Study of Combustion Characteristics of Coflowing Gas and Liquid Fuel

Energy & Fuels 2001, 15, 1369-1382

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Study of Combustion Characteristics of Coflowing Gas and Liquid Fuel Stream Tharwat w. Abou-Arab,† Saad A. El-Sayed,* Mohammed M. Shamloul, and Tarek M. Khass Zagazig University, Mechanical Power Engineering Department, El-Sharkia, Egypt Received October 20, 2000. Revised Manuscript Received May 28, 2001

One of the most important parameters combustion engineer looks for is the flame stability in combustion chamber. In this work, a trial was made to get a stable flame by using liquid fuel injected in gaseous fuel combustion, since liquid fuel combustion is more stable than gaseous fuel combustion. A test rig is designed and constructed to carry out the experiments. Two sets of experiments were carried out to investigate the effect of changing the injection location at constant percentage of liquid fuel (kerosene) of 20% and the effect of changing the percentage of the injected liquid fuel while the injection location was maintained at Xinj/D ) 0.377. It was found that the injection of liquid fuel leads to an increase in the maximum positive axial velocity and reduces the absolute value of the maximum negative velocity (recirculation zone). Also, a stable temperature distribution is noticed at an axial distance X/D ≈ 2.15 which is less than that of gaseous fuel combustion (LPG, X/D ≈ 2.91). The changing of injection location leads to a reduction in gas temperatures for Xinj/D ) 0.189, then increasing to reach a maximum values (which approximately the same values for combustion of LPG fuel alone) at Xinj/D ) 0.377. Any further increase in the injection location leads to a reduction in gas temperature especially at the upstream sections of the combustion chamber. It was found that a stable flame is found to be existed in all percentage of the liquid fuel injected.

Introduction Combustion of fuel blends with wide-ranging physical and chemical properties could lead to changes in ignition time,1 extinction limits,2 three-stage burning behavior in droplet combustion,3 characteristic burning velocity,4 and microexplosion phenomena. Previous work has shown that combustion of gas-liquid coburning mixtures leads to substantial increase in flame speed,5 with linear dependence on the fuel-air ratio and inverse proportionality with fuel mean drop size.6 Coburning effects were more prominent for small overall air-fuel ratios and mean droplet diameters. Direct heterogeneous burning of diesel-oil shale slurry containing up to 2.5 wt % (weight) solids has been studied experimentally by Ahmed et al.7 They used an air-atomized nozzle mounted on the bottom of a vertical * Corresponding author: Faculty of Technological StudiessMechancial Power & Refrigeration Dept, Zagazig University, P.O. Box 42325, ElSharkia 70554, Kuwait. † Permanent address: Mechanical Power Engineering Department, Cairo University, Giza, Egypt. (1) Niioka, T.; Mitani, T.; Sato, J. In Proceedings of the Twentieth International Symposium on Combustion; The Combustion Institute: 1984; pp 1877-1882. (2) Hamins, A. and Seshadri, K. In Proceedings of the Twentieth International Symposium on Combustion; The Combustion Institute: 1984; pp 1905-1913. (3) Wang, C. H.; Liu, X. Q.; Law, C. K. Combustion Flame 1984, 58, 175-182. (4) Milton, B. E.; Keck, T. C. Combust. Flame; 1984, 58, 13-19. (5) Nakabe, K.; Mizutani, Y.; Hirao, T.; Tanimurz, S. Combust. Flame 1988, 74, 39. (6) Richards, G. A.; Sojka, P. E.; Lefebvre, A. H. Trans. ASME, J. Eng. Gas Turbine Power 1989, 11, 84-90.

