Journal Articles
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Item Critical review of ribbed solar air heater and performance evaluation of various V-rib configuration(Elsevier Ltd, 2021) Nidhul, K.; Yadav, A.K.; Anish, S.; Kumar, S.The low heat transfer rate in a flat plate solar air heater (SAH) is due to the development of a laminar sub-layer near the heated absorber plate. Owing to this, the plate temperature rises substantially, instigating losses and thus reducing the efficiency. Extensive research has been carried out to mitigate this problem, of which passive technique emerged to be a promising solution. The passive method involves the application of fins/turbulators/ribs on the surface where the boundary layer develops so that it breaks the same. Various profiles and configurations of the ribs ranging from transverse to inclined and continuous to discrete have been presented concisely. Correlations on Nusselt number (Nu) and friction factor (f) for different rib configurations have been summarized in order so that it can be accessed for future research. 3-D CFD analysis is carried out to gain insight into the flow pattern of various V-ribbed SAH, and with the help of streamlines and contours, the findings are established. Furthermore, various exergy destruction has been studied in detail for different V-rib configurations, namely– V-rib, multiple V-rib, and multiple V-rib with the gap, and analyzed in detail for prospective studies. Exergetic performance study of these rib configurations indicates that the multiple ribs and the multiple rib-gap combinations enhance exergetic efficiency (ηex) by 12% and 31.6%, respectively, in comparison to V-rib SAH. © 2021 Elsevier LtdItem Machine learning approach for optimization and performance prediction of triangular duct solar air heater: A comprehensive review(Elsevier Ltd, 2023) Nidhul, K.; Thummar, D.; Yadav, A.K.; Anish, S.This paper presents a comprehensive review of various kinds of distinct artificial roughness employed in rectangular and triangular duct solar air heaters to aid prospective researchers in finding a critical gap in the domain of solar air heaters. A Machine Learning (ML) model is developed using 72 distinct rib combinations compiled to 454 datasets and trained using an Artificial Neural Network (ANN) to predict the performance of ribbed triangular duct Solar Air Heater (SAH). The developed ML model predicts the data with an average deviation of <3%. Owing to reasonably accurate predictions, the same could be increased when more cases (geometric or operating parameters) are added to the databases by retraining the ANN. Further, a second law analysis of the rib configurations features collector efficiency and entropy generation variation with Re for various rib parameters. For the Re range of 4000 to 18000, optimum parameters such as rib height, pitch, chamfer angle, and inclinations are obtained for triangular duct SAH. This could help design engineers obtain the performance parameters of ribbed triangular duct SAH with other artificial roughness designs, possibly with a combination of different geometrical and operating parameters, without having to perform tests. © 2023 International Solar Energy SocietyItem Transient analysis of subcritical/supercritical carbon dioxide based natural circulation loops with end heat exchangers: Numerical studies(Elsevier Ltd, 2014) Yadav, A.K.; Ram Gopal, M.; Bhattacharyya, S.Transient analysis of carbon dioxide based natural circulation loop (NCL) with end heat exchangers has been carried out. Subcritical and supercritical phases of CO2 are considered with operating pressures in the range of 50-100 bar for an operating temperature range of 323 K to 363 K. Studies are carried out for various loop tilt angles, different initial conditions, and different water mass flow rates. Results: are obtained for various inlet temperatures of water in the hot heat exchanger while keeping the inlet temperature of cooling water in the cold heat exchanger fixed. Effect of tilting the loop in XY and YZ planes on transient as well as steady state behaviour of loop are also studied. Validation of simulation results against experimental and numerical results reported in the literature in terms of modified Grashof number (Grm) and Reynolds number (Re) show good agreement. © 2014 Elsevier Ltd. All rights reserved.Item Effect of tilt angle on subcritical/supercritical carbon dioxide-based natural circulation loop with isothermal source and sink\(American Society of Mechanical Engineers (ASME) infocentral@asme.org, 2016) Yadav, A.K.; Ram Gopal, M.R.; Bhattacharyya, S.In recent years, a growing popularity of carbon dioxide (CO2) as a secondary fluid has been witnessed in both forced as well as in natural circulation loops (NCLs). This may be attributed to the favorable thermophysical properties of CO2 in addition to the environmental benignity of the fluid. However, an extensive literature review shows that studies on CO2-based NCLs are very limited. Also, most of the studies on NCLs do not consider the three-dimensional variation of the field variables. In the present work, threedimensional computational fluid dynamics (CFD) models of a NCL with isothermal source and sink have been developed to study the effect of tilt angle in different planes. Studies have been carried out employing subcritical (liquid and vapor) as well as supercritical phase of CO2 as loop fluid at