Crossflow Reynolds Number Research Articles

Effect of internal coolant crossflow on the film cooling performance of converging slot hole

The effect of crossflow Reynolds number on the film cooling characteristics was investigated on low speed flat-plate facility. The film cooling performance was measured by transient liquid-crystal technique and the flow feature was obtained by numerical simulation. Two film hole cases, converging slot hole case and cylindrical hole case, were carried out at two crossflow Reynolds numbers, Rec = 50,000 and 100,000 and three blowing ratios, M = 0.5, 1.0 and 2.0. For cylindrical hole, high Reynolds number deteriorates the film cooling effectiveness at blowing ratio of 0.5. However, better film coverages are observed at M = 1.0 and 2.0. At each blowing ratio, high crossflow Reynolds number produces higher heat transfer for cylindrical hole case. The converging slot hole case presents better film coverage in whole region. The film cooling effectiveness and heat transfer are not sensitive to the crossflow at each blowing ratio. Discharge coefficients of two holes are increased as increasing of blowing ratio, while the high crossflow Reynolds number deteriorates the discharge of flow. The numerical results indicate that the different performance of film cooling between two kinds of holes at crossflow condition is mainly attributed to the different development of in-tube vortex.

Experimental and Numerical Investigations on the Heat Transfer of Film Cooling With Cylindrical Holes Fed With Internal Coolant Cross Flows

Abstract The heat transfer coefficient of cylindrical holes fed by varying internal cross-flow channels with different cross-flow Reynolds numbers Rec is experimentally studied on a low-speed flat-plate facility. Three coolant cross flow cases, including a smooth case and two ribbed cases with 45/135-deg ribs, are studied at Rec = 50,000, and 100,000 with varying blowing ratios M of 0.5, 1.0, and 2.0. A transient liquid-crystal (LC) measurement technique is used to determine the heat transfer coefficient. At lower M, the heat transfer enhancement regions are asymmetrical for the smooth and 45-deg cases. The asymmetrical vortex is more pronounced with increasing cross-flow direction velocity, resulting in a more skewed distribution at Rec = 100,000. Conversely, the contours are laterally symmetric in the 135-deg case at varying Rec. A fork-shaped trend with a relatively high heat transfer coefficient appears upstream, and the increases in the heat transfer in the 135-deg cases are lower than those in the 45-deg cases. As M increases to 2.0, the vortex intensity increases, resulting in a stronger scouring effect upstream, especially at large Rec. The range and degree are affected by Rec at M = 2.0. The core of the heat transfer enhancement is skewed to the −Y side for both cases.

Numerical study on the performance of rounded corners on the top of electronic components on cooling effectiveness

The importance of this research is to explore and discuss the effects of rounding the top corners of electronic components which are subjected to a cross-flow and a perpendicular impinging jet on the cooling efficiency. Simulations were performed at a cross-flow Reynolds number of 3410 and three different impinging-to-cross-flow Reynolds number ratios (α=0.5,1and1.5). Four cubic geometries, based on the radius of the top corner, were investigated. The principal aim of this study is to find out the effects of the rounded corner on coherent structures and cooling improvement. The Shear Stress Transport (SST) K–ω model is implemented. Moreover, the assessment of this simulation is investigated by comparison with available experimental data. It should be emphasized that the high mesh resolution was handled where the wall-normal coordinate value is suitable (herein 0.01≤y+≤0.19 for the cube wall). The obtained numerical results are in good agreement with the experimental data. The flow features and coherent structures developed closer to the components considerably influence the wall heat transfer. Additionally, it is found that rounding top corners of the cube can improve the cooling efficiency for α=1and1.5 by more than 6% and 23% respectively when compared to a regular cube.

