A geometrically simple configuration of a gas bubble expanding in a bubbly medium is chosen to study multi-scale modeling approaches in simulating multiphase flows. Three solvers following the Eulerian-Lagrangian, multi-component compressible and analytical approaches have been studied for different bubbly medium void fractions. In the Eulerian-Lagrangian approach, an incompressible solver was coupled with a bubble dynamics model, while for the Eulerian two-fluid approach and the analytical model the surrounding fluid was assumed to be a homogeneous mixture. It was found that both two-fluid and analytical models match very well for the cases tested. The Eulerian-Lagrangian model shows that the dynamics of the bubbly medium and the expanding bubble are inherently coupled in a non-linear fashion. Presence of discrete bubbles also introduces localized perturbations in the void fraction as well as in the pressure distribution which cannot be captured by the homogeneous mixture models.

Calcium carbonate is predominantly present in cooling tower’s water and is usually the principal cause of hard water. This paper applies the modeling technique typically used for aerosol deposition to simulate the deposition process of calcium carbonate nano- and micro-particles suspended in turbulent cooling water flows. The mean turbulent velocity field and the fluctuating velocities are determined by the k - ε and RSM turbulence models by simulating the water flow in a typical heat exchanger horizontal tube. Commercial software (ANSYS FLUENT ™ 12.1.4) is used for turbulence mean flow modeling and the simulation of turbulence fluctuations is performed by stochastic models. Particle deposition velocities are obtained for the particles with diameters in the range 0.01–50 μm by the k - ε and RSM models and compared to the deposition velocities calculated from semi-empirical correlations to investigate the effect of the turbulence model on the deposition velocity. Results show that the proposed numerical model can predict deposition velocity of micro-particles in water accurately and can be useful in determining the range of particle diameters in which the highest deposition velocity occurs. However, for nano-particles, the model’s results do not agree with the correlations due to the higher lateral turbulence fluctuations calculated by ANSYS FLUENT ™ code. The proposed model can be useful for predicting fouling in industrial heat exchangers, for planning operations and cleaning schedules, and proposing efficient filtering processes for lowering deposition rate and cleaning costs.

Many structures such as automobile bodies, bridges and buildings are subjected to external forces due to the nature of the environment in which they exist. When the external excitation frequency is similar to the structure’s natural frequency a resonance effect is generated, increasing the energy and the amplitude of the oscillation, often causing catastrophic situations. This paper presents a method to decrease such resonant oscillations using magnetorheological dampers.

The flow and heat transfer characteristics of an impinging jet on a perpendicular flat surface are obtained by two dimensional numerical simulations of laminar and transitional flow regimes for the Reynolds number of Re = 300, 350, and 400 for a Prandtl number of Pr = 0.7. A fixed jet to plate spacing of H/W = 5 and a given heat flux on the plate surface are considered. Temporal evolution of velocity and temperature fields, Fourier spectra of the velocity temporal evolution and time average local and global Nusselt numbers are obtained for increasing Reynolds numbers for determining the time depending behavior and its effect on the heat transfer characteristics. Numerical simulation results demonstrate that self-sustained transitional periodic flow regimes arise from a laminar regime, when the Reynolds number is further increased to Re = 400 and that these regimes spread out to the whole domain with similar time dependent characteristics due to the flow incompressibility. Evaluations of time average local and global Nusselt numbers demonstrate the asymmetric Gaussian-type spatial distribution and the increase of both parameters when the flow evolves through the transitional periodic regime, with reasonable increases on the pumping power requirements.

This work presents the characterization of a thermal interface material consisting of an array of mercury micro droplets deposited on a silicon die. Three arrays were tested, a 40 × 40 array (1600 grid) and two 20 × 20 arrays (400 grid). All arrays were assembled on a 4 × 4 mm 2 silicon die. An experimental facility which measures the thermal resistance across the mercury array under steady state conditions is described. The thermal interface resistance of the arrays was characterized as a function of the applied load. A thermal interface resistance as low as 0.253 mm 2 K W −1 was measured. A model to predict the thermal resistance of a liquid-metal micro droplet array was developed and compared to the experimental results. The model predicts the deformation of the droplet array under an applied load and then the geometry of the deformed droplets is used to predict the thermal resistance of the array. The contact resistance of the mercury arrays was estimated based on the experimental and model data. An average contact resistance was estimated to be 0.14 mm 2 K W −1 .

Possibility of enhancement of oxygen diffusion in a gas diffusion layer of a fuel cell electrode is numerically investigated. Since oxygen is paramagnetic gas, it is attracted to a field of high magnetic flux density by the magnetizing force. If there exists gradient of the oxygen concentration and the gradient of magnetic flux density in the gas diffusion layer, air flow occurs in the layer. Numerical computations were conducted for air flow in the gas diffusion layer of the electrode under the magnetic field which is generated by parallel electric currents. Darcy model is used to represent the air flow in the layer and the oxygen concentration was solved. The effects of the permiability of the electrode, intensity of the electric current, the location of the electric wire and the thickness of the electrode on the enhancement of oxygen transfer were investigated. As a result, 5 to 20% of enhancement of oxygen transfer by using magnetizing force was observed.

