Effect of fuel volatility on spray combustion

Nineteenth Symposium (International) on Combustion/The Combustion Institute, 1982/pp. 511-518

E F F E C T OF F U E L VOLATILITY ON SPRAY C O M B U S T I O N THARWAT M. FARAG, MASATAKA ARAI AND

HIRO HIROYASU

Department of Mechanical Engineering University of Hiroshima Shitami, Saijo-cho, Higashi-Hiroshima, Japan The effect of fuel volatility on the lean blow-off limit and the temperature distribution in a swirl type combustor was studied. The flame stabilizing mechanism was also discussed. The fuels used were gasoline, kerosene and two types of heavy oils. Gasoline and heavy oil were used as the most and the least volatile fuels respectively. For all fuels used, the blow-off limit shifts to the lean side by increasing the swirl number of the flow, i.e. the flame stability increases. With a weak swirl, the fuel volatility has the higher effect on the blow-off limit than with the strong one where the established recirculation zone has the higher effect on the flame stability. In the absence of a swirl, only one main reaction zone is formed inside the flame around its axis. With a strong swirl, two main reaction zones are formed for kerosene and heavy oil flames, but only one reaction zone for a gasoline flame.

1. Introduction

times as an approach to study the lean blow-off of a bluff-body stabilized flame, and also obtained semi-empirical correlations for ignition, flame stabilization and emissions. However, in previous works, the liquid fuel was usually kerosene or a jet fuel. Therefore, at the present time, our understanding of the differences between high and low quality fuel combustion flames is insufficient to permit profitable conversion from high to low quality fuel. In this study, the lean blow-off limit, the combustion characteristics and the temperature distribution were measured for four different fuel qualities, of which gasoline and heavy oil were the most and the least volatile fuels, respectively. Since all the fuel sprays issuing from an air blast atomizer had the same injection momentum and burned in the same combustor, the effect of fuel volatility could be studied. Also, the effect of swirl on flame stabilization was assessed by measuring the sizes of the recirculation zones during combustion.

The relatively high cost of high quality fuels continues to affect the fuel economy of combustion devices. The growth rate of consumption for high quality fossil fuel can be reduced appreciably if some combustion devices use heavy oil and other residual oils of low quality. However, conversion from high to low quality fuel cannot be accomplished without regard for combustion stability and pollutant emissions. In burning a low quality fossil fuel, such as heavy oil, visbreaking oil and FCC process oil, especially in a small furnace, it is very important to realize the stable and high combustion efficiency. To achieve wide use of a low quality fossil fuel, it is necessary to examine the extent to which the flames are stable, i.e., do not blow off, in a small furnace. Recirculating flow is commonly used to increase flame stability and combustion intensity in conventional combustion devices. The effect of this recirculating flow on the stabilization of premixed gaseous flames has been discussed by many workers. 1)2)3) This same effect on spray flames, however, has been discussed by few workers, because there is little information about the aerodynamic interaction with the liquid spray and the swirling flow, especially in the recirculation zone. N. A. Chigier4/ et al. measured flight angles, velocities and diameters of burning droplets within a hollow-cone pressure jet spray flame stabilized by a disc. A. M. Mellor5)6~ used the characteristic

2. Experimental Apparatus The schematic diagram of the apparatus is shown in Fig. 1. This apparatus had three separate fuel lines for gasoline, kerosene and two types of heavy oil. Since the viscosity of heavy oil is very high at room temperature, the heavy oil line was heated to and kept at 90~ C by electric heaters. The combustor consisted of an air blast atomizer, a swirler, 511

514

TURBULENT COMBUSTION MEASUREMENTS II

Two main reaction zones were observed in this figure as the high temperature zone of the isothermal lines. The primary reaction zone was formed near the swirler and on the periphery of the recirculation zone where the fuel vapor, the small droplets of the spray, the recirculated hot gases and the fresh air mixed with one another. The secondary reaction zone was formed at a greater axial distance where the spray and the main flow of the combustion air came together. Figure 3(b) shows the results just before the occurrence of blow-off where the flame was unstable. In this case, the reaction zones mentioned above did not appear, and only one zone with a high temperature appeared in the forward flow zone near the atomizer, The size of the recireulation zone near the lean blow-off condition was almost similar to that of the cold state, because the total heat release in the combustion chamber was very small, and the flame was very short and small.

100

S=1.50 Mom. Flux=0.427 N 75

~ 50 o~ o~ tt.

2s

I 0

The effect of fuel volatility on the lean blow-off limit at swirl numbers S = 0 and 1.50, for example, are shown in Figs. 4 and 5 respectively. Increasing the swirl number for all fuels, decreased the fuel flow rates of the lean blow-off limit. In the absence of a swirl that decreased fuel volatility ( S = 0), the fuel flow rates of the lean blow-off

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FIG. 4. The lean blow-off limit at S = 0.

