Jet Impingement Heat Transfer

Impinging jets are widely used for their effective heat and mass transfer for several decades. Arrays of jet impingement have also been studied before due to its practical relevance to electronics cooling. A number of jet variations and jet-to-jet orientations have previously been studied, mainly to further improve the magnitude and uniformity of heat transfer. In recent years, swirling jets has also gained interest in heat transfer application due to their inherent mixing and spreading characteristics, which is believed to be an improvement on overall heat and mass transfer. As such, this paper numerically investigates an array of circular jets with and without swirl that impinges vertically onto a flat surface located at a fixed vertical distance H = 2D and at Reynolds number equals to 11,600, where D is the nozzle diameter. As the entire was symmetric, only quarter of the model was constructed for numerical analysis to save computational cost. In this case, numerical calculations were done via commercial software package ANSYS Fluent using SST k-ω turbulence model. Inlet conditions were taken from experimental data. The jet flows were in downward direction and gravity was not considered. This paper also compares numerical predictions with previously published literature for non-swirling and swirling jets.

The heat released from the flat surface is cooled by the use of jet impingement technique. The heat flux from the isothermally heated flat surface due to two-dimensional turbulent twin oblique confined slot-jet impingement is studied numerically using the ANSYS FLUENT software. Initially the flow and thermal fields for a normal confined slot-jet impingement are investigated using the RNG k-ε model and the SST k-ω model. The local Nusselt number distribution predicted by the SST k-ω model agrees notably better with the existing experimental data. Subsequently, the SST k-ω is employed to study the twin oblique impinging jet heat transfer problem. The study is conducted by varying the jet exit Reynold number, the jet to jet separation distance, jet-exit to target plate distance and the inclination angle of the jet to the impingement. Results indicate that impingement angle is reduced from 90o, the peak Nusselt number at the impingement surface is gradually reduced and its location slightly shifts away from the axis with slight decrease of the average Nusselt number for any combination of Re, L, H. The average Nusselt number is a direct function of the Reynold number and the impingement angle and is inverse function of the jet to target plate separation distance.

Conventional method of defining half velocity widths is applicable only for axisymmetric jets. Hence, geometry based definition of half velocity width is used for non-circular jets. Usefulness of this method becomes less when there is no symmetry based on geometry. Hence, a new half velocity width is proposed based on equivalent area method. Newly proposed half velocity width is computed for a conventional circular jet and a non-circular jet. The comparison of half velocity widths obtained using conventional method and newly proposed method shows good agreement with each other for circular jet. Geometry based half width and equivalent area based half velocity width agree in the near field for the non-circular jet. Equivalent area based method is found as better representation of half velocity width for non-circular turbulent jets.

The turbomachinery industry is continuously focusing on increasing the efficiency of its equipment. The gas turbine is one of the major pieces of equipment in the industry. It has extensive applications in such areas as power generation, jet propulsion, gas compression, etc. The efficiency and power output of the gas turbine depends on its inlet temperature. Therefore, the modern gas turbine inlet temperature began to exceed the permissible operating temperature of its blade material. Hence, various cooling methods such as dimple cooling, tubulated rib cooling, pin-fin cooling, and jet impingement cooling have been applied to ensure the performance and life of the turbine blade. This study considers the jet impingement cooling for the turbine blade internal cooling. The nozzle plate has 36 both in-line and staggered cylindrical holes. The experiment is completed with standoff distances of 3D and 5D, with the Reynolds number of up to 2000, where D is the nozzle diameter. The numerical study considered only 3D standoff distance , with Reynolds number of up to 8000. Different complex flow and heat transfer phenomena have been described.

A numerical simulation was performed to study the heat transfer performance of a hot fluid in a confined impinging jet on a flat heated surface. The tests were realized for the following ranges of the governing parameters: the jet thickness is 2m and the distance of horizontal jet to heated surface was set to 1 to 3 cm. Three different cases are considered in this analysis. They are H/D = 0.5,1. Fluids like Acetylene are compared with ?-Al2O3nan o fluid in this analysis. Tu rbulent models considered for this analysis are k-? and k-?. Out of these two k-? model gives more heat transfer characteristics. The plate is considered to be stationery. Horizontal Jet with convergent nozzle is considered and compared in this analysis. Finally it is found that surface nusselt number and surface heat transfer coefficient plays vital role for better heat transfer

This experimental study investigates the effects of internal geometry modifications on the performance of a curved Sweeping Jet actuator. The modifications are applied to the geometry of the feedback channel and the mixing chamber Coanda surface, and the resulting actuator properties are evaluated using time-resolved static pressure measurements inside the actuator and hot-wire measurements of the external flow. The major result is that small, localized modifications of the curved sweeping jet actuator geometry can lead to a complete change in the external flow regime, making the jet velocity distribution homogeneous, similar to the angled variant of the actuator. The Coanda surface shape is identified as the primary cause of the external jet adopting the bifurcated or homogeneous flow regime. The relationships between the sweeping frequency, jet deflection angle, required supply pressure, and pressure fluctuations are analyzed and discussed in detail. External flow behavior and coh...

