Lift-off and blow-off of methane and propane subsonic vertical jet flames, with and without diluent air

Abstract

The paper seeks to increase understanding of subsonic jet flame blow-off phenomena, through experimental studies that include the controlled introduction of air into the fuel jet. As the molar concentration of air in the jet flame gas, Aj, is increased the reaction zone becomes leaner, and the flame lift-off distance increases.Eventually, flame oscillations develop and are followed by flame blow-off. A jet mixing analysis enables the extent of the leaning-off of the mixture to be estimated. From this, the reduced mean flamelet burning velocity, ua, is found at the location of the pure fuel jet flame. The conditions for blow-off are correlated with the last measured stable values of the dimensionless flow number, Ub*, for methane and propane jet flames, with and without added air. Values of Ub* decline as the proportion of added air increases, more markedly so with methane. This is attributed to the leaning-off of the flame, and the associated decrease in the flame extinction stretch rate. As Ub* declines in value, with increasing air dilution, the emissions of unburned hydrocarbons just prior to blow-off increase. An underlying generality of the findings is revealed when ua is introduced into the expression for Ub*, and Aj is normalised by the moles of air required to burn a mole of fuel.

Generalised correlations of blow-off and flame quenching for sub-sonic and choked jet flames

“…6 Details of how a blow-off develops were studied experimentally by AP at the State Key Laboratory in Fire Science, Hefei. Subsonic jet flames of methane and propane were employed, with pipe diameters ranging between 3 and 8 mm, as described in [4]. Fig.…”

“…This was conducted at the State Key Laboratory of Fire Science, Hefei, described in [4], and at the Centre for Technological Risk Studies at the Polytechnic University of Catalonia, Barcelona, described in [28]. These data are also shown on this figure.…”

“…The increasing use of "fracking", with its associated flaring, with possible incomplete combustion and emissions of a potent greenhouse gas, emphasises the importance of further studies in this area. Jet flames can also arise from 4 high pressure, small diameter, fuel leakages, while hydrogen venting from malfunctioning nuclear reactors presents a formidable challenge.…”

“…All the flames considered are located on a cylindrical pipe, discharging pure fuel vertically, in the absence of any pilot flame. The presence of cross winds [1] can reduce combustion efficiency, as can the presence of inert gas, such as nitrogen and carbon dioxide [2], as well as air [3,4]. A review of the extensive experimental data on jet flame heights and lift-off distances is presented in [5].…”

Main geometrical features of horizontal buoyant jet fire and associated radiative fraction

“…The previous study indicated that the liftoff length is sensitive to exit velocity and exit diameter. 19 As the velocity increases, the intense mixing of ambient air and fuel will generate a higher strain rate in which eventually would lead to the flame quenching. The computation work shows that the smaller exit diameter gives better conditions for earlier flame blow off.…”