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• The flow and heat transfer characteristics of single and multiple-synthetic jet impingement cooling is presented. • The equivalent Reynolds and Nusselt numbers were determined for the purpose of describing multiple orifice configuration. • The merging jet process is presented and the merging point is determined. In this paper, the comparison between synthetic jet actuator with single axisymmetric orifice and multiple axisymmetric orifices is presented. The aim of this paper is to compare the flow and heat transfer characteristics of single and multiple synthetic jet impingements cooling. The multiple orifice configurations had a central orifice and a satellite orifices placed in the pitch circle. The synthetic jet actuator had central orifice with 0, 3, 4, 5 and 6 orifices placed in the diameters of the relative pitch circle p / d = 2.5, 3 and 4. A total of 13 various geometric configurations were tested experimentally. In the multiple orifice configurations, the total cross-section area was approximately equal to the cross-section area of the single orifice configuration. The input power delivered to the synthetic jet actuator was maintained constant and equal to 3.00 W. The synthetic jet actuator energetic efficiency, Reynolds number, equivalent Reynolds number, radial and axial distributions of pressure coefficient, and heat transfer characteristic were presented. The synthetic jet issuing from multiple orifice plate merged at some axial distance, creating a single axisymmetric jet. The merging jet axial distances were determined for all investigated configurations. In the present paper, the integral quantities averaged over all orifices cross section area, or over impingement plate much larger than total orifices area, were analyzed for the purpose of objective comparison. From all various investigated parameters obtained from five different and independent experimental methods, the following conclusion may be made: if the total orifice cross section area in the case of single and multiple orifice synthetic jet is similar and real power deliver to the SJA is maintained at the same level the resulting equivalent Reynolds number and equivalent dimensionless stroke length achieved similar values and the impinging jet equivalent Nusselt number also achieved comparable values and distributions. [ABSTRACT FROM AUTHOR]

Rising temperature of gas turbine inlet brings great challenge for blade temperature tolerance, thus high effective cooling structure for turbine blade is vital. In this paper, aerothermodynamic features including heat transfer and pressure drop of a Coriolis-applied rotating smooth U channel are studied in detail. Both experiment and numerical methods are utilized in the study. A rotating cooling structure experiment rig system has been established and validated. The experiments were conducted in the rotating cooling structure experiment rig. Besides, RANS was chosen in the simulation. In the experiment and numerical studies, Reynolds number is constant at 16000 and Rotation number ranges from 0 to 0.024. The results indicate that Coriolis force plays a dominant and positive role on heat transfer ability on trailing wall, and that bend outlet impingement effect is the primary factor compared to Coriolis force for the Nusselt number of smooth leading surface. Besides, Coriolis force at bend region probably suppresses the formation of K–H vortices, thus boosts the flow stability, reduce pressure loss of channel and weakens second heat transfer peak of impingement at the bend outlet. What's more, the total performance of the Coriolis-applied smooth U channel at Ro of 0.006 and 0.024 are 2.18 % and 8.51 % higher than the channel at Ro of 0, which means that rotation is good for the Coriolis-applied smooth U channel and that the channel has superior performance and is encouraging in future rotating blade cooling application. • A Coriolis-applied smooth U channel is proposed and investigated. • Detail heat transfer experiments on the channel were implemented. • Simulations were conducted to unveil the heat transfer and pressure drop mechanisms. • Rotation enhances overall heat transfer of the Coriolis-applied smooth U channel. • Rotation brings pressure loss of the Coriolis-applied smooth U channel decreased. [ABSTRACT FROM AUTHOR]

In this paper, the fluid flow behavior and cooling characteristics of a sweeping impingement jet were investigated. Also, the difference between free and confined impingement schemes was investigated. In addition, the conventional approach for the purpose of significantly reducing computational time by using a confined domain with a slip upper wall instead of an unconfined domain was evaluated. A 2D unsteady Reynolds averaged Navier-Stokes (URANS) simulation accompanied with the k-ω SST turbulence model is used in this study. The study has been conducted for a target wall with a constant heat flux of 3000 W/m2, jet-to-wall distance of 4, and a jet Reynolds number of 2500. The results show that the overall average cooling performance of the sweeping jet is better in the confined impingement scheme compared to that of the steady jet, while the steady jet is slightly better in the unconfined sweeping impingement scheme. Using a confined scheme with a slip upper wall does not reveal the complete thermal and flow behaviors, and the wall heat transfer distribution is very different from the unconfined domain. [ABSTRACT FROM AUTHOR]

