Scaling Considerations for Fire Whirls

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Scaling Considerations for Fire Whirls Progress in Scale Modeling, an International Journal Volume 1 Issue 1 Inaugural volume of PSMIJ Article 2 2020 Scaling considerations for fire whirls Forman A. Williams University of California San Diego, [email protected] Follow this and additional works at: https://uknowledge.uky.edu/psmij Part of the Architecture Commons, and the Risk Analysis Commons Right click to open a feedback form in a new tab to let us know how this document benefits you. Recommended Citation Williams, Forman A. (2020) "Scaling considerations for fire whirls," Progress in Scale Modeling, an International Journal: Vol. 1 : Iss. 1 , Article 2. DOI: https://doi.org/10.13023/psmij.2020.02 Available at: https://uknowledge.uky.edu/psmij/vol1/iss1/2 This Review Article is brought to you for free and open access by Progress in Scale Modeling, an International Journal. Questions about the journal can be sent to [email protected] Scaling considerations for fire whirls Category Review Article Abstract This brief report, based on a presentation made at the Eighth International Symposium on Scale Modeling, held in Portland, Oregon, in September of 2017, summarizes and evaluates different methods for classifying fire whirls and their scaling laws. It is indicated that a number of relevant non- dimensional parameters are known for fire whirls, and future scale-modeling experiments could provide useful additional information and insights. Keywords Fire whirl, Vortex breakdown, Rotation Cover Page Footnote This is Review Article in volume 1, 2020 This review article is available in Progress in Scale Modeling, an International Journal: https://uknowledge.uky.edu/psmij/vol1/iss1/2 Progress in Scale Modeling, an International Journal, Vol. 1 (2020) Article 2, pp. 1–4 https://doi.org/10.13023/psmij.2020.02 Scaling considerations for fire whirls Forman A. Williams Distinguished Emeritus Professor of Engineering Physics and Combustion, Department of Mechanical and Aerospace Engineering, University of California San Diego, La Jolla, CA 92093, USA E-mail: [email protected] Received May 16, 2020, Accepted May 23, 2020 Abstract This brief report, based on a presentation made at the Eighth International Symposium on Scale Modeling, held in Portland, Oregon, in September of 2017, summarizes and evaluates different methods for classifying fire whirls and their scaling laws. It is indicated that a number of relevant non-dimensional parameters are known for fire whirls, and future scale-modeling experiments could provide useful additional information and insights. Keywords: Fire whirl; Vortex breakdown; Rotation 2. Whirls above fuels in pools or on water. Introduction 3. Tilted fire whirls. It is an honor for me to contribute to this inaugural issue of the International Journal of Progress in Scale 4. Moving fire whirls. Modeling. There are so many different fields of 5. Whirls modified by vortex breakdown. scientific and engineering investigation that can benefit from activities in scale modeling that this journal may Scaling considerations will be addressed first, and be expected to experience a lively future. My comments then they will be applied to each of these five types here, however, will be narrow and specific. They separately. pertain only to a subset of my field of combustion, a particular class of fires that involve swirling motion, General scaling considerations for fire whirls namely, fire whirls. Although scale modeling can help to improve Fire whirls have been found to occur over a wide understanding even when performed purely intuitively range of conditions, and the different types that have (for example by simply building and testing something been encountered can be catalogued in different ways. small that appears to resemble something of interest One approach [1] is to define three different classes, which is too large to be studied conveniently), that is, fire whirls of type 1, which are centered over nevertheless quantitative designs of scale models, burning material, fire whirls of type 2, which move out through the identification and application of relevant from burning material then experience extinction in a non-dimensional parameters, usually will prove to be few seconds, and fire whirls of type 3, which move out more reliable and more revealing. Many years ago I into a protected area but remain burning for a much identified 28 different types of non-dimensional longer time. This classification was developed in scale parameters for combustion, a paper in the present modeling of the large and devastating fire whirl that issue by J. Quintiere lists 22 such parameters for fires in occurred in 1923 in the Hifukusho-ato area of Tokyo general, and 13 parameters have been defined during the great Kanto earthquake, claiming numerous specifically for fire whirls [2]. Yet, even 13 is too large a