Tham khảo tài liệu 'mechanical engineers data handbook episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | THERMODYNAMICS AND HEAT TRANSFER 115 Velocity compounded impulse turbine One row of nozzles is followed by two or more rows of moving blades with intervening rows of fixed blades of the same type which alter the direction of flow. Pressure I I ss bp PHP-DEE blades SZO3SEEED Two-row wheel Assume Pt P2 fc l and that all blades are symmetrical. Maximum efficiency mai cos al at P I in which case the steam leaves the last row axially. Impulse-reaction turbine In this case there is full admission . ỡ 360 . Both nozzles and moving blades are similar in shape and have approximately the same enthalpy drop. Referring to the figure Enthalpy drop io h for the fixed blades hj h2 for the moving blades . b h2 b h2 Degree of reaction R r r L h -h h0-h2 50 reaction Parson s turbine In this case the velocity diagram is symmetrical. . . 27iR_hCsina Mass flow rate m ---- -------- V where a blade outlet angle. Enthalpy drop per stage A i8 C2p 2cosa p cb where P Q and Cb 2nRmN. Stage power P8 mA i8 2p 2cosa p Stage efficiency n ---------7 s l p 2cosa p . . . _ . 2 cos2 a Maximum efficiency 5 when p cosa l cos2a 116 MECHANICAL engineer s data handbook Reheat factor and overall efficiency Referring to the condition curve on the h-s diagram A ia available stage enthalpy drop A iị isentropic stage enthalpy drop A ioa available overall enthalpy drop A 1OI isentropic overall enthalpy drop _ . A ia Stage efficiency rjs -rr ant Overall efficiency n. gA A 1O1 Reheat factor RF Ị. Gas turbines The gas turbine unit operates basically on the constant-pressure cycle particularly in the case of the closed cycle . In the open cycle air is drawn in from the atmosphere compressed and supplied to a combustion chamber where fuel is burnt with a large amount of excess air . The hot gases drive a turbine which drives the compressor and also provides useful work. The efficiency increases with compression ratio. The output power increases with both compression ratio and turbine inlet .
