Effects of High Ambient Pressure and Temperature on ...

ORGANIZED BY UNIVERSITI TENAGA NASIONAL, BANGI, SELANGOR, MALAYSIA; 28-30 AUGUST 2006

Effects of High Ambient Pressure and Temperature on the Autoignition of Blended fuel Droplets Q.S. Khan, S.W. Baek Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea, [email protected]

Keywords: Blended fuel, Droplet, Autoignition, HighPressure, High-Temperature

Abstract Autoignition delay times of blended fuel droplets of different initial compositions were experimentally determined at high ambient pressures and temperatures. Heptane and hexadecane are selected as volatile and non-volatile fuels respectively. The droplet was hanged at the tip of a fine quartz fibre and suddenly exposed to high ambient temperature by the help of falling electric furnace at desired ambient pressures under normal gravity. The ignition was detected visually by the use of high-speed photography. Since at atmospheric pressure (0.1 MPa) and lower ambient temperatures the partial microexplosion and droplet fragmentation was observed before autoignition, therefore its effect on ignition delay was also investigated. Results show that the ignition delay decreases with an increase in ambient temperature and ambient pressure. And there exists a characteristic minimum value of volatile component in the mixture for which ignition delay times reduces significantly. But that characteristic amount has different values at low and high ambient temperatures and pressures. Although in some cases droplet fragmentation happens before autoignition yet that behaviour was found to have negligible effect in autoignition delay times.

1 Introduction Spray combustion system is utilized in the extensive field of combustion engineering, including diesel engines, rocket engines, gas turbines and industrial furnaces where the fuel is atomized into a lot of fine droplets to enlarge the surface area for rapid evaporation and easy control of combustion rate. In order to elucidate the spray combustion phenomena, it is a basic requirement to have a profound knowledge of a single droplet behavior. In most of the practical systems, the droplet consists of the mixture of two or more pure liquids. The multicomponent droplets may consist of several species with completely different physical and chemical properties. The degree of volatility, boiling temperature, latent heat of vaporization, surface tension and heat capacity of each component play an important role in the interior thermo-fluid dynamics of the droplet. Therefore the pre-ignition vaporization period depends upon the individual component physical properties and has transient behavior depending

upon the component concentrations at the droplet surface. This pre-ignition vaporization period controls the physical delay and has a lot of importance. The single and multicomponent droplet vaporization and combustion is reviewed by Law [1] and Sirignano [2] quite extensively. Kadota et al. [3] has done the theoretical and experimental investigation regarding spontaneous ignition delay of pure fuel isolated droplets and studied the effects of ambient pressure, temperature and oxygen concentration. Whereas the droplet ignition characteristics were examined by Saitoh et al. [4] near the ignitable limit for pure component droplets using suspended droplet technique. Nakanishi et al. [5] has determined the effect of droplet diameter on ignition delay under supercritical environments for an isolated suspended droplet system using heptane and hexadecane separately. The effects of fuel boiling point and chemical type on the autoignition of single suspended droplet were investigated by Bergeron and Hallett [6] for different paraffins, naphthenes and aromatics. Although there is a lot of study about the evaporation, ignition and combustion of pure fuels as well as evaporation and combustion of multicomponent fuel droplets yet the data about the ignition of binary or multicomponent fuel droplets is scarce despite of its inevitable importance. Takei et al. [7] were the first to determine the effect of droplet diameter suspended on a fiber on the ignition of blended fuel droplet for different ambient temperatures at atmospheric pressure. Bergeron and Hallett [8] investigated the ignition delay time of single droplets of two component mixtures at atmospheric pressure experimentally and numerically. Wang et al. [9] have experimentally investigated the effects of temperature, pressure and oxygen concentration on the ignition delay of freely falling pure as well as binary droplets. But none of the binary droplet ignition studies has addressed the droplet microexplosive and fragmentation effect on ignition delay. The reason is that, at very high temperatures, ignition happens before the fragmentation starts. But in the present study in some low temperature and less-volatile component concentration dominant mixtures, fragmentation happens before ignition. The objectives of the present study are twofold. First is to investigate the role of droplet fragmentation on the ignition delay. And the second is to observe the effects of high ambient pressures on the binary droplet autoignition since no

