Jet Impingement Configuration Research Articles

Thermal and hydraulic performance of 3D printed jet impingement configuration for SiC power modules in aerospace propulsion inverters

Efficiently controlling the temperature of power electronic inverters is crucial for aerospace applications with high power density. Effective thermal management strategies could enhance the power output of power inverters to their maximum rated capacity. Jet impingement is a promising technology with outstanding heat transfer characteristics, making it a sophisticated aid for cooling innovation in power inverters. A numerical comparative study was conducted between jet impingement and traditional pin finned heat sink. In our case study, the pin fin could not effectively regulate the junction temperature to a level below 150 °C. By using a 3D printed jet impingement housing composed of polymer, the weight of the power module thermal management system could be decreased by 71 % compared to a metallic pin finned heat sink. Moreover, this approach aids in lowering the junction temperature to 129 °C, under the same boundary conditions. Additionally, a numerical study was conducted on power modules with thermal imbalance used in aerospace inverters. The study proposed various configurations of jet impingement to reduce temperature differences between power module switches, as well as minimize the pressure drop across the jet impingement housing. The primary objective is to minimize the temperature disparity between the unbalanced switches in order to improve the overall reliability and lifespan of the power module, while decreasing the pressure drop. Among all the options, the optimum design achieves a minimal temperature difference of 24 °C between the power module switches, while the pressure drop reaches 12 kPa.

The effects of outlet size, shape, and location on spatial temperature distribution reduction via direct jet impinging in power electronics

Power electronics are used daily in a wide range of applications. Overheating is a major problem associated with power electronic chips. Direct jet impingement on silicon-based chips has emerged as a promising technique for power electronics cooling. However, this technique results in variations in spatial temperature between the center of the jet and the jet outlet location. This paper introduces a jet outlet design that facilitates a more uniform spatial temperature distribution. To this end, a numerical model is developed for a jet impingement configuration featuring four top circular outlets and experimentally validated using a thermal test chip with a jet Reynolds number of 7000. The resultant data show that reducing the outlet size by 30% results in a 31% decrease in the spatial temperature variance between the stagnation point and hot spot locations.Five outlet configurations are also investigated: eight circular top outlets; eight circular side outlets; four square top outlets; eight square top outlets; and eight square side outlets. Analysis of the temperature profiles of these designs reveals that the configuration featuring eight top circular outlets produces the highest spatial temperature difference, while the configuration featuring four top circular outlets (1.4 mm diameter) produces the smallest difference. Specifically, the four top circular outlet design enables a 42 % reduction in the spatial temperature difference, with a 3.8 % decrease in the pressure drop between the main inlet and outlet. Additionally, the findings show that this configuration produces a higher increase in the hot spot heat-transfer coefficient compared to the eight top circular outlet configuration (from 18,500 to 22,300 w/m2 °C, an increase of 20.5 %), thus flattening the spatial heat-transfer coefficient radially.

Intensification of atmospheric freeze drying for thin food slices with impinging jet

Atmospheric freeze drying obviates the complexities and costs of maintaining a high vacuum for freeze drying. One of the main drawbacks of atmospheric freeze drying is the low sublimation rate, which is restricted by drying temperatures possible at ambient pressure to prevent food products from softening during dehydration. There is a strong need to intensify the process to reduce the total drying time. This study evaluated the feasibility of using impinging jets to enhance the mass transfer characteristics of the atmospheric freeze drying process. Atmospheric freeze drying experiments on thin lamb slices between −3 to −7 °C showed that the impinging jet configuration has a significant effect in improving the rate of mass transfer flux compared to the conventional cross-flow configuration. This was consistent across different thicknesses of the lamb slices, which would have presented different degrees of internal resistance to mass transfer. Given the amount of non-frozen water in the lamb slices at the drying temperature range evaluated, a scheduled switch from cold air atmospheric freeze drying to a mild hot air drying condition was explored. This strategy enhanced the removal of the remaining water content in the lamb slices, mainly the non-frozen water.

