Key terms Flashcards
What is Absolute Pressure
Gauge pressure + Atmospheric Pressure
Adiabatic Boundary
A boundary of a system that does not allow heat transfer.
Ambient Pressure, Temperature
The pressure, temperature of the surrounding atmosphere.
Boundary
The boundary of a system separates it from all other matter.
Carnot Cycle
A fully reversible cycle having the maximum possible efficiency.
Coefficient of Performance
An expression of efficiency for reversed heat engines- refrigerator or heat pump.
Critical point
A point on a property chart beyond which the substance does not experience a transition between liquid and vapour- critical pressure and critical temperature define this point.
Customary Temperature
Temperature measured on an arbitrarily chosen scale by a thermometer (usually in degrees C).
Cycle
A series of processes by which the substance returns to its original state.
Diathermal Boundary
A boundary that allows heat transfer across it.
Displacement Work
Work done by virtue of the displacement of the system boundary.
Dryness fraction (x)
The proportion of vapour in a liquid-vapour mixture.
Engine
A device which produces a work transfer while engaging in heat transfer with the surroundings.
Enthalpy (H)
A property derived from:
internal energy + (pressure x volume)
Usually in kJ.
Entropy (S)
A property related to the degree of disorder in a substance, usually in kJ/K.
Equilibrium
Said to exist when no spontaneous change takes place when a system is isolated from its surroundings.
Extensive Property
Any property which depends upon the quantity of substance (i.e. the extent of the system).
Gauge Pressure
Pressure above (or below for vacuum gauges) ambient as indicated by a pressure gauge, in kPa or bar.
Heat Pump
A device which takes energy from a cold source and transfers it to provide energy to a hotter region using work input.
Heat (Q)
Energy transfer under the influence of a temperature difference, usually in kJ.
Heat transfer rate (Q with a dot)
Heat transfer in unit time, usually in kW.
Intensive Property
A property which is independent of the quantity of substance (e.g. pressure, temperature)
Internal Energy (U)
Energy possessed by a substance by virtue of its phase and temperature, usually in kJ.
Isentropic Process
A process in which there is no change in the entropy of the substance- usually a reversible adiabetic process.
Kinetic Energy (KE)
Energy possessed by a body by virtue of its ordered motion, usually in kJ.
Perfect Gas
A gas for which pv/T, Cp and Cv are taken to be constants.
Potential Energy
Energy possessed by a substance by virtue of its height above a chosen datum.
Power
Rate of doing work, usually in kW.
Property
Identifiable characteristics of a substance such as temperature, pressure, internal energy.
Refrigerator
A device built to transfer energy from a cold body- it rejects to a higher temperature region, usually the atmosphere.
Reversed Engine
A refrigerator or heat pump.
Reversible Engine
An engine which gives the same energy transfers either as a forward or reversed engine- It has the highest possible efficiency.
Reversible Process
A process which may be reversed such that the process path passes through all the same states in either direction. OR a process that having once taken place can be reversed and in so doing leave no change in either system or surroundings.
Saturated Liquid (vapour)
A liquid (vapour) at the saturation temperature, i.e. the boiling point for the particular pressure.
Specific Gas Constant (R)
A constant relating properties for a particular gas; R=pv/T, usually in kJ/kg K.
Specific Heat Capacity (Cp, Cv)
The energy required to raise the temperature of unit mass through unit temperature rise at constant pressure or constant volume, usually in kJ/kg K.
Specific Property
The value of an extensive property per unit mass. E.g. Specific enthalpy, h= H/m.
Steady flow process
Process in which the mass flowrate is constant and the state at any location is constant with respect to time.
Superheated Vapour
Vapour above the saturation temperature.
Surroundings
All matter outside a system boundary.
System
A defined collection of matter.
Thermodynamic temperature (T)
Temperature defined by the efficiency of reversible engines, absolute temperature measured in Kelvin (K).
Triple Point
The state in which the solid, liquid and vapour phases exist in equilibrium- for H2O this is used as a datum for properties.
Universal gas Constant (R with squiggle)
The relationship pV/T, where V is the volume of 1 kmol of any gas, usually in kJ/kmol K.
Vapour
Part of the gaseous phase at or slightly above the saturation temperature.
Work (W)
Energy transfer which can be related to the movement of a force, usually in kJ.
1st law of thermodynamics and equation
Energy is neither created nor destroyed.
Q – W =∆U
Thermal Efficiency of a Heat Engine
η= (Qh-Qc)/Qh
COP of a refrigerator
COPref = Qc/(Qh-Qc)
COP of a heat pump
COPhp= Qh/(Qh-Qc)
how to show an engine is reversible (eqn)
η= 1/Chp where Chp= Qh/W
