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Energy transfer by heat

Energy transfer by work

Mechanical forms of work

The first law of thermodynamics

Energy balance

Energy change of a system

Mechanisms of energy transfer (heat, work, mass flow)

Energy conversion efficiencies

Efficiencies of mechanical and electrical devices (turbines, pumps, etc...)

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- 1. ENERGY, ENERGYTRANSFER AND ENERGY ANALYSIS Lecture 5 Keith Vaugh BEng (AERO) MEng Reference text: Chapter 3 - Fundamentals of Thermal-Fluid Sciences, 3rd Edition Yunus A. Cengel, Robert H. Turner, John M. Cimbala McGraw-Hill, 2008 KEITH VAUGH
- 2. OBJECTIVES } KEITH VAUGH
- 3. OBJECTIVESIntroduce the concept of energy and deﬁne itsvarious forms. } KEITH VAUGH
- 4. OBJECTIVESIntroduce the concept of energy and deﬁne itsvarious forms.Discuss the nature of internal energy. } KEITH VAUGH
- 5. OBJECTIVESIntroduce the concept of energy and deﬁne itsvarious forms.Discuss the nature of internal energy. }Deﬁne the concept of heat and the terminologyassociated with energy transfer by heat. KEITH VAUGH
- 6. OBJECTIVESIntroduce the concept of energy and deﬁne itsvarious forms.Discuss the nature of internal energy. }Deﬁne the concept of heat and the terminologyassociated with energy transfer by heat.Discuss the three mechanisms of heat transfer:conduction, convection, and radiation. KEITH VAUGH
- 7. OBJECTIVESIntroduce the concept of energy and deﬁne itsvarious forms.Discuss the nature of internal energy. }Deﬁne the concept of heat and the terminologyassociated with energy transfer by heat.Discuss the three mechanisms of heat transfer:conduction, convection, and radiation.Deﬁne the concept of work, including electricalwork and several forms of mechanical work. KEITH VAUGH
- 8. }KEITH VAUGH
- 9. Introduce the ﬁrst law of thermodynamics, energybalances, and mechanisms of energy transfer to orfrom a system. } KEITH VAUGH
- 10. Introduce the ﬁrst law of thermodynamics, energybalances, and mechanisms of energy transfer to orfrom a system. }Determine that a ﬂuid ﬂowing across a controlsurface of a control volume carries energy acrossthe control surface in addition to any energy transferacross the control surface that may be in the form ofheat and/or work. KEITH VAUGH
- 11. Introduce the ﬁrst law of thermodynamics, energybalances, and mechanisms of energy transfer to orfrom a system. }Determine that a ﬂuid ﬂowing across a controlsurface of a control volume carries energy acrossthe control surface in addition to any energy transferacross the control surface that may be in the form ofheat and/or work.Deﬁne energy conversion efﬁciencies. KEITH VAUGH
- 12. Introduce the ﬁrst law of thermodynamics, energybalances, and mechanisms of energy transfer to orfrom a system. }Determine that a ﬂuid ﬂowing across a controlsurface of a control volume carries energy acrossthe control surface in addition to any energy transferacross the control surface that may be in the form ofheat and/or work.Deﬁne energy conversion efﬁciencies.Discuss the implications of energy conversion onthe environment. KEITH VAUGH
- 13. FORMS OF ENERGYEnergy can exist in numerous forms such as thermal, mechanical,kinetic, potential, electric, magnetic, chemical, and nuclear, andtheir sum constitutes the total energy, E of a system. }Thermodynamics deals only with the change of the total energy.Macroscopic forms of energyThose a system possesses as a whole with respect to some outsidereference frame, such as kinetic and potential energies.Microscopic forms of energyThose related to the molecular structure of a system and thedegree of the molecular activity.Internal energy, UThe sum of all the microscopic forms of energy. KEITH VAUGH
