AERO40009 - Thermodynamics and Heat Transfer

Question 1

Define a system

Answer 1

Any collection of matter contained within prescribed (but often imaginary) boundaries in space.

Question 2

Define a surrounding

Answer 2

That matter which is external to the system.

Question 3

Define a closed system

Answer 3

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.

Question 4

Define an open system or a control volume

Answer 4

An imaginary boundary fixed in space encompasses the control volume.

Question 5

Define a cycle

Answer 5

A cycle is a process in which the end states are identical. It may have several distinct paths.

Question 6

Define heat

Answer 6

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.

Question 7

What is the zeroth law of thermodynamics?

Answer 7

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.

Question 8

Define displacement work

Answer 8

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.

Question 9

Sign convention for heat and work output

Answer 9

Heat to the system is positive.

And work output from the system is positive.

Question 10

Define specific heat capacity

Answer 10

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.

Question 11

Define the specific heat capacity at constant volume Cv

Answer 11

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.

Question 12

Define a cyclic heat power plant

Answer 12

A continuosly operating system across whose boundaries flow heat and work.

Question 13

Define a reversible process

Answer 13

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.

Question 14

Key words that identify a reversible process

Answer 14

• Heat transfer with a temperature difference.
• Friction
• Non-equilibrium
• Abrupt expansion
• Dissipation
• Mixing (friction)

Question 15

Key words that identify a reversible process:

Answer 15

• Frictionless
• Heat transfer at same temperature.
• “Slow process”
• Equilibrium or full resisted.

Question 16

What is the Kevin Planck statement?

Answer 16

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.

Question 17

What is the Clausius statement.

Answer 17

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.

Question 18

What is the kinetic theory of a gas?

Answer 18

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.

Question 19

Define the mean free path

Answer 19

The average distance travelled in between the collisions.

Question 20

Define work

Answer 20

The sole effect on the surroundings could have been the raising of a weight.

Question 21

What does it mean if a process is ‘slow’ and fully resisted?

Answer 21

• Process is in near equilibrium

- External force is equal and opposite to internal pressure force.

Question 22

What are intrinsic properties?

Answer 22

They are independent of the mass of the system.

Question 23

What is a perpetual motion machine of the second kind

Answer 23

Is a device which continuously produces a work output, whilst exchanging heat with only a single heat reservoir.

Question 24

Quasistatic

Answer 24

Happens so slowly that it appears to be static.

Question 25

Define an adiabatic process

Answer 25

It is a process that occurs without an heat energy being transferred.

Question 26

Define an isothermal process

Answer 26

The temperature of the system remains constant here.

Question 27

What is a isentropic process

Answer 27

One where there is no change in entropy. The 1-2 process is isentropic.

Question 28

How to calculate the change in energy due to a change in temperature?

Answer 28

∆ U = Cv ∆ T.
Where U = internal energy PER UNIT MASS.
If you want internal energy, MULTIPLY BY THE MASS!
∆ u = M Cv ∆ T.

Question 29

Important relations that are often useful

Answer 29

Cp T = U + Pv.

h = Cp T

Question 30

What is the first thing to do when you’ve been given a bunch of numbers?

Answer 30

MAKE A SHOPPING LIST!!!

Question 31

Why do we have to increase the throat area after we use afterburners?

Answer 31

This is because the T0 has increased, so we have to make the throat area bigger due to mass conservation.