Jet Reynolds Number Research Articles

Transformation of single drop breakup from binary to ternary and multiple in turbulent jet flows

By releasing liquid drops in turbulent jet flows, we investigated the transformation of single drop breakup from binary to ternary and multiple. Silicone oil and deionized water were the dispersed phase and continuous phase, respectively. The probability of binary, ternary, and multiple breakup of oil drops in jet flows is a function of the jet Reynolds number. To address the underlying mechanisms of this transformation of drop breakup, we performed two-dimensional particle image velocimetry (PIV) experiments of single-phase jet flows. With the combination of drop breakup phenomenon and two-dimensional PIV results in a single-phase flow field, these transformation conditions can be estimated: the capillary number ranges from 0.17 to 0.27, and the Weber number ranges from 55 to 111.

Intermittent statistics of the 0-mode pressure fluctuations in the near field of Mach 0.9 circular jets at low and high Reynolds numbers

The present paper reports an investigation of the statistical properties of pressure fluctuations in the near field of subsonic compressible jets. The data-base analyzed has been obtained numerically by DNS and LES of two single-stream circular jets, having diameter-based Reynolds numbers of 3125 and 100,000 and Mach number 0.9, respectively, initially laminar and highly disturbed. Pressure fluctuations are extracted from several virtual probes positioned in the near field of the jets and covering a region from 0 to 20 diameters in the axial direction and from 0.5 to 3 diameters in the radial. An azimuthal decomposition of the pressure fluctuations is performed, and the statistical analysis is applied to the axisymmetric 0-mode component and compared to the results obtained from the full original signals. The intermittent behavior is investigated by the estimation of standard statistical indicators, such as probability distribution functions and flatness factor, as well as through conditional statistics based on the application of the wavelet transform. It is shown that downstream of the potential core, intermittency estimated through the traditional indicators is relevant even at the lowest Re for the full signals, whereas it is apparently not significant for the 0-mode component. The wavelet analysis provides an estimation of intermittency scale-by-scale and allows for the calculation of a frequency-dependent FF. This approach reveals that the 0-mode component has a relevant degree of intermittency around the frequencies associated with the Kelvin–Helmholtz instability. The statistics of the intermittent events, in terms of their temporal appearance and energy content, are shown to be weakly sensitive to the jet Reynolds number and the universal behavior can be reproduced by simple stochastic models.

Direct numerical simulation of an evaporating turbulent diluted jet-spray at moderate reynolds number

The evaporation of liquid droplets dispersed in jet-sprays is involved in many industrial applications and natural phenomena. Despite the relevance of the problem, a satisfactory comprehension of the mechanisms that regulate the process has still not been achieved. Indeed, a wide range of turbulent scales and an enormous number of droplets are typically involved, causing the numerical and theoretical tackling of the problem to be challenging. In this context, we address the Direct Numerical Simulation (DNS) of a turbulent jet-spray at a bulk Reynolds number Re=10,000. We focus on the effect of the jet Reynolds number on the evaporation process and the preferential segregation of droplets. The analysis is conducted by comparing the outcomes of the present DNS with that from a lower Reynolds number one in corresponding conditions, Re=6,000. The problem is addressed by employing the point-droplet approximation in the hybrid Eulerian-Lagrangian framework. We present detailed statistical analyses of both the gas and the dispersed phases. We found that the mean droplet vaporization length grows as the bulk Reynolds number is increased from Re=6,000 to Re=10,000, while keeping all the remaining conditions fixed. We attribute this result to the complex interaction between the inertia of the droplets and the dynamics of the turbulent gaseous phase. In particular, at the higher Reynolds number, the effect of the faster turbulent fluctuations of the jet mixing layer, which tend to fasten the process, is compensated by the slower mass transfer from the liquid droplets to the surrounding environment. We also observe intensive preferential segregation of the dispersed phase that originates from the entrainment of dry environmental air into the mixing layer and is intensified by small-scale clustering in the far-field region. We show how the preferential segregation is responsible for a strongly heterogeneous Lagrangian evolution of the dispersed droplets. All these aspects contribute to the Reynolds-number dependence of the overall droplet evaporation rate. Finally, we discuss the accuracy of the d-square law, often used in spray modelling, for present cases. We found that using this law based on environmental conditions leads to a relevant overestimation of the droplet evaporation rate.