- 14. }Kinetic energy, KEThe energy that a systempossesses as a result of its motionrelative to some reference frame. KEITH VAUGH
- 15. }Kinetic energy, KE 1 2 KE = mV ( kJ )The energy that a system 2possesses as a result of its motionrelative to some reference frame. KEITH VAUGH
- 16. }Kinetic energy, KE 1 2 KE = mV ( kJ )The energy that a system 2possesses as a result of its motionrelative to some reference frame. V2 ke = 2 ( kg) kJ Per unit mass KEITH VAUGH
- 17. }Kinetic energy, KE 1 2 KE = mV ( kJ )The energy that a system 2possesses as a result of its motionrelative to some reference frame. V2 ke = 2 ( kg) kJ Per unit mass }Potential energy, PEThe energy that a systempossesses as a result of itselevation in a gravitational ﬁeld. KEITH VAUGH
- 18. }Kinetic energy, KE 1 2 KE = mV ( kJ )The energy that a system 2possesses as a result of its motionrelative to some reference frame. V2 ke = 2 ( kg) kJ Per unit mass }Potential energy, PE PE = mgz ( kJ )The energy that a systempossesses as a result of itselevation in a gravitational ﬁeld. KEITH VAUGH
- 19. }Kinetic energy, KE 1 2 KE = mV ( kJ )The energy that a system 2possesses as a result of its motionrelative to some reference frame. V2 ke = 2 ( kg) kJ Per unit mass }Potential energy, PE PE = mgz ( kJ )The energy that a systempossesses as a result of itselevation in a gravitational ﬁeld. pe = gz ( kg) kJ Per unit mass KEITH VAUGH
- 20. KEITH VAUGH
- 21. Mass ﬂow rate & & m = ρV = ρ AcVavg ( s) kg KEITH VAUGH
- 22. Mass ﬂow rate & & m = ρV = ρ AcVavg ( s) kgEnergy ﬂow rate & & E = me ( kJ s or kW ) KEITH VAUGH
- 23. Total energy of a system 1 2 E = U + KE + PE = U + mV + mgz ( kJ ) 2 KEITH VAUGH
- 24. Total energy of a system 1 2 E = U + KE + PE = U + mV + mgz ( kJ ) 2Energy of a system per unit mass V2 e = u + ke + pe = u + 2 + gz ( kg) kJ KEITH VAUGH
- 25. Total energy of a system 1 2 E = U + KE + PE = U + mV + mgz ( kJ ) 2Energy of a system per unit mass V2 e = u + ke + pe = u + 2 + gz ( kg) kJTotal energy per unit mass E e= m ( kg) kJ KEITH VAUGH
- 26. MECHANICAL ENERGYThe form of energy that can be converted to mechanical workcompletely and directly by an ideal mechanical device suchas an ideal turbine }Kinetic and Potential energies are the familiar forms ofmechanical energy KEITH VAUGH
- 27. MECHANICAL ENERGYThe form of energy that can be converted to mechanical workcompletely and directly by an ideal mechanical device suchas an ideal turbine }Kinetic and Potential energies are the familiar forms ofmechanical energy P V2 Mechanical energy of a emech = + + gz ρ 2 ﬂowing ﬂuid per unit mass KEITH VAUGH
- 28. Rate of mechanical energy of a ﬂowing ﬂuid& ⎛ P V 2 ⎞ &Emech = memech & = m ⎜ + + gz ⎟ ⎝ ρ 2 ⎠ KEITH VAUGH
- 29. Rate of mechanical energy of a ﬂowing ﬂuid& ⎛ P V 2 ⎞ &Emech = memech & = m ⎜ + + gz ⎟ ⎝ ρ 2 ⎠Mechanical energy change of a ﬂuid during incompressible ﬂow per unit mass P2 − P1 V22 − V12Δemech = ρ + 2 + g ( z2 − z1 ) ( kg) kJ KEITH VAUGH
- 30. Rate of mechanical energy of a ﬂowing ﬂuid& ⎛ P V 2 ⎞ &Emech = memech & = m ⎜ + + gz ⎟ ⎝ ρ 2 ⎠Mechanical energy change of a ﬂuid during incompressible ﬂow per unit mass P2 − P1 V22 − V12Δemech = ρ + 2 + g ( z2 − z1 ) ( kg) kJRate of mechanical energy change of a ﬂuid during incompressible ﬂow & ⎛ P2 − P1 V22 − V12 ⎞ &ΔEmech = mΔemech & = m ⎜ + + g ( z2 − z1 )⎟ ( kW ) ⎝ ρ 2 ⎠ KEITH VAUGH
- 31. ENERGY TRANSFER BY HEAT Heat The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature differential } KEITH VAUGH
- 32. ENERGY TRANSFER BY HEAT Heat The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature differential } Heat transfer per unit mass Q q= m ( kg) kJ KEITH VAUGH
