Jet Impingement Heat Transfer Research Articles

Flow field and heat transfer in the transitional type of turbulent round jet impingement

Jet impingement heat transfer in the transitional type involves the occurrence and disappearance of the secondary maximum heat transfer, which is challenging for numerical simulation. In the paper, the effects of the nozzle-plate spacing H/D on heat transfer and flow fields in the range of 5≤H/D≤7 within the transitional type are investigated. In this type, the secondary maximum heat transfer rate gradually vanishes. In addition, the transitional properties of jet impingement are further discussed. It is found that the heat transfer rate at the stagnation point shows an important relationship with the arriving stream Reynolds number and turbulence intensity. Additionally, three heat transfer modes, i.e., the peak (5<H/D≤5.5), swelling (5.5<H/D≤6.6), and linear modes (6.6<H/D≤7), are identified in the transitional type based on the analysis of the heat transfer rate, development of the intermittency, and the wall shear stress. For the latter two aspects, the laminar zone and the turbulence zone are discussed in detail for different H/D. In the peak mode, heat transfer rate is largely influenced by the transition process, resulting in a secondary peak. While in the swelling mode, the second peak evolves to a swelling and the effect of transition becomes weak. As a result, the influences of the laminar region will extend to downstream. However, in the linear mode, the swelling vanishes with a mild change of intermittency in the boundary layer and the sudden mutation of heat transfer mainly takes place in the stagnation region.

Influence of nozzle profile on submerged pipe jet impingement heat transfer

ABSTRACT Current work focuses on enhancing the performance of solar air heater by employing vertical submerged pipe jets featuring different profiles: circular, square, triangular, and rectangular. The study investigates parameters including jet diameter ratio d / D h ranging from 0.044 to 0.11 and the jet spacing ratio ( Sj / D h ) varying from 0.108 at 0.433 while keeping the streamwise ( X / D h ) and spanwise Y / D h pitch ratio fixed at 0.867. The Reynolds number Re ranges from 3,000 to 18,000. Initial results for all the profiles were obtained through CFD simulations, indicating that the circular pipe jet demonstrates superior heat transfer and thermo-hydraulic performance parameter (THPP). The experimental validation of circular profile pipe jet showed a maximum THPP of 2.69 achieved at a d / D h of 0.066 and Sj / D h of 0.217, with a Reynolds number of 15,000.

Prediction of heat transfer for a single round jet impingement using the GEKO turbulence model

Two-dimensional Reynolds-Averaged Navier–Stokes simulations are performed to study a single, round impinging jet heat transfer problem, utilizing the generalized k-omega (GEKO) turbulence model as a benchmark. The simulations are performed at a jet Reynolds number of 23,300 and a nozzle-to-plate distance of 2.0 where a second peak in surface Nusselt number is observed. The effects of the three primary (Csep, Cmix and Cnw) and three auxiliary (Cbf,l, Cbf,t and Cnw,sub) GEKO calibration parameters are investigated. The results indicate that Cmix has the most significant impact on the laminar-turbulent transition zone. A deep learning based regression model is developed and trained using the simulation outputs for fast predictions of the heat transfer curve. Using Csep=1.1, Cmix=−0.7, Cnw=2.0, Cnw,sub=2.25 and Cbf,t=3.0 along with laminar-to-turbulent transitional modeling values, provides the best agreement with experimental results from previous studies.