Double wall cooling of an effusion plate with cross flow and impingement jet combination internal cooling: Comparisons of main flow contraction ratio effects

The present study considers comparisons of hot-side effusion plate results for a mainstream flow passage with CR = 1 and for a mainstream flow passage with CR = 4. The coolant supply arrangement for both CR contraction ratio values includes simultaneous use of cross flow and an impingement jet array. For the effusion cooled/hot surface, presented are spatially-resolved distributions of surface adiabatic film cooling effectiveness, and surface heat transfer coefficients (measured using infrared thermography). These results are given for main flow Reynolds numbers Rems of 89,900 to 95,800. For this main flow Reynolds number range, four different combination values of crossflow Reynolds number and impingement Reynolds number are tested for each CR value, which are associated with four different values of initial blowing ratio BR. With this arrangement, crossflow Reynolds number is varied, as impingement jet Reynolds number is approximately constant. The resulting variations of surface adiabatic film cooling effectiveness and surface heat transfer coefficients are due to three competing phenomena, whose relative influences change with x/de location. These include increased turbulent mixing and transport result from effusion coolant jets, decreased magnitudes of local blowing ratio with streamwise location, and significant streamwise acceleration, which induces local boundary layer re-laminarization. When the CR = 4 arrangement is employed, these phenomena result in dramatic decreases of local and line-averaged heat transfer coefficients, and local and line-averaged adiabatic film effectiveness magnitudes, with streamwise development, for x/de > 60. Resulting values are then significantly lower than when CR = 1, provided comparisons are made at a particular streamwise location for each value of initial blowing ratio BR.

Experimental investigation of opposed rectangular impinging jets confined in an open cavity with vertical crossflow in a rectangular duct

An experimental investigation is carried out using stereoscopic time-resolved particle image velocimetry (TR-PIV) to study the dynamics of the flow structure of two turbulent planar water jets impinging normally against each other inside an open cubical cavity subjected to external lateral flow. The cavity depth is equal to the channel depth and both jets and the crossflow are kept at ambient temperature T0. The three-dimensional near field flow development has been investigated for fixed nozzle separation at crossflow Reynolds number defined with the channel hydraulic diameter of 3000 for three jets’ Reynolds numbers based on the inlet velocities and nozzle hydraulic diameter ranging from 1500 to 5000. In this arrangement, the jet impingement moves off center, the flow fields are asymmetric with respect to the cavity mid-span and exhibit self-sustained oscillations with a pronounced three-dimensional flapping motion. Based on the measured data in planes normal to the cavity floor, mean and instantaneous velocity and vorticity distributions and turbulent statistics have been analyzed and characterized. In addition, spectral analysis of the oscillating instability is performed to analyze the effects of the confinement boundaries and the interactions between the jets and the crossflow on the spatio-temporal dynamics of the flapping flow and the oscillatory motion of the vortex structures.

Numerical study of an impinging jet in cross-flow within and without influence of vortex generator structures on heat transfer

An impinging jet heat transfer in cross-flow within and without influence of a vortex generator pair (VGP) is studied using the unsteady Reynolds averaged Navier-Stokes (URANS) and the large-eddy simulation (LES). The jet Reynolds number is 15,000 and the cross-flow Reynolds number is 30,000. The elliptic-blending Reynolds stress model (EBRSM) is implemented and adapted to capture the effect of the jet close to the wall. A v2 − f model is also implemented to study the ability in predicting such a benchmark. Both models benefit from the elliptic relaxation equation in the entire computational domain. The URANS results are compared with the accurate results of the LES method and also the experimental data. The URANS method successfully presents the flow features of the impinging jet while underpredicts the enhancing heat transfer over the channel bottom wall. The URANS method fails to correctly predict the flow structures forming around the impinging region, because the method is more diffusive than the LES method. When manipulating VGP, a rectangular winglet vortex generator pair is placed in the cross-flow channel and upstream of the jet nozzle to enhance the impinging heat transfer. The VGP increases the Nusselt number at the impingement region. The structures generated by the VGP alter the effects of the cross-flow on the impinging heat transfer. There are Kelvin-Helmholtz instabilities at the shear layer of the jet and the cross-flow in the base flow (the flow without VGP). These instabilities are altered in the flow with VGP. A swirl component is added in the jet to study the effects on the heat transfer. The result shows that for a high or moderate level of swirl, the jet is diffused before the impinging.

