Fire Whirls Research Articles

An analysis of the stabilization of fire whirls

Stabilization of the flame base at the bottom Ekman boundary layer of the fire whirl under strong circulation is analytically and numerically studied. The problem is relevant for fire safety and the associated understanding of the generation mechanism of the newly discovered whirling flame, the so-called blue whirl. Based on the dynamic balance of the triple flame propagation speed and the local flow velocity at the bottom boundary layer, theoretical solutions of the liftoff position, rf, and the critical blowout limit represented by its Ekman number, EkBt, are derived. The theoretical results agree well with the numerical results both qualitatively and quantitatively. It is shown that the response of rf to the system Damköhler number, Da, forms a typical C-shaped curve and there are two solutions of rf for relatively large Da, with the one closer to the edge of the fuel pool being stable while the other being unstable. Explicit solution of the blowout limit shows that EkBt∝(SLS)4/3, where SLS is the stoichiometric planar flame speed. This is because not only the triple flame propagation speed linearly increases as SLS increases, but the flame front can also reach a position closer to the edge of the fuel pool where the flow velocity is lower. The effect of the transversal radial velocity gradient (TVG) at the Ekman boundary layer on the stabilization of the fire whirl is also investigated. The TVG makes the flame harder to liftoff and blowout and the effect of which can be described by a non-dimensional parameter, λ, which represents the ratio of the flame curvature and the TVG. The TVG normally plays a secondary role in the triple flame propagation due to the relatively large flame curvature of the triple flame.

Effects of circulation and buoyancy on the transition from a fire whirl to a blue whirl

Author(s): Hariharan, SB; Hu, Y; Gollner, MJ; Oran, ES | Abstract: The relative influence of circulation and buoyancy on fire whirls (FWs), blue whirls (BWs), and the transition between these regimes of a whirling flame is investigated using a combination of experimental data and scaling analyses. FWs are whirling, turbulent, cylindrical yellow (sooting) flame structures that form naturally in fires and are here created in laboratory experiments. In contrast, a BW is a laminar, blue flame (nonsooting) with an inverted conical shape. Measurements of the circulation and heat-release rate are combined with measurements of the flame geometry, defined by the flame width and the height, to provide characteristic length scales for these whirling-flame regimes. Using these, a nondimensional circulation (Γf) and a heat-release rate (Qf) were defined and shown to correspond to azimuthal and axial (buoyancy driven) momenta, respectively. The ratio R=Γf/Qf, a quantity analogous to the swirl number used to characterize swirling jets, was evaluated for FWs and BWs. For FWs, Rl1, so that axial momentum is greater than azimuthal momentum and the flame is dominated by buoyant momentum. For BWs, Rg1, so that the flame is circulation dominated. This is argued to be consistent with vortex breakdown being an important part of the transition of FWs to BWs. This work presents a basis for predicting when a BW will form and remain a stable regime.

Flow Field around a Fire Whirl over Line Fire

Flow Field around a Fire Whirl over Line Fire

Lifted flame in fire whirl: An experimental investigation

This paper presents an experimental study on the flame and flow characteristics of the lifted flame in a fire whirl. A small-scale rotating screen facility was used for experiments with finely controlled imposed circulation (Γ) and heat release rate (Q˙=2.0−8.0kW), by using methane as the fuel. The instantaneous three-dimensional velocity fields in the vertical plane passing through the flame centerline were measured by stereo particle image velocimetry (SPIV) technique. It is found that the flame of fire whirl can be lifted off the burner under relatively weak Γ, for which, the lifted flame is mainly blue partially premixed flame under a certain lower heat release rate (Q˙=2.0kW in this work), while when the heat release rate is beyond a certain value (Q˙≥4.0kW in this work), the lifted flame is structured by blue partially premixed flame at the bottom and yellow diffusion flame downstream. For these two different lifted flames, the flame length and the flame base diameter have significantly different dependence on Γ. Experimental data indicate that under a specified Q˙, the flame liftoff height firstly increases slowly and then decreases quickly with increasing Γ after reaching the maximum value. The maximum liftoff height decreases with Q˙ slightly and Γ dominates the variation of liftoff height. It is also found that the instantaneous axial velocity in the lifted fire whirl increases quickly in the bottom inflow layer, then decreases with height upstream of the flame base and increases again in the hot flame region. The flow field diverges as the instantaneous flame base is approached. The mean axial velocity conditioned on the instantaneous lifted flame base generally increases with Q˙ and is less than two times the maximum laminar flame speed. The lifted fire whirl and the triple flame share similar characteristics of the flame front and velocity fields.

