The Experiment of Angular Upward Flame Spread on PMMA Fuel - Literature review Example

Literature review The spread of the flame over a solid is a complex process. But since the flame spread rate can easily be measured, most experimental data of flame spread rate has been obtained over the years. However, there is still uncertainty as to which chemical and physical have dominant effect. For the flame to be propagated, certain heat transfers modes must be present for the heat to travel from the flame to the unignited material. Of potential importance are the motions of the gases at the leading edge of the flame, gas phase chemistry including pressure, oxygen concentration, inert diluents as well as inter-diffusion reactants. In addition, solid surface chemistry, geometry and thermal properties are also significant (National Research council (U.S.), Committee on fire safety aspects of polymeric materials, 1978). There are numerous materials that describe various theoretical and experimental observations models which explain flame spread process. Some of them include De is (1969); Hirano et al, 1974, Williams (1976) and others. De Ris (1969): Spread of Laminar Diffusion Flame De Ris and Markstein studied 2-dimension upward spread using cotton fabrics held at different inclinations. As the flame climbs upward turbulent flame climbs on the fabric, burning it out and therefore limiting the size of the flame and the rate of spread. With 150cm long fabric, the flame spread accelerated continually, increasing its turbulence following a short laminar period. They found out that the flame spread could have reached asymptotic rate if the fabric sample had been longer. The asymptotic rate ranges from 20 cm/s to 45cm/s, depending on the relative humidity and the inclination angle. They also found out that the transverse air flow had much effect in the absence of side walls, by shortening the rising flames, resulting in the reduction of the distance over which the fabric can be preheated. The spread rate is therefore lower than with side walls and achieves constant value faster. Therefore, the propagation direction and the inclination of the specimen had a powerful influence on the rate of flame spread. Hirano et al, 1974: Measured Velocity and Temperature Profiles Near Flames Spreading over a Thin Combustible Solid Hirano et al. used particle tracer techniques to analyze the velocity of the flame spread near the leading flame edge. The experiment was done on a sheet of paper at three different angles. They found out that the velocity of the downward flame spread on the vertical paper sheet was stable, but was not stable over a horizontal surface and on an inclined paper of 300 from the horizontal surface. The velocity also changes greatly with time. In all these cases, the velocity of the gas near the spreading flame was higher than the rate of flame spread, but close to the paper sheet surface. In downward flame spread on the inclined surface, a small vortex was sometimes observed ahead of the leading flame edge at the bottom side of the sheet. In the horizontal flame spread, the gas in the leading flame edge at the bottom surface flowed in the direction of the flame spread. The flow fields at leading edge were important in interpretation the mass and the heat transfers controlling the flame spread. The forward flow increased the heat transfer to the unburned area and increased the spread rate. Williams (1976) – Mechanism of Fire Spread Williams proposed a thermal theory of flame spread whereby the thermal runway condition of a gas phase reaction of PMMA vapor gives a criteria finding out the flame spreading speed. Mechanism involved in fire spread are described in a way that sacrifices accuracy for the reason of emphasizing general aspects of heat transfer, fluid flow, and chemical-kinetic phenomena. The heat balance across a surface of fire inception is represented by the following equation q = ρ s U f ∆h where q is the energy transferred across the separation line, S is the fuel density; and ∆h is the thermal enthalpy difference between the fuel at ignition and ambient temperature. The mechanism that controls the spread of flame differs with the surrounding conditions like the concentration of oxygen or the speed of the wind in relative to the flame direction. The rate at which the flame spread depends on the heat transfer from the flame into the unburned area. Spread of fire over PMMA Orloff, L., 1977 - Burning of large scale vertical Although Sibulkin and Hansen made a preliminary of studying flame spread over a small thermoplastic wall, Orlof et al made more comprehensive study of turbulent spread over a thick polymethyl methacrylate (PMMA) vertical wall with the height of 157 cm. To ensure true 2-dimension spread, they used cooled side walls. The velocity of fuel pyrolysis front increased from 0.05 cm/s to 0.6 cm/s. The flame height increased continuously over the pyrolysis front height. Using the measured flame height, measurements of the radiative flame heat transfer and transient temperature properties of the PMMA, the rate of flame spread were calculated, which agreed with the measured values. They concluded that the spread rate is influenced by radiative heat transfer from the turbulent flame to the wall above the pyrolysis. LIFT (lateral ignition and flame transport) related Quintire tried to come up with a simplified theory that will generalize all the results from the radiant panel rate from the flame spread apparatus. The test method was formulated by Robertson in 1979 and Quintire tried to analyze the flame results using mathematical models he developed. The model was created to allow transient flame spread that has external radiant heating and it was designed in a way that follows rockets (1974) analysis