Thermoelectric Generation with Impinging Nano-Jets

In this study, thermoelectric generation with impinging hot and cold nanofluid jets is considered with computational fluid dynamics by using the finite element method. Highly conductive CNT particles are used in the water jets. Impacts of the Reynolds number of nanojet stream combinations (between (Re1, Re2) = (250, 250) to (1000, 1000)), horizontal distance of the jet inlet from the thermoelectric device (between (r1, r2) = (−0.25, −0.25) to (1.5, 1.5)), impinging jet inlet to target surfaces (between w2 and 4w2) and solid nanoparticle volume fraction (between 0 and 2%) on the interface temperature variations, thermoelectric output power generation and conversion efficiencies are numerically assessed. Higher powers and efficiencies are achieved when the jet stream Reynolds numbers and nanoparticle volume fractions are increased. Generated power and efficiency enhancements 81.5% and 23.8% when lowest and highest Reynolds number combinations are compared. However, the power enhancement with nanojets using highly conductive CNT particles is 14% at the highest solid volume fractions as compared to pure water jet. Impacts of horizontal location of jet inlets affect the power generation and conversion efficiency and 43% variation in the generated power is achieved. Lower values of distances between the jet inlets to the target surface resulted in higher power generation while an optimum value for the highest efficiency is obtained at location zh = 2.5ws. There is 18% enhancement in the conversion efficiency when distances at zh = ws and zh = 2.5ws are compared. Finally, polynomial type regression models are obtained for estimation of generated power and conversion efficiencies for water-jets and nanojets considering various values of jet Reynolds numbers. Accurate predictions are obtained with this modeling approach and it is helpful in assisting the high fidelity computational fluid dynamics simulations results.

Heat Transfer Characteristics of a Coaxial Inverse Diffusion Flame Jet Impingement with an Induced Swirl

Flame jet has a wide range of applications in the industries and also in domestics field. The efforts have been put to enhance the heat transfer and to reduce the emissions from the premixed and inverse diffusion flame burners. Especially, the IDF burner suffers from lack of proper air and fuel mixing, the swirl generated motion from twisted tape would improve the combustion efficiency. Therefore, an aim of experiment is to study the heat transfer characteristics of an inverse diffusion flame (IDF) jet impinging on a flat surface in a coaxial tube burner with swirl. The twisted tape of 15mm pitch creates the swirl in the flame jet (Corresponding to the twist ratio of 3 and swirl number of 0.52). An effect of swirl at air jet Reynolds number of 1000 to 2500 and surface of the burner-to-impingement plate distance (H/da) varying from 2 to 20 is studied at fixed equivalence ratio (ϕ) of 1.1. An average heat flux and peak heat flux are studied for the region of 0<r/da<3 on an impingement plate. From an investigation, it is found that the swirling in the flame jet enhances the average heat flux by up to 179.2%. The maximum average heat flux is found at the optimal burner-to-target plate distance of 8.

Numerical investigation of a novel jet hole design for staggered array jet impingement cooling on a semicircular concave surface

Staggered array jet impingement cooling (JIC) on a semi-circular concave surface has investigated numerically. The main objective of this work is to examine elongated jet holes on heat transfer and flow characteristics of the staggered array JIC and to elucidate its applicability on cooling of a turbine blade. Jet holes were elongated to the target surface by the nozzles. Numerical computations were performed with different jet Reynolds numbers (5000≤Re≤25000), normalized confinement plate-to-target plate distances (1.0≤H/d≤8.0) and jet nozzle-to-target plate gap (0.5≤G/d≤6.0). Average Nusselt (Nu) number, local Nu number distributions on the surface, flow characteristics and Thermal Performance Factor (TPF) were comprehensively investigated. Numerical results were compared with the normal staggered array JIC. Results showed that local Nu number and average Nu number increases with decreasing G/d. The highest enhancement of heat transfer using elongated jet hole is obtained as 20.16% by decreasing G/d to 0.5 at H/d = 8.0. However, highest TPF is estimated as 1.11 at H/d = 8.0 by G/d = 2.0 for Re = 25,000. Decreasing of G/d provides more uniform heat transfer on the surface of interest in comparison with normal staggered array JIC. Consequently, elongating jet hole is a feasible heat transfer enhancement design for the staggered array JIC on a semicircular concave surface.