] Heavy Oil Gasoline ~Kerosene ~ Heavy Oil ~-C

3.2. Effect of Fuel Volatility on the Lean Blow-

Off Limit

I

1.0 2.0 Fuel Flow Rate Ivlf

3.0 g/s

FIG. 5. The lean blow-off limit at S = 1.50. limit increased, i.e. the flame stabilities decreased. With strong swirl, S = 1.50, where the recirculation zone was stable, the lean blow-off limit was slightly affected by fuel volatility. The fact that combustion region of kerosene was wider than that of gasoline, as shown in Fig. 5, is interesting. This is because, in the case of the heavy oil flame, combustion was heterogeneous due to the low evaporation rate, whereas the gasoline flame was homogeneous due to the high evaporation rate. Thus, the lean blow-off limit of the gasoline flame was more like a premixed gaseous flame than the kerosene or the heavy oil flame. In addition, the combustion range of a premixed flame is narrower than that of a diffusion flame. With strong swirl, S = 1.5, the combustion region of the C heavy oil was the narrowest for these fuels. It seems reasonable that the Sauter mean diameter of its spray is much larger than that of the other fuels, and, also, that its evaporation rate is lower. F o r example, at the fuel flow rate of My = 2.0 g/s, the designed Sauter mean diameter of the C heavy oil spray was 210 ~m, but that of the others was 80 Ixm. So it can be considered that this difference in the value of the Sauter mean diameter affected the lean blow-off limit so much. From Figs. 4 and 5, and the experimental results for the other swirlers, the following results were obtained. The lean blow-off limits of all fuels decreased when the swirl was increased. For weak swirls, S = 0 and 0.23, the fuel volatility had the

EFFECT OF FUEL VOLATILITY ON SPRAY COMBUSTION predominant effect on the lean blow-off limit, but for strong swirl, S = 0.50, 0.87 and 1.50, the fuel volatility had only a slight effect on the lean blowoff limit, and the recirculation zone established by the swirling flow had a large effect.

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3.3. Effect of Fuel Volatility on the Isothermal

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Lines and the Axial Temperature Gradient The isothermal lines of the gasoline, kerosene and heavy oil flames at S = 0 and the same burning conditions are shown in Fig. 6. Only one main reaction zone was formed downstream and around the flame axis. For the flame to be stable in the absence of the recirculation zone, the spray receives heat directly from the main reaction zone, and this heat must be enough for heating, evaporation and ignition. Therefore, a high axial temperature gradient is necessary to sustain a stable flame. Figure 7 shows the temperature gradient along the axis of the flame of Fig. 6. The temperature gradient along the flame axis was affected by many factors, e.g. the local heat release, the flow direction, the gas velocity and the heat flux due to convection and radiation. However, the temperature gradient along the axis can be considered to correspond mainly to the local heat release as long as the flow direction was forward. In this figure, because the burning conditions of the flows and the sprays were the same, the difference between the

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516

TURBULENT COMBUSTION MEASUREMENTS II

three axial temperature gradients was due to the different volatilities of the fuels. The axial temperature gradient of the gasoline flame had two peaks. The first peak appeared before the beginning of combustion; the second was in the mixing zone of the main part of the spray and the combustion air, i.e. the main combustion zone. Due to conduction and radiation, the axial temperature increased and ignition occurred at the axial distance of 200 - 250 mm. Because of the high volatility of gasoline, it evaporated quickly, but the mixing could not overtake the evaporation. Accordingly, the burning rate was slower; then the temperature increased once again in the main combustion zone. The axial temperature gradient distribution of the kerosene flame was similar to that of the gasoline flame, but the first peak only shifted downstream. The axial temperature gradient of the heavy oil flame had only one peak in the mixing zone of the spray and fresh air; this is due to the low evaporation rate of heavy oil. Figure 8 shows the isothermal lines atS = 0.50. In this case the kerosene and heavy oil flames had the two reaction zones, but the gasoline flame had only one reaction zone, because at S = 0.50 the mixing rate in the high-temperature recirculation zone was high enough to burn the evaporated fuel, and the evaporation rate of gasoline was also high enough to complete evaporation. The second reaction zones that appeared in the kerosene and heavy oil flames were due to the lower evaporation

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EFFECT OF FUEL VOLATILITY ON SPRAY COMBUSTION rates of these fuels. The axial temperature gradients of these flames are shown in Fig. 9. Since the flames in this condition were mainly controlled by the swirling flow, the locations of the peaks of these axial temperature gradients appear at the same axial position where the flow direction was forward as in Fig. 3. The effect of fuel volatility on the axial temperature gradient was related to the height of the peak. The height of the peak of the gasoline flame was twice the height of the heavy oil flame. 4. Conclusions