• Internal heat transfer of film-cooled leading edge is investigated. • Two jet impinging designs: the normal jet and the tangential jet are studied. • Mainstream flow influence decreases with increasing jet Reynolds number. • CFD simulations show a good agreement with experimental data. This paper investigates internal heat transfer of film-cooled leading edge with mainstream flow. The semi-cylindrical leading edge model receives coolant through impinging jets located at the neighborhood rectangular channel. The leading edge has three rows of cylindrical film cooling holes: row 1 located along the stagnation line (0°) and rows 2 and 3 at ±40° measured from the stagnation line. All film cooling holes are at an inclined angle of 25° relatives to the surface. There are two impinging jet designs in this study: the normal jet and the tangential jet. The normal jet has one row of normal jet impinging holes. After jets impinging on the inner wall of the semi-cylinder stagnation line, coolant spreads out through film cooling holes. The tangential jet has two rows of tangential jet impinging holes. Swirl flow is generated when jets enter the semi-cylinder from two sides of the semi-cylinder. Mainstream Reynolds number is about 100,000 based on the outside diameter of the leading edge cylinder, and the mainstream turbulence intensity is about 7%. Leading edge detailed internal heat transfer distributions are measured by using transient liquid crystal method. Three different coolant jet Reynolds numbers are tested (Re j = 5000, 10,000 and 15,000), corresponding to averaged blowing ratios about 0.77, 1.54, and 2.31. The experimental results provide useful information for the jet impingement cooling design, especially the leading edge region is under the conditions of mainstream flow and external film extraction. CFD simulations are performed to present the velocity field and compare the heat transfer results with experimental data. [ABSTRACT FROM AUTHOR]

• v 2 f turbulence model predict hydrodynamics and heat transfer of free surface liquid jet better compared to k − ϵ and k − ω models. • Lower jet to plate spacing (H / d) results in higher momentum before and after impact on impingement surface compared to high H / d. • For lower H / d , higher shear stress was observed throughout the impingement surface compared to higher H / d. • Nusselt number strongly dependent on jet to plate spacing for H / d ≤ 1. Numerical study is carried out to investigate hydrodynamic structures and heat transfer characteristics of vertical free surface liquid jet impingement on a horizontal flat plate. Different turbulent models (k − ϵ , k − ω , v 2 f) are compared to predict velocity field of the free surface jet, and the most suitable turbulence model is identified for prediction of flow physics of free surface jet impingement. A parametric study is performed to investigate the effect of Reynolds number (20000 − 60000), nozzle to plate spacing, H / d (0.5 − 2), and nozzle exit diameter (10.9, 15, and 23 mm) on the hydrodynamics, turbulence and heat transfer. The paper focuses on the underlying physics for jet impingement in the case of the lower jet to plate spacing, H / d (0.5 − 2). It was found that v 2 f turbulence model predicts flow field reasonably well compared to other turbulent models (k − ϵ and k − ω). Liquid jet radius before the impact and liquid film thickness after the impact have been examined. Velocity field, pressure and shear stress distribution on the flat plate are presented for free surface jet impingement. Further, turbulence intensity variation is examined and its effect on heat transfer is discussed. Jet to plate spacing, H / d = 0.5 resulted in around 20% higher Nusselt number compared to H / d ≥ 1 cases; and it is identified that a higher momentum before impact, compared to H / d ≥ 1, is the driving factor. Transition to turbulence is observed for higher Reynolds number and is explained by analyzing the variation in turbulence intensity along the flow streams of the free surface jet. [ABSTRACT FROM AUTHOR]

In a double-wall cooling system with multiple jet impingement the arrangement and size of effusion holes may change flow field and thereby change heat transfer characteristics. This paper stands in the view of internal cooling of turbine blades, and studied multiple-jet impingement heat transfer in double-wall cooling structures with narrow channels with pin fins and different-size effusion holes on the target wall. Five target plates were investigated including flat plate, pin fin plate and three pin fin plates with different-size effusion holes (effusion-to-jet diameter ratio of D e / D j = 0.5 , 1.0 a n d 1.5 ). Transient liquid crystal thermography experiments were conducted to explore the heat transfer characteristics on these target plates. The ratio of jet-to-plate spacing was fixed to be 1.5 and Reynolds numbers based on the jet diameter range from 15,000 to 30,000. The experimental results showed that the pin fins and effusion holes reduce the crossflow strength in downstream region, improve and uniform heat transfer on the whole target plate obviously. Compared with the flat plate, pin fin plate with effusion holes of D e / D j = 1.5 has highest averaged Nusselt number on the endwall. Numerical computations were carried out based on the experimental model, which revealed that the total heat transfer quantity on the pin fin plate with effusion holes of D e / D j = 1.5 can be increased by up to 51% comparing to that of the flat plate. Detailed interactional flow information between the wall jet flow, pin fins and effusion holes is expounded for the heat transfer improvement in the impingement-effusion structures. Moreover, conjugate heat transfer analyses were done to further investigate the overall cooling performance of the impingement-effusion structures. [ABSTRACT FROM AUTHOR]

