This type of heat transfer is characterized by high intensity and occurs in chemical technology, for example, when carrying out such processes as evaporation, distillation of liquids, in evaporators of refrigeration units, etc. The process of heat transfer during boiling is very complex and has not yet been sufficiently studied, despite great amount conducted research.
For the occurrence of boiling, it is necessary, first of all, that the temperature of the liquid be above the saturation temperature, and the presence of centers of vaporization is also necessary. Distinguish between boiling on the heating surface and boiling in the bulk of the liquid. The first type of boiling is due to the supply of heat to the liquid from the surface in contact with it. Boiling in the volume of a liquid is due to the presence internal sources heat or significant overheating of the liquid, which occurs, for example, with a sudden decrease in pressure (below equilibrium). The most important type of boiling in chemical technology is surface boiling.
To transfer heat from the wall to the boiling liquid, the wall must be overheated relative to the saturation temperature of this liquid. On fig. 11-9 shows a typical dependence of the heat transfer coefficient and specific heat load on temperature5
pressure at liquid boiling Δt= tst -tboil (tst, tboil - respectively, the wall temperature from the side of the boiling liquid and the boiling temperature). In region AB, the liquid overheating is small (Δt< 5 К), мало также число активных центров парообразования - микровпадин на поверхности стенки, в которых образуются зародыши паровых пузырьков, и интенсивность теплообмена определяется в основном закономерностями теплоотдачи свободной конвекции около нагретой стенки,При дальнейшем повышении Δt =tст -t увеличивается число активных центров парообразования, и коэффициент теплоотдачи резко возрастает (отрезок ВС на рис). Эту область называют пузырчатым, или ядерным, кипением.
The high intensity of heat transfer in the bubbly boiling regime is explained by the fact that the turbulence of the boundary layer y, the wall surface is proportional to the number and volume of vapor bubbles formed in microcavities on the heating surface. In areas close to the centers of vaporization), part of the liquid evaporates, forming vapor bubbles, which, rising and increasing in volume, entrain significant masses of liquid. The entrained and evaporated liquid is replaced by fresh flows, thus creating an intensive circulation of the liquid near the heating surface, which leads to a significant acceleration of the heat transfer process. At point C, the heat transfer coefficient reaches maximum value corresponding to the maximum specific heat load (point O). With a further increase in Δt, a sharp decrease in the heat transfer coefficient is observed. It is explained by the fact that at some - critical - value Δt = Δt cr coalescence (merging) of bubbles formed close to each other occurs. In this case, the value l in Fig. becomes smaller than the diameter of the vapor bubbles, and a vapor film appears near the wall surface, which creates additional thermal resistance to the heat transfer process. The heat transfer coefficient decreases sharply (tens of times). Of course, the resulting film of vapor is unstable, it is constantly destroyed and reappears, but in the end this seriously impairs heat transfer. This boiling mode is called film. It is quite obvious that the film boiling regime is extremely undesirable.
The values of temperature difference, heat transfer coefficient and specific heat load corresponding to the transition from bubble mode to film mode are called critical.
The steam bubble is formed in the microcavities of the heating surface. Having reached a certain diameter do, the bubble breaks away from the surface. On well-wetted surfaces, the bubble breaks away from the heating surface, having the shape of a ball. Rising, the bubble increases in volume due to the evaporation of liquid inside the bubble, flattens and takes the form of a mushroom with a complex ascent trajectory. In this case, continuous crushing and coalescence of bubbles occur. The moment of bubble separation corresponds to the state of equality of the Archimedean force acting on the bubble and the surface tension force of the liquid that keeps the bubble on the wall. If we assume that the bubble, when formed on the wall surface, has a shape close to spherical, then at the moment of separation, the value of do is expressed by the dependence
where pzh and pp are the density of the liquid and vapor, respectively; σ surface tension liquids at the interface; β-contact angle
Thus, the transport of heat during bubbly boiling consists of the transfer of heat from the wall to the liquid, and then the heat is transferred by the liquid inner surface bubbles in the form of heat of vaporization. The transfer of heat from the wall directly to the bubble is negligible, since the contact surface of the bubbles with the wall is very small, and the thermal conductivity of the vapor is also low. In order for the heat from the liquid to be transferred to the vapor bubbles, the liquid must have a temperature somewhat higher than the vapor temperature. Therefore, when boiling, the liquid is somewhat superheated relative to the temperature saturated steam above the surface of the boiling liquid.
