In order to deepen the understanding of the permeate characteristics of membrane distillation considering heat and mass transfers and concentration polarisation, a fully predictive mathematical model was developed. The first part that is devoted to the natural heat and mass transfer in the rectangular cavity, states the problem in Cartesian coordinates system, involves the use of a control-volume method and solves the full vorticity transport equation together with the stream function, mass and energy equations. The temperature fields and streamlines are plotted for various geometrical conditions to show some of the flow field characteristics. The second part was set up to simulate the phenomenon present in the membrane distillation process. The mass transfer was described with combined Knudsen and Maxwell-Stefan equations and coupled to the simultaneous heat transfer. The effect of parameters such as thickness of cavity, temperature and feed flow rate were investigated. The results were compared with the available data and the agreement is satisfactory.