Heat Transfer: Conduction

• Conduction, convection, and radiation are the three primary modes of heat transfer.

Thermal Conduction

• Thermal conduction involves the transfer of heat through a material by the movement of energy without any net movement of the material itself.
• It primarily occurs in solids where molecules are closely packed.
• Heat is transferred through the vibration of atoms and movement of free electrons.
• Thermal conductivity (k) is a measure of a material's ability to conduct heat.
• Materials with high thermal conductivity are good conductors of heat (e.g., metals).
• Materials with low thermal conductivity are good insulators (e.g., wood, plastic).
• Fourier's Law of Heat Conduction describes the rate of heat transfer through a material.
• Fourier's Law equation: Q = -kA(dT/dx), where Q is the heat transfer rate, k is the thermal conductivity, A is the cross-sectional area, and dT/dx is the temperature gradient.
• The negative sign indicates heat flows from high to low temperature.
• Thermal resistance (R-value) is the inverse of thermal conductance and measures a material's resistance to heat flow.
• R-value is calculated as thickness (L) divided by thermal conductivity (k), i.e., R = L/k.

Convection Currents

• Convection is heat transfer through the movement of fluids (liquids and gases).
• Convection occurs when a fluid is heated, becomes less dense, and rises, while cooler, denser fluid sinks.
• Natural convection occurs due to density differences caused by temperature variations.
• Forced convection occurs when a fluid is moved by external means, like a fan or pump.
• Examples of natural convection include the circulation of air in a room and the formation of sea breezes.
• Forced convection examples include heat sinks on computer processors and heating/cooling systems in cars.
• Convection heat transfer is described by Newton's Law of Cooling.
• Newton's Law of Cooling equation: Q = hA(Ts - Tf), where Q is the heat transfer rate, h is the convection heat transfer coefficient, A is the surface area, Ts is the surface temperature, and Tf is the fluid temperature.
• The convection heat transfer coefficient (h) depends on fluid properties, flow velocity, and geometry.

Radiation Spectrum

• Radiation is heat transfer through electromagnetic waves.
• It does not require a medium and can occur in a vacuum.
• All objects emit thermal radiation, with the amount and wavelength distribution dependent on their temperature.
• The electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
• Thermal radiation falls primarily in the infrared region of the spectrum.
• Blackbodies are ideal emitters and absorbers of radiation.
• Stefan-Boltzmann Law describes the total energy radiated by a blackbody.
• Stefan-Boltzmann Law equation: Q = σAT^4, where Q is the heat transfer rate, σ is the Stefan-Boltzmann constant (5.67 x 10^-8 W/m^2K^4), A is the surface area, and T is the absolute temperature in Kelvin.
• Emissivity (ε) is the ratio of radiation emitted by a surface to that emitted by a blackbody at the same temperature.
• Real objects have emissivities between 0 and 1.
• Absorptivity (α) is the fraction of incident radiation absorbed by a surface.
• For an object in thermal equilibrium, absorptivity equals emissivity (α = ε).
• Wien's Displacement Law relates the peak wavelength of emitted radiation to the temperature of the object.
• Wien's Displacement Law equation: λmax = b/T, where λmax is the peak wavelength, b is Wien's displacement constant (2.898 x 10^-3 m·K), and T is the absolute temperature.

Heat Transfer Applications

• Heat exchangers are devices designed to efficiently transfer heat between two or more fluids.
• Common applications include radiators in cars, condensers and evaporators in air conditioners, and industrial processes.
• Insulation is used to reduce heat transfer, conserving energy in buildings, pipes, and other systems.
• Materials with low thermal conductivity are used as insulators.
• Heating, ventilation, and air conditioning (HVAC) systems rely on all three modes of heat transfer to maintain comfortable indoor environments.
• Solar collectors use radiation to absorb solar energy, which can then be used for heating or electricity generation.
• Cooking involves all three modes of heat transfer: conduction through cookware, convection in ovens, and radiation from heating elements.
• Electronic devices require heat management to prevent overheating.
• Heat sinks and fans are used to dissipate heat generated by electronic components.
• Cryogenics involves the study and application of extremely low temperatures, often using liquid nitrogen or helium.
• Cryogenic applications include medical imaging (MRI), superconductivity research, and food preservation.
• Heat pipes are efficient heat transfer devices that use evaporation and condensation of a working fluid.
• Heat pipes are used in laptops, satellites, and other applications where efficient heat transfer is needed.
• The human body regulates its temperature through a combination of metabolic heat production, convection, radiation, and evaporation of sweat.
• Clothing acts as insulation, reducing heat loss to the environment.
• Thermos flasks minimize heat transfer through vacuum insulation (reducing conduction and convection) and reflective surfaces (reducing radiation).
• Power plants use heat transfer principles to generate electricity from various energy sources, such as fossil fuels, nuclear reactions, and renewable sources.
• Geothermal energy harnesses heat from the Earth's interior to generate electricity or for direct heating applications.
• Heat pumps transfer heat from a cooler space to a warmer space, using work to drive the process, often used for heating buildings.