Flow Impingement Research Articles

Investigation on flow mechanism and impingement heat transfer performance of combined fluidic oscillators

Two kinds of combined fluidic oscillators are designed by the structural innovation of the traditional fluidic oscillator. The working mechanism and flow characteristics of the combined fluidic oscillator are studied experimentally and numerically. The flow field characteristics of the combined fluidic oscillator are measured qualitatively and quantitatively. The inner flow fields of two combined fluidic oscillators are investigated by numerical simulation, and the different flow conversion mechanisms in the two combined fluidic oscillators are revealed. It is verified that the two combined fluidic oscillators can generate inphase and anti-phase sweeping jets. Experiment on heat transfer performance of the sweeping jet generated by the combined fluidic oscillator impinging on a flat plate under different nozzle-to-plate spacing is carried out. The experimental results show that the sweeping jet can improve the uniformity of the Nusselt number on the impinged target, and the improvement effect becomes more remarkable with the increase of the impingement distance. The research lays a foundation for the further study of the structural parameters and theoretical analysis of the combined fluidic oscillator, provides theoretical and experimental basis for the application of the combined fluidic oscillator.

EFFECT OF PARTICLE SHAPE ON HEAT TRANSFER AND ENTROPY GENERATION PERFORMANCE OF Al2O3/WATER NANOFLUID JET FLOW IMPINGING ON A CONVEX SURFACE

EFFECT OF PARTICLE SHAPE ON HEAT TRANSFER AND ENTROPY GENERATION PERFORMANCE OF Al2O3/WATER NANOFLUID JET FLOW IMPINGING ON A CONVEX SURFACE

Dual Synthetic Jets Actuator and Its Applications—Part III: Impingement Flow Field and Cooling Characteristics of Vectoring Dual Synthetic Jets

In order to understand the impingement flow field and cooling characteristics of vectoring dual synthetic jets (DSJ), an experimental investigation was performed to analyze the parameter effects. With the variation of the slot location, the vectoring angle of DSJ can be adjusted from 34.5° toward the left to 29.5° toward the right. The vectoring function can greatly extend the length of impingement region. There are three local peaks both for the local cooling performance (Nu) and the whole cooling performance (Nuavg). Although the peak Nu at a certain location of the slider is higher than that at the center, the corresponding Nuavg is lower. As for different driving frequencies, the vectoring angle reaches its minimum of 9.7° at 350 Hz, but the Nu is obviously improved. There is one local peak of Nuavg values at 350 Hz rather than three local peaks at 250 Hz and 450 Hz. The slot locations where the Nuavg of 250 Hz and 450 Hz reach maximum are different. With the increase in driving voltage from ±100 V to ±200 V, the vectoring angle drops from 46.9° to 22.2°, but both Nu and Nuavg are improved. The maximum Nuavg of each driving voltage occurs at the center location of the slider. The choking effect and the cross flow have dominated the vectoring angle and the cooling performance of impingement DSJ. Vectoring DSJ will give impetus to the thermal management of large-area electric devices in spaced-constrained cooling and removing dynamic hotspots.

Comparisons of cooling performance and flow characteristics of a combustor liner plate with compound angle and simple angle effusion holes

The present investigation considers a unique compound angle arrangement, never previously considered, with α = 30°, β = ±30°, such that the compound angle of the effusion holes changes from β = +30° to β = −30°, as adjacent rows of effusion holes are encountered. Also provided are data for comparison from a simple angle configuration with α = 30°, β = 0°. Coolant is supplied to both effusion hole arrays using arrays of impingement cooling jets. The results of the numerical simulations are obtained using the ANSYS FLUENT V 21.1 numerical code, with a shear stress transport (SST) k–ω turbulence model. Data are provided for main flow Reynolds numbers from 142,000 to 155,000, impingement flow Reynolds numbers from 7900 to 18,000, effusion flow Reynolds numbers from 10,400 to 23,600, and overall blowing ratios from 3.3 to 7.4. When β is changed from 0° to ±30°, area-averaged film cooling effectiveness values and area-averaged heat transfer coefficient magnitudes increase overall by approximately 2.89% and 11.7%, respectively. This is because the use of compound angle arrangements often inhibits the formation of the horseshoe-shaped vortex, and the resulting counter-rotating vortex legs, that often promote lift off of significant film concentrations from the test surface. The skewed and non-symmetric character of the horseshoe-shaped vortex is also increased by the use of compound angle arrangement, which also sometimes provides local improvements in surface thermal protection.

