DPM Droplet Heat and Mass exchanges: Are the droplet boiling?

Large Epic Simulations · September 5, 2017, 10:11

Hi everyone,

It's been a while since I'm looking to find an explanation about the differences between the DPM laws 2 (convective/diffusive evaporation) and laws 3 (boiling).

Specifically I'm probably misunderstanding the definitions of evaporation temperature (which is defined in the guide manual as a non-physical parameter) and the boiling temperature. I would assume that this last is the temperature along the gas-liquid saturation curve (depending on pressure).

Both of these temperatures defines a "switch condition" to let the code know which law should be taken in account.

Altought my interpretation of boiling droplet, is a liquid particle in which the vapour partial pressure is higher than the surrounding liquid one. I must admit that, for a not truly scientific perspective, this looks like an unlikely phenomena occurring in a small droplet (diameters ~10^-5 m).
Anyway, I have some references that uses the boiling law, so I'm here to ask you what are your opinions about it.

More precisely, my questions would be:

1) What are the substancial physical differences between these two laws?
2) Which values for the evaporation and boiling temperatures should be used?

Thank you in advance for your patience

Vincenzo_ · November 3, 2017, 12:34

Did you find your anwsers?

Large Epic Simulations · November 4, 2017, 01:51

Unfortunately not, but empirically, I've observed that law 2 (which corresponds to the d^2 Spalding law) produces a sort of discontinuity in the droplets quantities derivatives (diameter, temperature etc.).

I would say that this kind of behavior is given by the evaporation temperature switch conditions:
at the moment the particle reach this temperature, the evaporation rate term appears in the energy equation. As this term overcome the sensible exchange, the particle temperature fall-down in few timesteps.

This leads to a non-constant temperature evaporation, which is likely to occurs during flash evaporation.

I'm still wondering if this kind of behaviour is physically suitable.

On the other side, the droplet boiling law (law 3) produces a constant temperature mass loss, which mitigates derivatives discontinuities and leads to smoother profiles.

In conclusion my questions are still opened.

September 5, 2017, 10:11	DPM Droplet Heat and Mass exchanges: Are the droplet boiling?	#1
Large Epic Simulations Member Join Date: Jun 2017 Posts: 39 Rep Power: 8	Hi everyone, It's been a while since I'm looking to find an explanation about the differences between the DPM laws 2 (convective/diffusive evaporation) and laws 3 (boiling). Specifically I'm probably misunderstanding the definitions of evaporation temperature (which is defined in the guide manual as a non-physical parameter) and the boiling temperature. I would assume that this last is the temperature along the gas-liquid saturation curve (depending on pressure). Both of these temperatures defines a "switch condition" to let the code know which law should be taken in account. Altought my interpretation of boiling droplet, is a liquid particle in which the vapour partial pressure is higher than the surrounding liquid one. I must admit that, for a not truly scientific perspective, this looks like an unlikely phenomena occurring in a small droplet (diameters ~10^-5 m). Anyway, I have some references that uses the boiling law, so I'm here to ask you what are your opinions about it. More precisely, my questions would be: 1) What are the substancial physical differences between these two laws? 2) Which values for the evaporation and boiling temperatures should be used? Thank you in advance for your patience Last edited by Large Epic Simulations; September 6, 2017 at 09:23.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
mass flow in is not equal to mass flow out	saii	CFX	12	March 19, 2018 05:21
Multiphase flow - incorrect velocity on inlet	Mike_Tom	CFX	6	September 29, 2016 01:27
Error - Solar absorber - Solar Thermal Radiation	MichaelK	CFX	12	September 1, 2016 05:15
Question about heat transfer coefficient setting for CFX	Anna Tian	CFX	1	June 16, 2013 06:28
Water subcooled boiling	Attesz	CFX	7	January 5, 2013 03:32

November 3, 2017, 12:34		#2
Vincenzo_ New Member Vincenzo Join Date: Sep 2017 Posts: 7 Rep Power: 8	Did you find your anwsers?

November 4, 2017, 01:51		#3
Large Epic Simulations Member Join Date: Jun 2017 Posts: 39 Rep Power: 8	Unfortunately not, but empirically, I've observed that law 2 (which corresponds to the d^2 Spalding law) produces a sort of discontinuity in the droplets quantities derivatives (diameter, temperature etc.). I would say that this kind of behavior is given by the evaporation temperature switch conditions: at the moment the particle reach this temperature, the evaporation rate term appears in the energy equation. As this term overcome the sensible exchange, the particle temperature fall-down in few timesteps. This leads to a non-constant temperature evaporation, which is likely to occurs during flash evaporation. I'm still wondering if this kind of behaviour is physically suitable. On the other side, the droplet boiling law (law 3) produces a constant temperature mass loss, which mitigates derivatives discontinuities and leads to smoother profiles. In conclusion my questions are still opened.