Thermal capacitance of the material the duct is filled with such as air, bentonite, oil.

Symbol
$Q_{\mathrm{d_{\mathrm{fill}}}}$
Unit
J/m.K
Formulae
$1.0 \cdot 10^{-6} A_{\mathrm{d_{\mathrm{fill}}}} \sigma_{\mathrm{d_{\mathrm{fill}}}}$
Related
$A_{\mathrm{d_{\mathrm{fill}}}}$
$\sigma_{\mathrm{d_{\mathrm{fill}}}}$