CFD Study of Generation Process and Stability of a Fire Whirl in Large-Scale Fires

2019 ◽  
Author(s):  
Koyu Satoh ◽  
Domingos Viegas ◽  
Claudia Pinto ◽  
Ran Tu
Keyword(s):  
Forest Fires ◽  
Large Scale ◽  
Small Scale ◽  
Channel Height ◽  
Generation Process ◽  
Strong Wind ◽  
Cfd Simulations ◽  
Swirling Flame ◽  
Fire Whirl ◽  
The Stability

Abstract Large-scale urban and forest fires, especially earthquake-induced fires may produce huge fire whirls and cause serious damage, due to the involved tornado-like strong wind, together with radiation and swirling flame. If fire whirls are generated, the danger to firefighters increases extremely. Many small-scale experiments and CFD simulations on fire whirls have already been conducted and also our previous numerical studies examined the generation of a large fire whirl in an oil tank. However, details of large-scale fire whirls have not been clarified yet. In this study, developing the previous works, additional CFD simulations are conducted to examine the generation process and particularly the stability of fire whirls. Three schemes to generate fire whirls are employed, using the 15 × 15 PMMA fuel array in windy conditions and n-heptane burning in a steel pan placed centrally on the floor in a tall channel with staggered four corner gaps, also using a channel with a single corner gap. The numerical results showed that the relationship between the fire area and the wind blowing area is important on the fire whirl generation in the PMMA scheme and n-heptane fire burning scheme in a channel. In addition to the channel gap size to produce a maximum fire whirl, the effects of channel height and horizontal channel area upon the fire whirl are examined. The wall temperatures of the channel are important to keep the swirling motion stably, particularly the wall temperature at about 300°C can stabilize the fire whirl in a channel. Also multiple small fires placed surrounding the central swirling fire can increase the stability of the fire whirl, although too strong multiple fires may destroy the stability. These phenomena may be related to the real fire whirl generation in the natural environment.

scholarly journals Fractal properties of forest fires in Amazonia as a basis for modelling pan-tropical burnt area

2014 ◽  
Vol 11 (6) ◽  
pp. 1449-1459 ◽  
Author(s):  
I. N. Fletcher ◽  
L. E. O. C. Aragão ◽  
A. Lima ◽  
Y. Shimabukuro ◽  
P. Friedlingstein
Keyword(s):  
Forest Fires ◽  
Large Scale ◽  
Fire Spread ◽  
Small Scale ◽  
Burnt Area ◽  
Active Fire ◽  
Fractal Properties ◽  
Dynamic Global Vegetation Models ◽  
Vegetation Models ◽  
Global Vegetation

Abstract. Current methods for modelling burnt area in dynamic global vegetation models (DGVMs) involve complex fire spread calculations, which rely on many inputs, including fuel characteristics, wind speed and countless parameters. They are therefore susceptible to large uncertainties through error propagation, but undeniably useful for modelling specific, small-scale burns. Using observed fractal distributions of fire scars in Brazilian Amazonia in 2005, we propose an alternative burnt area model for tropical forests, with fire counts as sole input and few parameters. This model is intended for predicting large-scale burnt area rather than looking at individual fire events. A simple parameterization of a tapered fractal distribution is calibrated at multiple spatial resolutions using a satellite-derived burnt area map. The model is capable of accurately reproducing the total area burnt (16 387 km2) and its spatial distribution. When tested pan-tropically using the MODIS MCD14ML active fire product, the model accurately predicts temporal and spatial fire trends, but the magnitude of the differences between these estimates and the GFED3.1 burnt area products varies per continent.

