Impulse and Reaction Turbines

A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.

A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice.

Velocity is the measurement of the rate and direction of change in the position of an object. It is a vector physical quantity (both magnitude and direction are required to define it).

Velocity triangles for both Impulse and Reaction Turbines and the Force on the Blades

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Introduction

The vast majority of Turbines consist of a number of curved blades or cups which are attached to a wheel and move. This is actually not always true.

In Southern Sweden, there is a rudimentary wooden turbine which consisted of a wheel with a vertical axis and which was fitted with a number of flat wooden vanes set radially around the circumference. Water entered through three or four wooden "nozzles" of square cross section and after hitting the vanes fell down out of the machine. The efficiency of this arrangement is not known but it is unlikely to be high! It should be added that the head available to the mill was not high.

In Britain water power traditionally relied on Waterwheels.Two sorts were common. The undershot and the overshot. The efficiency of both were poor compared to a modern turbine.

This design worked on a modest head but suffered from the leakage from the wheel. The efficiency was further reduced by the need for the wheel to "push" water along the tail race. It would have been possible to site the wheel clear of the tail race but this would have exacerbate the leakage problem.

This was more efficient. Clearly as the wheel rotated water spilled from the cups and as a result the wheel did not make full use of the available head

It should be stated that the heads available in Britain, particularly in the Southern part, are in general not more than a few feet and quite unsuitable for many designs of modern turbine.

Velocity Triangles

To analyse the flow through moving curved vanes it is necessary to draw Velocity triangles.

The following symbols are used in their construction.

$V_r =$ The Relative Velocity and is tangential to the blades.

$v =$ Blade Velocity and is Added to $V_r$ .

$V =$ The Absolute Velocity and is the vector sum of $v$ and $V_r$

(Note: The arrows of $v$ and $v_r$ must follow each other around the triangle)

$V_w =$ The velocity of whirl (Component of $V$ in the direction of $v$ )

$V_f =$ The velocity of flow (Component of $V$ normal to direction of $v$ )

The suffix 1 refers to he outlet triangle.

$\alpha$ and $\beta$ are the inlet and outlet angles of absolute velocity.

$\theta$ and $\phi$ are the inlet and outlet angles relative to the blade velocity.

Axial Flow Turbines

At Low Speed the velocity triangles are as follows.

At High Speed, the outlet triangle remains the same but the inlet triangle is now.

Note: $v = v_1$ and $V_r = V_{r1}$ if there is no friction

Pelton Wheel ( Circumferential )

The two velocity triangles are for low and high flow. The inlet triangle is a straight line.

For both types of flow ( $a$ and $b$ ).

Jet velocity $= C_v\;\sqrt{2gH}$

Where $H$ is the head behind the nozzle and $C_v$ is the Velocity coefficient.

Weight of water per second, $W = w\,a\;V$

The blade speed, $v = \frac{\pi \,d\,N}{60}$

Force on the vanes = Mass of water/second multiplied to Change in velocity

$= \frac{W}{g}\left(V_w - V_{w1} \right)$

Work done on the vanes = Force multiplied to Velocity

$= \frac{W}{g}\left(V_w - V_{w1} \right)$

The Kinetic energy supplied $= \frac{W\,V^2}{2g}$

The efficiency, $\eta =$ Work done / K.E.supplied

$\therefore\;\;\;\;\;\eta = \frac{2\,v(V_1 - V_{w1})}{V^2}$

For a Pelton Wheel Only

$V_w = V$

$V_{r1} = V_r$ (If friction is ignored.)

$\therefore \;\;\;\;\;V_{r1} = V - v$

But: $V_{w1} + v_1 = V_{r1}\,cos\phi$

$= (V - v)\;cos\phi$

$\therefore\;\;\;\;\;V_{w1} = (V - v)\;cos\phi - v$

$\therefore\;\;\;\;\;\eta = \frac{2v}{V^2}\left(V_w - V_{w1}\right)}$

$= \frac{2v}{V^2}\left[(V - v) + (V - v)cos\phi \right]$

For the maximum $\eta$ at a given head and blade angle:

$\frac{d\eta }{dv} = 0$

Which occurs when $v = V/2$ i.e. The bucket speed is half the jet speed. This is a theoretical figure and in practice, due to frictional losses, the maximum efficiency is when $v\;\approx \;0.47\;V$

Example 1 [imperial]

Problem

The diagram shows a section of a Pelton wheel which has a 2 in. Diam. Jet which produces 2 cubic ft. of water per second. The blade speed is 40 ft/sec and due to friction $\;V_{r1} = 0.9\;V_r$ . ( 1 cubic foot of water weighs 62.4 lbs. and $g$ = 32.2 ft/s.).

Find the Kinetic Energy supplied by the jet per second and the efficiency of the turbine.

