| An experimental and numerical study on the combustion characteristics of turbulent diffusion flames of natural gas-hydrogen composite fuel is presented. Three mixtures (90–10%, 80–20% and 65–35% by volume) of natural gas and hydrogen were used. The results are compared with the combustion characteristics of a pure natural gas flame. The following parameters were measured: (i) flame stability (blowout velocity, and lift-off height at blowout condition), (ii) temperature field (radial profiles at three axial locations), (iii) composition profiles of stable species (CO 2, CO, NO, O2), (iv) composition profiles of intermediate species (OH, CH, H and O), and (v) visible flame length, flame radiation, emission indices and volumetric soot concentration. To study the flame stability five burners of 1mm, 2.3mm, 3.8mm and 4.5mm ID were used. Direct video photography, Schlieren imaging and acetone Planar Laser Induced Fluorescence (PLIF) imaging were used for flame stability and mixing study. For stable species concentration measurements, an uncooled quartz glass probe with chemilumenesce and infrared analyzers were used. Laser Induced Fluorescence (LIF) and Planar Laser Induced Fluorescence (PLIF) technique were used to measure radical concentrations. A combined LIF-Raman Spectroscopic procedure was carried out to quantify the LIF signals.; The following parameters were analyzed numerically: (i) cold jet mixing (axial and radial velocity, turbulent intensity, turbulent kinetic energy and local equivalence ratio), (ii) flame temperature, (iii) stable species (CO2, CO, NO, O2) and (iv) intermediate species concentrations (OH, CH, CN, H, and O). For the numerical computation, Favre-averaged Navier-Stokes equations with two-step reaction kinetics and the standard k-ϵ turbulence model were used. The fuel jet exit Reynolds number was kept constant at 8700 for flame structure measurements, computation, and measurements of global characteristics. The corresponding flame Froude number ranged between 0.85–1.18 depending upon the mixtures of natural gas and hydrogen.; Both blowout velocity and lift-off height at blowout condition increase non-linearly with the increase of hydrogen concentration in the mixture for all burner sizes. A general relation is also presented correlating blowout velocity, burner diameter, and hydrogen content in the fuel mixtures. The lift-off height at blowout decreases with the increase of hydrogen concentration in the mixture. The numerical prediction shows that with an increase of hydrogen concentration in the fuel mixture, the axial velocity decays faster while the axial turbulent fluctuations and local turbulent flame speed increase.; Experimental measurements and numerical predictions show that the local flame temperature increases with the increase of hydrogen content in the mixture; consequently the NO production increases in the flame. On the other hand, an increase of hydrogen content in the mixture increases OH, H and O radical concentrations, which increase CO and soot oxidation. Also, the increase in OH, H and O concentrations enhances the flame stability by increasing the laminar flame speed of the composite fuel. The visible flame length, radiative heat loss, and volumetric soot loading decrease with the increase of hydrogen concentration in the fuel mixture. The CO emission index decreases and NO emission increases with the increase of hydrogen content in the fuel mixture. |
