Confined Jet Impingement Research Articles

Magnetic field effect on the enhancement of nanofluids heat transfer of a confined jet impingement in mini-channel heat sink

The results of the magnetic fields effect on the heat transfer characteristics and pressure drop in a confined single jet impingement of mini-rectangular heat sink have been presented. The test section (heat sink) is fabricated from the brass block by the wire electrical discharge machine with the length∗width and the fin height of 50∗50mm and 2mm, respectively. The nanofluids mixture of de-ionized water and nanoscale TiO2 particles with various concentrations of 0.005%, 0.010%, 0.015% by volume are used as working fluid. Two different magnetic fields strength of 0.084, 0.28µT are generated by the Neodynium permanent magnetic bars. It is found that with the magnetic field effect, the Nusselt number tends to increase, compared to that without magnetic field effect, and the Nusselt number also increases with increasing the magnetic field strength. Due to the thin nanofluids concentration in this study, however, the nanofluids concentration has no significant effect on the pressure drop across the test section.

Axisymmetric wall jet development in confined jet impingement

The flow field surrounding an axisymmetric, confined, impinging jet was investigated with a focus on the early development of the triple-layered wall jet structure. Experiments were conducted using stereo particle image velocimetry at three different confinement gap heights (2, 4, and 8 jet diameters) across Reynolds numbers ranging from 1000 to 9000. The rotating flow structures within the confinement region and their interaction with the surrounding flow were dependent on the confinement gap height and Reynolds number. The recirculation core shifted downstream as the Reynolds number increased. For the smallest confinement gap height investigated, the strong recirculation caused a disruption of the wall jet development. The radial position of the recirculation core observed at this small gap height was found to coincide with the location where the maximum wall jet velocity had decayed to 15% of the impinging jet exit velocity. After this point, the self-similarity hypothesis failed to predict the evolution of the wall jet further downstream. A reduction in confinement gap height increased the growth rates of the wall jet thickness but did not affect the decay rate of the wall jet maximum velocity. For jet Reynolds numbers above 2500, the decay rate of the maximum velocity in the developing region of the wall jet was approximately −1.1, which is close to previous results reported for the fully developed region of radial wall jets. A much higher decay rate of −1.5 was found for the wall jet formed by a laminar impinging jet at Re = 1000.

Improvement of a novel heat pipe network designed for latent heat thermal energy storage systems

In the present work, the performance of a heat pipe network designed for a latent Thermal Energy Storage unit in a Concentrating Solar Power System was investigated numerically. A two-dimensional axisymmetric model was implemented to describe the vapor flow and heat transfer inside the vapor core. The network consists of a primary heat pipe and a concentric secondary heat pipe. The solar energy impinges on the disk shaped evaporator is transferred to the heat engine through adiabatic section. The excess heat is used to charge the phase change material via the secondary concentric heat pipe. The vapor flow leaving the adiabatic part of the primary heat pipe to the main condenser is similar to the confined jet impingement. As the flow impinges on the surface and spreads out radially, several recirculation zones have formed, resulting in non-uniform condensation on the condenser surface. The objective of the current work is to optimize the geometry of the heat pipe to alleviate flow separation and hence to improve the performance of the heat pipe. The effects of main condenser and secondary heat pipe entrance shapes on streamline contours, pressure and temperature distributions of the main condenser and secondary heat pipes were investigated. The impact of the primary heat pipe position was studied. Two configurations studied are center located adiabatic section and outward positioned adiabatic section. The performance of the heat pipe was evaluated by calculating the corresponding thermal resistances. For a case with tubular adiabatic section, the result showed that the condensers inlets shapes do not have significant effects on the recirculation zone configuration, but only shift up the pressure and temperature distributions of the main condenser and the secondary heat pipe. Among the various shapes studied for the main condenser inlet, tapered inlet results in the lowest thermal resistances for both the main and secondary condensers. It was also concluded that locating the adiabatic section of the primary heat pipe outwards reduces the size and quantity of the recirculation zones and dramatically increases the average temperatures of the condensers. Tapering the main condenser entrance eliminates the primary recirculation zone, resulting in a uniform temperature distribution inside the main condenser.

