THERMODYNAMICS - THEORY

THERMODYNAMICS - THEORY
		A control volume may involve one or more forms of work at the same time. If the boundary of the control volume is stationary, the moving boundary work is zero, and the work terms involved are shaft work and electric work. Another work form with the fluid is flow work.
		Flow Work (Flow Energy)
A Flow Element   Flow Work with Imaginary Piston Click to View Movie (68 kB)		Work is needed to push the fluid into or out of the boundaries of a control volume if mass flow is involved. This work is called the flow work (flow energy). Flow work is necessary for maintaining a continuous flow through a control volume. Consider a fluid element of volume V, pressure P, and cross-sectional area A as shown left. The flow immediately upstream will force this fluid element to enter the control volume, and it can be regarded as an imaginary piston. The force applied on the fluid element by the imaginary piston is:       F = PA The work done due to pushing the entire fluid element across the boundary into the control volume is       Wflow = FL = PAL = PV For unit mass,       wflow = Pv The work done due to pushing the fluid element out of the control volume is the same as the work needed to push the fluid element into the control volume.
		Total Energy of a Flowing Fluid
Total Energy of a Flowing Fluid Click to View Movie (52 kB)		The total energy of a simple compressible system consists of three parts: internal, kinetic, and potential energy.       E = U + KE + PE For unit mass,       e = u + ke + pe = u + v2/2 + gz where       e = total energy       u = internal energy       v = velocity of the system       z = the elevation of the fluid The fluid entering or leaving a control volume possess an additional energy, the flow work (Pv). Hence, the total energy of a flowing fluid becomes       θ = Pv + u + v2/2 + gz where       θ = methalpy, the total energy of a flowing fluid The definition of enthalpy gives       h = Pv + u Replacing Pv + u by h yields       θ = h + v2/2 + gz By using the enthalpy instead of internal energy, flow work is not a concern.
		The Steady-flow Process
Steady-flow Process Click to View Movie (60 kB)		Steady flow process is a process where: the fluid properties can change from point to point in the control volume but remains the same at any fixed point during the whole process. A steady-flow process is characterized by the following: No properties within the control volume change with time. That is mcv = constant       Ecv = constant No properties change at the boundaries with time. Thus, the fluid properties at an inlet or exit will remain the same during the whole process. They can be different at different opens. The heat and work interactions between a steady-flow system and its surroundings do not change with time.
		Mass and Energy Balance for Steady-flow Process
		The conservation of mass principle, which has been previously introduced, in rate format, is:        During a steady-flow process, the total amount of mass contained within a control volume does not change with time. That is,       dmsystem/dt = 0 Hence the conservation of mass principle gives the total amount of mass entering a control volume equal to the total amount of mass leaving it. In an equation format, it is
Mass and Energy balance for Steady-flow Process		(Total mass entering the control volume per unit time) = (Total mass leaving the control volume per unit time) or,        where       i = inlet       e = exit Also, the energy balance for a process, which has been previously introduced, in rate format, is:               For a steady-flow process, the total energy content of a control volume remains constant. That is,       dEsystem/dt = 0 Thus, the amount of energy entering a control volume in all forms (heat, work, mass transfer) must be equal to the amount of energy leaving it for a steady-flow process. In an equation format, it is (Rate of net energy transfer in by heat, work and mass) = (Rate of net energy transfer out by heat, work and mass) Or,        For a general steady-flow process, the energy balance can be written as        If the sign introduced previously for heat and work is used, the energy balance for a general steady-flow process can be rewritten as:

Thermodynamics Directory | Heat Transfer Directory

Thermodynamics Directory | Heat Transfer Directory

First Law of Thermodynamics

The First Law of Thermodynamics is a balance of the various forms of energy as they pertain to the specified thermodynamic system (control volume) being studied.

