5. Heat Energy ?
What is Energy
Energy is the capacity to do work.
Ex - Mechanical energy, chemical energy, electrical energy,
Nuclear energy and so on
What is Heat Energy
It is a form of energy in transit, the driving force for which is the
temp difference
6. TD vs. HT
One real example.
Suppose you want to heat 5 kg of water from 30 to 100 C. By thermodynamics,
you can know the amount of heat required.
Lets say Q = 1463 kJ is required.
Now, can you wait for a day Or can you wait for 5 hours to heat that quantity of
water to 100 C. Or If you don’t have any work , you can do that
But if you are a busy person and you want to heat within few minutes, so you have
to take help of heat transfer here in order to fix time.
Rate is fixed by knowledge of heat transfer.
Only amount is fixed by knowledge of thermodynamics.
If you want to heat within 15 mins, if you have to supply heat to the water at a rate of
1.63 kW.
If you want to heat within 30 mins , you can supply heat at a rate of 0.81 kW. This is
where Heat transfer makes a difference to thermodynamics
7. Heat Transfer
We are concerned, as to How and What rate heat transfer takes
place.
HT is based on the Law of Thermodynamics,
which states that heat flows from a body at higher temp to a
body at lower temp. Conversely, heat can not flow from a body
at lower temp to a body at higher temp, unless & until some
external device, like heat pump, is employed
It means, whenever there is temp difference between the two
bodies, heat energy will flow.
11. Conduction - 1
transfer of heat from one part of a substance to another part in the
same substance or one substance to another substance by direct contact
Different solids conduct different amounts of heat in a specific time.
(copper vs. glass)
Those materials that conduct heat well, are called thermal conductors
while those that conduct heat poorly, are known as thermal insulators
Most metals are excellent thermal conductors, while wood, glass, and
most plastics are common thermal insulators.
15. Assumptions of Fourier’s Law
1. Unidirectional heat flow (only one direction)
2. Steady state heat flow
3. Constant temp gradient
4. Constant thermal conductivity (k)
5. Both faces isothermal
6. No Heat generation (Qg)
7. Material is Isotropic & Homogeneous
16. Thermal Conductivity (k)
Those materials that conduct heat well, are called thermal conductors
while those that conduct heat poorly, are known as thermal insulators
17. What if – required other side of the temp is less?
Solution – Composite Walls
18.
19.
20. Examples of Composite walls
Walls of buildings
Walls of home refrigerators
Insulated pipe carrying steam
Walls of a furnace
Walls of a cold storage
Hot case for food