- 33. ENERGY TRANSFER BY HEAT Heat The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature differential } Heat transfer per unit mass Q q= m ( kg) kJ Amount of heat transfer when heat transfer rate is constant & Q = QΔt ( kJ ) KEITH VAUGH
- 34. ENERGY TRANSFER BY HEAT Heat The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature differential } Heat transfer per unit mass Q q= m ( kg) kJ Amount of heat transfer when heat transfer rate is constant & Q = QΔt ( kJ ) Amount of heat transfer when heat transfer rate changes with time & t2 Q = ∫ Qdt t1 ( kJ ) KEITH VAUGH
- 35. During an adiabatic process, a systemexchanges no heat with its surroundings KEITH VAUGH
- 36. ENERGY TRANSFER BY WORK Work The energy transfer associated with a force acting through a distance. } Heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negativeSpecifying the directions of Alternative is to use the subscripts in and out toheat and work indicate the direction KEITH VAUGH
- 37. ENERGY TRANSFER BY WORK Work The energy transfer associated with a force acting through a distance. } Heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negativeSpecifying the directions of Alternative is to use the subscripts in and out toheat and work indicate the direction Work done per unit mass W w= m ( kg) kJ KEITH VAUGH
- 38. KEITH VAUGH
- 39. Heat Vs. WorkBoth are recognised at the boundaries of asystem as they cross the boundaries. That is,both heat and work are boundary phenomena.Systems possess energy, but not heat or work.Both are associated with a process, not a state.Unlike properties, heat or work has no meaningat a state.Both are path functions (i.e., their magnitudesdepend on the path followed during a process aswell as the end states). KEITH VAUGH
- 40. KEITH VAUGH
- 41. Properties are point functions; but heat and workare path functions (their magnitudes depend on thepath followed KEITH VAUGH
- 42. Properties are point functions and have exact differentials (d). 2 ∫ dV = V2 − V1 = ΔV 1Properties are point functions; but heat and workare path functions (their magnitudes depend on thepath followed KEITH VAUGH
- 43. Properties are point functions and have exact differentials (d). 2 ∫ dV = V2 − V1 = ΔV 1 Path functions have inexact differentials (δ) 2 ∫ δ W = W12 1Properties are point functions; but heat and workare path functions (their magnitudes depend on thepath followed KEITH VAUGH
- 44. ELECTRICAL WORK }Electrical power in terms of resistanceR, current I, and potential difference V. KEITH VAUGH
- 45. ELECTRICAL WORK Electrical Work We = VN }Electrical power in terms of resistanceR, current I, and potential difference V. KEITH VAUGH
- 46. ELECTRICAL WORK Electrical Work We = VN } Electrical Power (rate form) & We = VI (W )Electrical power in terms of resistanceR, current I, and potential difference V. KEITH VAUGH
- 47. ELECTRICAL WORK Electrical Work We = VN } Electrical Power (rate form) & We = VI (W ) When potential difference and current change with time & 2 We = ∫ VI dt 1 ( kJ )Electrical power in terms of resistanceR, current I, and potential difference V. KEITH VAUGH
- 48. ELECTRICAL WORK Electrical Work We = VN } Electrical Power (rate form) & We = VI (W ) When potential difference and current change with time & 2 We = ∫ VI dt 1 ( kJ )Electrical power in terms of resistanceR, current I, and potential difference V. When potential difference and current remain constant We = VI Δt ( kJ ) KEITH VAUGH
- 49. MECHANICAL FORMS OF WORK There are two requirements for a work interaction between a system and its surroundings to exist } ✓ there must be a force acting on the boundary ✓ the boundary must move The work done is proportional to the force (F) and the distance traveled (s) KEITH VAUGH
