Flow Impingement Research Articles

RETRACTED: Thermal study on non-Newtonian fluids through a porous channel for turbine blades

The current paper aims to utilize non-Newtonian fluid and improve the cooling performance of turbine blades. To implement impinging fluid flow through a porous channel on a hot lower wall, in the first step, the rheology of non-Newtonian behavior is introduced. Then differential quadrature procedure is used to convert these highly nonlinear equations of motion to some simple algebraic expressions. There is a reasonable agreement between the present findings with previous research work. Finally, some vital parameters such as the cross-viscosity parameter and power law index are changed to evaluate how these factors improve the cooling performance of turbine blades. The findings show that a rising Prandtle number results in a 19% decrement in temperature pattern. For a constant cross-viscosity parameter, Reynolds number enhancement leads to wall friction augmentation of around 15%. Moreover, a 32% Nusselt number increment is observed by increasing the power law index for the same Reynolds number.

Experimental and Numerical Investigation of Flow Structure and Heat Transfer Behavior of Multiple Jet Impingement Using MgO-Water Nanofluids.

Nanofluids have attracted significant attention from researchers due to their ability to significantly enhance heat transfer, especially in jet impingement flows, which can improve their cooling performance. However, there is a lack of research on the use of nanofluids in multiple jet impingements, both in terms of experimental and numerical studies. Therefore, further investigation is necessary to fully understand the potential benefits and limitations of using nanofluids in this type of cooling system. Thus, an experimental and numerical investigation was performed to study the flow structure and heat transfer behavior of multiple jet impingement using MgO-water nanofluids with a 3 × 3 inline jet array at a nozzle-to-plate distance of 3 mm. The jet spacing was set to 3, 4.5, and 6 mm; the Reynolds number varies from 1000 to 10,000; and the particle volume fraction ranges from 0% to 0.15%. A 3D numerical analysis using ANSYS Fluent with SST k-ω turbulent model was presented. The single-phase model is adopted to predict the thermal physical nanofluid. The flow field and temperature distribution were investigated. Experimental results show that a nanofluid can provide a heat transfer enhancement at a small jet-to-jet spacing using a high particle volume fraction under a low Reynolds number; otherwise, an adverse effect on heat transfer may occur. The numerical results show that the single-phase model can predict the heat transfer trend of multiple jet impingement using nanofluids correctly but with significant deviation from experimental results because it cannot capture the effect of nanoparticles.

Numerical investigation on effects of rib shapes on oscillating flow and heat transfer in heat exchanger channels

Ribs are straightforward and cost-effective to improve the heat exchange efficiency, and the impact of cooling medium driven by the movement of the channel itself can also enhance the heat exchange capacity. This paper investigates the flow patterns of the gas–liquid two-phase flows in a smooth cubic and a cubic with semi-circular ribs by using a high-speed camera under different low-frequency oscillations. A numerical simulation method was used to study the impact of different ribs on each wall at various frequencies. The microscopic flow and heat transfer characteristics of the ribs were analyzed. Reflux occurred near ribs, and the fluid accumulated near the wall under the ribs with a liquid coverage ratio of approximately 79%. After the gas–liquid two-phase flow fully oscillated in the channel, the heat transfer contribution of the top wall to the entire channel reached the greatest value of approximately 40%, and the left and right walls contributed to the entire channel with 21% and 27%, respectively. An additional inertial force of the two-phase flow was sustained in the oscillating environment to form a periodic impingement flow on the rib and hence improve heat transfer. Due to the reflux near the rib, the local minimum values of the heat transfer coefficient appeared on the downwind of the rib. The difference in rib design affected the fluid motion and caused different heat transfer distributions on the top surface. These findings can support the application design of ribs in dynamic heat exchange channels for optimizing heat transfer.

