Manabendra Pathak

AN INVESTIGATION OF TURBULENT RECTANGULAR JET DISCHARGED INTO A NARROW CHANNEL WEAK CROSSFLOW

A computational investigation of the mean flow field of turbulent rectangular jets issuing into a narrow channel crossflow is presented. The length of the jet slot spans more than 55% of the crossflow channel bed, leaving a small clearance between the jet edge and sidewalls. A finite volume code employing the standard k-∈ model is used to predict the mean, three-dimensional flow field. The mean flow field is investigated for two velocity ratios (6 and 9). Important flow features, such as the formation of different vortical structures and their characteristics owing to different values of the velocity ratio, are discussed. Some predicted results are compared with the experimental data reported in the literature. The predicted mean and turbulent flow properties are shown to be in good agreement with the experimental data.

Numerical Simulation of Droplet Dynamics in Membrane Emulsification Systems

An emulsion is a two-phase liquid system of two immiscible liquids, where the liquid with lower mass fraction is dispersed in form of small droplets in other surrounding liquid of higher mass fraction. Emulsions are widely used to produce sol–gel, drugs, synthetic materials, and food products. Based on the size of droplet, emulsions can be classified as micro and macro emulsion. Karbstein and Schubert, (1995) have made a limiting droplet size of 0.1 μm, below which the emulsion is termed as micro emulsion and above that size the emulsion is termed as macro emulsion. Size and size distribution of droplets play important roles in the stability of emulsion. There are also other factors such as sedimentation, skimming, droplet aggregation and coalescence, which may affect the stability of the droplets. Thus for making a stable emulsion it is necessary to convert the dispersed phase into tiny droplets and stabilize them against coalescence. Some amount of energy is required in the process to break the dispersed phase into droplets. The amount of energy put in the dispersing phase also controls the resulting droplet size. The stability of newly formed droplets depends on how fast the used emulsifiers are able to occupy the newly created interfaces and how well they stabilize them. The common devices used to produce emulsions are rotor-stator-systems, stirrers and high-pressure homogenizers. During last two decades, new technologies of making emulsion have been developed. Compared to conventional method of emulsification such as rotor-stator method, these new techniques of emulsification have several advantages such as low energy consumption, controllable droplet size with proper distribution and easy scalability. These new methods are based on the microdroplet formation in micrometer sized channels. Three such new methods are T-junction emulsification, flow focusing emulsification, and membrane emulsification. In all these methods, controllable droplet formations are achieved by properly maintaining the combination of continuous and dispersed phase flow rate.

Distribution of Temperature as a Passive Scalar in the Flow Field of a Heated Turbulent Jet in a Crossflow

This article presents a computational investigation of the mean flow field of heated turbulent rectangular jets in a crossflow. The jet is discharged with a slightly higher temperature (about 6°C) than the crossflow. The computations are carried out for two values of jet-to-crossflow velocity ratio, 6 and 9. The commercial code FLUENT 6.2.16, employing the Reynolds stress transport model, is used to predict the mean flow field. The influence of the velocity field on the temperature distributions is discussed. A comparison of the predicted results is made with the available experimental data, and reasonably good agreement is observed.

Clogging behavior of an elongated bubble in uniform and diverging microchannel

A numerical investigation has been performed to make a comparative study of clogging and detachment behavior of an elongated bubble during adiabatic flow through a uniform and diverging cross-sectional microchannel. The adiabatic gas–liquid two-phase flow has been numerically solved using volume of fluid approach in a finite volume code. Simulations have been made to investigate the effects of wall wettability, surface tension, and flow velocity on the bubble dynamics for both the configurations of microchannel. The net pressure drop across the bubble in the diverging channel has been calculated analytically and the results have been compared with numerical results. In both types of channels, the bubble shape goes through a transient stage followed by a steady and stable shape. The change in bubble shape during transient phase depends on the surface wettability, flow velocity, and the confinement of the channels. The bubble attains the symmetrical shape earlier in diverging channel. The formation of liquid film between the bubble and the wall plays an important role in the clogging behavior and movement of the bubble. Wall wettability has less impact on the pressure drop characteristics in the diverging channel.

Computational investigations of solid–liquid particle interaction in a two-phase flow around a ducted obstruction

The effect of bed obstruction on the dispersion of solid particles in a two-phase flow was numerically investigated. An algebraic slip mixture model was used to model this flow and the re-normalization group k-ϵ model to resolve flow turbulence. Computations were performed for three different sizes of solid particles to investigate their effect on turbulence modulation. Due to the obstruction, a distortion of the self-similar mean velocity profile was observed including the typical stratified dispersion of solid particles in the region comprising the stagnation and wake regions. Its length increases with particle size. The stagnation region is characterized by high preferential deposition of particles, whereas the wake region has a low concentration gradient. The effect of wake vortices on particle dispersion decreases as the particle size increases. Turbulence intensity increases with the particle size, whereas relative turbulence modulation was observed in complex flow regions for larger-sized particles.

