GAS TURBINES

Introduction:-

The gas turbine is the engine at the heart of the power plant that produces electric current.A gas turbine is a combustion engine that can convert
natural gas or other liquid fuels to mechanical energy. This energy then drives a generator that produces electrical energy. It is electrical energy that moves along power lines to homes and businesses.

HOW GAS TURBINE POWER PLANTS WORK:-

The combustion (gas) turbines being installed in many of today's natural-gas-fueled power plants are complex machines, but they basically involve three main sections:

• The compressor, which draws air into the engine, pressurizes it, and feeds it to the combustion chamber at speeds of hundreds of miles per hour.
• The combustion system, typically made up of a ring of fuel injectors that inject a steady stream of fuel into combustion chambers where it mixes with the air. The mixture is burned at temperatures of more than 2000 degrees F. The combustion produces a high temperature, high pressure gas stream that enters and expands through the turbine section.
• The turbine is an intricate array of alternate stationary and rotating aerofoil-section blades. As hot combustion gas expands through the turbine, it spins the rotating blades. The rotating blades perform a dual function: they drive the compressor to draw more pressurized air into the combustion section, and they spin a generator to produce electricity.

To generate electricity, the gas turbine heats a mixture of air and fuel at very high temperatures, causing the turbine blades to spin. The spinning turbine drives a generator that converts the energy into electricity.

The gas turbine can be used in combination with a steam turbine—in a combined-cycle power plant—to create power extremely efficiently.

1. Air-fuel mixture ignites.
   • The gas turbine compresses air and mixes it with fuel that is then burned at extremely high temperatures, creating a hot gas.
2. Hot gas spins turbine blades.
   • The hot air-and-fuel mixture moves through blades in the turbine, causing them to spin quickly.
3. Spinning blades turn the drive shaft.
   • The fast-spinning turbine blades rotate the turbine drive shaft.
4. Turbine rotation powers the generator.
   • The spinning turbine is connected to the rod in a generator that turns a large magnet surrounded by coils of copper wire.
5. Generator magnet causes electrons to move and creates electricity.
   • The fast-revolving generator magnet creates a powerful magnetic field that lines up the electrons around the copper coils and causes them to move.
   • The movement of these electrons through a wire is electricity.

WHY V-SHAPED ENGINES ARE USED ?

The primary reason to use a V-engine is largely packaging. They're shorter and more compact than an inline engine.

However, you're getting to the really fun stuff and that's the bank angle and phasing. The first thing to consider is an engine as a purely rotational system. Imagine a one cylinder engine; the rotating crankshaft is massively out of balance. Even if the actual crankshaft was perfectly balanced, the motion of the piston and the forces it applies will never cancel out. This is why most one cylinder engines run a balance shaft; a weighted shaft geared into the crankshaft to mitigate the out-of-balance effects. A balance shaft can be run on other out of balance engines as well.

A V-engine, on the other hand, can be made to be inherently balanced. This means that no matter what speed the engine's running at, the forces from the rotating assembly will always cancel out. Most notably, a 90 degree V4 or V8 engine can be easily balanced and many race motorcycle engines use this configuration (Honda and Ducati in MotoGP). V6s and V12s run balanced at 60 degree bank angle.

Note that I say 'can' be better balanced. This is because you also have to look at your firing orders and phasing. Back to MotoGP, we can look at the Honda NSR500 2 stroke race bike of the '80s and '90s. They ran that engine as an even-fire engine (over 360deg of rotation for a 2 stroke or 720deg of rotation for a 4 stroke), the cylinders fire at even intervals. So if it's a 4 cylinder, they fire every 90 or 180 degrees. # of cylinders/cycle length=cylinder phasing. This is the most ideal way of running an engine and will provide the inherent balance 

However they also ran that engine as a big bang engine. By building new crankshaft and camshafts, they made it so that all pistons fired within 72 degrees of each other. This was done for non-engine reasons, tire dynamics actually, but it's a good example of how you can change phasing.

An interesting note: because a V12 is essentially two inline 6 engines stuck together they're naturally balanced regardless of bank angle. This is why V12 Jaguars have a reputation for smoothness. V6s, being two 3 cylinders stuck together, are inherently out of balance.

TYPES OF MANUAL GEARBOX

Types of Manual Gearbox:

Manual Transmission Gearbox:

In this type of transmission different speed ratio or gear ratio is selected by the driver manually. Some special skill of driving is required to operate this
type of gear box. According to their design, this is subdivided into three types.

(A.) Sliding mesh gear box:

When we talk about types of gearbox, this is one of the oldest type. It this, gears on the main shaft are moved right or left for meshing them with appropriate gears on the counter shaft for obtaining different speed. This type of gear box derives its name from the fact that the gears are meshed by sliding. One disadvantage of it is that, special skill is required to operate this gear box.

