Flow Impingement Research Articles

Experimental and numerical studies on multi-jet impingement flow characteristics in a double-wall structure under rotating conditions

Experimental and numerical studies on multi-jet impingement flow characteristics in a double-wall structure under rotating conditions

Effect of impinging flow on hydrothermal characteristics of a pipe-network mini channel heatsink

In the current work, a lattice-structured mini channel heatsink is subjected to an impinging flow (IF) in which the flow enters from the top and exits from the lateral sides. An experimental setup is built, and numerical simulations are performed. The hydrothermal performance of the heatsink subjected to the IF is investigated and compared with the same heatsink when a cross flow (CF) enters from one side and exits from another. The pressure drops, overall thermal resistances, and heat transfer coefficients are scrutinized for the mass flow rates between 2 and 5 gr.s−1. The cases with CF depict superior thermal performance at the same mass flow rates; however, the pressure drops are greater than those with the IF. The IF case leads to lower thermal resistances at the same pumping powers such that when the pumping power is approximately 0.75W, thermal resistance for the IF is 17.1% lower than CF. The heat fluxes proportional to the maximum base temperatures of the heatsink are calculated. For the critical base temperature of 75°C, the allowable heat flux of the heatsink is in the range of 2.24−4.33W/cm2 for the studied mass flow rates.

Effect of Different Flame Deflectors Plate Geometry on Supersonic Jet Impingement Flow and Heat Characteristics from Rocket Exhaust

The numerical investigation has been carried out on an under-expanded supersonic jet impingement on the perpendicular and inclined deflector plates of various shapes, such as flat, sinusoidal, and wedge types. The numerical study considers flat plates at Mach number of 2.2, and pressure ratio of 1.2, and results of flow and heat transfers are obtained using the Reynolds averaged Naiver Stokes (RANS) two-equation turbulence models. Various two-equation models (especially Standard k-ε, Reynolds Normalization Group k-ε, Realizable k-ε, Shear Stress Transport k-ω, and Baseline k-ω) were employed to predict the deflector’s surface temperature and pressure ratios. The pressure ratios are plotted with various two-equation turbulence models, and it is observed that among all turbulence models, the Shear Stress Transport k-ω shows close agreement with experimental results. Moreover, among the three deflectors, the wedge-shaped deflector shows improved performance in deflecting the rocket exhaust plume away from the surface. However, the maximum temperature on the deflector surface for a wedge-shaped deflector is 8.19% and 26% higher than the flat and sinusoidal-shaped deflectors, respectively.

New Jet impingement flow-scale sets wall approach, Proximity limits & wall-jet heat transfer

A new scale is here identified as dominant in laminar jet impingement flow and heat transfer, namely, the height of the stagnation zone, zw. This being the point at which the jet “senses” the approaching wall, which corresponds to the location of marginal static pressure rise above the impinged plate. It is shown that this new profile-specific scale emerges from the virtual origin concept in Glauert's wall-jet solution, and physically constrained expressions are derived for both. Reynolds analogy is then conducted for the cases of uniform-heat-flux and -wall-temperature. This leads to a new prediction of the wall-jet heat transfer, valid upon convergence to quasi-self-similarity – within a diameter or two of the stagnation point. The convergence distance depends on incoming velocity profile and flow rate, as captured by the new virtual origin expression. The analysis also reveals that the convergence to self-similarity is delayed in the uniform heat flux case, even though eventually heat transfer is about one-third higher.The new scale also led to a successful description of the deceleration during wall-approach, as an interpolation between near-wall (Homann solution) and far-field (inviscid flow) asymptotes. Consequently, the new scale is shown to give the proximity limits for nozzle-to-plate distance. This being the distance at which the incoming jet becomes distorted due to constriction and back-pressure. Here found to correspond to around 2/5 of the stagnation zone height.All these findings are confirmed and validated through simulations, experimental measurements and literature data; and simple expressions are derived for prediction and design purposes.

A vertebrobasilar junction aneurysm successfully treated with a combination of surgical clipping and flow diverter placement based on the results of computational fluid dynamics analysis: illustrative case.