Heat transfer and flow characteristics in symmetric and parallel wavy microchannel heat sinks with porous ribs

Based on the synergistic design concept, double-layered microchannel heat sinks with parallel and symmetric wavy porous fins are developed with the desire to simultaneously attain pressure drop reduction, heat transfer enhancement, and cooling uniformity improvement. Using a 3D fluid-solid conjugate model, the hydrodynamic and thermal details are numerically studied to compare two wavy configurations with the solid- and porous-fin designs. The results demonstrate that for wavy microchannel heat sinks, the porous-fin design can significantly enhance heat transfer performance and reduce pressure drop. The symmetric configuration yields a higher pressure drop reduction, whereas the parallel one provides a higher increment in thermal performance. As a result, using the porous design, two wavy configurations have nearly the same level of pressure drop penalty, but the parallel configuration contributes to higher thermal performance. The decreased flow rate of the channel due to the permeation of coolant fluids into porous ribs and the slip effect on the channel wall contribute to the pressure drop reduction. The permeation effect of coolant greatly restrains secondary flow characteristics induced by wavy walls which are responsible for the enhanced coolant mixing in conventional heat sinks. Consequently, the increased inlet flow velocity at a constant pumping power contributes to the improved thermal performance, namely the reduced thermal resistance and the increased Nusselt number. With a stronger coolant permeation owing to the jet-like impingement flow, the parallel configuration yields a slightly lower thermal performance compared to the symmetric configuration in the new design. Finally, the parametric analysis at a fixed pumping power is further carried out for optimizing the proposed design.

Moffatt eddies in electrohydrodynamics flows: numerical simulations and analyses

We study numerically a sequence of eddies in two-dimensional electrohydrodynamics (EHD) flows of a dielectric liquid, driven by an electric potential difference between a hyperbolic blade electrode and a flat plate electrode (or the blade–plate configuration). The electrically driven flow impinges on the plate to generate vortices, which resemble Moffatt eddies (Moffatt, J. Fluid Mech., vol. 18, 1964, pp. 1–18). Such a phenomenon in EHD was first reported in the experimental work of Perri et al. (J. Fluid Mech., vol. 900, 2020, A12). We conduct direct numerical simulations of the EHD flow with three Moffatt-type eddies in a large computational domain at moderate electric Rayleigh numbers ( $T$ , quantifying the strength of the electric field). The ratios of size and intensity of the adjacent eddies are examined, and they can be compared favourably to the theoretical prediction of Moffatt; interestingly, the quantitative comparison is remarkably accurate for the two eddies in the far field. Our investigation also shows that a larger $T$ strengthens the vortex intensity, and a stronger charge diffusion effect enlarges the vortex size. A sufficiently large $T$ can further result in an oscillating flow, consistent with the experimental observation. In addition, a global stability analysis of the steady blade–plate EHD flow is conducted. The global mode is characterised in detail at different values of $T$ . When $T$ is large, the confinement effect of the geometry in the centre region may lead to an increased oscillation frequency. This work contributes to the quantitative characterisation of the Moffatt-type eddies in EHD flows.

Drag and heat transfer in turbulent channel flow over shallow circular dimples: The shift of the deepest point of dimples

In this study, the flow characteristics and enhanced heat transfer performance of circular dimples in a channel flow were numerically analyzed using DNS and compared with a flat channel. The effect of shifting the deepest point of the dimples in a turbulent channel flow on their drag and heat transfer performance are discussed. The strength and extent of the induced recirculating flow is suppressed significantly when the deepest point is shifted downstream, enhancing the heat transfer performance of the dimpled wall. At the same time, the flow structure above the dimpled wall is also manipulated by the geometry changes. The flow impingement on the dimpled wall increases the drag; consequently, the power required to drive the flow is increased. A study is conducted to investigate the effect of the shift in the deepest point of circular dimples on its heat transfer performance while minimizing the drag increase.