Experimental investigation of oil film flow and droplets behavior in the annular channel of a cyclone separator

The separation of the gas and liquid in the gas-liquid cyclone separator is achieved through the swirling flow. The liquid film flow characteristics and the dynamics of droplets impacting the liquid film are very important for the separation performance, but these phenomena have not been well understood in the swirling flow. In this work, an experiment was conducted to investigate the liquid film flow and droplet impingement in the annular channel of the gas-liquid cyclone separator. Three regions of the annular channel could be divided based on the film flow patterns, which were the inlet region, the annular region, and the outlet region; and four typical droplet impact outcomes could be observed, which were splashing, coalescence, bouncing, and jet breakup. According to the characteristics of the liquid film and the outcome of droplet liquid film impingement, if the droplet splashing in the inlet region could be weakened and the length of the annular region was lengthened, it would be beneficial for gas-liquid separation. The impact outcomes were shown to undergo a transition from bouncing to coalescence as the droplet Weber number increased. Droplet coalescence was affected by the disturbance of gas flow and the evenness of the liquid film, so when the superficial velocity of the gas flow was fixed, the transition boundary was constant whatever the liquid volume fraction in the channel, and when the Reynolds number of the gas flow increased, the transition boundary of droplet Weber number increased first and then decreased. This work may provide a basis for a comprehensive understanding of the liquid film flow pattern and droplet behavior in the cyclone separator, and the results have the potential to be applied in other multiphase studies.

An experimental investigation of new roughness patterns (dimples with alternative protrusions) for the performance enhancement of solar air heater

Conventional solar air heater (SAH) has poor performance and in this work, an initiative has been taken to improve their overall performance with the help of a combined roughness pattern over the absorber plate. The experimental setup has been designed and fabricated having a rectangular duct with one heat-absorbing side (with roughness). The combination of dimple and protrusion roughness elements are arranged alternatively over the absorber plate for performance improvement of SAH. The precise placement of these roughness elements over the heat-absorbing side has been ensured with relative roughness pitch (RRP; P/e), print diameter to hydraulic diameter (d/D), and relative roughness height (RRH, e/D) which ranged from 10 to 20, 0.025–0.045, and 0.25 to 0.5, respectively. It is discovered with the experimental results that the thermal performance increases along with the pressure loss in the roughened SAH (rSAH). The heat flow increment is comparatively higher at the trailing and the leading edge of the dimple and the protrusion because of flow separation, reattachment, and flow impingement. The thermal performance is the highest while RRP, RRH, d/D are 10, 0.036, and d/D = 0.35, respectively. The proposed roughness pattern has resulted in a 2.1 and 1.95 times higher thermal augmentation and overall performance of SAH over the conventional.

A numerical study on the interaction of droplet collisions and air flow impact in cross-impinging spray

The spray impingement method has been widely used to effectively achieve better atomization of liquid fuel. The factors influencing the formation of impinged spray are analysed using computational fluid dynamics (CFD) modelling. Free spray and impinging sprays with three impinging distances are simulated using the Eulerian-Lagrangian multiphase method. The results are validated with experimental data. Meanwhile, some advanced methods are employed to obtain better results of simulation such as the AMR technique and adaptive collision mesh method. The spray widths in different impingement distances are compared to analyse the distribution of spray droplets. Upon the binary collision theory, the distribution of discrete droplets is attributed to the collisions of huge amounts of particles. In the process of spray impingement, the impact of air flow, generated by gas entrainment of spray, also influences the spray. As a supplement to the collisions of discrete droplets, the impingement of continuous air flow and their interaction is mainly discussed. The impact waves observed in the flow field demonstrate that the impingement of entrained air flow is similar to jet impingement.

Turbulent Flow and Heat Transfer Enhancement for Turbine Blade Trailing Edge Cooling With Gyroid-Type Triply Periodic Minimal Surfaces

Abstract The Gyroid is a type of triply periodic minimal surface (TPMS) that has interconnected, perfectly curved topologies and excellent thermomechanical properties. Due to its topological feature to enhance heat transfer and self-support structure, this study presents numerical investigations of the flow, heat transfer, and pressure loss in various Gyroid architectures in a wedge-shaped channel, representing a trailing edge cooling for gas turbine blades. The Gyroid structures are partly arranged near the outlet of the wedged channel or are fully filled within the wedged channel. The local and overall flow and heat transfer mechanisms with different Gyroid configurations are compared to the baseline pin fins within the Reynolds number range of 10,000–30,000. The results show that for the case with partly infilled Gyroid structures, the overall heat transfer enhancement is higher by 39–102%, and the related pressure loss is higher by 93–154% than the baseline pin fins. For the case with fully infilled Gyroid structures, the total heat transfer is superiorly higher by 157–188%, and the related pressure loss is higher by 248–555% than the baseline pin fins. For all the Gyroid configurations, obviously improved cooling uniformity is achieved on the whole trailing edge wall. The significant heat transfer enhancement in the trailing edge channel with Gyroid structures is mainly due to a much increased wetted area, the generated helical and impingement flow through the curved interconnected channels and improved flow distribution within the wedged channel.

