FULLY DEVELOPED LAMINAR HEAT TRANSFER IN CIRCULAR-SEGMENT DUCTS WITH UNIFORM WALL TEMPERATURE

Heat transfer to constant-property, fully developed, laminar flows in circular-segment ducts with uniform wall temperature (T) has been analyzed. Besides representing a compact surface, the segment duct geometry models the flow cross section of a circular tube with a straight-tape insert. Two variations in the T thermal boundary condition are considered: constant axial and circumferential wall temperature, and constant temperature on the curved surface but an adiabatic flat wall. These two conditions model the extremes of the fin effects of a straight-tape insert, i.e., 100% and zero fin efficiencies, respectively. Numerical solutions, obtained by using finite difference techniques, are presented for both the velocity and temperature fields. The isothermal friction factors are in excellent agreement with analytical solutions reported in the literature. The Nusselt number results for the two thermal boundary conditions are presented for different segment shapes, 0° ≤, 6 ≤, 90°, and they represent the lower limits of the heat transfer enhancement due to twisted-tape inserts.     

Experimental and thermal analysis of energy efficient novel design wet cooling towers

Waste heat is generally dissipated from process water to atmospheric air in cooling towers. In the present study, a novel design is used to extract more amount of heat without any additional energy input by incorporating secondary ambient air in an induced draft wet cooling tower. In addition, more fresh air is induced in the tower from the rain zone, which increases the effectiveness at any value of the water to air flow rate (L/G ratio). Moreover, tower characteristics, range, and evaporation loss were also increased due to the novel design. It is noteworthy that secondary fresh air increases effectiveness, heat rejection, and tower characteristics by 10.12%, 19.65%, and 26.11%, respectively, and decreases approach by 16.32% at 0.55 L/G ratio, 44°C inlet water temperature, 29.7°C dry bulb temperature, and 18.4°C inlet air wet bulb temperature.     

Dysprosium doped copper oxide (Cu1-xDyxO) nanoparticles enabled bifunctional electrode for overall water splitting

The production of hydrogen, a favourable alternative to an unsustainable fossil fuel remains as a significant hurdle with the pertaining challenge in the design of proficient, highly productive and sustainable electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, the dysprosium (Dy) doped copper oxide (Cu_1-xDy_xO) nanoparticles were synthesized via solution combustion technique and utilized as a non-noble metal based bi-functional electrocatalyst for overall water splitting. Due to the improved surface to volume ratio and conductivity, the optimized Cu_1-xDy_xO (x = 0.01, 0.02) electrocatalysts exhibited impressive HER and OER performance respectively in 1 M KOH delivering a current density of 10 mAcm^?2 at a potential of ?0.18 V vs RHE for HER and 1.53 V vs RHE for OER. Moreover, the Dy doped CuO electrocatalyst used as a bi-functional catalyst for overall water splitting achieved a potential of 1.56 V at a current density 10 mAcm^?2 and relatively high current density of 66 mAcm^?2 at a peak potential of 2 V. A long term stability of 24 h was achieved for a cell voltage of 2.2 V at a constant current density of 30 mAcm^?2 with only 10% of the initial current loss. This showcases the accumulative opportunity of dysprosium as a dopant in CuO nanoparticles for fabricating a highly effective and low-cost bi-functional electrocatalyst for overall water splitting.     

Double dispersed bioconvective Casson nanofluid fluid flow over a nonlinear convective stretching sheet in suspension of gyrotactic microorganism

Microorganisms play a vital role in understanding the ecological system. The motions of micororganisms are self‐propelled while the impact of thermophoresis and Brownian motion property of nanoparticle shows more challenges in biotechnological and medical applications. The present problem is based on the understanding of double‐dispensed bioconvection for a Casson nanofluid flow over a stretching sheet. Suction phenomenon is introduced at the surface of the stretching sheet along with the convective boundary condition. The convection and movement of the microorganisms are assisted by an applied magnetic field, nonlinear thermal radiation, and first‐order chemical reaction. The governing equations are highly coupled and thus we used the spectral quasilinearization method to solve the governing equations. The study of the residual errors on the systemic parameters had given a confidence with the present results. The final outcomes are displayed through graphs and tables. The thermal dispersion coefficient shows a positive response in the temperature while a similar response is observed for the concentration with solutal dispersion coefficient. The response is reversible for the heat transfer rate at the surface with thermal dispersion coefficient. The density of the motile microorganism at the surface decreases with increase in the Casson number, thermal dispersion coefficient, and solute dispersion coefficient, while an opposite phenomenon was observed with increase in the density ratio of the motile microorganism.     

Use of cellular confinement for improved railway performance on soft subgrades

Due to extensive right-of-way, railroads are inevitably subject to poor subgrade conditions and interrupted service for significant maintenance due to excessive deformations and loss of track geometry. Geocell confinement presents itself as a possible solution for improving performance of ballasted railroad embankments over weak subgrade. To investigate the efficacy of geocell confinement on ballasted railway embankments, a set of well-instrumented, large-scale cyclic plate loading tests and numerical simulations were performed on geocell-confined ballast overlaying a weak subgrade material. The agreement of results from tests and simulations served as a basis for simulating practical track geometry and performance for various geocell configurations and subgrades using three-dimensional (3D) finite element (FE) analyses. The study showed that geocell reinforcement significantly decreased track settlement, decreased subgrade deformations with lower and uniform distribution of vertical stresses on subgrade and inhibited lateral deformation and serviceability under cyclic loading. These results demonstrate that geocell confinement can be an effective alternative to subsurface improvement or shorter maintenance cycles, particularly on weak subgrades.     

