Buoyant Turbulent Diffusion Flames Research Articles

Thermal radiation and soot in buoyant turbulent diffusion flames under different oxygen concentrations: Measurements and implications to radiation modeling

To develop and validate numerical models for sooty fires, we have established a dataset of the thermal radiation and soot in 15 kW buoyant turbulent ethylene flames. The flames are stabilized on a water-cooled round burner with a 15.2 cm outer diameter (D) and 13.7 cm inner diameter at three oxygen concentrations (OC) of 15.2 %, 16.8 %, and 20.9 %. A two-color optical probe is used to measure the spectral radiative intensities at two wavelengths, from which soot volume fraction and temperature are determined. The overall mean soot volume fractions are consistent with results from laser induced incandescence and laser extinction measurements. For a given OC, the mean soot temperature and volume fraction conditioned on the radiative intensity greater than a threshold value (instrumental detection limit) are relatively independent of spatial location. When OC decreases from 20.9 % to 15.2 %, the conditional mean soot volume fraction decreases by a factor of two. However, the conditional mean soot temperature at different locations and OCs are within a narrow range (with a standard deviation of only 22 K). The effect of detection limit is discussed, and the results show that the correlation between soot volume fraction and temperature is weak with a sufficiently low detection limit. Based on the experimental findings, a simplified model for the turbulence-radiation interaction (TRI) is proposed for application in the numerical modeling of soot radiation. The model approximates the turbulent closure term for radiation by taking advantage of the fact that the soot temperature has a relatively unchanged mean value and a narrow quasi-normal distribution within the buoyant turbulent flame, regardless of the spatial location and oxygen concentration. Therefore, the soot emission power can be directly calculated from the mean soot volume fraction and conditional mean soot temperature in a decoupled manner.

Joint OH-PLIF and Mie scattering imaging of enhanced water mist suppression of buoyant fires

This study presents the first near-field measurements of interactions between reaction layers and water mist droplets suppressing a buoyant turbulent diffusion flame that is representative of a fire. Joint Mie scattering and OH-PLIF are employed at a high repetition rate of 400 Hz to image the droplet field and OH structure, which are deemed here to approximate the high-temperature reaction zone. Compressed natural gas (CNG) buoyant diffusion flames are stabilised on the Sydney Turbulent Buoyant Fire Burner, which is surrounded by a low-velocity air co-flow that is seeded with fine water mist droplets (d32 ∼ 50 µm). The water mist is doped with varying concentrations of metal alkali salts (NaHCO3, KHCO3) that release radical scavenging molecules, providing enhanced suppression capabilities through chemical inhibition in addition to the quenching effect from the water droplets. The spray boundary condition across the wind tunnel exit plane is quantified using Phase Doppler Interferometry. Flame extinction limits are measured by defining the critical condition at which the flame destabilises and lifts off the burner exit plane. A stability map is developed to quantify suppression efficacy in terms of the water mist flow rate and inhibitor concentration. Joint Mie-OH images of selected cases at varying extinction levels show that the addition of inhibitor introduces frequent breakages in the OH layers, marking regions of local extinction and reignition. Large mass fractions of inhibitor in the water, which, once evaporated, produces excess salt crystals in the local extinction zones that causes streaks of salt clouds that line both the lean and rich side of the flame. The chemical scavenging effects of the OH radical by NaHCO3 and KHCO3 is quantified by evaluating the reduction of the mean OH fluorescence signal in the flame at varying locations. Probability density functions for the OH area characterise local extinction in the flame due to the radical scavengers. The order of OH radical depletion and localised extinction in the flame due to the water mist follows: KHCO3 > NaHCO3 > H2O.

Two-zone subgrid combustion model for large eddy simulations of buoyant diffusion flames

This work presents the development of a novel two-zone subgrid combustion model (SCM) recently designed to incorporate the finite-rate kinetics in the large eddy simulations of buoyant turbulent diffusion flames in which turbulence might not be fully developed. The reaction rate is evaluated as the mass-weighted average of contributions by the higher-temperature flamelet zone and the lower-temperature surrounding. The volume fraction of the flamelet zone and the time scale responsible for the inter-zone transfer are determined by the expected thickness of the laminar diffusion flamelet existing in the grid cell, and by wrinkling its surface by the unresolved turbulent eddies. This approach is applicable to an arbitrary level of turbulence (quantified by the subgrid Reynolds number) thereby overcoming the limitation of the conventional eddy dissipation concept formulated for the high-Re flames. Advancing the previous study [1], the flamelet status at every time step is evaluated by solving transient equations of heat and mass transfer between the subgrid zones. Effective kinetic parameters for the simplified global reaction of fuel oxidation are derived from experimental data for autoignition delay in methane-air and n-heptane-air mixtures and are shown to capture transition from the low- to high-temperature measurement datasets including the NTC (negative temperature coefficient) interval. With these kinetic parameters, the SCM is validated against the measurements in the unconfined methane-air flame produced by the UMD line burner and then applied to predict the development and extinction of the heptane-air flame in an under-ventilated enclosure. In the first case, flame extinction is predicted at the oxygen concentrations consistent with the experimental values. In the second scenario, global low-frequency oscillations of the heat release rate are predicted prior to flame extinction.