Hiemenz flow with heat transfer in a slip condition micropolar fluid model: Exact solutions

The impingement of Hiemenz stagnation-point flow and a flexible surface which has linear stretching/shrinking is studied. The flow is governed by the wall's velocity, gyration velocity, transpiration and velocity slip parameters. The stretching sheet problem offers single solution, whereas for the shrinking sheet dual solutions are obtained in the region s∗ ≤ s < ∞ , and λ ≤ λ∗. The stretching/shrinking strength parameter (c) under different values produces numerous exact solutions. The existence of critical numbers explains the increasing or decreasing nature of solutions. Dual solutions are found for skin friction, couple stress friction and temperature gradient. In the process of boundary layer, increasing suction or injection decreases velocity. The effect of velocity slip indicates a decrease in linear wall velocity. It was found that the number and formation of solutions are dependent on a parameter measuring correlation of the external flow strength to the surface stretching/shrinking parameter which is a key player in determining exact solutions. Exact solutions for the uniform surface temperature and linearly growing wall temperature are derived for different special cases.

The effect of short pin fin aspect ratio on thermal characteristics of intermittent impinging jet; An experimental and numerical study

BackgroundPin fins are secondary surfaces extending from an object to enhance the heat transfer rate. Pin fin is one of the most effective practical techniques used in many industrial applications and thermal treatments especially for cooling electronic devices. Also, Pulsating flow is extremely used in the machinery industry and it can improve the heat transfer. Therefore, pulsating flow is combined with an extended surface to enhance the heat transfer efficiency. MethodsThe current paper investigates numerically and experimentally the intermittent impingement flow and consequence heat transfer characteristics in the presence of a pin-fin array over a flat plate. The RNG k − ɛ model is employed to simulate unsteady three-dimensional turbulent flow using CFD software. The influences of jet Reynolds number, pulsation frequency, jet-to-surface distance, and aspect ratio on the distribution of the area-time averaged Nusselt number are studied. 36 elliptical pin-fins considered at three rows on the surface, placed at r/ D = 3, 5, and 7 (36, 60, and 84 mm) from the central point of the jet. Considering the influence of the aspect ratio on the streamline and temperature patterns of the flow, numerical simulations were conducted in various pin-fin diameters of 4–12 mm. Significant findingsFrom the experimental data, the results indicated that there is a specific frequency that the heat transfer increases for Reynolds numbers in the range of 10,000–20,000. The results illustrated that the rate of heat transfer is reduced with rising the diameter of the pin fin for AR>1, whereas it is enhanced for AR<1. The area-time averaged Nusselt number is enhanced with rising aspect ratio at various pin fin heights. For Re=10,000, the heat transfer rate decreases with increasing the jet-to-surface distance in steady state while it increases for pulsating flow. Also, the maximum value of turbulent kinetic energy appears nearby the pin-fin owing to the formation of a horseshoe vortex. The vortex structures are strengthened and grow downstream, as the frequency increases.

Experimental decoupled-analysis of overall cooling effectiveness for a turbine endwall with internal and external cooling configurations

High-efficient composite cooling structures that are comprised of external and internal cooling techniques, are one of the key technologies to guarantee the aerothermal performance and reliability of a turbine vane, especially the endwall region due to unique complex flow structures. It is thereby critical to understand the flow mechanism and conjugate heat transfer characteristics in this region for further optimal design. In this paper, a representative endwall model which matches non-dimensional parameters to real turbine operating conditions is investigated. Thermal protection for the endwall is provided by internal jet impingement and external purge flow and discrete hole injection cooling. Comprehensive measurements of overall cooling effectiveness over the endwall, total pressure loss coefficient, and non-dimensional temperature at the cascade exit are performed by using an infrared (IR) thermography technique, a five-hole probe, and a temperature probe. Through decoupled analysis, heat transfer and flow characteristics of film cooling only, both jet impingement and film cooling, upstream purge cooling only, and combined cooling are analyzed, respectively, and their optimal ranges of cooling air are obtained. The measurements reveal that with film cooling only, only the regions around the film holes are covered. Using both film and jet impingement cooling, cooling coverage is expanded, and cooling effectiveness is much higher with a more uniform distribution pattern. Purge flow achieves thermal protection for upstream regions of endwalls, and the combined cooling scheme that involves all cooling sources provides the best cooling performance. Globally, those cooling schemes have no obvious effects on overall cascade aerodynamic performance but affect the development and distribution of secondary flows.

