Modes of Work Transfer

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Modes of Work Transfer

• P M V Subbarao
• Professor
• Mechanical Engineering Department
• I I T Delhi

ManyMore Needs/Happenings are Equivalent to
Displacement Work!!!
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Displacement Work
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Generalized Process in A Control Mass

• p p0 k (V-V0)
• p2 p02 k (V-V0)2
• Polytropic Nature of Fluid Process
• pVn Constant
• n Polytropic constant.
• n0 Constant Pressure Process.
• n? Constant Volume Process.

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The Polytropic Process by A Control Mass

• One in which the pressure-volume relation is
  given as
• pVnconstant
• n may have any value from minus to plus
  infinity.
• Work transfer during a polytropic process.

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• For a vapour (Real Gas) under going a process.

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Evolution of Wind Turbines

• Wind is a clean, safe, renewable form of energy.
• Although the use of wind power in sailing vessels
  appeared in antiquity, the widespread use of wind
  power for grinding grain and pumping water was
  delayed until
• the 7th century in Persia,
• the 12th century in England, and
• the 15th century in Holland.
• 17th century, Leibniz proposed using windmills
  and waterwheels together to pump water from mines
  in the Harz Mountains.
• Dutch settlers brought Dutch mills to America in
  the 18th century.
• This led to the development of a multiblade wind
  turbine that was used to pump water for
  livestock.
• Wind turbines were used in Denmark in 1890 to
  generate electric power.
• Early in the 20th century American farms began to
  use wind turbines to drive electricity generators
  for charging storage batteries.

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SOME TYPES OF WIND TURBINES HAWT
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SOME TYPES OF WIND TURBINES VAWT
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Schematic of Modern Wind Turbines
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Wind Flow Past A Locked Wind Turbine
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Work Transfer in A Wind Turbine Control Volume
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Work Transfer in a Flow Device Control Volume

• A Centrifugal Compressor

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Continuous flow system Pressure at inlet is pi
and pressure at exist is po always. Fluid with
Vi,Ti enters and leaves with Vo and To. The
volume is subjected to a changing pressure field..
Outlet po, To, Vo
p
How to define the infinitesimal work?
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Work Transfer in a Flow Device Control Volume
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Acquisition of Work from Constant Volume Flow
Process
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The Hydro Power House
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Wind generation for developing countries

• Unlike the trend toward large-scale grid
  connected wind turbines seen in the West.
• The more immediate demand for rural energy supply
  in developing countries is for smaller machines
  in the 5 - 100 kW range.
• These can be connected to small, localized
  micro-grid systems and used in conjunction with
  diesel generating sets and/or solar photovoltaic
  systems.
• The main area of growth being for very small
  battery charging wind turbines (50 - 150 Watts).
• In Inner Mongolia there are over 30,000 such
  machines used by herders for providing power for
  lighting, televisions, radios, etc.
• Other applications for small wind machines
  include water pumping, telecommunications power
  supply and irrigation.

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Polytropic work Transfer in A Flow Device
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Shaft Work
Energy transmission with a rotating shaft is very
common in engineering practice.
A force F acting through a moment arm r generates
a torque T of
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Spring Work
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Work Done on Elastic Solid Bars
For linear elastic springs, the displacement x is
proportional to the force applied
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Work Transfer in Nature
How can a baby snail come out of a egg shell?
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Who controls the Growth Shape Equilibrium Shape
of A Crystal
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Why cutting of a solid consumes power?
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Who decides the beauty of Table Cloth ???
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The Role of Surface Tension in Engineering

• When splitting a solid, the amount of energy
  required is 2sA, where 2A is the area created
  (one A on each side).
• This energy is less than that needed just to
  break the bonds, since there is atomic and
  electronic relaxation.
• The surface energy is always positive because the
  atoms are less bound at the surface.
• The surface tension can be defined as the
  reversible work of formation of a unit area of
  surface at constant T, V, m.
• The surface tension is the two-dimensional analog
  to the pressure.

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Thermodynamic Definition of Surface Tension
The relative change in internal energy of a
control mass w.r.t. change in surface area at
constant temperature, volume.
Law of a Nature
The tendency to minimize surface energy is a
defining factor in the morphology and composition
of surfaces and interfaces. This Is important
for Solids Liquids.
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Values of Surface Tension for Selected Materials

Material g J/m2
Tungsten (solid) 2.9
Iron (solid) 2.2
Iron (liquid) 1.9
MgO 1.2
Mercury (liquid) 0.5
Water 0.07
Acetic acid 0.03
Nitrogen (liquid) 0.01
Helium (liquid) 0.0003
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Work Associated with the Stretching of a Liquid
Film
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The Faradays Work An Amazing form of Work
Transfer

• Consider a conducting rod PQ moving at a steady
  speed V perpendicular to a field with a flux
  densityB.
• An electron (negative charge e) in the rod will
  experience a force ( Bev) that will push it
  towards the end P.

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Description of Work done by A Conductor

• The same is true for other electrons in the rod,
  so the end P will become negatively charged,
  leaving Q with a positive charge.
• As a result, an electric field E builds up until
  the force on electrons in the rod (unit length)
  due to this electric field ( Ee) balances the
  force due to the magnetic field

Force per unit charge on Rod of unit length
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For a rod of length L, define the EMF as
What happens when the EMF drives a current in an
external circuit? To do this, imagine that the
rod moves along a pair of parallel conductors
that are connected to an external circuit
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Electrical Loading of Conductor
The EMF will now cause a current to flow in the
external resistor R. This means that a similar
current flows through the rod itself giving a
magnetic force, BIL to the left.
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Quantification of the Faradays Work
L is now the separation of the two conductors
along which the rod PQ moves. An equal and
opposite force (to the right) is needed to keep
PQ moving at a steady speed.
In a time t, the rod moves a distance x
Vt Work done on the rod
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The Pairs of Work Transfers
Energy dissipated in R power x time EMF I
t Giving B I L V t EMF I t or,
as before, EMF B V L
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