The quartz crystal microbalance (QCM) sensor is used to study the dynamic characteristic of hydrophobic nanofiberous surfaces. The nanofibrous films of polymethyl methacrylate (PMMA), PMMA/ Polydimethylsiloxane (PDMS) and Polyacrylonitrile (PAN) were prepared with an electrospinning process for different hydrophobicity (wettability). The mechanical impedance analysis of DI water on a fibrous coated QCM surface is able to quantitatively characterize the hydrophobicity of these nanofibers surfaces. The two layers including a viscoelastic nanofiber film and a liquid layer result in a nonlinear combination of mechanical impedances. To simplify the analysis, an apparent viscosity was introduced in the analysis to account for the surfacial slip effect. The experimental results showed that the hydrophobic surface resulted in small mechanical impedance loading and low value of apparent viscosity, while the hydrophilic surface generated large mechanical impedance and gave high value of apparent viscosity.

An understanding of the fundamental mechanisms involved in the interaction between cavitation bubbles and structures is of importance for many applications involving cavitation erosion. Generally, the final stage of bubble collapse is associated with the formation of a high-speed reentrant liquid jet directed towards the solid surface. Local forces associated with the collapse of such bubbles can be very high and can exert significant loads on the materials. This formation and impact of liquid jet is an area of intense research. Under some conditions the presence of gravity and other nearby boundaries and free surfaces alters the jet direction and need to be understood, especially that in the laboratory, small scale tests in finite containers have these effects inherently present. In this work, experiments and numerical simulations of the interaction between a vertical wall and a bubble were carried out using Dynaflow ’s three-dimensional code, 3 DynaFS_Bem © , which models the unsteady dynamics of a liquid flow including the presence of highly non-linear time evolving gas-liquid interfaces. The numerical predictions were validated using scaled experiments carried out using spark generated bubbles. These spark bubble tests produced high fidelity test data that properly scale the fluid dynamics as long as the geometric non-dimensional parameters, gravity and time are properly scaled. The use of high speed cameras allowing framing rates as high as 50,000 frames per second to photograph the bubbles produced high quality observations of bubble dynamics including clear visualizations of reentrant jet formation inside the bubble. Such observations were very useful in developing and validating the numerical models. The cases studied showed very good correlation between the numerical simulations and the experimental observations and allowed development of predictive rules for the re-entrant jet characteristics, including jet angle and various definitions of the jet speed.

This paper reports numerical studies of steady two-dimensional natural convection in fluid-superposed porous layers heated locally from below. The numerical simulation is based on the Darcy-Brinkman-Forchheimer model for the porous layer and focuses on the parametric domain in which the flow is well established, i.e., the overall Rayleigh number is several orders of magnitude larger than the critical value. An emphasis is placed on revealing the effects of two dimensionless parameters on the overall Nusselt number: the porous layer-to-cavity height ratio (η = H m /H) and the heater-to-cavity base length ratio (δ = L H /L). Calculations cover η = 0.25, 0.5, 0.75, δ = 0.25, 0.5, 1, and overall Rayleigh numbers from 10 3 to 10 6 . For a fixed height ratio, overall Nusselt numbers increase with a decrease in the heater length. For a given heater length ratio, overall Nusselt number increases with an increase in the height of the overlying fluid layer. Recirculating flow is confined primarily to the overlying fluid layer with some penetration into the upper part of the porous layer. The present results represent an extension of the well studied problem of buoyant convection in superposed layers with a fully heated lower surface.

Laminar flow inside a circular duct of variable section in the axial direction is modeled, assuming that the working fluid is Newtonian, incompressible, with laminar flow, a permanent state, and constant properties. The results describe the behavior of the stream function, the velocity field, and the pressure field, and graphic results are presented for each of those functions. The method used to solve the problem makes use of regular perturbations around the shape factor ε parameter. This research can be used for the design of new technological devices important to industry, optimizing processes in which fluids are transported, energy is transferred, etc.

In the present work, results of electrohydrodynamic (EHD) flow field in a wire-plate air-oil droplets electrostatic separator under positive polarity are presented. Using Particle Image Velocimetry (PIV), the structure of EHD flow under fully developed primary laminar gas flow is investigated and corresponding flow patterns are studied. Velocity distribution results show that the transverse velocity induced by EHD flow depends significantly on applied voltage and the cross-section plane position of the separator. Detailed transverse velocity distribution profiles under different ratio between electrostatic force and inertial force of droplets ( Ehd/Re 2 ) are presented. Oscillating jets are observed under a relatively low Ehd/Re 2 ∼ 4 and counter rotating vortices around the wire for large Ehd/Re 2 > 90 are characterized in this experiment. Contrary to EHD flow patterns for fine solid particles, no von Karman vortex was observed downstream of the charged electrode wire.