The treatment of vertebrobasilar junction (VBJ) aneurysms is challenging. Although flow diverters (FDs) are a possible treatment option, geometrical conditions hinder intervention. VBJ aneurysms possess dual inflow vessels from the bilateral vertebral arteries (VAs), one of which is ideally occluded prior to FD treatment. However, it remains unclear which VA should be occluded. A 75-year-old male with a growing VBJ complex aneurysm exhibiting invagination toward the brainstem and causing perifocal edema required intervention. Preoperative computational fluid dynamics (CFD) analysis demonstrated that left VA occlusion would result in more stagnant flow and less impingement of flow than right VA occlusion. According to the simulated strategy, surgical clipping of the left VA just proximal to the aneurysm was performed, followed by FD placement from the basilar artery trunk to the right VA. The patient demonstrated tolerance of the VA occlusion, and follow-up computed tomography angiography at 18 months after FD treatment confirmed the disappearance of the aneurysm. Preoperative flow dynamics simulations using CFD analysis can reveal an optimal treatment strategy involving a hybrid surgery that combines FD placement and direct surgical occlusion for a VBJ complex aneurysm.

Direct numerical simulation of a pressurized thermal shock scenario at higher reynolds number

Direct Numerical Simulation (DNS) is performed for a pressurized thermal shock scenario in a reactor downcomer geometry. In the present work, a simplified representative geometry is adopted where cold fluid injected from a square duct is injected in a planar downcomer. A secondary hot inlet at the top of the downcomer is employed to enforce a downward flow of the injected emergency core cooling water, to mimic the effect of density driven flow. The Reynolds number of the impinging flow, based on bulk velocity and duct height, equals 12,500, while a unity Prandtl number fluid is used. The simulation is performed for two thermal boundary conditions, namely an isotemperature condition and an adiabatic condition, representing the two extreme scenarios of a conjugate heat transfer problem. The instantaneous and mean fields are first analysed in order to study the flow pattern and formation of vortical structures within the downcomer. The impinging flow is deflected radially outwards in the vicinity of the barrel wall, which then interacts with the surrounding hot fluid forming large vortical structures and bringing the cold fluid back to the vessel wall. The mixing of flow and penetration of colder temperatures in the downcomer is observed to be greatly influenced by interaction of these vortical structures. The descending cold plume in the downcomer is observed to form three branches separated by low-speed regions. The maximum values of turbulent kinetic energy are observed only in the vicinity of flow impingement, while high values of its production and dissipation are also observed within the shear layers of the descending plume. The maximum values of temperature fluctuations are found at the interface of the descending cold plume and surrounding hot fluid, while relatively high values are also seen at the locations of large vortical strucutres. Anisotropy in turbulence is also analysed using the componentality contour approach and a modified barycentric colour mapping scheme. Also reported herein is a comparison of mean and statistical profiles with those at lower Reynolds number reported in the literature. At higher Reynolds number, the cold impinging jet is deflected farther outwards in the downcomer. The locations of peak fluctuations in velocity and temperature are also farther away from the impingement. The magnitudes of fluctuating temperature and turbulent kinetic energy are observed to be higher for the present higher Reynolds number case. The peak Nusselt number at the barrel wall is observed to scale with a factor of Re1/2 in the present configuration.

Effect of Impingement Angle on Flame Stability in a Non-Premixed Methane/Oxygen Diffusion Flame Burner

ABSTRACT The focus of this research was understanding the effect of varying the secondary flow impingement angle of a single coaxial injector in an experimental non-premixed flame burner on flame stability. The burner was a horizontally mounted, rectangular combustion chamber with retractable spark plug for ignition. Reactants were gaseous oxygen and methane as the primary and secondary flow, respectively. Three injectors were designed and fabricated with exit impingement angles of 15, 30, and 45 degrees with the primary flow. The diffusion flame behavior and flame standoff distance length was observed through optical chamber side windows parallel to the axis of the flame. Stability maps of the diffusion flame behavior based upon equivalence ratio, reactant Reynolds numbers, and injector impingement angle were created. It was observed that the 15 degree impingement angle generated the most stable, anchored diffusion flames for high equivalence ratios and low Reynolds numbers. As impingement angle increased, stable, anchored flames transitioned from existing at fuel-lean highly turbulent primary flows to fuel-rich, highly turbulent flows as well as from fuel-rich, low primary flows to no ignition under the same conditions. Therefore, secondary flow impingement angle can drastically influence the location and range of stable, anchored methane/oxygen diffusion flames.

