# THERMODYNAMICS 03

version from 2018-03-28 12:16

## Section

Efficiencyis one of the most frequently used terms in thermodynamics, and it indicates how well an energy conversion or transfer process is accomplished.
Efficiency of a cooking appliancecan be defined as the ratio of the useful energy transferred to the food to the energy consumed by the appliance.
Efficiency of a water heateris defined as the ratio of the energy delivered to the house by hot water to the energy supplied to the water heater.
Efficiency of resistance heatersis 100 percent as they convert all the electrical energy they consume into heat.
Electrical workis work done on a system as electrons in a wire move under the effect of electromotive forces while crossing the system boundary.
Energy efficiency rating(EER) is the performance of refrigerators and air conditioners, and is the amount of heat removed from the cooled space in Btu's for 1 Wh (watt-hour) of electricity consumed.
Energy transport by massis the product of the mass of the flowing fluid and its total energy. The rate of energy transport by mass is the product of the mass flow rate and the total energy of the flow.
English systemwhich is also known as the United States Customary System (USCS), has the respective units the pound-mass (lbm), foot (ft), and second (s). The pound symbol lb is actually the abbreviation of libra, which was the ancient Roman unit of weight.
EnthalpyH (from the Greek word enthalpien, which means to heat) is a property and is defined as the sum of the internal energy U and the PV product.
Enthalpy departureis the difference between the enthalpy of a real gas and the enthalpy of the gas at an ideal gas state and it represents the variation of the enthalpy of a gas with pressure at a fixed temperature.
Enthalpy departure factoris the nondimensionalized form of the enthalpy departure. Entropy departure is the difference between the entropy of a real gas at a given P and T and the entropy of the gas at an ideal gas state at the same P and T .
Enthalpy of a chemical componentat a specified state is the sum of the enthalpy of formation of the component at 25°C, 1 atm, and the sensible enthalpy of the component relative to 25°C, 1 atm, which is the difference between the sensible enthalpy at the specified state ad the sensible enthalpy at the standard reference state of 25°C and 1 atm. This definition enables us to use enthalpy values from tables regardless of the reference state used in their construction.
Enthalpy of combustionhC is the enthalpy of reaction during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and pressure and represents the amount of heat released.
Enthalpy of formationis the enthalpy of a substance at a specified state due to its chemical composition. The enthalpy of formation of all stable elements (such as O2, N2, H2, and C) has a value of zero at the standard reference state of 25°C and 1 atm.
Enthalpy of reactionhR is defined as the difference between the enthalpy of the products at a specified state and the enthalpy of the reactants at the same state for a complete reaction.
Enthalpy of vaporization(or latent heat of vaporization) is the quantity hfg listed in the saturation tables.
Entropy(from a classical thermodynamics point of view) is a property designated S and is defined as dS =(dQ/T)int rev.
Entropy(from a statistical thermodynamics point of view) can be viewed as a measure of molecular disorder, or molecular randomness. The entropy of a system is related to the total number of possible microscopic states of that system, called thermodynamic probability p, by the Boltzmann relation, expressed as S = k ln p where k is the Boltzmann constant.
Entropy balance relation for a control volumeis stated as the rate of entropy change within the control volume during a process is equal to the sum of the rate of entropy transfer through the control volume boundary by heat transfer, the net rate of entropy transfer into the control volume by mass flow, and the rate of entropy generation within the boundaries of the control volume as a result of irreversibilities.
Entropy balance relation in generalis stated as the entropy change of a system during a process is equal to the net entropy transfer through the system boundary and the entropy generated within the system as a result of irreversibilities.
Entropy change of a closed systemis due to the entropy transfer accompanying heat transfer and the entropy generation within the system boundaries.
Entropy departure factoris the nondimensionalized form of the entropy departure.
Entropy generationSgen is entropy generated or created during an irreversible process, is due entirely to the presence of irreversibilities, and is a measure of the magnitudes of the irreversibilities present during that process. Entropy generation is always a positive quantity or zero. Its value depends on the process, and thus it is not a property.
Environmentrefers to the region beyond the immediate surroundings whose properties are not affected by the process at any point.
Equation of stateis any equation that relates the pressure, temperature, and specific volume of a substance. Property relations that involve other properties of a substance at equilibrium states are also referred to as equations of state.
Equilibrium implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings.
Evaporative coolersalso known as swamp coolers, use evaporative cooling based on the principle that as water evaporates, the latent heat of vaporization is absorbed from the water body and the surrounding air. As a result, both the water and the air are cooled during the process.
Evaporatoris a heat exchanger in which the working fluid evaporates as it receives heat from the surroundings.
Excess airis the amount of air in excess of the stoichiometric amount.
Exergy (availability or available energy)is a property used to determine the useful work potential of a given amount of energy at some specified state. It is important to realize that exergy does not represent the amount of work that a work-producing device will actually deliver upon installation. Rather, it represents the upper limit on the amount of work a device can deliver without violating any thermodynamic laws.
