Lower Jet Velocity Research Articles

Liquid breakup and droplets behavior of free triple-impinging jets with different impinging distance

The breakup characteristics and droplet behavior of free triple-impinging jets (FTIJs) were investigated using a high-speed camera. The liquid sheet and droplets breakup mechanism, liquid sheet breakup length, width, droplet diameter and velocity, were studied under different jet velocity and horizontal impinging distance and perpendicular impinging distance. The results revealed that with increasing jet velocity from 1.42 m/s to 6.61 m/s, different breakup modes were observed: closed-rim mode, open-rim mode, rimless mode, and wave or ligament mode. At low jet velocities, increasing impinging distance delayed the development of breakup mode. Based on experimental observations, the sheet breakup mechanism is categorized into two main regimes with four subregimes. Droplet breakup occurs in two stages: growth and necking, however, at high jet velocities complete breakup was achieved. Increasing jet velocity led to an increase in liquid sheet breakup length, width, and droplet velocity but a decrease in droplet diameter. Conversely, increasing impinging distance resulted in a decrease in liquid sheet breakup width and droplet radial velocity but an increase in droplet diameter. The effect of perpendicular impinging distance on axial velocity and breakup length was found to be more significant than that of horizontal impinging distance due to the effect of perpendicular nozzle. The findings indicate that the perpendicular jet greatly enhances atomization. This study provides a theoretical basis for designing and optimizing FTIJs.

Simulation of flow field based on constrained fluid for electrolyte jet control

In previous studies, jets were controlled by adjusting parameters such as velocity, pressure, size and shape of nozzle, and temperature. In this study, a new method of controlling electrolyte jets was proposed by introducing a constrained fluid. To analyze the effects of the constrained fluid on the jet flow field, the initial jet velocity and velocity ratio of the flow field were examined. Simulation results showed that the proposed method can avoid stray deposition caused when the electrolyte is spread on the substrate surface. The electrolyte jet could be controlled by the constrained fluid, indicating the characteristic of cyclical fluctuation. The initial jet velocity and velocity ratio had different impacts on the flow field; the former mainly affected the velocity and pressure distribution. The relationship between the velocity of flow field and initial jet velocity was observed to be linear. However, the relationship between pressure of the flow field and initial jet velocity was quadratic. The velocity ratio had a significant impact on the state of electrolyte jet. At low initial jet velocities, the electrolyte jet was not continuous at different velocity ratios. When the initial jet velocity increased, cyclical fluctuations in the electrolyte jet weakened under high velocity ratios. The electrolyte jet diameter also decreased linearly as the velocity ratio increased. The velocity and pressure of the flow field were quadratically related to the velocity ratio. This indicates that the constrained fluid had a considerable impact on the flow field and state of the electrolyte jet. Moreover, the electrolyte jet could be controlled effectively by adjusting the initial jet velocity and velocity ratio to obtain a stable and small electrolyte jet diameter.

Impacts of viscosity on bending behavior of the electrospun jet: Simulation model and experiment

Numerous experimental data indicate that higher viscosity solutions make it easier to obtain coarser fibers. However, the influences of viscosity on the whipping behavior of the jet and their subsequent impacts on the formation of terminal fiber during electrospinning remain topics of ongoing research. Simulation model and experiment presented in this study aim to solve these problems. The simulation model alters the dimensionless parameter Fve to predict the impacts of different viscosities on the bending behavior of the electrospun jet, while the experiment observes this behavior under different viscosities by regulating the solution concentration. The consistency of the simulation and experimental data implies the accuracy of the conclusions in this paper. Furthermore, the simulation model and experiment have demonstrated that a higher viscosity jet results in a smaller radius of the jet trajectory and lower jet velocity, which in turn leads to the formation of coarser fiber.

