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Transient Process = Unsteady Process

Transient Process

(or unsteady process) :

Transient processes can be very complex. Therefore, we will only consider processes in which we can assume uniform flow and uniform state.

Uniform Flow

Uniform State

Open
System

msys(t)

A process in which one or more properties or process variables change as a function of time.

The properties and flowrates of all inlet and outlet streams are constant over the cross-sectional area for flow and are also constant with respect to time.
(

, and

are constant)

The state of the mass within the system is uniform. At all times, the properties of the outlet stream are exactly the same as the properties of the system at that point in time. The properties of the system do change with time, but they are always the same as the properties of the outlet stream.

Lesson 5D: page 1 of 7

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Ch5, Lesson D, Page 1 - Transient Process = Unsteady Process

Prior to this lesson, we have only considered steady-state processes. Now, we will learn how to apply the mass conservation equation to transient processes.
A transient process is one in which one or more properties or process variables change with time.
This makes the problem a lot more complicated.
For starters, the 1st Law cannot be reduced to an algebraic equation. We must solve the ODE.
In many cases if one property or process variable changes with time, then many or ALL of them do.
The 1st Law quickly becomes very difficult to solve without the help of a computer.
This complex topic is not the focus of this course.
So, we will only consider some very special, but important processes for which the mass balance equation and the 1st Law simplify greatly.
A Uniform Flow process is one in which the properties and flow rates of all inlet and outlet streams remain constant
This means that the rates at which mass and energy flow in and out of the system are constant.
But if the rate at which mass or energy is flowing into the system is different from the rate at which it is flowing out, then the mass or energy inside the system will either increase or decrease.
But clearly, if the mass or energy within the system is increasing or decreasing, then the system is NOT operating at steady-state !
In addition to Uniform Flow, the processes we will study will also be Uniform State processes.
The easiest way to picture a uniform state process is to picture a stirred tank.
The stream that flows out of the tank has the same properties as the fluid in the tank.
The properties of the fluid in the tank can change with time, so the process is transient, but at all times the properties of the contents of the tank are the same as the properties of any and all effluent streams.
In this lesson, we will use the characteristics of Uniform-Flow, Uniform State processes to simplify the differential mass balance equation.
In the next lesson, we will simplify the 1st Law for Uniform-Flow, Uniform State processes.

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