It is known that introduction of the gas phase into the liquid flow leads to flow restructuring and can significantly change the key thermal-hydraulic flow parameters, for example, the heat transfer coefficient. This paper presents the results of an experimental study of an ascending bubble flow in a vertical assembly of 3×3 rods. Distilled water was used as a working liquid. The experiment was carried out at Reynolds numbers from 4000 to 11000 and volumetric gas flow rate ratio from 3 to 10%. In the experiment, the coefficients of heat transfer from the central rod heated by electric current to the flow were determined and it was shown that gas inclusions have the maximal effect on heat transfer at low Reynolds numbers and maximal distance from the spacer grid.