High-speed Flow Visualization Research Articles

Fluid-Structure Interactions of a Fin over a Compliant Panel in Supersonic Flow

The fluid-structure interactions of a blunt fin mounted over a compliant panel in Mach 2 flow were investigated. The test geometry consisted of a thin, flexible panel fixed on all edges with a blunt fin with a hemispherically rounded leading edge mounted in a manner that the leading edge of the fin overhung the panel. The blunt fin was also pivoted to several angles of attack with respect to the oncoming flow. This allowed for the study of the dynamic system created by the interactions between the compliant panel and the blunt fin geometries. Both rigid and compliant panel models were used in order to provide a comparison between the flow with and without the dynamics of the compliant panel. High-speed schlieren and surface oil flow visualizations showed that the time-averaged flow remained almost entirely unchanged between the rigid and compliant panels. The instantaneous flow fields, however, showed a highly dynamic system where the compliant panel induced oscillatory shock waves that were both stationary and freely moving. Simultaneous high-speed schlieren visualization and stereo digital-image correlation showed a strong linkage between the motion of the shock waves and the deformation of the panel. This was demonstrated through frequency analysis and modal decomposition of the panel deformation and shock motions.

Experimental investigation into thermal characteristics of subcooled flow boiling in horizontal concentric annuli for cooling ultra-fast electric vehicle charging cables

In an effort to reduce the charging time of electric vehicles (EVs), the thermal management of the charging system has emerged as one of the most urgent issues. The charging rate has been restricted to avoid thermal failure especially in charging cable. Recently, a direct contact cooling technique that utilizes subcooled flow boiling for charging cables has been proposed and has shown promising thermal management performance. However, there has been a lack of clear explanation regarding its thermal characteristics due to its intrinsically complicated flow features. This study investigates the thermal characteristics of subcooled flow boiling in a horizontally aligned concentric annular tube intended to simulate the direct contact cooling technique employed for charging cables. For the experimental investigation, the wall temperature measurements and high-speed flow visualization images acquired from the transparent test module annulus, with inner and outer diameters of 6.35 mm and 22.0 mm, were utilized. Three key axial thermal characteristics of subcooled flow boiling in annulus have been analyzed, and they are flow regime transition, variation of heat transfer mechanisms, and occurrence of critical heat flux (CHF). The transition of flow regimes and the variation of dominant heat transfer mechanisms in the axial direction are mutually influencing due to the coupled effects between the hydrodynamic and thermal characteristics of simultaneously developing flows. The flow regime changes from partially developed boiling (PDB) to fully developed boiling (FDB) as the intensities of nucleate boiling and bubble recondensation vary in the axial direction, while the dominant heat transfer mechanism shifts from single-phase convection to nucleate boiling, corresponding to the flow regime transition from PDB to FDB. The departure from nucleate boiling (DNB) type CHF is observed, manifested by the wavy interface propagating from the far downstream to the upstream region, and the intensities of nucleate boiling and bubble recondensation are also identified as the dominant parameters in determining the occurrence of CHF. Charging time estimation reveals that the proposed method can achieve an 80 % charge for 100-kWh EV batteries in 98 s, while maintaining the cable wire temperature below the safety limit of 80 °C. This strongly supports its potential as a promising thermal management technique for future ultra-fast charging systems.

Microscale heat transfer and phase change mechanisms of carbon dioxide at near-critical conditions

