MEEN 315 Final - True/False and Multiple Choice Question Bank

COP

(Qh/W) = (Qh/Qh-Ql) = (Th/Th-Tl) = (Win+Ql/Win) = (1+(Ql/Win))

Second Law of Thermodynamics:

- Determines a way to find u or h - Defines Kelvin Temperature Scale - Finds maximum efficiency of a process - Defines entropy - Determines direction for spontaneous process - Establishment of the conditions for equilibrium

Irreversibilies

- Heat Transfer over finite deltaT - Mixing of unlike components - Unrestrained expansion - Friction - Chemical reaction - Electric current flowing through resistance - Inelastic deformation - Magnetization or polarization with hystersis

Carnot Efficiency

1 - (Tl/Th)

Water as a saturated liquid with an initial temperature 40C exchanges thermal energy with the surroundings during a reversible process, until the mater reaches the saturated vapor state. What is the temperature of the surroundings

40C (to do reversible Heat Transfer)

Entropy is:

An absolute measurement of the disorder in molecular structure

A steady state (steady flow) condition exists in a system if:

At any position within the system, properties do not vary with time

T for an incompressible substance, how are the specific constant volumes Cv and Cp related?

Cp=Cv

Nozzles

Cross Sectional Area Decreases: P1 > P2 Purpose: Decrease Pressure and increase kinetic energy

Diffuser

Cross Sectional Area Increases: P2 > P1 Purpose: Increase the pressure of a fluid while slowing it down

Pressure and temperature are _________ in the saturation region, and as a result, another property called ________ can be used to define the state of the system in this region.

Dependent, quality

Extensive properties

Enthalpy Mass Volume Total Internal Energy

If a process is isentropic, then it is must be reversible and adiabatic

False

If a system's temperature has increased, it must have experienced heat transfer.

False

For a simple compressible substance, specific heat at constant volume is larger than specific heat at constant pressure.

False, Cp=Cv+R

The energy of an isolated system can only increase.

False, it can't change at all

The mass of a closed system can change, but its volume cannot change.

False, mass is constant, volume can change.

A process that violates the Second Law of thermodynamics also violates the First Law

False.

For a control volume at steady state, mass can accumulate in the control volume.

False.

If a closed system, containing a simple, compressible substance, is at equilibrium at some T and P, there can be only one phase present.

False.

The pressures listed in the thermodynamic tables are gage pressures.

False.

The volume of a closed system cannot change.

False.

A process for a closed system that violates the second law of thermodynamics necessarily violates the first law of thermodynamics.

False. A process can violate one without violating the other.

For heat pumps, the coefficient of performance is always greater than or equal to 1.

False. Could have experienced work transfer or transformation from KE or PE

The State Principle (State Postulate) reads: Two independent extensive properties are needed to completely specify the state of a simple, incompressible system.

False. Intensive and compressible.

Kinetic and potential energy cannot be negative

False. PE can be. KE cannot.

The Second Law is a statement of the conservation of entropy.

False. S is not conserved.

The change in the entropy of a system cannot be negative.

False. Stot = Ssys + Ssurr >= 0

Work and Heat are properties.

False. They are energy transfer mechanisms

A device that operates on a cycle and has net positive work and net positive heat transfer is called a heat pump.

False. This description is for a heat engine.

A significant increase in pressure can be achieved by introducing a restriction into a line through which a gas or liquid flows.

False. Throttles/valves decrease P

When mass crosses the boundary of a system, the accompanying energy transfer is accounted for solely by the internal energy carried in by the mass.

False; h=u+Pv

The work (or energy) needed to push mass into and out of a control volume is known as

Flow Work

In the definition of enthalpy, for an open system, the Pv term represents:

Flow work

For a quasi-equilibrium process the pressure in a system:

Has the same instantaneous value everywhere in the system

The term (m_dot*deltah) in a control volume equation:

Includes the rate of work due to pressure forces

If one property can be varied while another is held constant, the properties are:

Independent

Which thermodynamic property is the best measure of the molecular activity of a substance?

Internal energy

The Carnot cycle is of interest because:

It establishes the upper limit of a heat engine's efficiency

If a substance is in the vapor dome, which of the following is true?

Its pressure is equal to the saturation pressure

Compressed or subcooled liquid properties for a liquid at T and P might be approximately determined at the saturation values for the

Liquid at the same T

Primary Dimensions

Mass (kg, lbm) Length (m, foot) Time (s) Temperature (C/K, F/R)

An appropriate interpretation of the Kelvin-Planck Statement of the Second Law is:

No heat engine can transfer work without rejecting heat.

An appropriate interpretation of the Clausius Statement of the Second Law is:

No heat pump can transfer more heat than the work it accepts.

