PE Mechanical Thermal Fluids key terms

A combined property equal to internal energy plus the product of pressure and specific volume. In steady-flow devices it tracks the energy carried by fluid in and out, making it central to turbine, compressor, and heat exchanger analysis.

A property measuring energy dispersal that increases in any real process. It sets the ideal limit for work-producing cycles, since an isentropic device produces the maximum work or requires the minimum work input.

Energy is conserved: the net heat added to a system minus the net work it performs equals its change in internal energy. For open systems the same balance includes mass flow carrying enthalpy.

Real processes have a preferred direction and cannot fully convert heat to work. It justifies the existence of entropy, defines the Carnot efficiency ceiling, and rules out devices that would violate these limits.

An energy balance for a control volume with unchanging conditions over time: heat in minus work out equals mass flow times the change in enthalpy, kinetic energy, and potential energy between inlet and outlet.

A fixed region in space chosen for analysis, with mass, heat, and work crossing its boundary. Most power and refrigeration equipment is modeled this way rather than as a closed fixed mass.

The ratio of actual device performance to the ideal isentropic performance for the same inlet state and outlet pressure. Turbines fall below 100 percent; compressors require more work than ideal.

The maximum thermal efficiency any heat engine can achieve between two constant-temperature reservoirs, equal to one minus the ratio of cold to hot absolute temperatures. It is a ceiling, never attainable in practice.

Net work output divided by heat input for a power cycle. It measures how much of the purchased heat becomes useful work and is the primary performance index for Rankine, Brayton, and Otto cycles.

The ideal cycle for steam power plants: pump, boiler, turbine, and condenser, often with superheat and reheat. Its efficiency depends strongly on boiler pressure and condenser pressure.

The ideal gas-turbine cycle of compression, constant-pressure heat addition, expansion, and heat rejection. Efficiency rises with pressure ratio, and regeneration recovers exhaust heat to reduce fuel input.

Heating a vapor above its saturation temperature at a given pressure. In steam cycles it raises turbine inlet temperature, boosting efficiency and limiting moisture formation during expansion.

The temperature at which a pure substance changes phase at a given pressure. It rises with pressure, and the pairing with saturation pressure defines the boundaries of boiling and condensation.

The energy absorbed per unit mass when a saturated liquid becomes saturated vapor at constant temperature and pressure. It decreases as pressure rises and vanishes at the critical point.

The energy needed to raise a unit mass of substance one degree while pressure is held constant. It links temperature change to enthalpy change for ideal gases and appears in gas-cycle calculations.

The constant-pressure specific heat divided by the constant-volume specific heat, denoted k or gamma. It governs isentropic relations for ideal gases and sets the efficiency of Brayton cycles.

An idealized adiabatic process with no internal irreversibility, so entropy stays constant. It provides the reference path for computing ideal compressor work and ideal turbine output.

The absolute discharge pressure divided by the absolute inlet pressure of a compressor. Raising it increases gas-turbine efficiency but also raises discharge temperature and required work.

A heat exchanger that transfers heat from hot exhaust gas to the compressed air entering a combustor. It improves cycle efficiency when turbine exhaust is hotter than compressor discharge.

The ratio of actual heat transfer to the maximum thermodynamically possible heat transfer for given inlet conditions and flow rates. It gives performance without needing detailed internal geometry.

The temperature driving force used to size heat exchangers, computed from the terminal temperature differences in a logarithmic average. Counterflow arrangements yield higher values than parallel flow.

A combined conductance per unit area accounting for convection on both sides and conduction through the wall, including fouling. Multiplying it by area and temperature difference gives total heat rate.

The temperature difference divided by heat transfer rate for a component, analogous to electrical resistance. Resistances in series add, so the largest one dominates the overall thermal path.

The ratio of heat transfer from a finned surface to heat transfer from the bare base area it covers. Values below one mean the fin adds more resistance than it removes.

The ratio of internal conduction resistance to external convection resistance for a solid body. Small values justify a lumped-capacitance transient analysis with uniform internal temperature.

A transient cooling or heating model that treats a body as having one uniform temperature at each instant. Valid when internal temperature gradients are negligible, giving exponential temperature decay over time.

For a lumped system, the product of mass, specific heat, and the inverse of the convection conductance-area product. It sets how quickly a body approaches the surrounding fluid temperature.

A dimensionless convection heat transfer coefficient, equal to the convective coefficient times a characteristic length divided by fluid conductivity. It expresses how much convection exceeds pure conduction.

The ratio of momentum diffusivity to thermal diffusivity of a fluid. It indicates how velocity and temperature boundary layers compare and appears in most convection correlations.

The ratio of inertial to viscous forces in a flow, setting whether flow is laminar or turbulent. It is the key parameter for friction factor and convection correlations in ducts and over surfaces.

A dimensionless measure of pipe or duct resistance used in the Darcy-Weisbach pressure drop relation. It depends on Reynolds number and relative roughness, with different forms for laminar and turbulent flow.

A dimensionless factor applied to the velocity head to quantify pressure loss through fittings, valves, bends, and entrances. These losses add to friction losses in piping system analysis.

An energy relation along a streamline for steady, incompressible, frictionless flow linking pressure, velocity, and elevation. It is the basis for flow measurement and many hydraulic calculations.

The energy per unit weight a pump adds to a fluid, expressed in length units. It must overcome static lift, friction, and minor losses, and it sets the operating point on a pump curve.

A plot of required head versus flow rate for a piping network, combining static head with flow-dependent friction losses. Its intersection with the pump curve defines the actual operating point.

Scaling rules for centrifugal pumps and fans: flow varies with speed, head with speed squared, and power with speed cubed at constant impeller diameter. They support speed control and impeller trimming estimates.

The absolute pressure margin above vapor pressure at the pump suction, expressed as available versus required. Falling below the required value causes cavitation, noise, and impeller damage.

Formation and collapse of vapor bubbles in liquid when local pressure drops below vapor pressure. It erodes surfaces, degrades performance, and is avoided by maintaining adequate suction pressure margin.

The mass of fluid passing a cross section per unit time, equal to density times velocity times area. It is conserved in steady flow and anchors energy balances in open systems.

A statement that mass is conserved: for steady incompressible flow, velocity times area stays constant along a stream tube. It explains why area changes alter speed in nozzles and ducts.