The rate of heat transfer during boiling depends on many various factors(physical properties of the liquid, pressure, temperature difference, properties of the heating surface material, and many others), it is extremely difficult to take into account the influence of which on the process and reduce them into a single dependence. a complex of many quantities that affect the intensity of heat transfer during boiling
10. Radiant heat transfer. complex heat transfer. Can be carried through any medium due to energy transfer magnetic waves infrared part of the range. Radiant heat transfer is carried out during the transfer of matter through gaseous environment, which exists between the zone of more and less heated gas. In the 1st queue they lead between the TV bodies.
This is the equation for coverage factor=1. If the emitting surface completely surrounds the absorbed one,
When heat is transferred through a gaseous medium of radiation, the intensity of this transfer is referred to as moderate. T-x is carried out only under conditions of natural convection, i.e. along with radiant heat transfer there is convective heat transfer. The total intensity of heat transfer. Joint heat transfer due to radiant heat transfer and convection is called complex heat transfer.
End of work -
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Thermal processes and devices. Types of heat transfer and heat exchange pr. Transfer of heat from one body to another
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General form of criterion equations
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Vertical tubular film machines
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During boiling, as in all other heat transfer processes, the heat transfer equation (Newton's law) is used, which establishes the relationship between the temperature difference "wall - liquid" and the heat flux through the heat exchange surface:
where Q - heat flux, W; q=Q/F - surface density heat flow, W/m2; F - heat exchange surface (walls), m2; is the heat transfer coefficient averaged over the surface F, W/(m2K); - temperature of the heat exchange surface (wall), 0С; - liquid saturation temperature at a given pressure, 0С.
In this case, the overheating of the wall acts as a temperature difference:
where T f, max is the maximum superheat of the liquid, 0C.
Thus, the heat flux is proportional to the area F of the heat exchange surface and the temperature difference between the wall and the liquid.
Heat transfer coefficient
The heat transfer coefficient, W / (m2K), is a proportionality coefficient in Newton's law, characterizing the intensity of heat transfer. The value of the heat transfer coefficient at boiling depends on a large number various factors:
a) the physical properties of the liquid;
b) the purity of the liquid;
c) its temperature and pressure;
d) geometric shape, dimensions and spatial orientation of the heat exchange surface;
e) material and roughness (cleanliness of processing) of the surface;
f) liquid superheat values, etc.
Therefore, the determination of the heat transfer coefficient during boiling is a very difficult task. There are local (at a given point on the surface) and average value of the heat transfer coefficient over the heat transfer surface:
that is, the heat transfer coefficient is numerically equal to the heat flux transmitted through a unit heat exchange surface at a temperature difference of 10C (1 K).
Boiling modes (heat transfer)
The mechanism of boiling and the intensity of heat transfer depend on the magnitude of the overheating of the wall. There are three main boiling modes: bubble, transition and film.
In practice, the most common is the boiling of a liquid on a solid heat exchange surface through which thermal energy is supplied.
The boiling process is a special case of convective heat transfer, in which there is an additional transfer of the mass of matter and heat by vapor bubbles from the heating surface into the liquid volume.
bubble mode
The radius of the interfacial surface of the bubble-nucleus is proportional to the size of the microroughness forming it on the wall surface. Therefore, at the beginning of the bubble mode of boiling, with a slight overheating of the liquid, only major centers vaporization, since the bubbles-nuclei of small centers have a radius less than the critical one.
As the superheating of the liquid increases, smaller centers of vaporization are activated, so the number of bubbles formed and the frequency of their separation increase.
As a result, the intensity of heat transfer increases extremely rapidly (Fig. 3, region 2). The heat transfer coefficient reaches tens and even hundreds of thousands of W / (m2K) (at high pressures).
This is due to the high specific heat phase transition and intensive mixing of the liquid by growing and detaching vapor bubbles. Bubble boiling mode provides the most efficient heat transfer. This mode is used in steam generators of thermal and nuclear power plants, when cooling engines, structural elements of energy, metallurgical, chemical units operating at high temperatures. Heat transfer in the bubble mode is proportional to the amount active centers vaporization and the frequency of bubble detachment, which, in turn, are proportional to the maximum superheat 8 liquid and pressure. strength of this average coefficient heat transfer can be calculated by the formula of the form:
where C1, z, n are empirical constants; ?Tw - wall overheating, 0C; . - saturation pressure (external liquid pressure), bar.
The formula is used in calculations of nucleate boiling under boundary conditions of the first kind.
Rice. 3. Heat transfer curves during boiling: 1 - convective region without boiling; 2 - area of nucleate boiling; 3- transition region; 4 - area of film boiling; 5 - section of film boiling with a significant proportion of heat transfer by radiation; kr1, kr2 are the points of the first and second boiling crises, respectively.
