Closed Processes - TEST Tutorial

(The Closed Process daemons build upon the state daemons. Visit States pages first.)

States		Closed Steady	Open Steady
Open Process	Closed Cycle	Open Cycles	States-II
HVAC	Combustion	Equilibrium	Gas Dynamics

(Each section above is divided into two sub-sections - Manual and Applications.)
	Applications

(This page is yet to be completed)

Daemons>Closed Process> Manual

A closed process
is a journey of a closed system from a begin-state to a finish-state.

A closed process may involve a uniform or a non-uniform system.

Closed processes can also
be found in the Closed
Cycles, HVAC and
Combustion chapters.

The desired unknown may
be a b-State, f-State
or process variable.

a. Closed Process: In the Approach section, we have discussed the questions that need to be answered to classify a problem. As you read the problem description you define the system at hand (somewhat subjectively) by drawing an imaginary boundary around the object of interest. If there is no mass transfer across the boundary, the system is closed . Now suppose the snapshot of the system taken with the state camera (discussed in the States.Manual page) evolves from an initial or begin- image ( b-state) to a final or finish-image ( f-state ) as the system exchanges heat and work (called the process variables) with its surroundings. Such a transition from a b-state to a f-state by a closed system is called a closed process .

For instance, when a hot block of copper cools down to ambient temperature, or a gas in a piston-cylinder device expands to twice its original volume, a clear b-state and f-state serve as the process anchors. In these instances the system is uniform, that is any two points within the system can be described by a single unique state at any given time during the process. The daemons that handle such closed processed can be found in the following branch: Daemons. Systems. Closed. Process. Generic. Uniform.

In a non-uniform closed systems (a block of copper coming to thermal equilibrium when dropped in a tank of cold water, or gases in two different chambers mixing to form a uniform mixture), two or more states may be required to describe the b- and f-states of the composite system. Daemons that handle such problems appear under the branches Daemons. Systems. Closed. Process. Generic.NonUniformMixing and Daemons...NonUniformNonMixing .

Closed processes can also be found under the Daemons. Systems. Closed. Process. Generic. Specific branch, in the Closed Cycles , HVAC and Combustion chapters. They will be discussed in the corresponding chapters linked at the top. However, in all problems involving Closed Processes, be it a single process involving a non-uniform mixing system or a sequence of processes executed by a diesel cycle, a complete understanding of how to handle a single process involving a uniform system is essential.

In a closed process problem, partial information is given about the b- and f-States and the process . The task is to find the unknown variables using the balance equations and property relations for the given working fluid.

Fig. 1 Image of the Daemons.Process...IdealGas page. Except for two additional tab-panels,
Process and Exergy, the daemon looks identical to the corresponding State Daemon.

The anchor states.

b. The States Panel: The b-state and the f-state are the anchor states of any process and must be evaluated in most problems involving a closed process. Figure 1 shows the image of a generic process daemon with ideal gas as the working fluid model. The States panel, which is the default panel when the daemon starts, is identical to the States panel found in the corresponding volume state daemon. A comprehensive discussion of the States panel can be found on the Tutorial.States.Background page.

The number of anchor states for a given process depends on the type of the system and the nature of the process. In an uniform system, two anchor states suffice, while in a non-uniform (binary) system there can be up to four anchor states. Evaluate the anchor states as best as possible from the given information about the process.

Fig. 3 The Process-Analysis panel for a uniform closed process.

Process Analysis Panel.

Loading the anchor states.

The custom balance equations.

Only process variables appear on the Process-Analysis panel.

Based on what is known about the b- and f-states, the daemon attempts to calculate W_B.

The solution Approach.

c. The Uniform System: The Process Analysis panel for the daemon of Fig. 2 is shown in Fig. 3. The first three lines, the title, tab buttons and the global controls, remain unchanged except for the selected tab, which is now highlighted.

The global control panel (unit selection, Super-Calculate button etc.) remain the same as in Fig. 2. The process is identified by a letter (just like a State is identified by a number) , Process-A in this case. The b-state and the f-state selectors contain only those states which have been calculated. The Null value Process Type (if p=constant or Vol=constant etc.) is determined by the daemon once the anchor states are loaded.

