A fluid that contains species in the presence of a concentration gradient generates unusual phenomena when it is forced into pulsatile motion. For example, each species in the fluid has an enhanced mass transport due to pure molecular diffusion. This enhancement takes place even if there is no net total flow over a cycle of the pulsatile motion. When more than one species in dilute amounts is present in an otherwise pure fluid, called a carrier, each species is transported at a different rate thereby causing a partial separation of the species. This idea traces back to hyperventilation studies done over 40 years ago and to the implementation of the technology in hospital environments to provide life-support for patients under anesthesia. However, it is only in recent years that the underlying physics of oscillatory flow as applied to mass transfer have been understood and this may lead to promising application of the technique to novel means of enhancing separation in life support applications and for detection purposes. In this thesis, results from three-dimensional time accurate studies carried out using the commercial computational fluid dynamics code FLUENT are presented. These results simulate the separation of CO2 from He in an N2 environment (carrier). The model consists of two reservoirs/mixing chambers, an oscillating piston wall, and a connecting tube. Several cases are carried out reporting on separation enhancement as a function of the Womersley number and the ratio of tidal displacement to connecting tube diameter. Unlike previous studies which were undertaken using asymptotic analysis, the present models and results incorporate full entrance effects and 3D interactions. Results of this study will be useful as a guide for the design and miniaturization of an oscillating device for species separation in further research projects at the University of Central Florida. Observations showed that a molar fraction increase occurs during the species transport in the presence of a thermal boundary layer. This was accompanied by an imposed external forced temperature condition on the surface of the cylinder to create thermal diffusion, also known as the Soret or thermal diffusion effect, which refers to the separation of mixtures in a temperature gradient as means of change on the concentration gradient of the species ratio. Calculations were performed to analyze the effect of the heat transfer on the molar fraction of the species at a specific region of the model, called the measurement point. Various mathematical models and correlations were incorporated into a MATLAB computer code that predicted the concentration of the species in an entire cycle after steady state is reached and data can be exported from FLUENT.
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Master of Science in Mechanical Engineering (M.S.M.E.)
College of Engineering and Computer Science
Mechanical, Materials and Aerospace Engineering;
Length of Campus-only Access
Masters Thesis (Open Access)
Crain, Jennifer, "Oscillatory Flow As A Means Of Enhanced Species Separation: A Three Dimensional Time-accurate Cfd Analysis" (2006). Electronic Theses and Dissertations. 871.