The need for higher turbine efficiency has been constantly pushing the turbine inlet temperatures to elevated levels. Hot gas path temperatures are much higher than the typical blade material yield temperature. Efficient internal cooling technologies are required for safe operation of gas turbine. Several internal cooling technologies have been developed in order to enhance the heat transfer from relatively hotter walls of turbine blade. For mid-chord region of turbine blade, rib turbulators are typically installed in multi-pass channels. Rib turbulators trip the boundary layer, induce secondary flows which enhance near wall shear as well as enhance turbulent mixing when they interact with surrounding walls. Research has been carried out on several aspects of rib turbulated passages in order to achieve higher thermal hydraulic performance. Generally, rib turbulators are installed on two opposite walls of serpentine passages in order to enhance heat transfer from both pressure and suction sides of blade through coolant flowing through complicated paths. Typical arrangement of rib turbulators were parallel to each other or having some offset from each other. In the present study, an attempt has been made to arrange 45° angled ribs in a way that they form a Criss-Cross pattern. Two ribbed configurations with Criss-Cross pattern - Inline and staggered, have been studied where the baseline case was smooth duct with no rib turbulators. The effective rib-pitch-to-rib-height ratio (p/e) was 8.6 and rib-height-to-channel-hydraulic diameter ratio (e/d h ) was 0.1. The channel had a total length of 20 hydraulic diameters and the rib turbulators were installed at a distance of six hydraulic diameters from the inlet of the test section to allow flow development. Detailed heat transfer coefficients were measured using transient liquid crystal thermography employing 1D semi-infinite conduction model. Globally averaged Nusselt numbers are calculated from the detailed measurements and thermal hydraulic performance of configurations have been reported with respect to Reynolds number. The aim of this study was to develop a cooling configuration which has higher thermal-hydraulic performance compared to other traditional rib configurations. It has been found that the heat transfer characteristics of the inline and staggered configurations were similar to each other and ranged between three times D-B correlation to 2.7 times, for Reynolds number ranging from 30000 to 60000. Inline configuration had relatively lower frictional losses, however the thermal hydraulic performances of both the configurations were similar.

Flow patterns for water-butanol, water-toluene and water-hexane two-phase flows were visualized in the cross-shaped junctions of three square glass microchannels with hydraulic diameters of 200 μm, 400 μm and 600 μm. Typical flow pattern maps for water-butanol two-phase flow were developed based on Capillary number of the continuous phase and Weber number of the dispersed phase, and compared with a previous flow pattern map in the literature. Three main flow pattern groups were observed, including the tubing/threading regime group, the dripping regime and the jetting regime. The geometry confinement and fluid properties affected the viscous shear and interfacial tension forces and therefore their influence on flow pattern transitions was clarified. Besides, in the dripping regime, the dimensionless slug length can be scaled as a function of the flow rate ratio and the Capillary number of the continuous phase.

In order to obtain a fluid with both drag reducing and heat transfer enhanced abilities, we use viscoelastic fluid for drag reducing and stimulate the elastic turbulence for the sake of heat transfer enhancement. To further enhance the heat transfer, carbon powder particles were added to the viscoelastic fluid where Polyacrylamide Ethylene Glycol solution was served as the base fluid. Fluid properties were measured at different solution concentration, shear rate and temperature by the rotary rheometer and thermal conductivity meter including viscosity and heat conductivity coefficients. The results showed that the addition of the carbon powder could significantly change the viscoelastic fluid properties. As for the viscosity, the shear-thinning behavior still exists after adding the carbon powder. Moreover, the heat conductivity coefficients increase about 7.5% after adding the carbon powder and nearly linearly increase with the temperature and volume concentration of the carbon particle.

We develop a semi-analytical solution for the Nusselt number for fully-developed flow of liquid between parallel plates, one of which is textured with isothermal parallel ridges. The opposite plate is smooth and adiabatic. The liquid is assumed to be in the Cassie state on the textured surface, on which a mixed boundary condition of no slip on the ridges and no shear along menisci applies. An existing solution for the velocity field is valid. The thermal energy equation is subjected to a mixed isothermal-ridge and adiabatic-meniscus boundary condition on the textured surface. Given the nature of the isothermal boundary condition, the analysis concerns a three-dimensional developing temperature profile, and the results are obtained for a streamwise location that tends to infinity. We assume that the temperature field is governed by an infinite sum of the product of a function of the streamwise coordinate and a second function of the spanwise co-ordinates. The latter functions are eigenfunctions satisfying a two-dimensional Sturm-Liouville problem from which the eigenvalues follow. The fully-developed Nusselt number follows from the first eigenvalue.

