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Thesis (M.S.)--Ohio University, March, 1999. / Title from PDF t.p.

Algae-derived biodiesel is currently under investigation as a suitable alternative to traditional fossil-fuels. Though it possesses many favorable characteristics, algae remains prohibitively expensive to mass produce and distribute. The most economical means of growing algae are large-scale open pond raceways. These, however, suffer from low culture densities; this fact impacts the cost directly through diminished productivity, as well as indirectly by raising costs due to the necessity of dewatering low culture density raceway effluent. Algae, as a photosynthetic organism, achieves higher culture densities when sufficient light is provided. In open ponds this can be accomplished by frequently cycling algae to the raceway surface. The current work examined delta wing vortex generators as a means of instigating this cycling motion. In particular the impact of spacing and angle of attack was analyzed. These vortex generators were found to significantly increase vertical mixing when placed in a series, developing precisely the motion desired. Their impact on power requirements was also examined. Specifically it was shown that increases in spacing and decreases in angle of attack result in lower power consumption. It was demonstrated that the most efficient mixing generation is achieved by larger spacings and smaller angles of attack. The impact that these devices had on raceway turbulence as measured by dissipation rate was also investigated and compared to published values for algae growth. Raceways were found to be significantly more turbulent than standard algae environments, and adding delta wings increased these levels further.

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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Open channel raceway bioreactors are a low-cost system used to grow algae for biofuel production. Microalgae have many promises when it comes to renewable energy applications, but many economic hurdles must be overcome to achieve an economic fuel source that is competitive with petroleum-based fuels. One way to make algae more competitive is to improve vertical mixing in algae raceway bioreactors. Previous studies show that mixing may be increased by the addition of mechanisms such as airfoils. The circulation created helps move the algae from the bottom to top surface for necessary photosynthetic exchange. This improvement in light utilization allowed a certain study to achieve 2.2-2.4 times the amount of biomass relative to bioreactors without airfoils. This idea of increasing mixing in open channel raceways has been the focus of the Utah State University (USU) raceway hydraulics group. Computational Fluid Dynamics (CFD), Acoustic Doppler Velocimetry (ADV), and Particle Image Velocimetry (PIV) are all methods used at USU to computationally and experimentally quantify mixing in an open channel raceway. They have also been used to observe the effects of using delta wings (DW) in increasing vertical mixing in the raceway. These efforts showed great potential in the DW in increasing vertical mixing in the open channel bioreactor. However, this research begged the question, does the DW help increase algae growth? Three algae growth experiments comparing growth in a raceway with and without DW were completed. These experiments were successful, yielding an average 27.1% increase in the biomass. The DW appears to be a promising method of increasing algae biomass production. The next important step was to quantify vertical mixing and understand flow patterns due to two DWs side-by-side. Raceway channels are wider as they increase in size; and arrays of DWs will need to be installed to achieve quality mixing throughout the bioreactor. Quality mixing was attained for several paddle wheel (PW) speeds. Also, an optimal spacing between the DWs in an array was found to be the width of the DW. This optimal spacing allows for the best increase in vertical mixing along the width of the channel. Dimensional analysis was performed using experimental data to estimate vertical mixing index (VMI) results for data obtained by larger scale DW experiments. This rough analysis showed that the VMI may be estimated from small to large scale within 26.6% and 26.5% when equating Reynolds and Froude numbers, respectively. These results suggest that quality mixing would still be present at a larger DW scale.

Turbulent mixing plays an important role in the distribution of sunlight, carbon dioxide, and nutrients for algae in the raceway ponds. For large-scale raceway ponds the choice of mixing technology still needs to be evaluated in order to prevent algae sedimentation and to enhance light utilization efficiency. In open ponds, mixing the algae culture is of great significance in terms of input energy costs and particularly productivity. A very small amount of research has been performed previously using different vortex generators in the algal raceway ponds, but the quantification of mixing depth relationships is not defined well. By accepting the premise from the literature review that mixing increases algal production, delta wings were selected to study mixing characteristics in the raceway. The main objective of this research was to study algae-raceway hydrodynamics with an emphasis on increasing vertical mixing. A clear acrylic raceway was designed and constructed for flow visualization studies. Experimental investigations were performed to quantify the vertical mixing with and without delta wings in a lab-scale raceway at approximately the same power input to the paddle wheel. Velocity vector profiles and turbulence parameters were measured using an Acoustic Doppler Velocimeter (ADV) at various locations along the entire length of the raceway. The results indicated that the addition of delta wings increases the vertical mixing intensity or circulation of algae cells over the raceway depth. Vortices were observed in the raceway up to a distance of around 3 m downstream of the delta wing. This sort of systematic vertical mixing plays an important role to produce the flashing light effect (light-dark cycles) on algae mass culture. In addition, turbulence dissipation rates were evaluated to compare them with the published literature and to estimate the microscales using the Kolmogorov hypothesis. Also, an energy model was developed to operate the paddlewheel-driven raceway with the delta wing.

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