Thermorheological Characterization of Elastoviscoplastic Carbopol Ultrez 20 Gel

The temperature and concentration play an important role on rheological parameters of the gel. In this work, an experimental investigation of thermorheological properties of aqueous gel Carbopol Ultrez 20 for various concentrations and temperatures has been presented. Both controlled stress ramps and controlled stress oscillatory sweeps were performed for obtaining the rheological data to find out the effect of temperature and concentration. The hysteresis or thixotropic seemed to have negligible effect. Yield stress, consistency factor, and power law index were found to vary with temperature as well as concentration. With gel concentration, the elastic effect was found to increase whereas viscous dissipation effect was found to decrease. Further, the change in elastic properties was insignificant with temperature in higher frequency range of oscillatory stress sweeps.

A Comparative Study of Subcooled Flow Boiling in Uniform, Diverging Cross-Section and Segmented Finned Microchannels

In this work an experimental investigation has been performed to assess the heat transfer characteristics during subcooled flow boiling in diverging and segmented finned microchannels relevant to applications in electronics cooling systems. Experiments have been also performed in uniform cross-section microchannels to compare their performances with other two types of channels configurations. Arrays of microchannel consisting of 12 numbers of channels with rectangular cross-section have been fabricated on individual copper block for these three different geometrical configurations. Deionized water has been used as the working fluid in the experiment. Experiments have been performed mostly in subcooled boiling regions relevant to cooling of electronic components where bulk fluid was below the saturation temperature and the surface temperature was around the saturation temperature of the coolant. The heat transfer characteristics of all three configurations have been compared in terms of heat transfer coefficient and thermal instability during highly subcooled and developed subcooled flow boiling. Although diverging channel performs significantly better in saturated and superheated regions of boiling with high heat flux as observed in literature, in present work its performance has been found slightly better in subcooled boiling regions compared to uniform cross-section channel. Segmented finned channels have shown the best performance both in sensible heating and subcooled region and thus demonstrate high potential for electronics cooling applications.Copyright

Numerical Analysis of Droplet Dynamics Under Different Temperature and Cross-Flow Velocity Conditions

Numerical investigations have been performed to investigate the dynamics of a single liquid droplet formation in another immiscible cross-flow liquid for different values of cross-flow velocities and temperature difference between the two phases. The transient growth of the droplet and detachment time have been calculated by solving Navier-Stokes equation for two-phase flow using coupled level-set and volume of fluid method. The effect of temperature on surface tension has been incorporated in numerical simulation by modeling the surface tension as a linear function of temperature. The effects of cross-flow velocity and temperature in the evolution and detachment of the droplet have been investigated from the balance of different forces acting on the system. With the increase in cross-flow velocity, the diameter of the droplet decreases and droplet detachment time increases. While with the increase in cross-flow temperature, both the diameter and detachment time decreases.

Effect of pulse rate variation on blood flow through axisymmetric and asymmetric stenotic artery models

The present work reports numerical simulations of blood flow patterns and wall shear stress (WSS) distributions in stenotic arteries, modelled as straight tubes. Inflow waveforms have been generated for different pulse rates considering constant volumetric flow during each pulsation cycle and a two-element windkessel model has been used to specify the outlet pressure. It is noticed that the non-Newtonian shear thinning rheology of blood produces more accurate and realistic predictions of the flow field as compared to the Newtonian assumption. Further, the effects of variation of pulse rates on the spatial and temporal distribution of WSS and oscillatory shear index (OSI) have also been studied for both axisymmetric and asymmetric stenosis. The changes in the mean flow features due to changes in pulsation frequencies have also been reported.

Performance Analysis of Uniform and Expanding Cross-Section Microchannels for Single Phase and Flow Boiling Heat Transfer

In the present experimental work a comparative study has been done to investigate the effects of single phase and flow boiling heat transfer performance of straight, uniform and expanding cross-section microchannels. Two configurations of microchannels have been fabricated on individual copper substrate (25.7 mm × 12.02 mm). In both types of geometry, 12 channels of rectangular cross-section have been fabricated having cross-section is uniform and expanding. Width at the inlet is 400 μm for both of channels and depth is 750 μm. Subcooled, deionized water with inlet temperature of 27 °C has been used. Heat transfer analysis revealed that performance of diverging channel is minimal higher for flow boiling case only, at high heat flux and low mass flow rate. For single phase convective heating its performance is not appreciable.