Thomas M. Harms

A Kinetic Model for Simultaneous Saccharification and Fermentation of Avicel With Saccharomyces cerevisiae

This work describes a numerical model for predicting simultaneous saccharification and fermentation of Avicel, an insoluble crystalline cellulose polymer. Separate anoxic cultivations of 40 g/L glucose and 100 g/L Avicel were conducted to verify model predictions and obtain parameters to describe the reaction kinetics. Saccharification of Avicel was achieved with Trichoderma reesei cellulases from the enzyme preparation Spezyme CP with an enzyme loading of 10 FPU/g cellulose. Cultivations were supplemented with 50 IU/g cellulose of β‐glucosidase from Novozym 188 to prevent product inhibition by cellobiose. Saccharomyces cerevisiae MH‐1000 is a robust industrial strain and was used to ferment glucose to ethanol, glycerol, and carbon dioxide. The numerical model presented in this paper differs from previous models by separating the endoglucanase and exoglucanase enzyme kinetics and allowing for inhibitive site competition. Assuming all enzymes remain active and that each enzyme complex has a corresponding constant specific activity, the model is capable of predicting adsorbed enzyme concentrations with reasonable accuracy. Comparison of predicted values to experimental measurements indicated that the numerical model was capable of capturing the significant elements involved with cellulose conversion to ethanol. Biotechnol. Bioeng. 2011; 108:924–933.

Assessment of the wind power potential at SANAE IV base, Antarctica: a technical and economic feasibility study

This paper presents a study for the utilization of wind energy at the South African research station, SANAE IV, in Antarctica (71°40’ S 2° 50’ W). A procedure to evaluate the feasibility of utilising wind power for Antarctic stations is given. The analysis is based on the technical and economic aspects of installing and operating a wind turbine at remote locations. Special attention has been given to conditions encountered at Antarctica, like site accessibility, low temperatures, icing and snow, long transportation distances and environmental issues. The aspect of externalities is incorporated into the economic analysis. The Northern Power Systems NW100/19 wind turbine is found to be the best-suited wind turbine for use at SANAE IV, given the harsh climatic conditions, like frequent windstorms and extreme temperatures. The wind turbine features a yearly energy output of 430 MWh with a capacity factor of 0.49, at a mean wind speed of 10.8 m/s. The study shows that a wind turbine installation at SANAE IV is an attractive solution to reduce fuel consumption and therefore emissions of the diesel electric generators considerably. The use of a wind turbine at SANAE IV could lead to a savings in externalities of about R110 000, -per annum.

Outdoors modelling of snowdrift at SANAE IV Research Station, Antarctica

Abstract This paper describes the experimental modelling procedure and results to evaluate the snowdrift characteristics surrounding the research station of the South African National Antarctic Expedition, SANAE IV. For this purpose a 1:25 scale model of the research station was constructed and mounted on a large flat snow covered area in the vicinity of the base. The wind profiles of the prevailing winds were measured with a 6-m wind mast positioned near the base model and the characteristic surface roughness and shear velocity functions were derived and analysed. Snow buildup was measured behind the scale model as well as the existing research station and a comparison was evaluated between the two, based on a similarity analysis. The similarity analysis method and results are presented in this paper. It is found that the surface roughness and shear velocity functions derived from the wind velocity data compares favourably with available results in literature for similar measurements of wind profiles in drifting snow conditions. From these results an inner law velocity relationship is established with additional parameters to account for the effects of surface roughness and saltating snow. It is further shown that the scaling parameters employed for the snowdrift modelling provide acceptable results when applied to outdoors modelling of snowdrifting.

A CFD study of wind patterns over a desert dune and the effect on seed dispersion

In the Namib Desert seed distribution is greatly influenced by wind patterns. Existing literature regarding wind patterns over dunes focuses on two-dimensional simulations of flow over simplified dune structures. The three-dimensional geometries of the sand dunes suggests far more complex flow features exist, which are not captured by two-dimensional simulations. Computational fluid dynamics (CFD) was used to reproduce the three-dimensional near surface wind patterns around a dune with the aim to learn more about seed distribution. Field work included terrain mapping, wind speed, direction and temperature metering. The CFD results show the expected two-dimensional flow features of high pressure at the dune toe, low pressure at the crest and flow acceleration up windward slope. Also observed are some three-dimensional flow features such as a spiral vortex near the crest and transverse flow due to crest-line curvature of the dune. It was also observed how the wall shear stress differs due to the three-dimensional shape of the dune. The wall shear stress suggests that seed accumulation is more likely to occur behind trailing (down-wind) crest edges. Particle tracking showed how seeds tend to move over the dune crest and recirculate towards the crest on the lee-side. The study showed that adding the third dimension makes the simulations more complex, adds to computational requirements and increases simulation time but also provides vital flow information which is not possible with two-dimensional simulations.

