Heat and mass transfer (advanced course)

Main objectives:
1. Find out basics of convection, conduction and radiation
2. Understand main principles of heat travel
3. Learn main units of energy
4. Find out temperature scales
5. Learn about specific heats of liquids, gases and solids
Lesson 1.

Basics of Heat Transfer
Heat is the form of energy that can be transferred from one system to another as a result of temperature difference.
The science that deals with the determination of the rates of such energy transfers is the heat transfer.
Heat transfer is the exchange of thermal energy between physical systems, depending on temperature and pressure by dissipating heat.
There are three main heat transfer mechanisms including conduction, convection and radiation

Heat Transfer
Fig. 1. In the early nineteenth century, heat was thought to be an invisible fluid called the caloric that flowed from warmer bodies to the cooler ones.

Heat Transfer
Fig. 2. We are normally interested in how long it takes for the hot coffee in a thermos to cool to a certain temperature, which cannot be determined from a thermodynamic analysis alone.

Heat Transfer
Fig. 3. Heat flows in the direction of decreasing temperature

Heat Transfer

Heat Transfer

Heat Flux
Fig. 4. An example of
heat flux (heat transfer per unit time and per unit area)

Application areas of Heat Transfer
Heat transfer is commonly encountered in engineering systems and other aspects of life, and one does not need to go very far to see some application areas of heat transfer. In fact, one does not need to go anywhere. The human body is constantly rejecting heat to its surroundings, and human comfort is closely tied to the rate of this heat rejection. We try to control this heat transfer rate by adjusting our clothing to the environmental conditions.

Application areas of Heat Transfer
Many ordinary household appliances are designed, in whole or in part, by using the principles of heat transfer. Some examples include the electric or gas range, the heating and air-conditioning system, the refrigerator and freezer, the water heater, the iron, and even the computer, the TV, etc. Of course, energy-efficient homes are designed on the basis of minimizing heat loss in winter and heat gain in summer.
Heat transfer plays a major role in the design of many other devices, such as car radiators, solar collectors, various components of power plants, and even spacecraft.
The optimal insulation thickness in the walls and roofs of the houses, on hot water or steam pipes, or on water heaters is again determined on the basis of a heat transfer analysis with economic consideration

Application areas of Heat Transfer
Fig. 5. Heat Transfer around us

Conduction
Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. Conduction can take place in solids, liquids, or gases. In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion. In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons.

Conduction
Fig. 6. Heat conduction through a large plane wall of thickness ∆x and area A

Convection
Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion.
The faster the fluid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction

Convection
Convection is called forced convection if the fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. In contrast, convection is called natural (or free) convection if the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid.

Convection
Fig. 7. The cooling of a boiled egg by forced and natural convection

Radiation
Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an intervening medium.
In fact, energy transfer by radiation is fastest (at the speed of light) and it suffers no attenuation in a vacuum. This is how the energy of the sun reaches the earth.
All bodies at a temperature above absolute zero emit thermal radiation

Radiation
Fig. 8. Radiation heat transfer between a surface and the surfaces surrounding it.

Radiation
Fig. 9. A human body can also radiate heat outside if there is a temperature difference (as an example here the surrounding temperature is lower than the temperature of a person, say, lower than 300C).

Main Units of Energy

British Thermal Unit
The British thermal unit (BTU or Btu) is a traditional unit of work equal to about 1055 joules. It is the amount of work needed to raise the temperature of one pound of water by one degree Fahrenheit (Physical analogue: one four-inch wooden kitchen match consumed completely generates approximately 1 BTU). The British thermal unit (BTU or Btu) is a traditional unit of work equal to about 1055 joules.

Temperature scales

Celsius vs. Fahrenheit degrees

Specific heats of Gases, Liquids and Solids
Specific heat is defined as the energy required to raise the temperature of a unit mass of a substance by one degree
The specific heat at constant volume Cv can be viewed as the energy required to raise the temperature of a unit mass of a substance by one degree as the volume is held constant. The energy required to do the same as the pressure is held constant is the specific heat at constant pressure Cp.

Specific Heat
Fig. 10. Specific heat is the energy required to raise the temperature of a unit mass of a substance by one degree in a specified way.

Specific Heat
Fig. 11. The Cv and Cp values of incompressible substances are identical and are denoted by C.

Specific Heat

Specific Heat of Water
Fig. 12. Specific heat of water depending on temperature