Staggered array jet impingement cooling (JIC) on a semi-circular concave surface has investigated numerically. The main objective of this work is to examine elongated jet holes on heat transfer and flow characteristics of the staggered array JIC and to elucidate its applicability on cooling of a turbine blade. Jet holes were elongated to the target surface by the nozzles. Numerical computations were performed with different jet Reynolds numbers (5000 ≤ Re ≤ 25000), normalized confinement plate-to-target plate distances (1.0 ≤ H/d ≤ 8.0) and jet nozzle-to-target plate gap (0.5 ≤ G/d ≤ 6.0). Average Nusselt (Nu) number, local Nu number distributions on the surface , flow characteristics and Thermal Performance Factor (TPF) were comprehensively investigated. Numerical results were compared with the normal staggered array JIC. Results showed that local Nu number and average Nu number increases with decreasing G/d. The highest enhancement of heat transfer using elongated jet hole is obtained as 20.16% by decreasing G/d to 0.5 at H/d = 8.0. However, highest TPF is estimated as 1.11 at H/d = 8.0 by G/d = 2.0 for Re = 25,000. Decreasing of G/d provides more uniform heat transfer on the surface of interest in comparison with normal staggered array JIC. Consequently, elongating jet hole is a feasible heat transfer enhancement design for the staggered array JIC on a semicircular concave surface.

This work presents and discusses the results of an experimental investigation of heat transfer between the different plates of impinged jets. An unconfined jet of water is impinged on the mild steel copper and aluminium plate and the position of the plate to nozzle distance of 20mm for each plate.The experiment is focused on the verification of the jet effect on the distribution of local heat transfer coefficient on the impinged target surface. The effect of flow in jet to test plate distance are also examined at fixed intersect spacing. As the result obtained the heat transfer rate experienced by a aluminium plate was more followed by mild steel plate and copper plate, when direct jet of fluid is impinged on it. Also compare the nozzle to plate distance 10 mm from each plate.

Bioprinting offers great potential for the fabrication of three-dimensional living tissues by the precise layer-by-layer printing of biological materials, including living cells and cell-laden hydrogels. The laser-induced forward transfer (LIFT) of cell-laden bioinks is one of the most promising laser-printing technologies enabling biofabrication. However, for it to be a viable bioprinting technology, bioink printability must be carefully examined. In this study, we used a time-resolved imaging system to study the cell-laden bioink droplet formation process in terms of the droplet size, velocity, and traveling distance. For this purpose, the bioinks were prepared using breast cancer cells with different cell concentrations to evaluate the effect of the cell concentration on the droplet formation process and the survival of the cells after printing. These bioinks were compared with cell-free bioinks under the same printing conditions to understand the effect of the particle physical properties on the droplet formation procedure. The morphology of the printed droplets indicated that it is possible to print uniform droplets for a wide range of cell concentrations. Overall, it is concluded that the laser fluence and the distance of the donor–receiver substrates play an important role in the printing impingement type; consequently, a careful adjustment of these parameters can lead to high-quality printing.

Bioprinting offers great potential for the fabrication of three-dimensional living tissues by the precise layer-by-layer printing of biological materials, including living cells and cell-laden hydrogels. The laser-induced forward transfer (LIFT) of cell-laden bioinks is one of the most promising laser-printing technologies enabling biofabrication. However, for it to be a viable bioprinting technology, bioink printability must be carefully examined. In this study, we used a time-resolved imaging system to study the cell-laden bioink droplet formation process in terms of the droplet size, velocity, and traveling distance. For this purpose, the bioinks were prepared using breast cancer cells with different cell concentrations to evaluate the effect of the cell concentration on the droplet formation process and the survival of the cells after printing. These bioinks were compared with cell-free bioinks under the same printing conditions to understand the effect of the particle physical ...

Staggered array jet impingement cooling (JIC) on a semi-circular concave surface has investigated numerically. The main objective of this work is to examine elongated jet holes on heat transfer and flow characteristics of the staggered array JIC and to elucidate its applicability on cooling of a turbine blade. Jet holes were elongated to the target surface by the nozzles. Numerical computations were performed with different jet Reynolds numbers ( 5000 ≤ R e ≤ 25000 ) , normalized confinement plate-to-target plate distances ( 1.0 ≤ H / d ≤ 8.0 ) and jet nozzle-to-target plate gap ( 0.5 ≤ G / d ≤ 6.0 ) . Average Nusselt (Nu) number, local Nu number distributions on the surface, flow characteristics and Thermal Performance Factor (TPF) were comprehensively investigated. Numerical results were compared with the normal staggered array JIC. Results showed that local Nu number and average Nu number increases with decreasing G/d. The highest enhancement of heat transfer using elongated jet ...

