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Basic Thermodynamics

on Jul 04, 2010

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Basic ThermodynamicsPresentation Transcript

• Introduction
• Basic Thermodynamics Concepts
• Heat.
• System.
• State.
• Path .
• Process.
• Cycle.
• Property .
• Contd..
• Process - Any change that a system undergoes from one equilibrium state to another is called a process .
• Path - The series of state through which a system passes during a process is called a path
• Cycle - A process with identical end states is called a cycle .
• A review of basic thermodynamics: A refresher The ball represents mass exchange The arrow represents energy exchange
• Zeroth Law of thermodynamics
• The Zeroth Law deals with thermal equilibrium and provides a means for measuring temperatures.
• Difference between thermal equilibrium and Thermodynamic equilibrium.
• Zeroth Law of thermodynamics
• First Law of thermodynamics
• The first law is the law of conservation of energy.
• The algebric sum of the work transfers is proportional to the algebric sum of heat transfer.
• Limitations of First Law
• It does not place any distinction on the direction of the process under consideration.
• It will not help to predict, whether the system would undergo a change or no. It simply states that in a certain process heat and work are mutually convertible.
• Second Law of thermodynamics
• The Second law of clausis states that
• It is impossible to construct a device that operates in a cycle and produces no effect other than the removal of heat from a body at one temperature and the absorption of an equal quantity of heat by a body at a higher temperature.
• Second Law of thermodynamics contd..
• The Second law of Max Planck’s states that
• It is impossible to construct an engine working on a cyclic process whose sole purpose is to convert all the heat supplied to it into equivalent amount of work.
• Few Examples
• Some common examples.
• All processes in nature occur unaided or spontaneously in one direction. But to make the same process go in the opposite direction one needs to spend energy.
• Third Law of Thermodynamics
• It is impossible by any procedure no matter how idealized, to reduce any system to the absolute zero temperature in a finite number of operations .
• Summation of three laws
• You can’t get something for nothing
• To get work output you must give some thermal energy
• You can’t get something for very little
• To get some work output there is a minimum amount of thermal energy that needs to be given
• You can’t get every thing
• However much work you are willing to give 0 K can’t be reached.
• Definitions of Reversible Process
• A process is reversible if after it, means can be found to restore the system and surroundings to their initial states.
• Some reversible processes:
• Constant volume and constant pressure heating and cooling -the heat given to change the state can be rejected back to regain the state
• Reversible Process (contd…)
• Isothermal and adiabatic processes -the work derived can be used to compress it back to the original state.
• Elastic expansion/compression (springs, rubber bands)
• Some Irreversible Process
•
• Thermodynamic Processes
• A process in which the volume remains constant
• constant volume process. Also called isochoric process / isometric process
• A process in which the pressure of the system remains constant.
• constant pressure process. Also called isobaric process
• A process in which the temperature of the system is constant.
• constant temperature process. Also called isothermal process
• A process in which the system is enclosed by adiabatic wall.
• Rankine Vapor power cycle
• T-s diagram Rankine power cycle
• P-V diagram Rankine power cycle
• Rankine Cycle contd…
• Process 1-2: Water from the condenser at low pressure is pumped into the boiler at
• high pressure. This process is reversible adiabatic.
• Process 2-3: Water is converted into steam at constant pressure by the addition of heat
• in the boiler.
• Process 3-4: Reversible adiabatic expansion of steam in the steam turbine.
• Process 4-1: Constant pressure heat rejection in the condenser to convert condensate
• into water.
• The steam leaving the boiler may be dry and saturated, wet or superheated. The
• corresponding T-s diagrams are 1-2-3-4-1; 1-2-3’-4’-1 or 1-2-3”-4”-1.
• Thermal efficiency of rankine cycle
• Consider one kg of working fluid, and applying first law to flow system to various processes with the assumption of neglecting changes in potential and kinetic energy, we can write,
• δ q - δ w = dh
• For process 2-3, δw = 0 (heat addition Process), we can write,
• ( δ q )boiler= (dh )boiler =(h3-h2)
•
•