FLUID MACHINES

A fluid machine is a device which convert the energy stored by a fluid into mechanical energy or vice versa. The energy stored by a fluid mass appears in the form of potential, kinetic and intermolecular energy. The mechanical energy, on the other hand, is usually transmitted by rotating shaft. Machines using liquid (mainly water, for almost all practical purpose) are termed as hydraulic machines. In this chapter we shall discuss, in general, the basic fluid mechanical principle governing the energy transfer in a fluid machine and also a brief description of different kinds of hydraulic machines along with their performance.

FLUID                       TYPES OF TURBINE

Water                          Hydraulic Turbine
Steam                          Steam Turbine
Froen                          Vapour Turbine
Gas or air                    Gas Turbine
Wind                           Wind Mills

Similarly, fluid machines which convert shaft power to fluid power by raising the energy content per unit mass of the fluid are classified as follows:

Fluid                                                Types of Machine

Water and Other liquids                     Pumps

Air and Other Gases                          Fans and Propellers
(with slight pressure rise) 

Air and Other Gases                          Blowers and Compressors
(with higher pressure rise)



CLASSIFICATION OF FLUID MACHINES 


The turbines in general are classified in two ways:

• According to the direction of flow of water through the runner

• According to the action of water on the runner blades.

In order to classify machines according to the direction of flow of water through the
runner, three mutually perpendicular directions for flow of water are chosen


RADIAL FLOW MACHINE


The path of water particles is wholly or mainly in the plane of rotation. i.e., the water
enters the runner at the outer periphery, flows inwards in the radial direction and leaves at
a different radius as shown in figure 4.1(b).


AXIAL FLOW MACHINES

The water mainly flows through the runner in a direction parallel to the axis of rotation as shown in figure 4

MIXED FLOW MACHINES


The flow in the runner may not be merely in one direction.
turbines, water enters radially inwards and emerges out axially so that parallel to the axis of the shaft as shown in figure 4.1(d).

In mixed flow the discharge in According to the action of water on moving blades, the turbine way be placed in one of the two general categories: i Impulse ii Reaction.

In a hydroelectric power scheme, water in a very large quantity is stored in a high
level reservoir. In an impulse turbine, the water is brought to the turbine entrance through penstock pipes ending in one or more fixed nozzles. The entire pressure energy of water is converted into the kinetic energy of an unconfined jet. The jet of fluid then strikes the blades of the runner and loses practically all of its kinetic energy, i.e., the velocity of water at the exit of the runner is just sufficient to enable
it to move out the runner. The static pressure of water at the entrance to the runner
is equal to the static pressure at exit and the rotation of the wheel is caused purely due to the tangential force created by the impact of the jet, and hence an impulse turbine. The most common impulse turbine is called Pelton turbine.

Tangential flow machine

The water strikes the blades or buckets of the runner in a direction tangential to the path of rotation. The tangential direction is perpendicular to both axial and radial directions as shown in figure 4.1(c).


EXCHANGE OF ENERGY


•    A machine wherein rotary motion is obtained by centrifugal forces which result
from a change in the direction of high velocity fluid jet that issues from a nozzle.

A  hydraulic  turbine  uses  the  potential  and  kinetic  energy  of water  and  convert  it  into usable  mechanical energy.  The fluid  energy is available  in the  natural or artificial high level water reservoirs which are created by constructing dams at appropriate places in the flow  path  of rivers.  When  water  from  the  reservoir  is  taken to  the  turbine,  transfer  of energy  takes  place  in  the  blade  passages  of  the  unit.  The  mechanical  energy  made available at the turbine shaft is used to run an electric generator which is directly coupled
to the turbine shaft.

The power generated by utilizing the potential  and kinetic  energy of water has the
advantages of

•    High efficiency
•    Operational flexibility
•    Low wear and tear
•    Ease maintenance.

Despite the heavy capital cost involved  in constructing dams and reservoirs,  in running
pipelines  and  in  turbine  installation  (when  compared  to  an  equivalent  thermal  power plant)  different  countries  have tried to tap all their water power resources. Appropriate types of water turbines have been installed for most efficient utilization.