Obtain boundary face values for a cell
 

I have read the UDF manual multiple times and also searched the forum, but could not find a solution.

For a 2D domain (rectangular), for each cell inside the domain (away from the walls) I want to obtain the temperature of the bottom wall at their corresponding cell x-location. Is there a way to do this?

What I have currently done is to save the first-layer cell centroid temperature values and the temperature gradients to calculate the wall temperatures. (But for each cell, to obtain appropriate wall temperature, it would have to iterate through all the saved values of wall temperature to obtain correct x-location, thus very time consuming).

Probably not, because Fluent does not know the concept "bottom face", Fluent can work with unstructured meshes, so you have to tell Fluent which walls are bottom, and you can only do that by looping through all of them at least once.

And this smells like a XY question. You are asking X, but actually your goal is Y. Why do you want to see the temperature at the bottom boundary of each cell? What do you want to do with that information?

My apologies, probably I wasn't very clear in the description.

Essentially I would want to obtain the temperature at the bottom wall of the domain (not of each cell) at the corresponding x-location of each cell. Its for a frost model with a criteria depending on the value of the cold plate (bottom wall) inside a channel. So when calculating the mass transfer rate (from define_mass_trasnfer) for a cell, it needs a wall/plate temperature at its x-location on the wall. (Its not a fixed temperature plate and the temperature varies along the plate)