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Feng Z.-G.,UTSA | Michaelides E.E.,TCU | Mao S.,Los Alamos National Laboratory
Fluid Dynamics Research | Year: 2012

We investigate the hydrodynamic drag force on a viscous sphere in a fluid of different viscosities at small but finite Reynolds numbers when interfacial slip is present at the surface of the sphere. The sphere is small enough for it to retain its spherical shape, as is the case with most small droplets. By using a singular perturbation method, the exterior flow field of the droplet is decomposed into an inner region, where the viscous effects dominate, and an outer region, where the inertia is important. The interior flow of the viscous sphere is also solved analytically. By applying appropriate boundary conditions to the surface of the viscous sphere and matching the conditions between the inner and outer flow fields, stream functions up to the order of Re 2 log Re for both the exterior and the interior flow are obtained. Thus, an analytical expression for the drag force coefficient of the viscous droplet is derived. This general expression yields, as special cases, several other expressions that are applicable to spheres that translate rectilinearly under more restrictive conditions. One of the practical conclusions from this study is that the presence of interfacial slip can significantly reduce the drag force on a droplet. © 2012 The Japan Society of Fluid Mechanics and IOP Publishing Ltd. Source

Michaelides E.E.,Texas Christian University | Feng Z.,UTSA
Proceedings of the 25th International Conference on Efficiency, Cost, Optimization and Simulation of Energy Conversion Systems and Processes, ECOS 2012 | Year: 2012

Coal gasification is becoming a common method of producing synthetic gas to be used subsequently with reduced pollution products. The coal gasifiers are essentially chemical reactors where the coal particles are fluidized and react chemically with the gas. The modelling of the gasifiers involves both particle flow and heat transfer processes. Direct Numerical Simulation (DNS) methods for particulate flows have been welldeveloped in the last decade. Following the significant advances in computational power, DNS methods may now be employed to solve relatively complex particulate flow problems and provide us with scientific, reliable and validated information on fluid-particle interactions that will lead to the design optimization of gasifiers. While most of the DNS methods that have been developed pertain to isothermal flows, the effects of heat and mass transfer on the momentum and energy exchanges are very important in all practical applications. This because, for most practical applications of particulate flow-e.g. fluidized bed reactors, gasifiers, coal burners and chemical reactor columns-the heat and mass transfer processes influence significantly the flow and, in addition, are of primary interest to engineers. We have developed a new DNS method, which uses an extension of the Immersed Boundary Method (IBM) to track individual particles and calculate the Lagrangian motion as well as the heat transfer from them. The model enables us to examine the lift of hotter particles due to the buoyancy their temperature field creates; the temperature field around the particles; and the effect of the Reynolds and Grashof numbers on the flow and heat transfer of suspensions. This model is also of use in the determination of the boundary conditions of particles at solid boundaries, conditions that are essential to all two-fluid models. The simulations show that a significant slip exists along the longitudinal direction at the walls of the gasifiers and that the size of the particles is the primary parameter that determines this velocity slip. Source

Araujo P.R.,Greehey Childrens Cancer Research Institute | Yoon K.,Greehey Childrens Cancer Research Institute | Ko D.,UTSA | Smith A.D.,University of Southern California | And 4 more authors.
Comparative and Functional Genomics | Year: 2012

Translation regulation plays important roles in both normal physiological conditions and diseases states. This regulation requires cis-regulatory elements located mostly in 5′; and 3′; UTRs and trans-regulatory factors (e.g., RNA binding proteins (RBPs)) which recognize specific RNA features and interact with the translation machinery to modulate its activity. In this paper, we discuss important aspects of 5′; UTR-mediated regulation by providing an overview of the characteristics and the function of the main elements present in this region, like uORF (upstream open reading frame), secondary structures, and RBPs binding motifs and different mechanisms of translation regulation and the impact they have on gene expression and human health when deregulated. Copyright © 2012 Patricia R. Araujo et al. Source

Manjili Y.S.,UTSA | Vega R.,The Texas Institute | Jamshidi M.,ACE Inc
Intelligent Automation and Soft Computing | Year: 2013