Openfoam Simulations for Development of a Mist Flow Diverter in Aerosol Jet Printing

The Aerosol Jet® (AJ) direct-write technology enables material deposition for producing micron-scale fine features on various substrates with almost arbitrary geometry, which is of particular interest in manufacturing 3D conformal electronics among other applications. It operates by delivering an aerosol mist stream consisting of micron-size “ink” droplets, through an aerodynamic focusing nozzle with co-flowing sheath gas in a tapered channel, to the substrate several millimetres away with an impinging jet flow [1]. The ink pattern produced with AJ printing is essentially controlled in a “vector drawing mode” by the relative motion of the substrate with respect to the deposition nozzle. When a constant mist flow rate is fed into the deposition head, a mechanism for stopping the mist flow toward substrate must be implemented to prevent ink deposition in places where ink is undesired along the substrate moving toolpath. In other words, the mist stream toward substrate needs to be switched off at the time when the places of substrate where ink deposition is unwanted are moved under the AJ deposition nozzle, and to be switched on at the time when ink deposition is wanted. For the mist flow toward substrate, it can be considered as a function of time with on or off at certain time points. Here we describe a vacuum-boost diverter for rapid mist flow on-and-off switching in AJ printing, with OpenFOAM® simulations to aid its development and to offer fundamental understanding of its operational behaviour.

Numerical simulation of droplet impingement and film flow for three-dimensional wings

Abstract In order to investigate the three-dimensional effects on the flow characteristics of the thin water film for the three-dimensional wings, the numerical simulation of the droplet impingement and film flow on the MS-0317 wing is implemented based on the open-source package OpenFOAM. The simulation focuses on the effects of the angle-of-attack and the angle of sweepback. The movement and impingement of the droplets are calculated using the Lagrangian method, and the film flow is simulated using the thin film assumption and the finite area method. The simulation of the water film flow of the three-dimensional MS-0317 wing shows that there is a spanwise flow of the water film due to the three-dimensional effects. This suggests that more research should be conducted on the warm glaze ice with surface water film of three-dimensional ice accretion on three-dimensional geometries.

TURNING REGION HEAT TRANSFER IN FIVE, STATIONARY AND ROTATING, MULTI-PASS CHANNELS WITH VARIOUS ASPECT RATIOS

Abstract This work summarizes studies that experimentally investigated the effect of rotation on heat transfer in the 180-deg tip and hub turns of cooling channels with various aspect ratios (ARs). The studied AR ranges from 1:4 to 4:1, which is the typical range within turbine blades. In addition to the smooth surface case, the cases with 45-deg angled ribs and turning vanes are also included. For several designs, the effect of channel orientation with respect to the angle of rotation is also investigated. This work covers a wide range of Reynolds and buoyancy numbers. The rib turbulators are found to have higher heat transfer enhancement and larger disturbance on the flow impingement on the tip and hub walls in channels with a wider aspect ratio. The rotational effect is reduced by the presence of ribs and turning vanes, and it is also reduced in the blade-shaped channels with the angled rotation feature. The tip wall heat transfer is increased by rotation due to the pumping effect from the centrifugal force; however, the hub wall heat transfer is reduced. The effect of rotation is most prominent in the first pass of the channels and is gradually mitigated in the following pass after the turns. This work provides stationary and rotational heat transfer coefficients in the tip and hub turning regions in blades, and heat transfer correlations for a variety of cooling channel designs can be generated, which is of benefit to the gas turbine community.