Air/fuel mixing in jet flames

The paper examines eight diverse regimes in which fuels can mix and react with air. These comprise: (i) Lifted subsonic; and (ii) supersonic jet flames, with (iii) and without (iv) cross flows; (v) Rim-attached flames; (vi) Early Downwash flames; (vii) Downwash-attached jet flames; and (viii) Fire Whirls.Correlations of characteristics within these regimes are principally in terms of a dimensionless Flow Number, U*, Cross Flow Reynolds number, Rec, and, for Fire Whirls, a dimensionless Critical Velocity, CV. Boundaries of seven of the eight regimes are identified, through plots of U*, against Rec, and of the eighth through a plot of CV against U*. The circumstances of transitions between regimes are identified. The study involves a variety of CH4 cross flow flame measurements, in a wind tunnel. Cross flows can initially create a small lee-side flame downwash, due to the depression in pressure. With increasing fuel flow this might extend 1.3 m downwards from the horizontal tip of the vertical burner. Jet flames can attach to the downwash, which can become significant above Rec ≈ 2000. More extensive downwash might further delay blow-off. Regime boundaries are constructed on the U*/Rec diagram covering lifted flames, early downwash, and downwash-attached flames. The most powerful flames tend to be lifted, choked, flames, with cross flow, and fire whirls. Combustion becomes less efficient at high Rec and low U*, although CH4 was efficiently reacted.Experimental values of the ratio of fuel to air velocity, u/uc, of CH4 flames ranged between about 10 and 30 for lifted flames, and between 0.3 and 3.6, at blow-off, for rim-attached flames. The latter comprise an important category, often intermediate between lifted flames and downwash-attached flames.

Evolution from conical to cylindrical fire whirl: An experimental study

This paper presents the first experimental effort on the evolution of the flow field from conical to cylindrical fire whirl, by using a small-scale rotating screen facility with finely controlled imposed circulation (Γ) under a fixed heat release rate of 4.0 kW. Stereo particle image velocimetry (SPIV) technique was employed to measure the instantaneous and mean velocity fields. Two-celled vortex structure and central recirculation zone are found to form and move downwards with increasing Γ in the plume region of cylindrical fire whirl. Within the mean flame height, the mean vortex core radius decreases slightly with Γ for conical fire whirl, while expands linearly after the onset of cylindrical fire whirl. The radial profiles of tangential velocity have self-similarity within the mean flame height for both conical and cylindrical fire whirls. In the plume region of the cylindrical fire whirl, the radial distribution of tangential velocity is similar to that of the Rankine vortex, and the decay of maximum tangential velocity and the expansion of the vortex core are faster than those of the conical fire whirl. With increasing Γ, the unimodal profile of axial velocity is found to change into the plateau profile and the wakelike profile in the plume region of the conical fire whirl and the transition location advances upstream steadily. For both conical and cylindrical fire whirls, the centerline axial velocities of certain levels and the mean flame height have consistent variation trends with Γ. Within the mean flame height for cylindrical fire whirl, the centerline axial velocity falls sharply at the axial position where the radial profile of axial velocity changes from the plateau to the wakelike profile. The height of this axial position is found to decrease steadily with the increase of Γ.

Double S-type Pitot Tube for velocity field study of fire whirls

In this paper we present a pressure measurement instrument based on the S-type Pitot tube modified to measure the two velocity components of a high temperature flow assuming that the flow is locally two-dimensional. The development of this new device, which we designate as the Double S-type Pitot Tube, is related to the difficulty and the lack of precision of measurements with a standard S-type Pitot tube in flows with unknown directions like the case of fire whirls in laboratory experiments. The design construction and calibration method of the device are described. The pitch angle of the flow and the velocity coefficients of the probe are analysed experimentally in a wind tunnel calibration as well as the associated errors. The use of this sensor in a fire whirl laboratory test is shown and the results are compared with those of simple S-type Pitot tubes in the same test. The obtained results show the applicability and better performance of the novel device.