Optimization of CdO nanoparticles by Zr<Superscript>4+</Superscript> doping for better photocatalytic activity

Chemically controlled co-precipitation method has been adopted for the fabrication of pure and different wt% Zr doped CdO photocatalysts. Conventionally, the crystallite size and crystalline phase of CdO are in the midst of the parameters involved in the control of the photocatalytic activity. Aiming utterly at the size effect that modifies other attributes which are important to assess the photocatalytic activity of nanometric CdO, it was explored to synthesize CdO nanoparticles with controlled size, highly comparable morphology and analogous phase. The crystal structure and the crystallite size were estimated from the X-ray diffraction patterns and were confirmed through transmission electron microscope. The degree of crystallinity varied on Zr doping and the calculated crystallite sizes were in the range of 16–81 nm. The dopant ion Zr⁴⁺ have been detected through X-ray photoelectron spectroscopy (XPS) analysis signifying the dopant to substitute for cadmium (Cd²⁺) in the lattice of CdO. Particle size dependent optical band gaps calculated in the range 2.02–2.57 eV informed the viability of the materials to initiate photocatalytic reaction in the visible light region. Lesser recombination rate of the generated electrons and holes under light irradiation produced low intense photoluminescence peaks that displayed the appropriateness as photocatalysts. Zr⁴⁺ doping resulted in the enhancement of photocatalytic activity, evaluated by monitoring the degradation of methylene blue solution. 0.5 wt% Zr doped CdO nanoarticles calcined at 400 °C exhibited the highest photocatalytic activity with better percentage of color abatement (80.95%). The pseudo-first-order reaction rate became faster on Zr doping such that the rate constant is ~?0.4–0.5 h^?1 for Zr doped CdO while that for pure CdO is ~?0.3 h^?1.     

Convective heat transfer enhancement techniques of heat exchanger tubes: a review

Heat exchangers are widely used in almost all industrial activities. Turbulent promoters used in heat exchanger tubes are an effective way of enhancing the performance. This paper summarises various investigations using twisted tapes, wire coil ribs, baffles, and swirl flow generators. The main objective of this paper is to review various studies, in which different obstacle roughness elements are used to enhance the rate of heat transfer in the heat exchanger tubes. It has been found that a lot of experimental and analytical studies reported in the literature. On the basis of correlations developed by various investigators an attempt has been made to compare the thermal hydraulic performance of obstacles in heat exchanger tubes. In this work, a comparative study is also carried out to select the best obstacle roughness shapes for higher heat transfer rate and low pressure drop losses.     

Distributed adaptive state estimation and tracking by using active-passive sensor networks

Heterogeneous sensor networks (HSN) find a wide range of applications in the field of military and civilian environments, where sensor nodes are utilized to estimate the position of a target with both dynamics and control input being unknown for the purposes of tracking. In the HSN, nodes are considered active depending upon their ability to sense the target output while the others are taken passive. Accurate estimation requires local information exchange among the spatially located sensor nodes, so that the active nodes as well as the passive nodes converge simultaneously to the same value. The local information exchange among the nodes is dictated by a connected graph. By using the criterion of collective observability, a novel distributed adaptive estimation scheme is introduced via adaptive observer where the nodes are allowed to have different sensor modalities. Using the estimated information, a subset of active and passive nodes, referred to as mobile nodes, can track the moving target. By using a constant state feedback controller at each mobile node, the state and parameter estimation as well as the tracking errors are shown to be uniformly ultimately bounded. Simulation results verify theoretical claims.     

PDMS microfluidics: A mini review

Polydimethylsiloxane (PDMS) is hailed as one of the foundational materials for microfluidics. Though a silicone-based elastomer of many desirable properties, the emergence of microfluidic fabrication techniques, especially soft lithography, has elevated its status to an exceptional one. In this mini review, we look at the salient aspects that make PDMS so special in achieving such a coveted status in the microfluidics community. A methodical approach is followed to touch upon the application of PDMS in various aspects of microfluidics with the advantages, limitations, and some future directions. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48958.     

Pinecone biomass-derived activated carbon: the potential electrode material for the development of symmetric and asymmetric supercapacitors

Activated carbon, from biomass (pinecone), was synthesized by conventional pyrolysis/chemical activation process and utilized for the fabrication of supercapacitor electrodes. The pinecone-activated carbon synthesized with 1:4 ratio of KOH (PAC4) showed an increase in surface area and pore density with a considerable amount of oxygen functionalities on the surface. Moreover, PAC4, as supercapacitor electrode, exhibited excellent electrochemical performances with specific capacitance value ∼185 Fg⁻¹ in 1 M H₂SO₄, which is higher than that of nonactivated pinecone carbon and 1:2 ratio KOH-based activated carbon (PAC2) (∼144 Fg⁻¹). The systematic studies were performed to design various forms of devices (symmetric and asymmetric) to investigate the effect of device architecture and operating voltage on the performance and stability of the supercapacitors. The symmetric supercapacitor, designed utilizing PAC4 in H₂SO₄ electrolyte, exhibited a maximum device-specific capacitance of 43 Fg⁻¹ with comparable specific energy/power and excellent stability (∼96% after 10 000 cycles). Moreover, a symmetric supercapacitor was specially designed using PAC4, as a positive electrode, and PAC2, as a negative electrode, under their electrolytic ion affinity, and which operates in aqueous Na₂SO₄ electrolyte for a wide cell voltage (1.8 V) and showed excellent supercapacitance performances. Also, a device was assembled with poly(3,4-ethylene dioxythiophene) (PEDOT) nanostructure, as positive electrode, and PAC4, as a negative electrode, to evaluate the feasibility of designing a hybrid supercapacitor, using polymeric nanostructure, as an electrode material along with biomass-activated carbon electrode.