30 cm diameter segmented combustor. The heat transfer rate distribution to the water jacket and flame stability improved as the percentage of oil-shale in the diesel fuel was increased. Stable flames were observed for up to 20 wt % of oil shale. They concluded that (I) a stable flame was achieved for up to 20 wt. % of oil shale in diesel slurry; (ii) oil shale improves the flame stability when mixed with diesel. As the percentage of oil shale was increased, the flame front moved closer to the atomizer and the combustion efficiency increased; (iii) luminous oil-shale particles were observed at a down stream section along the combustor. These improved the radiation heat transfer distribution to the combustor water jacket and enhanced the combustion of large slurry droplets. In water heaters, turbulent flames are inherently associated with eddying flow structure and the reactive and diffusive characteristics of the duel fuel liquid-gas mixture were studied by Abou-Arab et al.8 In their study, a water heater was used to investigate the combustion and heat transfer characteristics of LPG, kerosene, and a 50% blend of LPG in kerosene. Air and fuel mass flow rates were varied, to alter residence time and heat release rate at different air-to-fuel ratios. Stability limits using three fuels were also evaluated. They concluded that LPG demonstrated higher combustion efficiency and heat release rate due to the (7) Ahmed, N. T.; Abou-Arab, T. W.; Al-Doss, T. K.; Khasawneh, B. Energy-MS037, 1994, 1-8. (8) Abou-Arab, T. W.; Othman, M. O.; Najjar, Y. H.; Ahmed, N. T. Fuel 1990, 69, 485-489.

10.1021/ef000232j CCC: $20.00 © 2001 American Chemical Society Published on Web 10/09/2001

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Figure 1. General layout of the test rig.

Figure 2. Radial distribution of axial velocity along the combustion chamber (LPG alone, ma ) 84.1 kg/h).

Abou-Arab et al.

Coflowing Gas and Liquid Fuel Stream

Energy & Fuels, Vol. 15, No. 6, 2001 1371

Figure 3. Axial velocity contours for both cold and hot conditions along the combustion chamber (LPG combustion and ma ) 84.1 kg/h).

premixed nature of the flame. Kerosene spray flame showed increasing combustion efficiency with increased air mass flow rate up to a certain level. Flame temperature and kinetics then decreased, with consequent drop in combustion efficiency. Stability limit were wider with kerosene due to the availability of unvapor-

ized fuel pockets. The LPG flame stood higher air mass flow rate, due to higher heat release rates with premixed flames. The present work has been directed toward the investigation of the variation of the main structure of the flow and thermal profiles of diffusion flames whose

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Figure 4. Radial distribution of temperature along the combustion chamber (LPG alone, ma ) 84.1 kg/h).

parameters are the air-fuel ratio and air mass flow rate for gaseous fuel combustion. Also, the injection location of liquid fuel with gaseous fuel combustion and percentage of injected liquid fuel were investigated. It could be concluded that injection of liquid fuel in gaseous fuel combustion is very effective in improving the flame stability. Experimental Test Rig A test rig is designed and constructed for carrying out the experiments. Figure 1 shows the general layout of the test rig.

Abou-Arab et al. It consists mainly of a combustion chamber, an air supply system, a gaseous fuel supply system, a liquid fuel supply system, a cooling water system, and an ignition circuit. All these systems are equipped with necessary controlling and measuring devices. A cylindrical combustion chamber is designed and constructed to carry out the experiments. It is fixed vertically to eliminate the gravitational effect. The inner wall of combustion chamber is made of stainless steel to ensure high resistance to corrosion, so that heat transfer properties of the wall not vary due to scaling and other effects. It is of 265 mm inner diameter, 4 mm thickness, and a length of 1200 mm. It is surrounded by a cylindrical shell, of 345 mm inside diameter, which is divided into six segments. Water flows through the annulus area between the wall and the shell. A diffuser of divergence angle 12° is placed prior to the combustion chamber to ensure minimum pressure loss.9 A nozzle having the same dimensions of diffuser is placed after the combustion chamber. Water is supplied to each segment besides to the diffuser and nozzle through inlet branched pipes. These branched pipes are taken from a main supply of water. The inlet branched pipes are provided with ball valves to control the amount of water flowing through each segment. Each segment as well as the diffuser and the nozzle has two taps of 8 mm inside diameter for measurements and one tap of 28 mm inside diameter used as an observation window. All different taps lie on the same axis as shown in Figure 1. The air supply system consists of a centrifugal blower driven by an electric motor of 2 kW power through a single V-belt. The amount of air is controlled by setting a gate at the blower inlet. This air gate consists of a movable wooden disk of 200 mm diameter that can be turned about a threaded stud fixed in the center of the inlet opening of the air blower. An orifice meter made from stainless steel sheet of 4 mPm thickness and chamfered with an angle of 30° is connected to a micromanometer for measuring the air mass flow rate supplied to

Figure 5. Temperature contours for LPG combustion along the combustion chamber (ma ) 84.1 kg/h).