Numerical description of axisymmetric blue whirls over liquid-fuel pools

This numerical investigation is focused on determining the structures of blue whirls, recently found to occur in laboratory investigations of fire whirls when the circulation becomes sufficiently large to produce a vortex breakdown that drastically shortens the fire whirl and correspondingly reduces residence times, so that the yellow flames turn blue. The computations address axisymmetric configurations for round pools of liquid fuels flush with and at the center of a larger solid horizontal disc, at the outer edge of which vanes of adjustable angles cause the entrained air to enter with a controllable azimuthal component of velocity. The nondimensionlized conservation equations employed include realistic Lewis numbers with temperature-dependent transport coefficients and a one-step chemical-kinetic approximation that correctly reproduces laminar burning velocities. Buoyancy and radiant energy transport from the flames to the liquid surface are both taken into account, the latter being found to be essential for the blue whirl. Along with the vaporization-equilibrium and energy-conservation boundary conditions at the fuel surface, inflow boundary conditions are provided by a recently developed solution for the boundary-layer flow over the solid disc, while zero-gradient outflow conditions are applied above the whirl. Controlling nondimensional parameters, besides Reynolds, Damköhler, and Froude numbers, are a ratio of radiant to convective energy flux and a ratio of azimuthal to inward radial flow velocity in the boundary layer at the edge of the disc. The computed conditions for the onset of the blue whirl, as well as the computed structure of the whirl itself, bear close resemblance to what was found experimentally.

The structure of the blue whirl revealed.

The blue whirl is a small, stable, spinning blue flame that evolved spontaneously in recent laboratory experiments while studying turbulent, sooty fire whirls. It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a previously unknown potential way for low-emission combustion. Here, we use numerical simulations to present the flame and flow structure of the blue whirl. These simulations show that the blue whirl is composed of three different flames-a diffusion flame and premixed rich and lean flames-all of which meet in a fourth structure, a triple flame that appears as a whirling blue ring. The results also show that the flow structure emerges as the result of vortex breakdown, a fluid instability that occurs in swirling flows. These simulations are a critical step forward in understanding how to use this previously unknown form of clean combustion.

A model for the constant-density boundary layer surrounding fire whirls

Abstract

The barely implicit correction algorithm for low-Mach-Number flows II: Application to reactive flows

In this paper, the Barely Implicit Correction (BIC) algorithm is applied to the simulation of low-Mach-number, reactive flows. The BIC algorithm is a solution procedure consisting of an explicit predictor step to solve the convective portion of the Navier-Stokes equations and an implicit corrector step which removes the acoustic limit on the integration time step. We show how to include reaction processes into the BIC integration procedure. The algorithm is then applied to four test problems with successively increased difficulty. First, a series of one-dimensional flames are computed, showing that BIC can reproduce the correct flame properties. Second, simulations of two-dimensional counter-flow diffusion flames show that the integration procedure can compute the structure of a strained diffusion flame. Third, triple flames are computed and the results show that BIC can compute complex and multi-dimensional heat release structures. Finally, simulations of a three-dimensional, unsteady fire whirl show that the algorithm can compute complex heat release structures in a turbulent vortex flow field. These results indicate that the algorithm is robust, stable for flows with combustion, and can be used to study a wide variety of reactive flows.

Structure and dynamics of concentrated mesoscale vortices in planetary atmospheres

Concentrated vortices are spatially localized structures with nonzero vorticity surrounded by a potential flow. These structures include a broad class of mesoscale vortices, such as dust devils, water vortices, and fire vortices, as well as larger-scale and more intense tornados. From a rather broad class of concentrated mesoscale vortices, dust devils are selected as the simplest and most easily observable structures owing to dust particles that play the role of tracers. The aim of this review is to present the main results of studies of dust devils in the atmospheres of Earth and Mars. In the framework of ideal fluid dynamics, a recently proposed model is discussed that allows describing vortex structures that are fully localized in space. The results of numerical modeling of vortex dynamics and an analytic model of vortex generation in a convectively unstable atmosphere are briefly discussed.

Capturing the Swirling Vortex and the Impact of Ventilation Conditions on Small-Scale Fire Whirls

The fundamental flow structure and temperature distribution of small-scale fire whirls, including tangential and axial velocities, temperature variation, and air entrainment in the lower boundary layer, were successfully captured using a generic fire field model with large eddy simulation (LES) turbulence closure. Numerical predictions were validated thoroughly against two small-scale experimental measurements, where detailed temperature and velocity distributions were recorded. Good agreement between numerical and experimental results was achieved. Normalization was also performed to compare the numerical predictions with the empirical correlations by Lei et al. (2015) developed from medium-scale fire whirl measurements. The transient development stages of small-scale fire whirls and the impact of air entrainment on the stability of the fire whirls were also investigated based on the validated numerical results. The numerical validations showed the potential of the current LES fire field model in capturing the dynamic behaviour of the fire whirl plume and performing a quantitative analysis on its onset criteria and combustion dynamics in future.

