CHAPTER 1 BASIC CONCEPTS

Transcription

1 GTU Paper Analysis CHAPTER 1 BASIC CONCEPTS Sr. No. Questions Jan 15 Jun 15 Dec 15 May 16 Jan 17 Jun 17 Nov 17 May 18 Differentiate between the followings; 1) Intensive properties and extensive properties, 2) Point function and path function, 3) Microscopic approach and macroscopic approach, 4) Pure substance and working substance Define State & property Define a thermodynamic system. Differentiate between open system, closed system and an isolated system. Define Critical Point Temperature, Thermodynamic Equilibrium, Dead State of a given System, Mole Fraction Define Heat, Show that heat is an inexact differential What do you mean by thermal equilibrium? Explain it with suitable example Explain p-v-t surface for a pure substance by defining i) Triple point, ii) Critical point and iii) Vapour dome 8. Explain quasi-static process Darshan Institute of Engineering & Technology 1

2 GTU Paper Analysis CHAPTER 2 FIRST LAW OF THERMODYNAMICS Sr. No. Questions Jan 15 Jun 15 Dec 15 May 16 Jan 17 Jun 17 Nov 17 May 18 Theory Derive equation for (a) filling of a tank and (b) emptying of tank. State the first law of thermodynamics, its applications and limitations. State the Steady Flow Energy Equation and explain how this equation can be applied for (i) Nozzle, (ii) Boiler, and (iii) Steam Turbine. Explain the significance of S.F.E.E. in engineering applications. OR State the SFEE for a single stream inlet and single stream leaving a control volume and explain the various terms in it. Also write SFEE for following Applications: (1) Cooling Tower (2) Centrifugal Pump (3) Expansion valve of refrigerator Justify that energy is property of the system Derive an expression for displacement work for a closed system and state its limitations. 04 Examples Steam at 6.87bar, 205 C enters in an insulated nozzle with a velocity of 50m/sec. It leaves at a pressure of 37bar and a velocity of 500m/sec. Determine the final enthalpy of the steam. Also clearly mentioned the applied assumptions. Use of steam table is permitted. The mass flow rate of steam into a steam turbine is 5 Kg/s and heat loss from the turbine is 8.5 KW. The steam is entering the turbine at the pressure of 2MPa, Darshan Institute of Engineering & Technology 1

3 GTU Paper Analysis temperature 350 C, Velocity 50 m/s, elevation 6 m and is leaving the turbine at a pressure of 0.1 MPa, quality of 100%, velocity of 200 m/s, elevation of 3 m. Determine power output of turbine. In steam power plant, steam pressure, temperature and velocity are 2 MPa, 400 C and 50 m/s respectively at inlet of steam turbine. At exit of steam turbine, steam pressure, dryness fraction and velocity are 15 kpa, 0.9 and 180 m/s respectively. Elevation difference between inlet and exit of steam turbine is 4 m. The power output of an adiabatic steam turbine is 5 MW. (1) Compare the magnitudes of Δh, Δke, and Δpe. (2) Determine the work done per unit mass of the steam flowing through the turbine. (3) Calculate the mass flow rate of the steam. 4. Steam enters an adiabatic turbine steadily at 3 MPa and 400 C and leaves at 50 kpa and 100 C. If the power output of the turbine is 2 MW. Determine (a) the isentropic efficiency of the turbine and (b) the mass flow rate of the steam flowing through the turbine. Neglect the change in potential and kinetic energies. 5. A domestic refrigerator is loaded with food and the door closed. During a certain period the machine consumes 1 kw h of energy and the internal energy of the system drops by 5000 kj. Find the net heat transfer for the system In a gas turbine unit, the gas flow through the turbine is 15 Kg/Sec. and the Power developed by the turbine is KW. The enthalpies of gases at inlet and Outlet are 1260 KJ/Kg and 400 KJ/Kg respectively, and the velocity of gases at the Inlet and outlet are 50 m/s and 110 m/s respectively. Calculate (i) the rate at which Heat is rejected to the turbine, and (ii) The area of the inlet pipe given that the Specific volume of gases at inlet is 0.45 m 3 /kg. 7. Determine the work required to compress steam isentropically from 1 bar to 10 bar, assuming that at the initial state the steam exists as (a) saturated liquid and (b) Darshan Institute of Engineering & Technology 2