This experimental study investigates the effects of internal geometry modifications on the performance of a curved Sweeping Jet actuator. The modifications are applied to the geometry of the feedback channel and the mixing chamber Coanda surface, and the resulting actuator properties are evaluated using time-resolved static pressure measurements inside the actuator and hot-wire measurements of the external flow. The major result is that small, localized modifications of the curved sweeping jet actuator geometry can lead to a complete change in the external flow regime, making the jet velocity distribution homogeneous, similar to the angled variant of the actuator. The Coanda surface shape is identified as the primary cause of the external jet adopting the bifurcated or homogeneous flow regime. The relationships between the sweeping frequency, jet deflection angle, required supply pressure, and pressure fluctuations are analyzed and discussed in detail. External flow behavior and coherence are characterized by phase-averaged, phase-locked velocity profiles and auto-correlation of the velocity signals.

Increasing heat transfer rates over smaller area and volumetric footprints are a challenge to electronics cooling. Conventional air-based cooling solutions require larger space for heatsink fins and impose challenges on chassis design. Those challenges of air cooling encourage a shift to a more efficient liquid cooling solution. The more efficient heat transfer performance in a small volume is attributed to high specific heat and latent heat associated with a liquid coolant. In this research, a bench level two-phase experimental setup was built to study the heat transfer and hydraulic performance of a jet impinging cold plate using a dielectric coolant HFE 7000. A copper block heater arrangement was used to mimic a computer chip with a footprint of 1“ × 1” (6.45 cm 2 ). Heat transfer performance and pressure drop were compared in single- and two-phase operation at three volumetric flow rates 1.0, 1.25, and 1.75 LPM corresponding to mass fluxes 2005, 2506, and 3509 kg/m 2 s respectiv...

Bioprinting offers great potential for the fabrication of three-dimensional living tissues by the precise layer-by-layer printing of biological materials, including living cells and cell-laden hydrogels. The laser-induced forward transfer (LIFT) of cell-laden bioinks is one of the most promising laser-printing technologies enabling biofabrication. However, for it to be a viable bioprinting technology, bioink printability must be carefully examined. In this study, we used a time-resolved imaging system to study the cell-laden bioink droplet formation process in terms of the droplet size, velocity, and traveling distance. For this purpose, the bioinks were prepared using breast cancer cells with different cell concentrations to evaluate the effect of the cell concentration on the droplet formation process and the survival of the cells after printing. These bioinks were compared with cell-free bioinks under the same printing conditions to understand the effect of the particle physical ...

Synthetic impingement is an attractive cooling mechanism due to the capability of achieving high heat transfer rates. The flow characteristics of synthetic jets are highly complex and consequently the heat transfer from the target surface is highly variable. The current research investigates the heat transfer characteristics of synthetic jet impingement cooling applied to thermal management of electronic systems. Synthetic jet is produced by a piston-cylinder arrangement. The major parameters identified to describe the synthetic jet heat transfer are Reynolds number, jet to plate spacing, orifice geometry and frequency of the jet. The cooling medium used is air. The experiments are performed for the jet Reynolds number (Re) of 12000, with dimensionless distance between the nozzle and plate surface(Z/D) ranging from 2 to 16 and orifice diameter (D) ranging from 3 to 5 mm. The results show that the average Nusselt number increases with frequency of the jet. The optimum Z/D corresponding to maximum heat transfer is high in the synthetic jet, as compared to that in a continuous jet. Synthetic jet introduces a stronger entrainment and more vigorous penetration in the surrounding fluid. In the case of circular orifice, the heat transfer characteristics of smaller diameter orifice are better at high Z/D ratios. The rectangular orifice has higher heat transfer characteristics as compared to circular and square jets for the same hydraulic diameter

در این پژوهش رفتار نوسانی جابجایی ترکیبی در برخورد جت به یک صفحه تخت دما ثابت در محیط متخلخل، موردمطالعه قرارگرفته است. علاوه‌براین تأثیر پارامترهایی همچون تخلخل، عدد گراشف اصلاح‌شده، عدد پرانتل و عدد دارسی بر انتقال حرارت بررسی‌شده است. برای این منظور مدل توسعه یافته برینکمن-فورچهیمر به‌کار رفته است. نتایج نشان می‌دهند که بین تخلخل، عدد پرانتل و عدد گراشف اصلاح‌شده با عدد ناسلت متوسط و دامنه نوسانات، رابطه‌ای مستقیم و بین عدد دارسی و عدد ناسلت متوسط رابطه‌ای معکوس برقرار است.