Xiaodong Zhou

Bio: Xiaodong Zhou is an academic researcher from University of Science and Technology of China. The author has contributed to research in topics: Heat flux & Ignition system. The author has an hindex of 13, co-authored 50 publications receiving 494 citations.

A floor field cellular automaton for crowd evacuation considering different walking abilities

Abstract: It has been hard to model a crowd evacuation process considering different walking abilities using a synchronous cellular automaton. That is because the cross and the overlaps of routes have to be taken into consideration and the conflicts resolution between pedestrians is more complex. However, the desired velocities of evacuees might be quite different due to the discrepancies of the physiological function, including age, gender, physical state, and the psychological behavior, such as the perception and reflection to the dangers. Additionally, an evacuee might change his desired velocity constantly to adapt to the changing evacuation environment. Thus, a multi-velocities floor field cellular automaton model was established in this paper. Using little CPU time, a dense crowd evacuation simulation with tiny varied velocity can be conducted very well. Significant discrepancies between the single-velocity evacuation and the multi-velocities evacuation were observed. The plateaus, where the exit flow rate is rather low, can be well predicted by a dimensionless parameter describing the congestion level of the evacuation system. The crowd evacuation time almost depends on the low desired velocity evacuees, though the proportion is not high. We also observed that faster evacuees make the evacuation system easily approaching to the jam.

Influences of low atmospheric pressure on downward flame spread over thick PMMA slabs at different altitudes

Abstract: This study addresses the influences of low atmospheric pressure on heat and mass transfer process of downward flame spread over thick PMMA (polymethyl methacrylate) slabs in quiescent air. Series of experiments were conducted at three altitudes: Hefei (1.0 atm), Xining (0.77 atm) and Lhasa (0.67 atm). Burning rate (mass loss rate), flame spread rate and flame height were investigated in this paper. From experimental results in reduced pressure, burning rates are correlated by the expression: m ˙ ∝ P 1.8 . At lower pressure, it was found that flames go quenching below a critical Damkohler number, which is caused by chemical kinetic change and reduction of total heat feedback from flame to solid fuel. For finite width samples, the flame spread rate derived from experiments increased with the sample thickness, which is different from the previous conclusions based on an assumption of infinite width. Power-law progressions of flame spread rate and flame height to pressure were produced, and linear relationships between the index and thickness of samples were obtained.

Estimating the longitudinal maximum gas temperature attenuation of ceiling jet flows generated by strong fire plumes in an urban utility tunnel

Abstract: This study is centred on longitudinal maximum gas temperature attenuation of ceiling jet flows generated by strong fire plumes in a utility tunnel. A theoretical correlation is derived by incorporating the approximate boundary layer thickness. To obtain related coefficients, a series of full-scale fire tests are also conducted in a utility tunnel under construction. Four Existing models for the estimation of longitudinal temperature decay are compared and analysed in detail. It is seen that none of them are applicable to make satisfactory predictions in current situations, while good agreement is observed between our theory and test data. Additionally, impinging conditions of strong plumes under different ceiling clearances above fuel are divided into three states. We find Stanton numbers in temperature attenuation were overestimated by Delichatsios, and they are reassessed under these states. In these one-dead-end cases, it is also found the end wall has a great impact not only on the upstream flow pattern, but also on the characteristics of downstream flow. As the fire moves away from the dead end, the downstream gas temperature decays more slowly and Stanton number becomes smaller. In an enclosed case, a new expression has also been provided following the same derivation procedure. The present work positively provides a tool for the assessment of thermal environment, heat exposure and estimation of fire detection time in engineering practice.