The droplet impacting on a substrate is widely encountered in daily life and industrial applications. In this paper, the impact dynamics and heat transfer are numerically simulated for both single droplet and binary droplets on a hot superhydrophobic substrate. The dimensionless numerical model is built up through the transient 2D axisymmetric model with a volume of fluid (VOF) model. The effect of the Weber number, Reynolds number, size ratio, contact angle on the spreading dynamics and heat transfer is investigated in details. It is found that the spreading factor and contact time increase with the increasing Weber number. Besides, the maximum spreading factor and contact time of binary droplets are larger than those of a single droplet under the same Reynolds and Weber numbers. The transient heat transfer rate of a single droplet is larger than that of binary droplets impingement due to larger spreading surface area-to-volume. The total input heat of a single droplet is generally larger than that of binary droplets except at large Weber numbers, and the equal-sized binary droplets have the larger total input heat than un-equal-sized binary droplets. For binary droplets impact on a hot superhydrophobic surface, the hot substrate can promote the spreading and retard the receding due to thermo-capillary effect, and thus will enhance heat transfer between the droplet and the hot substrate. These findings may be helpful in gaining insights into the dynamics and heat transfer of binary droplets impact on the hot substrate. [Display omitted] • Spreading factor and contact time of binary droplets are higher than single droplet. • Heat transfer of single droplet is higher than binary droplets. • Heat transfer is the best for equal-sized droplet. • Thermal-capillary effect promotes the spreading and retards the receding. [ABSTRACT FROM AUTHOR]

• Kagome truss structure of water-drop-shaped rods is proposed. • Flow and heat transfer characteristics of the composite structures are conducted. • The thermal performance of the new structure is better than the reference structure. • The best thermal performances of the composite structures with cross-flow are found. Jet array impingement providing high heat transfer are widely used in heat exchangers. However, the target plate is easily damaged impinged by high-speed air flow constantly. To solve this issue, Kagome truss structure is combined with jet array impingement to improve cooling performance in this paper. The flow and heat transfer characteristics of this composite structure in a heat exchanger unit with/without cross-flow were experimentally studied. Additionally, Kagome truss structures of circular and water-drop-shaped rods were used respectively in jet space and the results showed that the latter was beyond 7% higher than the former for the comprehensive thermal performance. When cross-flow was not added, the comprehensive thermal performance factor of the latter reached to a maximum of 1.656 at the Reynolds number of 5000. When cross-flow was added, the comprehensive thermal performance got worse at the cross-flow-to-jet velocity ratio of 1.0, while the best comprehensive thermal performances were found at the cross-flow-to-jet velocity ratio of 0.6 and at the mass flow rate proportion of cross-flow of 0.5, respectively. These results can provide a basic experimental data as a reference for designing new structures in advanced heat exchanger. [ABSTRACT FROM AUTHOR]

• The paper presents a novel approach for heat transfer enhancement using a cylindrical insert with impinging jet. • Twin jets are formed with velocity twice as high as that of a jet without an insert • The complex flow near the stagnationzone creates a plume like structure with high turbulent kinetic energy Periodic vortex shedding which forms when fluid moves over a circular cylinder creates a wake which causes fluctuating unsteady forces on structures. The oscillating wake may also serve to enhance heat transfer rates from a surface on which it impinges. In this paper we investigate the passive excitation of the impinging jet's velocity field using a cylindrical insert. The insert is placed just before where the jet issues from the circular pipe. The flow is examined using Large Eddy Simulations (LES). The jet's Reynolds number based on bulk inlet velocity is 23,000 and jet's outlet-to-target wall distance is two. It is found that using a cylindrical insert in the impingement pipe results in enhanced heat transfer rates for cooling applications. [ABSTRACT FROM AUTHOR]

Present paper considers hybrid-linked jet impingement which involves both parallel linked jets and serial linked jets. Eight configurations with different topologies were tested under five mass flow rates ranging from 0.4 to 0.8 kg/m 2 s. The equivalent Reynolds numbers range from 12,000 to 24,000 for serial linked jets. Both heat transfer data and pressure drop data were obtained from the steady state experiments. Due to the parallel linkage, the heat transfer for the parallel jets decreases with increasing total number of parallel jets, while the heat transfer for the serial jets stay constant regardless of the change in topology. Although the serial jets are inclined, the heat transfer level of serial jets are typically higher than parallel jets due to the concentration of mass flow rate. Case 1 with purely serial linked jets shows a heat transfer enhancement by 100–150% as compared to the traditional design Case 8. However, the pressure drop of such configuration is around 20 times higher than that of the traditional designs. Analytical modelling work shows that the overall averaged heat transfer is a weighted average of [( m / A )/ n ] 0.4 and ( m / A ) 0.4 , which represents the heat transfer induced by parallel jets and serial jets respectively. [ABSTRACT FROM AUTHOR]

