•A cavitation structure of double-hump PTLVC is firstly observed in mixed flow pump.•Double-hump PTLVC is classified: incepting, growing, merging, propagating stage.•Propagating velocity of the front ...hump of PTLVC reaches the maximum of 0.38 Utip.•Empirical function proposed for projected area, axial thickness, collapse position.
Tip leakage cavitation remains an unsolved problem that threatens the safe operation of hydraulic machines and plagues researchers worldwide. The objective of this work is to investigate the classification and spatial-temporal evolution of tip leakage cavitation, and even to provide additional insights into the flow physics. Experiments are conducted in a mixed flow pump installed on a closed-loop test rig. High-speed visualizations are performed to capture the flow patterns of tip leakage cavitation at rated flow rate. It is demonstrated that tip leakage vortex cavitation can be categorized as primary tip leakage vortex cavitation (PTLVC) and secondary tip leakage vortex cavitation (STLVC). A new tip leakage cavitation structure, named as the double-hump PTLVC, is firstly observed in the mixed flow pump under severe cavitation conditions. The spatial-temporal evolution of the double-hump PTLVC is classified into four stages: incepting stage, growing stage, merging stage and propagating stage. The averaged propagating velocity of the front hump of PTLVC increases with decreasing net positive suction head (NPSH), and reaches the maximum of 0.38 Utip in the present experiment. Three empirical functions are proposed to describe the relationship between projected area, the maximum axial thickness, circumferential collapse position and NPSH, respectively. It is found that for every 0.1 m drop in NPSH, the projected area increases by about 2.1%, the maximum axial thickness increases by about 2.7%, and the circumferential length of the PTLVC increases by about 3.5%, respectively.
•The effect of the orifice diameter of a novel passive pre-chamber (PC) on the combustion process was assessed.•The PC with a 1.2 mm orifice diameter showed the fastest combustion, followed by 1.5 ...and lastly 1.0 mm.•The SI engine equipped with the PC showed a higher equivalent flame front velocity than the typical operation.
Due to its potential to extend the lean limit or overcome knock-limited conditions on spark-ignited (SI) engines, the pre-chamber spark-ignition (PCSI) concept has gained attention as an efficient alternative to the conventional ignition architecture. Nevertheless, especially in engine-like conditions, the effect of the main geometrical parameters on combustion development is still not fully understood and might be a hindrance to the use of this technology. In this sense, the current study aims to evaluate the effect of the orifice diameter on the jet characteristics and main chamber combustion of a novel passive pre-chamber in an optically accessible single-cylinder SI engine, as well as to compare the PCSI with the typical SI ignition method. High-speed broadband chemiluminescence imaging was used to track the jet penetration and flame front propagation of three mixture compositions at various spark timings (ST). Additionally, based on the in-cylinder pressure, a heat-released analysis was performed to assess the overall combustion process. When compared with the typical SI, the pre-chamber concept presented a higher equivalent flame front velocity, which is also corroborated by the higher in-cylinder pressure rise rate and rate of heat released. For all mixtures tested, the 1.2 mm orifice diameter PC presented the shortest flame development angle (crank angle interval between ST and 10 % of mass fraction burned in the MC), followed by 1.5 and lastly 1.0 mm. When comparing different STs, the shortest flame development angle occurred at MBT condition. Despite the highest flame development angle, the PC 1.0 mm case presented the lowest main chamber combustion duration (CA10-90), which increases as the orifice diameter is increased.
The Roof-attached Vortex (RAV), Floor-attached Vortex (FAV) and Combined Submerged Vortices (CSV) induced by the closed sump of a tidal pumping station have become an intricate problem that urgently ...needs to be clarified in expanding the stable operation range of units. This work reveals the dynamic evolution laws of various vortices and their typical forms under different flow conditions based on a combination of high-speed visualization and scale-resolving simulation methods. The results show that the RAV is a single spiral vortex structure under low flow rates and a main-secondary vortex double spiral structure under high flow rate conditions. The increase in flow rate has a positive promoting effect on the growth of the FAV. The RAV and FAV frequency values of occurrence are in the range of 0.001–0.1 Hz and 0.007–0.06 Hz, respectively. The maximum frequency of the CSV is 0.03 Hz. In the symbiotic stage of the CSV, there is an interaction between RAV and FAV. The suction of the surrounding low-pressure system affects the FAV, with vertical vorticity propagating towards the side of the RAV. In addition, the tangential velocity distribution of RAV and FAV exhibit rigid-like rotation characteristics consistent with the Burgers vortex model.
•Typical morphology and evolution characteristics of various vortices in a closed sump have been revealed.•The frequency of multiple vortices occurring is statistically analyzed.•The instantaneous characteristics of dynamic parameters of the combined submerged vortices are explored.•The roof-attached and floor-attached vortex exhibit tangential velocity distribution characteristics like rigid-body rotation.
