Measurements of the Laminar Burning Velocity of Ethanol-Water-Air Mixtures

The increasing interest in renewable fuels introduces new challenges in the different fields of combustion applications. With the introduction of biochemical derived fuels and impurities in it, validation is necessary to determine whether existing methods and models are usable for these bio-fuels. This research focuses on the combustion characteristics of ethanol-water-air mixtures. By using a modified Heat Flux burner the adiabatic laminar burning velocity is determined in a wide range of fuel-air-ratios and elevated temperatures and water contents. The stabilization of the approximately one-dimensional flame on a porous brass burner plate is performed under atmospheric conditions. The experimental results are in reasonable agreement to literature data and in good agreement with the detailed combustion reaction models of Leplat et al. and the San Diego mechanism. Introduction Renewable liquid fuels which are easily miscible with fossil liquid fuels and which additionally offer similar energy densities compared to fossil liquid fuels have significant advantages in compliances compared to other renewable liquid fuels. Hence, fuels with these prerequisites can play major role in a future energy supply. Due to the nature of these renewable fuels they can act as, so called, drop-in fuels. Hence, the introduction of these kind of renewable fuels can benefit from the well-established liquid fuel infrastructure. Alcohols derived through biochemical processes from biomass have the potential to provide such a renewable liquid fuel pathway, due to the oxygenated nature of alcohols these fuels they can act as renewable component for fulfilling legislative regulations. One such a fossil fuel substitute is ethanol, when produced from non-food-stock plants or waste. Due to the perfectly miscible character of ethanol and water it is generally difficult or costly to extract all the water by distillation. Furthermore, ethanol will absorb water as moisture from the air fast due to its hydrophilic nature. Both effects combined give a so called hydrated ethanol when used in appliances, which consists of about 5% water by volume. By implementing ethanol as a fuel in combustion appliances it is clear that the properties related to the safety of burner devices will also change, e.g. ignition delay time, flame blow off and flame flash back. As a result, combustion properties have to be known for a wide operational range, since in practical appliances the fuel-air ratio, the pressure at which the combustion takes place, and the initial temperature of the combustible mixture may vary significantly from device to device. Among these combustion properties, the laminar burning velocity is one of the key parameters, which is a fundamental characteristic of the combustible mixture. Although a significant ∗Corresponding author: r.hermanns@owi-aachen.de Proceedings of the European Combustion Meeting 2015 amount of experimental data of the laminar burning velocity of all kind of fuels is available in literature, the results exhibit some significant inconsistency. Even the spread of the measured values often exceeds the reported experimental uncertainty [13, 14]. Hence, new and accurate data for different fuels is invaluable for the design of combustion devices and validation of various models and reaction mechanisms. This was picked up recently by Beeckmann et al. [3] who tried to define some reference compositions and boundary conditions in order to identify the uncertainties of the different measurement methods with the goal to increase the accuracy of the determined experimental laminar burning velocity data. The primary goal of the present study was to acquire new and accurate data of the laminar burning velocity of ethanol-water-air mixtures at elevated inlet temperatures. Additionally the experiments were performed to validate the adapted Heat Flux setup for liquid fuels and elevated inlet temperatures. In the next section the developed and dedicated liquid fuel Heat Flux setup for elevated inlet temperatures is described. Followed by a brief explanation of the determination of both the laminar burning velocity. Since the experimental results are compared with numerical results, the applied combustion reaction mechanisms and numerical methods are also presented. The presented measurements are compared with experimental data from literature when available. This work is concluded with a discussion of the found results. Experimental Details The presented experiments are performed with the Heat Flux method, which is a well established method to determine the adiabatic laminar burning velocity. The method is discussed thoroughly in previous publications [4, 7, 14, 20] and should be consulted for a detailed description of the basic principles of the Heat Flux method. The experimental setup allows the measurement Figure 1: Schematic setup of Heat Flux burner showing its main components. of the adiabatic laminar burning velocity by stabilizing a flat flame on a heated porous burner plate. The resulting flame can be assumed to be one-dimensional [21]. Additionally, de Goey [7] showed that the flame can be approximated as adiabatic due to the negligible heat loss of the flame. The present experimental setup is very close to the original Heat Flux burner design of van Maaren [20], which was improved by Bosschaart [4]. Figure 1 shows a schematic representation of the Heat Flux burner setup used in the present research. Compared to the original design it is extended with an evaporator to convert liquid fuel to the gaseous phase. In this figure, four main components can be identified, the fluid input devices, the evaporator, the plenum chamber, and the burner plate. Detailed explanation of these new components follows below in order of the flow through the system. The fuel-air-mixture is controlled by a mass flow controller and a high-performance liquid chromatography (HPLC) pump. In order to utilize liquid fuels, an accurate dosage system is necessary to provide a confident and steady fuel mass flow. In the present setup a HPLC pump, Type 305 by Gilson Inc., delivers a constant volumetric flow. The pump allows a fuel dosage of up to 25 mL/min with a minimal flow increment of 0.0025 mL/min. An additional pressure valve effectively damped the pumping fluctuations to a negligible level. The mass flow of the fuel is estimated by taking into account the temperature dependent density according to Golubev et al. [11]. The air flow is controlled by a mass flow controller. The error estimate of the air flow and fuel pump flow was identified to be 2.1 % and 1 % respectively of the set flow. The error estimate in density of the fuel itself is negligible. This adds to typical fuel equivalence ratio errors of approximately 3 %. The liquid fuel is evaporated prior to mixing with the oxidizer. The vaporizer is a porous metal mesh disk, electrically heated. The fuel enters on the bottom of the disk and distributes inside due to capillary motion. The porous metal mesh of the evaporator acts as a buffer volume to further steady the fuel flow. The complete evaporation of the ethanol is ensured by temperature control of the metal vaporizer. The metal disk was kept at 90±5 °C by manual control of the electric heater. The temperature of the air is preheated by passing a thermal oil thermostat, which also conditions the plenum chamber of the burner. The air enters the evaporator through a series of holes and generates a homogeneous mixture of fuel and air. The premixed mixture is led towards the plenum chamber surrounded by a cooling jacket to provide temperature conditioning to the fuel-air-mixture. Before leaving the plenum chamber through the burner plate, with an effective diameter of 29.28 mm, the unburnt mixture temperature is recorded by a thermocouple. By varying the temperature of the thermal oil circuit and the electric energy supplied to the vaporizer, the mixture temperature is being controlled. Typical temperature variations of the premixed unburnt mixture are within 1 K, measured upstream but close to the burner plate. Measurement Procedure Standard procedure to measure the adiabatic laminar burning velocity of a fuel-air-mixture is to record the temperature profile on the burner plate for various gas velocities slightly above and below the estimated laminar burning velocity. An analytic analysis of the heat balance of the burner plate by van Maaren [20] showed that the recorded temperature profiles have a parabolic shape: Tp (r) = Tc + Cr. (1) In this equation Tp is the plate temperature, Tc is a reference temperature measured at centre thermocouple, C the parabolic coefficient and r the radius at which the temperature is measured. Modelling details The experimentally determined laminar burning velocities are compared with numerical results. The freely propagating steady adiabatic premixed flames were modeled by using the multi-purpose flame code CHEM1D [6]. This code is based on a second-order exponential scheme to discretize the spatial derivatives in combination with