Exergy balancecan be stated as the exergy change of a system during a process is equal to the difference between the net exergy transfer through the system boundary and the exergy destroyed within the system boundaries as a result of irreversibilities (or entropy generation).
Exergy balance for a control volumeis stated as the rate of exergy change within the control volume during a process is equal to the rate of net exergy transfer through the control volume boundary by heat, work, and mass flow minus the rate of exergy destruction within the boundaries of the control volume as a result of irreversibilities.
Exergy destroyedis proportional to the entropy generated and is expressed as Xdestroyed = T0Sgen³ 0. Irreversibilities such as friction, mixing, chemical reactions, heat transfer through a finite temperature difference, unrestrained expansion, non-quasi-equilibrium compression, or expansion always generate entropy, and anything that generates entropy always destroys exergy.
Exergy of a closed system (or nonflow system)of mass m is X = (U - U0) + P0(V - V0) - T0(S - S0) +m/2 + mgz. On a unit mass basis, the exergy of a closed system is expressed as f= (u - u0) + P0(v - v0) - T0(s - s0) + /2 + gz where u0, v0, and s0 are the properties of the system evaluated at the dead state. Note that the exergy of a system is zero at the dead state since u = u0, v = v0, and s = s0 at that state. The exergy change of a closed system during a process is simply the difference between the final and initial exergies of the system.
Exergy of the kinetic energy(work potential) of a system is equal to the kinetic energy itself regardless of the temperature and pressure of the environment.
Exergy of the potential energy(work potential) of a system is equal to the potential energy itself regardless of the temperature and pressure of the environment.
Exergy transfer by heat Xheat is the exergy as the result of heat transfer Q at a location at absolute temperature T in the amount of Xheat = (1-T0/T)Q.
Exergy transfer by workis the useful work potential expressed as Xwork = W - Wsurr for closed systems experiencing boundary work where Wsurr = P0(v2 - v1) and P0 is atmospheric pressure, and V1 and V2 are the initial and final volumes of the system, and Xwork = W for other forms of work.
Exergy transport by massresults from mass in the amount of m entering or leaving a system and carries exergy in the amount of my, where y = (h - h0) - T0(s - s0) + /2 + gz, accompanies it. Therefore, the exergy of a system increases by my when mass in the amount of m enters, and decreases by the same amount when the same amount of mass at the same state leaves the system.
Exhaust valveis the exit through which the combustion products are expelled from the cylinder.
Exothermic reactionis a reaction during which chemical energy is released in the form of heat.
Extensive propertiesare those whose values depend on the size-or extent-of the system. Mass m, volume V, and total energy E are some examples of extensive properties.
Extensive properties of a nonreacting ideal-or real-gas mixture are obtained by just adding the contributions of each component of the mixture.
External combustion enginesare engines in which the fuel is burned outside the system boundary.
Externally reversible process has no irreversibilities to occur outside the system boundaries during the process. Heat transfer between a reservoir and a system is an externally reversible process if the surface of contact between the system and the reservoir is at the temperature of the reservoir.
Fahrenheit scale(named after the German instrument maker G. Fahrenheit, 1686-1736) is the temperature scale in the English system. On the Fahrenheit scale, the ice and steam points are assigned 32 and 212 °F.
Fanis a device that increases the pressure of a gas slightly and is mainly used to mobilize a gas.
Fanno lineis the locus of all states for frictionless adiabatic flow in a constant-area duct plotted on a T-s diagram.
Feed water heateris the device where the feedwater is heated by regeneration. This technique is used to raise the temperature of the liquid leaving the pump (called the feedwater) before it enters the boiler. A practical regeneration process in steam power plants is accomplished by extracting, or "bleeding," steam from the turbine at various points. This steam, which could have produced more work by expanding further in the turbine, is used to heat the feedwater instead.
First law of thermodynamicsis simply a statement of the conservation of energy principle, and it asserts that total energy is a thermodynamic property. Joule's experiments indicate the following: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process.
First law of thermodynamics for a closed systemusing the classical thermodynamics sign convention is Qnet, in - Wnet, out = DEsystem or Q - W =D E where Q = Qnet, in = Qin - Qout is the net heat input and W = Wnet, out = Wout - Win is the net work output. Obtaining a negative quantity for Q or W simply means that the assumed direction for that quantity is wrong and should be reversed.
Flow work, or flow energyis work required to push mass into or out of control volumes. On a unit mass basis this energy is equivalent to the product of the pressure and specific volume of the mass Pv.
Forced-draft cooling tower,or induced-draft cooling tower, is a wet cooling tower in which the air is drawn through the tower by fans.
Formal sign convention(classical thermodynamics sign convention) for heat and work interactions is as follows: 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 negative.
Four-stroke internal combustion engines are engines in which the piston executes four complete strokes (two mechanical cycles) within the cylinder, and the crankshaft completes two revolutions for each thermodynamic cycle.
Fuelis any material that can be burned to release energy.
Fuel-air ratiois the reciprocal of air-fuel ratio.
Fuel cells operate on the principle of electrolytic cells in which the chemical energy of the fuel is directly converted to electric energy, and electrons are exchanged through conductor wires connected to a load. They convert chemical energy to electric energy essentially in an isothermal manner.