•In situ observations of bubble dynamics in ultrasonic emulsification (USE).•Role of gas bubbles with different cavitation modes in USE is revealed.•The chaotic oscillation of gas bubbles is the ...governing cavitation mode in USE.•Increasing gas content in USE generates smaller and more stable emulsion droplets.
Ultrasonic emulsification (USE) assisted by cavitation is an effective method to produce emulsion droplets. However, the role of gas bubbles in the USE process still remains unclear. Hence, in the present paper, high-speed camera observations of bubble evolution and emulsion droplets formation in oil and water were used to capture in real-time the emulsification process, while experiments with different gas concentrations were carried out to investigate the effect of gas bubbles on droplet size. The results show that at the interface of oil and water, gas bubbles with a radius larger than the resonance radius collapse and sink into the water phase, inducing (oil–water) blended liquid jets across bubbles to generate oil-in-water-in-oil (O/W/O) and water-in-oil (W/O) droplets in the oil phase and oil-in-water (O/W) droplets in the water phase, respectively. Gas bubbles with a radius smaller than the resonance radius at the interface always move towards the oil phase, accompanied with the generation of water droplets in the oil phase. In the oil phase, gas bubbles, which can attract bubbles nearby the interface, migrate to the interface of oil and water due to acoustic streaming, and generate numerous droplets. As for the gas bubbles in the water phase, those can break neighboring droplets into numerous finer ones during bubble oscillation. With the increase in gas content, more bubbles undergo chaotic oscillation, leading to smaller and more stable emulsion droplets, which explains the beneficial role of gas bubbles in USE. Violently oscillating microbubbles are, therefore, found to be the governing cavitation regime for emulsification process. These results provide new insights to the mechanisms of gas bubbles in oil–water emulsions, which may be useful towards the optimization of USE process in industry.
•O/W emulsion preparation by ultrasound is more complex than believed until now.•High-speed camera recordings reveal additional intermediate W/O and O/W/O stages.•Mechanical effects driving ...ultrasonic emulsification are discussed.•The new ultrasonic emulsification model is proposed.
Today emulsion preparation is receiving a lot of scientific attention, since emulsions are playing an essential role in many of the big industries, such as food, pharmaceutical or cosmetic industry. One of the most promising techniques for emulsion preparation is ultrasound emulsification. The purpose of this study is to expand the knowledge on the ultrasonically assisted emulsification model, that has not been amended since 1978. The model explains that oil-in-water emulsion formation is a two-step process. Firstly, the surface of the oil phase is disturbed and separated by the acoustic waves. Secondly, cavitation implosions further disrupt and disperse oil drops. We have used a high-speed camera to closely observe oil-in-water emulsion formation. The images show, that the ultrasound emulsification process is profoundly more complex. While the first and the last step of emulsion formation are the same as believed until now, additional intermediate stages of water-in-oil and even oil-in-water-in-oil occur.
•Experiments with saturated water and ethanol boiling up to CHF were carried out.•Dynamics of vapor bubble growth and separation was studied in wide range of heat fluxes.•Evolution of microlayer ...regions and growth rate of dry spots were measured.•The behavior of two-phase systems near a heating wall at high heat flux was described.
In this paper the results of experimental study of vapor bubbles dynamics at pool boiling of various liquids in a wide range of heat fluxes up to q/qCHF ∼ 0.9 are presented. The experiments were performed at boiling of saturated water and ethanol at atmospheric pressure with the use of high-speed experimental techniques including video macro-visualization and IR thermography from the bottom side of a transparent heated sample. As a result, new data on the growth rate of vapor bubbles and dry spots in their base, evolution of the liquid microlayer region and unsteady temperature field of a thin film heater surface were obtained, and analysis of patterns of process at low heat fluxes was carried out. The usage of high-speed experimental techniques also allowed in this study to investigate the evolution of vapor bubbles with formation of vapor patterns and large agglomerates, to study dry spots dynamics, to estimate void fraction close to heating surface at fully developed nucleate boiling regime up to the critical heat flux depending on liquid properties. Obtained experimental information can be further used to construct more accurate physical models for the theoretical description of microcharacteristics, heat transfer and boiling trigger mechanisms at nucleate boiling of liquids with different physical properties.
•Cavitating flow over a foil is studied using experiments and numerical simulations.•The studied cavitating flow exhibits both large and small-scale shedding behaviors.•Three small-scale shedding ...mechanisms of cavitating structures are identified.•The small-scale shedding mechanisms are described in detail.
