Elbaz, A. M., M. S. MANSOUR, B. M. Akoush, M. Juddoo, A. M. Khedr, H. M. Al-Bulqini, M. F. Zayed, M. M. A. Ahmed, W. L. Roberts, and A. R. Masri, "Detailed investigation of the mixing field and stability of natural gas and propane in highly turbulent planar flames", Fuel, vol. 309, pp. 122222, 2022. AbstractWebsite

In most practical combustion devices, the actual combustion process occurs within different mixture inhomogeneity levels. Investigating the mixture fraction field upstream of the reaction zones of these flames is an essential step toward understanding their structure, stability, and emission formation. In this study, the mixture fraction fields were measured for turbulent non-reacting inhomogeneous mixtures immediately downstream from the slot burner exit, using Rayleigh scattering imaging. The slot burner had two concentric slots. The inner air slot can be recessed at distances upstream from the exit of the outer fuel slot, allowing various degrees of mixture inhomogeneity. Mixture fraction field statistics and the two-dimensional gradient were utilized to characterize the impact of the air-to-fuel velocity ratio, global equivalence ratio, fuel composition, Reynolds number, and the premixing length on the mixture mixing field, and thus flame stability. These impacts were evaluated by tracking the normalized mean mixture fraction and mixture fraction fluctuation transition across the regime diagram for partially premixed flames. The results showed that the air-to-fuel velocity ratio was the critical parameter affecting the mixture fraction field for the short premixing length. Stability results showed that the level of mixture inhomogeneity mainly influenced the flame stability. High flame stability is achieved if a large portion of the inhomogeneous mixture fraction is within the fuel flammability limits.

MANSOUR, M. S., A. M. Elbaz, W. L. Roberts, M. F. Zayed, M. Juddoo, B. M. Akoush, A. M. Khedr, H. M. Al-Bulqini, and A. R. Masri, "Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner", Proceedings of the Combustion Institute, vol. 38, no. 2, pp. 2597-2606, 2021. AbstractWebsite

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.

MANSOUR, M. S., M. S. Abdallah, N. K. Allam, A. M. Ibrahim, A. M. Khedr, H. M. Al-Bulqini, and M. F. Zayed, "Biogas production enhancement using nanocomposites and its combustion characteristics in a concentric flow slot burner", Experimental Thermal and Fluid Science, vol. 113, pp. 110014, 2020. AbstractWebsite

Biogas combustion is a very essential topic for the development of many industrial combustion systems and engines. This fuel can replace current fossil fuels used in burners, engines, and many other applications. Understanding the combustion characteristics of this fuel and its stability in highly turbulent flames of practical interest is the aim of this work. The percentage of CO2 in Biogas varies between 25% and 45%, which affects the combustion stability and flame structure. The present work shows that the generation of Biogas is improved by adding Ni-Co-Ferrite or Ni-ferrite nano-additives. In this work, we selected 25 flames of mixtures of natural gas and CO2, where the ratio of CO2 varies from 0% to 40%. The flames are generated in a concentric flow slot burner that produces planar two-dimensional flames. The stability characteristics and the flame structure were investigated. The flame structure is presented in the form of temperature profiles in some selected flames using fine wire thermocouple measurements. The stability characteristics are illustrated for two limits of lifted flames and blow out. The production rate of Biogas can be increased by almost 30% using nano-additives of Ni-Co-Ferrite or Ni-ferrite. The data show that the stability of the flames is affected significantly for the 40% CO2 mixture. Therefore, it is recommended to keep CO2 percentage up to 30% for stable turbulent Biogas flames. On the other hand, partially premixed flames are highly stable for a certain level of mixture inhomogeneity at a mixing length ratio of L/D = 16. At this level, the mixture fraction fluctuations are expected to be within the flammability limits range based on previous investigations in round jet configuration.

Elbaz, A. M., M. S. Senosy, M. F. Zayed, W. L. Roberts, and M. S. Mansour, "Highly stabilized partially premixed flames of propane in a concentric flow conical nozzle burner with coflow", Experimental Thermal and Fluid Science, vol. 95, pp. 2-10, 2018. AbstractWebsite

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MANSOUR, M. S., H. Pitsch, S. Kruse, M. F. Zayed, M. S. Senosy, M. Juddoo, J. Beeckmann, and A. R. Masri, "A concentric flow slot burner for stabilizing turbulent partially premixed inhomogeneous flames of gaseous fuels", Experimental Thermal and Fluid Science, vol. 91, pp. 214 - 229, 2018. AbstractWebsite

