Thermal Engineering Principles: Condensation, Boiling, and Radiation

Film and Dropwise Condensation

• Film Condensation:
  • A continuous liquid film forms on the surface as vapor condenses.
  • The film acts as a thermal resistance, reducing heat transfer efficiency.
  • Common in practical systems due to impurities and surface characteristics.
  • The heat transfer rate is generally lower compared to dropwise condensation.
• Dropwise Condensation:
  • Condensed vapor forms droplets instead of a film on the surface.
  • Offers higher heat transfer rates due to minimal thermal resistance.
  • Droplets grow, merge, and slide off, exposing fresh surface area for condensation.
  • Requires specific surface treatments to maintain, as it is less stable than film condensation.

Boiling Point Elevation in Evaporators

• Boiling Point:
  • The boiling point is the temperature at which a liquid’s vapor pressure equals the surrounding pressure, causing it to change into vapor.
• Boiling Point Elevation:
  • Boiling point elevation is the increase in the boiling point of a solvent when a non-volatile solute is added.
  • This occurs because the solute lowers the solvent’s vapor pressure, requiring more heat to reach boiling.
• Boiling Point Elevation in Evaporators:
  • In evaporators, boiling point elevation is important when concentrating solutions with dissolved solutes (e.g., salts or sugars).
  • Higher boiling points due to solute presence require additional energy, impacting evaporator efficiency.
  • This effect is particularly relevant in industries like food processing, chemical manufacturing, and desalination.

Fundamental Laws of Thermal Radiation

• Planck’s Law:
  • Describes the intensity of radiation emitted by a blackbody as a function of wavelength and temperature.
  • Shows that higher temperatures result in greater emission of radiation at shorter wavelengths.
• Stefan-Boltzmann Law:
  • States that the total energy radiated per unit surface area of a blackbody is directly proportional to the fourth power of its absolute temperature.
  • Formula: E = σT⁴, where E is the radiated energy, T is temperature, and σ is the Stefan-Boltzmann constant.
• Wien’s Displacement Law:
  • Indicates that the wavelength at which a blackbody emits maximum radiation is inversely proportional to its temperature.
  • Formula: λ_max = b/T, where λ_max is the peak wavelength, T is temperature, and b is a constant.
• Kirchhoff’s Law:
  • States that, for a body in thermal equilibrium, the emissivity of a material (ability to emit radiation) is equal to its absorptivity (ability to absorb radiation).
  • Implies that good emitters are also good absorbers of radiation at a given wavelength and temperature.

Key Concepts in Thermal Engineering

Kirchhoff’s Laws

• Kirchhoff’s Current and Voltage Laws: Although Kirchhoff’s laws primarily relate to electrical circuits, they have thermal analogies in heat transfer. Kirchhoff’s Voltage Law, for instance, can help understand how temperature differences drive heat transfer in a steady-state system.
• Kirchhoff’s Law of Thermal Radiation: In thermal engineering, Kirchhoff’s law of radiation states that for a body in thermal equilibrium, the emissivity is equal to the absorptivity. This is critical in studying heat exchange and radiation, as it implies that materials that are good absorbers are also good emitters of thermal radiation.

Carburetor

• A carburetor is a device in internal combustion engines that mixes air and fuel for combustion. In thermal engineering, it’s significant because it directly influences the air-fuel ratio, which affects the temperature and efficiency of combustion. Proper air-fuel mixture optimization in carburetors can ensure efficient fuel combustion, resulting in better thermal efficiency and temperature control.

Planck’s Law

• Planck’s Law describes the spectral distribution of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature. In thermal engineering, this law helps determine the intensity of radiation emitted at different wavelengths. It’s particularly relevant for analyzing heat transfer through radiation in high-temperature applications, such as furnaces or engines, where thermal radiation plays a significant role.

Fick’s Laws of Diffusion

• Fick’s Laws describe diffusion, which is the movement of particles (like molecules in fluids or gases) from high to low concentration regions. In thermal engineering, diffusion is key in understanding heat transfer by conduction, as heat in solids transfers partly through the diffusion of kinetic energy among particles. Fick’s Laws can also be applied in combustion, where the diffusion of reactants impacts the rate and efficiency of burning.