Numerical Investigation on Thermal Performance of Plate-Fin Heat Sink Designs Subjected to Parallel and Impinging Flow

The electronic industry has been working for decades to improve the cooling efficiency of heat sinks by creating more advanced, efficient cooling technologies. However, heat dissipation remains the major problem in this highly competitive sector. Plate-fin heat sinks with and without fillet profiles were investigated and two new proposed designs for plate-fin heat sinks with half-round pins attached to the fin were developed in this study. Numerical analysis was performed using ANSYS FLUENT R21 to evaluate the thermal performance of the proposed designs. For the element optimization, the grid independency test was performed to obtain the optimal number of elements. A constant heat flux of 18.750 kW/m2 was applied at the bottom plate of heat sinks as the input parameter and two different flow directions e.g., impinging flow and parallel flow at various mass flow rate was also applied to study the base temperature, thermal resistance and Nusselt number of these designs. The study has shown that plate-fin heat sinks with fillet profile and corrugated half-round pins (PFHS 4) subject to parallel flow and plate-fin heat sinks with fillet profile and symmetrical half-round pins (PFHS 3) subject to impinging flow exhibit better thermal performance over other configurations. Hence, these design configurations have a potential to be applied in the future.

Effects of the Topography of Sumatra on Tropical Cyclone Formation over the Indian Ocean

One-quarter of the world’s tropical cyclones (TCs) occur in the Indian Ocean (IO) basin.The mechanisms for TC initiation in the IO are varied, but one recently discovered process involves the flow around the steep topography of Sumatra. When the low-level flow impinges on Sumatra, it is blocked and the flow splits under typical environmental stratification. As a result, wake vortices commonly develop at northern and southern island tips of the island. For the case of easterly flow, these circulationssubsequently move downstream over the IO. The wake vortices emanating from the island tips are counter-rotating, but since Sumatra straddles the equator, the circulations are cyclonic in both hemispheres and thus have the potential for TC development. Using data from2.5yearsof observations from DYNAMO and YOTC, it is found that approximately 25% of the TCsthat occurred overIO basin during that periodwere initiated by Sumatra-induced wake vortices.Additional analysis of vortex statistics for the period 2008-17 has found that vortex counts are highest near Madden-Julian Oscillation (MJO) phase 1 when low-level easterlies are strongest across southern Sumatra. A secondary peak in vortex formation occurs during MJO phase 4 when low-level westerlies exist near the equator west of Sumatra. The latter finding suggests that MJO-related, low-level westerly surges on the equator impinging on Sumatracontribute to an increase in wake vortex development. Numerical simulations have shown that circulations farther upstream such aswestern Pacific remnant TCs and the Borneo vortex can influence the development of Sumatra wake vortices and their growth into TCs over the IO.

Influence of impingement flows with sand particles on the barrier properties of organic coatings

Influence of impingement flows with sand particles on the barrier properties of organic coatings

Abstract Number ‐ 56: Investigation of Aneurysmal Initiation Factor with Experimental Animal Model by Computational Fluid Dynamics Analysis