Patterning of colloidal particles on surfaces is an application that has evinced wide interest from the fluid mechanics community, due to its possible applicability in a number of engineering situations such as manufacture of photonic crystals[1], bioengineering tissues[2] and lab on chip technology[3], etc. Recently Kumar et al. had proposed the technique of rapid electrokinetic patterning (REP) [4], a hybrid opto-electric manipulation technique that can manipulate and pattern colloidal particles on an electrode surface. REP utilizes optical landscapes to create local gradients in temperature on an electrode substrate. This allows local changes in permittivity and conductivity of the fluid. Colloidal particles can then be dynamically patterned at the illuminated locations of the electrode surface. REP can be used for capturing selective group of particles and thus it serves as a sorting technique too [5].

In this study, a closed loop low-temperature wind tunnel was custom-built and uniquely used to investigate the anti-icing mechanism of superhydrophobic surfaces in regulated flow velocities, temperatures, humidity, and water moisture particle sizes. Silica nanoparticle-based hydrophobic coatings were tested as superhydrophobic surface models. During tests, images of ice formation were captured by a camera and used for analysis of ice morphology. Prior to and after wind tunnel testing, apparent contact angles of water sessile droplets on samples were measured by a contact angle meter to check degradation of surface superhydrophobicity. A simple peel test was also performed to estimate adhesion of ice on the surfaces. When compared to an untreated sample, superhydrophobic surfaces inhibited initial ice formation. After a period of time, random droplet strikes attached to the superhydrophobic surfaces and started to coalesce with previously deposited ice droplets. These sites appear as mounds of accreted ice across the surface. The appearance of the ice formations on the superhydrophobic samples is white rather than transparent, and is due to trapped air. These ice formations resemble soft rime ice rather than the transparent glaze ice seen on the untreated sample. Compared to untreated surfaces, the icing film formed on superhydrophobic surfaces was easy to peel off by shear flows.

The moving contact line is the line of intersection between two immiscible fluids as one displaces the other along a solid boundary (i.e. dynamic wetting). The displacement process has been the subject of a large amount of theoretical, numerical, and experimental research throughout the twentieth century [1], yet the fundamental mechanisms that govern contact line motion are not fully described. Theoretical study has dominated the published work on the moving contact line problem. This is due to the inability of previous experimental methods to elucidate flow behavior at small enough scales to differentiate between competing theories.

The presence of fully established elastohydrodynamic lubricating (EHL) film between piston skirt and cylinder liner during normal engine operation prevents adhesive wear, piston noise and slap. The absence of EHL in the initial engine start up fails to prevent the same. During normal engine operation, thermal loading due to combustion dominates piston skirts lubrication. However, in a few initial cold engine start up cycles, shear heating may be assumed to considerably affect the lubricant viscosity and other characteristics. This study undertakes a 2-D EHL modeling of piston lubrication by incorporating shear heating effects due to lubricant flow between skirts and liner surfaces. The EHL film profile is predicted by solving the 2-D Reynolds equation using inverse solution technique and the finite difference method in fully flooded lubrication conditions. The temperature distribution within oil film is given by using the 2-D transient thermal energy equation with heat generated by viscous heating. The numerical analysis is based on energy equation having adiabatic conduction and convective heat transfer with no source term effects. The study is extended to a number of engine lubricants having different viscosities to investigate the extent of adverse effects due to temperature rise on load carrying capacity of lubricants. Numerical simulations show that piston eccentricities, film thickness profiles, hydrodynamic and EHL pressures visibly change when using different viscosity grade engine lubricants. This study suggests that a lubricant of appropriate viscosity can be optimized keeping in view the vulnerability of piston skirts and cylinder liner to adhesive wear at the time of initial engine start up.

The present paper provides a numerical study of a parametric analysis of a bayonet tube with a special type of extended surface during the laminar-turbulent transition. The working internal fluid is air. Attention is focused on the heat transfer characteristics of the tube. The results constitute a systematic investigation of the effect of the extended surface located along the annulus of the bayonet on the overall heat transfer rate. The effects of the variation of some parameters related to the extended surface aiming to attain the maximum heat transfer with the minimum pressure drop are discussed. Comparisons between designs with and without extended surface are also made.