The principle difference between a turbine and compressor is:

One transfers work into the surroundings and the other transfer work from the surroundings

Pressure Equation

Pabs = Patm + Pgage Patm = 14.7 psi = 1 bar = 100 kPa

Manometer Equation

Ptank = Pa + (density*gravity*height)

Steady-Flow devices does not result in a drop in the pressure of the working fluid from the inlet to exit

Pump Diffuser

For a subcooled liquid or mildly compressed liquid, where do we look to find the properties?

SCL = CL = "f" properties at the same T, from the saturation tables

The energy required to raise the temperature of a unit mass of a substance by one degree is:

Specific heat

Which assumption underlies the continuity equation p1A1v1 = p2A2v2

Steady Flow

Intensive Properties

Temperature Specific volume Pressure Specific enthalpy Density Specific Internal Energy

Which of the following is NOT true regarding the Carnot cycle:

The coefficient of performance of a Carnot heat pump cannot exceed one

What is not correct for an ideal gas?

The pressure increase as the temperature increases.

A heat pump is:

The same as a refrigerator, with a different objective

System A undergoes a process between two fixed states reversibly and System B undergoes a process between the same two states irreversibly. The entropy change of the two systems is:

The same, because entropy is a property and the states are fixed (the same) for both processes.

Mass flow for a 1-D system depends on all of the following: except:

Total volume of the working fluid. (mass flow = density*cross sectional area*velocity)

A device that operates on a cycle and has net positive work and net positive heat transfer is called a heat engine:

True

A mixing chamber is a direct-contact heat exchanger.

True

A mixing chamber is a form of heat exchanger, where the fluids have direct contact

True

Air, through a mixture of oxygen and nitrogen, can be considered to be a pure substance

True

At the critical point, the internal energy of the saturated liquid is the same as the internal energy of the saturated vapor.

True

For any cycle, the net amounts of energy transfer by heat and work are equal

True

If a process is reversible and adiabatic, it is necessarily isentropic.

True

If a substance is in the vapor dome region, its temperature MUST be equal to the saturation temperature AND its pressure MUST be equal to the saturation pressure.

True

Pressure is an intensive property.

True

The First Law says that the net energy crossing the system boundary is equal to the change in energy inside the system.

True

The volume of a closed system can change, but its mass cannot change.

True

Work is required for thermal energy to be transferred from a low temperature body to a high temperature body

True

For a simple compressible substance, specific heat at constant pressure is larger than specific heat at constant volume.

True, Cp=Cv+R

All spontaneous processes are irreversible

True.

If a closed system undergoes a process for which the work is negative and the heat transfer is positive, the change in total energy must be positive.

True.

For a compressor, the enthalpy at the inlet is always smaller than the enthalpy at the exit.

True. -W = m(h2-h1) hout > hin

Entropy is always generated within a system when there is irreversibility present in the system.

True. Irreversibilities generate entropy.

Gage pressure indicated the difference between the absolute pressure of a system and the absolute pressure of the atmosphere.

True. Pabs = Patm + Pg

If a saturated liquid undergoes a reversible, adiabatic expansion to lower pressure, some of the liquids will vaporize

True. T and P have opposite trends

An ideal Rankine cycle consists of:

Two constant pressure and two isentropic processes

Air undergoes a three-process cycle in a piston/cylinder device, with a constant pressure process, a constant temperature process and a constant volume process.

W=0 for the constant V process

Assuming air is an ideal gas, its changes in internal energy and enthalpy

are functions of temperature only

Boundary Work is:

associated with a moving surface of a system

The work associated with a piston/cylinder arrangement is

boundary work

Convective heat transfer involves:

conduction and fluid motion

The point that connects the saturate liquid line to the saturated vapor line is called the:

critical point

For a steady state, steady flow process:

dEcv/dt=0 and dmcv/dt=0

For a fluid flowing through a constant cross-sectional area pipe under steady flow:

density*velocity - constant

A reversible, isothermal heat addition process:

does not generate entropy

A quasiequilibrium process is one in which all the states through which the system passes are very close to:

equilibrium states

A substance in a closed system, which is subject to boundary work, only undergoes a constant pressure process from state 1 to state 2. The heat transfer (q) equals"

h2 - h1; q = du +Pdv = dh

Flow through an idealized throttling device has:

h2=h1 and P2 < P1

Flow through an idealized valve has:

h2=h1 and P2 < P1

A system undergoing an adiabatic process

has no thermal energy interactions with its surroundings.

A process where, at any moment in time, a system's properties are homogeneous

is also called a equilibrium process

A real device

must satisfy both the First and Second Laws

An isentropic process

results in no change in the entropy of the substance

Energy can be exchanged between the system and the surroundings

through heat, work, and mass transfer

A closed thermodynamic system can not

transfer mass with its surroundings