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Questions and Answers
What is the primary purpose of thermal insulation?
What is the primary purpose of thermal insulation?
Metal is an example of a substance that is a poor thermal conductor.
Metal is an example of a substance that is a poor thermal conductor.
False
What does the variable 'k' represent in the conductive heat equation?
What does the variable 'k' represent in the conductive heat equation?
Thermal conductivity
The formula for conductive heat transfer is given by q = (k / s) A dT, which can also be written as U A dT. Here, q represents _____ transfer.
The formula for conductive heat transfer is given by q = (k / s) A dT, which can also be written as U A dT. Here, q represents _____ transfer.
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Match the following materials to their thermal conduction characteristics:
Match the following materials to their thermal conduction characteristics:
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What is convection primarily associated with?
What is convection primarily associated with?
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Forced convection occurs naturally without the aid of external forces.
Forced convection occurs naturally without the aid of external forces.
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What equation is used to describe the heat transfer per unit surface through convection?
What equation is used to describe the heat transfer per unit surface through convection?
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_____ convection is caused by density differences in a fluid due to temperature changes.
_____ convection is caused by density differences in a fluid due to temperature changes.
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Match the convection types with their descriptions:
Match the convection types with their descriptions:
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What is the typical convective heat transfer coefficient for forced convection of water and liquids?
What is the typical convective heat transfer coefficient for forced convection of water and liquids?
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The convective heat transfer coefficient for boiling water can reach as high as 100,000 W/(m2K).
The convective heat transfer coefficient for boiling water can reach as high as 100,000 W/(m2K).
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What is the temperature difference if the surface temperature is 50°C and the fluid temperature is 20°C?
What is the temperature difference if the surface temperature is 50°C and the fluid temperature is 20°C?
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The convective heat transfer coefficient for forced convection of liquid metals ranges from _________ (W/(m2K)).
The convective heat transfer coefficient for forced convection of liquid metals ranges from _________ (W/(m2K)).
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Match the following heat transfer applications with their corresponding typical convective heat transfer coefficients:
Match the following heat transfer applications with their corresponding typical convective heat transfer coefficients:
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Which of the following represents the convective heat transfer equation in the given example for a fluid flow?
Which of the following represents the convective heat transfer equation in the given example for a fluid flow?
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The average convection heat transfer coefficient for hot air blowing over a surface is always higher than that for water.
The average convection heat transfer coefficient for hot air blowing over a surface is always higher than that for water.
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What is the convection heat transfer coefficient mentioned for forced convection of air and gases?
What is the convection heat transfer coefficient mentioned for forced convection of air and gases?
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The surface area in the example problem of convection heat transfer is __________ (in m2).
The surface area in the example problem of convection heat transfer is __________ (in m2).
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Study Notes
Heat Conduction
- Conduction is the transfer of heat from a hotter part of an object to a colder part.
- Heat conduction in solids and liquids works by transferring energy through the structure of atoms or molecules.
- Metal and aluminum are good thermal conductors.
Thermal/Heat Insulation
- Thermal insulation reduces heat transfer between objects by creating a barrier.
- Insulators are substances that do not conduct heat well.
- Examples of insulators include plastic, fabric, and wood.
Thermal Conductivity (K)
- Thermal conductivity (k) is the ability of an object to conduct heat.
- Thermal conductivity of a material describes how well the material conducts heat.
- Heat conduction in solids and liquids involves transferring energy through the atomic or molecular structure.
Conductive Heat Equation
- Heat transfer through conduction can be described by the equation q = (k/s)A dT = U A dT
-
qis heat transfer. -
kis thermal conductivity of the material. -
sis material thickness. -
Ais the heat transfer area. -
Uis the heat transfer coefficient. -
dTis the temperature gradient.
Example of Conductive Heat Equation
- A plane wall made of iron (k = 70 W/m°C) with a thickness of 50 mm and surface area of 1 m by 1 m has a temperature difference of 90°C on one side and 20°C on the other side.
Convection
- Convection is heat transfer by the motion of a fluid (liquid or gas), primarily through currents.
- Natural convection is caused by density differences due to temperature variations in a fluid.
- Forced convection is induced by an external force (e.g., pump, fan).
- Both free and forced convection help to cool and heat engines and rooms.
Convective Heat Transfer Equation
- Heat transfer per unit surface by convection described by Newton's Law of Cooling: q = h A dT
-
qis heat transfer. -
his convective heat transfer coefficient. -
Ais the heat transfer area. -
dTis temperature difference.
Example of Convective Heat Transfer Equation
- Fluid flow over a plane surface (1 m by 1 m) with a surface temperature of 50°C and fluid temperature of 20°C, and a convective heat transfer coefficient of 2000 W/m²°C.
Radiation
- Radiation is heat transfer by electromagnetic waves.
- The higher the temperature of an object, the more thermal radiation it emits.
- The interaction of radiation with a material depends on the wavelength of radiation.
Interaction of Radiation to Materials
- Transmittance: radiation passing through a material.
- Absorptance: radiation converted into heat within the material.
- Reflectance: radiation reflected off the material's surface.
- Emittance: radiation emitted by the material, reducing its heat content.
Greenhouse Effect by Radiation
- The greenhouse effect is related to how different wavelengths of radiation interact with materials.
- Some solar radiation passes through glazing(like glass), while some is absorbed or reflected.
Thermal Mass
- Thermal mass is a material's ability to absorb, store, and release heat.
- Materials with high thermal mass have longer thermal lag (time to release stored heat).
- Thermal mass helps moderate internal temperatures in buildings.
Why is Thermal Mass Important?
- Thermal mass is important because it helps moderate the indoor temperature of a building.
- This is achieved by storing heat during the day and releasing it at night.
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Description
Test your understanding of heat conduction, thermal insulation, and thermal conductivity. This quiz covers the principles of how heat is transferred in solids and liquids as well as the properties of various materials. Assess your knowledge of conductive heat equations and examples of insulators.