Numerical analysis of jet impingement cooling with elongated nozzle holes on a curved surface roughened with V-shaped ribs

Numerical studies were performed in this study to analyze the effect of dimensionless elongated injection holes (G/d = 0.5, 2.0, 4.0, 8.0) on heat and flow characteristics on rib-roughened surfaces with an array of inclined impingement jets. Experimental and numerical data from existing literature were used to validate the numerical solution procedure’s heat transfer and flow characteristics in the semicircular test section. The turbulence equations were solved using the SST k-ω turbulence model by varying the Reynolds number from 5000 to 25,000. The curvature effect and staggered array pattern created an additional stagnation region between adjacent impinging jets on smooth surfaces, leading to a low heat transfer zone. Rectangular cross-sectional V-shaped ribs (VSR) were placed in regions where the stagnation point occurs to eliminate this disadvantage. The effect of different normalized rib heights (Hr/d = 0, 0.2, 0.3, 0.4) and rib angles (α = 30°, 45°, 60°, 90°) on the curved surface was also investigated to increase convective heat transfer performance and achieve more homogenous heat transfer distribution by relatively reducing the effect of thermal stresses. Flow properties, area-averaged and local Nusselt number variations on smooth and ribbed surfaces, and the thermal performance criterion (TPC) were investigated in detail. The results indicated an increase in overall heat transfer and a more evenly distributed measurement region compared to the conventional (unextended jet and smooth surface) jet impingement configuration. The most significant heat transfer enhancement from combining the elongated jets with VSR was 47.23% at Hr/d = 0.2 by reducing the G/d to 0.5. In addition, the highest TPC was determined as 1.07 on the proposed model with G/d = 2.0 and Hr/d = 0.2 at Re = 25,000.

Comprehensive analysis of heat transfer and pressure drop in square multiple impingement jets employing innovative hybrid nanofluids

This study presents a comprehensive analysis of heat transfer and pressure drop characteristics in square multiple impingement jets utilising a novel class of hybrid nanofluids. This study goes beyond the usual vertical impingement method by looking at the use of oblique impingement in a multiple impinging jet configuration with a hybrid nanofluid. Al2O3–Cu/water with different volume fractions (φhnf) such as 0.1%, 0.33%, 0.75%, and 1.0% are employed as a working fluid. The purpose of the study is to clarify the impact of the jet angle (β), the jet Reynolds number (Re), extended jet height (Ej), and different volume fraction (φhnf) on the heat transfer behaviours of the curved target surface. The jet Reynolds number varies from 8000 to 24,000, and five different jet angles (β = 15 °, 30°, 45°, 60°, 90 °) and three extended jet heights (Ej = 0.2H, 0.4H, and 0.6H) are adopted. Outcomes disclosed that the highest values of Re and φhnf greatly led to an increase in heat transfer rate and pressure drop of the system. It is uncovered that the heat transfer rate of binary hybrid nanofluids enhances with increasing volume fraction from for all jet angles and Re. Results also exposed that the angle of jet, which is 45°, gives a higher Nusselt number compared to other angles proposed in this study, and the maximum boost reaches 35%. Besides, despite the fact that reducing the height of the extended jet yields enhanced heat transfer rates in comparison to other methods, it concurrently results in an elevation in pressure drop. Finally, this research yielding insights that can be applied to improve the efficiency of heat transfer systems in practical applications.

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.

Numerical heat transfer investigation of a single impingement jet with combined V-ribs and preliminary optimization of V-rib configuration in initial crossflow

The heat transfer performance of a single jet impingement configuration with V-ribs on the impingement plate and V-ribs on the target plate was numerically investigated for turbomachinery applications. The geometrical parameters of the impingement wall ribs were kept constant. The effects of the height and the rib-to-jet distance of the target wall ribs were analyzed in detail. The studied jet Reynolds number was Re = 35,000, the separation distance was H/D = 3, and the velocity of jet-to-crossflow ratio was R = 1–5. Based on the numerical results, taking the heat flow rate as the objective, the rib geometry was optimized by the Differential Evolution (DE) algorithm. The optimized rib configuration was then numerically simulated to validate whether the DE algorithm can be utilized for similar applications. The results of the optimized configuration were compared to those of the sample cases to verify the effectiveness of the optimization. The pressure differences between the inlet of the main jet and the outlet, and between the inlet of the initial crossflow and the outlet were computed to evaluate the pressure loss introduced by the ribs. Thus, this work provides another perspective on designing high efficient anti-crossflow cooling configurations for turbomachinery applications.

