Faculty Publications
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Item Effect of exhaust gas recirculation rate on performance, emission and combustion characteristics of a common-rail diesel engine fuelled with n-butanol–diesel blends(Taylor and Francis Ltd. michael.wagreich@univie.ac.at, 2020) Lamani, V.T.; Yadav, A.; Gottekere, K.N.Increasing fears of fossil fuel attenuation and tough emission protocols compel the research community to explore alternative renewable fuels for diesel engines. Butanol is desirable among renewable fuels due to its properties favorable to diesel engines. This study focused on the suitability of exhaust gas recirculation (EGR) and optimum injection timing on the performance, combustion and exhaust emission characteristics of common-rail direct-injection (CRDI) engine fueled with n-butanol-blended diesel using experimental and computational fluid dynamics (CFD) simulation. Various EGR rates and injection timings are considered for different butanol–diesel blends (0, 10, 20 and 30%). Obtained simulation results are validated with experimental data and found to be in good agreement. For all EGR rates and blends, nitrogen oxide (NO) emission is reduced drastically, whereas carbon monoxide (CO) and soot emissions are decreased moderately, with increase in n-butanol–diesel blends. The CO and soot emissions increase with EGR rate due to oxygen deficiency as well. Brake thermal efficiency is reduced by approximately 1% for neat diesel (Bu0) with increase in EGR rates. Soot emission for Bu30 (15 ° Before top dead centre (BTDC) is decreased by 23, 25, 24 and 26% for 0, 10, 20 and 30% EGR rates, respectively, compared to Bu0 (12° BTDC). © 2017, © 2017 Informa UK Limited, trading as Taylor & Francis Group.Item Numerical and experimental investigation of melting characteristics of phase change material-RT58(Elsevier Ltd, 2020) Yadav, A.; Donepudi, T.; Siddani, B.S.The present study is focused on experimental and numerical analysis of unconstrained melting of Paraffin wax-RT58 in a horizontally placed cylindrical container. After the validation of numerical model with experimental results, numerical analysis is extended to constrained melting to investigate the process. The experiments are carried out at constant wall temperature maintained on the lateral surface of the cylinder. The influence of initial sub-cooling and lateral surface temperature on the melting rate is investigated. The melting process is better analyzed by the melting phase front and temperature contours as time progresses. The results show that the melting rate decreases by increasing the initial sub-cooling, and increases with increasing lateral surface temperature of the cylinder. In unconstrained melting, heat transfer by conduction governs the melting process initially, but later it is restricted to only the bottom part of the cylinder as the solid PCM at a higher density sinks due to effects of gravity. Heat transfer in the upper half of the cylinder is dominated by natural convection set up in the liquid PCM. In constrained melting, pure conduction phenomenon exists only in the beginning, and later conjugate heat transfer occurs. When subjected to similar boundary conditions, PCM melt-time is lower in unconstrained melting than in constrained melting. A correlation between melt-time and Stefan number is also developed. © 2019 Elsevier LtdItem Enhanced thermo-hydraulic performance in a V-ribbed triangular duct solar air heater: CFD and exergy analysis(Elsevier Ltd, 2020) Nidhul, K.; Kumar, S.; Yadav, A.; Anish, S.Computational fluid dynamics (CFD) and exergy analysis are conducted to investigate the impact of secondary flow produced by V-ribs on the overall performance of a triangular solar air heater (SAH) duct. For a fixed relative rib pitch (Rp = 10) and relative rib height (Rh = 0.05), the effect of rib inclination (?) is studied using CFD technique for varying Reynolds number (5000 ? Re ? 20000). Based on the CFD simulation results, empirical correlations capable of predicting Nu and f with an absolute variance of 8.7%, and 4.7%, respectively, are developed. Employing these correlations, exergetic performance analysis is carried out. Maximum effectiveness parameter (?) of 2.01 is obtained for ? = 45° at Re = 7500. The exergy analysis reveals that the entropy generated is lower for the ribbed triangular duct compared to the smooth duct with maximum enhancement in exergetic efficiency (?ex) as 23% for ? = 45°. The study is extended for the rectangular duct to compare the performance with the ribbed triangular duct SAH (? = 45°). Results show that ribbed triangular duct SAH (? = 45°) is superior over various configurations of the ribbed rectangular duct SAH at higher mass flow rates. © 2020 Elsevier LtdItem Efficient design of an artificially roughened solar air heater with semi-cylindrical side walls: CFD and exergy analysis(Elsevier Ltd, 2020) Nidhul, K.; Yadav, A.; Anish, S.; Arunachala, U.C.Solar air heater (SAH) with semi-cylindrical sidewalls and W-baffles is analyzed for energy and exergy efficiency in the turbulent flow regime. Computational fluid dynamics (CFD) analysis is carried out for a fixed baffle inclination (?) and varying the relative baffle height (Rh = e/D) and relative baffle pitch (Rp = P/D) in the range 0.046–0.115 and 0.46–1.15, respectively. For Reynolds number (5000 < Re < 17,500), the numerical methodology is substantiated using experimental and theoretical correlations obtained from the literature. Smaller vortices near the sharp corners are removed by rounding the sharp edges, allowing the flow of fluid from inside and horizontal walls of the duct towards the semi-cylindrical sidewalls. This increases the overall turbulent kinetic energy. A peak augmentation of 3.24 and 4.03 times is obtained for Nusselt number (Nu) and friction factor (f), respectively, in contrast to conventional SAH. With a maximum enhancement of 127% in the effectiveness parameter relative to smooth SAH, this novel SAH design is evidently energy efficient. Based on CFD results, new correlations are developed in terms of Rh and Rp, which predicts the values with an absolute deviation of 4% and 7.4%, respectively. With lower exergy destruction, maximum enhancement in thermal and exergetic efficiency is obtained as 40.7% and 95.4%, respectively, for the proposed SAH relative to conventional SAH. Upon comparison with ribbed rectangular duct SAH configurations, the present design with semi-cylindrical side walls outperforms at all flow Re. © 2020 International Solar Energy SocietyItem Comparative computational appraisal of supercritical CO2-based natural circulation loop: effect of heat-exchanger and isothermal wall(Springer Science and Business Media B.V., 2020) Thimmaiah, S.; Wahidi, T.; Yadav, A.; Mahalingam, A.Natural circulation loop (NCL) is a geometrically simple heat transfer device in which fluid flow occurs due to density gradient of loop fluid, induced by the temperature difference between the source and the sink. NCL has an inherent problem of instability caused by the combined effect of buoyancy, friction and inertia forces at varying operating conditions, and hence it requires an elegant solution of instability. The primary objective of the present work is to do a comparative study on the dynamic performance between two different configurations of NCL based on supercritical CO2, i.e. (i) NCL with isothermal heater and a cold heat-exchanger (ISO-CHX), and (ii) NCL with hot and cold heat-exchangers (HHX-CHX). To explore these NCLs, two-dimensional transient computational fluid dynamics studies have been carried out on the stability of supercritical CO2-based natural circulation loop. Results are obtained for different operating pressures and temperatures in the form of mass flow rate and velocity variation with respect to time. Results show the higher instabilities in both side heat-exchanger loop than an isothermal heater with heat-exchanger loop. At a lower rate of heat input at source in the HHX-CHX loop, the mass flow is bidirectional, whereas it is unidirectional in the ISO-CHX loop at all level of heat input. It is also observed that as pressure increases, flow instability also increases. Obtained results are validated with the published experimental and numerical data and found in good agreement. © 2020, Akadémiai Kiadó, Budapest, Hungary.