The first boiling crisis. transition mode
With a further increase in superheat (?Tw), the heat transfer intensity, reaching a maximum at critical point"cr1" begins to decrease (see Fig. 3 area 3) due to the merging of an ever-increasing number of bubbles into vapor spots. The area of vapor spots increases with increasing ΔTw and eventually covers the entire wall, turning into a continuous vapor film that conducts heat poorly.
Thus, there is a gradual transition from bubble to film boiling, accompanied by a decrease in the intensity of heat transfer. The beginning of such a transition is called first boiling crisis. A crisis is understood as a fundamental change in the mechanism of boiling and heat transfer.
The second boiling crisis. Film mode
With a further increase in overheating (ΔTw), the heat transfer intensity, having reached a minimum at the second critical point "cr2", again begins to increase in the region of the film boiling regime (see Fig. 3, regions 4 and 5). Such a change in the nature of the effect of overheating on heat transfer is called second boiling crisis.
In the film boiling mode, a continuous vapor film pushes the liquid away from the surface and the heat transfer conditions stabilize, while the heat transfer coefficient ceases to decrease, remaining practically constant. The heat flow, according to Newton's law (3), will again begin to increase due to an increase in the temperature difference?Tw. The intensity of heat transfer in the film boiling mode is very low, and this leads to a strong overheating of the heat exchange surface.
Boiling in large volume
The heat flux transferred from the surface to the boiling water can be unambiguously associated with the temperature difference between the wall and the liquid:
where is the heat flux;
wall temperature;
the average temperature of the liquid.
This dependence characterizes heat transfer from the heating surface to the liquid and is called the boiling curve (Figure 4).
Rice. four.
Five characteristic areas can be distinguished:
1. To the point. Convection area;
2. Between points and. Region of undeveloped nucleate boiling. It is characterized by an increase in the intensity of heat transfer due to the transfer of the resulting bubbles to the core of the flow;
3. Between points and. Area of developed nucleate boiling. It is characterized by a high intensity of heat transfer due to the transfer of the resulting bubbles to the core of the flow. The intensity increases as the bubble density increases;
4. Between points and. Region of unstable film boiling. It is characterized by "merging" of individual bubbles in the near-wall region. Due to the decrease in the centers of vaporization, as well as the growth of the vapor film at the heating surface, heat transfer decreases;
5. From the point. Region of stable film boiling. It is characterized by covering the heating surface with a continuous vapor film and, as a result, low heat transfer.
This curve can be obtained by increasing and maintaining the temperature of the heating wall. In this case, as the increase increases, five boiling regions are successively replaced.
In the case of increasing and maintaining the heat flux, the order of changing the boiling regimes will be different. First, the modes of convection of a non-boiling liquid (up to t.), surface boiling (between points i) and developed nucleate boiling (between points i) successively replace each other. With a further increase in the heat flux, the heating surface is quickly covered with a vapor film (from point to point), which is accompanied by an increase in temperature and through a short time, after reaching the steady state, boiling is characterized by a high wall temperature (from the point). This phenomenon is called a heat transfer crisis, and the heat flow at which a sharp increase in temperatures begins (-) is called the first critical heat flow, or, more often, simply, the critical heat flow.
If, after reaching the point, the heat flux begins to decrease, then the film boiling regime is maintained until the point is reached. In the case of a further decrease in the heat flux, the film boiling mode changes to bubble mode (from point to point), and the temperature of the heating surface decreases rapidly. The heat flux at which the film boiling mode changes to bubble (-) is called the second critical heat flux.
Heat transfer during liquid boiling is widely used in ship power engineering - this is the production of steam in the main and auxiliary boilers, nuclear reactors, evaporators sea water, in evaporators and air coolers of refrigeration units.
A distinction is made between boiling on a solid heat exchange surface, through which a heat flux passes, and boiling in a volume, when a heat flux is induced directly into the volume of a liquid.
In practice, the type of boiling of a liquid in contact with a heat exchange surface is much more common.
Boiling is a process of intensive formation of steam under the condition of a constant supply of heat. Boiling occurs when a liquid is slightly superheated, when the temperature of the liquid is above the saturation temperature at a given pressure. The amount of superheat required depends on the physical properties of the liquid, its purity, pressure, and also on the state of the surface through which the heat flows into the liquid. The purer the liquid, the more it needs to be superheated before boiling occurs. This is explained by the difficulty of the spontaneous formation of initial nucleating vapor bubbles due to the need to overcome the energy of mutual attraction of molecules in the liquid.
