1. Advantages of Rotating Machines Less balancing problems Low lubricating oil consumption Higher reliability

2. Three Processes Compression Combustion Expansion

3. Fuel/Air Ratio Max Fuel/Air ratio is governed by: Working temp. of highly stressed turbine blades. Critical value of strength of turbine material. Working life of the turbine required.

4. Turbine Performance Depends upon: Component efficiency Turbine working temp. Reduction in Comp. work Increase in Expansion work For a given pressure ratio, power required per unit quantity of fuel is proportional to the inlet temp.

5. Two System of Combustion Const. Volume Combustion: High thermal efficiency Higher mechanical difficulties Const. Pressure Combustion Continuous combustion process Can handle high mass flow rate.

6. Single & Twin Shaft Arrangements Single Shaft Arrangement For const. speeds or fixed loads Base loads Twin Shaft Arrangement Variable speed load e.g pipeline comp., marine propeller No reduction gear box Over speed problems are high

7. Simple Gas Turbine Cycle, The Brayton Cycle Assumptions: Min. friction and pressure losses Const. mass flow rate Working fluid, same composition Negligible K.E change at inlet and outlet of each component

8. Simple Gas Turbine Cycle, The Brayton Cycle

9. Cycle Efficiency

10. Key Parameters (t, r) Temperature Ratio (t) = T 3 /T 1 Pressure Ratio (r) = P 2 /P 1 = P 3 /P 1 As P 2 and P 3 are virtually same.

11. Thermodynamic Calculations t = T 3 /T 1 = 2.5 ; r = P 2 /P 1 = 3.5 T 1 = Ambient Temp. = 40 0 C = 313 K T 3 = t×T 1 = 2.5×313 = 782.5 0 C  800 0 C =1073K Since Therefore T 2 = × T 1 = 3.5 × 313 = 447.7K  450K

12. Cont…

15. Efficiency and Specific Work Efficiency depends on: Pressure ratio Nature of Gas Max. Specific Work depends on: Pressure ratio Max. cycle temperature

16. Efficiency and Specific Work

17. Centrifugal Compressor Most suitable for smaller gas turbines For handling small volume flows Shorter length than an equivalent axial comp. Better resistance to foreign object damage Less susceptibility to loss of performance by build-up of deposits on blade surface Ability to rotate over a wider range of mass flow at particular rotational speed

18. Materials Aluminum Alloy: For medium speeds Can build pressure ratios up to 4:1 Suitable for turbine inlet temp. in range 1000- 1200 K Titanium Alloy: For much higher speeds Can build pressure ratios up to 8:1 in a single stage

19. Uses Natural gas pipelines Small airplanes Air separation plants Large scale refrigeration plants

20. Principle of Operation

21. Cent. Comp consists of: A stationary casing Rotating Impeller: which imparts a high velocity to the air Diffuser: which are fixed diverging passages where air is decelerated with a consequent rise in static pressure

22. Principle of Operation Air is sucked into the impeller eye and whirled round at high speed by the vanes. Static pressure increases from eye to the tip of the impeller. The remainder of the pressure is obtained in the diffuser.

23. Cent. Compressor Losses Friction in impeller and diffuser Boundary layer separation due to poor design Owing to the action of the vanes in carrying the air around with the impeller, there will be slightly higher static pressure on the forward face of a vane than on the trailing face. The air tend to flow in the space b/w the impeller and casing, which causes a loss in efficiency.

24. Assumptions Air enters the impeller eye in axial direction Initial angular momentum of the air is zero To pass air smoothly through the eye, axial portion of the vanes must be curved. Inlet vane angles are given by relative velocities.

25. Design of Cent. Compressor

26. Design Assumptions Slip Factor:  = 0.9 Power Factor: = 1.04 Overall impeller diameter: D 2 = 0.4m Eye tip Diameter: D 1tip = 0.2m Hub diameter: D 1hub = 0.1m Isentropic Efficiency:  c = 0.75 Air mass flow: = 1 kg/s Isentropic Efficiency:  C = 0.75

27. Impeller Calculations

28. Cont…