Critical Conditions of Thermal Explosion, and Pre-Explosion Heating - Assignment Example

T1. Analyse thermal explosion in a vessel with cold walls, including the mechanism of self-accelerating reaction and nature of induction period? Thermal explosion in a vessel with cold walls has been analysed by researchers in the past. This includes the overall mechanism of self-accelerating reaction and nature of induction period. This occurs in a situation where there are diversities in kinetic and thermal properties. Thermal explosion is affected by various factors such as temperatures and even shape of affected instrument. (Beyler, 1986) The rate of heat exchange in a vessel with cold walls is usually quite high therefore leading to thermal explosion. The mechanism of self accelerating reaction in thermal explosion in such a case is influenced by a thermal regime that has a quasi stationery nature. The autocalyticity number in this case does not have a reliable effect. It is known to have effect only in the initial stages of a reaction. The temperatures in the reaction zone play an imperative role towards accelerating the thermal reaction in a vessel with cold walls. The nature of induction period in such a vessel highly depends on the heat release rate as compared to the overall rate which the heat is removed from the vessel’s environment. T2. Using Semenov diagrams, explain the effect of heat transfer on thermal heat balance in a vessel with cold walls, critical conditions of thermal explosion, and pre-explosion heating. Source: http://chem.kstu.ru or http://confportal.kstu.ru From the above diagram, the effect of thermal heat transfer on thermal heat balance in vessel with cold walls is quite clear. A very fast thermal heat transfer across the walls of cold vessel leads to thermal explosion. Critical conditions for thermal explosion in this case include reactions where there are high activation energies and thermal effects that are quite large. In situations where there is liberation of very little heat to the environment from the walls of cold vessel, thermal explosion definitely occurs. Two reactions that have diverse thermal effects and impurities in a vessel result in thermal explosion. Critical conditions for pre-explosion heating include heat, kinetic energy and high abundance of electrons. (Fleischmann and Parkes, 1997) T3. Define explosion, deflagration and detonation. Explain the development and main features of detonation. An explosion is a situation where there is overall release of gases, energy, and high temperatures after their rapid increase. This is usually accompanied by loud voice and creates shock wave. There are usually low and high explosives depending on various influential factors. Deflagration refers to a case where cold material is heated and ignited by hot burning material. This happens when thermal conductivity takes place by having a combustion that is subsonic in nature. (Peatross and Beyler, 1997) Detonation occurs in gases, liquid and solid explosives where exothermic front is released through a medium resulting in a shock front that eventually promulgates in front of the medium. There are various features of detonation. This includes their occurrence in systems that are highly confined in nature. In detonations, there is usually a lot of pressure when compared to that in deflagrations. Detonations are highly associated with dust suspensions, gases and even droplet fogs. In this case the gases consist of mixtures that are below limits of flammability. That is mixture of oxidant of composition and fuel. Detonation therefore develops when exothermic front is released through a medium like gas, solid and liquid explosives. T4. Review the main features of diffusion combustion. Explain the types of diffusion flames (momentum and buoyancy dominated fires). Compare the Froude number for these types of fire. Diffusion combustion occurs where fuel coalesces with an oxidiser through diffusion process. In this case, the combustion is highly affected by the rate of diffusion. Diffusion combustion is quite slow as compared to other types of combustion. It also produces a lot of soot since less oxygen or oxidiser leads to incompletion of the burning process. The flames released in this combustion are yellow in colour and they are less localised. There are various types of diffusion flames. These are defined by the type of fires that cause the resultant flames. In this case, there are in buoyancy and momentum dominated fires. Diffusion flames from buoyancy dominated fires are quite buoyant in nature hence buoyant diffusion flames. In this flame, the fuel leaves surface with extensive velocity. It has a jet like behaviour. Momentum diffusion flames burn at very high momentum. Froude number in buoyancy fires is usually higher than momentum diffusion fires. (Peatross and Beyler, 1997) T5. Analyse the burning rate of solids and mechanism of flame spread over solid surfaces. The burning rate in solids is normally the rate at which the entire surface recedes during combustion process. It has three phases; preheating, distillation and solid phase. Pre-heating is when solid is heated to flash point. Distillation is when oxygen mixes with evolved gases to produce some light and heat. This leads to flames. The last phase entails smouldering. (Rangwala, Buckley and Torero, 2007) The burning rate of solids is affected by various factors such as scrubbing of hot gases, the temperature, pressure and type of material under combustion. The mechanism of flame spread over solid surfaces consists of diverse processes. These include radiation, conduction and convection. The surface goes through radiation spread that enhances the flame to spread faster. The mechanism of flame spread across solid material is also enhanced by