Department of Mechanical Engineering

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

The onset of convection instability in a differentially heated layer consisting of gray and non-gray gaseous mixtures is studied numerically. The conditions investigated cover a wide range of Planck number values (Pl=κkT4σT03), from the conduction-dominated regime of Pl≫ 1 to the radiation-dominated regime of Pl≪ 1. The linear stability theory is applied to mass, momentum and energy balance equations and the resulting linear stability equations are solved by Chebyshev spectral collocation method. The divergence of radiative flux is solved by the finite-volume-based discrete ordinates method. The Spectral Line Weighted sum of gray gas (SLW) model is used to represent the fine spectral variation of absorption coefficient for a non-gray gas medium. The results indicate that the critical Rayleigh number (Rac) for the onset of convection increases with mean temperature (T) in the conduction-dominated regime at low values of T. In the radiation-dominated regime (Pl≪ 1), Rac decreases with T for gray media. If the medium is non-gray, the critical Rac reduces to even lower values (as compared with those of gray gases) due to the dependence of gas absorptivity on temperature T. A reduction in the wall emissivity value increases the stability of the fluid layer due to reflection of radiation from the wall, in the radiation-dominated regime. The reverse trend is seen for Pl≫ 1. The critical parameters also significantly depend on the concentrations of radiatively participating gases in the mixture. The temperature profile in the fluid layer transforms from a linear profile in conduction regime to a stratified profile with steep gradients near the walls, in the presence of non-gray participating gases. © 2021, Indian Academy of Sciences.

World University root node

Sadhana - Academy Proceedings in Engineering Sciences

Onset of natural convection in a differentially heated layer of gray and non-gray gas mixtures

Aircraft ejection systems are lifesaving devices used, in case of emergencies for military pilots. As per current literature the success rate of ejections happening in low altitudes (below 150 m) is 51.2%. The success rate of an ejection depends on various parameters such as height for parachute full deployment, ejection posture, spinal alignment, restraints etc. This paper proposes a multi objective optimization based on a novel injury parameter and the dynamic conditions of the aircraft prevailing during the time of emergency to choose the optimal impulse required for safe ejection. This injury parameter is capable of modelling the spine in different postures. A non-linear seat restraint interaction is also modelled to include the effect of varying slackness of seat restraints there by optimizing force applied for the different physical conditions of the pilot, which hasn’t been reported in literature yet. The Pareto solutions obtained thereof indicates the dangers associated with posture during the ejection process. The optimal solutions after decision making are used to train a two stage Artificial Neural Network architecture to choose optimal parameters for different ejection scenarios. The ANN proposed was seen to give good results which were along the generated Pareto fronts for different simulated aircraft conditions instantaneously. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature.

Optimization and Engineering

A two stage neural network for choosing optimal ejection parameters in low altitude seat ejection based on novel injury parameter

This paper proposes a numerical model of wicked heat pipes using a lumped parameter network. A lumped parameter network of a screen mesh wicked heat pipe is developed for the solid and fluid regions. Nodal analysis is applied to each node using the principles of conservation of mass, momentum, and energy. The resulting system of differential equations is solved using numerical integration. The lumped parameter model is validated using experimental results available from the literature. A parametric study is conducted to determine the effect of heat input, wick thickness, and effective wick porosity on the thermal resistance of the heat pipe. The interplay between heat input and pipe diameter of the heat pipe is investigated. A comparison of heat pipe with working fluid as water and self-rewetting fluids has been conducted. Self-rewetting fluids are found to give better performance for the wicked heat pipe. © 2020 Taylor & Francis Group, LLC.

