, thus causing ...
(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. 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 engineering problem in heat transfer applications, such as
nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...
s, where fuel plates must not be allowed to overheat. DNB may be avoided in practice by increasing the
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
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. 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 above the TS. 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 point. 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 The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
of the vapor is much less than that of the liquid, the convective
heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ). ...
and the heat flux reduces with increasing temperature difference.


See also

*
Boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Th ...
*
Cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...
*
Chemical engineering Chemical engineering is an engineering field which deals with the study of operation and design of chemical plants as well as methods of improving production. Chemical engineers develop economical commercial processes to convert raw materials int ...
* Fluid physics *
Heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
*
Leidenfrost effect The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this re ...
*
Sonoluminescence Sonoluminescence is the emission of light from imploding bubbles in a liquid when excited by sound. History The sonoluminescence effect was first discovered at the University of Cologne in 1934 as a result of work on sonar. Hermann Frenzel and ...


References

{{reflist Thermodynamic entropy Nuclear technology Cooling technology Heat transfer Transport phenomena