# THERMODYNAMICD 06

version from 2018-03-28 13:33

## Section

Reduced P TPR is the ratio of the p T to the critical pressure T.
Reference stateis chosen to assign a value of zero for a convenient property or properties at that state.
Refrigerantis the working fluid used in the refrigeration cycle.
Refrigeratoris a cyclic device which causes the transfer of heat from a low-temperature region to a high-temperature region. The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it.
Refrigerator coefficient of performanceis the efficiency of a refrigerator, denoted by COPR, and expressed as desired output divided by required input or COPR = QL/Wnet, in.
Refrigeratorsare cyclic devices which allow the transfer of heat from a low-temperature medium to a high-temperature medium.
Regenerationis a process during which heat is transferred to a thermal energy storage device (called a regenerator) during one part of the cycle and is transferred back to the working fluid during another part of the cycle.
Regenerator effectivenessis the extent to which a regenerator approaches an ideal regenerator and is defined as the ratio of the heat transfer to the compressor exit gas to the maximum possible heat transfer to the compressor exit gas.
Relative humidityis a measure of the amount of moisture the air holds relative to the maximum amount the air can hold at the same temperature. The relative humidity can be expressed as the ratio of the vapour pressure to the saturation pressure of water at that temperature.
Relative pressurePr is defined as the quantity exp(s°/R) and is a dimensionless quantity that is a function of temperature only since s° depends on temperature alone. Relative pressure is used in isentropic processes of ideal gases where variable specific heats are required.
Relative specific volume vr is defined as the quantity T/Pris a function of temperature only and Pr is the relative pressure. Relative specific volume is used in isentropic processes of ideal gases where variable specific heats are required.
Reversed Carnot cyclEis a reversible cycle in which all four processes that comprise the Carnot cycle are reversed during operation. Reversing the cycle will also reverse the directions of any heat and work interactions. The result is a cycle that operates in the counterclockwise direction.
Reversed Carnot cycleis the result of reversing all the process that comprise the reversible Carnot heat-engine cycle, in which case it becomes the Carnot refrigeration cycle.
Reversible processdefined as a process that can be reversed without leaving any trace on the surroundings. Reversible processes are idealized processes, and they can be approached but never reached in reality.
Reversible steady-flow workdefined as the negative of the integral of the specific volume-pressure product. The larger the specific volume, the larger the reversible work produced or consumed by the steady-flow device. Therefore, every effort should be made to keep the specific volume of a fluid as small as possible during a compression process to minimize the work input and as large as possible during an expansion process to maximize the work output.
Reversible workis defined as the maximum amount of useful work that can be produced (or the minimum work that needs to be supplied) as a system undergoes a process between the specified initial and final states. Reversible work is determined from the exergy balance relations by setting the exergy destroyed equal to zero. The work W in that case becomes the reversible work.
Saturated airair which can hold no more moisture. Any moisture introduced into saturated air will condense.
Saturated liquida liquid that is about to vaporize.
Saturated liquid linethe saturated liquid states connected by a line that meets the saturated vapour line at the critical point, forming a dome.
Saturated liquid-vapour mixtureis a mixture of the liquid and vapour phases that coexist in equilibrium.
Saturated liquid-vapour mixture region, or the wet regionis all the states that involve both the liquid and vapour phases in equilibrium and are located under the dome.
Saturated vapour is a vapour that is about to condense.
SAturated vapour lineis the saturated vapour states connected by a line that meets the saturated liquid line at the critical point, forming a dome.
Saturation pressurePsat is called the pressure at which a pure substance changes phase at a given temperature.
Saturation temperatureTsat is the temperature at which a pure substance changes phase at a given pressure.
Scramjet engineessentially a ramjet in which air flows through at supersonic speeds (above the speed of sound).
Secondary dimensions, or derived dimensionssuch as velocity, energy E, and volume V, are expressed in terms of the primary dimensions.
Second law distinction between heat transfer and workstates that an energy interaction that is accompanied by entropy transfer is heat transfer, and an energy interaction that is not accompanied by entropy transfer is work.
Second-law efficiency the ratio of the actual thermal efficiency to the maximum possible (reversible) thermal efficiency under the same conditions. The second-law efficiency of various steady-flow devices can be determined from its general definition, hII = (exergy recovered)/(exergy supplied).
Seebeck effect results whenwires made from different metals are joined at both ends (junctions), form a closed circuit, and one of the ends is heated. As a result of the applied heat a current flows continuously in the circuit. The Seebeck effect is named in honour of Thomas Seebeck, who made its discovery in 1821.
Sensible energythe portion of the internal energy of a system associated with the kinetic energies of the molecules.
Shaft workenergy transmitted by a rotating shaft and is the related to the torque T applied to the shaft and the number of revolutions of the shaft per unit time.
Simple compressible systema system in which there is the absence of electrical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineering problems.
Simple coolingthe process of lowering the temperature of atmospheric air when no moisture is removed.
Simple heatingthe process of raising the temperature of atmospheric air when no moisture is added.
Simultaneous reactionschemical reactions that involve two or more reactions occurring at the same time.
Sling psychrometera device with both a dry-bulb thermometer and a wet-bulb temperature mounted on the frame of the device so that when it is swung through the air both the wet-and dry-bulb temperatures can be read simultaneously.
Solid phasemolecules arranged in a three-dimensional pattern (lattice) that is repeated throughout. Because of the small distances between molecules in a solid, the attractive forces of molecules on each other are large and keep the molecules at fixed positions.
Solubilitythe maximum amount of solid that can be dissolved in a liquid at a specified temperature.
Sonic flowoccurs when a flow has a Mach number M =1.
Spark-ignition (SI) enginesreciprocating engines in which the combustion of the air-fuel mixture is initiated by a spark plug.
