virkeshwar kumar
@iitk.ac.in
Assistant Professor, Department of Mechanical Engineering
Indian Institute of Technology Kanpur
RESEARCH INTERESTS
Solidification, Melting, Casting, Welding, Interfacial interactions phenomena, Evaporation, Crystallization during evaporation, Boiling, Flow visualization, Transport Phenomena, Natural convection, Experimental Fluid Dynamics and Heat Transfer, Phase change process, Phase change materials.
Scopus Publications
Scopus Publications
Virkeshwar Kumar, Shyamprasad Karagadde, and Kamal Meena
Springer Nature Singapore
Pranjal Agrawal, Virkeshwar Kumar, Samantha McBride, and Susmita Dash
American Chemical Society (ACS)
Scaling or mineral fouling occurs due to the presence of dissolved minerals in water. Scaling is problematic in numerous industrial and household plumbing applications where water is used. The current methods of scale removal often utilize harsh chemicals that are not environmentally friendly. The evaporation of a saline droplet provides a platform to study the role of the substrate in the dynamics of crystallization during scaling. In the present work, we show out-of-plane growth of crystal deposits during the evaporation of saline droplets of aqueous potassium chloride on a heated smooth and microtextured hydrophobic substrate. These out-of-plane deposits, termed as "crystal legs", are in minimal contact with the substrate and can be easily removed from the substrate. The out-of-plane evaporative crystallization of saline droplets of different initial volumes and concentrations is observed irrespective of the chemistry of the hydrophobic coating and the crystal habits investigated. We attribute this general behavior of crystal legs to the growth and stacking of smaller crystals (size ∼10 μm) between the primary crystals toward the end of evaporation. We show that the rate at which the crystal legs grow increases with an increase in the substrate temperature. A mass conservation model is applied to predict the leg growth rate, which agrees well with the experiments.
Virkeshwar Kumar and Susmita Dash
American Chemical Society (ACS)
In the present work, we investigate the influence of substrate wettability and crystal morphology on the evaporative crystallization of saline droplets. On a superhydrophilic substrate, the evaporative crystals formed during the drying of a saline droplet of aqueous potassium nitrate are observed to be long and needle-shaped, oriented along the substrate. The crystal deposits form a flower-shaped pattern when the initial contact angle of the droplet increases to ∼72°. The orientation of the crystals along the triple contact line of the droplet controls the self-amplifying creeping growth of the salt crystals that eventually determines the overall evaporative patterns. The crystals change from being needle-shaped to globular salt deposits as the volume of liquid available for crystallization reduces. We demonstrate that the arrangement of the crystal with respect to the substrate and the droplet-air interface governs the rate of evaporation, growth, and morphology of the crystals.
Virkeshwar Kumar, Ketan Sakalkale, and Shyamprasad Karagadde
AIP Publishing
In this work, the effect of solute expansion coefficient on the natural convection and freezing front propagation is investigated by performing three-side cooled solidification experiments. Four different aqueous salt solutions, and different compositions thereof, were employed for experimentation. The mixtures were solidified to analyze the effect of solute expansion coefficients on the convection currents and the composition distribution in the bulk. The initial compositions were chosen such that all cases have the same primary solid fraction at eutectic temperature, for obtaining similar compositional changes in the bulk liquid at various stages. Similar cooling conditions were also maintained to ensure that the variation in convection strength is primarily caused by different solute expansion coefficients. A distinct observation of the free surface freezing before the bulk, termed bridging, is reported in certain cases. Further analysis revealed that the bridging could be attributed to a difference in solute convection caused by the solute expansion coefficient. Numerical simulations were performed to further ascertain the plausible initiation mechanisms for bridging. The predicted compositional and solid fraction distribution revealed lesser solute accumulation near the surface, for the lower solute expansion cases, and the resulting increase in the tendency of freezing at the top. An upper limit for the ratio of solutal to thermal Rayleigh numbers in the experimental conditions has been identified for the occurrence of bridging in high Prandtl number fluids.