Hypersonic shock impingement studies on a flat plate: flow separation of laminar boundary layers

Interactions between shock waves and boundary layers produce flow separations and augmented pressure/thermal loads in hypersonic flight. This study provides details of Mach 7 impinging-shock-flat-plate experiments conducted in the T4 Stalker Tube. Measurements were taken at flow conditions of Mach 7.0 (2.44 MJ kg $^{-1}$ ) and Mach 7.7 (2.88 MJ kg $^{-1}$ ) flight enthalpies with a range of freestream unit Reynolds numbers from $1.43 \times 10^{6}$ m $^{-1}$ to $5.01 \times 10^{6}$ m $^{-1}$ . A shock generator at $12^{\circ }$ or $16^{\circ }$ to the freestream created an oblique shock which impinged on a boundary layer over a flat plate to induce flow separation. The flow field was examined using simultaneous measurements of wall static pressure, heat transfer and schlieren visualisation. Measured heat transfer along the flat plate without the shock impingement indicated that the boundary layer remained laminar for all flow conditions. The shock impingement flow field was successfully established within the facility test duration. The onset of separation was observed by a rise in wall pressure and a decrease in heat transfer at the location corresponding to the stem of the separation shock. Downstream of this initial rise, an increased pressure and higher heating loads were observed. The heat-transfer levels also indicated an immediate boundary layer transition due to the shock impingement. The separation data of the present work showed good agreement with our previous work on shock impingement on heated walls (Chang et al., J. Fluid Mech., vol. 908, 2021, pp. 1–13). A comparison with the previous scaling indicated that the separation also relates to the pressure ratio and the wall temperature parameter.

The effect of covering structure in pantograph sinking platform on the aerodynamics of high- speed train

ABSTRACT The aerodynamic drag of pantograph system hinders the increase of train operating speed, which can be reduced by the covering structure proposed in this study. Exploring the drag reduction mechanism of covering structure is critical. An IDDES method based on the SST κ–ω turbulence model was utilized to study the effect of covering structure on aerodynamic performance of high-speed train. The results show that the covering structure reduces the overall drag by 5.6% for 3-car train-set since the impingement of incoming flow on the surface of sinking platform is alleviated. The aerodynamic drag of the middle car is significantly reduced while that of the tail car increases due to higher pressure on the train joint surface caused by the covering plates. The turbulence of the flow field surrounding sinking platform is suppressed by weakening the flow separations from cavity edge. Vortical structure changes occur to the vortices shed from the leading edge of cavity because the front covering plate delays the process of flow separation, further leading to the alteration of dominant frequencies for folded pantograph. The covering structure will decrease the oscillation of lift force by 24.74% and 37.14% for lifted and folded pantograph.

Investigation on the maximum ceiling temperature of the weak plume impingement flow in tunnel fires under longitudinal ventilation

This paper numerically investigated the characteristics of the fire plume behavior and the ceiling maximum temperature with increasing fire source height, in the longitudinal ventilated tunnel fires. Results show that with the continued increase of the longitudinal velocity, the fire plume gradually tilts to the downstream of the tunnel, causing significantly decrease of the ceiling temperature. With the elevation of the fire source, a circulation area forms at the leeward side of the fire source, the pulling effect of the circulation area and the accumulated smoke layer can protect the fire plume from the effect of ventilation airflow, which can weaken the fire plume inclination. To quantify the influence of the longitudinal flow, the dimensionless velocity v* is adopted and divided into three regions, i.e., v*= 0, 0 < v*≤ 0.19 and v*> 0.19. When 0 < v*≤ 0.19, a velocity influence factor f (v) is added to the original equation to represent the effect of ventilation velocity for increasing fire source locations and the ceiling maximum excess temperature varies as the 2/3 power of the dimensionless heat release rate and linear function of dimensionless ventilation velocity. When v* > 0.19, a concept of virtual fire source is proposed to quantify the effects of the inclined fire plume on the maximum temperature and the ceiling maximum excess temperature varies as the 2/3 power of the dimensionless heat release rate and exponential function of dimensionless ventilation velocity. Besides, the applicability and reliability of the predictive correlations are further verified by comparing to the experimental data of different scales.

