try this

We define the following fluid variables:

where :math:`\rho_g =` 

where :math:`\rho_g = ` fluid density (assumed to be constant in the absence of reactions)

where :math:`\varepsilon_g =` 

where :math:`\varepsilon_g = ` volume fraction of fluid (accounts only for displacement of fluid by particle, does not account for the EB walls)

where :math:`U_g =`

where :math:`U_g = ` fluid velocity

where :math:`p_g = ` fluid pressure