Virkeshwar Kumar and Susmita Dash
American Chemical Society (ACS)
Adulteration of milk poses a severe health hazard, and it is crucial to develop adulterant-detection techniques that are scalable and easy to use. Water and urea are two of the most common adulterants in commercial milk. Detection of these adulterants is both challenging and costly in urban and rural areas. Here we report on an evaporation-based low-cost technique for the detection of added water and urea in milk. The evaporative deposition is shown to be affected by the presence of adulterants in milk. We observe a specific pattern formation of nonvolatile milk solids deposited at the end of the evaporation of a droplet of unadulterated milk. These patterns alter with the addition of water and urea. The evaporative deposits are dependent on the concentrations of water and urea added. The sensitivity of detection of urea in milk improves with the dilution of milk with water. We show that our method can be used to detect a urea concentration as low as 0.4% in milk. Based on the detection level of urea, we present a regime map that shows the concentration of urea that can be detected at different extents of dilution of milk.
Virkeshwar Kumar, Atul Srivastava, and Shyamprasad Karagadde
Cambridge University Press (CUP)
Solidifying ternary systems can exhibit complex natural convection phenomena, particularly due to the presence of two porous zones (cotectic and primary mush), and the rejection of two differently dense solutes. The primary objectives of this study are to investigate the following: (i) the natural convection patterns in various compositional regimes of a typical ternary system, and (ii) the role of the combined existence of the microstructure (facets and dendrites) in the porous zone on natural convection, with a motivation to enhance the current understanding of the microstructure–convection relationships. A ternary mixture is chosen such that different compositions of the three primary solidifying components lead to the formation of distinct ice, dendritic and faceted solid structures that cover the complete span of microstructure–convection relationships. The observations of flow in different compositional regimes show convection occurring in the form of plumes, random mixing and double-diffusive layering, as well as combinations of these, which are governed by the type of coexisting microstructures. The study reveals the occurrence of Rayleigh–Taylor instability with varying amounts of the heavier component. The bulk liquid composition showed a tendency to cross the cotectic line, and thus also change the nature of primary solidifying structure from faceted to dendritic in cases where facets and dendrites were present in cotectic mush, and facets in primary mush. These insights are believed to elucidate the complex mechanisms of ternary solidification, as well as provide important real-time data for direct numerical simulations.
Aniket D. Monde, Oaj Chawla, Virkeshwar Kumar, Shyamprasad Karagadde, and Prodyut R. Chakraborty
AIP Publishing
Development and proposition of a numerical model to capture the shrinkage induced flow during directional solidification of a pure substance in a bottom cooled cavity are carried out. A novel numerical scheme involving fixed grid-based volume fraction updating is proposed to track the solid–liquid interface, considering the inclusion of the shrinkage effect. Directional solidification in bottom cooled orientation is of particular interest since shrinkage and buoyancy effects oppose each other. The results from the proposed numerical model indicated the existence of an unprecedented flow reversal phenomenon during the progression of the solidification process, caused by the opposing nature of shrinkage and buoyancy effects. The flow reversal phenomena predicted by the numerical model are validated by conducting experiments involving directional solidification of coconut oil in a bottom cooled cavity. Qualitative and quantitative measurements of the velocity field and interface growth are obtained using the particle image velocimetry technique and compared with three dimensional numerical results. Once the flow reversal phenomena are established through numerical and experimental evidences, case studies are performed, considering varying material properties, cold boundary temperatures, initial temperatures of the melt, and cavity heights to find the effect of each of these parameters on flow reversal phenomena. The parametric study also allowed us to check the robustness and consistency of the proposed model. The proposed model will serve as an important milestone toward the development of numerical models for capturing macro-scale shrinkage defects and prediction of composition heterogeneity during directional alloy solidification.
