Autoignition and Stabilization of Diesel–Propane Lifted Flames Issuing

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Autoignition and Stabilization of Diesel−Propane Lifted Flames Issuing into a Hot Vitiated Co-flow Zhijun Wu,*,† Qing Zhang,† Liguang Li,† Jun Deng,† Zongjie Hu,† and Robert W. Dibble‡ †

School of Automotive Studies, Tongji University, No. 4800 Cao’an Road, Shanghai 201804, People’s Republic of China Combustion Analysis Laboratory, University of California, Berkeley, Berkeley, California 94720, United States

‡

ABSTRACT: Turbulent lifted flames of diesel−propane blend fuels issuing into a vitiated co-flow are recorded with a highspeed camera. The spray characteristics, flame structures, ignition delays, and lift-off heights of jet flames of diesel−propane blend fuels are analyzed in this research. As an additive to diesel fuel, propane has little influence on the autoignition process of diesel from the perspective of chemical kinetics but it improves the atomization, evaporation, and turbulence of the fuel spray. The addition of propane is beneficial for studying the interaction of the chemical kinetics and fluid dynamics in turbulent lifted flames. The experimental results show that the propane fraction has different influences on the ignition delay in two temperature ranges. The ignition delay increases with the increase of the fraction of propane when the co-flow temperature is lower than 1080 K and decreases when the co-flow temperature is higher than 1117 K. Thus, a related mechanism controlling the ignition delay of the blends is proposed by combining the physical preparation process and chemical preparation process. A zero-dimensional model based on chemical kinetics supports this conclusion. For flame stabilization, a higher fraction of propane results in a higher lift-off height in the temperature range of 1043−1098 K, a lower lift-off height in the temperature range of 1117−1155 K, and a higher lift-off height in the temperature range of 1175−1194 K. A mechanism for stabilizing lifted flames from liquid blend fuels is proposed from the perspective of autoignition and turbulence.

1. INTRODUCTION Autoignition is the most important process in compression ignition engines and other technical devices and has been studied for several decades by many researchers.1−5 A wellcontrolled autoignition is necessary in some cases, such as homogeneous charge compression ignition (HCCI) engines and lean-premixed pre-vaporized (LPP) gas turbines,6,7 while it should be prevented under special circumstances, including preignition and knock in spark-ignition (SI) engines.8−10 To better control autoignition, it is necessary to work on the autoignition and flame stabilization mechanism in premixed or partially premixed combustion of both gas and liquid fuels. The strong interaction between turbulence and chemical kinetics plays an important role in the autoignition and flame stabilization of turbulent jet fuel, especially for the lifted flame under a relatively high jet velocity.11,12 The autoignition and stabilization of the lifted flame are usually controlled by the turbulent stretching, molecular transport process, chemical kinetics, and propagation velocity of the turbulent flame. Many experiments studying H2/N2 and CH4/air lifted flames have been performed in a hot vitiated co-flow burner.13−20 The flame temperature distribution, ignition delay, lift-off height, important species, and time-resolved parameters of the lifted flame are investigated in these experiments. Different numerical simulations were conducted on the basis of the results of these experimental data.21−25 Various models (e.g., Reynoldsaveraged Navier−Stokes, large eddy simulation for flow, probability density function, and conditional moment closure for combustion) and direct numerical simulation are employed. All simulation results were aligned with the experimental data, even though different simulation methods had been used. However, studies of liquid fuels have yet to be thoroughly © XXXX American Chemical Society

