Two-Phase Flows in Mini-/Micro-Gas Flow Channels of PEM Fuel Cells

In this paper, two-phase flow dynamics in a micro channel with various wall conditions are both experimentally and theoretically investigated. Annulus, wavy and slug flow patterns are observed and location of liquid phase on different wall condition is visualized. The impact of flow structure on two-phase pressure drop is explained. Two-phase pressure drop is compared to a two-fluid model with relative permeability correlation. Optimization of correlation is conducted for each experimental case and theoretical solution for the flows in a circular channel is developed for annulus flow pattern showing a good match with experimental data in homogeneous channel case.

CFD Modeling of Pressure Coefficients for a Natural Ventilation Study in an Experimental Low Rise Building

Mean wind pressure coefficient (Cp) is one of the major input data for natural ventilation study using building energy simulation approach. Due to their importance, they need to be accurately determined. In current engineering practice, tables and analytical Cp models only give mostly averaged results for simpler models and configurations. Considering the limitation of tables and analytical models, Computational Fluid Dynamics (CFD) could provide a means for an accurate and detailed assessment of Cp. In this paper, we make use of a relatively high resolution, detailed experiments done at Florida Intentional University to validate a CFD modeling of the pressure coefficients Cp. The results show that existing CFD model has a good agreement with experimental results and gives important information of distribution of Cp values over the surface. The local values of the Cp are investigated. In addition, the CFD derived Cp and discharge coefficient (Cd) values are utilized in semi-analytical ventilation models in order to get a more accurate value of ventilation rates.

Energy Benchmarking of Water and Wastewater Treatment, Distribution and Collection: A Case Study of Austin Water Utility

Nationally, 4% of electricity usage goes towards moving and treating water and wastewater. The energy intensity of the water and wastewater utility sector is affected by many factors including water source, water quality, and the distance and elevation that water must be transported. Furthermore, energy accounts for 10% or more of a utility’s total operating cost, suggesting that energy savings can account for significant cost savings. Better knowledge of where and when energy is used could support strategic energy interventions and reveal opportunities for efficiency. Accordingly, this investigation quantifies energy intensity by process and type, including electricity and natural gas, and explores the time-varying nature of electric energy consumption for potable water distribution using the Austin Water Utility (AWU) in Austin, Texas as a case study. This research found that most of energy consumed by the AWU is for pumping throughout the distribution network (57%) and at lift stations (10%) while potable water treatment accounts for the least (5%). Though the focus is site specific, the methodology shown herein can be applied to other utilities with sufficient data.

Value-Centric Approaches to Design, Operations, and Maintenance of Wind Turbines

Wind turbine maintenance is emerging as an unexpectedly high component of turbine operating cost and there is an increasing interest in managing this cost. Here, we present an alternative view of maintenance as a value-driver, and develop an optimization algorithm to maximize the value delivered by maintenance. We model the stochastic deterioration of the turbine in two dimensions: the deterioration rate, and the extent of deterioration, and view maintenance as an operator that moves the turbine to an improved state in which it can generate more power and so earn more revenue. We then use a standard net present value (NPV) approach to calculate the value of the turbine by deducting the costs incurred in the installation, operations and maintenance from the revenue due to the power generation. The application of our model is demonstrated using several scenarios with a focus on blade deterioration. We evaluate the value delivered by implementing blade condition monitoring systems (CMS). A higher fidelity CMS allows the blade state to be determined with higher precision. With this improved state information, an optimal maintenance strategy can be derived. The difference between the value of the turbine with and without CMS can be interpreted as the value of the CMS. The results indicate that a higher fidelity (and more expensive) condition monitoring system (CMS) does not necessarily yield the highest value, and, that there is an optimal level of fidelity that results in maximum value. The contributions of this work are twofold. First, it is a practical approach to wind turbine valuation and operation that takes operating and market conditions into account. This work should therefore be useful to wind farm operators and investors. Second, it shows how the value of a CMS can be explicitly assessed. This work should therefore be useful to CMS manufacturers and wind farm operators.

Second Law Analysis of Tree-Shaped Fins for the Heat Transfer Enhancement in a PCM Thermal Storage System

Latent heat thermal energy storage (LHTES) systems based on phase change materials (PCMs) are a promising option to be employed as effective energy storage devices. PCM allows one to achieve high energy storage density and almost constant temperature energy retrieval, however LHTES systems performance is limited by poor thermal conductivity of the PCMs which leads to unacceptably low melting and solidification rates. Thus, heat transfer enhancement techniques are required in order to obtain acceptable melting and solidification rates. The preliminary design of a shell-and-tube LHTES unit is investigated by means of computational fluid-dynamics (CFD). Three different fin designs are considered: a conventional radial fin, a constructal Y-shaped fin design and a non-constructal Y-shaped configuration previously investigated by the authors. The performances of each fin configuration are evaluated by means of a Second-law analysis. Moreover, local and global entropy generation rates are analyzed in order to show the main source of thermodynamic irreversibilities occurring in the system. The analysis indicates that solidification rate is significantly enhanced when Y-shaped fins are adopted in the LHTES unit, however the constructal Y-shaped geometry is not optimal since further improvements can be achieved by means of a Y-shaped fins with elongated secondary branches.