- 50. MECHANICAL FORMS OF WORK There are two requirements for a work interaction between a system and its surroundings to exist Work = Force × Distance We = VI Δt ( kJ ) } ✓ there must be a force acting on the boundary ✓ the boundary must move The work done is proportional to the force (F) and the distance traveled (s) KEITH VAUGH
- 51. MECHANICAL FORMS OF WORK There are two requirements for a work interaction between a system and its surroundings to exist Work = Force × Distance We = VI Δt ( kJ ) } ✓ there must be a force acting on When force is not constant the boundary ✓ the boundary must move & 2 We = ∫ VI dt ( kJ ) 1 The work done is proportional to the force (F) and the distance traveled (s) KEITH VAUGH
- 52. KEITH VAUGH
- 53. Shaft work is proportional to the torque applied and the number of revolutions of the shaft KEITH VAUGH
- 54. A force (F) acting through a momentarm (r) generates a torque (T) T T = Fr → F = r Shaft work is proportional to the torque applied and the number of revolutions of the shaft KEITH VAUGH
- 55. A force (F) acting through a momentarm (r) generates a torque (T) T T = Fr → F = rThis force acts through a distance (s) s = ( 2π r ) n Shaft work is proportional to the torque applied and the number of revolutions of the shaft KEITH VAUGH
- 56. A force (F) acting through a momentarm (r) generates a torque (T) T T = Fr → F = rThis force acts through a distance (s) s = ( 2π r ) nShaft work Shaft work is proportional to the torque applied and the number of revolutions ⎛ T ⎞ of the shaft Wsh = Fs = ⎜ ⎟ ( 2π rn ) = 2π nT ( kJ ) ⎝ r ⎠ KEITH VAUGH
- 57. A force (F) acting through a momentarm (r) generates a torque (T) T T = Fr → F = rThis force acts through a distance (s) s = ( 2π r ) nShaft work Shaft work is proportional to the torque applied and the number of revolutions ⎛ T ⎞ of the shaft Wsh = Fs = ⎜ ⎟ ( 2π rn ) = 2π nT ( kJ ) ⎝ r ⎠The power transmitted through the shaftis the shaft work done per unit time & & Wsh = 2π nT ( kW ) KEITH VAUGH
- 58. KEITH VAUGH
- 59. Spring WorkWhen the length of the spring changes by adifferential amount dx under the inﬂuence of aforce (F), the work done is δ Wspring = Fdx Elongation of a spring under the inﬂuence of a force KEITH VAUGH
- 60. Spring WorkWhen the length of the spring changes by adifferential amount dx under the inﬂuence of aforce (F), the work done is δ Wspring = FdxFor linear elastic springs, the displacement (x)is proportional to the force applied F = kx ( kN ) k: spring constant (kN/m) Elongation of a spring under the inﬂuence of a force KEITH VAUGH
- 61. Spring WorkWhen the length of the spring changes by adifferential amount dx under the inﬂuence of aforce (F), the work done is δ Wspring = FdxFor linear elastic springs, the displacement (x)is proportional to the force applied F = kx ( kN ) k: spring constant (kN/m)Substituting and integrating yields 1 Wspring 2 ( = k x2 − x12 2 ) ( kJ ) Elongation of a spring under the inﬂuence of a force x1 and x2: the initial and the ﬁnal displacements KEITH VAUGH
- 62. KEITH VAUGH
- 63. Work done on Elastic Solid Bars 2 2 Welastic = ∫ F dx = ∫ σ n A dx 1 1 ( kJ ) Solid bars behave as springs under the inﬂuence of a force KEITH VAUGH
- 64. Work done on Elastic Solid Bars 2 2 Welastic = ∫ F dx = ∫ σ n A dx 1 1 ( kJ ) Solid bars behave as springs under the inﬂuence of a force Work associated with the stretching of a liquid ﬁlm 2 Wsurface = ∫ σ s dA 1 ( kJ ) Stretching a liquid ﬁlm with a movable wire KEITH VAUGH
- 65. KEITH VAUGH
- 66. Work done to raise or to accelerate a bodyThe work transfer needed to raise a body isequal to the change in the potential energy ofthe body.The work transfer needed to accelerate a bodyis equal to the change in the kinetic energy ofthe body. The energy transferred to a body while being raised is equal to the change in its potential energy KEITH VAUGH
