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MET 220 > Exam 2 > Flashcards

Exam 2 Flashcards

1
Q

The equation (velocity*cross sectional area)/specific volume is used to compute?

A

mass flow

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2
Q

which of the following is NOT a valid unit for measuring pressure:

a. ) psi
b. ) mm Hg
c. ) kPa
d. ) in H2O
e. ) none of these

A

none of these

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3
Q

the SI unit for specific heat is

A

kJ/(kg-K)

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4
Q

an English unit for power is

A

Btu/hr

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5
Q

(TURE OR FALSE) a cycle is part of a process

A

false

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6
Q

(TURE OR FALSE) an isentropic process is adiabatic

A

false

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7
Q

what is the difference between mechanical efficiency (nm) and thermal efficiency(nt)?

A

nm applies to processes and nt applies to cycles

nm applies to first law of thermodynamics and nt applies to 2nd law or thermodynamics

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8
Q

what is the functional difference between a pump and a compressor?

a. ) no difference, same as 1st law equation for both devices
b. ) pump handles gases, compressor handles liquids
c. ) pump handles small volumetric flow, compressor handles large volumetric flow
d. ) pump handles large mass flow, compressor handles small mass flow
e. ) none of these

A

none of these

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9
Q

the term isobaric means

A

constant pressure

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10
Q

use the first law of thermodynamics to derive a simplified equation for a throttle that explains its performance based on inlet (1) and outlet (2) conditions.

A

0 = h2 - h1

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11
Q

use the first law of thermodynamics to derive a simplified equation for a nozzle that explains its performance based on inlet (1) and outlet (2) conditions

A

0 = 1/2(v2^2 - v1^2) + (h2 - h1)

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12
Q

an engine produces 8.0 kW of power with a thermal efficiency of 20%. What is the rate of heat rejection for this cycle?

A 
nth = W/Qh
0.2 = 8/Qh
Qh = 40
Ql/Qh = 0.8
Ql/40 = 0.8
Ql = 0.8 (40)
Ql = 32
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13
Q

Even though the Carnot Cycle has never been built, it is still useful because it

A

defines peak performance.

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14
Q

A Carnot heat pump supplies warm air to a building at 100 degrees F. What is the coefficient of Performance when to cold outside air is -10 degrees F?

A 
COP = Th/(Th-Tl)
COP = 560/(100-(-10))
COP = 560/110
COP = 5.1
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15
Q

A turbine for an ideal Rankine Cycle power plant decreases the pressure of the water but does not change its

A

entropy

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16
Q

what are the English units for “k” in the conduction equation Q = -k*A(delta T/delta x)

A

Btu - in/(hr - ft^2 - degrees F)

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17
Q

what new design information do heat transfer equations provide Thermodynamics equations?

a. ) temperature
b. ) pressure
c. ) flow
d. ) solid, liquid, gas
e. ) none of these

A

none of these, area and size

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18
Q

fill in the blank: ___ & ___ are used to estimate Nusselt #, which is then used to estimate the convection heat transfer coefficient

A

Reynolds # & Prandtl #

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19
Q

An empirical equation is?

A

determined be experiments

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20
Q

The inverse of specific volume (1/v) is?

A

density

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21
Q

The magnehelic pressure gage used in the hair dryer experiment measured velocity pressure in units of?

A

fpm

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22
Q

the flow of air in a duct would most likely be expressed in

A

cfm

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23
Q

the power supplied to an electric motor would likely be expressed in

A

kW

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24
Q

which of the following is energy (not power)?

a. ) hp
b. ) Btu/hr
c. ) therm
d. ) kW
e. ) none of these

A

therm

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25
Q

The English unit for specific heat (c) is?

A

Btu/(lbm-R)

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26
Q

mcdelta T has units of

A

energy

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27
Q

Enthalpy is a derived property that combines

A

internal and flow energy

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28
Q

(TRUE OR FALSE) hfg quantifies latent energy

A

true

29
Q

(TRUE OR FALSE) hf > hg for a fluid

A

false

30
Q

An open system has

A

mass flow

31
Q

sensible energy is “heat transfer by virtue of ____ change”

A

temperature

32
Q

A 1st law analysis of a diffuser shows that it converts

A

pressure to kinetic energy

33
Q

The term isentropic means

a. ) constant volume
b. ) constant pressure
c. ) constant temperature
d. ) constant mass
e. ) none of these

A

none of these

34
Q

use the first law of thermodynamics to derive a simplified equation for a turbine that explains its performance based on inlet (1) and outlet (2) conditions

A

w=h2-h1

35
Q

what are two modes of operation for a heat pump?

