Mark A. Havstad

Methanol steam reformer on a silicon wafer

A study of the reforming rates, heat transfer and flow through a methanol reforming catalytic microreactor fabricated on a silicon wafer are presented. Packed bed microchannel reactors were fabricated using silicon DRIE, followed by wafer bonding. The reactor bed was subsequently filled with catalyst particles. Thermal control is achieved through on-chip resistive heaters, whereby methanol steam reforming reactions were studied over a temperature range from 180-300 degC. Three simulations of varying complexity, including three-dimensional (3-D), quasi-3-D, and 1-D models, were developed. Comparison of the models with experimental results shows good agreement over a range of operating conditions. We found that Amphletts kinetics for methanol reforming provided accurate results, and that for our operating conditions the reforming reaction could be modeled without mass transport considerations. The 1-D model provided a rapid analytical tool to assess the performance of the microreactor. Use of such computationally efficient design tools provides an effective means to analyze the performance of microreactor designs prior to fabrication and test. Hence, reformer geometry, catalyst loading, and operating parameters can be optimized to afford the desired hydrogen output and conversion. Concepts for insulating the reactor while maintaining small overall size are further analyzed

Comparison of surface chemical kinetic models for ablative reentry of graphite

A general formulation for surface chemical reactions is used with a finite element heat conduction code to compare computations of the ablated mass flux from carbon bodies experiencing conditions representative of Earth reentry. Several credible models for the surface chemical kinetics are exercised with the formulation and are compared both to each other and to test data obtained by the Passive Nosetip Technology program in the mid-1970s. Sublimation of C 5 and C 7 is shown to be a concern for surface temperatures greater than about 3900 K. The best match between measurements and the calculations is obtained with surface chemical models that use the usual CO formation reactions and the sublimation of C 1 -C 3 but that also include CN formation and the sublimation of C 5 and C 7 . For surface temperatures above 3500 K and for similar assumptions for the equilibrium vapor pressure and evaporation coefficients of the sublimated species, the net reaction rate approach and the surface site occupation approach give similar ablated mass fluxes.

An accelerated multi-zone model for engine cycle simulation of homogeneous charge compression ignition combustion

We have developed an accelerated multi-zone model for engine cycle simulation (AMECS) of homogeneous charge compression ignition (HCCI) combustion. This model incorporates chemical kinetics and is intended for use in system-level simulation software. A novel methodology to capture thermal stratification in the multi-zone model is proposed. The methodology calculates thermal stratification inside the cylinder based on a single computational fluid dynamics (CFD) calculation for motored conditions. CFD results are used for tuning zone heat loss multipliers that characterize wall heat loss from each individual engine zone based on the assumption that these heat loss multipliers can then be used at operating conditions different from those used in the single CFD run because the functional form of thermal stratification is more dependent on engine geometry than on operating conditions. The model is benchmarked against detailed CFD calculations and fully coupled HCCI CFD chemical kinetics calculations. The results indicate that the heat loss multiplier approach accurately predicts thermal stratification during the compression stroke and (therefore) HCCI combustion. The AMECS model with the thermal stratification methodology and reduced gasoline chemical kinetics shows good agreement with boosted gasoline HCCI experiments over a range of operating conditions, in terms of in-cylinder pressure and heat release rate predictions. The computational advantage of this method derives from the need for only a single motoring CFD run for a given engine, which makes the method very well suited for rapid HCCI calculations in system-level codes such as GT-Power, where it is often desirable to evaluate consecutive engine cycles.

Detailed Chemical Kinetic Modeling of Iso-octane SI-HCCI Transition

We describe a CHEMKIN-based multi-zone model that simulates the expected combustion variations in a single-cylinder engine fueled with iso-octane as the engine transitions from spark-ignited (SI) combustion to homogenous charge compression ignition (HCCI) combustion. The model includes a 63-species reaction mechanism and mass and energy balances for the cylinder and the exhaust flow. For this study we assumed that the SI-to-HCCI transition is implemented by means of increasing the internal exhaust gas recirculation (EGR) at constant engine speed. This transition scenario is consistent with that implemented in previously reported experimental measurements on an experimental engine equipped with variable valve actuation. We find that the model captures many of the important experimental trends, including stable SI combustion at low EGR (-0.10), a transition to highly unstable combustion at intermediate EGR, and finally stable HCCI combustion at very high EGR (-0.75). Remaining differences between the predicted and experimental instability patterns indicate that there is further room for model improvement.

Transport in a Microfluidic Catalytic Reactor

A study of the heat and mass transfer, flow, and thermodynamics of the reacting flow in a catalytic microreactor is presented. Methanol reforming is utilized in the fuel processing system driving a micro-scale proton exchange membrane fuel cell. Understanding the flow and thermal transport phenomena as well as the reaction mechanisms is essential for improving the efficiency of the reforming process as well as the quality of the processed fuel. Numerical studies have been carried out to characterize the transport in a silicon microfabricated reactor system. On the basis of these results, optimized conditions for fuel processing are determined.