- 67. Non-mechanical forms of workElectrical workThe generalised force is the voltage (the electricalpotential) and the generalised displacement is theelectrical charge.Magnetic workThe generalised force is the magnetic ﬁeld strengthand the generalised displacement is the totalmagnetic dipole moment.Electrical polarisation workThe generalised force is the electric ﬁeld strengthand the generalised displacement is the polarisationof the medium. KEITH VAUGH
- 68. FIRST LAW OF THERMODYNAMICS The ﬁrst law of thermodynamics (the conservation of energy principle) provides a sound basis for studying the relationships among the various forms of energy and energy interactions. } The ﬁrst law states that energy can be neither created nor destroyed during a process; it can only change forms. KEITH VAUGH
- 69. FIRST LAW OF THERMODYNAMICS The ﬁrst law of thermodynamics (the conservation of energy principle) provides a sound basis for studying the relationships among the various forms of energy and energy interactions. } The ﬁrst law states that energy can be neither created nor destroyed during a process; it can only change forms. Energy cannot be created or destroyed; it can only change forms KEITH VAUGH
- 70. The First LawFor all adiabatic processes between two speciﬁedstates of a closed system, the net work done is thesame regardless of the nature of the closed systemand the details of the process. KEITH VAUGH
- 71. KEITH VAUGH
- 72. In the absence of any workinteractions, the energy change ofa system is equal to the net heattransfer KEITH VAUGH
- 73. In the absence of any work The work (electrical) done on aninteractions, the energy change of adiabatic system is equal to thea system is equal to the net heat increase in the energy of thetransfer system KEITH VAUGH
- 74. ENERGY BALANCE } KEITH VAUGH
- 75. ENERGY BALANCEThe net change (increase or decrease) in the total energy of thesystem during a process is equal to the difference between thetotal energy entering and the total energy leaving the system }during that process. KEITH VAUGH
- 76. ENERGY BALANCE The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system } during that process.⎛ Total energy ⎞ ⎛ Total energy ⎞ ⎛ Change in the total ⎞⎜ entering the system ⎟ − ⎜ leaving the system ⎟ = ⎜ energy of the system ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠ KEITH VAUGH
- 77. ENERGY BALANCE The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system } during that process.⎛ Total energy ⎞ ⎛ Total energy ⎞ ⎛ Change in the total ⎞⎜ entering the system ⎟ − ⎜ leaving the system ⎟ = ⎜ energy of the system ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠ Ein − Eout = ΔEsystem KEITH VAUGH
- 78. KEITH VAUGH
- 79. The work (boundary) done on anadiabatic system is equal to theincrease in the energy of thesystem KEITH VAUGH
- 80. The work (boundary) done on an The energy change of a systemadiabatic system is equal to the during a process is equal to the netincrease in the energy of the work and heat transfer betweensystem the system and its surroundings KEITH VAUGH
- 81. Energy change of a system, ΔEsystemEnergy change = Energy at ﬁnal state - Energy at initial state KEITH VAUGH
- 82. Energy change of a system, ΔEsystemEnergy change = Energy at ﬁnal state - Energy at initial state ΔEsystem = E final = Einitial = E2 − E1 KEITH VAUGH
- 83. Energy change of a system, ΔEsystemEnergy change = Energy at ﬁnal state - Energy at initial state ΔEsystem = E final = Einitial = E2 − E1 ΔE = ΔU + ΔKE + ΔPE KEITH VAUGH
- 84. Energy change of a system, ΔEsystemEnergy change = Energy at ﬁnal state - Energy at initial state ΔEsystem = E final = Einitial = E2 − E1 ΔE = ΔU + ΔKE + ΔPEInternal, kinetic and potential energy changes ΔU = m ( u2 − u1 ) 1 ( ΔKE = m V22 − V12 2 ) ΔPE = mg ( z2 − z1 ) KEITH VAUGH