Recent studies highlight the potential benefits of two-phase carbon dioxide (CO2) systems operating near their thermodynamic critical point. Significant heat transfer coefficients and resistance to critical heat flux from improved vapor phase properties can enable heat transfer and reliability enhancements. However, the physical mechanisms of phase change that govern heat transfer behavior in the near-critical regime of reduced pressures (0.85<PR<1, where PR=P/Pcrit) are not well understood. In the present study, subcooled microscale (hydraulic diameter, Dh=500μm) flow boiling experiments are conducted to elucidate the heat transfer behavior near the critical point and the underlying mechanisms that influence it. Experimental data reveals a deviation from the trends predicted by the Cheng correlation (Cheng, L et al., 2008, “New prediction methods for CO2 evaporation inside tubes: Part II—An updated general flow boiling heat transfer model based on flow patterns.” Int. J. Heat Mass Transfer), in particular, displaying a decreasing peak heat transfer coefficient as pressure increases. Heat transfer in the near-critical regime can be further partitioned into two regions: near-critical boiling (0.85<PR<0.96) that exhibits heat transfer highly dependent on surface temperature, and supercritical-like behavior (0.96<PR<1) exhibiting a strong heat transfer dependence on mass flux. High-speed side-view flow visualization reveals the dominant nucleation mechanism (heterogeneous or homogeneous) governs the heat transfer behavior and is adequately predicted using flow inlet conditions. The present study provides a method for predicting nucleation behavior, which, in turn, facilitates the prediction of heat transfer near the critical point. The continuous alteration of heat transfer in the near-critical regime as a function of PR is underscored by this mechanistic insight, bridging the gap between typical flow boiling (PR<0.85) and supercritical heat transfer (PR>1).

Coaxial air blast atomization of a particulate gel suspension jet

The objective of this study is to experimentally and theoretically explore the impact of particle volume fractions and nozzle thread structure on the coaxial air blast atomization of particulate gel suspension jets through the high-speed flow visualization technology. The four breakup modes of particulate gel suspension jets are confirmed, namely oscillation mode, membrane-type breakup, fiber-type breakup, and superpulsating submode. However, the fiber-type breakup of jets disappears as the particle concentration reaches 40%, primarily attributed to the numerous particles sharply promoting the instability on the free jet surface. The experimental statistical results demonstrate that an increase in particle concentration and the adoption of the threaded nozzle both result in an expansion of the jet spray angle. The breakup length exhibits a reduction trend with the increase of particle concentration. However, it remains nearly the same value as that in the case of volume fraction 20% when the particle concentration continues to increase, contributed by the surge of viscous dissipation counteracting the enhanced instability on the jet interface caused by a large number of particles. The breakup length of a pure gel jet is predicted through the linear instability analysis, which is further modified by the viscosity model of particle suspensions to derive the breakup lengths of particulate gel suspension jets with different concentrations. The theoretical predictions align well with the statistical results in the experiment. The research is poised to have potential implications for numerous industrial processes and engineering applications including gel propellants.

Aerodynamic breakup of gel suspension droplets loaded with aluminum particles

The persistent and frequent space explorations are affected by the performance of propellants, among which the closely related gel propellants have been widely used due to their unique characteristics. Currently, research efforts primarily concentrate on the metallization and atomized combustion of gel propellants, emphasizing the combustion performance of metallized gel propellants. However, little attention has been given to the secondary breakup process that occurs just before combustion. By utilizing high-speed flow visualization technology, this study examines the key aerodynamic breakup process of gel suspension droplets loaded with micro aluminum particles, experimentally and theoretically investigating the droplet dynamics induced by particle volume fraction. The experimental confirmation of the three breakup behaviors exhibited by gel suspension droplets, namely oscillation mode, bag-stamen breakup, and hemline breakup, is presented along with the construction of a phase diagram. The impact of heterogeneous effects resulting from an increase in particle volume fraction on the critical breakup Weber number is revealed, with a concentration of 9 % as the turning point. Within the range of moderate Weber numbers, the tail flick deformation of suspension droplets in airflow is observed and reported for the first time. At different particle concentrations, there keeps a linear correlation between the dimensionless tail length and Weber number. Spherical suspension droplets will deform into thin disks in the airflow, with a nearly constant deformation rate regardless of changes in particle concentration. Additionally, the relationships between characteristic times and droplet parameters are discussed through statistical analysis of the results. As Ohnesorge number and particle volume fraction increase, the characteristic times both show a monotonically increasing trend. Moreover, a model of unstable growth rate associated with the particle volume fraction is established through linear instability analysis, accurately predicting the variation in critical Weber number for gel suspension droplet breakup. The congruence between our theoretical framework and experimental findings not only enhances comprehension of the impact of micro aluminum particles on the secondary breakup of gel suspension droplets but also provides valuable guidance for the aerospace applications of metallized gel propellants.

