# Heat Transfer : Steady State Heat Transfer - PowerPoint PPT Presentation

1 / 9

Heat Transfer : Steady State Heat Transfer. Objectives. Section 6 – Thermal Analysis Module 4: Steady State Heat Transfer Page 2. U nderstand steady state heat transfer. Explore linear steady state analysis. Explore nonlinear steady state analysis. S tudy an example:

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

Heat Transfer : Steady State Heat Transfer

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

HeatTransfer:

### Objectives

Section 6 – Thermal Analysis

Page 2

• Understand steady state heat transfer.

• Explore linear steady state analysis.

• Explore nonlinear steady state analysis.

• Study an example:

• Thermal analysis of a heat sink assembly

Section 6 – Thermal Analysis

Page 3

• Steady state heat transfer occurs when temperatures, thermo-physical properties, surface properties and bulk motion of fluid are constant over time.

• Temperature has reached equilibrium.

Element under study

Heat In

Heat Out

Any machinery running at constant speed is a good example of steady state heat transfer.

Section 6 – Thermal Analysis

Page 4

### Linear Steady State Heat Transfer

• A linear steady state problem is approximated when the thermophysical properties (e.g., conductivity, density, viscosity) are not dependent on temperature. Moreover, radiation loss is unaccounted for.

• Thermal conductivity of a material in real life can vary significantly depending upon the material temperature.

• If the material properties are fixed, the problem is said to be linear steady state.

• If “k” in the above relationship is approximated as not dependent on temperature, then the problem is linear steady state.

• An example where a linear steady state heat transfer approximation can be used is heat loss from domestic hot water pipes when the temperature differential is low.

### Nonlinear Steady State Heat Transfer

Section 6 – Thermal Analysis

Page 5

• Nonlinear steady state heat transfer occurs when thermophysical properties are assumed to vary with temperature.

• This is closer to real life conditions; however more complex to solve.

• To solve such problems, an iterative scheme is used by assuming initial temperatures and evaluating through calculations later.

• The material properties are adjusted for new temperatures and calculated again until convergence is reached (for example, when heat entering and heat exiting the system boundaries are equal).

• Most examples of heat transfer are nonlinear, such as heat loss from a car engine block (engine temperature varies significantly).

If “k” in this equation is a function of temperature f(T), than the problem becomes nonlinear.

### Simplifying Heat Transfer Analysis

Section 6 – Thermal Analysis

Page 6

• Problems can be simplified by avoiding the need to calculate for conjugate heat transfer.

• For instance at low temperatures (<70⁰ C), radiation may be ignored.

• If the material properties do not vary significantly (for example, the percentage change is < 20%), the problem can be assumed to be linear steady state.

• Likewise, if the effect of body forces such as gravitational force is overwhelmed by forces responsible for bulk fluid flow, natural convection can be ignored.

• Geometrical symmetry, if present, should be used to avoid the need to create a full model.

### Example: Heat Sink Assembly

Section 6 – Thermal Analysis

Page 7

20°

• A heat sink assembly is a common design element in electronics such as desktop computers, laptops and audio systems.

• A two-part video presentation for this module using this example is available. The second part covers steady state heat transfer analysis.

B

C

Fins

(Aluminium)

(Copper)

40 Watts

Microprocessor

(Silicon)

### Summary

Section 6 – Thermal Analysis

Page 8

• Steady state heat transfer conditions are said to occur when a system is in equilibrium.

• For instance, the engine on a car travelling at a constant speed on a highway would more or less lose a fixed amount of heat every second.

• Steady state heat transfer can be linear or nonlinear.

• Nearly all real life examples are nonlinear; however, systems can be approximated as linear if temperature variation is insignificant.

### Summary

Section 6 – Thermal Analysis