Exam 2 Flashcards
The equation (velocity*cross sectional area)/specific volume is used to compute?
mass flow
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
none of these
the SI unit for specific heat is
kJ/(kg-K)
an English unit for power is
Btu/hr
(TURE OR FALSE) a cycle is part of a process
false
(TURE OR FALSE) an isentropic process is adiabatic
false
what is the difference between mechanical efficiency (nm) and thermal efficiency(nt)?
nm applies to processes and nt applies to cycles
nm applies to first law of thermodynamics and nt applies to 2nd law or thermodynamics
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
none of these
the term isobaric means
constant pressure
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.
0 = h2 - h1
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
0 = 1/2(v2^2 - v1^2) + (h2 - h1)
an engine produces 8.0 kW of power with a thermal efficiency of 20%. What is the rate of heat rejection for this cycle?
nth = W/Qh 0.2 = 8/Qh Qh = 40 Ql/Qh = 0.8 Ql/40 = 0.8 Ql = 0.8 (40) Ql = 32
Even though the Carnot Cycle has never been built, it is still useful because it
defines peak performance.
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?
COP = Th/(Th-Tl) COP = 560/(100-(-10)) COP = 560/110 COP = 5.1
A turbine for an ideal Rankine Cycle power plant decreases the pressure of the water but does not change its
entropy
what are the English units for “k” in the conduction equation Q = -k*A(delta T/delta x)
Btu - in/(hr - ft^2 - degrees F)
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
none of these, area and size
fill in the blank: ___ & ___ are used to estimate Nusselt #, which is then used to estimate the convection heat transfer coefficient
Reynolds # & Prandtl #
An empirical equation is?
determined be experiments
The inverse of specific volume (1/v) is?
density
The magnehelic pressure gage used in the hair dryer experiment measured velocity pressure in units of?
fpm
the flow of air in a duct would most likely be expressed in
cfm
the power supplied to an electric motor would likely be expressed in
kW
which of the following is energy (not power)?
a. ) hp
b. ) Btu/hr
c. ) therm
d. ) kW
e. ) none of these
therm
The English unit for specific heat (c) is?
Btu/(lbm-R)
mcdelta T has units of
energy
Enthalpy is a derived property that combines
internal and flow energy
(TRUE OR FALSE) hfg quantifies latent energy
true
(TRUE OR FALSE) hf > hg for a fluid
false
An open system has
mass flow
sensible energy is “heat transfer by virtue of ____ change”
temperature
A 1st law analysis of a diffuser shows that it converts
pressure to kinetic energy
The term isentropic means
a. ) constant volume
b. ) constant pressure
c. ) constant temperature
d. ) constant mass
e. ) none of these
none of these
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
w=h2-h1
what are two modes of operation for a heat pump?
heating or work
An engine uses 80 kW of heat to deliver 20 kW of useful work. How much heat is rejected to surroundings?
60
The ____ Cycle represents perfection in terms of heat engine performance.
Carnot
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?
7
How much electricity is needed to operate a heat pump with a COP of 3 that provides 3kW of useful cooling?
1kW
A boiler for an ideal Rankine Cycle power plant increases the temperature of the water but does not change its ____.
pressure
The energy transfer to water in the boiler for a power plant is?
sensible and latent
Which device in a power plant transfers the least energy?
a. ) pump
b. ) turbine
c. ) boiler
d. ) condenser
e. ) none of these
pump
Air removing heat form a light bulb is an example of ____ heat transfer
convection
Heat moving through a solid object is an example of?
conduction
Thermal efficiency of a heat engine
nth = output/input nth = Wnet/Qh nth = 1-Ql/Qh
Coefficient of Performance
COP = output/input COP = Qh/(Qh-Ql)
Carnot Heat engine
nth = output/input nth = Wnet/Qh nth = 1-Tl/Th
What is the difference between mechanical and thermal efficiency?
Thermal: amount of work that can be obtained
Mechanical: how efficiently the engine converts from one form of energy to another
What is 2nd Law efficiency?
the efficiency of a process taking into account the 2nd law of thermodynamics
What is entropy?
property based on heat transfer and temperature
defined in terms of dS = delta*Q/T
also quantifies magnitude of irreversibility in a process
causes of entropy change
heat transfer, irreversibilities, mass transfer
Rankine cycle
the ideal steam power plant cycle consisting of 4 processes: boiler, turbine, condenser, and feedwater pump
Thermodynamics vs, Heat Transfer
how much vs. how big
types of heat transfer
conduction, convection, radiation
conduction
heat transfer due to particle interaction with a substance or between two similar substances
Qcond = -kA(delta T/delta x)
convection
heat transfer between a surface and a moving fluid
Qconv= hA(Ts-Tinfinity)
radiation
transfer by electromagnetic wave
Qrad = epsilonsigmaA(Ts^4-Tsurr^4)
heat transfer coefficient “h”
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
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)
c
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)
b
(TRUE OR FALSE) from looking at the heat engine sketch, W = Qh-Ql. (see notecard 2)
true
which of the sketches correctly summarizes a heat engine? (see notecard 3)
c
which of the sketches correctly summarizes a heat pump? (see notecard 3)
d
Which of lines on the T-S graph represent an isobaric process? (see notecard 4)
B and D
Which of the lines on the T-S graph represent the turbine in a power plant? (see notecard 4)
c
Name pump, boiler, turbine and condenser in the diagram. (see notecard 4)
a: pump
b: condenser
c: turbine
d: boiler
isobaric
T does not change, entropy does
power plant schematic
(see notecard 5)
name each of the 3 states (see notecard 6)
1: compressed liquid
2: saturated liquid-vapor mixture
3: superheated vapor