Thermal analysis of baffle jetting in fuel rod assembly

Baffle jetting plays a significant role when it comes to safe operation of nuclear power plants. The baffle jetting phenomenon is the generation of horizontal flow impingement on fuel/control rods during the outward flow of the primary coolant into a nuclear reactor. To understand the flow and heat transfer characteristics under the baffle jetting conditions, large eddy simulations (LES) of flow around a 6×6 fuel rod assembly were conducted. Three Reynolds numbers based on jet width and inlet velocity were considered 5010, 10 000, and 20 000. A temperature difference of 5°C between the inlet fluid and the heated rods was considered to analyze the heat transfer characteristics within the assembly under baffle jetting. Various flow parameters were computed such as pressure coefficients along different rods, mean and fluctuating forces, Strouhal number, local and averaged Nusselt numbers. LES results were validated against experimental measurements and other numerical data. It was observed that the effect of the baffle jet was more significant on the first stream-wise row of rods with the stagnation points at the lower part of these rods. Furthermore, the averaged Nusselt number was found to be higher on rods in the stream-wise direction of the jet, rather than at other locations.

Liquid-cooled heat sink design for a multilevel inverter switch with considerations for heat spreading and manufacturability

Multilevel inverter switches (MIS) present a thermal management challenge by virtue of their small size yet high power operation. In this contribution, a water-cooled heat sink for an MIS was designed and optimized through a series of computational fluid dynamics simulations and surrogate modeling. The parameters under consideration were technical metrics (thermal resistance, pressure loss, and temperature uniformity) and manufacturing costs. The stark difference between air-cooled and liquid-cooled heat sinks was demonstrated in parallel modeling efforts. The heat sink consisted of a metallic baseplate with pin-fins and a separate resin manifold containing an array of nozzles for flow impingement and fluid extraction. The pin–fin baseplate was parametrically designed using three different fin densities: restricted array (RA), variable array semifull (VAS), and variable array (VA), where fin diameter and pitch were correlated. The manufacturability of each design iteration was determined by obtaining quotes from a vendor. Since all heat sinks had the same inlet manifold, the pressure drop was similar across heat sink designs, indicating that the fin distribution had a minimum effect on the overall pumping power required for circulating the flow through the heat sinks. Effective thermal resistance calculations illustrated a complex interplay between fin diameter and pitch spacing. The complexity was better visualized using a feedforward neural network (FNN) and an overall performance metric (PPTR). A sensibility analysis using the FNN indicated that the fin pitch was the dominant variable in the determination of the overall heat sink performance. The PPTR allowed the creation of a ranking system that matched the FFN outcome and indicated that the VA2D1 design was the most technically promising; nonetheless, once manufacturing cost was considered in the analysis, as PPTR-$, the RA1D1 was the most commercially viable as it satisfied technical cooling goals per the lowest unit cost.

Capturing wake capture: a 2D numerical investigation into wing–wake interaction aerodynamics

A wing generating lift leaves behind a region of disturbed air in the form of a wake. For a hovering insect, the wings must return through the wake produced by the previous half-stroke and this can have significant effects on the aerodynamic performance. This paper numerically investigates 2D wings interacting with their own wake at Reynolds numbers of 102 and 103, enabling an improved understanding of the underlying physics of the ‘wake capture’ aerodynamic mechanism of insect flight. We adopt a simple kinematic motion pattern comprised of a translational stroke motion followed by a complete stop to expose wake interaction effects. Representative stroke distance to chord ratios between 1.5 and 6.0 are considered, enabling different leading-edge vortex (LEV) attachment states. We also allow pitching rotation towards the end of stroke, leading to wake intercepting angles of 135°, 90°, and 45°, analogous to delayed, symmetric, and advanced pitching rotations of insect wings. It is shown that both vortex suction and jet flow impingement mechanisms can lead to either positive or negative effects depending on the LEV attachment state, and that stroke distances resulting in a detached/attached LEV lead to beneficial/detrimental wake interaction lift. Pitching rotation at the end of the stroke motion is found to induce a strong rotational trailing-edge vortex (RTEV). For advanced pitching, this RTEV serves to enable either a stronger flow impingement effect leading to positive wake interaction lift if the LEV is detached, or a less favourable vortex suction effect leading to negative wake interaction lift if the LEV is closely attached. The higher Reynolds number led to faster development of the wake vortices, but the primary wake interaction mechanisms remained the same for both Reynolds numbers.

