Kevin Drost

RANS Predictions of Turbulent Scalar Transport in Dead Zones of Natural Streams

Natural stream systems contain a variety of flow geometries which contain flow separation, turbulent shear layers, and recirculation zones. This work focuses on stream dead zones. Characterized by slower flow and recirculation, dead zones are naturally occurring cutouts in stream banks. These dead zones play an important role in stream nutrient retention and solute transport studies. Previous experimental work has focused on idealized dead zone geometries studied in laboratory flumes. This work studies the capabilities of computational fluid dynamics (CFD) to investigate the scaling relationships between flow parameters of an idealized geometry and the passive scalar exchange rate. The stream geometry can be split into two main regions, the main stream flow and the dead zone. For the base case simulation, the depth-based Reynolds number is 16,000 and the dead zone is 0.5 depths in the flow direction and 7.5 depths in the transverse direction. Dead zone lengths and the main stream velocity were varied. These flow geometries are simulated using RANS turbulence model and the standard k–ω closure. Scalar transport in dead zones is typically modeled as a continuously stirred tank with an exchange coefficient for the interface across the shear layer. This first order model produces an exponential decay of scalar in the dead zone. A two region model is also developed and applied to the RANS results. Various time scales are found to characterize the exchange process. The volumetric time scale varies linearly with the aspect ratio. The simulations showed significant spatial variation in concentration leading to many different time scales. An optimized two region model was found to model these different time scales extremely well.Copyright

Low Reynolds Number Flow Dynamics of a Thin Airfoil with an Actuated Leading Edge

Use of oscillatory actuation of the leading edge of a thin, flat, rigid airfoil, as a potential mechanism for control or improved performance of a micro-air vehicle (MAV), was investigated by performing direct numerical simulations at low Reynolds numbers. The leading edge of the airfoil is hinged at one-third chord length allowing dynamic variations in the effective angle of attack through specified oscillations (flapping). This leading edge actuation results in transient variations in the effective camber and angle of attack that can be used to alleviate the strength of the leading edge vortex at high angles of attack. A fictitious-domain based finite volume approach [Apte et al., JCP 2009] was used to compute the moving boundary problem on a fixed background mesh. The flow solver is three-dimensional, parallel, secondorder accurate, capable of using structured or arbitrarily shaped unstructured meshes and has been validated for a range of canonical test cases including flow over cylinder and sphere at different Reynolds numbers, and flow-induced by inline oscillation of a cylinder. Flow over a plunging SD7003 airfoil at two Reynolds numbers (1000 and 10,000) was computed and results compared with those obtained using AFRL’s high-fidelity solver [Visbal, AIAA J. (2009)] to show good predictive capability. To assess the effect of an actuated leading edge on the flow field and aerodynamic loads, two-dimensional parametric studies were performed on a thin, flat airfoil at 20 degrees angle of attack and Reynolds number of 14,700 (based on the chord length) with sinusoidal actuation of the leading edge over a range of reduced frequencies (k=0.57-11.4) and actuation amplitudes. It was found that high-frequency, low-amplitude actuation of the leading edge significantly alters the leading edge boundary-layer and vortex shedding and increases the mean lift-to-drag ratio. This study indicates that the concept of an actuated leading-edge has potential for development of control techniques to stabilize and maneuver MAVs in response to unsteady perturbations at low Reynolds numbers. The summer research at AFRL’s computational sciences division has resulted in several opportunities for future collaborations with AFRL scientists and researchers. At Oregon State, new projects for senior students are initiated to build and modify the existing physical setup and measure lift and drag coefficients.

High Flux Microscale Solar Thermal Receiver for Supercritical Carbon Dioxide Cycles

Characterization of a microchannel solar thermal receiver for a supercritical carbon dioxide (sCO2) is presented. The receiver design is based on conjugate computational fluid dynamics and heat transfer simulations as well as thermo-mechanical stress analysis. Two receivers are fabricated and experimentally characterized — a parallel microchannel design and a microscale pin fin array design. Lab-scale experiments have been used to demonstrate the receiver integrity at the design pressure of 125 bar at 750°C surface temperature. A concentrated solar simulator was designed and assembled to characterize the thermal performance of the lab scale receiver test articles. Results indicate that, for a fixed exit fluid temperature of 650°C, increase in incident heat flux results in an increase in receiver and thermal efficiency. At a fixed heat flux, efficiency decreased with an increase in receiver surface temperature. The ability to absorb flux of up to 100 W/cm2 at thermal efficiency in excess of 90 percent and exit fluid temperature of 650°C using the microchannel receiver is demonstrated. Pressure drop for the pin array at the maximum flow rate for heat transfer experiments is less than 0.64 percent of line pressure.Copyright

