Paper Example on Fire Characteristics: Gaseous Combustibles

Date:  2021-05-26 22:35:24
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Carnegie Mellon University
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Research paper
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When one wants to prevent unwanted gas explosions or fires disasters, he or she requires having knowledge of the characteristics of flames. The characteristics include the burning rates, ignition requirements, and limits of flammability. Gaseous combustibles include butane, natural gas, hydrogen, methane, and propane. They are the gasses that often burn when ignited and mixed with oxygen. The combustible gasses are in the market as byproducts of a company or after being leaked unwillingly into the air. The research paper has focused on types of gaseous flames, flammability limits, ignition of gasses, specific hazardous gasses, burning rates and limits of the gasses in the air.

Types of gaseous flames

There are various uses of gaseous flames such as used by welders, industries and at home for cooking. The major types of gaseous flames include premixed and laminar diffusion flames (Gann and Raymond, p.96). Pre-mixed flames occur when inert gasses, oxidants, and fuel mixtures mix before combustion commences on the molecular scale. Such flames include those found in gasoline-fueled combustion chambers, Bunsen flames and gas appliance stoves (Law, p.409). The structure of the flames has four regions namely the preheat, unburned, reaction and burned gas (Poinsot, p. 47). The unburned mixture goes to the zone of the flame, and when it reaches the flame, it is heated by radiation and conduction upstream (Law, p. 613). A chemical reaction later takes place when the temperatures become hot enough to support ignition in the reaction zone. The gasses later go into the burned gas zone where the temperatures and the concentration become constant again.

Diffusion flames occur when oxidizers and fuel enter the chamber of combustion separately. The mixture of the fuel and air is formed by diffusion and convection. At the position where the flame sits and ignition may happen, molecular diffusion occurs. The flame then advances downwards causing an increase of the mixing layer and the product is the formation of a uniform mixture (Law, p.270). The diffusion rate of the mixture is responsible for supporting ignition, and no oxygen is used in such flames. Fuel breaks down or pyrolysis of smaller radical and molecules occurs forming soot which produces a distinctive luminous flame. More oxygen is introduced into the flame zone allowing the pyrolysis products to react and ensure they pass through the stoichiometric level thus completing the reaction ((Gann and Raymond, p.134). The diffusion flames are thicker than the premixed flames, and their rate of combustion is restricted to the mixtures mass diffusion unlike in premixed flames where combustion in a chemical reaction (Law, p. 362).

There are different types of flames from gasses used in welding as illustrated in Figure 1. Neutral flame consists of oxygen and acetylene in equal portions. When one needs to obtain a neutral flame, he or she begins the combustion process with excessive acetylene making the inner core have a feather extension (Pease, p.448). Later he or she should increase the flow of oxygen or decrease the acetylene levels to make the feather disappear thus forming a neutral flame. It can be achieved by slowly opening the oxygen valve to create a visible and clear inner core, which decreases the acetylene flame. During combustion, the neutral flame attains more oxygen from the air to complete the combustion process. The gaseous flame is mainly used for welding cast iron, aluminum, mild steel and stainless steel and it consists of a luminous cone. The inner zone of the flame is bluish-white. The inner cores temperature is 3232oC while the envelope regions temperature is around 1260oC.

Carburizing flame has acetylene similarly to the neutral flame. It has an acetylene feather which is twice as big as the inner core. Oxygen concentration for the flame is lower than acetylene, and it requires a person to add carbon to the metal being welded. To create the flame, one forms a neutral flame and later opens the acetylene valve slowly and the flame formed is determined by the extent of the carburization of the flame. The flames temperature is 3149oC, and it causes the metal to melt. It occurs when metals absorb the carbon in the flame and emit it leaving the heat in the metal and boils it.The flame is common in high carbon steel welding and nonferrous alloys such as Monel and nickel. The outer and intermediate flames are used in soft solder and silver solder operations.

The oxidizing flame is produced when the concentration of oxygen is higher than the volume of nacetylee The first step is to make a neutral flame then the oxygen flow is increased to make the inner core shorter(Urben, p. 293). The oxidizing flame turns purple when properly adjusted and is distinct from the others due to its hissing sound. The flames temperature is 3482oC. The flame can melt metal into foam as well as make it spark showing that there is excess oxygen mixing with the metal which make it burn.

Home cooking gas contains propane. The gas needs a volume of 24:1 on air-to-gas Volume ratio. The ratio clearly outlines that very high amounts of oxygen are required to use the gas. There are two flames which include the blue and the yellow. When there is adequate oxygen, there is complete combustion hence a blue flame appears (Law, p. 695). However, lack of enough oxygen leads to the yellow flame, which is full of carbon monoxide emissions.

