| OCR Text |
Show BURNER DEVELOPMENT The basic design of the water-cooled liquid-fuel burners consisted of a fuel jet located within an adjustable fuel body sandwiched by two oxygen jets, located above and below the fuel body as shown in Fig.I. The reversed step separated the oxygen flows exiting from its nozzles and provided flame stabilisation in two recirculation zones formed on the top and bottom of the fuel jet. Pressure jet atomisation was used to form the fuel spray. Three nozzles having different spray angles of 25°, 40°, and 50°, were tested. The velocity of the oxygen jets was sufficient to provide some secondary atomisation. This nozzle produced a flat triangular sheet-like spray pattern which, coupled with the shear layer created between the liquid-fuel jet and each of the oxygen jets, resulted in a sheet of very finely atomised fuel droplets. The oxygen nozzles were designed to obtain smooth acceleration and resulted in an overlapping shape with the fuel spray. By moving the fuel body along the longitudinal direction with respect to the burner body, oxygen nozzle exit velocities could be changed smoothly. In this manner the flame characteristics could be altered from highly convective to more radiative. In this study a drop-size analyser, (Malvern 2600) was used to gather droplet sizing information under various operating conditions. A drop size range of 5.8 J.1m - 564 J.1m were measured using a 300 nun lens. The injector nozzles with spray angles of 25°, 40°, and 50° were used with water to characterise spray properties for different locations within the burner body. Tests were conducted to investigate the effect of oxygen on droplet mean diameter for different spray angles and different atomiser positions in~ide the burner. Water and air were used as a substitute for oil and oxygen with flows which simulate combustion conditions for 80 kW output. The data obtained was analysed in terms of histogram with 15 size classes and model - independent mode. Single jets were generated at flow rates of 0.13 litres/min at 298 K. Mean drop diameter and volume fractions were obtained with and without oxygen flow. Reproducibility tests showed that experimental measurements of mean drop diameters agreed within ± 8%. In addition, the effects of changing the degree of mixing of fuel with oxygen on the combustion performance were investigated. The liquid jet and its interaction with gas jets was analysed qualitatively using thermal imaging techniques. Finally, various prototype designs were tested to obtain the proper oxygen and fuel-jet configuration, flame-shaping characteristics and upper and lower oxygen canal cross-sectional ratios. CFD PREDICTIONS A 3-D numerical simulation was used to assess the potential of the burner. The flow-field predictions are obtained by solving the Reynolds-averaged conservation equations using the Simple algorithm for the pressurevelocity coupling. Turbulence closure was obtained using the k-E model. In this model spray combustion is described by simulating a two-phase flow with droplet size distribution data obtained from experimental measurements. Gas-phase combustion was represented using a four-step global reaction scheme: (1) CnH2n+2 + nH20 => nCO + (2n+l)H2 (2) (3) (4) The gas-oil fuel droplets were considered chemically reactive. The heat and mass transfer relationship taken into account includes (i) heating of droplets to a temperature less than vaporisation temperature (T vap)' (ii) droplet |