Fully resolved numerical simulation of particle-laden turbulent flow in a horizontal channel at a low Reynolds number

2012 ◽  
Vol 693 ◽  
pp. 319-344 ◽  
Author(s):  
Xueming Shao ◽  
Tenghu Wu ◽  
Zhaosheng Yu
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Large Scale ◽  
Horizontal Channel ◽  
Small Scale ◽  
Channel Height ◽  
Wall Region ◽  
Sediment Layer ◽  
Particle Settling ◽  
Volume Fractions

AbstractA fictitious domain method is used to perform fully resolved numerical simulations of particle-laden turbulent flow in a horizontal channel. The effects of large particles of diameter 0.05 and 0.1 times the channel height on the turbulence statistics and structures are investigated for different settling coefficients and volume fractions (0.79 %–7.08 %) for the channel Reynolds number being 5000. The results indicate the following. (a) When the particle sedimentation effect is negligible (i.e. neutrally buoyant), the presence of particles decreases the maximum r.m.s. of streamwise velocity fluctuation near the wall by weakening the intensity of the large-scale streamwise vortices, while increasing the r.m.s. of the streamwise fluctuating velocity in the region very close to the wall and in the centre region. On the other hand, the particles increase the r.m.s. of transverse and spanwise fluctuating velocities in the near-wall region by inducing the small-scale vortices. (b) When the particle settling effect is so substantial that most particles settle onto the bottom wall and form a particle sediment layer (SL), the SL plays the role of a rough wall and parts of the vortex structures shedding from the SL ascend into the core region and substantially increase the turbulence intensity there. (c) When the particle settling effect is moderate, the effects of particles on the turbulence are a combination of the former two situations, and the Shields number is a good parameter for measuring the particle settling effects (i.e. the particle concentration distribution in the transverse direction). The average velocities of the particle are smaller in the lower half-channel and larger in the upper half-channel compared to the local fluid velocities in the presence of gravity effects. The effects of the smaller particles on the turbulence are found to be stronger at the same particle volume fractions.

Generation mechanism of a hierarchy of vortices in a turbulent boundary layer

2019 ◽  
Vol 865 ◽  
pp. 1085-1109 ◽  
Author(s):  
Yutaro Motoori ◽  
Susumu Goto
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Energy Spectrum ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Generation Mechanism ◽  
Small Scale ◽  
Generation Process ◽  
Mean Flow ◽  
The Mean

To understand the generation mechanism of a hierarchy of multiscale vortices in a high-Reynolds-number turbulent boundary layer, we conduct direct numerical simulations and educe the hierarchy of vortices by applying a coarse-graining method to the simulated turbulent velocity field. When the Reynolds number is high enough for the premultiplied energy spectrum of the streamwise velocity component to show the second peak and for the energy spectrum to obey the$-5/3$power law, small-scale vortices, that is, vortices sufficiently smaller than the height from the wall, in the log layer are generated predominantly by the stretching in strain-rate fields at larger scales rather than by the mean-flow stretching. In such a case, the twice-larger scale contributes most to the stretching of smaller-scale vortices. This generation mechanism of small-scale vortices is similar to the one observed in fully developed turbulence in a periodic cube and consistent with the picture of the energy cascade. On the other hand, large-scale vortices, that is, vortices as large as the height, are stretched and amplified directly by the mean flow. We show quantitative evidence of these scale-dependent generation mechanisms of vortices on the basis of numerical analyses of the scale-dependent enstrophy production rate. We also demonstrate concrete examples of the generation process of the hierarchy of multiscale vortices.

Preliminary Study of Fire Spread in Cities and Forests, Using PMMA Specimen as a Fuel in CFD Simulations

2009 ◽  
Author(s):  
Koyu Satoh ◽  
Naian Liu ◽  
Qiong Liu ◽  
K. T. Yang
Keyword(s):  
Laboratory Experiment ◽  
Forest Fires ◽  
Three Dimensional ◽  
Fire Spread ◽  
Weather Data ◽  
Strong Wind ◽  
Cfd Simulations ◽  
Worst Case ◽  
Fire Propagation ◽  
Fire Spreading