A tomographic-PIV investigation of vapor-induced flow structures in confined jet impingement boiling

Tomographic particle image velocimetry (PIV) is used to study the effect of confinement gap height on the liquid flow characteristics in jet impingement boiling. This first application of tomographic PIV to flow boiling is significant given the complexity of confined two-phase jet impingement. A jet of subcooled water at a Reynolds number of 5,000 impinges onto a circular heat source undergoing boiling heat transfer at a constant heat input. Confinement gap heights of 8, 4, and 2 jet diameters are investigated. A visual hull method is used to reconstruct the time-varying regions of the vapor in the flow. The vapor motion is found to govern the liquid flow pattern and turbulence generation in the confinement gap. Time-averaged velocities and regions of turbulent kinetic energy in the liquid are highest for a confinement gap height of 8 jet diameters, with lower velocity magnitude and turbulence being observed for the smaller spacings. Coherent vortical structures identified with the λ2-criterion are found to occur most frequently near the moving vapor interface. The most intense regions of turbulent kinetic energy do not coincide with the location of coherent structures within the flow. Irrotational velocity fluctuations in the liquid phase caused by vapor bubble pinch-off are the primary cause of the high turbulent kinetic energy measured in these regions. At a gap height of H/d=2 the vapor plume is constrained as it grows from the heat source, causing bulk flow oscillations in the downstream region of the confinement gap.

Stereo-PIV measurements of vapor-induced flow modifications in confined jet impingement boiling

A single subcooled jet of water which undergoes boiling upon impingement on a discrete heat source is studied experimentally using time-resolved stereo particle image velocimetry (PIV). The impinging jet issues from a 3.75mm diameter sharp-edged orifice in a confining orifice plate positioned 4 orifice diameters from the target surface. The behavior at jet Reynolds numbers of 5,000 and 15,000 is compared for a constant jet inlet subcooling of 10°C. Fluorescent illumination allows for simultaneous imaging of both the flow tracers and the vapor bubbles in the flow. Flow structure, time-averaged velocities, and turbulence statistics are reported for the liquid regions within the confinement gap for a range of heat inputs at both Reynolds numbers, and the effect of the vapor generation on the flow is discussed. Vapor generation from boiling is found to modify the liquid velocities and turbulence fluctuations in the confinement gap. Flow in the confinement gap is dominated by vapor flow, and the vapor bubbles disrupt both the vertical impinging jet and horizontal wall jet flow. Moreover, vapor bubbles are a significant source of turbulence kinetic energy and dissipation, with the bubbly regions above the heated surface experiencing the most intense turbulence modification. Spectral analysis indicates that a Strouhal number of 0.023 is characteristic of the interaction between bubbles and turbulent liquid jets.

Topology Optimization, Additive Layer Manufacturing, and Experimental Testing of an Air-Cooled Heat Sink

Abstract Topology optimization of an air-cooled heat sink considering heat conduction plus side-surface convection is presented. The optimization formulation is explained along with multiple design examples. A postprocessing procedure is described to synthesize manifold or “water-tight” solid model computer-aided design (CAD) geometry from three-dimensional (3D) point-cloud data extracted from the optimization result. Using this process, a heat sink is optimized for confined jet impingement air cooling. A prototype structure is fabricated out of AlSi12 using additive layer manufacturing (ALM). The heat transfer and fluid flow performance of the optimized heat sink are experimentally evaluated, and the results are compared with benchmark plate and pin-fin heat sink geometries that are conventionally machined out of aluminum and copper. In two separate test cases, the experimental results indicate that the optimized ALM heat sink design has a higher coefficient of performance (COP) relative to the benchmark heat sink designs.

