Numerical study of flame structure in the mild combustion regime

In this paper, turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is studied numerically. This flame is under condition of the moderate or intense low-oxygen dilution (MILD) combustion regime and related to published experimental data. The modelling is carried out using the EDC model to describe turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. The flame structure for various O2 levels and jet Reynolds numbers are investigated. The results show that the flame entrainment increases by a decrease in O2 concentration at air side or jet Reynolds number. Local extinction is seen in the upstream and close to the fuel injection nozzle at the shear layer. It leads to the higher flame entertainment in MILD regime. The turbulence kinetic energy decay at centre line of jet decreases by an increase in O2 concentration at hot Co-flow. Also, increase in jet Reynolds or O2 level increases the mixing rate and rate of reactions.

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Actuation Studies for Active Control of Mild Combustion for Gas Turbine Application

Volume 4B: Combustion, Fuels and Emissions ◽

10.1115/gt2014-27138 ◽

2014 ◽

Cited By ~ 2

Author(s):

Emilien Varea ◽

Stephan Kruse ◽

Heinz Pitsch ◽

Thivaharan Albin ◽

Dirk Abel

Keyword(s):

Combustion Chamber ◽

Gas Turbine ◽

Active Control ◽

Gas Turbines ◽

Reverse Flow ◽

Air Flow ◽

Combustion Regime ◽

Nox Emissions ◽

Low Oxygen ◽

Mild Combustion

MILD combustion (Moderate or Intense Low Oxygen Dilution) is a well known technique that can substantially reduce high temperature regions in burners and thereby reduce thermal NOx emissions. This technology has been successfully applied to conventional furnace systems and seems to be an auspicious concept for reducing NOx and CO emissions in stationary gas turbines. To achieve a flameless combustion regime, fast mixing of recirculated burnt gases with fresh air and fuel in the combustion chamber is needed. In the present study, the combustor concept is based on the reverse flow configuration with two concentrically arranged nozzles for fuel and air injections. The present work deals with the active control of MILD combustion for gas turbine applications. For this purpose, a new concept of air flow rate pulsation is introduced. The pulsating unit offers the possibility to vary the inlet pressure conditions with a high degree of freedom: amplitude, frequency and waveform. The influence of air flow pulsation on MILD combustion is analyzed in terms of NOx and CO emissions. Results under atmospheric pressure show a drastic decrease of NOx emissions, up to 55%, when the pulsating unit is active. CO emissions are maintained at a very low level so that flame extinction is not observed. To get more insights into the effects of pulsation on combustion characteristics, velocity fields in cold flow conditions are investigated. Results show a large radial transfer of flow when pulsation is activated, hence enhancing the mixing process. The flame behavior is analyzed by using OH* chemiluminescence. Images show a larger distributed reaction region over the combustion chamber for pulsation conditions, confirming the hypothesis of a better mixing between fresh and burnt gases.

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Numerical and Experimental Study on Lift-Off Suppression Phenomenon of Subsonic Hydrogen Jet Diffusion Flame

2010 14th International Heat Transfer Conference, Volume 3 ◽

10.1115/ihtc14-22116 ◽

2010 ◽

Author(s):

Shuichi Torii

Keyword(s):

Experimental Study ◽

Diffusion Flame ◽

Fuel Injection ◽

Numerical Study ◽

Vacuum Pressure ◽

Jet Flame ◽

Injection Nozzle ◽

Circular Nozzle ◽

Lift Off

Experimental and numerical study is performed on subsonic hydrogen jet diffusion flame formed from the vertical circular nozzle. Emphasis is placed on the effect of the cavity height formed at the fuel injection nozzle tip on suppression of the flame lift-off. It is found that (i) an increase in the cavity height triggers and enhances a vacuum pressure, (ii) the air from the surroundings is transported naturally into the cavity to replenish the air entrained and consumed by the jet flame, and (iii) the vacuum pressure results in the mitigation of flame lift-off propensity.

