Xishi Wang

Visualization of Two Phase Flow inside an Effervescent Atomizer

Effervescent atomization is one of the twin-fluid atomization methods while it has better performance in terms of smaller drop sizes and/or lower injection pressures. In order to investigate the effects of the internal flow patterns on droplet characteristics, a new kind of effervescent atomizer was designed and manufactured. The bubble forming process was visualized with a high-speed camera, while the droplet size was characterized with a LDV/PDA system. The experimental results show that there are three regimes of the two-phase flows inside the discharge orifice, one is bubbly flow, another is annular flow while the other is the intermittent flow. The flow patterns transfered from bubbly flow to intermittent flow and then to annular flow with decreasing of the water flow rate. In addition, with increasing of the working pressure or decreasing of the water flow rate, the SMD (Sauter mean diameter) of the droplets decreased and the axial mean velocity increased.

Experimental Study on the Interaction of a Water Drop Impacting on Hot Liquid Surfaces

This article presents the results of experimental investigation of water drop impacting on surfaces of different fuel liquids, such as alcohol, kerosene, and molten-ghee, which are typical fuels for liquid pool fire study or typical fire source in historical buildings. Each of the liquids was heated to simulate the temperature conditions as it burns, although the temperature is still a little lower than the real one. The impacting processes were recorded using a high-speed digital camera with 1000fps. The results show that the liquid physical properties, such as surface tension, viscosity, and miscibility, have primary influences on the drop impact behavior except for drop Weber number, and liquid temperature mainly influences the size of the splashed drops and bouncing height of the jet. The drop liquid is ejected for water—alcohol impact, while the bulk liquid is ejected for water—kerosene impact. The drop immersed into the liquid when it impacts on molten-ghee surface, no splashing and drop rebounding occur in this case.

Fire Protection of Heritage Structures: Use of a Portable Water Mist System under High-altitude Conditions

In order to verify the application of water mist on fire protection of the Potala Palace in Tibet and deepen the knowledge of its suppression mechanisms under high-altitude conditions, a series of experiments are performed with a portable water mist fire protection system and with diesel oil, gasoline and, in Lhasa, ghee as fuels. All of the experimental tests are conducted with and without multicomposition (MC) additives. The experimental results show that the MC additive can evidently improve the extinguishing efficiency of water mist for a diesel fire, but the gasoline fire is a little difficult to extinguish. The effects of high-altitude conditions on fire suppression are discussed.In order to verify the application of water mist on fire protection of the Potala Palace in Tibet and deepen the knowledge of its suppression mechanisms under high-altitude conditions, a series of experiments are performed with a portable water mist fire protection system and with diesel oil, gasoline and, in Lhasa, ghee as fuels. All of the experimental tests are conducted with and without multicomposition (MC) additives. The experimental results show that the MC additive can evidently improve the extinguishing efficiency of water mist for a diesel fire, but the gasoline fire is a little difficult to extinguish. The effects of high-altitude conditions on fire suppression are discussed.

A New Type of BTP/Zeolites Nanocomposites as Mixed-phase Fire Suppressant: Preparation, Characterization, and Extinguishing Mechanism Discussion

A new type of nanocomposites consisting of solid 4A zeolites and gaseous fire extinguishing agent of 2-bromo-3,3,3-trifluoropropene (BTP) was fabricated in large scale, in which BTP anchored in the porous zeolites. Laboratory-scale fire extinguishment tests showed that the nanocomposites as additives can greatly improve the performance of conventional sodium bicarbonate (NaHCO3) dry powder for relatively shorter extinguishing time and smaller amount of agents required. Such an improvement could be ascribed to the simultaneous effect of solid NaHCO3, gaseous BTP desorbed from the zeolite when presented to higher temperature and the porous zeolite as heterogeneous inhibition of flame free radicals.A new type of nanocomposites consisting of solid 4A zeolites and gaseous fire extinguishing agent of 2-bromo-3,3,3-trifluoropropene (BTP) was fabricated in large scale, in which BTP anchored in the porous zeolites. Laboratory-scale fire extinguishment tests showed that the nanocomposites as additives can greatly improve the performance of conventional sodium bicarbonate (NaHCO3) dry powder for relatively shorter extinguishing time and smaller amount of agents required. Such an improvement could be ascribed to the simultaneous effect of solid NaHCO3, gaseous BTP desorbed from the zeolite when presented to higher temperature and the porous zeolite as heterogeneous inhibition of flame free radicals.

Experimental Study on Characteristics of Methane–Coal-Dust Mixture Explosion and Its Mitigation by Ultra-Fine Water Mist

This paper presents the results of an experimental investigation on the characteristics of methane–coal-dust mixture explosion and its mitigation by ultra-fine water mist. Four E12-1-K-type fast response thermocouples, two printed circuit board (PCB) piezotronic pressure transducers were used to obtain the temperature and pressure history, while a GigaView high-speed camera was used to visualize the processes. Different methane concentrations, coal-dust concentrations, diameters of coal particles, and volumes of ultrafine water mist were considered to investigate their effects on methane–coal-dust mixture explosion. The temperature of explosion flame, the maximum explosion overpressure, the maximum rate of overpressure rise, and the critical volume flux of ultra-fine water mist were experimentally determined. The results show that the characteristics of the methane–coal-dust mixture explosion and the mitigating effectiveness by ultra-fine water mist are influenced by the methane concentration, the coal-dust concentration, the coaldust diameter and the applied volume flux of ultra-fine water mist. For example, both the maximum explosion overpressure and rate of overpressure rise increased with increasing of coal-dust concentrations and methane concentrations. All of the test cases indicate that ultra-fine water mist can mitigate the mixture explosion and suppress the flame propagation efficiently from the images recorded by the high-speed video camera. [DOI: 10.1115/1.4005816]

