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Home > Boiling heat transfer
Boiling heat transfer  
 
 
      In the solid-liquid interface of evaporation, called boiling. When the surface temperature Ts over the corresponding liquid under pressure from the saturation temperature ts, which occurred in the process. Calories from solid to liquid surface mass, Newton’s law of cooling for the corresponding form (3-1)

      Hill which △ t = tw - ts called superheat. Tw for the wall temperature, ts liquid saturation temperature. This process is characterized by a steam bubble formation, they grow up from the surface. Steam bubble growth and its dynamic characteristics of overheating, and the surface characteristics such as surface tension, such as the thermal fluid between physical parameters have a very complicated relationship. Conversely, vapor bubbles formed the dynamic characteristics affects the fluid near the surface movement, the heat transfer coefficient thus have a strong impact. These impacts on the foreign detailed description is beyond the scope of the book.     

      Boiling the various conditions under which it occurs. For example, the pool boiling (or large containers boiling) means is that the liquid is generally static, in the vicinity of the surface movement is due to natural convection and the bubble growth and detachment caused due to the disturbance. Unlike this, the forced convection heat up, in addition to the movement of fluid due to natural convection and bubble caused the disturbance, also caused due to external forces. Boiling it is also in accordance with the cold or too saturated to the classification; During the cold (or partial) boiling, liquid temperature is below saturation temperature, solid thus formed on the surface of the bubble finally be in liquid condensation, on the contrary, saturated boiling, the temperature of the liquid over saturation temperature, Solid formed on the surface of the bubbles floating lift the liquid through the promotion, finally free from the surface to escape.    

(1)large containers saturated boiling and boiling curve

   Heating wall is immersed in the liquid free surface as a boiling large containers called boiling. At this point the bubble free floating or passing through the liquid free surface into one container space.

      The main liquid temperature reached saturation temperature ts, the wall temperature tw saturation temperature higher than the boiling called saturated boiling. In saturated boiling, with the wall superheat △ t = tw - ts and the higher there will be four completely different heat transfer area. Water in a saturated atmospheric pressure boiling under the curve (Figure 3 -1) representative. Abscissa map for the wall superheat face △ t (the number of coordinates), the longitudinal coordinates for the heat flux q. This four regional heat transfer characteristics are as follows :     

    Wall superheat hours(图3-1中△t<4℃)Boiling has not yet started, single-phase heat transfer subordinate to the law of natural convection.

      From the initial boiling point, the heating surface of certain specific points (called core] vaporization bubble. Initially, Vaporization core of the bubble without them interfering with one another, saying isolated bubble, its boiling scenes figure 3 2 (a) below. With △ not increase further, the core of vaporization, Bubble impact on each other. and synthesis gas block and column, a picture of figure 3 - 2 (b) below. In both areas, Bubble violent disturbance, heat transfer coefficient and heat flux density increases rapidly. As the core of vaporization heat transfer plays a decisive impact Boiling the two districts collectively known as the nuclear boiling (or bubbly boiling). nucleate boiling temperature difference is small, the heat transfer characteristics of strong, So general industrial applications are designed in this context. nucleate boiling finish at the Figure 3 -1 China and the peak heat flux point.       

      From the peak point further enhance △ t, heat transfer laws appeared unusual changes. Heat flux not only with the increase △ not increased, but less and less. This is because the bubble pool coverage in the heating surface, to the exclusion of steam process becomes more difficult. This continued until the arrival minimum heat flux qmin far. Boiling called this transition boiling is very unstable process. Figure 3 2 (c) of transition boiling pictures.     

      Qmin heat transfer from the law again with the shift. Then the surface has been heated to form a stable layer of steam, the steam from the rules to film, q With △ not increase. Despite called stable film boiling. Stable film boiling in physics filmwise condensation with a common thread, But because the heat is to be through a larger thermal resistance of the vapor membrane, rather than film, the heat transfer coefficient was much smaller than the condensation. Stable film boiling photographs shown in Figure 3 -2 (d).

      Above the peak heat flux oading significant, it was known as the critical heat flux (CHF Citical Heat Flux). Rely on controlled heat flux to change the status of the heating equipment, such as electric heater, heating of the cooling water nuclear reactors Once over the peak heat flux, the situation along the dotted line jump oading stable film boiling line, Hill jumped to nearly 1,000 ° C may lead to the burning of equipment, must be strictly monitored and controlled heat flux density, ensure security within the scope of the work. Because it could lead to more than equipment destroyed, burned oading known as points. In the vicinity of the burning point (than the heat flux oading slightly smaller), Figure in a 3 -1 manifested as a slow rise q nuclear boiling turning point denitrifying bacteria (Departure fi orll Nuclear unpersuasive, meaning from nucleate boiling), It oading close surveillance as a warning, is very reliable. For evaporative condenser wall temperature controlled equipment, such surveillance is important. Once over q turning point, this could lead to film boiling, the same wall temperature heat makes for greatly reduced

    These are boiling water saturation curve outlined. The different quality, different pressures, different parameters boiling boiling phenomena and the evolution of its laws are similar

Boiling heat transfer phenomena and analysis
图3-2 Different boiling point (hot metal filament)

(a)Isolated bubble (nucleate boiling)

(b)Gas block (nucleate boiling)

(c) Transition boiling

(d) Stable film boiling

2、Vaporization core analysis

     In the nuclear boiling, the bubble disturbance on heat transfer play a dominant role. Vaporization bubble formation in the core Department. Vaporization of the impact of the core factors and the number of vaporization of the core wall superheat the relationship between the dependent variable, will help to nucleate boiling heat transfer and its understanding of the law.