Mathematical modeling and analysis of steady state performance of a heat pipe network

In the current study, the thermal-fluid phenomenon inside a heat pipe network is simulated numerically. The heat pipe is specially configured to be implemented in thermal energy storage units for a concentrating solar power system. It composed of a main heat pipe and an array of secondary heat pipes. The applied thermal energy to the disc-shaped evaporator is transferred to the heat engine by the primary heat pipe. The extra heat is delivered to the phase change material through the concentric secondary heat pipes. The vapor flow leaving the adiabatic pipe section of the primary heat pipe to the disc-shaped condenser behaves similarly to the confined jet impingement. Therefore, the condensation is not uniform over the main condenser. In order to model the physical phenomenon inside the heat pipe network, a new numerical procedure was developed. The feature that makes the procedure distinguished from other available techniques is its ability to simulate non-uniform condensation of the working fluid in the condenser section. The vapor jet impingement on the condenser surface along with condensation was modeled by attaching a porous layer adjacent to the condenser wall. This porous layer acts as a wall and let the vapor flow to split out radially while it allows mass transfer through it. The effects on the pressure and temperature distributions from the heat input, portion of heat transferred to the phase change material, main condenser geometry and secondary heat pipe configurations have been investigated. The result showed that the temperature distributions of main and secondary heat pipes are greatly affected by the geometrical features. The formed recirculation zones in the main condenser region, which depend on geometrical parameters, make the performance of heat pipes convoluted. For the case with one secondary heat pipe, placing it away from the primary heat pipe provides higher temperatures at the primary and the secondary heat pipes. It also yields more uniform temperature at the main condenser. For cases with two and three secondary heat pipes, the temperature of the secondary heat pipes highly depends on the secondary heat pipes position and the ratio of heat transfer to the PCM.

Confined Jet Impingement With Boiling on a Variety of Enhanced Surfaces

Confined jet impingement with boiling offers unique and attractive performance characteristics for thermal management of high heat flux components. Two-phase operation of jet impingement has been shown to provide high heat transfer coefficients while maintaining a uniform temperature over a target surface. This can be achieved with minimal increases in pumping power compared to single-phase operation. To investigate further enhancements in heat transfer coefficients and increases in the maximum heat flux supported by two-phase jet impingement, an experimental study of surface enhancements is performed using the dielectric working fluid HFE-7100. The performance of a single, 3.75 mm-diameter jet orifice is compared across four distinct copper target surfaces of varying enhancement scales: a baseline smooth flat surface, a flat surface coated with a microporous layer, a surface with macroscale area enhancement (extended square pin fins), and a hybrid surface on which the pin fins are coated with the microporous layer. The heat transfer performance of each surface is compared in single- and two-phase operation at three volumetric flow rates (450 ml/min, 900 ml/min, and 1800 ml/min); area-averaged heat transfer parameters and pressure drop are reported. The mechanisms resulting in enhanced performance for the different surfaces are identified, with a special focus on the coated pin fins. This hybrid surface showed the best enhancement of all those tested, and resulted in an extension of critical heat flux (CHF) by a maximum of 2.42 times compared to the smooth flat surface at the lowest flow rate investigated; no increase in the overall pressure drop was measured.

Heat Transfer Augmentation for Electronic Cooling

Problem statement: The performance of electronic devices has been improving along with the rapid technology development. Cooling of electronic systems is consequently essential in controlling the component temperature and avoiding any hot spot. The study aims to review the present electronic cooling methods which are widely used in electronic devices. Approach: There are several methods to cool down the electronics components such as the pin-fin heat sink, confined jet impingement, heat pipe, micro heat sink and so on. Results: The cooling techniques can obviously increase heat transfer rate. Nonetheless, for active and passive cooling methods the pressure drop could extremely rise, when the heat transfer rate is increased. Conclusion: When the cooling techniques are used, it is clearly seen that the heat transfer increases with pressure drop. To avoid excessive expense due to high pressure drop, optimization method is required to obtain optimum cost and cooling rate.