Effects of Pulsation Intensities on Flame Characteristics of a Small Backward-Inclined Jet Flame in Crossflow

Abstract The effects of pulsation intensities on the flame characteristics of a 10 deg-backward-inclined jet flame in the crossflow were investigated in a wind tunnel. The jet and the crossflow Reynolds numbers were 1527 and 2165, respectively. The jet-to-crossflow momentum flux ratio was 0.10. A loudspeaker was used to acoustically excite the jet flame. The excitation Strouhal number was 0.73, while the jet pulsation intensities varied from 0 to 1.26. The flame behaviors were studied through photography techniques. The flame temperatures were measured using a fine-wire R-type thermocouple. The combustion-induced emissions were probed by a commercial multi-gas analyzer. The jet flames were categorized into five characteristic modes with increasing pulsation intensities. Mode I was characterized by a yellowish down-washed recirculation flame, a blue neck flame, and a yellow tail flame. Modes II and III featured a split yellow tail flame, a yellowish recirculation flame, and a blue neck flame. Mode IV was characterized by a blue down-washed recirculation flame and neck flame, as well as a split yellow tail flame. Mode V was identified by a single yellow tail flame and the absence of the down-washed recirculation flame. When the jet flames were excited beyond mode I, the combustion-induced pollutants of carbon monoxide and nitric oxide were significantly reduced. However, the excited jet flame in mode V displayed low temperatures in the near-tube region.

Computational analysis of span-wise hole locations on fluid flow and film cooling of internal channels with crescent ribs

Purpose This study aims to clarify the mechanism of film hole location at the span-wise direction of an internal cooling channel with crescent ribs on the adiabatic film cooling performance, three configurations are designed to observe the effects of the distance between the center of the ellipse and the side wall(Case 1, l = w/2, Case 2, l = w/3 and for Case 3, l = w/4). Design/methodology/approach Numerical simulations are conducted under two blowing ratios (i.e. 0.5 and 1) and a fixed cross-flow Reynolds number (Rec = 100,000) with a verified turbulence model. Findings It is shown that at low blowing ratio, reducing the distance increases the film cooling effectiveness but keeps the trend of the effectiveness unchanged, while at high blowing ratio, the characteristic is a little bit different in the range of 0 = x/D = 10. Research limitations/implications These features could be explained by the fact that shrinking the distance between the hole and side wall induces a much smaller reserved region and vortex downstream the ribs and a lower resistance for cooling air entering the film hole. Furthermore, the spiral flow inside the hole is impaired. Originality/value As a result, the kidney-shaped vortices originating from the jet flow are weakened, and the target surface can be well covered, resulting in an enhancement of the adiabatic film cooling performance.

Experimental Investigation on Effect of Cross-Flow Reynolds Number on Film Cooling Effectiveness

Film cooling effectiveness and discharge coefficient of cylindrical holes fed by different internal cross-flow channels with varying cross-flow Reynolds numbers are investigated. Three cross-flow cases—a smooth case and ribbed cases with 135-deg ribs and 45-deg ribs—are studied at three blowing ratios and two . Film cooling effectiveness contours are obtained by the transient liquid-crystal measurement technique. For the smooth case and 45-deg case, the asymmetrical vortex is more pronounced with increasing cross-flow direction velocity, resulting in more skewed film coverage at and . Helical motion within the film hole for the 45-deg case is more violent. On the contrary, laterally symmetric film-coverage contours are displayed for the 135-deg case, and the film cooling effectiveness at decreases slowly as flowing downstream. The influence of on lateral distribution is greater at . The lateral spread of film coverage at is wider than that at . At , there is almost no film coverage in the downstream region at . The jet is relatively attached greater to the surface, thus producing positive influence on the film cooling effectiveness at . The hole-exit velocity contour on the side mainly affects film coverage. The discharge coefficient increases as increases. The 45-deg case is the lowest, whereas the 135-deg case is the highest.

3D Printed Fouling-Resistant Composite Membranes.

Fouling remains a long-standing unsolved problem that hinders the widespread use of membrane applications in industry. This article reports the use of numerical simulations coupled with extensive material synthesis and characterization to fabricate fouling-resistant 3D printed composite membranes. The membranes consist of a thin polyethersulfone selective layer deposited onto a 3D printed flat and double sinusoidal (wavy) support. Fouling and cleaning of the composite membranes were tested by using bovine serum albumin solution in a cross-flow ultrafiltration setup. The transmembrane pressure was regulated at 1 bar and the cross-flow Reynolds number (Re) varied between 400 and 1000. In comparison to the flat membrane, the wavy membrane showed superior performance in terms of pure water permeance (PWP) (10% higher) and permeance recovery ratio (87% vs 53%) after the first filtration cycle at Re = 1000. Prolong testing showed that the wavy membrane could retain approximately 87% of its initial PWP after 10 complete filtration cycles. This impressive fouling-resistant behavior is attributed to the localized fluid turbulence induced by the 3D printed wavy structure. These results show that not only the lifetime of membrane operations could be favorably extended but also the operational costs and environmental damage of membrane-based processes could also be significantly reduced.

