eJournals International Colloquium Fuels 13/1

International Colloquium Fuels
icf
expert verlag Tübingen
101
2021
131

Laminar burning velocities of ethanol and butanol isomers

101
2021
Sebastian Feldhoff
The laminar burning velocity is an important parameter governing the properties of combustion. It can for ex-ample be used to validate reaction mechanisms. However, the laminar burning velocity is not easily measured. The heat flux method is one of the methods to measure the laminar burning velocity. The heat flux burner consists of a thin, brass burner plate with a hexagonal pattern of holes. The burner plate is heated in such way that the premixed mixture flowing though the burner plate is heated. Now the heat gained by the unburnt mixture compensates the heat loss of the flame. The advantage is that mixture velocities higher and lower than the laminar burning velocity can be stabilized on this burner. In contrast to other methods, it is possible to determine the laminar burning velocity at a state of a nearly adiabatic stretchless flame by means of interpolation. The heat flux experimental setup at OWI has been further improved to provide more accurate measurement data of the laminar burning velocity. Recently, a new test series has been started investigating burning velocities of different alcohols, such as ethanol and butanol with air. Recent investigations focus on butanol isomers. Burning velocities have been measured in a wide fuel-air ratio from 0.6 to 1.6 and compared to numerical calculations, which have been performed with Cantera. The study provides an overview of the laminar burning velocity depending on fuel composition and presents the data in comparison to numerical data.
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13th International Colloquium Fuels - September 2021 139 Laminar burning velocities of ethanol and butanol isomers Sebastian Feldhoff OWI Science for Fuels gGmbH Summary The laminar burning velocity is an important parameter governing the properties of combustion. It can for ex-ample be used to validate reaction mechanisms. However, the laminar burning velocity is not easily measured. The heat flux method is one of the methods to measure the laminar burning velocity. The heat flux burner consists of a thin, brass burner plate with a hexagonal pattern of holes. The burner plate is heated in such way that the premixed mixture flowing though the burner plate is heated. Now the heat gained by the unburnt mixture compensates the heat loss of the flame. The advantage is that mixture velocities higher and lower than the laminar burning velocity can be stabilized on this burner. In contrast to other methods, it is possible to determine the laminar burning velocity at a state of a nearly adiabatic stretchless flame by means of interpolation. The heat flux experimental setup at OWI has been further improved to provide more accurate measurement data of the laminar burning velocity. Recently, a new test series has been started investigating burning velocities of different alcohols, such as ethanol and butanol with air. Recent investigations focus on butanol isomers. Burning velocities have been measured in a wide fuel-air ratio from 0.6 to 1.6 and compared to numerical calculations, which have been performed with Cantera. The study provides an overview of the laminar burning velocity depending on fuel composition and presents the data in comparison to numerical data. 1. Introduction The adiabatic laminar burning velocity (LBV) is an important parameter governing the properties of combustion. It can for example be used to validate reaction mechanisms and is often needed in designing of different industrial and domestic burners. This is the reason that research has focused on the adiabatic burning velocity of several types of fuel-oxidizer mixtures. However, the adiabatic burning velocity is not easily measured. There are several experimental methods to measure laminar burning velocity: the Bunsen flame method, the spherically expanding flame method, the stagnation flame method, and the flat flame burner method, including the Heat Flux method. A detailed overview of different methods can be found in [1] and [2]. The heat flux burner consists of a thin, brass burner plate with a hexagonal pattern of holes. The burner plate is heated in such way that the premixed mixture flowing though the burner plate is heated. Now the heat gained by the unburnt mixture compensates the heat loss of the flame. The advantage is that mixture velocities higher and lower than the adiabatic burning velocity can be stabilized on this burner. In contrast to other methods, it is possible to determine the laminar burning velocity at a state of a nearly adiabatic stretchless flame by means of interpolation, rather than extrapolation. This has as advantage that the uncertainties due to extrapolation are circumvented. In this study the Heat Flux method has been applied to measure laminar burning velocities of ethanol as well as butanol isomers with air at atmospheric pressure. 