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ENGR 260 Section 6.5 – 6.8. Heat Pump. Refrigerator. Heat Engine. Kelvin-Planck Statement. The Second Law of Thermodynamics It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. Kelvin Planck Statement.
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Heat Pump Refrigerator Heat Engine
Kelvin-Planck Statement • The Second Law of Thermodynamics • It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.
Kelvin Planck Statement • Heat Engine must have a low temperature sink!
Clausius Statement of Second Law • It is impossible to construct a device that operates in a cycle and produces no effect other than to transfer heat from a low temperature body to a higher temperature body.
Clausius Statement • Heat Pumps and Refrigeration must have work input!
Perpetual Motion Machines Two Types: • PMM1 ~ violates the first law of thermo • PMM2 ~ violates the second law of thermo • Some PMMs actually violate both laws
Perpetual Motion Machines • PMM1 ~ violates the first law of thermo
Perpetual Motion Machines • PMM2 ~ violates the second law of thermo
Perpetual Motion Machines Quiz for Tuesday: Find an example of a perpetual motion machine Show it in class Tuesday Explain if it is a PMM1 or PMM2
Reversible and Irreversible Processes • Reversible process ~ a process that can be reversed without leaving any trace on the surroundings • the system and the surrounding are returned to their initial state • net work and heat exchange between process and surrounding is zero for the combined process
Reversible Processes Can a reversible process really occur? No! So why study them? 1) They are easy to analyze. 2) They are idealized models to predict theoretical limits of corresponding actual processes.
Reversible Processes • Deliver the most • Consume the least work
Reversible and Irreversible Processes • Reversible process ~ a process that can be reversed without leaving any trace on the surroundings • the system and the surrounding are returned to their initial state • net work and heat exchange between process and surrounding is zero for the combined process
Irreversibilities • Friction • Unrestrained expansion of a gas • Mixing of two fluids • Heat transfer through a finite temp differential • Electric Resistance • Inelastic deformation of solids • Chemical Reactions
Friction Energy supplied as work is converted to heat. Heat is transferred to bodies in contact. This is seen as a temperature rise. When reversed heat is not converted back to work.
Unrestrained expansion of a gas Only way to restore system: Compress to initial volume Transfer heat from gas to return to original temperature Involves transferring heat to work which violates 2nd law
Heat Transfer Violates 2nd Law Clausius Statement – cannot transfer heat from low temp body to high temp body without work
Clausius Statement • Heat Pumps and Refrigeration must have work input!
Internally/Externally Reversible • Internally Reversible: • No irreversibilities occur within system boundaries (quasi-equilibrium) • Externally Reversible: • No irreversibilities occur outside system boundaries • Totally Reversible: • No irreversibilites occur within system or its surroundings
Chapter 5 Example An adiabatic air compressor is to be powered by a direct-coupled adiabatic steam turbine that is also driving a generator. Steam enters the turbine at 12.5 MPa and 500oC at a rate of 25 kg/s and exits at 10 kPA and a quality of 0.92. Air enters the compressor at 98 kPa and 295 K at a rate of 10 kg/s and exits at 1 Mpa and 620 K. Determine the net power delivered to the generator by the turbine. Mass flow = 25 kg/sec Mass flow = 10 kg/sec hairout = 628.07 kJ/kg (Table A-17) hsteam in= 3343.6 kJ/kg (Table A-6) Quality = 0.92 hairin = 295.17 kJ/kg (Table A-17) hwaterout = 2392.5 kJ/kg (Table A-5)
Heat Engine Review • Heat engines are cyclic devices in that the working fluid returns to it original state at the end of each cycle. • Work is done by the fluid in part of the cycle and on the fluid during another part of the cycle. • Efficiency of a cycle is dependent on the processes that make up a cycle. • Efficiency can be maximized by using reversible processes.
Carnot Cycle • Proposed by a French engineer Sadi Carnot in 1824 • Theoretical heat engine • Comprised of four reversible processes. 2 isothermal and 2 adiabatic
Carnot Cycle • Consider a closed system containing gas in an adiabatic piston-cylinder assembly.
Reversible Isothermal Expansion • Process 1-2 • TH is constant. • Cylinder head in close contact with source at TH • Gas expands slowly doing work on surroundings • Reversible heat transfer process • Amount of heat transferred is QH
Reversible Adiabatic Expansion • Process 2-3 • Reservoir is removed, replaced with insulation • Gas expands doing work on surroundings • Temp drops from TH to TL • Frictionless piston and quasi-equilibrium • Reversible and adiabatic
Reversible Isothermal Compression • Process 3-4 • TL is constant. • Cylinder head in close contact with sink at TL • Piston is pushed with external force doing work • Reversible heat transfer process • Amount of heat rejected is QL
Reversible Adiabatic Compression • Process 4-1 • Reservoir is removed, replaced with insulation • Gas is compressed to original state • Temp rises from TL to TH • Frictionless piston and quasi-equilibrium • Reversible and adiabatic
Carnot Principles • The efficiency of a irreversible heat engine is always less than a reversible one operating between the same two reservoirs. • The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.