Effect of crossflow regime on the deposit and cohesive strength of membrane surface fouling layers

Acquiring knowledge of the properties of membrane fouling layers is crucial to mitigating fouling and developing cleaning strategies. The cohesive strength of these fouling layers, which determines the cleaning requirement of the membrane, is nevertheless rarely investigated. Here we introduced fluid dynamic gauging (FDG) to the crossflow microfiltration of a wood material, namely microcrystalline cellulose (MCC, nominal particle size 20 μm, 95% (in volume) of the particles are bigger than 5.4 μm and smaller than 56.4 μm), to study in situ the cohesive strength of the membrane surface fouling formed under different crossflow regimes. Using regenerated cellulose membrane with a nominal pore size of 0.2 μm, filtration experiments with FDG measurement show that the crossflow regime can lead to the formation of surface fouling layers with distinct cohesive strength. Fouling formed in turbulent/transitional crossflow (Reynolds number, Reduct = 4170) was stronger and its removal required more liquid shear stress compared to the layers formed in laminar crossflow (Reduct = 1560). The fouling layers that can withstand the minimum shear of 35 Pa from the FDG sensor with turbulent/transitional crossflow were, on average 294 ± 10 μm thick, in contrast to those formed in laminar crossflow, which were significantly thinner (144 ± 73 μm at 35 Pa shear stress, p < 0.05). On the other hand, turbulent/transitional crossflow reduced material deposition significantly (p < 0.05). After 1000 s filtration, 0.117 ± 0.003 kg m−2 MCC were found on the turbulent/transitional crossflow membranes, compare to 0.134 ± 0.005 kg m−2 in the laminar crossflow situation. Moreover, a similar permeate flux was observed in all experiments. Therefore, this work also highlights the necessity of developing membrane cleaning protocols based on the fouling layer properties, rather than on the permeate flux decline.

Experiments on a jet in a crossflow in the low-velocity-ratio regime

The hairpin instability of a jet in a crossflow (JICF) for a low jet-to-crossflow velocity ratio is investigated experimentally for a velocity ratio range of $R\in (0.14,0.75)$ and crossflow Reynolds numbers $Re_{D}\in (260,640)$. From spectral analysis we characterize the Strouhal number and amplitude of the hairpin instability as a function of $R$ and $Re_{D}$. We demonstrate that the dynamics of the hairpins is well described by the Landau model, and, hence, that the instability occurs through Hopf bifurcation, similarly to other hydrodynamical oscillators such as wake behind different bluff bodies. Using the Landau model, we determine the precise threshold values of hairpin shedding. We also study the spatial dependence of this hydrodynamical instability, which shows a global behaviour.

Dominant modes in hydromagnetic stability of channel flow with porous walls

A linear stability analysis of a plane channel flow with porous walls under a uniform cross-flow and an external transverse magnetic field is explored. The physical problem is governed by a system of combined equations of the hydrodynamic and those of Maxwell. The perturbed problem of base state leads to a modified classical Orr-Sommerfeld equation which is solved numerically using the Chebyshev spectral collocation method. The combined effects of the cross-flow Reynolds number and the Hartmann number on the dangerous mode of hydromagnetic stability are investigated.The study shows that, the magnetic field tends to suppress the instability occurred by cross-flow. This stabilizing effect becomes perceptible when the magnetic field produces a mode transition from walls mode to that of the center.

