Xieshang Yuan

Bio: Xieshang Yuan is an academic researcher from University of Science and Technology of China. The author has contributed to research in topics: Crosswind & Wind speed. The author has an hindex of 5, co-authored 8 publications receiving 74 citations.

Upslope fire spread over a pine needle fuel bed in a trench associated with eruptive fire

Abstract: Eruptive fire is a typical extreme fire behavior in wildland fuels, characterized by a sudden acceleration of fire spread over confined slopes in trench-like terrain (e.g. in a deep canyon). However, wildfire spread acceleration in trench configuration has received little study. This paper presents a systematic experimental study on the fire spread over a pine needle fuel bed in an inclined trench. The effects of aspect ratio (ratio of the trench side wall height to the trench width) and slope angle on the fire spread behaviors are investigated. It is indicated that flame attachment toward the fuel bed surface is induced by the combined effects of the slope and the air entrainment restriction due to the trench configuration. The flame attachment occurring at higher slopes with higher aspect ratios induces an acceleration of fire spread, causing the fire spread to undergo a rapid transition from a steady phase with lower rate of spread (ROS) to quick spread phase with much higher ROS. The fire spread acceleration induced by the flame attachment leads to remarkable enhancement of burning intensity, as verified by long flame depth and high mass loss rate during fire spread. The fuel consumption efficiency decreases almost linearly with increasing slope angle. Under higher slope angles (higher than 20○ in this work) the flame tilting induces hot gas flow ahead of the flame front, which enhances the convective heating. When flame attachment occurs, the spatial influence range of convective heating increases sharply. It is concluded that for eruptive fire, radiative heating and convective heating play a comparable role in fuel preheating. Experimental data suggest that when flame attachment occurs the convective heating is greatly enhanced and becomes an important mechanism of fuel preheating. This is inferred to be a major potential mechanism for eruptive fire in trench configuration.

Effect of Wind on Fire Whirl Over a Line Fire

Abstract: Fire whirls are often reported to occur in wildland and urban fires due to the effect of ambient wind. This paper presents an experimental study on the fire whirls over a line fire with cross wind, focusing on the occurrence frequency of fire whirls. The experimental observations indicated that the fire whirls induced by a line fire may spread beyond the line fire region with the effect of wind. For the effect of cross wind, it is indicated that the cross wind basically increases the occurrence frequency, while the velocity components parallel or perpendicular to the line fire have competitive effects. A scaling law is presented for the critical wind speed inducing fire whirls based on the experimental data in this work and literature. A method is proposed to estimate the magnitude of the fire whirl height under the critical wind speed.

Experimental Study on Flame Wander of Fire Whirl

Abstract: In this paper, the flame wander of fire whirl is investigated by experimental means. Small-scale fire whirls were produced by two split cylinders, and the data of vertical velocity measured by stereo particle image velocimetry are analyzed to track the flame displacement along the horizontal direction. Medium-scale fire whirls were produced by a fixed wall facility, in which a video camera was used to monitor the flame position along the horizontal direction, thereby the flame displacements are determined by image analyses. It is found that during each test the flame displacements at different heights vary synchronously, suggesting that the flame is wandering as a whole. The flame displacements at different heights involve different variation ranges. By using the flame displacement data, the appearance probabilities of flame along the horizontal direction are calculated for small- and medium-scale fire whirls. The results show that the probabilities follow the Gaussian distribution, suggesting that a fire whirl almost always wanders at the very vicinity of the pool center. Finally, it is verified that the frequency of flame wander linearly depends on the circulation of fire whirl, while the correlation formulations differ between small-scale and medium-scale fire whirls.

Flow induced by fire in a compartment

Abstract: Fifty-five full-scale steady-state experiments were conducted to study the flow induced by a simulated pool fire in a compartment under conditions characteristic of the developing fire period. The mass flow rate through the door or window opening and bounds on the fire plume entrainment rate are presented as a function of opening geometry, fire strength, and fire location. The characteristics of the measured opening flow rates are explained by a simple hydrostatic model based on temperature distribution. A good correlation between the measured results and the idealized flows, taking into account the complete temperature distribution, is demonstrated. Entrainment results for fires near walls are in reasonable agreement with results from free-standing plume models. Except for the smallest openings, fires in other locations entrain at a rate two to three times the rate predicted by these models. This phenomenon is attributed to room disturbances caused by the opening flow and is similar to the behavior of a fire plume in a cross wind.

