Thermal stresses in heterogeneous thick isotropic shells

The application of a thick shell theory in the design of nuclear pressure vessels is given. A derivation for thick shells of revolution with the inclusion effects of transverse shear stress and normal stress, in which the Young's modulus is considered dependent on temperature, is presented. The set of equations so obtained are solved by a numerical integration method. The example of a spherical shell is solved and the results compared with the case in which the Young's modulus is assumed to be constant. It is shown that under large temperature gradients through the thickness, the change in deflections and stresses can be significant, if the present derivation is used.     

Fermentative biohydrogen production: Evaluation of net energy gain

Most dark fermentation (DF) studies had resorted to above-ambient temperatures to maximize hydrogen yield, without due consideration of the net energy gain. In this study, literature data on fermentative hydrogen production from glucose, sucrose, and organic wastes were compiled to evaluate the benefit of higher fermentation temperatures in terms of net energy gain. This evaluation showed that the improvement in hydrogen yield at higher temperatures is not justified as the net energy gain not only declined with increase of temperature, but also was mostly negative when the fermentation temperature exceeded 25 °C. To maximize the net energy gain of DF, the following two options for recovering additional energy from the end products and to determine the optimal fermentation temperature were evaluated: methane production via anaerobic digestion (AD); and direct electricity production via microbial fuel cells (MFC). Based on net energy gain, it is concluded that DF has to be operated at near-ambient temperatures for the net energy gain to be positive; and DF + MFC can result in higher net energy gain at any temperature than DF or DF + AD.     

Simulation of nanoparticle synthesis in an aerosol flame reactor using a coupled flame dynamics–monodisperse population balance model

Flame aerosol synthesis is one of the commonly employed techniques for producing ultra fine particles of commodity chemicals such as titanium dioxide, silicon dioxide and carbon black. Large volumes of these materials are produced in industrial flame reactors. Particle size distribution of product powder is the most important variable and it depends strongly on flame dynamics inside the reactor, which in turn is a function of input process variables such as reactant flow rate and concentration, flow rates of air, fuel and the carrier gas and the burner geometry. A coupled flame dynamics–monodisperse population balance model for nanoparticle synthesis in an aerosol flame reactor is presented here. The flame dynamics was simulated using the commercial computational fluid dynamics software CFX and the particle population dynamics was represented using a monodisperse population balance model for continuous processes that predicts the evolution of particle number concentration, particle volume and surface area. The model was tested with published experimental data for synthesis of silica nanoparticles using different burner configurations and with different reactor operating conditions. The model predictions for radial flame temperature profiles and for the effects of process variables like precursor concentration and oxygen flow rate on particle specific surface area and mean diameter are in close agreement with published experimental data.     

Feasibility of biohydrogen production at low temperatures in unbuffered reactors

Feasibility of biohydrogen production by dark fermentation at two temperatures (22 °C and 37 °C) in unbuffered batch reactors was evaluated using heat-treated compost as inocula and sucrose as substrate, without any initial pH adjustment or inorganic nutrient supplements. Gas production was quantified by two different pressure release methods – intermittent pressure release (IPR) and continuous pressure release (CPR). Hydrogen production (47.2 mL/g COD/L) and sucrose-to-hydrogen conversion efficiency (53%) were both found to be highest at the lower temperature and IPR conditions. Hydrogen production was higher at the lower temperature irrespective of the pressure release condition. The high yield of 4.3 mol of hydrogen/mole of sucrose obtained in this study under IPR conditions at 22 °C is equivalent to or better than the literature values reported for buffered reactors. Even though literature reports have implied potential inhibition of hydrogen production at high hydrogen partial pressures resulting from IPR conditions, our results did not show any negative effects at hydrogen partial pressures exceeding 5.0 × 10⁴ Pa. While our findings are contrary to literature reports, they make a strong case for cost-effective hydrogen production by dark fermentation.     

Achieving End-to-end Fairness in 802.11e Based Wireless Multi-Hop Mesh Networks Without Coordination

To mitigate the damaging impacts caused by interference and hidden terminals, it has been proposed to use orthogonal channels in wireless multi-hop mesh networks. We demonstrate however that even if these issues are completely eliminated with perfectly assigned channels, gross unfairness can still exist amongst competing flows which traverse multiple hops. We propose the use of 802.11e’s TXOP mechanism to restore/enforce fairness. The proposed scheme is simple, implementable using off-the-shelf devices and fully decentralised (requires no message passing).     

