A combined experimental and CFD modeling study of the turbulent non-premixed natural gas on a laboratory scale has been performed. Effect of solid surface enhancement in combustion chamber on the flame temperature and NO emission was investigated. The solid surface called as filling material (FM) was cylindrical and was placed coaxially in the center of combustion chamber. The temperature and NO distribution in the combustion chamber were compared for different geometries of the filling material. The diameters of the filling materials were 25 and 30 cm with two lengths of 20 and 40 cm. Experimental study has been carried out on a fire tube water heater. The flame temperature on the center line of the combustion chamber, gas temperature and NO emission in the combustion chamber were measured. The actual geometry of the fire tube water heater and the burner were modeled and then analyzed by the FLUENT code. Turbulent diffusion flames were investigated numerically using a finite volume method for the solution of the conservation and reaction equations governing the problem. The measured values were specified as the boundary conditions. The elemental analysis of the natural gas was taken as a mixture of hydrocarbon and air was the oxidizer. The standard k-epsilon, model was used for the modeling of the turbulence phenomena in the combustor. The non-premixed combustion model was chosen. In the conserved scalar approach, turbulence effects were accounted for with the help of an assumed shape probability density function or PDF. The discrete ordinates (DO) radiation model was used for modeling of the radiative heat transfer in the combustion room. The model results were compared with the experimental results. The model results were in good agreement with the measurements. The filling material provided the recirculation of the cooler gases into the flame. The recirculation reduced the oxygen concentration in the flame and controlled the flame temperature. It was found that the filling material with the diameter bigger than the flame diameter increased the heat transfer rate in the back flow around the flame. (C) 2009 Elsevier Ltd. All rights reserved.