CHAPTER 1

The thermal conductivities of gases, such as air, vary significantly from those of pure metals, like copper. Pure crystals and metals exhibit the highest thermal conductivities, while gases and insulating materials have the lowest.

The thermal conductivity of materials typically changes with temperature, often increasing for solids and decreasing for gases as temperature rises.

The thermal conductivity of materials at room temperature can vary widely, typically ranging from about 0.01 W/m·K for insulating materials to over 400 W/m·K for metals.

The surface energy balance is valid for both steady and transient conditions and does not involve heat generation since a surface does not have a volume.

It is convenient to treat the conversion of nuclear, chemical, mechanical, and electrical energies into thermal energy as heat generation.

Forced convection occurs when a fluid is forced to flow over a surface by external means such as a fan, pump, or the wind.

A closed system consists of a fixed mass.

The combined heat transfer coefficient h combined includes the effects of both convection and radiation.

A surface contains no volume or mass, and thus no energy, allowing it to be viewed as a fictitious system whose energy content remains constant during a process.

Heat transfer is applied in various fields including HVAC systems, thermal management in electronics, food processing, power generation, and chemical processing.

A substance whose specific volume (or density) does not change with temperature or pressure.

The total energy E consists of the internal energy U.

Natural convection is when fluid motion is caused by buoyancy forces induced by density differences due to temperature variation in the fluid.

When radiation and convection occur simultaneously between a surface and a gas.

Enthalpy is defined as h = u + Pv, where u is internal energy and Pv represents the flow energy of the fluid.

Tiny balls that are in motion and possess kinetic energy.

The first law of thermodynamics states that energy can neither be created nor destroyed during a process; it can only change forms.

It is relevant because stationary systems don’t involve any changes in their velocity or elevation during a process.

They are identical for incompressible substances.

Forced convection involves external means to move the fluid, while natural convection relies on buoyancy forces due to temperature differences.

Heat transfer and work.

The term Pv represents the flow energy of the fluid, also known as flow work.

EES (Engineering Equation Solver) is a program that solves systems of linear or nonlinear algebraic or differential equations numerically and has a large library of built-in thermodynamic property functions.

The energy associated with the random motion of atoms and molecules.

The difference between the rates of radiation emitted by a surface and the radiation absorbed.

The net change in total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process.

It relates to the convection heat transfer coefficient, h, which is measured in W/m²·°C.

Yes, heat transfer processes that involve a change of phase of a fluid, such as boiling and condensation, are considered convection due to the fluid motion induced during these processes.

The fraction of the radiation energy incident on a surface that is absorbed by the surface, where 0 ≤ α ≤ 1. A blackbody has α = 1.

The amount of heat transferred per unit time.

The Stefan-Boltzmann constant is σ = 5.670 × 10^−8 W/m²·K⁴.

They depend on temperature only.

Reporting results with more significant digits than the given data falsely implies greater accuracy than exists.

A theory that posits heat is a fluidlike substance called caloric, which is massless, colorless, odorless, and tasteless.

Thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear.

Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) due to changes in the electronic configurations of atoms or molecules.

No, EES does not solve engineering problems; it only solves the equations supplied by the user.

Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, involving the combined effects of conduction and fluid motion.

The properties of the surfaces, their orientation relative to each other, and the interaction of the medium between the surfaces with radiation.

The emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.

The two main groups are rating problems and sizing problems.

A blackbody is an idealized surface that emits radiation at the maximum rate, representing the maximum amount of radiation that can be emitted from a surface at a specified temperature.

It depends on surface geometry, nature of fluid motion, properties of the fluid, and bulk fluid velocity.

The rate of heat transfer per unit area normal to the direction of heat transfer.

Conduction is the transfer of energy from more energetic particles of a substance to adjacent less energetic ones due to interactions between the particles.

James P. Joule, whose experiments published in 1843 convinced skeptics that heat was not a substance.

It is compared to thinking that a person who can use a wrench can work as a car mechanic.

Usually three significant digits.

The sum of all forms of energy, denoted as E (or e on a unit mass basis).

