PESIT Bangalore South Campus Hosur road, 1km before Electronic City, Bengaluru -100 Department of Basic Science and Humanities

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1 USN 1 P E PESIT Bangalore South Campus Hosur road, 1km before Electronic City, Bengaluru -100 Department of Basic Science and Humanities INTERNAL ASSESSMENT TEST 1 Date : 28/08/2017 Marks: 40 Subject & Code : Elements of Mechanical Engg. & 15EME14/24 Sec : F,G,H,I &J Name of faculty : TA,AK,SS,MA,CM Time : 11.30am 1pm Note: Answer FIVE full questions, selecting any ONE full question from each part. Marks PART 1 1 a What are renewable and non- renewable energy sources? Explain with examples 3 b Explain the working of wind turbine with a neat diagram 5 2 a Write any 5 major differences between renewable and non-renewable energy sources 5 b Why is it impossible to store solar energy in a natural pond? What are the modifications that have to be 3 done in a natural pond to store solar energy? PART 2 3 a What is enthalpy of evaporation? Explain the variation of enthalpy of evaporation with pressure and temperature with the help of a neat labeled T-H diagram. Define sensible heat and super heat. 4 a 2 kg of Water at 12 0 C at 1 bar is heated at constant pressure to steam at C. The boiling point of water at atmospheric pressure is C. Find i) sensible heat, ii) Enthalpy of evaporation iii) enthalpy of superheated steam iv) Degree of super heat. The specific heat of water and steam at constant pressure is 4.18 kj/kg and 2.25 kj/kg respectively. Draw a T-H diagram and mark all the given values on the diagram. Enthalpy of water at 12 0 C, 1 bar =57 kj/kg, Enthalpy of Saturated water at 1 bar= 424 kj/kg, Enthalpy of Saturated steam at 1 bar= 2674 kj/kg PART 3 5 a Explain the working of any water tube boiler with the help of neat labeled sketch. Why the area of cross 8 section of hot flue gas passage is decreasing along the flow in a Lancashire boiler? 6 a Explain i) economizer ii) Air preheater iii) super heater. Show the flow of air, water and flue gases in a 8 flow diagram and place these accessories in appropriate places. PART 4 7 a Compare impulse steam and impulse water turbine. 4 b Compare Kaplan and Francis turbine 4 8 a Draw an outline of any reaction water turbine and label the draft tube. What is the importance of draft 8 tube in a reaction turbine? Why it is not necessary to have a draft tube in an impulse turbine? PART 5 9 a Compare open and closed cycle gas turbines a What is the use of cooler in a closed cycle gas turbine? (i.e., will it work well without a cooler after the turbine?) b Why it is necessary to have Convergent-Divergent nozzle in a steam turbine instead of a convergent nozzle? justify your answer If you have finished answering the questions and there is still time left, you need not leave the hall in a hurry! Double check that your answers are correct and the representation is professionally outstanding BE I semester

2 Solutions: 1. (a) What are renewable and non-renewable resources of energy? Explain with examples. The renewable resources of energy are defined as the energy resources which are produced continuously in nature and are essentially inexhaustible atleast in the time frame of human societies. These energy resources replenish themselves naturally in a relatively short time and therefore will always be available. Examples : Solar energy, Wind energy, Tidal energy, Biomass energy etc The non-renewable resources of energy are defined as the energy resources which have been accumulated over the ages and not quickly replenishable when they are exhausted. These energy resources get exhausted eventually in the future. Examples : Coal, Petroleum, Natural gas, Nuclear fuels 1. (b)

3 2. (a) Factor Renewable Non-renewable (1) Exhaustibility Inexhaustible Exhaustible (2) Availability Abundantly and freely available in nature (3) Replenishment Are replenished naturally over a useful period of time at the same rate at which they are consumed. Not abundantly available Cannot be replenished over a desired period of time. (4) Environment friendliness Environment friendly Except in the case of biomass energy resource. Not environment friendly and can produce green house gases that contribute to global warming. (5) Cost Factor Initial cost for building the system is high. The running cost is low. Cost of production is low. But since depletion rate is high, demand is increasing and supply is decreasing. This is eventually increasing cost of usage.

4 (6) Nature of availability (7) Regional restriction & dependency factor Intermitently available No regional restriction. Almost available in all countries. This reduces dependency. Continuously available Non-renewable resources (like petroleum) available only in selected countries. Hence high dependency. 2. (b) In a natural pond, nearly about 30% of solar radiation reaches to a depth of 2 meters. The solar radiation (heat energy) is absorbed by water at the bottom of the pond. The warm water due to its low density rises to the surface and loses its heat to the atmosphere without serving any purpose or beneficial effects. However, if this warm water is allowed to remain at the bottom of the pond by some mechanism, it can be piped to a boiler where it is heated further to produce steam that can drive a turbo-generator to produce electricity, or the hot water can be used for various practical applications. A simple mechanism to achieve this concept is by artificially creating a pond over large area and adding salt, either sodium chloride or magnesium chloride at the bottom of the pond. The salt dissolves in water making the water too heavy to rise to the surface of the pond. Higher salt contents (salinity) in water will increase the density of the water allowing it to stay at the bottom of the pond. The hot water can thus be used for various applications.

