LOAD AND RESISTANCE FACTOR DESIGN

CIVIL_ENGINEERING

• Load and Resistance factor design (LRFD), Ultimate Design, or Limit State design

If the major part of factor of safety is applied on the service loads to increase loads called factored loads. The material strength is divided by the minor remaining part of factor of safety. The design method is called load and resistance factor design (LRFD), Ultimate design, or Limit State design.

• Overload factor

The factor of safety by which any load is increased for load and resistance factor design is calledoverload factor.

• Resistance factor or Reduction factor (phi)

The reciprocal of factor of safety or reduction factor by which the material strength is slightly adjusted is called Resistance factor.

• Limit state

Limit state is defined as the limiting stage in the loading after which the structure cannot fulfill its function due to strength considerations.

Analysis of structures for loads is performed considering the structure to be within elastic range. However, inelastic behavior, ultimate failure modes and redistribution of forces after elastic range are considered in this method. This is more realistic design as compared with the old allowable stress design.

• Nominal strength (Rn)

Nominal strength is defined as the strength of astructure or its component determined by usingformulas given in specifications.

• Required strength (Ru)

Any particular load effect increased by the load factors is called required strength (Ru).

• Design strength

The nominal strength is reduced by the resistancefactor is called design strength.

Hence, the deign equation in case of LRFD reduces to the following equation:

Ru less than or equal to phi Rn

TYPES OF VISCOUS FLOWS

• Types of Viscous Flows :

The viscous flows may be classified into two types depending upon the factor, whether the viscosity is dominating or not.

• Laminar Flow :

It is the flow in which the viscosity of the fluid is dominating over the inertia forces.

Such a flow, which takes place at very low velocities, is known as a laminar flow.

A laminar flow can be best understood by considering the liquid moves in the form of concentric cylinders sliding within one another. Concentric cylinders are those having the same center but with different diameters.

It is a theoretical flow and does not come in contact with the engineers.

• Turbulent Flow :

It is a flow in which the inertia forces are dominating over the viscosity.

Such a flow, which takes place at very high velocities, is known as a laminar flow.

Turbulent flow can be best understood by considering concentric cylinders diffuse or mix with each other and the flow is a disturbed one.

It is a practical flow which comes in contact with the engineers.

CRITICAL VELOCITY

The velocity at which the flow changes from laminar toturbulent is known as Critical Velocity.

It is further divided into two types.

• Lower critical velocity :

A velocity at which laminar flow stops.

Or

The velocity at which the flow enters from laminar totransition period is known as lower critical velocity.

There is a transition period in between laminar and turbulent flow. It has been experimentally found that when a laminar flow changes into turbulent, it does not change abruptly. But there is transition period between two types of flows. This experiment was first performed by Prof. Reynold Obsorne in 1883.

• Upper critical velocity :

A velocity at which turbulent flow starts.

Or

A velocity in which flow enters from transition period toturbulent flow is known as upper or higher critical velocity.

CENTRIFUGAL PUMP

Pump :

It is a machine which converts mechanical energy into hydraulic energy to move liquid from one place to another place.

Centrifugal pump :

A machine which converts mechanical energy into hydraulic energy by the action of centrifugal force is known as centrifugal pump.

It is exactly the reverse of francis reaction turbine. In a reaction turbine, the water at high pressure is allowedto enter the casing. This high pressure of water gives mechanical energy at its shaft. Whereas in pump, the mechanical energy is provided to shaft. Water enters the impeller which is attached to the rotating shaft. This increases the pressure energy of the outgoing fluid. the water enters the impeller radially and leaves the vanes axially.

The centrifugal pump is so called because the pressure increases within its rotor due to centrifugal action. It consists of an impeller rotating within a case. Fluid enters the impeller in the center portion, called the eye. Then it flows outwardly through the vanes of the impeller. Then it moves centrifugally outward. The fluid receives energy from the vanes, resulting in an increase in both pressure and absolute velocity.

FUNCTIONS AND USES OF PUMP

Uses of Pumps:

• A pump is used to raise pressure of liquids.
• It can handle any kind of liquid, even contaminated with solid particulates to some extent.
• In power plants, pumps are used to handle ash slurry made with water, which is highly erosive in nature.
• In chemical industries, it is used to handle corrosive fluids.

Performance of the pump is affected by the increase in the viscosity of the liquid and the presence of contamination in it.

Function of pumps:

• Function of the pump is to raise pressure of fluid and impart desirable velocity. So that fluid may be transported from one place to another. More is the velocity the more is the flow rate and hence less time required for transportation. However we know that head loss is directly proportional to the square of flow velocity. It therefore requires pressure of the fluid at the outlet of the pump to be more to overcome these losses.
• The main purpose of the pump is to raise the pressure, not the velocity of the fluid.

IRROTATIONAL FLOW

If the net rotation of the fluid element is equal to zero, then the flow is known as irrotational flow.

Or

The flow in which the element of the moving fluid suffers no net rotation from one instant to the next, with respect to the given frame of reference.

Or

The flow in which fluid particles do not rotate about their own axes and retain their original orientations, is called an irrotational flow.

• In this flow, the fluid element is subjected to much more distortion.
•  As long as the algebraic average rotation is zero, the motion is irrotational.

Example

In an irrotational flow, if a match stick is thrown on the surface of the moving fluid, it does not rotate about its axis but retains its original orientation.

ROTATIONAL FLOW

A flow in which net rotation of the fluid element is not equal to zero, is known as rotational flow.