Virkeshwar Kumar, Atul Srivastava, and Shyamprasad Karagadde
Begell House
Virkeshwar Kumar, G. S. Abhishek, Atul Srivastava, and Shyamprasad Karagadde
Cambridge University Press (CUP)
Abstract In this study, identical experiments of bottom-cooled solidification fluidic mixtures that exhibit faceted and dendritic microstructures were performed. The strength of compositional convection, created due to the rejection of a lighter solute, was correlated with the solidifying microstructure morphology via separate Rayleigh numbers in the mushy and bulk-fluid zones. While the bulk fluid in dendritic solidification experienced a monotonic decrease in the temperature, solidification of the faceted case revealed an unconventional, anomalous temperature rise in the bulk liquid after the formation of a eutectic solid. Based on the bulk-liquid temperatures, three distinct regimes of heat transfer were observed in the liquid, namely, convection-dominated, transition and conduction-dominated. The observations were analysed and verified with the help of different initial compositions and cooling conditions, as well as other mixtures that form faceted morphology upon freezing. The observed temperature rise was further ascertained by performing an energy balance in an indicative control volume ahead of the solid–liquid interface. The plausible mechanism of permeability-driven flow causing a gain in the temperature of the liquid during freezing was generalized with the help of a semi-analytical investigation of a one-dimensional system comprising solid, porous mush and liquid regions. The analytical scaling relations for fluid velocity and vorticity, for the faceted and dentritic cases, revealed contrasting vorticity values, which are much larger in low permeability (faceted case) and cause enhanced mixing in the bulk. The study sheds new insights into the role of microstructural morphology in governing the transport phenomena in the bulk liquid.
Virkeshwar Kumar, Atul Srivastava, and Shyamprasad Karagadde
AIP Publishing
Stratified double-diffusive layers (DDLs) in fluidic mixtures such as oceans, magma, and latte typically contain alternating low gradient mixing regions separated by high gradient interfaces. The prior knowledge is restricted to the formation of layers, but the existence of DDLs, under prolonged freezing conditions, as well as in multicomponent mixtures, is not yet understood well. In this work, a new observation depicting the existence of a life-cycle for a double-diffusive layer is revealed with the help of real-time observations of unidirectional freezing of multicomponent mixtures. The observations showed a systematic occurrence of the onset, formation, disappearance, and recurrence of the DDLs when freezing conditions prevailed for longer durations of time. The results also include first-ever observations of compositional stratification in a ternary mixture, which depends on the regimes and nature of buoyant convection. The ternary experiments also demonstrated the formation of DDLs much closer to the solidifying mush, which shed light on retaining the stratified layers in the frozen state. Furthermore, the hypothesized life-cycle of the DDL was mapped to the regimes of occurrence and the nonexistence of DDLs in the mixture phase diagrams of binary and ternary systems, with a threshold composition difference and the corresponding critical Rayleigh number. This distinction of the regimes on the phase diagram shows a striking correlation with a reduced ternary phase diagram of igneous rocks, thus providing a suitable basis for explaining the formation of layered rocks.Stratified double-diffusive layers (DDLs) in fluidic mixtures such as oceans, magma, and latte typically contain alternating low gradient mixing regions separated by high gradient interfaces. The prior knowledge is restricted to the formation of layers, but the existence of DDLs, under prolonged freezing conditions, as well as in multicomponent mixtures, is not yet understood well. In this work, a new observation depicting the existence of a life-cycle for a double-diffusive layer is revealed with the help of real-time observations of unidirectional freezing of multicomponent mixtures. The observations showed a systematic occurrence of the onset, formation, disappearance, and recurrence of the DDLs when freezing conditions prevailed for longer durations of time. The results also include first-ever observations of compositional stratification in a ternary mixture, which depends on the regimes and nature of buoyant convection. The ternary experiments also demonstrated the formation of DDLs much closer to th...
