Experimental Investigation On The Effects Of Hydrogen Addition And Dilution Gas On Thermal Characteristics Of Methane/Air Premixed Flames

With the growing crisis of the energy shortage and environmental issues,improving fuel efficiency and reducing the generation of hazardous substances arebecoming the most important goal of combustion research. Methane is considered oneof the most ideal clean gas fuel, but methane remains relatively slow burning velocities.Hydrogen has higher burning velocities, so methane/air flame enriched by hydrogen hasbecome an effective way to improve the combustion efficiency of methane. Dilution hasbecome another major research focus, because adding the dilution gas can reduce themaximum combustion temperature, thereby reducing the generation of NOxpollutants.In order to provide technical support for the efficient utilization of methane gas fuel, weresearched the effects of hydrogen addition and dilution gas on thermal characteristicsof methane/air premixed flames based on McKenna laminar flame test. The content andthe main results of this paper are as follows:1. This paper presented an experimental study on the measurements of premixedlaminar methane/air flames with and without hydrogen addition. The premixed flameswere stabilized on a McKenna flat fame burner at atmospheric pressure. The travelingthermocouple approach was used to measure the axial flame temperature profiles overranges of equivalence ratios and hydrogen enriching ratios. by solving the continuityequations of mass, energy, and species which were applied to a flat flame. Some ofimportant flame properties, such as the peak temperature, the average temperature, theflame thickness, the thickness of reaction zone and combustion efficiency, werepresented to characterize the effect of hydrogen enrichment on laminar flamepropagation. It is shown that the presence of hydrogen in laminar flame can promoteflame reaction to some extent. With an increase of hydrogen addition fraction in fuel,the peak rate of heat release and combustion efficiency show increases, while theaverage temperature gives decrease. The analysis of the heat release profiles suggestedthat hydrogen addition has significant effect on the early part of flame heat releaseprofile. The flames enriched by hydrogen show linear approximations in the plots of thelogarithmic heat release rate against the reciprocal of flame temperature.2. This paper presented an experimental study on the measurements of premixedlaminar methane/air flames with dilution gas, the dilution gas using in this paper isnitrogen and carbon dioxide, respectively. Research showed that, adding two kinds of dilution gas could reduce the flame spreading. With an increase of dilution gas fractionin fuel, the peak temperature, the peak rate of heat release and combustion efficiencyshow decreases, while the thicknesses of flame gives decrease. Using theone-dimensional premixed laminar burner-stabilized flame model as well, the influenceof dilution gas on flame NO, NO2, CO concentration were simulated. The resultsshowed that, NO, NO2concentration decreased with increasing dilution content, whilethe concentration of CO was increased. Moreover, it was found that the CO2dilutioneffect is higher than N2.On the one hand, the heat capacity of CO2is higher than N2onthe other hand, CO2as the main product of combustion, participating in the reaction inthe combustion process caused the reaction of CO+OH CO2+H moving to the left, andreducing the H concentration, so it hindered the flame combustion more greatly.3. A modeling for one dimensional premixed laminar burner-stabilized flame hadbeen implemented with GRI-Mech3.0detailed kinetic reaction mechanism, based onthe measured temperature profiles of these premixed flames. And then an analysis onthe heat release was performed for each reaction. It suggested that the reaction OH+H2H+H2O gradually increases its contribution to the early heat release with theincrease of hydrogen enrichment, which is due to the fact of hydrogen addition resultingin an increased concentration of radical H in flame. The promoted H formationaccelerates the flame burning velocity, and then thins the thicknesses of flame andreaction zone to a great extent.