Steady natural convection heat transfer from vertical array of equally-spaced isothermal horizontal cylinders has been investigated experimentally and numerically. Experimental study was carried out at different ambient temperatures in a conditioned room which can be maintained at a stable required value and inside a sufficiently designed test cabin. The ambient and cylinders’ surface temperatures varied 20°C to 30°C and 30°C to 60°C respectively. The experimental apparatus was designed to adjust different operating parameters such as number of cylinders, cylinders’ surface temperatures, distance between the cylinders and environmental temperature. Each cylinder surface temperature can be accurately adjusted to the desired temperature by means of specially designed measurement and control system. Copper test cylinders have length of 1 m and outer diameter of 4.8 mm. The uncertainty analysis method proposed by Kline and McClintock was used and explained elaborately. Detailed information and algorithm of numerical method are given to ease the understanding of the numerical part of the study. The problem was solved numerically by means of a CFD program in 2D. Average Nusselt numbers are given based on the experimental data for single and each two horizontal cylinders. Heat transfer rates obtained from experimental and numerical studies for upper and lower cylinders were compared with each other. The deviation of experimental and numerical heat transfer rates are in a good agreement and stay in the range ± 20%. It is seen that heat transfer from the lower cylinder is close to the single cylinder case. However, higher temperature of the passing air reduces the heat transfer from the upper cylinder for S/D = 2.

Light Vegetable oils are a promising alternative among the different diesel fuel alternatives. Using Light Vegetable oils in diesel engine is not a new idea. The Vegetable oils have high energy content. However, the high viscosity, poor volatility and cold flow characteristics of vegetable oils can cause some problems such as severe engine deposits, piston ring sticking and thickening of lubrication oil due to long-term use in diesel engines. Diesel fueled engine have the disadvantage of producing Smoke, Particulate Matter and Nitrogen Oxides and are now subjected to increasingly severe legislation of Emission norms. The required levels are difficult to achieve through engine design alone. Even with high-grade fuels, catalytic systems are being extensively investigated to reduce the diesel engine emission. But there are still difficulties in operation of these. This leads to replacement of diesel fuel with renewable fuels has been set target worldwide to reduce the diesel exhaust pollution. The energy of the light vegetable oil can be released more efficiently with the concept of low heat rejection (LHR) engine. The aim of the study is to apply LHR engine for improving the engine performance and reducing the emission when light vegetable oil (turpentine oil) is used as an alternate fuel. The work was carried out in two stages. In first Stage, the turpentine oil (20, 40, 60, 80 & 100, v/v) with diesel blends used in direct injection diesel engine and to identify best blend with respect to performance and emission. In second Stage, the work has been carried out by the converting direct injection diesel engine in to a LHR engine and the effects of different blends of turpentine oil (20, 40, 60, 80 & 100, v/v) with diesel fuel used in LHR engine and its performance, emission and combustion characteristics have been investigated experimentally. From the experimental investigation, the combination of LHR engine with blended fuels shows the better performance when compared to diesel engine. The smoke density decreases for the diesel engine (without LHR) whereas with the effect of LHR, Oxides of Nitrogen will be reduced and heat release rate also reduces.

This paper addresses the concept of thrust augmentation through bubble injection into an expanding-contracting nozzle. Two-phase models for bubbly flow in an expanding-contracting nozzle are developed, in parallel with laboratory experiments, and used to ascertain the geometry configuration for the nozzle that would lead to maximum thrust enhancement upon bubble injection. For preliminary optimization of experimental setup’s design, a quasi 1-D approach is used. Averaged flow quantities (such as velocities, pressures, and void fractions) in a cross-section are used for the analysis. The mixture continuity and momentum equations are numerically solved simultaneously, along with equations for bubble dynamics, bubble motion, and an equation for conservation of bubble number. Various geometric parameters such as the exit and inlet areas, the area of the bubble injection section, the presence of a throat and its location, the length of the diffuser section and the length of the contraction section are varied, and their effects on thrust enhancement are studied. Investigation on the effect of the injected void fraction is also carried out. The key objective function of the optimization is the normalized thrust parameter, which is the difference between the thrust with the bubble injection and the thrust before the bubble injection, normalized by the inlet momentum. An approximate analytical expression for the normalized thrust parameter was also derived starting from the mixture continuity and momentum equations. This analytical expression involved flow variables only at three locations; inlet section, injection section, and outlet section, and the expression is simple enough to produce a quick concept design of the diffuser-nozzle thruster. The numerical and analytical approaches are verified against each other and the limitations of the analytical approach are discussed.

The objective of the present study is to develop an Adaptive Network-based Fuzzy Inference System (ANFIS) model to predict the unsteady lift coefficients of an airfoil. The airfoil performs a flapping motion in Low-Reynolds-Number (LRN) flow regime. Computational Fluid Dynamics (CFD) simulations of the flow field are conducted and the corresponding unsteady lift coefficients are used as the input data to ANFIS. The results show that the ANFIS model is capable of predicting the lift coefficients with very good accuracy, which could be of great value in the preliminary design stages.