## Fluids

#### how can i use the expression function to achieve the function of PID or other algorithm.

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• lvxk
Subscriber

Hello teacher,

i want to achiece the function that given the temperature to solve the power of heater,i am not good at the programme,

the aim temperature of outlet is 230℃，I want to solve the heater power of this system useing the expression function，if you have a case，can u let me look look，i will express my sincere thanks for your attention.

• Rob
Ansys Employee
Is this a steady or transient flow system? The easiest method will be to calculate (on paper) the heat you need to add to achieve the desired temperature and then increase/decrease the heat source until it's reached. From there, an expression (or UDF) linking the heat input to the output temperature should work: the UDF is probably safer as you can control the heat flux more readily.
I don't have any examples, but if you look in the Tutorials (click on Help to access) you'll find source terms are used and you can then adjust the approach to suit your application.
• lvxk
Subscriber
it is a steady flow system´╝îbut I use the UDF with ADJUST Function to compute it ´╝îthe temperature is high but the heater power solved is low´╝îi can not find the reason´╝îthe aveT is actual temperature´╝îthe dTS is the adjusted power´╝îthe Tsource is donated to the source term´╝îI find when The heater power is low´╝îbut the Temperature is high ´╝îi write down the related term´╝îI do not know the reason why it has this phenomenon´╝îI appreciate it for your reply. and the other question is how can ues the "TEXT"to sheet the "Tsource" automaticly

• lvxk
Subscriber
the Tsource is write in THe UDF "MESSAge Function"
• Rob
Ansys Employee
It's solving in one iteration so I very much doubt the model is converged. Check the heat fluxes to see if they balance. Remember the source is per cubic metre so if the volume is small the value can be very high but not do much to the flow temperature.