Contents

# Heat-Transfer interview questions and answers

## Q: What is heat transfer?

A: Heat transfer is the process by which thermal energy is transferred from one body 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 movement of the material itself. It occurs when there is a temperature gradient within the material.

## Q: What is thermal conductivity?

A: Thermal conductivity is a measure of a materialโs ability to conduct heat. It is defined as the amount of heat that passes through a material of unit thickness and unit area in unit time when there is a temperature difference of one degree between the two faces of the material.

## Q: What is convection?

A: Convection is the transfer of heat through a fluid (liquid or gas) by the movement of the fluid itself. It occurs when there is a temperature gradient within the fluid.

## Q: What is natural convection?

A: Natural convection is the type of convection that occurs due to the density differences that arise from temperature differences within a fluid. As a result, fluid is heated and rises, while cooler fluid sinks.

## Q: What is forced convection?

A: Forced convection is the type of convection that occurs when a fluid is forced to flow over a surface by an external means such as a fan or a pump.

A: Radiation is the transfer of heat through electromagnetic waves. It does not require a medium and can occur through a vacuum.

## Q: What is Stefan-Boltzmann law?

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

## Q: What is emissivity?

A: Emissivity is a measure of a materialโs ability to emit radiation. It is defined as the ratio of the radiation emitted by a material to the radiation emitted by a blackbody at the same temperature.

## Q: What is heat exchanger?

A: A heat exchanger is a device that is used to transfer thermal energy between two or more fluids at different temperatures.

## Q: What are the different types of heat exchangers?

A: The different types of heat exchangers include shell-and-tube, plate-and-frame, and finned-tube heat exchangers.

## Q: What is the purpose of fins in a finned-tube heat exchanger?

A: Fins increase the surface area of the tube, which increases the rate of heat transfer between the fluid in the tube and the fluid outside the tube.

## Q: What is a heat pipe?

A: A heat pipe is a passive heat transfer device that is used to transport heat from one location to another. It typically consists of a sealed tube that is partially filled with a working fluid. Heat is absorbed by the evaporator section of the tube, which causes the working fluid to vaporize and travel to the condenser section of the tube, where it releases heat and condenses back into a liquid.

## Q: What is a thermocouple?

A: A thermocouple is a temperature sensor that is used to measure temperature by detecting a voltage generated by a junction of two dissimilar metals when there is a temperature difference between them. It is commonly used in industrial applications, such as in furnaces and power plants.

## Q: What is a heat transfer coefficient?

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

## Q: What is the Nusselt number?

A: The Nusselt number is a dimensionless number that is used to describe the convective heat transfer between a fluid and a surface. It is defined as the ratio of the convective heat transfer coefficient to the thermal conductivity of the fluid.

## Q: What is the Reynolds number?

A: The Reynolds number is a dimensionless number that is used to describe the flow of a fluid. It is defined as the ratio of inertial forces to viscous forces in the fluid.

## Q: What is the Prandtl number?

A: The Prandtl number is a dimensionless number that is used to describe the relative importance of momentum diffusivity and thermal diffusivity in a fluid. It is defined as the ratio of the kinematic viscosity to the thermal diffusivity of the fluid.

## Q: What is a phase change material?

A: A phase change material is a material that can store thermal energy by undergoing a phase change, such as from a solid to a liquid or from a liquid to a gas. They are used in many applications, such as in thermal energy storage and in refrigeration systems.

## Q: What is heat transfer enhancement?

A: Heat transfer enhancement refers to the techniques used to increase the rate of heat transfer between a fluid and a surface. These techniques can include the use of turbulence promoters, surface coatings, and the use of nanofluids.

## Q: What are the different types of heat exchanger fouling?

A: Heat exchanger fouling refers to the accumulation of unwanted deposits on the heat transfer surfaces. The different types of fouling include mechanical fouling, corrosion fouling, and biological fouling.

## Q: What is a heat transfer coefficient in heat exchangers?

A: The heat transfer coefficient is a measure of the rate of heat transfer between the fluids in a heat exchanger. It is affected by the fluid flow rate, the fluid properties, and the geometry of the heat exchanger.

## Q: What is a heat exchanger effectiveness?

A: The heat exchanger effectiveness is a measure of the efficiency of a heat exchanger. It is defined as the ratio of the actual heat transfer rate to the maximum possible heat transfer rate.

## Q: What is a thermal boundary layer?

A: A thermal boundary layer is the thin layer of fluid that is in contact with a solid surface in a fluid flow. It is characterized by a gradient in temperature and is important in determining the rate of heat transfer between the solid surface and the fluid.

## Q: What is heat transfer by phase change?

A: Heat transfer by phase change refers to the transfer of thermal energy that occurs when a material changes phase, such as from a liquid to a gas or from a solid to a liquid. It is an important process in many engineering applications, such as in power plants and refrigeration systems.

