Learning Forum

[pc_singh]

nI am getting WARNING: Invalid cp (-2.235390e+03 J/kgK) for gadolinium at temperature 1.000000 K error. I have compiled specific heat udf. nCan someone correct the udf below? Please Suggest Changes Required.nDEFINE_SPECIFIC_HEAT(CP, T, Tref, h, yi)n{ntreal CP_0, CP_1, CP, CP_dot5, B, Time;nt/*Setting up the magnetic field*/ntTime = CURRENT_TIME;ntfor(;n    {n    tif(Time<=4.2)n      ttbreak;n    tTime=Time-4.2;n  t}n            if(Time<=0.1)n    tttB=7*Time+0.3;n  tttif(Time<=2.1 && Time>0.1)n    tttB=1;n  tttif(Time>2.1 && Time<=2.2)n    tttB=-7*Time+15.7;n  tttif(Time>2.2)n    tttB=0.3;tn           /*Calculating CP*/n            if(T<=288.60 )n                    CP_0=0.017275*T*T-7.201999*T+984.900000004;nt            elsent            {nt            if(T<=292.4866)nt            tCP_0=-27.98559*T+8431.999941;nt            elsent            tCP_0=0.02421075*T*T-16.2260213*T+2916.68679;n             t}n             if(T<=287.0577)n                    CP_dot5=54.7513501*exp(0.00615364381*T);nt            elsent            {nt         if(T<=292.4247)nt            tCP_dot5=0.14988*T*T-94.2584*T+15031.3045;nt            elsent            tCP_dot5=0.150563*T*T-94.47235*T+15032.60325;n             t}tn             if(T<=287.4382)n                    CP_1=-0.0408*T*T+23.65002936*T-3125.9999;nt            elsent            {nt         if(T<=298.1055)nt            tCP_1=-0.1694534*T*T+96.069998*T-13314;nt            elsent            tCP_1=0.1209901*T*T-76.7138035*T+12379.99999;n             t}tn             if(B<=0.5)n             t    CP=CP_0+(CP_dot5-CP_0)*(B/0.5);n             telsen             t    CP=CP_dot5+(CP_1-CP_dot5)*((B-0.5)/0.5);n             t Message(The value of CP is %g
,CP);nn                *h = CP*(T-Tref);nt            return CP;tn}    n

[Amine Ben Hadj Ali]

Have you checked the values of Cp outside Fluent?nYou can first help the solver by limiting the temperature for the first iteration to a reasonable narrow interval.n

[pc_singh]

Thank you for your kind reply.nHow can I limit the temperature for the first iteration?n

[Rob]

Solution - Controls - Limits nI'd set the minimum temperature to be 20K or so below what you're expecting to get it going. Do NOT consider the model to be converged if you're bouncing off the limiter! n

[pc_singh]

Sir, still getting same error.nnWARNING: Invalid cp (-8.863752e+00 J/kgK) for gadolinium at temperature 20.000000 K

[Rob]

If you put 20K into the cp formula what do you get? I also suggested 20K BELOW your expected minimum temperature, not 20K temperature. n

[pc_singh]

sir I have checked it on 280K and 290K too, but got same error.nBy manual calculation, I am getting positive value at 20K.n

[Amine Ben Hadj Ali]

Further details required about the case: mixture properties or single component flow? Are you seeing the error after initialization? (Please do not attach pictures just include them in the answer)n

[pc_singh]

Case scenario : Flow of water between parallel plates of Gadolinium( Gd). The specific heat of Gd is property of both the temperature and magnetic field. No sir, I am seeing this error during simulation.n

[Rob]

In Excel plot the whole temperature range to see what's going on.

[pc_singh]

after simulation?n

[Rob]

No, plot the relationship you've put into the UDF. Whenever you need 9+ significant figures in a relationship there's a good chance something will go wrong. n

[pc_singh]

Ok sir.n