Dynamics of regime transition of autoignitive jet flames from conventional to MILD combustion

Turbulent combustion of jet flames in a hot diluted coflow of combustion products has been studied across a range of jet Reynolds numbers for propane, ethylene, and an ethylene-propane blend as fuels. The study revealed a transition from conventional autoignitive combustion to a regime of Moderate or Intense Low-oxygen Dilution (MILD) combustion, which is characterized by a nearly invisible flame. The flames studied are luminous at low jet velocities and become MILD at higher jet velocities. Planar Laser-Induced Fluorescence (PLIF) of formaldehyde (CH2O), a key intermediate species in hydrocarbon combustion, is combined with CH*-chemiluminescence imaging and Rayleigh scattering to investigate the phenomena. The transition to MILD combustion is found to accompany a broadening of the formaldehyde region, indicating a broader low-temperature reaction zone. As the transition is approached, the appearance of holes in the chemiluminescent front is also observed. Correspondence between the location and structure of these holes with regions of formaldehyde in the flame are illustrated. These regions are proposed to be signatures that precede the transition to a fully MILD flame. In other words, the transition to MILD combustion begins at a local level and the MILD region spreads to engulf the entire combusting mixture. Scalar gradients were imaged to further unravel the local turbulence/chemistry interactions that yield the MILD inception. The measured scalar gradients were found to be commensurate with scalar gradients obtained from chemical kinetic simulations at the stoichiometric mixture fractions, while being an order of magnitude larger at other locations; this suggests that the inception of MILD combustion occurs near the stoichiometric region.

Computational study of the impacts of the nozzle configurations on passive pre-chamber engine combustion

The pre-chamber (PC) nozzle configurations are vital in the narrow-throat pre-chamber design. However, their impacts on the jet and combustion characteristics are poorly understood. In the current study, various parameters of nozzle configurations, including the number of rows, nozzle position, and the jet included angle were evaluated on a well-calibrated numerical model of a passive PC engine fueled with methane. The unique buffering effect of the bottom cavity below the orifice in the PC nozzle was discovered and investigated for the first time. Results show that the PC with double-row orifices exhibits slower general combustion than the single-row for the significant flow bypass from the upper-row orifices that generate low jet velocity. A higher orifice position favors combustion due to less interaction between the flame and the piston. Pressure accumulation at the nozzle was discovered to be the key factor affecting jet velocity. Jet velocity increases with the enlarged pressure accumulation decreasing the effective flow area of the orifices. However, it starts to decrease when the pressure accumulation becomes excessive and the obstruction to the flow outweighs the enhancement to the velocity. For single-row PCs without the bottom cavity, the pressure accumulation is enhanced as the jet included angle increases (from 111° to 180°) and the angle of 157° achieves the highest jet velocity, although the pressure difference decreases as the included angle increases. For single-row-orifice PCs with a bottom cavity, the buffering effect of the cavity produces close pressure accumulation to maintain similar jet characteristics when the jet included angle varies. Comparative analysis shows that the included angle of 157° achieves the fastest main chamber combustion mainly because the proper relative position of the PC jet and piston favors turbulence and flame spread.

Investigation of control effects of end-wall self-adaptive jet on three-dimensional corner separation of a highly loaded compressor cascade

To overcome the limitations posed by three-dimensional corner separation, this paper proposes a novel flow control technology known as passive End-Wall (EW) self-adaptive jet. Two single EW slotted schemes (EWS1 and EWS2), alongside a combined (COM) scheme featuring double EW slots, were investigated. The results reveal that the EW slot, driven by pressure differentials between the pressure and suction sides, can generate an adaptive jet with escalating velocity as the operational load increases. This high-speed jet effectively re-excites the local low-energy fluid, thereby mitigating the corner separation. Notably, the EWS1 slot, positioned near the blade leading edge, exhibits relatively low jet velocities at negative incidence angles, causing jet separation and exacerbating the corner separation. Besides, the EWS2 slot is close to the blade trailing edge, resulting in massive low-energy fluid accumulating and separating before the slot outlet at positive incidence angles. In contrast, the COM scheme emerges as the most effective solution for comprehensive corner separation control. It can significantly reduce the total pressure loss and improve the static pressure coefficient for the ORI blade at 0°-4° incidence angles, while causing minimal negative impact on the aerodynamic performance at negative incidence angles. Therefore, the corner stall is delayed, and the available incidence angle range is broadened from −10°–−2° to −10°–4°.This holds substantial promise for advancing the aerodynamic performance, operational stability, and load capacity of future highly loaded compressors.

