Abstract
The purpose of this research programme was to investigate the mode of combustion of Low Pressure Air Atomised Burners since published information on this type of burner is almost non existent. For the necessary experimental work a combustion rig capable of burning 100 litres/hr. of oil was constructed. The rig is a small scale version of those which exist at the International Flame Research Foundation IJmuiden, Holland, except that the combustion space is cylindrical and not rectangular. The combustion chamber's dimensions are 0.61 m. in internal diameter and 3.1 m. in length, with 0.155 m. thick refractory walls. The extent of combustion unmixedness and flame length was determined by means of gas analysis probes. The temperature and emissivity of the flame at six points was measured by a twin-beam total radiation pyrometer. The degree of oil atomisation was estimated using a specially developed liquid nitrogen probe. Such fundamental measurements,plus the more conventional operating measurements, on different oil flames enabled the length of any particular flame and the radiation along it to be measured, for any set of fuel/air input conditions. Using a Stordy-Hauck Type 783 LPA burner, five fuels namely, gas oil, kerosine, 950-sec. fuel oil, 3500-sec. fuel oil and CTF-50 were burnt. For the gas oil experiments, five different diameter nozzles were employed. The minimum excess airs which could be achieved were 3% for every fuel except CTF-50, where it was 5%. In every case all the combustion air as well as the fuel were introduced through the same nozzle and therefore no 'secondary air' existed as such. It is worth pointing out, however, that the orifice design was very complex, resulting in part of the air issuing with the fuel from a central core whilst the remaining air issued through a concentric nozzle around this core. Both streams then mix and eject through an orifice, the diameter of which is assumed to be the 'nozzle diameter'. The present analysis consists of two approaches. Firstly, a straightforward empirical regression analysis based on the observation that the flame length varied linearly with jet momentum, excess air nozzle diameter and the carbon hydrogen ratio of the fuel. This results in the general correlation. The second approach to the problem is by using the theoretical analysis of Bragg which assumes that the flame length is governed, purely, by the reaction rate of the fuel and air. This results in the correlation. [mathematical equations].