Open Systems Flashcards
How does the change in total energy happen in an open system?
When both the mass and energy cross the boundary of an open system
What is an open system known as?
Control Volume
Control Volumes usually enclose a device that involves mass flow such as…
-Nozzels / diffusers
-Compressors
-Turbines
What is the definition of mass conservation?
Mass like energy is a conserved property, and cannot be created or destroyed during a process
What is the definition of Mass Flow Rate?
The amount of mass flowing through a cross section per unit time
What is the definition for mass balance for steady flow?
During a steady flow process, the total mass contained within a control volume remains constant
What is the definition for Mass Continuity?
For steady flow, there is no accumulation of mass in the control volume
What is the definition of Flow Work?
The work required to push or pull mass into or out of a control volume
What is the definition of Steady?
Does not change with time
Steady Flow system
-Fluid flowing through CV in a steady manner
-All properties within CV remain constant with time
-Heat and work interactions with the surroundings do not vary with time
-Volume, mass and total energy content of CV must be constant
What is a nozzel?
A device that increases the velocity of the fluid but reduces its pressure
What is a diffuser?
A device that increases the pressure of the fluid but reduces its velocity
Assumptions for Nozzles and Diffusers:
-Heat transfer to surroundings usually kept small
-No work done on or by the flow in the device
-Change in PE is very small
-The change in enthalpy is equal and opposite to the change in kinetic energy as the flow passes through the nozzle or diffuser
Turbines
-Compressor: work consuming device (negative work)
-For both, work done on/ by fluid on the rotating shaft
-Flow for both devices can be considered steady
Turbine / Compressors Assumptions:
-Heat transfer to surroundings usually kept small
-Change in KE is generally negligible (not always)
-Change in PE is very small