Mechanism of jet velocity affecting surface quality based on electrode dynamics under temperature field

Electrolyte jet machining (EJM) is a rapid method for preparing metal surface microstructures. The local temperature changes at the anode interface caused by electrochemical heat and jet velocity are to be determined, and the mechanism of the effect of different jet velocities on the surface quality of the anode is not clear. Therefore, an EJM simulation model considering temperature was developed, and the etching profiles were compared with and without the effect of temperature. The simulation results show that considering the temperature field can improve the prediction accuracy of the simulation. In addition, simulation and experimental results show that high electrochemical heat is favourable for increasing the conductivity of the electrolyte and generating a high-quality surface. Enhanced convective heat dissipation at high jet velocities reduces the conductivity of the electrolyte in the machining area, generating low-quality surfaces. It is worth noting that excessively low jet velocity can accumulate anode electrolysis products at the interface, affecting machining accuracy. This paper analyzes the mechanism of the influence of different jet velocities on the surface quality of an anode in combination with the electrode dynamics under the influence of the temperature field. This analysis is crucial for achieving high surface quality in EJM.

Two-phase flow and heat transfer on a cylinder via low-velocity jet impact

Combining the benefits of jet impact and phase-transition heat transfer, jet impact boiling has been widely adopted in industrial scenarios. Notably, owing to limitations of high-velocity jets, such as high impact stress and pumping pressure in liquid circuits, low-velocity jet impacting deserves more attention and explorations. In this paper, under an atmospheric environment with wall heating temperatures ranging from 36 °C to 128 °C, the two-phase flow and heat transfer characteristics (local surface temperature, boiling curves and liquid loss rate) of low-velocity jet impacting on a cylindrical surface are investigated by experiment, followed by the analysis on the effects of jet outlet velocity and impact height. The findings reveal that, with the increase of heat transfer rate, the heat transfer regimes in sequence are non-phase transition (single-phase convection and air-liquid two-phase convection) and nucleate boiling. The influences of jet impact height and outlet velocity on local surface temperatures are pronounced at non-phase transition stage. In impact region, along with average superheat, the growth rates of heat transfer rate and liquid loss rate increase significantly from non-phase transition to nucleate boiling stage, and slow down at anaphase of nucleate boiling stage. Interestingly, the increase in outlet velocity delays the onset of slowdown in growth of liquid loss rate to some extent. Overall, the heat transfer rate and liquid loss rate are significantly enhanced at higher jet outlet velocity of 0.60 m/s, but not affected significantly by impact height.

Relative size of the effective regurgitant orifice to right ventricular cavity contributes significantly to the underestimation of 2D PISA method in tricuspid regurgitation quantitation

Abstract Background 2D proximal isovelocity surface area (PISA) method underestimates tricuspid regurgitation (TR) severity largely due to the distortion of proximal flow geometry under low jet velocity. Previous studies demonstrated that the confining wall geometry also strained the streamline distribution in the proximal flow field. This could add to the variability when applying 2D PISA in TR as TR presents with wider range of effective regurgitant orifice area (EROA). Purpose We aim to examine (1) the impact of ventricular wall constraint to proximal flow region on TR quantitation and (2) its relative contribution to the underestimation of 2D PISA compared with the distortion of proximal flow field under low jet velocity. Methods 150 pts with more than mild TR underwent comprehensive echocardiographic session. The standard 2D PISA EROA (EROASD)was derived as per guideline with angle correction applied for tethering leaflets. The underetimation caused by low jet velocity was corrected by multiplying the standard EROA (EROASD) with VOrifice/(VOrifice-VNyquist), where the VOrifice stands for the peak regurgitant velocity and the VNyquist represents the color Doppler aliasing velocity (EROAVo-VN). The volumetric EROA and vena contracta area serves as two independent reference methods. The ventricular wall constraint was quantified as the ratio of reference EROA to right ventricular end-diastolic volume (RVEDV). Results A consistent significant underestimation by standard 2D PISA methods, either corrected for low jet velocity or not, was observed as a function of the EROA (Figure 1). In Bland-Altman analysis, correction for low jet velocity lessened the systematic underestimation but still leaves minor while could-be-important error (VCA, bias = 19.6 and 15.1 mm2; volumetric EROA, bias = 10.1 and 5.6 mm2 for EROASD and EROAVo-VN respectively). The accuracy of both methods to differentiate severe TR was similar but the cut-off values were lower compared with guideline recommendation (AUC = 0.905 and 0.903, cut-off = 0.26 and 0.29 cm2 for EROASD and EROAVo-VN respectively). In multi-linear regression model (Table 1), the difference between standard 2D PISA EROA and reference EROA (mean of volumetric EROA and vena contracta area) was significantly related to etiology, the jet velocity-related correction coefficient, and the reference EROA/RVEDV ratio. The ventricular wall constraint contributes to the underestimation of 2D PISA to a greater extent than the low jet velocity (beta = -0.269 and 0.592 for VOrifice/(VOrifice-VNyquist) and reference EROA/RVEDV respectively). Conclusion Correction for low jet velocity lessens the extent of underestimation of TR severity by 2D PISA method but still leaves important measurement error. The confining wall geometry to proximal flow region contributes significantly to the understimation but is not accounted for in current recommendation. This could be especially important in massive and torrential TR.