(B.) Constant mesh gear box:

This is one of the famous type used in twenty century. It this gearbox, all the gears are in constant mesh with each other all the time. The gears on the main shaft rotates freely without rotating the main shaft. Constant mesh gear box consists two dog clutches. These clutches are provided on the main shaft, one between the clutch gear and the second gear and the other between the first gear and reverse gear. When the left side dog clutch is made to slide left by means of gearshift lever, it meshes with the clutch gear and the vehicle runs on top speed. If this clutch slide right and mesh with second gear, than the vehicle runs on second gear speed. So in constant mesh gear box we can change the gear ratio by shifting the dog clutch. This type of gear box is more popular than sliding mesh because it creates low noise and less wear of gears.

(C.) Synchromesh gear box:

One big problem occur in constant mesh gear box is that when the driver engage the dog clutch, the main shaft and gear to be meshed running at different speed. So when engage this gear cause wear and tear of dog clutch. This problem is solved by a synchromesh gear box. This gear box is same as the constant mesh gear box except dog clutch is replaced by synchromesh devices.

Synchromesh gear devices works on the principle that two gears to be engaged are first brought into frictional contact which equalizes their speed after which they are engaged readily and smoothly. The synchromesh looks like as the cone clutch where the outer surface of cone consist the frictional surface. This type of gear box is widely used in automobile.

  

Underwater Welding:-

Underwater welding is performed while the welder is submerged, often at elevated barometric pressures.  This introduces a variety of challenges
that require specialized skills and training that are taught at CDA Technical Institute (formerly Commercial Diving Academy).  Because of the adverse conditions and inherent dangers associated with underwater welding (also known as wet welding) divers must be trained to an exceptionally rigorous standard with highly specialized instruction.

Wet Welding

Welding underwater can be acheived by two methods: wet welding & dry welding. Wet welding entails the diver to perform the weld directly in the water. It involves using a specially designed welding rod, and employs a similar process used in ordinary welding. 

Here are advantages to wet welding:

• Cheap and fast
• high tensile strength
• ease of access to weld spot
• no habitat
• no construction

Dry Welding / Hyperbaric Welding

Another method of welding underwater is hyperbaric welding or dry welding. Hyperbaric welding is the process by which a chamber is sealed around the structure that is to be welded. It is then filled with a gas (typically mixture of helium and oxygen, or argon), which then forces the water outside of the hyperbaric sphere.

This allows for a dry environment in which to perform the weld. Here are some advantages to dry welding:

• welder / diver safety
• higher weld quality
• surface monitoring
• non-destructive testing

AIR CONDITIONING SYSTEM OF AN AUTOMOBILE

We all know about air conditioning system. The automotive air conditioning system is also works on the reverse Brayton or Rankine cycle.
As like all air conditioning system the aim of automotive air conditioning is to control the temperature and humidity of the atmospheric air and circulate the same in the automobile. The automotive air conditioning system consists a refrigeration system, air circulation and distribution system and a control system. The refrigeration system cool down the air which include many other parts like compressor, condenser etc. The air circulating system circulate this cooled air into the car which include blower, air duct etc. The controlling system used to control the temperature of the car by sensing it and control the refrigerating system.  

Principle of Car Air Conditioning System:

The automotive air conditioning system includes the compressor, condenser, evaporator, receiver-dehydrator and connecting lines which includes expansion valve, orifice tube, suction throttling valve, positive operating absolute valve, evaporator pressure regulator valve, thermal sensor, high pressure cut off switch and cycling compressor switch. In old days Freon 12 was used as refrigerant but now it is replaced by alternative refrigerant like R134a. The layout of refrigerating system is as shown in figure.

Working of Car Air Conditioning System:

The working of automotive air condition system is similar to all other air conditioning system.  The refrigerant vapour from the evaporator is compressed to high pressure by the compressor. The compressor is driven by the engine through a belt drive. It is connected by a electromagnetic clutch witch serve engage and disengage the compressor required. A variable displacement AC compressor is sometime used to match compressor capacity to varying cooling requirement.  Refrigerant pressure and temperature increases in the compressor and convert it into vapour form. This high pressure and temperature refrigerant vapour from the compressor then discharge to the condense, which is a heat exchanger situated in front of vehicle. In the condenser the refrigerant liberate heat and convert into liquid form. Sometime the ram air is not sufficient so an extra engine or electric driven fan is used to cool down the refrigerant. This cooled but high pressure refrigerant allow to pass form dehydrator to extract any moisture. Dry refrigerant liquid is then made to pass through expansion valve mounted at the inlet side of the evaporator. The expansion valve allows the refrigerant liquid to expand to low pressure in the evaporator. The process of expansion to low pressure makes the refrigerant to evaporate and thereby cool the evaporator.  A sensing devices, called temperature tube signals the diaphragm in the expansionvalve to vary orifice size depending upon the refrigerant temperature at the evaporator outlet, thus achieving automatic temperature control. The evaporator is similar in construction to the condenser.

