Question

4. (20%) In two phase flow problem, The Nukiyama first identify different regimes of pool boiling using power-controlled heating apparatus for demonstrating the boiling curve as below. E- C- 2 e fq A D- k ΔΤgr Please describe the four different boming regimes and their physical mechanisms at -AmA-C, C-E, E-, and describe physical meaning of the A, C, D point 4

Answer 1

The four regimes are

1. Natural or free convection,
2. Nucleate boiling,
3. Transition boiling,
4. Film boiling

Natural convection boiling Nucleate boiling Transition boiling Film boiling Maximum (critical) Bubbles! collapse in the liquid heat flux, 4mus 10% E - - - W/m2 1 1 10 sealing 1 B 104 DI AL ! Bubbles! rise to the free surface! Leidenfrost point,min - 1 1 - 102 -5 10 --30 1000 100-120 =T -T°C AT

(1)Natural or free convection 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.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'.

(2)Nucleate boiling

If the liquid isoverheated 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.The 2 stages of nucleate boiling are 1.Bubbles form and collapse on the surface itself 2.Bubbles form on the heated surface, but get condensed in the liquid after detaching from the surface 3.Bubbles form, break away from the heated surface with increasing frequency and intensity.

(3) Transition boiling.

As the temperature increases beyond the critical heat flux point, the rate of bubble generation exceeds the rate of bubble detachment from the heater surface. Bubbles from an increasing number of sites merge to form continuous vapor films over portions of the surface, further decreasing the contact area between the heating surface and the saturated liquid. These vapor films are not stable, however: they can detach from the surface, leading to restoration of contact with the liquid and resumption of nucleate boiling.Since the filim formed by vapour in this stage is unstable this stage is called transition boiling.

(4) Film Boiling.

Further heating causes a vapour film to 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'.