Heat Transfer Characteristics of Two-Phase Impinged-Jet Flow and Channel Flow

In the present study, the lateral heat transfer and fluid flow characteristics between two-phase impinged-jet flow and two-phase channel flow were compared. The effects of volumetric quality (0 − 0.9) on the lateral variation of the Nusselt number were considered. The working fluids are air and water. A circular jet of 470 mm long with a 5.86 mm inner diameter is used. The water jet Reynolds number of 3030 is obtained. The results show that the lateral Nusselt number of the two-phase channel flow is 30 − 40% higher than that of the two-phase impinged-jet flow in the bubble-impact region due to the movement of high-velocity bubbles. Additionally, from the flow visualization, most of the bubbles are observed in the two-phase channel flow and increase the lateral Nusselt number in the bubble-impact region, while most of the bubbles are burst and escaped to the environment air in the jet-impact region and few bubbles float on the hydraulic jump in the two-phase impinged-jet flow, which neither touch the heating surface nor enhance the heat transfer enhancement.

Experimental and Numerical Investigation of Gas-Focused Liquid Micro-Jet Velocity

Compressible multiphase numerical simulations of gas-focused micro-jets are compared with the experimental data obtained with the dual pulse imaging laser-induced fluorescence drop velocimetry. Such jets, originating from a 3D printed gas dynamic virtual nozzle into a low-vacuum (150 Pa) environment, are increasingly being used for sample delivery in serial femtosecond crystallography. The distance traveled by a detaching drop from the jet is measured between the two consecutive illumination pulses with a known time delay at the positions 200 µm and 450 µm from the nozzle. Additionally, the high-speed camera images are used to analyze the shape of the jet. An axisymmetric, compressible, Newtonian two-phase helium-water mixture model is numerically solved within the framework of the volume of fluid and the finite volume method. The experimental and the computational studies are performed with a constant volumetric liquid flow rate of 14 μl/min and the gas mass flow rate in the range from 4.6 mg/min to 20 mg/min. The related jet Reynolds number ranges from 120 to 220 and Weber number from 30 to 150. The maximum difference between the measurements and the results of the numerical model in terms of the droplet velocity and jet diameter is within 10 %. The study provides new information on the jet velocities for micron-sized gas-focused nozzles. The validated numerical model can be used as a design tool for the nozzles dedicated to the specific needs of the femtosecond crystallography experiments.

On the formation and sustenance of the compressible vortex rings in starting axisymmetric jets: A phenomenological approach

The present work numerically investigates the formation and sustenance of a primary vortex ring (PVR) and counter-rotating vortex rings (CRVRs) in laminar, axisymmetric, continuously blowing compressible jets. The effect of jet Reynolds number (Rej), jet Mach number (Mj), and pressure-ratio (pjpa) on these compressible vortex rings and their flow dynamics is explored. The core bow shock is formed in the inviscid core of the transonic/supersonic jets for sustaining the supersonic flow field in the PVR. The embedded shock (ES) and vortex-induced shock (VIS) are also found to sustain the evolving PVR in the supersonic jets. The effect of Mj and pjpa on the circulation and pinch-off of the PVR is also investigated. For over-expanded and perfectly expanded supersonic jets, Kelvin–Helmholtz vortices (KHVs) entrained into the PVR interact with the ES to cause a shock–vortex interaction, which results in the appearance of multiple VISs. It is established that the different events of interaction of KHV with the ES inside the PVR after the pinch-off are a manifestation of two factors: (i) the constraint imposed on the PVR by quasi-time-invariance of circulation around a material curve enclosing the PVR and (ii) the requirement of spatially cyclic property distribution inside the PVR. The shock-shear layer–vortex interaction in under-expanded jets is also investigated using vorticity analysis and oblique shock theory. For under-expanded jets, apart from the Mach-reflection timescale, the CRVR pattern formation is also governed by the initial strength of the slip-stream, which, in turn, is regulated by all three parameters, i.e., Rej, Mj, and pjpa.