A smart decision-making framework based on genetic algorithms (GA) and fuzzy logic is proposed for control and energy management of micro-grids. Objectives are to meet the demand profile, minimize electricity consumption cost, and to modify air pollution under a dynamic electricity pricing policy. The energy demand in the micro-grid network is provided by distributed renewable energy generation (coupling solar and wind), battery storage and balancing power from the electric utility. The fuzzy intelligent approach allows the calculation of the energy exchange rate of the micro-grid storage unit as a function of time. Such exchange rate (or decision-making capability) is based on (1) the electrical energy price per kilowatt-hour (kWh), (2) local demand (load), (3) electricity generation rate of renewable resources (supply), and (4) air pollution measure, all of which are sampled at predefined rates. Then, a cost function is defined as the net dollar amount corresponding to electricity flow between micro-grid and the utility grid. To define the cost function one must consider the cost incurred by the owner of the micro-grid associated to its distribution losses, in addition to its demand and supply costs, in such a way that a positive cost translates to owner losses and a negative cost is a gain. Six likely scenarios were defined to consider different micro-grid configurations accounting for the conditions seen in micro-grids today and also the conditions to be seen in the future. GA is implemented as a heuristic (DNA-based) search algorithm to determine the sub-optimal settings of the fuzzy controller. The aforementioned net cost (which includes pricing, demand and supply measures) and air pollution measures are then compared in every scenario with the objective to identify best-practices for energy control and management of micro-grids. Performance of the proposed GA-fuzzy intelligent approach is illustrated by numerical examples, and the capabilities and flexibility of the proposed framework as a tool for solving intermittent multi-objective function problems are presented in detail. Micro-grid owners looking into adopting a smart decision-making tool for energy storage management may see an ROI between 5 and 10. © 2013 TSI® Press. Source