Исследование течения, создаваемого скользящим разрядом, с помощью методов компьютерного зрения

A quantitative study has been made of the flow with shock waves generated in air by a sliding surface discharge lasting less than one microsecond. The high-speed flow was visualized using the shadowgraph method, the process was recorded at a rate of 124 000 frames/s, the exposure time was 1 μs. The aim of this work is to study the dynamics of the two discontinuities: the cylindrical shock wave and the contact surface generated by the discharge. Each experiment allowed several hundred images to be taken of a short-lived gas-dynamic process lasting up to 1 ms. A YOLOv8 convolutional neural network was trained and used to determine the positions of the discontinuities. A data set of 984 markups was labeled. The model on the mAP50 metric achieved 0.887 and the mAP50-95 was 0.557. The model was used to automatically measure the vertical dimensions of the contact discontinuity. It expands at times up to 0.4 - 0.8 ms to a vertical size of 5 - 11 mm. The x-t plots and the velocities of the cylindrical shock waves were measured. It is shown that up to 1 ms after the discharge, the flow development is due to the blast wind motion behind the shock wave. It is shown that the use of computer vision can significantly speed up the analysis of high-speed flow visualizations and the extraction of quantitative information.

High-Speed Flow Visualization by a Nanosecond Volume Discharge during Shock Wave Diffraction on an Obstacle

We study the spatial structure of nonstationary inhomogeneous supersonic airflows as shock wave diffraction on an obstacle occurs in a shock tube of a rectangular cross section. The Mach numbers of shock waves were 2.7–4.4 at initial air pressures of 10–30 Torr. The supersonic flow in the discharge chamber was visualized by high-speed shadowgraphy and by the registration of radiation of combined volume discharge by photo camera and by ICCD camera. In experiments, a combined volume discharge with a current duration of ~ 500 ns was initiated 40–150 μs after the initial shock wave have passed an obstacle. It has been established that the radiation of the volume phase of discharge lasts 400–700 ns, and the displacement of the flow during this time does not exceed 0.6 mm. A correlation is established between the spatial distribution of discharge radiation and the low-density local areas determined as a result of two-dimensional Navier-Stokes based numerical simulation of the flow. As visualized by the glow of the discharge, the shape of the shock wave front is in good agreement with the results of shadowgraphy at different stages of diffraction and with the numerical simulation results.

Experimental Investigations on High-Speed Aerodynamic Heating over Inflatable Decelerators

Heat transfer measurements and high-speed schlieren flow visualization were performed over a cylinder with an aft-attached inflatable decelerator configuration at Mach 3.9. Back wall temperature measurements using thermocouples were used to derive the heat flux distribution over the decelerator surface. A strong nonlinear rise in peak heat flux with angle of attack is observed and windward–leeward effects are quantified. Using appropriate flow and geometric symmetries, heat maps are generated over the regions of the decelerator, which are critical for its thermal design. The effect of fore-end bluntness in terms of peak heat fluxes and integrated heat loads is investigated. The results indicate a possibility of flying at a small angle of attack to reduce the integrated heat loads while simultaneously generating some lift. Tests up to an angle of attack of 20 deg show that even though the peak heating rises strongly, the overall integrated heat loads on the surface of the decelerator remain manageable. A local minimum is seen in peak heat flux levels on the surface at angles of attack of 12–16 deg.

