— This paper presents the analysis and generalization of the results of studies of turbulence in flame and in the vicinity of a combustion source during model steppe and crown fires performed at the Basic Experimental Complex of Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences, in the period from 2019 to 2022. The spectra of air temperature changes and the scales of induced atmospheric turbulence in the vicinity of the front of a model fire are obtained. The ranges of air temperature pulsation frequency corresponding to the inertial and dissipative sections of the energy spectrum are found for a steppe fire; dissipative processes start running at an altitude of 10 m at wavenumbers with log k > 1.58 and the corresponding pulsation frequency f > 3 Hz; no dissipative processes are observed at an altitude of 3 m. During a model crown fire, turbulent processes in the atmosphere correspond to the inertial part of the energy spectrum at an altitude of 10 m; dissipative processes hardly manifest themselves.

In this study, a theoretical formulation of the ignition and combustion of the wood layer by burning and smoldering firebrands has been considered. The effect of the firebrands’ length, distances between firebrands and their geometrical parameters on the heat exchange with the wood layer and the ignition process were analyzed. With a decrease in firebrand size, ignition of wood is possible with a decrease in the distance between the firebrands. With an increase in firebrand size at the same distance between them, the ignition regime becomes possible albeit with a longer delay time Δt. With a decrease in the distance between the firebrands, the ignition of wood is possible with an increase in Δt. As a result of mathematical modeling of the process, the following processes are noted: the heat stored in firebrands of small sizes is insufficient to initiate the ignition process; the temperature in the wood layer, due to conductive heat exchange, slightly increases at first, before beginning to decrease as a result of heat exchange with the surrounding air and the wood layer; intensive heat exchange with the environment of small size firebrands leads to the end of firebrand smoldering and its cooling; and, if the firebrand size reaches a critical value, then the pyrolysis process begins in the area adjacent to it.

Представлен анализ и обобщение результатов исследований турбулентности в пламени и в окрестности очага горения при модельных степных и верховых пожарах в экспериментах, проведенных в период с 2019 по 2022 г. на Базовом экспериментальном комплексе Института оптики атмосферы СО РАН. Получены спектры изменения температуры воздуха и масштабы индуцированной атмосферной турбулентности в окрестности фронта модельного пожара. Установлены диапазоны частот пульсации температуры воздуха, соответствующие инерционному и диссипативному участкам энергетического спектра, для модельного степного пожара. Показано, что диссипативные процессы начинают формироваться на высоте 10 м при волновых числах k таких, что lgk > 1,58 и соответствующей частоте пульсаций ƒ > 3 Гц; на высоте 3 м диссипативные процессы не наблюдаются. При модельном верховом пожаре на высоте 10 м турбулентные процессы в атмосфере соответствуют инерционному участку энергетического спектра, а диссипативные процессы практически не проявляются. This paper analyzes and generalizes the results of studies of turbulence in flame and in the vicinity of the combustion source during model steppe and crown fires in the period from 2019 to 2022 at the Base Experimental Complex of Institute of Atmospheric Optics SB RAS. The spectra of air temperature changes and the scales of induced atmospheric turbulence in the vicinity of the front of a model fire are obtained. For a steppe fire, the air temperature pulsation frequency ranges corresponding to the inertial and dissipative sections of the energy spectrum are found; dissipative processes begin to form at an altitude of 10 m at wave numbers with lgk > 1.58 and the corresponding pulsation frequency ƒ > 3 Hz; at an altitude of 3 m, dissipative processes are not observed. During a model crown fire, turbulent processes in the atmosphere correspond to the inertial part of the energy spectrum at an altitude of 10 m, and dissipative processes practically do not manifest themselves.

The study investigates the transition process from a ground fire to a crown fire. The studies were carried out in seminatural conditions on the territory of the Base Experimental Complex (BEC) of the IAO SB RAS. Previously, reconstruction of the forest canopy was carried out. The emergence of a crown fire occurred due to the transition of a ground fire to the bottom layers and further to the crowns of trees. Non-contact diagnostic methods (IR thermography) were predominantly used to measure combustion front parameters.

The processes of combustion and flame propagation in various technological devices and during wildfires are carried out, specifically, under turbulence conditions. Turbulent combustion is a non-stationary process of turbulent mixing of combustion products with a fresh mixture and its ignition due to the temperature increase. This article compares estimates of the turbulence scale in a flame during the combustion of forest fuels in laboratory conditions and in a test site.

В результате математического моделирования проанализировано поведение образца древесины в условиях точечного источника теплового воздействия. Моделировался случай падения частиц на горизонтальную поверхность образца древесины. С увеличением числа частиц, режим зажигания становится возможным при большем расстоянии между ними. С уменьшением размеров частиц зажигание древесины возможно только при меньшем расстояния между ними. При этом зажигание небольшими частицами возможно только при увеличении их количества.