Estimation of Melt Pool Dimensions, Thermal Cycle, and Hardness Distribution in the Laser-Engineered Net Shaping Process of Austenitic Stainless Steel

Laser engineered net shaping (LENS) and other similar processes facilitate building of parts with freeform shapes by melting and deposition of metallic powders layer by layer. A-priori estimation of the layerwise variations in peak temperature, build dimension, cooling rate, and mechanical property is requisite for successful application of these processes. We present here an integrated approach to estimate these build attributes. A three-dimensional (3-D) heat transfer analysis based on the finite element method is developed to compute the layerwise variation in thermal cycles and melt pool dimensions in the single-line multilayer wall structure of austenitic stainless steel. The computed values of cooling rates during solidification are used to estimate the layerwise variation in cell spacing of the solidified structure. A Hall–Petch like relation using cell size as the structural parameter is used next to estimate the layerwise hardness distribution. The predicted values of layer widths and build heights have depicted fair agreement with the corresponding measured values in actual deposits. The estimated values of layerwise cell spacing and hardness remain underpredicted and overpredicted, respectively. The slight underprediction of the cell spacing is attributed to the possible overestimation of the cooling rates that may have resulted due to the neglect of convective heat transport within the melt pool. The overprediction of the layerwise hardness is certainly due to the underprediction of corresponding cell spacing. The application of Hall–Petch coefficients, which is strictly valid for wrought and annealed grain structures, to estimate the hardness of as-solidified cellular structures may have also contributed to the overprediction of the layerwise hardness.     

A population balance model for flocculation of colloidal suspensions by polymer bridging

A detailed population balance model for flocculation of colloidal suspensions by polymer bridging under quiescent flow conditions is presented. The collision efficiency factor is estimated as a function of interaction forces between polymer coated particles. The total interaction energy is computed as a sum of van der Waals attraction, electrical double layer repulsion and bridging attraction or steric repulsion due to adsorbed polymer. The scaling theory is used to compute the forces due to adsorbed polymer and the van der Waals attraction is modified to account for presence of polymer layer around a particle. The irregular structure of flocs is taken into account by incorporating the mass fractal dimension of flocs. When tested with experimental floc size distribution data published in the literature, the model predicts the experimental behavior adequately. This is the first attempt towards incorporating theories of polymer-induced surface forces into a flocculation model, and as such the model presented here is more general than those proposed previously.     

Challenges of the Omicron (B.1.1.529) Variant and Its Lineages: A Global Perspective

The SARS-CoV-2 virus has shown increased ability to mutate over the past two years, especially in the regions of the spike protein and receptor binding sites. Omicron (B.1.1.529) is the fifth variant of concern (VOC) after the emergence of the Alpha, Beta, Gamma, and Delta VOCs of SARS-CoV-2. This new variant has now circulated in 128 countries and according to the Global Initiative on Sharing All Influenza Data (GISAID), these 128 countries have shared 650,657 Omicron genome sequences as of 26 January, 2022. In this article, we highlight the real challenges of Omicron and its different lineages.     

Modeling dark fermentation for biohydrogen production: ADM1-based model vs. Gompertz model

Biohydrogen production by dark fermentation in batch reactors was modeled using the Gompertz equation and a model based on Anaerobic Digestion Model (ADM1). The ADM1 framework, which has been well accepted for modeling methane production by anaerobic digestion, was modified in this study for modeling hydrogen production. Experimental hydrogen production data from eight reactor configurations varying in pressure conditions, temperature, type and concentration of substrate, inocula source, and stirring conditions were used to evaluate the predictive abilities of the two modeling approaches. Although the quality of fit between the measured and fitted hydrogen evolution by the Gompertz equation was high in all the eight reactor configurations with r² ∼0.98, each configuration required a different set of model parameters, negating its utility as a general approach to predict hydrogen evolution. On the other hand, the ADM1-based model (ADM1BM) with predefined parameters was able to predict COD, cumulative hydrogen production, as well as volatile fatty acids production, albeit at a slightly lower quality of fit. Agreement between the experimental temporal hydrogen evolution data and the ADM1BM predictions was statistically significant with r² > 0.91 and p-value <1E-04. Sensitivity analysis of the validated model revealed that hydrogen production was sensitive to only six parameters in the ADM1BM.