Experimental and numerical investigations on jet impingement heat transfer with different nozzles and enhanced surfaces

Hot air anti-icing systems are essential in aviation engines, preventing ice formation on inlet components during high-altitude flights. A critical factor in enhancing these systems' effectiveness is the application of jet impingement technology. In order to further enhance the performance of jet impingement heat transfer in this application scenario, the research assesses the impact of nozzle type and surface protrusion type on the anti-icing system's performance by integrating experimental and numerical approaches. Findings reveal that circular nozzles outperform swirl and satellite nozzles in heat transfer efficiency in the application of aviation anti-icing system with high jet Reynolds number. The average Nusselt number, indicative of heat transfer rate, can be increased by up to 10.54 % with protrusions. For surface protrusions, sequentially-arranged cylinder protrusions are more effective at Reynolds numbers between 8000 and 48000. Cross-arranged circular truncated cone protrusions do better at higher Reynolds numbers, from 48000 to 80000 which is the range relevant to the hot air anti-icing system. Comparing concave and flat plates with the same type of protrusions, the former shows superior heat transfer performance. The study also establishes correlations for convective heat transfer coefficients and attenuation coefficients on the concave plate with sequentially-arranged cylinder protrusions. By applying the cross-arranged circular truncated cone protrusions, which proved to be the best in our experiments, to the engine nacelle's anti-icing section, the numerical results indicate a 6.21 % increase in anti-icing thermal efficiency and an 8.31 % decrease in air bleed capacity, all under the same heat transfer conditions. The use of protrusion structures in the anti-icing system of an engine nacelle boosts thermal efficiency, lowers air bleed capacity, and reduces the fuel penalty, thereby optimizing overall performance.

Numerical analysis of heat transfer and fluid flow structures of jet impingement on a flat plate with different shapes of roughness elements

Jet impingement is a heat transfer technique utilized in different applications in industry. It is used for cooling the turbine blade and the combustor wall because of its high cooling rate. The heat transfer (HT) and flow structure of a circular air-impinging jet over a flat target plate with a circular row of 48 roughness elements are numerically investigated using renormalization group k–ε turbulence model. Different shapes of roughness elements, namely cylindrical roughness elements (CREs), droplet roughness elements (DREs), and hemispherical roughness elements (HREs), are examined. The effects of Reynolds number changed from 7,000 to 35,000, and the roughness elements location relative to jet diameter ( s / d ) of 1.5, 2, 2.5, and 3 at a constant jet-to-target plate distance ( H / d ) of 2 are studied. Temperature distribution, local Nusselt number ( Nu ), average Nusselt number (Nuavg ), average Nusselt number ratio (Nuavg,r ), velocity distribution, turbulence kinetic energy, streamline contours, and static pressure ( p ) over the roughness elements are evaluated and discussed. The results showed that the existence of roughness elements has a significant effect on the thermal and hydraulic characteristics. The Nuavg,r for the CREs, DREs, and HREs is up to 1.61, 2.02, and 1.2, respectively. All roughness element shapes increase the flow velocity in the narrow area between the elements and increase the wetted area. CREs augment the HT rates by increasing the turbulence intensity, while DREs reduce the drag force effects.

A developed transition simulation model for turbulent plane jet impingement heat transfer

Jet impingement has been widely used due to its high heat transfer rate, but numerical simulation of this flow is highly challenging because of complex flow phenomena. In this paper, a turbulence model based on the physics of jet impingement is developed using the shear stress transport (SST) four-equation transition model and the SST with cross-diffusion correction (SSTCD) model. The proposed method is evaluated for plane jet impingement under various nozzle-plate spacings (2.6, 4 and 6) and different Reynolds numbers ranging from 10,200 to 20,000 in terms of heat transfer and flow fields. The results show that the developed model has the ability to capture the second peak of heat transfer and skin friction at low nozzle-plate spacing. Even at high nozzle-plate spacing, this approach also gives accurate trends for the above two features. However, without the cross-diffusion correction, the transition model provides too high energy and low velocity peaks. With the developed model in hand, the effects of pressure gradient on heat transfer and skin friction coefficient are further investigated. It is found that when the adverse pressure gradient becomes strong, the position of the secondary peak of skin friction occurs earlier than that of the secondary peak of heat transfer rate. Conversely, if the adverse pressure gradient disappears, the positions of these features coincide.

