TURBINES & AXIAL-FLOW MACHINES UNIT - 3 Section (12.5)
HYDEL TURBINES (Dynamic machine to extract energy from fluid) 1. Reaction (Static pressure changes) : Converts both Flow & Kinetic energy (a) - Radial or (Francis turbine) (b) - Axial flow or propeller turbine (Kaplan, Bulb) - Mixed flow (Francis turbine) 2. Impulse: (Static pressure unchanged) Converts only Kinetic energy (a) -Tangential flow on buckets (Pelton)
1a.Francis turbine (Radial) Runner
1b. Axial flow (Kaplan) Turbine (Reaction)
1b. Axial flow (Bulb) Turbine (propeller type) (Reaction) Used for very low head high volume flow Ideal for tidal power plant
1c. Francis Turbine (Mixed flow) (Reaction)
2. Impulse turbine (PELTON WHEEL) Converts kinetic energy alone
IMPULSE TURBINE High velocity jet discharging at atmosphere pressure hit buckets turning the turbine Momentum change is through change in flow direction No Pressure Change As jet velocity is depends on the head (v ~ H 0.5 ). So it is unsuitable for low head. It requires high head Ideal Specific speed is 1-10 rpm
Velocity diagram of impulse turbine
Power developed (Impulse turbine) Jet velocity V 1 = C v (2gH) 0.5 Tangential velocity at entry, V t1 =V 1 = v 1 +u at exit V t2 = u v 2 cos β 2; Euler Eq. Energy transfer/mass = (u V t1 -u. V t2 ) Substituting P = (ρq)u[v 1 +u (u v 2 cos β 2 )] = (ρq)u(v 1 u)(1+ k.cos β 2 ) where k = (v 2 /v 1 ). Ideally velocity does not change in impulse turbine blades giving k =1, but some loss may occur Efficiency of runner = Power/Kinetic energy of jet = (ρq)u(v 1 u)(1+cos β 2 )/[(ρq) V 12 /2] = (2u/ V 12 )(V 1 u) (1+cos β 2 )
Radial/Mixed flow Turbine (FRANCIS TURBINE) In a reaction turbine (hydraulic, steam, or gas), a part of the head is converted into KE in stationery guide vanes. Static pressure changes in the runner Used for low head high flow application It can be mixed flow or radial type It gives high efficiency but is unsuitable for high head Specific speed for such turbine is 10-110
AXIAL FLOW MACHINES for air/gas (12.2.2)
An axial flow machine
Axial Flow Machines Here fluid flows parallel to the machine axis Propeller, air circulator, table fan are examples of axial flow pump/fan It is also used extensively in turbines Axial flow machines gives high flow at low head. For high head series of impellers (blades) are mounted in series. Guide or static vanes are used in-between to guide the flow for optimum efficiency Its impellers are designed to give constant axial velocity at all radius but varying head.
Axial flow machines Fluid enters and leaves at the same radial distance giving u 1 = u = u 2 ; V n1 =V n = V n2 Eq.(8) is valid here. So we can write 2 u2( u2 Vn2 cot β2) u uvn cot β2 (14) H = g Eq. 14 suggests that head varies from axis to periphery with u. Axial flow fans are designed to have same velocity at all radius. Can you suggest how to do this? = g g
Axial flow turbine Power developed ~ Flow rate x Head available Axial flow turbine allow large flow rate and therefore work with low velocity or low head Guide vanes gives free vortex type whirl to water (V t ~ R -1 ) To handle large flow rate blades are large. Blade velocity (u ~ R) increases with radius. But fluid velocity (V t ~ R -1 ). To cater for this difference the runner blades are twisted making larger angle at the tip For maximum efficiency blades are designed such that tangential component of exit velocity, V t1 is zero P = ρ Q(u 2 V t2 u 1 V t1 ) = ρ Qu 2 V t2 Specific speed is 400-800 rpm
Multi blade Reaction Turbine
Turbine selection The BHP output depends on Q, H, n & D Specific speed N sp compares output with Head N sp = N(rpm) (BHP) 0.5 /[{Hft)) 1.25 ] Optimum efficiency of turbine depends on its specific speed
Problem on Pelton wheel A pelton wheel driven by two similar jets transmits 3750 kw to the shaft when running at 375 rpm. The head from the reservoir level to the nozzles is 200 m and there 10% loss in head for flow through the pipelines and nozzles. The jets are tangential to a 1.45m diameter circle. The relative velocity decreases by 10% as the water traverses the buckets, which are so shaped that they would, if stationery, deflect the jet through 165 0. Neglecting windage losses, find: (a) the efficiency of the runner and (b) the diameter of each jet [Ans: 93.3%, 157 mm]
Home work An axial flow fan has a hub diameter of 1.5 m and tip diameter of 2 m. It rotates at 18 rad/s and, when handling 5 m 3 /s of air, develops a theoretical head equivalent to 17 mm of water. Determine the blade outlet and inlet angles at the hub and at the tip. Assume that the velocity of flow is independent of radius and that the energy transfer per unit length of blade is constant. Air density = 1.2 kg/m 3 and water density = 1000 kg/m 3 [11.4 0, 15.1 0, 19.5 0, 48.6 0 ] (Douglas p-719)