DETAILED NUMERICAL MODELING OF A MICROCHANNEL REACTOR FOR METHANE-STEAM REFORMING

Numerical modeling of methane-steam reforming is performed in a microchannel with heat input through Palladium-deposited channel walls corresponding to the experimental setup of Eilers [1]. The low-Mach number, variable density Navier-Stokes equations together with multicomponent reactions are solved using a parallel numerical framework. Methane-steam reforming is modeled by three reduced-order reactions occurring on the reactor walls. The surface reactions in the presence of Palladium catalyst are modeled as Neumann boundary conditions to the governing equations. Use of microchannels with deposited layer of Palladium catalyst gives rise to a non-uniform distribution of active reaction sites. The surface reaction rates, based on Arrhenius type model and obtained from literature on packed-bed reactors, are modified by a correction factor to account for these effects. The reaction-rate correction factor is obtained by making use of the experimental data for specific flow conditions. The modified reaction rates are then used to predict hydrogen production in a microchannel configuration at different flow rates and results are validated to show good agreement. It is found that the endothermic reactions occurring on the catalyst surface dominate the exothermic water-gas-shift reaction. It is also observed that the methane-to-steam conversion occurs rapidly in the first half of the mircochannel. A simple one-dimensional model solving steady state species mass fraction, energy, and overall conservation of mass equations is developed and verified against the full DNS study to show good agreement.Copyright

Design of A Microchannel Based Solar Receiver/Reactor for Methane-Steam Reforming

This study investigates use of solar thermochemical processing of clean fuels using biomass products (in particular CH4, H2O). To address technological feasibility of a microchannel-based solar receiver/reactor, a combined numerical and experimental study of methane-steam reforming is carried out on a single microchannel with Palladium-deposited channel walls and heat input to facilitate endothermic heterogeneous reactions producing syngas. A simple one-dimensional model solving steady state species mass fraction, energy, and overall conservation of mass equations is developed, calibrated and validated against concurrent experimental data [1, 2]. Methane-steam reforming is modeled by three reduced-order reactions occurring on the reactor walls. The effects of the total heat input, heat flux profile, and inlet flow rate on production of hydrogen are investigated to assess the effectiveness of the microchannel configuration for production of hydrogen. A coupled shape-constrained optimization and Monte-Carlo...

Microscale Enhancement of Heat and Mass Transfer for Hydrogen Energy Storage

The document summarized the technical progress associated with OSU’s involvement in the Hydrogen Storage Engineering Center of Excellence. OSU focused on the development of microscale enhancement technologies for improving heat and mass transfer in automotive hydrogen storage systems. OSU’s key contributions included the development of an extremely compact microchannel combustion system for discharging hydrogen storage systems and a thermal management system for adsorption based hydrogen storage using microchannel cooling (the Modular Adsorption Tank Insert or MATI).

