AIRPAK - How to properly account for sensible heat and latent heat for indoor ventilation simulation

Engineers are given information on the total cooling load of a space due to occupants that identifies both the sensible heat (SH) and latent heat (LH) components. what values should be entered as source terms in the three-dimensional volume representing the occupied region?
to properly account for both effects, the user should specify both an energy source and an h2o mass source.

the energy source should be equal to the combination of the SH and the mdot_h2o*enthalpy of the h2o being added (it is incorrect to set the energy source equal to SH + LH and this will apply too much energy to the ventilated space.

the h2o mass source should be the rate at which water vapor is being added to the occupied space by the occupants. if the engineer is supplied with on the LH value, then he can calculate the amount of water vapor mass using the following equation ...

mdot_h2o = LH / h_fg

now, considering the energy source term again ... this input is not intuitively easy to figure out since not very many people are familiar with the reference temperature T_ref that Fluent/Airpak use for computing enthalpies relative to a reference enthalpy. in Fluent ...

T_ref= 25 deg C = 298.15 K = 77 deg F

thus, assuming that the water vapor from the occupants enters the space at 98.6 deg F, the portion of the energy source due to latent heat effects is ...

mdot_h2o*enthalpy = (LH / h_fg) * c_p*(98.6-77)

where c_p is the specific heat and thus the total energy source is

SH + mdot_h2o*enthalpy

so you should include simulation of the water_vapor to properly account for both effects. the bulk of the latent heat is actually rejected in the air handling unit and does not contribute to sensible energy addition to the ventilated space.

Show Form
No comments yet. Be the first to add a comment!