Fuel 90 (2011) 27732781

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Laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixturesJ.P.J. van Lipzig a, E.J.K. Nilsson b, L.P.H. de Goey a, A.A. Konnov b,a b

Department of Mechanical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands Division of Combustion Physics, Department of Physics, Lund University, Lund, Sweden

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a b s t r a c tMeasurements of the adiabatic laminar burning velocities of n-heptane, iso-octane, ethanol and their binary and tertiary mixtures are reported. Non-stretched ames were stabilized on a perforated plate burner at 1 atm. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss from the ame to the burner is zero. Initial temperatures of the gas mixtures with air were 298 and 338 K. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be better than 1 cm/s. These new measurements were compared with the literature data when available. Experimental results in lean ethanol + air mixtures are systematically higher than previous measurements under similar conditions. Good agreement for n-heptane + air ames and for iso-octane + air ames was found with the experiments performed in counter-ow twin ames with linear extrapolation to zero stretch. 2011 Elsevier Ltd. All rights reserved.

Article history: Received 6 February 2011 Received in revised form 18 April 2011 Accepted 19 April 2011 Available online 1 May 2011 Keywords: n-Heptane Iso-octane Ethanol Burning velocity

1. Introduction Adiabatic laminar burning velocity is a fundamental characteristic of each combustible mixture, which is invaluable both for design of combustion devices and for understanding of underlying combustion chemistry, validation of models, formulation of surrogate fuels, etc. Although in practical applications pressure and initial temperature of the mixture are often higher than standard values, the laminar burning velocities measured at atmospheric pressure and initial temperature of 298 K are convenient for comparison of experimental data acquired in different laboratories using different experimental methods. They also serve as reference values in the analysis of pressure and temperature dependences for laminar and turbulent ames. In spite of numerous and continuous efforts, the laminar burning velocities of the neat components of primary reference fuels, n-heptane and iso-octane, still exhibit signicant inconsistency, and spread of the measured values often exceeds reported experimental uncertainty [1]. n-Heptane and iso-octane are always considered as major components of surrogate fuels for gasoline; therefore primary reference fuels (mixtures of n-heptane and iso-octane) with different research octane number (RON) have also been investigated, albeit only in a few studies. Laminar burning velocities have been determined for the primary

Corresponding author. Address: Division of Combustion Physics, Department of Physics, Lund University, P.O. Box 118, S-221 00 Lund, Sweden. Tel.: +46 46 222 9757; fax: +46 46 222 4542. E-mail address: Alexander.Konnov@forbrf.lth.se (A.A. Konnov).0016-2361/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.04.029

reference fuels with RON = 90 [2], RON within 8595 [3] and with RON = 87 [4]. Contemporary environmental concerns caused increasing interest in alternative fuels among which ethanol is regarded as possibly the best candidate to augment or even replace oil-based fuels. Ethanol can easily be blended with hydrocarbons and as a mixture of 85% of ethanol with gasoline (E85) is commercially available in Americas and Europe. Formulation of surrogate fuels for these mixtures clearly requires fundamental experimental data including laminar burning velocities for binary and tertiary mixtures. These data, however, are very scarce. Glder [5] measured burning velocities of iso-octaneethanol blends with up to 20% of ethanol by liquid volume in the fuel over the range of equivalence ratios from 0.75 to 1.4, at initial temperatures from 350 to 600 K at atmospheric pressure. Broustail et al. [6] determined burning velocities of, among others, iso-octaneethanol blends containing 25, 50 and 75 vol.% of ethanol in the mixtures over the range of equivalence ratios from 0.8 to 1.4, at initial temperature of 393 K at atmospheric pressure. No data are available for n-heptaneethanol mixtures. The goal of the present study therefore was to acquire new accurate data on laminar burning velocities of n-heptane, iso-octane, and mixtures of the two with each other and with ethanol. In addition measurements were performed on pure ethanol mixtures with the aim to validate a new experimental setup and the experimental procedure. In the following the new dedicated Heat Flux installation is described and possible experimental uncertainties are assessed. Then new measurements are presented and compared with experimental data from the literature when available.