It is important to examine the behavior of forest fires and city fires to mitigate the property damages and victims by fires. There have been many previous studies on forest fires where the fire spreading patterns were investigated, utilizing artificial satellite pictures of forest fires, together with the use of corresponding weather data and GIS data. On the other hand, large area city fires are very scarce in the world, particularly in modern cities where high-rise concrete buildings are constructed with sufficient open spaces. Thus, the examples of city fires to be referred are few and detailed investigations of city fires are limited. However, there have still been existing old cities where traditional houses built with flammable material such as wood, maybe historically important, only separated with very small open spacing. Fires may freely spread in those cities, once a big earthquake happens there and then water supply for the fire brigade is damaged in the worst case along with the effect of strong wind. There are some fundamental differences between the forest fires and city fires, as the fuel may distribute either continuously or discretely. For instance, in forest fires, the dead fallen leaves, dry grasses and trees are distributed continuously on the ground, while the wooden houses in cities are discretely distributed with some separation of open spacing, such as roads and gardens. Therefore, the wooden houses neighboring the burning houses with some separation are heated by radiation and flames to elevate the temperatures, thus causing the ignition, and finally reaching a large city fire. The authors have studied the forest fire spread and are planning to start a laboratory experiment of city fire spreading. In the preliminary investigation, a numerical study is made to correlate with the laboratory experiment of city fire propagation, utilizing the three-dimensional CFD simulations. Based on the detailed experimental analysis, the authors are attempting to modify the three dimensional CFD code to predict the forest fires and city fires more precisely, taking into account the thermal heating and ignition processes. In this study, some fundamental information on the city fire propagation has been obtained, particularly to know the safe open spacing distances between the houses in the cities and also the wind speed.

Development of Efficient Small Scale Axial Turbine for Solar Driven Organic Rankine Cycle

2016 ◽  
Author(s):  
Ayad Al Jubori ◽  
Raya K. Al-Dadah ◽  
Saad Mahmoud ◽  
Khalil M. Khalil ◽  
A. S. Bahr Ennil
Keyword(s):  
Large Scale ◽  
Renewable Energy Sources ◽  
Cycle Performance ◽  
Working Fluid ◽  
Small Scale ◽  
Cfd Simulations ◽  
Axial Turbine ◽  
Working Fluids ◽  
Wide Range ◽  
Inlet Conditions

Recently, the increase in fossil fuel consumption and associated adverse impact on the environment led to significant interest in renewable energy sources like solar. This paper presents a new methodology that integrates the ORC cycle analysis with modeling of an efficient small scale subsonic axial turbine at low temperature heat sources using wide range of organic working fluids like R123, R134a, R141b, R152a, R245fa, R290 and isobutene. The work involves detailed turbine analysis including 1D mean line approach, extensive 3D CFD simulations and ORC cycle analysis at inlet total pressure ranging from 2–5 bar corresponding to temperature range from 275K–365K to achieve the best turbine and cycle performance. This work provides a more reliable data base for small scale organic working fluids instead of using the map of large scale gas turbine. The numerical simulation was performed using 3D RANS with SST turbulence model in ANSYS-CFX. Using iterative CFD simulations with various working fluids with subsonic inlet conditions, Mach number ranging from 0.6–0.65, results showed that using working fluid R123 for a turbine with mean diameter of 70mm, the maximum isentropic efficiency was 82% and power output 5.66 kW leading to cycle efficiency of 9.5%.

scholarly journals Rock Mass Characterization by UAV and Close-Range Photogrammetry: A Multiscale Approach Applied along the Vallone dell’Elva Road (Italy)

Geosciences ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 436
Author(s):  
Maria Migliazza ◽  
Maria Teresa Carriero ◽  
Andrea Lingua ◽  
Emanuele Pontoglio ◽  
Claudio Scavia
Keyword(s):  
Rock Mass ◽  
Large Scale ◽  
Small Scale ◽  
Stability Conditions ◽  
Multiscale Approach ◽  
Rock Mass Characterization ◽  
Geometrical Data ◽  
Close Range ◽  
The Stability ◽  
Kinematic Mechanisms