News Article
Site: http://www.nature.com/nature/current_issue/

The driving range of an adsorbed natural gas (ANG) vehicle is determined primarily by the volumetric usable CH capacity of the adsorbent, which is defined as the difference between the amount of CH adsorbed at the target storage pressure (generally 35–65 bar) and the amount that is still adsorbed at the lowest desorption pressure (generally 5.8 bar)8, 9, 10. With few exceptions11, adsorbents that have been investigated in the context of natural gas storage exhibit classical Langmuir-type adsorption isotherms, where the amount of CH adsorbed increases continuously, but at a decreasing rate, as the pressure is raised (Fig. 1a). Consequently, it has proved difficult to develop adsorbents with the higher usable capacities needed for a commercially viable ANG storage system9. In pursuit of a new strategy for boosting usable capacity, we endeavoured to design an adsorbent with an ‘S-shaped’ or ‘stepped’ CH adsorption isotherm, where the amount of CH adsorbed would be small at low pressures but rise sharply just before the pressure reaches the desired storage pressure (Fig. 1b). Stepped isotherms have been observed for many flexible metal–organic frameworks that exhibit ‘gate-opening’ behaviour, whereby a non-porous structure expands to a porous structure after a certain threshold gas pressure is reached, but none of these materials have exhibited characteristics beneficial for CH storage applications12, 13, 14, 15, 16. If, however, a responsive adsorbent could be designed to expand to store a high density of CH at 35–65 bar, and to collapse to push out all adsorbed CH at a pressure near 5.8 bar, then it should be possible to reach higher usable capacities than have been realized for classical adsorbents. The metal–organic framework Co(bdp) was selected as a potential responsive adsorbent for methane storage, owing to its large internal surface area and its previously demonstrated high degree of flexibility17. In its solvated form, this framework features one-dimensional chains of tetrahedral Co2+ cations bridged by μ2-pyrazolates to form a structure with square channels with edge lengths of 13 Å. The N adsorption isotherm of the evacuated framework at 77 K exhibits five distinct steps, which have been attributed to four structural transitions as the framework progresses from a collapsed phase with minimal porosity to a maximally expanded phase with a Langmuir surface area of 2,911 m2 g−1 (ref. 18). To investigate the ANG storage potential of Co(bdp), a high-pressure CH adsorption isotherm was measured at 25 °C (Fig. 1c). There is minimal CH uptake at low pressures and a sharp step in the adsorption isotherm at 16 bar. Although there is hysteresis in the desorption isotherm, the hysteresis loop is closed by 7 bar, such that there is less than 0.2 mmol g−1 of CH adsorbed at pressures below 5.8 bar. The step in the CH isotherm is fully reproducible over at least 100 adsorption–desorption cycles (Extended Data Fig. 1), and can be attributed to a reversible structural phase transition between a collapsed, non-porous framework and an expanded, porous framework at transition pressures that are ideal for ANG storage. To determine the specific structural changes responsible for the stepped CH adsorption isotherm of Co(bdp), in situ powder X-ray diffraction experiments were performed under various pressures of CH at 25 °C. Under vacuum, only one crystalline phase is observed in the diffraction pattern, consistent with the complete conversion of Co(bdp) to a collapsed phase upon desolvation. From 17 bar to 23 bar, there are substantial changes to both the positions and intensities of the diffraction peaks, as peaks corresponding to the collapsed phase decrease in intensity and peaks corresponding to a new expanded phase increase in intensity (Fig. 2a). During desorption, this expanded phase is fully converted back to the collapsed phase between 10 bar and 5 bar. Owing to the anisotropic peak widths and complex peak shapes that result from paracrystallinity effects19, analysis of the powder diffraction data is not trivial, but ab initio structure solutions followed by Rietveld refinements (Extended Data Fig. 7) were successfully performed using the diffraction data at 0 bar and 30 bar to provide crystal structures of the collapsed and expanded phases of Co(bdp) (Fig. 2d). As discussed in the Supplementary Information and shown in Extended Data Fig. 8, paracrystallinity arises from highly correlated shifts of the positions of Co-pyrazolate chains in the crystallographic a–b plane, whereby neighbouring chains exhibit average displacements of approximately 0.5 Å from their average periodic positions. Importantly, this minor systematic disordering has no effect on the accuracy of the average crystal structures or the calculated crystallographic densities of each phase. Additionally, a substantial diffuse-scattering component is present in the experimental diffraction patterns, particularly at high CH loadings. Although most of the diffuse scattering can be attributed to the thick-walled quartz glass capillaries used as sample holders in the diffraction experiments at high CH pressures (Supplementary Fig. 11), there may also be some diffuse scattering that is intrinsic to Co(bdp), which could arise from minor local disorder or from scattering by adsorbed CH molecules. Even though the density of the collapsed phase (1.50 g cm−3) is nearly double that of the expanded phase (0.77 g cm−3), the Co2+ ions adopt a similar pseudotetrahedral geometry in both structures. During the phase transition, the angles between the planes of the