fatalities, which was an example of type 3. Over the number, in that it is impossible to arrange a scale model years, however, an increasing variety of fire whirls have to maintain that many parameters identical to those of been encountered, motivating broadened classifica- the fire whirl of interest. Reasoned judgment therefore tions. is essential for the scale modeling of fire whirls; One possible approach to classification is to place fire decisions must be made concerning which parameters whirls into five different categories: are likely to be most important, and non-dimensional 1. Whirls generated by fuel distribution in wind. parameters deemed to be of lesser importance then © 2020 The Author(s). This is an open access article published under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided that the original author(s) and publication source are credited and that changes (if any) are clearly indicated. https://uknowledge.uky.edu/psmij/ – 1 – PSMIJ, Vol. 1 (2020) Article 2, pp. 1–4 F.A. Williams Fig. 1. Illustration of a fire whirl generated by an L-shaped fuel distribution in wind, a photograph of a scale model. purposely are not forced to be the same in the model as The most eye-catching aspect of fire whirls is their they are at full scale. This is the heart of the art of scale large visible flame lengths. This length will be denoted modeling. by H, the flame height for vertical whirls (although the Burning rates clearly are important in fire whirls. same notation will be retained for tilted fire whirls), its They generally are determined by the rates of heat non-dimensional scaling parameter being H/L. For fires transfer from the flames to the condensed fuel. With L in general H/L scales with a fuel Froude number, = a characteristic length scale and g the acceleration of /( ) / , being approximately proportional to a gravity, D denoting a gas-phase thermal diffusivity near power of1 2that scaling parameter ranging from 2/3 at the fuel surface and the kinematic viscosity there, a large L to 2/5 at small L under turbulent conditions. relevant scaling parameter for a heat-transfer rate This scaling, however is violated by the swirl in fire controlled by natural convection is the Rayleigh whirls. In addition, for a round horizontal fuel source of number, = ( )/( ) , where h is a shorthand radius R, under laminar-flow conditions without swirl, notation for an appropriate3 fractional driving enthalpy taking L = R flame-height correlations for gaseous and difference for theℎ heat-transfer process; the heat- liquid fuels often are based on a Peclet number, defined transfer rate, expressed by a Nusselt number, increases as Pe = RVo/D, according to H/R = Pe/(4f), where f with Ra, the variation depending on geometry and size denotes the stoichiometric fuel-to-air mass ratio (with turbulent flow tending to dominate at large sizes). (typically taking non a small value in this formula). In terms of the gas density and the mass burning Although this would apply to laminar momentum- rate per unit base area of the fuel , an effective controlled gaseous jets, it also has been derived for average normal component of the gas ′′velocity at the laminar round buoyancy-controlled fuel jets, both with surface of the burning fuel can be defined as = and without swirl, and it correlates data for small ( / ). This is determined by the average rate of fuel liquid-pool fires, but there are significant departures gasif′′ ication per unit area, which (through an energy from this scaling at high swirl. In general, flame-height balance ) is the ratio of a heat transfer rate per unit area scaling for fire whirls needs much further investigation, to the energy per unit mass needed to convert the clarifying differences between laminar and turbulent condensed fuel to gas, and although this burning rate conditions without swirl and explaining combined increases with Ra, the specific relationships vary effects of buoyancy and swirl, which are not yet well greatly, depending on what fuel is burning, so that Vo understood. scaling with Ra is strongly fuel-dependent. Moreover, A representative component of velocity along the the swirl in the flow generally exerts a large influence, axis of a fire whirl may be considered to be the velocity as does the fuel orientation, and there are fire whirls in V at an axial distance H from the fuel source. For vertical which radiant energy transfer is dominant, further buoyancy-controlled fires, the order of magnitude of V complicating the scaling problem. Burning-rate scaling is (Hg)1/2, but it may be larger than that at high swirl. thus can become the most challenging scaling aspect of Ambient wind can influence fire whirls, and a fire whirls, to such an extent that, for some purposes, it convenient non-dimensional measure of an average can be useful to decouple the problem by supplying ambient wind velocity U is reasonably defined as U/V.
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