- 85. Mechanisms of Energy Transfer, Ein and EoutEnergy can be transferred to or from a system in three forms ✓ Heat transfer ✓ Work transfer ✓ Mass ﬂow KEITH VAUGH
- 86. Mechanisms of Energy Transfer, Ein and EoutEnergy can be transferred to or from a system in three forms ✓ Heat transfer ✓ Work transfer ✓ Mass ﬂow } The energy content of a control volume can be changed by mass ﬂow as well as heat and work interactions KEITH VAUGH
- 87. Mechanisms of Energy Transfer, Ein and Eout Energy can be transferred to or from a system in three forms ✓ Heat transfer ✓ Work transfer ✓ Mass ﬂow } The energy content of a control volume can be changed by mass ﬂow as well as heat and work interactions Ein − Eout = (Qin − Qout ) + (Win − Wout ) + ( Emass, in − Emass, out ) = ΔEsystem ( kJ ) 14 2 4 3 123 Net energy transfer Change in internal, kinetic,by heat, work and mass potential, etc... energies KEITH VAUGH
- 88. Mechanisms of Energy Transfer, Ein and Eout Energy can be transferred to or from a system in three forms ✓ Heat transfer ✓ Work transfer ✓ Mass ﬂow } The energy content of a control volume can be changed by mass ﬂow as well as heat and work interactions Ein − Eout = (Qin − Qout ) + (Win − Wout ) + ( Emass, in − Emass, out ) = ΔEsystem ( kJ ) 14 2 4 3 123 Net energy transfer Change in internal, kinetic,by heat, work and mass potential, etc... energies Expressed in the Rate form dEsystem & Q = QΔt & & Ein − Eout = ( kW ) 14 2 4 3 dt & W = W ΔtRate of net energy transfer 123 by heat, work and mass Rate of change in internal, kinetic, potential, etc... energies ⎛ dE ⎞ ΔE = ⎜ ⎟ Δt ⎝ dt ⎠ KEITH VAUGH
- 89. The energy balance can be expressed in a per unit mass basis ein − eout = Δesystem ( kg) kJ KEITH VAUGH
- 90. The energy balance can be expressed in a per unit mass basis ein − eout = Δesystem ( kg) kJThe energy balance can also be expressed in a differential form KEITH VAUGH
- 91. The energy balance can be expressed in a per unit mass basis ein − eout = Δesystem ( kg) kJThe energy balance can also be expressed in a differential form δ Ein − δ Eout = dEsystem or δ ein − δ eout = desystem KEITH VAUGH
- 92. The energy balance can be expressed in a per unit mass basis ein − eout = Δesystem ( kg) kJThe energy balance can also be expressed in a differential form δ Ein − δ Eout = dEsystem or δ ein − δ eout = desystemFor a closed system undergoing a cycle, the initialand ﬁnal states are identical, then the energy balancefor a cycle simpliﬁes to Ein = EoutThe energy balance for a cycle can be expressed interms of heat and work interactions KEITH VAUGH
- 93. The energy balance can be expressed in a per unit mass basis ein − eout = Δesystem ( kg) kJThe energy balance can also be expressed in a differential form δ Ein − δ Eout = dEsystem or δ ein − δ eout = desystemFor a closed system undergoing a cycle, the initial For a cycle ΔE = 0and ﬁnal states are identical, then the energy balance thus Q = Wfor a cycle simpliﬁes to Ein = EoutThe energy balance for a cycle can be expressed interms of heat and work interactions & & Wnet , out = Qnet , in or Wnet , out = Qnet , in for a cycle KEITH VAUGH
- 94. ENERGY CONVERSION EFFICIENCIESEfﬁciencyis one of the most frequently used terms in thermodynamics,and it indicates how well an energy conversion or transferprocess is accomplished } Desired output Performance = Required output KEITH VAUGH
- 95. ENERGY CONVERSION EFFICIENCIESEfﬁciencyis one of the most frequently used terms in thermodynamics,and it indicates how well an energy conversion or transferprocess is accomplished } Desired output Performance = Required outputEfﬁciency of a water heaterThe ratio of the energy delivered to the house by hot water tothe energy supplied to the water heater. KEITH VAUGH
