Date of Award:

2014

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Engineering and Technology Education

Advisor/Chair:

Bryard Wood

Abstract

A promising feedstock for biofuels is microalgae. The most economical means of cultivating microalgae is via open raceway ponds. However, a large gap in economic feasibility exists between algae-based biofuels and traditional petroleum fuels. Recent research at Utah State University has focused on increasing biomass growth by implementing Delta Wings (DWs). DWs are placed facing the incoming flow, with a 40 degree angle of attack to create large vortices which travel downstream. The trailing vortices increase vertical mixing, which in turn increases algal growth.

Past researchers at USU quantified vertical mixing with new metrics, optimized various raceway operating conditions, and established a positive correlation between the newly defined metrics and algal growth. Research was performed with the aid of a small-scale clear acrylic raceway. Both stereo particle image velocimetry (SPIV) and acoustic Doppler velocimetry (ADV) were used to estimate the recently defined mixing metric: the vertical mixing index (VMI). The focus of this work is to ascertain additional preferred operating conditions, in particular those unique to large scale raceways, with the aid of a computational fluid dynamics (CFD) model validated by experimental data. Three case studies are presented herein, which analyze the DW vertical position (VP), array spacing ratio (ASR), and the projected height to depth ratio (PHDR). The criteria for these studies are the VMI and power consumption.

While it was previously assumed that vertically centering the DW centroid was optimal, the first case study revealed the ideal VP to be far lower. The lowest possible VP allows the trailing vortices to travel further downstream, resulting in increased vertical mixing. The second case study entails modeling complete arrays of DWs with various spacing. This model was the first to account for an increase in the number of allowable DWs with a decrease in array spacing. The ASR study revealed the ideal array spacing to be approximately half a DW, as opposed to the initial estimate of a full DW. The third case study confirmed the largest allowable DW to be superior.

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