Heat is the form of energy that can be transferred from one system to another as a result of temperature difference.

No, unlike conduction and convection, heat transfer by radiation does not require the presence of an intervening medium.

It means no change with time at a specified location.

A blackbody absorbs the entire radiation incident on it, which means its absorptivity (α) is equal to 1.

The energy required to raise the temperature of a unit mass of a substance by one degree.

Rating problems deal with the determination of the heat transfer rate for an existing system at a specified temperature difference.

The faster the fluid motion, the greater the convection heat transfer.

Radiation is usually significant relative to conduction or natural convection, but negligible relative to forced convection.

Emissivity (ε) measures how closely a surface approximates a blackbody, where ε = 1 for a perfect blackbody and 0 ≤ ε ≤ 1 for real surfaces.

In gases and liquids, conduction is due to the collisions and diffusion of molecules during their random motion.

T_s represents the surface temperature.

The work done per unit time.

It implies greater accuracy than actually exists, which should be avoided.

The sum of all microscopic forms of energy within the system.

Thermodynamics is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another.

Heat transfer by radiation occurs at the speed of light.

Heat is the form of energy that can be transferred from one system to another as a result of temperature difference.

The amount of mass flowing through a cross section of a flow device per unit time.

Sizing problems involve determining the size of a system to transfer heat at a specified rate for a specified temperature difference.

Thermal diffusivity is a measure of how fast heat diffuses through a material, expressed in m²/s.

The rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference.

Internal energy may be viewed as the sum of the kinetic and potential energies of the molecules.

Specific heat at constant volume (c_v) and specific heat at constant pressure (c_p).

In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction.

The net rate of radiation heat transfer between these two surfaces is determined by the surrounding conditions and the properties of the surfaces.

T_∞ represents the temperature of the fluid sufficiently far from the surface.

In solids, conduction occurs due to the combination of vibrations of molecules in a lattice and the energy transport by free electrons.

It describes the rate of heat conduction through a solid, which is directly proportional to its thermal conductivity.

We are interested in thermal radiation, which is emitted by bodies because of their temperature.

Thermodynamic analysis is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another.

Heat Transfer deals with the determination of the rates of energy transfers as well as variation of temperature.

The volume of a fluid flowing through a pipe or duct per unit time.

Sensible heat is the kinetic energy of the molecules.

It indicates that the material is a good heat conductor.

The two approaches are experimental (testing and taking measurements) and analytical (by analysis or calculations).

A material with high thermal conductivity or low heat capacity will have a large thermal diffusivity.

They depend on two independent properties such as temperature and pressure.

The rate of heat conduction is proportional to the temperature difference across the layer and the heat transfer area, and inversely proportional to the thickness of the layer.

An example of convection heat transfer is heat transfer from a hot surface to air.

A measure of the ability of a material to conduct heat.

The transfer of energy as heat is always from the higher-temperature medium to the lower-temperature one.

No, it is not a property of the fluid; it is an experimentally determined parameter.

Problem Statement

Devices such as water heaters and car radiators that involve mass flow in and out of a system.

Yes, all bodies at a temperature above absolute zero emit thermal radiation.

Latent heat is the internal energy associated with the phase of a system.

The science that deals with the determination of the rates of energy transfers is heat transfer.

It indicates that the material is a poor heat conductor or insulator.

The advantage is that it deals with the actual physical system, and the desired quantity is determined by measurement.

The slope of the temperature curve on a T-x diagram.

At low pressures, all real gases approach ideal gas behavior, and their specific heats depend on temperature only.

A large thermal diffusivity indicates that heat propagates quickly into the medium.

Schematic

Fourier’s law states that the rate of heat transfer through a material is proportional to the negative gradient of temperature and the area through which heat is being transferred.

The transfer of energy as heat is always from the higher-temperature medium to the lower-temperature one.

Heat transfer stops when the two mediums reach the same temperature.

The experimental approach is expensive, time-consuming, and often impractical.

Thermal conductivity.

Radiation is a volumetric phenomenon, but it is usually considered a surface phenomenon for solids.

