UDF for linear temperature gradient
 

Hi
I am trying to apply a UDF for linear temperature gradient across a surface. The left wall of the surface is at 700K, while the right wall is at 300K. Length of surface is 10mm.
The UDF goes this way:
#include "udf.h"

DEFINE_PROFILE(temperature_profile, thread, position)
{
real x[ND_ND]; /* this will hold the position vector */
real y;
face_t f;

begin_f_loop(f, thread)
{
F_CENTROID(x, f, thread);
y = x[0];
F_PROFILE(f, thread, position) = 500-40*y;
}
end_f_loop(f, thread)
}
However, this temperature profile doesn't show up in the results.
Can someone please send me a correct UDF for this?