- 96. Q Amount of heat released during combustionηcombustion = = HV Heating value of the fuel burnedHeating value of the fuelThe amount of heat released when a unit amount of fuel atroom temperature is completely burned and thecombustion products are cooled to the room temperature.Lower heating value (LHV)When the water leaves as a vapour.Higher heating value (HHV)When the water in the combustion gases is completelycondensed and thus the heat of vaporisation is alsorecovered. KEITH VAUGH
- 97. The efﬁciency of space heating systems of residential and commercial buildings is usually expressed in terms of the annual fuel utilisation efﬁciency (AFUE). This accounts for the combustion efﬁciency as well as other losses such as heat losses to unheated areas and start-up and cool down losses.The deﬁnition of the heating value of gasoline KEITH VAUGH
- 98. GeneratorA device that converts mechanical energy to electrical energy.Generator efﬁciencyThe ratio of the electrical power output to the mechanicalpower input.Thermal efﬁciency of a power plantThe ratio of the net electrical power output to the rate of fuelenergy input. KEITH VAUGH
- 99. GeneratorA device that converts mechanical energy to electrical energy.Generator efﬁciencyThe ratio of the electrical power output to the mechanicalpower input.Thermal efﬁciency of a power plantThe ratio of the net electrical power output to the rate of fuelenergy input. & Wnet , electric Overall efﬁciency ηoverall = ηcombustionηthermalηgenerator = HHV × mnet & of a power plant KEITH VAUGH
- 100. GeneratorA device that converts mechanical energy to electrical energy.Generator efﬁciencyThe ratio of the electrical power output to the mechanicalpower input.Thermal efﬁciency of a power plantThe ratio of the net electrical power output to the rate of fuelenergy input. & Wnet , electric Overall efﬁciency ηoverall = ηcombustionηthermalηgenerator = HHV × mnet & of a power plantLighting efﬁciencyThe amount of light output in lumens per W of electricityconsumed. (refer to text page 88) KEITH VAUGH
- 101. Using energy-efﬁcient appliances conserve energy.It helps the environment by reducing the amount ofpollutants emitted to the atmosphere during the combustionof fuelThe combustion of fuel produces ✓ carbon dioxide, causes global warming ✓ nitrogen oxides and hydrocarbons, cause smog ✓ carbon monoxide, toxic ✓ sulphur dioxide, causes acid rain The efﬁciency of a cooking appliance represents the fraction of the energy supplied to the appliance that is transferred to the food KEITH VAUGH
- 102. Efﬁciencies of mechanical and electrical devices Mechanical energy output Emech, out Emech, loss ηmech = = = 1− Mechanical energy input Emech, in Emech, in KEITH VAUGH
- 103. Efﬁciencies of mechanical and electrical devices Mechanical energy output Emech, out Emech, loss ηmech = = = 1− Mechanical energy input Emech, in Emech, inThe effectiveness of the conversion process between the mechanicalwork supplied or extracted and the mechanical energy of the ﬂuid isexpressed by the pump efﬁciency and turbine efﬁciency, KEITH VAUGH
- 104. Efﬁciencies of mechanical and electrical devices Mechanical energy output Emech, out Emech, loss ηmech = = = 1− Mechanical energy input Emech, in Emech, inThe effectiveness of the conversion process between the mechanicalwork supplied or extracted and the mechanical energy of the ﬂuid isexpressed by the pump efﬁciency and turbine efﬁciency, & & Mechanical energy increase of the fluid ΔEmech, fluid W pump, u η pump = = = Mechanical energy input & , in Wshaft & W pump KEITH VAUGH