Self-similarity of obliquely impinging jet heat flux fields in a range of Mach numbers from 0.1 to 1.0

This study reveals self-similarity (Mach-number-invariance) of the normalized time-averaged surface heat flux fields measured using temperature sensitive paint (TSP) in an obliquely impinging jet in a range of the nozzle exit Mach numbers from 0.1 to 1.0. Further, this self-similarity is also found in the normalized root-mean-square (RMS) fields of the heat flux fluctuation and the normalized skin friction fields extracted from the surface temperature and heat flux fields. To elucidate the origin of this self-similarity, a semi-empirical model is proposed for the maximum surface temperature difference used for normalization, indicating that the Mach number effect on the impinging jet heat transfer is mainly represented by the recovery temperature at the impingement point.

Experimental and numerical study of combined impinging jets at low nozzle-plate distance for confined and unconfined impingement plate

This study investigates the heat transfer and flow dynamics of combined impinging jets at low nozzle-plate distances. The combined jets comprise (i) circular and quadruple helical jets and (ii) circular and double helical jets. A conventional jet is generated from the only circular part of the combined jet. Experiments are conducted with a constant total flow rate (Qt = 85 LPM) and low nozzle-plate distances (H/D = 0.1, 0.2, 0.3, 0.4) for confined and unconfined jet cases. Flow rate ratios (Q*=0.176, 0.235, 0.352, 0.588, 0.823) are used for different swirl intensities. A numerical study was performed using ANSYS Fluent software. The results show that the numerical analysis is consistent with the experimental research. The combined impingement jet shows a potential to improve the low heat transfer occurring at the stagnation zone at low nozzle-plate distance in the swirl jet, leading to non-uniform heat transfer. The confined jet case at the low nozzle-plate distance reduces the heat transfer at the stagnation point and over the entire plate as regards the unconfined jet case. Increasing the H/D distance in the combined impinging jet improves the heat transfer in the confined jet case but reduces it in the unconfined case. Correlations are given for heat transfer prediction depending on H/D and Q* for unconfined and confined cases of the combined jet. Increasing the flow rate ratio and the nozzle-plate distance increases the heat transfer uniformity for the confined and unconfined jet cases. In contrast, the unconfined jet case is better regarding heat transfer uniformity. The confinement plate creates a negative pressure on the impingement plate in all conditions. In contrast, the unconfined jet case is precisely the opposite.

Experimental investigation of metal foam embedded surface thermal characteristics using air-jet impingement with different orifice geometry

Experimental investigation of metal foam embedded surface thermal characteristics using air-jet impingement with different orifice geometry

Sweeping jet impingement heat transfer on concave surface: Influence of trapped vortex ring revealed by generalized k-ω (GEKO) model