Development of a novel subgrid combustion model for buoyant turbulent diffusion flames

A novel approach is presented to incorporate finite rate reactions in modeling of turbulent diffusion flame, with the existence of unresolved flamelets inside the CFD grid cells taken into account. This approach, called SCM (subgrid combustion model), is applied in large eddy simulations of the UMD line burner flames. With the global kinetic parameters fitted for the measurement data and detailed simulations of autoignition in stoichiometric methane-air mixture, as well as for the extinction strain rate in the opposed non-premixed jets, the SCM model is shown to replicate the mean temperature profiles in unsuppressed flame with mean total heat release of 50 kW. For flame suppression via dilution of the oxidizer flow by nitrogen, the lift-off and extinction of non-anchored flame are predicted in agreement with the experimental data. The SCM is designed as a tool to allow for the finite rate kinetics effects in large eddy simulations of buoyant turbulent diffusion flames, with the focus on weakly turbulent regions, in which applicability of conventional combustion models is limited.

Experimental study on the buoyant turbulent diffusion flame height of various intermittent levels

This work studies the buoyant turbulent diffusion flame characteristics considering various flame intermittent levels and mean flame projection area. Although many investigations have been carried to reveal the mean flame height evolution of the buoyant turbulent diffusion flame, the turbulent diffusion flame height of various intermittent levels, occurs in the un-controllable (unplanned) fire accident (such as the gas fuel pipeline/tank leakage). The underlying evolutions of the buoyant turbulent diffusion flame with the heat release rate and intermittent levels have not been fully elucidated. Experiments are conducted by various burner dimensions under varied heat release rates. The flame geometries/heights at various intermittent levels are measured and quantified comprehensively by two cameras from the front- and side- view. It shows that the mean flame height declines as burner area/dimension increases. All experimental data for mean flame height in the present work and previous work are well correlated by the modified normalized heat release rate derived from the new characteristic length. Results also show that the flame height at various intermittent levels decreases with the intermittent level. A model of the flame heights at various intermittent levels is proposed considering the intermittent level and mean flame height to predict the experimental data in this work and previous work well. Finally, a flame projection area model is proposed based on the assumption of the isosceles triangle flame projection geometry. This work provides huge amount experimental data and prediction models to describe buoyant turbulent diffusion flame behaviours, which is meaningful for evaluating thermal failure probability and laying of gaseous fuel transportation pipelines.

On the structure of buoyant fires with varying levels of fuel-turbulence

This paper employs a novel burner to study the effects of fuel-generated turbulence on the spatial and temporal structure of buoyant turbulent diffusion flames which are representative of large fires. Fuel-turbulence levels are increased using a perforated plate that issues high-velocity jets, enabling shearing of the fuel stream. The perforated plate may be recessed to control the turbulence level at the jet exit plane. It is shown that the exit plane axial velocity fluctuations can be increased from 0.135 m/s to 1.813 m/s. Varying the levels of fuel-turbulence in the burner allows for the control of key processes defining buoyant fires such as the spatial and temporal flame structure and flame instability modes. These processes are characterised by high-speed simultaneous imaging of planar laser-induced fluorescence of the OH radical (OH-PLIF) and Mie scattering from soot particles. Increasing the fuel-turbulence level deforms the flame, which promotes non-radial lateral entrainment into the flame sheet. This results in a sharp increase in the tilting of the near-field flame sheet along the vertical flame axis. Strong angular entrainment forces are shown to overcome the diffusive and thermal expansive forces at the flame neck, which leads to a strained asymmetric sinuous flame pinch-off instability, followed by separation of the flame base. Sinuous pinch-off instabilities occur at a greater frequency than the symmetric varicose pinch-off instabilities observed for flames with low fuel-turbulence. The asymmetric stretching of the flame neck inhibits the formation of the classical puffing instability formed with an axisymmetric plume that defines classically buoyant flames. Probability density functions calculated for the flame front curvature and flame surface area are shown to monotonically broaden in the near-field region of the flame due to lateral entrainment effects. The transition to buoyancy-driven turbulence also shifts to an increasingly more upstream location. This burner, with its well-defined boundary conditions and novel data, forms a platform for advancing capabilities to model complex fire phenomena including turbulence-buoyancy interactions.