Numerical results are presented for laminar impinging flow and heat transfer with a non-Newtonian inelastic fluid in a planar two dimensional geometry. Bifurcation diagrams are computed to characterize flow separation and reattachment in steady flow. For a range of rheological parameters calculations show that the dimensionless wall jet heat transfer rate Q may be correlated as R P 1.3 where R P is the reattachment coordinate of the primary vortex scaled with the jet half-width, thus quantifying the extent of enhancement with shear thinning. For Re = 200 the unsteady time periodic flow is computed for both fluids and employed in the heat transfer calculations. The Newtonian flow Nusselt numbers at the stagnation point and in the wall jet region, although periodic, show an oscillation in magnitude less than 10% of the mean and time averages similar to steady flow. For the shear thinning fluid the wall jet Nusselt number displays an oscillation amplitude of about half the mean value, and the Nusselt number profile shows considerably improved uniformity over a length scale extending several nozzle widths into the wall jet region. However, unlike steady flow, heat transfer rates are not significantly increased in the oscillatory flow regime.

Capturing particles, mainly bio-particles, such as viruses and spores, from the air into a liquid is critical for air purification and analysis of the sampling systems. In the conventional impinger sampling systems, the air bubbles carrying the particles are injected into a liquid column. The particles are captured from the air into the liquid as the bubbles travel to the surface due to the buoyancy forces. This paper focuses on the numerical simulation of a newly developed microfluidic air sampling system. In this system, a microscale liquid column, equipped with an array of microchannels as pressure controlled gas injection points, is used for gas bubbling process for air sampling purpose. The air bubbles containing the particles are injected into the bottom of the microscale column filled with a liquid (such as water). As the microscale bubbles (of about 250∼500 microns) travel to the surface, a shear driven flow will develop within the bubbles and the airborne particles will follow the flow toward the boundary of the bubble. The inertia force of particles will result in the departure of particles from the flow into the liquid. The fraction of the particles departed from air bubble and trapped in the liquid represents the collection efficiency of the air sampler. For tracking the liquid/gas interface, the Volume-of-Fluid (VOF) method along with Youngs’ algorithm for geometric reconstruction of the free surface is used. The particle trajectories are predicted by integrating the force balance on each particle, which is based on a Lagrangian frame of reference. It is found that in addition to the particle departure during the bubble rising, a considerable amount of particles are trapped in the liquid during the air injection and bubble formation which may be due to the high velocity of the injected air and oscillation of the bubble interface. It is worth noting that this is the first time that the numerical simulation is performed to understand the whole air sampling process which was never addressed in the past. As a comparison, an experimental study was conducted to measure the collection efficiency. The fluorescent polystyrene latex particles with different diameters (diameters: 0.5, 1, 2 microns) were used in the experiment. The experimental results were compared with that of numerical simulation. Both the experiments and computations revealed that a collection efficiency of 90% can be achieved by the microfluidics based impinger, which is much higher than that of conventional, macro-scale, impinger systems. In addition to verifying the existing experimental work, this numerical simulation method provides a valuable tool to design and improve new generations of air sampling systems.

Longitudinal and transverse pressure-driven vortices induced by shear instabilities behind vortex generators play a crucial role in convective transport phenomena. The proven influence of these turbulence promoters on heat and mass transfer enhancement has led to their incorporation in modern multifunctional heat exchangers/reactors. The purpose of this work is to demonstrate experimentally the effects of hydrodynamics on the transfer processes accompanying such flows. The high-efficiency vortex (HEV) is an innovative static mixer and a low-energy-consumption heat exchanger designed to exploit these types of vortices. Results obtained in turbulent flow with embedded vorticity in an HEV static mixer are compared with numerical results in the literature. Both numerical and experimental results confirm the high energy efficiency of the HEV static mixer flow.