4 GTU Paper Analysis saturated vapor. Neglect changes in kinetic energy and potential energy. What conclusion do you derive from this example? One kg of gas is confined to a constant volume tank. Initial pressure and volume are 4 bar and 0.21 m 3 respectively. When a heat energy of 82 kj is supplied to the system, the final temperature of the gas becomes 127 C. Find: i) Work done, ii) Change in internal energy, iii) Specific heat at constant volume. Assume R = 300 N-m/kg-K. One kg of air having an initial volume of 0.3 m 3 is heated at constant pressure of 2 bar until the volume is doubled. Calculate i) heat added; ii) work done; iii) initial and final temperature of air. Take Cp = 003 kj/kg K and R = kj/kg K. Darshan Institute of Engineering & Technology 3

5 Dec- 15 Jan- 15 Jun - 15 May-16 Jun- 17 Jan- 17 Nov- 17 May-18 Chapter 3 Second Law of Thermodynamics GTU Paper Analysis (New Syllabus) Sr. No. Questions Define following terms Theory (1) Heat Engine (2) Thermal Energy Reservoir (3) Refrigerator(4) Heat Pump Prove that all reversible engines operating between operating between same temperatures limits have are equally efficient. OR Prove that all reversible engines working between the two constant temperature reservoirs have the same efficiency. State Kelvin-Plank Statement of Second Law of thermodynamics and show that violation of Kelvin-Plank statement leading to violation of Clausius statement. OR Write the limitation of first law of thermodynamics. Explain the second law of thermodynamics by Clausius statement and Kelvin-Plank statement State and explain the Perpetual motion machines of Second Kind 04 5 State the comparisons of First law and Second law of thermodynamics Evaluate the following statements: Heat pump provides a thermodynamic advantage over direct heating. Kelvin temperature scale is independent of the peculiar characteristics of any particular substance. 7 Identify the reasons for the impracticability of Carnot cycle Define following terms: Heat Engine, Thermal Energy Reservoir,Refrigerator OR Heat Engine Explain Carnot theorem. OR State and prove Carnot s theorem for Heat Engine and also write statement of Carnot s 03 Darshan Institute of Engineering & Technology, Rajkot. 1

6 theorem in the view of refrigerator and Heat Pump GTU Paper Analysis (New Syllabus) 10 Discuss limitations of Carnot cycle Define Heat, Show that heat is an inexact differential Explain thermodynamic temperature scale Examples Two Carnot engines work in series between the sources and sink temperatures of 550K and 350K. If both engines develop equal power, derive the formulae to find out the intermediate temperature and determine the intermediate temperature also. Two Carnot engines work in series between the sources and sink temperatures of 550 K and 350 K. If both engines develop equal power determine the intermediate temperature. A heat pump is used to heat the house in winter. A house requires 50 KJ/s heat for heating in winter which is delivered by heat pump from outside air. Work required to operate the heat pump is 8 KW. Calculate the Co-efficient of Performance of heat pump and heat abstracted from outside. A heat engine receives 999KW of heat at constant temperature of C. The heat is rejected at The possible heat rejected are: (a) 850 KW, (b) 490 KW, (c) 400 Kw. State which of the results report a reversible Cycle or irreversible cycle or impossible result. A reversible heat engine operates between two reservoirs at temperatures of 600 C and 40 C. The engine drives a reversible refrigerator which operates between reservoirs at temperature of 40 C and -20 C. The heat transfer to the heat engine is 2000 kj and the net work output of the combined engine refrigerator plant is 360 kj. Evaluate the heat transfer to the refrigerant and the net heat transferred to the reservoir at 40 C. A domestic refrigerator maintains a temperature of -12 C. The ambient air temperature is 35 C. If heat leaks into freezer at the continuous rate of 2kJ/s determine the least power necessary to pump this heat out continuously. A heat engine receives heat at the rate of 1500kJ/min and gives an output of 8.2 KW. Determine: (i) The thermal Efficiency (ii) The rate of heat rejection. Darshan Institute of Engineering & Technology, Rajkot. 2