increase in oxygen mass fraction. (Chen and Sun, 2006) T6. Analyse emissivity of opaque surface and grey body, and determine the total black body emissive power and total grey body emissive power at a given temperature. Emissivity relates to the overall ability of a material to emit heat. It is used to measure a material's ability to radiate absorbed energy. The emissivity of opaque surface is affected by specular reflectance. In this case, when reciprocity relationship is adhered to, an incident ray on opaque surface results in emissivity. Surface radiation of opaque surface is absorbed. There is selective absorption in situation where visible light strikes an opaque surface. When an object absorbs all radiation that is electromagnetic in nature that falls on it, it is referred to as a black body. In black bodies, there is absorption and re-emmition of radiation. (Tewarson, 1995) Infrared wavelengths are emitted by black bodies when at room temperature. When temperatures exceed hundred degrees Celsius, they start to emit wavelengths that are highly visible. They tend to appear, orange and red. Then the emmition turn yellow, white and finally blue as temperatures increase. At room temperature, a grey body never absorbs all incident radiation. A grey body has less emissive power at room temperature. T7. Analyse the radiating gases produced in combustion, define the mean beam length and explain the use of emissivity charts. There are various radiating gases that are produced in combustion. There is complete and incomplete combustion and in each case different radiating gases are produced. Complete combustion leads to release of nitrogen dioxide, carbon dioxide and sulphur dioxide. This is where oxygen is the oxidising agent. Carbon monoxide is commonly released when the combustion is incomplete. Mean beam length is where an inhomogeneous medium that is three dimensional in nature has radiative exchanges that are described as volume to volume, surface to surface and volume to surface. Emissivity charts are very essential in showing ability of diverse materials to release heat. The charts are used when making pyrometers. They are also useful to engineers who constantly check out the heat released by various objects that they deal with. Temperatures of ventilation systems are checked by sailors and other professionals using the concept of emissivity charts. They are commonly used by radiologists in their professional training. (Babrauskas and Grayson, 1992) T8. Critically analyse the effect of ventilation on the composition of smoke using equivalence ratio. Compare over ventilated and under ventilated combustion. Ventilation has various effects on composition of smoke. Typically, a room that is highly ventilated allows adequate amounts of combustion. This results in the produced smoke having carbon dioxide in situation where carbon was the main material. Less ventilation means less oxygen hence smoke consists of carbon monoxide which is quite harmful to human health. Ventilated combustion is complete and does not release harmful gases as compared to under-ventilated combustion that is incomplete and leads to production of toxic gases. (Smith, 2010) Equivalence ratio of this correlation is shown below Fuel to oxidizer ratio =M (fuel)/M (ox) = N (fuel)/N (ox) (Fuel to oxidiser ratio) st M (fuel)/M (ox) st N (fuel)/N (ox) N is the moles, M is the mass and st is stoichiometric conditions. (Peatross and Beyler, 1997) T9. Analyse smoke optical properties: absorption, scattering and extinction of light. Examine the role of extensive (extinction coefficient, optical density per meter, optical density per meter) and specific characteristics (mass extinction coefficient, mass optical density per meter, smoke potential) and their relationships. Smoke optical properties include absorption, scattering and extinction of light. Scattering occurs where molecules in the smoke go through diverse fluctuations. Scattering is normally accompanied by absorption. This is where energy that is not useful is converted to another form. When energy is removed from smoke, it results in light extinction. Extinction coefficient is used to measure how the article absorbs light in a specific wavelength. Optical density per meter is used to estimate overall visibility. Mass extinction coefficient shows how substance has overall absorption of light at specific wavelength per molar concentration and per certain mass unit. Smoke potential is overall potential of burning object to release smoke. All the above factors have direct relationship on smoke potential. (Peatross and Beyler, 1997) T. 10 Characterise the main sources of heat release and heat loss in a typical compartment fire. Analyse the difference between burning rates in open space and in a compartment. Heat release in compartment fire plays a great role to its full development. The main channel through which heat is released in compartment fire is ventilation. The more ventilated the compartment, the more heat is released. The main sources of heat loss in a typical compartment fire include the door, the walls, floor and through the ceiling. The rate at which a material if consumed by fire is called burning rate. (Smith, 2010) There are various key issues in burning rate; they include the overall loss of mass rate and the rate at which heat is released. The difference between burning rates in open space and in compartment can be traced in the two factors. The heat release rate in an open fire is quite high therefore leading to high burning rate. In compartment, there is minimal heat release rate hence slow burning rate. In open space, the existence of oxygen that tends to accelerate the burning rate as compared to compartment fire where there is minimal oxygen hence low burning rate. (Smith, 2010) Read More