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Chapter 3 Section 4

Chapter 3 Section 4. Rate in Thermal Systems. Objectives. Define heat flow rate & its SI & English units. Describe the heat transfer processes of conduction, convection & radiation. Solve heat transfer problems using the heat conduction equation. Heat Flow Rate.

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Chapter 3 Section 4

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  1. Chapter 3 Section 4 Rate in Thermal Systems

  2. Objectives • Define heat flow rate & its SI & English units. • Describe the heat transfer processes of conduction, convection & radiation. • Solve heat transfer problems using the heat conduction equation.

  3. Heat Flow Rate • Heat flow rate = heat transferred/ time • Q =Q/t

  4. Units • Heat flow rate can be measured in a number of different units. • Basically, any unit of energy divided by any unit of time can be a heat flow rate.

  5. Energy or heat British thermal units (BTU) Calories (cal) Joules (J) Foot – pounds (ft-lbs) Time Hours (hr) Minutes (min) Seconds (s) Units – cont.

  6. Heat conduction • Heat conduction is the transfer of energy (heat) between different parts of a body due to a temperature difference. • It is caused by molecules of a higher kinetic energy colliding with slower molecules and speeding them up. • Thermal conductivity is a measure of a substance’s ability to conduct heat.

  7. Heat conduction rate • The rate of heat conduction through a substance depends upon: • Its thermal conductivity • Its cross sectional area • Its thickness and; • The temperature difference.

  8. Heat conduction rate • Q = kADT / Dxwhere: • Q = heat flow rate • K= thermal conductivity • A = cross sectional area • Dx = thickness • DT = temperature difference

  9. Other forms of heat transfer • Convection – heat transfer by moving large quantities of fluid (liquid or gas) • Radiation – caused by electromagnetic radiation (light). No medium is required.

  10. Summary • Heat flow rate is the amount of thermal energy transferred per unit of time. • Heat is transferred from high temperature areas to low temperature areas. • Heat conduction is the transfer of heat within an object due to a temperature difference within the object. • Thermal conductivity is a measure of an object’s ability to conduct heat. • Convection is the transfer of heat by moving fluids (liquid or gas). • Radiation is the transfer of heat by electromagnetic waves.

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