If there is a dissolved gas (for example, air) or small suspended particles in the liquid, the boiling process begins almost immediately after the liquid reaches its saturation temperature. Gas bubbles, as well as solid particles in the liquid, serve as ready-made initial nuclei of the vapor phase.
The value of the required overheating is also reduced if the heat exchange surface (walls and bottom of the vessel, pipe walls), through which the heat flow enters the liquid, has microroughness. When a heat flux is supplied through such a surface, the formation of bubbles is observed at individual points of the surface. These points are called VAPOR CENTERS. In this case, the boiling process begins in the liquid layers that are in contact with the heat exchange surface and have the same temperature with it. The formation of vapor bubbles occurs in the superheated boundary layer of the liquid and only in the centers of vaporization. Steam bubbles grow, break away from the surface and float.
But not all bubbles are capable of further growth, but only those whose radius exceeds the value of the critical radius of the vapor nucleus Rmin. The value of Rmin depends on the surface temperature and sharply decreases with increasing wall temperature. Therefore, an increase in the heat load, which causes an increase in the surface temperature, leads to an increase in the number of active centers of vaporization, and the boiling process becomes more intense.
All the heat entering the liquid is spent on the formation of steam:
where r is the heat of vaporization, J/kg.
G"" - the amount of steam formed during boiling, kg / s.
The nature of the development and detachment of bubbles from the heat-exchange surface largely depends on whether the liquid wets the surface or does not. If the boiling liquid wets the heating surface, then the steam bubbles have a thin leg and easily come off the surface. If the liquid does not wet the surface, then the vapor bubbles have a wide stem and only the upper part of the bubble comes off.
Rice. 14.1. The shape of the vapor bubbles on the wetted (a)
and non-wetted (b) surfaces
The growth of bubbles before separation and their movement after separation cause intensive circulation and mixing of the liquid in the boundary layer, which sharply increases the heat transfer from the heating surface to the liquid. This mode of boiling is called bubbly. In nucleate boiling, the contact area of the bubble leg with the heat exchange surface is small and therefore the heat flux is transferred to the liquid almost without restrictions and is spent on vaporization and a slight increase in temperature in the volume of the liquid (for example, for water at atmospheric pressure, the overheating in the volume is usually 0.2 ... 0 .4 °C). For practice, nucleate boiling is of the greatest interest.
Heat removal in bubble boiling mode is one of the most advanced methods of cooling the heating surface. He finds wide application in nuclear reactors, when cooling jet engines, when the heat exchange surface works with high density heat flow.
In the bubble boiling mode, steam is produced in the steam generators and the main and auxiliary boilers are operated.
The intensity of nucleate boiling depends on the value of the specific heat load q, W/m 2 supplied to the heat exchange surface. However, the heat flux cannot be increased indefinitely. With an increase in the heat flux, the number of active centers of vaporization continuously increases, and there are so many of them that individual bubbles can merge into a vapor layer, which periodically breaks, and the resulting vapor breaks into the volume of the boiling liquid. This mode of boiling is called film boiling. The appearance of a film instead of individual bubbles is called the first boiling crisis. For water at atmospheric pressure, the boiling crisis occurs at a heat flux density q = 1.2 10 6 W / m 2, this heat flux corresponds to the critical value of the temperature difference Dtcr = 25 ... 35 ° C.
The reason for the boiling crisis is as follows. The coalescence of bubbles that did not have time to break away from the heat exchange surface, the formation of a vapor film change the conditions of heat exchange between the liquid and the wall. The wall, to which the heat flow is supplied, ceases to be washed by the liquid, since it is separated from the liquid by a vapor film, and therefore the heat flow entering the wall, only a small part of it is transferred to the steam due to the low thermal conductivity of the steam, the rest of the heat flow is spent on heating the wall. The wall temperature rises by hundreds of degrees in a fraction of a second. And if the wall is made of refractory material, the crisis ends with a new steady state- film boiling at very high temperature heat exchange surface, and, accordingly, at a new, very high value of the temperature difference Dt between the temperature wall and the saturation temperature, which remains constant, since its value depends only on the pressure value. Bubble boiling mode (Fig. 14.2, a) and film (Fig. 14.2, b) is shown in Fig. 14.2.
Rice. 14.2. Boiling modes: a - bubble, b - transitional, c - film
The figure also captures (see Fig. 14.2, b) is the moment of transition from bubble to film boiling. In the film boiling mode, heat transfer from the heating surface to the liquid is carried out by thermal conduction and convective heat transfer in the vapor film, as well as radiation through the vapor film. As the temperature of the heating surface increases (and, accordingly, Dt increases), all most of heat is transferred to the liquid by radiation. The intensity of heat transfer in the film boiling regime is low. The vapor accumulating in the vapor film periodically breaks off in the form of large bubbles in pulsations.
