Lecture 7 : Heat transfer I Flashcards
DEFINITION
What is specific heat capacity and its units?
quantity of heat gained/lost by a unit mass product to accomplish a unit change in temperature, without change in state.
units: kJ /(kg.K) or kJ/(kg.°C)
DEFINITION
What is thermal conductivity, k?
What is its units?
The amount of heat that will be conducted through a unit thickness of object per unit time, if a unit temperature gradient exists.
- per unit time : because Watt = joules/second
Units: W/(m.k) = W/(m.°C)
DEFINITION
What is thermal diffusivity, α ?
Thermal diffusivity is the ratio of heat conducted by a material relative to the heat it absorbs/stores.
What is the equation for thermal diffusivity, α and its units?
α = k / (ρ.c_p) , units = m^2/s
ρ : density
c_p : specific heat capacity
[MODES OF HEAT TRANSFER]
How is heat transferred through conduction?
Is there physical movement of object undergoing heat transfer?
In conduction, when a molecule gets heated up, it gains kinetic energy and vibrates, and this energy is passed onto the neighboring molecule.
No physical movement, movement occurs at a molecular level
[MODES OF HEAT TRANSFER]
Heat transfer through conduction occurs between which 2 medium?
Solid to solid
[MODES OF HEAT TRANSFER]
What is the equation for rate of heat transfer, q, through conduction? What law is it called? State units for each component.
Fourier’s law : q = -kA (dT/dx)
where
q : rate of heat flow in direction of heat transfer by conduction
k : thermal conductivity (W/m.K or W/m.°C)
A: surface area normal/perpendicular to direction of heat transfer (m^2)
dT : temperature difference (K/°C)
x : thickness (length) of wall (m)
[MODES OF HEAT TRANSFER]
How is heat transferred through convection?
Is there physical movement of object undergoing heat transfer?
In convection, exchange of heat occurs through a moving fluid (liquid/gas)
There is physical movement of fluid.
[MODES OF HEAT TRANSFER]
Heat transfer through convection occurs between which 2 medium?
Solid and fluid
(solid transfers heat to fluid for the lecture content)
[MODES OF HEAT TRANSFER]
What are the 2 types of convection and their definition?
- Forced convection: convection that occurs due to the use of mechanical means such as a pump or fan to induce movement of fluid
- Free convection : convection that occurs due to density differences caused by temperature gradients within the system.
[MODES OF HEAT TRANSFER]
What is the equation for rate of heat transfer, q, through convection? What is the law called? State units for each component.
Newton’s law of cooling : q = hA(T_s - T_∞)
q: rate of heat transfer (W)
h: convective heat transfer coefficient (W/m.K or W/m.°C)
A: surface area (m^2)
T_s : temperature at solid surface (K/°C)
T_∞: ambient temperature (K/°C)
- stabilised temperature far away from heated solid surface
T_s - T_∞ : temperature gradient
[MODES OF HEAT TRANSFER]
In convective heat transfer, a high value of h indicates?
High value of convective coefficient indicates high rate of heat transfer
[MODES OF HEAT TRANSFER]
How is heat transferred through radiation?
Transfer of heat occurs between 2 surfaces by emission and absorption of electromagnetic waves (of light), also known as photons
[MODES OF HEAT TRANSFER]
A physical medium is required for heat transfer through radiation. True or False?
False, no physical medium is required, and can occur in perfect vacuum.
[MODES OF HEAT TRANSFER]
Are
(i) Solid
(ii) Liquid
(iii) Gas
good absorbers of radiation?
Solid : opaque to thermal radiation, meaning that they dont allow the electromagnetic waves to pass through. But whether it is a good absorbers depends on the colour of surface
- black : perfect absorbers (black body)
- shiny surfaces : poor absorbers (reflect all the light and thus, electromagnetic waves off)
Liquid : good absorbers of radiation
Gas : transparent to radiation, meaning they allow electromagnetic waves to pass through and are poorest absorbers of radiation out of solid, liquid and gas.