2. Heat Flux Method The main parts of the Heat Flux burner are shown in Figure 1. One of the main burner elements is a 2 mm thick brass burner plate with an effective diameter of approximately 30 mm and a uniform perforation. A conditioned heating jacket operates at a constant temperature, which is set to keep a temperature difference between unburnt gas and the heating jacket between ΔT = 60 K and ΔT = 75 K. A plenum chamber is surrounded by a cooling jacket, which maintains the temperature of the plenum chamber at a temperature equal to that of the unburnt gas mixture. To measure a radial temperature profile in the burner plate, fifteen thermocouples are attached to the burner plate. 140 13th International Colloquium Fuels - September 2021 Laminar burning velocities of ethanol and butanol isomers Figure 1: Schematic view of the Heat Flux burner To stabilize an adiabatic flame on top of the burner plate the net heat flux should be equal to zero. For gas velocities U G lower than the laminar burning velocity S L the net heat flux is positive and the burner plate has a higher temperature than the heating circuit. This results in a radial heat flux from the burner plate center to the edge and the burner plate center achieves the highest temperature. The net heat flux is negative, when the gas velocity is higher than the LBV. In this case the temperature at the center of the burner plate is the lowest measured temperature, due to the reverse direction of the radial heat flux. For a flat temperature profile, the net heat flux is zero, which means that all the heat transferred from the flame to the burner plate is transferred then to the unburnt gas mixture. This situation corresponds to adiabatic flame conditions and the corresponding gas velocity U G equals to the laminar burning velocity S L . 2.1 Temperature profile along the burner plate The temperature profile over the burner plate is used to determine if there is a net heat gain or heat loss by the mixture. In the adiabatic state, the heat loss is equal to the heat gain by the burner plate. Assuming that the conductivity of the burner plate is not temperature (and thus r) dependent, solving the energy equation leads to the following equation for the temperature distribution along the burner plate: In this equation, T p is the temperature as a function of the radius r. q is the net heat flux from the unburnt gas mixture to the plate, λ the conduction coefficient of the burner plate, and h is the height of the burner plate. The temperature along the burner plate is described by a parabola, with the symmetry axis at the center of the burner plate [3]. For gas velocities U G lower than the laminar burning velocity S L the net heat flux q is positive and the burner plate has a higher temperature than the heating circuit. This results in a radial heat flux from the burner plate center to the edge and the burner plate center achieves the highest temperature (Figure 2 mid). The net heat flux is negative, when the gas velocity is higher than the laminar burning velocity. In this case the temperature at the center of the burner plate is the lowest measured temperature, due to the reverse direction of the radial heat flux (Figure 2 top). Figure 2: Temperature profile and flame pattern over the burner plate at various U G (schematic) For a flat temperature profile, the net heat flux q is zero, which means that all the heat transferred from the flame to the burner plate is transferred then to the unburnt gas mixture. This situation corresponds to adiabatic flame conditions and the corresponding gas velocity U G equals to the laminar burning velocity S L . Additionally, in the adiabatic case the temperature of the burner plate should be equal to the heating jacket temperature [4]. 