Thermal Conductivity:
Thermal Conductivity of a material is defined from the above definition.
Heat flux of a material for unit temperature gradient for unit length is known as its thermal conductivity. Thermal Conductivity has the units of watts per meter per Celsius degree when heat flow is expressed in watts. The numerical values of thermal conductivities indicate how fast heat will flow in a given material.
Content:
5. Experimental Determination of thermal conductivity
1.Basic concept of Thermal conductivity:
In general,
Here Potential is driving force for the transfer (In case of heat transfer it is temperature difference).
Conductance is defined as,
Therefore, Flow α (conductance X Potential)
In general conductance is directly proportional to the Area and inversely proportional to the length.
This can be written as,
where ‘k’ is thermal conductivity of material.
From the above equation thermal conductivity is defined as below.
Definition of thermal conductivity:
When the conductance is reported for a quantity of material 1 ft thick material with heat flow area 1 ft2, in 1 hr time, and with a temperature difference of 1oF, it is called thermal conductivity ‘k’.
Finally heat flow equation can written as,
2. Thermal Conductivity of gases:
Thermal conductivity evaluated experimentally. In general, the thermal conductivity is a strong function of temperature. The faster the molecules move, the faster they will transport energy. The thermal conductivity of a gas varies square root of the temperature (It may be recalled that the velocity of sound in gas varies with the square root of the temperature; this velocity is approximately the mean speed of the moles). Over a wide range of pressures thermal conductivity of gases considered to be constant.Table 1: Thermal conductivities of some gases
| Gases | Thermal conductivity (W/m .K) |
| Hydrogen | 0.175 |
| Helium | 0.141 |
| Air | 0.024 |
| Water vapour (saturated) | 0.0206 |
| CO2 | 0.0146 |
3. Thermal Conductivity of liquids:
The physics of mechanism of thermal-energy conduction in liquids is quantitavely the same as in gases; however, the situation is more complex because the molecules are more closely special and molecular force fields exert a strong influence on the energy exchange in the collision process.
Typical values of thermal conductivities are:
Table 2: Thermal conductivities of some liquids
| Liquids | Thermal conductivity (W/m .K) |
| Mercury | 8.21 |
| Water | 0.556 |
| Ammonia | 0.540 |
| Lubricating oil, SAE 50 | 0.147 |
| Freon 12, CCl2F2 | 0.073 |
4. Thermal Conductivity of solids:
Thermal energy may be conducted in solids by two modes: lattice vibration and transport by free electrons.
In good electrical conductors a rather large number of free electrons move about in the lattice structure of the material. Just as these electrons may transport electric charge; they may also carry thermal energy from a high-temperature region to a low temperature, as in the cases of gases.
In fact these electrons are frequently referred to as the electron gas.
Energy may also be transmitted as vibrational energy in the lattice structure of the material. In general, however, this lattice mode of energy transport is not as large as the electron transport, and for this reason good electrical conductors are almost always good heat conductors. V.Z. Cu, Al, and Ag, and electrical insulators are usually good heat insulators. A notable exception is diamond which is an electrical insulator; but which can have a thermal conductivity five times as high as silver or copper. It is this fact enables a jeweller to distinguish between genuine diamonds and fake stones. A small instrument is available to a thermal heat pulse. A true diamond will exhibit a far more rapid response than the non-genuine stone.
| Solids | Thermal conductivity (W/m .K) |
| Silver | 410 |
| Copper | 385 |
| Aluminium | 202 |
| Nickel | 93 |
| Iron | 73 |
| CS, 1% C | 43 |
| Lead | 35 |
References:
1. “Heat Transfer”, J.P. Holman, Pages: 6-10
2. “unit operations of chemical engineering”, Warren.l. Mccabe and Julian C. Smith, Pages: 291-292.
3. “Process Heat Transfer”, D.Q. Kern, Pages:6-15
4. “Heat Transfer a basic approach”, Necati Ozisik.
No comments:
Post a Comment