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Effects of CO2 Dilution on Methane Ignition in Moderate or Intense Low-oxygen Dilution (MILD) Combustion: A Numerical Study

Chinese Journal of Chemical Engineering ◽

10.1016/s1004-9541(11)60238-3 ◽

2012 ◽

Vol 20 (4) ◽

pp. 701-709 ◽

Cited By ~ 4

Author(s):

Zhenjun CAO ◽

Tong ZHU

Keyword(s):

Numerical Study ◽

Low Oxygen ◽

Mild Combustion

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Numerical study of the effect of turbulence on rate of reactions in the MILD combustion regime

Combustion Theory and Modelling ◽

10.1080/13647830.2011.561368 ◽

2011 ◽

Vol 15 (6) ◽

pp. 753-772 ◽

Cited By ~ 32

Author(s):

Amir Mardani ◽

Sadegh Tabejamaat ◽

Mohammadreza Baig Mohammadi

Keyword(s):

Numerical Study ◽

Combustion Regime ◽

Mild Combustion

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Experimental and Numerical Investigation of a Hydrogen Combustion Chamber Under Various Inlet Conditions

Volume 4A: Combustion, Fuels and Emissions ◽

10.1115/gt2014-25472 ◽

2014 ◽

Author(s):

Heidemarie Malli ◽

Kurt Eckerstorfer ◽

Oliver Borm ◽

Peter Leitl

Keyword(s):

Combustion Chamber ◽

Development Stage ◽

Combustion Regime ◽

Hydrogen Combustion ◽

Emission Characteristics ◽

Low Oxygen ◽

Combustion Chambers ◽

Mild Combustion ◽

Combustion Technology ◽

Inlet Conditions

Flameless combustion, MILD (moderate or intense low oxygen dilution) combustion and HiTAC (high temperature air combustion) all refer to a combustion regime characterized by high temperatures and a high dilution of reactants. In most cases, this is achieved by recirculating exhaust gases. This leads to comparatively low oxygen concentrations, a largely uniform temperature field and to a drastically reduced NOx formation. Up to now, the application of this combustion technology for gas turbine combustion chambers is still in an early development stage. Most investigations of flameless or MILD combustion chambers have been carried out for methane or certain fuel blends. Since this combustion technology has already successfully demonstrated low NOx emissions without the need of premixing with its potential risks of flashback and autoignition, it might be a promising technology for hydrogen burning combustion chambers. The scope of this paper is to investigate a hydrogen combustion chamber for its NOx emission characteristics and for its use in the flameless or MILD combustion regime. Thus, the influence of different inlet parameters (excess air ratio, thermal input of hydrogen, inlet velocity of the combustion air, pressure inside the combustion chamber) on the emission characteristics of the combustion chamber are examined experimentally. Additionally, for one operating point, a two–dimensional numerical simulation of the combustion chamber was carried out.

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Numerical Study of the Effect of Nozzle Configurations on Characteristics of MILD Combustion for Gas Turbine Application

Journal of Energy Resources Technology ◽

10.1115/1.4033141 ◽

2016 ◽

Vol 138 (4) ◽

Cited By ~ 10

Author(s):

Xiaowen Deng ◽

Yan Xiong ◽

Hong Yin ◽

Qingshui Gao

Keyword(s):

Gas Turbine ◽

Numerical Study ◽

Combustion Efficiency ◽

Low Noise ◽

Peak Temperature ◽

Pollutant Emissions ◽

Operating Point ◽

Low Oxygen ◽

Mild Combustion ◽

Jet Velocity

The MILD (moderate or intense low-oxygen dilution) combustion is characterized by low emission, stable combustion, and low noise for various kinds of fuel. This paper reports a numerical investigation of the effect of different nozzle configurations, such as nozzle number N, reactants jet velocity V, premixed and nonpremixed modes, on the characteristics of MILD combustion applied to one F class gas turbine combustor. An operating point is selected considering the pressure p = 1.63 MPa, heat intensity Pintensity = 20.5 MW/m3 atm, air preheated temperature Ta = 723 K, equivalence ratio φ = 0.625. Methane (CH4) is adopted as the fuel for combustion. Results show that low-temperature zone shrinks while the peak temperature rises as the nozzle number increases. Higher jet velocity will lead to larger recirculation ratio and the reaction time will be prolonged consequently. It is helpful to keep high combustion efficiency but can increase the NO emission obviously. It is also found that N = 12 and V = 110 m/s may be the best combination of configuration and operating point. The premixed combustion mode will achieve more uniform reaction zone, lower peak temperature, and pollutant emissions compared with the nonpremixed mode.

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