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)

Foot-pounds(ft•lb)

British Thermal Units (BTU)

joules (J)

calories (cal) SI Units

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 conductorsWood has low thermal conductivity because it is a poor heat conductor.

Heat Conduction Rate

Heat Conduction =

Rate

thermal conductivity

x cross-sectional area

x temperature difference

thickness

Q = (k) x (A) x (t)

(x)

•(-)

Convection

Transfer of heat by movement of a fluid.

Low Temperature

Warm Fluid rises

Cool fluid sinks

High Temperature

Natural convection - earths atmosphere

Forced convection – fan or pump is used

Human circulatory system

Ponds and lakes “turning over”

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.

SummaryHeat 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.