The effects of fuel volatility and swirl on flames were studied by measuring the lean off limits and the temperature distributions. the results, the following conclusions have reached:

spray blowFrom been

(1) The fuel flow rate of the lean blow-off limits of all fuels decreases with an increase in the swirl. (2) With weak swirls, S = 0 and 0.23, fuel volatility has the predominant effect on the lean blowoff limit, but with the strong swirls, S = 0.50, 0.87 and 1.50, it has only a slight effect on the lean blow-off limit. (3) In the absence of the recirculation zone, only one reaction zone is formed. The two peaks of the axial temperature gradient appear in the gasoline and kerosene flames, but only one peak appears in the heavy oil flame. The peak position of the axial temperature gradient shifts downstream with decreasing fuel volatility. (4) In the presence of the recirculation zone, two main reaction zones appear in the kerosene and heavy oil flames, but only one reaction zone, i.e., the primary reaction zone appears in the gasoline flame.

517

Acknowledgments The authors wish to thank K. Fujii and M. Tabata for their cooperation on this work. The authors also acknowledge the financial support of the Grantin-Aid Scientific Research of the Ministry of Education, Japan.

REFERENCES 1. BAFUWA, G. C. AND MACCALLUM, N. R. L.: European Symposium on Combustion, p. 565, The Combustion Institute, 1973. 2. KREMER, H., MINX, E. AND RAWE, R.: European Symposium on Combustion, p. 536, The Combustion Institute, 1973. 3. MANHEIMER-TIMNAT,Y., SEGAL, A. AND WOLFSHTEIN, M.: European Symposium on Combustion, p. 571, The Combustion Institute, 1973. 4. CmGIER, N. A., McCREATH, C. G. AND MAKEPEACE, R. W.: Combustion and Flame, 23, 11, (1974). 5. PLEE, S. L. AND MELLOR, A. M.: Combustion and Flame, 35, 61, (1979). 6. MELLOR,A. M.: Progress in Energy and Combustion Science, 6, 347, (1980). 7. BEI~R, J. M. AND CHICIER, N. A.: Combustion Aerodynamics, p. 112, Applied Science Publishers, 1974. 8. HmOVASU, H., KADOTA,T. AND ARAI, M.: The 3rd World Hydrogen Energy Conference Tokyo, Japan, p. 1199, International Association for Hydrogen Energy, 1980. 9. HmOYASU,H., ARM, M., KADOTA,T. AND YOSO, J.: Bulletin of the JSME, 23-184, 1655, (1980). 10. NUKIYAMA, S. AND TANASAWA,Y.: Trans. Soc. Mech. Engrs. Japan, 5, 68, (1939).

COMMENTS N. Hay, University of Nottingham, U.K. The experiment appears to have been conducted at atmospheric pressure. Have the authors done any experiments at high pressure? At high pressure there will be no fl0w into the furnace from the atmosphere and the reverse flow region will change into a recirculation region. Measurements at over-atmospheric pressure would be of interest from the point of view of gas turbine combustors, while the atmospheric pressure tests are obviously simulating the boiler furnace situation.

Author's Reply. We have not yet measured the lean blow-off limit, the recirculation zone or the temperature distribution at high pressure environment. The aim of this research is to examine the effect of fuel volatility. However, we have already done the experiment for the evaporation rate measurement, droplet size measurement and ignition delay measurement at high pressure environment for diesel combustion and high pressure gas turbines.

518

TURBULENT COMBUSTION MEASUREMENTS II

O. Gulder, National Research Council, Canada. I have two questions: a) How did you measure the droplet SMD for four fuels? Have you determined the droplet size distribution as well? b) From one of your figures I estimated that the blow-off limit for kerosene corresponds to approximately an air/fuel ratio of 250. This value seems a little bit high. Would you comment on this?

Author's Reply. a) We measured the mean droplet size and the size distribution for four fuels by Laser diffraction method. (Malvern Instrument Co.) b) I have no comment. However, I suppose that the operating condition is a little bit different, or the quality of kerosene is different.

R. Bilger, University of Sydney, Australia. You speak of the recirculation region when in fact you have measured the boundaries of the reverse flow region. Is this correct? It is most important to be precise in such terminology. Author's Reply. The recirculation zone fundamentally consists of reverse flow and forward flow regions. But the reverse zone has the major part of the recirculation zone, and the aerodynamic effect of the recirculation zone is mainly caused by the reverse flow. Further, with the flow in the combustor having too high turbulence, it is difficult to separate the recirculating forward flow from the main forward flow issued from the swirler. Then, for the convenience of the consideration, we have measured the boundary between the reverse and the forward flow as the recirculation boundary.