Bursting bubbles and the formation of gas jets and vortex rings

Bubble bursting is important in air–sea interactions, food science, and industry, but the process of the pressurized gas escaping from inside the bursting bubble is not well understood. The fluid dynamics of gas jets and vortex rings produced by the bursting of 440 µm to 4 cm diameter smoke-filled bubbles resting at an air–water interface is investigated using high-speed stereophotogrammetry. The initial speed of the gas jet released from the bubbles increases with parent bubble size until the Bond number reaches unity and subsequently increases more slowly. The slow, low Reynolds number jets characteristic of small bubbles are attributed to high film retraction speeds which produce relatively large holes in the bubble cap, and these jets roll up into spherical, slow-growing vortex rings which travel short distances. However, the low film retraction speeds characteristic of larger bubbles produce high speed, high Reynolds number jets emitted through relatively small holes which roll up into highly oblate, fast-growing, far-traveling vortex rings. The tiniest bubbles eject only a thin stem-like jet which does not form a vortex ring. Finally, a simple scaling relationship relating the gas jet Reynolds number to the square root of the parent bubble Bond number is proposed.

LES investigation of a Passively Excited Impinging Jet

Periodic vortex shedding which forms when fluid moves over a circular cylinder creates a wake which causes fluctuating unsteady forces on structures. The oscillating wake may also serve to enhance heat transfer rates from a surface on which it impinges. In this paper we investigate the passive excitation of the impinging jet's velocity field using a cylindrical insert. The insert is placed just before where the jet issues from the circular pipe. The flow is examined using Large Eddy Simulations (LES). The jet's Reynolds number based on bulk inlet velocity is 23,000 and jet's outlet-to-target wall distance is two. It is found that using a cylindrical insert in the impingement pipe results in enhanced heat transfer rates for cooling applications.

Effects of pin-fin shape on cooling performance of a circular jet impinging on a flat surface

The present study aims to investigate the effects of pin-fin shape on flow and heat transfer from a single impinging jet on a heated flat surface. Four pin-fin shapes (square, rectangular, circular and elliptical) with identical wetted area were symmetrically located in a circular arrangement. The numerical simulations have been performed by the SST k–ω model for three different jet Reynolds numbers (23,000, 35,000 and 55,000). Numerical results indicate that the pin shape has significant effect on the velocity field and the extension of the recirculation zone around the pins. In addition, the experiments have been conducted on the smooth and roughened surfaces to confirm the priority of elliptical pin-fin in cooling the heated flat plate. The elliptical pin-fins provide maximum velocities and weak back flow region around the pin. The experimental data and numerical results reveal that applying the elliptical pin-fins can significantly decrease the local temperatures in both stagnation and wall jet regions. The average Nusselt number over the surface roughened by elliptical pins increased by about 47–54% in comparison with the flat surface depending on the Reynolds number.

Heat transfer rate of swirling impinging jets issuing from a twisted tetra-lobed nozzle

Heat transfer augmentation by swirling impinging jets delivered through twisted tetra-lobed nozzles (T-LT) is reported. Impinging jets delivered through a circular nozzle (CT: non-swirling jet) and a tetra-lobed nozzle (LT: non-swirling jet) were also tested for comparison. Experimental results showed that Nusselt numbers of impinging jets delivered through all nozzles were increased with Re and decreasing L/Dh. At a given jet Reynolds number (Re) and a jet-to-plate spacing (L/Dh), the impinging jets delivered through the twisted tetra-lobed nozzles and tetra-lobed nozzle yielded superior heat transfer to that delivered by a circular nozzle. For the jets delivered through the twisted tetra-lobed nozzles, the stagnation Nusselt number (Nu) was decreased while the radial heat transfer distribution was improved with decreasing twist ratios of the nozzle. For the investigated range, the jet delivered through the twisted tetra-lobed nozzle with twist ratio of 4.0 yielded a maximal average Nusselt number. Furthermore, the twisted tetra-lobed nozzles (T-LT) with twist ratios of y/Dh = 2.0, 3.0, 4.0 and 5.0 generate higher heat transfer rates than those of the circular nozzle (CT: non-swirling jet) by around 27.7%, 25.0%, 21.6% and 19.3%, respectively.

