Evaporation/Condensation

Heat flux on surfaces from evaporation and condensation can be applied. Evaporation and condensation are leading to cooling and heating respectively.

It is assumed that the Relative Humidity (RH) on the surface is 100%. This is obtained by keeping the surface wet, by wet covering (evaporation) or by means of a steam curing chamber (condensation). If a surface is not kept wet, the analysis can be used to assess, whether requirements concerning rate of evaporation or total evaporation, are met.

The Heat Flux is determined by the rate of evaporation/condensation on the surface together with the Heat of Evaporation/Condensation for water:

Where:

	Heat Flux from evaporation/condensation
H	Heat of Evaporation/Condensation for water
	Parameter for intersection
	Parameter for slope
	Wind speed
	Difference in water vapor pressure between surface and surroundings

H is depending on the surface temperature and is determined by b4cast during the calculation.

The water vapor pressure on the surface is determined by the actual surface temperature and the assumption that RH is 100%.

The water vapor pressure in the surrounding is determined by the ambient temperature and the ambient RH.

The amount of evaporated/condensed water is also calculated during the analysis.

The necessary user input is:

- p₁, p₂ in Setup Window (e.g. wet concrete surface p₁ = 1.125e-4 kg/m²/h/Pa, p₂ = 8.25e-5 kg s/m³/h/Pa)

- boundary condition for wind speed

- boundary condition for ambient temperature

- boundary condition for ambient Relative Humidity

- boundary condition for Humidity Transmission Coefficient turning wetting or steam chamber on/off. Or turning off when the unprotected surface gets covered. On is indicated with the value 1.0 while off is indicated with the value 0.0.

Please notice that the calculation is based on the surface temperature of the structure.

That means that no shield or convective transmission coefficient (representing a shield) can be present while the Humidity Transmission is turned on. A convective transmission coefficient representing the thin layer of air, expressed by a wind model, is accepted, because the evaporation/condensation takes place through the air layer. If an insulation is still to be included together with evaporation/condensation, it must be modelled as a (thin) volume.

After the analysis the amount of evaporated/condensed water is found on the tab with temperature and maturity results.