Effect of oxygen concentration on the velocity in buoyant turbulent diffusion flames

To better understand fire dynamics of buoyant turbulent diffusion flames under different oxygen concentrations (OCs, volumetric basis), we have recently conducted measurements on the flame temperature, radiation, and soot temperature and volume fraction under normal and reduced OCs. In the present study, particle image velocimetry (PIV) technique was employed to investigate the effect of OC (20.9%, 16.8%, and 15.2%) on the velocity field of the same flames with a theoretical heat release rate (HRR) of 15 kW. The statistics of the velocity field, such as mean and standard deviation of velocity, turbulent kinetic energy, Reynolds stress, and turbulent viscosity, were investigated and compared among different OCs. Results show that OC has minimal effect on the velocity distribution at heights close to the flame length. However, at lower heights above burner (HABs), turbulent mixing is reduced under reduced OCs, and the velocity shows an increased value at the flame centerline but with a narrower profile along the radial direction. Finally, a comparison with previous data was conducted and analyzed to further explore the fire dynamics. This study improves the physical understanding of related fire dynamics under reduced OCs and provides a comprehensive dataset for turbulent fire modeling validation.

Assessment of flame height of two unequal turbulent diffusion flames: Experimental interpretation and quantification

Multiple fires burning is a fairly common phenomenon in wildland fires and urban process industries. The burning of multiple fires is much more dangerous than single fire due to the merging of flames. This work aims to study the flame height of two unequal buoyant turbulent diffusion flames, which has not been well revealed. Total 168 experimental tests were performed using two square gas burners with different side lengths of 10 cm and 15 cm as the fire sources. The heat release rates (HRRs) in fire group and burner spacing (S) were changed. Results showed that for a given HRRs group, with the increase of S, the flame height presents a decay trend first, followed by a rise trend, and finally it maintains unchanged. This non-monotonic trend is explained from the view of interaction mechanisms. Three states of fully merging, intermittent merging and non-merging of two flames are divided to illustrate and quantify the flame height. By introducing a normalized parameter λ to characterize the air entrainment difference, a correlation for flame height of intermittent merging is developed. For the fully merging and non-merging, the equations for flame heights are respectively established based on the assumed equivalent single flame. The accuracy of the proposed model for flame height is validated using experimental data of this work and in literatures. And the application scope of model is expanded to the flame height in merging state of two identical fires.

Buoyant turbulent diffusion flame heights of free-, wall- and corner air entrainment conditions: Experiments and global model based on mirror approach

This paper investigates buoyant turbulent diffusion flame height of gaseous fuel combustion in various air entrainment conditions: free-, wall- and corner flames. Experiments are carried out using five square burners with side lengths of 0.3 m, 0.25 m, 0.2 m, 0.15 m and 0.1 m, and five rectangular burners with side dimensions of 0.15 m × 0.3 m, 0.125 m × 0.25 m, 0.1 m × 0.2 m, 0.075 m × 0.15 m, 0.05 m × 0.1 m, to produce corner-, wall- and free flames. The heat release rates (HRR) are in the range of 6.62–26.46 kW for the rectangular burners and 2.21–35.28 kW for the square burners to have 172 experimental conditions comprehensively. It is found that for the same HRR and burner size, the flame height (ℓf) of corner restriction is larger than the flame height of wall flame, and ℓf of wall flame is larger than that of free flame. For a given HRR and air entrainment condition, the flame height decreases with the burner dimension increase. Results also show that the effect of wall or corner on the air entrainment can be represented by the “mirror-approach” with an acceptable deviation (−12.5%~13.5%). At the same time, for free and wall flames, there is a transition of dimensionless flame heights from two-dimensional air entrainment to three-dimensional air entrainment. For corner flame, most of the data fall in the two-dimensional air entrainment condition. Combined with comprehensive data of previous reports together, flame height formulas and critical non-dimensional heat release rates for the transition points are proposed for these three types of air entrainment conditions globally by using the perimeter as the characteristic length scale based on “mirror-approach”.