Jet impingement-based heat transfer enhancement is widely employed due to the significant mass, momentum, and energy it accompanied. While the steady jet with high energy flux is commonly favored, it still exhibit few defects, including heat transfer deterioration away from the stagnation point and mutual flow interference in the jet arrays, which are particularly critical in demanding applications. Recently, the sweeping jet has been proposed to resolve the above challenges owing to its self-excited and self-sustaining unsteady oscillation. However, current research predominately focuses on impinging on the flat surface, neglecting impinging on the concave surface, such as internal cooling at the gas turbine blade leading-edge as well as the hot gas de-icing at the aircraft wing leading-edge. Understanding the heat transfer characteristics in these scenarios is crucial as they notably differ from those on flat surfaces. Therefore, this paper investigates the heat transfer characteristics and the related essential flow dynamics of a sweeping jet with a Reynolds number of 2577 impinging on a concave surface, and those of a steady jet under the same conditions are compared. The jet-to-wall distance and the curvature of the impingement surface are 4 and 10 times the jet's hydraulic diameter, respectively. To correctly capture the three-dimensional characteristics of the sweeping jet, this research employs the Reynolds stress model for turbulence modeling and the Generalized k-ω (GEKO) model for better calibration of the jet's spreading and dissipation rates, as well as the characteristics of trapped vortex generated due to the confined concave flow channel. The study initially focuses on the average Nusselt number on the impingement surface, revealing a 15.2% increase in heat transfer intensity of the sweeping jet compared to the steady jet, and the sweeping jet covers a larger area with enhanced uniformity. Subsequent analysis on streamlines in two perpendicular planes and Q criterion iso-surfaces in the spatial domain shows the formation of a trapped vortex ring with a constant location around the jet, restricting the effective cooling range to its inner side and thus resulting in a steep temperature rise on the two lateral sides of the vortex ring. The stability of the trapped vortex is enhanced by the curved impingement wall in the oscillation plane, which on the one hand forces the high-temperature fluid that has exchanged heat with the wall to repeatedly wash the same location, and on the other hand significantly reduces the jet oscillation angle compared to a free sweeping jet. Consequently, the heat transfer intensity of the sweeping jet is lower in the main oscillation plane than in the non-oscillation plane. Analysis of the near-wall flow parameters indicates that the sweeping jet generates higher velocity and turbulence kinetic energy in the wall jet zone far away from the stagnation point compared to the steady jet, which results in a higher local heat transfer intensity. The unsteady transverse oscillation of the sweeping jet disturbs the wall jet zone and disrupts the formation of the wall boundary layer, causing a higher velocity gradient at the wall and hence the lower thicknesses of the velocity boundary layer and the thermal boundary layer than those of the steady jet. In conclusion, this study reveals the flow and heat transfer features of the sweeping jet impingement on confined concave surface, with the influence of the trapped vortex ring mattering significantly. The findings provide a reference for designing a more effective jet arrangement. Future work will focus on investigating the effects of jet Reynolds number and impingement distance variations on the trapped vortex ring, and how to mitigate the adverse effects of the trapped vortex ring on impingement heat transfer.

Effect of Strip-Fin Height on Jet Impingement Heat Transfer in a Rectangular Channel at Two Jet-to-Target Surface Spacings

Abstract An experimental investigation of heat transfer performance in a rectangular impingement channel featuring staggered strip-fins was completed. Four configurations were considered to study the effects of varying the strip-fin height (H/d = 1.5 and 2.75) at two jet-to-target surface spacings (z/d = 3 and 6) on the heat transfer, pressure loss, and crossflow magnitude for a long impingement channel with in-line, 4 × 12 impinging jets. Also, the effect of the reference temperature choice, either jet inlet temperature or local bulk temperature, for calculating the local heat transfer coefficients was considered. The regionally averaged heat transfer coefficients were measured at seven Reynolds numbers, based on the jet diameter (10k–70k) utilizing the copper plate experimental method. The empirical correlations were expressed for the area averaged Nusselt number estimation of impingement channels with strip-fin or pin-fin roughness elements. The results showed that the long strip-fin channel with z/d = 3 provided the best thermal performance. The discharge coefficients are similar for all configurations between Rejet = 10k and 50k. The results are compared with the impingement channels with conventional pin-fins. They show that strip-fin channels provide lower pressure drop with marginally better heat transfer coefficients compared to the conventional pin-fin channels. However, when the channel weight is considered, strip-fins would increase the roughness material volume more than the conventional pin-fins.

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.

Experimental investigation of liquid jet impingement heat transfer at superhydrophobic surfaces

This paper presents an experimental exploration of thermal transport due to a water jet impinging on post patterned superhydrophobic surfaces and provides the first experimental data for this problem. The surface is heated to temperatures much lower than the saturation value. Results are obtained for a smooth hydrophobic surface and superhydrophobic surfaces with varying microfeature pitch (w = 8, 16, and 24 µm) and cavity fraction (Fc = 0.56 and 0.85) and are compared to results derived from a previously published analytical model. The jet Reynolds number varied from 1.1 × 104 to 1.7 × 104 and the nominal surface heat flux varied from 2.5 × 104 to 4.9 × 104 W/m2. Results obtained for all superhydrophobic surfaces show a significant decrease in the local and average Nusselt numbers (up to 30 % reduction) compared to impingement on a smooth surface. Further, the reduction is a strong function of the surface post and cavity geometric parameters. The effective temperature jump length is determined for all scenarios considered, representing the first experimental measurements of temperature jump length for heat transfer at superhydrophobic surfaces. Functional relationships showing how the average Nusselt number and the non-dimensional temperature jump length depend on the superhydrophobic surface parameters are also presented.

