Podcast
Questions and Answers
What is the primary cause of fluid motion in free convection?
What is the primary cause of fluid motion in free convection?
- Surface tension of the fluid
- Viscosity of the fluid
- Buoyancy forces due to density differences (correct)
- External forces applied to the fluid
Heat transfer by radiation requires a medium for transfer.
Heat transfer by radiation requires a medium for transfer.
False (B)
Besides solids, what other phases of matter participate in thermal radiation?
Besides solids, what other phases of matter participate in thermal radiation?
Liquids and gases
The heat transfer coefficient, 'h', is experimentally derived and depends on surface geometry, fluid properties, and the nature of fluid ________.
The heat transfer coefficient, 'h', is experimentally derived and depends on surface geometry, fluid properties, and the nature of fluid ________.
Match the type of heat transfer with its typical value of 'h' (heat transfer coefficient):
Match the type of heat transfer with its typical value of 'h' (heat transfer coefficient):
What is the term for an idealized surface that emits the maximum possible radiation at a given temperature?
What is the term for an idealized surface that emits the maximum possible radiation at a given temperature?
The emissivity of a blackbody is equal to 0.
The emissivity of a blackbody is equal to 0.
What does the Stefan-Boltzmann law describe?
What does the Stefan-Boltzmann law describe?
The fraction of radiation energy incident on a surface that is absorbed by the surface is called the ______.
The fraction of radiation energy incident on a surface that is absorbed by the surface is called the ______.
Match the following materials with their corresponding emissivity values at 300 K:
Match the following materials with their corresponding emissivity values at 300 K:
According to Kirchhoff's law, when the temperature and wavelength are the same, what is the relationship between a surface's emissivity and absorptivity?
According to Kirchhoff's law, when the temperature and wavelength are the same, what is the relationship between a surface's emissivity and absorptivity?
What does the term 'absorptivity' (α) refer to in the context of radiation heat transfer?
What does the term 'absorptivity' (α) refer to in the context of radiation heat transfer?
The Stefan-Boltzmann constant is approximately equal to ______ W/m²·K⁴.
The Stefan-Boltzmann constant is approximately equal to ______ W/m²·K⁴.
Net radiation heat transfer (Q_rad) is calculated as the sum of emitted radiation (Q_emit) and absorbed radiation (Q_abs).
Net radiation heat transfer (Q_rad) is calculated as the sum of emitted radiation (Q_emit) and absorbed radiation (Q_abs).
Blackbody radiation is the radiation emitted by a real surface.
Blackbody radiation is the radiation emitted by a real surface.
What is the primary difference between heat transfer and work as forms of energy transfer?
What is the primary difference between heat transfer and work as forms of energy transfer?
The rate of work done per unit time is termed ______.
The rate of work done per unit time is termed ______.
In the context of heat transfer from a person in a breezy room, which of the following is most relevant when calculating the total heat transfer?
In the context of heat transfer from a person in a breezy room, which of the following is most relevant when calculating the total heat transfer?
Convection heat transfer coefficient is measured in units of W/m·oC.
Convection heat transfer coefficient is measured in units of W/m·oC.
What does the thermal conductivity of a material signify in the context of heat transfer?
What does the thermal conductivity of a material signify in the context of heat transfer?
Match the following terms with their respective definitions in the context of energy transfer:
Match the following terms with their respective definitions in the context of energy transfer:
Flashcards
Maximum radiation emitted
Maximum radiation emitted
The maximum rate of radiation a surface can emit at a given absolute temperature.
Blackbody
Blackbody
An ideal surface that emits radiation at the maximum rate possible.
Absorptivity (a)
Absorptivity (a)
The fraction of incident radiation energy absorbed by a surface. It varies between 0 and 1.
Emissivity (e)
Emissivity (e)
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Rate of radiation absorption
Rate of radiation absorption
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Kirchhoff's law
Kirchhoff's law
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Blackbody radiation
Blackbody radiation
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Radiation from real surfaces
Radiation from real surfaces
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Free Convection
Free Convection
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Conduction
Conduction
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Convection
Convection
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Radiation
Radiation
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Work
Work
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Power
Power
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Heat transfer rate
Heat transfer rate
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Thermal conductivity
Thermal conductivity
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Thermal resistance
Thermal resistance
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Study Notes
Fundamentals of Thermodynamics GET 227, Lecture 7
- Course is instructed by Dr. Petrus Nzerem, Engr. Abdulmojeed Oluogun, and Engr. Seun Jesuloluwa
- Focuses on energy transfer by heat, convection, conduction, and radiation
Energy Transfer by Heat
-
Conduction: Heat transfer through solids, liquids, or gases.
- The rate of heat conduction (Qcond) through a layer of constant thickness is given by Qcond = k1A(ΔT)/Δx, where:
- k1 is the thermal conductivity of the material
- A is the area normal to the heat transfer direction
- ΔT is the temperature difference
- Δx is the thickness of the layer
- The rate of heat conduction (Qcond) through a layer of constant thickness is given by Qcond = k1A(ΔT)/Δx, where:
-
High k1 values indicate good heat (and electrical) conductors, while low k1 values indicate poor conductors. Specific examples of materials and their thermal conductivities are listed in Table 8.1
-
Convection: Heat transfer by the movement of fluids (liquids, gases).
- Convection heat transfer rate (Qconv) is given by Qconv = hA(Ts - Tf), where:
- h is the convection heat transfer coefficient
- A is the surface area through which heat transfer takes place
- Ts is the surface temperature
- Tf is the bulk fluid temperature
- Convection can be forced or natural (free) depending on the method of fluid movement
- Typical values of h are in Table 9.1
- Convection heat transfer rate (Qconv) is given by Qconv = hA(Ts - Tf), where:
-
Radiation: Heat transfer through electromagnetic waves.
- Thermal radiation occurs due to temperature difference
- Does not require a medium for transfer
- The maximum rate of radiation emitted from a surface (Qemit,max) is given by Qemit,max = εσAT4, where:
- ε is the emissivity of the surface
- σ is the Stefan-Boltzmann constant (5.67 × 10-8 W/m2·K4)
- A is the surface area
- T is the absolute temperature
-
Table 9.2 provides emissivity values for specific materials
Energy Transfer by Work
-
Work: Energy transfer associated with a force acting through a distance
- Can be defined with respect to the states 1 and 2 using the equation W = Fds
- There are two requirements for work to be done:
- Force must act on the boundary
- The boundary must move
-
Shaft Work (Wsh): Work done through a shaft connected/rotating shaft. The amount of work is proportional to the torque applied (T) and the number of revolutions (n). Equation is Wsh = 2πnτ
-
Electrical Work (We) & Electrical Power (We):
- We = VI -- Work is dependent on voltage and electric charge
- We = I2R -- work, considering the resistance
- We =VI -- electrical power or Work per time.
Other Important Concepts
- Point Functions: Have exact differentials
- Path Functions: Do not have exact differentials.
- Sign Conventions for Heat and Work
- Examples related to heat transfer in various scenarios (e.g., a person in a breezy room, heat transfer through a wall, etc.) and problems related to calculation of work, power and other related values are provided in the Lecture
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