5 3 (a) With increase in both pressure and saturation temperature the enthalpy of evaporation is seen to decrease as seen in the T-H diagram. 4. (a)

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7 5. (a)

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9 6. (i) Economizer: It is a device used to heat the feed water (water that enters the water drum) by extracting the heat from the gases that are passing out of the chimney. It is placed between the boiler exit and the chimney entrance. (ii) Air-preheater: Its function is to extract the heat from the gases that are passing out of the chimney. The heat extracted is utilized for heating the air that is supplied for the combustion of fuel in the furnace. It is placed between the economizer and the chimney entrance. (iii) Superheater: Steam from steam space in the boiler drum is sent through the t tbues into the U shaped superheater tubes which is placed in the combustion chamber. Since the superheater tubes are placed in the combustion chamber they will be exposed to the hot flue gases and hence get superheated. 7. (a) Impulse Steam Turbine The working fluid is Steam. The production and supply of steam is regulated from the boiler. Operation temperatures is very high. Higher output speeds are achieved Chances of erosion and corrosion are high because of high steam Temperatures. Impulse steam turbines are generally axial flow turbines. Example DeLaval turbine Impulse Water Turbine The working medium is water.

10 Water is stored in dams/reservoirs and supplied to turbines from high head, and the supply is regulated by the spear. Comparatively lower output shaft speeds are produced. Chances of erosion of shafts are less and only if impurity particles are present in the incoming supply of water. Impulse Water turbines are generally tangential flow turbines. Example Pelton Wheel Turbine. 7. (b) Kaplan Francis Low head turbine Medium Head Turbine High Discharge Medium Discharge Axial Flow Mixed Flow Movable Blades are attached to the The runner blades are attached to the Hub/boss Runner wheel Runner blades in the Hub are Guide blades in the guide wheel are adjustable while guide blades are fixed adjustable and runner blades are fixed Used to generate moderate power Used to generate relatively larger output. output power. 8. (a) In turbines, water enters from very high head i.e. very high pressure. As the water passes through turbine vanes, it gives its energy to vanes and pressure of water decreases to sub-atmospheric values so that we can have high energy output because of higher pressure difference across turbine. Now we just can't make turbine open to atm. because of low pressures otherwise air entrapment could cause problems like cavitation. Now in order to again have water exiting at atmospheric pressure, we need to raise water pressure. To achieve this, we use draft tube and because of its tapering outward shape, it acts as diffuser since area of flow increases. Thus we have water coming out from turbine at atm. pressure.

11 Another function of draft tube is that it also acts as collector for water coming out of turbine and directs it to reservoir. In an impulse turbine there is no variation of pressure anywhere inside the turbine and hence a draft tube is not required. 9. (a) OPEN CYCLE CLOSED CYCLE Consists of compressor, combustion chamber and turbine only. Working fluid is air and the combustion products. Working substance is continuosly replaced in every cycle. Consists of compressor, heat exchanger and turbine and cooler. Working fluid can be air or any other fluid with adequate thermodynamic properties. Working substance is continuosly recirculated

12 Exhaust from turbine is exited into the atmosphere. No cooling water is required as the exhaust is expelled into atmosphere. Exhaust from turbine is sent to the cooler and fed back into the cycle. Large amounts of cooling water is required in the cooler. There is heat work and mass transfer between system and surroundings. There is only heat and work transfer. High grade fuels should be used. Reduced size of plant for given output Possibility of turbine blades damage is higher. Any type of fuel can be used since combustion occurs in separate combustion chamber. Large size of plant due to presence of heat exchanger and cooler components. Wear and tear of turbine blade is very much reduced 10. (a) Cooler is a component which specific to only closed cycle gas turbines. In closed cycle gas turbines since the working fluid is operating in a thermodynamic cycle it has to be continuosly brought to specific conditions of temperatures and pressures after passing through each component. The cooler receives turbine exhaust and cools it to temperatures suitable to be introduced into the compressor. Each component (turbine, compressor, heat exchanger) is designed to work within a given range of temperature and pressure values. Without a cooler the exhaust from turbine will directly be sent to the compressor and it will damage the compressor. With reference to the T-S diagram it can also be seen that the presence of cooler reduces the compressor work. 10. (b) In a steam turbine the nozzle is that part which converts the pressure energy into kinetic energy. In a convergent nozzle the area of cross section goes on decreasing. According to continuity equation as the are decrease the velocity of flow increases and the pressure decreases, discharge remaining same. Where as in a CD nozzle it is the same as that of a convergent nozzle in the initial convergent part where the Mach no. is less than 1. At the throat section the mach no. is equal to 1. Whereas

13 in the divergent section since the Mach no is seen to be greater than 1 the velocity and area of cross section is related by the equation, (1 M 2 ) dv V = da A. Therefore much higher and efficient conversion of pressure energy to kinetic energy is possible compared to a convergent one.