Experimental and machine learning studies of thermal impinging flow under ceiling induced by hydrogen-blended methane jet fire: Temperature distribution and flame extension characteristics

Hydrogen-blended natural gas has a winged used in such thermal and combustible systems, the ceiling thermal characteristics have important parameters of impinging jet configurations. In this paper, a series of experiments were carried out to study the ceiling flame extension characteristics and temperature profile in a thermal impinging flow of methane jet diffusion flame with hydrogen addition. It was found that: for the given volume flow rate of methane fuel, the ceiling temperature increases although ceiling flame extension area (length) decreases, with the volume flow of hydrogen mixing ratios increasing. Then, a physical and mathematical model, combined influence of flame shape hypothesis based on physically the unburnt fuel, hydrogen addition, heat release rate, the effects of source-ceiling height and nozzle diameter is estimated to predict the flame extension area of methane jet diffusion flames with different hydrogen mixing ratios. Moreover, the fire plume temperature between buoyant plume and intermittent flame increased with the increase in hydrogen addition due to the fire plume temperature increased. The temperature profile models of the ceiling jet are proposed for methane diffusion flames with different hydrogen mixing ratios. Furthermore, to perform the calculation quickly and facilitate rapid thermal engineering design, this paper used machine learning methods (GABP neural network) to derive the volume flow rate of gas leak from the temperature data of the ceiling detected by thermocouples. The results show that the AI prediction for the heat release rates has a good correlation with the verification experimental value. The results of this paper may demonstrate the potential for industrial combustion systems and fire safety engineering applications of methane jet diffusion flame with hydrogen addition.

A direct numerical simulation study for confined non-isothermal jet impingement at moderate nozzle-to-plate distances: Capturing jet-to-ambient density effects

A direct numerical simulation (DNS) campaign is deployed for a series of confined downward oriented, non-isothermal turbulent impinging jet configurations. A baseline Reynolds number of 9960 is obtained through a precursor DNS pipe flow simulation (Reτ=505). Three jet temperature configurations (confinement height to nozzle diameter of three) enter a cylindrical domain that share ambient and impingement plate temperatures (298.15K). The range of jet temperatures are crafted such that the ratio of inlet to ambient density varies from unity to 0.52, showcasing the effect of density disparity on flow characteristics such as core collapse, radial mixing of momentum and energy, near-wall stagnation behavior, wall-jet profiles, and large-scale vortical structures. Surface quantities provided include mean radial heat flux and wall-shear stress profiles, and heat flux histograms at select radial stations. Results showcase increased radial normal stresses for higher temperature jets that support increased mixing, resulting in large-scale recirculation structures that are smaller, while retaining similar normalized radial wall profiles for shear stress, heat flux and pressure. Radial plots for wall shear stress and Nusselt number showcase strong radial decay as compared to previous configurations that share similar jet and ambient temperatures. For the 373.15 K case, a Gaussian-like histogram for heat fluxes at the impingement plate transitions to a log-normal profile as radial distances increase. In contrast, the 573.15 K configuration displays a bi-modal heat flux characteristic at the impingement plate, and in similar manner to the moderate temperature counterpart, transitions to a log-normal profile at larger radial distances.

Measurement of alumina film induced ablation of internal insulator in solid rocket environment

This study investigates the ablation of internal insulation induced by the deposition of an alumina film with different lateral film speeds. A sub-scale test solid rocket motor (SRM) was designed in an impinging jet configuration to form an alumina film on the sample and to encourage the lateral movement of the film by a high-speed wall jet. Fifteen static fire tests of the test SRM were conducted with six different jet velocities (Vjet = 100 m/s, 150 m/s, 200 m/s, 268 m/s, 330 m/s, and 450 m/s) that indirectly affected the velocity of the wall jet and the deposition rate of alumina droplets. The ablation velocity was deduced from the difference in the sample thickness after a test using a coordinate measuring machine. The droplet deposition mass flux and wall jet velocity were obtained via two-phase flow simulation with the same jet velocity and effective pressure. As a result, the characteristics of alumina-induced ablation and the changes in ablation with jet velocities were obtained. The area within 0.8 × jet diameter was focused upon, where the ratio of ablation velocity to incoming alumina mass was constant for each jet velocity, and showed a similarity in jet structure. When the ablation velocity was increased from 2.05 to 9.98 mm/s with increasing jet velocity, the ratio of the ablation velocity and alumina mass flux decreased from 1.07×10−4 to 0.49×10−4 m3/kg as Al2O3–C reactions became less efficient with a reduced residence time of the film. Because the decrease in residence time by the wall jet is more pronounced for slow reactions involved in Al2O3–C reactions, fast reactions in Al2O3–C reactions are less affected and result in a convergence of the volumetric rate of ablation per unit mass of alumina.

