Heat Transfer methods and modes in which heat is transferred

Heat Transfer

1. Heat Transfer- Applications
Everywhere in life (and beyond?)
1.Cooking
2.Comfort
3.Food processing
4.Cell functioning-- cell preservation
5.Electrical/electronic equipment
6.Automobiles
7.Machining processes
8.Casting, welding, material processing
9.Climate, oceanic currents
10.
11.
12.….
Transformer
Inside Igloo Additive manufacturing
Aging of a galaxy
Global warming leads to
extreme weather- why?

2. Heat Transfer – Engineering Approach
Understanding heat, energy, work
Understanding modes of heat transfer –conduction, convection, radiation
Physics, constraints, mathematical models
Applying over laws of thermodynamics- 1
st
law energy balance, 2
nd
law feasibility of the
process
Combining with momentum/energy equations – fluids & solids
Devices and processes- modeling and analysis
Differential calculus, numerical methods, graphical tehcniquesDifferential calculus, numerical methods, graphical tehcniques

3. Heat and Heat Transfer
Heat is energy in transit – heat is realized during its transfer only.
Thermal energy is not heat, it is the energy associated with the molecular motion
inside any matter
Energy transferred from a thermodynamic system by mechanisms other than
thermodynamic work of transfer of matter is heat transfer.
Further, heat transfer is due to temperature difference – (higher to lower)
I.e, direction of heat flow is in the direction of temperature reduction.
Else, it violates Claussius statement:
Original statement- Heat can never pass from a colder to a warmer body without some other
change, connected therewith, occurring at the same time
In connection with a cyclic process: It is impossible to construct a device which operates on a cycle
and produces no other effect than the transfer of heat from a cooler body to a hotter body.
 

4. Heat Transfer and Entropy Generation
Heat transfer through finite temperature difference is irreversible and hence generates
entropy
Heat transfer from colder to hot body with no work input results in negative entropy
generation
T
L
T
H
Q
Consider no change in states, entropy of both bodies remain
same as they are temperature reservoirs
Mathematical form of second law:
0
1 1
gen
L H
gen
H L
Q Q
S
T T
S Q
T T
  
 
    
 
So, 0 as per assumptions
gen H L
S T T 
Therefore, second law justifies direction of heat transfer
NOT POSSIBLE!!NOT POSSIBLE!!

5. Energy Conservation - First Law
Energy of a system = thermal energy + kinetic energy (KE)+ potential energy (PE)

molecular level
energy
mechanical energy
Energy of a flow = enthalpy + mechanical energy
First law of thermodynamics for a process in a control volume
The rate of increase of thermal and mechanical energy stored in the control volume (CV) must equal the rate at which
thermal and mechanical energy enters the CV minus the rate at which thermal and mechanical energy leaves the CV
plus the rate at which heat is added to the cv minus the rate of work done by the cv
cv in out cv cv
E E E Q W   
   
In case of no velocity or height change: CV CV
E U
 
Flow energy due to entry and exit
2
/ /
/
1
2
in out in out
in out
E m h V gz
 
  
 
 
 

6. Steady State Flow – Specific Heat
Q

(h,V,z)
in (h,V,z)
out
First law, no boundary work- 2 21 1
0
2 2
in out
m h V gz m h V gz Q
   
      
   
   
 
Assuming no change in
mechanical energy
  0
in out
m h h Q  

 or,
out in
Q m h h 

So, heat given per unit mass
 
out in p out in
q h h c T T    Assuming constant cp or
small change in t
So, we can write
p
dq c dT

7. Syllabus and grading policies

8. Heat Transfer - Modes
Heat transfer is (thermal/flow) energy in transit due to a spatial temperature difference
If there is temperature difference there must be heat transfer.
However, the rate of heat transfer between two different temperature objects may be
augmented or reduced by moderating over the modes of heat transfer
Insulation is used to obtain negligible rate of heat transfer
Modes of heat transfer: conduction, convection & radiation

9. Modes of Heat Transfer -- Conduction
qq
When a temperature gradient exists in a stationary
medium (solid or fluid), the mode of heat transfer is
named as conduction
1 2
T T
q is the rate of heat transfer per unit time per unit area in the
direction of temperature gradient. It is known as heat flux (W/m
2
)

10. Modes of Heat Transfer -- Convection
Heat transfer between a surface and a moving
fluid when they are at different temperature
q
s
T T


When an external work-absorbing device is used to move the fluid – forced
convection
When the fluid moves due to density difference and buoyancy – natural
convection

11. Modes of Heat Transfer -- Radiation
Surfaces at finite temperature emit energy in form of
electromagnetic wave. Hence intervening medium is not
required for transfer of energy between two surfaces.
This mode is known as radiation.

12. Modes of Heat Transfer may co-exist
All these three modes of heat transfer may co-exist (in any combination) during any real
heat transfer process