Prediction of Flame Liftoff Height of Diffusion/Partially Premixed Jet Flames and Modeling of Mild Combustion Burners

Abstract

In this article, a new flame extinction model based on the k=e turbulence time scale concept is proposed to predict the flame liftoff heights over a wide range of coflow temperature and O 2 mass fraction of the coflow. The flame is assumed to be quenched, when the fluid time scale is less than the chemical time scale (Da < 1). The chemical time scale is derived as a function of temperature, oxidizer mass fraction, fuel dilution, velocity of the jet and fuel type. The present extinction model has been tested for a variety of conditions: (a) ambient coflow conditions (1 atm and 300 K) for propane, methane and hydrogen jet flames, (b) highly preheated coflow, and (c) high temperature and low oxidizer concentration coflow. Predicted flame liftoff heights of jet diffusion and partially premixed flames are in excellent agreement with the experimental data for all the simulated conditions and fuels. It is observed that flame stabilization occurs at a point near the stoichiometric mixture fraction surface, where the local flow velocity is equal to the local flame propagation speed. The present method is used to determine the chemical time scale for the conditions existing in the mild= flameless combustion burners investigated by the authors earlier. This model has successfully predicted the initial premixing of the fuel with combustion products before the combustion reaction initiates. It has been inferred from these numerical simulations that fuel injection is followed by intense premixing with hot combustion products in the primary zone and combustion reaction follows further downstream. Reaction rate contours suggest that reaction takes place over a large volume and the magnitude of the combustion reaction is lower compared to the conventional combustion mode. The appearance of attached flames in the mild combustion burners at low thermal inputs is also predicted, which is due to lower average jet velocity and larger residence times in the near injection zone.

Investigation of Jet-Flame Blowout with Lean-Limit Considerations

“…Because the jet exit diameter was the same for each experimental case, it follows that in order for ε to be constant for a given fuel (ρ 0 ) 1/2 /u must also be constant. To calculate u for the flames with co-flow analyzed in Moore et al [11] (where the methane is undiluted at the jet exit), an effective velocity is used where C = 40 [9] and all subscripts cf refer to characteristics of the co-flow. Using this velocity gives a range of ε from 3.9 to 4.9 and an average ε of 4.35, a value in good agreement with previous studies.…”

A review of MILD combustion and open furnace design consideration

“…The advantages of MILD combustion are already reaching industry: Danon (2011) reported an increase in demand for expertise on the implementation of MILD combustion, especially for large-scale furnaces equipped with multiple burners. MILD combustion has been achieved experimentally (Cabra et al, 2003(Cabra et al, , 2005Derudi and Rota, 2011;Derudi et al, 2007aDerudi et al, , 2007bDerudi et al, , 2007cErtesvag and Magnussen, 2000;Hasegawa et al, 2002;Kraus and Barraclough, 2012;Kumar et al, 2007;Li and Mi, 2010;Li et al, 2010aLi et al, , 2010bMi et al, 2010;Mörtberg et al, 2007;Oldenhof et al, 2010Oldenhof et al, , 2011Özdemir and Peters, 2001;Rafidi and Blasiak, 2006;Sabia et al, 2007;Weber et al, 2000;Yuan and Naruse, 1999;Zhenjun, Tong and Chaohua, 2010) and numerically (Awosope et al, 2006;Cabra et al, 2003Cabra et al, , 2005Coelho and Peters, 2001;Ertesvag and Magnussen, 2000;Frassoldati et al, 2010;Galletti et al, 2009;Kim et al, 2005;Kraus and Barraclough, 2012;Kumar et al, 2007;Oldenhof et al, 2010;Park et al, 2003;Szegö, 2010;Zhenjun et al, 2010) in premixed, partially-premixed and non-premixed combustio...…”

“…For the MILD flame, to determine the lift-off height a certain threshold level for an averaged quantity is defined. The use of variables such as temperature (Kumar, Paul and Mukunda, 2007), OH concentration (Cabra et al, 2003;Ertesvag and Magnussen, 2000) and luminescence (Cabra et al, 2005) were proposed.…”

Blow-out limits of nonpremixed turbulent jet flames in a cross flow at atmospheric and sub-atmospheric pressures

“…4, where the flame has a relative large angle θ [close to 90] from the jet direction, and the blow-out is mainly dominated by the cross flow). It was reported that the effective velocity [6][7][8], which combines the effects from the fuel jet and the co-flow, can be calculated as eff e e co co…”

“…Broadwell et al [5] proposed a large-scale mixing model and defined as a criterion of the ratio between the chemical reaction time and the turbulent mixing time to characterize the stability of turbulent diffusion flames. The effect of coflow air on the blow-out limit was also investigated [6][7][8][9][10][11][12][13][14][15] and an effective velocity was proposed. The effect of coflow air temperature was also studied [16,17].…”