Comparison of Different Gasoline Alternative Fuels in Terms of

Article pubs.acs.org/EF

Comparison of Different Gasoline Alternative Fuels in Terms of Laminar Burning Velocity at Increased Gas Temperatures and Exhaust Gas Recirculation Rates Tobias Knorsch,*,†,‡ Andreas Zackel,† Dmitrii Mamaikin,† Lars Zigan,†,‡ and Michael Wensing†,‡ †

Department of Engineering Thermodynamics, and ‡Erlangen Graduate School in Advanced Optical Technologies, Friedrich-Alexander-University (FAU) Erlangen-Nuremberg, Am Weichselgarten 8, D-91058 Erlangen, Germany ABSTRACT: The butanol isomers n-butanol and isobutanol as well as ethanol are among the biofuels most likely to be used for engine combustion and are likely to become more relevant as surrogate fuels or blend components in the future. To use the potentials of alternative fuels, the combustion mechanisms and, thus, burning behavior should be known. A key parameter for flame kinetic studies and combustion simulation is the laminar burning velocity. However, reproducible measurements of flame speeds of gasified liquid fuels are difficult. To reduce the scattering of experimental results to acceptable levels, there is a common research interest of several European institutes using so-called heat flux burners. In this work, a self-designed liquid fuel evaporation system has been combined with a precise droplet generator, which allows for measurements of adiabatic laminar burning velocity at elevated gas temperatures. The one-dimensional (1D) flame appears very stable, even close to its flammability limits. The boundary conditions are chosen to model exhaust gas recirculation (EGR) with different temperatures and compositions of the mixtures. The measurement data are compared to the corresponding results of other research groups, and the uncertainty of the burning velocity is estimated.

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compensated by heating a double-walled heating jacket, which is mounted around the burner plate14 (Figure 1). From this, the fuel−air

INTRODUCTION The laminar burning velocity (sL) is a physicochemical parameter dependent upon several external conditions, such as fuel, temperature, pressure, equivalence ratio, etc. It is an important parameter for the design of burners and internal combustion (IC) engines. In terms of simulations, this property is a key chemical data for validation of combustion models. For substitute fuels of conventional gasoline (e.g., heptane and isooctane), the flame velocity is known; however, investigation of modern blends with biofuels has just started to be enhanced with novel techniques. The measurement of sL using conventional methods is quite complicated, and therefore, the values for sL are scattering. To measure sL, various types of burners, such as the counterflow burner,1 spherical bomb combustor,2−7 flat-flame burner,8 and heat flux burner,9−15 have been developed. Within this work, an in-house-developed evaporation system16 for heat flux burners is used, which was enhanced using a droplet generator instead of injector and mass flow controllers in this work. Furthermore, for this paper, a broad measurement series of the laminar burning velocity of different standard and novel gasoline fuels and alternative fuels is carried out. For simulation of an IC engine with charge stratification and exhaust gas recirculation (EGR), a wide range of sL depending upon equivalence ratio, temperature, and EGR must be covered. The test bench enables the measurement of accurate, reproducible adiabatic laminar flame velocities of liquid fuels up to 473 K gas temperature.

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Figure 1. Perforated burner plate of the Heat-flux-burner plate in combination with mounting positions of the thermocouples. mixture is heated as soon as it passes the brass outlet grid.11,14 The state can be assumed as quasi-adiabatic8 because the flame still loses heat. However, this heat loss is just compensated by setting a higher outlet temperature. The heating jacket temperature is permanently conditioned to 50 K above the mixture gas temperature. The perforated burner outlet grid is responsible for achieving a stable one-dimensional (1D) flat flame. More details on the burner principle can be found in refs 9 and 14−16. A liquid fuel evaporation system (“HeatFluxER”16) is combined with a high-precision piezo-actuated droplet generator in this work. The droplet generator allows for a more precise fuel metering compared to the automotive fuel injector used in ref 16. It provides a constant chain of monodisperse fuel droplets with a diameter of 40− 100 μm, which can be adjusted by replacing the nozzle plate (bore hole diameters of 20−50 μm) of the droplet generator. The small droplet size supports a fast evaporation and, hence, the formation of a

