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    A Comparison of Three Different Flow Solvers For Simulating Steam Condensation Inside a Nozzle
    (Springer, 2023-04) Dasgupta, Mani Sankar; Yadav, Shyam Sunder
    In this work the condensation process of steam inside a converging diverging nozzle is simulated, the experiments on which were performed by Moses and Stein [1]. Three different flow solvers are used for this purpose, namely, Ansys Fluent, Ansys CFX and the open source flow solver OpenFOAM. The aim of the current work is to narrow down the choice of the flow solver which can finally be used for simulating non-equilibrium condensation of Carbon Dioxide inside ejectors. The pressure distribution predicted inside the nozzle by the three solvers follow closely the experimental one. However, the location and amount of pressure rise due to the condensation onset are differently predicted by the flow solvers. Only one plateau is observed in the pressure curve experimentally while at least two different plateaus are predicted numerically. For the vapor temperature and supercooling, the three solvers predict similar values before the condensation onset. After start of condensation, Ansys Fluent and OpenFOAM give similar results while Ansys CFX predicts at least 10 degree higher values. Ansys CFX predicts a wider nucleation zone compared to the two other solvers. The highest discrepancy being displayed by the solvers appears in the distribution of droplet diameter and droplet number density. However, the three solvers predict similar trend for the liquid mass fraction distribution inside the nozzle
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    Effect of Motive Nozzle Exit Position in a R-744 Two-Phase Ejector
    (Springer, 2022-01) Dasgupta, Mani Sankar; Yadav, Mani Sankar
    Ejector expansion transcritical refrigeration systems, in general, have an advantage over conventional transcritical refrigeration systems. In this paper, a CFD-based investigation of transonic flow phenomenon for R-744 inside a three-dimensional model of an ejector is presented. Real-gas thermophysical properties of R-744 such as the density, dynamic viscosity, and thermal conductivity are defined as a function of pressure and temperature based on NIST Refprop. Three different motive nozzle exit positions are investigated. A motive nozzle tip position that is near to the mixing section is found to have higher pressure and lower Mach number due to better mixing.
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    Classical nucleation theory based simulations of non-equilibrium condensation of carbon dioxide inside converging-diverging nozzles
    (Begell House, 2021) Dasgupta, Mani Sankar; Yadav, Shyam Sunder
    In the current work, we perform numerical simulations of the phase change process of Carbon Dioxide inside three different converging diverging nozzles, the experimental data on which is available in open literature. The simulations are performed with the classical nucleation theory based non-equilibrium phase change solver available in Ansys CFX with the thermophysical properties of CO2 obtained from NIST Refprop. We focus on the supercooling levels attained by the fluid and the distribution of the liquid mass fraction of CO2 during its high speed expansion inside the nozzles. The nozzle shape, expansion rate and fluid inlet conditions have a strong influence on the supercooling levels and the maximum liquid mass fractions obtained inside the nozzles. The results show much lower supercooling levels attained by CO2 (~ 2K) inside the Claudio Lettieri nozzle, the inlet state for which is near to the critical point. The supercooling attained by the vapor inside the Gyarmathy nozzle is around 22.5 K, the inlet state for which is far from the critical point. The case with the Nakagawa nozzle fails to converge properly.
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    A comparison of the equilibrium and the droplets based non-equilibrium compressible phase change solvers for condensation of carbon dioxide inside nozzles
    (Global Digital Central, 2022) Dasgupta, Mani Sankar; Yadav, Shyam Sundar
    In the current work, we simulate the condensation of supercritical CO2 during its high speed flow inside two different converging-diverging nozzles. We use the homogeneous equilibrium method and the classical nucleation theory based non-equilibrium phase change model for this purpose. The simulation results indicate significant influence of the nozzle inlet condition, nozzle shape and the fluid thermophysical behaviour on the nonequilibrium conditions prevailing inside the nozzles. We observe very low, 0.15 K, supercooling for the flow of CO2 inside the Claudio Lettieri nozzle compared to the supercooling of 3 K observed for the Berana nozzle. Very high nucleation rate ( 1035 nucleation per m3 per second) is observed before the throat of the nozzles which remains confined to a very small axial distance. The nucleation rate takes much smaller values ( 107 nucleation per m3 per second) in rest of the nozzle. A maximum of 70 nano meter sized droplets with number densities of the order of 1021 droplets per m3 are predicted inside the nozzles. Liquid mass fraction values between 0.2 to 0.4 are predicted by the solvers inside the nozzles. These results will be useful to the engineering community involved in the design and fabrication of CO2 based systems.