Questions and Answers: Engineering Design - Assignment Example

Questions & Answers 1. Give a definition of combustion. In layman’s terms, combustion is defined as the burning of fuel to produce heat and light. In scientific terms, it is a series of chemical reactions between a fuel and an oxidant which produces heat and light. The compound that is burnt reacts with an oxidizing agent and the products formed after the reaction are the compounds of elements in the fuel with the oxidizing element. 2. Fire safety in buildings is concerned with achieving two fundamental objectives, name them. Fire safety in buildings is concerned with achieving two fundamental objectives: To reduce the loss of life in, or in the neighborhood of, building fires. To reduce the property or financial loss in, or in the neighborhood of, building fires. 3. What are the functional requirements of Approved Document B? B1 ‘Means of warning and escape’ B2 ‘internal fire spread (lining)’ B3 ‘internal fire spread (structure)’ B4 ‘external fire spread’ B5 ‘access and facilities for the Fire service’ 4. Give a definition for the term “means of escape from fire” Means of escape in case of fire means the provision of a safe route(s) from any part of a building to a final exit or place of safety, enabling the person to escape from fire or smoke by his / her own unaided efforts 5. What information do you need to know when design means of escape? Following information is necessary when designing means of escape: Time of evacuation Travel distances No. of occupants Calculation of exit widths Calculation of minimum no. of exits 6. Why is it important to perform fire tests? Fire tests are important as these are important to determine whther or not fire protection products meet minimum performance criteria. 7. Why is it important to carry out more than one experiment to test the same parameter? It is important to carry out more than one experiment to test the same parameter, so as to ensure safety rules and regulations, which if miscalculated can yield huge losses of lives and property. 8. Explain the term Trench Effect. Trench effect came into light in 1987, in the famous but unfortunate event of King’s Cross Fire in London. It is the process, in which various circumstances act in tandem or combination, whose combined effect is climbing up of fire on an inclined surface. Various reasons have been provided for as explanation of Trench effect. Most accepted one is that it occurs because of combination of Coanda effect and Flashover effect. Coanda effect- The term is after Romanian scientist Henri Coanda, who propagated Coanda effect as “the tendency of fluid jet to stay attached to curved surface. Here a fast moving stream of air experiences a sudden fall in pressure. Hence pressure gradient is created between areas near the wall and far from the wall. Consequence of this phenomenon is that fast moving stream is bent towards the surface and sticks to it. Flashover effect- When a space or surface is heated to a point that the gases which the various areas of the surface emit themselves get auto ignited after reaching their burning point, whole of the surface starts burning. Prior to Flashover, gases which are inflammable are emitted but the temperature is not high enough to make gases auto ignite. Hence, when flames get attached to inclined surfaces on account of Coanda effect, they make surface emit inflammable gases which themselves get ignited once temperature reaches their ignition point. 9. Define the terms Upper and Lower Flammability limits. Flammability limits are also known as Explosive limits or Flammable limits. These are proportions of various combustible gases in a certain mixture, between whose limits mixture is flammable .Now mixtures which have constituents such as combustible gas, oxidizing and inert gas are flammable under certain conditions. Lower Flammability limit- It is leanest mixture which is still flammable that is mixture with smallest fraction or proportion of combustible gas. For example, LFL for Hydrogen fuel is 4%. This means in air above Hydrogen percentage of 4%, combustion is possible, if ignited. But below this limit, no combustion will take place even if ignition is also provided. Upper Flammable limit- It is the richest flammable mixture. For example, UFL for same fuel Hydrogen is 75 %. This means mixtures with up to 75% of Hydrogen are combustible when ignited, but above this limit, it is not possible to ignite them even when ignition source is present. Increasing the proportion of Inert gas in the mixture lowers the combustible gas percentage; hence Lower Flammability limit is raised. Similarly Upper Flammability limit is lowered on addition of inert gas. To calculate these limits of various combustible gases, following formula is used, which was given by Le-Chatelier. Flammability limits are influenced by Temperature as well as Pressure. High temperature lowers Lower Flammability limit and raises Upper Flammability limit. Pressure increases both limits. 10. There are five main factors which will affect the development of fire growth within a compartment. They can be divided into 2 categories: those that are concerned with the compartment itself and those are concerned with the fuel. Please name 4 of these factors. The 2 factors concerned with the compartment are: Available area over which combustion can occur Flashpoint of gases emitted Factors specific to fuel are as follows: Flammability of the fuel, measured as LOI Lower and Upper Flammability Limits 11. Discuss and explain stages of fire development. There are 4 stages of Fire development. These are as follows- Incipient Stage-Here no visible smoke or flame is present, also very little amount of heat is produced. But one important characteristic of this stage is the presence of large amount of invisible combustion particles which may be smell able. This stage develops slowly. Smouldering Stage-Here smoke with little amount of heat is produced , but with no flame Flame stage-Now the flame is visible and more heat is generated. Often less or no smoke, particularly with flammable liquids and gas fires is also evident. Heat stage-Large amounts of heat, flame, smoke and toxic gases are produced. The transition from the previous stage can be very fast. 12. Combustible and flammable fuels involved in fires have been broken down into 5 categories. Identify them. These are: Chemical fuels Bio fuels Fossil fuels Nuclear fuels Alternative fuels 13. Name 4 parameters that you can determine using the cone calorimeter. The 4 parameters that could be determined using Cone Calorimeter are as follows- Ignition Time Mass Loss Combustion Products Rate of Heat release. 