EXPERIMENTAL SECTION

The laminar flame velocity is not measured directly with the heat flux burner. It is derived using several measurable characteristics of the kinematic equilibrium. The natural heat loss of the flame is © 2014 American Chemical Society

Received: April 2, 2013 Revised: January 16, 2014 Published: January 17, 2014 1446

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⎛ 3u D2 ⎞1/3 d = ⎜⎜ D ⎟⎟ ⎝ 2fG ⎠

homogeneous mixture without possible pulsations. The gap between two single droplets is as large as the droplet diameter. For a directed flow to the burner and to prevent condensation of the air−fuel mixture, the double-walled tube is heated above the boiling temperature (currently up to 473 K) by heating thermostats and an air-flow heater. The evaporator is located upstream of the heat flux burner. To enhance the mixture preparation, a resonator and path extension spiral are applied in the evaporator. The droplet chain is directly impinging on a double-walled hot surface. The incoming air is regulated by a mass flow controller (MFC). Another MFC is used for nitrogen supply, which, in turn, is mixed to the air flow for EGR simulation. The total air flow (with or without EGR) is also checked by a rotameter behind the MFC. Thus, a key advantage of the applied optimized system is the possibility of reproducible precise air and fuel mass flow and, thus, measurement data. Figure 2 shows the main parts of the experimental setup.

(4)

The fuel mass flow is based on a study of gravimetrical injection mass measurements with variable pressure. Thus, by adjusting the pressure and measuring the fuel mass of each pressure point for a specific time, the calibration function can be obtained. Because of differences in viscosity, density, and other physicochemical properties for each fuel, a gravimetrical mass calibration is carried out. For this work, a temperature difference from the initial gas temperature to the burner outlet grid of 50 K is kept constant at all times to compensate for the heat loss11,14,15 of the flame. Eight thermocouples (TCs) are mounted in the burner grid (Figure 1), in the applied setup 250 μm below the grid plane (diameter of a TC equals the diameter of a single hole). For both reading and logging the temperature profiles along the grid radius, a 10-channel TC data logger is applied. Detailed information about the laminar burning velocity measuring technique and calculations can be found in a previous publication.16 To determinate sL, different flow velocities uflow are applied. All relevant temperature profiles across the burner outlet profile are read and saved for subsequent processing. In the next step, the temperature profiles are fitted by the following function:18

T (r ) = Tcenter + Cr 2

(5)

where C stands for the parabolic coefficient and r is the radius of the burner outlet grid.18 Figure 3 shows exemplarily measured temperature

Figure 2. Setup of the evaporator “HeatFluxER” and burner.

To obtain a continuous, homogeneous flow of fuel without pulsations caused by injections, a high-precision droplet generator is used, which is basically a tube with a small nozzle pinhole at the tip. The mechanism of generating droplets is based on the Rayleigh breakup of laminar fluid jets. This principle is used to create a controlled decay of the jet by the usage of defined interference waves, which are transferred to the fluid jet by the piezoceramic vibrating element. The stability criterion implies that a jet decays into monodisperse droplets if the wavelength of deformation on the jet surface is higher than the circumference of the jet. The applied interference wave, which influences the fluid jet, produces droplets of the same size and predictable diameter d, if the following condition applies to the excitation frequency f G:17 0.3uD 0.9uD ≤ fG ≤ πD πD

Figure 3. Temperature profiles with parabolic fit functions (isooctane, 343 K, and Φ = 1.0).

profiles and the correlating parabolic fit functions for a wide range of mixture velocities. The exactly horizontal function is challenging to measure. Thus, positive and negative parabolic functions are measured, and the horizontal functions can be found by interpolation. The parabolic coefficient18 q C=− (6) 4kh

(1)

where D is the pinhole diameter and uD is the jet velocity. By building the ratio of the fuel volume flow V̇ fuel and the pinhole cross-sectional area S, the jet velocity can be determined:

uD =

̇ Vfuel S

is dependent upon the heat transfer q, the heat conductivity k, and the burner grid height h. More details can be found in ref 18. The achieved parabolic coefficients out of Figure 3 and eq 6 are compared to the set flow velocities. In Figure 4, the parabolic coefficients marked red are not included in the fit function for the interpolation of the laminar burning velocity because they typically show up as soon as the flame starts to lift off for outflow velocities higher than the desired laminar burning velocity. At a certain outflow velocity above the laminar burning velocity, the flame lifts off from the grid and is not flat any more. For these lift-off points (in this case, C > 0.025), the temperatures are quite low and start fluctuating because the flame is not stable anymore. Thus, these values (marked red in Figure 4) are not included in the determination of sL, and no discontinuity of the fit function occurs.