Facade flame height and horizontal extending distance from opening of compartment fire with external sideward wind

Abstract: The present study investigated facade flame height and horizontal extending distance from opening of compartment fire with external sideward wind (parallel to facade). These two important parameters determine essentially the critical conditions that how far the flame can touch and ignite the combustible above or especially beside the opening with external sideward wind, which have not been quantified in the literature. Experiments were carried out employing a 0.4 m cubic compartment fire model with an attached facade (width: 1.2 m; height: 1.6 m) subject to external sideward wind provided by a wind tunnel. A propane porous burner was set inside the compartment as fire source. The facade flame height and horizontal extending distance were measured for 203 test conditions involving eight opening sizes (or ventilation factors: A H , A and H is area and height of the opening, respectively) at various heat release rates with external sideward wind speeds up to 3.0 m/s. It was found that the facade flame height just decreased with increase in sideward wind speed. However, the flame horizontal extending distance increased for relative large openings, but first increased then decreased for relative small openings, with increase in sideward wind speed in this range. A non-dimensional analysis was performed based on the physical mechanism of air entrainment change and tilting of the flame caused by external sideward wind. These facade flame quantities were shown to correlate well to two proposed non-dimensional numbers based on the analysis, i.e., the ratio of air entrainment caused by the sideward wind to that induced by flame buoyancy itself with no wind, and the wind Froude number representing flame tilting caused by the sideward wind. The experimental data and proposed correlations in the present study provide an essential base for quantifying facade fire characteristics under external sideward wind conditions.

Combustion dynamics of large-scale wildfires

Abstract: A fact often overlooked is that large-scale wildfires, although occurring infrequently, are responsible for the overwhelming majority of fire-related suppression costs, economic losses, and natural resources damages. Fortunately, the increasingly severe problems of large-scale wildfires worldwide have been receiving ever-growing academic attention. The high-intensity burning behaviors in wildfires stem from the significant interaction of combustion with heat transfer and atmospheric flow under complicated fuel, meteorology, and topography conditions. Therefore, mitigating measures against large-scale wildfire disasters have grown into a challenging research focus for combustion scientists. Research over the past century has resulted in incrementally enhanced insights into the mechanisms of combustion dynamics underlying the various erratic behaviors in large-scale wildfires, with theories and models of fire accelerations developed and validated. These advances are expected to improve the efficacy of large-scale wildfire predictions significantly. Nevertheless, the physical interpretation of the acceleration of large-scale wildfires is far from adequate and complete. This paper intends not to make a comprehensive review of the entire wildfire research field, but to depict an overall pattern of the essential factors that lead an initial small-scale spreading flame to a large-scale wildfire beyond control. It is outlined that the complicated transformation of fuel preheating mechanisms determines the growth of surface fire spread, while varied large-size flame fronts and unique spread modes induced in specific fire environments play an essential role in fire spread acceleration. Additionally, multiple fires burning and merging often act as crucial steps for accelerating surface fire spread, generating large-size flames, and triggering unique spread modes. These major potential factors strike the energy balance of a low-intensity wildfire and push it to a high-intensity state. Several issues regarding intensely burning behaviors in large-scale wildfires are selected for in-depth discussions, for which an overview of the progress and challenges in research is presented. It is concluded that the fundamental exploration targeted at developing application tools capable of dealing with large-scale wildfires remains at its early stages. Opportunities for innovation are abundant, yet systematic and long-term research programs are required.

Interpretation on fire interaction mechanisms of multiple pool fires

Abstract: This work intends to interpret the fire interaction mechanisms of multiple pool fires (MPF). A series of n-heptane and ethanol MPF tests with fuel pan diameters D of 0.2 m, 0.15 m, 0.05 m and different S/D (S: fire spacing) are performed. It is revealed that the air entrainment and convection of the center fire, and the leaning of the outer fire depend on the difference of the burning rates between the center and outer fires. Besides, it is indicated that the fire merging criterion depends on fuel type. For the typical fuels used in this work, compared to ethanol MPF, n-heptane MPF has a larger critical S/D attributed to its higher burning rate and flame height. The results also indicate that for MPF with no air entrainment restriction, the mass burning flux and heat feedbacks of the center fire monotonically increase with decreasing S/D. However, if air entrainment is restricted, the heat feedback rates and fractions non-monotonically depend on S/D. Furthermore, an under-ventilated burning phenomenon of MPF is found and illustrated as a result of air entrainment restriction. It is indicated that the under-ventilated burning induces the spill of excess unconsumed fuel vapors, influences the near-field radial velocity profile of MPF, and weakens the conduction heat feedback of the center fire. Finally, for the center fire of convection-dominated MPF, a modified mass burning flux correlation is derived based on the stagnant layer theory, which is verified to reasonably agree well with the experimental data.