A few initial cold engine start up cycles at low idling speeds do not prevent wear due to the absence of a fully established elastohydrodynamic lubrication (EHL) film between the piston skirts and the cylinder liner. It happens when the thermal loading due to combustion may be ineffective initially, and shear heating becomes significant as a result of the sliding motion of the piston. This study models the 2-D piston skirts EHL at the idling speeds in the initial engine start up by using a high-viscosity grade engine oil and incorporating the shear heating effects. The 2-D heat transfer equation is used with no source term effects to study the temperature changes and their effects on the viscosity of a Newtonian lubricant at the different idling speeds in the initial start up of an internal combustion engine. The 2-D Reynolds equation is solved numerically to generate the hydrodynamic pressures as the function of 720 degrees crank rotation cycle. Under the flooded lubrication conditions the inverse solution technique is employed to generate the hydrodynamic pressures in the EHL regime. The numerical analysis at the two different idling initial engine start up speeds is presented based on the 2-D heat equation having adiabatic conduction and convective heat transfer with no source term effects. Viscous dissipation coupled with the piston motion, the pressure fields generation, the temperature effects on the viscosity of the lubricant and the subsequent oil film thickness profiles in the contact region are examined. The influence of the low-temperature shear heating on the hydrodynamic and EHL film thickness at the time of initial engine start up are investigated. This study suggests that by using a high-viscosity grade oil in the idling speed engine start up the film temperature rises non-uniformly due to shear heating in the hydrodynamic and EHL regimes. The low temperature rise affects the pressure and temperature dependent oil viscosity, and the secondary transverse eccentric displacements of the piston. Resultantly, the piston skirts lubrication is affected despite the initial engine start up at the idling speeds.

The present work presents how scaling analysis can be applied into multiphysics and multicoupled problems related to welding processes. The formation of the weld pool surface depression in high current and velocity Gas Tungsten Arc Welding (GTAW) is dominated by the gas shear acting on the weld pool. Considering this dominant force the weld penetration was estimated and compared to experimental results. Plastic deformation and heat flow are coupled phenomena in Friction Stir Welding (FSW), the maximum temperature was estimated using scaling analysis and compared with experimental and numerical results reported in the literature. Although the simplicity of the scaling models, they are capable of capturing correct trends and order of magnitudes of the unknown estimations in a problem. Moreover, they are capable of determining the dominant forces that act on the process studied.

The aim of this study is to analyze the phenomena of heat transfer enhancement between two periodic sinusoidal walls for a single gas flow. The experimental set-up is characterized by a few geometrical parameters: amplitude of wall waviness (A); channel height (H); fin wavy-channel width (ω); fin period (l) and total wavy length (L). Combination of these ones is reduced to: the wall aspect ratio γ, the cross-section aspect ratio α and the channel spacing ratio ε. The Reynolds number defined on the hydraulic diameter and the bulk velocity is greater than 4000. A constant heat flux is maintained on the second lateral wall. For Re = 5700, we observe an entrance region from the first to the fourth period; beyond, the velocity profiles are autosimilar. A shear layer is generated just downstream of the crest and develops in its wake up to the concavity area. Thermal experimental approach is performed by local measurements of convective heat transfer coefficient along the walls, within the viscous sublayer. The heat transfer profile presents an increasing from the crest of 15%, and the maximum is located at the first quarter of the period, close to the separated point. Beyond, the value of heat transfer decreases of 50% and the minimum is located close to the reattachment point. Then the heat transfer increases up to the next crest. The same phenomenon is observed in the next periods of the channel. To explain theses results, we calculate the turbulence terms obtained from the classical equations of fluid mechanics. The turbulence production (P) presents a maximum in the core of the shear layer, where the Reynolds constraints and the heat transfer are maxima. A good correlation is obtained between turbulence production and heat transfer. The flow pattern (mean, fluctuating and turbulence terms) are performed with PIV technique in order to analyze the vortices that develop in the shear layer, based on 1000 pairs of images.

The turbulent flow and the heat transfer in a unitary cell of a cross corrugated plate pattern heat exchanger has been studied using Chen’s high-Re k-ε model, Suga’s low-Re k-ε model, the RSM and the V2F model at a Reynolds number of 4930. The ability of these models in predicting the mean Nusselt number and Fanning friction factor has been investigated. The V2F model predicted higher heat transfer and friction factors than the other models. It was observed that the upper and lower flow in the unitary cell interact throw a shear process. This in turn initiates a complex secondary flow pattern which promotes the heat transfer. The V2F model predicted the strongest shear process. This may explain the fact that it also predicted the highest values of heat transfer and friction factor compared to the other models. The shear flow also caused high levels of turbulent kinetic energy in the centre of the unitary cell. The observed secondary motion is believed to be an efficient means of increasing the heat transfer coefficient with limited pressure drop penalty. It is also demonstrated that despite the geometrical complexity, high quality computational grids can be created and thereby details of the flow and heat transfer phenomena can be studied. The RSM appeared to be instable and gave results similar to Chen’s k-ε model. Therefore, its use is not motivated for such applications.

