Power Plant Engineering - Thermodynamics and Cycles Review

Vocabulary flashcards covering core concepts, properties, laws, and gas/steam power cycles from Power Plant Engineering Days 1 through 4.

Thermodynamics

The study of heat and work.

Pure substance

A working substance whose chemical composition remains the same even if there is a change in phase (e.g., water).

Ideal gas

A working substance which remains in a gaseous state during its operating cycle and whose equation of state is $PV = mRT$ (e.g., air).

Pressure

In general, the ratio of force per unit area.

Gage pressure

The pressure reading from the gage pressure instrument which is higher or lower than the atmospheric pressure.

Vacuum pressure

A pressure reading that is less than atmospheric pressure.

Absolute pressure

The pressure measured relative to absolute zero pressure, equal to the sum of gauge pressure and atmospheric pressure: $P_{abs} = P_g + P_{atm}$.

Temperature

The degree of hotness or coldness of a substance or body.

Pyrometer

An instrument used to measure high temperature gases.

Density

In thermodynamic terms, the mass per unit volume of a substance ($w = \frac{m}{V}$).

Specific volume

The volume per unit mass of a substance ($v = \frac{V}{m}$), which is also the reciprocal of density.

Specific gravity

The ratio of the density of a substance to the density of a standard fluid (water for liquid and air for gas).

Internal Energy ($U$)

The heat energy due to the movement of molecules within a substance brought about by its temperature.

Flow Work

The energy necessary to push a mass into or out of a control volume, given by pressure times specific volume ($W_f = P v$).

Enthalpy ($H$)

The total heat content of a system, equal to the sum of internal energy and flow work ($h = U + P v$).

Stagnation Enthalpy

Enthalpy given by the equation $h_s = h_o + \frac{v_o^2}{2000}$, where $v_o$ is initial velocity and $h_o$ is ordinary enthalpy.

Entropy ($S$)

A measure of unavailable energy or molecular randomness, calculated as heat transferred divided by temperature ($S = \frac{Q}{T}$).

Potential Energy ($P$)

An energy produced due to a change in elevation ($P = mgh$).

Kinetic Energy ($KE$)

An energy produced due to the mass and velocity of a body ($KE = \frac{1}{2} m v^2$).

Work ($W$)

The product of the displacement of a body and the component of force in the direction of displacement ($\text{Work} = \text{Force} \times \text{Distance}$).

Zeroth Law of Thermodynamics

States that when two bodies are in thermal equilibrium with a third body, they are in thermal equilibrium with each other and hence are at the same temperature.

First Law of Thermodynamics

States that one form of energy may be converted into another form, meaning all energy entering a system equals all energy leaving.

Second Law of Thermodynamics (Kelvin-Planck Statement)

States that it is impossible to construct a heat engine which operates in a cycle and receives a given amount of heat from a high temperature body and does an equal amount of work.

Third Law of Thermodynamics

States that the absolute entropy of a pure crystalline substance in complete internal equilibrium is zero at zero degrees absolute.

Boyle's Law

Ideal gas law stating that at constant temperature, pressure and volume are inversely proportional ($P_1 V_1 = P_2 V_2$).

Charles's Law

Ideal gas law stating that at constant pressure or constant volume, temperature varies directly with pressure or volume ($\frac{T_1}{P_1} = \frac{T_2}{P_2}$ and $\frac{V_1}{T_1} = \frac{V_2}{T_2}$).

Reversible Process

An ideal thermodynamic process occurring with no friction loss.

Adiabatic Process

A process in which there is no heat gain and no heat loss ($Q = 0$), as the system is perfectly insulated.

Constant Volume Process

A thermodynamic process occurring at constant volume ($V_1 = V_2$); also called isometric, isochoric, or isovolumic process.

Constant Pressure Process

A thermodynamic process occurring at constant pressure ($P_1 = P_2$); also called an isobaric process.

Constant Temperature Process

A thermodynamic process occurring at constant temperature ($T_1 = T_2$); also called an isothermal or hyperbolic process.

Isentropic Process

A reversible adiabatic process where entropy remains constant ($\Delta s = 0$) and governing equation is $P V^k = c$.

Polytropic Process

An internally reversible process governed by the relationship $P V^n = c$, where $n$ is the polytropic index.

Carnot Cycle

The most efficient thermodynamic cycle, consisting of two constant temperature (isothermal) processes and two isentropic processes.

Otto Cycle

A thermodynamic power cycle modeling a spark-ignition type engine, featuring constant volume heat addition and rejection.

Diesel Cycle

A thermodynamic power cycle modeling a compression-ignition type engine, featuring constant pressure heat addition and constant volume heat rejection.

Brayton Cycle

An open cycle gas turbine system consisting of an isentropic compressor, constant pressure combustor, isentropic turbine, and generator.

Sub-cooled liquid

A liquid at a temperature lower than its saturation temperature for a given pressure, so it is not about to vaporize; also known as compressed liquid.

Saturated liquid

A liquid that exists at its boiling temperature and is about to vaporize with any addition of heat.

Saturated vapor

A vapor at its condensation temperature that is about to condense with any removal of heat.

Superheated vapor

A vapor at a temperature higher than its saturation temperature for a given pressure, so it is not about to condense.

Critical point

The state point on a phase diagram at which the properties of saturated liquid and saturated vapor become identical.

T-s Diagram regions for Pure Substance

Diagram illustrating the subcooled region, wet/mixture region, superheated region, saturated liquid line, saturated vapor line, and critical point.

Quality ($x$)

The ratio of the mass of vapor to the total mass of liquid and vapor mixture ($x = \frac{m_v}{m_L + m_v}$).

Moisture ($y$)

The ratio of the mass of liquid to the total mass of vapor and liquid mixture ($y = 1 - x = \frac{m_L}{m_L + m_v}$).

Rankine Cycle

The most common steam power cycle consisting of a boiler, turbine, condenser, and pump, using water as its working fluid.