Ignition

Ignition of gasses is the use of a flame or spark. The process of gas ignition is piloted ignition, and it involves the following process. There has to be a formation of highly reactive species that destabilize a fuel molecule. A piloted ignition occurs where it is obtained from a flame or spark. Auto-ignition later occurs (Urben, p.50). It consists of thermal ignition and non-piloted ignition. The process involves heating a fuel without the spark or flame. The next step is the ionization of the molecules in the gap. It occurs when a spark skips a gap between an anode and cathode thus ionizing the molecules within the gap. Free electrons, positive ions and parts of fuel molecules are created from the ionization step (Gaydon, p.248). The third step involves the a different system from the thermal equilibrium. The minimum energy for electrical ignition is different to the surroundings temperature, and it is not sensitive to the temperature of the environment. It, therefore, requires one to judge it depending on the gaseous fuel being used, spark energy, volume ratio of the air and fuel mixture as well as focusing on a stoichiometric mixture of combustible air and gas unlike focusing on the lean mixture. The next step involves identifying the different energies of spark ignition in Stoichiometric mixtures. It is important to consider that the advanced unsaturation of a hydrocarbon molecule makes it easy to combust (Ma, p.7). All hydrocarbons that are saturated, combust in the air while mixtures of oxygen and fuel ignite sparks easily than those with air and fuel.

Additionally, the least amount of ignition energy required to combust air and fuel gas is inversely proportional the absolute pressures square. The introduction of free radicals in an existing flame can enable gas mixtures to be lit. Gas and air mixtures can be combusted when they contact a hot surface thus raising the temperature of the mixture. Ethane has been found to be harder to combust than acetylene while carbon disulfide can be easily ignited than hydrogen through thermal ignition.

Most of the flammable mixtures can be combusted through a small content of energy with a huge density of power that is more than 1MW/cm3. The ignitions total energy requirements may, however, be exceedingly large when the source energy is diffuse. A reaction is started when a flammable mixture is inflamed to a high temperature that could continue with the adequate promptness of the ignition of the mixture. The time lag is the time elapsing between the time appears and the temperature of the mixture rises. It decreases when temperatures increase during ignition (Mannan and Frank, p.2431). The types of data on ignition temperature include the impact of temperature on time lag can be considered for delays below 1 second which apply to systems whose time of contact between the flowing mixture and the heated surface is extremely short making them unsatisfactory when the time of contact is indefinite. The minimum temperature, spontaneous ignition, or auto ignition is the least temperature needed for ignition to occur. One can determine it in an apparatus that is adequately big to reduce any effects from wall quenching.

Limits of flammability

A combustible mixture of gas and air can be burned with a diverse range of concentrations whether it is subjected to a catalytic surface at ordinary temperatures or elevated temperatures. Homogenous combustible mixtures of air and gas are however flammable because they can propagate flames freely within a restricted range of compositions (Beyler, p.540). For instance, small amounts of methane present in the air can be oxidized readily on a surface that is heated. The flame will, however, disseminate from an ignition source at ambient pressures and temperatures if the mixture around it comprises not less than five and not more than fifteen volume percent methane (Law, p. 363). The most concentrated mixtures in the case are referred as the combustible-rich limit or upper limits while those that are the most dilute are known as the combustible lean limit or lower limits.

Experiments can be used to determine the limiting mixture compositions between non-flammable and flammable mixtures. The following formulas are used in the experiments

LT,P =1/2(Cgn + C1f),

UT,P= 1/2(Cgf + C1n)

Where LT, P is the lower limit of flammability while UT, P is the upper limit of flammability at particular pressure and temperature. Cgn represents the highest concentrations of fuel in non-flammable oxidants while C1n represents the least concentrations of fuel in oxidants that are non-flammable. C1f represents the least concentrations of fuel inflammable oxidants while Cgf represents the greatest concentrations of fuel in oxidants that are flammable. There are factors that affect the rate that the flame disseminates through a flammable mixture and they include the pressure, temperature, and the composition of the mixture. The rate is at the highest level when it is near the stoichiometric mixtures, and it is the least when it is at the limits of flammability.

The Bureau of Mines apparatus was first used for room temperature and atmospheric pressure measurements. Later, it was modified to analyze reduced pressures by to be used for reduced pressures by integrating a spark gap igniter. The improvement, however, had a limitation because there was no adequate spark energy to be used in the determination of limit of flammability. Limits of flammability can be referred to limit mixtures that are self-regulating of the strength of ignition source and offer a portion of the flames ability to disperse from the source of ignition.

There is a need for more energy to establish an upper limit than for a lower limit. Therefore, when the strength of the source is sufficient, flame caps are yielded when one ignited the mixture just outside the range of flammable makeups (Hurley et al., p.530). The lower limit decreases as the temperature increases hence the flame caps in a uniform mixture only disseminate a short distance from the source of ignition. When temperatures are elevated adequately, a non-flammable mixture could, therefore, be lit for some time. Practically, when determinations of flammability limit and flame propagation are made in small tubes, complications may arise because heat is transferred through conduction, radiation, and con...

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