Influence of Skewing of Contact Face on Performance of Wave Rotor Refrigerators and Superchargers

As a novel generation of rotational gas wave machines, wave rotor machines such as wave rotor refrigerators (WRR) and wave rotor superchargers (WRS) are unsteady flow devices. In their passages two gas streams (with different pressure or even different phases) comes into direct contact can exchange energy due to the movement of shock waves and expansion waves. A detailed study shows that, when rotor channels open to the high pressure port gradually, the contact face in rotor channels inevitably skews, which is always accompanied with reflection of shockwaves. This causes very large energy dissipation and influences adversely on the refrigeration performance of WRR or the supercharging performance of WRS. In this work, factors such as centrifugal forces, Coriolis forces, gradual channel opening and gradual channel closing, etc, which influence the wave transportation and skewing of shock waves and contact faces are studied by means of computational fluid dynamics and experiments. The skewing of contact faces causes uneven distribution of velocity and large local loss. With rotation Mach number smaller than 0.3, the skewing of contact face can be alleviated. To reduce the adverse influence of rotation Mach number, a smaller rotor channel width or higher rotational speed is necessary. The rotation effect plays an important role for the skewing of gas discontinuities. Both the centrifugal and Coriolis forces of wave rotor cannot be ignored with the Rossby number of 1.3∼3.5. To reduce the skewing loss of contact face, a lower rotational speed seems necessary. The rotation speed of wave rotors has dialectical influences on the skewing of shock waves and contact faces. The jetting width of high pressure port is the key factor of the gradual opening of rotor channels. A feasible way to reduce skewing losses of gas waves is to optimize the ratio between high pressure port width and channel width. The validation experiments have got at least 3∼5% rise of isentropic efficiency for WRRs.

Effect of Adhesive Flaws on the Strength of Adhesive Joints

The influence of interior and surface flaws in Z-shaped adhesive joints of carbon/epoxy wind turbine blades is examined using finite element method. Contour integral method is used for evaluating the stress intensity factors at the flaw tips, while the strength of the joint is assessed through the crack initiation and propagation simulation. The effect of adhesive shear modulus has also been investigated.

FEM Simulation of Swelling Elastomer Seals in Downhole Applications

Petroleum exploration and development industry is witnessing a rapid growth in the use of swelling elastomers. They are being used in new applications aimed at enhanced oil recovery through slimming of well design, zonal isolation, water shutoff, etc. Initially developed as a problem-solving strategy (for repair of damaged or deteriorating wells), swelling elastomers are now targeting major savings in cost and time through reduction in borehole diameter, reduced casing clearance, cementless completions, etc. Due to material and geometric nonlinearity, modeling and simulation of swelling elastomer applications becomes quite complex. In this work, finite element simulation has been carried out to study swelling elastomer seal performance in downhole petroleum applications using the software ABAQUS. A hyperelastic model (that most closely resembles swelling elastomer behavior) is used for simulation of seal behavior. A series of experiments have been designed and performed to determine necessary material properties of a water-swelling elastomer as it gradually swells when exposed to saline water of two different concentrations at 50°C (to emulate field conditions of medium-depth oil wells). A large number of simulations are carried out to investigate sealing behavior against water salinity and swelling time. Sealing pressure at the contact surface between elastomer and formation (or outer casing) is studied for variations in seal length, seal thickness, compression ratio, water salinity, and swelling period. Results show that seal contact pressure increases with amount of swelling, seal length, and compression ratio; higher salinity environment results in lower sealing pressure; and more contact pressure is generated in the case of rock formation as compared to steel outer casing.

Parametric Analysis of Single-Phase and Two-Phase Models of a Microfluidic Direct Methanol Fuel Cell

We perform numerical simulations of single-phase and two-phase models of a direct methanol microfluidic fuel cell (μ-DMFC). The focus of this study is on the parametric analysis of a single channel of the system, for specific sets of operating conditions, in order to map the dependence of the cell performance with respect to the geometrical parameters. Different geometries, ranging from 500 μm to 4 mm in width, and 500 μm to 4 cm in length, along with membrane thicknesses from 50 μm to 500 μm, were considered. The mathematical models are given in terms of the Navier-Stokes, the Butler-Volmer and the Maxwell-Stefan equations, along with Darcy’s equation for the flow across the membrane. The difference between the single- and two-phase flow models lies upon the specific constitutive equations used. For each geometry and operating condition, the two-dimensional equations were solved by a finite element method. The conditions of operation include: flow rates and inlet weight fractions of methanol at the anode and oxygen the cathode. The results from this analysis, presented as polarization curves and power densities, indicate that fuel-cell systems with higher flow rates and inlet weight fraction of methanol achieve the best performance. However, when the concentration of methanol exceeds 2M the cell performance is negatively impacted due to crossover. Comparison of the results indicates that the two-phase model has a more restrictive domain for both the geometrical parameters and operating conditions.

Examining Effects of Sulfur Poisoning on Ni/YSZ Solid Oxide Fuel Cell Anodes Using Synchrotron-Based X-Ray Imaging Techniques

Fuel flexibility is widely considered one of the most significant advantages of solid oxide fuel cells (SOFC). However, the presence of small amounts of sulfur or other impurities in the gas stream can have a serious impact on cell performance [1–10]. Under certain conditions, hydrogen sulfide (H2S), even at the ppm level, can lead to the formation of bulk nickel-sulfides within the conventional Ni–yttria-stabilized zirconia (Ni-YSZ) anode of SOFC’s [9]. Understanding the distribution of these sulfides is critical to describing their effects on the electrochemical activity of the cell.