Confinement Effects on Coaxial Jet Diffusion Flame

ABSTRACT A combined experimental and numerical study has been conducted to investigate the characteristics of confined coaxial jet diffusion flames in air coflow, using methane as fuel. The main concerns were focused on the unequal fuel velocity (0.1–72 m/s)/air velocities (0.045–0.53 m/s) and confinement ratio (δ*, defined as the ratio of the confinement tube diameter and the jet diameter), and most of the flow remains laminar while the ranges of δ* and gas velocities vary widely. The flame height linearly increases with the central fuel jet velocity, until the flame tip-opening or liftoff occurs, which is similar to that of the free jet flame. The critical fuel jet velocity for flame liftoff decreases with the ratio of air velocities to δ * . The critical fuel jet velocity of flame tip-opening increases with δ*, and the critical volumetric flow rate of air increases to a maximum, then decreases, indicating a combined effect of the oxygen supplement and the mixing with the fuel. Under smaller δ* with high jet flow velocity, a circular plane flame is observed before flame extinction. The formation of a recirculation zone in a confinement with combustion reaction has a significant influence on flame behaviors. A critical nondimensional Craya-Curtet number is used to predict the onset of flow recirculation with combustion reaction. At a certain coflow air velocity, the critical number initially increases and then remains constant with δ*, while at a certain δ*, the critical number decreases with air velocity. When the Craya-Curtet number is higher than the critical point, recirculation cannot be established, resulting in the formation of a tip-closed flame. With the decrease of the Craya-Curter number, the recirculation evolves to the upstream of the flame, then, the flame may be lifted off, along with the recirculation zone moving downstream. With the effect of recirculation, the stoichiometric mixture fraction contour shrinks toward the central axis where there is a higher local velocity. Therefore, the flame will be lifted off under a lower jet velocity in a smaller δ * . With the increase of the jet velocity, the Craya-Curtet number further decreases, and the recirculation becomes much stronger than the upstream inflow, resulting in the flame being compressed into a circular plane flame at a small δ* with a relatively high jet flow velocity or blowout directly at a relatively large δ * .

Effect of ammonia addition on combustion characteristics of hydrogen/air using passive turbulent jet ignition

Hydrogen (H2) and ammonia (NH3) are ideal carbon-free fuels, and the application of NH3/H2 in internal combustion engines offers the potential to reduce carbon emissions. And turbulent jet ignition (TJI) is an advanced ignition strategy that can effectively enhance the combustion. This work aims to experimentally investigate the effect of NH3 addition on the combustion characteristics of H2/air mixtures under passive TJI conditions. Combustion images and instantaneous pressure were collected for processing and analysis. The effect of NH3 substitution and equivalence ratio were investigated. The results indicate that the peak combustion pressure decreases with the addition of NH3, accompanied by longer combustion duration and ignition delay, as well as lower jet velocity and flame area growth rate. The hot jet will be quenched by the strong heat transfer, causing the change in the ignition mechanism as the NH3 substitution ratio increases. In addition, compared to lean conditions, the combustion appears to be less sensitive to the equivalence ratio on the rich side and always shows high jet velocity and flame propagation speed. However, a relatively small amount of NH3 addition changes the ignition mechanism on the rich combustion side compared with lean conditions, resulting in a longer ignition delay.