Geostructural rock mass surveys and the collection of data related to discontinues provide the basis for the characterization of rock masses and the study of their stability conditions. This paper describes a multiscale approach that was carried out using both non-contact techniques and traditional support techniques to survey certain geometrical features of discontinuities, such as their orientation, spacing, and useful persistence. This information is useful in identifying the possible kinematics and stability conditions. These techniques are extremely useful in the case study of the Elva valley road (Northern Italy), in which instability phenomena are spread across 9 km in an overhanging rocky mass. A multiscale approach was applied, obtaining digital surface models (DSMs) at three different scales: large-scale DSM of the entire road, a medium-scale DSM to assess portions of the slope, and a small-scale DSM to assess single discontinuities. The georeferenced point cloud and consequent DSMs of the slopes were obtained using an unmanned aerial vehicle (UAV) and terrestrial photogrammetric technique, allowing topographic and rapid traditional geostructural surveys. This technique allowed us to take measurements along the entire road, obtaining geometrical data for the discontinuities that are statistically representative of the rock mass and useful in defining the possible kinematic mechanisms and volumes of potentially detachable blocks. The main purpose of this study was to analyse how the geostructural features of a rock mass can affect the stability slope conditions at different scales in order to identify road sectors susceptible to different potential failure mechanisms using only kinematic analysis.

Terrain-Forced Flows

2000 ◽  
Author(s):  
C. David Whiteman
Keyword(s):  
Large Scale ◽  
Air Flow ◽  
Windward Side ◽  
Leeward Side ◽  
Small Scale ◽  
Mountainous Terrain ◽  
Jet Streams ◽  
Thermally Driven ◽  
Degree Of Stability ◽  
The Stability

Winds associated with mountainous terrain are generally of two types. Terrain-forced flows are produced when large-scale winds are modified or channeled by the underlying complex terrain. Diurnal mountain winds are produced by temperature contrasts that form within the mountains or between the mountains and the surrounding plains and are therefore also called thermally driven circulations. Terrain-forced flows and diurnal mountain winds are nearly always combined to some extent. Both can occur in conjunction with small-scale winds, such as thunderstorm inflows and outflows, or with large-scale winds that are not influenced by the underlying mountainous terrain. Terrain forcing can cause an air flow approaching a mountain barrier to be carried over or around the barrier, to be forced through gaps in the barrier, or to be blocked by the barrier. Three factors determine the behavior of an approaching flow in response to a mountain barrier: •the stability of the air approaching the mountains, •the speed of the air flow approaching the mountains, and •the topographic characteristics of the underlying terrain. Unstable or neutrally stable air (section 4.3) is easily carried over a mountain barrier. The behavior of stable air approaching a mountain barrier depends on the degree of stability, the speed of the approaching flow, and the terrain characteristics. The more stable the air, the more resistant it is to lifting and the greater the likelihood that it will flow around, be forced through gaps in the barrier, or be blocked by the barrier. A layer of stable air can split, with air above the dividing streamline height flowing over the mountain barrier and air below the dividing streamline height splitting upwind of the mountains, flowing around the barrier (figure 10.1), and reconverging on the leeward side (section 10.3.2). A very stable approaching flow may be blocked on the windward side of the barrier (section 10.5.1). Moderate to strong cross-barrier winds are necessary to produce terrain-forced flows, which therefore occur most frequently in areas of cyclogenesis (section 5.1) or where low pressure systems (figure 1.3) or jet streams (section 5.2.1.3) are commonly found. Whereas unstable and neutral flows are easily lifted over a mountain barrier, even by moderate winds, strong cross-barrier winds are needed to carry stable air over a mountain barrier.

