Heat Transfer and Thermodynamics

 The subject of heat transfer, or more generally the transport of energy, is of importance to all engineers and scientists, for it is energy, initially derived from the sun, on which the world runs. If we were to cut off the radiation from the sun we would soon find the world to be an uninhabitable sphere, and if we misuse the energy that is currently available to us a similar result may occur for other reasons.

 The branch of science that deals with the relation between heat and other forms of energy, including mechanical work in particular, is called thermodynamics. Its principles, like all laws of nature, are based on observations and have been generalized into laws that are believed to hold for all processes occurring in nature because no exceptions have ever been found. For example, the first law of thermodynamics states that energy can be neither created nor destroyed but only changed from one form to another. It governs all energy transformations quantitatively, but places no restrictions on the direction of the transformation. It is known, however, from experience that no process is possible whose sole result is the net transfer of heat from a region of lower temperature to a region of higher temperature. This statement of experimental truth is known as the second law of thermodynamics.

  Whenever a temperature gradient exists within a system, or whenever two systems at different temperatures are brought into contact energy is transferred. The process by which the energy transport takes place is known as heat transfer. The thing in transit, called heat, cannot be observed or measured directly. However, its effects can be identified and quantified through measurements and analysis. The flow of heat, like the performance of work, is a process by which the initial energy of a system is changed.

 All heat transfer processes involve the exchange and/or conversion of energy. They must, therefore, obey the first as well as the second law of thermodynamics. At first glance, one might therefore be tempted to assume that the principles of heat transfer can be derived from the basic laws of thermodynamics. This conclusion, however, would be erroneous, because classical thermodynamics is restricted primarily to the study of equilibrium states including mechanical, chemical, and thermal equilibriums, and is therefore, by itself, of little help in determining quantitatively the transformations that occur from a lack of equilibrium in engineering processes. Since heat flow is the result of temperature nonequilibrium, its quantitative treatment must be based on other branches of science. The same reasoning applies to other types of transport processes such as mass transfer and diffusion.

 Classical thermodynamics deals with the states of systems from a macroscopic view and makes no hypotheses about the structure of matter. To perform a thermodynamic analysis it is necessary to describe the state of a system in terms of gross characteristics, such as pressure, volume, and temperature, that can be measured directly and involve no special assumptions regarding the structure of matter. These variables (or thermodynamic properties) are of significance for the system as a whole only when they are uniform throughout it, that is, when the system is in equilibrium. Thus, classical thermodynamics is not concerned with the details of a process but rather with equilibrium states and the relations among them. The processes employed in a thermodynamic analysis are idealized processes devised to give information concerning equilibrium states.

6. Answer the questions:

1. What is the subject of heat transfer? 2. What does the world run on? 3. What branch of science is called thermodynamics? 4. How many laws of thermodynamics are described in the text? 5. What does the first law of thermodynamics state? 6. What statement is known as the second law of thermodynamics? 7. What kind of process is known as heat transfer? 8.Сan heat be observed or measured directly? 9. How can the effects of heat be identified and quantified? 10. What do all heat transfer processes involve? 11. What must all heat transfer processes obey? 12. What is classical thermodynamics primarily restricted to? 13. What kinds of equilibrium are mentioned in the text?