The Effect of Heat Transfer on Thermal Heat Balance in a Vessel with Cold Walls - Assignment Example

Title: Fluid Dynamics Dated: July 26, 2010 T1. Analyse thermal explosion in a vessel with cold walls, including the mechanism of self-accelerating reaction and nature of induction period? Answer: Thermal explosion is rapid increase in the volume and release of energy and heat in an extreme manner usually associated with the generation of high temperature. Gases are also released during the explosion into the surroundings with adverse effects on the system and on the rate of the reaction. Thermal stresses are the major concerns in the reactor systems due to the magnitude of the stresses and the external conditions of the walls of the vessels. Cold walls of the vessels provide a medium for the release of heat from the system into the surroundings during the explosions and finally release of heat takes place and therefore lowering of the temperature. Cold walls of the vessels develop a system for the explosions that create an uneven flow of heat in various parts of the vessels, heat travels from high temperature zone to the lower temperature. Self accelerating reaction helps in the maintenance of reactions in a state of equilibrium to influence the process and the reaction progress for any surrounding temperature profile like isothermal, periodic temperature variations, stepwise flow of heat and energy and temperature shocks. The nature of induction period plays a significant role in the momentum and occurrence of reactions during the state of explosions. Higher the threshold levels for the initiations of the reactions with the availability of fuel, oxygen and heat, grater are the discharge of heat from the system into the surroundings and vice versa. Occurrence of explosions in the natural systems and in the vessels observes the fundamental principles as are relating to thermodynamics for the sustainability and continuity of the thermal reactions with the exchange of heat. The external media like cold walls determine the pace of chemical reactions and the mechanisms of self accelerating reactions and the nature of the induction period. 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. Answer: Semenov developed diagrams to explain the effect of heat transfer on thermal heat balance in a vessel with cold walls for the explanation of thermal explosions. Cold walls provide a surrounding for the heat explosions with varied degrees for the flow of heat from the systems into the surroundings. There are three important conditions for the occurrence of explosions as Semenov has explained with the help of diagrams. These are the temperature T, volume of the vessel and the thickness of the vessel walls. Critical conditions as required for the completion of reactions are the availability of fuel, oxygen and heat as ignition agent. Thermodynamics of the heat reaction depends on the heat gain (W), density in terms of kg/m3, exothermicity in the shape of (J/kg), heat capacity of the overall system as in the shape of J/kg K, pre-exponential factor, activation energy as J/mol. Pre-explosion heating as explained by Semenov with the help of diagrams has helped in the calculation of heat lost as “W”, convection coefficient, and surface of the reacting system. Cold walls of the system provide a platform for the flow of heat during the explosions. Three different scenarios like the heat production is less than the heat lost, heat production is the same as the heat lost during the explosions and finally the heat production is greater than that of the heat lost. During the condition that is the heat production is less than that of the heat lost, the reactants enter into the system at a low temperature and in this position the heat production curve lies above the heat lost curve for reaching a stable temperature for the overall system. In the second condition that is the heat production is the same as that of the heat lost, all the three temperatures that is stable temperature, critical temperature and ignition temperature are the same. In the final stage that is production is greater than the heat lost, the heat lost flux is always on the higher side than that of the heat gain flux and whatever may be the temperature of the reactants at the time of the initiation stage, a thermal explosion will definitely take place. T3. Define explosion, deflagration and detonation. Explain the development and main features of detonation. Answer: An explosion is defined as a rapid increase in the volume ad release of energy in extreme manner along with the generation of high temperature and release of gases. A thermal explosion creates a shock wave within the system and in the surroundings. A nuclear weapon is a kind of explosive weapon which drives its destructive force from the nuclear reactions like fusion or fission or a combination of both of these reactions. Deflagration is a kind if technical term which describes subsonic combustion that usually proceeds through thermal conductivity. The process involves hard burning material heats along with the next layer of cold material which ignites. Most of the fire as found in daily life which observe flames to explosion is an example of the deflagration. Deflagration differs from donation which is a kind of supersonic that propagates through shock compression. It has also been observed that deflagrations are easier to control than that of the detonations. Detonation involves an exothermic reaction with the release of heat in an accelerated form within the medium that drives a shock front as in the shape of a set of reactions. These set of reactions propagate directly in a direct format like solid and liquid explosives. The development of the flames in the detonations observes both conventional mechanisms and non-conventional procedures. The velocity of detonations in liquid as well as solid explosives is much greater than that in the gaseous materials. Exothermic waves are subsonic in detonation with maximum pressure to the vessels as in the form of surroundings of the explosive systems. 