Chapter 1: Overview of Heat Transfer


1.1 What is Heat Transfer?

Thermal energy is related to the temperature of matter. For a given material and mass, the higher the temperature, the greater its thermal energy. Heat transfer is a study of the exchange of thermal energy through a body or between bodies which occurs when there is a temperature difference. When two bodies are at different temperatures, thermal energy transfers from the one with higher temperature to the one with lower temperature. Heat always transfers from hot to cold.

Table 1 shows the common SI and English units and conversion factors used for heat and heat transfer rates. Heat is typically given the symbol Q, and is expressed in joules (J) in SI units. The rate of heat transfer is measured in watts (W), equal to joules per second, and is denoted by q. The heat flux, or the rate of heat transfer per unit area, is measured in watts per area (W/m2), and uses q" for the symbol.

Table 1. Units and Conversion Factors for Heat Measurements
  SI Units English Units
Thermal Energy (Q) 1 J 9.478710-4 Btu
Heat Transfer Rate (q) 1 J/s or 1 W 3.4123 Btu/h
Heat Flux (q") 1 W/m2 0.3171 Btu/h ft2
1.2 Three Modes of Heat Transfer

There are three modes of heat transfer: conduction, convection, and radiation. Any energy exchange between bodies occurs through one of these modes or a combination of them. Conduction is the transfer of heat through solids or stationery fluids. Convection uses the movement of fluids to transfer heat. Radiation does not require a medium for transferring heat; this mode uses the electromagnetic radiation emitted by an object for exchanging heat.

1.2.1 Conduction

Conduction is at transfer through solids or stationery fluids. When you touch a hot object, the heat you feel is transferred through your skin by conduction. Two mechanisms explain how heat is transferred by conduction: lattice vibration and particle collision. Conduction through solids occurs by a combination of the two mechanisms; heat is conducted through stationery fluids primarily by molecular collisions.

In solids, atoms are bound to each other by a series of bonds, analogous to springs as shown in Figure 1.1. When there is a temperature difference in the solid, the hot side of the solid experiences more vigorous atomic movements. The vibrations are transmitted through the springs to the cooler side of the solid. Eventually, they reach an equilibrium, where all the atoms are vibrating with the same energy.

Solids, especially metals, have free electrons, which are not bound to any particular atom and can freely move about the solid. The electrons in the hot side of the solid move faster than those on the cooler side. This scenario is shown in Figure 1.2. As the electrons undergo a series of collisions, the faster electrons give off some of their energy to the slower electrons. Eventually, through a series of random collisions, an equilibrium is reached, where the electrons are moving at the same average velocity. Conduction through electron collision is more effective than through lattice vibration; this is why metals generally are better heat conductors than ceramic materials, which do not have many free electrons.


Figure 1.1 Conduction by lattice vibration


Figure 1.2 Conduction by particle collision

In fluids, conduction occurs through collisions between freely moving molecules. The mechanism is identical to the electron collisions in metals.

The effectiveness by which heat is transferred through a material is measured by the thermal conductivity, k. A good conductor, such as copper, has a high conductivity; a poor conductor, or an insulator, has a low conductivity. Conductivity is measured in watts per meter per Kelvin (W/mK). The rate of heat transfer by conduction is given by:

 
(Eq. 1.1)

where A is the cross-sectional area through which the heat is conducting, T is the temperature difference between the two surfaces separated by a distance Δx (see Figure 1.3). In heat transfer, a positive q means that heat is flowing into the body, and a negative q represents heat leaving the body. The negative sign in Eqn. 1.1 ensures that this convention is obeyed.


Figure 1.3 Heating curve of water

1.2.2 Convection

Convection uses the motion of fluids to transfer heat. In a typical convective heat transfer, a hot surface heats the surrounding fluid, which is then carried away by fluid movement such as wind. The warm fluid is replaced by cooler fluid, which can draw more heat away from the surface. Since the heated fluid is constantly replaced by cooler fluid, the rate of heat transfer is enhanced.