Heat transfer characteristics of water jet impingement on high-temperature steel plate by angular nozzle

The average jet flow rate, vapor volume fraction inside the nozzle, and the distribution of pressure, shear force, turbulence intensity, temperature, and Nusselt number on the heat transfer surface were simulated when water jet impinged on a 900 °C steel plate by a straight cone nozzle and an angular nozzle under the condition of 0.7 MPa inlet pressure. Results indicate that the strong cavitation effect of the angular nozzle makes its jet impingement heat transfer intensity superior to the straight cone nozzle. Although the average jet flow rate of the angular nozzle is 3.33% lower than that of the straight cone nozzle, its shear force, turbulence intensity and Nusselt number at the stagnation point are 18.43%, 20.43%, and 18.81% higher than those of the straight cone nozzle. Experiments also show that the jet impingement heat transfer intensity of the angular nozzle is better than that of the straight cone nozzle. Its maximum cooling rate at the stagnation point increased by 13.16%, and the time to reach the maximum cooling rate decreased by 60%, compared with the straight cone nozzle. It is hoped that the results can help improve the performance of online cooling equipment for hot-rolled steel.

Machine learning algorithms to study the relative roles of jet convection and natural convection in the presence of surface roughness elements on enhancement of jet impingement heat transfer

Based on data-driven approaches, eight machine-learning algorithms have been proposed to characterize the heat transfer performance of turbulent jet impingement cooling. These algorithms are designed to cool high-heat-flux surfaces modified with single protrusions, multi-protrusions, or V-grooves. To train these algorithms, an experimental dataset was used, which included 184 experiments with Reynolds numbers ranging from 10,000 to 33,000, nozzle exit-to-plate distances ranging from 2 to 10 jet diameters, and two different heat inputs of 60 W and 90 W. Multiple linear regression algorithms in Python programming were used to model jet impingement cooling, showing that the proposed machine learning models provided more accurate predictions with a 20.4 % to 66.5 % relative residual improvement over the existing conventional regression model based on performance metrics like R2 and relative residual. This improvement was leveraged to quantify natural convection's impact on the effectiveness of heat transfer enhancement due to different types and sizes of roughness elements in high-temperature applications. The research found that as the protrusion surface area increased from 0.02 % to 0.188 % compared to the flat plate, the effectiveness of heat transfer enhancement decreased from 2.69 % to 8.37 % at similar operating conditions. These results revealed that natural convection, which opposes forced jet impingement, was strengthened with increased heat input values, roughness element surface area, and the local turbulence generated. The higher natural convection weakened the enhancement afforded by jet impingement on the modified plates relative to the flat plate. Additionally, the capabilities of machine learning algorithms were explored to unlock the full potential of the jet impingement technique and to overcome the associated complexity in jet impingement applications for next-generation electronics cooling. Finally, the Decision Tree Classifier algorithm was applied to classify the performance of the surface roughness element as either effective or ineffective.

