Axial temperature profile in vertical buoyant turbulent jet fire in a reduced pressure atmosphere
Highlights
► Data achieved in a reduced pressure through unique experiments at high altitude. ► Difference from those obtained in normal pressure clarified. ► Virtual origin of jet fires correlated non-dimensionally globally for both pressures. ► Axial temperature profiles correlated non-dimensionally with virtual origins.
Introduction
A jet fire usually forms due to the leakage and ignition of hydrocarbon gas fuel or two-phase flow, in case of pipe or safety valve broken, or similar accidental scenarios. Jet fires can pose serious adverse impact, such that there have been extensive investigations into jet fire characteristics in the past decades on temperature profile [1], [2], [3], flame height [4], [5], [6], [7], [8], [9], [10], lift-off [11], [12], [13], [14], [15], [16], [17], [18], [19], radiation [20], [21], [22] and soot volume fraction [23], [24], [25] for their importance in practical significance. A series of semi-empirical theories and correlation models have been developed and proposed in the literatures. Axial temperature profile is an important parameter in characterization of a vertical turbulent diffusive jet fire.
The behavior of a turbulent diffusion jet flame is dominated by entrainment either buoyancy-controlled (where the flow is governed by buoyancy) or momentum-controlled (where the nozzle fuel flow velocity is the characteristic velocity). An indicator is that the flame height increases with increase in the fuel supply flow rate in the buoyancy-controlled regime, but in contrast it is only determined by the nozzle diameter and does not change with fuel supply rate in the momentum-controlled regime. So, the behavior of a jet fire transfers from buoyancy-controlled to momentum-controlled with the increase in nozzle flow velocity (or nozzle fuel supply rate) to be even sonic.
Gomez-Mares et al. [3] has studied the axial temperature distribution for sonic (momentum-controlled) jet fire (flame length larger than 4 m and up to 9 m) with empirical equations correlated. McCaffrey [7] has proposed till now the most classical three-region-theory in characterizing the vertical temperature profile of a buoyant turbulent diffusive fire plume, based on correlation of measured data from a 30 cm methane pool-type porous bed burner. In his theory, the vertical temperature profile is divided into three regions: the continuous flame region, the intermittent flame region and the buoyant plume region, as described by following global non-dimensional expression:where η and κ are constants with different values for the three regions as listed in Table 1.
The above equation has been validated by extensive experiments for pool fires [26], [27], [28] and porous gas burners with low (or nearly zero) initial fuel momentum. Imamura and Sugawa [29] has shown that there are still limitations of its application to turbulent jet diffusive fires as the initial fuel momentum from the nozzle is not negligible. This effect can be accounted for by a parameter, named as virtual origin Z0, a hypothetical point source to revise the vertical height Z by requiring that at the flame tip location (Z − Z0) the calculated temperature using the plume equation is equal to the flame temperature [e.g., 29].
Although there have been extensive works and experimental measurements on characteristics of a vertical jet fire, they are all carried out and convinced to be only applicable for normal (standard) pressure atmosphere. There is, however, need to extend these correlations for conditions at low pressure such as at high altitudes, for example, Tibet. In a reduced pressure atmosphere, the local air and oxygen density should both decrease proportionally to the ambient atmospheric pressure. The change of their density will affect both combustion and entrainment, which in turn dominate the axial temperature profile of a vertical turbulent diffusive jet fire. Some earlier works have been reported for pool fire characteristics in the reduced pressure atmosphere at high altitude in Tibet [26], [27], [28]. Their characteristics are found to be remarkably different from those in the normal pressure atmosphere. It should be also noted that the burning of a liquid pool fire, where complex thermal feedback and fuel evaporation process is incorporated, is definitely different from burning of gaseous jet fires. As the entrainment changes, the flame height should also change accordingly. It has been observed that the flame is higher in the reduced pressure atmosphere [26], [27], [28]. The virtual origin, as an important physical parameter relating closely to the flame height, thus should also change and needs to be clarified, for correlating the axial temperature profile of a vertical buoyant turbulent diffusive jet fire in a reduced pressure atmosphere.
In this work, a series of experiments are conducted for buoyancy-controlled turbulent jet diffusion flame (flame length up to 1.2 m) correspondingly in both normal pressure (Hefei City: 50 m and 1 atm) and a reduce pressure (Lhasa City: 3650 m and 0.64 atm) atmosphere at high altitude. The axial temperature profiles are measured for different size nozzle jet fires at different propane fuel supply rates, and are further compared for these two different atmospheric pressures with their differences clarified. The virtual origins of the jet fires are correlated for both these two atmospheric pressures. The axial temperature profiles are then correlated non-dimensionally with vertical location accounting for the virtual origins. The classical three-region-theory is checked for its global applicability in the reduced pressure atmosphere.
Section snippets
Experimental facility
Fig. 1 shows the experimental apparatus and measurement setup to study the axial temperature distributions of vertical buoyant turbulent diffusive jet fires in a still air. The facility consists of a fuel supply system, thermocouples and nozzles made of stainless steel. The nozzle diameters are 4 mm, 5 mm, 6 mm, 8 mm and 10 mm with thickness of 2.5 mm and length of 30 cm. The propane gas fuel flow rate is regulated by a throttle valve and measured by a controlled volumetric flow meter. The real mass
Results and discussion
The measured axial temperature profiles above the nozzle orifice are presented. Subsequently, the virtual origins for the buoyant turbulent diffusive jet fires are deduced based on temperature data [31], and further correlated with Flame Froude Number (Frf) proposed by Delichatsios [4]. The axial temperature profiles are then further correlated non-dimensionally with vertical positions accounting for the virtual origins in these two atmospheric pressures based on the classical
Conclusions
This paper investigates the axial temperature profile in vertical turbulent buoyant diffusive jet fires in both reduced- (0.64 atm) and normal pressure (1 atm) atmosphere through experiments carried out correspondingly in both Lhasa and Hefei City at two altitudes. Major findings include:
- (1)
The maximum temperature in the continuous flame region is higher in normal pressure than that in the reduced pressure, while the temperature in the buoyant plume region is higher in the reduced pressure. This is
Acknowledgements
This work was supported by National Nature Foundation of China under Grant No. 51036007, National Basic Research Program of China under Grant No. 2012CB719702, Fundamental Research Funds for the Central Universities, and Program for New Century Excellent Talents in University under Grant No. NCET-09-0914.
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