investigated. Special attention should be paid to spray breakup, atomization, and evaporation in the research on liquid fuels. For liquid fuels, the lifted flame structure of turbulent dilute methanol sprays with either air or nitrogen as a carrier was studied, and OH and CH2O distributions were measured.26 The lifted flame of a jet gasoline−diesel blend fuel was analyzed, and the “simultaneous autoignition temperature” concept was proposed.27 A related simulation coupling the eddy dissipation concept (EDC) model and a simplified reaction mechanism had good agreement with the previous experiment.28 However, more efforts should be invested to the study of liquid lifted flames, especially for the mechanisms of ignition delay control and flame stabilization. Furthermore, previous studies were mainly based on different boundaries of combustion;27,28 this study draws comparisons of ignition delays and lift-off heights based on physical properties of fuels other than boundaries of combustion. In the present study, the diesel−propane blends are injected into a hot co-flow generated from a lean premixed hydrogen/air flat flame at atmospheric pressure. There are two reasons for the selection of diesel−propane blends. First, by changing the physical properties of the blends, the contrast in the lifted flames can be obtained under the same boundaries of combustion. Propane has a much longer ignition delay and is less active at the beginning of combustion, which means that the addition of propane has little influence on the autoignition of diesel from the perspective of the chemical reaction process. The simulation in section 3.3 supports this conclusion. Received: May 2, 2016 Revised: October 9, 2016

A

DOI: 10.1021/acs.energyfuels.6b01042 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels However, the flash boiling effect of propane greatly increases the turbulent velocity fluctuation, improves the atomization and evaporation, and accelerates the mixing process of the fuel and oxidant from the co-flow.29 Second, propane has many practical applications and is a good prospect as an alternative fuel. To be specific, the propane additive improves the atomization of diesel and, thus, reduces the emissions of soot and hydrocarbon.30 In addition, propane has a higher latent heat of evaporation and facilitates low-temperature combustion (LTC). The study focuses on (i) the autoignition mechanism of liquid fuels obtained by changing the physical properties of fuels and combining the chemical kinetics and physical preparation process and (ii) lift-off height in a high co-flow temperature range (1175−1194 K), which is inconsistent with ignition delay. Thus, visualization research is performed on the lifted flames. The spray and flame structures, ignition delays, and lift-off heights by propane fraction and co-flow temperature are measured. The comparison of the data of four fuels indicates the interaction results of fluid dynamics and chemical kinetics and reveals the mechanisms that control the ignition delay and flame stabilization of liquid fuels.

injected into a known volume accumulator chamber, and then the piston is pushed up to exhaust the air in the chamber and the connecting pipe. Next, the propane is forced into the pressure chamber with nitrogen at 2 MPa after connecting the upside-down propane cylinder with the accumulator chamber and opening the valve. The fuel line is then shifted to the injector. Finally, the accumulator chamber is shaken to ensure the homogeneity of the blends. During these processes, air is eliminated and propane is maintained in the liquid state. 2.2. Fuel Injection and Combustion System. As shown in Figure 2, the fuel injection control and data collection system consist

2. EXPERIMENTAL SECTION 2.1. Preparation of Diesel−Propane Blend Fuel. Fuels of four propane fractions are represented by P0, P10, P20, and P30, where P stands for propane and the numbers represent the volume fraction of propane. Some of the physical and chemical properties of the fuels under normal temperature and pressure are shown in Table 1.

Figure 2. Sketch of the blend spray system. of an accumulator, high-pressure pipes, solenoid valves, a personal computer (PC) controller, and a high-speed camera. The capture rates selected are 2000 frames per second (FPS) for spray and 5000 FPS for flame. In this paper, the Cabra buner is used to provide the hot co-flow.31 The flow rate of hydrogen and air are adjusted to achieve the required co-flow velocity and temperature. The parameters of the co-flow and injector are shown in Table 2. The region of interest is a cylindrical

Table 1. Properties of Diesel and Propane diesel density (kg/m3) dynamic viscosity (mPa s) latent heat of evaporation (kJ/kg) cetane number boiling point (°C) low calorific value (MJ/kg)

propane

860

489.3 (l)

21.7 (g)

1.723−6.883

95.13 × 10−3 (l)

8.823 × 10−3 (g)

260

Table 2. Important Parameters for the Diesel−Propane Lifted Flames

426

co-flow temperatures (K) 45 283−338 42.5

−2 −42.07

mass flow rate of co-flow (g/s) injection duration (s) injection pressure (MPa) orifice diameter (mm)