Introduction Hemodynamics has been reported to be involved in the pathophysiology of the initiation of intracranial aneurysms. Computational fluid dynamics (CFD) simulation has been widely used to investigate what kind of hemodynamics is related to intracranial aneurysm development or rupture. However, in the case of humans, it is difficult to obtain cerebrovascular images before the aneurysm formation because most of them are detected incidentally by magnetic resonance angiography (MRA) or computed tomography angiography (CTA). Therefore, the mechanism of aneurysm initiation has not been elucidated yet. In this study, CFD analysis was conducted with experimental animal models. Furthermore, immunostainings and Elastica van Gieson (EvG) staining were also performed to investigate the relationship between hemodynamics and aneurysm initiation. Methods We selected the experimental aneurysm model of a rat proposed by Shimizu et al.[1] With this model, aneurysms can be induced with a certain probability at the artificial bifurcation site created by end‐to‐side anastomosis of bilateral common carotid arteries and induced hypertension. Among the 50 rats surgically manipulated, 5 de novo cases and 14 cases without aneurysm initiation defined as stable cases were included in this study. The other 31 cases were excluded because of death or lack of imaging quality. MRA was conducted weekly until 4 weeks after manipulation. Acquired images were processed for the CFD analysis and various kinds of hemodynamic parameters were calculated. Among them, we regarded dimensionless wall shear stress divergence (WSSD*) and pressure loss coefficient (PLc) as the hemodynamic parameters previously suggested to be related to aneurysm initiation[2]. Vessels were collected after 2 weeks of follow‐up, and we observed endothelial cells (EC), smooth muscle cells (SMC), and the internal elastic lamina (IEL) at the apex of the bifurcationby immunostaining and EvG staining. Results The CFD results showed that a high WSSD* area existed at the apex of bifurcation in both de novo cases and stable cases. The averaged WSSD* of the de novo cases was 18.2% higher than that of the stable cases (de novo: 0.577 ± 0.243, stable: 0.488 ± 0.235). Likewise, the average PLc of the de novo cases was 11.5% higher than that of the stable cases (de novo: 8.22 ± 1.76, stable: 7.37 ± 2.49). The immunostaining images demonstrated that EC and SMC were more degenerated in de novo cases than in stable cases. Furthermore, the EvG staining images showed that IEL degenerated only in the de novo cases. Therefore, because WSSD* and PLc indicate a stretching force and flow impingement on the vessel wall, respectively, the elevation of those parameters may contribute to damage of EC, SMC, and IEL known as pathological features of an intracranial aneurysm. Conclusions CFD analysis and tissue observation were conducted with the vessels of the experimental animal model (5 de novo cases and 14 stable cases). Our results indicate that high WSSD* and high PLc are the factors related to aneurysm initiation.

Abstract Number ‐ 57: Investigation on Hemodynamic Force Related to Thin‐walled Regions in Intracranial Aneurysm by Using CFD

Introduction Although thin‐walled regions (TWRs) in an intracranial aneurysm have a risk of rupture due to contact with surgical instruments, imaging modalities cannot accurately evaluate the thickness of aneurysm walls. The surgical operation will be able to perform safely by identifying the location of TWRs before the treatment. In previous studies, computational fluid dynamics (CFD) has been used to investigate the relationship between hemodynamics and TWRs. However, a quantitative method has not been employed to evaluate the location of TWRs. This study aims to clarify the relationship between hemodynamics and TWRs by comparing the results of CFD analysis with quantitatively defined TWRs. Methods We identified 70 aneurysms (MCA: 48, ACA: 20, ICA: 2) treated with craniotomy and clipping. CFD analysis was conducted to evaluate the pressure difference (PD), wall shear stress (WSS), and wall shear stress divergence (WSSD) on the aneurysm wall. High regions were defined as the regions with values above the 90th percentile for each parameter, and Low regions were defined as the regions with values below the 10th percentile for each parameter. In this study, 4 regions (HighPD, HighWSS, LowWSS, and HighWSSD) and 6 regions obtained by combining two of these regions were defined as regions of interest (RoI). Because TWRs generally indicate intense red, the comprehensive red value (cR value) was defined by using the RGB color model to evaluate the intensity of redness. The cR value was calculated for each pixel of the intraoperative images, and TWRs were defined using the cR value. Comparing the results of CFD analysis and identified TWRs, the percentage of the area of TWRs in RoI was calculated as the occupancy ratio. Results Table 1 shows the mean occupancy ratio for each defined RoI. The mean occupancy ratio in RoI by a single parameter is higher in the order of HighPD, HighWSSD, HighWSS, and LowWSS regions. In HighPD regions, which show the highest occupancy ratio, the impingement flow to the aneurysm wall is considered to make the aneurysm wall thinner. In HighWSSD regions, the aneurysm wall seems to have thinned due to the tensile force along the wall surface caused by blood flow. The mean occupancy ratio of RoI obtained by the combination of two regions is lower than that of HighPD regions. However, in some cases, TWRs that could not be identified by HighPD regions can be detected by HighPD or HighWSSD regions. Conclusions The mean occupancy ratio for RoI was higher in the order of HighPD, HighWSSD, HighWSS, and LowWSS regions. Therefore, it was suggested that these hemodynamic parameters are related to TWRs. Furthermore, in some cases, TWRs that the RoI of a single parameter could not identify can be detected using RoI obtained by the combination of two parameters.

