Nozzles and Diffusers
Nozzles and diffusers regulate flow velocity by either increasing or decreasing flow speed. Flow speed can be altered very easily by changing the cross-sectional area of a channel. Decreasing the outlet area will increase flow speed, creating a nozzle. Likewise, increasing the outlet area will decrease flow speed, acting as a diffuser.
Nozzle and diffuser summary
| Conservation Item | Condition | Result |
|---|---|---|
| Heat | Adiabatic | \dot{Q}=0 |
| Work | Zero Work | \dot{W}=0 |
| Enthalpy | Non-negligible | |
| Kinetic | Non-negligible | |
| Potential | Typically negligible | \Delta PE=0 |
| Mass Conservation | \rho_1v_1A_1=\rho_2v_2A_2 | |
| Total Energy | 0=h_2-h_1+\frac{1}{2}(v^2_2-v^2_1) |
Turbines, Compressors, and Pumps
Turbines, compressors, and pumps convert between fluid pressure and shaft work. Turbines will provide work output in exchange for decreasing fluid pressure. Likewise, compressors and pumps will require work input to increase fluid pressure. Compressors are used to increase the pressure of gases, while pumps will increase the pressure of liquids.
Turbine, compressor, and pump summary
| Conservation Item | Condition | Result |
|---|---|---|
| Heat | Adiabatic | \dot{Q}=0 |
| Work | Non-zero work | |
| Enthalpy | Non-negligible | |
| Kinetic | Typically negligible | \Delta KE=0 |
| Potential | Typically negligible | \Delta PE=0 |
| Mass Conservation | \dot{m_i}=\dot{m_e} | |
| Turbine Pressure | Decreases | P_2 < P_1 |
| Compressor/Pump Pressure | Increases | P_2>P_1 |
| Total Energy | -\dot{W}=\dot{m}(h_2-h_1) |
Throttling Valves
A throttle is typically some form of abrupt restriction to fluid flow. As a result, a very quick pressure drop will occur. There is no output work and negligible heat transfer for a throttling process. Therefore the process is assumed to be a pressure drop at constant enthalpy.
Throttle valve summary
| Conservation Item | Condition | Result |
|---|---|---|
| Heat | Adiabatic | \dot{Q}=0 |
| Work | Zero work | \dot{W}=0 |
| Enthalpy | Negligible | \Delta H=0 |
| Kinetic | Typically negligible | \Delta KE=0 |
| Potential | Typically negligible | \Delta PE=0 |
| Mass Conservation | \dot{m_i}=\dot{m_e} | |
| Pressure | Decreases | P_2 < P_1 |
| Total Energy | h_1=h_2 |
Heat Exchangers
Heat exchangers transfer thermal energy from one fluid or fuel to another without mixing. Ideally, the heat lost by one fluid will be entirely gained by the other. Heat exchangers come in many forms, differing in efficiency, however, for simplicity we will assume the aforementioned ideal condition for the time being. Boilers, condensers, and evaporators are all forms of heat exchangers which may or may not use another fluid to heat/cool the main fluid line.
Heat exchanger summary
| Conservation Item | Condition | Result |
|---|---|---|
| Heat | Transferred between fluids | |
| Work | Zero work | \dot{W}=0 |
| Enthalpy | Non-negligible | |
| Kinetic | Typically negligible | \Delta KE=0 |
| Potential | Typically negligible | \Delta PE=0 |
| Mass Conservation | Per fluid line | \dot{m}_i=\dot{m}_e |
| Total Energy | \dot{m_1}(h_2-h_1)=\dot{Q}_1=-\dot{Q}_2\dot{m_3}(h_3-h_4) |
Mixing Chambers
Mixing chambers combine multiple streams of a fluid and may output a fluid via multiple outlets. The resulting enthalpy of the output fluid will be the weighted average of the input fluid enthalpies.
Mixing chamber summary
| Conservation Item | Condition | Result |
|---|---|---|
| Heat | Adiabatic | \dot{Q}=0 |
| Work | Zero work | \dot{W}=0 |
| Enthalpy | Non-negligible | |
| Kinetic | Typically negligible | \Delta KE=0 |
| Potential | Typically negligible | \Delta PE=0 |
| Mass Conservation | \sum\dot{m}_i=\sum\dot{m}_e | |
| Total Energy | \sum\dot{m}_ih_i=\sum\dot{m}_eh_e |
Steam Power Cycle
The basic steam power cycle includes four components, each of which is required for the cycle to function. The purpose of a steam power cycle is to convert thermal energy into usable work, which may be further converted into electrical energy using a generator. The purpose of each component is as follows:
- Boiler: Increases temperature and enthalpy of fluid, typically turning the fluid from a liquid to a superheated vapour.
- Turbine: Converts enthalpy from the fluid into usable work.
- Condenser: Turns vapour into a liquid to allow flow through pump. Utilization of a pump increases efficiency of the overall cycle.
- Pump: Facilitates flow through the cycle and increases fluid pressure before heating.
Refrigeration Cycle
The refrigeration cycle can be thought of the steam cycle in reverse in many ways. The goal of any refrigeration cycle is to move heat from one space to another by inputting work. Each component is explained as follows:
- Evaporator: Transfers heat from an external area to the refrigerant. Changes refrigerant phase from liquid to gas.
- Compressor: Facilitates flow through cycle and increases refrigerant pressure.
- Condenser: Transfers heat from the refrigerant to a space. Changes refrigerant phase from a gas to a liquid.
- Expansion Valve: Decreases pressure of refrigerant to more easily transform the refrigerant to a gaseous state in the evaporator.