Upper part-load instability in a reduced-scale Francis turbine: an experimental study

Francis turbines with medium or high specific speeds may experience a particular type of instability in the upper part load in which the precessing vortex has an elliptical shape. The occurrence of the upper part-load instability (UPLI) is accompanied by large-amplitude pressure fluctuations at a distinct frequency between 2 and 4 times the runner rotational speed. This paper experimentally investigates UPLI for a reduced-scale Francis turbine. To investigate the causal factors of this instability, draft tube pressure measurements, particle image velocimetry, and high-speed flow visualizations have been performed at several operating points under cavitation and cavitation-free conditions. It is shown for the first time that for an operating point within the UPLI range, the vortex always features a circular section in cavitation-free conditions, which is preserved even after the initial appearance of cavitation. It is only below a certain Thoma number that the vortex section turns into an ellipse and shows an abrupt increase in pressure fluctuations. Analysis of the phase-averaged velocity fields reveals that a concentrated vortex with a large precession radius is a prerequisite for UPLI, while the instantaneous velocity fields clearly illustrate the asymmetric velocity distribution around the elliptical vortex. The existence of a breathing mode and the intermittent formation of two side vortices along the elliptical vortex rope are also evidenced by high-speed flow visualizations. These results provide a much deeper insight into the flow structures that favor the development of UPLI and help delimit its thresholds to higher precision, and thus, prevent its occurrence during turbine operations.

Flow and acoustic characteristics of convergent-divergent nozzle with and without wedges

The advancements in flight have increased over the years to achieve and provide comfortable flights to passengers. In addition to shorter travel time, passengers expect good comfort and an eco-friendly environment with no noise around them. Most of the present turbofan engines are producing much noise due to the supersonic jets created at the nozzle. By increasing the mixing rate at the exit, it is possible to reduce the noise and increase the performance. The enhanced flow mixing can be achieved by using many methods such as active and passive control methods including co-flow jets and tabs. Here in this research, the selected method to provide the controlled jet was using wedges. The significance of using this method is its mixing efficiency by following the principle of entrainment of atmospheric air into the exit jet. The wedges used here act as a vortex generator. These produced vortices weaken the shocks and the shear layer thus the potential core length is reduced. The CFD analysis and experimental analysis were also done and the results were compared. The experimental analysis was carried out without and with a wedge nozzle. In experimental analysis 2 bar and 4.74, bar gauge pressure was used and for both cases and the Mach Number was 1.8. The high-speed jet facility schlieren flow visualization and acoustic analysis were carried out. In CFD analysis flow characteristics of 2 bar gauge pressure such as contours and graphs were obtained for both with and without wedge nozzle. From the results, it is clear that the second shock-cell length was reduced with the use of wedges.

Effect of turbulence modulation caused by thread structure on coaxial air-blast atomization

This study aims at investigating the influence of turbulence modulation caused by a thread structure on coaxial air-blast atomization by means of high-speed flow visualizations and droplet particle size techniques. The medium in the central channel of an atomizer is water while the annular channel is airflow. The results show that the thread structure added to the inner surface of an annular channel plays an important role in atomization effect. The generated liquid ligaments on the jet present more structures, which grow shorter and breakup faster than that without thread. To compare the difference in jet breakup length and droplet diameter caused by the thread structure, we establish a new breakup length model and then use the ratio of structure factors to describe the change in the droplet diameter. The results in this experiment confirm the improvement of atomization performance brought by optimization design of the thread structure.

Impact of pressure drop oscillations and parallel channel instabilities on microchannel flow boiling and critical heat flux

