Heat-Transfer viva questions and answers ~ MECHTECH GURU

# Heat-Transfer viva questions and answers

## Q: What is heat transfer?

A: Heat transfer is the process by which thermal energy is transferred from one object or system to another due to a difference in temperature.

## Q: What are the three modes of heat transfer?

A: The three modes of heat transfer are conduction, convection, and radiation.

## Q: What is conduction?

A: Conduction is the transfer of heat through a material without any motion of the material itself.

## Q: Give an example of conduction.

A: A metal spoon in a hot cup of tea will conduct heat from the hot liquid to the handle of the spoon, making the handle hot to the touch.

## Q: What is convection?

A: Convection is the transfer of heat by the movement of a fluid or gas, such as air or water.

## Q: Give an example of convection.

A: A pot of boiling water on a stove, where heat is transferred from the bottom of the pot to the water by conduction, and then the hot water rises and cooler water sinks in a circular pattern due to convection.

A: Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation.

## Q: Give an example of radiation.

A: The warmth felt from standing in front of a fire or from the sun on a warm day is due to the transfer of heat through radiation.

## Q: What is thermal conductivity?

A: Thermal conductivity is a measure of how well a material conducts heat.

## Q: What is the equation for Fourier's law of heat conduction?

A: The equation for Fourier's law of heat conduction is Q = -kA(dT/dx), where Q is the rate of heat transfer, k is the thermal conductivity of the material, A is the cross-sectional area through which heat is being transferred, and (dT/dx) is the temperature gradient.

## Q: What is the difference between thermal conductivity and thermal resistance?

A: Thermal conductivity is a measure of how well a material conducts heat, while thermal resistance is a measure of how much a material impedes the flow of heat.

## Q: What is heat transfer coefficient?

A: Heat transfer coefficient is a measure of the rate of heat transfer between a fluid and a solid surface.

## Q: What is the difference between natural convection and forced convection?

A: Natural convection is the flow of a fluid due to buoyancy forces caused by temperature differences, while forced convection is the flow of a fluid due to an external force, such as a fan or a pump.

## Q: What is the Stefan-Boltzmann law?

A: The Stefan-Boltzmann law states that the total amount of thermal radiation emitted by a surface is proportional to the fourth power of its absolute temperature.

## Q: What is the Nusselt number?

A: The Nusselt number is a dimensionless number used in convective heat transfer calculations that relates the rate of heat transfer to the fluid flow properties.

## Q: What is the Prandtl number?

A: The Prandtl number is a dimensionless number used in convective heat transfer calculations that relates the kinematic viscosity to the thermal diffusivity of a fluid.

## Q: What is the Reynolds number?

A: The Reynolds number is a dimensionless number used in fluid mechanics that describes the ratio of inertial forces to viscous forces and is used to predict flow patterns in fluids.

## Q: What is heat exchanger?

A: A heat exchanger is a device that transfers heat from one fluid to another fluid without mixing the two fluids.

## Q: What are some common types of heat exchangers?

A: Some common types of heat exchangers include shell and tube, plate and frame, and finned tube heat exchangers.

## Q: What is the difference between a parallel flow and counterflow heat exchanger?

A: In a parallel flow heat exchanger, both fluids flow in the same direction, while in a counterflow heat exchanger, the fluids flow in opposite directions.

## Q: What is the effectiveness-NTU method used for in heat exchanger design?

A: The effectiveness-NTU method is used to determine the heat transfer rate and the temperature change of each fluid in a heat exchanger.

## Q: What is a thermal boundary layer?

A: A thermal boundary layer is the thin layer of fluid adjacent to a solid surface where the temperature of the fluid changes from the solid surface temperature to the bulk fluid temperature.

## Q: What is a heat sink?

A: A heat sink is a device used to absorb and dissipate heat generated by an electronic device or system.

## Q: What is a thermocouple?

A: A thermocouple is a temperature sensor that works by generating a voltage in response to a temperature difference between two different metals.

## Q: What is a heat pipe?

A: A heat pipe is a device that uses the evaporation and condensation of a working fluid to transfer heat from one location to another.

## Q: What is a blackbody?

A: A blackbody is an idealized object that absorbs all incident radiation and emits thermal radiation according to its temperature, and has an emissivity of 1.

## Q: What is the Stefan-Boltzmann constant?

A: The Stefan-Boltzmann constant is a physical constant that relates the amount of thermal radiation emitted by a blackbody to its temperature, and has a value of approximately 5.67 x 10^-8 W/(m^2 K^4).

