Research Work

Ph.D. Thesis Abstract:

Investigation of fluid flow in various forms compressible/incompressible, rarefied/non-rarefied, choked/non-choked through various test sections micro/macro, smooth/textured/rough, straight/complex is of great interest. Each of these complexities brings about new flow physics and therefore needs to be carefully looked into. The main objective of this thesis is to focus on the frictional resistance experienced by the gaseous flow in rough and textured surfaces at the micro-scale. Three different sets of experimental campaigns and two sets of numerical campaigns have been carried out. The first experimental campaign deals with the first-ever mass flow rate, pressure, and temperature measurements for a three-dimensional microchannel of aspect ratio 0.49 in the slip flow regime. The objective is to inspect the occurrence of subsonic choking phenomenon. The nature of the choked state is carefully scrutinized and found to be more adiabatic than isothermal. The frictional resistance is examined at various inputs of mass flow rates. Any kind of pressure loss due to pressure measurements at end manifolds on overall pressure drop is calculated using existing empirical correlations. The second experimental campaign deals with fabricating ridges of different lengths protruding outward from the microchannel surface and comparing them with smooth microchannel. The key interest is to check if subsonic choking can be delayed in the channel by the creation of such micro-ridges, which has been explored for the first time. Subsonic choking is observed to occur for ridges of the shortest length, but not when the length of the ridge is increased. These micro-ridges have also been studied numerically, to scrutinize the influence of geometry, Tangential momentum accommodation coefficient, Reynolds number and Knudsen number on Poiseuille number. The second numerical campaign deals with Taguchi analysis of microchannels with cavities to find the textured configuration with minimum flow friction and pumping power requirement. It is established that the texture height ratio (h/H) is the most dominant control factor and the texture shape is the least dominant factor. The third experimental campaign deals with random roughness enhancement in stainless steel micro-tubes by chemical etching. The Poiseuille number is compared for the etched versus unetched tubes to check for any significant impact of roughness enhancement on the gas flow. The etching of micro-tubes increases the Poiseuille number for gas flow. This thesis would be useful in industrial cooling applications for heat transfer enhancement, choking delay for mass transfer enhancement, high altitude vehicles, long distance gas transport, vacuum, and biomedical devices.