on vertical downward spread. The work by Quintire is not the same as the ones carried out by Williams and de Ris since it was also based on fire parameters like the rate of flame transfer and length. He used a simple theoretical model in analyzing the experimental results on the rate of lateral flame spread and the time of piloted ignition in an externally imposed flux. He managed to show that the rate of flame spread is given by Vf-1/2 =C (qo,ig-qe) Where C- is material constant qo,ig – is the minimum heat flux needed for piloted ignition A research conducted by Harkleroad and his team has analytical approach including parameters and solutions resulting from transient heat conditions in semi solid materials. The data used in the experiment was generated using apparatus designed by Robertson. The rate at which the flame spread and ignition events are measured against incident radiation and time of exposure. The materials used in the test include PMMA and aircraft materials. In 1999, Delichatsios tried to interpret the measurements and results obtained from the LIFT experiment. The result from the research shows that a parameter deduced from the obtainable protocol wasn’t a material property but it could be affected by external heat flux applied during the test. Opposed Turbulent Flow Flame Spread Studies on the opposed flow flame spread caused opposed gas flow, normally due to convection induced by external force like ventilation and wind, can be put in two mechanisms: heat transfer from the flame to fuel and the gas chemical reaction. Eichorn, R., et al studied Damkohler number which describes the kinetic effect of the gas and the rate of a non-dimensional flame, which describes the process of heat transfer. Damkohler number is the ratio of time required for fuel particle to travel the preheated gas region to the time needed for fuel to react with the oxidizer. The non-dimensional flame spread rate is the ratio of heat flow in the solid ahead of the flame to the heat transferred from the flame to the fuel. The main modes heat transfer is conduction, radiation and convection. Conduction takes place in solids and gases but convection is through the gas only. Radiation heat transfer occurs in all direction in direct and indirect path. The three mechanisms are all involved in the flame spread process concurrently, therefore, by determining the dominant mode of the heat transfer a relative simple and complete analysis of the process can be developed. Wichman, et al, 1982 and Kashiwagi, 1986: Flame spread in an opposed flow and downward flame spread in air Wichman, et al, 1982 and Kashiwagi, 1986 studied the relative significance of each heat transfer modes in the flame spread process. They found out that the temperature distributions in solid and gas phase measured for different conditions and the heat flow by each mechanism are obtained by balancing energy in the solid and gas. They concluded that the thickness of sample sheet affect the dominant heat transfer mode. Sheets with thickness of less than 2 mm, heat transfer are dominated by the heat conduction through the gas, while sheets thicker than 20 mm are dominated by the heat conduction through the solid. The radiation contributes to the overall heat flow, but for small fires, the radiation between the flame and the fuel is not significant because of the small view factor between the flame and the fuel. Fernandez-Pello, et al., 1978: Fernandez-Pello, et al urged that the leading flame edge in opposed flow situation is considered to be in a near-extinction due to low fuel concentration and low temperature as a result of convection effect of the opposing gas flow. Under such conditions, the chemical reactions are extremely sensitive to the external parameters, which lead to the conclusion that the flame leading edge region is controlled both by the chemical kinetics and heat transfer. Concurrent Turbulent Flow Flame Spread This is where the oxidizer gas flow in the same direction as the flame and therefore aiding further spread of the flame. Because of its potential harzard nature, the concurrent flow flame spread controlling mechanism has been studied using various experimental analyses. Early studies focused on the upward wall fire problem, which involves flame spreading over a vertically surface. Kroesser, F. et al., 1970 - Laminar free-convective burning of fuel surfaces Kroesser, F. et al. conducted an experiment on small scale fires, and observed that the characteristics of typical laminar flow and the experimental results agree each. They found out that the rate of upward laminar flame spread decrease with the quarter power of the height. The pyrolysis height and flame increase with the square of time. The laminar flame spread, which transition to turbulent flows, applies to flame of over 100 mm high. The main controlling factor is the transfer of heat by convection through the gas, while the radiation is ignored due to small size of the samples. Markstein, G. H. and De Ris, J. 1973: Upward fire spread over textiles Markstein and De Ris did large scale experiments of flame spread over a vertical wall with a thick PMMA sheet and cotton fabric. They found out that the heat transfer from flame to the solid surface is the main factor in the flame process. As the generated flow develops to a turbulent from laminar flame, the dimension also increases. They observed that in the turbulent region, thermal radiation is dominant in the transfer of heat from flame to the solid surface, but the convection transfer most of the energy in the laminar region. The power law of the flame spread used in cotton textile obtained from the experimental data is in the form of Vp = βIpn, where Vp is the pyrolysis rate, Ip is pyrolysis length, n and β are constants within 0 Read More