Laser induced incandescence measurement of soot in ethylene buoyant turbulent diffusion flames under normal and reduced oxygen concentrations

There is a lack of published soot measurement data in buoyant turbulent flames under reduced oxygen conditions. In this work, the laser induced incandescence (LII) technique is used to investigate the soot volume fraction (fv) and soot sheet thickness in buoyant turbulent diffusion flames in atmospheres with different oxygen concentrations (OC, oxygen molar fraction in oxidizer). The experiments establish an extensive database of quantitative fv spatial distribution in ethylene flames under three OCs (20.9%, 16.8%, and 15.2%) and two heat release rates (HRR = 10 kW and 15 kW). Mean, fluctuation, and probability density function of fv, as well as local soot intermittency are studied and compared across different conditions. The results show that the maximum instantaneous fv in normal air can be ~8 ppm; but the corresponding mean fv only reaches up to 0.58 ppm due to turbulence and flame intermittency. As the oxygen concentration is reduced from 20.9% to 15.2%, the mean fv decreases, and this effect is more significant at lower heights above the burner (e.g., ~75% reduction in the mean fv at a height of half a burner outer diameter). Soot is found to be distributed unevenly in the flame, appearing in sheet-like regions with irregular and complicated shapes. The soot sheet thickness is quantified to better understand soot-turbulence interactions. This soot dataset, along with other accompanying radiation and flow field measurements, provides a comprehensive understanding of the dynamics of turbulent buoyant flame dynamics under normal and vitiated conditions, and can be used to develop and validate models for soot formation and radiative heat transfer in such flames.

Predicting radiative heat fluxes from two buoyant turbulent diffusion flames from burning propane under cross wind

Experimental investigations were carried out to estimate the radiative heating that occurs downstream of wind-driven buoyant turbulent diffusion flames produced by twin square propane burners aligned parallel to the cross wind. During the experiments, the heat release rate, cross wind velocity, and burner edge spacing were changed. Considering the effect of wind on flame shapes, a new triangular prism flame model was established to predict the radiant heat flux received by downstream objects from two buoyant turbulent diffusion flames under cross wind. The flame radiations to downstream ground were measured by five radiometers, and the relevant parameters of the flame model were analyzed. The results show that the radiative heat flux decays with increasing flame spacing and intensifies with increasing wind velocity. A piecewise function for the flame drag length in various flame merging stages is established. The effect of wind velocity and flame spacing on flame emissivity is relatively limited, and flame emissivity is formulated as a function of the heat release rate. The view factor between the triangular prism flame model and external targets is calculated based on the contour integral method. Finally, prediction results between the proposed triangular prism flame model and conventional models are compared against experimental data. The results confirm that the new triangular prism flame model can better predict the thermal radiation from two interacting flames in the wind.

Numerical simulations of soot-radiation from buoyant turbulent wall-bounded diffusion flames

Buoyant, turbulent diffusion flames anchored on a slot burner in the presence of an isothermal, low-temperature (333 K) wall have been numerically simulated. A laminar smoke point based subgrid (flamelet) radiation model has been applied toward estimation of overall radiant fractions and radiant power distributions of the ethylene fires. The model uses a turbulent micro-scale strain rate and an enthalpy defect that influence the flamelet soot radiation. In the presence of the isothermal, low-temperature wall, the radiant fractions of the fires are observed to decrease along with a reduction of their peak radiant power output. Predicted flame heights, radiant fractions and radiant power distributions are found to reproduce observed experimental trends and exhibit quantitative agreement with measurements. Comparison of radiant power distributions are also made for fires adjacent to high-temperature (1000 K) isothermal and adiabatic walls and peak radiant power output for both scenarios are found to be higher compared to the isothermal, low-temperature wall-bounded fire.

Experimental study on pulsation frequency of free-, wall- and corner buoyant turbulent diffusion flames

This work studies the flame pulsation frequency f of free-, wall- and corner buoyant turbulent diffusion flames. Five square gaseous burners with the burner side length of 10, 15, 20, 25 and 30 cm are used to produce the three types of flames. In addition, five rectangular gaseous burners with dimensions of 30 cm × 15 cm, 25 cm × 12.5 cm, 20 cm × 10 cm, 15 cm × 7.5 cm, 10 cm × 5 cm (aspect ratio of 2:1 with half surface area of the square burners) are also used to produce wall flames. Propane is used as fuel. Results show that heat release rates have no significant effect on f. The flame pulsation frequency of free flames is larger than that of wall flames, which is further larger than that of corner flames. For all types of flames, the flame pulsation frequency decreases with the increasing of the burner size. The classic 1/2 power law correlation, i.e., f~(g/D)0.5 for flame pulsation frequency of free flame can well describe the data of all types of flames (square, rectangular; free flame, wall flame and corner flame) on the basis of the “mirror” approach, and the perimeter-equivalent diameter is used as the appropriate length scale.

