EME Basics

System

      Definition

• System: A quantity of matter in space which is analyzed during a problem.
• Surroundings: Everything external to the system.
• System Boundary: A separation present between system and surrounding.

 Classification of the system boundary:-

• Real solid boundary
• Imaginary boundary

 The system boundary may be further classified as:-

• Fixed boundary or Control Mass System
• Moving boundary or Control Volume System

          The choice of boundary depends on the  problem being analyzed.

Fig 9.1   System and Surroundings

Classification of Systems

Types of System

    Control Mass System (Closed System)

1. Its a system of fixed mass with fixed identity.
2. This type of system is usually referred to as "closed system".
   
   
3. There is no mass transfer across the system boundary.
4. Energy transfer may take place into or out of the system. 

Fig 9.2   A Control Mass System or Closed System

    Control Volume System (Open System)

1. Its a system of  fixed volume.
2. This type of system is usually referred to as "open system” or a "control volume"
3. Mass transfer can take place across a control volume.
4. Energy transfer may also occur into or out of the system.
5. A control volume can be seen as  a fixed region across which mass and energy transfers are studied. 
6. Control Surface- Its the boundary of a control volume across which the transfer of both mass and energy takes place.
7. The mass of a control volume (open system) may or may not be fixed.
8. When the net influx of mass across the control surface equals  zero then the mass of the system is fixed and vice-versa.
9. The identity of mass in a control volume always changes unlike the case for a control mass system (closed system).
10. Most of the engineering devices, in general, represent an open system or control volume.

Example:-

• Heat exchanger - Fluid enters and leaves the system continuously with the transfer of heat across the system boundary.
• Pump - A continuous flow of fluid takes place through the system with a transfer of mechanical energy from the surroundings to the system.

 

 

Fig 9.3  A Control Volume System or Open System

     Isolated System

1. Its a system of fixed mass with same identity and fixed energy.
2. No interaction of mass or energy takes place between the system and the surroundings.
3. In more informal words an isolated system is like a closed shop amidst a busy market.

Fig 9.4   An Isolated System

Conservation of Mass - The Continuity Equation

Law of conservation of mass

The law states that mass can neither be created nor be destroyed. Conservation of mass is inherent to a control mass system (closed system).

• The mathematical expression for the above law is stated as:

∆m/∆t = 0,    where m = mass of the system

• For a control volume (Fig.9.5), the principle of conservation of mass is stated as

Rate at which mass enters = Rate at which mass leaves the region + Rate of accumulation of mass in the region

OR

Rate of accumulation of mass in the control volume
                                                        + Net rate of mass efflux from the control volume = 0      (9.1)

Continuity equation

The above statement  expressed analytically in terms of velocity and density field of a flow is known as the equation of continuity.

Fig 9.5    A Control Volume in a Flow Field

A Control Volume in a Flow Field
[ See the Explanation Below ]

Flow Out

Flow In

Explanation:

The given box represents a fixed control volume, with an inlet and an outlet for a flowing fluid. The amount of fluid entering is more than the fluid leaving.Hence there is an accumulation of fluid inside the control volume.

Rate at which mass enters = Rate at which mass leaves the region + Rate of accumulation of mass in the region
OR
Rate of accumulation of mass in the control volume + Net rate of mass efflux from the control volume = 0