Learning Forum

[annes]

Hi all, n I simulate the moisture loss of a pile of potatoes through which humid air flows. I simulate the situation via a transient 2D model with porous zone. In the solid part of the porous zone I defined a UDS (=moisture content of the potatoes in kg water/kg potato) in the solid part (=the potatoes) of the porous zone (=the pile of potatoes).nI defined a source term for the UDS (=moisture lost via evaporation) in the form of a UDF. In this UDF the source term is related to the temperature and h2o mass fraction in the fluid part (=humid air) of the porous zone. Th humid air is modeled via species transport.nI patch the UDS value in the solid part (0., the h2o mass fraction in the fluid part (=0.01065) and the temperature of the solid (288.15K) and fluid (286.15K) part of the porous zone. nWhen I do some hand calculations the value for the source term is realistic: per m³ potatoes 0.14g of water is lost per second. However, when I run the simulation the UDS value drops immediately to 0 after only 1e-4s. nCan anyone help me with this? nThanks in advance for your time! nAnne-Sophien

[Rob]

Why are you using a scalar and water vapour, or have I missed something? n

[annes]

I want to keep track of the moisture loss of the potatoes, which are the solid part of the porous zone. Therefor I defined a UDS that stands for the moisture content of the potatoes. I thought I couldn't do that with species transport. Is there a better way to do this? n

[Amine Ben Hadj Ali]

Are you solving for that UDS as transport equation: if no I guess a simple user memory is enough. And here you need to clearly describe the law used for evaporation of the moisture content and how you have formulated it. Moreover: are you accounting for the porosity in your source terms calculations?n

[annes]

Yes, I am using the UDF as a transport equation. I tried multiplying with 1-porosity (=volume fraction of solid/potatoes in the porous zone=1-0.36) but the drop in UDS value was the same. nWould it be better to use an evaporation-condensation model? nThis is my UDF for evaporation:nDEFINE_SOURCE(potato_UDS_evap_moisture,cell,thread,ds,eqn)n{n Domain *d=Get_Domain(1);n Thread *t1 = Lookup_Thread(d,IDfluid);n Thread *t2 = Lookup_Thread(d,IDsolid);n real densa=C_R(cell,t1);n real Ta=C_T(cell,t1);n real Tp=C_T(cell,t2);n real vel=sqrt((C_V(cell,t1)*C_V(cell,t1))+(C_U(cell,t1)*C_U(cell,t1)));n real aw=0.95-0.28*(0.8-C_UDSI(cell,t2,0));n real C1=0.0091E3;n real C2=-7.5129E3;n real C3=3875.1E3;n real hfg=C1*pow(Tp,2)+C2*Tp+C3;n real ht=alphac1*0.06+((alphac2*pow(vel,0.67))/pow(0.06,0.33));?kjn real hm=ht/(densa*cpa);n real molmass_mixture=(C_R(cell,t1)*R*Ta)/((C_P(cell,t1)+101325)/1000);n real molfr_h2o=C_YI(cell,t1,0)*(molmass_mixture/18.02);n real pw=molfr_h2o*((C_P(cell,t1)+101325)/1000);n real rhova=(pw*18.02)/(R*Ta);n real pwsp=exp((-5800.2206/Tp)-5.516256-(0.048640239*Tp)+(0.000041764768*pow(Tp,2))-(0.000000014452093*pow(Tp,3))+(6.5459673*log(Tp)));n real rhovp=aw*pwsp*(Mw/R*Tp);n real m=hm*Aspec*(rhovp-rhova);n real source=-m*(1-0.36);n return source;n}n

[annes]

Does anyone have an idea of what could be wrong?n

[Amine Ben Hadj Ali]

Just a comment: How sure are you about the linking between the two threads t1 and t1 if you are on cell thread t (DEFINE_SOUCE passed one): in other words it can be possible that the UDF won't work well especially in parallel where threads may be located on different partitions.nnI will avoid using UDS and resolving the solid part and stick to a porous media and work with UDM's to store absorption rates and mass sources. Another UDM can keep track of the water content which will then decrease or increase locally.n

[Amine Ben Hadj Ali]

A good paper: https://www.researchgate.net/publication/49591372_Modeling_and_Simulation_of_Fixed_Bed_Adsorption_Column_using_Integrated_CFD_Approachn

[annes]

Thanks Amine, I will look into that! n