Damp material conductivity

[corus]

Before a material dries out I need to estimate the thermal conductivity. The conductivity of water is less than the conductivity of the materal itself when dry, but I have the feeling that the nett effect of the material being damp is to increase the thermal condcutivity. Would this be correct and is there any evidence for that? Can the overall conductivity of the damp material be estimated by the volume of water it contains?

corus

 

[torpen]

Hello,

The beginnings of your answer can be found in the following empirical formula :

k_wet = k_dry + w * k_wet

where,
w (kg/m3): water content
k_wet (W.m2/kg/K): proportional factor


Some values are given below :

k_wet w
expanded polystyrene 0.00013 < 2
extruded polystyrene 0.00010 < 2
fiber of cellulose 0.0005 3

light Concrete : k_wet w
300 kg/m3 0.0009 7
600 kg/m3 0.0009 < 14


And there is the notion of apparent effective conductivity k_ap_wet :

k_ap_wet = k_wet + rho*L*D_T_v

where,
L (J/kg): Latent heat of vaporization
rho (kg/m3): density of water
D_T_v (m2/s/K): no isothermal specific diffusivity in vapor state due to the temperature T


Bibliography :

LANGLAIS (C.), SILBERSTEIN (A.), SANDBERG (P.I.).
Effects of moisture on the thermal performance of insulating materials,
in Moisture Control in Building. H.R. TRESCHEL, Editor,
ASTM Manual Series MNL 18.
ASTM Publication code Number (PCN) 28-078094-10 (1994).

Regards,

Torpen

 

[EdStainless]

Corus, Even though the material has a higher Tc than water it must be less than fully dense. This low density conditiion will not have very good heat transfer. In this case I would approximate the Tc simply by the law of mixtures.

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[corus]

Many thanks, and particularly torpen. This brick is heated and the water is driven off as it boils. I think this action would increase the nett conductivity but at least the formulae and reference given will be of great help. The effect of the increase in density due to the water in the material should slow the rate of temperature increase below 100C, but I'm not sure if this is offset by an increase in conductivity.

corus