Effect of finite dimension on downward flame spread over PMMA slabs: Experimental and theoretical study

Abstract: This work investigates experimentally and theoretically the heat and mass transfer process of downward flame spread over finite width polymethyl methacrylate (PMMA) slabs in quiescent air. A series of experiments with different sample dimensions, 3, 4, 5 and 6 mm thick and 1.5–12 cm wide, are conducted and a solid phase flame spread model is developed to predict the behaviors during the process. Being different from the 2-D condition, an inverse ‘V’ shape leading edge, accelerated mass loss rate and spread rate exist due to the lateral combustion of samples and enhanced oxygen diffusion. The angle of leading edge increases with increasing width for 3 mm slabs, whereas no significant change occurs for thicker samples. The analytical model indicates that the mass loss rate and spread rate are functions of thermal parameters, leading edge angle and geometry of material. For fixed thickness, a linear relationship exists between the mass loss rate and width, while the spread rate is inversely proportional to the width of sample. The model also supplies two methods to estimate the heat flux ahead of leading edge and on the pyrolysis surface: by measured mass loss rate or flame spread rate. The validity of the proposed model is verified by the good agreement between the experimental and predicted results.

Pyrolysis and spontaneous ignition of wood under time-dependent heat flux

Abstract: This work investigates experimentally and theoretically the effect of time-dependent incident heat flux (HF), which is more reasonable in fire-like environment, on thermal degradation process of wet pine wood. A feedback method was utilized to generate a time-dependent heat flux by controlling the output power of radiative heater. Both quadratic and linear heat fluxes were studied in this study. Comparison between measured heat fluxes and designed values indicates that the method provides high accuracy. Measurements of temperature distribution at different depths of material, ignition time and mass loss rate were implemented in the tests to examine the effects of time-dependent heat fluxes. Additionally, analytical model and numerical model were developed to predict the pyrolysis behaviors, and good agreement exists between the experimental and simulational results. Results showed that the heat penetration layer is restricted to a thinner depth for HF with higher increasing rate. A linear relationship was found between ignition time and HF parameters, which is also validated by experimental data and reexamined by constant heat flux circumstance. Mass loss rate was affected significantly by the changed heat flux compared with constant scenario. Furthermore, critical mass flux, which keeps almost unchanged, can be employed as ignition criterion due to the fact that the ignition temperature increases with increasing heat flux, which also certifies the conclusions of other researchers.

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)

Sfpe Handbook Of Fire Protection Engineering

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The effect of environmental factors and dust accumulation on photovoltaic modules and dust-accumulation mitigation strategies

Abstract: The mitigation of environmental effects on clean-energy technology is an area of increasing interest. Photovoltaic (PV) modules have been widely used in small and large-scale applications for many years. However, they are not yet competitive with other electrical energy-generation technologies, especially in environments that suffer from dust, airborne particles, humidity and high ambient temperatures. This paper presents a review of the effect of climatic conditions on PV module performance, in particular, the effect of dust fouling. Research to date indicates that dust deposition has a considerable effect on PV module performance as it reduces the light transmissivity of the PV module surface cover. Studies on the ways in which dust is deposited on PV module surfaces are reviewed, as understanding this process is essential to develop effective mitigation approaches. Module performance is also adversely affected by high ambient temperature, humidity and lack of rainfall. The current review summarizes the past, current and promising future approaches towards mitigating environmental effects, in particular dust fouling. Electrostatic cleaning methods and micro/nanoscale surface functionalization methods both have the potential to counteract the negative effects of dust deposition, with the combination of the two methods showing special efficacy, particularly in arid regions.

Experimental study of air entrainment mode with natural ventilation using shafts in road tunnel fires

Abstract: A set of burning experiments with n-heptane pool fire were conducted to investigate the air entrainment mode with natural ventilation using shafts in road tunnel fires. One criterion was proposed to determine the critical shaft height of the plug-holing occurrence in our previous work whereas this study quantificationally analyzes the effect of plug-holing on the smoke exhaust efficiency. The disturbance of smoke exhausting on the smoke–air interface, which causes different amounts of fresh air exhausted directly and indirectly, is investigated. Results show that using vertical shafts to discharge smoke leads to a strong mixing process between the smoke layer and the fresh air. Consequently, about 2/3 of the smoke exhausting rate of the shaft is air. Moreover, some of the entrained air mixes into the smoke layer downstream the shaft, resulting, in a small reduction amount of the spilling smoke, which also proves an inefficient smoke exhausting process. Hence, for the sake of improving effectiveness of natural ventilation, some significant improvements are needed in practice.