Heat transfer process for penetration flow into a branch pipe causing thermal fatigue at a T-junction

The heat transfer process between a fluid and a wall at a T-junction was investigated for impinged penetration flow in which the hot mainstream flow impinged on the branch pipe wall and penetrated into the branch pipe on the downstream side. The temperature distributions of the fluid near the branch pipe wall and the pipe inner surface were simultaneously measured using thermocouples. The fluid temperature close to the branch pipe wall fluctuated as a result of the hot mainstream flow penetrating into the branch pipe, and these fluctuations caused temperature fluctuations on the pipe inner surface. However, the temperature fluctuation range on the inner surface did not exceed 40% of the range of the fluid temperature fluctuation, because fluctuations were attenuated due to the heat transfer process from fluid to structure. The power spectrum method was used to the heat transfer coefficient on the pipe inner surface. The evaluated heat transfer coefficient was 1.62 times larger than that calculated using the Dittus-Boelter equation. The temperature fluctuation range on the inner surface was successfully estimated by leveraging the derived heat transfer coefficient and the measured fluid temperature.

Numerical study of turbulent flows around a cubic obstacle blown from a variable geometry jets diffuser

This research focuses on utilizing numerical simulation to analyse how modifications in the air jet diffuser shape impact the cooling efficiency of electronic parts. The main aim of this study is to understand the physical and thermal mechanisms involved in the process. The study consists in numerically predicting the physical and thermal field of a cubic-shaped obstacle placed in the centre of a square subjected to a resulting flow field created by a transverse flow and a perpendicularly oriented impacting jet. The computations were done at a Reynolds number of 3 410, analyzing three perpendicular impinging jets with the ratio of impinging and cross flow Reynolds numbers α = Rej /ReH having the values of 0.5, 1, and 1.5. The k–ω SST turbulence model was used in this investigation. The effectiveness of the methodology that was put into action was evaluated by referring to the findings derived from the experiments conducted by Masip and his team. Once the methodology was validated, we studied the effect of changing the geometric shapes of the impinging jet diffusers on the cooling efficiency. Three geometrical shapes of air diffusers were tested (circular, swirling and lobed). We noted that there is a direct correlation between flow morphology and cooling efficiency. The impact of the ratio α was analysed and found to have a significant effect on the cooling efficiency. As this ratio increases, the quantity of heat transfer increases in all three air jet diffuser cases. In relation to the lobed air jet, it was observed that the Nusselt number exhibits greater values when compared to the other air jet diffuser cases. Moreover, it was noted that the lobed air jet diffuser can enhance the heat transfer efficiency for α = 1.5 by more than 16.3 % compared to the circular air jet diffuser.

Sınırlandırılmış Çarpan İkiz Jetlerin Akış Yapısı ve Isı Transferi Karakteristikleri Üzerine Bir İnceleme

An investigation into the heat transfer performance and flow characteristics of circular twin jets was conducted using both numerical and experimental methods. The jets emanated from the lower surface and were confined to impinge perpendicularly on the upper surface. To simulate the bidimensional impinging twin jet flow field with powerful turbulence and heat transfer, Realizable k-ε and Standard k-ω turbulence models were employed. Reynolds numbers ranging from 30000 to 50000 were used in both simulations and experiments, with jet-to-jet spacing ranging from 0.5 to 2 and nozzles to target impingement plate spacing in the same range. Sub-atmospheric pressure regions were detected on the impingement surface in experimental measurements for spacing up to 1, whereas numerical results suggested that they could be found at all nozzle to target impingement plate spacings studied. A correlation was discovered between the peaks in coefficients of heat transfer on the target surface and the sub-atmospheric regions in pressure distributions.

SPH-FEM Analysis of Effect of Flow Impingement of Ultrasonic Honing Cavitation Microjet on Titanium-Tantalum Alloy Surface.

To investigate the machining effect of ultrasonic honing microjets on a titanium-tantalum alloy surface, a cavitation microjet flow impingement model was established using the smoothed particle hydrodynamics-finite element method (SPH-FEM) coupling method including the effects of wall elastic-plastic deformation, the ultrasonic field and the honing pressure field. Simulation analysis was conducted on a single impact with different initial speeds and a continuous impact at a constant initial speed. The results showed that the initial speed of the microjet needed to reach at least 580 to 610 m/s in order to obtain an obvious effect of the single impact. The single impact had almost no effect at low speeds. However, when the microjet continuously impacted the same position, obvious pits were produced via a cumulative effect. These pits were similar to that obtained by the single impact, and they had the maximum depth at the edge rather than the center. With the increase in the microjet's initial speed, the total number of shocks required to reach the same depth gradually decreases. When the number of impacts is large, with the increase in the number of impacts, the growth rate of the maximum pit depth gradually slows down, and even shows no growth or negative growth at some times. Using the continuous impacts of the microjet by prolonging the processing time can enhance titanium-tantalum alloy machining with ultrasonic honing for material removal.