The customized balance equations and a schematic, depicting the process for the uniform system, are displayed on this analysis panel.

d. The Process Variables: A close look at the balance equations suggests that the variables can be divided into three kinds: (a) the state variables from the b- and f-State ; (b) the boundary temperature T_B (in most situations it is the ambient temperature); and (c) the heat transfer, Q , into the system, the work, W , delivered by the system, and S_gen , the entropy generated during the process.

Unlike the state properties, the variables Q, W and S_gen depend on the particular path the the system takes in traversing between the anchor states. These path-dependent variables, therefore, carry the footprint of the process and are called the process variables. The balance equations of Fig. 3 provide a bridge between the difficult-to-measure process variables and the easy-to-evaluate state properties of the anchor states, b-State and f-State.

The work transfer, W, is divided into two parts, W_B for boundary work and W_O for all other kinds - mostly electrical and shaft - of work. The daemon attempts to evaluate the boundary work based on simple assumptions. For instance, if pressure remains unchanged between the b- and f- states, a constant pressure assumption is made. You can always override W_B calculated by the daemon, if necessary.

e. Solution Procedure: The solution procedure that is emphasized in example after example in the Archive, Slide Show and the Applications page is simple. (a) Evaluate the anchor states as best as possible. (b) On the Process-Analysis panel, choose a process name (Process-A, for instance), and select from the calculated states the b- and f-states. (c) Enter the known process variables (for instance Q=0 in an adiabatic process, S_gen=0 in an internally reversible frictionless process). (d) Press the Enter key (or the Calculate button) and Super-Calculate to update all variables.

A detailed report, a spreadsheet friendly table of properties and a few lines of what is called the TEST-Codes are produced on the I/O panel. The TEST-Codes can be saved for later use. In a later session, the solution can be regenerated by pasting the TEST-Codes (Fig. 4) on the I/O panel and clicking the Load and Super-Calculate button. In the Archive you will find a library of TEST-Codes for many practical problems.

Fig. 4 The TEST-Codes produced on the I/O panel by the Super-Calculate operation..

Load the dead State and Calculate.

f. The Exergy (Availability) Analysis: Once a closed process involving a uniform system has been analyzed, an availability or exergy analysis can be carried out on the Exergy panel provided a designated dead-State is evaluated first. The exergy balance equation along with relevant definitions are shown on the exergy panel (Fig. 5). To calculate all the exergy variables, load the dead state and Calculate. The process that is visible on the Process panel, is analyzed and all the exergy related variables are calculated.

Fig. 5 The Exergy panel showing the exergy balance equation and all the related variables.

Change a variable, Calculate and Super-Calculate

g. Parametric Studies: Once a closed process has been set up, it is quite simple to evaluate the effect of changing one or more variables on the problem. Simply change the variable of interest, be it a state or process variable, Calculate and Super-Calculate . All variables in each panel are updated.

You will find a number of closed process examples on the Applications page, Slide Show and the Archive. Try them all!

Fig. 6 The Process-Analysis panel for a non- uniform closed process.

The only difference in using the daemons for the non-uniform systems is the composite nature of the b- and f-states.

h. The Non-Uniform Systems: The Process Analysis panel for a non-uniform mixing system is shown in Fig. 6. The system schematic and governing balance equations reflect the composite nature of the b- and the f-states. There are four anchor state selectors as opposed to two in case of a uniform system. MOst of the previous discussions on the uniform system (sections c-g) apply equally well for this daemon.

You will find a number of uniform and non-uniform closed process examples on the Applications page, Slide Show and the Archive.

Non-Uniform Non-Mixing Systems

i. Non-Uniform Non-Mixing Process Daemons: A number of models to handle various combinations of working substances in a non-uniform system are offered by TEST. The starting page for these daemons is ..Closed.Process.Generic.NonMixing-NonUniform. All these daemons work in a fairly uniform manner. If you learn to use one, you will have no problems with any other. It is recommended that this page be read in conjunction with the examples on the companion page (Applications).