7 Dec-15 Jan-15 Jun-15 May-16 Jun- 17 Jan- 17 Nov- 17 May-18 GTU Paper Analysis (New Syllabus) Chapter 4 Entropy Sr. No. Questions Theory Prove that entropy is a property of system With usual notations prove that OR With usual notations prove that ΦδQ/T <= Explain Clausius theorem Explain principle of increase of entropy for an isolated system. OR State the principle of increase of entropy. List the four application of entropy principle. 03 State the Principle of increase of entropy. Explain application of entropy principle with Any two 5. suitable examples. OR What is entropy principle? With the help of it prove that adiabatic mixing of two fluids is irreversible Explain in brief the characteristics of entropy Define: Entropy Derive the expression for the entropy change for pure substance. 9. Define the 3rd law of thermodynamics. 03 Examples 300 kj/s of heat is supplied at a constant fixed temperature of 290 C to a heat engine. The heat rejection takes place at 8.5 C. The following results were obtained : Darshan Institute of Engineering & Technology, Rajkot. 4.1

8 (i) 215 kj/s are rejected. (ii) 150 kj/s are rejected. (iii) 75 kj/s are rejected. Classify which of the result report a reversible cycle or irreversible cycle or impossible results. 5 kg of water at 0 is exposed to reservoir at 98. Calculate the change of entropy of water, reservoir and universe. Assume that specific heat of water is KJ/KgK. A steam power plant operates between boiler temperature of 160 C and condenser temperature of 50 C. Water enters the boiler as saturated liquid and steam leaves the boiler as saturated vapour. Assuming the isentropic expansion in turbine. Enthalpy of water entering boiler = 687 kj/kg. Enthalpy of steam leaving boiler = 2760 kj/kg Condenser pressure = N/m 2. Verify the Clausius inequality for the cycle. 2 kg of N 2 at 155 and 0.25 m 3 is expanded to 0.40 m 3 at constant pressure, and then expanded isothermally to volume of 0.6 m 3. Assume that specific heat at constant volume is kj/kgk and gas constant is kj/kgk. Calculate the overall change of entropy of the process. An iron cube at a temperature of 400 C is dropped into an insulated bath containing 10 kg water at 25 C. The water finally reaches a temperature of 50 C at steady state. Given that the specific heat of water is equal to 4186 J/kg K. Find the entropy changes for the iron cube and the water. Is the process reversible? If so why? 1 kg of ice at 0 C is mixed with 12 kg of water at 27 C. Assuming the surrounding temperature as 15 C, calculate the net increase in entropy and unavailable energy when the system reaches common temperature : Given: Specific heat of water = 4.18 kj/kg K; specific heat of ice = 1 kj/kg K and enthalpy of fusion of ice (latent heat) = 335 kj/kg. GTU Paper Analysis (New Syllabus) Darshan Institute of Engineering & Technology, Rajkot. 4.2

9 Dec- 15 Jan- 15 Jun- 15 May-16 Jun- 17 Jan- 17 Nov- 17 May-18 GTU Paper Analysis (New Syllabus) Chapter 5 Energy Sr. No. Questions Define the following terms: Theory 1) Dead state of a given system, 2) Availability, 3) Irreversibility, 4) Enthalpy, 5) Entropy 6) High Graded Energy Explain the concept of decrease in available energy when heat is transferred through a finite temperature difference with the aid of T-S diagram. What is irreversibility? State various types of irreversibilities and explain them. OR Write a short note on reversibility and irreversibility Identify the cause of irreversibility What is irreversibility? State various types of irreversibilities and explain them. 6. Explain the available energy referred to finite heat source How can you improve the first law efficiency and second law efficiency? 03 5 kg of air at 550 K and 4 bar is enclosed in a closed system. Examples (i) Determine the availability of the system if the surrounding pressure and temperature are 1 bar and 290 K respectively. (ii) If the air is cooled at constant pressure to the atmospheric temperature, determine the availability and effectiveness. Calculate the available energy in 40 kg of water at 75 C with respect to the surroundings at 5 C, the pressure of water being 1 atm. Darshan Institute of Engineering & Technology, Rajkot. 5.1