Jet impingement cooling is usually used in zones with extremely high thermal loads. The major factor that limits impingement heat transfer uniformity is the crossflow. The impingement cooling with return hole is applied to reduce the negative effect of crossflow on the target surface heat transfer in present paper. The influences of jet-to-plate spacing (H / D), impingement target surface curvature (α), temperature ratio (TR) and Reynolds number on flow field and heat transfer performance are investigated in detail. The results indicate that the correlation between the overall area average Nusselt number of target surface and H / D , the overall area average Nusselt number decreases by approximately 5.76% with H / D increases by one unit. Increasing the impingement target surface curvature is beneficial to expand the spanwise heat transfer. The overall area average Nusselt number increases by 295.5% for Reynolds number varying from 10,000 to 50,000 at TR = 0.910, and the overall area average Nusselt number increases by 8.68% for the TR varying from 0.668 to 0.910 at Re = 30,000. The results indicate that impingement cooling with return hole can achieve high heat transfer coefficient and uniform heat transfer distribution. [ABSTRACT FROM AUTHOR]

Swirling flows have applications in several areas which includes enhanced heat transfer and combustion. In these applications, swirling jets typically impinge on a target surface either orthogonally or at an angle. This paper presents an experimental and numerical investigation on the effects of jet-to-target spacing (z/d – 0.5 to 4) on heat transfer characteristics of orthogonally impinging swirling jets in comparison to geometrically similar cylindrical jet. Further, obliquely impinging jets (at z/d = 4) with anti-clockwise swirl (when viewing target surface from the jet) have been studied. Transient liquid crystal thermography experiments were carried out for above configurations at jet Reynolds number of 18,000 for orthogonal impingement and for Re ranging from 9000 to 18,000 for obliquely impinging swirling jet. The experimentally obtained local heat transfer when studied in conjunction with the flow-field predictions reveal that swirling jets can result in high heat transfer compared to conventional cylindrical jets at low z/d, where the area coverage of swirling jet is smaller than cylindrical jet. With increasing z/d, swirl jet impingement heat transfer showed significant reduction (while increasing coverage) while cylindrical jet impingement primary heat transfer zone was not very sensitive towards it. This is attributed to increased expansion of swirling jet exiting the nozzle at higher z/d, while cylindrical jet retaining its potential core for the z/d range investigated here. Obliquely impinging swirling jet had reduced stagnation zones due asymmetric arrangement of swirl passages with respect to target surface. [ABSTRACT FROM AUTHOR]

The main aim of this paper is to investigate the effect of nozzle geometry on the flow and heat transfer from a pulsed jet into a concave surface. Experiments have been carried out for pulsed circular jet and numerical simulations were done for circular, elliptical, rectangular and square geometries. Numerical solution was performed for the frequency ranges of 25 H z to 100 H z , the jet Reynolds number of 7000 , the dimensionless nozzle distance to concave surfaces of 2 and 5. Numerical results show a consistent agreement with experimental results and previous works. Accordingly, the geometry of nozzle directly affects the air entrainment ratio. In addition, with the increase of aspect ratio of the nozzle, average Nusselt number (Nu ave) decreases. By pulsating the inlet jet with pulse frequency of 100 H z , the average Nu number for circular/square and elliptical/rectangular jets increases 22 % and 15 % , respectively. Generally, the pulsation causes a reduction and an increase of the Nu ave at low and high frequencies, respectively, compared to that of the steady jet. At low frequencies, the Nu ave of circular jet decreases significantly in comparison to that of the square and elliptical jets. However, at high frequencies, circular jet shows higher Nu ave compared to other ones. As the distance between the jet and the concave surface increases, the effect of the nozzle shape on Nu ave decreases. [ABSTRACT FROM AUTHOR]

• IR Thermography is applied to measure heat transfer from impinging swirling jets. • The swirl number and impingement distance influence the shape of the heat transfer distribution. • The Reynolds number affects essentially the magnitude of the heat transfer rates. • Four different regimes are identified as a function of the swirl degree and impingement distance. • Empirical correlation laws for the area-averaged Nusselt number in these regimes are derived. This paper reports on a parametric study of the impingement heat transfer from swirling jets issuing from a circular nozzle. The swirl velocity component is imparted to the flow via a tangential entry into a recirculation chamber located upstream of the nozzle; the degree of swirl is varied by changing the angle of the channels leading to the chamber with respect to the radial direction. The effects of the dimensionless impingement distance (H / D = 1 , 2, 3, 4, 6, 8, 10, 12, 14 with D being the nozzle diameter), the swirl number (S = 0 , 0.078 , 0.22 , 0.37 , 0.54) and the Reynolds number (Re = 20,800 , 31,100 , 41,500) on the spatial distribution of the heat transfer are investigated by means of infrared thermography and the heated thin foil sensor. The present results show that the swirl number and the impingement distance significantly influence the structure of the heat transfer distribution, while the Reynolds number affects essentially the magnitude. Four distinct ranges with different behaviours can be identified by distinguishing between weakly or non swirling jets (0 ≤ S ≤ 0.22) and moderately swirling jets (0.37 ≤ S ≤ 0.54) and between short and long impingement distances (1 ≤ H / D ≤ 4 and 6 ≤ H / D ≤ 14 respectively). A comparative assessment of the performance of the swirling jets is carried out by focusing on the area-averaged Nusselt number distributions. It is found that in the range of short impingement distances and over relatively small target areas an enhancement of the heat transfer rates can be obtained at a small extent by adding a weak swirl with no detrimental effect on the uniformity of the distribution and at a larger extent by a moderate swirl with a deterioration of the uniformity. For long impingement distances no enhancement is observed, although swirl yields a significant reduction of the non-uniformity. Finally, correlation laws of the area-averaged Nusselt number as a function of the control parameters are derived in the four regimes identified. [ABSTRACT FROM AUTHOR]