pyrazolate rings and the Co–N bonds decrease as the framework expands (Extended Data Fig. 6). In addition, the central benzene ring of the bdp2− ligand twists out of the plane of the two pyrazolates by 25° in the collapsed structure, resulting in edge-to-face π–π interactions with four neighbouring benzene rings that probably provide most of the thermodynamic driving force for the collapse of Co(bdp) at low pressures (Fig. 2e)20. These close contacts between neighbouring bdp2− ligands lead to no accessible porosity, and thus no CH adsorption, in the collapsed phase. The usable CH capacity of Co(bdp) at 25 °C is 155 cm3 STP cm−3 (v/v) for adsorption at 35 bar and 197 v/v for adsorption at 65 bar, which are the highest values of usable CH capacity reported so far for any adsorbent under these conditions. A recent computational analysis of a database containing over 650,000 classical adsorbents predicted a theoretical-maximum 65-bar usable capacity of 196 v/v (ref. 9); however, all adsorbents in this large-scale computational screening were rigid, and the potential utility of flexible adsorbents for CH storage was not considered. The Co(bdp) usable capacities reported here are a result of the transition from the expanded to the collapsed phase leading to near complete CH desorption by 5.8 bar. For comparison, the highest previously measured 35-bar and 65-bar usable capacities for any adsorbent are 143 v/v and 189 v/v, as obtained for the metal–organic frameworks HKUST-1 and UTSA-76a, respectively7, 8, 21. Both of these Cu paddlewheel-based frameworks have high densities of CH adsorption sites with a near-optimal (for maximizing the usable CH capacity for ambient temperature adsorption at pressures between 35 bar and 65 bar) binding enthalpy of −15 kJ mol−1 to −17 kJ mol−1, but display Langmuir-type adsorption isotherms that leave a substantial amount of unusable CH adsorbed at 5.8 bar. One major, and often overlooked, challenge in developing adsorbents for natural gas storage, or indeed for any gas storage application, involves managing the exothermic heat of adsorption and endothermic heat of desorption, both of which reduce the usable capacity of an adsorbent. These heat effects can be substantial, with temperature changes of as much as 80 °C observed during testing of prototype activated carbon-based ANG systems, and result in large reductions in the usable CH capacity22, 23. On-board thermal-management systems are essential to minimizing the negative impacts of the heats of sorption, but these engineering controls take up already limited space on a vehicle and add considerable cost and complexity24. Responsive adsorbents, such as Co(bdp), offer the possibility of managing heat intrinsically within a material, rather than through an external system, by using the enthalpy change of a phase transition to partially, or perhaps even fully, offset the heats of sorption. For Co(bdp), the expansion of the framework during adsorption is endothermic, because energy is needed to overcome the greater thermodynamic stability of the collapsed phase. As a result, some of the enthalpy of CH adsorption should go towards providing the heat needed for the transition to the expanded phase, lowering the overall amount of heat released compared to adsorption in the absence of a phase transition. Similarly, the transition to the collapsed phase is exothermic, and some of the heat released by the framework as it collapses should offset the endothermic desorption of CH . In classical porous materials, low-coverage differential CH adsorption enthalpies are generally −12 kJ mol−1 CH to −15 kJ mol−1 CH for adsorbents that do not have any strong CH binding sites and are closer to −15 kJ mol−1 to −25 kJ mol−1 for adsorbents with the highest volumetric CH capacities7, 8. For the steepest region of the CH adsorption isotherm of Co(bdp), the differential enthalpy is considerably lower, at just −8.4(3) kJ mol−1 (where the uncertainty corresponds to ±1 standard deviation), because the endothermic framework expansion partially offsets the exothermic heat of adsorption (Fig. 3c). After the transition to the expanded Co(bdp) phase is complete, the differential enthalpy approaches −13 kJ mol−1, which is consistent with weak CH physical adsorption in the absence of a phase transition to mitigate heat. To confirm the accuracy of the calculated differential enthalpies, the heat released during CH adsorption was directly measured by performing variable-pressure microcalorimetry experiments. As shown in Fig. 3c, the differential enthalpies obtained from calorimetry are in excellent agreement with those calculated from the variable-temperature adsorption isotherms. The total amount of heat released when increasing the pressure of CH adsorbed in Co(bdp) from 5.8 bar to 35 bar, as would occur during refuelling of an ANG vehicle, is calculated by integrating the differential-enthalpy curve with respect to the amount of CH adsorbed. The 73.4 kJ of heat released per litre of Co(bdp) represents a 33% reduction relative to the 109 kJ l−1 of heat released by HKUST-1 under the same conditions, even though the amount of CH adsorbed in Co(bdp) is 8% greater. We further calculate that 93.9 kJ l−1 of heat would be released for hypothetical CH adsorption in a rigid Co(bdp) framework—28% higher than when adsorption occurs with a phase transition to provide heat mitigation25. By chemically modifying Co(bdp), we hypothesized that it might be possible to obtain a new flexible framework with a similar stepped CH isotherm, but a higher-energy phase transition that could provide even greater intrinsic heat management. Because