Flow boiling regimes question 1

[rjw57]

I am reviewing some of my heat transfer texts and finding them woefully inadequate in explaining flow boiling. I have reviewed pool boiling and the correlations associated with it. I am comfortable with understanding where I am on a pool boiling curve and the variables which affect the shape / placement of the curve on the pool boiling graph. I even am comfortable with doing a pool boiling calculation. Where I fall down is in understanding how to know, for flow boiling, where I am on this same curve (I understand that the shape changes particularly for nucleate boiling and the location of DNB/CHF). I have looked at a number of correlations for nucleate boiling, but don't understand how I know when I am in a nucleate boiling regime (versus say transition or film boiling). If it is a matter of excess temperature (Twall - Tsat), I don't see how the flow boiling correlations I have reviewed accommodate this factor, except for to maybe calculate it implicitly using said correlation and iterate until things balance. Any insight into this issue would be greatly appreciated.

Thanks,
rjw57

 

[srfish]

Have you reviewed the text by Kakac and Liu ? It was published in 1998 and is named "Heat Exchangers Selection, Rating, and Thermal Design". They have an extensive section on flow boiling on pages 220-247.

There is little discussion in the literature of problem areas in evaluating flow boiling. Some situations such as stratified flow or mist flow will cause a big drop in heat transfer. Another problem is the one you mention which is high temperature difference (Twall -Tsat). As a long time thermal design engineer,I have used a maximum temperature difference of 80 F. for hydrocarbons. This has been used for years without a problem.

 

[davefitz]

As a folowup to srfish, S. Kakac has a newer text " Boilers evaporators and condenser" 1991 , J Wiley + Sons, isbn 0-471-62170-6 , chapter 6 and 7. . In particular, the CHF correlations by Dorschuck and dryout by Kieffer, Kohler, Hein and Wittchow are considered to be the mopst reliable correlations for flow boiling of water.

For insight into their methods , see papers by Keifer, Kohler and Hein ( Siemen KWU Erlangen) as published in 1985-1995 in the In'tl journal of multiphase flow.

 

[rjw57]

Thanks srfish & davefitz

I will look into these publications shortly. I have just found some vertical and horizontal flow boiling correlations (described as flow pattern mapping)in a text I own which are attributed to Kattan-Thome-Favrat and are applicable to a large range of regimes in both vertical and horizontal flows. The correlations are different for horizontal vs. vertical flows of course. Are either of you familiar with this work?

Thanks,
rjw57

 

[srfish]

I am not familar with Kattan-Thome-Favrat. The correlation I have used is Chen.

Good Luck

 

[davefitz]

The KWU correlations ahve been extended to horizontal and inlcined tubes, as explained in the published papers by Kiefer- Kohler-Hein in In't J Multiphase flow.