Combustion of turbulent inhomogeneous mixtures of air and fuel is common in many practical systems providing improved stability for both gaseous and liquid fuels. Understanding the structure and stability of turbulent flames in this mode has been the aim of many research groups who employed special burner designs to control the fuel and air mixing process. In this work, a modified design inspired by the Wolfhard-Parker slot burner was developed for planar turbulent flames with inlet conditions that are overall lean yet either compositionally inhomogeneous or partially premixed. The new burner is referred as the Concentric Flow Slot Burner (CFSB). The stability characteristics and flame structure are investigated for methane and natural gas fuels using planar laser induced fluorescence of C2Hx and high speed PLIF-OH. The effects of the jet equivalence ratio, the level of inhomogeneity, and the Reynolds number are investigated in this work. The data show that the flames with inhomogeneous mixture are more stable than fully premixed flames. Lean flames are stabilized in the CFSB burner. Stability is significantly improved by the use of a hollow truncated rectangular pyramid nozzle at the burner exit. The reaction zone structure varies significantly in the current burner from thin structures in rich flames to distributed with thick preheat zones in lean flames. The effect of the level of inhomogeneity on the reaction zone structure is presented and discussed. The new CFSB burner is able to generate a wide range of turbulent planar flames spanning the entire range from non-premixed to fully premixed flames. In addition, the high stability level of the burner allows for the study of highly turbulent flames of practical interest.

Elbaz, A. M., M. F. Zayed, M. Samy, W. L. Roberts, and M. S. Mansour, "The flow field structure of highly stabilized partially premixed flames in a concentric flow conical nozzle burner with coflow", Experimental Thermal and Fluid Science, vol. 73, pp. 2 - 9, 2016. AbstractWebsite

The stability limits, the stabilization mechanism, and the flow field structure of highly stabilized partially premixed methane flames in a concentric flow conical nozzle burner with air co-flow have been investigated and presented in this work. The stability map of partial premixed flames illustrates that the flames are stable between two extinction limits. A low extinction limit when partial premixed flames approach non-premixed flame conditions, and a high extinction limit, with the partial premixed flames approach fully premixed flame conditions. These two limits showed that the most stable flame conditions are achieved at a certain degree of partial premixed. The stability is improved by adding air co-flow. As the air co-flow velocity increases the most stable flames are those that approach fully premixed. The turbulent flow field of three flames at 0, 5, 10m/s co-flow velocity are investigated using Stereo Particle Image Velocimetry (SPIV) in order to explore the improvement of the flame stability due to the use of air co-flow. The three flames are all at a jet equivalence ratio (Φj) of 2, fixed level of partial premixing and jet Reynolds number (Rej) of 10,000. The use of co-flow results in the formation of two vortices at the cone exit. These vortices act like stabilization anchors for the flames to the nozzle tip. With these vortices in the flow field, the reaction zone shifts toward the reduced turbulence intensity at the nozzle rim of the cone. Interesting information about the structure of the flow field with and without co-flow are identified and reported in this work.

MANSOUR, M. S., A. M. Elbaz, W. L. Roberts, M. S. Senosy, M. F. Zayed, M. Juddoo, and A. R. Masri, "Effect of the mixing fields on the stability and structure of turbulent partially premixed flames in a concentric flow conical nozzle burner", Combustion and Flame, vol. 175, pp. 180 - 200, 2017/1//. AbstractWebsite

AbstractThe mixing field is known to be one of the key parameters that affect the stability and structure of partially premixed flames. Data in these flames are now available covering the effects of turbulence, combustion system geometry, level of partially premixing and fuel type. However, quantitative analyses of the flame structure based on the mixing field are not yet available. The aim of this work is to present a comprehensive study of the effects of the mixing fields on the structure and stability of partially premixed methane flames. The mixing field in a concentric flow conical nozzle (CFCN) burner with well-controlled mechanism of the mixing is investigated using Rayleigh scattering technique. The flame stability, structure and flow field of some selected cases are presented using LIF of OH and PIV. The experimental data of the mixing field cover wide ranges of Reynolds number, equivalence ratio and mixing length.
The data show that the mixing field is significantly affected by the mixing length and the ratio of the air-to-fuel velocities. The Reynolds number has a minimum effect on the mixing field in high turbulent flow regime and the stability is significantly affected by the turbulence level. The temporal fluctuations of the range of mixture fraction within the mixing field correlate with the flame stability. The highest point of stability occurs at recess distances where fluid mixtures near the jet exit plane are mostly within the flammability limits. This paper provides some correlations between the stability range in mixture fraction space and the turbulence level for different equivalence ratios.

Mansour, M. S., A. M. Elbaz, and M. F. Zayed, "Flame Kernel Generation and Propagation in Turbulent Partially Premixed Hydrocarbon Jet", Combustion Science and Technology, vol. 186, issue 4-5, pp. 698-711, 2014.
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