## Q: What is a heat transfer coefficient?

A: A heat transfer coefficient is a measure of the rate of heat transfer between a fluid and a solid surface. It is defined as the amount of heat transferred per unit area per unit time per unit temperature difference.

## Q: What is the purpose of insulation in heat transfer applications?

A: The purpose of insulation is to reduce heat transfer between two objects or systems by minimizing heat flow through a material.

## Q: What is the difference between conductive and convective heat transfer?

A: Conductive heat transfer occurs when heat flows through a material without any motion of the material itself, while convective heat transfer occurs when heat is transferred through the motion of a fluid or gas.

## Q: What is the heat equation?

A: The heat equation is a partial differential equation that describes how temperature changes over time and space due to the flow of heat.

## Q: What is the Biot number?

A: The Biot number is a dimensionless number used to describe the relative importance of conduction and convection in a heat transfer process.

## Q: What is the Fourier number?

A: The Fourier number is a dimensionless number used to describe the rate of heat transfer in a system during a transient process.

## Q: What is the Peclet number?

A: The Peclet number is a dimensionless number used to describe the relative importance of convection and diffusion in a fluid flow process.

## Q: What is thermal radiation?

A: Thermal radiation is the transfer of heat energy through electromagnetic waves emitted by an object due to its temperature.

## Q: What is the emissivity of a material?

A: The emissivity of a material is a measure of how effectively it emits thermal radiation relative to a blackbody, and is expressed as a value between 0 and 1.

## Q: What is a temperature gradient?

A: A temperature gradient is the change in temperature over a distance, typically expressed in units of temperature per unit length.

## Q: What is a heat exchanger fouling?

A: Heat exchanger fouling is the accumulation of unwanted deposits, such as scale or biological material, on heat transfer surfaces, which can reduce the efficiency of the heat exchanger.

## Q: What is a thermal shock?

A: A thermal shock is a rapid change in temperature that can cause mechanical stress and material damage to a system.

## Q: What is a heat transfer fluid?

A: A heat transfer fluid is a material used to transfer heat between two systems, typically by circulating the fluid through a heat exchanger.

## Q: What is a convection coefficient?

A: A convection coefficient is a measure of the heat transfer rate per unit surface area per unit temperature difference between a fluid and a solid surface.

## Q: What is a heat flux?

A: A heat flux is the rate of heat transfer per unit area of a surface, typically expressed in units of watts per square meter.

## Q: What is a boundary layer?

A: A boundary layer is the layer of fluid adjacent to a solid surface where the flow of the fluid is influenced by the surface, and is typically characterized by a gradient in velocity, temperature, and other properties.

## Q: What is heat transfer by conduction?

A: Heat transfer by conduction is the transfer of heat through a solid material by molecular vibration and collisions, without any bulk motion of the material.

## Q: What is heat transfer by radiation?

A: Heat transfer by radiation is the transfer of heat through electromagnetic waves that propagate through a vacuum or through a medium.

## Q: What is heat transfer by convection?

A: Heat transfer by convection is the transfer of heat between a fluid and a solid surface by the motion of the fluid, which can be natural or forced.

## Q: What is a heat pipe?

A: A heat pipe is a passive heat transfer device that uses a working fluid to transfer heat from one location to another, typically by evaporating the fluid at a hot surface and condensing it at a cooler surface.

## Q: What is a thermocouple?

A: A thermocouple is a temperature sensor that consists of two dissimilar metals joined at one end, which generates a voltage proportional to the temperature difference between the two ends.

## Q: What is a thermistor?

A: A thermistor is a temperature sensor that uses the change in resistance of a semiconductor material with temperature to measure temperature.

## Q: What is a pyrometer?

A: A pyrometer is a temperature measuring device that uses the thermal radiation emitted by an object to determine its temperature, typically by measuring the intensity of the radiation at a specific wavelength.

## Q: What is a double-pipe heat exchanger?

A: A double-pipe heat exchanger is a type of heat exchanger consisting of two concentric pipes, with the hot fluid flowing through the inner pipe and the cold fluid flowing through the annular space between the pipes.

## Q: What is a shell-and-tube heat exchanger?

A: A shell-and-tube heat exchanger is a type of heat exchanger consisting of a cylindrical shell with a bundle of tubes inside, through which the hot and cold fluids flow in separate channels.

## Q: What is a plate heat exchanger?

A: A plate heat exchanger is a type of heat exchanger consisting of a stack of metal plates with alternate channels for the hot and cold fluids.

## Q: What is a counterflow heat exchanger?

A: A counterflow heat exchanger is a type of heat exchanger in which the hot and cold fluids flow in opposite directions, which maximizes the temperature difference and improves the efficiency of the heat transfer.