Graph 14.3 shows bubble and film boiling regimes. It can be seen from the graph that there is no smooth transition from one mode to another. If we increase the heat flux density, this leads to an increase in the intensity of heat transfer, but at the same time, the surface temperature (and, accordingly, Dt) also slightly increases. Increasing the heat load over allowable limit causes a boiling crisis. This crisis transition in Fig. 14.3 is shown by an arrow and occurs as a jump from the nucleate boiling curve to the film boiling line at the same value of the heat load qcr1. Usually, the boiling crisis ends with the straightening (burning out) of the heating surface.
Rice. 14.3. Dependence of the critical thermal load on ∆t
However, if the surface is not destroyed, and the film boiling mode is established, then a decrease in the heat flux density will not give quick results, and the film mode will be saved. With a decrease in the heat flux, the process will occur along the line of film boiling. And only if we reduce the load to the value qcr2, will there be prerequisites for changing the regime. This regime change also has a crisis character and is called the second boiling crisis. When the heat load is reduced to the value qcr2, the liquid at some points begins to touch the heat exchange surface, which increases the heat removal from the surface, which leads to a rapid cooling of the heating surface. There is a change of regimes and nucleate boiling is established. This reverse transition is also carried out by "jumps" along the arrow from the film-like curve to the nucleate boiling line at qcr2. For water at atmospheric pressure, the value critical density in this case, the heat flux is equal to qcr2 = 25000 W / m 2.
So, both transitions: from bubble to film and back are of a crisis nature. They occur at heat fluxes qcr1 and qcr2, respectively. Under these conditions, the transition mode of boiling cannot exist stationary, because the transition occurs almost instantly, in a fraction of a second.
In practice, the boiling of a liquid moving inside pipes or channels of various shapes is widely used. Due to the motion of a fluid in a limited volume, new features arise. The development of the process is influenced by the speed of the forced movement of the liquid or steam-water mixture and the structure of the two-phase flow. The nature of the movement of a mixture of water and steam inside the pipes is shown in (Fig. 14.4)
Rice. 14.4. The nature of the movement of the steam-water mixture in the pipes
Depending on the steam content, the speed of the mixture and the location of the pipes in space, the nature of the movement can be in the form of a homogeneous emulsion (see Fig. 14.4a) or in the form of independent flows of water and steam (see Fig. 14.4 b, 14.4d).
If the pipe is located vertically, then an independent steam flow will move along the pipe axis, in the center, and a water film will move along the periphery, along the pipe wall. At horizontal arrangement pipes, steam moves at the top of the pipe, water - at the bottom.
Experimental data on boiling were summarized by D.A. Labuntsov. He proposed a criterion equation for calculating heat transfer during nucleate boiling.
where is the Nusselt criterion characterizing the heat transfer during boiling at the wall-liquid interface;
The Reynolds criterion characterizing the state of inertia forces and viscosity forces during boiling;
Characteristic linear size proportional to the separation diameter of the bubble, m;
Boiling speed, m/s;
Cp is the heat capacity of the liquid, kJ/(kg K);
r is the heat of vaporization, kJ/kg;
s - surface tension, N/m;
r", r"" - density of liquid and vapor at a given saturation temperature, kg/m 3 ;
Ts- absolute value saturation temperature, K.
The values of the constants C and n are taken equal to:
The values of all physical parameters included in the similarity criteria should be taken at a given saturation temperature. Due to the complexity and cumbersomeness of calculations to determine the heat transfer coefficient using the criterion equation (14.2), in practice, to calculate the heat transfer coefficient in the nucleate boiling mode, the dependence obtained by boiling water by M.A. Mikheev:
where q is the surface heat flux density, W/m 2 ;
p - absolute vapor pressure, Pa.
Bubble boiling is characterized by a high intensity of heat transfer and, accordingly, the possibility of removing significant heat fluxes from a unit surface, limited only by the value of the critical heat flux qcr1. The value of qcr1 under natural convention on horizontal pipes and tiles can be determined from the formula:
In the film boiling mode, the boiling liquid is separated from the heating surface by a vapor film. Therefore, the surface temperature tc is much higher than the saturation temperature ts. Due to the high temperatures of the heat exchange surface, radiant heat transfer occurs between it and the liquid. The intensity of convective heat transfer during film boiling is determined by the thermal resistance of the vapor film. The nature of vapor movement in the film and its thickness depend on the size and shape of the heating surface and its location in space. Calculation of heat transfer during film boiling on horizontal pipes can be carried out according to the dependence
All physical parameters in this formula (with the exception of the liquid density r") refer to the right phase. They should be chosen according to the average vapor temperature
For film boiling on the surface of vertical pipes, the experimental data are summarized by D.A. Labuntsov:
The physical properties of the steam here should also be chosen according to the average temperature of the steam.