Stability of a Compressible Laminar Wall-Jet With Heat Transfer

1996 ◽  
Vol 118 (4) ◽  
pp. 824-828 ◽  
Author(s):  
O. Likhachev ◽  
A. Tumin
Keyword(s):  
Heat Transfer ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Small Scale ◽  
Wall Jet ◽  
Boundary Layer Equations ◽  
Incompressible Boundary ◽  
Flow Approximation ◽  
Ambient Gas ◽  
The Stability

The flow of a plane, laminar, subsonic perfect gas wall jet with heat transfer through the wall was investigated theoretically. For the case under consideration the entire surface was maintained at a constant temperature which differed from the temperature of the ambient gas. The velocity and temperature distribution across the flow were calculated for a variety of temperature differences between the ambient gas and the surface. The boundary layer equations representing these flows were solved by using the Illingworth-Stewartson transformation, thus extending the classical Glauert’s solution to a thermally non-uniform flow. The effects of heat transfer on the linear stability characteristics of the wall jet were assessed by making the local parallel flow approximation. Two kinds of unstable eigenmodes coexisting at moderate Reynolds numbers are significantly affected by the heat transfer. The influence of cooling or heating on the stability of the flow was expected in view of the experience accumulated in incompressible boundary layers, i.e. heating destabilizes and cooling stabilizes the flows. Cooling of the wall affects the small scale disturbances more profoundly, contrary to the results obtained for the large scale disturbances.

The growth of meteorological disturbances

1949 ◽  
Vol 45 (1) ◽  
pp. 92-98 ◽  
Author(s):  
H. Bondi
Keyword(s):  
Heat Conduction ◽  
Temperature Gradient ◽  
Lower Limit ◽  
Time Scale ◽  
Large Scale ◽  
Ideal Gas ◽  
Small Scale ◽  
Sharp Distinction ◽  
The Subject ◽  
The Stability

1. Introduction. A considerable amount of attention has been paid to the problem of determining the conditions which decide whether a liquid heated from below is stable or unstable. The motion consequent upon the disturbance of an unstable ideal gas does not, however, seem to have been treated so far, and this problem forms the subject of the present paper. Heat conduction and viscosity are at first neglected, and we are therefore dealing with the small motions of a gas slightly disturbed from a position of equilibrium under the influence of gravity. The condition for the stability of such a gas is well known, namely, the temperature gradient must be less than the adiabatic gradient. Furthermore, it is known that there is a sharp distinction between slow large-scale (meteorological) and rapidly varying small-scale (acoustical) phenomena. The present paper confirms these points and derives the time scale of meteorological phenomena. Heat conduction and viscosity are then shown to set a lower limit to the dimensions of such disturbances, while the effect of the earth's rotation is shown to be negligible.

scholarly journals A Study on the Power Reserve of Distributed Generators Based on Power Sensitivity Analysis in a Large-Scale Power System

Electronics ◽  
2021 ◽  
Vol 10 (7) ◽  
pp. 769
Author(s):  
Dongmin Kim ◽  
Jung-Wook Park ◽  
Soo Hyoung Lee
Keyword(s):  
Power System ◽  
Nuclear Power ◽  
Large Scale ◽  
Renewable Energy Sources ◽  
Droop Control ◽  
Generation Process ◽  
Power Generators ◽  
Distributed Generators ◽  
Proper Power ◽  
The Stability

Converter-based generators (CBGs) that use renewable energy sources (RESs) are replacing traditional aging coal and nuclear power generators. Increasing the penetration of CBGs into the entire power generation process reduces both the inertia constant of the power system and the total amount of power reserves. Additionally, RESs are very intermittent and it is difficult to predict changes in them. These problems, due to CBGs using RESs, pose new challenges to net–load balancing. As a solution, this paper proposes a virtual multi-slack (VMS) droop control that secures the stability and efficiency of system operation by controlling the output of CBGs distributed in various regions. The VMS droop control makes it possible to increase the inertia constant of the power system and to respond quickly and appropriately to load changes through the proposed VMS droop control based on power sensitivity. It is also proposed that the process selects proper power reserves of CBGs for stable VMS droop control. To verify the effectiveness of the proposed VMS droop control and the proper power reserve selection method for CBGs, several case studies were performed using a real Korean power system.

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