Natural convection (or free convection) refers to a case where the fluid movement is created by the warm fluid itself. The density of fluid decrease as it is heated; thus, hot fluids are lighter than cool fluids. Warm fluid surrounding a hot object rises, and is replaced by cooler fluid. The result is a circulation of air above the warm surface, as shown in Figure 1.4.


Figure 1.4 Natural convection

Forced convection uses external means of producing fluid movement. Forced convection is what makes a windy, winter day feel much colder than a calm day with same temperature. The heat loss from your body is increased due to the constant replenishment of cold air by the wind. Natural wind and fans are the two most common sources of forced convection.

Convection coefficient, h, is the measure of how effectively a fluid transfers heat by convection. It is measured in W/m2K, and is determined by factors such as the fluid density, viscosity, and velocity. Wind blowing at 5 mph has a lower h than wind at the same temperature blowing at 30 mph. The rate of heat transfer from a surface by convection is given by:

 
(Eq. 1.2)

where A is the surface area of the object, Tsurface is the surface temperature, and T is the ambient or fluid temperature.

1.2.3 Radiation

Radiative heat transfer does not require a medium to pass through; thus, it is the only form of heat transfer present in vacuum. It uses electromagnetic radiation (photons), which travels at the speed of light and is emitted by any matter with temperature above 0 degrees Kelvin (-273 C). Radiative heat transfer occurs when the emitted radiation strikes another body and is absorbed. We all experience radiative heat transfer everyday; solar radiation, absorbed by our skin, is why we feel warmer in the sun than in the shade.

The electromagnetic spectrum classifies radiation according to wavelengths of the radiation. Main types of radiation are (from short to long wavelengths): gamma rays, x-rays, ultraviolet (UV), visible light, infrared (IR), microwaves, and radio waves. Radiation with shorter wavelengths are more energetic and contains more heat. X-rays, having wavelengths ~10-9 m, are very energetic and can be harmful to humans, while visible light with wavelengths ~10-7 m contain less energy and therefore have little effect on life. A second characteristic which will become important later is that radiation with longer wavelengths generally can penetrate through thicker solids. Visible light, as we all know, is blocked by a wall. However, radio waves, having wavelengths on the order of meters, can readily pass through concrete walls.

Any body with temperature above 0 Kelvin emits radiation. The type of radiation emitted is determined largely by the temperature of the body. Most "hot" objects, from a cooking standpoint, emit infrared radiation. Hotter objects, such as the sun at ~5800 K, emits more energetic radiation including visible and UV. The visible portion is evident from the bright glare of the sun; the UV radiation causes tans and burns.

The amount of radiation emitted by an object is given by:

 
(Eq. 1.3)

where A is the surface area, T is the temperature of the body, σ is a constant called Stefan-Boltzmann constant, equal to 5.6710-8 W/m2K4, and ε is a material property called emissivity. The emissivity has a value between zero and 1, and is a measure of how efficiently a surface emits radiation. It is the ratio of the radiation emitted by a surface to the radiation emitted by a perfect emitter at the same temperature.

The emitted radiation strikes a second surface, where it is reflected, absorbed, or transmitted (Figure 1.5). The portion that contributes to the heating of the surface is the absorbed radiation. The percentage of the incident radiation that is absorbed is called the absorptivity, α. The amount of heat absorbed by the surface is given by:

 
(Eq. 1.4)

where I is the incident radiation. The incident radiation is determined by the amount of radiation emitted by the object and how much of the emitted radiation actually strikes the surface. The latter is given by the shape factor, F, which is the percentage of the emitted radiation reaching the surface. The net amount of radiation absorbed by the surface is:


Figure 1.5 Interaction between a surface and incident radiation

 
(Eq. 1.5)

For an object in an enclosure, the radiative exchange between the object and the wall is greatly simplified:

 
(Eq. 1.6)

This simplification can be made because all of the radiation emitted by the object strikes the wall (Fobject→wall = 1).

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