A phenomenological model of near-field fire entrainment

A phenomenological model of air entrainment into combusting regions of axisymmetric fire plumes is developed and compared with some existing experimental entrainment data. The model considers engulfment of ambient air into a fire plume due to the action of the toroidal vortices that are formed periodically close to the source of an axisymmetric fire. It is shown that the diffusive entrainment rates of air towards a wrinkled flame sheet representing the turbulent diffusion flame severely underpredicts the entrainment rates by at least two orders of magnitude, suggesting that the turbulent large-scale engulfment is the primary mechanism of entrainment. By describing the entrainment process as periodic engulfment of ambient air around the toroidal vortex rings and the frequency of formation and passage of these vortex rings in combusting regions of fire plumes, it is shown that the modeled entrainment process gives good agreement with the experimentally determined fire plume entrainment or plume mass fluxes. This model accounts for the physical processes of unsteady fire plume dynamics, including the effects of pulsations for the first time. The model predicts the plume mass flux to grow linearly with height above the fire source and to be independent of the fire heat release in the visible flame region. The predicted scaling with respect to the source diameter appears to be in the range suggested by the experimental data. Overall, this new model provides a better foundation for the scaling of entrainment rates in the near-field of fire plumes. The methodology can be extended to predict entrainment rates in other periodic unsteady vortex dominated flows, such as pulsating buoyant non-reacting plumes.     

Compartment fire phenomena under limited ventilation

Fire behavior of heptane pool fires were investigated in a small-scale 40 cm cubic compartment with wall vents at the ceiling (top vent) and the floor (bottom vent). The measurements included pressure, mass loss, temperature, heat flux, and gas mole fraction. Flame oscillations, ghosting, and burning at the air inlet were seen. The regime of limited ventilation was examined to study the effect of extinction and the influence of oxygen. A theory based on a critical flame temperature showed that extinction depends on heating as well as oxygen concentration. A complete uniform property model was developed and its solution agrees qualitatively with the measurements.     

Smoke measurements in large-and small-scale fire testing — Part II

A new extinction photometer design has been developed that will provide the stability and accuracy necessary for taking smoke measurements in fire testing.     

Study on air entrainment coefficient of one-dimensional horizontal movement stage of tunnel fire smoke in top central exhaust

When tunnel fire smoke is exhausted using a top central exhausting system, the air entrainment at the smoke-air interface has a significant effect on the overall mass flux. Using theoretical analysis, the air entrainment coefficient in one-dimensional horizontal smoke movement stage in central exhausting was obtained. Both heat release rate and smoke extraction rate have effects on the horizontal entrainment coefficient. The experimental results show that the entrainment coefficient in central exhausting is between 0.01 and 0.03 and the horizontal entrainment coefficient increases to 0.0181 with increasing heat release rate and becomes relatively stable afterwards. The entrainment coefficient increases with the smoke extraction rate increasing as the smoke extraction process will enhance the smoke momentum in the tunnel.     

Smoke measurements in large- and small-scale fire testing — Part I

A new extinction photometer design has been developed that will provide the stability and accuracy necessary for taking smoke measurements in fire testing.     

A comparison of gas velocity measurements in a full-scale enclosure fire

Gas velocity measurements were conducted in the doorway of an enclosure containing a natural gas fire. Two independent measurement techniques, Stereoscopic Particle Image Velocimetry (SPIV) and bi-directional impact-pressure probes, were utilized for comparison – the first such comparison for a fire-induced flow in a full-scale structural fire. Gas velocities inferred from the bi-directional probe measurements were consistently greater than SPIV measurements in a region of the flow between the floor and the flow interface. The comparison revealed that a measurement bias exists in the bi-directional probe technique. Estimates of the relative magnitude of the bias were inferred from the results.     

Experimental study of the effects of a thermal plume entrainment mode on the flow structure: Application to fire

The aim of this work is to experimentally simulate two heat sources of fire, one placed at ground level, the other at a height above the ground, in order to determine the effect of source air entrainment on the resulting flow structure. The development of a free thermal plume was examined by generating a plume produced from an electrically heated disk at a constant temperature. We first studied the behavior of a thermal plume induced by a disk embedded in a horizontal plate placed at the level of the ground. This configuration ensured a regular lateral entrainment of air to the plume. The analysis of the average fields as well as the axial evolution of velocity and temperature showed an important widening of the profiles that encouraged a faster spread of the plume. We then determined the structure of the thermal plume generated by the same source placed at a height above the ground, ensuring simultaneous vertical and lateral entrainment of air into the resulting plume.     

Predicting the structural response of a compartment fire using full-field heat transfer measurements

Inverse heat transfer analysis (IHT) was used to measure the full-field heat fluxes on a small scale (0.9 m×0.9 m×0.9 m) stainless steel SS304 compartment exposed to a 100 kW diffusion flame. The measured heat fluxes were then used in a thermo-mechanical finite element model in Abaqus to predict the response of an aluminum 6061-T6 compartment to the same exposure. Coupled measurements of deflection and temperature using Thermographic Digital Image Correlation (TDIC) were obtained of an aluminum compartment tested until collapse. Two convective heat transfer coefficients, h =35 W/m²-K and h =10 W/m²-K were examined for the thermal model using the experimentally measured heat fluxes. Predictions of the thermal and structural response of the same compartment were generated by coupling Fire Dynamics Simulator (FDS) and Abaqus using the two values for h, h =35 W/m²-K and h from convection correlations. Predictions of deflection and temperature using heat fluxes from IHT and FDS with h=35 W/m²-K agreed with experimental measurements along the back wall. The temperature predictions from the IHT-Abaqus model were independent of h, whereas the temperature predictions from the FDS-Abaqus model were dependent on h.     