10 Chapter 6 Vapor Power Cycles GTU Paper Analysis Sr. No. Questions Jan 15 Jun 15 Dec 15 May 16 Jan 17 Jun 17 Nov 17 May Theory State various methods to improve efficiency of Rankine cycle. With suitable diagrams, explain any two of them. OR What is the effect of regeneration? On the (1) specific output, (b) mean temperature of heat addition (c) cycle efficiency and (d) steam rate OR Draw ideal simple Rankine cycle with reheating on T-s and h-s diagram. Identify the reheating process and locate the increase in work done due to reheating in both graph. With help of T-s diagram, explain the effects of variables on efficiency of the Rankine cycle. OR Explain the effect of superheating of steam on the performance of Rankine cycle. Explain Carnot vapor cycle. State and explain required modifications with help of suitable diagrams to make the cycle feasible. Draw Rankine cycle on P-v, T-s and h-s diagrams and derive an expression for its thermal efficiency with and without pump work. OR Draw Rankine cycle on three usual thermodynamic co-ordinates Enlist the various components used in Rankine cycle based power plant Why steam power plants are popular than gas turbine plant in electric power generation? What do you understand by the mean temperature of heat addition? For a given T2, show how the Rankine cycle efficiency depends on the mean temperature of heat addition. 04 Darshan Institute of Engineering & Technology 6.1

11 GTU Paper Analysis 8. What do you understand by steam rate and heat rate? What are their units? State the comparisons of Carnot vapour cycle and Rankine cycle Examples In a Rankine cycle, the steam at inlet to the turbine is saturated at pressure of 35bar and exhaust pressure is 0.2bar. Determine: 1) the pump work, 2) the turbine work, 3) the Rankine efficiency, 4) the quality of steam at the end of expansion. Assume flow rate of 9.5kg/sec. Use of steam table is permitted. Dry and saturated steam at pressure of 10.5 bar is supplied to a turbine and expanded isentropically to a pressure 0.5 bar. Calculate thermal efficiency of Rankine cycle. Consider a steam power plant operating on the ideal Rankine cycle. Steam enters the turbine at 3 MPa and 350 C and is condensed in the condenser at a pressure of 10 kpa. Determine (1) the thermal efficiency of this power plant, (2) the thermal efficiency if steam is superheated to 600 C instead of 350 C. In a steam power cycle, the steam supply is at 15 bar and dry and saturated. The condenser pressure is 0.4 bar. Calculate the Carnot and Rankine efficiencies of the cycle. Neglect pumps work. A steam turbine power plant operating on ideal Rankine cycle, receives steam at 30 bar, 350 C at the rate of 2 Kg/s and it exhausts at 0.09 bar. Calculate the followings: i) Net power output, ii) Steam rate, iii) Heat rejection in condenser in KW, iv) Rankine Cycle efficiency. In a single heater Regenerative cycle the steam enters the turbine at 30 bar, 400 C and the exhaust pressure is 0.1 bar. The feed water heater is direct contact type which operates at 5bar. Find (a) efficiency (b) steam rate and (c) mean temperature of heat addition with regeneration. Neglect pump work. 7. A simple Rankine cycle works between pressures 28 bar and 0.06 bar, the initial condition of Darshan Institute of Engineering & Technology 6.2

