2030727 Thermodynamics

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The Zeroth Law of Thermodynamics This law states that if object A is in thermal equilibrium with object B, and object B is in thermal equilibrium with object C, then object C is also in thermal equilibrium with object A. This law allows us to build thermometers. For example the length of a mercury column (object B) may be used as a measure to compare the temperatures of the two other objects. The First Law of Thermodynamics Conservation of Energy The principle of the conservation of energy sta
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  The Zeroth Law of ThermodynamicsThis law states that if object   A is in thermal equilibrium with object   B      , and object   B is in thermal equilibrium with object  C  , then object  C  is also in thermal equilibrium with object   A   . This law allows usto build thermometers. For example the length of a mercury column (object B ) may be used as ameasure to compare the temperaturesof the two other objects.   The First Law of Thermodynamics Conservation of Energy The principle of the conservation of energystates that energy can neither be created nor destroyed  . If a systemundergoes a process by heat and work transfer, then the net heat supplied, Q , plus the net work input, W , is equal to the change of  intrinsic energyof the working fluid, i.e.where U1 and U2 are intrinsic energy of the system at initial and final states, respectively.The special case of the equation applied to a steady-flow systemis known as steady-flow energy equation. Applying this general principle to a thermodynamic cycle, when thesystem undergoes a complete cycle, i.e. U1 = U2 , results in:where: Q= The algebraic sum of the heat supplied to (+) or rejected from (-) the system.   W= The algebraic sum of the work done by surroundings on the system (+) or bythe system on surroundings (-).  Applying the rule to the power plant shown in figure below,  gives: Q = Qin - QoutW = Win - Wout   Qin + Win - Qout - Wout = 0  where, Qin = Heat supplied to the system through boiler, Win = Feed-pump work, Qout = Heat rejected from the system by condenser, Wout = Turbine work. The Second Law of Thermodynamics The second law of thermodynamics states that noheat enginecan be more efficient than areversible heat engine working between two fixed temperature limits (Carnot cycle   ) i.e. the maximum thermal efficiency is equal to the thermal efficiency of the Carnot cycle :or in other words  If the heat input to a heat engine is Q      , then the work output of theengine, W  will be restricted to an upper limit  Wmax    i.e.It should be noted that real cycles are far less efficient than the Carnot cycle due tomechanical friction and other irreversibility. Exergy or Availability Exergy of asystemis defined as the theoretical maximum amount of work that can be obtained from the system at a prescribed state (P, T, h, s, u, v) when operating with areservoir at the constant pressure and temperature P0 and T0. The specific exergy of anon-flow system is:and for a steady flow system:  where, u=Specific internal energy,h=Specific enthalpy, v=Specific volume,s=Specific entropy,C=Velocity,Z= Height of the system measured from a fixed datum, g= Gravity constant. Heat Engine Heat engine is defined as a device that converts heat energy into mechanical energy or more exactly a system which operates continuously and onlyheat  and work  may pass across its boundaries .The operation of a heat engine can best be represented by athermodynamic cycle. Someexamples are:Otto,Diesel, Brayton, Stirling andRankine cycles. Forward Heat Engine LTER= Low Temperature Energy Reservoir   HTER= High Temperature Energy Reservoir A forward heat engine has a positive work output such as Rankine or Brayton cycle.Applying the first law of thermodynamicsto the cycle gives: Q1 - Q2 - W = 0 The second law of thermodynamics states that the thermal efficiencyof the cycle, , has an upper limit (the thermal efficiency of the Carnot cycle), i.e.It can be shown that: Q1 > W  which means that it is impossible to convert the whole heat input to work and Q2 > 0  which means that a minimum of heat supply to the cold reservoir is necessary. Reverse Heat Engine LTER= Low Temperature Energy ReservoirHTER= High Temperature Energy Reservoir A reverse heat engine has a positive work input such as heat pump and refrigerator.Applying the first law of thermodynamicsto the cycle gives: - Q1 + Q2 + W = 0 In case of a reverse heat engine the second law of thermodynamicsis as follows: It is impossible to transfer heat from a cooler body to a hotter body without any work inputi.e.  W > 0 Turbine Turbines are devices that convert mechanical energy stored in a fluid into rotationalmechanical energy. These machines are widely used for the generation of electricity. Themost important types of turbines are:steam turbines, gas turbines,water turbines and wind turbines. Steam Turbine Steam turbines are devices which convert theenergystored in steam into rotationalmechanical energy. These machines are widely used for the generation of electricity in anumber of different cycles, such as: ã Rankine cycle ã Reheat cycle   ã Regenerative cycle  
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