46.33

1043, 1061, 1080, 1098, 1117, 1136, 1155, 1175, and 1194 33 0.1 16 0.16

region, 200 mm in diameter and 350 mm in height (from the nozzle to the downstream distance), in which the temperature field is most stable. A vortex flowmeter and rotameter are used to measure the flow rate of air and hydrogen. The temperatures are measured by thermocouples, and the radiation factor is considered by the Shaddix method.32 The measured data have maximum uncertainties as follows: 1% for the flow rate of air, 1.5% for the flow rate of hydrogen, and 0.75% for the co-flow temperatures. Table 3 shows the compositions of the co-flow under the selected temperatures. Z is the mole fraction of the species in the co-flow. V is the volume flow rate of the co-flow. Slight differences are found between the compositions under these temperatures. 2.3. Image Post-processing and Data Evaluation. An automated image processing code is developed with MATLAB software to convert the massive number of images obtained from the high-speed camera to quantitative data. For flame, the grayscale (Gs) is calculated from the color information (R, G, and B) of the images by the following equation:

The preparation process of blend fuels is shown in Figure 1. The gas space of the propane cylinder is charged with nitrogen at 2 MPa to ensure the liquid state of propane and to push the propane out when the cylinder is placed upside down. A certain volume of diesel is

Gs = 0.4R + 0.3G + 0.4B

Figure 1. Sketch of the blend preparation system. B

DOI: 10.1021/acs.energyfuels.6b01042 Energy Fuels XXXX, XXX, XXX−XXX

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earlier and presented in the literature.33 The results indicate that the breakup, atomization, and evaporation of the blends change greatly with the addition of propane. Figure 4a shows the movement of the spray front in the centerline over time. The spray penetration increases over time, and the spray with a higher propane fraction penetrates a shorter distance at the same time. The velocity of the spray is obtained by the derivation of the distance with respect to time. Figure 4b shows that the jet velocity declines at the same height as the fraction of propane increases. There are two reasons for this result. The gas resistance is relatively higher when the density of blends with a higher propane fraction is lower, and the inertia is smaller. The flash boiling caused by propane evaporation accelerates the breakup, atomization, and evaporation of the blends and results in a smaller droplet size. Droplets of a smaller size have a larger gas resistance relative to their momentums. Under a higher co-flow temperature, such as 600 K, the transient spray penetration decreases over time occasionally, which is due to the fast evaporation of small droplets. However, the spray characteristic under a higher coflow temperature will not be presented in detail in this paper. 3.2. Flame Structures. Figure 5 shows the flame structure from the start of injection to steady combustion. The co-flow temperature of 1136 K is selected. The jet fuel passes through a visible laser above the nozzle, and thus, t = 0 ms is defined in the timeline. The sporadic flame then appears, the flame zone extends, and the steady lifted flame is finally formed. The autoignition occurs earlier as the percentage of propane increases (the spark is blurred after zooming out the picture). The largest flame width (40.7 mm) and flame cone angle (28.8°) occur in the pictures of P20 fuel when the lifted flame is steady. Analysis suggests that the flash boiling of propane reduces the size of fuel droplets. The resistance from the axial direction that accelerates the flow fluctuation and the turbulent mixing process is relatively large. Therefore, the flame is more likely to develop radially. The result is consistent with the spray characteristic in room temperature mentioned in section 3.1. It is notable that the pictures of P30 flame show the narrowest width (29.2 mm) and flame cone angle (18.9°), even compared to pure diesel (33.3 mm and 23.2°). Figure 6 shows the quantitative data obtained by the average of the 30 flame images after the flame stabilizes. For the P30 flame, the widths of up- and downstream flames are close, forming a cylindrical flame area, instead of a cone. It is suggested that the density of liquid propane is much less than that of diesel according to Table 1, and thus, propane consumes less oxygen than diesel at the same volume. When propane reaches 30%, the oxygen in the cylindrical area is sufficient to support combustion, so that the diffusion flame will not propagate farther in the radial direction. 3.3. Ignition Delay. Figure 7 shows the ignition delays of the four fuels investigated under co-flow temperatures at intervals of approximately 20 K. The ignition delay is defined as the duration from injection (0 ms) to the first bright spot (regardless of the laser spot) in the camera. The data points at 1043 K are unsuitable for quantitative analysis because the liftoff heights exceed 200 mm, which is the boundary of the stable and constant temperature field. The experiment shows that the ignition delays of all blend fuels decrease with increasing the coflow temperature. A high co-flow temperature accelerates the chemical reaction rate and provides enough activation energy in a short time. In addition, for liquid fuels, a higher co-flow temperature accelerates the evaporation and the mixing process