12 8. 9. steam being dry saturated. Calculate the cycle efficiency, work ratio and specific steam consumption. (Use of Steam table is Permitted) A steam power plant operates on a theoretical reheat cycle. Steam at boiler at 150 bar, 550 C expands through the high pressure turbine. It is reheated at a constant pressure of 40 bar to 550 C and expands through the low pressure turbine to a condenser at 0.1 bar. Draw T-S and h-s diagrams and find: (i) Quality of steam at turbine exhaust, (ii) Cycle efficiency, (ii) steam rate in kg/kwh. (Mollier diagram is permitted) A steam power plant operating on Rankine cycle receives steam from a boiler at 5 MPa and 350 C. It is exhausted to condenser at 10 kpa. Calculate: i) Energy supplied per kg of steam generated in a boiler, ii) Quality of steam entering the condenser, iii) Rankine cycle efficiency considering feed pump work and iv) Specific steam consumption. GTU Paper Analysis Darshan Institute of Engineering & Technology 6.3

13 GTU Paper Analysis Chapter 7 Gas Power Cycles Sr. No. Questions Jan 15 Jun 15 Dec 15 May 16 Jan 17 Jun 17 Nov 17 May - 18 Theory State various methods to improve efficiency of Brayton cycle. With suitable diagrams, explain any two of them. OR What is regeneration in gas turbine plant? How it improves thermal efficiency of simple open cycle Gas Turbine Plant. Explain it with the help of schematic diagram and T-S Diagram of the cycle. OR Enlist the various components used in intercooling and reheating gas cycle based power plant. OR Draw the schematic diagram of Simple Gas Turbine cycle with intercooling. Draw T-S diagram of the cycle. Compare Otto, Diesel and Dual cycle for i) Same compression ratio and heat supplied ii) Same maximum pressure and temperature. How actual Brayton cycle differs from the theoretical cycle? Explain with the help of T-S diagram. What are the advantages of the Brayton cycle over the conventional heat engine cycles? OR State the thermodynamic process of open cycle gas turbine power plant What are the air standard assumptions? Justify: For the same compression ratio and work capacity, Brayton cycle is more suitable than Otto cycle in gas turbine power plants. 03 Darshan Institute of Engineering & Technology 7.1

14 6. Justify: In Otto cycle, the compression ratio cannot, however, be increased beyond a certain limit. 03 GTU Paper Analysis 7. Discuss limitations of Carnot Cycle Define: Cycle, Air standard Efficiency, Compression ratio, Break thermal efficiency Draw Bryton cycle and derive expression for optimum pressure ratio Derive equation for efficiency of Diesel cycle. 4. Examples In an I C Engine working with the Otto cycle, the cylinder diameter is 250mm and a stroke is 375mm. If the clearance volume is m 3, and the initial pressure and temperature are 1bar and 50 C, calculate the air standard efficiency and mean effective pressure of the cycle. The maximum cycle pressure is limited to 25bar. An engine working on Diesel cycle has cylinder bore of 190 mm and piston stroke of 230 mm. The clearance volume is 290 cm 3. The fuel injection takes place at constant pressure for 6% of the stroke. Determine the air standard cycle efficiency. An air-standard Diesel cycle has a compression ratio of 20, and the heat transferred to the working fluid per cycle is 1800 kj/kg. At the beginning of the compression process, the pressure is 0.1 MPa and the temperature is 15 C. Consider ideal gas and constant specific heat model. Determine: (1) The pressure and temperature at each point in the cycle. (2) The thermal efficiency. (3) The mean effective pressure. A Diesel engine takes in air at pressure 1 bar and temperature 30 C. The pressure at the end of the compression is 30 bar and the cut off is 6% of the stroke. Calculate: (i) The compression ratio (ii) The percentage clearance (iii)the heat supplied in kj/kg Darshan Institute of Engineering & Technology 7.2

