Rate in Thermal Systems

Objectives

Define Heat flow rate and its SI and English units of measure.

Describe the heat transfer processes of conduction, convection and radiation.

Solve heat transfer rate problems using the heat conduction equation.

Basics of heat flow

Always flows from regions of higher temperature to regions of lower temperature.

Always caused by a difference of temperature.

Heat is the exciting of molecules.

Rate Review

Heat Flow Rate

• seconds (s), minutes (min) or hours (h)  calories (cal) joules (J) SI Units British Thermal Units (BTU) Foot-pounds(ft•lb) English Units

Heat Conduction

The process of transferring heat from one molecule to another.

The collision of vibrating molecules.

The transfer of thermal energy arising form a temperature difference between adjacent parts of a body.

Thermal Conductivity (k)

The measure of a material’s ability to conduct heat.

Metals have a large thermal conductivity because they are good heat conductors Wood has low thermal conductivity because it is a poor heat conductor.

Heat Conduction Rate

Heat Conduction = Rate thermal conductivity x cross-sectional area x thickness temperature difference • Q = ( ) (k) x (A) x (  t) (  x)

Convection

Transfer of heat by movement of a fluid.

Low Temperature

Natural convection -

earths atmosphere Ponds and lakes “turning over” Warm Fluid rises Cool fluid sinks

Forced convection

– fan or pump is used High Temperature Human circulatory system

Radiation

Transfer of energy by electromagnetic waves.

All objects radiate electromagnetic energy.

The energy radiated by an object depends on the objects temperature, surface area, and material composition of the surface.

Radiation transfers energy from one body to another through empty space – it does not use a medium.

Energy from the sun to all the planets is done through radiation.

Summary

Heat flow rate is the amount of thermal energy transferred per unit of time.

Heat is transferred from a high temperature object or region to a low temperature object or region.

Heat conduction is the transfer of thermal energy within an object due to a temperature difference between adjacent regions of the object.

The thermal conductivity of a material is a measure of its ability to conduct heat.

Convection is a transfer of heat by movement of fluid. Convection can be natural or forced.

Radiation is a transfer of energy by electromagnetic waves.