## Q: What is a thermosyphon?

A: A thermosyphon is a passive heat transfer device that uses natural convection to transfer heat. It typically consists of a closed loop system that is partially filled with a working fluid. Heat is absorbed at one end of the loop, which causes the fluid to vaporize and travel to the other end of the loop, where it releases heat and condenses back into a liquid.

## Q: What is a heat transfer coefficient in radiation heat transfer?

A: The heat transfer coefficient in radiation heat transfer is a measure of the rate of heat transfer between a surface and its surroundings due to radiation. It depends on factors such as the emissivity of the surface, the temperature of the surface, and the temperature of the surroundings.

## Q: What is a heat transfer coefficient in conduction heat transfer?

A: The heat transfer coefficient in conduction heat transfer is a measure of the rate of heat transfer between two solids in contact. It depends on factors such as the thermal conductivity of the solids, the thickness of the solids, and the temperature difference between the solids.

## Q: What is the difference between steady-state and transient heat transfer?

A: Steady-state heat transfer occurs when the temperature distribution in a system does not change with time, while transient heat transfer occurs when the temperature distribution in a system changes with time. Steady-state heat transfer is typically easier to analyze and design for, while transient heat transfer is important in applications where the temperature changes over time, such as in cooling or heating processes.

## Q: What is a heat exchanger?

A: A heat exchanger is a device that is used to transfer thermal energy between two or more fluids at different temperatures. They are used in many industrial and household applications, such as in refrigeration systems, air conditioning systems, and power plants.

## Q: What is a convection heat transfer coefficient?

A: The convection heat transfer coefficient is a measure of the rate of heat transfer between a fluid and a surface due to convection. It is affected by factors such as the fluid flow rate, the fluid properties, and the geometry of the surface.

## Q: What is a finned heat exchanger?

A: A finned heat exchanger is a type of heat exchanger that has fins attached to the heat transfer surfaces. The fins increase the surface area available for heat transfer and can improve the heat transfer efficiency of the system.

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

A: A double-pipe heat exchanger is a type of heat exchanger that consists of two concentric pipes. One fluid flows through the inner pipe, while the other flows through the annular space between the two pipes. They are commonly used in low flow rate applications.

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

A: Natural convection occurs when a fluid flows due to buoyancy forces caused by temperature differences in the fluid. Forced convection occurs when a fluid flows due to an external force, such as a pump or a fan. Forced convection is typically more efficient at transferring heat than natural convection.

## Q: What is a counterflow heat exchanger?

A: A counterflow heat exchanger is a type of heat exchanger where the two fluids flow in opposite directions. This configuration maximizes the temperature difference between the two fluids, which can improve the efficiency of the heat transfer process.

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

A: A parallel-flow heat exchanger is a type of heat exchanger where the two fluids flow in the same direction. This configuration is simpler and less expensive than a counterflow heat exchanger, but it typically has lower efficiency.

## Q: What is thermal conductivity?

A: Thermal conductivity is a measure of how easily a material can conduct heat. It is defined as the rate at which heat flows through a unit area of a material per unit temperature gradient.

## Q: What is convective heat transfer?

A: Convective heat transfer refers to the transfer of thermal energy between a fluid and a surface due to the motion of the fluid. It can occur through either forced convection or natural convection.

## Q: What is heat flux?

A: Heat flux is a measure of the rate at which heat flows through a unit area of a surface. It is typically measured in units of watts per square meter.

## Q: What is the difference between conduction, convection, and radiation heat transfer?

A: Conduction heat transfer occurs when heat flows through a material without any motion of the material itself. Convection heat transfer occurs when heat is transferred by the motion of a fluid. Radiation heat transfer occurs when thermal energy is transferred by electromagnetic radiation.

## Q: What is a thermal insulator?

A: A thermal insulator is a material that resists the flow of heat. It is typically used to prevent heat from escaping from a system or to keep heat out of a system.

## Q: What is a heat sink?

A: A heat sink is a device that is used to absorb and dissipate thermal energy. They are commonly used in electronics and other systems where excess heat needs to be removed.

## Q: What is a thermal boundary condition?

A: A thermal boundary condition is a specification of the temperature or heat flux at the boundary of a system. It is an important parameter in determining the temperature distribution and heat transfer rate in the system.

## Q: What is a heat pipe?

A: A heat pipe is a passive heat transfer device that transfers thermal energy from one point to another through the evaporation and condensation of a working fluid. They are commonly used in applications where high heat fluxes need to be transferred over relatively long distances.

## Q: What is a thermal contact resistance?

A: Thermal contact resistance is a measure of how much a contact interface between two solids resists the flow of heat. It is an important parameter in determining the overall thermal resistance of a system.