Impingement Atomization of Carbopol Gels

Experiments were performed on the atomization of Carbopol–water gels of compositions in the range 0.10–0.30 gellant wt.% on a like-on-like jet impingement configuration. Analysis indicates two primary modes of sheet breakup: perforation driven and wave driven. While perforation-driven breakup is not seen for the lowest gellant concentration case, 0.10 wt.%, the wave-driven breakup is present in all cases within the range of investigated jet velocities. Based on the observations, a mechanism involving feedback between the impingement point and the sheet vertex, and a separation front initiated in between the impingement point and the sheet vertex are proposed for the wave-driven breakup.

Experimental and Numerical Investigations of Extended Jet Effects on Multiple-Jet Impingement Heat Transfer Characteristics

Abstract A jet impingement system equipped with extended jet holes was experimentally evaluated in this study. To reproduce a strong crossflow condition, the jet impingement configuration featured a multiple-jet array of 16 × 5 rows in the streamwise and pitchwise directions, respectively, and the crossflow was exhausted from one open side of the impingement channel only. The transient thermochromic liquid crystal (TLC) technique was used to measure heat transfer on the target surface for different extended lengths of 1.0–2.5 jet hole diameters with Reynolds numbers from 1.0 × 104 to 3.0 × 104. To provide complementary flow physics for elaborating the heat transfer patterns observed from the experiments, well-validated numerical simulations were carried out. Comparisons with a baseline jet hole configuration showed that the extended jet holes helped to significantly improve the heat transfer levels as well as to generate a more uniform distribution pattern by suppressing the crossflow. Despite an aerodynamic penalty, the extended jet holes provided much higher heat transfer levels at equal pumping power consumption. The flow fields obtained by the numerical simulations revealed that the jets issued from the extended jet holes were straighter and had less mixing with the crossflow, resulting in a higher jet momentum impinging onto the target. Most importantly, it was found that the dominated flow mechanism of the extended jet holes was to prevent the jets from being redirected by the crossflow in strong crossflow conditions, rather than reducing the jet-to-target distance.

Spectral proper orthogonal decomposition of coupled hydrodynamic and acoustic fields: Application to impinging jet configurations (draft)

A large variety of flow configurations involve an aero-acoustic coupling and the complexity of the interaction can make the analysis difficult. In the purpose to decompose these flows in simpler phenomena, the spectral proper orthogonal decomposition (SPOD) algorithm can be used. Compared to POD, this method adds up a discrete Fourier transform before the singular value decomposition. A theoretical framework for the weighting of the flow field variables used in the SPOD has been proposed to identify hydrodynamic modes by Schmidt et al. However, in flow configurations where both the hydrodynamic and acoustic fields interact, without additional data processing, only hydrodynamic modes that are generally dominants would be identified by the SPOD. This paper proposes a general framework based on a few pre-processing steps applied to the input aero-acoustic fields to facilitate the identification of both the hydrodynamic and acoustic fields by the SPOD, the purpose being to analyze how the hydrodynamic waves lead to the acoustic ones and the possible feedback process. This framework is applied to large-eddy simulations of supersonic ideally expanded impinging planar jet of infinite extent characterized by different nozzle-to-plate distances and incidence angles of the flat plate. The analysis shows that the SPOD algorithm is able to capture two important aero-acoustic feedback loops of impinging jet configurations: the one closed by sound waves traveling upstream (related to the jet impingement on the flat plate) and the one characterized by neutral wave modes of the jet shear layer itself. The analysis has also shown that the symmetrical/anti-symmetrical acoustic patterns radiated with respect to the jet main axis and due to the jet impingement are mainly correlated to the axial velocity fluctuations structures in the jet for the normally impinging cases. For the cases with flat plate incidence, the acoustic radiated is mainly correlated to vertical velocity fluctuations structures developing in the upper shear layer of the jet.

