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Questions and Answers
What is the SI unit for thermal conductivity?
What is the SI unit for thermal conductivity?
- Watt/m.°K (correct)
- Watt/m4.°K
- Watt/m2.°C
- Watt/m2.°K
Direct contact heat exchangers are generally avoided because of which of the following reasons?
Direct contact heat exchangers are generally avoided because of which of the following reasons?
- High heat transfer coefficients
- Increased energy efficiency
- Risk of contamination (correct)
- High operational costs
According to Duhring's rule, the boiling point of a solution is a linear function of which property of water?
According to Duhring's rule, the boiling point of a solution is a linear function of which property of water?
- Density
- Boiling point at the same pressure (correct)
- Pressure
- Viscosity
Water is commonly used as a coolant in heat exchange equipment primarily due to its:
Water is commonly used as a coolant in heat exchange equipment primarily due to its:
The overall resistance for heat transfer through a series of flat resistances is calculated as which of the following?
The overall resistance for heat transfer through a series of flat resistances is calculated as which of the following?
The rate of emission of radiation by a body depends on which of the following factors?
The rate of emission of radiation by a body depends on which of the following factors?
At what value of Prandtl number is conduction considered negligible in the turbulent core of a fluid flowing through a heated pipe?
At what value of Prandtl number is conduction considered negligible in the turbulent core of a fluid flowing through a heated pipe?
Which of the following materials typically has maximum thermal conductivity at the same temperature?
Which of the following materials typically has maximum thermal conductivity at the same temperature?
What does the Biot number indicate regarding heat transfer in a solid object?
What does the Biot number indicate regarding heat transfer in a solid object?
In fully developed liquid flow, the Nusselt number is dependent on which two factors?
In fully developed liquid flow, the Nusselt number is dependent on which two factors?
What is the significance of the Peclet number in heat transfer?
What is the significance of the Peclet number in heat transfer?
Which property primarily influences the thermal conductivity of a material?
Which property primarily influences the thermal conductivity of a material?
What role does temperature gradient play in natural convection heat transfer?
What role does temperature gradient play in natural convection heat transfer?
In the context of heat exchangers, the Overall Heat Transfer Coefficient (U) is influenced by which of the following?
In the context of heat exchangers, the Overall Heat Transfer Coefficient (U) is influenced by which of the following?
Which of the following statements correctly describes conduction in solids?
Which of the following statements correctly describes conduction in solids?
Which process is NOT associated with heat transfer in multiple effect evaporators?
Which process is NOT associated with heat transfer in multiple effect evaporators?
Which factor does NOT influence radiation heat transfer rates?
Which factor does NOT influence radiation heat transfer rates?
The variation of thermal conductivity of a metal with temperature follows which of the following expressions?
The variation of thermal conductivity of a metal with temperature follows which of the following expressions?
The units of 'a' in the expression for thermal conductivity variation are?
The units of 'a' in the expression for thermal conductivity variation are?
According to the relationship between rate, driving force, and resistance, which statement is correct?
According to the relationship between rate, driving force, and resistance, which statement is correct?
Film condensation is favored on which type of surface?
Film condensation is favored on which type of surface?
Convective heat transfer, where heat is transferred via the movement of warmed matter, is described by which law?
Convective heat transfer, where heat is transferred via the movement of warmed matter, is described by which law?
The Dietus-Boelter equation cannot be applied to molten metals primarily due to its low?
The Dietus-Boelter equation cannot be applied to molten metals primarily due to its low?
In heat exchangers, what primarily determines the overall heat transfer coefficient?
In heat exchangers, what primarily determines the overall heat transfer coefficient?
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Study Notes
Thermal Conductivity
- Expressed in SI units as Watt/m.°K
- Important property in heat transfer, indicating how well heat can flow through a material.
Heat Exchangers
- Indirect contact heat exchangers are preferred over direct contact ones due to:
- Higher heat transfer coefficients.
- No risk of contamination.
- Absence of mist formation.
- Lower equipment cost.
Duhring's Rule
- The boiling point of a solution is a linear function of the boiling point of water at the same pressure.
Characteristics of Water as a Coolant
- Water is favored due to:
- Abundance and a high heat capacity.
- Low density.
- Low viscosity.
- High fluidity.
Resistance in Heat Transfer
- The overall resistance for heat transfer through flat resistances is the sum of the individual resistances.
Radiation Emission
- The rate of emission does not depend on the wavelength of radiation but is influenced by:
- Surface temperature.
- Nature of the surface.
- Shape and porosity.
Prandtl Number
- Conduction becomes negligible in the turbulent core of a fluid flowing through a heated pipe at a specific Prandtl number.
Maximum Thermal Conductivity
- At the same temperature, the material with the maximum thermal conductivity is important in heat exchange applications.
Natural Convection
- Heat transfer occurs by natural convection due to temperature-induced variations in:
- Viscosity.
- Density.
- Thermal properties of the fluid.
Biot Number
- A metal wire with a diameter of 0.01 m and thermal conductivity of 200 W/m.K in a fluid stream (h = 100 W/m².K) shows a Biot number that indicates the ratio of internal to external resistance.
Kinematic Viscosity and Thermal Diffusivity
- The ratio of kinematic viscosity to thermal diffusivity is known as the Prandtl number.
Mass and Heat Transfer Analogy
- The analogy is not applicable in cases of:
- Viscous heating or chemical reactions.
- Different velocity profiles or uneven eddy diffusivities.
Radiation Heat Transfer
- Does not depend on factors such as:
- Type of absorbing surface.
- Distance from the heat source.
- Surface area and temperature of the heat source.
Driving Force in Heat Transfer
- Correct formulas relating rate and resistance include:
- Rate = Driving force x Resistance.
- Driving force = Rate x Resistance.
Film Condensation
- Promoted on clean and smooth surfaces, which aids heat transfer efficiency.
Convective Heat Transfer
- Described by Newton's law of cooling, which addresses heat transfer through movement of warmed matter.
Dietus-Boelter Equation
- Inapplicable for molten metals due to their very low Prandtl number, limiting effective heat transfer modeling.
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