Effects of wind speed and heat release rate on the vortex strength and size of fire whirls without an inner core of flame

Dust-devil-like fire whirls without an inner core of flame cause considerable damage due to the strong winds during wildfires and urban fires. The purpose of this study is to clarify the effect of the crosswind speed (U) and heat release rate (HRR) on the circulation, size, and velocity of this type of fire whirl. A U of 0.06–0.54 m/s was investigated at HHRs of 0.84 kW and 2.23 kW. The particle image velocimetry measurements clarified the velocity fields of the vortices and complicated flow structures downwind of a long narrow fire source. The absolute value of the circulation of the vortex increases when the wind speed decreases in the range of 0.28 m/s ≤ U ≤ 0.54 m/s. The radius and velocity of the vortex have the identical tendency. This change in absolute value of the circulation with the crosswind speed is determined by the vortex radius and not by the average vorticity in the vortex. The absolute value of the circulation of the vortex increases when the HRR increases. This relationship is determined by the increase in average vorticity in the vortex and not by the radius of the vortex. This relationship does not depend on the crosswind speed.

Fire-whirl Movement along Line Fires – A Separated-flow Rotating-cylinder Analogy

ABSTRACT A model is proposed for estimating how fire whirls may begin to move along line fires and the velocities at which they may propagate steadily against the direction of the prevailing wind, instead of moving downwind, as most moving fire whirls do. The analysis is based on theoretical considerations of two-dimensional, inviscid, piecewise-irrotational flow containing a vortex line and vortex sheets. Comparisons of the predictions with available experimental results seem to be qualitatively consistent, but theoretical questions are identified that motivate further considerations.

Numerical Study of the Comparison of Symmetrical and Asymmetrical Eddy-Generation Scheme on the Fire Whirl Formulation and Evolution

A numerical study of the fire whirl formation under symmetrical and asymmetrical entraining configuration is presented. This work aims to assess the effect of eddy-generation configuration on the evolution of the intriguing phenomenon coupled with both flow dynamics and combustion. The numerical framework implements large-eddy simulation, detailed chemistry to capture the sophisticated turbulence-chemistry interaction under reasonable computational cost. It also adopts liquid-based clean fuel with fixed injection rate and uniformed discretisation scheme to eliminate potential interference introduced by various aspects of uncertainties. The result reveals that the nascent fire whirl formulates significantly rapidly under the symmetrical two-slit configuration, with extended flame height and constrained vortex structure, compared with the asymmetrical baseline. However, its revolution orbit gradually diverges from domain centreline and eventually stabilises with a large radius of rotation, whereas the revolution pattern of that from the baseline case is relatively unchanged from the inception of nascent fire whirl. Through the analysis, the observed difference in evaluation pathway could be explained using the concept of circular motion with constant centripetal force. This methodology showcases its feasibility to reveal and visualise the fundamental insight and facilitate profound understanding of the flaming behaviour to benefit both research and industrial sectors.

The behaviour of wood crib fires under free burning and fire whirl conditions

The dynamics of wood crib fires were investigated under fire whirl (FW) and free burning (FB) conditions in a small-scale apparatus. For open-packed cribs, the burning rates and fire spread rates of the FB and FW cribs were almost identical. However, as the number of sticks per layer (n) increased from 3 to 7, the burning rate for FW experiments increased 1.0–1.3 times that of the FB cribs and the spread rate for FW experiments was 0.92–1.66 times that of the FB cribs for the same geometric configuration. Likewise, for open-packed cribs the average flame height of the FB and FW cribs were very similar, but as n increased the average flame height for FW tests increased from 1.0 to 1.69 times that of FB cribs for the same configuration, which was almost the same increase as that for the fire spread rates. Temperature measurements along the centreline above the crib showed distinctly different profiles for FB and FW crib fires. The ratio of the absolute temperature in the continuous flame region of a FW to that of a FB crib of the same configuration increased from 0.99 to 1.2 as n increased.

What Do You Get When You Cross a Thunderstorm with a Wildfire?

Lightning, fire vortices, and black hail are some of the frightening features of fire-fueled storms, which may become more common in the future.

Calculation of gas flow rates in concentrated fire vortices

The article presents the results of numerical simulation of the generation of free fire vortices in the laboratory without the use of special twisting devices. A. Yu. Varaksin, the corresponding member of the Russian Academy of Sciences, in his experimental studies has described the principal possibility of physical modeling of the occurrence of concentrated fire vortices. In the model of a compressible continuous medium for the complete system of Navier — Stokes equations, an initial-boundary value problem has been proposed that describes complex three-dimensional unsteady flows of a viscous compressible heat-conducting gas in ascending swirling heat flows. We has constructed approximate solutions of the complete Navier — Stokes system of equations and has determined velocity characteristics of threedimensional unsteady gas flows initiated by local heating of the underlying surface by nineteen heat sources, using explicit difference schemes and the proposed initial-boundary conditions.

What are fire whirls? | Things to Know

What are fire whirls? | Things to Know