different operating pressures and temperatures. Results are obtained for a range of tilt angles of the loop, and a significant effect is observed on heat transfer, mass flow rate, and stability of the loop. It was also found that changing the orientation of the loop could be an elegant and effective solution to the flow instability problem of NCLs.Item Optimum Operating Conditions for Subcritical/Supercritical Fluid-Based Natural Circulation Loops(American Society of Mechanical Engineers (ASME) infocentral@asme.org, 2016) Yadav, A.K.; Bhattacharyya, S.; Ram Gopal, M.R.Natural circulation loop (NCL) is simple and reliable due to the absence of moving components and is preferred in applications where safety is of foremost concern, such as nuclear power plants and high-pressure thermal power plants. In the present study, optimum operating conditions based on the maximum heat transfer rate in NCLs have been obtained for subcritical as well as supercritical fluids. In recent years, there is a growing interest in the use of carbon dioxide (CO2) as loop fluid in NCLs for a variety of heat transfer applications due to its excellent thermophysical environmentally benign properties. In the present study, three-dimensional (3D) computational fluid dynamics (CFD) analysis of a CO2-based NCL with isothermal source and sink has been carried out. Results show that the heat transfer rate is much higher in the case of supercritical phase (if operated near pseudocritical region) than the subcritical phase. In the subcritical option, higher heat transfer rate is obtained in the case of liquid operated near saturation condition. Correlations for optimum operating condition are obtained for a supercritical CO2-based NCL in terms of reduced temperature and reduced pressure so that they can be employed for a wide variety of fluids operating in supercritical region. Correlations are also validated with different loop fluids. These results are expected to help design superior optimal NCLs for critical applications. © 2016 by ASME.Item Tuning the self-assembly and photophysical properties of bi-1,3,4-thiadiazole derivatives through electron donor-acceptor interactions and their application in OLEDs(Royal Society of Chemistry, 2017) Yadav, A.K.; Pradhan, B.; Ulla, H.; Nath, S.; De, J.; Pal, S.K.; Satyanarayan, M.N.; Ammathnadu Sudhakar, A.S.We report several shape anisotropic molecules that contain two centrally placed 1,3,4-thiadiazole units, which vary from each other with respect to the number and length of the flexible chains at the termini. The number, position and length of the peripheral chains connected to the termini showed an impact on the thermal behavior of these compounds. The compounds with two terminal tails exhibited an enantiotropic smectic C phase, whereas the compounds with four terminal tails turned out to be crystalline. Surprisingly, among the compounds with six terminal tails, only the compound with a longer terminal chain exhibited a columnar phase with oblique symmetry. It is also to be noted that only compounds with six terminal chains exhibited gelation in long chain hydrocarbons. The xerogel of the hexacatenar with six n-decyloxy chains showed an entangled network of nanofibers of several micrometers in length. The aggregation behavior of the hexacatenar in the hydrocarbon solvent is mainly supported by the attractive ?-? interactions of the aromatic cores and the van der Waals interactions offered by the peripheral flexible tails. The emission behavior is dependent on the number of peripheral tails and not on the length. Furthermore, one of the hexacatenars exhibited solvatochromic emissive behavior. This molecular design helps in the development of long molecular nanowires with a central conducting core and insulating peripheral sheath, which will be helpful for the application in organic electronic devices. The application potential of the columnar liquid crystal material was tested by the fabrication of organic light emitting diodes (OLEDs) either as a single emissive material or as a guest material in a host polymer. Higher efficiency and brightness were noticed in the host guest OLED, which exhibited a technologically important bright blue emission. © 2017 The Royal Society of Chemistry.Item Influence of low-temperature combustion and dimethyl ether-diesel blends on performance, combustion, and emission characteristics of common rail diesel engine: a CFD study(Springer Verlag service@springer.de, 2017) Lamani, V.T.; Yadav, A.K.; Gottekere Narayanappa, K.G.Due to presence of more oxygen, absence of carbon-carbon (C-C) bond in chemical structure, and high cetane number of dimethyl ether (DME), pollution from DME operated engine is less compared to diesel engine. Hence, the DME can be a promising alternative fuel for diesel engine. The present study emphasizes the effect of various exhaust gas recirculation (EGR) rates (0–20%) and DME/Diesel blends (0–20%) on combustion characteristics and exhaust emissions of common rail direct injection (CRDI) engine using three-dimensional computational fluid dynamics (CFD) simulation. Extended coherent flame model-3 zone (ECFM-3Z) is implemented to carry out combustion analysis, and k-?