Question

When prescribing a boundary condition for mass transfer at a solid-gas interface, why do we need to specify the side of the surface (whether the solid or the gas side)? Why did we not do it in heat transfer?

Short answer

Answer: In mass transfer, specifying the side of the surface (solid or gas side) is important because mass transfer relies on different substance concentrations and properties between the solid and gas phases. Thus, it ensures correct boundary conditions are applied for accurate results. In heat transfer, there is no need to specify the side of the surface because heat transfer is primarily driven by temperature differences, not substance properties or phases. All substances, regardless of their phase, will have the same temperature at the boundary.

Step-by-step solution

Understanding boundary conditions in mass transfer

In mass transfer, boundary conditions are essential to model the behavior of a system correctly. These conditions describe how the mass transfer occurs at the interface between the solid and the gas phase, including the concentration or flux of substances at the surface. Since mass transfer at the interface involves substances from both phases, it is crucial to specify which side of the surface we are referring to.

Significance of specifying the side of the surface in mass transfer

The mass transfer phenomenon is heavily dependent on the concentration gradients and the properties of the substances involved, which can significantly vary between the solid and gas phases. Thus, specifying the side of the surface (solid or gas side) is necessary as it ensures that the correct boundary condition is applied. It avoids confusion during problem-solving and helps in obtaining accurate results.

Understanding boundary conditions in heat transfer

In heat transfer, boundary conditions usually involve specifying the temperature or heat flux at a particular surface. The temperature is a scalar property, and heat flow is governed by temperature gradients. Unlike mass transfer, where the solid and gas phases have different substance concentrations, the heat transfer is mainly driven by the difference in temperature and not the substance properties or their phases.

Reason for not specifying the side of the surface in heat transfer

Since heat transfer is primarily driven by temperature differences, the focus is not on which phase the heat is being transferred through. Therefore, there is no need to specify the side of the surface when describing boundary conditions for heat transfer as all substances, regardless of their phase, will have the same temperature at the boundary.