Процессы горения и распространения пламени в различных технологических устройствах и при природных пожарах, реализуются, как правило, в условиях турбулентности. Турбулентное горение представляет собой нестационарный процесс турбулентного смешения продуктов сгорания со свежей смесью и воспламенение ее вследствие повышения температуры. В данной работе приводится сравнение оценок масштабов турбулентности в пламени, при горении растительных горючих материалов в лабораторных условиях и в условиях полигона.

The probability of structural ignition is dependent both on physical properties of materials and the fire exposure conditions. In this study, the effect of firebrand characteristics (i.e., firebrand size, number of firebrands) on wood ignition behavior was considered. Mathematical modeling and laboratory experiment were conducted to better understand the conditions of wood ignition by a single or group of firebrands with different geometry. This model considers the heat exchange between the firebrands, wood layer and the gas phase, moisture evaporation in the firebrands and the diffusion gases of water vapor in the pyrolysis zone. In order to test and verify the model, a series of experiments to determine probability and conditions for ignition of wood-based materials (plywood, oriented strand board, chipboard) caused by wildland firebrands (pine twigs with a diameter of 6–8 mm and a length of 40 ± 2 mm) were conducted. The experiments investigated the firebrand impact on the wood layer under different parameters, such as firebrand size and quantity, wind speed, and type of wood. The results of experiments showed that the increase in wind speed leads to the increase in probability of wood ignition. Based on the received results, it can be concluded that the ignition curve of wood samples by firebrands is nonlinear and depends on the wind speed and firebrand size as well as their quantity. At the same time, there is no ignition of wood samples in the range of wind speed of 0–1 m/s. The ignition of wood is possible with a decrease in the distance between the firebrands with a decrease in the firebrand length. This result agrees more closely with the model.

В докладе представлены некоторые результаты многолетних экспериментальных исследований влияния модельных степных пожаров на характеристики атмосферы. Установлено локальное изменение метеопараметров и параметры наведенной атмосферной турбулентности, определены характерные газообразные продукты пиролиза и горения, которые попадают в атмосферу в результате природных пожаров. Полученные результаты позволят осуществлять оценку влияния массовых пожаров на экологию регионов, глобальные атмосферные и климатические процессы.

The article presents the results of a series of mesoscale experiments to study wildfires and their impact on the atmosphere. A change in meteorological parameters, a significant increase in fluctuations of the refractive index, speed of sound and temperature in the vicinity of the experiment, which are a reflection of the occurrence of turbulent processes in the atmosphere, were established, experimental data were obtained on changes in the concentration of methane and carbon dioxide as a result of a fire, and data were obtained on the mechanism of occurrence of crown fire.

A study of the conditions of ignition of a layer of wood as a result of exposure to burning particles formed during a fire has been carried out. Research results show that the ignition process is influenced by the wind speed, the size and number of particles falling on the underlying surface.

The paper presents experimental investigation results of turbulent vortices scales in diffusion flames. Comparison of obtained data on the basis of digital tracer visualization and thermography was carried out. A good correlation is obtained between the sizes of large vortex structures in the velocity field, recorded using the PIV method, and the size of temperature inhomogeneities, recorded by flame thermography.

The physicochemical processes associated with combustion and flame propagation in various technological devices and in natural fires are realized under conditions of turbulence. Traditionally, in experimental studies of combustion processes, thermocouples are used to determine the temperature fields in a flame. The methods of thermography allow us to abandon the use of thermocouples and at the same time obtain information on the temperature distribution with good spatial and temporal resolution. In this paper we present the results of an investigation of the influence of sound perturbations on the combustion process. As combustible materials liquid hydrocarbon fuels (gasoline, kerosene, diesel fuel) and vegetable combustible materials (a mixture of field combustible materials, pine needles, pine wood, cedar wood) were used. To measure the flow velocity in a turbulent flame, the contactless optical method of flow diagnostics was used-particle image velocimetry (PTV) method. The intensity of the IR radiation of the flame and the temperature distribution in the flame were registered with the JADE J530SB thermal imager. As a result, a good agreement was reached on the estimation of the size of the vortex structures obtained using PTV and the dimensions of the temperature inhomogeneities recorded by means of IR thermography in a flame.

The paper describes experimental results of determining scales of turbulent eddies in diffusion flames. The results are obtained by analyzing a set of 2D cross-sections cut from 3D semi-transparent flames. The comparison of data obtained by using the particle image velocimetry (PIV) method and infrared (IR) thermography is presented to demonstrate similarity between hydro- and thermodynamic parameters.