Thermo‐structural design of ZrB2–SiC‐based thermal protection system for hypersonic space vehicles

The leading edges of hypersonic space vehicle experience high temperature and stress due to prevailing aerothermodynamic conditions of extreme heat flux and pressure. The design of thermal protection system (TPS) to protect the metallic airframe structure can ensure longer life and reliability under flight conditions. The effective design of TPS system requires the precise quantitative understanding of thermo‐mechanical stresses and deformation, which demands careful computational study under flight simulated conditions. In the above perspective, TPS design for a leading edge exposed to Mach 7 hypersonic flow for 250 seconds has been carried out by performing finite element‐based thermo‐structural analysis with pressure and heat flux estimated from computational fluid dynamics analysis (CFD). The fidelity and robustness of CFD scheme is established using grid independence and convergence analysis. CFD analysis effectively captures the formation of bow shock around the leading edge and stagnation region near its nose. For finite element analysis, high‐quality structural elements have been generated using HyperMesh to precisely model the thermo‐structural behavior of TPS. In our computational analysis, TPS is modeled as a three‐layered system with outermost layer of ZrB₂‐SiC, middle layer of phenolic cork and innermost layer of Ti‐alloy. The analytical values of spatial variation of temperature, stress components, and displacement across the TPS have been critically analysed to rationalise specific structural configuration for better thermo‐structural stability. Together with temporal variation of temperature, the implication of such computational results has led us to propose a new design for TPS. The proposed TPS is capable of containing the stress and displacement within 32 MPa and 0.58 mm, respectively, when the leading edge is exposed to shock induced aero‐thermal heating as high as 2.11 MW/m² and pressure of 72.8 kPa for a hypersonic cruise flight of 500 km range.     

Tuning Fluidic Resistance via Liquid Crystal Microfluidics

Flow of molecularly ordered fluids, like liquid crystals, is inherently coupled with the average local orientation of the molecules, or the director. The anisotropic coupling—typically absent in isotropic fluids—bestows unique functionalities to the flowing matrix. In this work, we harness this anisotropy to pattern different pathways to tunable fluidic resistance within microfluidic devices. We use a nematic liquid crystalline material flowing in microchannels to demonstrate passive and active modulation of the flow resistance. While appropriate surface anchoring conditions—which imprint distinct fluidic resistances within microchannels under similar hydrodynamic parameters—act as passive cues, an external field, e.g., temperature, is used to actively modulate the flow resistance in the microfluidic device. We apply this simple concept to fabricate basic fluidic circuits, which can be hierarchically extended to create complex resistance networks, without any additional design or morphological patterning of the microchannels.     

A generalized approach to model oxygen transfer in bioreactors using population balances and computational fluid dynamics

In many biological processes, increasing the rate of transport of a limiting nutrient can enhance the rate of product formation. In aerobic fermentation systems, the rate of oxygen transfer to the cells is usually the limiting factor. A key factor that influences oxygen transfer is bubble size distribution. The bubble sizes dictate the available interfacial area for gas-liquid mass transfer. Scale-up and design of bioreactors must meet oxygen transfer requirements while maintaining low shear rates and a controlled flow pattern. This is the motivation for the current work that captures multiphase hydrodynamics and simultaneously predicts the bubble size distribution.Bubbles break up and coalesce due to interactions with turbulent eddies, giving rise to a distribution of bubble sizes. These effects are included in the modeling approach by solving a population balance model with bubble breakage and coalescence. The population balance model was coupled to multiphase flow equations and solved using a commercial computational fluid mechanics code FLUENT 6. Gas holdup and volumetric mass transfer coefficients were predicted for different superficial velocities and compared to the experimental results of Kawase and Hashimoto (1996). The modeling results showed good agreement with experiment.     

Slurry Erosion Performance of Ni-Al2O3 Based Thermal-Sprayed Coatings: Effect of Angle of Impingement

In this paper, slurry erosion performance of high velocity flame-sprayed Ni-Al₂O₃ based coatings was evaluated. The coatings were deposited on a hydroturbine steel (CA6NM) by varying the content of Al₂O₃ in Ni. Using jet-type test rig, erosion behavior of coatings and bare steel was evaluated at different impingement angles. Detailed investigation of the surface morphology of the eroded specimens was undertaken using SEM/EDS to identify potential erosion mechanism. A parameter named “erosion mechanism identifier” (ξ) was used to predict the mode of erosion. It was observed that the coating prepared using 40 wt.% of Al₂O₃ showed a highest resistance to erosion. This coating enhanced the erosion resistance of the steel by 2 to 4 times. Spalling in the form of splats and chunks of material (formed by interlinking of cracks) along with fracture of Al₂O₃ splats were identified as primary mechanisms responsible for the loss of coating material. The erosion mechanism of coatings and bare steel predicted by ξ was in good agreement with that observed experimentally. Among different parameters, $ \left( {K_{\text{IC}}^{2} H} \right)^{1/3} $ , a function of fracture toughness (K _IC) and hardness (H) showed excellent correlation with erosion resistance of coatings at both the impingement angles.     

