Liao Guang-xuan

Experimental Study on the Effectiveness of the Extinction of a Pool Fire with Water Mist

In order to study the effectiveness of the extinction of a pool fire with one nozzle water mist, a series of experiments were conducted under different conditions in an open space. Fire sources are small-scale circular stainless steel pans of 130 and 200 mm in diameter with the fuel of alcohol and kerosene. Before and after the application of water mist, some K-type thermocouples along the pool centerline and a digital camera were used to measure the flame temperature and visualize the process of the fire extinction. A thermogage and a turbine flux sensor were used to measure the flame radiant heat flux and the application rate of water, respectively. A Particle Image Velocimetry and Sizing (PIVS) technique was developed to characterize the water mist. The experimental results show that both the flame of alcohol and kerosene cannot be extinguished in many cases when the pressure is lower than 0.4 MPa. In addition, the distance between the flame and the nozzle, and the application rate of water are two main factors which influence the effectiveness of the extinction of a pool fire when the working pressure is lower and the impulse injection of water mist can improve the effectiveness of fire extinction. However, it should be noted that the results are limited to the experimental conditions because it is still difficult to get the functional relationships of pool fire extinction mechanisms with water mist.

Preliminary Study on the Interaction of Water Mist with Pool Fires

In this paper, some preliminary measurements and observations were conducted to investigate the interaction of water mist with pool fires. The fire source is a small-scale circular stainless steel pan with heptane, ethanol and kerosene as fuel. Water mist was generated by a downward-directed pressure nozzle that was positioned on a square steel plate 300 mm over the fuel sample and was operated at pressures of 0.5 MPa. The water mist characteristics were determined by a LDV/APV system. The radiation spectra of the flames were measured by a monochromator and the radiant heat flux was obtained by a thermogage before and after application of water mist. Thermography was used to visualize the thermal field of the flame. The experimental results show that the interaction mechanisms of kerosene flame are different from those of ethanol and heptane.

Study of Liquid Pool Fire Suppression with Water Mists by Cone Calorimeter

An experimental study of the pool fire suppression with water mists was performed in this paper. Spatially resolved droplet size, velocity distributions, and volume flux of the fine water mist has been obtained by LDA/APV system. A method to correct for the presence of droplets is presented and used to determine the heat release rate using a cone calorimeter system, the effects of water mist application on the heat release rate for liquid pool fires were studied. It was found that the volume flux of the water spray mostly determined the liquid pool fire-behavior. For the current experimental conditions, the flame was enlarged when lower volume flux of water is used, and suppressed significantly when higher volumes of water are used. Two opposing mechanisms compete when water mists are applied, which is more complex than the halon agents, such as 1301. The combustion enhancement becomes insignificant when water mists with the proper characteristics such as adequate volume flux are applied to the diffusion flame in a confined space.

Experimental Study on Cooling a Hot Solid Surface with Water Mist

In this paper, a series of experiments are performed to investigate the hot solid surface cooling with water mist under different conditions, such as different initial surface temperatures and mist characteristics (droplet size, droplet velocity, etc.), while these parameters are simultaneously varied. A 150 150 10mm 3chrome-plated stainless steel plate is heated from below by a 1500-W electric cooker continuously. A downward pressure nozzle is positioned on a square steel plate, 1000mm above the surface to inject water mist, the 0.5mm K-type thermocouples are jointed on the surface to obtain its temperature, and a TVS-2000ST Thermography is used to visualize the transient thermal behavior of a water mist and obtain the temperature distribution of the surface. The characteristics of the water mist are obtained by a modified PIVS (Particle Image Velocimetry and Sizing) technique previously. Based on the experiments, the effects of mist parameters and the initial surface temperature on droplet evaporative cooling are analyzed. Further discussion is carried out by considering the relationship between these parameters and the droplet Weber number, the results show that an inverse relationship between the cooling efficiency and mist droplet Weber number existed in this study.

Extinction Limit of Diesel Pool Fires Suppressed by Water Mist

This work describes an experimental and theoretical investigation of extinction limit of diesel pool fires suppressed by water mist. A downward-directed nozzle produced a fine water spray over small-scale opposed diesel pool fires. The fire extinction limit is obtained from minimum nozzle injection pressure or maximum fire size when the fire extinguishment took place. For the conditions tested, it is observed that there are two distinct regions in the relationship between the injection pressure and fire size, i.e., a fire suppression region and a fire intensification region. The transition of the two regions is dependent on the effective water flux and the plume-spray thrust ratio. It is also revealed that the fuel surface cooling is the dominant mechanism for diesel pool fires at extinction limit in the tests. The failure to cause fire extinction is mainly due to the insufficient water flux applied to remove enough heat from the fuel surface.