Heat Transfer Experiments in a Confined Jet Impingement Configuration Using Transient Techniques

A confined jet impingement configuration has been investigated in which the matter of interest is the convective heat transfer from the air flow to the passage walls. The geometry is similar to gas turbine blade cooling applications. The setup is distinct from usual cooling passages by the fact that no crossflow and no bulk flow directions are present. The flow exhausts through two staggered rows of holes opposing the impingement wall. Hence, a complex 3-D vortex system arises, which entails a complex heat transfer situation. The transient thermochromic liquid crystal (TLC) method was used in previous studies to measure the heat transfer on the passage walls. Due to the nature of these experiments, the fluid as well as the wall temperature vary with location and time. As a prerequisite of the transient TLC technique, the heat transfer coefficient is assumed to be constant over the transient experiment. Therefore, it is the scope of this article to qualify this assumption and to validate the results at discrete locations. For this purpose, fast response surface thermocouples and heat flux sensors were applied, in order to gain information on the temporal evolution of the wall heat fluxes. The linear relation between heat flux and temperature difference could be verified for all measurement sites. This validates the assumption of a constant heat transfer coefficient. Nusselt number evaluations from independent techniques show a good agreement, considering the respective uncertainty ranges. For all investigated sites, the Nusselt numbers range within ±9% of the values gained from the TLC measurement.

Confined jet impingement of liquid nitrogen onto different heat transfer surfaces

Jet impingement of liquid nitrogen owns many applications in the cryogenic cooling aspects, such as, cooling of high-power chips in the electronic devices and cryoprobes in the cryosurgery. In the present study, we systematically investigated the confined jet impingement of liquid nitrogen from a tube of about 2.0 mm in diameter onto the heat transfer surfaces of about 5.0 mm in basement diameter with different heat transfer surface geometries and conditions, i.e., flat surface, hemispherical surface and flat surface with a needle. The effects of many influential factors, such as, the geometry of the heat transfer surface, jet velocity, distance between the nozzle exit and heat transfer surface, heat transfer surface condition, and some other, on the heat transfer were investigated. The heat transfer correlations were also proposed by using the experimental data, and it was found that the heat transfer mechanism of liquid impingement in the confined space was dominated by the convective evaporation rather than the nucleate boiling in the present case. The critical heat flux (CHF) of the confined jet impingement was measured and the visualization of the corresponding flow patterns of the confined jet impingement of liquid nitrogen was also conducted simultaneously to understand the heat transfer phenomena.

Unsteady Mixed Convection of a Confined Jet in a Fluid-Superposed High Porosity Medium

Numerical study of a confined jet impingement cooling of a fluid-superposed porous medium heated from below is conducted to investigate the oscillatory mixed convection. The effects of the Rayleigh number (2 × 105 ≤ Ra ≤ 1 × 106) and the Darcy number (1 × 10−5 ≤ Da ≤ 5 × 10−4) on the heat transfer are investigated for different Péclet numbers. It is found that, the average Nusselt number increases with the increase in Darcy number or Rayleigh number. The values of average Nusselt number are found to oscillate with time for some combination of Rayleigh numbers (Ra ≥ 4 × 105) and Péclet numbers (200 ≤ Pe ≤ 1000), at which the oscillatory convection occurs. The oscillation of average Nusselt number is investigated for different porous medium height and porous medium-to-fluid heat capacity ratio.

Multiple flow patterns and heat transfer in confined jet impingement

The flow field of a 2-D laminar confined impinging slot jet is investigated. Numerical results indicate that there exist two different solutions in some range of geometric and flow parameters. The two steady flow patterns are obtained under identical boundary conditions but only with different initial flow fields. Two different exit boundary conditions are investigated with two commercial software packages to eliminate artificial or computational effects. The different flow patterns are observed to significantly affect the heat transfer. A flow visualization experiment is carried out to verify the computational results and both flow patterns are observed. The bifurcation mechanism is interpreted and discussed.

A Model for the Relative Velocity of a Particle in an Impingement Dryer: Application to Heat Transfer

A simplified mathematical model for the relative gas-particle motion in a confined jet impingement dryer is developed. Model predictions based on an unsteady momentum balance are in good agreement with the observed cycling motion of a spherical particle. The model is applied to coriander seeds submerged in a flow field of superheated steam. It is found that relative motion occurs in unsteady turbulent regime, and that steady settling velocity of particles is never achieved. Model results are applied to correlate experimental heat transfer data of an impingement dryer. Experimental Nu numbers compare fairly well with correlations for fluidized systems.