Specific gravity, or relative densitydefined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C, for which the density is 1000 kg/m3).
Specific heatis defined as the energy required to raise the temperature of a unit mass of a substance by one degree. In general, this energy will depend on how the process is executed.
Specific heat at constant pressureas the energy required to raise the temperature of the unit mass of a substance by one degree as the pressure is maintained constant. Cp is a measure of the variation of enthalpy of a substance with temperature. Cp can be defined as the change in the enthalpy of a substance per unit change in temperature at constant pressure.
Specific heat at constant volumeis the energy required to raise the temperature of the unit mass of a substance by one degree as the volume is maintained constant. Cv is related to the changes in internal energy. It would be more proper to define Cv as the change in the internal energy of a substance per unit change in temperature at constant volume.
Specific heat ratiodefined as the ratio Cp/Cv.
Specific heats for solids and liquidsor incompressible substances, are equal.
Specific propertiesextensive properties per unit mass. Some examples of specific properties are specific volume (v=V/m) and specific total energy (e= E/m).
Specific volumethe reciprocal of density and is defined as the volume per unit mass.
Specific weightis the weight of a unit volume of a substance and is determined from the product of the local acceleration of gravity and the substance density.
Spray ponda pond where warm water is sprayed into the air and is cooled by the air as it falls into the pond. Spray ponds require 25 to 50 times the area of a cooling tower because water loss due to air drift is high.
Spring workthe work done to change the length of a spring.
Stable form of an elementthe chemically stable form of that element at 25° C and 1 atm. Nitrogen, for example, exists in diatomic form (N2 ) at 25° C and 1 atm. Therefore, the stable form of nitrogen at the standard reference state is diatomic nitrogen N2 , not monatomic nitrogen N.
Stagnation (or total) temperaturethe temperature an ideal gas will attain when it is brought to rest adiabatically.
Stagnation enthalpythe total energy of a flowing fluid stream per unit mass and represents the enthalpy of a fluid when it is brought to rest adiabatically with no work. The stagnation enthalpy equals the static enthalpy when the kinetic energy of the fluid is negligible.
Stagnation pressurethe pressure a fluid attains when brought to rest isentropically. For ideal gases with constant specific heats, the stagnation pressure is related to the static pressure of the fluid through the isentropic process equation relating pressure and temperature.
Stagnation propertiesproperties of a fluid at the stagnation state. These properties are called stagnation temperature, stagnation pressure, stagnation density, etc. The stagnation state and the stagnation properties are indicated by the subscript 0.
Standard reference statefor the properties of chemical components is chosen as 25°C (77°F) and 1 atm. Property values at the standard reference state are indicated by a superscript (°) (such as h°and u°).
Standard-state Gibbs function changeis the difference between the sum products of the stoichiometric coefficients and the Gibbs function of a component at 1 atm pressure and temperature T for the products and reactants in the stoichiometric reaction.
Statea system not undergoing any change gives a set of properties that completely describes the condition of a system. At this point, all the properties can be measured or calculated throughout the entire system.
State postulatethe number of properties required to fix the state of a system: The state of a simple compressible system is completely specified by two independent, intensive properties.
Stationary systemsclosed systems whose velocity and elevation of the center of gravity remain constant during a process.
Stationary systemssystems that do not involve any changes in their velocity or elevation during a process.
Statistical thermodynamicsapproach to thermodynamics more elaborate than classical thermodynamics, is based on the average behaviour of large groups of individual particles.
Steady-flow devicesfor long periods of time under the same conditions.
Steady-flow processdefined as a process during which a fluid flows through a control volume steadily. That is, the fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process.
Steam generatorthe combination of a boiler and a heat exchanger section (the superheater), where steam is superheated.
Steam power plantan external-combustion engine in which steam (water) is the working fluid. That is, combustion takes place outside the engine, and the thermal energy released during this process is transferred to the steam as heat. A turbine in the power plant converts some of the energy of the steam into rotating shaft work.
Stoichiometric (theoretical) reactionthe balance reaction equation for a chemical equilibrium reaction.
Stoichiometric air or theoretical airthe minimum amount of air needed for the complete combustion of a fuel. When a fuel is completely burned with theoretical air, no uncombined oxygen will be present in the product gases.
Stoichiometric coefficientsthe mole numbers in the stoichiometric (theoretical) reaction.
Stoichiometric combustion or theoretical combustionthe ideal combustion process during which a fuel is burned completely with theoretical air.
Strokethe distance between the top dead center and the bottom dead center is the largest distance that the piston can travel in one direction within a cylinder.
Subcooled liquida temperature less than the saturation temperature corresponding to the pressure.
Sublimationthe process of passing from the solid phase directly into the vapour phase.
Sublimation linethe solid and vapour regions on the phase diagram.
Subsonic flowwhen a flow has a Mach number M <1.
Superheated vapoura vapour that is not about to condense (not a saturated vapour). A superheated vapour has a temperature greater than the saturation temperature for the pressure.
Superheated vapour regionall the superheated states located to the right of the saturated vapour line and above the critical temperature line.
Supersaturated steamsteam that exists in the wet region without containing any liquid. This phenomenon would exist due to the supersaturation process.
Supersaturationthe phenomenon owing to steam flowing through a nozzle with the high velocities and exiting the nozzle in the saturated region. Since the residence time of the steam in the nozzle is small, and there may not be sufficient time for the necessary heat transfer and the formation of liquid droplets, the condensation of the steam may be delayed for a little while.
Supersonic flowwhen a flow has a Mach number M >1.
Surface tensionis the force per unit length used to overcome the microscopic forces between molecules at the liquid-air interfaces.
Surroundingsis the mass or region outside the thermodynamic system.
Surroundings workis the work done by or against the surroundings during a process.