Primary service outage and secondary service performance in cognitive radio networks

In this work, we have used stochastic geometry to analyze the effects of the radio environment and the secondary service sensing error on the primary service outage probability. We have also obtained a closed form expression for the primary service outage probability in a Rayleigh fading channel. Furthermore, an approximation was formulated for a general fading channel. The results obtained were used to drive an expression for the secondary service successful transmission density. Finally, we formulated an optimization problem to achieve highest density of secondary service successful transmissions while keeping the primary service outage probability in check by adjusting sensing error and secondary service node density to their optimal values. Copyright © 2014 John Wiley & Sons, Ltd.     

Prediction of the Strength of Energetic Materials Using the Condensed and Gas Phase Heats of Formation

The strength of energetic materials is one of the principal parameters to express their performance. Two new methods were introduced for the prediction of the strength of C_aH_bN_cO_d energetic materials through the Trauzl test. They are based on elemental composition and the condensed or gas phase heats of formation of energetic compounds. The model is based on the gas phase heat of formation, uses the group additivity method and requires only the molecular structure of the desired energetic compound. These methods provide more reliable predictions as compared to the best available theoretical methods. Some of the benefits of these new models are their accuracy, precision, simplicity, and low price.     

A novel approach to determine the Biot’s coefficient using X-ray computed tomography

Linear poroelastic effective stress law is often used to relate in situ total stresses to pore fluid pressure in underground formations. The indirect estimations of effective stress coefficient (Biot coefficient) based on porosity relationships are often not accurate, and therefore the Biot coefficient measurements on retrieved samples using hydro-mechanical laboratory testing is considered. In this study, we propose a new indirect technique to infer the Biot coefficient from micro X-ray computed tomography (XRCT) images of porous granular samples whenever images are available using a simple principle of continuum mechanics. The performance of the method was assessed through a series of experiments conducted on sandstone and unconsolidated sand. The proposed method enables calculating the anisotropy of the Biot coefficient of the sample in three dimensions. We also conducted the conventional hydro-mechanical experiment to validate the obtained Biot coefficient results from the XRCT technique. The proposed technique shows promising results on estimation of the Biot coefficient. The analysis of the images confirms that the method performs well when the connectivity between grains (skeleton structure) can be extracted from XRCT images with enough resolution. The method is also able to (a) estimate the Biot coefficient of both consolidated and unconsolidated sand structure and (b) map the anisotropy of Biot coefficient in three dimensions.

     

Evolutionary based multi-criteria optimization of an integrated energy system with SOFC,gas turbine,and hydrogen production via electrolysis

The aim of this study is to exploit the waste heat of a biomass-based solid oxide fuel cell (SOFC)–model (a)–in a gas turbine (GT) to enhance the power generation/exergy efficiency (model (b)). Moreover, surplus power which is generated by the GT is transferred to a proton exchange membrane electrolyzer (PEME) for hydrogen production (model (c)). Parametric study is performed to investigate the influence of the effective parameters on performance and economic indicators. Eventually, considering exergy efficiency and total product cost as the objective functions, the proposed models are optimized by multi-objective optimization method based on genetic algorithm. Accordingly, the optimum solution points are gathered as Pareto frontiers and subsequently favorable solution points are ascertained from exergy/economic standpoints. Results of parametric study indicate that model (b) is the best model as it has higher exergy efficiency and lower total product cost. Moreover, model (c) may be a more suitable model compared to the model (a) because of higher exergy efficiency and capability of hydrogen production. The results further show that, at the best final solution point, the exergy efficiency and total product cost of the model (b) would be 33.22% and 19.01 $/GJ, respectively. Corresponding values of exergy efficiency and total product cost of the model (c) are 32.3% and 20.1 $/GJ. Moreover, the rate of hydrogen production of the model (c) is 8.393 kg/day, at the best solution point. Overall, the integration methods are promising techniques for increasing exergy efficiency, reducing total product cost and also for hydrogen production.     

Criteria for Predicting Transitions in Free Convection Heat Transfer From Isothermal Convex Bodies in Fluids With Any Prandtl Number: A New Analytical Model

This article presents criteria for predicting transitions in free convection from isothermal convex bodies in fluids with any Prandtl number (conduction–laminar and laminar–turbulent transitions). Laminar–turbulent transition results for vertical plates happening in the vicinity of Grashof number 10⁹ in fluids with any Prandtl number and other geometries show that this criterion based on Grashof number is a better choice compared to that of the Rayleigh number, as the Prandtl number yields a large change in the transition Rayleigh number. Besides, for convex geometries other than vertical plates, the laminar–turbulent transition occurs at values slightly less than Grashof number 10⁹ owing to longitudinal vortices bringing about separation over their surface and helping laminar–turbulent transition to arise sooner. On the other side, the results of the conduction–laminar transition criterion for different convex bodies disclose that the Rayleigh number is superior to the Grashof number. Likewise, the present results for five different geometries over a wide range of Prandtl numbers are compared with the available experimental data, and excellent agreement is found. This reveals that the proposed criteria is powerful enough to predict the two transition phenomena in free convection heat transfer from isothermal convex bodies for a wide range of convex geometries in fluids with any Prandtl number.     

