I don't think you need a UDF. This is my understanding to your case .. correct me if I am wrong. You have a small cylinder "heater" heating a larger tank.You have a heat generation inside the heater only.All the system starts from the room temperatureYou want to turn the heat generation off, when the heater wall temperature reaches 90 CIf this is the case, you don't need UDFIf the volume does not exist, this means you are not simulating solids ... Can you show the geometry and a screen shot of the boundary conditions?https://forum.ansys.com/discussion/comment/112672#Comment_112672

What you described is correct. nI have a tank with a diameter of 50 cm and length of 40 cm. nThe heater is a cylinder with diameter of 5 cm and length of 13 cm. nI need to heat the stationary water in the tank using the electric heater (heat generation). when the temperature of water reaches 90C the heater should turn off. nIn the attached file, you can see the geometry and the boundary conditions.nThe walls are insulated with heat flux 0. In the cell boundary condition, I set a source term a constant value 18500 W/m3.nIf you want I send you the case by email. nThank you for your help. nArrayn