Virkeshwar Kumar, Atul Srivastava, and Shyamprasad Karagadde
AIP Publishing
Density variations arising from thermal and compositional gradients in multi-component fluids can lead to natural convection flows. The double-diffusive layer is one such flow phenomenon, commonly observed in oceanic and phase change systems. The solidification of high Prandtl number fluids offers a suitable platform to study multi-diffusive convection owing to continuously evolving temperature and compositional fields. In this work, an experimental investigation was conducted to study the influence of transport phenomena on the double-diffusive layer formation, by performing full-field measurements of concentration and flow velocities during bottom-cooled solidification of a hyper-eutectic aqueous mixture. Using a Mach-Zehnder interferometer, the first-ever real-time, quantitative observations of solutal mixing, plume formation, and the evolution of the double-diffusive layers by forming a stepped compositional distribution have been reported. In addition, the associated flow velocities were measured using the particle image velocimetry technique which clearly characterizes the compositional and thermal natural convection patterns along the vertical and horizontal directions, respectively. The study revealed a life-cycle for the existence of the double-diffusive layers, wherein they undergo onset, development, and disappearance depending on the initial composition, and identified critical Rayleigh numbers for each of these stages. The experimental observations were further supported with analytical scale estimates of the critical length, time, and velocities of the system. The quantitative results elucidate the conditions, including a newly hypothesized threshold composition difference, which led to the formation as well as the disappearance of the layers.Density variations arising from thermal and compositional gradients in multi-component fluids can lead to natural convection flows. The double-diffusive layer is one such flow phenomenon, commonly observed in oceanic and phase change systems. The solidification of high Prandtl number fluids offers a suitable platform to study multi-diffusive convection owing to continuously evolving temperature and compositional fields. In this work, an experimental investigation was conducted to study the influence of transport phenomena on the double-diffusive layer formation, by performing full-field measurements of concentration and flow velocities during bottom-cooled solidification of a hyper-eutectic aqueous mixture. Using a Mach-Zehnder interferometer, the first-ever real-time, quantitative observations of solutal mixing, plume formation, and the evolution of the double-diffusive layers by forming a stepped compositional distribution have been reported. In addition, the associated flow velocities were measured usi...
Virkeshwar Kumar, Atul Srivastava, and Shyamprasad Karagadde
ASME International
Natural convection during solidification of liquids is known to impact the freezing characteristics and also lead to defect formation. In this study, we report the findings of real-time interferometric observation of bottom-cooled solidification of pure water in a cubical cavity. The results show first quantitative evidence of full-field thermal history during solidification, clearly depicting the anomalous expansion of water below 4 °C. Furthermore, based on the strength of natural convection, characterized by the Rayleigh number, we identify and report four distinct regimes of solidification, namely—conduction dominated, early convection, front instability, and sustained convection. A critical Rayleigh number that initiates instability in the solidifying front has been proposed, which is significantly different from conventional calculations of Rayleigh number relating to the initiation of flow. The study shows full-field quantitative evidence of a well-known phenomenon and provides a further understanding of flow driven nonhomogeneities in the solidifying interfaces.
Virkeshwar Kumar, Atul Srivastava, and Shyamprasad Karagadde
Begell House
Virkeshwar Kumar, Mitesh Kumawat, Atul Srivastava, and Shyamprasad Karagadde
AIP Publishing
In a wide variety of fluidic systems involving thermal and compositional gradients, local density changes lead to the onset of natural convection that influences the process itself, for example, during phase-change phenomena and magmatic flows. Accurate knowledge of the flow characteristics is essential to quantify the impact of the flow of the processes. In this work, the first-ever demonstration of flow reversal during bottom-up solidification of water using full-field thermal and flow measurements and its direct impact on the solidifying interface is presented. Based on prior optical interferometric measurements of full-field temperature distribution in water during solidification, we use the particle image velocimetry technique to quantify and reveal the changing natural convection pattern arising solely due to the density anomaly of water between 0 °C and 4 °C. The independently captured thermal and flow fields show striking similarities and clearly elucidate the plausible mechanism explaining the fo...