What Is True About Heat Transfer?

Heat transfer refers to the transfer of thermal energy between systems due to a temperature difference. There are three main mechanisms of heat transfer: conduction, convection, and radiation.

Conduction is the transfer of heat between substances that are in direct contact with each other. It occurs when faster moving hot molecules collide with slower moving cold molecules, transferring kinetic energy. Metals are good conductors of heat.

Convection is the transfer of heat by the movement of fluids. It occurs when hotter and less dense fluids rise and colder denser fluids sink, causing circulation. Convection can be natural, driven by temperature differences, or forced, driven by an external source like a pump or fan.

Radiation is the transfer of heat via electromagnetic waves directly between substances without requiring a medium. All objects emit thermal radiation related to their temperature.

Heat transfer is a fundamental process in the fields of engineering and science. Understanding heat flow is critical in applications like engines, chemical processing, electronics cooling, and climate modeling. Detailed knowledge of heat transfer mechanisms allows engineers to optimize designs for efficient heating or cooling.

Table of Contents

Conduction Heat Transfer

Conduction is the transfer of heat between substances that are in direct contact with each other. It occurs when heat energy is transferred by collisions between neighboring molecules or electrons within a material. The molecules and electrons with higher energy collide with and transfer some of their energy to the neighboring particles that have lower energy.

On a microscopic scale, conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring particles. The classical kinetic theory of gases describes this energy transfer. Metals are good conductors of heat because they contain many free electrons that can transfer thermal energy rapidly through the metal. Insulating materials like glass and plastics have very few free electrons, inhibiting heat transfer by conduction.

Several factors affect the rate of heat transfer by conduction:

Temperature difference – A greater temperature difference between objects results in faster heat transfer.

Cross-sectional area – More contact area allows for faster conduction between objects.
Material – Metals and dense materials conduct heat better than insulators like wood or plastic.
Thickness – Thinner materials allow for faster conduction across the material.

Molecular vibration – Higher molecular vibration and mobility in a material leads to better heat conduction.

Understanding heat conduction allows engineers to choose or design materials that either promote or inhibit the conduction of heat in applications like insulation, heat sinks, cooking surfaces, and more.

Convection Heat Transfer

Convection is the transfer of heat by the movement of fluids from one place to another. Fluids can be liquids or gases like air. When a fluid is heated up, it expands and becomes less dense than the surrounding fluid. This less dense fluid rises and is replaced by cooler, denser fluid which is then heated, creating a continuous circulation driven by the temperature difference, known as natural or free convection. Forced convection occurs when the fluid motion is created by an external source instead of density differences, like with a fan or pump.

Convection plays an important role in many engineering applications. Natural convection allows heat to escape from hot surfaces like computer chips and car engines into the surrounding air. Forced convection is used in heat exchangers to improve heat transfer between fluids. Convection also occurs inside radiators, ovens, nuclear reactors, and even the atmosphere as hot air rises and cold air sinks. Understanding convection is key for designing cooling and heating systems.

The convection heat transfer coefficient represents the effectiveness of convection and depends on fluid properties, velocity, surface geometry, and orientation. Convection coefficients are larger for forced convection compared to natural since fluid motion is greater. Enhancing convection heat transfer allows engineers to design more compact, efficient systems.

Thermal Radiation

Thermal radiation is the transfer of heat through electromagnetic waves or photons. All matter with a temperature above absolute zero emits thermal radiation. Unlike conduction and convection, which require matter-to-matter contact for heat transfer, thermal radiation can transfer heat across vacuum or empty space.

The main difference between thermal radiation and conduction/convection is that radiation does not rely on the movement of fluids or direct contact between materials. Radiation occurs when electromagnetic waves carry energy away from an emitter. The amount of radiation emitted depends on the object’s temperature and emissivity.