(2)

where the fuel volume flow is the ratio of the fuel mass flow and fuel density. ̇ = Vfuel

ṁ fuel ρfuel

(3)

The resulting droplet diameter should be as small as possible for a fast conversation to the gas phase and can be calculated with the following equation:17 1447

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Table 1. Uncertainties of System Components component

Figure 4. Interpolation of the laminar burning velocity from the parabolic coefficient fit function (isooctane, 343 K, and Φ = 1.0). In addition, the points that are very far from the correct velocity do not affect the final results because only some measurements with too high outlet velocity are sufficient, as shown in Figure 5. Thus, to

medium

rotameter

air

thermostat (Lauda, 5L) thermostat (Lauda, 10L)

thermal oil thermal oil

digital manometer (Keller) digital balance (Kern) thermocouples

nitrogen

mass flow controller (Bronkhorst) mass flow controller (Bronkhorst)

air nitrogen

parameter

uncertainty

combustion air volume flow heating ring temperature evaporator/ chamber temperature fuel injection pressure fuel amount

1.6% Rd at V > 50%

temperature at the burner grid combustion air volume flow

0.05% Rd + 1 K

inert gas volume flow

0.2 K 0.2 K 10 mbar 1 mg

0.5% Rd + 0.1% FS 0.5% Rd + 0.1% FS

uneven height of the soldering point of the microprobe pair inside the 0.5 mm stainless-steel shaft of the TC might be present (see Figure 5). At a radial distance of 9 mm, a deviation from the fit function is obvious. This can cause an uncertainty up to 1.5%, as indicated in Figures 5 and 6 with error bars. The presence of a very long mixing pipe and conditioned doublewalled evaporator and plenum chamber of the burner enhances the homogeneity of the gasified fuel and oxidizer. However, to prove the mixture homogeneity and its impact on the flame measurements of formaldehyde (CH2O) as a marker of the flame are carried out within this work. For this, a pulsed frequency-tripled Nd:YAG (Quantel Brilliant, 355 nm, 40 mJ) is used to excite CH2O. The laser light is formed to a laser light sheet and focused on the flame above the burner grid. The height of the laser light sheet is 15 mm. The fluorescence signal of the formaldehyde is detected by an intensified charge-coupled device (CCD) camera (ICCD, PCO DiCam, 100 ns detection). Furthermore, a 360 nm long pass filter combined with a 492 nm short pass filter (AHF AG) is mounted in front of the ultraviolet (UV) lens of the camera (B. Halle Berlin, f = 100 mm, UV enhanced for 195−495 nm). The formaldehyde signal is presented as both single and mean images of 10 single shots for half of the burner grid (see Figure 7). An inverted image furthermore enhances the

Figure 5. Temperature profiles with parabolic fit functions (isooctane, 373 K, and Φ = 1.1). determine the laminar burning velocity with reasonable effort, it is sufficient to use only a smaller range of velocities, where the fit function behaves almost linearly (compare Figure 4 to Figure 6). The uncertainty of the laminar burning velocity is caused by the uncertainty of all system components, which are listed in Table 1. The maximum uncertainty of sL was calculated with an error propagation from those individual uncertainties to be ∼0.8 cm/s. All TCs are pushed down in the grid by a 250 μm punch. However, an

Figure 7. (Left) Mean image and (right) single image of CH2O for isooctane, 373 K, and Φ = 1.0. representation of the signal distribution depth. The fluorescence images prove that the flame front is flat and very stable (i.e., no oscillations) in the area where the TCs are placed. This proves the homogeneity of the mixture and flow profile outside the grid.