Schooling benefits from a system of active and passive hydrofoils

Fishes school to swim more efficiently while not much of fish schooling is understood from the perspective of fluid–structure interactions. To understand the benefits of fish schooling, we investigate thrust, efficiency, and wake structure of a two-hydrofoil system where the upstream hydrofoil is forced pitching (active) and the downstream hydrofoil is free pitching (passive). The streamwise and lateral separations between the two hydrofoils are 0.109c and 1.0D, respectively, where c is the chord length and D is the diameter of the leading edge. The active hydrofoil pitches with normalized amplitudes A∗=0.55 – 0.80 and normalized frequencies StD=0.23–0.33. It is found that the upstream active hydrofoil undergoes up to 38% thrust enhancement in the presence of the downstream passive hydrofoil while the downstream passive hydrofoil achieves the same thrust as the upstream active hydrofoil under certain conditions. The maximum combined thrust and efficiency achieved for the two-hydrofoil system are 95% and 180% higher than those for a single isolated hydrofoil. These are some reasons for fish swimming in school. Three distinct flow structures (vortex impingement flow, vortex trapping flow, and vortex splitting flow) are identified in the A∗– StD domain, where the vortex trapping flow provides the greatest thrust and efficiency enhancement.

Thermo-chemo-mechanical influences of impingement flow on the degradation of organic coatings in the underwater zone of offshore wind turbines

Thermo-chemo-mechanical influences of impingement flow on the degradation of organic coatings in the underwater zone of offshore wind turbines

An Improved Particle Impact Model by Accounting for Rate of Strain and Stochastic Rebound

Abstract A methodology for informing physics-based impact and deposition models through the use of novel experimental and analysis techniques is presented. Coefficient of restitution (CoR) data were obtained for Arizona Road Dust (ARD), AFRL02 dust, and each component of AFRL02 impacting a Hastelloy X plate at a variety of flow temperatures (295–866 K), surface temperatures (295–1255 K), particle velocities (0–100 m/s), and impact angles (0–90 deg). High speed particle shadow velocimetry (PSV) allowed individual impact data to be obtained for more than 8 million particles overall, corresponding to 20 combinations of particle composition, flow temperature, and surface temperature. The experimental data were applied to an existing physics-based particle impact model to assess its ability to accurately capture the physics of particle impact dynamics. Using the experimental data and model predictions, two improvements to the model were proposed. The first defined a velocity-dependent “effective yield strength,” designed to account for the effects of strain hardening and strain rate during impact. The second introduces statistical spread to the model output, accounting for the effect of randomizing variables such as particle shape and rotation. Both improvements were demonstrated to improve the model predictions significantly. Applying the improved model to the experimental data sets, along with known temperature-dependent material properties such as the elastic modulus and particle density, allowed the temperature dependence of the effective yield strength to be determined. It was found that the effective yield strength is not a function of temperature over the range studied, suggesting that changes in other properties are responsible for differences in rebound behavior. The improved model was incorporated into a computational simulation of an impinging flow to assess the effect of the model improvements on deposition predictions, with the improved model obtaining deposition trends that more closely match what has been observed in previous experiments. The work performed serves as a stepping stone towards further improvement of physics-based impact and deposition models through refinement of other modeled physical processes.

Assessment of liquid and gas impingement cooling fluids with numerical solution for better steel austempering

Impingement jet heat transfer was studied using liquid and gas fluids to determine better cooling fluid. The materials used were rectangular steel plates of 230 mm by 120 mm by 12 mm, single jet diameters of 10–40 mm with impingement gaps of 115–155 mm. A computational fluid dynamics software application ANSYS 2020R1 was employed for simulation, and lumped thermal mass analysis was used for experimental modeling. The experimental results showed an increase in heat transfer coefficient with increased pipe diameters and with a corresponding increase in impingement gaps and flow rate. This revealed 265.4–383.9 W/m2 K and 85.3–109 W/m2 K at diameter 10 mm and 336.5–365 W/m2 K and 109.0–137.5 W/m2 K at diameter 40 mm for both water and air, respectively. Numerical simulation revealed heat flux of 22518–38.94 W/m2 and 7570.2–4.25 W/m2 and 6742.8–27.1 W/m2 and 4155.6–6.1 W/m at diameters 10 mm and 40 mm, respectively. This confirmed that water remains a better cooling fluid with a 6.2% difference at the diameter of 10 mm and a 0.1% difference at the diameter of 40 mm. An acceptable error margin of 4 to 18% upon the comparison of empirical analysis with numerical simulation is obtained. The above suggests a better cooling rate for the microstructure of the steel using water against air.

