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98-Mar-B5 May 2015 Page 6 of 19 QUESTION 4 CURTIS TYPE IMPULSE TURBINE Refer to the Examination Paper Attachments Page 12 Steam Turbine Velocity Diagram (one stage only) for nomenclature of velocities and angles, Steam exits the nozzles and enters the first stage moving blades of a velocity compounded two stage (Curtis) impulse turbine at 1411 m/s. The nozzle angle is 20º and the fixed blade exit angle is the same as its inlet angle, that is, the fixed blades are symmetrical. The moving blades of both stages are also symmetrical but with different angles. Assume zero fluid friction in nozzles and blades. (a) Determine a blade velocity to give optimum work (minimum exit kinetic energy). (b) Draw to scale, as recommended below, the velocity diagrams for the two stages. (c) Determine all the actual and relative steam velocities and blade angles and show them on the diagrams. (d) Calculate the work done by each stage, in kJ/kg of steam. (e) Calculate the total power output for a steam flow of 100 kg/s. (1) Calculate the blade efficiency. Note: The scale drawing should be to a large enough scale for accurate measurements (a scale of 1 mm = 10 m/s is suggested). While calculation of velocities by trigonometric ratios is acceptable it is longer and more time consuming (10 marks)
98-Mar-B5 May 2015 Page 12 of 19 QUESTION 4 STEAM TURBINE VELOCITY DIAGRAM Nomenclature for velocity vectors and angles O Vs1 VR1 0 0 Vg Y 8 VR2 Vs2 Ve VS1 VRA Absolute steam velocity entering moving blades Relative steam velocity entering moving blades Vg Moving blade velocity VR2 Vsz Relative steam velocity leaving moving blades Absolute steam velocity leaving moving blades