CFD Assessment of Car Park Ventilation System in Case of Fire Event

This scientific article presents the results of computational fluid dynamics (CFD) simulations conducted using OpenFOAM to evaluate the effectiveness of a jet fan ventilation system in managing the dispersion of smoke resulting from a car fire incident within an underground car park spanning a total area of 21,670 m2, situated in Tabriz, Iran. The primary objective of the study is to determine the velocity fields and evaluate visibility conditions within a 10 m radius to gauge the efficiency and effectiveness of the system. The study employs a smoke concentration production rate of 5.49 × 10−4 kg/m3s for simulations involving fire scenarios. A total of 17 fire scenarios are examined, each extending 30 m in all directions from the initial location. The research findings demonstrate that the placement of jet fan components plays a significant role in the system’s efficiency, with fans positioned near the ceiling leading to back-layering. To mitigate this issue, the recommended design solution involves the strategic installation of multiple jet fan arrays in specific zones with the addition of 10 extra jet fans, effectively curbing lateral smoke dispersion. Furthermore, the analysis of air flow rates shows that when jet fans direct an excessive airflow towards the exhaust shafts (which have a designated flow rate of 22.5 m3/s), recirculating flows occur, leading to the dispersion of smoke throughout the car park. Consequently, the utilization of low-velocity jet fans (11.2 m/s) proves to be more effective in clearing smoke compared to high-velocity jet fans (22.3 m/s). The study also emphasizes the importance of optimal positioning of supply and exhaust shafts to achieve effective smoke control, highlighting the need for placing them on opposite walls or minimizing airflow turns. Additionally, the research underscores the significance of fire resistance in jet fan units, as their failure during fire incidents can have severe consequences.

A numerical study on the structure and dynamics of fuel-rich premixed H2–air jet flames above a divergent micro-nozzle

In this study, the flame structures of a fuel-rich premixed H2–air mixture issued from a divergent micronozzle were numerically studied and compared with their counterparts. The flame dynamics of the divergent micronozzle at relatively low jet velocities were also analyzed. A flame pattern of “dual flames with a smaller one surrounded by a larger one” was discovered for both the straight and divergent micro-nozzles at high jet velocities. This flame structure began to appear at a lower jet velocity (Vin) as the divergence angle (θ) decreased. Specifically, it first appeared at Vin = 7.5 and 20 m/s for θ = 0° and 5°, respectively. In addition, a flame mode consisting of dual flames with a smaller one inside the nozzle occurred only for divergent micronozzles under low and medium jet velocities. This flame structure could emerge at a higher jet velocity as the divergence angle increased. For example, it could occur until Vin = 5 m/s and 17.5 m/s for θ = 1° and 5°, respectively. A global map of all flame patterns was drawn based on the results obtained by varying the jet velocity and divergence angle. Moreover, periodic flame dynamics appeared when the jet velocity was less than the critical value (<1.7 m/s). Specifically, a small flame (i.e., secondary flame) split from the jet flame (i.e., main flame) and propagated towards the nozzle inlet; however, its propagation direction was inverted and it finally extinguished owing to the heat loss to the nozzle wall. The next cycle was initiated after a short duration. The analysis revealed that the periodic flame dynamics are a result of the thermal interactions between the flame and nozzle wall.

Experimental investigation of various energy-absorbing layer materials and sodium alginate viscosities on the jet formation in laser-induced-forward-transfer (LIFT) bioprinting

Laser-induced-forward-transfer (LIFT) bioprinting technology has been viewed as a regenerative medicine technology because of its high printing quality and good cell viability. To stabilize the jet to achieve high-quality printing, an energy-absorbing layer (EAL) can be introduced. In this study, three materials (graphene, gelatin, and gold) were utilized as the EAL. The effect of each EAL on the jet generation process was investigated. Besides, the effect of graphene EAL thickness was addressed for various experimental conditions. The jet generation process using sodium alginate solutions with different concentrations (1 and 2 wt. %) was also discussed to investigate the effect of viscosity. The time sequence images of the formed jets utilizing three EALs showed that both graphene EAL and gelatin EAL can promote the formation of jet flow. For the gold EAL, no jet flow was observed. This study provides experimental verifications that the interaction between laser and EAL materials can result in different jets due to various dominant interaction mechanisms. For example, strong absorption in the infrared range for the graphene EAL, strong scattering loss for the gelatin EAL, and strong absorption in the ultraviolet range but weak absorption in the infrared for the gold EAL. We also observed the holes left on the EAL after the printing was completed. The thermal effect is dominant to create regular and round shape holes for the graphene EAL, but it changes to the mechanical effect for the gold EAL because of the existence of irregular and unorganized holes. In addition, we identified the existence of an input laser energy threshold value for a certain thickness graphene EAL. More laser energy is required to break down thicker graphene EALs, which will result in a higher initial jet velocity. Furthermore, we explored the effect of sodium alginate (SA) solution's viscosity on the generated jet. We found that a high-viscosity SA solution can result in a low initial jet velocity, a short jet, and small droplets on the receiving substrate. The findings from this study help determine the mechanisms of EAL–laser interaction with different EAL materials in the LIFT process. This work aims to facilitate the development of new EAL and bioink to achieve stable jet formation and high printing quality in future LIFT bioprinting.

Engine Exhaust Stub Sizing for Turboprop Powered Aircraft

Turboprop engines are widely used in the commuter or light transport aircraft (LTA) turboprop engines, because they are more fuel efficient than the propeller, which has a low jet velocity, at flight velocities below 0.6 Mach. For short distances, turboprop engines are more fuel efficient than jet engines, because the light weight assures a high power output per unit of weight. In addition, turboprops are known for their efficiency at medium and low altitudes. Turboprop engines require an exhaust stub (or nozzle) to duct the engine exhaust flue gas outboard of the aircraft. The design of these exhaust stubs is dictated primarily by the aircraft configuration. During the exhaust stub design, full flow at bends and in diffusing sections must be realized by following the established practice for the design of internal flow ducts. Otherwise, the flow will separate from the wall, causing unnecessary pressure loss and reducing the effective flow area. This paper discusses some of the many variations in exhaust stub design, and examines how they influence the performance of the engine, the performance of the aircraft, and the manufacturing aspect. The authors carried out a detailed analysis on the influencing parameters, such as the location, orientation, flange dimension, and geometric effective area of exhaust port. On this basis, the authors determined the jet temperature at exhaust stub exit and temperature at exhaust stub exit plane and nacelle midsection were determined at both static and cruise condition, laying the data basis for further analysis on the exhaust temperature effects over the nacelle and aircraft surfaces.

Effect of Jet Velocity on the Formation of Moderate or Intense Low-Oxygen Dilution Combustion of Pulverized Coal.

Moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal is regarded as a new combustion technology with great potential due to its advantages of reducing NO x emission and improving the uniformity of heat flux in the furnace. Increasing the jet velocity has been proved to be an important technical means to achieve MILD oxy-coal combustion, especially without a high level of preheat, but the transition mechanism of coal combustion modes with the increase of jet velocity is not clear enough. In this work, a high-velocity coal-laden jet combustion system on the basis of a flat-flame burner was designed to study the effect of jet velocity on the formation of MILD combustion of pulverized coal. The combustion mode transition of the pulverized coal jet is revealed by analyzing the flame structure, temperature, radiation, and reaction intensity through a variety of optical measurement methods. Theoretical criteria for combustion mode were applied to predict the formation of MILD combustion and were validated by the experimental data. In the environment with an ambient temperature of 1600 °C, the transition velocity of the coal jet into the MILD combustion regime is about 50 m/s and about 100 m/s at 5 and 15% O2 molar fraction, respectively. The dilution effect, jet entrainment, and turbulent mixing in the high-velocity jet, as the key factors to achieve an MILD combustion regime, were analyzed theoretically and experimentally. The dilution effect inherent in the jet significantly reduces the reactant concentration and ultimately reduces the reaction intensity and flame brightness while the entrainment of the jet promotes the radial dispersion of particles and the flame uniformity, which is dominant at lower jet velocities. Strong turbulent mixing promotes the ignition and volatile combustion, which is dominant at higher jet velocities.

Impingement and breakup characteristics of free opposed impinging jets with unequal nozzle diameter

