Thermodynamics Flashcards
Define the term primary quantity?
Properties that cannot be broken down any further, they are not made up from anything apart from themselves.
State 3 primary quantities with names, symbols and units?
Mass m kg
Displacement L m
Time t s
Temperature T K
Electrical current I A
Define the term derived quantities?
A combination of two or more primary quantities.
State 3 derived quantities with names, symbols and units?
Area A m2
Volume V m3
Velocity v m/s
Acceleration a m/s2
Force F N or N/m2
Pressure P Pa
Energy Q,W or U Nm or J
Power P W or J/s
Define the term intensive properties?
Properties that are independent of mass of substance
State 2 intensive properties?
Pressure
Temperature
Explain the term extensive properties?
Properties that are dependent upon the mass of the substance
State 2 extensive properties?
Volume
Energy
Explain the term specific properties?
Extensive properties expressed per unit mass
State 3 specific properties names, symbols and units?
Specific heat energy transfer q J/kg
Specific work energy transfer w J/kg
Specific volume v m3/kg
Explain thermal expansion?
Heating a material causes the atoms in the material to vibrate more therefore taking up more space and so the material expands to accommodate this
State 3 modes of thermal expansion?
Linear
Superficial
Volumetric
What 3 things does the change in size of an object due to expansion depend on?
The material its made from
The temperature change its subjected to
The original size
Explain the coefficient of linear expansion?
The amount by which a unit length of material will expand when its temperature is raised by one degree, units are Celsius or Kelvin = alpha
Explain the coefficient of superficial expansion?
The amount by which the unit area of a material increases when the temperature is raised by one degree, beta = 2 x alpha
Explain the coefficient of volumetric expansion?
The amount by which the unit volume of a material increases when the temperature is raised by one degree, gamma = 3 x alpha
Explain energy and state its unit?
The ability to do work
Measured in joules
Define the units of energy?
The resultant of a force of 1 Newton travelling 1 metre in the same direction
1 J = 1 Nm
Explain specific heat capacity and what it is measured in?
The amount of heat energy required to raise the temperature of 1kg of a substance by 1 degree or 1 kelvin, symbol is c, measures in J/kgK
State the 3 methods of heat transfer?
Conduction
Convection
Radiation
Explain conduction?
The transfer of heat energy by vibrating atoms in solids
Explain convection?
The transfer of heat energy in fluids via convection currents caused by less dense particles rising
Explain radiation?
The transfer of heat energy by means of electromagnetic waves that can pass through gas and a vacuum
Explain the zeroth law?
The only direction in which energy can move by heat transfer is from a warmer object to a cooler object. The higher the temperature difference, the greater the rate of heat transfer
Explain the 1st law?
Energy cannot be created nor destroyed, but can only be changed from one form to another
Name 3 types of heat exchangers used in the RN?
Shell and tube
Concentric tube
Plate type
What are the 2 types of flow in a heat exchanger?
Counter current
Parallel
Explain counter current flow?
(practice diagrams)
The cooling medium and the fluid to be cooled are travelling in opposite directions
Explain parallel flow?
(practice diagrams)
The cooling medium and the fluid to be cooled are travelling in the same direction
Explain calorific value?
Amount of heat energy released per kg of the fuel when completely burned
What are the 2 types of thermodynamic systems?
Open system
Closed system
What is a boundary?
The surface of separation between the system and its surroundings
Explain a closed system?
Only energy (heat and work) is allowed to cross the boundary. The fluid itself remains within the boundary. Eg engine piston in a cylinder
Explain an open system?
Energy can cross the boundary but the boundary has breaks in it to allow the fluid to enter and leave the system (flow through it). Eg turbines
Explain what is meant by state?
The pressure, temperature and volume of a gas at a particular point of time
What is a process?
The changing of a gas to one state to another
What is a cycle?
A series of processes that return the working fluid to its original state
Explain the 1st law of thermodynamics applied to a cycle?
When a system undergoes a thermodynamic cycle then the net heat transfer between the system and its surroundings plus the net work transfer between the system and its surroundings must equal zero
Q + W = 0
What is Boyle’s law?
For a fixed mass of gas the pressure is inversely proportional to volume providing the temperature remains constant
What is Charles’ law?
For a fixed mass of gas the volume is proportional to temperature providing the pressure remains constant
What is the combined gas law?
Pressure, temperature and volume are all affected by a change of state. Therefore, Boyle and Charles’ laws can be combined for a fixed mass of gas
What does isothermal mean?
Temperature remains constant
What does isobaric mean?
Pressure remains constant
What does isochoric mean?
Volume remains constant
What does adiabatic mean?
Without loss or gain of heat
Explain the difference between polytropic, isothermal and adiabatic processes?
Isothermal and adiabatic represent the two extremes with regard to heat transfer.
For the process to be isothermal, heat must be removed from the system. This needs to happen over a period of time making this process theoretically infinitely slow.
For the process to be adiabatic, there must be no heat transfer even though the temperature increases. No heat transfer takes no time, therefore adiabatic process are theoretically infinitely fast.
In reality, the process is rarely completely adiabatic or isothermal but somewhere in between. This is where the polytropic process models reality
What does isentropic mean?
Constant entropy, a process which is reversible and has no losses
What does entropy mean?
A property of a fluid that remains constant during a truly reversible process
What is the 2nd law of thermodynamics in relation to heat exchangers?
In a parallel flow heat exchanger, coolant will continue to increase in temperature as long as their is a temperature difference, the coolant will eventually become the same temperature as the hot flow, at which point the energy transfer will stop
What is the 2nd law of thermodynamics in relation to heat engines?
It is impossible to construct a heat engine that works from a single heat source. There must be a temperature difference between the heat source and the engine for it to produce work output
What are the differences with a forced aspirated engine?
Peak pressure is significantly higher, requiring a stronger engine
Positive work area increased
Pumping loop gives positive work