      Now widely recognized that fell off the wall, cracks are most likely to become the core of vaporization. They fell off the residual gas (including steam), as a result of the surface tension of liquid reasons, it is difficult to completely evicted they become new breed bubble favorable places. Below we vaporization of the core for the formation of an analysis. Assuming there is fluid in a spherical bubble, as shown in figure 3 -3, and the surrounding liquid in thermal equilibrium and balance. As the role of surface tension, the pressure within the bubble pv certainly greater than the pressure outside the bubble pl. Under equilibrium conditions, the pressure inside and outside the bubble should be in the role of liquid-vapor interface on the surface tension balance that πR2 (pv - pl) πR γ = 2 (3-2) where, γ for the length of the liquid-vapor interface surface tension (N / m). If neglected fluid pressure, the effect would be pl can be considered to approximate the boiling pressure on the environment, pl ≈ ps. And the thermal balance within the bubble require the temperature steam under pressure for pv the saturation temperature tv. The same interface and outside temperature, ti = tv, outside the bubble is necessarily liquid overheating superheat for tv-ts. Adherent Department is the largest liquid superheat tw - ts and fell off Department residual gases, Au wall joints of the first to meet the conditions of bubble formation

(3-3)
Therefore, the bubbles are generated on the wall.

     Balance the bubble is very unstable. Bubbles with a radius of less than Type (3-3) indicated in the radius and surface tension than pressure, steam condensation inside the bubble, bubble collapse. Greater than the radius-only (3-3) shown in radius, the interface liquid evaporation, air bubbles can only grow. In summary, we can see a certain degree of overheating of the surface conditions, the wall-only meeting (3-3) in those conditions. can become the core of vaporization.

      With the wall superheat increased pressure pv - ps increasingly high value. By type (3-3), the equilibrium state of bubble radius R will be reduced. Therefore, the wall temperature tw increases, the wall increasingly small gas fell off the depositors will become the core of vaporization, so few with the vaporization of the core wall of overheating of increase.       

      SAIC heating on the surface of the core and the formation of bubbles in the liquid growth and movement rules of the study, It will grasp the basic boiling heat transfer mechanism and the development of enhanced boiling heat transfer surface is of great significance. Existing forecast of boiling heat transfer model is based on the nucleation theory and bubble dynamics of understanding to establish some kind of years. 20 is the end of the 1950s on the vaporization of core First, on the surface, some small pits on the formation of this basic viewpoint The establishment has led the 1970s in the 20th century on boiling heat transfer enhancement surface development work. Vaporization of the core group of recent research results can be found in the literature.

(3)Forced convection boiling brief

      Boiling in containers, produced by heating the surface bubbles, bubbles float up by the buoyancy effect, So fluid movement driven mainly by the buoyancy. Boiling forced convection (forced convection boiling), fluid flow is the direct liquid movement and the common buoyancy effects. And the forced convection similar, forced convection boiling can be divided into forced convection boiling external and internal forced convection boiling. The latter are generally referred to as two-phase flow (two-phase flow). it is based on the direction of the flow of liquid from steam to rapidly become characteristic.

(1)External forced convection boiling

      On a heating plate external flows, its heat flux can use standard forced convection relationship has been estimated that at the beginning of boiling. With heating temperature increase, nuclear boiling emergence of flux density increase. If the steam generation rate is not high and that supercooled liquid, Bergles and Rohsenow proposed pure forced convection boiling pool and the combination to estimate the total heat flux .

(2)Boiling tube (two-phase)

      Pipe forced convection heat up, mixed with the steam flow, there many different forms of two-phase structure, heat transfer mechanism is more complicated. As an example, Figure 3 -4 showed a firm boiling tube may arise type of flow and heat transfer types. Exiles end of the saturated liquid is heated wall, reach a certain location on the surface begins to produce air bubbles. At this time have not yet reached the mainstream liquid saturation temperature, at too cold state, then boiling too cold boiling. Continue to live so heated flow reached saturation temperature, saturation nuclear boiling zone. Saturated nucleate boiling region experiencing bubbly flow and bulk flow (bubble converge into pieces, also known as the slug flow). Steam rose to a certain level, the FAW block further consolidation in the form of steam-center, and to squeeze out liquid surface, annular film, called annular flow. At this point heat transfer into human convection film boiling zone. Annular heating liquid film evaporation, gradually thinning, the film eventually disappear, and anti-steam direct contact with the wall. Disappear as film evaporated. At this time, the deterioration due to heat transfer, will galloping wall temperature, causing a threat to security. The flow of wet steam heating to make the quality of people in the last steam into single-phase heat transfer area. Wang tube heat up, the gravity field of two-phase structure and affect the appearance of new characteristics, So, the location of impact of boiling within one of the factors. Boiling in the pipes, the most important parameter is the impact of the volume containing vapor (steam degrees), the mass flow rate and pressure.