APPLIED THERMODYNAMICS AND
HEAT TRANSFER EXAM QUESTIONS
WITH CORRECT ANSWERS
Transparent - ANSWER-Allowing all light to pass through.
Translucent - ANSWER-Allowing some light to pass through.
Opaque - ANSWER-Light does not transmit through the substance.
Opaque Solids (e.g. metals, wood, bricks, rigid insulation) - ANSWER--Radiation
generated within the substance never reaches the surface
-Radiation striking the surface only penetrates a few microns.
Semi-transparent Materials (e.g. glass, salt crystals, gases at elevated temperatures) -
ANSWER-Radiation can travel through the material.
Radiation depends on: - ANSWER--the media
-geometry
-surface temperature
-wavelengths
Radiation from an object - ANSWER-Surfaces emit energy by virtue of the fact that they
have temperature.
Emissivity - ANSWER-The property (condition of the surface) which governs how much
energy can be emitted.
Incident Radiation - ANSWER-Radiation energy directed at a surface (from another
body that has temperature).
Absorptivity - ANSWER-The property (condition of the surface) which governs how
much energy is absorbed.
-wavelength dependent
i.e. a surface may absorb more short wavelength radiation than long wavelength
radiation.
Black Body Radiation - ANSWER-A 'black body' is a material that is a perfect emitter
and perfect absorber to all wavelengths.
(Emissivity = 1, Absorptivity = 1)
-not limited to just 'black' in the visible spectrum
- Allows us to calculate a maximum energy exchange rate
, Four Radiative Properties - ANSWER-1. Emissivity
2. Absorptivity
3. Reflectivity
4. Transmissibility
Radiation through Glass - ANSWER-1. Radiation emitted from surfaces at room
temperature are in the infrared spectrum
2. Glass selectively filters
-visible spectrum passes through
-long-wavelength infrared radiation and short wavelength UV is blocked
Result: Glass allows solar radiation to enter a building but inhibits interior surfaces from
dissipating energy by radiation... greenhouse effect.
Qnet_rad - ANSWER-Can predict Qnet_rad when:
1. The area of the emitter is small compared to the surroundings
2. The emitter is completely enclosed
3. The gas separating objects does not interfere with the radiation
Heat Exchangers - ANSWER-Exchange heat but no mass exchange occurs between
streams.
Area Density - ANSWER-Ratio of heat transfer surface area to volume.
Heat Exchange Characteristics - ANSWER-Heat transfer based on the circulating
media:
-liquid / liquid: easy and efficient
-liquid / gas: moderate
-gas / gas: difficult and poor efficiency
(use fins to increase surface area)
Energy Balance - ANSWER-Uses delta T's for a single stream to predict the rate of heat
exchanged.
Log Mean Temperature Difference (delta Tlm) - ANSWER-Equivalent temperature
difference used in heat transfer calculations to account for varying temperatures across
the length of the heat exchanger.
Used for simple parallel flow or counter flow heat exchangers.
Condensers and Boilers - ANSWER-One of the temperature versus position profiles is
flat.
Condenser: phase change in the 'hot' stream holds Thot constant.
Boiler: phase change in the 'cold' stream holds Tlow constant.
Can model as either counter or parallel flow since both methods give same delta Tlm.
Tube - ANSWER-Represented by "t" (t1, t2)
HEAT TRANSFER EXAM QUESTIONS
WITH CORRECT ANSWERS
Transparent - ANSWER-Allowing all light to pass through.
Translucent - ANSWER-Allowing some light to pass through.
Opaque - ANSWER-Light does not transmit through the substance.
Opaque Solids (e.g. metals, wood, bricks, rigid insulation) - ANSWER--Radiation
generated within the substance never reaches the surface
-Radiation striking the surface only penetrates a few microns.
Semi-transparent Materials (e.g. glass, salt crystals, gases at elevated temperatures) -
ANSWER-Radiation can travel through the material.
Radiation depends on: - ANSWER--the media
-geometry
-surface temperature
-wavelengths
Radiation from an object - ANSWER-Surfaces emit energy by virtue of the fact that they
have temperature.
Emissivity - ANSWER-The property (condition of the surface) which governs how much
energy can be emitted.
Incident Radiation - ANSWER-Radiation energy directed at a surface (from another
body that has temperature).
Absorptivity - ANSWER-The property (condition of the surface) which governs how
much energy is absorbed.
-wavelength dependent
i.e. a surface may absorb more short wavelength radiation than long wavelength
radiation.
Black Body Radiation - ANSWER-A 'black body' is a material that is a perfect emitter
and perfect absorber to all wavelengths.
(Emissivity = 1, Absorptivity = 1)
-not limited to just 'black' in the visible spectrum
- Allows us to calculate a maximum energy exchange rate
, Four Radiative Properties - ANSWER-1. Emissivity
2. Absorptivity
3. Reflectivity
4. Transmissibility
Radiation through Glass - ANSWER-1. Radiation emitted from surfaces at room
temperature are in the infrared spectrum
2. Glass selectively filters
-visible spectrum passes through
-long-wavelength infrared radiation and short wavelength UV is blocked
Result: Glass allows solar radiation to enter a building but inhibits interior surfaces from
dissipating energy by radiation... greenhouse effect.
Qnet_rad - ANSWER-Can predict Qnet_rad when:
1. The area of the emitter is small compared to the surroundings
2. The emitter is completely enclosed
3. The gas separating objects does not interfere with the radiation
Heat Exchangers - ANSWER-Exchange heat but no mass exchange occurs between
streams.
Area Density - ANSWER-Ratio of heat transfer surface area to volume.
Heat Exchange Characteristics - ANSWER-Heat transfer based on the circulating
media:
-liquid / liquid: easy and efficient
-liquid / gas: moderate
-gas / gas: difficult and poor efficiency
(use fins to increase surface area)
Energy Balance - ANSWER-Uses delta T's for a single stream to predict the rate of heat
exchanged.
Log Mean Temperature Difference (delta Tlm) - ANSWER-Equivalent temperature
difference used in heat transfer calculations to account for varying temperatures across
the length of the heat exchanger.
Used for simple parallel flow or counter flow heat exchangers.
Condensers and Boilers - ANSWER-One of the temperature versus position profiles is
flat.
Condenser: phase change in the 'hot' stream holds Thot constant.
Boiler: phase change in the 'cold' stream holds Tlow constant.
Can model as either counter or parallel flow since both methods give same delta Tlm.
Tube - ANSWER-Represented by "t" (t1, t2)