Numerical solution of the three-dimensional fluid flow in a rotating heterogeneous porous channel

A numerical solution to the problem of the three-dimensional fluid flow in a long rotating heterogeneous porous channel is presented. A co-ordinate transformation technique is employed to obtain accurate solutions over a wide range of porous media Ekman number values and consequent boundary layer thicknesses. Comparisons with an approximate asymptotic solution (for large values of Ekman number) and with theoretical predictions on the validity of Taylor-Proudman theorem in porous media for small values of Ekman number show good qualitative agreement. An evaluation of the boundary layer thickness is presented and a power-law correlation to Ekman number is shown to well-represent the results for small values of Ekman number. The different three-dimensional fluid flow regimes are presented graphically, demonstrating the distinct variation of the flow field over the wide range of Ekman numbers used

Thermal and Structural Issues of Target Injection into a Laser-Driven Inertial Fusion Energy Chamber

Abstract Inertial fusion energy (IFE) targets injected into fusion chambers must withstand the demanding acceleration forces and the intense thermal environment of the fusion chamber. For indirect targets, the ultrathin capsule support membrane is the target component that is most sensitive to acceleration forces. Maintaining the deuterium-tritium (DT) temperature, to prevent a significant increase in DT vapor pressure, is the most critical thermal requirement. Secondarily, material selection of the high-temperature laser entrance hole window is required. This paper briefly describes how these requirements are satisfied for a laser-driven IFE plant design.

Integration Strategies for Efficient Multizone Chemical Kinetics Models

Three integration strategies are developed and tested for the stiff, ordinary differential equation (ODE) integrators used to solve the fully coupled multizone chemical kinetics model. Two of the strategies tested are found to provide more than an order of magnitude of improvement over the original, basic level of usage for the stiff ODE solver. One of the faster strategies uses a decoupled, or segregated, multizone model to generate an approximate Jacobian. This approach yields a 35-fold reduction in the computational cost for a 20 zone model. Using the same approximate Jacobian as a preconditioner for an iterative Krylov-type linear system solver, the second improved strategy achieves a 75-fold reduction in the computational cost for a 20 zone model. The faster strategies achieve their cost savings with no significant loss of accuracy. The pressure, temperature and major species mass fractions agree with the solution from the original integration approach to within six significant digits; and the radical mass fractions agree with the original solution to within four significant digits. The faster strategies effectively change the cost scaling of the multizone model from cubic to quadratic, with respect to the number of zones. As a consequence of the improved scaling, the 40 zone model offers more than a 250-fold cost savings over the basic calculation.

LIFE: a sustainable solution for developing safe, clean fusion power.

AbstractThe National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) in California is currently in operation with the goal to demonstrate fusion energy gain for the first time in the laboratory—also referred to as “ignition.” Based on these demonstration experiments, the Laser Inertial Fusion Energy (LIFE) power plant is being designed at LLNL in partnership with other institutions with the goal to deliver baseload electricity from safe, secure, sustainable fusion power in a time scale that is consistent with the energy market needs. For this purpose, the LIFE design takes advantage of recent advances in diode-pumped, solid-state laser technology and adopts the paradigm of Line Replaceable Units used on the NIF to provide high levels of availability and maintainability and mitigate the need for advanced materials development. The LIFE market entry plant will demonstrate the feasibility of a closed fusion fuel cycle, including tritium breeding, extraction, processing, refueling, accountability, and safety, in a steady-state power-producing device. While many fusion plant designs require large quantities of tritium for startup and operations, a range of design choices made for the LIFE fuel cycle act to reduce the in-process tritium inventory. This paper presents an overview of the delivery plan and the preconceptual design of the LIFE facility with emphasis on the key safety design principles being adopted. In order to illustrate the favorable safety characteristics of the LIFE design, some initial accident analysis results are presented that indicate potential for a more attractive licensing regime than that of current fission reactors.

The Effect of Permeability Variations on the Flow in a Heterogeneous Porous Channel Subject to Rotation

A significant effect of permeability variations on the three-dimensional fluid flow in a heterogeneous porous channel subject to rotation is presented. The results of numerical solution to the governing equations confirm for the more general case the conclusions from earlier analytical investigations, which suggest that permeability functions be classified corresponding to whether their variation is monotonic or not, and to whether their vertical gradient is positive or not. Unicellular and multiple vortex solutions are obtained for the secondary flow in the plane perpendicular to the imposed axial flow, while their direction is dictated by the corresponding class of permeability function as applicable. The impact of rotation on the imposed axial flow is shown to be significant as well, leading to different axial flow fields depending again on the class of permeability function used. In particular, the rotation impacts significantly in creating axial flow deficiencies in some regions on the cross section