“…A look at the governing equation (1)- (4) reveals that constitutive relations are required for T, q, f , , and r. Less obvious is the fact that in many practical problems involving competing effects, such as temperature and concentration, the body force b, which in problems dealing with natural convection oftentimes depends on the temperature and is modeled using the Boussinesq assumption (see [28]), now might have to be modeled in such a way that it is also a function of concentration (see for example, Equation (2.2) of [15]). Furthermore, in many problems involving chemical reactions, there is usually a (heat) source term, Q, in Equation (4) that also has to be constitutively modeled (see [12,18]). Ignoring these effects, along with the influence of radiation, we will now briefly discuss the three necessary constitutive relations, namely equations for T, q, and f , for closure.…”