15 GTU Paper Analysis 5. (iv) The heat rejected in kj/kg (v) Mean effective pressure in bar An engine operating on Diesel cycle has maximum pressure and temperature of 45bar and 1500 C. Pressure and temperature at the beginning of compression are 1bar and 27 C. Determine air standard efficiency of the cycle. Take adiabatic index γ = In an air standard Diesel cycle, the compression ratio is 16, and at the beginning of isentropic compression, the temperature is 15 C and the pressure is 0.1MPa. Heat is added until the temperature at the end of the constant pressure process is 1480 C. Calculate (a) the cut-off ratio, (b) the heat supplied per kg of air, and (c) the cycle efficiency. In an ideal Brayton cycle, air from the atmosphere at 1atm, 300K is compressed to 6atm and the maximum cycle temperature is limited to 1100K by using large air fuel ratio. If the heat supply is 100MW, find (a) the thermal efficiency of the cycle, (b) work ratio, (c) power output, (d) exergy flow rate of the exhaust gas leaving the turbine. 8. Find the required air-fuel ratio in a gas turbine whose turbine and compressor efficiencies are 85% and 80%, respectively. Maximum cycle temperature is 875 C. The working fluid can be taken as air (Cp = 0 kj/kg K, γ = 4) which enters the compressor at 1 bar and 27 C. The pressure ratio is 4. The fuel used has calorific value of kj/kg. There is a loss of 10% of calorific value in the combustion chamber. 9. A closed cycle ideal gas turbine plant operates between temperature limits of 800 C and 30 C and produces a power of 100 kw. The plant is designed such that there is no need for a regenerator. A fuel of calorific kj/kg is used. Calculate the mass flow rate of air through the plant and rate of fuel consumption. Assume Cp = 1 kj/kg K and γ = An air standard Otto cycle is required to operate between the temperature limits of 300 K and 1800 K. Estimate the optimum compression ratio and the corresponding thermal efficiency. Assume, γ =4. Darshan Institute of Engineering & Technology 7.3

16 Dec-15 Jan-15 Jun-15 May-16 Jun- 17 Jan- 17 Nov- 17 May-18 GTU Paper Analysis (New Syllabus) Chapter 8 Properties of Gases & Mixtures Sr. No. Questions Theory Derive Vander waal s equation. OR Write a short note on Vander Waal s equation. OR What is the Vander waal s equation of state? State its importance and derive it 04 State Amagat s law and mole fraction State and explain Dalton s law of partial pressure and Avogadro s law. OR State the statements of Dalton s Law and Gibbs-Dalton Law. Explain in detail Dalton s law of partial pressures. OR State and explain the Gibbs-Dalton law of partial pressures. OR Explain briefly Dalton s law, Gibbs-Dalton law and Amagat s law for perfect gas mixture Discuss deviation of real gas from ideal gas. OR What is an ideal gas equation? What assumptions are made in deriving the ideal gas equation of state from kinetic theory of gas? OR Prove the Ideal Gas equation. Draw the generalized compressibility chart. OR State the law of corresponding states and define the compressibility factor and explain its significance Explain adiabatic mixing of perfect gases What is an equation of state? State its different type and which type is focused in the present subject. 03 Darshan Institute of Engineering & Technology, Rajkot. 8.1

17 GTU Paper Analysis (New Syllabus) 8. State the Boyle s law, Charle s and Avogadro s law for perfect gas. 03 Examples A piston cylinder device initially contains 0.5 m3 of nitrogen gas at 400 kpa and 27 C. An electric heater within the device is turned on and is allowed to pass a current of 2 A for 5 min from a 120- V source. Nitrogen expands at constant pressure, and a heat loss of 2800 J occurs during the process. Consider nitrogen gas as ideal gas and nitrogen has constant specific heats. Take characteristics gas constant for nitrogen is 0.297kJ/kgK. Take Cp = 039 kj/kgk for nitrogen at room temperature. Determine the final temperature of nitrogen. A cylinder of 0.1 m3 volume is filled with kg of C8H18 at K. Assuming that C8H18 obeys the Vander Walls equation of state, calculate the pressure of the gas in the cylinder. The Vander walls Constant a and b for C8H18 are 789 Pa (m3/mol) 2 and m3/mol, respectively. 04 Darshan Institute of Engineering & Technology, Rajkot. 8.2