the combustion chamber. The formula of the curve fitting of the air orifice meter calibration is given by

ma ) 68.2264∆Po0.576496

((1.510179 kg/h)

(1)

where ma ) air mass flow rate (kg/h) and ∆Po ) pressure drop across the air orifice meter (mm H2O). At the end of the air supply pipe, a swirler is mounted before the diffuser inlet to create a recirculation zone which aids the flame to be stabilized. The swirler is simply axial cascade guide vanes. The tangential velocity component at the swirler exit depends on the angle of the vane setting. The swirler is made of 1 mm sheet stainless steel and composed of eight blades welded to the inner and outer race at an angle of 30°. The hub diameter of the swirler is 34 mm, and the outer race diameter is 122 mm. The dimension of the swirler leads to a swirl number of 0.4083.10 Gaseous and liquid fuels are supplied to the combustion chamber through two pipes of 15 and 5 mm inside diameter, which they fitted coaxially in the center of the diffuser inlet. The liquid fuel is supplied through the inner pipe which is ended by a fuel nozzle, while the gaseous fuel is supplied through the annulus area between the two pipes. The fuel supply systems are shown in Figure 1. Two cylinders of LPG are connected together by means of a Y connection. Each cylinder has a pressure regulator and supplied with a pressure gauge. A digital gas flow meter with accuracy of ( 1% is used for measuring the flow rate. The exit from the Y connection is first connected to the digital flow meter then to the outer fuel pipe through a hose of 6 mm inside diameter. The outer fuel (gaseous) pipe is welded in the elbow, and the gas enters the combustion chamber at the diffuser inlet. The liquid fuel supply circuit consists of a fuel tank of 265 mm inside diameter and a length of 600 mm. Volume of the tank is equal to 33 L, and a fuel pump is driven by an electric motor of 1 HP at constant speed of 1430 rpm. The liquid fuel pipe line is provided with a filter mounted before the inlet to the pump. The amount of liquid fuel can be controlled by two control ball valves, one on the bypass line and the other on the liquid fuel feed line to the combustion chamber. The liquid fuel feed line which is ended with a fuel nozzle is supplied with a pressure gauge. A fuel nozzle is used to help the fuel to be atomized. It is of 4 mm inside diameter and has six holes of 0.25 mm diameter. The liquid fuel pipe can be moved axially through the combustion chamber. Calibration tests are carried out on the fuel nozzle to determine the quantity of fuel in kg/h at different pressure gauge readings. The curve fitting relationship between liquid mass flow rate and pressure gauge reading can be written as follows:

ml ) -0.143035 + 3.61534p - 0.168903p2

(2)

where p ) pressure gauge reading (kg/cm2 ) and ml ) injected liquid fuel mass flow rate (kg/h). The combustion chamber is cooled by water. The heat carried away by the water divided by the heat liberated from burning of the fuel equals to the combustion chamber efficiency. Therefore, the water temperature and water mass flow rate must be measured accurately. Since the water enters each section individually, its flow rate must be measured in each section. Eight orifice meters of diameter 10 mm and thickness of 4 mm are used for this purpose. The orifice has a diameter ratio of 0.5 and two taps are located at 20 mm upstream and 10 mm downstream the orifice are used to measure the differential pressure across the orifice by using (9) Lefebvre, A. H. Gas Turbine Combustion; McGraw-Hill: New York, 1983. (10) Khalil, K. H.; El-Mahallawy, F. M.; Moneib, H. A. The Bulletin of the Faculty of Engineering; Cairo University: Egypt, 1975.

Figure 6. (a) Radial distribution of axial velocity along the combustion chamber (Cold condition, ma ) 200.14 kg/h). (b) Radial distribution of axial velocity along the combustion chamber (LPG alone, ma ) 200.14 kg/h). a U-tube manometer. The manometer uses mercury as the sensing fluid. The calibration is done by using the direct method. In this method the calibration is done by comparing unknown flow with a primary standard. The relation between water mass flow rate and pressure drop across the water orifice meter was obtained as

mw ) 2.29∆Po0.475492

((0.374 kg/min)

(3)

where mw ) water mass flow rate (kg/min) and ∆Po ) pressure drop across the water orifice meter (mmHg). Measurements of both axial and tangential velocities were done by using a calibrated three-hole probe. The calibration of the three-hole probe was done in both square and circular ducts. The following equation was obtained by curve fitting:

K ) 1.02828 + 1.06768 × 10-5 Rep Rep )

Vad νa

((0.02529881) (4) (5)

where Rep ) Reynolds number around the three-hole probe, d

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Figure 7. Axial velocity contours for both cold and hot conditions along the combustion chamber at different mass flow rates (LPG combustion and AF ) 70).


