Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering Summary of Energy Topics Chapter 1: Thermodynamics / Energy Introduction Chapter 2: Systems & Processes Chapter 3: Work, Energy, Temperature & Heat Chapter 4: Work Processes of Closed Systems Chapter 5: Thermodynamic Properties Chapter 6: Steam Tables Chapter 7: Ideal Gases Chapter 8: Conservation of Mass & Energy Chapter 9: 1 st Law of Thermodynamics Chapter 10: Steady Flow Energy Equation Chapter 11: Heat Engines and Reversibility Chapter 12: 2 nd Law of Thermodynamics Chapter 13: Entropy Chapter 14: General Energy
Chapter 8: Conservation of Mass & Energy The principle of the conservation of mass OR energy states that mass OR energy cannot be created or destroyed, but only changed in form. Einstein's equation E=mc 2 relates mass and energy (c is the velocity of light in this equation). However, in almost every case in engineering thermodynamics no detectable conversion between mass and energy occurs and therefore the principle of conservation of mass and energy holds separately. Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering
Chapter 8: Conservation of Mass & Energy DEFINITIONS A steady flow rate is a flow rate that does not vary with time. A non-steady (or unsteady) flow rate is one that varies with time. The term transfer indicates that an extensive physical or thermodynamic property passes from one system to another. A net change is the change that remains after all changes have occurred. A net effect (effect = something that causes change) is the combined effect of all effects. Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering
Chapter 8: Conservation of Mass & Energy MASS BALANCE EQUATION Since mass is conserved, a mass balance equation for a general system Or as mass rate: where the symbol t represents time. Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering
Chapter 8: Conservation of Mass & Energy MASS BALANCE EQUATION Closed systems have no rate of change of mass Open systems mass rate is: where; c n is normal velocity of fluid/gas and is uniform over the area A Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering
Chapter 8: Conservation of Mass & Energy ENERGY BALANCE EQUATION E system + E surroundings = constant E system + E surroundings = 0 Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering
Chapter 8: Conservation of Mass & Energy ENERGY BALANCE EQUATION Substituting u + pv for specific enthalpy, h,
Chapter 8: Conservation of Mass & Energy EXAMPLE 8.1: Mass Example Water enters a boiler at the mass rate of 1.5 kg/s. Dry saturated steam leaves with a velocity of 17 m/s in a pipe of 40 mm diameter at a pressure of 60 bar. What is the rate of change of the mass of water substance within the boiler? SOLUTION At 60 bar or 6MPa; Dr. Owen Clarkin School of Mechanical & Manufacturing Engineering