Numerical Study and Structural Optimization of Impinging Jet Heat Transfer Performance of Floatation Nozzle

A floatation nozzle can effectively transfer heat and dry without touching the substrate, and serves as a vital component for heat transfer to the substrate. Enhancing the heat transfer performance, and reducing its heat transfer unevenness to the substrate play an important role in improving product quality and reducing thermal stress. In this work, the effects of key structural parameters of the floatation nozzle on the heat transfer mechanism are systematically investigated by means of a numerical simulation of computational fluid dynamics. The findings demonstrate that the secondary vortex structure induced by the floatation nozzle with effusion holes increases heat transfer performance by 254.3% compared with the nozzle without effusion holes. The turbulent kinetic energy and temperature distribution between the jet and the target surface are affected by the jet angle and slit width respectively, which change the heat transfer performance of the float nozzle in different degrees. Furthermore, to improve the comprehensive heat transfer performance of the floatation nozzle structure, taking into account the average heat transfer capability and heat transfer uniformity, the floatation nozzle’s design is optimized by the application of the response surface method. The comprehensive heat transfer performance is increased by 26.48% with the optimized design parameters. Our work is of practical value for the design of floatation nozzles with high heat transfer performance to improve product quality in industrial systems.

Numerical analysis of concentric jet impingement heat transfer to the concave surface of the cone

This work includes numerical investigation of concentric jet impingement heat transfer to the concave surface of 30° apex angle conical surface. Heat transfer results are represented in terms of nondimensional heat transfer coefficient Nusselt number along the slant edge of the cone. Turbulence intensity, static pressure distribution along slant edge of the cone, and velocity contours in the domain are also presented for a detail understanding of the flow behavior. Different geometrical parameters like diameter of pipe (10 mm, 14 mm, and 20 mm) and L/D (4.25, 6.25, and 10.25) are considered for heat transfer analysis. Jet Reynolds number is varied between 25000 and 82000 for each diameter and L/D case. It is observed that Nusselt number value is minimum close to the apex of the cone due to stagnation region, then increases with increase in the distance from the apex of the cone and becomes maximum due to maxima in the turbulence intensity and decreases with increases in the distance. Nusselt number value decreases with increase in the L/D. Nusselt number increases with increase in the jet Reynolds number. There is an insignificant influence of the diameter of pipe on Nusselt number for identical jet Reynolds number and L/D.

Heat transfer of movable air jet impingement over flat plate: experimental implementation

Heat transfer of movable air jet impingement over flat plate: experimental implementation

Structure optimization of straight cone nozzle and its effect on jet impingement heat transfer

In order to reduce the pressure loss of the nozzle and boost the efficiency of jet impingement heat transfer, this article takes the flow coefficient as the target variable and uses response surface methodology to optimize the straight cone nozzle’s internal structure. The optimal structural parameters obtained are as follows: the contraction angle is 30° and the lengths of the contraction section and the outlet section are 5 and 2 times of the outlet diameter, respectively. Besides, the influence of nozzle structural parameters on the heat transfer characteristics of a single water jet impinging on a high-temperature steel plate are simulated by ANSYS-Fluent. According to the results, the optimized nozzle has the largest flow coefficient, and the average Nusselt number on the steel plate is also the largest under the same inlet pressure.

Converging Jet Impingement for Enhanced Liquid Cooling

Abstract Jet impingement cooling is an advanced thermal management technique for high heat flux applications. Standard configurations include single, axisymmetric jets with orifice, slot, or pipe nozzles. This choice in nozzle shape, number of jets, and jet inclination greatly influences the turbulence generated by fluid entrainment due to differences in initial velocity profiles and location of secondary stagnation points. Regarding high power electronics with integrated jet impingement schemes, turbulence, and heat transfer rates must be optimized to meet the extreme cooling requirements. In this study, the heat transfer rates of dual inclined converging jets are investigated experimentally. Emphasis is placed on the comparison of different jet schemes with respect to geometrical parameters including nozzle pitch, incline angle, and nozzle-to-target plate spacing. A parametric experimental investigation is performed as a point of comparison using a modular, additively manufactured jet setup. Computational simulations are used to evaluate the effect of jet configuration on stagnation pressure associated with maximum heat transfer rates, and an empirical Nusselt number model is provided. The results show the effects of convergence height on jet behavior and the associated impacts on heat transfer.