Jet impingement is exceedingly operational for numerous field applications including electronics cooling and process industry. Its implementation comfort makes it common for many applications. The paper deals with investigation of heat transfer characteristics of a cooling flat surface. It is cooled by air jet impinged at an inclination of 15–75° to target plate. For understanding variation in cooling performance, comprehensive experiments are performed with different configurations of inclined jets on hot surface. For simplicity in using air, it is recommended for verity of applications as cooling fluid. Air Inclined jet with Reynolds Number in the range of 2000 ⩽ Re ⩽ 20,000 is examined for the circular Inclined jet. The target to inclined jet perpendicular height ( H ) is varied from 0.5 ⩽ H / D ⩽ 6.8 for understanding effect of H on cooling performance of different locations on target plate. The investigation leads to equations for average Nusselt Number for inclined non-confined air jet for cooling applications. [ABSTRACT FROM AUTHOR]

In this paper, extensive experimental investigation of the heat transfer characteristics of jet impingement on a variable-curvature concave surface in a wing leading edge was conducted for aircraft anti-icing applications. The experiments were carried out over a wide range of parameters: the jet Reynolds number, Re j , from 51,021 to 85,340, the relative tube-to-surface distance, H / d , from 1.736 to 19.76, and the circumferential angle of jet holes on the piccolo tube, θ , from −60° to 60°. In addition, jet impingements with single one, two and three rows of aligned jet holes were all investigated. Experimental results revealed the effects of various parameters on the performance and characteristics of jet impingement heat transfer in the specific structure adopted here, and our insufficient understanding on the corresponding physical mechanism was presented. It was found that the jet impingement heat transfer performance was enhanced with the increase of jet Reynolds number. For single one row of jet holes, an optimal H / d of 4.5 was determined under Re j = 51,021 and d = 2 mm, for which the jet impingement achieved the best heat transfer performance. For two and three rows of aligned jet holes, the Nu x curves in the chordwise direction exhibited much different shapes due to different intensity of the interference between adjacent air jets. This work contributes to a better understanding of the jet impingement heat transfer on a concave surface in a wing leading edge, which can lead to optimal design of the aircraft anti-icing system. [ABSTRACT FROM AUTHOR]

• Flow and heat transfer characteristics of jet array impingement with effusion holes are investigated. • A total of 300 simulations are tested for five hole arrangements. • The radial acceleration, turbulence, and jet potential core strongly affect impingement heat transfer. • Predictions of previous Nusselt number correlations for jet array impingement are evaluated. • A new Nusselt number correlation applicable to five hole arrangements is proposed. Impingement/effusion cooling systems have been studied extensively over the past few decades to constantly improve cooling performance as the turbine inlet temperature of modern gas turbine engines has increased. However, a Nusselt number correlation is less developed for an impingement/effusion cooling system with effusion holes in its impingement plate than for jet array impingement without effusion holes. Moreover, Nusselt number correlations, available for a wide range of geometric and flow parameters with various hole arrangements, are sparse for gas-turbine casing cooling systems. Therefore, this study investigated the effects of these parameters on the flow and heat transfer characteristics of an impingement/effusion cooling system that uses a jet array of effusion holes to cool a gas-turbine casing. The circular holes in the parallel jet and impingement plates were arranged in five different hole arrangements. Air forced through holes in the jet plate struck the impingement plate then discharged through the effusion holes of the impingement plate. Three-dimensional simulations were performed to examine the effects of hole pitch, distance between the jet and impingement plates, ratio of total jet area to heat transfer area, and Reynolds number based on the jet hole diameter on impingement heat transfer. A total of 300 simulations were run for the five hole arrangements to obtain the local and average Nusselt numbers on the impingement plate. Our results show that the radial acceleration and turbulence near the impingement plate, and the length of the jet potential core strongly affect the impingement heat transfer. This study also evaluated previous Nusselt number correlations for jet array impingement. This paper presents a new correlation to predict the average Nusselt number of jet array impingement applicable to the five hole arrangements. [ABSTRACT FROM AUTHOR]

