Lizhong Yang

Bio: Lizhong Yang is an academic researcher from University of Science and Technology of China. The author has contributed to research in topics: Materials science & Heat flux. The author has an hindex of 19, co-authored 102 publications receiving 1261 citations.

Simulation of the kin behavior in building occupant evacuation based on Cellular Automaton

Abstract: Individual behavior of occupants, which is quite complex, greatly affects the route choice, the evacuation time and the efficiency. The phenomenon of the kin behavior often occurs during evacuation. In this paper, a two-dimensional Cellular Automata model is applied to simulate the process of evacuation with respect to the kin behavior. Many interesting phenomena during real evacuation, such as incoherence, jamming, gathering, backtracking and waiting, are simulated by the variation of the building structure, the occupants' arrangement, the kin attraction and the route selection. We conclude that the psychology of going with the crowd is not always harmful; the conclusions about the kin behavior in one-exit case are the same as those in two-exits case; the number of the sub-groups and the members in each sub-group influence the evacuation efficiency a lot; the typical phenomena of waiting and backtracking in real evacuation do reduce the evacuation efficiency. We have preliminarily found that the proper kin behavior is beneficial to improve the evacuation efficiency.

Flame extension length and temperature profile in thermal impinging flow of buoyant round jet upon a horizontal plate

Abstract: This paper investigates the flame extension length and temperature profile in a thermal impinging flow, induced by a buoyant turbulent round jet impingement upon a horizontal plate, focusing on the scenarios that flame heights are comparable to or larger than plate-source height. Results show that the measured flame extension lengths in this work with a wider range [(Hf−H)/D from 0 to 14.5] can be still well approached by the You and Faeth's correlation obtained from limited data range [(Hf−H)/D from 0 to 5.8]. New correlations to predict the flame extension length are also developed based on physically the unburnt fuel (at impingement) estimation with cylindrical- or ellipse flame shape hypothesis. The change of maximum temperature at the impingement zone along with plate-source height is found to be well described by a three-regime law. A new correlation is proposed, with modified characteristic plume radius, to collapse the horizontal temperature decay profile with Hf/H up to 4.13.

Numerical study on the optimization of smoke ventilation mode at the conjunction area between tunnel track and platform in emergency of a train fire at subway station

Abstract: To cope with fires in a subway station, ventilation systems are usually installed, which includes an air supply system and a smoke exhaust system. In case of a train fire, the operation of these ventilation systems needs to be studied in order to get optimal control of smoke propagation and provide better environmental conditions for personnel evacuation. In this paper, CFD simulations are carried out by Fire Dynamics Simulator (FDS) to study the effectiveness of different ventilation modes in case of a train fire in a subway station. The temperature and visibility contours are computed as to compare the performance of various ventilation modes for subway stations with full-seal Platform Screen Door (PSD) or half-height safety door. Results show that appropriate activation of the air supply system can improve the efficiency of the ventilation system in smoke control, and vice versa. It is better to activate the lobby air supply system and meanwhile close the platform air supply system. As for the exhaust system, it is necessary to activate the platform exhaust system and the Over Track Exhaust (OTE) system, and it is better to deactivate the Under Platform Exhaust (UPE) system. The optimization strategy of the ventilation mode for subway stations with full-seal PSD is similar to that for subway stations with half-height safety door. With the help of the additional smoke barrier, smoke propagation in a subway station can be well controlled. The results in the paper may serve as a useful reference for the smoke control design in case of subway train fires.

Thermal buoyant smoke back-layering flow length in a longitudinal ventilated tunnel with ceiling extraction at difference distance from heat source

Abstract: Tunnel is a special long-narrow building structure. The thermal buoyancy-driven upstream reversing smoke back-layering flow produced by the burning of heat source is harmful for human evacuation. This paper reveals the effect of the distance between ceiling extraction (opening) distance and heat source on the thermally-driven smoke back-layering flow length beneath the ceiling in a tunnel with combination of ceiling extraction and longitudinal ventilation, which has not previously been reported. Experiments are conducted in a reduced-scale tunnel [72 m (length) × 1.5 m (width) × 1.3 m (height)]. A porous gas burner is used to provide steady heat source and its relative distance from the ceiling extraction (opening) is varied. It is found that the thermally-driven smoke back-layering flow length increases as the heat source-ceiling extraction (opening) distance increases. A new model is theoretically deduced to predict the smoke back-layering flow length including the factor of heat source-ceiling extraction (opening) distance, by accounting for the energy loss due to extraction which is calculated based on the local longitudinal temperature profile estimation. The predictions by the proposed model are in good agreement with the experimental results.

