Subhrajit Dey

Numerical simulation of a dual-source supersonic plasma jet expansion process : continuum approach

Expanding thermal plasma (ETP) is a versatile technology for thin film deposition process with directional plasma flux and high deposition rates. This process involves expansion of supersonic plasma jets through a steep pressure ratio into a chamber maintained at near vacuum. Usually the plasma jets deviate from chemical and thermal equilibrium and the continuum approach is insufficient to describe the phenomena. In the current work, the continuum approach based Navier?Stokes equations have been implemented to study and understand the jet expansion process in a typical dual-arc plasma deposition reactor. The numerical predictions have been compared against in-house experimental data obtained by thermocouple measurements. For the range of back pressures (6?200?Pa) considered, it was observed that the jet core is supersonic and transitions to a subsonic zone downstream without the formation of any Mach disc for the prevalent operating parameters. Indications of thick and smeared barrel shocks were however observed in the computed flow-thermal fields. The modelled fluid was assumed to be a perfect gas with temperature dependent specific heats, thermal conductivity and viscosity coefficients, with constant Prandtl number of order unity. The radial spreads of the jets increase with increasing pressure ratio thus leading to enhanced interactions within reduced distances downstream of the nozzle exit. The jet core Mach number also increases, but moderately, with decreasing backpressure. It is concluded that within reasonable accuracy, continuum approach based calculations are able to capture most of the important phenomena involved in compressible, high-temperature, supersonic jet expansion processes which are essential in designing chambers relevant to the mentioned processes.

Jet Impingement Melting With Vaporization: A Numerical Study

The objective of this work was to develop a computational model for better understanding of the process of producing contrast agent, used in Magnetic Resonance Imaging (MRI). Contrast agents are used to provide high-resolution anatomical and functional information to identify tumor growth for prostrate and breast cancers. The production process for the contrast agents involves melting and dissolution of the imaging agent (maintained at very low temperature ∼1K to retain its polarization capability) by injecting the jet of alkaline solvent. Dissolution should happen in minimum time to allow for time required to inject contrast agent into the patient with sufficient time to travel to the targeted organ. This process involves multi-phase, multi-species and chemically reacting fluid dynamics. The intricacy and complexity of the melting process and very small time scales (order of a few milliseconds) poses practical challenges of collecting enough experimental data for the better understanding of such processes. It creates a need for looking at these kinds of processes from a numerical point of view. A computational model was developed in commercial software to capture the relevant physics involved in the flow-thermal process. System was analyzed to guide design changes with the objective of minimizing the melting time of imaging agent. Model predictions were validated against experiments and sensitivity studies were carried out pertaining to operating parameters such as solvent flow rate, temperature and other geometrical parameters. The predictions from model gave an insight into the process. It was found that melting time is not only a function of operating conditions and geometrical parameters but also a function of nature of the multiphase flow. Other than solid and molten phase, vapor phase (vaporization of the alkaline solvent) also coexist in the system under certain operating conditions; which further complicates the process. Desired operating conditions and geometrical changes were recommended to minimize the melting time. It is believed that current findings and numerical modeling approach could be utilized in other similar processes.Copyright

Numerical Sensitivity Studies on Nucleation of Droplets in Steam Turbine

This work presents the results of canonical test cases that highlight the importance of nucleation bulk surface tension factor (NBTF) on CFD predictions for condensing flows in steam turbines. Numerical simulations are carried out on nozzle and cascade geometries to explore modeling effects on the condensation of water vapor. The recent Euler-Euler approach [10] for modeling homogeneous condensation provides better results than the equilibrium assumption. Modeling of the nucleation rate plays a significant role in the non-equilibrium approach and it depends on the free surface energy of each droplet. NBTF is introduced in the classical homogeneous condensation nucleation rate expression to control the intensity of the homogeneous condensation event [3, 10]. It is observed that the NBTF controls the location of the condensation front, degree of super cooling, wetness fraction and droplet size. In addition, no unique value of NBTF is found in the range of simulations to match the experimental observations. Finally, by increasing the value of NBTF from 0.7 to 1.0 for a particular nozzle case, the location of condensation front is shown to be delayed by 60 mm and super cooling increased by 20%. This in turn will affect quantities such as the flow angle, pressure at the blade row exit and the thermodynamic loss which are relevant for the turbine designer.Copyright

Aerodynamic Performance Assessment of Part-Span Connector of Last Stage Bucket of Low Pressure Steam Turbine

Modern steam turbines often utilize very long last stage buckets (LSB’s) in their low-pressure sections to improve efficiency. Some of these LSB’s can range in the order of 5 feet long. These long buckets (aka “blades”) are typically supported at their tip by a tip-shroud and near the mid span by a part span shroud or part span connector (PSC). The PSC is a structural element that connects all the rotor blades, generally at the mid span. It is primarily designed to address various structural issues, often with little attention to its aerodynamic effects. The objective of the current work is to quantify the impact of PSC on aerodynamic performance of the last stage of a LP steam turbine by using detailed CFD analyses. A commercial CFD solver, ANSYS CFX™, is used to solve the last stage domain by setting steam as the working fluid with linear variation of specific heat ratio with temperature. A tetrahedral grid with prismatic layers near the solid walls is generated using ANSYS WORKBENCH™. The results show a cylindrical PSC reduces the efficiency of the last stage by 0.32 pts, of which 0.20 pts is due to the fillet attaching the PSC to the blade. The results also show insignificant interaction of the PSC with the bucket tip aerodynamics. The work presents a detailed flow field analysis and shows the impact of PSC geometry on the aerodynamic performance of last stage of steam turbine. Present work is useful to turbine designer for trade-off studies of performance and reliability of LSB design with or without PSC.Copyright

Cooling Enhancement of Radiators Using Dimples and Delta Winglets

Cooled exhaust gas recirculation and lower intake manifold temperature (post compressor) are used to meet emission regulations for a turbocharged intercooled diesel engine. This places a significant demand on the cooling load and space constraint on the radiator of the engine. A typical radiator is a cross-flow fin-tube heat exchanger with coolant water flowing inside the tube and ambient air taking out heat from the fin and tube surfaces. The major resistance to heat transfer in this configuration is offered by the air-side heat transfer co-efficient. The current study focuses on enhancing convective cooling rates on air side in a typical radiator which helps in taking additional load of EGR cooling with minimal increase in space and radiator fan power. Published literature clearly indicates that specific geometrical structures such as delta winglets and dimples, when placed in a convective flow path, act as vortex generators. This ability helps in disturbing/disrupting a steady thermal boundary layer, resulting in enhanced convective heat transfer. Detailed CFD simulations have been carried out to study the individual and combined effect of dimples and delta winglets on the heat transfer rates in a typical radiator geometry. Delta winglets on the fins indicated significant heat transfer enhancement but with increased pressure drop. Dimples on the tubes also led to enhanced heat transfer rates, but with a comparatively lesser increase in the pressure drop. A combination of delta winglets on the fins and dimples on the tubes increased the heat transfer rates substantially (+40%) with a minimal increase in pressure drop compared to the baseline case.Copyright