Table 3. Compositions of Co-flow in Different Co-flow Temperatures temperature (K)

ZH2O

Z O2

Z N2

Vtotal (m3/h)

1043 1061 1080 1098 1117 1136 1155 1175 1194

0.063 0.065 0.067 0.069 0.07 0.073 0.075 0.077 0.078

0.175 0.173 0.173 0.170 0.169 0.166 0.165 0.163 0.162

0.762 0.761 0.761 0.761 0.761 0.761 0.760 0.760 0.760

113.6 113.7 113.8 113.9 114 114.2 114.3 114.4 114.5

The weight of the blue color is greater than that used in normal methods to emphasize the light blue from premixed combustion in the flame base. The grayscale threshold of the flame or ignition is 5 in the codes. The definition principle that we used in setting the threshold is the minimum value that can filter the background interference. The small-scale noise is filtered by the “bwareaopen” function, which removes all connected components with less than 20 pixels. The ignition delay, lift-off height, and flame kernel are determined by this method. To validate the reproducibility of experimental data, two sets of spray data and three sets of flame data under identical experimental conditions are compared. The relative differences in the maximum spray penetration, spray angle, ignition delay, and lift-off height are all less than 5%. There is a 0.2 ms error for all experimental ignition delays, owing to the capture rate.

3. RESULTS AND DISCUSSION 3.1. Spray Characteristics. Figure 3 shows the spray characteristics of the fuels captured at room temperature (300 K). Fuel with a higher propane fraction presents a larger spray cone angle. The spray cone angles are obtained by the average of 30 images for each case, and they are 13.34°, 18.76°, 20.05°, and 23.94° for sprays P0, P10, P20, and P30, respectively. Some of the more quantitative results of the spray were obtained

Figure 3. Images of the spray development process of the blends. C

DOI: 10.1021/acs.energyfuels.6b01042 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 4. (a) Spray penetration over time and (b) spray front velocities at different heights.

Figure 7. Ignition delay versus background temperature.

tP30. These mean that the autoignition delay is increased at a low co-flow temperature and reduced at a high co-flow temperature by the addition of propane. Chemical ignition delays (CIDs) of P0 and P100 in Figure 7 show the calculated stoichiometric CID (defined as the time of maximum heat release) of pure diesel and propane controlled by chemical kinetic changes with initial temperature. A zerodimensional closed homogeneous model is employed, and the chemical preparation process is quantified. Here, diesel and propane are mixed with the co-flow compositions (in Table 3) under the corresponding temperatures. The ignition delay of diesel in the figure is calculated by a detailed chemical reaction mechanism of n-heptane because the cetane numbers of diesel and n-heptane are close.34 The ignition delay of propane is calculated by a C1−C4 reaction mechanism.35 The chemical preparation process becomes faster as the temperature increases, and that of n-heptane is much shorter than that of propane under the same initial background temperature. The comparison shows the difference in the real ignition delay and the CID. The real ignition delay is longer than the CID under a low temperature for P0 fuel. The reason is that the atomization, evaporation, and mixing process with oxygen proceed slowly under that temperature. Figure 8 shows the main species in propane and n-heptane stoichiometric oxidation reaction at 1098 K. The same reaction mechanisms are adopted in the calculation of CIDs of P0 and P100. The decomposition reaction paths of n-heptane are more various than those of propane, and the reaction rates are much faster. The ignition delay of n-heptane is 7.86 ms, and elementary reactions before this time point are compared.

Figure 5. Images of lifted flames after injection.