State Evaluation As an example of this class of daemons, let us choose the SL/SL model. You will notice that the daemon is remarkably similar to the Uniform Process daemons discussed in section 2a. On close inspection you will notice that there are only three differences: 1. The Exergy Panel has been dispensed with to simplify the daemon. 2. On the States Panel button strip there are two choices for working fluid. 3. A new state variable called Model has been added.

When you select a solid or liquid from the first choice (Model-1: SolidLiquid Model), you will see that the variable Model is set to 1. Similarly if a substance is chosen from the second choice, Model is set to 2. Suppose the non-uniform system is made of a block of copper and some liquid water. Since both the working substances are of the same model (SL model in this case) in this case, you can select Copper from either of the choices. Suppose we choose Copper from the left choice and Water(L) from the right one. You will notice that as you choose a substance, the background color of the choice turns yellow (showing the selected material) and the variable Model is set to either 1 or 2 (First or Second model). To evaluate a state with copper as the working fluid, select a state number, say, State-1, select Copper from the left choice, enter known properties, and Calculate (or press the Enter key). To evaluate a state with liquid water as the working fluid, select a state number, say, State-2, select Water(L) from the right choice, enter known properties, and Calculate (or press the Enter key). Now that you have chosen the left choice for Copper and the right choice for Water, this order must be maintained in subsequent states, otherwise Super-Calculate will not work properly . For State-3 with copper for instance, the first choice must be Copper. Note that every time you calculate a new state, the material must be selected either directly from the choice or setting the Model variable to 1 or 2. Everything else should work just like the Uniform Process daemons.

If the two subsystems are made of the same material (example: two copper blocks at two different temperatures are brought into thermal contact, to find the equilibrium temperature and entropy generation), only one of the choices can be used and the other ignored in setting up the states. The PC/SL model, thus, can be used for any combination of the following: (a) steam and copper block, (b) steam and R-12 (no mixing, obviously), (c) steam and steam, and (d) copper and copper.

If the two models happen to be the phase-change (PC) and ideal gas (IG) model respectively, the steps for evaluating a state remain unaltered. The set of properties displayed by the daemon is a superset of all the PC and IG state variables. For instance, the property quality has no meaning in the IG model, but is still displayed.

Because the PG/PG, IG/IG and RG/RG models also allow mixing, the state properties include the mass fracation x_A and mole fraction y_A of the gas-A. To specify a mixture as a pure gas selected from the left choice (gas-A) simply set x_A or y_A to be one. A zero value, on the other hand, will make the mixture a pure gas-B (selected from the right choice).

Process Evaluation If the states are completely known, a process analysis is quite straightforward. Simply load the four anchor states - two b-states and two f-states. Enter the known process variables, W and Q, and Calculate. Algebraic expressions are not allowed in the Process Panel.

Iterative Solution Suppose two blocks of solids are brought in thermal contact and we are to find the final equilibrium temperature. If State-3 and State-4 are the fA and fB states, we have two unknowns, T3 and T4. In some other systems (two gas chambers in thermal contact), it may appear that there are four unknowns - p3, T3, p4 and T4. In such situations, an iterative approach works quite well. In this, you leave Q as an unknown (even though it is supplied), enter T4 as '=T3' in State-4, guess T3, and Super-Calculate Q. Improve the guess successively until Q approaches the given value.

What-If Scenario Unless the solution is iterative in nature a parametric study can be performed on any variable of interest by simply changing it, pressing the Enter key, and then using Super-Calculate. To change the pair of working substances, however, changes have to be made in two states one with Model=1 (the first choice) and the other with Model=2 (the second choice).

I/O Panel The I/O Panel can be used as in most other daemons as a calculator that understands property symbols, and a place for displaying detailed solution report generated by Super-Calculate, and loading TEST-Codes.

TEST-Codes The TEST-Codes work as described in the Uniform Process daemons.

Miscallaneous Can the non-uniform daemon be used to solve uniform process problems? Sure! There are two drawbacks, however. First, there is no Exergy Panel in these daemons and second, for every state the working fluid must be selected (in a Uniform Process daemon, you select the material only once since there is no confusion about the working fluid when you start up a new state).

Examples: The companion Applications page provides at least one complete example to lead you step-by-step to the solution. To see what types of problems these daemons are capable of solving visit the Problems>Chapter-4 page.


	Applications