Week 1-3 Flashcards
What is heat transfer? What is heat flux?
Heat transfer is the energy in transit as a consequence of a temp. Difference in a system.
Heat flux is the rate of energy flow driven by the temp. Difference through a surface per unit area. Measured in W/m^2
- What does a heat pump do?
- What are heat exchangers?
- Moves heat from low temp region to high temp region.. against natural direction. (Refrigerator)
- Used to transfer heat between process flows .e.g. calcium looping CCS plant
What is conduction
Heat transfer within or through a stationary medium (solid, liquid) which contains, or across which the exists, a temperature gradient.
What is convection? What is natural/free convection?
Heat transfer that occurs, between a moving fluid and a stationary object which are a different temperatures
Free/natural convection (Uo=0) is convection motion due to density gradients
When does forced convection happen
It is when freestream velocity (Uo) is > 0
Convection motion in this case is provided by external sources, for example a fan blowing air through a CPU heat sink
What is radiation
Heat transfer between two or more surfaces, at different temps. In the absense of an intervening medium
What does Fourier’s law state
States that the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right To that gradient, through which heat flows
Explain Case 1 ‘ steady state conduction through a homogenous solid’
When k= constant.
No other energy/gains losses I.e. all energy entering at 1 flows through to 1 hence qx’’ is independent of x
By integrating we get qx’’ = -k(🔼T/L)
Explain case 2 ‘ thermal conductivity varies with temperature’
Assume k= aT + b …
So qx’’ = -k(🔼T/L) where k is a/2(T2+T1) + b
With reference to Fourier’s law describe the conditions that must be satisfied in order for the temp. Distribution to be linear in a wall
Fourier law states that temp gradient is dT/dx = -k(qx’’ /k)
For temp distribution to be linear, dT/dx must be constant, and therefore (qx’’ /k) must be constant too. Heat flux will be constant under one dimensional steady state conditions and k will be constant throughout the wall if it’s made of homogenous material which has a thermal conductivity that is independent of temp.
What does k and qx’’ and qx represent in Fourier’s law
K: thermal conductivity (Wm-1K-1). Characteristic of the material concerned requiring experimental determination.
qx’’ = Heat flux (Wm-2). In x direction, Perpendicular to cross sectional area
qx = heat loss (W)
What is newtons law of cooling equation and what does h, Ts,To and q’’ represent
q’’ = h(Ts-To)
q’’ : connective heat flux (Wm-2)
h: heat transfer coeff/ film coeff. (Wm-2K-1)
h depends on the surface geometry, thermodynamic/transfer properties, and nature of the fluid motion I,e. Whether it’s laminar or turbulent.
Name the 6 parameters of heat transfer coefficient
Viscosity η , thermal conductivity k, specific heat c, density ρ, mean velocity U, characteristic length scale associated with the solid in contact with the fluid ℓ.
What is the law of energy conservation ( first law of thermodynamics)
The rate at which thermal and mechanical energy enters a control volume minus the rate at which the energy leaves the control volume must equal the rate at which energy is stored in the control volume
What is the equation for general form of the energy conversion law for an arbitrary shaped control volume
εin + εg - εout = εst
ε g = rate of energy generation within the CV per unit
ε st = energy store within the control volume
These two are usually volumetric phenomena e.g. nuclear fission, chemical reactions
In and out usually surface phenomena e.g. conduction, convection and radiation
