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STEADY HEAT TRANSFER AND THERMAL RESISTANCE NETWORKS

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Presentation on theme: "STEADY HEAT TRANSFER AND THERMAL RESISTANCE NETWORKS"— Presentation transcript:

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


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