First, go expression under setup .. right click --- newnnwrite the name as T_avnin the expression write Average(Temperature, ['heater_wall'], Weight = 'Area) and save the expressionnWhen Defining the heat source, click on the arrow on the right and select expression and write IF(T_av < 90 , 18500, 0) and save the expressioow, Fluent will calculate the average heater_wall temperature and if it is below 90, the heat generation will be 18500. If the temperature goes above 90, the heat generation will be 0n