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a. Open Process: Detailed discussion of Closed
Processes can be found in Daemons>Closed
Process>Manual. A process, - defined as the
migration of a system from an initial or beginning-state, b-state, to a final or final-state,
f-state - can also be executed by an open system. In addition
to heat
and work , an open system can also exchanges mass with its surroundings.
For instance, when a propane cylinder is charged, or steam escapes a pressure cooker, a clear b-state and a f-state serve as the process anchors signaling a process . In addition, depending on the problem, an inlet or an exit port, identified by the i-state or e-state, characterizes the mass transfer for the open system. As the state of the fluid in the cylinder changes, so does the inlet or exit states. A simplifying assumption, called the uniform-state uniform-flow assumption, treats the states at the inlet or exit as invariant during the process. This is quite reasonable for many situations; for instance, the state of propane in the supply line may not be affected much during the charging of a cylinder as long as the supply comes from a large reservoir. Similarly, when water boils inside a pressure cooker, the steam exiting through the valve is saturated vapor and its state does not change, as the pressure inside remains constant, until the last drop of liquid vaporizes. Clearly, the system boundary must be carefully drawn to ensure that the i or e state is on the appropriate side of the supply (or exit) valve. The open process daemons appear under the branch Daemons. Systems. Open. Process on the TEST-Map. Generally in an open process problem,
partial information is given about the i- , e- , b- and f-States, and the process variables . The task is to find the unknown variables using the
balance equations and property relations for the given working fluid. |
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| Fig. 1 Image of the process panel of an open process daemon. Note the location of the i and e states. |
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b. The Open Process Variables:
There are three kinds of
variables that appear in the balance equations for an open process (see
Fig. 1): (a) The state properties from the i- , e- , b- and f-states . These include
the system (total or flow) property m (=rho*Vol or rho*A*Vel*t), extrinsic property j and thermodynamic
property s. (b) The boundary temperature T_B (in most situations,
it is the ambient temperature); (c) The total heat transfer, Q (Qdot*t), into
the system, the total work, W_ext (=Wdot_ext*t), delivered by the system through all means
(mostly electrical or shaft) other than flow, and S_gen (=Sdot_gen*t),
the total entropy generation within the control volume during the process.
While m , j and s at the i- , e- , b- and f-States , and T_B (see Fig. 1) are all state properties, the variables
m_i, m_e, Q, W_ext and S_gen depend on the particular process the open system executes
in taking the system from the b-State to the f-State. The balance equations of Fig.
1 provide a bridge between the difficult-to-measure device variables and
the easy-to-evaluate state properties of the anchor states, b-state, f-state, i-state and e-state. |
| c. Process Panel: The process panel for an open-process daemon is shown
in Fig. 1.
The global control panel remains unaffected. On the local control panel, there are four state selectors for the i-, e-, b-, and f-states . The default state of a port, State-Null , is equivalent to having a port plugged. For a charging problem only i-state is relevant while for a discharge problem only e-state needs to be loaded. The device is identified by a letter (as in a closed process), Device-A being the default device. The boundary temperature T_B is given a default
value of 25 deg-C, which can be overridden, if necessary. For adiabatic
devices, the value of T_B is inconsequential as can be inferred from the
balance equations of Fig. 1. d. Solution Procedure: The solution procedure
is straightforward (a) Evaluate the anchor states, the b- and f-states and the i- or e-state as best as
possible. (b) On the Analysis panel, choose a device name (Device-A, for instance),
and select from the calculated states the anchor states. (c) Enter the known
device variables (for instance Q=0 for an adiabatic device). (d) Press
the Enter key (or the Calculate button) and Super-Calculate to
update all variables. To specify a completely evacuated state (vacuum)
enter pressure and mass as zero. Sometimes an iterative solution is necessary
when more than one anchor states are unknown. For instance, in the filling
of a tank, suppose the mass that enters is known. m_f, therefore, can be expressed
in terms of m_b, which, say, is unknown. Without a value for m_b, the b-state
and, hence, e_b, may be an unknown too. If Q and W_ext are given, make one
of them, say, Q, as an unknown and guess the value of m_b. Super-Calculate
to evaluate Q. Repeat until the known Q is reproduced. It is not as hard as
it sounds since the A detailed report, a spreadsheet friendly table of properties and a few lines of TEST-codes are produced in the I/O panel. TEST-codes can be saved for later use. In a later session, the solution can be regenerated by pasting the TEST-codes into the I/O panel and clicking the Load and Super-Calculate buttons. |
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e. Parametric Studies: Once an open 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 open process examples on the Example page, VisualTour and the Problems pages. |
| Copyright 1998-: Subrata Bhattacharjee |