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RESULTS AND DISCUSSION Calibration Procedure. Prior to the adiabatic burning velocity study, it is necessary to calibrate the experimental setup to reach the most precise measurement results. Because of different fuel densities, viscosities, and other physicochemical properties, the calibration process has to be carried out for each fuel right before the measurement procedure. To achieve a stable flame, the smallest possible size of droplets is desired. With respect of this information and considering the fueldependent heat of vaporization and, therefore, different

Figure 6. Interpolation of the laminar burning velocity from the parabolic coefficient fit function (isooctane, 373 K, and Φ = 1.1). 1448

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evaporation time, different nozzle pinhole diameters of the droplet generator are used for different fuels (35 and 50 μm). As an example for the calibration study, isooctane with a nozzle pinhole diameter of 35 μm is used in Figure 8. The constant

Figure 9. Fuel mass flow calibration functions for n-butanol.

laminar burning velocities of isooctane and literature values shown in Figure 10 are in the range of currently available Figure 8. Fuel mass flow calibration function for isooctane.

droplet chain is injected in a tightly closed test bottle, which is afterward gravimetrically measured (measurement time of 300 s each) by a digital balance. The measurements are repeated 10 times for each fuel pressure. Several independent measurements have been carried out and repeated to ensure a constant and reproducible measurement of the obtained function. Fuel pressure describes the difference between the injection pressure in the “HeatFluxER” and the atmospheric pressure. The gravimetrical measurements of fuel mass have been carried out for different fuel pressure points. Using eqs 3 and 4 and Bernoulli’s equation

Δp =

Figure 10. Isooctane 423 K adiabatic burning velocities of the present work compared to refs 19 and 20.

ρfuel uD2

(7) 2 the dependency of second order between fuel pressure and fuel mass flow was derived. On the basis of the resulting calibration points, the fit calibration function of second order has been created for each fuel after the calibration measurements and verified daily before the measurements. The viscosity of the investigated fuels and, therefore, the fuel mass flow are affected by the injection temperature. Thus, several calibration procedures with corresponding gas temperature conditions for each measurement have to be carried out as the nozzle tip is heated by the hot air flow. As an example, the calibration measurement results of n-butanol with two temperature conditions are presented in Figure 9. Measurements of Isooctane and n-Heptane Flames. In this section, both fuels are compared to other references before all flame velocities are presented, with variable gas temperature at the end. The first fuel to be combusted and tested with the modified “HeatFluxER” is isooctane. The high stoichiometric amount of air lst allows for the use of a nozzle plate with a diameter of only 35 μm. The injected fuel amount is completely evaporated and homogeneously mixed with the preheated air, which is shown by constant resulting temperatures once a stable flame is achieved (see also Figure 7). A total of 12 different equivalence ratios are being applied (0.7− 1.6). In general, the variation of the equivalence ratio exhibits the typical parabolic function of the laminar burning velocity, depending upon the equivalence ratio. The resulting adiabatic

simulation using the flamelet approach19 and analytical model.20 n-Heptane has a similar stoichiometric amount of air and boiling point to isooctane. Thus, the nozzle plate used for isooctane could also be applied for n-heptane. The measured laminar burning velocities exactly match the trends of recent measurements12,21 (Figure 11) at a similar temperature range. This proves that the concept of the heat flux setup with evaporation unit works very well. Figure 11 shows a

Figure 11. n-Heptane adiabatic burning velocities of the present work compared to refs 12 and 21 with a similar gas temperature. 1449

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comparison of the measurements of n-heptane for the lowest applied temperature. The dependency of the laminar burning velocity upon the reactivity of the alkanes at specific initial gas temperatures can also be seen for the two fuels n-heptane and isooctane. Isooctane C8H18, owning more C atoms than n-heptane, does not burn as fast as n-heptane C7H16 with an equal boiling point. The molecular structure of n-heptane is simpler compared to the branched isooctane and, thus, shows a higher reactivity. Measurements of Butanol Isomers and Ethanol Flames. In comparison to the tested fuels thus far, n-butanol has a lower stoichiometric amount of air lst, meaning that a higher mass flow of fuel is needed at a certain air flow amount. Hence, another nozzle plate with a larger diameter has to be used, leading to larger fuel droplets. In addition, the heat of vaporization value is about twice as high as that for isooctane or n-heptane but a third lower than that for ethanol. As a result of this, for an initial gas temperature of 348 K, the evaporation time is quite long for complete droplet evaporation. Thus, no stable flame could have been achieved with this setup for lower gas temperatures of this fuel. However, for higher initial gas temperatures and, thus, more engine-relevant temperatures (373 and 423 K), valid measurements can be conducted. Nine different equivalence ratios (0.7−1.5) are applied (Figure 12).