## Q: What is a thermocouple?

A: A thermocouple is a temperature sensor that works by measuring the voltage difference between two dissimilar metals. They are commonly used in temperature measurement applications, such as in industrial processes and in HVAC systems.

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

A: Conductive heat transfer occurs through the direct transfer of heat between two materials in contact with each other. Convective heat transfer occurs through the motion of a fluid, which can carry thermal energy from one place to another.

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

A: The Stefan-Boltzmann law is a fundamental law of thermodynamics that describes the rate at which thermal radiation is emitted by a surface. It states that the total thermal radiation emitted by a surface is proportional to the fourth power of the surface temperature.

## Q: What is a blackbody?

A: A blackbody is an idealized object that absorbs all incident electromagnetic radiation and emits radiation in a manner that is solely determined by its temperature. It is often used as a reference for radiation heat transfer calculations.

## Q: What is emissivity?

A: Emissivity is a measure of how well a surface emits thermal radiation compared to a perfect blackbody at the same temperature. It is typically expressed as a value between 0 and 1.

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

A: Thermal conductivity is a measure of how easily a material can conduct heat, while thermal diffusivity is a measure of how quickly heat can diffuse through a material. Thermal diffusivity is a function of both thermal conductivity and the materialโs specific heat capacity.

## Q: What is a thermal boundary layer?

A: A thermal boundary layer is a region near a solid surface where the temperature of a fluid is affected by the temperature of the surface. It is an important factor in convective heat transfer, as it can greatly affect the rate of heat transfer between a fluid and a surface.

## Q: What is a thermoelectric device?

A: A thermoelectric device is a device that converts thermal energy directly into electrical energy, or vice versa. They are commonly used in applications such as power generation and refrigeration.

## Q: What is a finned heat exchanger?

A: A finned heat exchanger is a type of heat exchanger that uses fins to increase the surface area for heat transfer between the two fluids. The fins can be either attached to the tube or plate or be a part of the tube or plate.

## Q: What is a heat transfer coefficient?

A: The heat transfer coefficient is a measure of how easily a fluid can transfer thermal energy to a solid surface. It is defined as the rate of heat transfer per unit area per unit temperature difference between the fluid and the solid surface.

## Q: What is a phase-change material?

A: A phase-change material is a substance that can store thermal energy by undergoing a phase change (e.g. solid to liquid, liquid to gas) at a particular temperature. They are commonly used in thermal energy storage applications.

## Q: What is the difference between laminar and turbulent flow?

A: Laminar flow is a type of fluid flow where the fluid moves in smooth, parallel layers with no mixing between the layers. Turbulent flow is a type of fluid flow where the fluid moves in an irregular, chaotic manner, with mixing occurring between the layers.

## Q: What is a heat exchanger fouling?

A: Heat exchanger fouling refers to the accumulation of unwanted material (e.g. dirt, corrosion products, biological matter) on the heat transfer surfaces of a heat exchanger. It can significantly reduce the efficiency of the heat exchanger and increase energy consumption.

## Q: What is a regenerator?

A: A regenerator is a type of heat exchanger that is used to recover thermal energy from a fluid that is leaving a system. The recovered thermal energy is then used to preheat the incoming fluid, reducing energy consumption and increasing system efficiency.

## Q: What is a heat transfer fluid?

A: A heat transfer fluid is a fluid that is used to transfer thermal energy from one location to another. Common examples include water, ethylene glycol, and refrigerants.

## Q: What is the Nusselt number?

A: The Nusselt number is a dimensionless number that characterizes the heat transfer between a fluid and a solid surface. It is defined as the ratio of the convective heat transfer coefficient to the conductive heat transfer coefficient.

## Q: What is a thermal storage system?

A: A thermal storage system is a system that stores thermal energy for later use. They can be used to shift the timing of energy use from high-demand periods to low-demand periods, reducing energy costs and improving system efficiency.

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

A: A heat transfer enhancement technique is a technique that is used to increase the rate of heat transfer between two fluids. Common techniques include the use of fins, turbulators, and surface coatings.

## Q: What is a heat exchanger?

A: A heat exchanger is a device that is used to transfer thermal energy from one fluid to another, or from a solid surface to a fluid. They are used in a variety of applications, including heating and cooling systems, power generation, and chemical processing.

## Q: What is a convective heat transfer coefficient?

A: The convective heat transfer coefficient is a measure of how easily a fluid can transfer thermal energy to or from a solid surface. It is dependent on the properties of the fluid, the velocity of the fluid, and the geometry of the solid surface.

## Q: What is thermal conductivity?

A: Thermal conductivity is a measure of how easily a material can conduct heat. It is defined as the rate of heat transfer per unit area per unit temperature gradient.