International Conference on Energy and Environment 2006 (ICEE 2006)

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ignition study has been done so far for blended droplets above 0.3 MPa ambient pressures. In order to accomplish the present study, n-heptane (boiling point 371.5 K) and n-hexadecane (boiling point 560.6 K) were selected as two miscible fuels with completely different volatilities. Isolated suspended droplet of different blends of the two fuels was autoignited under normal gravity. The ambient temperature has been varied from 700 to 1100 K while the ambient pressure range was from 0.1 to 1.5 MPa. Droplets of approximately same sizes (1-1.3mm diameter) were formed and the ignition was detected by the help of high-speed photography (500 frames per second).

2 Experimental Setup A schematic of the experimental apparatus is shown in Fig. 1.

means the heat leakage from furnace to outside is negligible. Also, since a thin quartz fiber is used for droplet suspension, the heat transfer between droplet and fiber is minimized. The ignition delay is defined as the time interval from exposure of droplet to high ambience to the first appearance of flame. The detailed description of the experimental setup, levels of accuracies, and procedure is provided by Ghassemi et al. [10].

3 Results and Discussions 3.1 Ignition of pure fuels Figure 2 shows the effect of temperature on the ignition delay times of heptane and hexadecane at an ambient pressure of 0.5 MPa. The ignition delay decreases for both fuels as an increase in temperature as expected. At higher ambient temperature, ignition delay of heptane levels off. The reason is that the pre-evaporation heating time (one of the important parameter in physical delay) for heptane is too little as compared to hexadecane and at higher temperature the chemical delay also becomes smaller. But in the case of hexadecane, even at moderately higher temperatures, most of the delay is due to the physical component because of longer heating time despite of the fact that the post heating vaporization rate for hexadecane progresses more quickly as an increase of temperature as compared to heptane, Ghassemi et al. [10]. 6.0

Pressure 0.5 MPa

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Figure 1: A sketch of the experimental setup 1) Pressure vessel, 2)

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Guide bar, 3) Furnace entrance 4) Electric furnace, 5) Quartz glass window on furnace, 6) Temperature controller, 7) Lever, 8) Air vessel, 9) Quartz glass window on pressure vessel, 10) Backlight source, 11) Quartz fiber, 12) Droplet, 13) Shock absorber, 14) Droplet maker, 15) Plunger micro pump, 16) High-speed camera

Figure 2: Effect of temperature on ignition delay of heptane and hexadecane

A droplet hanging on a fine quartz fiber (0.125mm diameter) was subjected to the hot environment by a freely falling electric furnace, thereby resulting in autoignition. Air is used as oxidizing gas and after each run the chamber is purged with fresh air to keep oxygen concentration alike. This unit is enclosed within a pressure vessel installed with glass windows which enable to observe the process. The whole process is observed using a high-speed camera. Due to flexible feature of the furnace design the ambient gas temperature steps up from low to high stage in a short time. It

Figure 3 gives the comparison between ignition delay of heptane and hexadecane droplets as a function of pressure at an ambient temperature of 873 K. The effect of increase in ambient pressure was found to be directly proportional to the vaporization rates in the case of heptane while for the case of hexadecane it was reverse [10]. Despite of that finding, the sharp decrease in ignition delay of hexadecane at increasing pressure shows that increase in ambient pressure has significant impact in reducing the chemical delay of hexadecane. While for the case of heptane, an increase in


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pressure enhances vaporization rate hence further lowers the physical delay. 5.0

Temperature 873 K


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Heptane Hexadecane

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Figure 3: Effect of pressure on ignition delay of heptane and hexadecane