Leading-edge cavitation is responsible of the generation of transient cavities, usually made of clouds of bubbles. These transient cavities travel downstream to high-pressure regions and collapse violently, leading to noise and vibration as well as erosion. In the present paper, the focus is on the mechanisms generating transient cavities to better understand the starting point of the erosion process. The case studied is the cavitating flow over a NACA0009 hydrofoil which is investigated using experiments and numerical simulation. In the experimental part, which is conducted in EPFL high-speed cavitation tunnel, the shedding behavior is studied using high-speed visualization (HSV). In the numerical part, the cavitating flow is simulated using an incompressible solver coupled with isothermal homogeneous two-phase mixture cavitation model and Implicit Large Eddy Simulation (ILES) turbulence modelling. Owing to high speed visualization and numerical simulations, we identified two shedding mechanisms of transient cavities: (i) A primary shedding, characterized by a periodic generation of large cloud cavities and (ii) a secondary shedding of small-scale horse-shoe vortices, which are revealed for the first time. These small-scale structures, which are believed to play a major role in the erosion process, result from a complex interaction between the sheet cavity, the cloud cavity and re-entrant jets of different types. Furthermore, the detailed comparison between HSV and simulation confirms that the current numerical approach is capable of capturing the two types of shedding mechanisms very well.
A study on diesel spray characteristics is essential for the improvement of diesel combustion. However, information on diesel spray characteristics, especially those by the effect of injector needle ...lift, are lacking. In this study, the near-nozzle spray characteristic in a wide range of injection duration was investigated to understand the spray dynamics, which is vital to increase engine thermal efficiency in an engine with a multi injection strategy. The investigation has been done by conducting a high-speed visualization using X-ray, which is generated in Advanced Photon Source (APS) at Argonne National Laboratory, to obtain a phase-contrast image of a diesel spray. The needle lift inside the injector was also visualized and analyzed. The results show that the needle lift has a strong dominance on the spray angle that the spray angle is suppressed when the needle lift reaches a certain point regardless of the injection pressure and injection duration. The nozzle-exit spray velocity is strongly correlated with the needle lift and injection pressure. The spray velocity-decrease is significant when the needle lift is low and is independent of injection pressure and injection duration. The internal flow area is found to has an important role in determining the flow state.
•High speed visualization of plain and microchannel surfaces with tapered manifold.•Flow pattern map using superficial liquid and vapor velocities is presented.•Underlying mechanism for high heat ...transfer with OMM geometry is explained.•Comparison of bubble ebullition cycle for closed microchannel and OMM geometry.•Flow stability improves with taper and is unaffected by the rapid bubble growth.
The ever increasing demand of high heat removal from compact form factor devices has generated considerable interest in advanced thermal management techniques. Flow boiling in microchannels has the ability to provide high heat dissipation due to the utilization of the latent heat of vaporization, while maintaining a uniform coolant temperature. Recently, a number of studies have introduced variable flow cross-sectional area to augment the thermal performance of microchannels. The open microchannel with manifold (OMM) configuration provides stable high heat transfer performance with very low pressure drop. In the current work, high speed images are obtained to gain an insight into the nucleating bubble behavior and flow patterns at high heat fluxes including critical heat flux (CHF). The flow patterns are plotted as a function of superficial gas and liquid velocity. The resulting map indicates significant departure from the earlier work on macroscale tubes and confined microchannels. A mechanistic description of the heat transfer mechanism is also presented and the underlying differences between flow boiling in closed microchannels and open microchannels with tapered manifold configuration are highlighted. Furthermore, bubble ebullition cycle in pool boiling is compared with the tapered geometry utilizing plain and microchannel surfaces.
•The influence of operating parameters on heat transfer performance and instability was studied.•The dissipated heat flux up to 117 W/cm2 for a large heating area of 30 × 40 mm2 was achieved.•Four ...flow patterns were observed and the flow reversal occurred when the R is higher than unity.•An improved predictive model for heat transfer coefficient was proposed.
Flow boiling in parallel microchannels is an efficient method to deal with the high heat flux removal challenge for electronic modules. In this work, the visualized experiments on the R134a flow boiling inside a high-heat-flux microchannels heat sink were performed. The heat sink contains 27 parallel microchannels with a hydraulic diameter of 0.89 mm and a length of 30 mm. The experiments were conducted over a base heat flux between 2 and 117 W/cm2 (based on the footprint area), mass velocity of 282–1033 kg/m2 s, the saturation temperature ranged from 27.2 to 35.5 °C, exit vapor quality up to 0.45 and an inlet sub-cooling approximately 3 °C. The effects of mass velocity, saturation temperature, and heat flux, on the flow boiling characteristics were analyzed and discussed. Results demonstrated that the heat transfer coefficient (HTC) rise with the increase of mass flux under middle and high heat flux. At the maximum mass velocity of 1033 kg/m2 s, the flow boiling HTC reached 51456 W/m2 K. Moreover, the pressure drop increased with higher mass and heat flux, while it decreased with increasing saturation temperature. Additionally, under high heat flux conditions, the two-phase flow became more stable, and the wall surface temperatures exhibited greater uniformity when higher mass velocity or saturation temperature was applied. The visualization results illustrated that there were four flow patterns and reverse flow occurred when R exceeded unity. Finally, the experimental results were evaluated with six previous heat transfer correlations. An improved predictive model for flow boiling HTC was developed and good agreement with present data was obtained with MAE of 9.08 %. A two-phase micro-channels cold plate for high-power density electronic components could be rationally designed and optimized with reference to this work.
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