Gas temperature measurements using fibre Bragg grating during fire experiments in a tunnel

Gas temperature measurements with thermocouples are affected by radiation. This effect means that the temperature measured by a thermocouple can be far from the actual gas temperature. To study this effect and to compare the thermocouple readings with the readings of a fibre Bragg grating (FBG) sensor, fire experiments were performed in a model-scale tunnel. In this paper the results from two such fire experiments are presented. The temperatures measured using thermocouples with different diameters and the FBG sensor are compared and discussed. The “true” gas temperature, corrected for radiation effects from the thermocouples, was calculated from the results from the thermocouples with different diameters. The results show that the temperature measured by the FBG sensor is closer to the “true” gas temperature than is the case for any of the thermocouples used in these tests.     

多仓式双层水泥库

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Experimental measurements,integral modeling and smoke detection of early fire in thermally stratified environments

Building fires go through a series of stages. They start with a fire plume period during which buoyant fire smoke rises to the ceiling. A second stage is the following enclosure smoke-filling period. In this paper, the first stage is the subject, especially for the fire plume behavior in thermally stratified environments in large volume spaces. In NFPA 92B, Morton's integral equation was introduced for calculating the maximum plume rise, and beam smoke detectors were recommended for smoke detection design. In this work, experiments and CFD simulations were conducted in a small-scale enclosure and a large space to investigate early fire movements in temperature-stratified ambients. The results show that in a thermally stratified environment, the axial temperature and velocity of a fire plume decrease more quickly along the vertical axis than in uniform environment, and in some cases the fire plume ceases to rise. The previous integral equation was shown to underestimate the actual maximum height of a fire smoke plume, and also was unable to explain the differences of the maximum heights of low-density and high-density smoke plumes with the same stratification and outlet conditions. The integral equation was improved by introducing two correction factors, and extended for non-linear temperature stratified environments. A light section smoke detection method with three space-protected area was suggested and discussed.     

Air entrainment into balcony spill plumes

New plume formulations have been developed for assessing air entrainment into thermal plumes that spill over a balcony edge into an atrium void.     

Experimental measurements of the thermal effectiveness of two types of protective clothing for fire fighters

Experiments were conducted to quantitatively assess and compare the thermal effectiveness of two types of lower torso protective clothing, commonly known as a day clothing set and a night clothing set, as a function of the moisture content and tightness of fit. Results were obtained giving the temperature distribution through the clothing sets. Note: Allan T. Kirkpatrick is Assistant Professor, Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado. Henry, Curtis and Alan Adelgren are undergraduate students, Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado.     

Compartment Masonry Walls in Fire Situations

A series of analytical studies were conducted to investigate the behaviour of masonry walls supporting a reinforced concrete slab under a number of fire scenarios. The main purpose is to use various types of sub-assembly in predicting the structural behaviour in fire. The analysis of sub-frames rather than complete structures is attractive since they have the potential to reduce computation time dramatically, which could be of significance if large numbers of results need to be generated for design documents. The analyses are all performed using a finite element program, called MasSET (Masonry Subject to Elevated Temperatures), whose most recent development has been the capability to take into account the interface element between the masonry wall and the slab. Separation, sliding, and opening and closing of initial gaps at the interface between the masonry wall and the reinforced concrete slab are accounted for by adjusting the properties of interface elements.     

空间建筑防火分区方法

现阶段,设计空间建筑防火分区,划分标准相对单一,且缺乏有效的理论依据作为支撑,为此,提出了基于火灾风险评估的防火分区方法,并指出大空间建筑在设计防火分区过程中存在的问题,详细介绍了基于火灾风险评估防火分区方法的具体应用和实施,最后借助FDS软件对实例进行了仿真,验证该方法具有充足的理论依据和实践价值,可供相关工程建设借鉴和参考.     

电梯轿箱空气品质及改善方案

本文针对电梯轿箱空气品质较差这一状况,分析了轿箱内空气品质较差的原因,并提出通过与前室防烟加压送风系统共用风道,对电梯井道进行机械送新风来改善电梯轿箱空气品质.     

An analytical investigation of near-field plates

This paper describes an analytical approach to modeling near-field plates, which are non-periodic grating-like structures that can focus electromagnetic waves to subwavelength dimensions. The analysis provides additional insight into the operation and design of such plates that focus cylindrical waves to subwavelength resolutions. Explicit expressions for the current density induced on the plate and its impedance profile are derived. The analytical expressions are validated numerically.     

欧盟的消防力量和火灾

“总有一天,到那时,……所有的欧洲国家,无须丢掉你们各自的特点和闪光的个性,都将紧紧地融合在一个高一级的整体里;到那时,你们将构筑欧洲的友爱关系……”——维克多·雨果