Turbines

 TURBINES

 A hydraulic machine is a device in which mechanical energy is transferred from the

liquid flowing through the machine to its operating member (runner, piston and others) or

from the operating member of the machine to the liquid flowing through it.

 Hydraulic machines in which, the operating member receives energy from the liquid

flowing through it and the inlet energy of the liquid is greater than the outlet energy of

the liquid are referred as hydraulic turbines.

 Hydraulic machines in which energy is transmitted from the working member to the

flowing liquid and the energy of the liquid at the outlet of the hydraulic machine is less

than the outlet energy are referred to as pumps.

 It is well known from Newton’s Law that to change momentum of fluid, a force is

required. Similarly, when momentum of fluid is changed, a force is generated. This

principle is made use in hydraulic turbine.

 In a turbine, blades or buckets are provided on a wheel and directed against water to alter

the momentum of water. As the momentum is changed with the water passing through

the wheel, the resulting force turns the shaft of the wheel performing work and generating

power.

 A hydraulic turbine uses potential energy and kinetic energy of water and converts it into

usable mechanical energy. The mechanical energy made available at the turbine shaft is

used to run an electric power generator which is directly coupled to the turbine shaft

 The electric power which is obtained from the hydraulic energy is known as Hydro- electric energy. Hydraulic turbines belong to the category of roto- dynamic machinery.

The hydraulic turbines are classified according to type of energy available at the inlet of

turbine, direction of flow through vanes, head at the inlet of the turbines and specific

speed of the turbines.

According to the type of energy at inlet:

 Impulse turbine: - In the impulse turbine, the total head of the incoming fluid is converted in to a large velocity head at the exit of the supply nozzle. That is the entire available energy of the water is converted in to kinetic energy. Although there are various types of impulse turbine designs, perhaps the easiest to understand is the Pelton wheel turbine. It is most efficient when operated with a large head and lower flow rate.

Reaction turbine: Reaction turbines on the other hand, are best suited -for higher flow rate and lower head situations. In this type of turbines, the rotation of runner or rotor (rotating part of the turbine) is partly due to impulse action and partly due to change in pressure over the runner blades; therefore, it is called as reaction turbine. For, a reaction

turbine, the penstock pipe feeds water to a row of fixed blades through casing. These

fixed blades convert a part of the pressure energy into kinetic energy before water enters

the runner. The water entering the runner of a reaction turbine has both pressure energy

and kinetic energy. Water leaving the turbine is still left with some energy (pressure energy and kinetic energy). Since, the flow from the inlet to tail race is under pressure, casing is absolutely necessary to enclose the turbine. In general, Reaction turbines are medium to low-head, and high-flow rate devices. The reaction turbines in use are Francis and Kaplan  

According to the direction of flow through runner:

 Tangential flow turbines: In this type of turbines, the water strikes the runner in the direction of tangent to the wheel. Example: Pelton wheel turbine.

 Radial flow turbines: In this type of turbines, the water strikes in the radial direction. accordingly, it is further classified as, inward flow turbine: The flow is inward from periphery to the centre (centripetal type). Example: old Francis turbine.

b. Outward flow turbine: The flow is outward from the centre to periphery (centrifugal type). Example: Fourneyron turbine.

 Axial flow turbine: The flow of water is in the direction parallel to the axis of the shaft. Example: Kaplan turbine and propeller turbine.

 Mixed flow turbine: The water enters the runner in the radial direction and leaves in axial direction. Example: Modern Francis turbine.

According to the head at inlet of turbine:

 High head turbine: In this type of turbines, the net head varies from 150m to 2000m or even more, and these turbines require a small quantity of water. Example: Pelton wheel turbine.

 Medium head turbine: The net head varies from 30m to 150m, and also these turbines require moderate quantity of water. Example: Francis turbine.

 Low head turbine: The net head is less than 30m and also these turbines require large quantity of water. Example: Kaplan turbine.

According to the specific speed of the turbine

 The specific speed of a turbine is defined as, the speed of a geometrically similar turbine that would develop unit power when working under a unit head (1m head). It is prescribed by the relation, 

Ns = N √p/H^(5÷4)

 Low specific speed turbine: The specific speed is less than 50. (varying from 10 to 35 for single jet and up to 50 for double jet ) Example: Pelton wheel turbine.

Medium specific turbine: The specific speed is varies from 50 to 250. Example:

Francis turbine.

 High specific turbine: the specific speed is more than 250. Example: Kaplan turbine.