A comprehensive investigation on the thermal and toxic hazards of large format lithium-ion batteries with LiFePO4 cathode.

Abstract: Toxic gases released from lithium-ion battery (LIB) fires pose a very large threat to human health, yet they are poorly studied, and the knowledge of LIB fire toxicity is limited. In this paper, the thermal and toxic hazards resulting from the thermally-induced failure of a 68 Ah pouch LIB are systematically investigated by means of the Fourier transform infrared spectroscopy (FTIR) and 1/2 ISO full scale test room. The LIBs with higher state of charge (SOC) are found to have greater fire risks in terms of their burning behavior, normalized heat release rate, and fire radiation, as well as the concentration of toxic gases. Specifically, the thermal hazards are evaluated by combining the effects of convective and radiative heat. The major toxic gases detected from the online analysis are CO, HF, SO2, NO2, NO and HCl. Furthermore, Fractional Effective Dose (FED) and Fractional Effective Concentration (FEC) models are used to quantitatively assess the overall gas toxicity. Results show that the effects of irritant gases are much more significant than those of asphyxiant gases. HF and SO2 have much greater toxicity than the other fire gases. The maximum FEC value is approaching the critical threshold in such fire scenarios.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Abstract: To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Convection Heat Transfer

Abstract: Geometry: shape, size, aspect ratio and orientation Flow Type: forced, natural, laminar, turbulent, internal, external Boundary: isothermal (Tw = constant) or isoflux (q̇w = constant) Fluid Type: viscous oil, water, gases or liquid metals Properties: all properties determined at film temperature Tf = (Tw + T∞)/2 Note: ρ and ν ∝ 1/Patm ⇒ see Q12-40 density: ρ ((kg/m) specific heat: Cp (J/kg ·K) dynamic viscosity: μ, (N · s/m) kinematic viscosity: ν ≡ μ/ρ (m/s) thermal conductivity: k, (W/m ·K) thermal diffusivity: α, ≡ k/(ρ · Cp) (m/s) Prandtl number: Pr, ≡ ν/α (−−) volumetric compressibility: β, (1/K)

Modeling crowd evacuation of a building based on seven methodological approaches

Abstract: Crowd evacuation of a building has been studied over the last decades. In this paper, seven methodological approaches for crowd evacuation have been identified. These approaches include cellular automata models, lattice gas models, social force models, fluid-dynamic models, agent-based models, game theoretic models, and approaches based on experiments with animals. According to available literatures, we discuss the advantages and disadvantages of these approaches, and conclude that a variety of different kinds of approaches should be combined to study crowd evacuation. Psychological and physiological elements affecting individual and collective behaviors should be also incorporated into the evacuation models.

Cellular automaton model for evacuation process with obstacles

Abstract: A bidimensional cellular automaton model is used to simulate the process of evacuation of pedestrians in a room with fixed obstacles. A floor field is defined so that moving to a cell with lower floor field means approaching an exit door. The model becomes non-deterministic by introducing a “panic” parameter, given by a probability of not moving, and by a random choice to resolve conflicts in the update of pedestrian positions. Two types of exit doors are considered: single (where only one person can pass) and double (two persons can pass simultaneously). For a double door, the longest evacuation time turns out to occur for a very traditional location of the door. The optimum door position is determined. Replacing the double door by two single doors does not improve evacuation times noticeably. On the other hand, for a room without obstacles, a simple scaling law is proposed to model the dependence of evacuation time with the number of persons and exit width. This model fails when obstacles are present, as their presence introduces local bottlenecks whose effect outweighs the benefits of increasing door width beyond a certain threshold.

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