Figure 13. Isobutanol adiabatic burning velocities of 423 K compared to data by Gu et al.22

The behavior of ethanol matches the trend of measurements by Broustail et al.7 (Figure 14) for the lean and rich part of the values very well.

Figure 14. Ethanol adiabatic burning velocities at 423 K compared to data by Broustail et al.7

Influence of the Initial Mixture Gas Temperature. Currently, there is a lack of data of the laminar flame speed at increased temperatures for different fuels. Figures 15−19 show the influence of the initial gas temperature for all tested fuels. However, the maximal possible temperature for the applied setup is currently 423 K, which is limited by the thermostats and the autoignition temperature of some of the fuels (e.g., n-

Figure 12. n-Butanol adiabatic burning velocities of the present work compared to refs 7 and 22 at 423 K.

As an example, a comparison of the results of 423 K to different references is shown in Figure 12. Again, the results are in good agreement with recently published refs 7 and 22 at similar gas temperatures. Because isobutanol and n-butanol are both butanol isomers, their stoichiometric amount of air lst is equal; therefore, the nozzle plate is kept constant. The heat of vaporization of isobutanol is very similar to n-butanol. Thus, a stable isobutanol flame cannot be achieved for an initial gas temperature of 348 K either. However, the measurements of 373 and 423 K can be confidently conducted for isobutanol too. Figure 13 shows the laminar burning velocities for isobutanol in comparison to the obtained results for n-butanol for an initial gas temperature of 423 K. For each of the applied equivalence ratios (0.7−1.5), the laminar burning velocity of isobutanol is slightly lower than that for n-butanol (about 3 cm/s), which corresponds to the measurements by Gu et al.22 The lower values for isooctane can be addressed to the branched molecular structure and, thus, lower reactivity.

Figure 15. Temperature-dependent flame velocities of isooctane. 1450

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Figure 19. Temperature-dependent flame velocities of ethanol.

Figure 16. Temperature-dependent flame velocities of n-heptane.

exhaust gas is lead back to the fresh air intake in these engines. To simulate the content of EGR in the fresh air, nitrogen as an inert gas is used within this study. However, the ignitability is a limiting factor. As a first tested fuel for this setup, ethanol is tested for EGR rates up to 20% at a constant gas temperature (see Figures 20 and 21). As exhaust gas, pure nitrogen is mixed

Figure 17. Temperature-dependent flame velocities of n-butanol.

Figure 20. Ethanol at variable EGR rates at a constant gas temperature of 373 K.

with the carrier gas (air) by mass flow controllers before the evaporation unit. It is found that, for increasing EGR rates, the laminar burning velocity decreases almost linearly and up to 50% at 20% EGR rate. For high EGR rates, the flammability is slightly decreased for both lean and rich mixtures. The range of the equivalence ratio is reduced to 0.8−1.6. Figure 18. Temperature-dependent flame velocities of isobutanol.

heptane). All fuels show increased laminar burning velocities for increased gas mixture temperatures. The standard model fuels for gasoline, n-heptane and isooctane, present very different laminar burning velocities. Flame velocities of the high-reactive n-heptane fuel can even be measured at a fuel/air ratio of 0.6 to this high-temperature operating point. Ethanol delivers the highest quasi-adiabatic laminar burning velocities of all tested fuels, followed by nheptane and n-butanol. Influence of EGR. To decrease the formation of thermal nitric oxides (NOx), EGR is widely applied in modern turbocharged gasoline direct-injection engines. A fraction of the

Figure 21. Ethanol at variable EGR rates at a constant gas temperature of 423 K. 1451