Radial flow turbines

 Radical flow turbines are those turbines in which the water flows in radial direction. The

water may flow radically from outwards to inwards or from inwards to outwards.

 If the water flows from outwards to inwards through the runner, the turbine is known as inward radial flow turbine. If the water flows from inwards to outwards, the turbine is known as outward radial flow turbine.

 Reaction turbine means that the water at inlet of turbine possesses kinetic energy as well as  pressure energy.

 The main parts of a radial flow reaction turbine are:

 Casing: - The water from penstocks enters the casing which is of spiral shape in which area of cross section of casing goes on decreasing gradually. The casing completely surrounds the runner of the turbine.

 Guide mechanism: - It consists of stationary circular wheel all round the runner of the turbine. The stationary guide vanes are fixed on guide mechanism. The guide vanes allow the water to strike the vanes fixed on the runner without shock at inlet.

 Runner: - It is a circular wheel on which a series of radial curved vanes are fixed. The surfaces of the vanes are made very smooth. The radial curved are so shaped that the water enters and leaves without shock.

Draft tube: - The pressure at the exit of the runner of reaction turbine is generally less

than atmospheric pressure. The water exit cannot be directly discharged to the tail race.

A tube or pipe of gradually increasing area is used for discharging water from the exit

of turbine to the tailrace. This tube of increasing area is called draft tube.

Axial flow turbines

 If the water flows parallel to the axis of the rotation of the shaft, the turbine is known as

axial flow turbine.

 If the head at the inlet of the turbine is the sum of pressure energy and kinetic energy and

during the flow of water through runner a part of pressure energy is converted into kinetic

energy, the turbine is known as reaction turbine.

 For the axial flow reaction turbines, the shaft of the turbine is vertical. The lower end of

the shaft is made larger which is known as hub. The vanes are fixed on the hub and hence

hub acts as runner for axial flow reaction turbine.

 The following are the important type of axial flow turbines:

1. Propeller turbine

2. Kaplan turbine

 When the vanes are fixed to the hub and they are not adjustable, the turbine is known as

propeller turbine.

 If vanes on hub are adjustable the turbine is known as a Kaplan turbine. This turbine is

suitable where a large quantity of water at low heads is available.

Overall Mass Balance and Continuity Equation

  Overall Mass Balance and Continuity Equation 

In fluid dynamics, fluids are in motion. Generally, they are moved from place to 

place by means of mechanical devices such as pumps or blowers, by gravity head, or by 

pressure, and flow through systems of piping and/or process equipment. 

The first step in the solution of flow problems is generally to apply the principles 

of the conservation of mass to the whole system or any part of the system. 

InpUT – OUTPUT = ACCUMULATION 

At steady state, the rate of accumulation is zero 

∴ INPUT = OUTPUT 

 In the following Figure a simple flow system is shown where fluid enters section 

 1 with an average velocity (u1) and density (ρ1) through the cross-sectional area (A1). 

The fluid leaves section 2 with an average velocity (u2) and density (ρ1) through the 

cross-sectional area (A2). 

Thus, 

At steady state   ṁ1=ṁ2

 Q1 ρ1 = Q2 ρ2

 u1 A1 ρ1 = u2 A2 ρ2

For incompressible fluids at the same temperature [ρ1 = ρ2] 

∴ u1 A1 = u2 A2

Diesel combustion Engines

The processes that occur during diesel combustion are still somewhat mysterious, in spite of extensive attempts at photography and other, more advanced, optical diagnostics. The environment in the diesel combustion chamber appears designed

to resist detailed investigation. It is small, hot, and subject to high pressure and intense vibration. When windows are installed

in the chamber to observe the combustion, soot blocks the passage

of light and quickly deposits on the windows, obscuring further investigation. The combustion involves gas, liquid, and solid

phases as well as complex physical processes and chemical reactions. In spite of this complexity, researchers generally agree about the sequence of processes that occurs in the chamber.

Diesel combustion is the process that occurs when a fuel blend, chosen for its readiness to auto-ignite, is injected into a volume of turbulent air that has been compressed to a high temperature and pressure. The fuel does not ignite immediately.

