Expt5 Pelton

ME 413 Expt #5

PERFORMANCE CHARACTERISTICS OF PELTON WHEEL

Purpose: To investigate the performance characteristics of an Impulse Turbine (Pelton wheel), and compare with its ideal efficiency curve. Apparatus: Armfield Hydraulic Bench, Armfield F1-25 Pelton turbine demonstration unit, stopwatch and tachometer. The turbine demonstration unit sits on top of the hydraulic bench, which circulates water from its reservoir through the turbine. The flow rate is controlled by the spear valve (2), and can be measured by means of a stopwatch and the volumetric gage on the bench. The total water head (in meters) at the turbine inlet is indicated by the pressure gage (1). The slide rod and the lock screw (6) adjust the tension in the belt below the spring balances (5). Rotation speed (in RPM) is measured by a digital tachometer counting the frequency of the marking on the surface of the brake drum on the back side of the turbine. For Lab Report: Sketch a schematic diagram of the apparatus Review of Theory Apply the continuity equation and energy equation across a Pelton turbine to show that the ideal power available is Pi    g  Q  H

where , Q = fluid density and flow rate, respectively. H = total head change across the turbine, which is approximately same as the static head change if the velocity head and elevation head changes are small. Ideal Efficiency Sketch a diagram of a Pelton bucket. Apply the Bernoulli equation to show that the ideal jet V j  2  g  H velocity is: Apply the momentum equation, neglecting viscous effects, to show that the ideal power output from the Pelton wheel is: P    Q  u  (V j  u )  (1  cos  )

where  = 180o u  2  n  r

is geometric deflection angle of the bucket, is the linear velocity of the bucket and r is the radial distance of the jet from the turbine axis.

Hence, the ideal efficiency is: where

 

u Vj

  2  (1  cos  )    (1   )

is the velocity ratio, also called the peripheral-velocity factor.

Actual Efficiency The efficiency of the turbine can be found experimentally:  

Pb Pi

where Pb  2  n  T is the output power applied on the brake, and is the torque acting on the brake at rotor speed n with the brake force F on T  F R the brake drum with radius R. Procedure 1. Turn on the pump on the hydraulic bench and fully open the control valve. 2. With the friction belt on the brake drum completely disengaged, adjust the needle valve at the exit of the nozzle so that the total pressure head is about 20 m. 3. Record the time required to collect 40 l of water in the collection tank of the hydraulic bench. 4. Measure the rotation speed of the turbine with the tachometer. 5. Tighten the friction belt so that the difference in the tension of the two spring balances is about 1 N. 6. Repeat step 4 and 5 until the maximum reading on the spring balance is reached, or the rotation speed is zero. Data Radius from Turbine center to water jet: r = 5.5 cm.

Brake drum Radius: R= 3 cm.

Head at inlet: H = ______ m

Flow Rate: Q = _______ l/sec

Rot. Speed (RPM) n (Hz) Friction F (N) Results 1. Plot both the ideal and actual efficiency curves versus u/Vj and compare. 2. Calculate the dimensionless flow coefficient, pressure coefficient and power coefficient defined as follows: CQ 

Q n  D3

C p 

p   n2  D2

CP 

Pi   n3  D5

3. Plot actual efficiency, pressure coefficient and power coefficients versus flow coefficient. Problem Find the best flow coefficient for this turbine, and predict the flow rate, pressure head and the power output for a geometrically similar turbine which is 10 times larger in dimension and rotating at one-fifth of the speed.