First Law of Thermodynamics Flashcards
First Law of Thermodynamics
Energy cannot be created or destroyed but can change form
-Provides a basis for studying the relationships among various forms of energy and energy interactions
Energy balance
DeltaE(system) = E(in) - E(out)
Mechanisms of energy transfer
- Heat (Q)
- Work (W)
- Mass flow (m dot)
State the assumptions for a closed-system problem (3)
- System is perfectly thermally insulated
- System pressure is constant
- All processes are quasi-equilibrium processes
Steady-flow process
A process which a fluid flows through a control volume steadily
- total volume, mass, total energy content (E) of the control volume must remain constant
- Wb = 0
- heat and work interactions between a steady flow system and its surroundings do not change with time
Nozzles
Devices that increase the velocity of a fluid, at the expense of pressure
Diffusers
Devices that increase the pressure of a fluid, by decreasing its velocity
Turbines
In gas, steam, or hydroelectric power plants, turbines drive the electric generator
-produces work (w(dot) > 0)
Compressors, pumps, and fans
Devices used to mobilise a fluid and/or to increase its pressure
-Work is supplied to these devices ( w(dot) < 0)
Throttling valves / isenthpic device
-Any kind of flow restricting device that causes a significant pressure drop in the fluid
-Also a large drop in temperature
h1=h2
Mixing chamber
The section where a mixing process takes place
- physical contact
- same pressure
- enthalpy change depends on fluid temperature change
Heat exchangers
Devices where two moving fluids exchange heat without mixing
- no physical contact
- can be different pressures
- v.important: choice of control volume
Assumptions for steady-flow problems
- No moving boundary work
- Negligible change in potential and/or kinetic energies (depending on device)
