Transition Boiling Regime

 

 

 

The transition boiling regime is probably the least studied region of the boiling curve because: (1) transition boiling is less important, practically speaking, than nucleate boiling, and (2) the mechanism of transition is very complicated and experimental investigation is very difficult. In this region, the liquid periodically contacts the heating surface creating large amounts of vapor and forcing the liquid away from the surface. The combined result is an unstable vapor film or blanket. Then the film collapses, allowing the liquid to contact the heating surface again. It is a region with alternate nucleate and film boiling, and is bypassed if the boiling curve experiment is performed with a controlled-heat flux setting. It is only obtainable by experiments with controlled surface temperature, which are very rare in practical applications. The boiling curve in Pool Boiling Regimes (Fig. 1) shows that the excess temperature ΔT for transition boiling at one atmosphere is about 30 to 120 °C. The limited conditions conducive to this regime in turn limit its practical appeal to many investigators. However, investigation of transition boiling can help the experimenter to avoid burnout caused by excessive surface temperature. It is also possible to reduce the heat flux fluctuations that result from unstable dry regions in contact with the heating surface.

 

 

Witte and Lienhard (1982) reported the existence of a hysteresis in transition boiling and suggested that there are two distinct boiling curves: the nucleate-transition boiling curve, obtained by increasing the excess temperature from nucleate boiling, and the film-transition boiling curve, created by decreasing excess temperature from film boiling (see Fig. 1). If the liquid wets the heating surface very well, which is generally presumed to be the case, the boiling curve does not progress immediately into the transition regime once the excess temperature exceeds its value corresponding to the critical heat flux. Witte and Lienhard (1982) suggested that the low contact angle made it easier for the liquid to spread over the surface, which promotes vigorous nucleation instead of the expected transition boiling. The curve also deviated from the conventional pool boiling curve when film boiling was initiated first, and the curve was then traversed by decreasing the excess temperature. As the point of minimum heat flux is approached, vapor is not produced as rapidly. The vapor layer becomes less stable, and the film breaks down. The lower the contact angle (i.e., the better the wetting characteristics of the liquid), the higher the temperature or heat flux at which the curve departs from film boiling. In this case, the boiling curve departs from film boiling without first reaching the minimum heat flux.

 

Recent experimental work in transition boiling did not support the existence of a hysteresis, because there was no evidence of two distinctive boiling curves. In an experimental work by Hohl et al. (1996), the entire boiling curve for FC-72 under steady and transient conditions was measured. The heater surface was a horizontal circular flat plate of 34 mm in diameter, coated with a 20 μm nickel layer. The surface temperature was controlled via feedback control. The typical boiling curve is shown in Fig. 2. As is evident, the boiling curves are identical regardless of the direction of the temperature change. The contradicting evidence with regard to the existence of hysteresis in the boiling curve could be due to different geometric configurations and the temperature control system. Another possible cause is contamination, since some combinations of liquid and solid surface can affect wettability. At this time, there is no universal criterion for the existence of hysteresis in transition boiling.

 

Heat transfer in the transition boiling regime is very challenging to predict because it is the most complex regime in pool boiling. Contradiction about the existence of a hysteresis makes accurately predicting heat transfer even more difficult. Transition boiling can be viewed as a combination of unstable nucleate boiling and unstable film boiling at different locations, or at different times at the same location. Therefore, the heat flux for transition boiling can be expressed as (Berenson, 1962)

where F is the average proportion of the heating surface in contact with liquid at any given moment. It can be estimated by the following correlation:

 where ΔT_CHF is the excess temperature at critical heat flux (CHF), q''_max. The heat flux during the vapor contact can reasonably be assumed to be equal to the minimum heat flux, i.e.,

 [[Image:poiling_p_(6).png|thumb|400 px|alt= Boiling curves of saturated FC 72 | Figure 3 Boiling curves of saturated FC 72 (Hohl et al., 1996

 

The heat flux during the liquid contact,

, is time-dependent. Its time-averaged value can be correlated as

 

 The minimum heat flux, q''_min, in the above correlations needs to be determined in the next subsection.

 

<참고> 열교환기 설계시 Transition Boiling Regime은 가능한 피해야 한다. Tube Inlet Nozzle과 Shell Inlet Nozzle의 방향이 반대가 되도록 설계하면 Transition Boiling Regime을 피하는데 도움이 될 것이다.