Realistic simulation of fuel economy and life cycle metrics for hydrogen fuel cell vehicles

The present work contributes an engineered life cycle assessment (LCA) of hydrogen fuel cell passenger vehicles based on a real‐world driving cycle for semi‐urban driving conditions. A new customized LCA tool is developed for the comparison of conventional gasoline and hydrogen fuel cell vehicles (FCVs), which utilizes a dynamic vehicle simulation approach to calculate realistic, fundamental science based fuel economy data from actual drive cycles, vehicle specifications, road grade, engine performance, fuel cell degradation effects, and regenerative braking. The total greenhouse gas (GHG) emission and life cycle cost of the vehicles are compared for the case of hydrogen production by electrolysis in British Columbia, Canada. A 72% reduction in total GHG emission is obtained for switching from gasoline vehicles to FCVs. While fuel cell performance degradation causes 7% and 3% increases in lifetime fuel consumption and GHG emission, respectively, regenerative braking improves the fuel economy by 23% and reduces the total GHG emission by 10%. The cost assessment results indicate that the current FCV technology is approximately $2,100 more costly than the equivalent gasoline vehicle based on the total lifetime cost including purchase and fuel cost. However, prospective enhancements in fuel cell durability could potentially reduce the FCV lifetime cost below that of gasoline vehicles. Overall, the present results indicate that fuel cell vehicles are becoming both technologically and economically viable compared with incumbent vehicles, and provide a realistic option for deep reductions in emissions from transportation. Copyright © 2016 John Wiley & Sons, Ltd.     

Energy and exergy analyses of hydrogen production via solar-boosted ocean thermal energy conversion and PEM electrolysis

Energy and exergy analyses are reported of hydrogen production via an ocean thermal energy conversion (OTEC) system coupled with a solar-enhanced proton exchange membrane (PEM) electrolyzer. This system is composed of a turbine, an evaporator, a condenser, a pump, a solar collector and a PEM electrolyzer. Electricity is generated in the turbine, which is used by the PEM electrolyzer to produce hydrogen. A simulation program using Matlab software is developed to model the PEM electrolyzer and OTEC system. The simulation model for the PEM electrolyzer used in this study is validated with experimental data from the literature. The amount of hydrogen produced, the exergy destruction of each component and the overall system, and the exergy efficiency of the system are calculated. To better understand the effect of various parameters on system performance, a parametric analysis is carried out. The energy and exergy efficiencies of the integrated OTEC system are 3.6% and 22.7% respectively, and the exergy efficiency of the PEM electrolyzer is about 56.5% while the amount of hydrogen produced by it is 1.2 kg/h.     

Modeling and simulation of a horizontal pulsed sieve-plate extraction column using axial dispersion model

In this research, the impact of pulsation intensity and phase flow rates on the volumetric overall mass transfer coefficients based on the continuous phase (K_oca) and the axial dispersion coefficients of phases in a horizontal pulsed sieve-plate column has been investigated using axial dispersion model. The toluene-acetone-water and butyl acetate-acetone-water systems with acetone transfer in both directions were used. In this study, the flow regime transition from pseudo-dispersion regime to emulsion regime has been characterized. Two new correlations have been proposed for prediction of K_oca and E_c.     

Experimental study of a heavy fuel oil atomization by pressure-swirl injector in the application of entrained flow gasifier

The available SMD(Sauter mean diameter) correlations on pressure-swirl injectors predict droplet sizing very different from each other, especially for heavy fuels. Also there was a lack in the literature for comparing available correlations. So an experimental study was conducted on a heavy fuel oil(HFO) spray, Mazut 380. A pressure swirl injector was designed and fabricated. The experiments for Mazut at 40 °C and 80 °C were compared with the results for water, including spray half cone angle, breakup length and mean droplet diameter,at different injection pressures. Lower spray angle, higher breakup length and larger droplets were observed for lower injection pressures and higher liquid viscosity. SMD was about 75 μm for water and about 87 μm for Mazut at 80 °C. The results for droplet mean diameter were also compared with correlations from previous studies on pressure swirl atomizers. The SMD results show that for water spray, LISA method was in good agreement,also Babu and Ballester correlations were successful when high viscosity fluid was injected.     

Performance evaluation of a continuous flow Photo-Impinging Streams Cyclone Reactor for phenol degradation

In this study, an impinging streams cyclone reactor has been utilized as a novel apparatus in photocatalytic degradation of phenol. Degussa P25 TiO₂ nano particles have been applied as the photocatalyst under UV radiation. The operating parameters including catalyst loading, pH, initial phenol concentration and light Intensity have been found to affect the efficiency of the photocatalytic degradation process within this photoreactor. Photocatalytic degradation of phenol has been also investigated in a continuous flow impinging streams system. The results have shown a higher efficiency and an increased performance capability of the present reactor in comparison with the conventional processes.