Microchannel flow boiling heat sinks that leverage the highly efficient heat transfer mechanisms associated with phase change are a primary candidate for cooling next-generation electronics in electric vehicles. In order to design flow boiling heat sinks for such practical applications, one key obstacle is an understanding of the conditions for occurrence of dynamic two-phase flow instabilities, to which microscale flow boiling is particularly susceptible, as well as their impact on heat transfer performance. While mapping the operational regimes of these instabilities has been well-studied, with numerous stability criteria available, their impact on the heat transfer performance of heat sinks in practical applications is not understood. This work seeks to assess the impact of pressure drop oscillations and parallel channel instabilities on the surface temperature and critical heat flux in parallel microchannel heat sinks. This is achieved through measurement of time-averaged steady-state temperatures and pressures, combined with high-frequency pressure signals and high-speed flow visualization. These data are compared across three controlled flow configurations that comprise a condition of stable flow boiling, a condition where only parallel channel instabilities can occur, and a third where both pressure drop oscillations and parallel channel instabilities can occur. Experiments are performed using the dielectric refrigerant HFE-7100 in 2 cm long parallel microchannel heat sinks with square-cross-section channels (0.25, 0.5, 0.75, and 1 mm widths) at three mass fluxes (100, 400, and 1600 kg/m2s). Across this range of conditions, the time-averaged surface temperature and critical heat flux were remarkably insensitive to the occurrence of these instabilities despite the significant hydrodynamic events and transient flow patterns observed.

Linear instability analysis of a shear thinning gelled jet with yield stress in coaxial atomization.

This study aims to experimentally and theoretically investigate the roles of a threaded nozzle and yield stress in the coaxial air blast atomization of a gelled jet by means of rheological tests and high-speed flow visualization techniques. Similar to the breakup types of Newtonian fluids, a gelled jet will gradually exhibit an oscillation mode, membrane-type breakup, fiber-type breakup and a superpulsating submode as the airflow velocity increases. The transition conditions for each type are determined and plotted in the dimensionless map of We-Oh. For the jet spray angle, the maximum occurs when We = 600 and is clearly enlarged by the gas-side thread in the nozzle. The numerical results of the linear stability analysis indicate that a smaller dominant unstable growth rate is acquired by swelling the yield stress, while the higher dominant unstable growth rate and truncated wavenumber appear as the rotational airflow is acquired through the gas-side threaded nozzle.

Experimental research on surface acoustic wave microfluidic atomization for drug delivery

This paper demonstrates that surface acoustic wave (SAW) atomization can produce suitable aerosol concentration and size distribution for efficient inhaled lung drug delivery and is a potential atomization device for asthma treatment. Using the SAW device, we present comprehensive experimental results exploring the complexity of the acoustic atomization process and the influence of input power, device frequency, and liquid flow rate on aerosol size distribution. It is hoped that these studies will explain the mechanism of SAW atomization aerosol generation and how they can be controlled. The insights from the high-speed flow visualization studies reveal that it is possible by setting the input power above 4.17 W, thus allowing atomization to occur from a relatively thin film, forming dense, monodisperse aerosols. Moreover, we found that the aerosol droplet size can be effectively changed by adjusting the input power and liquid flow rate to change the film conditions. In this work, we proposed a method to realize drug atomization by a microfluidic channel. A SU-8 flow channel was prepared on the surface of a piezoelectric substrate by photolithography technology. Combined with the silicon dioxide coating process and PDMS process closed microfluidic channel was prepared, and continuous drug atomization was provided to improve the deposition efficiency of drug atomization by microfluidic.

Experimental heat transfer results and flow visualization of vertical upflow boiling in Earth gravity with subcooled inlet conditions – In preparation for experiments onboard the International Space Station

