Guangxuan Liao

Interaction of water mists with a diffusion flame in a confined space

Abstract This paper describes the study of the interaction of water mists with a diffusion flame in a confined space with proper ventilation control. Water mist was generated by a single pressure nozzle and diffusion flames were produced from ethanol and pine samples, respectively. The LDV/APV system was employed to determine the water mist characteristics. The Cone Calorimeter was used to measure the heat release rate, oxygen and carbon monoxide concentrations and other important parameters of the interaction under various conditions. The test results showed that water mist suppressed the diffusion flame in the confined space through oxygen displacement, evaporative cooling and heat radiant attenuation, and enhanced the combustion through expansion of the mixture and chain reaction as well. Suppression played the dominating role when the water mists with enough volume flux were applied to the diffusion flame in confined space. The poorer was the ventilation, the easier the suppression. The water mists had a more complex effect on the solid sample than the liquid, and affected the smoke release rate and movement.

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 of back-draft in a compartment with openings of different geometries

Abstract This paper presents the results of reduced-scale experimental tests to study back-draft in a reduced-scale compartment (1.2 m × 0.6 m × 0.6 m), fitted with six different geometries for the opening on one end-wall and with two geometries for the opening in the ceiling. The experimental variables included the flow rate, the time during which the fuel was burned, and the geometries of the inlet. The quantities recorded before the back-draft included temperature and the concentrations of oxygen, carbon dioxide, and carbon monoxide. To quantify the effect of back-draft, the gas velocities in the inlet and also the pressures in the compartment were measured. The effects of different inlet geometries on the occurrence of back-draft are discussed. This study shows that the mass fraction of total hydrocarbons (i.e, the un-burned methane in this study), whose critical value varies with the geometry of the inlet, is a key parameter determining the occurrence of back-draft.

Wavelet-based image denoising in (digital) particle image velocimetry

Mallat algorithm, which analyzes the evolution of the wavelet transform maxima across scales based on wavelet transform, is applied in image denoising in particle image velocimetry (PIV) in this paper. An improved interrogation method for PIV images based on cross-correlation with discrete window offset, which makes use of a translation of the second interrogation window and rebuilds it considering rotation and shear is also presented. The displacement extracted from PIV images is predicted and corrected by means of an iterative procedure. In addition, the displacement vectors are validated at each intermediate of the iteration process. The method of image denoising in PIV based on wavelet transform is compared with averaging filter, Wiener filter and median filter by interrogation of synthetic and real PIV images and the results are discussed.

Experimental study on the premonitory phenomena of boilover in liquid pool fires supported on water

Abstract An experimental study of boilover phenomena in oil-tank fires supported on water has been performed. It is found that a typical process of liquid fuel burning on water consists of three basic stages: a quasi-steady period, a boilover premonitory period and a boilover period. Basic characteristics during the boilover process such as the flame structure, the fuel burning aspects, and the thermal structure of the oil-water layer at each stage have been examined. In particular, phenomena such as water seething at the oil-water interface and combustion micro-explosion noise emission, which appear only in the boilover premonitory period, have been examined and analysed in detail. By correlative analysis of these sub-processes, it can be concluded that boilover premonitory noise is certainly emitted before the occurrence of boilover and is quite closely connected with the temperature distribution of the oil-water layer before boilover occurs. This conclusion is helpful in understanding the mechanisms of boilover phenomena and offers a practical means to predict the occurrence of boilover.

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.

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.

Study and prediction of boilover in liquid pool fires with a water sublayer using micro-explosion noise phenomena

An experimental simulation was conducted to investigate the mechanisms of boilover phenomena in liquid pool fires with water sublayers. Observations of fire behavior and instrumentation of the local temperature history in oil/water layers revealed that a typical boilover process can be divided into three phases, i.e. a quasi-steady period, premonitory period and boilover period. Our attention was mainly focused on the premonitory phenomena of boilover, which is generally considered as one of the most important factors leading to the occurrence of boilover. Experimental examinations demonstrated that boilover only happens after the fuel/water interfacial temperature has reached the boiling point of water, and that it was the violent seething of water at the interface which brought about boilover. The emission of micro-explosion noise, one of the most prominent premonitory phenomena of boilover was examined in detail and found to be a result of the water boiling. The investigation on the premonitory micro-explosion noise of boilover illustrated that it is a possible means for early and remote detection of the occurrence of boilover in liquid pool fires. However, in a real fire situation, the micro-explosion is always contaminated by the environmental noise. Hence, pattern recognition techniques should be used to differentiate the micro-explosion noise from the unwanted background noise, and a predictive model should be used to evaluate the status of oil burning and predict the occurrence of boilover. A set of noise features and a practical model have been presented for these purposes.

A Comparative Experimental Study on Heat Release Rates of Charring and Noncharring Solid Combustible Materials

Two commonly used solid combustible materials in China, PMMA(Polymethyl Methacrylate) and Red Pine of Northeast China, are selected as typical noncharring and charring material respectively. Bench-scale experiments were carried out to measure the heat release rates of these two materials under various external radiant heat fluxes. A comparison of similarity and difference was made between the Heat Release Rate (HRR) of PMMA and that of Red Pine and the influence of external radiant fluxes on HRR was analyzed. Bubble layers and char layers formed near the heated surface of burned noncharring and charring materials, respectively, make them differ greatly in pyrolysis and combustion processes as indicated by different HRR profiles, which are closely related to the materials original structural properties and greatly influenced by external radiant heat flux.

Study on correlation between fire fighting time and fire loss in urban building based on statistical data

AbstractIn this paper, the correlation between fire fighting time and fire loss (burned area of urban building) based on fire statistical data in Jiangxi Province (China) from 2000 to 2010 was studied. The results showed the probability distribution of fire fighting time met lognormal distribution. In the probability density function, the expectation of the distribution represents the average level of fire fighting time. We found the average fighting time of warehouse and workshop fires were higher than the average level of whole building fires. In addition, the probability distribution of the burned area in each fire fighting time interval also followed power function, which was valid in the case of the fire fighting time within 4 hours. Furthermore, the absolute value of the exponent of the function is positively correlated with the small-scale fires and negatively with the large-scale fires. The value decreased with the increase of the fire fighting time, indicating that the fire control ability became...