EGR 2500 THERMODYNAMICS TEST 3
CHAPTER 16-18.1 EXAM QUESTIONS
WITH CORRECT ANSWERS
What is the relationship between insulation thickness and amount of heat loss?
Between the temperature of the room and amount of heat loss? Between surface area
and amount of heat loss? - ANSWER-The thicker the insulation, the lower the heat loss.
When the temperature of the room is lowered, tank loses heat at a higher rate.
A larger tank has a larger surface area and a larger rate of heat loss
The rate of heat transfer through the wall is _____________ when delta T is
______________, but _______________ when wall thickness is ____________. -
ANSWER-doubled ; doubled ; halved ; doubled
Rate of heat conduction through a plane layer is ______________ to temperature
difference and heat transfer area, but _____________ to the thickness. - ANSWER-
proportional ; inversely proportional
What is thermal conductivity? - ANSWER-Measure of the ability of a material to conduct
heat ; Rate of heat transfer through a unit thickness of material per unit area per unit
temperature difference.
In what direction is heat conducted? - ANSWER-In the direction of decreasing
temperature
When does the temperature gradient become negative? - ANSWER-When temperature
decreases with increasing "X"
What does a high value of thermal conductivity indicate? What does a low value of
thermal conductivity indicate? - ANSWER-High value indicates that material is a good
heat conductor
Low value indicates that material is a poor conductor or a good inductor
What is important to note about the thermal conductivity of materials? - ANSWER-It is
often a function of temperature, but since this would complicate heat transfer analysis,
we evaluate thermal conductivity at average temp and treat it as a constant
What important assumption is normally made in heat transfer analysis? - ANSWER-A
material is assumed to be isotropic (uniform properties in all directions)
, What does the heat capacity of a material represent? How is it calculated? - ANSWER-
It represents the heat storage capability of a material per unit unit volume.
Calculated by density * specific heat
What does thermal diffusivity represent? How is it calculated? - ANSWER-It represents
how fast heat diffuses through a material.
Calculated by Heat conducted / Heat stored (thermal conductivity / heat capacity)
How does the size of diffusivity affect the propagation of heat into a medium? -
ANSWER-Larger the diffusivity (high conductivity, low heat storage).
Small diffusivity means that heat is mostly stored by the material and a small amount is
conducted further.
What is convection? - ANSWER-Mode of energy transfer between a solid surface and
the adjacent moving fluid or gas. Involves combined effects of conduction and fluid
motion.
The faster the fluid motion, the __________________ the convective heat transfer -
ANSWER-greater
What happens to heat transfer between a solid surface and the adjacent fluid in the
absence of bulk fluid motion? What effect does bulk fluid motion have? - ANSWER-In
the absence of any bulk fluid motion, heat transfer between a solid surface and the
adjacent fluid is by pure conduction.
Bulk fluid motion enhances the heat transfer between the surface and the fluid.
What is the difference between natural and forced convection? - ANSWER-Convection
is natural or free if fluid motion is caused by buoyancy forces induced by density
differences caused by variations of the temp of the fluid.
Convection is forced if the fluid is forced to flow over a surface by external means (fan,
pump, wind)
What is the relationship between the rate of convection heat transfer and temperature
difference? - ANSWER-Rate of convection heat transfer is proportional to the
temperature difference, as expressed by Newton's Law of Cooling
What are the meanings of each variable in the equation for Newton's Law of Cooling?
Qconv = hAs(Ts - T∞) - ANSWER-h = convection heat transfer coefficient
As = surface area through which convection takes places
Ts = surface temperature
CHAPTER 16-18.1 EXAM QUESTIONS
WITH CORRECT ANSWERS
What is the relationship between insulation thickness and amount of heat loss?
Between the temperature of the room and amount of heat loss? Between surface area
and amount of heat loss? - ANSWER-The thicker the insulation, the lower the heat loss.
When the temperature of the room is lowered, tank loses heat at a higher rate.
A larger tank has a larger surface area and a larger rate of heat loss
The rate of heat transfer through the wall is _____________ when delta T is
______________, but _______________ when wall thickness is ____________. -
ANSWER-doubled ; doubled ; halved ; doubled
Rate of heat conduction through a plane layer is ______________ to temperature
difference and heat transfer area, but _____________ to the thickness. - ANSWER-
proportional ; inversely proportional
What is thermal conductivity? - ANSWER-Measure of the ability of a material to conduct
heat ; Rate of heat transfer through a unit thickness of material per unit area per unit
temperature difference.
In what direction is heat conducted? - ANSWER-In the direction of decreasing
temperature
When does the temperature gradient become negative? - ANSWER-When temperature
decreases with increasing "X"
What does a high value of thermal conductivity indicate? What does a low value of
thermal conductivity indicate? - ANSWER-High value indicates that material is a good
heat conductor
Low value indicates that material is a poor conductor or a good inductor
What is important to note about the thermal conductivity of materials? - ANSWER-It is
often a function of temperature, but since this would complicate heat transfer analysis,
we evaluate thermal conductivity at average temp and treat it as a constant
What important assumption is normally made in heat transfer analysis? - ANSWER-A
material is assumed to be isotropic (uniform properties in all directions)
, What does the heat capacity of a material represent? How is it calculated? - ANSWER-
It represents the heat storage capability of a material per unit unit volume.
Calculated by density * specific heat
What does thermal diffusivity represent? How is it calculated? - ANSWER-It represents
how fast heat diffuses through a material.
Calculated by Heat conducted / Heat stored (thermal conductivity / heat capacity)
How does the size of diffusivity affect the propagation of heat into a medium? -
ANSWER-Larger the diffusivity (high conductivity, low heat storage).
Small diffusivity means that heat is mostly stored by the material and a small amount is
conducted further.
What is convection? - ANSWER-Mode of energy transfer between a solid surface and
the adjacent moving fluid or gas. Involves combined effects of conduction and fluid
motion.
The faster the fluid motion, the __________________ the convective heat transfer -
ANSWER-greater
What happens to heat transfer between a solid surface and the adjacent fluid in the
absence of bulk fluid motion? What effect does bulk fluid motion have? - ANSWER-In
the absence of any bulk fluid motion, heat transfer between a solid surface and the
adjacent fluid is by pure conduction.
Bulk fluid motion enhances the heat transfer between the surface and the fluid.
What is the difference between natural and forced convection? - ANSWER-Convection
is natural or free if fluid motion is caused by buoyancy forces induced by density
differences caused by variations of the temp of the fluid.
Convection is forced if the fluid is forced to flow over a surface by external means (fan,
pump, wind)
What is the relationship between the rate of convection heat transfer and temperature
difference? - ANSWER-Rate of convection heat transfer is proportional to the
temperature difference, as expressed by Newton's Law of Cooling
What are the meanings of each variable in the equation for Newton's Law of Cooling?
Qconv = hAs(Ts - T∞) - ANSWER-h = convection heat transfer coefficient
As = surface area through which convection takes places
Ts = surface temperature