Effects of pulsation intensity on the flow and dispersion of pulsed dual plane jets

The effects of jet pulsation intensity on the flow and dispersion characteristics of dual parallel plane jets were experimentally studied. The jet pulsations were generated by acoustic excitations. The laser-light-sheet-assisted smoke flow visualization method was used to study the flow evolution processes. The lateral spreading characteristics of the jets were measured using the binary edge detection method. A hotwire anemometer was used to measure the velocity pulsations, mean velocities, turbulence intensities, and Lagrangian turbulent length and time scales. The dispersion characteristics of the jet fluids were measured by the tracer-gas concentration detection method. The jet Reynolds number and excitation Strouhal number were fixed at 600 and 0.106, respectively. The jet pulsation intensities were varied from 0 to 1.6. As the jets were pulsed, the turbulence intensity, lateral spread width, and jet fluid dispersion capability markedly increased compared with those of the non-pulsed jets. The larger the pulsation intensity was, the higher the turbulence intensity, lateral spread width, and jet fluid dispersion. The Lagrangian time and length scales of the turbulent eddies decreased as the jet pulsation intensity increased because the mushroom-shaped structures generated by the jet pulsations broke up into small turbulent eddies caused by the strong vortex-stretching effect.

Modeling subfilter soot-turbulence interactions in Large Eddy Simulation: An a priori study

A new LES model for subfilter soot-turbulence interactions is developed based on an a priori analysis using large-scale DNS data of temporally evolving non premixed n-heptane jet flames at a jet Reynolds number of 15,000. In this work, soot formation is modeled in LES by solving explicit transport equations for soot moments, and the unclosed filtered soot moment source terms are closed by a presumed PDF approach. Due to the strong intermittency of soot fields, a previous modeling approach assumes the presumed PDF to be bimodal accounting for sooting and non-sooting subfilter regions but neglects any sub-structure of the soot distribution. In this work, the modeling framework is improved by a new presumed PDF model that explicitly accounts for the sub-structure of the sooting mode, which is modeled by a log-normal distribution. The previous and new models are assessed by means of their prediction of the filtered source terms and the filtered intermittency, and the log-normal distribution is found to significantly reduce modeling errors, in particular, for the coagulation source term. Introducing a log-normal distribution for the PDF of the sooting mode involves a large amount of additional model parameters, such as the width of the distribution and correlation coefficients among different soot moments, so model assumptions to reduce the number of model parameters are discussed by means of the DNS data. The conclusions are found to be robust with respect to a variation in the global Damköhler number in the DNS datasets. The final model formulation only requires solving two additional transport equations in LES compared to previous models, while significantly improved model predictions are obtained for the coagulation source term which is import for predicting the number of soot particles.

Nanoscale heat transfer investigation of an array of impinging jet systems with different working fluids under crossflow with and without pin fins

AbstractIn the current numerical study, the thermal and flow field performance of an array of confined multiple jets with air, water, and water‐Al2O3 nanofluid in the maximum crossflow configuration over the target plate with and without pin fins is investigated. The numerical results are validated with the experimental data; it is found that a reasonable prediction related to heat transfer can be made. For this study, steady‐state Reynolds‐averaged Navier‐Stokes simulations with the shear‐stress transport turbulence model in ANSYS Fluent were performed. The simulations are performed with volumetric concentration to 3% and the jet's Reynolds number Re = 15 000 to 35 000. In all cases, the jet outlet‐to‐target plate distance is 3. It is found that the increase in values of the volumetric concentration of nanoparticles results in a decrease of the Nusselt number and an increase of the convective heat transfer coefficient. This is because there is an increase in thermal conductivity of the working fluid with the increase in the volumetric concentration of nanoparticles for the same Reynolds number. About 81.5% and 89.1% enhancement in the average heat transfer flux values is observed for flat and pin fin‐roughened target plates, respectively, for .