The impingement cooling is used to protect turbine components under high thermal load. This paper conducts a numerical study of heat transfer characteristics of turbine leading edge with lamilloy cooling structure. Target surface curvature and jet-to-surface spacing under various crossflow Reynolds number conditions are considered. Meanwhile, the target surface is dimpled to enhance heat transfer capability. Besides, staggered film holes are considered in the computational domain. The results show that the crossflow induces a counter rotating vortex pair that greatly increase the near wall turbulence and local heat transfer. The vortex pair forms the bound for a U-shaped high heat transfer region caused by the jet impingement. The target surface curvature tends to enlarge the relative area of the U-shaped region. The increased crossflow strength weakens the jet cooling effect, the total heat transfer performance is therefore decreased. Large jet-to-surface spacing enhances the impingement cooling when crossflow is weak but the total heat transfer will not recover as crossflow Reynolds number get higher compared with small and medium spacing cases. • Computational fluid dynamics (CFD) models used to simulate turbine leading edge internal heat transfer. • Impingement cooling configured with dimpled surface and film holes. • Compound influence of jet-to-surface spacing, surface curvature under crossflow. [ABSTRACT FROM AUTHOR]

Impingement cooling is an important turbine blades cooling technique, but it is often not as efficient as required due to the deflection of downstream jets caused by crossflow. In this paper, a novel corrugated target surface is proposed to convert downstream oblique impingement into orthogonal impingement for higher heat transfer. To prove this design, the flow and heat transfer characteristics of impingement cooling with conventional flat target surface (Baseline case), full-corrugated (FC) target surface, and semi-corrugated (SC) target surface are compared over the jet Reynolds number ranging from 15,000 to 45,000. Results show that the Nusselt number of full-corrugated target surface is almost the same as that of flat target surface at the region corresponding to the first five jets, but its peak value and uniformity are significantly improved at the region corresponding to the last four jets, especially under high jet Reynolds numbers. In addition, with the increase of corrugation depth (H), the total heat transfer capacity and area-averaged Nusselt number increase, but this comes at the cost of increased friction loss. Comprehensively evaluating heat transfer and friction loss, only the FC with H = 0.8 D case always has better thermal performance than Baseline case under all computed conditions, and the maximum improvement can reach 5.8%. Last but not least, the SC with H = 0.8 D case is proposed based on the FC with H = 0.8 D case to further reduce the friction loss, and results show that its thermal performance is improved significantly due to the decreased friction loss and almost unchanged heat transfer. [ABSTRACT FROM AUTHOR]

• Introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. • Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone. • Impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets. • Impinging inverse flame jet is a desirable option for impingement heating applications. This paper presents an experimental study to explore the feasibility of utilizing a swirling inverse diffusion flame (IDF) jet for impingement heating. The investigation concentrates on the impinging flame structure and impingement heat transfer. The effects of air swirling, the air jet Reynolds number, Re a , the global equivalence ratio, Φ, and the non-dimensional burner-to-plate distance, H/d a , on the impinging flame shape and impingement heat transfer are investigated experimentally. The Experiments show that introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. Under-swirling produces no evident heat transfer enhancement due to insufficient air/fuel mixing, while over-swirling reduces heat transfer rate because of the dilution of the entrained cold ambient air. Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone around a stoichiometric condition. The investigation shows that the impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets with blue appearance, ultra-low NO x emission, high flame temperature, good self-stabilization capability, and enhanced heat transfer rate. Therefore, it is a desirable option for impingement heating applications. [ABSTRACT FROM AUTHOR]

• Detailed parametric study of novel concentric microroughness configurations subjected to array jet impingement • Configuration with largest pin-fin height produced maximum pin-fin effectiveness of 3.23 • Prescribed correlation for pin-fin effectiveness based on 64 configurations Array jet impingement when combined with strategically placed short height (<1.25 mm) roughness elements on the target wall results in enhanced heat dissipation at high fin-efficiency (>95%) without much penalty on pumping power. Advances in metal-powder-based additive manufacturing enables the printing of roughness scales (< 0.5 mm) with acceptable accuracy. This paper presents an experimental and numerical study on the characterization of heat transfer performance of concentric-shaped micro pin-fins subjected to array jet impingement. The experimental study served the dual purpose of establishing manufacturability of such configurations and validating the computational model. Three concentric micro-pin fin configurations were additively manufactured with AlSi10Mg alloy through Direct Metal Laser Sintering (DMLS) with dimensions between 0.25 - 2.25 mm on the target wall subjected to a 5 × 5 array of jets with normalized jet-to-jet spacing of X/D jet =Y/D jet = 3 and normalized jet-to-target spacing Z/D jet of 1. The resultant Nusselt number (Nu) and fin effectiveness (ε) values have been reported for Reynolds number ranging between 3,000 and 12,000. The three target plate configurations produced effectiveness between 1.8 - 2.4 and the configuration with maximum wetted surface area (wall thickness of 250 µm) and void fraction produced highest effectiveness. Further, a validated numerical model was used to perform a parametric study on geometrical parameters of roughness element viz. pin-fin spacing (S), inner diameter (D 1), outer diameter (D 2) and height (H) at a Re Djet = 6,000. Significant enhancement in ε was observed with H being the dominant parameter affecting the heat transfer. The effect of void fraction (α), which denotes the free flow area between pin-fin elements (A b –A pin) was also explored. There exists an α opt at which maximum heat transfer was achieved. A correlation for fin effectiveness is proposed as a function of different non-dimensional variables derived from the parametric study and its predictive capabilities were found to be within ±10%. [ABSTRACT FROM AUTHOR]

