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Identify the unique vocabulary associated with thermodynamics through the precise definition of basic concepts to form a sound foundation for the development of the principles of thermodynamics. • Review the metric SI and the English unit systems. • Explain the basic concepts of thermodynamics such as system, state, state postulate, equilibrium, process, and cycle. • Review concepts of temperature, temperature scales, pressure, and absolute and gage pressure.
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Thermodynamics: An Engineering Approach
Yunus A. Cengel, Michael A. Boles McGraw-Hill, 20 11
Mass and Volume Flow Rates
Conservation of Mass Principle The conservation of mass principle for a control volume : The net mass transfer to or from a control volume during a time interval t is equal to the net change (increase or decrease) in the total mass within the control volume during t. These equations are often referred to as the mass balance and are applicable to any control volume undergoing any kind of process.
Mass Balance for Steady-Flow Processes During a steady-flow process, the total amount of mass contained within a control volume does not change with time ( m CV = constant). Then the conservation of mass principle requires that the total amount of mass entering a control volume equal the total amount of mass leaving it. For steady-flow processes, we are interested in the amount of mass flowing per unit time, that is, the mass flow rate. Multiple inlets and exits Single stream Many engineering devices such as nozzles, diffusers, turbines, compressors, and pumps involve a single stream (only one inlet and one outlet).
Special Case: Incompressible Flow The conservation of mass relations can be simplified even further when the fluid is incompressible, which is usually the case for liquids. Steady, incompressible Steady, incompressible flow (single stream) There is no such thing as a “conservation of volume” principle. For steady flow of liquids, the volume flow rates, as well as the mass flow rates, remain constant since liquids are essentially incompressible substances.
Total Energy of a Flowing Fluid The total energy consists of three parts for a nonflowing fluid and four parts for a flowing fluid. h = u + Pv The flow energy is automatically taken care of by enthalpy. In fact, this is the main reason for defining the property enthalpy.
Energy Transport by Mass When the kinetic and potential energies of a fluid stream are negligible When the properties of the mass at each inlet or exit change with time as well as over the cross section
Mass and Energy balances for a steady-flow process A water heater in steady operation. Mass balance Energy balance
Under steady operation, shaft work and electrical work are the only forms of work a simple compressible system may involve. Energy balance relations with sign conventions (i.e., heat input and work output are positive) when kinetic and potential energy changes are negligible The units m 2 /s 2 and J/kg are equivalent. At very high velocities, even small changes in velocities can cause significant changes in the kinetic energy of the fluid.
Nozzles and Diffusers Nozzles and diffusers are commonly utilized in jet engines, rockets, spacecraft, and even garden hoses. A nozzle is a device that increases the velocity of a fluid at the expense of pressure. A diffuser is a device that increases the pressure of a fluid by slowing it down. The cross-sectional area of a nozzle decreases in the flow direction for subsonic flows and increases for supersonic flows. The reverse is true for diffusers. Energy balance for a nozzle or diffuser:
Turbines and Compressors Turbine drives the electric generator in steam, gas, or hydroelectric power plants. As the fluid passes through the turbine, work is done against the blades, which are attached to the shaft. As a result, the shaft rotates, and the turbine produces work. Compressors , as well as pumps and fans , are devices used to increase the pressure of a fluid. Work is supplied to these devices from an external source through a rotating shaft. A fan increases the pressure of a gas slightly and is mainly used to mobilize a gas. A compressor is capable of compressing the gas to very high pressures. Pumps work very much like compressors except that they handle liquids instead of gases. Energy balance for the compressor in this figure:
Mixing chambers In engineering applications, the section where the mixing process takes place is commonly referred to as a mixing chamber. Energy balance for the adiabatic mixing chamber in the figure is:
Heat exchangers Heat exchangers are devices where two moving fluid streams exchange heat without mixing. Heat exchangers are widely used in various industries, and they come in various designs. A heat exchanger can be as simple as two concentric pipes. Mass and energy balances for the adiabatic heat exchanger in the figure is: The heat transfer associated with a heat exchanger may be zero or nonzero depending on how the control volume is selected.
