Flashcards in Thermodynamics & Refrigeration Cycles Deck (100):
What is thermodynamics the study of?
The study of energy, its transformations, and its relation to states of matter.
What are the methods used to calculate thermodynamic properties?
The First and Second law of thermodynamics.
What are two common refrigeration cycle methods of thermal energy transfer?
Compression and Absorption Refrigeration Cycles.
What is a thermodynamic system?
A region in space or a quantity of
matter bounded by a closed surface.
What is the surroundings?
Everything external to the system.
What separates the system from the surroundings?
The system boundary.
Does the system boundary have to be real or imaginary?
The system boundary can be both real or imaginary.
Can the system boundary be movable or fixed?
The system boundary can be either movable or fixed.
What is entropy?
It measures the molecular disorder of the system.
What forms can energy be categorized into when producing an effect?
Stored or transient forms.
What are the types of stored energy?
Thermal (Internal), Potential, Kinetic, Chemical, and Nuclear Energy.
What is internal energy?
Energy caused by the motion of molecules
and/or intermolecular forces.
What is potential energy?
Energy caused by attractive forces existing
between molecules, or the elevation of the system.
What is kinetic energy?
Energy caused by the velocity of molecules.
What is chemical energy?
Energy caused by the arrangement of atoms composing the molecules.
What is nuclear energy?
Derives from the cohesive forces holding
protons and neutrons together as the atom’s nucleus.
What are the types of transient energy?
Heat, Work, Mechanical or Shaft Work, and Flow Work.
What is heat?
The mechanism that transfers energy across the boundaries of systems with differing temperatures.
In what direction does heat always go?
Always toward the lower temperature.
When is heat considered positive?
When energy is added to the system.
What is work?
The mechanism that transfers energy across the boundaries of systems with differing pressures (or force of any kind).
In what direction does work always go?
Always toward the lower pressure.
When is work considered positive?
When energy is removed from the system or the system does work on the surroundings.
What is the shaft work?
The energy delivered or absorbed by a mechanism, such as a turbine, air compressor, or internal combustion engine.
What is flow work?
The work done by the fluid just outside the system on the adjacent fluid entering the system to force or push it into the system.
When does flow work occur?
When fluid is entering or exiting the system.
What is a property of the system?
An observable characteristic of the system.
What is the state of the system?
Specifying the minimum set of independent properties.
What are the most common thermodynamic properties?
Temperature, pressure, specific volume, density.
What are other used thermodynamic properties?
Entropy, enthalpy, and stored forms of energy.
What is enthalpy equal to?
The sum of internal energy and flow work.
What's a process?
A change in state that can be defined as any change in the properties of a system.
How is a process described?
A process is described by specifying the initial and final equilibrium states, the path (if identifiable), and the interactions that take place across system boundaries during the process.
What is a cycle?
A process or a series of processes wherein the initial and final states of the system are identical.
What do the values at the conclusion and at the beginning of a cycle have in common?
At the conclusion of a cycle, all the properties have the same value they had at the beginning.
What is a pure substance?
A homogeneous and invariable chemical composition.
If the phase changes in a pure substance does the composition change?
It can exist in more than one phase, but the chemical composition is the same in all phases.
What is a subcooled liquid?
If the temperature of the liquid is lower than the saturation temperature for the given pressure.
What is a compresses liquid?
The pressure is greater than the saturation pressure for the given temperature.
What is a saturated liquid?
If a substance is liquid at the saturation temperature and pressure.
When does a substance have quality?
When a substance exists as part liquid and part vapor at the saturation temperature.
What is the quality equal to?
The ratio of the mass of vapor to the total mass.
Are pressure and temperature independent properties for saturated substances?
No. Pressure and temperature of saturated substances are not independent properties.
What is a saturated vapor?
If a substance exists as a vapor at saturation temperature and pressure.
What is a superheated vapor?
When the vapor is at a temperature greater than the saturation temperature or the pressure is less than the saturation pressure.
Are the pressure and temperature of a superheated vapor independent properties?
Yes, the pressure and temperature of a superheated vapor are independent properties, because the temperature can increase while pressure remains constant.
What is the first law of thermodynamics?
The Law of Conservation of Energy
What is the Law of Conservation of Energy?
The difference between the energy going into a system and energy going out of the system is equal to the energy gained or lost by the system.
What properties are considered for the energy going into a system?
The internal energy, flow work, kinetic energy and potential energy.
What properties are considered for the energy already stored in the system?
The internal energy, kinetic energy, ad potential energy.
For the general case of multiple mass flows with uniform properties in and out of the system, the conservation law is equal to?
The difference between sum of the mass flows going into the system to the sum of the mass flows going out of the system plus the heat rate minus the net work is equal to the energy gained or lost by the system or the difference between the final and initial energy states of the system.
What is a steady flow process?
Steady flow signifies that all quantities associated with the system do not vary with time.
What does the first law become for steady flow processes?
The energy entering the system is the energy leaving the system.
What does the first law become when the system is closed?
The heat rate minus the net work is equal to the difference between the final and initial energy states of the system.
What is the second law of thermodynamics?
The second law of thermodynamics differentiates and quantifies processes that only proceed in a certain direction (irreversible) from those that are reversible.
What is an irreversibility?
The energy lost due to factors in the system when going from one state to another.
How is cycle performance improved?
Reducing total irreversibility in a cycle improves cycle performance.
When does a cycle obtain maximum efficiency?
When there is no irreversibilities.
What is the second law of thermodynamics equal to for an open system?