Figure 6. Flame width and cone angle.

with oxygen. The above two factors lead to a faster ignition delay decrease. However, an interesting phenomenon is noticed that the ignition delays of four fuels are compared in different co-flow temperature ranges. The blend fuel with a higher propane fraction has a longer ignition delay in the co-flow temperature range (1043−1080 K), which is tP30 > tP20 > tP10 > tP0. The curve representing blends of the higher propane fraction intersects with the lower propane fraction curve after the coflow temperature exceeds 1080 K. In the co-flow temperature range (1117−1194 K), the blend fuel with a higher propane fraction has a shorter ignition delay, which is tP0 > tP10 > tP20 > D

DOI: 10.1021/acs.energyfuels.6b01042 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 8. Main species in the autoignition process at 1098 K. The orders of magnitude of some species are enlarged to fit the graph. (a) All species in C3H8 oxidation, 10. (b) In C3H8 combustion, CO, 4 × 102; CO2, 4 × 104; OH, 4 × 105; H, 106; and O, 107 and in n-C7H16 combustion, OH, 4; and O, 10.

ature. As mentioned above, the real ignition delay is mainly controlled by the factor whose preparation is longer. The chemical preparation process dominates the autoignition delay when the co-flow temperature is lower than a certain value, which is close to 1100 K. In addition, under a co-flow temperature higher than 1100 K, the decrease in the ambient temperature caused by the addition of propane does not have much influence on the ignition delay. However, the physical mixing process proceeds much faster by the flash boiling of propane. It is reasonable that propane reduces the ignition delay, and the ignition delay is mainly decided by the physical preparation process in the high co-flow temperature range (1117−1194 K). 3.4. Lift-off Height and Flame Stabilization. The lift-off height is an important parameter of lifted flame stabilization and the consequence of interactions of fluid dynamics and chemical kinetics. Figure 9 shows the lift-off heights of the four

Figure 8a shows that propane consumes only 5.7%, whereas there is barely any n-heptane left at 7.86 ms. The mole fractions of intermediate species C2H4, C3H6, and H2 in the propane reaction are less than those in the n-heptane reaction by approximately 1 order of magnitude at the same time point. Figure 8b shows the mole fraction of some active species and oxidation products. The concentrations of these important species indicating the degree of combustion vary widely for the two fuels. It is concluded that the addition of propane has little influence on the autoignition process of n-heptane from the perspective of chemical kinetics. The CIDs of the blend fuels are supposed to be controlled by the diesel autoignition process. During the overall process of autoignition, the jet spray is not ignited immediately, owing to the quenching at the nozzle exit. The flame quenching is caused by the drastic endothermic evaporation of the liquids. When the spray penetrates downstream, the turbulent stretch decreases and the combustible gas mixture is generated after the process of heating, atomization, evaporation, and mixing. Under the same co-flow temperature, the ignition delay is controlled by two main factors: (i) the physical preparation process, which is mainly controlled by the co-flow temperature and fuel properties in this experiment and consists of the evaporation of the liquid fuels and the mixing of the fuel and oxidant, and (ii) the chemical preparation process, which is the temperaturedependent decomposition and oxidation process before strong exothermic reactions and is affected only by the co-flow temperature in this experiment because the additive propane is ignored. When the two factors are taken together, the real ignition delay is mainly controlled by the slow factor. Furthermore, the addition of propane affects the processes in two ways. One is that the flash boiling effect accelerates the breakup, atomization, evaporation, and mixing process of the fuel drops and, thus, accelerates the physical preparation process. The other is that the faster evaporation process and relatively higher latent heat of evaporation slightly lower the ambient temperature and, thus, slow the chemical preparation process. The experiment shows the final results, which are decided by two interaction pathways. The higher the propane fraction, the longer the ignition delays are under a low co-flow temperature. The opposite situation occurs under a high co-flow temper-

Figure 9. Lift-off height versus co-flow temperature.

fuels under different co-flow temperatures. Each of the points is the average value of 100 images after the flame is steady. Similar to the ignition delay, the values higher than 200 mm are unsuitable for quantitative analysis. The heights of three points (P10, P20, and P30 at 1043 K) in the figure exceed the scope of the corresponding photos and are, therefore, meaningless. The lift-off heights of the four fuels all decrease with increasing the co-flow temperature. It is obvious that a higher temperature accelerates the autoignition process and results in a E