Turbulent structures in a supersonic plane mixing layer at the convective Mach number of Mc=1.2 are studied using spatially developing DNS. High-resolution compact upwind-biased schemes developed by Deng & Maekawa (1996)[1] are employed for spatial derivatives. The numerical results indicate that the turbulent structures are generated after transition in the mixing layer, which are similar to the plane jet turbulent shear layer. The mixing layer Reynolds number based on the vorticity thickness reaches 6500. Unlike low Mach number mixing layers with a roller-like structure, hierarchic structures with hairpin packet-like structure and its cluster vortices are observed in the turbulent mixing layer. The effect of the turbulent hierarchic structure on scalar mixing is investigated using the DNS database. The visualized scalar field associated with vortical structure evolution of the turbulent mixing layer shows that the intermittent hairpin packet-like structure and its cluster govern a large-scale scalar mixing in the shear layer. The turbulent fine structure of pair vortices also plays an important role for scalar mixing. Furthermore, dilatational fields of the mixing layer show intense areoacoustic phenomena associated with the turbulent structure evolution.

Flame Stabilization in a high-speed premixed environment requires the presence of a mechanism to stabilize the flame. Bluff bodies or geometrical flame holders introduce a recirculation zone that anchor the flame. The current study considers the influence of equivalence ratio and the boundary layer state at the trailing edge of the flame holder on the flowfield and combustion characteristics. It was found that the recirculation zone is shortened as the equivalence ratio increases towards unity. A secondary shear region emerges downstream of the recirculation zone and is caused by the accelerated low-density combustion products. The emergence of the secondary shear region moves upstream with increasing equivalence ratio. Tripping the boundary layer causes a dramatic reduction in the length of the recirculation zone, and the secondary shear region is greatly augmented. Visualizations show that tripping the boundary layer resulted in a greatly disturbed flame near the trailing edge and large flame scales. Flowfield measurements suggest that the heat release is increased by approximately 50% when the boundary layer tripped.

The direct numerical simulation methodology was employed to analyze the unsteady features of a three-dimensional separated flow and heat transfer around a downward step in a rectangular channel, and to clarify systematically the channel expansion ratio effects upon them. Numerical calculations were carried out using the finite difference method. The Reynolds number Re based on the mean velocity at inlet and the step height was varied from 300 to 1000. The channel expansion ratio ER is 1.5, 2.0 and 3.0 under a step aspect ratio of 36.0. It is found that the flow is steady upto Re = 500 but becomes sensibly unsteady at Re = 700 for all the three expansion ratios. In the case of ER = 2.0, the separated shear layer is most unstable. In the case of ER = 1.5, the longitudinal vortices formed near the side walls of channel are strongest. Nusselt number reaches its maximum in the reattachment flow region and also in the neighborhood of the side wall, and their locations depend greatly upon ER and Re .

Convective heat transfer to an impinging air jet is known to yield high local and area averaged heat transfer coefficients. Such jets are of interest in the cooling of electronic components and of turbine blades and in manufacturing processes such as grinding. The current research is concerned with the measurement of heat transfer to an impinging air jet over a wide range of test parameters. These include Reynolds numbers, R e , from 10000 to 30000 and nozzle to impingement surface distance, H / D , from 0.5 to 8. The current research reports both mean and fluctuating heat transfer distributions up to 6 diameters from the geometric centre of the jet. The heat transfer results are compared to local velocity data. At low nozzle to impingement surface spacings the heat transfer distributions exhibit peaks at a radial location that varies with both R e and H / D . These peaks are shown to be due to an abrupt increase in turbulence in the wall jet boundary layer. At certain test configurations vortices that initiate in the shear layer impinge on the surface and move along the wall jet before being broken down into smaller scale turbulence. The effects of the vortical flow on the heat transfer mechanisms in an impinging jet flow are discussed.

Present paper deals with modelling of a laminar flow of shear-thickening non-Newtonian fluid. First, approach with the Quadratic model to rheological modelling of such a fluid is presented. Secondly, the laminar flow of non-Newtonian fluid in a pipe bend is studied by numerical means to obtain the pressure loss coefficient prompted by disagreement between the empirical correlations and numerically obtained results. Finally, the comparison between two rheological models is presented.