Chemical (bond) energy is the internal energy associated with the atomic bonds in a molecule.

A small value of thermal diffusivity means that heat is mostly absorbed by the material, with little heat conducted further.

Heat is conducted in the direction of decreasing temperature.

Thermal conductivity measures a material's ability to conduct heat, defined as the amount of heat that passes through a unit thickness of the material per unit time for a unit temperature difference.

Heat transfer is applied in various fields including engineering, environmental science, and energy systems.

Assumptions and Approximations

Heat can be transferred in three different modes: conduction, convection, radiation.

A simple experimental setup.

Heat transfer stops when the two mediums reach the same temperature.

The analytical approach is fast and inexpensive.

It ensures that heat transfer in the positive x direction is a positive quantity.

Nuclear energy is the internal energy associated with the bonds within the nucleus of the atom itself.

The historical background of thermodynamics includes the development of laws governing energy and heat transfer, starting from the 19th century.

Thermal diffusivity is a measure of how quickly a material can conduct thermal energy relative to its ability to store thermal energy.

Physical Laws

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

The area normal to the direction of heat transfer.

The results obtained are subject to the accuracy of the assumptions, approximations, and idealizations made in the analysis.

Newton’s law of cooling states that the rate of heat loss of a body is directly proportional to the difference in temperature between the body and its surroundings, provided this temperature difference is small.

Internal energy is the total energy contained within a system, including kinetic and potential energy of the molecules.

Properties

Thermodynamics provides the principles that govern energy transformations, while heat transfer focuses on the movement of thermal energy between systems.

All modes of heat transfer require the existence of a temperature difference.

The Stefan-Boltzmann law states that the total energy radiated per unit surface area of a black body is proportional to the fourth power of the black body's absolute temperature.

Enthalpy is a measure of the total heat content of a system, defined as the internal energy plus the product of pressure and volume.

Calculations

Simultaneous heat transfer mechanisms refer to the occurrence of conduction, convection, and radiation occurring at the same time in a system.

Thermal energy is the energy associated with the temperature of a system, while other forms of energy, such as kinetic or potential energy, are related to the motion or position of objects.

Modeling in engineering involves creating mathematical representations of physical systems to predict their behavior under various conditions.

The basic mechanisms of heat transfer are conduction, convection, and radiation.

Reasoning, Verification, and Discussion

Engineering software packages are used to model and solve complex heat transfer problems, providing tools for simulation and analysis.

Conduction, convection, and radiation.

Specific heat is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius.

Fourier's law states that the rate of heat transfer through a material is proportional to the negative gradient of temperature and the area through which heat is flowing.

The Engineering Equation Solver (EES) is a software tool used for solving complex engineering problems, particularly those involving thermodynamics and heat transfer.

Energy transfer refers to the movement of energy from one system or object to another, often occurring through heat transfer mechanisms.

Opaque solids.

Newton's law of cooling describes the rate at which an exposed body changes temperature through convection with its environment.

Significant digits are important in heat transfer calculations to ensure the precision and accuracy of the results, reflecting the reliability of the measurements and calculations.

Conduction and radiation.

The energy balance for closed systems states that the change in internal energy is equal to the heat added to the system minus the work done by the system.

The Stefan-Boltzmann law states that the total energy radiated per unit surface area of a black body is proportional to the fourth power of its absolute temperature.

Yes, on its surfaces exposed to a fluid or other surfaces.

The energy balance for steady-flow systems indicates that the energy entering the system equals the energy leaving the system, assuming no accumulation.

Identifying simultaneous heat transfer mechanisms is crucial for accurately analyzing and solving real-world heat transfer problems.

Conduction and possibly radiation.

One should be aware that heat losses can lead to increased energy costs and reduced efficiency in thermal systems.

Surface energy balance involves accounting for energy exchanges at the surface of a system, including heat transfer and work interactions.

Convection equals conduction plus fluid motion.

Various heat transfer problems include calculating heat loss, designing heat exchangers, and analyzing thermal insulation effectiveness.

By radiation.

They do not interfere with radiation.

Liquids are usually strong absorbers of radiation.