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Understanding Heat Flow Rate in Thermal Systems: Processes and Equations

This chapter delves into the concept of heat flow rate in thermal systems, defining its measurement in various SI and English units. It explores the heat transfer processes of conduction, convection, and radiation. The chapter outlines how heat conduction occurs due to temperature differences and introduces the heat conduction equation for problem-solving. Key factors influencing heat conduction rate, such as thermal conductivity, cross-sectional area, thickness, and temperature difference, are examined. Convection and radiation processes are also discussed, illustrating their roles in heat transfer.

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Understanding Heat Flow Rate in Thermal Systems: Processes and Equations

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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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