# i am loading image of solar still - trying to figure out how to find certain b.c

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March 13, 2012, 07:26
i am loading image of solar still - trying to figure out how to find certain b.c
#1
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amrik singh
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i have to fill the rectangular basin with water and gas mixture(air + water vapour) 0.20 and 0.80 volume fraction of water and mixture .... and maintain water up to height 0.20 mm and as evaporation takes place so i have to maintain the level of water .so plz explain me how this would be taken
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 March 13, 2012, 17:37 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 You will have to explain what you are doing better first. Is the bottom of the domain filled with liquid water, which then evaporates into air/water vapour in the top section? What do the little pipes I see do?

March 14, 2012, 07:04
explanation
#3
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amrik singh
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on the top of the glass radiation falls and absorbed by the water in the basin at the bottom .water temperature increases and water starts evaporating their is temperature difference between glass and bottom so water condenses on the glass and slowly drops move down and collected in a trace not visible in figure and taken out through outlet pipe as shown in figure .and water enters in the basin through inlet as shown now first of all i have to fill water in the basin up to some height say it 30cm just below the smallest wall .and to make an expression so that after evaporation(water level decreases) water level should maintained up to 30 cm on account of pressure how should i do this...
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 March 14, 2012, 07:10 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 This is a very complex simulation. You have radiation, free surface multiphase, evaporation, condensation, thin film flows.... Do you have experience in modelling complex CFD models like this? If not then the place to start is to get each of the physics "modules" working independantly and matching a benchmark/analytical solution before combining them all together. I hope you have some time available as this is a lengthy process.

 March 14, 2012, 07:20 ok thanks #5 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 i have completed all the tutorial the only problem is that how to fill it with water .......shold i go for fluent or cfx which one is more better

 March 14, 2012, 17:16 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 CFX and Fluent cna both do this sort of simulation. Use which ever one you prefer.

 March 28, 2012, 04:59 hey friends #7 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 in the above problem of solar still(cfx) when i was defining the domain with two material (2 phase) 1 gas mixture 2water liquid then i am unable to turn on the radiation model can any body tell how should i tackle this problem as radiation from the top of glass is required when i am considering only single material(single phase ) then radiation option is available

 March 29, 2012, 05:40 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 Just because there is radiative heating does not mean you need a radiation model. If all the radiation does is to heat the walls then it is probably easier to just apply a heat source on the walls. If the radiation heats the fluid in a simple fashion you might be able to apply a simple volumetric heat source. How does the radiation heat the system?

 March 30, 2012, 00:27 thanks mr #9 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 i just want to know one more thing how should i prepare material for mixture of vapour +air i am unable to kindly explain some steps i am unable to find it in ipaws library also

 March 30, 2012, 05:00 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 Do the CFX tutorials on multiphase flow to see how to set this up. If you do not know how to do multiphase flow yet then forget about the IAPWS libraries for now. Wait until you understand multiphase flow a bit before adding that complexity.

 March 31, 2012, 07:15 hi #11 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 how to apply simply volumetric heat source to fluid not to wall iam unable to and one more thing for evaporation in vof model is it necessary to import evaporation_drop ccl or not how to transfer mass in vof model

 May 6, 2012, 04:16 hi #12 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 as shown in figure the domain consist of two fluid gas mixture (air + water vapour) and liquid water the top of the domain temperature is fixed and at the bottom i have to apply 40 degree celcius i have to run the simulaion for 10 minute . if i am running for 2 seconds it converges but if i give time step of ten minutes it give error gas mixture temperature is out of range and solver stops, so can any body help me how to divide 10 minutes and what time step shold i give and what time intervel and i have to create subdomain and to write continuity equation for gas phase gradient.(RgPgVg)+Slg=0 Rg = volume fraction of gas Pg density of gas Vg=velocity vector similarly for water suffix l is used for liquid liquid phase gradient.(RlPlVl)+Sgl=0 can any body tell me how to write

 May 6, 2012, 04:28 #13 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 if i give total time 2 sec and time step 0.025sec and time intervel 0.1 sec then it converges but i have to give the total time 10 minute how shold i run the problem for 10 minute can any one explain me what time step shold i take and what time intervel also i have given total time 10 min time intervel 0.5 min and time step =0.125 minthe solver stop running

 May 6, 2012, 04:47 #14 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 if i give total time 2 sec and time step 0.025sec and time intervel 0.1 sec then it converges but i have to give the total time 10 minute how shold i run the problem for 10 minute can any one explain me what time step shold i take and what time intervel also i have given total time 10 min time intervel 0.5 min and time step =0.125 minthe solver stop running

 May 6, 2012, 06:40 #15 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 Use adaptive time steps, converging on 3-5 (or maybe 5-8 for multiphase flows) coeff loops per iteration. Then it will find its own time step size and grow and shrink as required.

 May 6, 2012, 06:43 #16 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 how to use adaptive time step i am new in cfx can u explain some steps

 May 7, 2012, 01:02 #17 New Member   amrik singh Join Date: Feb 2012 Posts: 16 Rep Power: 7 as shown in figure the domain consist of two fluid gas mixture (air + water vapour) and liquid water the top of the domain temperature is fixed and at the bottom i have to apply 40 degree celcius i have to run the simulaion for 10 minute . if i am running for 2 seconds it converges but if i give time step of ten minutes it give error gas mixture temperature is out of range and solver stops, so can any body help me how to divide 10 minutes and what time step shold i give and what time intervel and i have to create subdomain and to write continuity equation for gas phase gradient.(RgPgVg)+Slg=0 Rg = volume fraction of gas Pg density of gas Vg=velocity vector similarly for water suffix l is used for liquid liquid phase gradient.(RlPlVl)+Sgl=0 can any body tell me how to write

 May 7, 2012, 07:04 #18 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 Well clearly 2s time step size is more appropriate than 10 minutes. How long do you want to run this for? I have no idea what your second paragraph is talking about.

May 7, 2012, 08:08
#19
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amrik singh
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as shown in figure domain have 2 fluid, gas mixture and liquid water. water volume fraction is 0.20 and gas mixture fraction is 0.80
1.now at the top named condensing cover temperature is fixed, say 30 degree celcius
2.bottom i have to apply temperature, say 40 degree celcius
now due to this temperature difference and due to action of buoyancy the water start evaporating
so i have to calculate the rate of evaporation in 10 minutes.
or we can say that how much water is collected in 10 minutes
i have to go for quasi steady state process
so please tell me how should i do this problem whether go for transient or steady state
if i have to solve this prblem in transient then please tell me how to give time step and total time
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 May 7, 2012, 17:25 #20 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,808 Rep Power: 107 Why are you doing the analysis? What are you trying to learn from the simulation? Your questions are very basic. If you do not know the answers to these questions yourself then I do not hold much hope that you can do what will be a complex multiphase simulation with phase change.

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