Title

Transport phenomena in micro/miniature channels for two phase flow heat transfer devices including heat pipes and fuel cells

Date of Completion

January 2003

Keywords

Engineering, Mechanical

Degree

Ph.D.

Abstract

A number of applications where small sizes cause transport phenomena not present in conventional size devices have become important for the development of economically viable products. The relationship of device and system performance to the essential parameters associated with the participating processes must be well known in order to implement efficient design methodologies. Three associated problems are presented in detail in this thesis. ^ A mathematical model of annular film condensation in a miniature tube is developed. The liquid flow is coupled with the vapor flow along the liquid-vapor interface through the interfacial temperature, heat flux, shear stress and pressure jump conditions due to surface tension effects. The model predicts the shape of the liquid-vapor interface along the condenser and leads to the conclusion that there is complete condensation, with a single continuous region of vapor in the two phase flow region, at a specific distance from the condenser inlet for circular tubes of 1 to 3 mm diameter. ^ Measurements of heat transfer and pressure drop were conducted for water undergoing complete condensation in a family of horizontal tubes with diameters from 1.7 to 4 mm. Average heat transfer coefficients are reported for tubes of 1.7, 2.5, 3.2 and 4.0 mm diameter, over a range of mass fluxes from 10 to 25 kg/m2s, at saturation temperatures of 60, 70, 80, and 90°C. Flow visualization experiments reveal a skewed annular liquid film over nearly the entire condensation length. ^ Finally, a model of diffusion is used in conjunction with experimental data from polymer electrolyte membrane (PEM) fuel cells to examine the resistance to mass transport caused by the gas diffusion layer (GDL) on the cathode side while operating at elevated temperatures. Using the Stefan-Maxwell formulation for a multi-component gas, a one-dimensional model is derived for diffusion in a porous media with two distinct layers. The diffusional resistance in the microporous layer of the GDL is always 70–80% of the total resistance. By examining the initial conditions for the model, the effect of humidification on oxygen transport in a PEM fuel cell operating at ambient pressure and elevated temperature are determined. ^