Boiling is the process of vaporization that occurs at the boiling (saturation) temperature in the thickness of the liquid. In this case, the heat of the phase transition is absorbed, as a result of which, to maintain the process, it is necessary to continuously supply heat, i.e. boiling is associated with heat transfer. When boiling, the vapor phase is formed in the form of bubbles. In a heated non-boiling liquid, in the absence of forced flow, heat is transferred through the boundary layer by free convection and heat conduction. During boiling, the transfer of the mass of matter and heat from the boundary layer to the volume of liquid is also carried out by vapor bubbles, which, rising up, cause intense mixing of the liquid and turbulence of the boundary layer. Since heat is usually supplied through the heat exchange surface, bubbles also appear on this surface. If the surface is immersed in a large volume of liquid, the forced movement of which is absent, then such a process is called boiling in large volume. In thermal power engineering, boiling processes are most often encountered on the heating surface (pipe surfaces, boiler walls, etc.).
boiling modes. There are two boiling regimes: a bubble regime, when steam is formed on the surface in the form of separate periodically emerging bubbles, and a film boiling regime, when the number of bubbles near the surface becomes so large that they merge into a single vapor film, through which heat from the heated surface is transferred to volume of liquid thermal conductivity. Since the coefficient of thermal conductivity of steam is about 30 times less than that of water, the thermal resistance of thermal conductivity through the vapor film increases sharply, which can lead to burnout of the heat exchange surface. Therefore, this mode is not allowed in thermal power plants.
Conditions necessary for the boiling process to occur. For the occurrence of boiling, two conditions are necessary and sufficient: the presence of overheating of the liquid relative to the saturation temperature at liquid pressure and the presence of centers of vaporization, which can be various inclusions in the liquid ( particulate matter and gas bubbles), as well as depressions and depressions on the heat exchange surface, which is associated with roughness.
Let the liquid be in a vessel with a heated bottom. If the liquid is boiling, then the temperature of the vapor above the liquid is . The temperature in the liquid itself is always slightly higher. As you approach the heated bottom, the temperature practically does not change. Only in the immediate vicinity of the bottom does it sharply increase to .
It follows from the figure that the greatest overheating () is observed at the heat exchange surface, but there are also centers of vaporization in the form of roughness. This explains why bubbles form precisely on the heat exchange surface.
In order for the bubble to develop, i.e. increases in volume due to the evaporation of liquid from the surface of the bubble into it, the vapor pressure in it must be greater than the pressure due to the surrounding liquid and the force of surface tension.
The saturation pressure and saturation temperature are related by a rigid relationship: than more pressure, the higher the saturation temperature. From this it becomes clear why one of the conditions for the occurrence of boiling (the formation of vapor bubbles) is overheating of the liquid. The volume of the bubble increases until the buoyant force tending to tear it off is greater than the forces holding it to the surface. The size of the bubble at the time of its separation is characterized by the separation diameter. The detached bubble moves upward, continuing to increase in volume. At the liquid-vapor interface, the bubble bursts.
Since bubbles arise, grow and detach on the heat exchange surface, they thereby destroy the boundary layer, which is the main thermal resistance. Therefore, heat transfer during boiling is a highly intensive process. For water, for example, the coefficient reaches (10 ... 40) 10 3 W / (m 2 × K).
During the boiling process, the heat exchange surface contacts partly with the vapor phase, partly with the liquid phase. But 
, so the heat is transferred liquid medium, i.e. goes to its overheating, and only then the superheated liquid evaporates from the surface of the bubbles into them.
The figure shows the dependence of the coefficient on 
(liquid overheating).
The following boiling regions can be distinguished. At low temperature differences, heat transfer is determined mainly by the conditions of free convection, since the number of forming bubbles is small and they do not significantly affect the boundary layer - this is the region of convective boiling I. In this region, the heat transfer coefficient is proportional to . As liquid overheating increases, less and less roughness can serve as vaporization centers, and this leads to an increase in their number, and, in addition, the frequency of bubble separation in each vaporization center increases. This causes an increase in circulation in the boundary layer, as a result of which the heat transfer increases sharply. A developed bubble boiling regime sets in (region II). proportional.
