i'm going to analyse one heat transfer problem in heat exchanger by using fluent with more accuracy mode. so for what are all enable while iterating?

For heat transfer, I suggest you always 1) use the double precision solver, and 2) make sure the energy under-relaxation factor is set to exactly 1.0 (especially if analyzing a conjugate heat transfer problem). In prior years, I have seen an invalid conjugate result with energy under-relaxation as high as 0.98, which quickly altered and corrected itself when energy under relaxation was changed from 0.98 to 1.0. Hope this helps.