Workings

To find the force in the $X$ direction:

Jet $V$ : $V = \frac{Q}{area} = \frac{2}{ \displaystyle\frac{\pi }{4}\times \displaystyle\frac{1}{36}} = 91.7 ft/sec.$

And: $V_w\;=91.7\,ft/sec$

$v_r = V - v = 91.7 - 40 = 51.7ft/sec$

And: $V_{r1} = 0.9V_r = 46.53ft/sec$

From the outlet triangle:

$(v_1 - V_{w1}) = V_{r1}\times cos45^0 = \frac{46.53}{\sqrt{3}} = 32.9ft/sec.$

And: $V_{r1} = 40 - 32.9 = 7.1ft/sec.$

Also: $V_{f1} = 32.9ft/sec$

In the $X$ direction the Force on the vanes = mass of water $X$ change of velocity.

$= \frac{2\times 62.4}{32.2}\times \left(91.7 - 7.1 \right) = 328 lbs.$

Force on vanes in the $Y$ direction $= \frac{2\times 62.4}{32.2}\left(0 - 32.9 \right) = 127.5lbs.$

The resultant force $= \sqrt{328^2 + 127.5^2} = 352lbs.$

The resultant is at $\tan^{-1}\;\frac{127.5}{328}$ i.e. at $21^0\,14'$ to the direction of motion.

Work done per second on the vanes = The force in the $X$ direction times blade velocity:

$= 328\times 40 = 13,120\;ft\,lbs/sec$

Solution

The Kinetic energy supplied by the jet per second $=\frac{W\,V^2}{2g} = \frac{2\times 62.4\times 91.7^2}{2\times 32.2} = 16,280\;ft\,lbs/sec.$

Thus, the efficiency of the turbine $= \frac{13220}{16280} = 80.6\%$

Turbine with Curved Vanes and an Inward Radial Flow ( Francis or Gerard Turbine)

The following diagram shows the velocity triangles for both low and high speed

Let:

The weight of water/second striking the vanes be $W$ lb/sec.

Tangential momentum/second at entry $= \displaystyle\frac{W}{g}V_w$ .

Moment of momentum at entry = $\displaystyle\frac{W}{g}V_w\times v$ .

Moment of momentum at outlet = $\displaystyle\frac{W}{g}(V_{w_1}\times v_1)$ .

Then the Torque on the vanes equals the change of moment of momentum per second

$=\frac{W}{g}\left(V_w\,v - V_{w1}\,v_1 \right)$

The work done per second on the vanes equals the Torque times the angular velocity

$= \frac{W}{g}\left(V_w\,v - V_{w1}\,v_1 \right)\Omega$

But: $\Omega_r = v$ and $\Omega_r_1 = v_1$

work done/second $= \displaystyle\frac{W}{g}\left(V_w\,v -V_{w1}\,v_1\right)$

This is the Euler equation which can be applied to any type of turbine or centrifugal pumps.

Example 1 [imperial]

Problem

Derive an expression for the hydraulic efficiency of a turbine in terms of the tangential velocities of the runner, the velocities of whirl at inlet and outlet and $H$ the supply head. Take all velocities in the direction of the runner as positive.

An inward flow reaction turbine discharges radially and the velocity of flow is constant and equal to the velocity of discharge from the suction tube.

Show that the hydraulic can be expressed by:

$\eta =1/1+\frac{\displaystyle\frac{1}{2}\tan^2\alpha}{1-(\tan\alpha/\tan\theta)}$

Where $\alpha$ and $\theta$ are the guide vane angles at inlet.

Workings

Please refer to the velocity triangles in the diagram.

The Force = The rate of change of Momentum
The available tangential force at the wheel entry $=\frac{W}{g}\times v_{w1}\;lb.$ And the available useful power at the wheel entry $=\frac{W}{g}\times v_{w1}U_1\;ft.lb/sec.$ Similarly the useful power expelled at exit $=\frac{W}{g}\times v_{W2}\times U_2$

But the power available $=WH\;ft.lb./sec.$

The power given to the wheel $=\frac{W}{g}\left ( v_{W1}U1-v_{W2}U_2 \right )\;ft.lb./sec.$ $\therefore \;\;\;\;\eta=\frac{ v_{W1}U1-v_{W2}U_2 }{gH}$

Since $v_2$ is radial $v_{W2}=0$

The work done per lb. of water $=v_{W1}\times \frac{U_1}{g}$

From the vector triangles it can be seen that:

$\frac{v_f}{v_{W1}}=\tan\alpha$ i.e. $v_{W1}=\frac{v_f}{\tan\alpha}$ Also: $\frac{v_f}{U_1-v_{W1}}=\tan(180^0-\theta)=-\tan\theta$

From the above equations:

$U_1=v_f\times \frac{\tan\theta-\tan\alpha}{\tan\theta\tan\alpha}$ The work done per lb.