Figure 4-a shows the droplet at t = 0.0 seconds and just exposed to high ambient temperature environment. Figure 4-b to 4-d shows the thermal expansion of droplet, a characteristic feature of low-volatile higher-boiling point fuels (hexadecane in the current case). Figure 4-e shows the bubbles inside of the droplet. While figure 4-f to 4-n demonstrates the rupturing, dilatating and fragmenting droplet. In figure 4-o, ignition takes place. In all of the previous blended fuel ignition studies, the ignition was always happened before fragmentation. So it was suspected in the current study that fragmentation that was happened prior to ignition may influence the ignition delay time, but it did not. The reason is that the sole effect of fragmentation is to supply more volatile fuel component from inside of the droplet. Fragmentation prior to ignition was found at relatively lower ambient temperature condition. For that ambient temperature case, the limiting ignition delay time was chemical delay because sufficient amount of fuel vapors needed for ignition are already available due to evaporation, but unable to overcome the activation energy (chemical delay). Hence further supply of volatile component through fragmentation was proven useless.

3.2 Ignition of blended fuels

3.2.1 Effect of ambient temperature

Binary fuel mixtures of different volumetric compositions of heptane and hexadecane were allowed to autoignite and their delay times were measured. In an earlier study of bicomponent droplet vaporization, partial microexplosion and droplet fragmentation phenomena was observed at atmospheric pressures [10]. But the droplet disruption was not reported prior to ignition in the previous binary droplet studies because the ambient temperature was fairly high. But in the comparatively lower ambient temperature case, and in the mixtures consisting of low volatile component greater than 50 % by volume, droplet dilatation and fragmentation starts before ignition. Figure 4 shows the sequential photographs of a bi-component droplet consisting of heptane and hexadecane 30 and 70 % by volume respectively and exposed to an ambient temperature of 973 K at 0.1 Mpa.

Figure 5 shows the variation of ignition delay time of binary fuel with heptane volume fraction versus ambient pressure for several environmental temperatures. Ignition delay time decreases with an increasing heptane volume fraction at all ambient temperatures. 5.0

Pressure 0.1 MPa


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T=873 K T=973 K T=1073 K

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Figure 5: Ignition delay of binary mixture at different temperatures at 0.1 MPa pressure

Figure 4: Sequential photographs of binary droplet fragmentation and ignition

After reaching a characteristic value of more volatile fuel component concentration, the ignition delay times drop sharply. This can be explained on the basis of bi-component droplet gasification mechanism that consists of three stages. During the first stage, the more-volatile component from the droplet surface gasifies since it has almost no heating period and the gasification is dominated by the more-volatile component in a thin layer at the droplet surface. The droplet


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The figures share a common feature of a characteristic value of more-volatile component concentration at which the ignition delay time suddenly drops and levels off with morevolatile component concentration. At either side of this characteristic value, the ignition delay approximately follows either of the two pure component ignition values. This characteristic value of more-volatile component concentration shifts towards lesser concentration at elevated temperatures. And this shifting takes place gradually if we increase temperature at constant pressure.

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Temperature 1073 K


P=0.5 MPa P=1.0 MPa P=1.5 MPa

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Figure 7: Effect of ambient pressure on the ignition of binary mixture at T=973 K

Figures 6-8 demonstrate the effects of high ambient pressure on the ignition delay for various component concentrations at different ambient temperatures.

Temperature 873 K

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Temperature 973 K

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temperature will be low, being mostly controlled by the boiling point of the more-volatile component, while the bulk of the vaporizing species will also be the more-volatile component. At this time, the ignition characteristics of the more-volatile component will control the ignition. In case, if chemical or physical delay at the present ambient conditions is not overcome, ignition will not happen. Now in the second stage, the heating-up of less-volatile component at the surface will start since liquid phase transport is extremely slow process and more-volatile component cannot be supplied from the droplet interior. This heat-up process will continue until the less-volatile component at the droplet surface reaches its boiling point and steady state vaporization starts in the third stage that is now controlled by the less-volatile component gasification rate. The results of fig. 5 can be explained recognizing the above discussion about three-stage gasification mechanism. At higher temperature, the ignition delay times are level off for the higher or equal concentrations of more-volatile fuel. The reason is that for these conditions, the more-volatile component gasifies from the droplet surface reaches the ignitable limit before allowing the droplet to enter into the second stage of three-staged behavior. But when the more-volatile component concentration is lesser, the droplet has to wait for heating up of less-volatile component and that heating time is responsible for the physical delay in that case. For the lower temperature, the level off point shifts towards the higher concentration of more-volatile component which means, more volatile component is needed to gasify from the droplet surface to achieve ignition in the first stage, otherwise if the droplet enters to second stage, the ignition delay has to account for additional heating of less-volatile component.