The same circulation and distribution system as used for ventilation and heating system. The air from the blower is forced to flow over the evaporator coils which remove the latent heat of vaporization, moisture and impurities form the air. Condenser moisture with the impurities fro the evaporator drips into a try form where the same are drained out. This system is microprocessor based and depending upon input signals outside air temperature, inside air temperature, mode selection and desired temperature setting, automatically controls the air mixer valve, re-circulation valve etc.

Precaution while using car AC system:

1. Do not use AC fresh air mode open always, since this mode is for occasional use just to release the air in circulation for a long time.

2. Never operate AC with heater on.

3. Never run AC without refrigerant otherwise compressor may seize.

4. Do not leave AC joints open which may cause the moisture to enter the system.

5. Do not charge the refrigerant in the AC system before flushing.

6. Never switch on the AC at high speeds, which may result in the seizing of compressor.

Tips for maintenance of car AC system:

1. Clean the condenser during normal servicing.

2. Maintain correct refrigerant level. Less refrigerant would result in less cooling, while excessive would also result lesser cooling and excessive tripping.

3. Top up the compressor with adequate oil.

4. Clean the evaporator periodically.

5. Maintain proper belt tension. A loose belt will slip whereas overtight belt would cause noise and premature compressor failure.

MERCHANT CIRCLE

For establishing the relationship between measurable and actual forces Merchant’s circle diagram will be used.

• Merchant circle diagram is used to analyze the forces acting in metal cutting.
• The analysis of three forces system, which balance each other for cutting to occur. Each system is a triangle of forces.

Assumptions made in drawing Merchant’s circle:

1. Shear surface is a plane extending upwards from the cutting edge.
2. The tool is perfectly sharp and there is no contact along the clearance force.
3. The cutting edge is a straight line extending perpendicular to the direction of motion and generates a plane surface as the work moves past it.
4. The chip doesn’t flow to either side, that is chip width is constant.
5. The depth of cut remains constant.
6. Width of the too, is greater than that of the work.
7. Work moves with uniform velocity relative tool tip.
8. No built up edge is formed.

The three triangles of forces in merchant’s circle diagram are

1. A triangle of forces for the cutting forces,
2. A triangle of forces for the shear forces,
3. A triangle of forces for the frictional forces.
   

Figure 1: Forces acting on a cutting tool

Let F = Frictional force
N = Normal to frictional force
F_s = Shear force
F_sn = Normal to shear force
F_c = Cutting force or tangential component of force
F_t =Thrust force or feed force
β = Friction angle
μ = Coefficient of friction = tanβ
F_c and F_t are along and normal to the direction of velocity.

Figure 2: Merchant’s circle diagram

Let R = resultant force

Then resultant force is given by the formula

R = (F_c² + F_t²)^0.5 

R = Diameter of merchant’s circle

F_t, F_c forces are defined based on actual machining conditions

• From the above merchant’s circle diagram it is found that there are three right angled triangles are present and all the three right angled triangle possessing common hypotenuse (largest side opposite to right angle in a right angled triangle).
• Merchant’s circle is used for establishing relationship between measurable and actual forces.

PROPERTIES OF FLUID

In this post we have written the fluid properties namely mass density, specific weight, specific volume, specific gravity, viscosity, vapor pressure, compressibility and surface tension.

Mass Density:

Mass Density (ρ) is the property of a fluid is the mass per unit volume.

Specific Weight:

Specific Weight (w) of a fluid is the weight per unit volume.

Specific Volume:

Specific Volume (v) of a fluid is the volume of the fluid per unit mass.

Specific Gravity or Relative Density:

Specific Gravity (s) of a fluid is the ratio of the mass density of a fluid to the mass density of a standard fluid.

Viscosity:

Viscosity is property by virtue of which it offers resistance to the movement of one layer of fluid over the adjacent layer.

Vapor Pressure:

When a liquid is confined in a closed vessel, the ejected vapor molecules accumulated in the space between free liquid pressure and top of the vessel exert a partial pressure on the liquid surface. This pressure in liquid is known as vapor pressure.

Compressibility:

The normal compressive stress of any fluid element at rest is known as hydro static pressure which arises as a result of innumerable molecular collisions in the entire fluid. The degree of compressibility of a substance is characterized bybulk modulus of elasticity (B).

Surface Tension:

Surface is a measure of fluid tendency to take a spherical shape, caused by mutual attraction of the liquid molecules.