A time period elapses, called the ignition delay, during which the fuel must vaporize, mix with air, and undergo preflame chemical reactions that produce the chemical species necessary for spontaneous ignition. Because the air temperature is above the thermodynamic critical point of many of the fuel components, vaporization takes place very quickly. In some engines, vapori-zation is complete within a few millimetres of the injection nozzle.After sufficient time has elapsed, ignition will occur spon-taneously in regions of fuel-air mixture that have fuel-air ratios close to the stoichiometric, or chemically correct mixture. Combustion proceeds very rapidly because of the backlog of prepared or nearly prepared fuel-air mixture formed during the ignition delay period. The rapidly rising temperature and pressure in the cylinder accelerate the combustion in an uncontrolled manner until the backlog is depleted. The fuel in the spray core is still too rich to burn, and the fuel in the periphery of the spray is too lean to burn, so combustion slows down and is controlled

by the rate at which the air is entrained and a combustible mixture formed. The first phase of combustion, where prepared fuel burns quickly, is known as the premixed phase and the second phase is known as the diffusion or mixing-controlled phase. The rate of burning during the mixing-controlled phase depends on the air motion and fuel spray momentum. The burning rate starts quite high because there is considerable excess air and the fuel spray entrains air rapidly. After the end of fuel injection, particularly at high loads when there is not as much excess air as with light loads, the burning rate decreases gradually

to zero. Each of the important features of the diesel combustion process will be discussed in this chapter. First, some basic combustion

theory will be presented. Then, the ignition delay and fuel-air mixing processes will be discussed. Finally, combustion system design issues will be presented before moving to a discussion of diesel fuels and their effect on diesel combustion, emissions,

and performance.

Basic combustion theory

Combustion is the chemical reaction that converts the energy

contained in the fuel to the internal energy of product gases.

The internal combustion engine serves as a mechanism to convert

this internal energy into useful work. This section discusses the

basic chemical reactions that relate to diesel combustion and

how the reactions associated with chemical equilibrium and

chemical kinetics influence combustion. A brief discussion of

hydrocarbon combustion is also included.

4.1.1.1 Stoichiometric combustion

Although diesel engines never intentionally run with the chemi-

cally correct, or stoichiometric, amount of air, it is useful to

compare the actual fuel to air ratio to the stoichiometric amount

as a measure of air utilization. Since diesel fuel composition

varies considerably, it is desirable to have a laboratory analysis

of the fuel that gives its composition. Method D5291 from the

American Society for Testing and Materials (ASTM) can give

the percentages of hydrogen and carbon in the fuel. If an average

molecular weight is also available, an equivalent hydrocarbon

molecule can be determined. Universal Oil Products Method

375-86 can be used to estimate the fuel molecular weight using

the fuel viscosity, density, and distillation curve1

.

A typical No. 2 diesel fuel will have a molecular weight of

183, a carbon mass fraction of 86.57%, and a hydrogen mass

fraction of 13.43%. For a hydrocarbon molecule of the form

CxHy, x and y need to be determined to match this measured

data. Because MWcarbon =12.0111 and MWhydrogen = 1.00797,

then

412.0111) + XL00797) = 183 (4.1)

In 1 kg of fuel, there is 0.8657 kg of carbon/12.0111 = 0.0721

kmol of carbon and 0.1343 kg of hydrogen/1.00797 = 0.1332

kmol of hydrogen. Thus,

ylx = 0.1332/0.0721 (4.2)

This system of two equations and two unknowns can be solved

to get x= 13.2 and y = 24.4. The equivalent diesel fuel molecule

1

s

 C13.2^24-4.

The stoichiometric reaction for this fuel can be obtained by

atom balances to be:

Ci32H244 + 91.90 (0.21 O2 + 0.79 N2) => 13.2 CO2

+ 12.2 H2O+ 72.60 N2 (4.3)

The molar air-fuel ratio is 91.90 kmol air/kmol fuel, which can

be converted to a mass basis as follows:

91 9Q kmol air x 28.97 kg air ^ kmolfuel

kmol fuel kmol air 183 kg fuel

=

 14

-

55m <4

-

4

kg fuel >

The equivalence ratio is defined as the actual fuel-air ratio

divided by the stoichiometric fuel-air ratio. If an engine using

the fuel described above were running with a 30:1 air-fuel

ratio, then its equivalence ratio would be

0=|^= 1/30 =Q485

FIX) stoich 1/14.55

This ratio indicates that the engine is using less than half of the

air supplied for combustion. Diesel engine air utilization is

generally limited to 0 < 0.7. Higher equivalence ratios cause

excessive smoke emissions. This can make it difficult for naturally

aspirated diesel engines to develop as much power per unit of

displacement as spark ignited engines.