Energy & Fuels, Vol. 15, No. 6, 2001 1375 ) three-hole probe diameter (d ) 6 mm), νa ) kinematic viscosity of air (m2/s), and Va ) velocity of air on the probe head (m/s). Measurements of the gas temperatures along the combustion chamber were done by using a bare wire thermocouple thermometer of type S (0.1 mm diameter). It has an accuracy of (0.1% of the reading. The readings were then corrected for the radiation error as in ref 11. The error in gas temperature does not exceed (10 °C. Water temperatures were also measured by using a thermocouple of type T with an error (0.1 °C.

Experimental Results and Discussion

Figure 8. Radial distribution of temperature along the combustion chamber (LPG alone, ma ) 200.14 kg/h).

Two sets of experiments were carried out. The first set was carried out for gaseous fuel combustion only, while the second set was carried out for combustion of both gaseous fuel alone and liquid fuel is injected during gaseous fuel combustion. A. Gaseous Fuel Combustion. Velocity and temperature measurements were taken in these set of experiments. The following factors were investigated:

Figure 9. Temperature contours for LPG combustion along the combustion chamber at different air mass flow rates (AF ) 70).

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Figure 10. Radial distribution of axial velocity along the combustion chamber (20% kerosene and 80% LPG, ma ) 200.14 kg/h, AF ) 70.2, and Xinj/D ) 0.377).

Abou-Arab et al.

(1) the effect of air-fuel ratio under constant air mass flow rate, (2) the effect of air mass flow rate under constant airfuel ratio. A.1. Effect of Air-Fuel Ratio. The air-fuel ratio can be changed either by changing the air mass flow rate and keeping the fuel mass flow rate constant or by changing the fuel mass flow rate and keeping the air mass flow rate constant. In this work, the air mass flow rate of 84.1 kg/h is kept constant and three air-fuel ratios of 27.75, 45.47, and 70.2 are investigated. Figure 2 shows a sample of axial velocity distribution along the combustion chamber, when ma ) 84.1 kg/h and AF ) 70.2 compared with cold conditions. Figure 3 shows the contours of axial velocity for all experiments of this set. From Figures 2 and 3, it can be seen that (A) Combustion of LPG gave an increase in axial velocity compared with cold conditions; this is attributed to the increase in temperatures resulting from the

Figure 11. Effect of injection location of injected liquid fuel on axial velocity contours along the combustion chamber (ma ) 200.14 kg/h, 20% kerosene and AF ) 70).


Figure 12. Radial distribution of temperature along the combustion chamber (20% kerosene and 80% LPG, ma ) 200.14 kg/h, AF ) 70.2, and Xinj/D ) 0.377).


combustion, and hence, lower value of density which is inversely proportional to the axial velocity. (B) Referring to the effect of air-fuel ratio, it can be seen from the same figure that the increase in air-fuel ratio slightly decreases the axial velocity. (C) In the upstream sections, the axial velocity reaches a maximum value near the combustion chamber wall then decreases suddenly to zero, then it becomes of negative value in the recirculation zone (see Figure 2). (D) The length of flame (identified by zero axial velocity line) is increased with the increase in airfuel ratio because increasing the air-fuel ratio decreases the gas mass flow rate (i.e. decreasing Reynolds number, which is inversely proportional to the flame length). Figure 4 shows a sample of gas temperature distribution along the combustion chamber when ma ) 84.1 kg/ h, AF ) 70.2. It can be noticed from the figures that,

Figure 13. Effect of injection location of liquid fuel on temperature contours along the combustion chamber (ma ) 200.14 kg/h, AF ) 70, and kerosene ) 20%).

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Figure 14. Radial distribution of axial velocity along the combustion chamber (30% kerosene and 70% LPG, ma ) 200.14 kg/h, AF ) 70.2, and Xinj/D ) 0.377).