First Law of Thermodynamics - The First Law of Thermodynamics states: Energy can neither be created nor destroyed, only altered in form. For any system, energy transfer is associated with mass and energy crossing the control boundary, external work and/or heat crossing the boundary, and the change of stored energy within the control volume. The mass flow of fluid is associated with the kinetic, potential, internal, and "flow" energies that affect the overall energy balance of the system. The exchange of external work and/or heat complete the energy balance. The First Law of Thermodynamics is referred to as the Conservation of Energy principle, meaning that energy can neither be created nor destroyed, but rather transformed into various forms as the fluid within the control volume is being studied. The energy balance spoken of here is maintained within the system being studied. The system is a region in space (control volume) through which the fluid passes. The various energies associated with the fluid are then observed as they cross the boundaries of the system and the balance is made. As discussed in previous chapters of this module, a system may be one of three types: isolated, closed, or open. The open system, the most general of the three, indicates that mass, heat, and external work are allowed to cross the control boundary. The balance is expressed in words as: all energies into the system are equal to all energies leaving the system plus the change in storage of energies within the system. Recall that energy in thermodynamic systems is composed of kinetic energy (KE), potential energy (PE), internal energy (U), and flow energy (P_L); as well as heat and work processes. Σ (all energies in) = Σ (all energies out) + Δ(energy stored in system) For most industrial plant applications that most frequently use cycles, there is no change in storage (i.e. heat exchangers do not swell while in operation). In equation form, the balance appears as indicated on Figure 14.
where:


Heat and/or work can be directed in to or out of the control volume. But, for convenience and as a standard convention, the net energy exchange is presented here with the net heat exchange assumed to be into the system and the net work assumed to be out of the system. If no mass crosses the boundary, but work and/or heat do, then the system is referred to as a "closed" system. If mass, work and heat do not cross the boundary (that is, the only energy exchanges taking place are within the system), then the system is referred to as an isolated system. Isolated and closed systems are nothing more than specialized cases of the open system. In this text, the open system approach to the First Law of Thermodynamics will be emphasized because it is more general. Also, almost all practical applications of the first law require an open system analysis.
An understanding of the control volume concept is essential in analyzing a thermodynamic problem or constructing an energy balance. Two basic approaches exist in studying Thermodynamics: the control mass approach and the control volume approach. The former is referred to as the Le Grange approach and the latter as the Eulerian approach. In the control mass concept, a "clump" of fluid is studied with its associated energies. The analyzer "rides" with the clump wherever it goes, keeping a balance of all energies affecting the clump.

The control volume approach is one in which a fixed region in space is established with specified control boundaries, as shown in Figure 15. The energies that cross the boundary of this control volume, including those with the mass crossing the boundary, are then studied and the balance performed. The control volume approach is usually used today in analyzing thermodynamic systems. It is more convenient and requires much less work in keeping track of the energy balances. Examples of control volume applications are included in Figures 16-18.



The forms of energy that may cross the control volume boundary include those associated with the mass (m) crossing the boundary. Mass in motion has potential (PE), kinetic (KE), and internal energy (U). In addition, since the flow is normally supplied with some driving power (a pump for example), there is another form of energy associated with the fluid caused by its pressure. This form of energy is referred to as flow energy (Pν-work). The thermodynamic terms thus representing the various forms of energy crossing the control boundary with the mass are given as m (u + Pν + ke + pe).

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The Second Law of Thermodynamics Or Energy is Forever, but Not Exactly