DOI: 10.1021/acs.energyfuels.6b01042 Energy Fuels XXXX, XXX, XXX−XXX

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flame propagation, and the stable lift-off height is restricted by the quenching distance at a high co-flow temperature, which is affected by turbulence in this experiment. This conclusion for liquid fuels is different from the single gas fuel jet flame stabilization mechanism proposed by Patwardhan et al.23 The latter does not consider several physical processes of liquid fuels, including atomization and evaporation. At low and medium co-flow temperatures (1043−1155 K), the phenomenon that the flame base approaching upstream after autoignition is not found and the final lift-off height is even higher than the initial autoignition position. This is inferred to be a result of continuous injection. The lifted flame in this experiment is provided by a continuous spray that brings a series of influences. The ambient temperature slightly drops over time, and the jet velocity increases at the same height, owing to gas flow caused by previous spray. The reactant mixture goes farther during the same ignition delay period. The change rule of the lift-off height is similar to that of the ignition delay under low and medium temperatures, and it is supposed that the lift-off is controlled by autoignition. In addition, the lift-off height of the P30 fuel flame is highest in almost all temperature ranges. It could be that excess propane extends the application range of the mechanism under a high co-flow temperature and eliminates the medium co-flow temperature area. In addition, by combination of the distinctive flame structure in Figure 5, there may be a different stabilization mechanism for blends of a high propane fraction.

lower lift-off height. The curves of P0, P10, and P20 are compared. In the low-temperature range (1043−1098 K), the lift-off height of P0 fuel is the lowest and the curves of P10 and P20 fuels are close. In the medium-temperature range (1117− 1155 K), the P0 curve intersects the P10 and P20 curves and the lift-off height of P0 fuel is higher than those of the other two fuels. In the high-temperature range (1175−1194 K), the lift-off height of P0 fuel decreases rapidly and becomes the lowest again. To sum up the comparison, the addition of propane causes the lift-off height to be higher, lower, and higher again in three co-flow temperature ranges from low to high. Differences are found between the variation trend of the ignition delay and lift-off height at a high co-flow temperature. Therefore, further analysis is needed of the phenomenon in which the addition of propane increases the lift-off height in the high co-flow temperature range. Figure 10 shows the position of the flame base of P0 and P10 fuels with co-flow temperatures 1080 K (in low- and medium-

4. CONCLUSION The fuels of this experiment are carefully designed and compared under the same test conditions to eliminate interference factors, such as co-flow temperatures. The physical and chemical preparation processes can be analyzed and compared directly from the fuel properties and a zerodimensional simulation. The results presented in this paper lead to the following conclusions: (1) The flash boiling effect caused by propane improves the atomization and evaporation of diesel fuel. (2) Under identical conditions, the ignition delay is mainly controlled by a chemical kinetic factor under a low coflow temperature (1043−1080 K) and by the physical preparation process under a high co-flow temperature (1117−1194 K). (3) Lift-off is controlled by autoignition under low and medium co-flow temperatures (1043−1155 K) and by turbulence under a high co-flow temperature (1175− 1194 K). (4) Jet blends with a propane volume fraction of 30% have a distinctive flame structure and the highest lift-off height in an identical co-flow temperature.

Figure 10. Variation of the flame base after ignition.

temperature ranges) and 1194 K (in the high-temperature range). The appearance of the laser spot is set as 0 ms, and images are selected every 4 ms. The pictures are clipped to clarify the flame base, and the lift-off position is marked with a white line. htop (hbtm) is the distance from the nozzle to the top (bottom) boundary of the local images. The lift-off heights of the flames of the two fuels decrease over time under a co-flow temperature of 1194 K, whereas there is no obvious change rule for the co-flow temperature of 1080 K. Under a high co-flow temperature (1175−1194 K), the flame base continues approaching upstream after autoignition. Excessive turbulent stretching at the nozzle exit quenches the flame, and the lift-off height is so low in this temperature range that it is restricted by the quenching distance. The quenching distance increases with increasing the turbulence intensity caused by the boiling effect of propane. In this case, a higher propane fraction results in a higher flame stabilization point. Therefore, the phenomenon that the flame base moves upstream at a high co-flow temperature is caused by turbulent

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-02169589205. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (91441125 and 51576141).

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DOI: 10.1021/acs.energyfuels.6b01042 Energy Fuels XXXX, XXX, XXX−XXX