Some examples of thermal radiation in everyday life include:

The heat you feel from a fireplace across the room

The warmth of the sun’s rays on your skin
Heat lamps keeping food warm in a buffet line
Infrared cameras detecting body heat

CO2 lasers using thermal radiation to cut and weld materials

Thermal radiation does not require a medium like air or water to transfer heat. It can travel through transparent media or a vacuum. This makes radiation the only heat transfer method that allows transfer of heat in space between a warm planet and cold space, for example. Understanding thermal radiation is key for designing satellites, space suits, and other technologies for space exploration.

Heat Transfer Coefficients

Heat transfer coefficients are used to characterize heat transfer between a fluid and a solid surface and determine the rate of heat transfer. The heat transfer coefficient depends on the conditions of the fluid flow and the nature of the fluid and solid materials involved.

For conduction heat transfer, the heat transfer coefficient is called thermal conductivity (k). It indicates the ability of a material to conduct heat. Metals typically have high thermal conductivity while insulators like rubber and plastics have low values. The thermal conductivity of a material is primarily dependent on the composition and temperature.

For convection heat transfer, the heat transfer coefficient is called the convection coefficient (h). It depends on the physical properties of the fluid and the velocity of flow. Forced convection with turbulent flow has higher coefficients compared to natural convection. Typical values of the convection coefficient are 10-100 W/m2K for gases, 100-20,000 W/m2K for liquids, and 50-1000 W/m2K for boiling and condensation.

For thermal radiation heat transfer between surfaces, the heat transfer coefficient is called radiative heat transfer coefficient (hr). It depends on the material’s surface properties like emissivity. Typical values range from 5-25 W/m2K.

Heat Transfer Equations

Heat transfer equations describe the rate of heat transfer between objects or systems. The main heat transfer mechanisms are conduction, convection and radiation. Each has its own basic heat transfer equation that defines the relationship between the heat transfer rate, the temperature difference, and properties of the materials involved.

The heat conduction rate is proportional to the negative temperature gradient and the area through which heat flows. It is defined by Fourier’s Law:

q = -kA (dT/dx)

Where q is the heat transfer rate (W), k is the thermal conductivity (W/m·K), A is the cross-sectional area (m2), and dT/dx is the temperature gradient (K/m).

For convection heat transfer, Newton’s Law of Cooling states that the convective heat transfer rate is proportional to the temperature difference between a surface and the fluid:

q = hA(Ts – T∞)

Where h is the convective heat transfer coefficient (W/m2·K), A is the surface area (m2), Ts is the surface temperature (K), and T∞ is the fluid temperature (K).

The heat transfer rate by radiation is described by the Stefan-Boltzmann Law:

q = εσA(Ts4 – Tsurr4)

Where ε is the emissivity, σ is the Stefan-Boltzmann constant (5.67×10-8 W/m2·K4), A is the radiating area (m2), Ts is the absolute temperature of the surface (K) and Tsurr is the absolute temperature of the surroundings (K).

These basic equations can be combined and modified to model heat transfer in more complex scenarios.

Combined Heat Transfer

In many real-world situations, heat transfer occurs through a combination of conduction, convection and radiation simultaneously. For example, in a building, heat transfers from the outdoors to indoors via conduction through the walls, convection due to air leakage, and radiation through windows. In a pot of boiling water on the stove, heat transfers via conduction from the burner to the pot, convection within the water, and radiation from the hot burner to the pot. In engineering systems like engines and heat exchangers, multiple heat transfer modes are usually present.

When multiple modes act simultaneously, the overall heat transfer can be significantly higher than each mode acting independently. This is because the modes can interact and amplify one another. For instance, radiation can heat up a surface, increasing temperature gradients and thereby increasing conduction and convection effects. Accurately calculating the total heat transfer requires summing the individual conductive, convective and radiative components. If a particular mode is neglected, the analysis may underestimate the overall heat transfer.

Therefore, when analyzing heat transfer processes, it is important to consider the combined effects of conduction, convection and radiation. Each mode needs to be evaluated and their interactions accounted for. Summing the individual modes will provide the most accurate prediction of total heat transfer.

