Mass Transfer

What is Mass Transfer?

Mass transfer is the physical phenomenon for which a net movement of generic particles is observed from one location to another. The mass transfer may occur in single-phase or over-phase boundaries in multiphase systems. Three basic types of mass transfer are distinguished: (1) diffusion in a quiescent medium, (2) convective mass transfer, and (3) mass exchange between phases.

The first case is the simplest case where the mass transfer occurs in a medium at rest. The driving force is the difference of concentrations in adjacent regions of the medium, and the mechanism is molecular diffusion. Under the statistical character of molecule motion, the substance flows from a high concentration region to a low concentration one tending to equalize the entire volume. However, in the second case, the mass is transferred due to the bulk motion of the base fluid. For example, the flow of liquid water transports molecules or ions dissolved in the water. Similarly, the flow of air transports molecules existing in air, including both concentrated species (e.g., oxygen and nitrogen) and dilute species (e.g., carbon dioxide). The last case, i.e., mass exchange between phases, is mainly categorized into three formats: evaporation, condensation, and cavitation.


Evaporation is a simultaneous heat and mass transfer process. This phenomenon occurs at a saturated liquid surface, where the vapor diffuses into the unsaturated gas. During evaporation, the gas directly above the surface of the liquid is saturated with moisture via molecular diffusion; in the absence of gas movement, it spreads further into the bulk gas by diffusion.


When a liquid and its vapor are in contact, molecules pass from liquid to vapor and liquid. Condensation occurs when the number of molecules entering the liquid phase exceeds that of leaving molecules. Under these circumstances, the temperature of the vapor in the immediate vicinity (a few mean free paths) of the vapor-liquid interface is higher than that of the liquid. The interface temperature drops increase with increasing condensation rate and decreasing pressure. Still, in most circumstances (an exception being the case of liquid metals), this is very small, and equilibrium conditions at the interface can be assumed.


Cavitation is defined as the formation of the vapor phase of a liquid when it is subjected to reduced pressure at a constant ambient temperature. Thus, it is boiling in a liquid due to pressure reduction rather than heat addition. However, the basic physical and thermodynamic processes are the same in both cases. A liquid is said to cavitate when vapor bubbles form and grow due to pressure reduction. When the phase transition results from hydrodynamic pressure changes, a two-phase flow composed of a liquid and its vapor is called a cavitating flow.

Mass transfer simulations in ANSYS Fluent software

The ANSYS FLUENT multiphase mass transfer model accommodates mass transfer between species belonging to different phases. Instead of a matrix-type input, multiple mass transfer mechanisms must be input. Each mass transfer mechanism defines the phenomenon from one entity to another. An entity is a particular species in a phase or the bulk phase itself if it does not have a mixture material. The mass transfer phenomenon could be specified either through the inbuilt unidirectional “constant-rate” mass transfer or through user-defined functions. ANSYS FLUENT loops through all the mass transfer mechanisms to compute each species’ net mass source/sink in each phase. The net mass source/sink of a species is used to calculate species and mass source terms. ANSYS FLUENT will automatically add the source contribution to all relevant momentum and energy equations based on assuming that the momentum and energy are carried along with the transferred mass. For other equations, the transport due to mass transfer needs to be explicitly modeled by the user.

As discussed in the previous paragraph, mass transfer effects in the framework of ANSYS FLUENT’s general multiphase models (i.e., Eulerian multiphase, mixture multiphase, or VOF multiphase) can be modeled in one of three ways:

  • Unidirectional constant rate mass transfer (not available for VOF calculations)
  • UDF-prescribed mass transfer
  • mass transfer through cavitation
  • mass transfer through evaporation-condensation

MR-CFD, an expert in the field of mass transfer simulations

With several years of experience simulating a wide range of problems in various CFD fields using Fluent software, the MR-CFD team is ready to offer extensive modeling, meshing, and simulation services. Simulation Services for mass transfer simulations are categorized as follows:

  • Diffusion of CO emission in a multistory car parking
  • Evaporation of water in a high-temperature pond
  • Surface evaporation in desalination systems
  • Dehumidification and emission removal in the surgery room
  • Cylindrical perforated dryer
  • Dehumidification from seeds
  • Cavitation in pumps and spillways (Straight Drop, ogee, shaft, chute, Side channel, siphon labyrinth)

Our services are not limited to the mentioned subjects, and the MR-CFD team is ready to undertake different and challenging projects in the mass transfer field ordered by our customers. You can consult with our experts freely and without charge at first, and then order your project by sending the problem details to us using the following address.

[email protected]

By entrusting your project to the MR-CFD team, you will not only receive the related project’s files (Geometry, Mesh, Fluent files). Also, you will be provided with an extensive tutorial video demonstrating how you can create the geometry, mesh, and define the needed settings in the Fluent software all by yourself. And these all come with post-technical support from the MR-CFD team.

MR-CFD experts are ready to fulfill every Computational Fluid Dynamic (CFD) needs. Our service includes industrial and academic purposes considering a wide range of CFD problems. MR-CFD serves in three main categories: ANSYS Fluent Consultation, ANSYS Fluent Training, and ANSYS Fluent Project Simulation. MR-CFD company has gathered experts from various engineering fields to ensure the quality of CFD services. Your CFD project would be done in the shortest time, with the highest quality and reasonable cost.

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