| OCR Text |
Show condition under which combustion became unstable. Fig.3 shows the proportion of dilution with nitrogen and the relationship between the diluted air temperature and the possible combustion range. Fig.4 shows direct photographs of flames at every point of oxygen mole fraction in diluted air and diluted air temperature in the same experiment. When the air temperature is higher than the self ignition temperature of the fuel, combustion occurs spontaneously if the fuel diffuses, is mixed and enters the possible combustion range. Similarly, if it is supposed that a self ignition temperature corresponding to the degree of dilution of air exists, stable combustion might be thought to occur without a flame stabilizing mechanism if the diluted air temperature is higher than this self ignition temperature. This observation reveals that compared with ordinary combustion, the combustion with high temperature diluted air is characterized by a remarkable increase in the flame volume, a decrease in the flame luminous emissivity, stable combustion in the downstream part distant from the fuel nozzle, namely lifted flame, etc. From these observation results, it might be judged that a flame which burns stably in a wide space is formed under the conditions of high temperature diluted air although the local heat generation rate is low, and conventional flame is a thermal concentrated flame which has local hot spot, but high temperature air combustion flame is a thermal dispersed flame which has flat leveled temperature distribution. NOx GENERATION, DATAS AND ANALYSIS The CO and NOx in combustion gas were measured under the same conditions as with the test in the stable combustion range. The result of this measurement is shown in Fig.5. Fig.5 shows we can keep 30-40ppm of NOx in the lower oxygen region over 1,000'C air temperature. From the consideration of 0 ppm line of CO, low NOx is not caused by i11 combustion but by or under complete combustion. It can be seen from the figure that a range in which the generation of CO and NOx can be simultaneously controlled by the conditions of high temperature and low oxygen concentration exists, which are the basic conditions for high temperature air combustion. BASIC EXPERIMENT Because the air temperature becomes as high as 800'c to 1,200'C in high temperature air combustion, it is expected that the flame structure and combustion mechanism cannot be explained from the present knowledge. It is necessary, therefore, to observe in detail the combustion process in which high temperature air is used without being influenced by conventional data. In order to consider the effect of mixing on the amount of NOx emission during high temperature air combustion, experiments were conducted by changing the intensity of recirculating currents formed by the air flowing into the combustion chamber and the injection position of the fuel supplied. The flow rate of combustion air was changed in the range from 1.5 LN/S to 4.0 LN/S at the four fuel injection positions shown in Fig.6 with the flow rate of injected fuel kept at 0.06 LN/S. Furthermore, the air temperature was changed in the range from 1,150'C to 800'C The range of change of overall equivalent ratio calculated from the supplied air volume and air volume is shown in Fig.6. The combustion gas at the outlet of combustion chamber was sampled using a water cooled gas sampling probe with a suction port of 0.5 mm and the NOx concentration was measured by an NOx analyzer (made by Best Sokki, BCL611A, atmospheric pressure chemiluminescence method). However, the measured NOx concentration is a dry concentration '• that is,samples do not include steam. The amount of NOx emission was indicated by an emission index(g/kg fuel) defined by grams of emission per kg of supplied fuel. - 3 - |