3. Numerical simulation The numerical simulation of the laminar burning velocity is performed using the Cantera software. The basis of the software is the calculation of the flame by solving the chemical and kinetic equations of combustion. Due to the nearly adiabatic and one-dimensional combustion on the Heat Flux burner, the flame can be considered independent of the heat losses to the burner plate and can therefore be simulated as a free flame. This also allows the comparison between the experimental and calculated 13th International Colloquium Fuels - September 2021 141 Laminar burning velocities of ethanol and butanol isomers values. Furthermore, the calculations are used to estimate the experimental parameters, especially the gas velocity ranges, and to calculate the adiabatic burning rate. The flame has been modeled as a freely-propagating, premixed flame (1-D) with air as the oxidizer at atmospheric pressures and an unburnt gas temperature of 338 K. All calculations of the laminar burning velocity are carried out with two existing detailed chemical kinetic mechanisms a) cloudflame [5] and b) CRECK [6] to which the experimental data is compared with for validation. 4. Results and discussion To evaluate the performance of the Heat Flux method including the recently made improvements on reliability, several sets of experiments have been carried out. In the first place, the laminar burning velocity of ethanol was determined as it serves as a well-known reference to evaluate the accuracy of the experimental setup. The results show a good agreement with the obtained numeric data in the range of ϕ = 0.6 - 1.6 (equivalence ratio). Both mechanisms show very similar results. In the range of ϕ = 1.0 - 1.4 the experimentally obtained data differ slightly from numerical results, however, the overall expectations were met (Figure 3). The maximum LBV for ethanol is about 51.6 cm s -1 at ϕ = 1.1 (experimental). Figure 3: Laminar burning velocity of ethanol-air-mixtures: experimental and numerical data The following experiments were related to butanol isomers (1-Butanol and 2-Butanol) at the same thermodynamical conditions. It was found, that burning velocities of both butanol isomers were lower compared to ethanol (Figure 4). The maximum LBV for 1-butanol is ca. 47.8 cm s -1 and for 2-butanol ca. 46.1 cm s -1 at ϕ = 1.1 (experimentals). This is due to the chemical bounds inside the molecules and was shown in literature before [7]. The difference is low at equivalence ratios < 1.0 and increases towards higher equivalence ratios. This behaviour can also be found in the results of the numerical simulations for both 1-butanol and 2-butanol (Figures 5 and 6). However, the obtained experimental data is in line with the numerical data. Figure 4: Laminar burning velocity of ethanol and butanol air-mixtures: experimental data Figure 5: Laminar burning velocity of 1-butnaol-airmixtures: experimental data and numerical data Figure 6: Laminar burning velocity of 2-butnaol-airmixtures: experimental data and numerical data 5. Conclusion The focus within this study was the determination of the adiabatic laminar burning velocity of ethanol and butanol isomers with the Heat Flux method. It was found that the adiabatic laminar burning velocity is highly depen- 142 13th International Colloquium Fuels - September 2021 Laminar burning velocities of ethanol and butanol isomers dent on fuel composition and is less for butanol isomers compared to ethanol. Furthermore, the difference in S L of 1-butanol vs. 2-butanol shows that the molecules’ geometrical structure is affecting the burning velocity, which has been shown by other researchers. In general, the experimental data is in good agreement with all the results of the numerical simulations. References [1] Egolfopoulos, F.N.; Hansen, N.; Ju, Y.; Kohse-Höinghaus, K.; Law, C.K.; Qi, F.: “Advances and challenges in laminar flame experiments and implications for combustion chemistry”, Prog. Energy Combust. Sci. 43 (2014) 36-67. doi: 10.1016/ j. pecs.2014.04.004. [2] Nilsson, E.J.K.; Konnov, A.A.: “Flame Studies of Oxygenates”, in: F. Battin-Leclerc (Ed.), Clean. Combust. Green Energy Technol., London, 2013: pages 231-280. doi: 10.1007/ 978-1-4471-5307- 8_10. [3] Bosschaart, K.J.: “Analysis of the Heat Flux Method for Measuring Burning Velocities”, PhD. Thesis, Eindhoven University of Technology. [4] De Goey, L.P.H.; van Maaren, A.; Quax, R.M.: “Stabilization of adiabatic premixed laminar flames on a flat flame burner”, Combustion Science Technology, Volume 92, pages 201-207. [5] S. Mani Sarathy, Patrick Oßwald, Nils Hansen, Katharina Kohse-Höinghaus. Alcohol combustion chemistry, Progress in Energy and Combustion Science, 2014, pages 0360-1285. https: / / doi. org/ 10.1016/ j.pecs.2014.04.003 [6] Pelucchi, M., Cavallotti, C., Ranzi, E., Frassoldati, A., Faravelli, T., Relative Reactivity of Oxygenated Fuels: Alcohols, Aldehydes, Ketones, and Methyl Esters, Energy & Fuels, 30(10), pages 8665-8679 (2016), DOI: 10.1021/ acs.energyfuels.6b01171 [7] Xiaolei Gu, Zuohua Huang, Si Wu, Qianqian Li, Laminar burning velocities and flame instabilities of butanol isomers-air mixtures, Combustion and Flame, Volume 157, Issue 12, 2010, pages 2318- 2325, ISSN 0010-2180, DOI: 10.1016/ j.combustflame.2010.07.003