3D printed composite membranes with enhanced anti-fouling behaviour

The fabrication of three dimensional (3D) printed composite membranes by depositing a thin polyethersulfone (PES) selective layer onto ABS-like 3D printed flat and wavy structured supports is presented here for the first time. The 50 mm disk supports were printed using an industrial 3D printer with both flat and double sinusoidal, i.e. wavy, surface structures. The thin selective layers were deposited onto the 3D supports via vacuum filtration. The resulting flat and wavy composite membranes were characterised and tested in terms of permeance, rejection, and cleanability by filtering oil-in-water emulsions of 0.3−0.5 vol% through a cross-flow (Reynolds number, Re = 100, 500 and 1000) ultrafiltration set-up under a constant transmembrane pressure of 1 bar. Results showed that pure water permeance through the wavy membrane was 30% higher than the flat membrane for Re = 1000. The wavy 3D printed membrane had a 52% higher permeance recovery ratio compared to the flat one after the first filtration cycle, with both membranes having an oil rejection of 96% ± 3%. The wavy 3D composite membrane maintained some level of permeation after 5 complete filtration cycles using only water as the cleaning/rinsing agent, whereas the flat one was completely fouled after the first cycle. Cleaning with NaOCl after the sixth cycle restored ~70% of the initial permeance for the wavy membrane. These results demonstrate that 3D printed wavy composite membranes can be used to significantly improve permeation and cleanability performance, particularly in terms of reducing fouling build-up, i.e. the main obstacle limiting more widespread adoption of membranes in industrial applications.

Assessment of laminar-turbulent transition models for Hypersonic Inflatable Aerodynamic Decelerator aeroshell in convection heat transfer

Hypersonic Inflatable Aerodynamic Decelerator (HIAD) has shown its great potential for future planetary explorations. However, the HIAD surface deflections could both promote boundary-layer transition early and augment heating levels sharply, which poses challenges for survivability of a Thermal Protection System (TPS). The goal of this work is to assess different transition models for the prediction of hypersonic transitional flows over scalloping deformed HIAD surface and to seek the critical factor for their capabilities to provide a reference for the application and advancement. Three representative transition models are considered: γ-Reθ, k-ω-γ and kT-kL-ω. The results show that the undulating surface causes flow separations and reattachments in valleys and strong crossflow along the leeward. k-ω-γ model can correctly predict both the shapes and locations of the transition onsets. The transition zone predicted by γ-Reθ model is much small, while kT-kL-ω is incapable of transition prediction for such a complex and irregular configuration. Moreover, this study reveals that the crossflow instability plays a dominant role in the transition. The crossflow Reynolds number Recf, whose distributions are approximately consistent with the transition zone, could be a feasible crossflow strength indicator for the transition onsets on the undulating surface. Once k-ω-γ model excludes effects of crossflow instability, it predicts incorrect transition results for the deformed surface of HIAD. Besides, a detailed analysis shows that both the crossflow instability mode and the first disturbance mode in valleys are the major contributors to the transition on the leeward surface. Near the leeward ray, the transition is mainly determined by the first disturbance mode.

Numerical predictions of flow and heat transfer of film cooling with an internal channel roughened by crescent ribs

This study explores the effects of crescent ribs mounted in an internal cooling channel on the external adiabatic film cooling performance to evaluate the advantage of crescent ribs in gas turbine blade cooling. Three ribs including a transverse rib, a crescent rib concave to the stream-wise direction, and a crescent rib convex to the stream-wise direction are considered. For a fixed mainstream flow Reynolds number, two cross-flow Reynolds numbers and two blowing ratios are taken into account. The results show that the case with a crescent rib concave to the stream-wise direction provides higher film cooling effectiveness both at the lower cross-flow Reynolds number and at the higher cross-flow Reynolds number with higher blowing ratio while the case with a crescent rib convex to the stream-wise direction performs worst at any condition. It is found that the longitudinal vortices produced by crescent ribs concave to the stream-wise direction can promote the cooling air entering the film hole while that induced by the other crescent ribs are nonbeneficial for the cooling air entering the hole. The results indicated that a lower spiral intensity leads to counter-rotating vortices with lower intensity and thus results in better cooling effectiveness. However, it makes a narrow coverage of the target surface by the coolant, which leads to disadvantages upon the cooling performance. The relative merits among the cases at different cross-flow Reynolds numbers and blowing ratios are investigated based on the dominating mechanism.