The change in entropy of the system during the process is equal to the mass entering the system minus the mass leaving the system entropy plus the change caused by reversible heat transfer between
system and surroundings at the given temperature, plus entropy caused by irreversibilities which is always positive.
What is the entropy equal to if the process can be considered to operate steadily with no change in time?
The change in entropy of the system is zero.
What is the irreversibility rate?
The rate of entropy production caused by irreversibilities in the process.
What happens when the temperature of the surroundings is equal to the system temperature. When calculating the irreversibilities?
The entropy caused by the reversible heat transfer between system and surroundings at the given temperature becomes zero.
What is the work of irreversibilities equal to?
The absolute ambient temperature multiplied by the sum of irreversibilities in all processes in the cycle.
What does work of irreversibilities cause in a power cycle and what is needed counteract this work?
A reduction of work produced by the power cycle (work output) is caused and more resources are needed to meet the required work produced by the power cycle.
What does work of irreversibilities cause in a refrigeration cycle and what is needed to counteract this work?
A reduction in work input is produced and more work input is needed to reduce the temperature of the refrigerant.
In terms of work, what is the irreversibility rate equal to?
The difference between the actual and reversible work divided by the temperature.
What do refrigeration cycles do?
Refrigeration cycles transfer thermal energy from a region of low temperature to one of higher temperature.
What is the coefficient of performance?
The performance of a refrigeration cycle.
What is the coefficient of performance equal to?
Amount of heat removed divided by the net energy supplied by external sources.
For a mechanical vapor compression cycle in what form of work is the net energy supplied?
The net energy supplied is usually in the form of mechanical or electrical work, and may include work to the compressor and fans or pumps.
For an absorption refrigeration cycle in what form of work is the net energy supplied?
In an absorption refrigeration cycle, the net energy supplied is usually in the form of heat into the generator and work into the pumps and fans.
Can the work supplied to an absorption refrigeration cycle be neglected? Why?
In many cases, work supplied to an absorption system is very small compared to the amount of heat supplied to the generator, so the work term is often neglected.
What is the coefficient of performance of a refrigerator that has a completely reversible cycle?
Has the maximum possible C.O.P a refrigeration cycle would have operating under the same conditions as one that is not completely reversible.
What is the refrigeration efficiency?
The ratio of the cop of an actual cycle to the cop of an ideal or completely reversible cycle.
What does thermodynamic analysis require?
Usually requires values for internal energy,
enthalpy, and entropy.
What are intensive properties?
Properties independent of the quantity of the substance.
What is the Clapeyron equation used for?
If vapor pressure and liquid and vapor density data are known at saturation, then changes in enthalpy and entropy can be found.
Can the Clapeyron equation be used for phase changes?
Yes it can be used for evaporation or condensation.
What is a volatile liquid?
A liquid that evaporates easily.
Do volatile liquids have higher vapor pressure?
What is the bubble point?
The point when a pure liquid mixture begins to bubble (evaporate) and reaches the bubble point curve.
What is the bubble point curve?
A curve that provides bubble points for various liquid mole fractions.
What happens to the mole fraction in the liquid as it keeps vaporizing?
The mole fraction the liquid decreases.
How is the temperature effected in the evaporating process?
The temperature increases as the mole fraction of the liquid decreases.
What is the mole fraction of the vapor after all the liquid mixture has been evaporated?
The mole fraction in the vapor equals the starting mole fraction in the all-liquid mixture.
What is the dew point?
The point when a pure vapor begins to condense and reaches the dew point curve.
What is the dew point curve?
A curve that provides condensation points for various vapor mole fractions.
What is a temperature glide?
When the composition shifts, the temperature required to boil (or condense) this mixture changes as the process proceeds.
What is a zeotropic mixture?
A mixture that experiences a temperature glide.
What is a azeotropic mixture?
If the dew-point and bubble-point curves intersect at any point other than at their ends, the mixture exhibits azeotropic behavior at that composition.
If a container of liquid with a mole fraction at the azeotropic composition were boiled. What are the mole fractions of the liquid and vapor equal to and what happens to the composition of the vapor liquid mixture?
Vapor would be formed with an identical mole fraction as that of the liquid. The addition of heat at constant pressure would continue with no shift in composition and no temperature glide.
What is a temperature concentration diagram?
A diagram that show the relationship between the temperature and mole fraction of a liquid and vapor mixture.
What is the Carnot Cycle?
It is a completely reversible cycle, that is a perfect
model for a refrigeration cycle operating between two fixed temperatures, or between two fluids at different temperatures and each with infinite heat capacity.
What are the two important properties for reversible cycles?
(1) no refrigerating cycle may have a coefficient of performance higher than that for a reversible cycle operated between the same temperature limits, and
(2) all reversible cycles, when operated between the same temperature limits, have the same coefficient of performance.
What is the process for the Carnot refrigeration cycle?
(1-2) Isentropic compression.
(2-3) Heat is rejected at constant ambient temperature.
(3-4) Isentropic Expansion
(4-1) Heat is withdrawn at constant temperature from the region to be refrigerated.
What is the net change of entropy of any refrigerant in any cycle?
It is always zero.
How can a pure refrigerant or azeotropic mixture be used to maintain constant temperature during phase changes?
By maintaining constant pressure.
What process do a refrigerant go through in a expansion valve?
A throttling process.
What is a throttling process?
Its an adiabatic process where enthalpy stays constant there is no change in potential or kinetic energy through the expansion valve but there is a pressure drop.