REGIMES OF POOL BOILING AND FLOW BOILING ;
Regimes of Boiling
Let us consider a heating surface (a wire or a flat plate) submerged In a pool of water which is at its saturation temperature. If the temperature of the heated surface exceeds the temperature of the liquid, heat energy will be transferred from the solid surface to the liquid. From Newton's law of cooling, we have
where Q /A is the heat flux, Tw is the temperature of the heated surface and Ts, is the temperature of the liquid, and the boiling process will start.
(i) Pool Boiling - Pool boiling occurs only when the temperature of the heated surface exceeds the saturation temperature of the liquid. The liquid above the hot surface I s quiescent and its motion n ear the surface is due to free convection.
Fig. 5.13 Temperature distribution in pool boiling at liquid-vapour interface
Bubbles grow at the heated surface, get detached and move upward toward the free surface due to buoyancy effect. If the temperature of the liquid is lower than the saturation temperature, the process is called 'subcooled or local boiling'. If the temperature of the liquid is equal to the saturation temperature, the process is known as 'saturated or bulk boiling'. The temperature distribution in saturated pool boiling is shown in Fig5.13. When Tw exceeds Ts by a few degrees, the convection currents circulate in the superheated liquid and the evaporation takes place at the free surface of the liquid.
(ii) Nucleate Boiling - Fig. I 1.5 illustrates the different regimes of boiling where the heat flux Q / Ais plotted against the temperature difference (Tw -Ts )When the temperature Tw increases a little more, vapour bubbles are formed at a number of favoured spots on the heating surface. The vapour bubbles are initially small and condense before they reach the free surface. When the temperature is raised further, their number increases and they grow bigger and finally rise to the free surface. This phenomenon is called 'nucleate boiling'. It can be seen from the figure (5.14) that in nucleate boiling regime, the heat flux increases rapidly with increasing surface temperature. In the latter part of the nucleate boiling, (regime 3), heat transfer by evaporation is more important and predominating. The point A on the curve represents 'critical heat flux'.
Fig. 5.14 Heat Flux - Temperature difference curve for boiling water heated by a wire (Nukiyama's boiling curve for saturated water at atmospheric pressure)
(L is the Laidenfrost Point)
(iii) Film Bolling –
point A, a vapour film forms and covers the entire heating surface. The heat transfer takes place through the vapour which is a poor conductor and this increased thermal resistance causes a drop in the heat flux. This phase is film boiling'. The transition from the nucleate boiling regime to the film boiling regime is not a sharp one and the vapour film under the action of circulating currents collapses and rapidly reforms. In regime 5, the film is stable and the heat flow rate is the lowest.
(iv) Critical Heat Flux and Burnout Point - For T beyond 550oC (region 6) the temperature of the heating metallic surface is very high and the heat transfer occurs predominantly by radiation, thereby, increasing the heat flux. And finally, a point is reached at which the heating surface melts - point F in Fig. 11.5. It can be observed from the boiling curve that the whole boiling process remains in the unstable state between A and F. Any increase in the heat flux beyond point A will cause a departure from the boiling curve and there would be a large increase in surface temperature.