Investigation of impingement heat transfer in double-wall cooling structures with corrugated impingement plate at small Reynolds numbers

This study investigated an impingement/effusion cooling scheme with a corrugated jet plate applicable to a high-temperature exhaust cooling nozzle. The primary objective was to evaluate the effect of the corrugated wall jet on the impingement heat transfer and flow loss in a narrow target chamber under a low jet Reynolds number ranging from 450 to 2700. Sinusoidal curves with different wavelengths were employed to model the corrugated jet plate, where impingement holes were located at the trough of the plate. The location of the impingement holes creates a staggering pattern relative to the effusion holes. The corrugated jet plates with relative ripple amplitudes (A/d) considered were 0.2, 0.5 and 0.8, and the results were compared with those of a flat jet plate. Three relative average jet-to-plate spacings, Have/d, were selected for use: 1.6, 2.6, and 3.6. The heat transferred over a smooth target surface by multiple jets was measured using the transient liquid crystal (TLC) method. Results show that the amplitude of the corrugated plate was found to profoundly influence the heat transfer of the target surface. The use of corrugated jet plates yielded a better heat transfer performance at a relatively high Re and low impinging distance. The optimal heat transfer performance was achieved at A/d of 0.5, which was 28 % higher than the flat plate. The discharge coefficient for the corrugated jet plate was 2–9.4 % lower than that of the flat jet plate at all Have/d and Re values, and the minimum discharge coefficient was reached at A/d of 0.2. Furthermore, internal impingement heat transfer and flow resistance characteristics of the corrugated impingement/effusion were correlated according to the experimental data.

Hydrophobicity induced drag reduction: Perspectives from the slip length

Hydrophobicity has been developed in many areas, whose potentials in drag reduction at microscale have attracted numerous attentions for expanding the practical applications in fields of on chip devices, materials synthesis, and enhanced heat transfer. In this article, we select polydimethylsiloxane (PDMS) as the base material, whose hydrophobic modifications have been well developed. Among them, hydrofluoric acid treated one shows great performance and leads us to two types of microchannels, the straight and U-shaped, with enhanced hydrophobicity (from 91° to 106°). The coefficients of the pressure drop are experimentally measured with the Reynolds number ranging from 0 to 300. The results illustrate that the drag reduction rate reaches at 37.8% for the straight microchannel and 26.8% for the U-shaped microchannel. With the increase in the Reynolds number, the drag reduction effect stays almost constant for the straight channel, while it decreases gradually for the U-shaped channel. The flow impingement induced by a centrifugal force has an important impact on the slip effect that grows with the Re. Next, we adopt the numerical method and the micro-particle imaging velocimetry measurement to analyze the drag reduction effect from perspectives of the slip length. We successfully derive the slip length model correlating the drag reduction effect. Our results not only achieve substantial drag reduction in PDMS microchannels, but also provide a quantitative correlation between hydrophobicity and drag reduction, offering a feasible strategy for extensive applications at microscale, such as fluid actuation, bio-chip analysis, and highly efficient cooling system.

Spatial distribution characteristics of splashing rainfall caused by nappe flow impingement on a plunge pool

Splashing is the main atomization source for a jet impinging on a liquid layer. Increased understanding of splashing rainfall characteristics can help to reduce the hazard of flood discharge atomization in hydraulic engineering. In this study, the spatial distribution of splashing rainfall caused by a nappe flow impinging on a downstream water cushion was experimentally investigated. Effects of the main hydraulic factors of impingement velocity, unit discharge, and water-cushion depth on splashing were investigated. The shape of splashing rainfall contours was approximately elliptical in horizontal planes. Maximum rainfall intensity was in the surrounding impingement region, and rainfall intensity decreased with an increase in the distance between the impingement center point and measurement points. Splashing rainfall intensity increased with increases in impingement velocity and unit discharge, whereas the opposite was observed with an increase in plunge pool depth. A gamma distribution described rainfall intensity distribution in the longitudinal and vertical direction, whereas a Gaussian distribution described intensity in the transverse direction. A series of empirical relations were proposed.