14. Explain what is meant by term ‘ignitability’ of a material. It is the easiness with which process of Ignition could be started in presence of ignition source. A liquid exhibits the characteristic of ignitability if it has a flash point less than 140ºF. 15. Explain what is meant by positive pressure ventilation. During normal process of breathing, air travels on account of negative pressure inside the lungs from outside. It happens with the help of Diaphragm and thoracic cavity which when expands create pressure gradient between lungs and outside i.e. lung pressure lower than atmospheric pressure. Hence air following the principle of movement from high pressure point to low pressure point goes into lungs. Whereas in Positive Pressure Ventilation, works by forcing air into lungs, thereby increasing pressure inside the airway relative to outside. Application to Firefighting- It is used to blow smoke out of room or any structure. It is achieved by forcing air into a building using a fan. These fans force air into structure to create a slightly higher pressure inside structure relative to external atmospheric pressure. This pressure gradient drives gases, heat and other products of combustion outside the structure through some hole or pipe. Some of the dangerous products of fire are gases like CO i.e. Carbon Monoxide, which could be extremely harmful to the people inside fire. Moreover PPV also clears air and hence visibility is improved. 16. Explain the difference between terms Heat and Temperature. Heat is a measurement of Total energy in a substance. That total energy is sum of Kinetic energy and Potential energy of molecules. Whereas Temperature is a number related to Kinetic energies of molecules. Hence Temperature is not energy. But it is a number .if measured in degrees Kelvin, is proportional to average Kinetic energies of molecules. Hence if average Kinetic energy is doubled, so is the Temperature. 17. Why is it important to investigate the cause of fires. Investigation of causes of fires is very important because it can serve numerous purposes. For example, in 1987 in famous King’s Cross fire, many people died. Investigation found that the cause of fire was smoking. Hence it was banned thereafter. By investigating proper actions could be taken to avoid the accidental fires. Also it has assumed importance after insurance related disputes have increased owing to large sums of money, where the property etc has been damaged by fire. 18. Explain the difference between diffusion flame and premixed flame. It is the type of Combustion in which, oxidizer combines with the fuel through the process of Diffusion. Hence a direct consequence is that flame speed is limited to or determined by speed of diffusion. This causes flames to burn slower and produce more soot. In a Premixed flame, Oxidizer is mixed with the fuel before the mixture reaches burning zone. This produces a thin flame front as all the reactants are readily available. 19. Explain what the flashpoint of the fuel is. Flashpoint of a fuel is defined as the lowest temperature at which the fuel is combustible. If the ignition source is removed at this temperature, vapor will stop burning. A Flashpoint is defined to minimize the risk during normal storage and handling because another way of looking at it is it is the indication how easy a chemical may burn. For example Flash point of Ethyl alcohol (Ethanol) is 55 degree F. 20. Why are vapors important to the burning process? Vapors are very important for burning process, because in case of flammable liquids, it is not the liquid which burns when provided ignition, it is the vapor cloud above the liquid that burns if the vapor's concentration is between the Lower and Upper Flammable limit of the flammable liquid. 21. Explain the limiting oxygen index test. Limiting Oxygen Index (LOI) test is a widely used research and quality control tool for determining the relative flammability of polymeric materials. A numerical index, the ‘LOI’, is defined as the minimum concentration of oxygen in an oxygen – nitrogen mixture, required to just support downward burning of a vertically mounted test specimen. Hence, higher LOI values represent better flame retardancy. The test method is generally reproducible to an accuracy of + 0.5% and although originally designed for testing of plastics, the method has been used extensively for evaluating the relative flammability of rubbers, textiles, paper, coatings and other materials. 22. What are the limitations of the limiting oxygen index test? This test is also known as critical oxygen index or COI. The basic tenet of this method is higher the LOI, safer is the material. Now considering Oxygen constitutes around 20.95% of air, so any material with LOI less than 20/95, will easily burn in the air. But for a material with LOI greater than 100, self sustained combustion is not possible. These values are not physically meaningful. Further, classification of materials according to LOI into various categories is specific to this test only, i.e. a material specified as flammable in this method may not qualify as same in another method. 23. Explain the terms ‘piloted ignition’ and ‘auto ignition’. Piloted ignition is the accidental start of combustion by means of contact of gaseous material with an external high energy source such as spark, glowing wire etc. Auto ignition is the start of combustion even in the absence of any external source of ignition 24. Calculate the mean, range, standard deviation and uncertainty in the mean and standard deviation for the following observations: 1 2 3 4 5 6 7 8 9 10 44.80 42.15 42.97 43.60 43.88 44.80 42.79 45.10 41.62 43.74 Mean= Sum of observations /Total no. of observations = Range=Spread between lowest and highest observation =45.10-41.62 =3.48 Standard deviation= Sum of squared deviations/no. of observations =  Where Xm is the mean of observations, n total no of observations =1.167792 Uncertainty of mean  Where n is student t factor, s is standard deviation. =0.67 at t value of 1.833 =0.83 at t value of 2.262 Uncertainty of Standard deviation 25. Plot and interpret the graph using values given below taken from Bang box experiment. 2 6 10 12 14 16 18 20 0 20 40 80 120 150 110 90 Interpretation- In the graph above, fuel used in the test is Acetone, whose value is changed and the impact on height attained is measured. As the drops of Acetone are increased, first height attained is increased upto a limit, then after the optimal combination is reached,as we increase no of drops of fuel,height attained decreases. References- http://www.firesafe.org.uk/html/guides/basicmoe.htm http://safetyguide.web.cern.ch/SafetyGui… http://www.desastres.org/pdf/ventilapp.pdf http://www.britannica.com/EBchecked/topic/163113/diffusion-flame http://www.engineeringtoolbox.com/flash-point-fuels-d_937.html http://www.fireproof.co.uk/POOLED/ARTICLES/BF_PSART/VIEW.ASP?Q=BF_PSART_196268 http://www.uow.edu.au/~mnelson/review.dir/oxygen.html Read More