Abou-Arab et al.

within the first zone of the flame, large temperature variations are occurred. On moving downstream along the combustion chamber, the variation of temperature becomes slight, and uniform radial temperature distributions are observed. The decrease in air-fuel ratio leads to an increase in the temperature values and the points of maximum temperature along the radial distance is shifted toward the combustion chamber wall. The axial distance at which the temperature profiles become uniform is decreased with the increasing the air-fuel ratio. Uniformity occurred at X/D ) 2.91 for AF ) 27.75 and X/D ) 2.15 for other air-fuel ratios. This may give an indication that the flame length increases with the decrease in the air-fuel ratio. The effect of air-fuel ratio on temperature distribution along the combustion chamber is shown in Figure 5. It can be seen that decreasing air-fuel ratio increases the values of temperature all over the combustion chamber. This is because decreasing the air-fuel ratio at constant air mass flow rate increases the fuel mass flow

Figure 15. Effect of injected percentage of kerosene on axial velocity contours along the combustion chamber (ma ) 200.14 kg/h, AF ) 70, and Xinj/D ) 0.377).


Figure 16. Radial distribution of temperature along the combustion chamber (30% kerosene and 70% LPG, ma ) 200.14 kg/h, AF ) 70.2, and Xinj/D ) 0.377).

rate and consequently increases the total heat released from the combustion of fuel responsible for the temperature distribution along the combustion chamber. A.2. Effect of Air Mass Flow Rate. To study the effect of air mass flow rate on combustion characteristics of gaseous fuel combustion, the air-fuel must be constant. Four experiments of different air mass flow rates of 84.1, 112.52, 167.97, and 200.14 kg/h and AF approximately equals 70. Figure 6a,b shows a sample of radial distribution of axial velocity along the combustion chamber of this set for both cold and hot conditions, respectively, at ma ) 200.14 kg/h and AF ) 70.2. It is shown that the values of axial velocities along the combustion chamber increase with increasing the air mass flow rate. The effect of air mass flow rate in axial velocity contours along the combustion chamber is shown in Figure 7. It can be seen that the axial velocity increases with a noticeable value as a result of increasing air mass flow rate, because the axial velocity is linearly proportional to the value of air mass flow rate. It can also be seen that, as air mass flow rate increases the flame length decreases as explained before, while its width is increased (width of the flame is identified by the maximum diameter of recirculation zone). This is because as air mass flow rate is increased, a turbulent flame came intoexistence, and consequently, shorter flame arose. Figure 8 shows the radial distribution of the gas temperature along the combustion chamber at ma ) 200.14 kg/h and AF ) 70.2, while the temperature contours along the combustion chamber are shown in Figure 9. It can be seen from the last two figures that as the air mass flow rate is increased, the value of the temperature in the downstream sections is increased. This is because the increase in air mass flow rate increases the turbulence, and consequently, good mixing between the emerged fuel jet and the swirling air and then high combustion efficiency and high temperature is obtained. B. Injection of Liquid Fuel during Gaseous Fuel Combustion. Two sets of hot experiments were carried (11) Khalil, M. B.; Farag, S. A.; El-Mahalawy, F. M. Thesis for the Degree of Master in Mechanical Power Engineering, Cairo University, Egypt, 1975.


