A. Ozer Arnas
Precise thermodynamics education is a requirement to discuss issues that one faces in global warming, energy conversion, and other energy-related topics that affect sustainability of the environment in the global sense. For this reason, learning, understanding, and meaningful and relevant application of topics in thermodynamics are required. To accomplish this, educating students at the undergraduate and graduate levels in classical, statistical, and non-equilibrium thermodynamics becomes important. Here a short synopsis of fundamentals of classical thermodynamics is discussed with the intent of bringing clarity to the laws of thermodynamics and their application in design of experiments, applications in other fields such as heat transfer, and physical interpretation of the mathematical relations that are so useful in explaining why certain things happen in thermodynamics and nature. Nature is the ultimate customer.
For all scientist and engineers, the courses that end in -ics must be studied and understood well with their correct and precise application, such as mathematics, physics, chemical kinetics, mechanics, and the like. Of course, thermodynamics is one portion of mechanics that is very important in the education of all engineers but particularly mechanical and chemical engineers. It relates natural phenomena to some order and disorder. From a thermal energy point of view, therefore, thermodynamics is the science that dictates what happens in nature and what not and why. Thus to better understand nature, the implications of energy usage on the environment, and sustainability of what we enjoy today, we need to study precise thermodynamics.
Definitions in thermodynamics must be precise to make everything that follows correct. Thus, in nature, there are only three types of systems. The closed system is one for which the mass within the boundaries remains a constant, such as a tank or a piston-cylinder arrangement. The filling/emptying system is one that a tank with a valve characterizes where mass can either enter or leave the tank. Both
I. Dincer et al. (eds.), Global Warming, Green Energy and Technology,
DOI 10.1007/978-1-4419-1017-2_2, © Springer Science+Business Media, LLC 2010
cannot occur simultaneously. If a system has mass entering and leaving the boundaries, then it is called open.
There are only eight thermodynamic properties. The measurable ones are pressure, p, volume, V, and temperature, T. As a consequence of the first law of thermodynamics, internal energy, U, is introduced followed by the second law of thermodynamics and the introduction of entropy, S. Then three convenience properties are defined in terms of the five above: enthalpy H = U+pV, the Gibbs' function G = H— TS, and the Helmholtz potential F = U — TS. The rest should be called physical properties of a system such as the mass m.
In thermodynamics, equilibrium is required to solve a problem. By definition, when two systems reach the same temperature T, they are called to be in thermal equilibrium. When they reach the same pressure p, they have mechanical equilibrium, and when they have the same electrochemical potential ^ , they have chemical equilibrium. When all three happen simultaneously, then thermodynam-ic equilibrium exists.
One of the more important statements in thermodynamics is the state principle. This principle is important not only in thermodynamic analyses but in applied areas like heat transfer. This principle states that any two independent and intensive thermodynamic properties would define any of the others and fix a ther-modynamic state, and the situation is unique no matter which choice is made. A lack of understanding of this principle may lead to published work which has no meaning at all, Chawla (1978). Thus once a thermodynamic state is defined then a characteristic line that connects any two such states is called a thermodynamic process. A sequence of thermodynamic processes ending up at the initial thermo-dynamic state is called a thermodynamic cycle.
These equations that apply may be obtained using the Reynolds' Transport Theorem. Its use is common to all of these conservation laws and will be done systematically for each one of them. Considering at time t, Bsystem(t) = BCV(t) and at time t+At, Bsystem(t+At) = [BCV(t+At) + Bout - Bin], where the amount of B that exited the control volume is given as Bout = b(mout) = b[(p)( V out)(Aout)(At)] and Bin=
b(min) = b[(p)( V in)(^in)(At)] is the amount of B that entered the control volume. Subtracting the first term from the second and dividing by At results in
Bsystem (t + At)- System (t) = B„(t + At) - B„ (t) + b[p)(Vout )(A„at )(At)] - b[(p)(Vin )(Am )(&t)]
Taking the limit of this equation as At approaches 0 yields the simplified equation i=d^cv+bp dt dt
V out Aout V in Ajn
Rewriting the outflow and inflow terms for the entire control surface in this equation gives dBsys = d^CV + f pbV ■ ndA
dt dt ics where the dot product, (V ■ n), is defined as (V n cos 9), where 0 is the angle between
V to n . The total amount of extensive property B, (B=m b), in the control volume is determined by integrating over the entire control volume as BCV = £ pbdV.
Upon substitution and combination, the Reynolds' Transport Theorem is obtained dB d as —iOL = — f pbdV + f pb(V • n)dA where the left-hand term is the time rate dt dt Jcv^ Jcs^
of change of extensive property (B) in the system, the first term on the right-hand side of the equation is the time rate of change of intensive property (b) in the control volume, and the second term on the right-hand side of the equation is the net flow of intensive property (b) across the control surface. A positive value indicates that net flow across the control surface is out of the control volume while a negative value indicates that net flow across the control surface is into the control volume.
There are two conservation equations, that of mass and energy - the first law of thermodynamics. For the case of conservation of mass, m = constant.
Closed system: Open system: Filling system: Emptying system:
Applying the Reynolds' Transport Theorem to the first law of thermody-
vii./ d r r namics, the result becomes—= — I pedV + I pe(V• n)dA.
This state ment is very similar to money and banking; the difference is that nature does not permit overdraw of funds, namely one cannot use what is not there, whereas the bank just charges us money for overdraft. At steady state, therefore, SQ — SW = dU . For a cycle, <SQ = <jSW resulting in the fact that dU = d (property) =0. This is a very important result which will be used later. Application of the above theorem to various systems in nature results in the first law of thermodynamics as
Closed system: Open system:
Was this article helpful?
Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.