Heat Exchangers

Heat exchangers are devices designed to transfer heat between two or more fluids, gases, or solid surfaces. They have a variety of important applications and uses in many industries and systems.

Some of the major types of heat exchangers include:

Shell and tube heat exchangers – This consists of a series of tubes enclosed in a cylindrical shell. One fluid flows inside the tubes while the other flows outside the tubes through the shell. This is one of the most common types used in various industries.

Plate heat exchangers – These are constructed of a series of thin metal plates sealed between top and bottom covers. The plates are fitted with gaskets which create alternating flow channels. Fluids flow between the plates allowing heat transfer between them. Plate heat exchangers offer high efficiency and compact size.
Air cooled heat exchangers – These use air to remove heat from a hot fluid like water or gas flowing through finned tubes. Fans blow ambient air over the finned tubes. They are a common choice for applications like power plants, oil refineries, natural gas processing, and chemical plants.
Regenerators – These involve a matrix of material with a large heat capacity that alternately absorbs heat from a hot fluid and then gives up heat to a cold fluid via alternate flow switching. It enables reuse of heat from hot exhaust gases. They are commonly used in applications like gas turbines.

Some major applications and uses of various heat exchangers include: process plants, power stations, petroleum refineries, natural gas processing, air conditioning, space heating, refrigeration, and automobiles. They play an indispensable role in efficiently transferring heat between fluids to improve thermal efficiency in these systems.

Heat Transfer Enhancement

There are various methods used to enhance heat transfer and improve the efficiency of heat exchangers and other thermal systems. Some common techniques include:

Extended Surfaces

Adding extended surfaces like fins and pins increases the surface area for heat transfer. This allows more heat to be dissipated from a surface through convection or radiation. Fins are commonly used on engine cylinder heads, heat sinks on computer chips, and condensers in HVAC systems.

Surface Roughness

Increasing surface roughness promotes turbulence and mixes fluid layers, improving convective heat transfer. Methods like sandblasting, machining, or chemical etching can be used to intentionally roughen metal surfaces. Surface texturing has been shown to increase heat transfer coefficients by 30-400% in some cases.

Vortex Generators

Vortex generators are small fin-like objects placed on a surface to create vortices and turbulence. This breaks up the boundary layer and improves fluid mixing and heat transfer. Vortex generators are often used on airplane wings, turbine blades, heat exchanger tubes, and other applications.

Coatings and Treatments

Applying specialized coatings or surface treatments can also enhance heat transfer. For example, black coatings increase radiative heat transfer rates, hydrophobic coatings improve pool boiling, and porous metallic coatings provide extended surface area. Overall, enhancing surfaces can often increase heat transfer with minimal additional costs.

Applications of Heat Transfer

Heat transfer plays a critical role in many engineering systems and everyday applications. Understanding heat transfer principles allows us to effectively design and analyze heating and cooling processes and systems.

Some of the key applications that rely heavily on heat transfer include:

HVAC Systems

Heating, ventilation, and air conditioning (HVAC) systems are designed to maintain comfortable temperatures and air quality in buildings. HVAC engineers apply heat transfer principles to determine heating and cooling loads, size equipment, design ductwork, and ensure effective climate control. Factors like heat conduction through walls and windows, air convection currents, and heat radiated from equipment and people all impact HVAC system design.

Electronics Cooling

Effective thermal management is essential for reliable electronics operation. Compact modern electronics generate substantial heat fluxes that must be dissipated to prevent overheating failure. Heat sinks, fans, heat pipes, and other cooling technologies rely on heat transfer principles to maintain safe junction temperatures.

Insulation

Insulation slows the rate of heat transfer through building envelopes and industrial systems. Insulation materials work by trapping air pockets to provide thermal resistance. Correct insulation thickness and properties are determined by analyzing heat conduction rates through materials. Proper insulation saves energy and costs in heating and cooling applications.

Many other areas like power generation, chemical processing, transportation, food storage, textiles, and more depend on heat transfer analysis and thermal systems design. Overall, comprehending heat transfer fundamentals enables impactful thermal engineering across industries.