• New correlation between stagnation point nu and local re numbers during synthetic jet impingement heat transfer has been proposed. • The axial distribution of flow parameters under free and impinging synthetic jet reaches maximum at x / d ≈ 2. • The real (unmodified) axial distribution of stagnation nu number has been estimated. The paper presents the results of experimental investigations of heat transfer coefficients, velocity and pressure distributions of synthetic jets for two cases: constant Reynolds number and variable Stokes number and vice versa. Measurements were made for synthetic jet Reynolds number in the range of Re =5440–11,475, dimensionless stroke length L 0 =7.49–38.5 and dimensionless jet-to-plate spacing x / d = 0–20. The synthetic jet time-averaged and momentum velocity as well as the pressure for each case are presented. The centerline turbulence intensity and the Euler number are also reported. The centerline heat transfer coefficient distributions are compared with the distributions of the various velocity and pressure parameters. A new correlation between the local Nusselt number and the local Reynolds number is presented. Under synthetic jet impingement cooling the maximum of the stagnation point heat transfer coefficient appears mostly at axial distance of x / d ≈ 5. The location of the maximum Nusselt number at x / d ≈ 5 is caused by the modification of the heat transfer distribution due to the recirculation of hot air. With the decrease of the heat power released into the control volume by the impingement plate (heater) this modifying effects becomes less pronounced thus the maximum of the heat transfer coefficient shifts towards the location of about x / d = 2. The maxima of various flow parameters are generally observed at about x / d = 2 what was also reported by other independent researchers. Thus, it may be concluded that the purely mechanical phenomena does not full coincide with thermal ones in the case of synthetic jet impingement cooling. [ABSTRACT FROM AUTHOR]

• A combined experimental and numerical study for single and a row of threedimensional wall jets have been performed. • Four low Reynolds number turbulence closure models are accessed to select the suitable model that predicts the fluid flow and heat transfer characteristics of the three-dimensional wall jets accurately. • Measurement of flow characteristics like mean velocity profiles, growth of half jet widths, and velocity decay has also been performed. • For a fixed mass flow rate, at a given diameter, a smaller number of jets have higher heat transfer performance than that of the closely spaced jets. • Correlations are developed to predict local and average Nusselt numbers accurately. This paper reports the results of experimental and numerical studies to understand the heat transfer characteristics of three-dimensional wall jets exiting from a single and a row of circular jet openings over an unheated flat plate. Infrared thermography is employed to obtain the temperature distribution over the target surface, and the semi-infinite approximation methodology is used to estimate the heat transfer coefficients. Single wire constant temperature hot-wire anemometer is used to measure the flow characteristics. Additionally, computational studies have been performed to select a suitable low Reynolds number turbulence closure models among the following models, namely (i) Spalart Almaras (SA) (ii) Realizable k-ε with enhanced wall treatment (RKE-ewt) (iii) k- ω SST and (iv) Reynolds Stress Model with enhanced wall treatment (RSM-ewt) that predicts the experimentally obtained results for heat transfer characteristics accurately. Based on the investigations carried out, it is observed that RKE-ewt turbulence closure model is not only accurate but also faster in the prediction of heat transfer coefficient. Further, it is also seen that for a fixed mass flow rate, and at a given diameter of the jet opening, widely spaced jets show higher heat transfer coefficients. Furthermore, for a row of three-dimensional wall jets correlations based on the numerical results are developed for the Nusselt number for a Reynolds number range of 5000 to 15000. [ABSTRACT FROM AUTHOR]