Dissipation element analysis of non-premixed jet flames

Abstract

Time-resolved thermographic analysis of the near-wall flow of a submerged impinging water jet

Time-resolved infrared thermography through infrared-transparent windows provides a novel measurement approach for non-invasive quantitative analysis of near-wall fluid flows. Here, we apply this approach to study a water flow induced by an axisymmetric submerged impinging jet in the immediate vicinity of the impingement plate. The variations in thermal radiation emitted from a thin near-wall layer result from passive scalar mixing of the weakly heated jet with ambient water. We analyze the magnitudes and turbulent behavior of the thermal fluctuations captured at a frame rate of 300 Hz for various jet impingement configurations and a wide range of jet Reynolds numbers (4000–35000). The results suggest that changes in root mean square and power spectra of the thermal signal are linked to changes in mixing intensity and flow structure in the near-wall region extending up to 16 nozzle diameters from the stagnation point.

Numerical Investigation of Heat Transfer and Pressure Force from Multiple Jets Impinging on a Moving Flat Surface

In this paper extensive numerical investigation of the heat transfer characteristics and the pressure force of jet impingement from the single row and multiple rows on a fixed and moving flat surface are reported. The computations were carried out over a wide range of parameters: relative nozzle-to-surface distance (H/d) from 0.5 to 6, relative nozzle to nozzle distances (S/d) from 4 to 10, jet angle from 45° to 90°, relative velocity ratio (Vplate/Vj) i.e. ratio of surface velocity to jet velocity from 0 to 1. The jet Reynolds number (Re) of 2,500, 3,400, 10,000, 20,000, and 23,000 and the number of jet rows of 1, 2, 4, and 8 have been used. It was found that the numerical accuracy by SST k-ω model is reasonably high to allow for a discussion of the main flow and heat transfer characteristics. The jet impingement heat transfer performance is generally enhanced with the increase of jet Reynolds number and jet angle and with the decrease of surface distance (H/d), jet distance (S/d) and the relative velocity ratio (Vplate/Vj) within the range examined. The pressure force coefficients on the impingement surface are relatively insensitive to Re number and the velocity ratio within the range examined, while it has highly dependent on H/d, S/d and jet angle. For multiple rows of aligned jet holes, the flow pattern exhibited a different shape due to the different intensity of the interference between adjacent air jets. The effect of multiple rows with regards to the impact on average Nu and pressure force coefficient for different geometry variations such as Re, H/d, S/d, VR and ɵ is negligible compared to the single row by approximately 9 and 13% in average respectively. Based on the computed results, equations of dimensionless parameters are correlated.

The effect of fuel composition and Reynolds number on soot formation processes in turbulent non-premixed toluene jet flames

The soot formation processes in three different turbulent prevaporized non-premixed toluene jet flames stabilized on a jet-in-hot-coflow (JHC) burner were investigated in this study. The jet Reynolds number and the stoichiometric mixture fraction were varied in order to manipulate the flow time scales and the chemistry, respectively. Time-resolved laser-induced incandescence (TiRe-LII), non-linear two-line atomic fluorescence of indium (nTLAF), and OH planar laser induced fluorescence (PLIF) were simultaneously applied to yield spatially resolved and instantaneous fields of soot volume fraction, primary particle size, temperature, and OH. The mean distributions of the detected quantities are used to identify major differences among the flames. The highest soot loading is observed for the low Reynolds number and low stoichiometric mixture fraction flame. However, this flame features also the lowest temperature and primary particle size. Based on these observations, the simultaneously detected data sets and flamelet computations are employed to elucidate differences in the soot formation pathways in the flames. The analyses reveal that the high soot loading causes greater heat losses in the low Reynolds number and low stoichiometric mixture fraction flame. This has a significant impact on the soot formation pathways and causes a reduction in the particle size.