Pallab Sinha Mahapatra

Passive droplet manipulation on open surfaces can be achieved by creating a wettability gradient on surfaces, which is essential in the fabrication of low cost biological and biochemical chips. We performed 3D numerical simulations to analyze the droplet motion on a broad range of wettability gradient surfaces. We found that the droplet shape evolves with time to maintain a minimum energy state, and the surface energy of the droplet is identical at a particular non-dimensional time (t∗) for different wettability gradient surfaces. Although the droplet is at various locations at a fixed t∗, the shape of the droplet is found to be identical. The physics behind this interesting phenomenon of identical droplet shape formation is explored. A co-relation for t∗ is proposed to get the dependency of t∗ on various geometrical parameters and fluid properties. Three distinct regimes of the droplet identical shape on different wettability gradient surfaces are shown using a regime plot. Along with the identical droplet shape phenomena, the detailed understanding of the dynamics of the droplet shape evolution on different wettability gradient surfaces gives an insight for better open surface passive manipulation.

Purpose: This study aims to enhance natural convection heat transfer for a porous thermal cavity. Multi-frequency sinusoidal heating is applied at the bottom of a porous square cavity, considering top wall adiabatic and cooling through the sidewalls. The different frequencies, amplitudes and phase angles of sinusoidal heating are investigated to understand their major impacts on the heat transfer characteristics. Design/methodology/approach: The finite volume method is used to solve the governing equations in a two-dimensional cavity, considering incompressible laminar flow, Boussinesq approximation and Brinkman–Forchheimer–Darcy model. The mean-temperature constraint is applied for enhancement analysis. Findings: The multi-frequency heating can markedly enhance natural convection heat transfer even in the presence of porous medium (enhancement up to ∼74 per cent). Only the positive phase angle offers heat transfer enhancement consistently in all frequencies (studied). Research limitations/implications: The present research idea can usefully be extended to other multi-physical areas (nanofluids, magneto-hydrodynamics, etc.). Practical implications: The findings are useful for devices working on natural convection. Originality/value: The enhancement using multi-frequency heating is estimated under different parametric conditions. The effect of different frequencies of sinusoidal heating, along with the uniform heating, is collectively discussed from the fundamental point of view using the average and local Nusselt number, thermal and hydrodynamic boundary layers and heatlines.

Milk adulteration is a common problem in developing countries, and it can lead to fatal diseases in humans. Despite several studies to identify different adulterants in milk samples, the effects of multiple adulterants remain unexplored. In this work, a three-dimensional (3D) paper-based microfluidic device is designed and fabricated to simultaneously detect multiple chemical adulterants in milk. This device comprises a top cover, a bottom cover, and a middle layer composed of transportation and a detection zone. By making cuts on the middle layer’s support, the device’s flow path is characterised by optimum and uniform velocity. For the first time, seven adulterants (urea, detergents, soap, starch, hydrogen peroxide, sodium-hydrogen-carbonate, and salt) are detected in the milk sample simultaneously with specificity evaluation and detailed color interference analysis. Only 1–2 mL of sample volume is required to detect 7 adulterants at one time. We have used only 10 μ L of the reagent’s volume for the colorimetric reaction and found the results within a few seconds. Observation reveals that the limit of detection (LOD) of the adulterants lies in the range between 0.05 % (vol./vol.) to 0.2 % (vol./vol.) using the colorimetric detection technique. The unknown quantity of the added adulterants is measured using the calibration curves obtained from the experiments results. The repeatability and reproducibility of the process, sensitivity, and the linear range of detection of the calibration curves and the statistical study of the color intensity data are thoroughly analysed herein. In any resource-limited setting, this simple, portable, and user-friendly 3D microfluidic device is expected to be used for testing liquid foods before consumption.

We investigate the evolution of the thermal field during evaporation, a fundamental aspect of evaporating sessile drops. With numerous reports in the literature investigating the contact line dynamics, we aspire to identify generalized features in the evolution of the thermal field and ultimately correlate these with the contact line dynamics. Considering a broad range of experimental parameters such as substrate wettability, substrate temperature, initial volume of the drop, and ambient relative humidity results in a wide range of evaporation rates, in turn affecting the strength of internal convective flows. Infrared thermography is utilized to extract the thermal field at the liquid–vapor interface, and optical imaging is used to record the evolution of drop shape during evaporation. We observe that the onset and presence of a convective cell as a cold spot at the interface highlights a non-axisymmetry in the thermal field. In consequence, a hitherto unreported asymmetry in the internal flow field is observed, as evidenced by the particle image velocimetry. Among the multitude of experiments conducted, we report four distinct trends in the evolution of interfacial temperature difference depending on the presence and duration of the presence of the convective cell, which are elucidated by discussing the evolution of maximum and minimum temperatures at the interface. The interplay between heat conducted into the drop and heat released due to evaporation can result in a momentary decrease in temperature of the drop, which is not reported previously. Lastly, a theoretical estimate for the temperature difference within the drop is extracted using vapor diffusion model and energy balance during evaporation. Comparison of this theoretical temperature difference with experimental observations highlights the influence of internal convective flows in homogenizing the thermal field within the drop.