Research on the flow field and film cooling effectiveness of the endwall with swirling film cooling

In order to obtain the effect of endwall secondary flow on the swirling film cooling, a geometric model of cascade is established to research the endwall swirling film cooling and swirling flow induced by prismatic jet impingement configurations. Numerical simulation is applied to investigate three jet flow configurations on the endwall film cooling performance at the compound angles of film hole γ = 0° and 30° and blowing ratios M = 0.5–2.0. The influence of complex vortex structures near endwall for jet flow is analyzed in detail; the strong transverse cross flow near the endwall is the main reason affecting the film cooling effectiveness. The variation laws of endwall film cooling effectiveness with the compound angle of film hole, jet flow configuration, and the blowing ratio are obtained. As the blowing ratio increases, the spanwise average film cooling effectiveness increases first and then decreases. While the blowing ratio is M =1.0, the endwall film cooling effectiveness is the best. Increasing the compound angle of the film hole leads to a decrease in the endwall cooling effectiveness. The spanwise average cooling effectiveness of γ = 30° decreases by 35% compared to the γ = 0°.

A robust physics-based method to filter coherent wavepackets from high-speed schlieren images

A complete understanding of jet dynamics is greatly enabled by the accurate separation of acoustically efficient wavepackets from their higher-energy convecting turbulent counterparts. Momentum potential theory (MPT) has proven highly effective in filtering the desired acoustic component irrespective of operating conditions or nozzle complexity. However, MPT is a data-intensive method predicated on the knowledge of fluctuation quantities in the entire flow field; as such, it has to date been applied only to numerically obtained data. This work develops an approach to extend its application to extract coherent wavepacket data from high-speed schlieren images. Pixel intensities from the schlieren image are mapped to a scaled surrogate for the line-of-sight integrated density gradient. The linear relation between the irrotational scalar MPT potential and time derivatives of density fluctuations is then exploited to perform the filtering. The effectiveness of the procedure is demonstrated using experimental as well as numerical schlieren images representing a wide range of imperfectly expanded free and impinging jet configurations. When combined with spectral proper orthogonal decomposition (SPOD), the method yields modes that accurately capture (i) the Mach wave radiation from a military-style jet, (ii) the mode shapes of the feedback tones in an impinging jet and (iii) the screech signature in twin rectangular jets, without recourse to user adjusted parameters. This technique can greatly enhance the use of high-speed diagnostics for real-time monitoring of the near-field acoustic content for potential feedback control. Additionally, the general nature of the approach allows a straightforward application to other flows, such as cavity and airfoil flow-acoustic interactions.

Experimental investigation of the cryogenic [formula omitted]-cooling performance for an impinging jet configuration for different inflow conditions

This work focuses on the cryogenic impingement cooling performance depending on the given inflow conditions. An experimental setup is presented which allows a detailed observation of the time dependent temperature distribution in a solid body via a thermal imaging camera during the cooling process. The space- and time-dependent heat transport from the solid to the impinging fluid is calculated from the gained temperature data using the so-called inverse global integration method (IGIM). This method allows an accurate determination of the heat flux density without the necessity of a precise calculation of the sensible temperature gradients. An investigation of the LN2-impingement conditions such as mass flow rates and fluid density is conducted to bridge the gap between the inflow conditions and the resulting heat fluxes. Moreover, a variation of the working pressure and the nozzle geometry is also performed to assess the main dependencies of the cooling performance. Thus, some important conclusions regarding the heat flux and its governing physical phenomena can be drawn. In order to simplify the application of the gathered information, the corresponding heat transfer coefficients are calculated as a function of the time dependent surface temperature and the set inflow conditions. It is shown that the usage of a constant value results in a high inaccuracy because the heat flux density and the heat transfer coefficient show no trivial correlation for arbitrary impingement conditions.

A novel strategy to enhance the cooling effectiveness in a confined porous jet impingement using non-Newtonian fluids

Jet impingement configuration is commonly employed in the thermal management of the gas turbine engines, rocket launchers, and high-density electrical equipment cooling to remove a large amount of heat. Owing to such overwhelming applications, significant research efforts have been devoted to enhance the cooling performance of jet impingement configuration. In this work, the influence of shear-thinning behaviour in overall heat transfer characteristics have been investigated as a function of pertinent dimensionless numbers as 10−6 ≤ Da ≤ 10−2 (Darcy number), 1 ≤ Pe ≤ 102 (Péclet number), 0 ≤ Ri ≤ 4 (Richardson number), 0.4 ≤ n ≤ 1 (shear-thinning index). The numerical results on local thermal non-equilibrium (LTNE) parameter, streamlines and isotherms, local and average Nusselt number are reported for a range of power-law index, Péclet number and Richardson number. All in all, shear-thinning fluids offer 50–60% enhancement in Nusselt number with reference to the Newtonian fluid at a higher Darcy number. A correlation for average Nusselt number is put forward to enable its use for process design calculations.