An improved cross-correlation method for (digital) particle image velocimetry

An improved method that brings enhancement in accuracy for the interrogation of (digital) PIV images is described in this paper. This method is 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. 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 present improved cross-correlation method is compared with the conventional one in accuracy by interrogation of synthetic and real (digital) PIV images and the interrogation results are discussed.

Study on fire extinguishing performance of ultrafine water mist in a cup burner

Two simplified models for predicting minimum extinguishing concentration (MEC) of ultrafine water mist (UFM) (<10 μm) were developed based on limiting oxygen concentration (LOC) and combustion limit temperature (CLT), respectively. Experiment was conducted using a modified cup burner which can reduce the surface adsorption of UFM. Two typical liquid fuels, n-heptane and ethanol, were used in the experiment. Tests using the same scenario were repeated 20 times or 10 times according to the variance of extinguishing time. The average and the standard deviations of extinguishing time were used to evaluate the fire extinguishing performance of UFM. Experimental results agree well with the model based on LOC, and disagree with the model based on CLT. The disagreements were explained by analyzing flow behavior of UFM. It was concluded that the primary mechanism of fire extinguishment with UFM was oxygen dilution.

Experimental study on fire smoke movement in a multi-floor and multi-room building

Fire smoke movement of multi-floor and multi-room (MFMR) fire was studied at the model test building in State Key Laboratory of Fire Science (SKLFS). The ingredient, temperature, air pressure difference and air velocity of smoke were measured and analyzed. Meanwhile, the hazard of smoke ingredient to exposed occupants was analyzed based on the national standard, Occupational Exposure Limit for Hazardous Agents in the Workplace (GBZ2-2002). The experimental results showed that the maximum temperature difference in MFMR fire was located along the vertical height from the fire source. With the spreading and diffusion of smoke, the temperature of smoke layer would tend to be no difference. In the fire of woodpile and kerosene, the main smoke ingredients such as SO2, CO and CO2 would first exceed human’s average physiological limit, while smoke ingredients such as NO and NO2 would come behind. Because of the higher fluctuation range and frequency of air pressure difference of smoke in multi-layer building fire, the fire smoke would spread around everywhere of the passageway and made the human evacuation more difficult.

Experimental Study of the Fire-extinguishing Effectiveness of 1-Bromo- 3,3,3-Trifluoropropene/Nitrogen Mixtures:

The search for a new fire-extinguishing agent with all the desirable properties of Halon 1301 has not been successful. In this study, the authors have pursued the development of 1-bromo-3,3,3-trifluoropropene (BTP), which has a very low extinguishing concentration, but with its high boiling point, it is not a suitable replacement for Halon 1301. However, mixtures of BTP in an inert gas could produce fire-extinguishing agents with many of the desirable properties of Halon 1301. The fire suppression effectiveness of BTP/nitrogen mixtures is studied. The results show that addition of a small amount of BTP in nitrogen reduces the extinguishing concentration of nitrogen considerably, with BTP/nitrogen mixtures showing strong synergistic interactions at low concentrations of BTP.

Influence of Potassium Nitrate on the Combustion Rate of a Water-based Aerosol Fire Extinguishing Agent

A water-based aerosol fire extinguishing agent (WAE) is a safe and practical alternative to halons. It comprises an oxidizer, a fuel and a catalyst. The WAE combusts and generates large amounts of inert gases and vapor, which are released into the fire scenario, extinguishing the fire. KNO3 is used as an oxidizer to adjust the combustion rate and to ensure highly effective fire extinguishing ability and safe application of WAE. Generally, the combustion rate of WAE increases with an increase of KNO3%. However, within a specific range of KNO3%, the combustion rate decreases initially and then increases with an increase of KNO3%, called the ‘N’ effect in this study. The competition between these two effects leads to a decrease of the activation energy of the thermal decomposition of WAE as KNO3% increases along with an inhibiting effect of the liquid mixture that is generated during the combustion of the WAE. In addition, the relationship between the combustion rate and the fire extinguishing time is also discussed in this study.