PLUME STRUCTURES IN NATURAL CONVECTION WITH TRANSPIRATION

We present plume structures at high Rayleigh numbers (Ra&lt;sub&gt;H&lt;/sub&gt; &amp;#126; 10&lt;sup&gt;11&lt;/sup&gt;) and Schmidt numbers (Sc &amp;#126; 675), in convection driven by density differences across a horizontal membrane, when vertical transpiration velocities in the range of 0.002 cm/s &amp;le; V&lt;sub&gt;o&lt;/sub&gt; &amp;le; 0.065 cm/s are imposed. The density differences across the membrane are created by having brine with higher concentrations above the membrane and ammonium chloride with lower concentrations below it, with the bottom fluid being forced upward across the membrane by gravitational heads to create transpiration. Planar laser induced fluorescence (PLIF) visualizations in a horizontal plane grazing the top of the membrane, with refractive index matching, brings out the plume structures and their evolution with Ra&lt;sub&gt;H&lt;/sub&gt; and V&lt;sub&gt;o&lt;/sub&gt;. The plume structures show complex dendritic patterns of line plumes with plume-free patches; the plume-free patches decrease in area and number, with decrease in Ra&lt;sub&gt;H&lt;/sub&gt; and V&lt;sub&gt;o&lt;/sub&gt;. At low V&lt;sub&gt;o&lt;/sub&gt;, these plume-free patches have aligned plumes around them, while at larger V&lt;sub&gt;o&lt;/sub&gt;, such aligned plumes are not seen. We expect these patches to be due to the impingement of large-scale flow at low V&lt;sub&gt;o&lt;/sub&gt;, while they could be due to the modification of boundary layer instability at large V&lt;sub&gt;o&lt;/sub&gt;. These plumes become closer with increase in Ra&lt;sub&gt;H&lt;/sub&gt;, while they become more separated with increase in V&lt;sub&gt;o&lt;/sub&gt;. We quantify this variation by measuring the total plume length (L&lt;sub&gt;p&lt;/sub&gt;) by skeletonizing the plume structures and calculate the mean plume spacing (&amp;lambda;) from L&lt;sub&gt;p&lt;/sub&gt;; &amp;lambda; show nontrivial, non-power law dependence on Ra&lt;sub&gt;H&lt;/sub&gt; and V&lt;sub&gt;o&lt;/sub&gt;, different from those observed for the case of no throughflow and low throughflows.

Experimental investigation on heat transfer and pressure drop characteristics of confined jet impingement boiling on hybrid-structured surface

In this study, confined jet impingement boiling experiments are carried out using ammonia for the heat dissipation of high-heat-flux hotspot. A hybrid-structured surface with triangular prism-convex structure in the stagnation zone and microchannel structure in the wall jet zone is designed for heat transfer enhancement. Utilizing the advantages of jet impingement and microchannel flow boiling, the temperature of the heating surface is maintained below 86.5 °C when subjected to a hotspot heat flux of 1367 W/cm2, verifying the promising cooling performance of jet boiling on hybrid-structured surface. Effects of jet velocity (0.34–7.07 m/s), heat flux (752, 1064, and 1367 W/cm2), saturation temperature (22, 26, and 30 °C), and inlet condition (subcooled, near-saturated, and two-phase state) are also investigated. Increasing jet velocity and saturation temperature are beneficial to the heat transfer of jet boiling in the central stagnation zone. For high jet velocity flow (V > 2.3 m/s), the junction-to-fluid thermal resistance is the lowest at heat flux of 753 W/cm2; while for low jet velocity flow (V < 2.3 m/s), the best cooling performance is obtained at heat flux of 1367 W/cm2. The pressure drop increases with jet velocity and inlet vapor quality, and shows no significant dependence on heat flux and saturation temperature.