Heat Transfer Engineering

Numerical Modeling of a Wicked Heat Pipe Using Lumped Parameter Network Incorporating the Marangoni Effect

Understanding the droplet-hot wall interaction is crucial in industrial applications such as spray cooling, desalination and refrigeration, and IC engines. The present study focusses on simulations of two consecutively impinging concentric droplets on a heated surface in ambient conditions. A numerical model with fluid-solid coupling is implemented in opensource CFD software OpenFOAM considering contact line evaporation and dynamic contact line motion. The implemented numerical solver is assessed by validating it against the sessile droplet evaporation case available in the literature, followed by in-house experimental results of a single droplet impact. Consequently, the validated solver is used to conduct the simulations of consecutive droplet impingement on a hot surface, considering the droplet flow rate (i.e., the time interval between the two droplets) and surface thickness as parameters. The droplet flow rate is chosen in the order of 102−103 droplets per second (time interval is in the range of 3 - 100 milliseconds), and two variants of the surface thickness of 0.025 and 2 mm are used. The spread and heat transfer dynamics of each droplet are calculated in terms of spread factor, dimensionless input and evaporation heat transfers, respectively, and compared at all parametric conditions. The study reveals that the selected droplet flow rate affects the spread dynamics of the two droplets, resulting in various droplet heat transfer patterns. Moreover, it is observed that a surface with higher thickness results in more droplet heat transfer due to large thermal inertia. The numerical results are in good agreement with theoretical calculations of maximum spread factor and corresponding droplet heat transfer. © 2020 Elsevier Ltd

International Journal of Heat and Mass Transfer

Influence of flow rate and surface thickness on heat transfer characteristics of two consecutively impinging droplets on a heated surface

The morphological and hydrodynamic behavior of bubbles in air–water plumes was studied by employing Particle Tracking Velocimetry (PTV) techniques. Individual bubbles were identified by analyzing the instantaneous high-speed camera images of the plume. The bubble velocities were then obtained by Lagrangian tracking of each bubble, based on its size and location. The nature of bubble coalescence and breakup was also investigated based on the change in the projected area of the parent and daughter bubbles. The effect of the bubble position and the air inflow rate on the bubble properties like diameter, aspect ratio, velocity, number flux, etc., and on the coalescence and breakup behavior was investigated. A novel correlation is proposed to estimate the bubble aspect ratio as a function of bubble Reynolds number and Eotvos number, for a wide range of flow conditions, and is compared with existing correlation from the literature. © 2020 Elsevier Ltd

Department of Metallurgical and Materials Engineering

Chemical Engineering Science

Morphology and flow behavior of buoyant bubble plumes

The current research work focuses on spray characterization in a model Lean Premixed and Pre-vaporized (LPP) atomizer that aims to minimize NOx emission in gas turbine combustors. The injector allows cross-stream atomization of two radially injected liquid jets from a central hub due to the crossflowing air within a surrounding annular region. Both non-swirling and swirling air flows were considered. Optical measurement of spray characteristics is reported for a range of aerodynamic Weber number, (Weg ≈ 40–140) and momentum flux ratio, (MFR ≈ 3–7) that ensured that the jet–wall interactions leading to liquid films were avoided prior to completion of the jet breakup. Planar laser sheet imaging of the spray illustrated significantly wider dispersion of the spray droplets and modification in the overall spray structure due to introduction of air swirl upstream of the liquid jets within the atomizer. The droplet size and all three velocity components were measured using a Phase Doppler Particle Analyzer (PDPA) technique. The SMD was found to significantly reduce with increase in Weg for swirling as well as non-swirling crossflows. The tangential and radial velocity of droplets increased under the presence of swirling air as compared to non-swirling crossflow. The proper orthogonal decomposition (POD) analysis of the spray images was done which revealed the dominant structures in spray. The second and third POD modes were found to depict the alternate feature of region with concentrated droplet number density. Spray fluctuations due to unsteady jet breakup become dominant feature over flapping of spray structure for higher Weg. © 2021, Springer Nature Singapore Pte Ltd.

