Heat Transfer Fundamentals

Heat Transfer Fundamentals

• Heat flows in the direction of decreasing temperature
• Heat transfer study concerns the mode of heat transfer, rate of energy transfer, and temperature variation in a medium
• Thermodynamics deals with equilibrium states, while heat transfer is a non-equilibrium phenomenon

Areas of Practical Engineering Applications

• Design of heat exchangers (e.g., radiators, condensers)
• Heat treatment of metals
• Human comfort
• Refrigeration and air-conditioning units
• Rating and sizing of heat transfer systems

Modes of Heat Transfer

• Three modes of heat transfer: conduction, convection, and radiation
• Each mode has a distinctive controlling law
• Heat transfer occurs in the direction of lower temperature

Heat Transfer by Conduction

• Conduction heat transfer occurs in a stationary medium (solid or fluid)
• Energy exchange is due to atomic or molecular activity in the presence of a temperature difference
• In solids, heat transfer is a combination of lattice vibrations and free movement of electrons
• In liquids and gases, conduction is primarily by collisions and diffusion of molecules

Fourier's Law of Conduction

• The rate of conduction heat transfer is given by Fourier's law (Eq. 1)
• 𝑄 = heat transfer rate by conduction (W), A = area (m2), k = thermal conductivity (W/m.℃), 𝑑𝑇/𝑑𝑥 = temperature gradient (℃/m)
• Assumptions: steady-state conditions, one-dimensional heat flow, no internal heat generation, and constant temperature gradient

Thermal Conductivity, k

• Materials are classified as thermal conductors or insulators based on their thermal conductivity
• Thermal conductivity depends on material structure, moisture content, material density, pressure, and temperature
• Generally, thermal conductivities for pure metals are the highest, but decrease with the inclusion of impurities

Heat Transfer by Convection

• Convection heat transfer occurs as a result of bulk fluid motion over a surface at a different temperature
• Convection heat transfer is categorized into natural (free) and forced convection
• Natural convection is induced by buoyancy effects, while forced convection occurs when the fluid motion is initiated by external means

Newton's Law of Cooling

• The rate of convection heat transfer is defined by Newton's law of cooling (Eq. 2)
• 𝑄 = heat transfer rate by convection (W), A = area (m2), h = heat transfer coefficient (W/m².℃), Ts = surface temperature, 𝑇∞ = fluid temperature
• The convection heat transfer coefficient, h, depends on fluid properties, nature of fluid flow, and surface geometry

Heat Transfer by Radiation

• Thermal radiation is energy emitted by matter at finite temperature
• All bodies at temperatures above absolute zero radiate heat
• Radiation energy is transported by electromagnetic waves (or photons)
• Radiation heat transfer occurs efficiently in a vacuum

Stefan-Boltzmann Law

• The rate at which energy is released per unit area (W/m²) is termed the surface emissive power, E
• The maximum radiation heat transfer from a surface (blackbody) is defined by the Stefan-Boltzmann law (Eq. 3)
• 𝑄 = heat transfer rate by radiation (W), A = area (m2), 𝜎 = Stefan-Boltzmann constant (W/m².K⁴), Ts = surface temperature (K), 𝜀 = emissivity (0 ≤ 𝜀 ≤ 1)