## Q: What is a parallel flow heat exchanger?

A: A parallel flow heat exchanger is a type of heat exchanger in which the hot and cold fluids flow in the same direction, which reduces the temperature difference and lowers the efficiency of the heat transfer.

## Q: What is a crossflow heat exchanger?

A: A crossflow heat exchanger is a type of heat exchanger in which the hot and cold fluids flow perpendicular to each other, typically through a set of parallel plates or tubes.

## Q: What is heat transfer enhancement?

A: Heat transfer enhancement is the improvement of the rate of heat transfer in a fluid system by modifying the fluid flow, the geometry of the heat transfer surfaces, or the properties of the fluid.

## Q: What is a heat transfer coefficient enhancement technique?

A: A heat transfer coefficient enhancement technique is a method used to improve the heat transfer coefficient in a fluid system, such as adding fins, roughening the surface, or using additives to alter the properties of the fluid.

## Q: What is the critical Reynolds number?

A: The critical Reynolds number is the value of the Reynolds number at which the flow transitions from laminar to turbulent.

## Q: What is laminar flow?

A: Laminar flow is a type of fluid flow in which the fluid particles move in parallel layers with minimal mixing between them, and in which the flow is characterized by low turbulence and low Reynolds number.

## Q: What is turbulent flow?

A: Turbulent flow is a type of fluid flow in which the fluid particles move in a chaotic and random manner, with high levels of mixing and energy dissipation, and in which the flow is characterized by high turbulence and high Reynolds number.

## Q: What is heat transfer coefficient correlation?

A: Heat transfer coefficient correlation is an empirical formula or equation that relates the heat transfer coefficient to the relevant fluid and system parameters, and is used to predict the rate of heat transfer in a particular application.

## Q: What is thermal conductivity?

A: Thermal conductivity is a measure of the ability of a material to conduct heat, and is defined as the amount of heat transferred per unit time per unit area per unit temperature gradient.

## Q: What is thermal resistance?

A: Thermal resistance is a measure of the resistance of a material or system to heat transfer, and is defined as the ratio of the temperature difference between the two sides of a material or system to the rate of heat transfer.

## Q: What is heat capacity?

A: Heat capacity is a measure of the amount of heat required to raise the temperature of a substance by one degree, and is defined as the ratio of the heat added or removed to the corresponding change in temperature.

## Q: What is latent heat?

A: Latent heat is the amount of heat required to change the phase of a substance (such as from solid to liquid or from liquid to gas) without a change in temperature.

## Q: What is the Stefan-Boltzmann law?

A: The Stefan-Boltzmann law is a fundamental law of thermodynamics that relates the rate of thermal radiation emitted by a surface to its temperature and its emissivity.

## Q: What is the Planck's law?

A: Planck's law is a fundamental law of physics that describes the spectral distribution of thermal radiation emitted by a black body at a given temperature.

## Q: What is black body?

A: A black body is an idealized object that absorbs all incident radiation and emits thermal radiation with a spectral distribution determined solely by its temperature, and which is used as a reference for measuring the properties of other materials and surfaces.

## Q: What is thermal boundary resistance?

A: Thermal boundary resistance is the resistance to heat transfer at the interface between two materials, typically due to differences in thermal properties and/or surface roughness.

## Q: What is fouling in heat exchangers?

A: Fouling in heat exchangers refers to the buildup of deposits or coatings on the heat transfer surface, which can reduce the heat transfer rate and increase energy consumption.

## Q: What is the effectiveness-NTU method?

A: The effectiveness-NTU method is a mathematical method used to analyze and optimize the performance of heat exchangers, based on the concept of heat exchanger effectiveness and the number of transfer units.

## Q: What is multi-pass heat exchanger?

A: A multi-pass heat exchanger is a type of heat exchanger in which the fluid flows through the heat transfer surface multiple times in a serpentine or other pattern, to increase the heat transfer rate and the temperature difference between the two fluids.

## Q: What is cross-flow heat exchanger?

A: A cross-flow heat exchanger is a type of heat exchanger in which the two fluids flow perpendicular to each other, typically separated by a solid barrier or heat transfer surface, to maximize the heat transfer area and the temperature difference between the two fluids.

## Q: What is counter-flow heat exchanger?

A: A counter-flow heat exchanger is a type of heat exchanger in which the two fluids flow in opposite directions, typically separated by a solid barrier or heat transfer surface, to maximize the temperature difference between the two fluids and the heat transfer rate.

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