With a further increase in the temperature difference (), the number of bubbles becomes so large that they begin to merge, as a result of which an increasing part of the surface will come into contact with the vapor phase, the thermal conductivity of which is lower than that of liquids. Therefore, heat transfer, having reached a maximum, will begin to decrease (transitional mode III) until a continuous vapor film is formed that separates the liquid from the heating surface. This mode of boiling is called film boiling (region IV). AT last case coefficient is practically independent of .
The figure shows the experimentally obtained dependence of the heat transfer coefficient on the heat flux density
when water boils in a large volume under conditions of free convection.
It follows from the figure that with an increase in the heat flux density, the heat transfer coefficient increases (section O - A). This section corresponds to the bubble boiling regime. Upon reaching
heat flux density \u003d W / m 2, the heat transfer coefficient decreases sharply (line A - D) - the bubble mode is replaced by a film one. Section D–D corresponds to the film regime. The phenomenon of the transition of the bubble mode of boiling into the film is called
first boiling crisis (). During the transition from the bubble regime to the film regime, the temperature difference increases significantly. The reverse transition from film to nucleate boiling occurs at a heat flux density 
W / m 2 (line B - C), which is about 4 times less. The phenomenon of transition from film boiling to bubble boiling is called the second boiling crisis (). The section of the curve A - B characterizes the transition mode, here both bubble and film modes on different parts of the heating surface.
Distinguish between the boiling of a liquid on a solid heat exchange surface, to which heat is supplied from the outside, and boiling in the bulk of the liquid.
When boiling on a solid surface, the formation of a vapor phase is observed in some places on this surface (according to H. Kuhling, the heat transfer coefficient á - boiling water - a metal wall is in the range from 3500 to 5800 W / (m 2 ⋅K).
During volumetric boiling, the vapor phase arises spontaneously (spontaneously) directly in the bulk of the liquid in the form of individual vapor bubbles. Volumetric boiling can occur only with significant overheating. liquid phase relative to the saturation temperature at a given pressure. For example, significant superheat can be obtained by rapidly depressurizing the system.
From the mechanism of heat transfer during convection of a single-phase liquid, the mechanism of heat transfer during nucleate boiling differs in the presence of an additional transfer of the mass of matter and heat by vapor bubbles from the boundary layer into the volume of the boiling liquid.
For the boiling process to occur, two conditions must be met:
The presence of overheating of the liquid relative to the saturation temperature;
The presence of centers of vaporization.
Liquid superheat has maximum value directly at the heated heat exchange surface, since there are centers of vaporization in the form of individual wall irregularities, air bubbles, dust particles, etc.
Boiling, in which steam is formed in the form of periodically emerging and growing bubbles, is called nucleate boiling.
With an increase in the heat flux to a certain value, individual vapor bubbles merge, forming a continuous vapor layer near the wall, which periodically breaks through into the liquid volume. This regime is called film boiling.
Heat transfer during nucleate boiling of a liquid under conditions of free motion
Heat transfer coefficient according to D.A. Labuntsov:
α kip st. dv.= C ⋅ λ ⋅ Re n⋅ Pr 1/3 /l , W/m 2 ⋅K,
where: l is the characteristic linear size of the vapor bubble at the moment of nucleation, in m.
The physical parameters included in the similarity criteria are determined at saturation temperature.
The values of the constants at boiling water are:
at Re ≤ 0.01, C = 0.0625, n = 0.5;
at Re > 0.01, C = 0.125, n = 0.65.
The dependence is valid in the range of values of the quantities:
Re = 10 -5 ÷ 10 +4 ; Pr = 0.86 ÷ 7.6; W ≤ 7 m/s;
and at volumetric steam content – â ≤ 70% for wide range saturation pressures (up to near-critical pressures).
Heat transfer coefficient according to M.A. Mikheev:
α kip st. dv.\u003d 33.4∆t 2.33 ⋅ R 0.5, W / m 2 ⋅K,
where P is the water pressure in bar.
The dependence is applicable for water in the pressure range 1 ÷ 40 bar (0.1-4.0 MPa).
Heat Transfer during Bubble Boiling under Conditions of Forced Convection in Pipes
In this case, the intensity of heat transfer is determined by the interaction of the pulsating motion of the liquid due to vaporization and perturbations penetrating from the volume of the liquid due to forced convection. Interpolation formula D.A. Labuntsov for heat transfer from nucleate boiling under conditions of forced convection in pipes has the form:
α/α w= 4α w/4α w + α q /α q, where:
α g is the heat transfer coefficient calculated according to the developed boiling formulas (when the speed does not affect heat transfer);
α w is the heat transfer coefficient calculated using the formulas for convective heat transfer of a single-phase liquid (when q does not affect heat transfer).