A

heating or work

36
Q

An engine uses 80 kW of heat to deliver 20 kW of useful work. How much heat is rejected to surroundings?

A

60

37
Q

The ____ Cycle represents perfection in terms of heat engine performance.

A

Carnot

38
Q

A Carnot heat pump supplies warm air to a building at 100 degrees F. What is the Coefficient of Performance when the cold outside air is 20 degrees F?

A

7

39
Q

How much electricity is needed to operate a heat pump with a COP of 3 that provides 3kW of useful cooling?

A

1kW

40
Q

A boiler for an ideal Rankine Cycle power plant increases the temperature of the water but does not change its ____.

A

pressure

41
Q

The energy transfer to water in the boiler for a power plant is?

A

sensible and latent

42
Q

Which device in a power plant transfers the least energy?

a. ) pump
b. ) turbine
c. ) boiler
d. ) condenser
e. ) none of these

A

pump

43
Q

Air removing heat form a light bulb is an example of ____ heat transfer

A

convection

44
Q

Heat moving through a solid object is an example of?

A

conduction

45
Q

Thermal efficiency of a heat engine

A 
nth = output/input
nth = Wnet/Qh
nth = 1-Ql/Qh
46
Q

Coefficient of Performance

A 
COP = output/input
COP = Qh/(Qh-Ql)
47
Q

Carnot Heat engine

A 
nth = output/input
nth = Wnet/Qh
nth = 1-Tl/Th
48
Q

What is the difference between mechanical and thermal efficiency?

A

Thermal: amount of work that can be obtained
Mechanical: how efficiently the engine converts from one form of energy to another

49
Q

What is 2nd Law efficiency?

A

the efficiency of a process taking into account the 2nd law of thermodynamics

50
Q

What is entropy?

A

property based on heat transfer and temperature
defined in terms of dS = delta*Q/T
also quantifies magnitude of irreversibility in a process

51
Q

causes of entropy change

A

heat transfer, irreversibilities, mass transfer

52
Q

Rankine cycle

A

the ideal steam power plant cycle consisting of 4 processes: boiler, turbine, condenser, and feedwater pump

53
Q

Thermodynamics vs, Heat Transfer

A

how much vs. how big

54
Q

types of heat transfer

A

conduction, convection, radiation

55
Q

conduction

A

heat transfer due to particle interaction with a substance or between two similar substances
Qcond = -kA(delta T/delta x)

56
Q

convection

A

heat transfer between a surface and a moving fluid

Qconv= hA(Ts-Tinfinity)

57
Q

radiation

A

transfer by electromagnetic wave

Qrad = epsilonsigmaA(Ts^4-Tsurr^4)

58
Q

heat transfer coefficient “h”

A

measure of capability for heat flow from surface to fluid
as h increases, Q increases
varies with type of fluid, flow conditions and surface properties

59
Q

Which line on the T-S graph shows a process where superheated steam enters a condenser at point 1 and exits as a saturate mixture at point 2? (see notecard 1)

A

c

60
Q

which line on the T-S graph shows a process where a saturated vapor enters a condenser at point 1 and exits as a subcooled liquid at point 2? (see notecard 1)

A

b

61
Q

(TRUE OR FALSE) from looking at the heat engine sketch, W = Qh-Ql. (see notecard 2)

A

true

62
Q

which of the sketches correctly summarizes a heat engine? (see notecard 3)

A

c

63
Q

which of the sketches correctly summarizes a heat pump? (see notecard 3)

A

d

64
Q

Which of lines on the T-S graph represent an isobaric process? (see notecard 4)

A

B and D

65
Q

Which of the lines on the T-S graph represent the turbine in a power plant? (see notecard 4)

A

c

66
Q

Name pump, boiler, turbine and condenser in the diagram. (see notecard 4)

A

a: pump
b: condenser
c: turbine
d: boiler

67
Q

isobaric

A

T does not change, entropy does

68
Q

power plant schematic

A

(see notecard 5)

69
Q

name each of the 3 states (see notecard 6)

A

1: compressed liquid
2: saturated liquid-vapor mixture
3: superheated vapor

MET 220 (2 decks)

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