Content : 4.1 Brayton cycle-p.v. diagram and thermal efficiency. 4Marks Classification of gas turbines. 4.2 Construction and working of gas turbines i) Open cycle ii) Closed cycle gas Turbines, P.V. and T.S diagrams. 10Marks Turbojet and turboprop engine. (no numerical) Introduction : Gas turbine is a rotary type internal combustion thermal prime mover. It s working principle is an improved version of the windmill which was used to achieve an efficient working of the turbine, the movement of gas (or) air is properly controlled and then directed on blades fixed to turbine runner. Classification of gas turbine : 1) According to path of working substance 1) Closed cycle gas turbine 2) Open cycle gas turbine 3) Semi-closed gas turbine 2) According to process of heat absorption 1) Constant pressure gas turbine 2) Constant volume gas turbine. 3) On the basis of working cycle of gas turbines 1) Joule s (or) Brayton cycle (constant pressure gas turbine) 2) Atkins cycle (constant volume gas turbine) 4) On the basis of combustion process 1) Continuous combustion (or) constant press type 2) The explosion (or) constant volume type 5) On the basis of action of expanding gases 1) Impulse turbine 2) Impulse reaction turbine 6) On the basis of direction of flow a) Axial flow b) Radial flow 7) On the basis of fuel used 1) Liquid fuel gas turbine 2) Gaseous fuel gas turbine R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 1
3) Solid fuel gas turbine 8) On the basis of application 1) Stationary 2) Automotive 3) Locomotive 4) Marine 5) Aircraft gas turbine. Open cycle gas turbine : It works on Brayton (or) Joule cycle which is shown on P-V and T-S diagram. Also important components of open cycle gas turbine shown below : (OR) 1) In this type of gas turbine liquid (or) gaseous fuels are used for power generation. The basic components are shown in figure above. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 2
2) Initially, atmospheric air is allowed to pass through rotary compressor in which its Pressure and temperature is increased, isentropically. 3) Then this compressed air is passed through combustion chamber in which fuel is injected for combustion purpose. After combustion of fuel in combustion chamber the heat is added under constant pressure condition the temperature of compressed air is further increased. 4) Now high pressure and temperature gases are expanded in gas turbine which is helpful to run the gas turbine or blades (generally of reaction type) 5) This gas turbine is directly connected to electric generator to produce Electricity and finally exhausted into the atmosphere. 6) This type of gas turbine works on open cycle because here working fluid is used only once. After single use it is thrown into atmosphere. 7) Here inlet and outlet both the ends are open to atmosphere hence termed as open cycle gas turbine. It is also called as continuous combustion gas turbine 1) Process (2-3) constant pressure process : (Heating) of air In this process at constant pressure heat addition takes place in heating chamber. Q A = Q 2-3 = m x C p x (t 3 t 2 ) 2) Process (3-4) Isentropic expansion process. During this process no heat addition (or) rejection. Here pressure and temperature drop takes place from P 3 to P 4 and t 3 to t 4 3) Process (4-1) constant pressure process (cooling) of air In this constant pressure heat rejection takes place i.e. cooling of air in cooling chamber. Heat rejection = Q R = Q 4-1 = m x C p x (t 4 t 1 ) 4) Process (1-2) Isentropic compression : During this process no heat addition (or) rejection. But during comp ression pressure increases from P 1 to P 2 and temperature also increases from t 1 to t 2. Work done by turbine per m Kg. of air is W T = m x C p x (t 3 t 4 ) Work done by air compressor per m Kg. of air Wc = m x C p x (t 2 t 1 ) R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 3
Net work done = W = W T W C (if mass = 1 Kg.) = C p x (t 3 t 4 ) Thermal efficiency (n TH ) = Net work done Heat addition = Heat received _ Heat rejected Per cycle per cycle = Heat received/cycle = m C p (t 3 t 2 ) m C p (t 4 t 1 ) m C p (t 3 t 2 ) N air std = 1 _ (t 4 t 1 ) (t 3 t 2 ) Closed cycle gas turbine : In above figure shows a closed cycle gas turbine which consists of compressor, heating chamber gas turbine which drives the generator, compressor and a cooling chamber. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 4
In this turbine air is compressed isentropically and then passed into heating chamber. The compressed air is heated with the help of some external source and made to flow over turbine blades. The gas while flowing over the blades gets expand from the turbine gas is passed to cooling chamber where it is cooled at constant pressure with the help of circulating air is circulated through compressor. This cycle contains two constant pressure and Isentropic expansion and compression. This cycle is works on Joule s (or) Brayton s cycle which is shown below : 1) Process (2-3) constant pressure process : (Heating) of air In this process at constant pressure heat addition takes place in heating chamber. Q A = Q 2-3 = m x C p x (t 3 t 2 ) 2) Process (3-4) Isentropic expansion process. During this process no heat addition (or) rejection. Here pressure and temperature drop takes place from P 3 to P 4 and t 3 to t 4 3) Process (4-1) constant pressure process (cooling) of air In this constant pressure heat rejection takes place i.e. cooling of air in cooling chamber. Heat rejection = Q R = Q 4-1 = m x C p x (t 4 t 1 ) 4) Process (1-2) Isentropic compression : During this process no heat addition (or) rejection. But during compression pressure increases from P 1 to P 2 and temperature also increases from t 1 to t 2. Work done by turbine per m Kg. of air is W T = m x C p x (t 3 t 4 ) Work done by air compressor per m Kg. of air Wc = m x C p x (t 2 t 1 ) Net work done = W = W T W C (if mass = 1 Kg.) = C p x [(t 3 t 4 ) (t 2 -t 1 )] Thermal efficiency (n TH ) = Net work done Heat addition R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 5