Comparative analysis of cross flow and jet impingement techniques of heat sink in electronics cooling

New and advanced technologies lead to the miniaturization of electronic devices with high energy densities. Though this is commendable progress in electronic technology, cooling these systems has become arduous due to the high heat flux generation from these devices. Traditional cooling systems are lagging due to these challenges, which brings up the demand for novel cooling techniques for this purpose. This research paper focused on the comparison of cross flow and jet impingement cooling techniques used in the heat sink, the peak temperature raised and the pressure drop across the heat sink is reported. The computational Fluid Dynamics (CFD) technique is used to investigate the flow and heat transfer characteristics. Among the cases studied, it is identified that the cross-flow shows superior thermal attributes as compared to the jet impingement method, due to which the base temperatures of pin fins in the cross-flow is noticed an average of 14.5° C lower than that of jet impingement.

Effect of Hybrid Nanofluids Concentration and Swirling Flow on Jet Impingement Cooling

Nanofluids have become increasingly salient in heat transfer applications due to their promising properties that can be tailored to meet specific needs. The use of nanofluids in jet impingement flows has piqued the interest of numerous researchers owing to the significant heat transfer enhancement, which is vital in the technological dependence era in every aspect of life, particularly in engineering applications and industry. The aim of this current work is to investigate the effect of hybrid nanofluids concentration and swirling flow on jet impingement cooling through experimental approach. The hybrid nanofluids are prepared through a two-step method and the characterization process is carried out to study the stability and morphological structure of the sample prepared. The prepared hybrid nanofluids are then used as a cooling agent to evaluate the heat transfer performance of jet impinging system. The experimental investigation compares the performance of swirling impinging jets (SIJs) with conventional impinging jets (CIJs) under various jet-to-plate distance (H/D) ratios and nanofluid concentrations. The effects of adding surfactant on nanofluids are also examined. The heat transfer performance of ZnO/water and CuO/water mono-nanofluids are used as comparison to ZnO-CuO/water hybrid nanofluid. The results show that the thermal performance of ZnO-CuO/water hybrid nanofluid is better than that of the mono-nanofluids. Furthermore, as the mass fraction increases, the heat transfer rates improve. The effect of heat transmission by swirling impinging jets is better than that of conventional impinging jets under similar operating conditions. At H/D = 4, Re = 20,000 and hybrid nanofluid concentration at 0.1% under SIJ is observed to have the highest overall Nusselt number.

Method of variable-depth groove on vortex and cavitation suppression for a NACA0009 hydrofoil with tip clearance in tidal energy

Tidal energy has attracted great attention due to its massive potential and eco-friendly nature. Performance of ducted turbines used in tidal energy conversion is significantly influenced by tip clearance and associated leakage flow. In this work, a method of variable-depth (VD) groove is developed to suppress the tip leakage vortex (TLV) and the tip leakage vortex cavitation (TLVC) around a NACA0009 hydrofoil, and different variable-depth laws are proposed to improve the suppressing effect. The flow pattern around the hydrofoil is investigated on basis of numerical method, which is validated by experiment results. Results show that this groove method not only effectively suppresses primary TLV by high-velocity flow impingement, but also reduces the primary TLVC by increasing local pressure in vortex core regions. In addition to the intrinsic primary TLV and secondary TLV, some local small-scale vortices are witnessed in vicinity of the groove. Among the VD grooves, the divergent groove shows an optimal effect on suppressing vortex and cavitation in general. Increasing the divergent groove depth at hydrofoil pressure side causes the shrinkage of the primary TLV but a larger scale of the additional vortices in vicinity of the groove. The suppressing effect of the divergent groove on the TLVC reaches a peak when the pressure side depth is increased to 50% of the tip clearance size. The lift coefficient of the hydrofoil is sensitive to the groove pressure side depth, by decreasing which can effectively improve the lift-drag performance of the hydrofoil.