Ting Wang Thermo 1 Final Review

UNO Ting Wang ENME 2770: Engineering Thermodynamics 1

1 Pascal = __ N/m²

1 N/m²

1 bar = __ Pa

10^5 Pa

1 atm = __ psi ~ __ bars

14.7 psi

1.013 bars

1 torr = __ mmHg

1 mmHg

When pressure decreases, the water’s freezing temperature:

a) increases

b) decreases

c) does not change

a

When the pressure increases, the water vapor’s condensing temperature:

a) increases

b) decreases

c) does not change

a

When the pressure is reduced below the saturated pressure at a given temperature, the liquid will:

a) boil

b) flash

c) does not change

b

Which equations are not correct for an ideal gas?

a) PV = mRT

b) Pv = nRT

c) PV = nRT

d) PV = n(universal R)T

c) pv = nRT

b and c

In the P-T (phase) diagram, what is the major difference between a substance contracting when freezing and another expanding on freezing. Show these two diagrams.

The curve between liquid and solid has a negative slope for a substance expanding during freezing and a positive slope for a substance contracting during freezing.

Define quality for a liquid-vapor mixture.

Quality is the percentage of vapor in a 2-phase mixture.

Write an equation to show how to use the property values of saturated liquid and saturated vapor to find internal energy (u) of liquid-vapor mixture with a quality x.

u = u_f + x * (u_g -u_f)

Why would a liquid on the left side of a vapor dome be called a “compressed” liquid?

The actual pressure is greater than the saturated pressure

Why would a liquid on the left side of a vapor dome be called a “subcooled” liquid?

The actual temperature is less than the saturated temperature

What are the 4 polytropic processes? And what are their n values?

1. isobaric: n=0 → P=C

2. isothermal: n=1 → T=C

3. isometric: n=infinity → V=C

4. isentropic: n=k → S=C, adiabatic, reversible

What is the function of a generalized compressibility factor (Z) chart?

to show deviation of areal gas from the ideal gas via the compressibility factor Z

Write the equations for Z, P_R, and T_R from the compressibility chart

Z = (RT) / (PV)

P_R = P / P_C

T_R = T / T_C

After a subsonic flow goes through a diffuser, the pressure:

a) increases

b) decreases

c) does not change

a

True/False

The Clausius statement says that it is impossible to transfer energy from a cooler body to a hotter body.

False

True/False

The perpetual motion machine of the third kind does not violate either the first or second laws of thermodynamics.

True

An inventor claims his invention delivers a thermal efficiency of 100%. This claim:

a) violates the 1st law

b) violates the 2nd law

c) violates both the 1st and 2nd laws

d) doesn’t violate either law

b

The black serpentine tube behind the refrigerator functions as a(n):

a) evaporator

b) condenser

c) compressor

d) expander

b

The indoor heat exchanger of a heat pump system functions as a(n):

a) evaporator

b) condenser

c) compressor

d) expander

b

The indoor heat exchanger of an air conditioning system functions as a(n):

a) evaporator

b) condenser

c) compressor

d) expander

a

Indicate changes (increase, decrease, or remains the same) of the following parameters after a supersonic flow goes through an adiabatic divergent duct:

velocity

Pressure

mass flow rate

total enthalpy

velocity: increases

Pressure: decreases

mass flow rate: does not change

total enthalpy: does not change

List the conditions that lead to the Bernoulli equation from the energy conservation equation.

1. no work

2. no heat transfer

3. no temperature change

4. steady state

What are the 7 values (or aspects, features) of the thermodynamic 2nd law.

1. determine whether any process is possible or not

2. predict which direction a spontaneous process will go

3. determine the theoretical max value for the: work that could be obtained, efficiency of a heat engine, COP of a refrigerator or heat pump

4. evaluate quantitatively the factors that preclude attaining the max coefficient

5. determine a thermodynamic temperature scale that is independent of physical properties, leading to Q_H/Q_C = T_H/T_C

6. correlate physical properties

7. quantify the grade of energy using the concept of entropy

What is the Kelvin-Planck 2nd law of thermodynamics?

It is impossible for any device operating in a cycle to absorb heat from a single reservoir and produce an equivalent amount of work.

efficiency =/= 100%

What are the two main purposes for employing a diffuser and a nozzle respectively?