Temperature measurement of a turbulent buoyant ethylene diffusion flame using a dual-thermocouple technique

High-frequency temperature measurements were carefully conducted for a 15 kW buoyant turbulent ethylene diffusion flame over a 15.2 cm diameter gas burner with air co-flow. A dual-thermocouple probe, consisting of two fine-wire thermocouples with 25 μm and 50 μm wire diameters, was used to determine a compensated turbulent gas temperature. A sensitivity analysis shows that temperatures resolved using this dual-thermocouple technique are less sensitive to changes in thermocouple bead size, therefore, uncertainty is greatly reduced even when soot deposition on the thermocouple bead occurs in sooty flames. Mean and root-mean square (rms) fluctuations of gas temperature were recorded in a two-dimensional plane across the flame centerline. The mean gas temperature monotonically decreases away from the flame centerline at most flame heights, except for 1 diameter above the burner, where a temperature dip is observed. The rms temperature peaks shift from the edge of the flame to the center as the height increases. This is due to the enhanced mixing between fuel and air, which is further shown using probability density functions of the local gas temperature. A systematic temperature dataset with high spatial resolution is established for sooty flames, which is valuable for future soot and radiation model validation.

Gas temperature rise and flame length induced by two buoyancy-controlled propane burners aligned parallel to the cross wind

This paper investigates the gas temperature rise and flame length of buoyant turbulent diffusion flames under cross wind, which does exist in fuel leakage fire accidents. Two identical propane burners with the same heat release rate were employed as fire sources. The burner spacing, heat release rate and cross wind velocity were changed. The results show that with the increase of the cross wind velocity, the gas temperatures in downwind region are strongly increased, resulting in a strong heat transfer to nearby energy loads. On the other hand, the non-monotonic trend of the flame length is caused by the cross wind effect. In addition, a segmented function predicting the flame length under cross wind is developed for merging and non-merging flames, which involves the heat release rate, burner spacing, pool size and cross wind velocity. The findings of this work have potential practical applications for optimal arrangements of energy loads, which allow predictions to be made regarding the possible threat and the establishment of the necessary safety distance to prevent the damage of multiple fires.

Size, effective density, morphology, and nano-structure of soot particles generated from buoyant turbulent diffusion flames

With a global gas flaring volume of ∼140 billion cubic meters, flares are an important source of particulate emissions; however, very little is known about the physical and morphological properties of these particle emissions. To study these properties, a laboratory pipe flare producing a buoyant turbulent diffusion flame was used which allowed controlled experiments on flames up to ∼3 m tall. Three flare diameters (38.1, 50.8, and 76.2 mm) were used in this study with fuel exit velocities of 0.5, 0.9, and 1.5 m/s. ‘Light’, ‘medium’, and ‘heavy’ fuel compositions (consisting of C1 to C4 alkanes, carbon dioxide, and nitrogen in concentration representative of flares in the Alberta, Canada upstream oil and gas sector) were used, where heavier compositions refer to a greater concentration of higher order alkanes. Size distributions of soot particles were measured using a scanning mobility particle sizer. Mass-mobility relationship and effective density of particles were determined using a tandem arrangement of a differential mobility analyzer, a centrifugal particle mass analyzer and a condensation particle counter. Morphology and nano-structure of the particles were studied using transmission electron microscopy and Raman spectroscopy, respectively. Results showed that the particle median diameter and concentration increased as the fuel composition was changed from light to medium to heavy. On the other hand, particle morphology, measured by the relationships between particle mass vs. mobility (or effective density) and primary particle size vs. particle aggregate size, was independent of fuel composition, flow rate, or flare size and was in good agreement with previously reported values for that of soot particles from different internal combustion engines. Previously developed relations between effective density and primary particle size work well for the soot particles of this study. Raman spectroscopy indicated slightly lower D1/G ratios (more graphitic content) for the heavier fuels.

