| OCR Text |
Show 3 Introduction Radiative heat transfer during natural gas combustion can be influenced by a number of controllable parameters including furnace volume, flame temperature and concentration of combustion products such as C02 and H20. In addition to these parameters it has been shown that the presence of soot has a profound effect on the radiative behaviour of flames in industrial furnaces (1-2). Soot loading can significantly enhance radiative heat transfer resulting in decreased flame temperatures and reduced thermal NOx formation rates. Previous investigations (3 -4) have shown that parameters, such as parent fuel molecular structure flame temperature diluents of fuel or oxidiser and pressure, can influence the formation of soot in industrial flames. In some studies (5-6) the effect of fuel preheat on flame temperature and soot formation was studied. Schalla and McDonald (5) concluded that for variations of initial fuel temperature from 303K to 463K there was no effect on smoke point. However studies by Delichatsios and Orloff (6) showed that preheating fuel to temperatures above I273K - I500K can facilitate the formation of soot particles which contributes to significant increases in flame radiation. In this paper the temperature sensitivity of the fuel on soot and NOx formation are investigated using a vertically-oriented natural gas turbulent diffusion flame. Measurements of radiant fraction soot volume fraction and NOx emission indices were made for initial methane temperatures varying between 298K - 873K. Experimental Experimental measurements were performed on an o~.'ygen enriched methane diffusion flame at various degrees of methane preheat. In order to isolate the effects of fuel preheating from other factors, it was decided that the adoption of the experimental strategy of maintaining a constant burner nozzle exit velocity, u, was appropriate. This would have the effect of keeping the mixing time scales (Eqn.(1)) and the buoyancy forces, represented by the Froude number (Eqn.(2)) constant. [:] oc [~] (1) z/ Fr=gd (2) The actual value of u of 70 m/s was chosen so that the exit nozzle flow would remain in the turbulent regime, and the flame would remain stable, for preheat temperatures ranging from 300 to 600 degrees centigrade. Experimental Facility The burner employed in the experimental work briefly consisted of a heated central burner tube mounted within an outer low turbulence shroud (Fig. 1). The stainless steel burner tube is 1/8 inch OD with a wall thickness of 0.006 inches and a length of 18 inches. The burner tube is directly electrically heated with contact points at the base and at the burner tip. By insulating the burner tip contact (OD 0.27 inches) from the rest of the burner assembly via burner tube guides made out of machineable ceramic and quartz tubing, current was made to flow only through the burner tube causing it to act as the heating element for the methane. By employing this direct electrical heating technique methane preheat temperatures upto 900 K have been achieved. This whole assembly is centrally mounted in a shroud which, via a porous plate a honey comb and sets of fine wire meshes yields a smooth outer flow. This low turbulence shroud was fed with compressed air and cylinder oxygen to achieve the desired oxygen enrichment level. The burner was mounted on an x)'z traverse so that different locations in the flame may be subject to measurement without disturbing the optical systems. |