Since 2012, researchers at the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) and NASA Glenn Research Center have been collaborating on a long-term effort to study flow boiling and condensation in microgravity. The ultimate goal has been to develop the Flow Boiling and Condensation Experiment (FBCE) for the International Space Station (ISS). Based on the findings from prior flow boiling experiments both at different orientations in Earth gravity and onboard parabolic flights simulating short durations of microgravity, a final refined experiment design, construction, and operating procedure have been arrived at for long-duration microgravity flow boiling experiments onboard the ISS. This study investigates flow boiling of n-Perfluorohexane with subcooled inlet in a rectangular channel of dimensions 114.6 mm heated length, 2.5 mm width, and 5 mm height. These pre-launch experiments (Mission Sequence Testing) were conducted in vertical upflow orientation in Earth gravity using the same experimental rig that was launched to the ISS in August 2021. The various operating parameters varied are heating configuration (single- and double-sided), mass velocity (180 – 3200 kg/m2s), inlet subcooling (+0 – 32 °C, encompassing both highly subcooled and near-saturated inlet conditions), and inlet pressure (119 – 191 kPa). High-speed video flow visualization images are presented to explain the two-phase interfacial physics within the channel's heated section. Heat transfer results in terms of flow boiling curves, streamwise profiles of wall temperature and heat transfer coefficient, and averaged heat transfer coefficients are analyzed and parametric effects elucidated. Severe temporal thermodynamic equilibrium is observed for near-saturated inlet at very low velocities. Nucleate boiling degradation starts at larger heat fluxes for single-sided heating than double- sided at low mass velocities with highly subcooled inlet, and conversely at high mass velocities with near-saturated inlet. Nucleate boiling degradation can be delayed to higher heat fluxes by highly subcooling the inlet and increasing mass velocity. The entire local heat transfer coefficient profiles are degraded at higher heat fluxes for near-saturated inlet, but only the downstream part for highly subcooled inlet. This study also confirmed reliability of the upcoming ISS experimental data for subcooled inlet conditions and the collected Earth-gravity data will be used for comparison against the ISS data.

Transient Flow Boiling and Maldistribution Characteristics in Heated Parallel Channels Induced by Flow Regime Oscillations

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m² into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m², boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (&#x007E;7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations.

Effect of double cross sectional ratio on performance characteristics of pulsating heat pipes

Effect of double cross sectional ratio on performance characteristics of pulsating heat pipes is experimentally investigated. The cross sectional ratios in the evaporator and condenser sides are different from each other, and this novel gradually constricted geometry provides to boost flow circulation and to maintain a stable performance regardless of orientation. Tests are performed for different values of inclination angle (0°, 30°, 60° and 90°) and filling ratio (20, 40 and 60%). Also, results of originally structured closed loop flat plate pulsating heat pipe (novel design, ND) are compared with those of the conventional pulsating one with uniform cross section (conventional design, CD). Measurements are supported by high speed flow visualization. It is shown that the ND shows better thermal performance than the CD, and when appropriate filling ratio (40%) is provided, the ND presents orientation-independent running. A turn of the ND is divided into three different zones, and a pressure-balance equation is presented following a detailed flow analysis. Rapid bubble growth instability and annular flow pattern characterized by thin film evaporation are two important characteristics observed for the ND.

Flow boiling heat transfer enhancement under ultrasound field in minichannel heat sinks

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.

Flow Boiling Heat Transfer Enhancement Under Ultrasound Field in Minichannel Heat Sinks

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.

Onset of Boiling, Heat Transfer, and Flow Patterns of Flow Boiling on the Superhydrophobic Porous Copper Surface in a Microchannel

Abstract To explore the effect of microstructured porous surface on enhancing the heat transfer of flow boiling in a microchannel, the microporous copper surface was fabricated with microscale pores in ranges of 1–5 μm, which presented super-hydrophobicity. Subcooled flow boiling experiments were carried out to study the hydraulic and thermal transport performance in a single narrow rectangular microchannel. High-speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux and mass flux on heat transfer performance and flow patterns for originally hydrophilic bare copper surface and superhydrophobic microporous structured surface. The onset of boiling (ONB) characteristics of both test surfaces was compared with predictive correlations with a good agreement and verified by the captured flow pattern images. Benefit from the superhydrophobic wettability provided by its microscale porous structures and a large number of potential nucleation sites, the required wall superheats, and imposed heat fluxes of onset of boiling both decreased for the modified surface. The flow patterns on the two surfaces varied, which resulted in the different trends of heat transfer coefficient (HTC) with mass fluxes and heat fluxes. Because of the strengthened bubble departure process, the enhancement of the porous surface compared to the original bare surface gradually increased with mass fluxes. The average two-phase heat transfer coefficients of the superhydrophobic porous copper surface were enhanced for up to 74.84%, due to the earlier onset of boiling, higher frequency of bubble nucleation, and strengthened boiling intensity.