- 105. Efﬁciencies of mechanical and electrical devices Mechanical energy output Emech, out Emech, loss ηmech = = = 1− Mechanical energy input Emech, in Emech, inThe effectiveness of the conversion process between the mechanicalwork supplied or extracted and the mechanical energy of the ﬂuid isexpressed by the pump efﬁciency and turbine efﬁciency, & & Mechanical energy increase of the fluid ΔEmech, fluid W pump, u η pump = = = Mechanical energy input & , in Wshaft & W pump & & & ΔEmech, fluid = Emech, out − Emech, in KEITH VAUGH
- 106. Efﬁciencies of mechanical and electrical devices Mechanical energy output Emech, out Emech, loss ηmech = = = 1− Mechanical energy input Emech, in Emech, inThe effectiveness of the conversion process between the mechanicalwork supplied or extracted and the mechanical energy of the ﬂuid isexpressed by the pump efﬁciency and turbine efﬁciency, & & Mechanical energy increase of the fluid ΔEmech, fluid W pump, u η pump = = = Mechanical energy input & , in Wshaft & W pump & & & ΔEmech, fluid = Emech, out − Emech, in Mechanical energy output & Wshaft , out & Wturbine ηturbine = = = & & Mechanical energy decrease of the fluid ΔEmech, fluid Wturbine, e KEITH VAUGH
- 107. Efﬁciencies of mechanical and electrical devices Mechanical energy output Emech, out Emech, loss ηmech = = = 1− Mechanical energy input Emech, in Emech, inThe effectiveness of the conversion process between the mechanicalwork supplied or extracted and the mechanical energy of the ﬂuid isexpressed by the pump efﬁciency and turbine efﬁciency, & & Mechanical energy increase of the fluid ΔEmech, fluid W pump, u η pump = = = Mechanical energy input & , in Wshaft & W pump & & & ΔEmech, fluid = Emech, out − Emech, in Mechanical energy output & Wshaft , out & Wturbine ηturbine = = = & & Mechanical energy decrease of the fluid ΔEmech, fluid Wturbine, e & & & ΔEmech, fluid = Emech, in − Emech, out KEITH VAUGH
- 108. The mechanical efﬁciency of a fan is the ratioof the kinetic energy of air at the fan exit tothe mechanical power input KEITH VAUGH
- 109. & ⎛ mV22 ⎞ & ΔEmech, fluid ⎜ ⎝ 2 ⎟ ⎠ ηmech, fan = = & Wshaft , in & Wshaft , inThe mechanical efﬁciency of a fan is the ratioof the kinetic energy of air at the fan exit tothe mechanical power input KEITH VAUGH
- 110. & ⎛ mV22 ⎞ & ΔEmech, fluid ⎜ ⎝ 2 ⎟ ⎠ ηmech, fan = = & Wshaft , in & Wshaft , in ⎝( ⎛ 0.50 kg )( 2 m ⎞ s 12 s ⎠ ) ηmech, fan = 2 50WThe mechanical efﬁciency of a fan is the ratioof the kinetic energy of air at the fan exit tothe mechanical power input KEITH VAUGH
- 111. & ⎛ mV22 ⎞ & ΔEmech, fluid ⎜ ⎝ 2 ⎟ ⎠ ηmech, fan = = & Wshaft , in & Wshaft , in ⎝( ⎛ 0.50 kg )( 2 m ⎞ s 12 s ⎠ ) ηmech, fan = 2 50WThe mechanical efﬁciency of a fan is the ratioof the kinetic energy of air at the fan exit to ηmech, fan = 0.72 or 72%the mechanical power input KEITH VAUGH
- 112. & Mechanical power output Wshaft , outηmotor = = Pump efﬁciency Electric power input & Welect , in & Electrical power output Welect , outηgenerator = = Generator efﬁciency & Mechanical power input Wshaft , in & & W pump, u ΔEmech, fluid Pump-motorη pump−motor = η pumpηmotor = = & Welect , in & Welect , in overall efﬁciency & Welect , out & Welect , out Turbine-generatorηturbine−gen = ηturbineηgenerator = = & & Wturbine, e ΔEmech, fluid overall efﬁciency KEITH VAUGH
- 113. Forms of energyEnergy transfer by heatEnergy transfer by workMechanical forms of workThe ﬁrst law of thermodynamics Energy balance Energy change of a system Mechanisms of energy transfer (heat, work, mass ﬂow)Energy conversion efﬁciencies Efﬁciencies of mechanical and electrical devices (turbines, pumps, etc...)Energy and environment Ozone and smog Acid rain The Greenhouse effect - Global warming KEITH VAUGH

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