Week 3 - Flow Path Terms Flashcards
What is transient conduction?
Fluctuation of heat flux at one boundary of a solid material finds its way to another boundary, being diminished in magnitude and shifted in time due to the material’s thermal inertia.
Governed by conductivity, k (W/m.K), density, ρ (kg/m3) and specific heat capacity, C (J/kg.K).
What is surface convection?
The process by which heat flux is exchanged between a surface and the adjacent air layer
The governing parameter is the convection coefficient, hc (W/m2.K), which depends on surface-to-air temperature difference, surface roughness, direction of heat flow and surface dimensions.
What is long-wave radiation?
Function of surface temperature, emissivity, the extent to which surfaces are in visual contact (represented by a view factor), and the nature of the surface reflection (diffuse,specular or mixed).
Long-wave radiation exchange between external surfaces and the sky vault, surrounding buildings and ground can result in a substantial lowering of surface temperatures, especially under clear sky conditions and at night.
What is short-wave radiation?
Short-wave energy arrives at a surface directly from the sun and diffusely after atmospheric scatter and terrain reflections.
By transient conduction finds its way through the fabric where it will contribute to the inside surface heat flux at some later time via convection and long-wave radiation.
The governing thermo-physical properties include shortwave absorptivity, transmissivity and reflectivity.
What is air-flow? (3 air-flow paths)
Three air flow paths: infiltration, zone-coupled flows and mechanical ventilation.
- Infiltration is the leakage of air from outside (through cracks and the fabric) and via the ingress of air through intentional openings referred to as natural ventilation.
- Zone-coupled air flow is caused by pressure variations and by buoyancy forces resulting from the density differences associated with the temperatures of the coupled air volumes.
- Mechanical ventilation is the supply of air to satisfy a fresh air requirement.
What is heat injection?
The heat gains from lighting installations, occupants, small power equipment, IT devices etc.
Modelling requires knowledge of the heat (radiant and convective) and moisture emissions.
What is moisture?
Dampness and mould growth are major problems affecting a significant proportion of houses.
Fluctuations in moisture levels within the building’s fabric can be problematic, leading to interstitial condensation or causing variations in material thermo-physical properties and, thereby, adversely affecting thermodynamic performance.
