Whether the boiling phenomenon corresponds to pool boiling or forced circulation boiling, there are three definite regimes of boiling associated with progressively increasing heat flux. These regimes have been identified in Fig. 12.8 which shows the relationship between heat flux (or film coefficient) and the temperature excess (ts – tsat); ts is the temperature of the hot surface and tsat is the saturation temperature corresponding to the pressure at which the liquid is being evaporated.
A rise in the slope of the curve indicates an increasing heat flux with increasing temperature excess and the boiling regime is stable. When the slope decreases, the boiling regime is unstable and must be avoided. These different regimes can be illustrated by considering an electrically- heated horizontal wire submerged in a pool of liquid at saturation temperature. The heat flux is easily controlled by voltage drop across a wire of fixed resistance.
Regime # 1. Evaporation Process with No Bubble Formation (Interface Evaporation):
The boiling takes place in a thin layer of liquid which adjoins the heated surface. The liquid in the immediate vicinity of the wall becomes superheated, i.e., temperature of the liquid exceeds the saturation temperature at the given pressure.
The superheated liquid rises to the liquid-vapour interface where evaporation takes place. The fluid motion is determined primarily by free convection effects. The heat transfer rate increases, but gradually, with growth in a temperature excess.
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Temperature excess, (ts – tsat) C Fig. 12.8. Boiling curve for water: surface heat flux as a function of excess temperature.
Regime # 2. Nucleate Boiling:
When the liquid is overheated in relation to saturation temperature, vapour bubbles are formed at certain favourable spots called the nucleation or active sites; these may be the wall surface irregularities, air bubbles and the particles of dust. The bubbles grow to certain size influenced by pressure, temperature and surface tension at the liquid-vapour interface.
Depending on temperature excess, the nucleate boiling essentially consists of the following stages:
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(a) Bubbles form and collapse on the surface itself
(b) Bubbles form on the heated surface, but get condensed in the liquid after detaching from the surface
(c) Bubbles form, break away from the heated surface with increasing frequency and intensity. The liquid is, however, quite hot and the bubbles do not condense in it. They rise to the liquid surface and are directly expelled to the vapour space and that helps rapid evaporation.
The mechanism and the cycle of bubble formation has been illustrated in Fig. 12.9.
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(i) Liquid next to the heated surface becomes superheated
(ii) Vapour nucleus of sufficient size is created to initiate the formation of bubble
(iii) Bubble grows in size and pushes the layer of superheated liquid away from the heated surface
(iv) Top of the bubble comes into contact with the cooler liquid and that has a tendency to arrest the bubble growth
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(v) Inertia of the liquid and the bubble has caused the bubble to grow to a size and position that it loses more heat to the cooler liquid than it gains by conduction from the heated surface
(vi) Bubble begins to collapse and the cooler liquid gains velocity to fill in the bubble volume
(vii) Bubble suffers a total collapse and inertia of the cooler liquid brings it into contact with the heating surface
(viii) Eventually the cooler liquid gets heated above the saturation temperature, and another cycle for bubble formation and its collapse begins.
The nucleation boiling is thus characterised by the formation of bubbles at the nucleation sites and the resulting liquid agitation. The bubble agitation induces considerable fluid mixing and that promotes substantial increase in the heat flux and the boiling heat transfer coefficient.
Regime # 3. Film Boiling:
The bubble formation is very rapid; the bubbles blanket the heating surface and prevent the incoming fresh liquid from taking their place. Eventually the bubbles coalesce and form a vapour film which covers the surface completely. Insulating effect of the vapour film (its low thermal conductivity) overshadows the beneficial effect of liquid agitation and consequently the heat flux drops with growth in temperature excess.
Within the temperature range 50 < Δt < 150, conditions oscillate between nucleate and film boiling and this phase is referred to as transition boiling, unstable film boiling or partial film boiling. Eventually the temperature differences are so large that radiant heat-flux becomes significant, rather controlling factor and the heat flux curve begins to rise upward with increasing temperature excess. That marks the region of stable film boiling. The phenomenon of stable film boiling is referred to as “Leidenfrost effect”.
Critical Heat Flux – Burnout Point:
The boiling crisis or the burnout point A on the boiling curve represents the point of maximum heat flux at which transition occurs from nucleate to film boiling. The maximum heat flux is called the critical heat flux and the corresponding temperature excess is termed as the critical temperature difference. For water evaporating at atmospheric pressure, the burnout occurs at temperature excess slightly above 55 K and has heat flux of the order of 1.58 × 106 W/m2.
The boiling process remains in the unstable state beyond the burnout point A until situation corresponds to point B on the boiling curve. An increase in the temperature excess beyond burnout is accompanied by decrease in the heat transfer capability of the surface.
The result is a continued increase in the surface temperature. At point B an equilibrium is established between the energy input into the surface and the heat flux away from the surface. The boiling conditions get stabilized but then the surface temperature has already become very large. And if the surface temperature exceeds the temperature limit of the wall material, burnout (structural damage and failure) of the wall occurs.