Experimental Study on the Effects of Low Ambient Pressure Conditions at High Altitude on Fire Suppression with Water Mist

In order to study the effects of low ambient pressure conditions at high altitude on fire suppression with water mist, a water mist fire suppression experimental facility was built in Lhasa, Tibet, a province of China. Using this facility, water mist extinguishing tests were carried out on gasoline, diesel oil, molten ghee, and wood crib fires, and the results compared with the similar tests conducted in Hefei. Water mist spray patterns under normal and low atmospheric pressures were characterized with a laser sheet method. Fuel mass burning rates were measured in both Lhasa and Hefei to investigate differences and influences on fire suppression. The experimental results showed that the burning rates of the above fuels are lower in Lhasa than in Hefei, and that mist droplets tended to congregate along the spray cone edge as the atmospheric pressure decreased. For most of the test cases, the fire suppression performance of water mist in Lhasa is a little better than that in Hefei, which may be largely caused by decreasing of the mass burning rate at lower atmospheric pressure conditions.

Experimental and numerical study on attenuation of thermal radiation from large-scale pool fires by water mist curtain

Full-scale experiments and numerical simulations were conducted to study the thermal radiation attenuation from large fires by water mist curtain with low and intermediate pressures. Fire dynamics simulator (version 6) was used for numerical simulations. A novel multi-injector nozzle was designed to generate a homogeneous water mist curtain with low water consumption. Water mist characteristics of one of the injectors were measured by Shadowgraphy technology. A 1 × 1 m diesel pool fire was considered as the fire source. The experimental results show that the water mist curtain has high thermal radiation attenuation efficiency, for example, about 82.7% radiant heat flux could be attenuated by the water mist curtain with 2 MPa working pressure and flow rate of 13.3 L/min. Comparisons with the experimental results show that the calculated radiant heat flux and temperature are slightly underestimated.

Experimental determination of atmospheric pressure effects on flames from small-scale pool fires

A small steel box, whose size was 1.0 m × 1.0 m ×1.0 m, was built for studying the flame characteristics of small pool fires. The pressure inside this box could be altered by a vacuum pump as a laboratory test rig. The fuels including gasoline and n-heptane were tested. The flame height and the flame pulsation frequency of the small-scale pool fires were experimentally determined. The results show that the flame height is proportional to the ambient pressure, and it will decrease with reducing ambient pressure. The flame pulsation frequency also decreased with reducing ambient pressure, and the laminar flame occurs when the ambient pressure is low enough. Some theory models were developed to explain this phenomenon in experiments.

Experimental study on the performance of transition metal ions modified zeolite particles in suppressing methane/air coflowing flame on cup burner

This study investigated the performance of three kinds of transition metal ions (Mn2+, Cu2+, and Zn2+) modified zeolite 4A particles in suppressing methane/air coflowing flames on cup burner. Ion-exchange method was employed to incorporate different amount of transition metal ions into parent zeolite 4A. Fire suppression effectiveness of zeolites containing varied molar percentage of metal ions was tested by cup-burner method. Results showed that transition metal ions modified zeolites outperformed the parent zeolite 4A with a ranking order of Mn2+ > Cu2+ > Zn2+. Performance of the zeolite particles was affected by the percentage of transition ions loaded, which dominated the number of active sites and Brunauer–Emmett–Teller surface area of the particles. Increasing molar percentage of metal ions loading increased the capacity to scavenge flame radicals, but decreased Brunauer–Emmett–Teller surface area of the particles. There existed an optimized molar percentage of transition metal ions loading where the best fire-suppressing effectiveness was exhibited.

Experimental study on the autoignition characteristics of pure and oil-impregnated transformer insulating paper board

In order to deepen the knowledge of transformer fire dynamics and its fire prevention, an experimental study on autoignition and combustion characteristics of pure insulating transformer paper board (PIPB) and oil-impregnated transformer insulating paper board (OIIPB) were performed. Ignition time, heat release rate (HRR), O2, CO and CO2 concentrations were obtained by a cone calorimeter under different external radiant heat fluxes. The results show that two peaks of HRR occurred during the burning process of PIPB, where three peaks occurred for the OIIPB. The first peak of the OIIPB occurred earlier than that of PIPB, which indicates that OIIPB is more ignitable, but the maximum HRR of PIPB is higher than that of OIIPB, which indicates that the combustion intensity of PIPB is larger. The PIPB has a relatively higher CO and CO2 yield and O2 consumption. The results demonstrate that both the tested 3-mm-thick PIPB and OIIPB samples in this study can be considered as thermally thick materials. Autoignition time of OIIPB and PIPB under external heat fluxes could be predicted based an empirical model, and the critical heat fluxes were obtained based on the models and the ignition data.