## Q: What is a heat pipe?

A: A heat pipe is a type of heat transfer device that uses a working fluid to transfer thermal energy from one location to another. They are commonly used in electronics cooling and aerospace applications.

## Q: What is a refrigerant?

A: A refrigerant is a fluid that is used in refrigeration and air conditioning systems to transfer thermal energy from one location to another. Common examples include hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs).

## Q: What is a thermocouple?

A: A thermocouple is a temperature sensor that consists of two dissimilar metals that are joined at one end. A voltage is generated at the junction of the two metals that is proportional to the temperature difference between the junction and a reference temperature.

## Q: What is a radiation heat transfer?

A: Radiation heat transfer is a type of heat transfer that occurs through the emission and absorption of electromagnetic radiation. It does not require a medium to transfer thermal energy.

## Q: What is a blackbody?

A: A blackbody is an idealized object that absorbs all incident electromagnetic radiation and emits radiation at a characteristic spectrum that depends only on its temperature. It is commonly used as a theoretical model for radiation heat transfer.

## Q: What is a heat exchanger network?

A: A heat exchanger network is a collection of heat exchangers that are interconnected in such a way as to optimize the thermal energy transfer within a system. They are commonly used in process industries to minimize energy consumption and improve process efficiency.

## Q: What is a heat pump?

A: A heat pump is a device that transfers thermal energy from a lower temperature source to a higher temperature sink, using mechanical work as the driving force. They are commonly used for space heating and cooling in residential and commercial buildings.

## Q: What is thermal resistance?

A: Thermal resistance is a measure of how easily heat can flow through a material. It is defined as the temperature difference per unit of heat flow.

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

A: A double pipe heat exchanger is a type of heat exchanger that consists of two concentric pipes, one inside the other. The fluid to be heated or cooled flows through the inner pipe, while the heating or cooling fluid flows through the annular space between the two pipes.

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

A: A multi-pass heat exchanger is a type of heat exchanger where the fluid flows through the exchanger multiple times before exiting. This allows for a greater amount of heat transfer to occur.

## Q: What is a plate heat exchanger?

A: A plate heat exchanger is a type of heat exchanger that uses a series of thin, corrugated plates to increase the surface area for heat transfer between the two fluids. They are commonly used in HVAC systems and industrial applications.

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

A: A shell and tube heat exchanger is a type of heat exchanger that consists of a shell (a large cylindrical vessel) with a bundle of tubes running through it. One fluid flows through the tubes while the other flows over the outside of the tubes, allowing for heat transfer to occur.

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

A: A counter-flow heat exchanger is a type of heat exchanger where the two fluids flow in opposite directions, allowing for a greater temperature difference and therefore greater heat transfer to occur.

## Q: What is a co-current heat exchanger?

A: A co-current heat exchanger is a type of heat exchanger where the two fluids flow in the same direction. This can lead to a lower overall temperature difference between the fluids and therefore lower heat transfer rates compared to counter-flow heat exchangers.

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

A: A cross-flow heat exchanger is a type of heat exchanger where the two fluids flow perpendicular to each other. They are commonly used in HVAC systems and in applications where one fluid is a gas and the other is a liquid.

## Q: What is a thermal fluid heater?

A: A thermal fluid heater is a type of industrial heater that uses a fluid to transfer thermal energy to a process. The fluid is typically a high-temperature oil or glycol, and the heater may use a variety of fuel sources, including natural gas, electricity, and biomass.

## Q: What is a phase change material (PCM)?

A: A phase change material is a substance that can store thermal energy by undergoing a change in phase, such as melting or freezing. They are commonly used in building materials and thermal energy storage systems.

## Q: What is a thermal energy storage system?

A: A thermal energy storage system is a technology that allows for the storage of thermal energy for later use. This can be achieved through the use of phase change materials, hot water storage tanks, or other methods.

## Q: What is a radiant heating system?

A: A radiant heating system is a type of heating system that uses thermal radiation to transfer heat from a hot surface to the surrounding environment. They are commonly used in residential and commercial buildings.

## Q: What is a cooling tower?

A: A cooling tower is a device that is used to remove waste heat from a process or building by evaporating water into the surrounding air. They are commonly used in HVAC systems and power plants.

## Q: What is a heat recovery ventilator (HRV)?

A: A heat recovery ventilator is a type of ventilation system that recovers heat from outgoing air and uses it to preheat incoming air. They are commonly used in residential and commercial buildings to improve indoor air quality and energy efficiency.

## Q: What is a thermal imaging camera?

A: A thermal imaging camera is a device that uses infrared radiation to produce an image of the temperature distribution on a surface. They are commonly used in building inspections, electrical inspections, and in the medical field.