one-dimensional chains are known to form with tetrahedral Fe2+ ions bridged by μ2-pyrazolates26, we anticipated that it might be possible to synthesize an isostructural iron analogue of Co(bdp). By heating FeCl and H bdp in a mixture of N,N-dimethylformamide (DMF) and methanol, we indeed obtained Fe(bdp) as yellow, block-shaped crystals. X-ray analysis of a DMF-solvated crystal (Extended Data Fig. 6) confirmed that Fe(bdp) is isostructural to Co(bdp). Fe(bdp) has a stepped high-pressure CH isotherm at 25 °C (Fig. 1d), suggesting that this new compound also undergoes a reversible phase transition between a collapsed and expanded framework. Although the total CH uptake is comparable to that of Co(bdp), the adsorption and desorption steps occur at the considerably higher pressures of 24 bar and 10 bar, respectively, suggesting that replacing Co with Fe increases the energy of the phase transition. In situ powder X-ray diffraction experiments from 0 bar to 50 bar of CH (Fig. 2b) and subsequent Rietveld refinements afforded the collapsed and CH -expanded crystal structures of Fe(bdp). Although the collapsed phase is nearly identical to that of Co(bdp), with edge-to-face π–π interactions and no accessible porosity, the volume of the expanded Fe(bdp) phase at 40 bar is 9% greater than that of Co(bdp) (Fig. 2f). In contrast to Co(bdp), we observe a second transition for Fe(bdp) at pressures above 40 bar, wherein Fe(bdp) slightly expands to a framework with nearly perfect square channels (Extended Data Fig. 6). In spite of its greater expansion and lower crystallographic density, the usable CH capacity of Fe(bdp) is still higher than all known adsorbents at 150 v/v and 190 v/v for 35 bar and 65 bar adsorption, respectively. Although Fe(bdp) and Co(bdp) have similar usable capacities, the initial Fe(bdp) phase transition offsets more heat, and only 64.3 kJ of heat is released per litre of adsorbent during CH adsorption at 35 bar, which is 12% lower than for Co(bdp) and 41% lower than for HKUST-1. This is a direct consequence of the larger increase in the enthalpy of Fe(bdp) (8.1 kJ mol−1) than of Co(bdp) (7.0 kJ mol−1) during the phase transition, which mitigates more heat of adsorption, thereby providing a greater source of intrinsic thermal management. This result demonstrates how a slight variation in the metal–organic framework can be used to improve its intrinsic thermal management, and it is very likely that similar effects will prove possible through alteration of the bdp2− bridging ligand. Examining the temperature dependence of the CH isotherms of Co(bdp) and Fe(bdp) (Extended Data Figs 2, 3) reveals another advantage of these materials, involving a reduction in the effect of cooling during desorption. Consistent with other gate-opening metal–organic frameworks, the CH adsorption and desorption steps in Co(bdp) and Fe(bdp) shift to lower pressures at lower temperatures (Fig. 3a, b). As long as the temperature stays above 0 °C in Co(bdp) or −25 °C in Fe(bdp), however, the transition to the collapsed phase occurs above 5.8 bar, and the usable CH capacity will not be affected by cooling (Supplementary Tables 2, 3). This property has practical benefits for driving in cold-weather climates and should further reduce the overall thermal management required in an ANG system. Recent work27, 28, 29 has shown that it is possible to induce a phase transition in flexible metal–organic frameworks by applying external mechanical pressure. With this in mind, we proposed that applying moderate mechanical pressure could provide a means of further tuning the CH adsorption and desorption step pressures in Co(bdp) and Fe(bdp) and of increasing the energy of the phase transition to offset more heat. To investigate this concept, high-pressure CH adsorption isotherms were measured for Co(bdp) at different levels of applied uniaxial mechanical pressure. At higher mechanical pressures and higher compaction densities, both the adsorption and desorption isotherm steps shift to higher CH pressures, which is consistent with an increase in the energy of the phase transition (Fig. 4). In addition, the isotherm hysteresis loop remains open until higher CH pressures, with hysteresis observed to pressures of at least 70 bar for the highest applied mechanical pressure. Because hysteresis at a given pressure implies that a phase transition is still occurring30, this result suggests that some Co(bdp) crystallites are expanding at much higher CH pressures when under an applied external mechanical pressure. Because Co(bdp) crystallites in a bulk powder will be at different orientations with respect to the direction of uniaxial compression (Extended Data Fig. 4), there will be a distribution of local mechanical pressures experienced by different crystallites. Crystallites that experience higher external pressures will have a greater free energy change associated with the phase transition and will open at higher pressures31. Overall, these results present the prospect of using mechanical work, such as provided through an elastic bladder, as a means of thermal management in a gas-storage system based on a flexible adsorbent. Designing new flexible adsorbents with stronger gas binding sites and higher-energy phase transitions provides a promising route to achieving even higher usable capacities and greater intrinsic heat management in a next generation of gas-storage materials. Moreover, improved compaction and packing strategies should allow further reductions to external thermal-management requirements and optimization of the overall storage-system performance.

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