1. STEADY HEAT TRANSFER AND THERMAL RESISTANCE NETWORKS
Dr. Şaziye Balku

2. STEADY HEAT CONDUCTION IN PLANE WALLS
Heat transfer
- temperature gradient
- not in the direction where no change in temperature
normal to the wall surface
no significant heat transfer in other directions
If T in and outside remain constant
Steady and one-dimensional
Dr. Şaziye Balku

3. Energy balance for the wall
rate of
heat transfer into the wall
heat transfer
out of the wall
rate of change
of the energy
of the wall - =
If there is no heat generation
steady operation; since there is no change in the temperature of the wall with time at any point
The rate of heat transfer through the wall is constant
Dr. Şaziye Balku

4. FOURIER’S LAW OF HEAT CONDUCTION
and A constant, then
constant also
Temperature through the wall varies linearly with x. Temperature distribution in the wall under steady conditions is a straight line.
Dr. Şaziye Balku

5. THERMAL RESISTANCE (W) (0C / W)
Depends on the geometry and the thermal properties of the medium
Electrical conductivity
Voltage difference across the resistance
Electrical resistance
Dr. Şaziye Balku

6. NEWTON’S LAW OF COOLING FOR CONVECTION HEAT TRANSFER RATE
Convection heat transfer coefficient
(0C / W)
Convection resistance of surface
Dr. Şaziye Balku

7. RADIATION
Dr. Şaziye Balku

8. THERMAL RESISTANCE NETWORK
The thermal resistance network for heat transfer through a plane wall subjected to convection on both sides and the electrical analogy
Dr. Şaziye Balku

9. ONE DIMENSIONAL STEADY HEAT FLOW
Rate of
heat convection
into the wall
heat conduction
through the wall =
Rate of
heat convection
from the wall
adding the numerators and denominators
Dr. Şaziye Balku

10. Thermal resistance network through a two-layer plane
Dr. Şaziye Balku

11. Total Thermal Resistance
Dr. Şaziye Balku

12. Thermal Contact Resistance
(W/m2 0C)
hC: thermal contact conductance
(m2 0C/ W)
Dr. Şaziye Balku

13. Thermal contact resistance is inverse of thermal contact conduction,
Depends on
Surface roughness,
Material properties,
Temperature and pressure at interface,
Type of fluid trapped at interface
Dr. Şaziye Balku

14. Effect of metallic coatings on thermal contact conductance
For soft metals with smoot surfaces at high pressures
Thermal contact conductance
Thermal contact resistance
Dr. Şaziye Balku

15. THERMAL RESISTANCE NETWORKS
Resistances are parallel
Dr. Şaziye Balku

16. COMBINED SERIES-PARALLEL ARRANGEMENT
Dr. Şaziye Balku

17. HEAT CONDUCTION IN CYLINDERS AND SPHERES
Steady-state heat conduction
Heat is lost from a hot-water pipe to the air outside in the radial direction.
Heat transfer from a long pipe is one dimensional
Dr. Şaziye Balku

18. A LONG CYLINDERICAL PIPE STEADY STATE OPERATION
constant
Fourier’s law of conduction
Dr. Şaziye Balku

19. FOR SPHERES
including convection
Dr. Şaziye Balku

20. CRITICAL RADIUS OF INSULATION
CYLINDER
show
Thermal conductivity
External convection heat transfer coefficient
Dr. Şaziye Balku

21. CHOSING INSULATION THICKNESS
max
Before insulation check for critical radius
Dr. Şaziye Balku

22. HEAT TRANSFER FROM FINNED SURFACES
Two ways of increasing
increase h
increase As
By adding fins
(Car radiators)
Dr. Şaziye Balku

23. Energy Balance on Volume Element (fin)
rate of heat conduction into the element at x
rate of heat conduction from the element at x+Δx
rate of heat convection from the element + =
Dr. Şaziye Balku

24. (fin) At constant AC and k Solution is; Boundary condition x = 0
Boundary condition x = 0
Dr. Şaziye Balku