• Experimental and numerical parametric investigation for enhancement in heat transfer from the heated cylinder. • Effect of mist loading fraction, Reynolds number and nozzle-to-surface spacing on the enhancement in heat transfer are studied. • High enhancement in heat transfer with high mist loading fraction, low nozzle-to-surface spacing and low Reynolds number. The paper presents the study of experimental as well as the numerical study of air-water mist jet impingement cooling over a heated cylinder in the non-boiling region. The studies are conducted for various mist loading fraction, f = 0.0025, 0.0050, 0.0075 and 0.01; Reynolds number, Re mix. = 8000, 10,500, 13,000 and 15,500; and surface-to-nozzle spacing, H/d = 30, 40, 50 and 60. Enhancement in the heat transfer (η) for mist jet impingement as compared to air jet impingement is obtained from experimental and numerical analysis. The numerical study also helped in understanding the distribution and tracking of droplets in the computational domain. High enhancement in the heat transfer is observed with high mist loading fraction. Also, the lowest Reynolds number yielded the highest enhancement in heat transfer with comparison to the higher Reynolds number. At various surface-to-nozzle spacing, the enhancement in heat transfer is high for low surface-to-nozzle spacing near the stagnation zone to a certain point, after which point the enhancement reverses as higher enhancement in heat transfer is observed for high surface-to-nozzle spacing. As high as 408% and 775% enhancement in the heat transfer at the stagnation point is observed for f = 0.01, Re mix. = 8000and H/d = 30 during experimental and numerical analysis respectively. The correlation has been proposed to estimate the enhancement in heat transfer at the stagnation point. [ABSTRACT FROM AUTHOR]

In this paper, a combined internal and external blade cooling system is conducted numerically to study the effect of different nozzles arrangements on flow and heat transfer performance. The applied operating conditions are different values of Reynolds number ranging from 5000 to 25,000 and different temperature ratios varied from 0.5 to 0.85. Results indicate that Nusselt number increases with the increase of Reynolds number at constant temperature ratio for all cooling models. The staggered normal cooling model achieves the highest overall Nusselt number with a maximum increase of 14.3% compared to the other cooling models. Overall film cooling efficiency increases with the increase in the corresponding blowing ratios. The staggered normal cooling model achieves superior overall cooling performance compared with the other cooling models. A correlation for overall Nusselt number is derived as a function of Reynolds number and temperature ratio. [ABSTRACT FROM AUTHOR]

The paper presents experimental studies on heat transfer phenomena which occurs during the impingement cooling with the use of synthetic jet. The synthetic jet was created by a special actuator consisting in a plexiglass cavity and a loudspeaker. An air was a working fluid. The effect of synthetic jet actuator geometry and supply parameters on heat transfer enhancement has been investigated. The experiment was performed with the use of classical thermal balance method. Heat transfer coefficients at a stagnation point have been obtained. Measurements were carried out in the range of excitation frequency f = 5 ÷ 600 Hz and of root mean square voltage E = 1, 2, 3, 4, 6 V. Changes of actuator geometry included: orifice diameters d = 9, 15, 24 mm, orifice thickness t = 3, 5, 20 mm and cavity depth H = 20, 40, 60 mm. On the basis of measurements results the correlations for the stagnation point Nusselt number of an axisymmetric impinging synthetic jet characterized by large Reynolds numbers have been obtained. Nusselt number has been determined in the function on scaling parameters: Reynolds number, jet-to-surface spacing x/d and the dimensionless stroke length L 0. Reynolds numbers were ranging from 3600 to 22950, x/d = 1 ÷ 20 and L 0 = 0.84 ÷ 170.5. The scaling parameters have significantly extended the range of research so far. The obtained Nusselt number correlations provide heat transfer calculations at large Reynolds numbers and are expected to have practical applications. [ABSTRACT FROM AUTHOR]

• Infrared thermography to resolve transient jet behaviour. • Impinging heat transfer analysis. • Non-swirling (S = 0) and highly swirling jets (S = 1.05). • Correlations presented. A multitude of research has already been undertaken to examine the effects of imparting swirl (S), Reynolds number (Re), and nozzle-to-plate distance (H/D) on the steady-state heat transfer characteristics of turbulent impinging jets. However, no studies to date have compared the transient development of such jets in both swirling and non-swirling conditions. The current paper addresses this gap by using highly resolved (time series) imaged (infrared) data in conditions spanning Re = 11,600, 24,600, and 35,000. The experiments are based on an electrically heated foil (0.025 mm) with jets over S = 0–1.05 and nozzle-to-plate-distances of H/D = 2, 4, and 6. For non-swirling impinging jets, at greater Reynolds numbers, the location of the highest Nusselt number remains fixed for all the time steps in contrast to jets with low Reynolds number. The transient characteristics for H/D = 4 are distinct compared to H/D = 2 and 6, with impingement heat transfer taking more time to reach steady-state for H/D = 4. It is also seen that heat transfer distribution over the impingement plate changes significantly over the small interval of time. For swirling jets, the impingement plate heat transfer characteristics develop simultaneously for the stagnation and wall jet region. The peak Nusselt number shifts to the wall jet region as the intensity of swirl increases. Two correlations (first for non-swirl and low-swirl, later for moderate to high swirl) are proposed to predict the time needed to reach steady-state for Re = 35,000. [ABSTRACT FROM AUTHOR]