Computation of thermal properties of CMSP shell and tube heat exchanger using different types of helical baffles

This paper investigates the flow and thermal properties of a combined multiple shell pass (CMSP)-shell and tube heat exchanger (STHE) with the provision of unilateral ladder-type helical baffle (ULHB) and continuous helical baffle (HB) in the outer shell pass of the heat exchanger. Two CMSP-STHEs with ULHB and HB, respectively, are compared with the traditional STHE having segmental baffles (SG-STHE) using the computational fluid dynamics method. The computational outcomes are validated with the empirical correlations of the Kern and Esso method. The Reynolds-averaged Navier–Stokes-based standard k–ω turbulence model accurately predicts the heat transfer (HT) rate and pressure drop. The computed results of HT rate, pressure drop, and logarithmic mean temperature difference corresponding to various mass flow rates (m) for three STHEs are presented. The results show that the overall HT rate of CMSP (ULHB)-STHE and the CMSP (HB)-STHE at the same mass flow rate are nearly 28.3% and 14.8% larger than that of traditional SG-STHE, respectively. Furthermore, the overall area-weighted average pressure drop (ΔP) of CMSP (HB)-STHE is smaller than that of SG-STHE by 26.5% at the same mass flow rate (m) and for CMSP (ULHB)-STHE it is larger by 2% than that of traditional STHE. Based on the above results, it is concluded that the CMSP (ULHB)-STHE is a suitable replacement for the conventional SG-STHEs.     

Light-induced enhancement of memory effect in self-assembled pyrene nanostructures

Journal of Materials Science - A one-pot self-assembly method for the growth of one-dimensional nanoribbons of pyrene with high yield is reported. Nitric acid-treated pyrene molecules are...     

Catalytic activity of a reusable polymer‐anchored nickel(II)–phenanthroline complex on the oxidation of various organic substrates

A polymer‐anchored nickel(II)–phenanthroline complex [polyNi(II)–phen] was synthesized and used effectively as a reusable catalyst in various oxidation reactions in the presence of tert‐butylhydroperoxide as an oxidant in acetonitrile medium. The catalyst was characterized with elemental analysis, atomic absorption spectrometry (AAS), thermogravimetric analysis, scanning electron microscopy, and spectrometric methods such as diffuse reflectance spectroscopy, ultraviolet–visible spectroscopy, and Fourier transform infrared spectroscopy. The study of the effects of the time, temperature, oxidant, catalyst concentration, molar ratio of substrate to oxidant, and solvent in the oxidation of styrene individually gave the optimized reaction conditions. Under optimized conditions, the catalyst exhibited good conversions for the oxidation reactions of various olefins, alkanes, aromatic alcohols, and thioethers. The catalyst was easily recovered by simple filtration and reused for more than five times with consistent catalytic activity. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

An experimental and theoretical understanding of a UV photodetector based on Ag nanoparticles decorated Er-doped TiO2 thin film

Glancing angle deposition (GLAD) was employed to synthesise plasmonic Silver (Ag) nanoparticles (NPs) on the chemically prepared Erbium-doped Titanium dioxide (Er:TiO₂) thin films (TFs). The impact of using Ag NPs on the morphological, optical, and electrical aspects of Er:TiO₂ TFs were sequentially analysed. From the field emission scanning electron microscopy (FESEM) image, the Ag NPs appeared spherical and uniformly distributed on the Er:TiO₂ TFs. The size (diameter) of the maximum number of Ag NPs was ~15 nm (calculated from FESEM image). Energy dispersive X-ray (EDX) spectra assured the presence of Ag NPs on the TFs. X-ray diffraction (XRD) pattern for Ag NPs decorated Er:TiO₂ TFs closely resembled the face centred cubic crystal structure of Ag NPs and body centred tetragonal Ag–O compound. The optical spectroscopy (UV–visible diffuse reflectance and photoluminescence) elucidated that the absorption of light was significantly enhanced in the UV–visible spectral range for the TFs in which Ag NPs were sandwiched between Er:TiO₂ TF layers (Er:TiO₂/Ag NPs/Er:TiO₂). The Schottky contact-based Au/Er:TiO₂/Si photodetector (PD) and Au/Er:TiO₂/Ag NPs/Er:TiO₂/Si (plasmonic) PD were constructed. The plasmonic PD offered a better photo-responsivity of ~4.5 fold higher as compared to Er:TiO₂ TF-based PD upon 380 nm illumination under ?6 V bias. An increase in detectivity and a decrease in noise equivalent power was observed for the plasmonic device compared to Er:TiO₂ device in the UV region. A theoretical approach had been adopted to calculate the wavelength-dependent responsivity for both devices. Further, the important parameters like photoconductive gain, electron transit time and electron mobility were calculated by simulating the experimental responsivity curves of the devices. These parameters exhibited improvement in the UV regime for the plasmonic PD. The fast temporal response with short rise and decay time proves the excellent efficiency of the plasmonic UV PD.     

Combined gradient‐stochastic optimization with negative circular masks for large deformation topologies

Topology optimization of large deformation two‐dimensional continua is presented using a combined gradient‐stochastic search with negative circular masks. The possibility of generating perfect black and white topologies is explored while attaining the efficiency of first‐order and second‐order search algorithms. The design region is modeled with honeycomb tessellation to thwart the known connectivity singularities such as the checkerboards and point flexures. Mask shrinkage is incorporated for ease in density transition between gradient and stochastic steps. Notches at continuum boundaries are moderated through multiple use of a simple boundary smoothing method. A neo‐Hookean elasticity model is employed to simulate the material nonlinearities in large displacement continua. With examples on stiff beams and large deformation compliant mechanisms, it is illustrated that perfectly binary, connected and smooth topologies can be obtained within a few hundred design evaluations.Copyright © 2012 John Wiley & Sons, Ltd.