Rational design of solid interfaces with an ability to prevent frost formation is crucial in many industrial systems such as refrigeration, air-conditioning, cryogenics, etc. In this work, we fabricated two hierarchically textured aluminum-based superhydrophobic surfaces by simple steps such as chemical etching and dip/spray coating. We used perfluorooctyltriethoxysilane (PFOTES) and commercial Glaco solution as a hydrophobic coating material. The fabricated substrates have shown excellent superhydrophobicity with a low contact angle hysteresis of below 5°. The frost formation on both the coatings was different despite having similar macroscopic wettability. We analyzed the surface topography and its effects on frost formation comprehensively. PFOTES coated substrate has shown a sweeping mode of drop drainage, whereas the Glaco coated substrate showed the droplet jumping mode. Rapid ejection of condensed microdroplets from the Glaco coated substrate is delayed the frost formation due to the significant reduction in the drop contact time. This study elucidates the dependence of nanostructured coatings over the microstructures during frost formation. The insights from this study are helpful for the fabrication of anti-frosting surfaces for industrial applications.

This paper reports the results of an experimental study to investigate the performance comparison between two Pulsating Heat Pipes namely, a Flat Plate Pulsating Heat Pipe (FPPHP) and a Capillary Tube Pulsating Heat Pipe (CTPHP). The comparison is made based on the flow regimes and the corresponding thermal performances at heat inputs varying from 20 W to 180 W with filling ratios of 40%, 60%, and 80%. Experiments are performed in the vertical bottom heating mode with ethanol as the working fluid. The pressure inside the PHPs and temperatures at the evaporator and condenser region are measured along with a recording of the internal flow regimes using a high-speed camera. Slug-plug flow is observed to be the dominant flow regime in both the PHPs. However, the amplitude of oscillations is found to be higher in CTPHP as compared to FPPHP. The reduction in thermal resistance of FPPHP and CTPHP due to the presence of working fluid is about 83% and 35% of the corresponding thermal resistances without any working fluid respectively. CTPHP shows better thermal performance than FPPHP due to the presence of lateral conduction arising in the latter which has a detrimental effect on the slug-plug oscillations.

In the present work, a numerical model has been developed using ANSYS Fluent 14.5 to simulate the combustion phenomenon in a partially premixed, swirl-stabilized, LPGfueled gas turbine combustor. In a practical gas turbine combustor, pulsations in the flow at the air side cannot be avoided which can lead to thermoacoustic instabilities. The primary objective of the study is to numerically analyze the effect of such pulsations on the fluid flow and combustion process inside the combustor. Different parameters like static temperature, progress variable and product formation rate are compared at the outlet plane of the combustor. The effect of change in the parameters like amplitude and frequency of the sinusoidal air flow input has also been investigated in the present study. It is observed that the solution changes from periodic to quasi-periodic at a higher amplitude condition. The numerical model was qualitatively validated against experiments performed on a laboratory-scale premixed, swirlstabilized, gas turbine combustor.

The thermal performance and the internal flow regimes of a closed-loop flat plate pulsating heat pipe (FPPHP) are experimentally investigated. Reports on the FPPHP using the ethanol-water mixtures as working fluids are scarce in the literature. The binary mixtures with different boiling point components are suitable for a wide range of heat fluxes. Therefore, the results are reported for the ethanol-water mixtures of ratios 3:1, 1:1, and 1:3, and the corresponding pure liquids filled in the range of [40–80] % with power inputs given from 40 to 200 W. The effect of different condenser cooling modes, such as forced convective water cooling, forced convective air cooling, and natural convective air cooling on the thermal performance of the FPPHP, is also reported. With the increase in the power input, the observed flow characteristics in the FPPHP channels are: no oscillations, slug-plug oscillations, droplet oscillations, and the evaporator dry out. The binary mixtures with increased ethanol content give better slug-plug flow oscillations with smaller thermal resistances and fewer evaporator drying out instances than the pure working fluids. For power inputs of less than 120 W, the ethanol:water mixture ratio of 3:1 at all filling ratios gives a larger slug departure frequency in the evaporator. The smallest thermal resistance measured is 0.1 K/W, a decrease of 27% over pure ethanol. For power inputs greater than 120 W, the mixture ratio of 1:1 at all filling ratios performs better with continuous droplet oscillations. The smallest thermal resistance measured at the 80% filling ratio is 0.12 K/W, a decrease of 22% over pure ethanol. When the condenser cooling mode is changed to air cooling, the evaporator temperatures reach around 100 °C for power inputs greater than 40 W and 80 W for natural and forced air convection. Thus, the FPPHP filled with ethanol-water mixtures with the water-cooled condenser gives a stable flow regime and better thermal performance for a long-range of power inputs.

In this decade, paper-based microfluidics has gained more interest in the research due to the vast applications in medical diagnosis, environmental monitoring, food safety analysis, etc. In this work, we presented a set of experiments to understand the physics of the capillary flow phenomenon through paper strips. Here, using the wicking phenomenon of the liquid in porous media, experimentally, we find out the capillary height of the liquid in filter paper at different time intervals. It was found that the Lucas-Washburn (L-W) model, as well as the evaporation model, fails to predict the capillary rise accurately. However, the detailed numerical solution shows a better similarity with the experimental results. We have also shown the different regimes of the wicking phenomenon using scaling analysis of the modified L-W model. The capillary rise method was applied to detect the added water content in milk. We used milk as a liquid food and found the added water content from the change in the capillary height at different concentrations of milk. Finally, results obtained from the paper-based device were verified with the commercially available lactometer data.