The evaporation model available in FLUENT is based on the Knudsen diffusion flux at the interface coupled with the Clausius-Clapeyron equation to account for vapour-liquid equilibrium.

The evaporation and condensation frequency denotes the probability of vapour molecule escaping from and absorbing in the interface. This probability value for obvious reason should lie in the range of 0 to 1.0.

Many work is published to determine this frequency factor which span over this range and interestingly a lot of disparity has been found in these reported values mostly due to the temperature dependency of these frequency factors.

It needs to be noted that this is a thermodynamic evaporation model which does not take care of the surface effect in the boiling phenomena. Another important thing is, this model works on the basis of temperature driven mass transfer process and as a result, no evaporation would be found if both the surface and fluid temperature is below the saturation temperature.

Hi Sahar,

I dont have the answer for your question regarding the frequency. But I am also using almost the same conditions for boiling off water in a vertical tube with heat flux. but in my case it started vapor generation at 0.3 sec.

Does vapour generation is sensible?i mean,does the outlet quality change?

in my case,inlet quality is 1% and outlet one is about 1.175% after 1second, while i give the heat flux to increase quality from 1% to 5%.

Yes vapor generation is sensible. I am heating water inside a container with pressure outlet. The results are quiet sensible.

The evaporation model available in FLUENT is based on the Knudsen diffusion flux at the interface coupled with the Clausius-Clapeyron equation to account for vapour-liquid equilibrium.

The evaporation and condensation frequency denotes the probability of vapour molecule escaping from and absorbing in the interface. This probability value for obvious reason should lie in the range of 0 to 1.0.

Many work is published to determine this frequency factor which span over this range and interestingly a lot of disparity has been found in these reported values mostly due to the temperature dependency of these frequency factors.

It needs to be noted that this is a thermodynamic evaporation model which does not take care of the surface effect in the boiling phenomena. Another important thing is, this model works on the basis of temperature driven mass transfer process and as a result, no evaporation would be found if both the surface and fluid temperature is below the saturation temperature.

The rate of evaporation is affected by the pressure of the gas phase and its composition. The rate of evaporation will increase with the heavy pressure of the gas above the liquid.

In Russ:

На скорость испарения влияет давление газовой фазы и ее состав. Скорость испарения увеличится с увеличением давления тяжелого газа над жидкостью. Левъ

The rate of evaporaton of fluid is incresed by an increase in temperature, an increase in area of exposed surface, a decrease in humidity of the surrondig air, the presence of a draught, and decrease in external pressure.

Please note that temperature of luid is related to the freqencey of molecules of fluid. As temperaure increases, numer of collisions made by molecule in unit time decreases casuing increase of frequency.This is actual interpretation of the data.    

thanks for your fruitful helps guys.

Measured data for the evaporation and condensation coefficients, based on the Knudsen equation (called β in Fluent Theory Guide), showed that evaporation coefficient is usually higher (about factor 10). At higher pressure the difference increases. But I have to mention that measured data has a quite high variance according to measurement technique. For ambient conditions I used values of 0.1 (ev.) and 0.8 (cond.).

Sahar Sharifi

I am modelling the same thing as you. Have you found an answer to the frequency of evaporation ?

I have been told that 1000-10 000 works, but I do not get good results.

I am also considering a steady state simulation, you think that's possible ?

Christos, i am also modelling evaporation with reacting species, but in my case if i change evaporation coefficient to 100, my problem face temperature divergance limit

there is no exact number for the coeff and you have to tune that for each case that you are going to simulate that but based on my experiences when you set the coeff in big number not thing will be changed (I am speaking about evaporation coeff) of course you must not set the coefficient at big number you have to change it during the iteration from low to high!