Numerical simulation of thermal mixing of liquid sodium in a Y-junction

Numerical simulations are conducted to investigate the flow and heat transfer behaviour of liquid sodium in a Y-junction. Three thermal hydraulic parameters – the fluid momentum ratio, the temperature difference between the inlets of the two branches, and the inlet Reynolds number – are studied to understand their impact on flow development, mixing characteristics, and heat transfer in the main pipe. Results indicate that the momentum ratio is a key factor affecting flow development and temperature distribution. When the momentum ratio lies outside a certain range, flow impingement on the opposite wall occurs. If the ratio falls within this range, two jet flows merge into a turn/deflecting slab-shaped jet, accompanied by two pairs of counter-rotating vortices in the main pipe. The Reynolds number has a negligible impact on flow and thermal mixing patterns for the turbulent flows considered. However, lower Reynolds numbers result in stronger velocity variation and weaker temperature differences at the cross-section in the mixing area. Lastly, the inlet temperature difference has a very limited impact on flow and thermal mixing, and changes in physical property variables negligibly affect velocity and temperature distributions in flow and thermal mixing.

Effect of Build Orientation and Heat Treatment on Erosion Behavior of Al10SiMg Fabricated Using Laser Powder Bed Fusion

Laser Powder Bed Fusion (LPBF) is a form of additive manufacturing that can directly make metal components and has begun to be employed in a variety of industrial situations. Moreover, the erosion characteristics of SLM-produced items have been documented relatively infrequently. In the current work, the erosion behavior of a heat-treated Al-Si-10% Mg alloy generated via laser powder bed fusion (LPBF) has been investigated. The samples were printed in three directions (horizontal (0o), vertical (90o), and inclined (45o) on the build platform using the Direct Metal Laser Sintering (DMLS) system (EOS M 290) machine. Al%Si-10%Mg samples were given treatment (T5, T6). Optical and scanning electron microscopes (SEM) were used to examine the microstructure under both printed and heat-treated conditions. Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) were used to investigate the alloy's elemental composition and compound production. The erosion test was executed in each of the three orientations, plus heat-treated samples and as-printed samples. The selected input parameters were impingement angle and flow velocity. The erosion rate was examined and contrasted across orientations for printed and heat-treated conditions to establish the best orientation. The post-eroded surface was analysed using SEM, EDS, and XRD to evaluate possible erosion processes. The key findings were: T5 heat-treated samples sustain well and have the least mass loss; Horizontal (0°) oriented samples undergo less erosion compared to other orientations; T6 heat-treated samples show maximum erosion on printed samples, and hence it is not recommended. T5 heat treatment is proven to be superior in providing high erosion resistance, followed by printed and T6 heat-treatment, respectively. The heat treatment effects are more significant than the orientation effects in determining the erosion resistance of the Al10SiMg alloy. The erosion mechanisms, such as ploughing, melting, and cratering, were identified.

Flow and thermal fields modeling in jet impingement configurations using a Reynolds stress turbulence closure within the RANS and Sensitized-RANS framework