Diffuser - increase pressure, decrease velocity

Nozzle - decrease pressure, increase velocity

Provide two examples of applications for a diffuser and a nozzle.

Diffuser:

1. Infront of a jet engine to convert high-velocity air stream to high pressure

2. Iin the back of a land-based gas turbine to recover exhaust gas kinetic energy as increased pressure

Nozzle:

1. Behind a jet engine to produce thrust

2. to produce low pressure suction to suck water as in a jet pump

Explain the function of a throttling process.

allows us to reduce temperature more than you would normally be able to

Give an example of a device that uses a throttling process.

Deep freezers → like for dry ice

What is the wet-vapor Carnot refrigeration/Heat pump cycle? What are the COP equations?

1→ 2: Compressor: isentropic (Q=0, W_in)

2→ 3: Condenser: isothermal/isobaric (W=0, Q_H,out)

3→ 4: Turbine: isentropic (Q=0, W_out)

4→ 1: evaporator: isothermal/isobaric (W=0, Q_C,in)

COP (ref) = Q_C / (Q_H - Q_C) = T_C / (T_H - T_C)

COP (heat) = Q_H / (Q_H - Q_C) = T_H / (T_H - T_C)

What is the gas Carnot power cycle? What is the efficiency equation?

1→ 2: isentropic compression (Q=0, W_in)

2→ 3: isothermal expansion (Q_in, W_out)

3→ 4: isentropic expansion (Q=0, W_out)

4→ 1: isothermal compression (Q_out, W_in)

Eff = 1 - (Q_out / Q_in) = 1 - (T_C / T_H)

What is the wet-vapor Carnot power cycle? What is the efficiency equation?

1→ 2: Compressor; isentropic (Q=0, W_in)

2→ 3: Boiler; isothermal/isobaric (W=0, Q_in)

3→ 4: Turbine; isentropic (Q=0, W_out)

4→ 1: Condenser; isothermal/isobaric (W=0, Q_out)

Eff = 1 - (Q_out / Q_in) = 1 - (T_C / T_H)

What is the gas refrigerant/heat pump Carnot power cycle? What are the COP equations?

1→ 2: isentropic compression (Q=0, W_in)

2→ 3: isothermal compression (Q_H,out, W_out)

3→ 4: isentropic expansion (Q=0, W_out)

4→ 1: isothermal expansion (Q_C,in, W_in)

COP (ref) = Q_C / (Q_H - Q_C) = T_C / (T_H - T_C)

COP (heat) = Q_H / (Q_H - Q_C) = T_H / (T_H - T_C)

How does a vortex tube work?

Pressurized gas is injected tangentially into a swirl chamber and accelerated to a high rate of rotation. The gas then travels down a tube in a vortex. Due to a conical nozzle at the end of the tube, only the outer most layer of compressed gas can escape. The rest of the gas is forced to return in an inner vortex.

What is relative pressure? What is it useful for?

the ratio of pressure to some reference condition, it can be found in the steam tables

it makes finding property values (pressure) much easier and faster

P2 / P1 = (P_R @ T_2) / (P_R @ T_1)

What is availability (Exergy)?

a measure of departure of the state of the system from that of the environment

maximum available energy to the system

What is Dead state?

When the system becomes equalized with the environment

What is irreversibility?

I = W_max - W_act = T_o (entropy production)

What is exergy destruction and losses?

represents lost work potential, is also called irreversibility or lost work.

(+) for any irreversible process

(0) for any reversible process

What does (1 - T_o / T_b) represent?

This represents the amount of available energy when multiplied by Q_j.

(T_o / T_b) is a ratio of unavailable energy to total energy

What is the difference between 2nd law efficiency and 1st law efficiency?

2nd law efficiency unavailable energy into consideration whereas 1st law efficiency does not.

What does each term mean in the below equation?

delta s = s(T_2) - s(T_1) - R ln(P_2 / P_1)

• delta s → change in entropy

• s(T_2) - s(T_1) → entropy difference at the same pressure (1 atm)

• R ln(P_2 / P_1) → pressure correction

The only processes that can occur are those for which the entropy of an isolated system _____.

increases