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(7) Broustail, G.; Halter, F.; Seers, P.; Moreac, G.; MounaimRouselle, C. Fuel 2013, 106, 310−317. (8) Gregor, M. A.; Dreizler, A. Meas. Sci. Technol. 2009, 20, 065402. (9) van Maaren, A.; de Goey, L. P. H. Combust. Sci. Technol. 1994, 99, 105−118. (10) de Goey, L. P. H.; van Maaren, A.; Quax, R. M. Combust. Sci. Technol. 1993, 92, 201−207. (11) van Maaren, A.; Thung, D. S.; de Goey, L. P. H. Combust. Sci. Technol. 2011, 96, 327−344. (12) van Lipzig, J. P. J.; Nilsson, E. J. K.; de Goey, L. P. H.; Konnov, A. A. Fuel 2011, 90, 2773−2781. (13) van Lipzig, J. P. J.; Nilsson, E. J. K.; de Goey, L. P. H.; Konnov, A. A. Proceedings of the 5th European Combustion Meeting; Cardiff, U.K., June 28−July 1, 2011. (14) Bosschaart, K. J.; de Goey, L. P. H. Combust. Flame 2003, 132, 170−180. (15) Konnov, A.; Meuwissen, R. J.; de Goey, L. P. H. Proc. Combust. Inst. 2011, 33, 1011−1019. (16) Knorsch, T.; Demmelmeyer, M.; Wensing, M.; Leipertz, A. Fuel Process. Technol. 2013, 107, 119−125. (17) Acikel, Ü .; Durst, F. Assembly and Operating Instructions of Monodisperse Droplet Generator MTG-01-G2; FMP Technology GmbH: Erlangen, Germany, 2011. (18) Bosschaart, K. J.; de Goey, L. P. H. Combust. Flame 2004, 136, 261−269. (19) Ewald, J. A level set based flamelet model for the prediction of combustion in homogeneous charge and direct injection spark ignition engines. Ph.D. Dissertation, RWTH Aachen University, Aachen, Germany, 2006; pp 133−140. (20) Metghalchi, M.; Keck, J. C. Combust. Flame 1982, 48, 191−210. (21) Kelley, A. P.; Smallbone, A. J.; Zhu, D. L.; Law, C. K. Proc. Combust. Inst. 2011, 33, 963−970. (22) Gu, X.; Huang, Z.; Wu, S.; Li, Q. Combust. Flame 2010, 157, 2318−2325.

CONCLUSIONS Gas-temperature-dependent adiabatic laminar burning velocities were measured for gasoline alternative single-component fuels. Two standard gasoline model fuels, isooctane and nheptane, were measured and compared to three key biofuels, nbutanol, isobutanol, and ethanol. The measured flame speeds of the most common gasoline model fuels, isooctane and nheptane, agree very well with values reported in the literature. n-Heptane showed much higher flame velocities than isooctane, which confirm the measurements of other groups. The reproducibility is very high, which can be seen in the small deviations and also in the high reproducibility of the measurements. Small uncertainties below 1.5% are estimated. This was confirmed by formaldehyde laser-induced fluorescence measurements of the flame structure, showing a very homogeneous distribution in the area of the TC positions and a stable flame. Isobutanol, owning a lower boiling point, presented lower burning velocities than n-butanol, which can be addressed to the branched molecular structure. The burning velocity is highest for ethanol, followed by n-heptane and nbutanol. First results were shown for EGR rate effects on the laminar flame speed for ethanol. The flammability for a 20% EGR rate is limited for both lean and rich mixtures. The adiabatic laminar burning velocity decreases almost linearly with increasing EGR rates. For all fuels, an increasing burning velocity for higher gas temperatures could be detected. Measurements of binary and ternary mixtures of the fuels at variable EGR rates are in preparation.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +49-9131-8529765. Fax: +49-9131-8529901. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the Bavarian Research Foundation (BFS) for financing the project “WiDiKO” (AZ-932-10), in which this work was carried out. Further acknowledgements are dedicated to the German Research Foundation (DFG) for financing parts of this work (GZ: INST 90/688-1) and funding the Erlangen Graduate School in Advanced Optical Technologies (SAOT) within the framework of the German Excellence Initiative. The authors also thank Dr. Stefan Voss and Prof. Dr. Christian Hasse from the Technische Universität (TU) Bergakademie Freiberg, Prof. Dr. L. P. H. de Goey from the TU Eindhoven, and Dr. Roy Hermanns from the Oel-Waerme-Institut (OWI) Aachen for the technical support and consulting during the measurements.

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REFERENCES

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