Chapter 5: Mass and Energy Analysis of Control Volumes Flashcards
what are closed system?
mass of system remains constant during process
what are control volumes?
mass can cross boundaries, and so we must keep track of the amount of mass entering and leaving the control volume
what kind of systems are diffusers?
steady-flow systems
does both volume and mass flow rates need to be conserved in a steady-flow system?
no, only mass needs to be conserved in a steady-flow process, volumes flow rates can change. In a steady-flow system, only mass and energy flow need to remain constant
does the volume flow rate change in a steady-flow system for incompressible substances?
no, for liquids and water, since they are not compressible, the volume flow rate remains nearly constant. However, the velocity can still change (do not get confused)
what is flow work/flow energy?
work required to push mass into/out of the control volume, necessary for keeping continuous flow in control volume process
this must always be considered in a flowing fluid
flow work = Pv (pressure x volume) in kj/kg
in a steady-flow system, do the heat and work interactions between system and surrounding vary or remain constant with time?
they remain constant with time
How can energy be transferred in a steady-flow system for control volume?
through heat, work or mass ONLY
what is a nozzle?
device that increases velocity of fluid by decreasing pressure
what is a diffuser?
a device that decreases the velocity of fluid by increasing the pressure
What type of energy is responsible in nozzles & diffuser?
Kinetic energy (large changes in velocity), conversion of kinetic to internal energy in device
Heat transfer (Q), Work & PE are all zero
What is a turbine?
device that drives electric generators. As fluid passes through turbine, work is done against the blades, which are attached to the shaft. As a result, the shaft rotates and the turbine produces work
- produce power output
- heat transfer negligible since usually well insulated
- PE = 0
- The fluid velocities are very high, significant change in its kinetic energy. However, this change is usually very small relative to the change in enthalpy, and is often disregarded.
What is a compressor?
device used to increase pressure of fluid, specifically compressing gas to very high pressures. Work is supplied to these devices from an external source through a rotating shaft.
- compressors involve work inputs
- heat transfer often negligible, UNLESS intentional cooling
- KE & PE = 0
What is a fan?
device used to increase pressure of fluid, specifically to increase the pressure of a gas slightly and is mainly used to mobilize a gas.
Work is supplied by external source through rotating shafts
- PE = 0
- change in velocity, often change in KE small compared to enthalpy so disregarded
What is a pump?
device used to increase pressure of fluid, specifically liquids instead of gases.
Work is supplied from external source through rotating shaft
- KE & PE = 0
what is a throttling valve?
any kind of flow-restricting devices that cause a significant pressure drop in the fluid. produce a pressure drop without involving any work, but instead by a large drop in temperature
- commonly used in refrigeration and air-conditioning applications (due to temp drop)
- considered adiabatic system (unless large exposed surface area)
- isenthalpic process
- no work, no PE, increase in KE often insignificant
- If the flow energy increases during the process, internal energy decreases, temp drops. If the product flow energy decreases, the internal energy and the tempof a fluid will increase.
what is an isenthalpic process?
enthalpy stays constant (h1 = h2)
what are mixing chambers?
where mixing of 2 streams of fluid occur
- conservation of mass (mass flow)
- usually well insulated (no heat transfer, Q)
- no work, no PE, no KE
what are heat exchangers?
two moving fluid stream exchange heat, WITHOUT MIXING
- no work, no PE, no KE
- typically well insulated outside, heat exchange between fluids IN device
- When the entire heat exchanger is selected as the control volume, Q becomes zero, since the boundary for this case lies just beneath the insulation and little or no heat crosses the boundary. If only one of the fluids is selected as the control volume, then heat will cross this boundary as it flows from one fluid to the other and Q will not be zero.
What kind of processes do pipes & duct satisfy?
steady-flow process & control volume