-f model is employed for turbulence modeling. Results show that in-cylinder pressure marginally decreases with employing EGR compared to without EGR case. As EGR rate increases, nitrogen oxide (NO) formation decreases, whereas soot increases marginally. Due to better combustion characteristics of DME, indicated thermal efficiency (ITE) increases with the increases in DME/diesel blend ratio. Adverse effect of EGR on efficiency for blends is less compared to neat diesel, because the anoxygenated region created due to EGR is compensated by extra oxygen present in DME. The trade-off among NO, soot, carbon monoxide (CO) formation, and efficiency is studied by normalizing the parameters. Optimum operating condition is found at 10% EGR rate and 20% DME/diesel blend. The maximum indicated thermal efficiency was observed for DME/diesel ratio of 20% in the present range of study. Obtained results are validated with published experimental data and found good agreement. © 2017, Springer-Verlag Berlin Heidelberg.Item Transient analysis of subcritical/supercritical carbon dioxide based natural circulation loop with end heat exchangers: experimental study(Springer Verlag service@springer.de, 2017) Yadav, A.K.; Ramgopal, M.; Bhattacharyya, S.Carbon dioxide (CO2) based natural circulation loops (NCLs) has gained attention due to its compactness with higher heat transfer rate. In the present study, experimental investigations have been carried out to capture the transient behaviour of a CO2 based NCL operating under subcritical as well as supercritical conditions. Water is used as the external fluid in cold and hot heat exchangers. Results are obtained for various inlet temperatures (323–353 K) of water in the hot heat exchanger and a fixed inlet temperature (305 K) of cooling water in the cold heat exchanger. Effect of loop operating pressure (50–90 bar) on system performance is also investigated. Effect of loop tilt in two different planes (XY and YZ) is also studied in terms of transient as well as steady state behaviour of the loop. Results show that the time required to attain steady state decreases as operating pressure of the loop increases. It is also observed that the change in temperature of loop fluid (CO2) across hot or cold heat exchanger decreases as operating pressure increases. © 2017, Springer-Verlag Berlin Heidelberg.Item Performance, emission, and combustion characteristics of twin-cylinder common rail diesel engine fuelled with butanol-diesel blends(Springer Verlag service@springer.de, 2017) Lamani, V.T.; Yadav, A.K.; Gottekere, K.N.Nitrogen oxides and smoke are the substantial emissions for the diesel engines. Fuels comprising high-level oxygen content can have low smoke emission due to better oxidation of soot. The objective of the paper is to assess the potential to employ oxygenated fuel, i.e., n-butanol and its blends with the neat diesel from 0 to 30% by volume. The experimental and computational fluid dynamic (CFD) simulation is carried out to estimate the performance, combustion, and exhaust emission characteristics of n-butanol-diesel blends for various injection timings (9°, 12°, 15°, and 18°) using modern twin-cylinder, four-stroke, common rail direct injection (CRDI) engine. Experimental results reveal the increase in brake thermal efficiency (BTE) by ~ 4.5, 6, and 8% for butanol-diesel blends of 10% (Bu10), 20% (Bu20), and 30% (Bu30), respectively, compared to neat diesel (Bu0). Maximum BTE for Bu0 is 38.4%, which is obtained at 12° BTDC; however, for Bu10, Bu20 and Bu30 are 40.19, 40.9, and 41.7%, which are obtained at 15° BTDC, respectively. Higher flame speed of n-butanol-diesel blends burn a large amount of fuel in the premixed phase, which improves the combustion as well as emission characteristics. CFD and experimental results are compared and validated for all fuel blends for in-cylinder pressure and nitrogen oxides (NOx), and found to be in good agreement. Both experimental and simulation results witnessed in reduction of smoke opacity, NOx, and carbon monoxide emissions with the increasing n-butanol percentage in diesel fuel. © 2017, Springer-Verlag GmbH Germany.Item Computation of fluid flow in double sided cross-shaped lid-driven cavities using Lattice Boltzmann method(Elsevier Ltd, 2018) Bhopalam, S.B.; Arumuga Perumal, D.A.; Yadav, A.K.This work implements Lattice Boltzmann method to compute flows in double-sided cross-shaped lid-driven cavities. Firstly, a complicated geometry which is a symmetrized version of the staggered lid-driven cavity namely, the double-sided cross-shaped lid-driven cavity with antiparallel uniform wall motion is studied employing Single as well as Two Relaxation time models. The streamline patterns and vorticity contours obtained for low to moderate Reynolds numbers (150–1000) are compared with published results and found to be in good accordance. Next, this code is extended to simulate flows in a double-sided cross-shaped lid-driven cavity with parallel uniform wall motion. The effect of three dimensionality is also studied for low Reynolds numbers. Lattice Boltzmann method is then used to investigate the oscillating double-sided cross-shaped lid-driven cavity with antiparallel and parallel wall motions. The movement and formation of primary and secondary vortices have been well captured with the variation of Reynolds numbers and oscillating frequencies for uniform and oscillating wall motions. Reasonable agreements with the established results have been observed for the double-sided cross-shaped cavity with uniform wall motions, while new results have been obtained in the case of oscillating wall motions. © 2018 Elsevier Masson SAS