Application of Taguchi Method for the Analysis of a Multiple Air Jet Impingement System with and without Target Plate Motion

Multiple air jet impingement is a complex heat transfer process involving several parameters which interfere with the cooling and heating performance. Due to its wide applicability in engineering applications, the influence of these parameters in jet impingement efficiency has been extensively analyzed by several researchers. However, the majority of the studies developed experiments and numerical simulations to determine the effect of each parameter individually. Considering that the heat transfer performance depends on the correct combination of the multiple jet impingement parameters, the implementation of a Design of Experiments (DoE) seems to be more effective since this method allows a comparative parameter analysis as well as an optimization of the experiments and numerical simulations. In this study, a DoE based on the Taguchi method and an Analysis of Variance (ANOVA) is applied to conduct an experimental study on multiple air jets impinging a surface. This work focuses on jet-to-jet spacing (S), nozzle-to-plate distance (H), Reynolds number, and target plate geometry. Taking into consideration these four parameters, the experiments are conducted with the target plate static and in motion and the results obtained in both cases are compared. Even if the static target plate is the most studied case, the surface motion is identified in several engineering applications, such as reflow soldering and drying processes. The results show that the heat transfer of a multiple jet impingement configuration is enhanced by S=3D, H=2D for a staggered configuration, and a higher Reynolds number for both static and moving plates.

Experimental Characterization of Supersonic Single- and Dual-Impinging Jets

Supersonic single-impinging jet flows are known to exhibit strong unsteadiness levels and acoustic signatures due to large-scale resonant motions. However, the flowfield characteristics of two such jets operating in tandem, where properties are influenced by jet–jet interaction and coupling, are relatively unknown. The present study reports on the experimental investigation of the flowfield associated with two underexpanded, impinging jets operating at a nozzle pressure ratio of 2.65 and discharged from identical converging nozzles with exit diameters of 25.4 mm. Comparisons with a single-impinging jet operating at the same conditions are provided through flow visualizations, near-field acoustics, and surface pressure measurements. Fountain flow produced by the interaction of wall jets (a unique feature of dual-impinging jets) is found to be relatively strong at short impingement heights and contributes to additional loads on the ground surface. Unsteadiness in dual jets is weaker than that in a single jet at conditions involving resonance, and the fountain upwash plays an important role in the process. Although the feedback mechanism that drives the resonance in both impinging jet configurations is similar and the corresponding instability mode shapes are retained, there are differences in the strengths of the instability modes between the two configurations.

Optimal design to control rotor shaft vibrations and thermal management on a supercritical CO2 microturbine

Supercritical carbon dioxide (SCO2) Brayton cycle microturbine can be used for the next generation of solar power. In order to comprehensively optimize the supporting system and cooling device parameters of Brayton cycle shafting, the concept of chaos interval is introduced by chaotic mapping, and the CIMPSO algorithm is proposed to optimize the multi-objective rotor system model with nonlinear variables.The results show that the resonance amplitude of the optimized model is effectively attenuated, and the critical speed point is far away from the working speed, which shows the robustness of the optimization algorithm. Finally, based on arbitrary several sets of optimization solutions and empirical parameters, the finite element model of shafting is established for simulation, and the results show that the optimized solution has certain guiding significance for the design of the rotor system.The cooling device is designed and simulated by CFD method based on the optimal solution set. Both the inlet boundary conditions of given pressure (1 MPα) and given mass flow rate (0.1 kg/s) numerical calculations were carried out to characterize the cooling performance, for different jet impingement configurations (Hr/din = 0.0125 ∼ 5).Several sets of analyses show the strong effects of the jet-to-target spacing (Hr/din) on the rotor thermal performance at a given diameter (din) of the nozzle. Average temperature (Tc) at the free end of the rotor show that, as jet-to-target distance decreases (0.0125 ≤ Hr/din ≤ 0.33), the heat dissipation efficiency of the cooling device with the given pressure boundary condition tends to decrease, while the conclusion is opposite when the inlet boundary condition is set to the given mass flow rate. And there is an interval for the optimal combination (Hr/din) to promote the cooling efficiency.