Experimental investigation of nitrogen addition effect on combustion characteristics of buoyant turbulent diffusion flame

Experimental study was carried out on the combustion characteristics of buoyant turbulent diffusion propane flame with fuel volume fraction ranging from 0.2 to 1.0 by nitrogen addition. The combustion characteristics reported include total flame length, flame diameter, soot free length fraction (SFLF), axial temperature distribution, maximum axial temperature and flame radiation fraction. Results show that the total flame length and flame diameter are insensitive to nitrogen addition. Soot free length fraction was found to be proportional to (1+S)1S-0.5fv,fuel-1. The axial temperature near the burner decreases while increases at higher height with nitrogen addition. The maximum axial temperature increases at first then decreases with nitrogen addition. The different trends in axial temperature characteristics are brought by two competitive effects between cooling effect and declined radiative heat loss due to nitrogen addition. The flame radiation fraction decreases with nitrogen addition due to soot reduction and is found to be proportional to (1+S)-0.25S0.125fv,fuel0.25 based on theoretical analysis and experimental results. The correlation result is compared to the previous data indicating good applicability.

An experimental study and analysis on maximum horizontal extents of buoyant turbulent diffusion flames subject to relative strong cross flows

The present study experimentally investigated the non-monotonous evolutions as well as the maximum lengths of the flame base extent (attached to the ground) and horizontal flame extent (deflected flame tip) of buoyant turbulent diffusion flames subject to relative strong cross flows of air. These transition behaviors and quantities have not been quantified in the literature, meanwhile are essential for quantifying this boundary layer diffusion combustion characteristics associated with complex fluid dynamics and air entrainment behavior, as well as for the estimation of hazardous impact of the flame upon the downstream areas. Four square burners with dimensions of 8, 10, 15 and 20 cm were used in the present work employing propane as fuel supplied with various heat release rates. The flame base extent and the horizontal flame extent were measured under cross flows provided by a wind tunnel with the cross flow air velocity with range of 0 m/s to a relative strong level, 6.0 m/s. It is shown in the results that the flame base extent and horizontal flame extent first increases with cross flow air speed at relative lower range, which is in accordance with observations reported in the literatures. However, a new phenomenon is found that once the cross flow air velocity is beyond a critical value, both these two horizontal extents of the flame decreased when the air speed is further increased. The observed two-regime behavior is then analyzed based on three dimensionless parameters including Froude number representing the counteraction of the cross flow with the buoyant flow, dimensionless heat release rate, and the ratio of gaseous fuel density to the ambient air density. New correlations are proposed to describe the two-regime evolution behavior for flame base extent and horizontal flame extent with increasing cross flow air speed, as well as maximum extents that can be reached. These new observations and proposed correlations provide an essential base to quantify the buoyant turbulent diffusion flame boundaries in relative strong wind conditions.

Erratum to: “Natural Buoyant Turbulent Diffusion Flame near a Vertical Surface”

In the original publication, the title was misspelled. It should read “Buoyant Turbulent Diffusion Flame near a Vertical Surface ” instead of “Natural Buoyant Turbulent Diffusion Flame near a Vertical Surface.” It should also read “buoyant turbulent diffusion flame” instead of “natural buoyant turbulent diffusion flame” everywhere else in the text and “the upward flame spread rate” instead of “the velocity of the upward flame spread rate” in the abstract. The original Russian text was translated by the journal.

Flame length of buoyant turbulent slot flame

This work presents a modeling and experimental analysis on the flame length of buoyant turbulent slot diffusion flame. An approximate predictive correlation of flame length is derived based on the governing equations. The role of initial momentum and buoyancy on flame length is quantitatively described by a modified Froude number Frm. The physical model indicates that the flame length of a turbulent slot flame is proportional to (Frm)1/3 and (Frm)0 respectively when the flame is buoyancy-dominated and momentum-dominated. The results, for the first time, give a physical verification on the previous scaling laws on buoyant turbulent slot flame in literature. Slot flame tests are conducted using two burners (for which the width W and length L are respectively 0.9 mm × 20.5 mm and 5.5 mm × 86 mm) and four kinds of high-purity hydrocarbon gas fuels (methane, acetylene, ethane and propane). Experimental data show that the radial temperature profiles of buoyant turbulent slot flames can be well described by a Gaussian function normalized by the temperature radius, which is independent of axial position and flame scale. Moreover, experimental data verify that the physical model applies for different fuel supply rates, types of fuels and flame scales. Additionally, the results indicate that the turbulent slot flame is buoyancy-dominated when Frm < 105, whereas the non-buoyant scenario is approached when Frm > 107. Finally, it is indicated that the scale effect of turbulent slot flame is closely associated with the flame temperature under different flame scales.