On the use of a fluid’s elasticity for deliberate rise of Taylor cells in a rotating micro-filter separator

The effect of radial throughflow on the instability of circular Couette flow is numerically studied for a viscoelastic fluid obeying the Giesekus model. An exact solution has been obtained for the base flow using the perturbation method with the cross-flow Reynolds number serving as the small parameter. The stability of the base flow to infinitesimally small, normal-mode, axisymmetric perturbations is studied using the linear temporal stability theory. An eigenvalue problem is obtained which is solved numerically using the pseudo-spectral, Chebyshev-based collocation method. The numerical results show that for small cross-flow Reynolds numbers, there exists a critical Weissenberg number at which the flow is at its most stable state. For sufficiently large cross-flow Reynolds numbers, however, it is predicted that the flow becomes monotonically less stable when the Weissenberg number is increased. These results suggest that elasticity can be used as an efficient means for the deliberate rise of Taylor cells in rotating micro-filter separators for self-cleaning purposes of the clogged pores.

Quantitative description of droplet dispersion of hollow cone spray in gaseous crossflow

In this study, we experimentally investigate the large-scale vortex pair formed by droplets in the spanwise direction of the flow field of a hollow cone spray injected transversely into a gaseous crossflow. Experiments are conducted in a square channel for a wide range of spray and crossflow conditions. The spatial and velocity distributions of the spray droplets for different cross-sections of the flow field in terms of different flow conditions are measured through flow visualization. Three parameters, namely, vortex vorticity, depth of vortex core, and distance between both vortex cores, are used to characterize the counter-rotating vortex pair (CVP) formed by droplets. The crossflow Reynolds number, (initial atomized) droplet Reynolds number, and number of droplets injected per unit time are all found to significantly influence the features of droplets CVP. We newly define the spray-to-crossflow momentum flux ratio (J∗) based on the injected momentum flux of initial atomized droplets. Accordingly, we develop a set of correlations for predicting the features of droplets CVP based on experimental measurements and the Buckingham π theorem. The results show that these correlations well predict all measured conditions. The results of this study should provide insights into the dynamics of spray droplets in a crossflow and an understanding of the large-scale mixing between a hollow cone spray and a crossflow.

Analytic Approximate Solution for a Flow of a Second-Grade Viscoelastic Fluid in a Converging Porous Channel

The problem of a two-dimensional steady flow of a second-grade fluid in a converging porous channel is considered. It is assumed that the fluid is injected into the channel through one wall and sucked from the channel through the other wall at the same velocity, which is inversely proportional to the distance along the wall from the channel origin. The equations governing the flow are reduced to ordinary differential equations. The boundary-value problem described by the latter equations is solved by the homotopy perturbation method. The effects of the Reynolds and crossflow Reynolds number on the flow characteristics are examined.

Numerical Evaluation of Heat Transfer from Rotating Discs Separated by Sealing Elements

Process waste heat recovery has been gaining attention in last decades in order to use resourcesefficiently. Even though the mechanisms of heat transfer in static systems such as cross flow heatexchangers are well understood, the situation is totally different for the heat transfer mechanisms insystems consisting of parallel, rotating discs. The situation becomes even more complex when sealingelements prevent fluid from following the natural path in rotational direction. In this work ReynoldsAveraged Navier-Stokes (RANS) simulations using the k-omega-SST turbulence model were performedfor various disc radii, rotational and cross flow velocities as well as disc spacings. Simulations wereexecuted for rotational Reynolds numbers Re! ranging from 94,094 to 435,623 and cross flow Reynoldsnumbers ReU between 3,993 and 36,052. In this paper we show the correlation of resultingflow structures, temperature distribution and resulting heat transfer from multiple rotating discs. Resultsfrom simulations of several different geometrical setups of the discs for various operating pointsshow a maximum of convective heat transfer at a given point that is dependent on the cross flowvelocity, disc diameter, disc spacing and rotational velocity. An increase in air cross flow velocity aswell as larger disc spacing show positive effects on the convective heat transfer in contrast to higherrotational velocity and larger discs. Even though the simulations show some limitations of classicalmeans to increase heat transfer from rotating discs, the influence of sealing elements and resultingflow structures in the gap have not been investigated. It is very likely that the thermal performancecan be further increased by optimizing shape and arrangement of these elements which is subject tofurther research.