HEAT TRANSFER AND FLOW FIELDS IN CONFINED JET IMPINGEMENT

Confined jet impingement of air and different liquids is reviewed. The influence of confinement on the heat transfer and fluid mechanics of governing parameters such as jet diameter and geometry, Reynolds number, jet-to-target separation distance, exit velocity profile and turbulence levels, fluid properties, and target size is discussed. This review deals primarily with turbulent jets. Single- and multiple-jet impingement are considered, including predictive correlations for stagnation-point and area-averaged heat transfer. The velocity and turbulence fields in confined jet impingement are also described, with particular reference to their effect on the heat transfer obtained. The important role of flow visualization in understanding confined flow fields is demonstrated. Augmentation of jet impingement heat transfer by employing surface enhancements is also considered. The review includes discussions of numerical modeling efforts.

Preface

The eleventh volume of the Annual Review of Heat Transfer marks the beginning of this successful serial publication in the new millennium and continues with the fine tradition of reviewing and communicating new, important developments in the general field of heat transfer. The chapters presented in the past decade of the Annual Review deal with a significant, emerging topic by starting from fundamental physical considerations and then proceeding in a logical fashion to the forefront of recent developments and future challenges. The contributing authors are both young and well-established in these emerging fields. Volume 10 illustrated well this special characteristic of the Annual Review, providing informative chapters on bioheat and mass transfer modeling, plasma chemical thermophysics, radiation scattering from rough surfaces, molecular dynamics in microscale thermophysics, turbulent heat transfer modeling, and the initiation and growth of solidification shrinkage voids. To further illustrate the special characteristic of the Annual Review, the present volume provides new, emerging topics such as laser-liquid interactions, surface impacting of molten droplets, rapid solidification, nucleation and boiling in micro-structures, optical measurement of surface temperature, confined jet impingement, oscillatory convection and advanced heat exchangers with external fields. As we begin this volume in the new millennium, the editor is deeply grateful to the participating authors for their excellent contributions, to Begell House for providing invaluable editorial assistance, and to the heat transfer community for its continuing support and input. Thank you all of you for your dedication during the past decade.

Flow-Field Prediction in Submerged and Confined Jet Impingement Using the Reynolds Stress Model

The flow field of a normally impinging, axisymmetric, confined and submerged liquid jet is predicted using the Reynolds Stress Model in the commercial finite-volume code FLUENT. The results are compared with experimental measurements and flow visualizations and are used to describe the position of the recirculating toroid in the outflow region which is characteristic of the confined flow field. Changes in the features of the recirculation pattern due to changes in Reynolds number, nozzle diameter, and nozzle-to-target plate spacing are documented. Results are presented for nozzle diameters of 3.18 and 6.35 mm, at jet Reynolds numbers in the range of 2000 to 23,000, and nozzle-to-target plate spacings of 2, 3, and 4 jet diameters. Up to three interacting vortical structures are predicted in the confinement region at the smaller Reynolds numbers. The center of the primary recirculation pattern moves away from the centerline of the jet with an increase in Reynolds number, nozzle diameter, and nozzle-to-target plate spacing. The computed flow patterns were found to be in very good qualitative agreement with experiments. The radial location of the center of the primary toroid was predicted to within ±40 percent and ±3 percent of the experimental position for Re = 2000–4000 and Re = 8500–23000, respectively. The magnitude of the centerline velocity of the jet after the nozzle exit was computed with an average error of 6 percent. Reasons for the differences between the numerical predictions at Re = 2000–4000 and experiments are discussed. Predictions of the flow field using the standard high-Reynolds number k-ε and renormalization group theory (RNG) k-ε models are shown to be inferior to Reynolds stress model predictions.

Flow Field Effects on Heat Transfer in Confined Jet Impingement

Flow Field Effects on Heat Transfer in Confined Jet Impingement