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Laminar burning speed and flame structure of syngas/oxygen/helium and syngas/air/edg premixed flame

Experimental studies have been performed in conjunction with a novel differential based multi-shell model to investigate the flame structure and measure laminar burning speed of H2/CO/oxidizer/diluent premixed flames at high pressures. The laminar burning speed of the combustion process is extracted from the pressure rise during flame propagation. This paper focuses on synthetic gas (syngas) as the fuel, which is a mixture of H2 and CO, and investigates the effect of dilution with helium and EDG (Extra diluent gases), with a composition of 14% CO2 and 86% N2. In this experiment three different EDG concentration (0%, 5% and 10%) has been used. The experiments were performed in two constant volume spherical and cylindrical chambers. The cylindrical chamber was set up in a Z-shaped Schlieren shadowgraph system equipped with a high speed CMOS camera, capable of taking pictures up to 40,000 frames per second, that was used to study the stability of the flame. The experiments have been done for laminar burning speeds of smooth H2/CO/O2/He and H2/CO/air/EDG flames over a wide range of temperatures (298K up to 500K), pressures (from sub-atmospheric up to 6atm), equivalence ratios (0.6-3) and three different hydrogen concentration of 5%, 10% and 25% respectively. Extra diluent gases (EDG) lower the laminar burning speeds but do not have significant effect on the stability compared to syngas/air in the stoichiometric case. Substitution of nitrogen in the air with helium, exactly with the same percentage, increases the range of temperature and pressure of stable flame as well as the laminar burning speed. INTRODUCTION Synthetic gas, also known as syngas, is primarily a mixture of hydrogen and carbon monoxide gas along with various other higher-order hydrocarbons. Syngas is considered an alternative fuel since it can be created through various sources such as biomass gasification, reactions that involve natural gas and coal, as well as the recycling of stationary turbine byproducts. With the advent of integrated gasification combined cycle (IGCC) technology, syngas can be created from coal cleaner with lower emissions as well as creating carbon free fuel. Thus, the development and research pertaining to syngas fuels are becoming more relevant amid growing concerns about pollutants and carbon emissions. Syngas is considered as a strong candidate to replace many fuels currently in use, therefore it is imperative to fully understand and characterize how syngas behaves in various conditions. The laminar burning speed adequately characterizes a fuel and provides a good indicator of how a fuel performs. It is widely used and contains information about a mixture’s exothermicity, diffusivity, and reactivity. It is also important to study the laminar burning speed in a high pressure environment as well as with different diluents, since those are normally gas turbine-relevant conditions. One typical form of diluent is the inert gas used in exhaust gas recirculation (EGR) technique commonly used in automobile engines, which is primarily a mixture of carbon dioxide and nitrogen and other products of combustion. There is a wide assortment of literature on the laminar burning speed of syngas fuels with and without diluent. Hassan et al. [1] measured the laminar burning velocities of various hydrogen to carbon monoxide ratios (3:97, 5:95, 10:90, 25:75, 50:50), sub-atmospheric to elevated pressures (0.5-4 atm), atmospheric temperature, and wide equivalence ratio (0.6-5.0) in a spherical combustion chamber. Sun et al. [2] used a dualcylindrical chamber to extract laminar burning speed data at atmospheric temperature from many different H2/CO ratios (1:99, 5:95, 25:75, 50:50), elevated pressures (up to 40 atm), and equivalence ratios (0.5-5.0). Sun et al. also replaced nitrogen with helium as the diluent in order to increase the stability of flames, which allowed them to obtain data for much higher pressures. Burke et al. [3] provided an in-depth study that explored the differences between experimenting in a spherical chamber versus a cylindrical chamber, and recommended that researchers using the constant-pressure methodology of extracting laminar burning speed restrict their data to less than 30% of the radius of the inner cylindrical wall. Vu et al. [4] compared the effects of CO2, N2, and He as diluents on the cellular instabilities in syngas flames in a cylindrical chamber at elevated pressures for a 50:50 hydrogen to carbon monoxide ratio, and found that He suppresses instabilities best and reduces the laminar burning speed the least. However, Vu et al.’s range of equivalence ratios was limited to slightly lean to slightly rich (0.8-1.4). Burbano et al. [5] used the burner method to extend the data on the effects of CO2 and N2 dilution on laminar burning speed and stability over a wider equivalence ratio (0.6-4.3). Lapalme and Seers [6] investigated the effect of initial temperature (up to 450K) as well as carbon dioxide and methane dilution on the laminar burning velocities of syngas flames, as well as provided a correlation based on their data. Han et al. [7] measured laminar burning velocities for various CO2 diluent percentages (10%-40%) at elevated temperatures and pressures for equivalence ratios of φ = 0.8 and φ = 1.0 using a dualcylindrical setup. Askari et al. [8] measured the laminar burning speeds of H2/CO/air flames using a new differential-based multi12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics