High-speed Flow Visualization Research Articles

Image-Based Analysis of Interactions Between a Steady Jet and an Impulsively Started Jet

Ignition overpressure (IOP) is a principal dynamic load experienced by a launch vehicle during liftoff. The IOP arises due to the sudden ignition of the solid rocket boosters (SRBs) of a launch vehicle. The present work attempts to simulate the ignition of an SRB with an impulsively started supersonic jet generated using a quick-open valve. High-speed schlieren flow visualization is used to document the evolution of an impulsively started jet flowfield that included the characteristic precursor shock wave (that simulates the blast wave causing the IOP) and the following vortex ring. The dynamic interactions between the impulsively started supersonic jet and an established steady supersonic jet from another nozzle () in close proximity are also investigated. Both the precursor shock wave and the vortex ring interact strongly with the established steady-jet column as they proceed downstream from the nozzle exit, thereby introducing new sources of acoustic waves. When a generic jet blast deflector is introduced downstream of the nozzles, the precursor shock wave reflected from it, propagating upstream toward the nozzle exit. The reflected shock wave is recognized as the source of IOP experienced by the launch vehicle. The results show that the quick-open valve presents a unique way to generate impulsively started jets for understanding transients associated with the ignition of the SRB leading to the IOP, without the complexity associated with the use of open-ended shock tubes for carrying out such simulations.

Favourably regulating two-phase flow regime of flow boiling HFE-7100 in microchannels using silicon nanowires

High performance miniaturized electronic devices require enhanced, compact and reliable thermal management system. As an efficient compact space cooling technique, flow boiling in microchannels has recently gained wide acceptance. However, weak buoyancy effects and microgravity in avionics and numerous space systems operations hinder the performance of flow boiling microchannel thermal management system due to poor bubble departure capacity and unfavorable development of flow regimes. Here we report the flow boiling silicon nanowires (SiNWs) microchannels which can favorably regulate two-phase flow regimes by enhancing explosive boiling, minimizing bubble departure diameter, and smoothing flow regime transition. Extensive experimental investigations along with high speed visualizations are performed. The experiments are performed with the dielectric fluid HFE-7100 in a forced convection loop for wide range of heat and mass fluxes. High speed flow visualizations have been employed at up to 70 k frames per second (fps) to understand the boiling mechanism in terms of bubble dynamics, flow patterns, and flow regime developments for SiNWs microchannels. These studies show that SiNWs reduce intermittent flow regimes (slug/churn), improve rewetting and maintain thin liquid film at wall. Therefore, flow boiling in SiNW microchannels is promising to thermal management owing to its high heat transfer rate with low pressure drop and negligible microgravity sensitivity.

Experimental investigation of gas-liquid flow in a vertical venturi installed downstream of a horizontal blind tee flow conditioner and the flow regime transition

A venturi device is commonly used as an integral part of a multiphase flowmeter (MPFM) in real-time oil-gas production monitoring. Partial flow mixing is required by installing the venturi device vertically downstream of a blind tee pipework that conditions the incoming horizontal gas-liquid flow (for an accurate determination of individual phase fraction and flow rate). To study the flow-mixing effect of the blind tee, high-speed video flow visualization of gas-liquid flows has been performed at blind tee and venturi sections by using a purpose-built transparent test rig over a wide range of superficial liquid velocities (0.3–2.4 m/s) and gas volume fractions (10–95%). There is little ‘homogenization’ effect of the blind tee on the incoming intermittent horizontal flow regimes across the tested flow conditions, with the flow remaining intermittent but becoming more axis-symmetric and predictable in the venturi measurement section. A horizontal (blind tee) to vertical (venturi) flow-pattern transition map is proposed based on gas and liquid mass fluxes (weighted by the Baker parameters). Flow patterns can be identified from the mean and variance of a fast electrical capacitance holdup measured at the venturi throat.

Flow behavior of concentrated tricalcium phosphate suspensions in oil through injection for softgel encapsulation

Softgel encapsulation is an important manufacturing process used to produce convenient oral dosage forms of pharmaceuticals and dietary supplements. An important step in this process involves the injection of a fill material into a gelatin lined mold, and poor flow of the fill material during the injection may lead to undesired defects. We investigated the effect of fill composition on the injection behavior of an oil-based tricalcium phosphate (TCP) suspension containing lecithin by rheological measurements, capillary breakup extensional rheometry (CaBER), and high-speed flow visualization experiments. Shear rheology for TCP suspensions in soybean oil at concentrations of 27.8 wt% and 55.6 wt% showed shear thinning behaviors both with and without the addition of lecithin. Adding up to 0.5 wt% lecithin led to decreases in shear viscosity, while additions up to 1 wt% caused a slight increase. Results from CaBER and breakoff studies indicated that the length at which flow breakoff occurs was closely related to the Ohnesorge number, a dimensionless parameter representing the relative importance of viscous to surface-tension forces during breakoff, with higher values correlated to longer breakoff lengths. An optimal lecithin concentration exists for minimized TCP suspension viscosity which shortens flow breakoff times and improves softgel capsule seaming quality.

Combined visualisation of cavitation and vortical structures in a real-size optical diesel injector

A high-speed flow visualisation set-up comprising of combined diffuse backlight illumination (DBI) and schlieren imaging has been developed to illustrate the highly transient, two-phase flow arising in a real-size optical fuel injector. The different illumination nature of the two techniques, diffuse and parallel light respectively, allows for the capturing of refractive-index gradients due to the presence of both interfaces and density gradients within the orifice. Hence, the onset of cavitation and secondary-flow motion within the sac and injector hole can be concurrently visualised. Experiments were conducted utilising a diesel injector fitted with a single-hole transparent tip (ECN spray D) at injection pressures of 700–900 bar and ambient pressures in the range of 1–20 bar. High-speed DBI images obtained at 100,000 fps revealed that the orifice, due to its tapered layout, is mildly cavitating with relatively constant cavity sheets arising mainly in regions of manufacturing imperfections. Nevertheless, schlieren images obtained at the same frame rate demonstrated that a multitude of vortices with short lifetimes arise at different scales in the sac and nozzle regions during the entire duration of the injection cycle but the vortices do not necessarily result in phase change. The magnitude and exact location of coherent vortical structures have a measurable influence on the dynamics of the spray emerging downstream the injector outlet, leading to distinct differences in the variation of its cone angle depending on the injection and ambient pressures examined.Graphic abstract

An experimental study on the heat transfer and wettability characteristics of micro-structured surfaces during water vapor condensation under different pressure conditions

In this study, condensation characteristics of water vapor on micro-structured surfaces at different pressures and subcooling temperature has been investigated experimentally. This work represents a basic study in order to design a secondary evaporator section in a refrigeration cycle. A set of surfaces has been manufactured over copper substrates with one sample used as unstructured (smooth) reference surface and two micro-structured surfaces with longitudinal grooves having V-shape and square cross-sections. As the second step, the surfaces have been modified by using a polymer coating to achieve stronger hydrophobicity at the surface and hence to influence wettability such that increased heat transfer rates should be reached. The polymer coating has been created with a dip coating process by applying a mixture of perfluoroalkyl triethoxysilane. Concerning the wettability, a high-speed flow visualization study has been performed for the interpretation of heat transfer results. Experimental results showed that an increase in the droplet contact angle by applying the polymer coating over surfaces, while the largest droplet contact angle was obtained (130.9°±2.0°) for the surface with V-shaped micro-structuring compared to other surfaces. The comparison of heat transfer performance reveals an enhancement in heat transfer coefficient for the coated version of unstructured, square-grooved and V-grooved surface by 34.5%, 61.8%, and 73.4%, respectively.

Two-Phase Heat Transfer and Flow Regimes in Pin Fin-Enhanced Microgaps—Effect of Pin Spacing

Abstract An experimental investigation of the flow boiling of dielectric refrigerant R245fa is conducted in microgaps with enhancement features. A silicon microgap of height 200 μm populated with pin fin arrays of diameter 150 μm with spacing 200 μm in both horizontal and vertical directions is examined. For five different test conditions and in a wide range of mass flux from 781 to 5210 kg/m2s, and inlet temperatures in the range of 13–18 °C, average single-phase and two-phase heat transfer coefficients, pressure drop, and exit vapor quality are reported. Three major flow patterns are observed in the pin finned area using high-speed flow visualization at frame rate of 2229 fps: foggy, bubbly, and slug flow. Based on the experimental data, a flow regime map is constructed.

Experiments on water vapour condensation within supersonic nozzle flow generated by an impulse tunnel

An impulse facility for analysis of water vapour nozzle flows using both shock tunnel and Ludwieg tube operating modes has been developed and tested at the University of Southern Queensland. Unique high-speed flow visualisation of the water vapour condensation shock has been acquired in the throat region of a nominally two-dimensional convergent-divergent nozzle with a 120 mm2 throat area. This paper presents the facility performance and the time-resolved visualisation results for the nozzle flow, and the results are analysed with the aid of image post-processing tools and quasi-one-dimensional (Q1D) thermodynamic calculations. The experiments have produced qualitative and quantitative data on the flow conditions at four different relative humidity values from 25% to 100%, and over a range of nozzle supply temperatures between 293 K and 343 K. The direct measurement of the pressures and wave speeds within the tunnel resulted in a good agreement with the ideal gas Q1D calculations, particularly for the first incident shocks and expansion waves. Differences progressively increased with the number of reflections, due to the effect of viscous dissipation and the non-ideal end-wall interaction caused by the presence of the nozzle inlet. Moreover, flow visualizations of the location and orientation of both the weak disturbance waves and the condensation shock enabled the investigation of the supersonic water vapour condensation within the nozzle of this impulse facility. Based on these measurements, the nozzle flow downstream of the condensation shock was demonstrated to have a lower Mach number relative to the dry nitrogen flow case. This phenomenon is due to the phase change heat release, which appeared to be more significant when the condensation shock intensity was higher. Although further development of the apparatus is possible, the impulse test-rig configuration that we have developed permits a relatively high flow rate of test gas for a short period of time, leading to significantly reduced operating expenses relative to a continuous flow facility with the same mass flow rate. It is therefore suggested that future studies further improve and test the impulse facility techniques for studying condensation phenomena as a potential alternative to high-power, continuous flow facilities.

Subcooled Flow Boiling on Micro-Porous Structured Copper Surface in a Vertical Mini-Gap Channel

Abstract An experimental investigation of subcooled flow boiling in a rectangular mini-gap channel with the dimension of 0.5 mm × 5 mm was conducted with deionized water as the working fluid. Fabricated by electroless plating method and high-temperature treatment, the copper-based hydrophobic micro-porous surface was utilized in the experiments. High-speed flow visualization was conducted to picture the flow patterns during the experiment. The mass fluxes were in the range of 200–400 kg/m2s, and the wall heat fluxes were spanned from 35 to 350 kW/m2. The onset of flow boiling, heat transfer coefficient, and pressure drop were discussed with the variation of heat fluxes and mass fluxes, the trends of which were analyzed along with the flow patterns. Because of the numerous nucleation sites on micro-porous surface, the superheat required for the onset of boiling are of small amounts of about 2 K. Due to the intense nucleation process, the boiling curves appeared to be a negative slope after the onset of boiling, which was more obvious in the lower mass flux conditions. In the high heat flux conditions, heat transfer coefficients under lower mass flux condition were higher because the intense nucleation process occurred and the elongated bubble flow along with the film evaporation heat transfer was formed. The film evaporation heat transfer inside the elongated bubble is more efficient to release the latent heat than the nucleate boiling. However, the appearance of the elongated bubble flow would attribute to higher pressure drop and severer pressure drop fluctuation due to its expansion toward upstream.

In Vitro Investigation of an Electrically Actuated Antireflux Valve for a Multiuse Contrast Agent Injection System.

OBJECTIVE. The purpose of this study was to determine the capability of a newly developed antireflux valve for a multiuse contrast agent injection system. MATERIALS AND METHODS. Multiuse contrast agent injection systems require an antireflux valve to eliminate the risk of cross-patient blood contamination. An electrically actuated antireflux valve developed for this purpose can control risk of contamination because it is under forced control and surveillance. In this study, the injection system was connected to a pulsatile circulation system that reproduced an aortic flow environment in vitro. The transvalvular pressure difference was measured, and flow dynamics during valve opening and closing were visualized by high-speed flow visualization. A total of 30 injection conditions were tested, which included two catheters (6- and 2-French diameter), five contrast agent concentrations (100%, 90%, 50%, 10%, and 0%), and a wide range of flow rates (0.1-25.0 mL/s). RESULTS. The transvalvular pressure difference and flow dynamics during valve opening and closing were characterized. Just before valve opening, a minimum transvalvular pressure difference of 0.20 MPa (1500 mm Hg) was confirmed. The positive pressure difference prevented regurgitation during valve opening. During valve closing, the front of the backflow was monitored continuously, and we confirmed that the front failed to reach the valve before complete closure. CONCLUSION. This study provided proof of concept for an electrically actuated antireflux valve to be used in a multiuse contrast agent injection system.

Experimental fluid dynamics characterization of a novel micropump-mixer.

The application of lab-on-a-chip systems to biomedical engineering and medical biology is rapidly growing. Reciprocating micropumps show significant promise as automated bio-fluid handling systems and as active reagent-to-sample mixers. Here, we describe a thorough fluid dynamic analysis of an active micro-pump-mixer designed for applications of preclinical blood analysis and clinical diagnostics in hematology. Using high-speed flow visualization and micro-particle image velocimetry measurements, a parametric study is performed to investigate the fluid dynamics of six discrete modes of micropump operation. With this approach, we identify an actuation regime that results in optimal sample flow rates while concomitantly maximizing reagent-to-sample mixing.

Time-resolved characterization of microchannel flow boiling during transient heating: Part 1 – Dynamic response to a single heat flux pulse

Microchannel flow boiling is an attractive approach for the thermal management of high-heat-flux electronic devices that are often operated in transient modes. In Part 1 of this two-part study, the dynamic response of a heated 500 μm channel undergoing flow boiling of HFE-7100 is experimentally investigated for a single heat flux pulse. Three heat flux levels exhibiting highly contrasting flow behavior under constant heating conditions are used: a low heat flux corresponding to single-phase flow (15 kW/m2), an intermediate heat flux corresponding to continuous flow boiling (75 kW/m2), and a very high heat flux which exceeds critical heat flux and would cause dryout if applied continuously (150 kW/m2). Transient testing is conducted by pulsing between these three heat flux levels and varying the pulse duration. High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are synchronized to high-speed flow visualizations to characterize the boiling dynamics during the pulses. At the onset of boiling, the dynamic response resembles that of an underdamped mass-spring-damper system subjected to a unit step input. During transitions between single-phase flow and time-periodic flow boiling, the wall temperature temporarily over/under-shoots the eventual steady operating temperature (e.g., by up to 20 °C) thus demonstrating that transient performance can extend beyond the bounds of steady performance. It is shown that longer duration high-heat-flux pulses (up to ~50% longer in some cases) can be withstood when the fluid in the microchannel is initial boiling, relative to if it is initially in the single-phase flow regime, despite being at an initially higher heat flux and wall temperature prior to the pulse.

Retracted: “Symmetric and Asymmetric Disturbances in the Rayleigh Zone of an Air-Assisted Liquid Sheet: Theoretical and Experimental Analysis,” [ASME Journal of Fluids Engineering, 2020, 142(7), p. 071302; DOI: 10.1115/1.4045998

AbstractThe temporal analysis of symmetric (dilatational) and asymmetric (sinusoidal) perturbations at the interface of a water sheet in a coflowing air stream focuses on low gas Weber number region (Weg < 0.4), namely, Rayleigh breakup zone. The motive for this investigation is to acquire a better insight of breakup phenomena involved, rather than technical relevance, by utilizing Kelvin–Helmholtz instability. Accordingly, perturbations are introduced on the basic flow whose stability is to be examined by the method of normal (Fourier) modes. The temporal growth-rate of perturbations is traced to extract the wavenumber associated with maximum growth-rate. Thus, the critical wavelength, in conjunction with the phase velocity of the disturbance will facilitate to obtain the corresponding breakup frequency of the liquid sheet. The analytical findings on liquid sheet breakup frequency with increasing Weber number ratio exhibit the dominance of symmetric wave over asymmetric wave. It also shows independent evolution of breakup frequency with respect to Weber number ratio for the respective perturbation modes, which appears to be a pointed profile. That is, the frequency contour for dilatational mode dips, whereas it rises for the sinusoidal mode and at the Weber number ratio of 0.518 the crossover occur. The theoretical results were substantiated by high-speed flow visualization studies that discern the coexistence of low-frequency (primary) and high-frequency (intermediate) breakup events. Furthermore, the empirical average frequency data track reasonably well with the dilatational instability.

Experimental Characterization of a Liquid Jet Emanating From An Effervescent Atomizer

Abstract The study focuses on experimental characterization of the primary atomization associated with an effervescent atomizer. Unlike the existing designs available in the literature that inject air perpendicular to the liquid flow direction, the present atomizer design utilizes coflowing air configuration. In doing so, the aerodynamic shear at the liquid–gas interface create instability and enhance the subsequent jet breakup. Both integrated and intrinsic properties of the liquid jet were extracted by utilizing high-speed flow visualization techniques. The integrated property consists of breakup length, while the intrinsic property involves primary and intermediate breakup frequencies. The primary instability is characterized by low-frequency sinusoidal mode, whereas the intermediate instability consists of high-frequency dilatational mode. Dimensionless plots of these parameters with Weber number ratio leads to a better collapse of data, thereby generating appropriate universal functions. The combined diagram of frequencies converge with increasing relative velocity. This may be due to the dominance of energy consuming sinusoidal wave as the aerodynamic shear increases.

Visualization of two-phase gas-liquid flow in a radial centrifugal pump with a vaned diffuser

The operation of centrifugal pumps with gas-liquid mixtures is a common problem in the oil and gas industry. The main problem is related to the accumulation of gas inside the Electric Submersible Pump (ESP) impellers, which causes degradation of the pump pressure-rise. Literature about global performance parameters of centrifugal pumps operating with gas-liquid flows is limited, and studies focused on understanding the complex flow patterns occurring inside impellers are even scarcer. In this scenario, this work presents an experimental work developed to carry out, simultaneously, head evaluation and high-speed flow visualization in a two-stage centrifugal pump with radial-type impellers and a vaned diffuser. The pump casing and the first stage impeller were replaced by equivalent pieces reproduced with great fidelity using a transparent material in order to allow flow visualization while minimizing the effect of non-original pump parts on performance. Results show flow pattern transitions in the impeller channels when the rotating speed, the inlet gas flow rate and the liquid flow rate are changed, whereas only a single flow pattern was observed in the diffuser for the whole range of tested operating conditions. In addition, increasing the rotating speed causes, in general, a decrease of the bubble diameters inside the impeller, improving the pump's ability to handle higher gas flow rates and ultimately extending the operational window of the pump. This analysis can provide a source of data that can be useful to support theoretical models or to validate numerical simulations.

Effect of angle of attack on vortex dynamics in laminar separation bubbles

The dynamics of coherent structures that develop within laminar separation bubbles over a NACA 0018 airfoil at Rec = 100 000 and Angles of Attack (AOA) of 0°, 5°, 8°, and 10° are investigated experimentally using a high-speed flow visualization technique and vortex tracking via embedded microphones. The results identify vortex shedding within the separation bubble for all the cases investigated. The vortices form upstream of mean transition location and break down in the vicinity of mean reattachment, with irregular vortex merging events detected in the aft portion of the separation bubble. As the mean size of the separation bubble decreases with increasing angle of attack, vortex shedding characteristics also change appreciably, with shedding frequency increasing and characteristic streamwise wavelength decreasing. However, vortex roll up and salient aspects of vortex development take place within approximately the same regions downstream of separation location in terms of percentage of the bubble length. For all the cases examined, significant cycle-to-cycle variability is observed in salient vortex characteristics, being particularly pronounced in the aft portion of the bubble where vortex merging occurs and significant spanwise deformations of vortex filaments are expected. Merging of the shear layer vortices is shown to occur irregularly between the mean transition and reattachment location, with merging events separated by a relatively wide range of shedding cycles. As the angle of attack increases from AOA = 5° to 10°, the fraction of primary vortices involved in merging increases up to about 25%. However, enhancement in vortex merging is also noted at lower AOA where shedding occurs close to the trailing edge, which is speculated to be linked to upstream scattering of periodic pressure fluctuations induced at the trailing edge.

A Qualitative Inter-Relation Between Drop Impacts on a Solid Surface and the Interaction of a Droplet Placed Inside a Superheated Pool

The phenomenon of drop impacts on a solid surface and the interaction of a cold droplet with a superheated liquid pool are both of great relevance in many practical and engineering situations. Most of the reported research in the literature on these phenomena focus on the impact of a droplet on a solid surface or a liquid surface and there are hardly any study on the interaction of a stationary cold droplet inside a hot pool of liquid, a phenomenon observed quite often. This study reports high speed flow visualization of a drop impact on a Teflon-coated solid surface at different liquid properties and impact Reynolds number. Simultaneously, high speed imaging of the interaction of a cold droplet with the superheated olive oil pool has also been carried out. The images show a striking resemblance between the flow dynamics of the two phenomena. A comparison between the two phenomena is made and suggestions are made for the possible underlying physics.

Heat transfer and pressure drop characteristics of water flow boiling in open microchannels

The open microchannel configuration can mitigate several drawbacks of closed microchannel for flow boiling heat transfer, but the studies related to flow boiling in open microchannels are limited up to now. In this study, experiments were conducted to investigate the heat transfer and pressure drop characteristics during flow boiling of deionized water (DI water) in open microchannels and to analyze the effects of operating conditions and channel dimensions. The experiments were carried out with the inlet subcooling ΔTsub = 20, 35, 50 °C at different mass fluxes ranging from 174 to 374 kg/m2 s for varied applied heat flux 214.9–1355.1 W/cm2. Two open microchannel samples with different geometry dimensions were tested. High speed flow visualizations were realized to illustrate the flow pattern transitions during flow boiling in open microchannels. Experimental results showed that the onset of stratified flow (OSF) denoted an alternation of dominant heat transfer mechanism at which the heat transfer coefficient (HTC) was largest during the flow boiling. The effects of operating conditions and channel dimensions on the HTC and two-phase pressure drop in open microchannels were mainly observed in the stratified flow regimes, during which nucleate boiling and convective evaporation jointly controlled the flow boiling process. In addition, the open microchannel with smaller size but a great number of channels had a better heat dissipation capability along with a higher pressure drop in stratified flow regimes.

Visualization of Rotating Cavitation Oscillation Mechanism in a Turbopump Inducer

An experimental investigation has been conducted on rotating cavitation in a turbopump inducer. Previous research suggested that incidence angle variation leads to rotating cavitation. This study, using particle image velocimetry (PIV) method, provides the first measurement of the incidence angle distribution near the tip region of an inducer blade's leading edge with and without rotating cavitation. Without rotating cavitation, the incidence angle near the leading edge of the following blade is positive. Under rotating cavitation, the tip leakage vortex cavitation regions on the blades become uneven, forming large and small tip leakage vortex cavitation regions. A large tip leakage vortex cavitation on the leading blade increases the axial and absolute tangential velocities near the following blade's leading edge. Thus, the incidence angle on the following blade becomes negative, suppressing tip leakage vortex cavitation. Conversely, a small tip leakage vortex cavitation on the leading blade increases the incidence angle and promotes tip leakage vortex cavitation on the following blade. Due to such suppression and promotion mechanisms, the tip leakage vortex cavitation region on each blade oscillates in sequence, seemingly but not actually propagating in the forward direction. High-speed camera flow visualization has been used to confirm the same oscillation mechanism of rotating cavitation.

Flow boiling characteristics in a novel minichannel with a step on each corner

ABSTRACTIn the present study, flow boiling in a single minichannel with a longitudinal step on its each corner is studied experimentally. Bubble/flow dynamics, and thus, underlying physical phenomenon is investigated through high-speed flow visualization experiments. The results are also compared with those of the conventional single rectangular minichannel (plane channel) in terms of heat transfer and total pressure drop. Experiments are performed for different values of the mass flux and wall heat flux at a constant inlet temperature. It is obtained that the stepped channel improves heat transfer performance up to 13.2% compared to the plane channel. During the visualization studies, five flow patterns are observed: bubbly flow, slug flow, churn flow, annular flow, and inverted annular flow.