Numerical Design of a Planar High-Flux Microchannel Solar Receiver

This paper discuses the design of several micro-channel solar receiver devices. Due to enhanced heat transfer in micro-channels, these devices can achieve a higher surface efficiency than current receiver technology, leading to an increase in overall plant efficiency. The goal is to design an efficient solar receiver based on use of super-critical carbon-dioxide and molten salt as heat-transfer fluids. The super-critical Brayton cycle has shown potential for a higher efficiency than current power cycles used in CSP. Molten salt has been used in CSP applications in the past. The required inlet and outlet temperatures of the fluid are 773.15 K and 923.15 K for carbon-dioxide and 573.15 K and 873.15 K for molten salt. These temperature values are determined by the power cycles the devices are designed to operate in. The required maximum pressure drop is 0.35 bar for carbon-dioxide and 1 bar for molten salt. These pressure values are intended to be a practical goal for maximum pressure drop. The super-critical carbon-dioxide power cycle requires an operating pressure of is 120 bar. Finally, each device must withstand any mechanical and thermal stresses that may exist. Devices presented range in size from 1 cm2 to 4 cm2 and in heat transfer rates from 200 W to 400 W. The size of the device is based on the output capacity of the solar simulator which will be used for testing. For carbon-dioxide, three designs were developed with varying manufacturability. The low risk design features machined and welded parts and straight parallel channels. The medium risk design features machined and diffusion bonded parts and straight parallel channels. The high risk design features a circular micro-pin-fin array created using EDM and is constructed using diffusion bonding. The absence of high operating pressure for molten salt made structural design much easier than for carbon-dioxide. Conjugate heat-transfer simulations of each design were used to evaluate pressure drop, receiver efficiency, and flow distribution. Two and three dimensional structural analyses were used to ensure that the devices would withstand the mechanical and thermal stresses. Based on the numerical analyses, a receiver efficiency of 89.7% with a pressure drop of 0.2 bar were achieved for carbon-dioxide. The design was found to have a structural safety factor of 1.3 based maximum mechanical stress occurring in the headers. For molten salt, an efficiency of 92.1% was achieved with a pressure drop of 0.5 bar.Copyright

Parameterization of Mean Residence Times in Idealized Rectangular Dead Zones Representative of Natural Streams

AbstractThree-dimensional Reynolds averaged Navier-Stokes modeling, validated against experimental data, is used to parameterize the flow features and time scales in idealized rectangular cavities for a wide range of width-to-length ratios, 0.4≤W/L≤1.1, and Reynolds number based on the depth, 5,000≤RD≤20,300, representative of isolated dead zones in small natural streams. The flow features for this parameter range are similar to open cavity flows and consist of a mixing layer spanning the entire length of the dead zone together with a single main recirculation region. The Langmuir time scale (ratio of dead-zone volume to discharge) based on the assumption of a well-mixed dead zone is found to be a function of the mean rotation time scale (inverse of average rotation rate) within the dead zone, the momentum thickness of the upstream boundary layer, and the dead-zone width. The entrainment coefficient, used to relate the exchange velocity to the average free- stream velocity, is shown to be directly related...

Numerical Design of a High-Flux Microchannel Solar Receiver

This computational study investigates design of microchannel based solar receiver for use in concentrated solar power. A design consisting of a planar array of channels with solar flux incident on one side and using supercritical carbon dioxide as the working fluid is sought. Use of microchannels is investigated as they offer enhanced heat transfer in solar receivers and have the potential to dramatically reduce the size and increase the performance. Designs are investigated for an incident heat flux of 1 MW/m2, up to 3.3 times that of current solar receivers [1], resulting in significant reduction of size and cost. The goal is to design a microchannel receiver with inlet and outlet temperatures of the working fluid of 500°C and 650°C, operating pressure of 100 bar, pressure drop less than 0.35 bar and surface efficiency greater than 90% defined by radiation and convection losses to the environment. Three micro-channel designs are considered: rectangular cross section with high and low aspect ratio (designs A and B) and rectangular cross section with an array of micro pin-fins of various shape spanning the height of the channel (design C). Numerical simulations are performed on individual channels and on a unit cell of the pin-fin design. Structural analysis is performed to ensure that the design can withstand the operating pressure and thermal stresses. The effects of flow maldistribution and header system in an array of channels are also investigated. Preliminary results show that all three designs are capable of meeting the requirements, with the pin-fin design having the lowest pressure drop and highest efficiency.© 2013 ASME

Enhancement of Heat and Mass Transfer in Mechanically Contstrained Ultra Thin Films

Oregon State University (OSU) and the Pacific Northwest National Laboratory (PNNL) were funded by the U.S. Department of Energy to conduct research focused on resolving the key technical issues that limited the deployment of efficient and extremely compact microtechnology based heat actuated absorption heat pumps and gas absorbers. Success in demonstrating these technologies will reduce the main barriers to the deployment of a technology that can significantly reduce energy consumption in the building, automotive and industrial sectors while providing a technology that can improve our ability to sequester CO{sub 2}. The proposed research cost