Or

The flow in which the fluid particles also rotate about their own axes while flowing, is called rotational flow.

• Distortion in this case is less than irrotational flow.

Example

In a rotational flow if a match stick is thrown on the surface of the moving fluid, it will rotate about its own axes.

DEFINITION OF DRAG AND LIFT

Drag Force

When there is relative motion between the body and the fluid and the body is completely immersed in a homogeneous fluid , then the body may be subjected to two forces.

If the force is parallel to the motion then it is called drag.

The drag force on a submerged body can be viewed as having two components.

1. Pressure drag force.
2. Friction or surface drag.

Lift Force

If the force is normal to the relative motion of the body and the fluid in a homogeneous fluid, then it is called lift force.

Pressure on a body immersed in a moving liquid

When a solid body is held in the path of a moving fluid and is completely immersed in it, the body will be subjected to some pressure or force. Conversely, if a body is moving with a uniform velocity through a fluid, then fluid will exert some resistance to moving body, or the body has to exert some force to maintain steady movement.

A little consideration will show that if the plate or body is immersed in a liquid parallel to the direction of flow, it will experience less force as compared to the position when plate is held at right angle to the direction of flow.

It has been experimentally found that when a plate is held at some angle with the direction of the flow, the streamlines of the liquid gets deflected.

The pressure exerted by the streamlines is the same when the fluid is at rest and the body is moving uniformly through it or the body is at rest and fluid moves through it.

When the plate is held at some angle, it is subjected to some pressure. As this pressure is acting at right angle to the plate. So it will have two components.

1. In the direction of the flow of the liquid which is called drag.
2. At right angle to the flow of liquid which is called lift.

TURBO MACHINES

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Definition of Turbo Machines | Types | Impulse Turbine

Friday, August 17, 2012 6:30

Posted in category Fluid Mechanics 2

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Turbo machines :

All those machines in which there is a change in whirl or angular momentum are called turbo machines.

Types of Turbo machines 

There are two types of turbo machines.

1. Turbines
2. Pumps

Types of Turbines 

There are two types of turbines.

• Impulse turbine
• Reaction turbine

Types of pumps 

Pumps are of the following types.

• Centrifugal pump
• Reciprocating pump

Impulse Turbine 

An impulse turbine is a turbine which runs by the impulse of water.

In an impulse turbine, the water from a dam is made to flow through pipeline, and then through guide mechanism and finally through nozzles. In such a process the entire available energy of the water is converted into kinetic energy. Nozzles are kept close to the runner. When water passes through nozzles, then the water enters the running wheel in the form of jets. The jets have an impact on the buckets which are fixed to the outer periphery of the wheel.

The jet of water have a high impact on the buckets having high velocity, and after flowing over vanes, leaves with the low velocity. Thus imparting energy to the runner. The pressure of water at entering and leaving the vanes is atmospheric pressure.

PELTON WHEEL AND PARTS OF PELTON WHEEL

Pelton wheel

Pelton wheel is an impulsive turbine used mainly for high head hydro electric schemes.

Pelton wheel has the following main components.

• Nozzle,
• Runner and buckets,
• Casing
• Breaking jet

Nozzle

Nozzle of a pelton wheel is a circular guide mechanism which guides the water to flow at the desired directions.

It also regulate the flow of water. A conical or spear needle operates inside the nozzle in axial direction. The main purpose of the nozzle is to regulate the flow of water through nozzle.

When the needle is pushed forward into the nozzle, it reduces area if jet. As a result the quantity of water through the jet is also reduced. Similarly if the spear is pushed back out of nozzle, it increases the area of nozzle and discharge increases.The movement of spear is regulated by hand or by automatic governing arrangement.

Sometimes it is very essential to close the nozzle suddenly. It is necessary when there is a sudden decrease of load on turbine. To close the nozzle, if we use spear then it will cause the  pipe to burst due to high pressure generation. In order to avoid such a mishap, an additional nozzle is provided through which the water can pass without striking the buckets. This needle is known as bypass needle.

Sometimes, a plate known as deflector plate is provided to the nozzle, which is used to deflect water jet.

• Runner and buckets

Runner of a pelton wheel consists of circulating disc fixed with horizontal shaft.

On the periphery of the runner, a number of buckets are fixed at a uniform distance. Bucket is a hemispherical cup or bowl with a divider in the middle. This divider is known as splitter. It split the water jet in two equal parts.

The surface of the bucket is made very smooth. For low heads, the buckets are made of cast iron. For high heads, buckets are made of stainless steel, bronze or other alloys. For impure water, buckets are made of special alloys.

The buckets are generally bolted to the runner disc. Sometimes the buckets and disc are cast as a single unit. Sometimes few buckets are damaged and need replacement. They can only be replaced if they are bolted with the runner disc.

Runner of pelton wheel. Click on the image to enlarge

• Casing

Steel coverings provided over the runner of pelton wheel is known as casing. It does not play any hydraulic function.

But is necessary to provide the runner against accident. It has the following functions.

• It prevents accidents
• Minimize the wind losses
• stops splashing of water
• Facilitates to collect water
• Transmission of water to the tail race.

The casing is generally made of cast or fabricated parts.

• Braking jet 

Whenever the turbine has to be brought to rest the nozzle is completely closed. But the runner of pelton wheel goes on revolving due to inertia. To bring the runner  to rest in short time, a small nozzle is provided in such a way that it will direct the jet of water on the back of buckets. It acts as a brake for reducing the speed of the runner.