The Second Law of Thermodynamics
Or Energy is Forever, but Not Exactly

		Keeping It Simple (and Clear) Teachers and Learners: The Second Law of Thermodynamics is probably the most misunderstood principle of physics. Because of the confusion and pervasive misinformation regarding this principle, I've dragged my feet shamelessly when it came to dealing with it in this website. Well, I can't undo all the misinformation with one little web site. So I won't try. If you are a beginner start here; and if you are not going to be an engineer or scientist, stay here. It is probably all you will need to understand the basic energy change results the 2nd law predicts. Your head won't get filled with confusing non-thermodynamic and incorrect analogies. We don't need the silly, and wrong, examples of messy desks. Such things are not predicted by the 2nd Law of Thermo. Honest. Other than the next few paragraphs, we won't discuss entropy on this page (but someday its page will come). Thermodynamic entropy is a measurable property of matter, not a vague predictor of universal decay. We don't need to discuss measurements of disorder. We never need disorder. What are those units of disorder anyway? Disorderites? Messyisms? Gobbledygooks? Being a physical property, entropy has units. The units are energy divided by absolute temperature (We are talking about classical thermodynamics here - not getting into statistical thermo). I don't believe there are units of disorder in any field, though it sounds like it could be a good legal term for court room judges ("You are guilty of generating 3.5 units of disorder in my court room"). All of the various 2nd Law definitions listed in text books result from the basic energy change results I describe in these pages. But why start backwards? Start with the basic energy changes the 2nd Law describes. Very simple. Read the following at least 3 times: It is only about energy. It is only about energy changes. It is only about the condition of the energy before and after the change. To be sure, there are interesting concepts about organizational disorder, probability, complexity, and things getting messy (and the propagation of misinformation about thermodynamic entropy). But it is incorrect to create metaphors and anlogies from thermodynamic entropy to explain those concepts, and it only misleads beginners. Relax, you don't need them to explain this concept to students. The Second Law of Thermodynamics absolutely does NOT say everything tends toward disorder (or decay)! It is not a universal law of messiness. It is only about energy changes. Isn't that nice? We can all relax. My messy desk and your wrinkled shirt are not predicted or measured by entropy formulas and the 2nd Law of thermodynamics.
		Picture 1: The pressure in the volume on the left is higher than the pressure on the right. The pressure energy in the left side can be thought of as more "concentrated" than the pressure energy on the right side. Both sides take up the exact same amount of space (or volume), but there is more pressure energy in the left side. More pressure energy in the same space means it is more concentrated.
		Picture 2: The valve has just been opened. Immediately, air on the higher pressure left side starts to flow to the right side, because the pressure is lower there. Just like air escaping from a baloon. Energy is flowing from more concentrated to less concentrated.
		Picture 3: The air kept flowing through the opening until the pressure on both sides was equal. See the pressure guages? They show the same pressure on both sides. The pressures are now equal - no difference in concentration levels. There is no more air flow through the opening. We have reached equilibrium. The total energy hasn't changed (First Law), but it is more "spread out" or less concentrated now (2nd Law).
		Picture 1: Two tanks of water. The water on the left side is hotter than the water in the tank on the right side. There is only a thin piece of sheet metal, or maybe some glass, separating the water, so heat (thermal energy) can easily flow from one side to the other. Thermal Energy is more concentrated in the hotter water. A cubic inch of water on the left side, has more thermal energy in it than a cubic inch of water on the right side.
		Picture 2: The thermal energy continued to flow from the left side into the right side. The temperature on the right steadily increased, while the temperature on the left side got steadily cooler. Eventually the temperatures on both sides became the same, as shown by the cartoon thermometers above. Equilibrium has been reached. The concentration of thermal energy is the same on both sides, so there is no more energy flow.

How Everything Happens
Energy makes everything happen, and every time something happens, there is an energy change. There are two important natural "laws of energy" that describe what happens to the energy involved in every change. We call them "laws" because countless observations and thousands of experiments have shown them to always predict what will happen.

Ponder that for a moment - how everything happens. It means we don't understand much, if we don't understand both the first and second laws of Energy.


These next few pages will give you an overview of the famous, but often misunderstood, 2nd Law.

Beyond the First Law
The First Law of Thermodynamics tells us energy is conserved. The total amount never changes. But something does change. I will call it "re-usability", for now. It's not an official text book word, but pretty good for communicating the basic idea.

Remember that there has to be an energy transfer for something to happen; energy changes form or moves from place to place (heat flow, for example). As energy moves and changes, the total amount of energy stays the same, constant forever as far as we know.

That sounds good doesn't it?
Energy is forever.

But wait! If it's forever, why are all these do-gooders telling us we need to conserve energy by using less? Can't we just keep using it over and over? Why shouldn't everyone drive to work alone in a 300 horsepower car?

The Rest of the Story...
Alas, my friends, there is always a rub, and when it comes to energy, the rub is described by the Second Law of Thermodynamics. The first law would be quite happy to let us re-use energy over and over. The first law is happy as long as energy is conserved. It's the happy law.

The second law may seem a little less happy to some. It describes the aftermath of every energy change that makes something happen. The second law says that each time energy gets transferred or transformed, some of it, and eventually all of it, gets less useful. That's the truth. It gets less useful, until finally, it becomes mostly useless (at least as far as its ability to make things happen is concerned).

All of the energy we use ends up, sooner or later, as what we engineers like to call "low-grade" energy. This low-grade energy is only good for warming the air around us a little bit. We can't use it to do things we consider useful, like generate electricity or make a car go. Inevitably, most of it gets radiated out into the vast cold universe, lost to us forever.

To understand this, it is helpful to start with another aspect of the Second Law. Let's call it "the direction energy moves" aspect. 






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