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. Answer: Combustion which involves the combination of oxidizers and fuel in diffusion shape is called as diffusion combustion. Speed of the flame is limited by the rate of diffusion which is further determined by the quantity and quality of the fuel and the type of the oxidizer used for formation of the flames in the diffusion combustion. The soot produced by the diffusion flames is greater than that of the premixed flames with same quantity of reactants in the production process. There are three major types of the flames as based on the momentum and buoyancy dominated flames as premixed flames which are further divided into laminar and turbulent flames, non- premixed diffusion flames and partially premixed flames. Froude number of these flames is based on the types of the reactants with right amount of oxidizers to consume the fuel for the completion of the process. Froude number is greater for the partially premixed flames than that for the non-premixed flames and similarly the Froude number is the lowest for the premixed flames. Fuel rich system which has an excess quantity of fuel has greater Froude number than that of a system with an excess quantity of oxygen in a fuel lean system. Standard air composition is applied for the calculation of the combustion of the fuels and the formation of the flames as a product of the combustion process. Excess in any shape like an excess in the air ratio, excess oxygen and similar other terminologies are applied for the determination of Froude numbers in the diffusion flames and their formation. T5. Analyse the burning rate of solids and mechanism of flame spread over solid surfaces. Answer: The burning rate of solids is the measure of the linear combustion of a substance or compound like solid bodies. The burn rate is calculated in terms of length over time as in the form of "inches/second" or "mm/second". The rate is the property of the combustible substances which determines the combustion rate of the solid substances and the rate can be measured at different pressures for specific results. The spread of flames of the solid burning substances depends on the variable factors like temperature, pressure and wind velocity and is calculated with the help of the pressure chart as in relation to the temperature chart. The most common method used for the measurement of the flame for the solid substances is the Strand Burner or Crawford Burner. The length of flame and the burn rate determines the type of the flame and the combustion nature like neither detonates nor deflagrates on the basis of distance travelled during the burning of the solid substances along with the production of the flames. 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. Answer: The emissivity that is ε or e is the relative ability of the body and its surface to emit energy in the shape of radiation. It is also expressed as the ratio of energy as radiated by a specific material to the energy as radiated by a black body at the same given temperature. It is also the measure of the material's ability to radiate the absorbed energy at a given temperature and time. The emissivity of an opaque surface is greater than that of the grey body at constant temperature. The emissive power of a true black body would always be equal to that of 1 and similarly the emissive power of a grey is less than 1 at the given temperature so it could not be expressed in terms of units. The reflective position of the body has an inverse relationship with that of its emissivity. T7. Analyse the radiating gases produced in combustion, define the mean beam length and explain the use of emissivity charts. Answer: The combustion process leads to the burning of the substances with the assistance of heat as the igniting agent, oxygen as the oxidizing agent and the release of heat. Oxygen has the ability to burn with flames and in order to obtain a flame from the liquid or solid, the process involves oxygen as in the gasified form. Fire emerges as in the luminous shape and the radiating gases release heat to the surroundings of the system. Mean beam length is the measure of the light beam as in the shape of narrow projection of light as the energy radiating form the source and into the beam as in terms of mean of the beam length. The emissivity charts help in the calculation of the beam length of the substances and the level of the radiation as is released during the combustion process. Many lightening devices use artificially produced light beam as like lamp or parabolic reflector for the measurement of beam length of these bodies. T8. Critically analyse the effect of ventilation on the composition of smoke using equivalence ratio. Compare over ventilated and under ventilated combustion. Answer: Ventilation plays a significant role in the process of combustion and on the thickness of the smoke as the product of the combustion process. Ventilation provides necessary oxygen for the completion of the combustion process. Ventilation lowers the thickness of the smoke in the system and helps in the spread of smoke into the surroundings of the system with net positive impacts on the overall combustion process at a given temperature and pressure provided all other reactants are available in adequate supply. 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. Answer: The optical properties of smoke like absorption, scattering and extinction of light are the result of type of the combustion material and the burning conditions of the whole process. The absorption of smoke increases with the dilution of the smoke with the external conditions for the combustion process and the heat level as in its threshold level. The scattering of smoke depends upon the intensity of the combustion process along with ventilation as available for the process. The extinction of light and its relationship with the smoke determines the pace and direction of the smoke as a product of the combustion process. The role of extensive properties like extinction coefficient, optical density per meter, optical density per meter are essential for the maintenance of the specific characteristics of smoke. The properties of smoke for example mass extinction coefficient, mass optical density per meter, smoke potential are having their relationships with the overall process of combustion and smoke formation along with its direction in the system and the surrounding. T10. 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. Answer: The major source of heat release in the surrounding of the combustion process is media as is associated with the system. Heat releases into surrounding as in the shape of radiation from higher temperature zone to the lower temperature area with an objective to establish equilibrium with minimum temperature differences among different components of the system. In a typical compartment fire, the heat releases with a target to establish a minimum gradient of heat difference within the system. Burning rates in open space are higher than that of in a compartment as the rate is influenced by the availability of the oxidizing agents are high in the open space than that of the compartment. Smoke emission, heat release and release of other gasses from the burning process are all on the higher side in the open space than that of compartment. However, controlled compartments with adequate heat and gaseous arrangement if provided are the encouraging signs for the burning process in the compartments as compared to that of the open spaces for burning process and the burning rates for the fuels to burn. References: 1. A. A. Putnam and W. C. Dennis (1953) "Organ-pipe oscillations in a flame-filled tube," Fourth Symposium (International) on Combustion, the Combustion Institute, pages 566-574. 2. E. C. Fernandes and M. V. Heitor, “Unsteady flames and the Rayleigh criterion” in F. Culick, M. V. Heitor, and J. H. Whitelaw, ed.s, Unsteady Combustion (Dordrecht, the Netherlands: Kluwer Academic Publishers, 1996), page 4. 3. Omaye ST. (2002). "Metabolic modulation of carbon monoxide toxicity". Toxicology 180 (2): 139–150. doi:10.1016/S0300-483X(02)00387-6. PMID 12324190. Read More