$=\frac{{v_{f}}^{2}}{g}\times \frac{\tan\theta-\tan\alpha}{\tan\theta\tan^2\alpha}$

(3)

If there are no guide vane losses:

$H= W+\frac{{v_{f}}^{2}}{2g}$ $\eta=\frac{W}{W}+\displaystyle\frac{{v_{f}}^{2}}{2g}}$

$\therefore \;\;\;\;\eta=\frac{1}{1+\displaystyle\frac{{v_{f}}^{2}}{2g}/W}$

(4)

(where $W=$ Work done per lb.)

Solution

From Equations (#1) and (#2)

$\eta=\frac{1}{1+\displaystyle\frac{\frac{1}{2}\tan^2\alpha\tan\theta}{\tan\theta-\tan\alpha}}=1/1+\frac{\displaystyle\frac{1}{2}\tan^2\alpha}{1-(\tan\alpha/\tan\theta)}$

Example 2 [imperial]

Problem

State briefly the reasons for fitting a draft tube to a reaction turbine and sketch three common types.

In a vertical Francis Turbine, the runner speed is 380 r.p.m. and the available head across the turbine is 150 ft. The inlet runner is 6 ft.above the tail race level and the area and diameter of the runner at inlet are 2.6 sq.ft. and 3 ft respectively. The guide and runner vane angles at inlet are shown in the diagram.

The water enters a draft tube without whirl 5.25 ft. above the tail-race and the draft tube diameter here is 1 ft. 10 in. At outlet the draft tube diameter is 2 ft. 4 in. If the frictional losses in the runner amount to 9 ft.lb/lb and in the draft tube 5 ft.lb./lb. and the overall efficiency is 0.9 X the hydraulic efficiency.

Find:

The B.H.\b{P}. of the Turbine.

The hydraulic efficiency.

The pressure head at the inlet to the runner in ft.lb./lb.

The pressure head at the entry to the draft tube in ft.lb./lb.

Workings

The reasons for fitting a draft tube are that it allows the turbine to be mounted above the tail race without there being a significant loss of head a loss of head. In addition because of the gradual increase in cross section the discharge velocity from the turbine is not all wasted as some is converted into useful a pressure head.

Three types of draft tubes are shown in the following sketch. From the left these are a simple cone, spreading and elbow types.

For the calculations please refer to the following two diagrams:

$U_1=R_1\times \frac{2\piN}{60}=\frac{1.5 \pi\times380}{30}=59.7\;ft./sec.$ $\frac{v_1}{\sin110^0}=\frac{U_1}{\sin55^0}$ i.e. $v_1=\frac{59.7\times 0.94}{0.82}=68.5\;ft./sec.$ $v_{W1}=v_1\cos15^0=68.5\times 0.996=66.2\;ft./sec.$ The work done per lb. $=\frac{U_1v_{W1}}{g}-\frac{59.7\times 66.2}{g}=123\,ft.lb./lb,$ Thus the Hydraulic Efficiency $=\eta_h=\frac{\text{Work done per lb.}}{H}=\frac{123}{150}=0.82$ The overall Efficiency, $\eta_o=0.9\times0.82=0.738$ $W=wA_1v_{f1}=wA_1v_1\sin15^0$ $=62.4\times 2.6\times 68.5\times 0.259=2880\;lb./sec.$

Therefore the B.H.P.generated is given by:

$\frac{\eta_0\times WH}{550}=\frac{0.738\times 2880\times 150}{550}=580\; b.h.p.$

For the Draft tube

$Q=\frac{W}{w}=Av$ = A Constant and therefore if $v_d$ is the exit velocity from the draft tube: $\frac{2880}{62.4}=\frac{\pi}{4}\times \left ( \frac{22}{12} \right )^2\times v_{f2}=\frac{\pi}{4}\times \left ( \frac{28}{12} \right )^2\times v_d$ $\therefore \;\;\;\;v_{f2}=17.45\;ft./sec.$ And: $v_d=10.75\;ft./sec.$

The total head, $H=$ Guide loss + Rotor loss +Work done + Draft-tube loss + $+\frac{{v_{d}}^{2}}{2g}$ i.e. $150=\text{Guide loss}+9+123+5+\frac{10.75^2}{2g}$ Therefore, Guide loss $=11.14\,ft.$

If suffix 1 refers to the runner inlet and suffix 2 refers to the draft-tube inlet then:

$H_1=\frac{p_1}{w}+\frac{{v_{1}}^{2}}{2g}+z_1$

Also:

$h_2=H_1$ - Work done/lb. - Rotor loss $=\frac{p_2}{w}+\frac{{v_{f2}}^{2}}{2g}+z_2$ i.e. $150-11.14-123-9=\frac{p_2}{w}+\frac{17.5^2}{2g}+5.25$ $\therefore \;\;\;\;\frac{p_2}{w}=-3.14\;ft.$ i.e. $3.14ft.$ below atmospheric pressure