A weakly compressible free-surface flow solver for liquid-gas systems using the volume-of-fluid approach

This paper presents a weakly compressible volume-of-fluid formulation for modelling immiscible high density ratio two-fluid flow under low Mach number conditions. This follows findings of experimental analyses that concluded the compressibility of the gas has a noteworthy effect on predicted pressure loads in liquid-gas flow in certain instances. With the aim of providing a more accurate numerical representation of dynamic two-fluid flow, the solver is subsequently extended to account for variations in gas densities. A set of governing equations is proposed, which accounts for the compressible properties of the gas phase in a manner which allows for a computationally efficient numerical simulation. Furthermore, the governing equations are numerically expressed so that they allow for large variations in the material properties, without introducing notable non-physical oscillations over the interface. For the discretisation of the governing equations an edge-based vertex-centred finite volume approach is followed. The developed solver is applied to various test cases and demonstrated to be efficient and accurate.

Simplified control-volume finite-element method

Localized vector algebra treatment of nonorthogonality is applied to two-dimensional quadrilateral control volumes using Cartesian base vectors in a primitive variable formulation of the Navier-Stokes equations for steady incompressible laminar flow. With optional grid-aligned, locally analytic interpolation, a simplified control-volume finite-element scheme is presented. Discretization of source terms, determination of interface convection-diffusion fluxes, pressure correction factors, and geometric quantities are described briefly. Results of three test cases provide useful initial insights into the performance of the method. The conclusion is reached that a simple finite-volume-based approach to nonorthogonality has been achieved.

TECHNICAL NOTE DERIVATION OF A MODIFIED POWER-LAW APPROXIMATION

The accuracy of the power-law approximation of the exponential functions appearing in the solution of the one-dimensional convection-diffusion equation is improved by an additional term derived in this note. An application in which this modification leads to a significant improvement in accuracy is described. Time trials, however, indicate that the use of partial exponential approximations can be faster than both the original and modified power-law approximations.

Fundamentals of four-quadrant axial flow compressor maps

Abstract The current paper describes the experimental determination of a complete four-quadrant map for a three stage low-speed axial flow compressor. Non-dimensional pressure rise and torque coefficients and efficiency characteristics were determined. Six possible modes of operation were identified in the four quadrants. The characteristic for a compressor with a rotational speed of zero rpm, in terms of pressure rise coefficient and flow coefficient, is S-shaped and forms the dividing line between two alternate modes of operation which exist in the second and fourth quadrants. First quadrant operation is only possible during positive rotation, and third quadrant operation during negative rotation, as the zero-speed S-curve passes through the origin and so does not pass through these quadrants. Consequently, only one mode of operation is possible in each of these two quadrants. A system of notation for describing the various modes of operation has also been developed. Second quadrant operation for positive compressor rotation, and fourth quadrant operation with negative rotation are highly dissipative conditions of operation. First and third quadrant operation are compressor-like modes. Second quadrant negative rotation and fourth quadrant positive rotation operation are turbine-like modes.

Preliminary performance analysis of a transverse flow spectrally selective two-slab packed bed volumetric receiver

A new volumetric receiver concept has been investigated, based on an adaptation of the spectrally selective, two-slab packed bed volumetric receiver concept of Flamant et al. Both slabs comprise spheres of identical size – borosilicate for the transparent slab 1 and SiC for the opaque slab 2 – which are ordered in a hexagonally close-packed bed. The flow direction has been changed from parallel to the incident radiation and perpendicular to the window, to parallel to the window and perpendicular to the incident radiation (transverse flow). The gap between the window and slab 1 has been removed, so the bed is held in place by the sidewalls, the floor and the window, allowing arbitrary orientation and dispensing with the need for beam-down operation. The receiver has been subjected to constant solar radiative load of approximately 70 suns, and the effect of variations in flowrate, the degree of air preheating as well as the thickness of slab 2 on the outlet air temperature distributions have been measured. ...

Performance outlook of the SCRAP receiver

A combined cycle (CC) concentrating solar power (CSP) plant provides significant potential to achieve an efficiency increase and an electricity cost reduction compared to current single-cycle plants. A CC CSP system requires a receiver technology capable of effectively transferring heat from concentrated solar irradiation to a pressurized air stream of a gas turbine. The small number of pressurized air receivers demonstrated to date have practical limitations, when operating at high temperatures and pressures. As yet, a robust, scalable and efficient system has to be developed and commercialized.A novel receiver system, the Spiky Central Receiver Air Pre-heater (SCRAP) concept has been proposed to comply with these requirements. The SCRAP system is conceived as a solution for an efficient and robust pressurized air receiver that could be implemented in CC CSP concepts or standalone solar Brayton cycles without a bottoming Rankine cycle.The presented work expands on previous publications on the thermal mod...