The free opposed impinging jets with unequal nozzle diameter is developed to improve mixing efficiency at unequal flowrates for liquid–liquid reactions. The liquid sheet breakup characteristics and droplet behavior are investigated using particle image velocimetry. Water, 40w% glycerol-water (40G), and 50% glycerol-water (50G) under different jet velocities and nozzle diameters are studied for comparison, the correlation equations are obtained and the errors are analyzed. When the jet velocity increases, the liquid sheet breakup mode exhibited a closed-rim mode, open-rim mode, rimless mode, wave or ligament mode, and fully developed mode for water and 40G under a nozzle diameter of 1.5–2 mm, and the first four modes were observed with the high viscosity liquid (50G) or under a large nozzle diameter (2–3 mm). As the jet velocity increases, the dimensionless breakup length increases sharply at the closed-rim and open-rim modes, and then decreases at the rimless mode. The sheet thickness, Sauter mean diameter, average droplet diameter, and droplet velocity decrease, and the droplet size becomes more uniform with increasing jet velocity. However, the opposite variation trend is observed with increasing viscosity and nozzle diameter, and the effect is more significant at u < 4.25 m/s. The droplet average diameter increases at u = 2.12 m/s and 3.18 m/s and decreases at u ≥ 4.25 m/s downwards along y-axis due to the coalescence and secondary breakup of droplets. The droplet velocity becomes lower as droplets move downwards along y-axis from −40 mm to −50 mm due to the dissipation of momentum. The results reveal that the increase of viscosity and nozzle diameter has a negative effect on liquid sheet and droplet behavior at low jet velocities. The rimless mode is the key mode affecting the transition of the sheet breakup mechanism, and after this mode, the negative effects of viscosity and nozzle diameter weaken due to the high momentum, which promotes the breakup of the liquid sheet and droplets. It will provide theoretical support for the mixing mechanism with unequal flowrates, and is helpful to expand the application of free opposed impinging jets.

Self-regulation of black hole accretion via jets in early protogalaxies

ABSTRACT The early growth of black holes (BHs) in high-redshift galaxies is likely feedback regulated. While radiative feedback has been extensively studied, the role of mechanical feedback has received less scrutiny to date. Here, we use high-resolution parsec-scale hydrodynamical simulations to study jet propagation and its effect on 100 M⊙ BH accretion in the dense, low-metallicity gas expected in early protogalaxies. As the jet propagates, it shocks the surrounding gas forming a jet cocoon. The cocoon consists of a rapidly cooling cold phase at the interface with the background gas and an overpressured subsonic phase of reverse shock-heated gas filling the interior. We vary the background gas density and temperature, BH feedback efficiency, and the jet model. We found that the width of the jet cocoon roughly follows a scaling derived by assuming momentum conservation in the jet-propagation direction and energy conservation in the lateral directions. Depending on the assumed gas and jet properties, the cocoon either stays elongated to large radii or isotropizes before reaching the Bondi radius, forming a nearly spherical bubble. Lower jet velocities and higher background gas densities result in self-regulation to higher momentum fluxes and elongated cocoons. In all cases, the outward cocoon momentum flux balances the inward inflowing gas momentum flux near the Bondi radius, which ultimately regulates BH accretion. The time-averaged accretion rate always remains below the Bondi rate, and exceeds the Eddington rate only if the ambient medium is dense and cold, and/or the jet is weak (low velocity and mass loading).

Numerical investigation of the effect of reactive gas jets on the flame acceleration and DDT process

Jet obstacles can quickly induce the deflagration to detonation transition (DDT) process and reduce the thrust loss of an engine. However, there are few studies on the use of combustible premixed gases as the reactive jet obstacle. Based on the OpenFOAM platform, a detailed numerical investigation of the flame acceleration and DDT process is carried out with different initial jet velocities and the number of jet obstacles. The results show that, although the stronger flame generated by the reactive jet obstacles will reduce the virtual blockage ratio to a greater extent, the turbulence and combustion heat release effect generated by them still significantly promote the flame acceleration. In terms of flame acceleration, initially, increasing the initial jet velocity has no obvious effect on the flame acceleration. Later, turbulence and combustion heat release begin to dominate the flame acceleration and significantly promote the flame acceleration. Since the jets in this study adopt the same combustible premixed gases, increasing the number of reactive jet obstacles downstream does not improve the upstream flame acceleration. In DDT, increasing the initial velocity of the reactive jet can improve the detonation initiation, but the effect of the obstacle number on DDT is greatly affected by the initial jet velocity. Specifically, at a low initial jet velocity, more jet obstacles can significantly promote the detonation initiation, while at a high initial jet velocity, the enhanced turbulence caused by the jet is the dominant factor for the flame acceleration. Therefore, compared with the number of jet obstacles, the detonation initiation process of the premixed gases is more sensitive to the change in the initial jet velocity.

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.