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For example, in fig. 6, the ignition curve at 0.5 MPa and at 873 K, the characteristic heptane value lies between 30 and 50 %, but for the same pressure value in fig. 7 at 973 K, the characteristic value lies between 15 and 30 % and in fig. 8, the same pressure value lies between 5 and 20%. Which means as temperature increases, the chemical delay time for


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more-volatile component reduces and a nominal concentration is enough to cause ignition during the first stage of gasification.

4 Conclusions The present study about the ignition of binary fuel droplets reveals the following facts: 1. The ignition delay times at different ambient temperatures and pressures can be explained with the help of its physical and chemical component. 2. For pure fuels, there is an overall decrease in ignition delay at higher ambient temperatures and pressures, and tends to level off at higher values of pressures and temperatures. 3. Droplet fragmentation may appear before ignition but found to have no significant effect on overall ignition delay. 4. There exists a characteristic value of more-volatile component in a binary mixture at which the ignition delay sharply decreases and tends to approach the pure component ignition delay. 5. The characteristic value tends to shift for lower value of more-volatile component with an increase in ambient temperature.

[7] M. Takei, T. Tsukamoto and T. Niioka, Ignition of Blended-Fuel Droplet in High-Temperature Atmosphere, Combustion and Flame, vol. 93, pp. 149-156, 1993. [8] C. A. Bergeron and W. L. H. Hallett, Autoignition of Single Droplets of Two-Component Liquid Fuels, Combustion Science and Technology, vol. 65, pp. 181-194, 1989. [9] C. H. Wang, K. H. Shy and L. C. Lieu, An Experimental Investigation on the Ignition delay of Fuel Droplets, Combustion Science and Technology, vol. 118, pp. 63-78, 1996. [10] H. Ghassemi, S. W. Baek and Q. S. Khan, “Experimental Study on Binary Droplet Evaporation at Elevated Pressure and Temperature”, 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, AIAA 2005-353.

Acknowledgements The present work was supported by the Combustion Engineering Research Center at the Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, which is funded by the Korea Science and Engineering Foundation.

References [1] C. K. Law, Recent Advances in Droplet Vaporization and Combustion, Progress in Energy and Combustion Sciences, vol. 8, no. 3, pp. 171-201, 1982. [2] W. A. Sirignano, Fuel Droplet Vaporization and Spray Combustion Theory, Progress in Energy and Combustion Sciences, vol. 9, no. 4, pp. 291-322, 1983. [3] T. Kadota, H. Hiroyasu and H. Oya, Spontaneous Ignition Delay of a Fuel Droplet in High Pressure and High Temperature Gaseous Environments, Bulletin of the JSME, vol. 19, no. 130, pp. 437-445, 1976. [4] T. Saitoh, S. Ishiguro and T. Niioka, An Experimental Study of Droplet Ignition Characteristics near the Ignitable Limit, Combustion and Flame, vol. 48, pp. 2732, 1982. [5] R. Nakanishi, H. Kobayashi, S. Kato and T. Niioka, “Ignition Experiment of a Fuel Droplet in High-Presure HighTemperature Ambient”, Twentyfifth Symposium (International) on Combustion, pp. 447-453, 1994. [6] C. A. Bergeron and W. L. H. Hallett, Ignition Characteristics of Liquid Hydrocarbon Fuels as Single Droplets, Canadian Journal of Chemical Engineering, vol. 67, pp. 142-149, 1989.


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