Lecture Notes in Mechanical Engineering

Spray Characterization and Structure Analysis in a Model LPP Atomizer

Water injection is one of the promising strategies for modern downsized GDI engines to meet performance and emission requirements under wide operating conditions. However, the benefits of water injection strongly depend upon water evaporation and distribution of water vapours inside the combustion chamber. In this regard, water injector location and orientation are two significant geometrical parameters which decide the in-cylinder positioning of the water spray, evaporation, and vapour distribution. Therefore, this CFD study presents the effects of water injector location, and orientation on the in-cylinder air-water interaction, water evaporation, vapour distribution, performance and emission characteristics of a wall-guided GDI engine. It also presents the effects of water injector location and orientations on water evaporation rate, water droplet distribution, heat release rate, IMEP, and exhaust emissions. In this study, the spark-plug is considered as located in the middle of the cylinder head, and the water injector is mounted in between the exhaust valves. Four locations of the water injector (21 mm, 25 mm, 29 mm, and 33 mm from the spark-plug) are considered for the investigation. Also, the injector orientation is varied from 12° to 28°, in the step of 4°. The engine is operated under naturally aspirated and full-load conditions at an engine speed of 2000 rev/min. Analysis of results shows that the water evaporation rate increases when more amount of water is sprayed in the region where the velocities of water droplets are in the opposite direction of that of the in-cylinder air. It is found that the water injector located at 29 mm with the orientation of 24° is found to be optimum configuration, where the IMEP improved by about 9.2%, while NOx and soot emissions reduced by about 48.2% and 22.6% respectively compared to those of the case of no_water. © 2020 Karabuk University

Fulltext

Engineering Science and Technology, an International Journal

Effect of in-cylinder air-water interaction on water evaporation and performance characteristics of a direct water injected GDI engine

In this study, researchers have explored real-world driving conditions and developed emission factors for 58 passenger cars using on-board emission measurement technique while driving on five different routes in Delhi. The measured average emission factors of CO, HC, and NO were 3.99, 0.34, and 0.54 g/km for diesel vehicles, 7.26, 0.17, and 0.62 for petrol vehicles respectively. Road, traffic, vehicle type, and driving characteristics affect the quantity of emissions released. However, speed and acceleration significantly impact emission rates increasing with the increase in speed and acceleration. Also, emissions were minimal at 40–60 kmph and −0.5–0.5 m/s2. The estimated city-wide CO, HC, and NO emissions were 60.8, 4.8, and 9.72tonnes/day. These results demonstrate the importance of monitoring the real-world exhaust emissions given the substantial difference between test cycle measurements used for compliance testing of new vehicles. © 2021 Elsevier Ltd

Department Of Civil Engineering

Transportation Research Part D: Transport and Environment

Characteristics of real-world gaseous exhaust emissions from cars in heterogeneous traffic conditions

Low stiffness bearings are useful to reduce the force transmitted from the vibrating rotor to the surrounding support structure. However, having low stiffness requires us to cross the low rigid body critical frequency while accelerating to operating rpm. In this work the stiffness of the bearing is changed online during operation by using an active magnetic bearing instead of a conventional constant stiffness rolling element bearing. This methodology is shown for a rigid rotor using both simulation and experimental techniques. During acceleration phase, a high stiffness is maintained, which gives us high critical frequency. After acceleration to operating rpm, the stiffness of the bearing is reduced at run time so that the bearing again becomes a soft support. In this work, a thrust magnetic bearing of variable stiffness is used to show that by changing the stiffness at run time, we can avoid crossing the rigid body critical frequency and hence reduce the amplitude of resonant vibrations. © Springer Nature Singapore Pte Ltd 2021.

Effecting critical frequency shift in rotors using active magnetic bearings

Focusing of ultrasound waves is critical to a number of clinical and industrial applications including biomedical and underwater imaging, nondestructive evaluation and material processing. This paper discusses the use of a novel 'add-on' gradient refractive index (GRIN) metamaterial structure made of concentric shells, to focus ultrasonic waves generated by conventional transducers. Analysis based on the Huygen's principle and numerical simulations is used to design the geometric and material properties of the proposed structure, whose working is demonstrated through experiments. Varying the shell material or thickness is shown to offer an elegant and straightforward way to tailor the focal spot inside the target material. The concentric-shell GRIN lens proposed here has a simple design, and has a potential to be used in dynamic focusing without advanced lenses or electronic steering. © 2021 Elsevier B.V.