The flow and thermal field characteristics underlying turbulent convective heat transfer in two reference configurations, representing a jet exiting a planar slot and a circular tube and impinging on a heated wall, are studied computationally. The performance of a conventional Reynolds Stress Model (RSM) and its scale-resolving variant, employed in the context of a time-accurate Reynolds-Averaged Navier–Stokes (RANS) simulation, is investigated in conjunction with different modeling approaches for the turbulent heat flux uj′θ′¯, whose gradient represents the turbulence-induced heat source in the temperature transport equation. In this so-called Sensitized-RANS modeling strategy, employing an improved eddy-resolving instability-sensitive Reynolds Stress Model (IISRSM, Jakirlić and Maduta (2015)), the unresolved ‘sub-scale’ turbulent correlations represent the solutions of a set of corresponding transport equations for the entire stress tensor ui′uj′¯. The turbulent scalar-flux models follow from the classical gradient-diffusion approach with the diffusion-like coefficients formulated as a function of both the scalar eddy-viscosity and the anisotropic Reynolds stress tensor. The latter model expressions also take into account various non-linear relationships in terms of turbulent stress components. The computational results of the fully-developed channel flow, featured only by the mean shear and constant wall heat flux, representing a preliminary investigated flow configuration, agree very well with the corresponding DNS (Direct Numerical Simulation) database for both RSM versions in conjunction with all the turbulent heat flux models adopted. However, the study of the two flow impingement cases with the different structure of the impinging jet and nozzle-to-wall distance variations, characterized by a much more complex flow straining, using the considered conventional Reynolds stress model shows significantly different results for the Reynolds stress and thermal fields compared to the available DNS data, especially in the immediate impact region manifested by a significant overprediction of the turbulence intensity. In contrast, the IISRSM-relevant results for the mean flow and temperature fields, the associated integral characteristics, and the corresponding turbulence quantities are in very good agreement with the reference DNS data, due to the much higher predictive capabilities of this eddy-resolving turbulence model compared to its baseline counterpart.

Clocking effects on aerodynamics of a gas turbine low pressure axial turbine, part 2: Unsteady simulation

Unsteady, viscous compressible flow is numerically simulated within first two stages of a low pressure turbine of a gas turbine engine under different clocking of its stator blades rows. Zonal DES turbulence modeling has been used for flow simulations. Time-dependent wake flow trajectory is modeled and visualized within the blades passages. All characteristics of the passage wake flow, including its V-shape motion, re-orientation, elongation, expansion, and stretching, are properly simulated. Unsteady flow field results together with those of general aerodynamic performance of the optimum and worst clocking cases are presented and compared with each other. Analyses of the flow simulation results showed that the optimum clocking of the blades rows occurs while the upstream wake flow impinges on the downstream stator blade leading edge region. Final results demonstrated that under the optimum clocking of the second stator, the inlet total pressure of the second rotor blades row and its output power increase by 0.1% and 0.336%, respectively. Consequently, efficiency of the second stage increases by 0.35%. In addition, total output power of the whole two stages increases by 0.237%.

Experimental study on the temperature distribution of impingement flow in a double slope roof generated by jet fire

The double slope roof is a commonly used building structure due to its adequate drainage and stable structural features. The thermal impinging on the double slope roof generated by the jet fire caused many casualties and property damage, whose thermal hazard has not been quantified in literatures. The temperature distribution at different source-ridge heights (H), heat release rates (HRR), and roof angle (θ) was measured in the experiment, with a total of 125 sets of experimental conditions. Results show that the ceiling maximum temperature rise increases with the roof angle decreases. A maximum temperature rise prediction model was established based on the analysis of the free plume length beneath the roof, which considering the effect of θ. Moreover, the model's accuracy was validated by comparing previous experimental data. The mean deviation between the proposed model and the experimental results is less than 5.7 %. The rate of temperature decay beneath the ridge decreased as the θ decreased. When the H is smaller, and the HRR is larger, the influence of the θ on the temperature decay profile is more significant. A new correlation has been proposed to correlate the temperature decay profile of a double slope roof with different θ by introducing flame extension length to modify the characteristic radius b. The model prediction agreed well with the experimental data, with a maximum error of around 20 %. New findings are helpful in guiding the building fire prevention and detection design.

An RSM approach to optimize the thermal performance of novel type vortex generators

In this study, a comparative analysis of crimp degree (α), attack angle (β), and pitch (Pt) optimization variables is performed for the novel type vortex generator (NTVG) structure. The central composite design is used to the computational fluid dynamics (CFD) technique for thermal performance optimization.The results of response surface analysis indicate that small β, proper α, and Pt contribute to the improvement of the heat transfer performance, while the flow loss decreases as they increase. According to the RSM approach combined with the particle swarm algorithm, the heat transfer performance of NTVGs is enhanced by 19%, the friction factor is improved by 30%, and the thermal performance factor is increased to 109.1%. The CFD analysis shows that the strength of the inner vortex around the optimized NTVG structure is weakened by the reduced flow impingement, resulting in a considerable reduction in the friction factor with the maintenance of heat transfer.