Microfluidic transport is finding increasing application in a number of emerging technologies. At these scales, classical analysis shows that the required fluid driving pressure is inversely proportional to the hydraulic diameter to the fourth power. Consequently, generating fluid motion at these physical scales is a challenge. There is thus considerable incentive for developing strategies to reduce the frictional resistance to fluid flow. A novel approach recently proposed is fabrication of micro-ribs and cavities in the channel walls which are treated with a hydrophobic coating. This reduces the surface contact area between the flowing liquid and the solid wall, yielding walls with no-slip and shear-free regions at the microscale. The shear-free regions consist of a liquid-vapor meniscus above the cavities between micro-ribs. Reductions in the flow resistance are thus possible. This paper reports results of an analytical and experimental investigation of the laminar, fully-developed flow in a parallel plate microchannel whose walls are microengineered in this fashion. The micro-ribs and cavities are oriented parallel to the flow direction. The channel walls are modeled in an idealized fashion, with the shape of liquid-vapor meniscus approximated as flat and characterized by vanishing shear stress. Predictions are presented for the friction factor-Reynolds number product as a function of relevant governing dimensionless parameters. Comparisons are made between the smooth-wall classical channel flow results and predictions for the microengineered channel walls. Results show that significant reductions in the frictional pressure drop are possible. Reductions in frictional resistance increase as the channel hydraulic diameter and/or micro-rib width are reduced. The frictional pressure drop predictions are in good agreement with experimental measurements made at dynamically similar conditions, with greater deviation observed with increasing relative size of the shear-free regions.

This third in a series of four papers (Parts I–IV) presents the equations used for the initial evaluation of a pump’s ability to suspend solids and extends those equations to establish the minimum local velocity required to suspend those solids. This minimum velocity was used in a finite difference model in Part I to predict the ability of a pump to suspend, or slurry, solids that had settled on the bottom of a nuclear waste tank. To slurry waste, the Advanced Design Mixer Pump (ADMP) discharges a fluid jet that impinges on, shears, and then suspends the waste. Prior to the pump’s installation in a waste tank, the local velocity at a point in the flow required to suspend solids was found from available equations, material properties, and empirical data for similar pumps. Also, the computational fluids dynamics (CFD) model was validated in Part II by comparing it to flow rates measured in a full scale test facility where the ADMP was operated. The CFD fluid model could then be used to predict flow rates throughout the actual waste tank where the pump was to be installed, and the ability of the pump to adequately slurry the waste could be shown. All that needed to be done was to compare the local velocity of the fluid required to shear the waste into suspension to the velocities modeled throughout the waste tank. In short, this paper validates the theoretical and experimental basis for the derivation of a minimum velocity required for the flow stream to shear the waste into suspension. The final installment to this series of papers (Part IV) validates the application of the CFD model, by concluding that a nuclear waste tank is effectively cleaned to the wall throughout most of tank, using the ADMP.

Experiments were conducted to investigate the effectiveness of a porous manifold in the formation and maintenance of thermal stratification in a liquid storage tank. A thermal storage tank with a capacity of 315 liters and a diameter-to-height ratio of 2 was used for the experiment. The porous manifold used was made from rolling up a nylon screen into the shape of a tube. Stratification was observed at a Richardson number as low as Ri = 0.615. Flow visualization was also performed to confirm the effectiveness of the porous manifold in the promotion and maintenance of stable thermal stratification. From the results of flow visualization, one can conclude that a porous manifold is able to reduce the shear-induced mixing between fluids of different temperature, and thus is able to promote and maintain a stable stratification.

The effect of periodic perturbation on a jet in a cross-flow (JICF) is reviewed. In the first part of the paper, flow visualization results from several past works are discussed. Beginning with a description of the characteristic vortex systems of a JICF it is shown that specific perturbation techniques work by organizing and intensifying specific vortex systems. Oscillatory blowing works primarily through an organization of the shear layer vortices. A mechanical perturbation technique is found to organize the wake vortices. In the second part of the paper, results of an ongoing experiment involving another mechanical perturbation technique are discussed. It involves two tabs at the orifice exit whose asymmetry in placement is reversed periodically. It directly modulates the counter-rotating vortex pair (CVP). Effects of the perturbation for an array of three adjacent orifices are explored. The flowfield data show an improvement in mixing compared to the unperturbed case.