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2. Experimental details The Heat Flux method for the stabilization of adiabatic premixed laminar ames on a at ame burner used in the present work was originally proposed by de Goey et al. [7]. This method was extensively used for measuring laminar burning velocities of gaseous fuels, e.g. [811] and has recently been extended towards liquid fuels [12]. Detailed description of the method and associated experimental uncertainties for gaseous fuels were given elsewhere [10,13]. Specic features for liquid fuels were also addressed [12] and, therefore, are only shortly outlined here. A new experimental rig very similar to that used by Konnov et al. [12] has been constructed and certied in the present work. Certication revealed some previously overlooked design peculiarities that systematically affected measured burning velocities. These peculiarities are discussed in the next section. The experimental setup for the adiabatic ame stabilization using the Heat Flux method is shown in Fig. 1 and cross-section of the perforated plate burner in Fig. 2. A 2 mm thick burner plate perforated with small holes (0.5 mm in diameter) is attached to the burner outlet, see Fig. 2. The top view of the perforated burner head, which is 30 mm in diameter, is shown in Fig. 3. The burner head has a heating jacked supplied with thermostatic water to keep the temperature of the burner plate constant. During the experiments this temperature was xed at 368 K. The plenum chamber has a separate cooling system supplied with water at a temperature of 298 K, which could be increased up to 358 K. This xes the initial temperature of the fresh gas mixture. The heating jacket keeps the burner plate edges at a certain temperature higher than the initial gas temperature, thus warming up the (unburned) gases owing through. Conductive heat transfer of the ame to the burner plate cools the gas ow on its turn. If the ame is stabilized under sub-adiabatic conditions, the gas velocity is lower than the adiabatic ame burning velocity and the sum of the heat loss and heat gain is higher than zero. Then, the center of the burner plate is hotter than the heating jacket. If the unburned gas velocity is higher than the adiabatic burning velocity (super-adiabatic conditions), the net heat ux is lower than zero and the center of the burner plate is cooler than the heating jacket. By changing the ow rate of the gas mixture an appropriate value of the gas velocity can be found to nullify the net heat ux. In this case the radial temperature distribution in the burner plate is uniform and equal to the temperature of the heating jacket. Experimentally, the series of thermocouples attached to the burner plate as shown in Fig. 2 allows measuring the temperature distribution in it. These thermocouples are placed at different radii from the center as shown in Fig. 3. The ow rate at which the net heat ux is zero is shown to be an adiabatic ame burning velocity [7,8]. A mixing panel shown in Fig. 1 was assembled to provide controlled ow of the vaporized fuel and air of the required equivalence ratio. The key part of this mixing panel is the Cori-Flow liquid mass-ow controller (MFC) connected to the Controlled Evaporator Mixer (CEM), both from Bronkhorst B.V. The liquid fuel ow from the fuel reservoir, pressurized by nitrogen, is metered by the Cori-Flow MFC and fed to the CEM. Part of the air ow controlled by the gas MFC-1 is used as a carrier gas to facilitate vaporization at temperatures up to 473 K. Another part of the air ow controlled by the gas MFC-2 and mixed downstream is varied to provide the required mixture composition.

variable ow ratio of the air between MFC-1 and MFC-2; inuence of the CEM operating temperature; fuel purity; dissolution of nitrogen in the liquid fuel; hygroscopic nature of the liquid fuel.

These ve sources were assessed experimentally [12] and corresponding procedures were repeated for the present installation. It was proved that the ratio of the ows via MFC-1 (carrier gas for the CEM) and MFC-2 (balance) does not affect measured burning velocity within expected accuracy of the measurements. This conrms, in turn, the correctness of the MFC calibration. No inuence of the CEM operating temperature was observed when it was set well above boiling temperatures of the pure fuels and their mixtures (specically at 423 K in the present work); no condensation of the fuel was detected in the transparent pipeline before the burner. This ensures complete vaporization and absence of aerosol in the mixture; the accuracy and stability of the CEM was within 0.2% of the liquid ow setpoint. Neat fuels (n-heptane, iso-octane, and ethanol) were used as delivered from Merck KGaA in sealed bottles. Mixtures were prepared in the lab shortly before experiments. The purity of n-heptane and of iso-octane was better than 99% and 99.5% respectively. The purity of ethanol was better than 99.9% with less than 0.1% water. The purity could potentially deteriorate due to hygroscopy (mostly for ethanol) when relling the fuel reservoir and due to dissolution of nitrogen, which pressurizes it. The relling was very short in time and the amount of water that could be dissolved in the ethanol during it was estimated to be less than 0.01% [12]. Mixing of ethanol with other fuels took longer time, yet the amount of water that could be absorbed is believed to be less than the stated purity of ethanol itself (