Thermodynamics Unit 2

Compressor

A device that increases the pressure of a fluid

turbine

A machine for producing power in which a wheel or rotor is made to revolve by a fast-moving flow of water, steam, gas, or air.

Total Energy

All energy, potential and kinetic, within a specific system. It consists of three parts for a nonflowing fluid and four parts for a flowing fluid.

specific heat ratio

Cp/Cv

The CP of an ideal gas can be determined from a knowledge Cv and R.

Cp= Cv+R

Specific heat is not a property and therefore depends on the process.

False, specific heat is a property independent of process

deltaU= Cavg(T2-T1)

For small temperature intervals

nozzle

Increases the velocity of a fluid at the expense of pressure

Ideal Gas Law

PV=nRT, it describes the behavior of an ideal gas. Some gases can be treated as an ideal gas with negligible error.

engineering convention

Q in - W out = delta E

boundary work

The expansion and compression work associated with gas in a piston cylinder. Area under a PV diagram curve represents the boundary work. Equal in magnitude.

Enthalpy

The heat content of a system at constant pressure

A polytropic process accounts for expansion and compression of a gas within a system.

True - polytropicprocess applies when a system experiences expansion or compression.

Average velocity in pipes

We must take the average velocity across the entire cross section of the pipe because velocity varies from zeros at the walls to some maximum value at or near the centerline of the pipe.

compressibility factor

Z=PV/RT, the farther away Z is from unity the more the gas deviates from ideal gas behavior

incompressible substance

a substance whose specific volume (or density) is constant. Specific heats depend on temperature only.

throttling valves

any kind of flow-restricting devices that cause a significant pressure drop in the fluid

mass in - mass out = change in mass of control volumee

conservation of mass principle

deltaH = deltaU = CavgdeltaT

constant pressure process

deltaH = VdeltaP

constant temperature process

closed system with constant pressure

delta U + Wb = delta H

energy balance/ first law relation

energy in - energy out = change in energy of the system. For a cycle, delta E = 0 and Q = W.

h = u +RT

enthalpy, also a function of temperature

An example of unsteady flowis the filling of a barrel where the total mass within the control volume changes with time, while for steady flowthe mass remains constant within the control volume. Thus, for steady-flow processes the control volume can be analyzed as a closed system.

false

At steady flow and for a single exit and single inlet, the volumetric flow rateat the exit and the inlet must be equal.

false

Flow work is required in open systems to account for the energy in the flowing fluid entering and leaving the CV. In the energy equation, flow work per unit mass is combined with specific internal energy to determine specific enthalpy h = u + Pv, indicating that specific enthalpy only exists for an open system.

false

The specific heat at constant pressure and the specific heat at constant volume are different for an incompressible substance.

false

Diffusers

increases the pressure of a fluid by slowing it down

U

internal energy, a function of temperature, not of pressure or specific volume. (Joule experiment)

polytropic process

is a process in which pressure and volume are often related by PVn= C, where n and C are constants, during expansion and compression processes of real gases. For the special case of n = 1, the boundary work is equivalent to the isothermal process for an ideal gas

conservation of mass principle

is expressed as net mass transfer to or from a control volume during a time interval that is equal to the net change (increase or decrease) in the total mass within the control volume during the time interval.

steady flow devices

operate for long periods of time under the same conditions.

mass flow rate

the amount of mass flowing through a cross section per unit time

Cp

the change in enthalpy of a substance per unit change in temperature at constant pressure

Cv

the change in internal energy of a substance per unit change in temperature at constant volume.

Specific Heat

the energy required to raise the temperature of a unit mass of a substance by one degree. It depends n how the process is executed. We are interested in specific heat at constant volume and specific hear at constant pressure.

conservation of mass

the principle stating that matter is not created or destroyed during a chemical reaction. For closed systems it is implicitly used, but for open systems the mass change must be kept track of.

Steady flow process

the total rate of mass entering a control volume is equal to the total rate of mass leaving it. Control volume does not leave with time

volume flow rate

the volume of a fluid flowing through a cross section per unit time

A control volume (CV), or open system, must account for energy transfer by work, heat, and energy accompanying mass as it enters and exits.

true

Assuming ideal gas behavior, u and h are properties that depend on temperature only.

true

During a steady flow process, volume flow rates are not necessarily conserved although mass flow rates are

true

Expansion against a vacuum involves no work and thus no energy transfer.

true

Flow work, or flow energy, can be considered the work (or energy) necessary to push mass into or out of a control volume.

true

For a closed system undergoing a quasi-equilibrium compression process where the pressure is constant, boundary work and the change in internal energy can be equated to a change in enthalpy

true

For a control volume undergoing a steady-flow process, heat and work interactions between the system and its surroundings do not change with time.

true

For a steady-flow process, the fluid properties at an inlet or exit remain constant (do not change with time). However, the property values may differ between inlets and exits.

true

For an ideal gas undergoing a polytropic process, the exponent (n) having a value equal to one corresponds to an isothermal process.

true

For an ideal gas, the difference between the specific heat at constant pressure (cp) and the specific heat at constant volume (cv) is the gas constant, R

true

In the absence of acceleration, the force applied on a fluid by a piston is equal to the force applied on the piston by the fluid

true

Properties at inlet and outlet of steady flow process remain constant and heat and work interactions do not change with time. Change in energy is zero.

true

The boundary work done during a process depends on the path followed as well as the end states.

true

The conservation of mass principle, applied to a CV, states that the time rate of change of mass within a CV plus the net mass flow through the control surface is equal to zero.

true

The net work done during a cycle is the difference between the work done by the system and the work done on the system.

true

Using the engineering convention, the heat transfer to the system and work done by the system are treated as positive quantities.

true

j/kg = m^2/s^2

true

Constant volume and constant pressure specific heats are identical for incompressible substances.

true, Cv = Cp = C

The boundary work for a gas experiencing a process at constant volume is zero.

true, No change in volume, therefore no movement of boundary, therefore no boundary work.

deltaU = cv*deltaT

valid for any kind of process

flow work

work required to push the mass into or out of the control volume = pV