out to investigate the effect of liquid fuel injection on the flow characteristics of LPG fuel combustion. These experiments were carried out at constant air mass flow rate of 200 kg/h and constant air-fuel ratio of 70. One set is carried out to investigate the effect of changing the injection location of liquid fuel, while the other set is carried out to investigate the effect of varying the percentage of injected liquid fuel (kerosene). B.1. Effect of Injection Location. Three experiments were carried out to investigate the effect of injection location with 20% liquid fuel and 80% gaseous fuel by mass at Xinj/D ) 0.189, 0.377, and 0.566. Figure 10 shows the radial distribution of axial velocity along the combustion chamber in the case of Xinj/D ) 0.377. Comparing this figure with Figure 6b, the following comments can be made: (1) The location of maximum axial velocity in all sections is almost not changed. (2) The size of recirculation zone is changed as a result of liquid fuel injection. (3) The absolute value of the maximum negative velocity is decreased as a result of liquid fuel combustion although air mass flow rate and constant air-fuel ratio (i.e., constant fuel mass flow rate) are held constant, while the value of maximum positive axial velocity is increased. This is because the injection of liquid fuel occurs at the combustion chamber axis; consequently, the positive axial velocity for liquid fuel at the entrance with negative axial velocity of the combustion product of LPG fuel leads to a decrease in the value of the maximum negative velocity. (4) Flame stability is improved as shown from the axial velocity distribution as a result of liquid fuel combustion, since the liquid fuel combustion is more stable than gaseous fuel combustion. Figure 11 illustrates the effect of injection location of liquid fuel on axial velocity contours along the combustion chamber. The axial velocity contours for LPG combustion only is also shown in the same figure for the purpose of comparison. The upper half of each graph is for LPG fuel combustion only, while the lower half of the graph is for combustion of 20% kerosene and 80% LPG fuel at different injection locations of liquid fuel for the same air mass flow rate of 200.14 kg/h and the same air-fuel ratio of 70. From the figure, it can be seen that the length of flame (identified by zero axial velocity) in the case of binary combustion at Xinj/D ) 0.189 and 0.377 is greater than the flame length in the case of LPG combustion only because the injected liquid fuel stretches the flame, while the flame length in the case of Xinj/D ) 0.566 is approximately equals to that in LPG combustion and shorter than the other two cases. In the downstream sections the axial velocity is approximately has the same value because the gas mass flow rate is constant and the temperature is slightly changed. The radial distributions of temperature along the combustion chamber in the case of Xinj/D ) 0.377 is shown in Figure 12 as a sample of this set. The figure is drawn on a basis of comparison form with LPG combustion only. The following conclusions can be obtained. (1) The location of maximum temperature is not changed, but its value is decreased as a result of liquid fuel injection.

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Figure 17. Effect of the percentage of injected kerosene on temperature contours along the combustion chamber (ma ) 200.14 kg/h, AF ) 70, and Xinj/D ) 0.377).

(2) The distribution of temperature is smoother than in the case of gaseous fuel combustion in the downstream sections. (3) It can also be seen that the temperature is increased in some of the down stream sections, which may be due to the complete combustion of liquid fuel in these sections. A discussion of this point will be covered later. Figure 13 shows the contours of temperature along the combustion chamber for the above experiments. It can be seen that the temperature in the case of Xinj/D ) 0.189 and 0.566 are approximately the same but lower than the values in the cases of both Xinj/D ) 0.377 and LPG combustion only. This can be explained as follows: (A) In the case of Xinj/D ) 0.189, the preheat zone of liquid kerosene is small, then droplets that emerges from the liquid fuel nozzle may be unvaporized, and consequently, the droplets may be incompletely burned.

(B) In the case of Xinj/D ) 0.377, the preheat zone of liquid kerosene is increased and the droplets of fuel is emerged in the flame zone which is responsible of completing the vaporization of the liquid fuel droplets and hence high temperature is obtained. (C) In the case of Xinj/D ) 0.566, the preheat zone is increased but the droplets emerging from the liquid fuel nozzle with a high velocity do not exposed to the flame for a long time, this means that the droplets may not complete its combustion and hence low temperature is obtained. B.2. Effect of Percentage of Liquid Fuel Injected. To study the effect of the percentages of liquid fuel injected, three experiments are carried out at Xinj/D ) 0.377 at kerosene percentages of 20, 25, and 30 of total mass of fuel. The air mass flow rate was kept constant at 200 kg/h and AF ) 70. Figure 14 shows the radial distribution of axial velocity along the combustion chamber



Figure 18. Effect of injection location on the heat transfer to cooling water.

Figure 19. Effect of injected kerosene percentage on the heat transfer to cooling water.

when kerosene percentage was 30 as a sample of this set of experiments. It can be seen that in the upstream sections the value of the maximum positive axial velocity is slightly increased with the increase in percentage of kerosene injected, while the absolute value of maximum negative velocity decreases with the increase in percentage of kerosene injected. Figure 15 shows the effect of liquid fuel percentage on axial velocity contours along the combustion chamber. It can be seen that the axial velocity in downstream sections for all percentages of liquid kerosene injected has approximately the same value and less than its value in the case of LPG combustion only. This is because the axial velocity is linearly proportional to mg/Fg. Since the total mass flow rate is constant and the temperature in the case of combustion of two fuels is changed so slightly, the axial velocity is approximately constant. The radial distribution of temperature at 30% kerosene is shown in Figure 16 as a sample of this set of experiments. Figure 17 shows the effect of percentage of injected kerosene on

