I advice you to read UDF (user defined function) in Ansys-Fluent, here could reach good solutions here:

 http://jullio.pe.kr/fluent6.1/help/pdf/udf/fl61udf.pdf

This is an example of heat generation that grows lineary to 1,4 MW/m3 in 240 seconds. The maximum time is 2280 s. hope it helps.

#include "udf.h"

#include "prop.h"

float fmod(float a, float b) { return (a - ((int)(a/b))*b); }

real

sub_heat_release(cell_t cell, Thread *thread, real dS[], int eqn)

{

real x = RP_Get_Real("flow-time");

real t;

dS[eqn] = 0.0;

if (x < 0.000000e+000) {

t = 0.000000e+000;

} else if (x > 2.280000e+003) {

t = 0.000000e+000;

} else {

if (x < 2.400000e+002) t = x*5833;

else t = 1400000;

}

return (t);

}

First of all should be created relationship between heat generation and time (heat generation as function of time (H.G. =f (t)), then put this relationship in UDF as input boundary condition as shown in attach file.

I advice you to read UDF (user defined function) in Ansys-Fluent, here could reach good solutions here:

 http://jullio.pe.kr/fluent6.1/help/pdf/udf/fl61udf.pdf

A user-defined function, or UDF, is a function that you program that can be dynamically loaded with the FLUENT solver to enhance the standard features of the code. UDFs are written in the C programming language.