The udf of specific heat can load but leading the case divergence

Kobe Peng · July 7, 2018, 12:21

Hello, everyone, i'm Kobe Peng.

I'm sorry that my English is not very well, i try to describe my question clearly.

I'm running a transcritical heat transfer case in which the transcritical fluid flows through a tube then it is heated.

I need to define the property of specific heat, and i have written, compiled and loaded the udf of cp but the case diverged after hundreds of interations.

Then I tried defining the specific by piecewise linear in FLUENT, and it did work!

I have checked the udf and the piecewise polynomial in it didn't have mistakes.

So, I am confused why once i use the udf then the case diverges.

If u guys know why, please tell me!

Thank you so much!

Kobe Peng · July 7, 2018, 12:27

Here is the udf
#include "udf.h"
#include "math.h"

/*cp*/
DEFINE_SPECIFIC_HEAT(ac_spec,T,Tref,h,yi)
{
real spec;
{
if(T>=300)
spec = 265124.8831155-4464.82375371177*T+32.9231274086636*pow(T,2)-0.136039343632479*pow(T,3)+0.000339294051264547*po w(T,4)-5.09716373722217e-07*pow(T,5)+4.26496354634277e-10*pow(T,6)-1.53193177603194e-13*pow(T,7);
else if(T>=230&&T<300)
spec = 90820232.6991246-2323424.32743038*T+25493.1862659987*pow(T,2)-155.469810184518*pow(T,3)+0.569025894531809*pow(T, 4)-0.00124972693433267*pow(T,5)+1.52481826353193e-06*pow(T,6)-7.97246697067391e-10*pow(T,7);
else if(T>=208&&T<230)
spec = 61783405144.5027-2134905306.97295*T+31366437.2097949*pow(T,2)-254307.268382587*pow(T,3)+1229.96834217446*pow(T,4 )-3.55139768453823*pow(T,5)+0.00567176410739463*pow( T,6)-3.86692837907887e-06*pow(T,7);
else if(T>=185&&T<208)
spec = -107434137534.901+3791580172.91008*T-57290594.522828*pow(T,2)+480412.690572312*pow(T,3)-2414.4280687518*pow(T,4)+7.27206290466646*pow(T,5)-0.0121532810731963*pow(T,6)+8.69337002128947e-06*pow(T,7);
else if(T>=100&&T<185)
spec = -225762.297493954+12310.0037428643*T-282.185198740609*pow(T,2)+3.576418647329*pow(T,3)-0.0270693354002758*pow(T,4)+0.000122408921466647*p ow(T,5)-3.06380049304191e-07*pow(T,6)+3.2776859907096e-10*pow(T,7);
else
spec = 4000;
}
*h=spec*(T-Tref);
return spec;
}

`e` · July 8, 2018, 19:11

First, there's a couple of spaces in your code such as "po w" on line 10 and "p ow" on line 18. Next, why have you written the coefficients with such a high precision? Perhaps rewrite with scientific notation with ~6 significant figures, e.g. 2.65124e5 for the first number. The nested if statements do not require the upper bound (e.g. T < 300) because of the preceding if condition (e.g. T >= 300). Then, have you checked with a plot that these equations are providing the expected heat capacity as a function of temperature, and is this relation a continuous function? Lastly, are the units from your equations in Kelvin or degrees Celsius? Fluent uses Kelvin for the units of temperature.

July 7, 2018, 12:21	The udf of specific heat can load but leading the case divergence	#1
Kobe Peng New Member Join Date: Jul 2018 Posts: 2 Rep Power: 0	Hello, everyone, i'm Kobe Peng. I'm sorry that my English is not very well, i try to describe my question clearly. I'm running a transcritical heat transfer case in which the transcritical fluid flows through a tube then it is heated. I need to define the property of specific heat, and i have written, compiled and loaded the udf of cp but the case diverged after hundreds of interations. Then I tried defining the specific by piecewise linear in FLUENT, and it did work! I have checked the udf and the piecewise polynomial in it didn't have mistakes. So, I am confused why once i use the udf then the case diverges. If u guys know why, please tell me! Thank you so much!

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July 7, 2018, 12:27		#2
Kobe Peng New Member Join Date: Jul 2018 Posts: 2 Rep Power: 0	Here is the udf #include "udf.h" #include "math.h" /cp/ DEFINE_SPECIFIC_HEAT(ac_spec,T,Tref,h,yi) { real spec; { if(T>=300) spec = 265124.8831155-4464.82375371177T+32.9231274086636pow(T,2)-0.136039343632479pow(T,3)+0.000339294051264547po w(T,4)-5.09716373722217e-07pow(T,5)+4.26496354634277e-10pow(T,6)-1.53193177603194e-13pow(T,7); else if(T>=230&&T<300) spec = 90820232.6991246-2323424.32743038T+25493.1862659987pow(T,2)-155.469810184518pow(T,3)+0.569025894531809pow(T, 4)-0.00124972693433267pow(T,5)+1.52481826353193e-06pow(T,6)-7.97246697067391e-10pow(T,7); else if(T>=208&&T<230) spec = 61783405144.5027-2134905306.97295T+31366437.2097949pow(T,2)-254307.268382587pow(T,3)+1229.96834217446pow(T,4 )-3.55139768453823pow(T,5)+0.00567176410739463pow( T,6)-3.86692837907887e-06pow(T,7); else if(T>=185&&T<208) spec = -107434137534.901+3791580172.91008T-57290594.522828pow(T,2)+480412.690572312pow(T,3)-2414.4280687518pow(T,4)+7.27206290466646pow(T,5)-0.0121532810731963pow(T,6)+8.69337002128947e-06pow(T,7); else if(T>=100&&T<185) spec = -225762.297493954+12310.0037428643T-282.185198740609pow(T,2)+3.576418647329pow(T,3)-0.0270693354002758pow(T,4)+0.000122408921466647p ow(T,5)-3.06380049304191e-07pow(T,6)+3.2776859907096e-10pow(T,7); else spec = 4000; } h=spec*(T-Tref); return spec; }

July 8, 2018, 19:11		#3
`e` Senior Member Join Date: Mar 2015 Posts: 892 Rep Power: 18	First, there's a couple of spaces in your code such as "po w" on line 10 and "p ow" on line 18. Next, why have you written the coefficients with such a high precision? Perhaps rewrite with scientific notation with ~6 significant figures, e.g. 2.65124e5 for the first number. The nested if statements do not require the upper bound (e.g. T < 300) because of the preceding if condition (e.g. T >= 300). Then, have you checked with a plot that these equations are providing the expected heat capacity as a function of temperature, and is this relation a continuous function? Lastly, are the units from your equations in Kelvin or degrees Celsius? Fluent uses Kelvin for the units of temperature.