Figure 20. Effect of both injection location of kerosene (Xinj/ D) and its percentage on the combustion chamber efficiency.

the temperature contours along the combustion chamber compared with LPG combustion only. It can be seen that the temperature is decreased with the increase in percentage of kerosene injected except in the first section. This is may be due to incomplete combustion of liquid fuel with the increase of its mass flow rate. The values of temperatures in the case of 20% kerosene is higher than the other two percentages and approximately the same as in the case of LPG combustion only. Heat Transfer. Figure 18 shows the effect of injection location on the heat transfer to the cooling water when kerosene percentage was 20%, while Figure 19 shows the effect of kerosene percentage on the heat transfer to the cooling water when Xinj/D is kept constant at 0.377. It can be seen from the two figures that the heat transfer to the cooling water was maximum at the first section, because higher temperature is existed in this section. This value of heat transfer to the cooling water is then decreased as the axial distance increases. The minimum value of qw occurred at X/D ) 4.5 then

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increases. This may because some of liquid fuel completes its combustion in the down stream section consequently higher temperatures and higher values of heat transfer to the cooling water. Combustion Chamber Efficiency. The combustion chamber efficiency can be calculated by dividing the heat transfer to the cooling water on the heat added from the fuel as follows:

ηc

)

100Qw ml(LHV)l + mg(LHV)g

(6)

where Qw ) total heat transfer to the cooling water (W), ml ) liquid fuel mass flow rate (kg/s), mg ) gaseous fuel mass flow rate (kg/s), ηc )combustion chamber efficiency (LHV)l ) lower heating value of liquid fuel (kerosene) ) 42.8 MJ/kg,12and (LHV)g ) lower heating value of gaseous (LPG ) 60% C4H10 and 40% C3H8) ) 45.973 MJ/kg. Figure 20 shows the effect of both injection location of liquid fuel and kerosene percentage on the combustion chamber efficiency. It can be seen from the figure that injection of liquid fuel during gaseous fuel combustion increases the combustion chamber efficiency. This may be because the burning velocity of a spray is significantly affected by turbulence.13 It is also established that burning velocity increases as the mass fraction of kerosene increased.5 Another reason may because the liquid kerosene has a higher heating value than that of LPG. Conclusions From the experiments carried out, the conclusion can be divided into two parts. (A) Injection Location of Liquid Fuel. (1) The absolute value of maximum negative axial velocity is decreased as a result of liquid fuel injection, while the maximum positive axial velocity is increased. (2) The length of flame is shortened as the location of liquid fuel injection is increased. (12) Bolz, R. E.; Tuve, G. L. Handbook of Tables for Applied Engineering, Science, Chemicals; New York, 1970. (13) Mizutani, Y.; Nishimato, T. Combust. Sci. Technol. 1972, 16, 1.

(3) Increasing the location of injection of liquid fuel from the burner exit (Xinj/D ) 0) leads to an increase in the value of temperature reaching a maximum at Xinj/D ) 0.377. This maximum value approximately equals to that of LPG combustion. Any further increase in the injection location leads to a decrease in the values of temperature due to incomplete combustion. (B) Percentage of Liquid Fuel Injected. (1) The increase in injected liquid fuel percentage leads to a decrease in the value of temperature in the upstream section (X/D ) 0.6644) and an increase in its values in the downstream sections because of incomplete combustion. (2) For all percentages of liquid fuel injected, the axial velocity in the downstream sections (above X/D ) 2.15) has approximately the same value but less than that value in the case of LPG combustion only. Nomenclature AF ) air-fuel ratio by mass D ) combustion chamber diameter d ) three-hole probe diameter K ) three-hole probe coefficient LHV ) lower heating value, MJ/kg m ) mass flow rate, kg/s P ) pressure gauge reading, kg/cm2 q ) heat transfer rate, W Ro ) inner radius of combustion chamber at any axial location, m Re ) Reynolds number V ) velocity, m/s X ) axial distance, m Greek Symbols ∆Po ) pressure drop across the orifice, mm H2O or mmHg ηc ) combustion chamber efficiency ν ) dynamic viscosity, m2/s F ) density, kg/m3 Subscript a ) air g ) gaseous fuel inj ) injection l ) liquid fuel w ) water EF000232J

Study of Combustion Characteristics of Coflowing Gas and Liquid Fuel

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