Dependency applicable:
In the range of values α q /α w from 0.5 to 2.0, (when the value of this ratio is less than 0.5 - α w = α, and at a greater 2.0 - α q = α);
With average volumetric vapor content not exceeding 70% (in this case, the heat transfer coefficient refers to the temperature difference t c - t n).
Heat transfer during film boiling of a liquid
Film boiling occurs in the presence of a large number centers of vaporization, in which the vapor bubbles merge, forming a continuous layer of vapor near the heat exchange surface, periodically breaking through into the liquid volume. In this case, the liquid is separated from the heated surface by a vapor layer. The heat flow to the interface passes through a low thermally conductive layer of steam. During film boiling of a liquid under conditions of free motion, the value of the heat transfer coefficient changes little with a change in the value of the heat flux.
Through the vapor film, in addition to heat due to convection and thermal conductivity, radiant heat also passes. Therefore, the heat transfer coefficient at film boiling is influenced by the radiation of the heat exchange surface, the radiation of the liquid surface, and the radiation of vapors. The share of radiant heat transfer increases sharply as the superheat of the liquid increases. Both forms of heat transfer are convective heat transfer and radiation - mutually influence each other. It manifests itself in the fact that the vapor formed due to radiation leads to a thickening of the vapor film and a corresponding decrease in the intensity of heat transfer due to convection and thermal conductivity.
At film boiling saturated liquid the heat flux removed from the heating surface is spent not only on the evaporation of liquid layers located at the boundary of the vapor film. Part of the heat removed is also used to superheat the vapor in the film, since the average temperature of the vapor inside the film is higher than the saturation temperature.
During film boiling of a subcooled liquid, the heat that passes through the vapor film from the boiling surface is partially transferred to the bulk of the liquid by convection. The intensity of convective heat transfer into the liquid volume depends on the subcooling and the liquid circulation rate.
In once-through boilers, process water enters in a subcooled state, and exits in the form of superheated steam. In such a boiler, as the steam-water mixture flows, the heat transfer coefficient changes: according to the laws of convection of a single-phase flow at the inlet section; according to the laws of convection and boiling of the bubble regime in the intermediate section; according to the laws of boiling of the film regime at the outlet section. With film boiling, the heat transfer is much less than with bubble boiling. However, at high pressures absolute value heat transfer becomes significant. Therefore, there is no burnout of the boiler pipes (surface burnout); the state of the heating surface remains controlled in this case as well.
Heat transfer coefficient at laminar motion vapor film on a vertical wall according to V.P. Isachenko:
α \u003d С 4 √ (λ 3 n⋅ r ⋅ ρ n (ρ and − ρ n) ⋅ g /(µ n⋅ ∆t ⋅ H)) , W/(m 2 ⋅K),
at t \u003d t n (water saturation temperature) and speed at the interface - W gr \u003d 0, constant factor C = 0.667;
with a speed gradient dw= 0, constant factor C = 0.943.
In the first case, the liquid is motionless, in the second case, the velocity of the liquid is equal to the velocity of the vapor at the interface.
The heat transfer coefficient for the laminar motion of the vapor film during boiling at outer surface horizontal cylinder according to V.P. Isachenko:
α \u003d С 4 √ (λ 3 n⋅ r ⋅ ρ n (ρ and − ρ n) ⋅ g /(µ n⋅ ∆t ⋅ d)) , W/(m 2 ⋅K),
In this case, C is equal to 0.53 (the liquid is stationary) and 0.72 (the velocity of the liquid is equal to the velocity of the vapor at the interface), respectively.
The given dependences of heat transfer during the laminar motion of a vapor film take into account the heat transfer over the film cross section by thermal conduction. The radiant (radiative) component of the heat transfer coefficient (α p) must be determined separately (see section 7.3.4.)
Heat transfer coefficient for turbulent motion of a vapor film during boiling on a vertical wall according to D.A. Labuntsov:
α = С ⋅ (λ/H)(Gr ⋅ Pr) G 1 /3 W / (m 2 ⋅K),
where: as applied to film boiling, the force that determines the movement of vapor in the film is equal to g*(ρ and − ρ n); constant factor C = 0.25; physical properties refer to the average temperature of the vapor film (as indicated by the index "G").
The Grashof criterion has the form Gr = (gl 3 /ν n 2)*(ρ and − ρ n)/ρ and
The dependence is applicable at (Gr ⋅ Pr) G ≥ 2 ⋅ 10 7 .
