AERO40009 - Thermodynamics and Heat Transfer Flashcards
Define a system
Any collection of matter contained within prescribed (but often imaginary) boundaries in space.
Define a surrounding
That matter which is external to the system.
Define a closed system
No mass flows across the system boundary.
But the system boundary may and usually does move. Heat and work are transferred across the system boundary, but no mass is transferred across the boundary.
Define an open system or a control volume
An imaginary boundary fixed in space encompasses the control volume.
Define a cycle
A cycle is a process in which the end states are identical. It may have several distinct paths.
Define heat
It is the form of energy transfer that occurs across the boundary of a system when there is a temperature difference between the system and its surroundings. Is measured in Joules.
What is the zeroth law of thermodynamics?
Is system A in in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then system A and C are in thermal equilibrium with each other.
Define displacement work
This mainly arises from displacement of system boundaries by an external force. It requires an external force (or at least a component of an external force) that acts NORMAL to at least part of the system boundary AND movement of at least part of the boundary, acted on by the force, PARALLEL to the direction of the force.
Sign convention for heat and work output
Heat to the system is positive.
And work output from the system is positive.
Define specific heat capacity
It is the ratio of the quantity of heat added or removed from a unit quantity of a given material, to its change in temperature.
Define the specific heat capacity at constant volume Cv
Is the proportionality constant relating changes in internal energy to changes in temperature.
In a fixed volume, all the heat goes to increase the internal energy because the gas itself cannot expand.
For an ideal gas, the changes in internal energy are proportional to the product of Cv and delta T.
Define a cyclic heat power plant
A continuosly operating system across whose boundaries flow heat and work.
Define a reversible process
A process which is without friction, or rapid heat transfer and is always in local equilibrium.
Means that the motion is fully resisted at system boundaries.
Key words that identify a reversible process
- Heat transfer with a temperature difference.
- Friction
- Non-equilibrium
- Abrupt expansion
- Dissipation
- Mixing (friction)
Key words that identify a reversible process:
- Frictionless
- Heat transfer at same temperature.
- “Slow process”
- Equilibrium or full resisted.
What is the Kevin Planck statement?
It is impossible to devise a cyclic system which produces no effect other than to deliver work and receive heat from a single reservoir.
AKA - There is no system on Earth that is fully 100% efficient. There must always be some sort of waste.
ONLY APPLIES TO A CYCLIC SYSTEM.
What is the Clausius statement.
No continuous cyclic process is possible, which causes heat to flow from a reservoir at a low temperature to one at a higher temperature with no other effect. ONLY APPLY TO A CYCLIC SYSTEM.
What is the kinetic theory of a gas?
We assume that the molecules of gas:
- behave like hard spheres which move in a continuous, random motion;
- are distributed uniformly throughout the container;
- exert no force on each other except when they collide, and so
- move in straight line paths between collisions, which occur either with other
molecules or the walls of the container.
Define the mean free path
The average distance travelled in between the collisions.
Define work
The sole effect on the surroundings could have been the raising of a weight.
What does it mean if a process is ‘slow’ and fully resisted?
- Process is in near equilibrium
- External force is equal and opposite to internal pressure force.
What are intrinsic properties?
They are independent of the mass of the system.
What is a perpetual motion machine of the second kind
Is a device which continuously produces a work output, whilst exchanging heat with only a single heat reservoir.
Quasistatic
Happens so slowly that it appears to be static.
Define an adiabatic process
It is a process that occurs without an heat energy being transferred.
Define an isothermal process
The temperature of the system remains constant here.
What is a isentropic process
One where there is no change in entropy. The 1-2 process is isentropic.
How to calculate the change in energy due to a change in temperature?
∆ U = Cv ∆ T.
Where U = internal energy PER UNIT MASS.
If you want internal energy, MULTIPLY BY THE MASS!
∆ u = M Cv ∆ T.
Important relations that are often useful
Cp T = U + Pv.
h = Cp T
What is the first thing to do when you’ve been given a bunch of numbers?
MAKE A SHOPPING LIST!!!
Why do we have to increase the throat area after we use afterburners?
This is because the T0 has increased, so we have to make the throat area bigger due to mass conservation.