= Heat received _ Heat rejected Per cycle per cycle = Heat received/cycle = m C p (t 3 t 2 ) m C p (t 4 t 1 ) m C p (t 3 t 2 ) N air std = 1 _ (t 4 t 1 ) (t 3 t 2 ) Closed cycle gas turbine Open cycle gas turbine 1 Combustion of fuel is external. 1 Combustion of fuel is internal. 2 Gas from turbine is passed into cooling chamber. 2 Gas from turbine is exhausted to atmosphere. 3 Any type of fluid is used. 3 Only air can be used. 4 Turbine blades cannot be contaminated. 5 Working fluid circulated continuously. 4 Turbine blades get contaminated. 5 Working fluid replaced continuously. 6 Maintenance cost is high. 6 Maintenance cost is low. 7 Mass of installation per KW is more. 8 Any type of fuel is used. 9 Heat exchanger is used. 10 This system required more space. 7 Mass of installation per KW is less. 8 Pure form of fuel should be used. 9 Heat exchanger is not used. 10 This system required less space. 11 Since exhaust is cooled by circulating water, it is best suited fo stationary installation, marine use. 11 Since turbine exhaust is discharged into atmosphere, it is best suited for moving Vehicle like Aircraft. 12 Figure 12 Figure Advantages of closed cycle gas turbine : 1) Use of higher pressure throughout the cycle which is useful for reduce size of plant. 2) No outside air is used for compressing so there is no problem of dust and dirt. 3) Also there is no need of filtration of incoming air. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 6
4) Any type of fuel can be used for combustion purpose. 5) It is not necessary that air is used as working fluid any other fluid having better thermodynamic property can be used. 6) Working fluid circulated continuously. 7) It avoids erosion of turbine blade due to contaminated gases. 8) The exhaust gas from the turbine is passed into cooling chamber. 9) Mass of installation per KW is more. 10) Maintenance cost is low 11) Longer life. Disadvantages : 1) Weight of system is high compared to open cycle. 2) Large amount of water is required for cooling in cooler. 3) System should be air tight when working substance other than air is used. 4) If load on system increases then performance of system is poor. Brayton cycle on P-V diagram and T-S diagram R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 7
Turbojet Engine: In the above figure shows turbojet engine which consist of diffuser, compressor, combustion chamber, turbine and nozzle. 1) At the entrance air enters at the velocity equal to that of aircraft through diffuser where the pressure of air increases above atmospheric. 2) It is further compressed in compressor (rotary-axial or radial type) to higher pressure which driven by turbine then it is supplied to combustion chamber. 3) The liquid fuel is injected under pressure and sprayed into chamber using pump through nozzles and burnt at constant pressure and temperature of air raises. 4) Then products of combustion are expanded in axial flow turbine. 5) The products of combustion after expansion through the turbine are discharges through nozzle as gases throughout from nozzle into atmosphere pressure drops With increase in velocity. It provides necessary power to drive the compressor. The discharge nozzle in which expansion of gases is completed and thrust of Propulsion is produced. The velocity in the nozzle is greater than flight velocity. It works according to third law of Newton. Advantages : 1) Simple construction 2) No vibrations 3) High speed 4) Pilot s view from cock pit is better. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 8
Disadvantages : 1) Trust at take off is low. 2) Expensive material is required. 3) More noise 4) Huge fuel is required. Turboprop engine : In the above figure shows a turboprop system employed in aircrafts. 1) Working of turboprop engine is similar as that of turbojet engine. 2) It consist of propeller It is used to give small additional trust in forward direction. 3) Compressor It takes atmospheric air and compresses it to high pressure it is rotary axial type compressor. 4) The overall efficiency of turboprop is improved by providing the diffuser before the compressor as shown. The pressure rise takes place in the diffuser. This pressure rise take due to conversion of kinetic energy of the incoming air (equal to aircraft velocity) into pressure energy by diffuser. This type of compression is known as ram effect. 5) Then air enters into combustion chamber in which fuel is supplied with a spark hot gases are generated in it. 6) Hot air is expanded at turbine (80%) which is again axial flow type and remaining 20% expansion in nozzle. 7) Hot air/gasses are throughout through nozzle with sudden drop in pressure and increase in velocity which causes engine to move. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 9
8) Here total power developed by turbine is used to run compressor as well as propeller. The forward motion of air craft is given party by propeller and partly by jet produced by nozzle. Advantages : 1) Low specific weight. 2) Simplicity in design. 3) It has high propulsion efficiency at speed below 600 Km./hr. 4) The overall efficiency of the turbo prop is more. R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 10
Question Bank : 1 Give the classification of gas turbine on basis of working principle. 2 Give the classification of gas turbine 4m 3 Classify gas turbine on basis ofa) Cycle of operation. b) Thermodynamic cycle c) Application d) Fuel used. 4 5 Identify the application of gas turbine in aviation industries. Explain Brayton cycle on PV and TS diagram./ Draw Brayton cycle on P-V diagram and T-S diagram 2 m 6 Give four advantages and disadvantages of closed cycle gas turbine 7 8 Difference between closed cycle and open cycle gas turbine Difference between closed cycle and open cycle gas turbine On the basis of i) Line diagram showing construction ii) Working principle iii) Working pressure iv) Maintenance v) Corrosion deposits of dust and dirt 9 Draw neat labeled sketch of turbo prop engine. 10 11 Draw a schematic diagram of turbojet engine and explain its working. Write its application. Classify atmospheric jet engine. Explain turbojet engine. 8 m 12 Draw a schematic diagram of turboprop engine and explain its working. Also represent it on PV diagram. 13 Describe with neat sketch construction and working of closed cycle gas turbine. 14 Describe with neat sketch construction and working of open cycle gas turbine. 8 m 8 m R.K.Yadav/Automobile Engg Dept/New Polytechnic Kolhapur. Page 11