According to the RPI model, heat flux from wall to two phase flow is partitioned into three components and each component has its own effect on heat transfer coefficient. It is recommended to use UDFs for heat fluxes. It gives you better results and you have the ability to modify each part e.g. bubble departure frequency.  Bubble departure frequency can be deemed as the reciprocal of the summation of bubble waiting time and bubble growth time. Higher waiting time reduces the bubble departure frequency and results in lower quenching and evaporation heat transfer rate. This may reduce the two phase heat transfer coefficient.

The quenching heat flux (Qq) is defined as:

• Qq = A2*hq*(Tw - Tl)

where: A2 is the fraction of the whole wall surface influenced by the vapor bubbles, hq is the quenching heat transfer coefficient, Tw is wall temperature and Tl is the liquid temperature.

The bubble waiting time (tw) is correlated to quenching heat transfer coefficient (hq) as:

• hq = 2*k*f*(tw/(π*a1))^1/2

where: k is the liquid thermal conductivity, f is the bubble detachment frecuency and a1 is the liquid temperature conductivity coefficient. The bubble detachment frecuency is correlated to bubble waiting time in the following way:

• f = 0.8/tw

hq is directly proportional to f, while depends on the square root of tw. Therefore higher values of tw provides smaller values of f and smaller values of hq. Which means that the quenching heat flux (Qq) decrease.  Finally, increase the value of bubble waiting time coefficient decrease the wall heat transfer coefficient and simultaneously decreases the departure of bubbles.

I hope it will be helpful, Ronny.

thanks for your notes Ronny and Ali

I don't have idea about UDF now.

First,I prefer using Fluent experimental correlations.

your theories about quenching heat flux is true. But in my case the total wall heat transfer coefficient increase with increasing the bubble waiting time coefficient!!

many parameters doens't affect the wall heat transfer coeff, except bubble waiting time!

I use tolubinski-kostanchuk correlation for bubble departure diameter. should I use other correlations for Boiling?which is the best?

In Fluent theory guide quenching heat flux is defined as:

Qq=Cwt*(2Kl/(πγlT)^0.5)*(Tw-Tl)*Ab

which Cwt is the bubble waiting time coeff, and increasing this coeffT increase the quenching heat flux. 

Does this correlation correspond to yours? if not How can i justify increasing the wall heat heat transfer coefficient?

thanks in advance

The nucleate site density is frequently signified by a correlation based on the wall superheat (Kocamustafaogullari and Ishii, 1985).

In boiling modeling, its important that y+ be 250. its mean that near the wall mesh must be large. because bubble need to grows.

thanks for your helps

bubble departure diameter and nucleate site density can affect the problem. but still i didn't find the answer of my question,that how increasing the bubble waiting time coeff, increase the total wall heat transfer ??

When the Bubble departure diameter would be smaller, Frequency of bubble formation is enhanced and consequently increases the evaporation contribution and heat transfer coefficient.

I like to know your result. if it is possible send me your results.

thanks.

thanks Mr Rahimian

your deduction is true. the smaller departure diam result in higher frequency of bubble departure, and finaly the evaporation heat flux increase.

but how about the effect of bubble waiting time?this coeff correct the waiting time between departures of consecutives bubbles.higher value of It means that bubbles depart later.but what is the influence of this on heat transfer coefficient?

thanks 

I think that with lower waiting time, the quenching and evaporation heat transfer increaseand frequency of bubble departure is enhancedthis means that we have higher heat transfer coefficient. But when we have at beginning of boiling, the liquid phase heat transfer is effective than quenching and evaporation heat transfer, therefore the effect of waiting time coeff decrease. Hence, I think with growing boiling along heat channel the effect of waiting time on heat transfer coefficient become greater.

 T in tolubinski-kostanchuk correlation for bubble departure diameter is the waiting time. and show that with increase T the frequency of bubble departure is decreased. we don't have Cwt in this correlation. check this correlation again.

thanks for your reply Mr Rahimian

there isn't any Cwt in bubble departure diameter model. But Fluent guide suggest that quenching heat flux is multiplied by a coefficient which is a corrector of bubble waiting time,as I mention in previous answers.also Fluent notes that is should be a constant but it can be different for each problems. So it can be valuable for quenching heat flux.

I think, I should work on  bubble departure diameter and get the periodic time and check it with available experimental data, rather than changing this coefficient.

thanks

Dear Ms. Sarifi,

As I know, the bubble departing frequency is the inverse of the sum of waiting and growing period of the bubbles. It is noticed on the literature that the ratio of waiting period to total period is equal to 0.75. According to my observations, this value is false. I have found it between 0.1 and 0.9 at various conditions. I am trying to correlate this value to other knowing parameters but the research is still in progress.

Regards