Jet Reynolds Number Research Articles

Multi-objective optimization of multiple impinging jet system through genetic algorithm

In this study, a pneumatic contactless levitation system utilizing impinging jet arrays is investigated numerically and the optimum design set is proposed using a multi-objective genetic algorithm known as elitist Non-Dominated Sorting Genetic Algorithm (NSGA-II). Steady and turbulent impinging jet flow is characterized by jet Reynolds number in the range of 3000-15000. Preliminary validation tests have been carried out for multiple impinging jet flow to establish an accurate numerical model. In order to assess enhancements in the contactless levitation system, levitation capacity and energy consumption are selected as objective functions depending on the optimization parameters. Design parameters are nondimensionalized distance between sequential jet nozzles (2 < s/Djet < 6), the aspect ratio of the cylindrical pillars (1 < ARP < 5) and the jet Reynolds number. Cylindrical pillars are implemented to the original design to prevent jet-to-jet interaction and to maintain a passive flow control. The optimization algorithm is performed through 20 generations with a population number of 50. Optimum solution set represented by Pareto Front suggests trade-off designs between the objective functions which provide useful results for the design of the contactless conveying system. The optimum design points are observed at uniformly distributed jet Reynolds numbers, a singular optimum solution for s/Djet=2 and two unique solutions for ARP as 1.6 and 4.2.

Effect of annular flow pulsation on flow and mixing characteristics of double concentric jets at low central jet Reynolds number

This study investigated the effects of annulus pulsation on the flow characteristics and mixing properties of double-concentric jets at a low central jet Reynolds number. The central jet Reynolds number was fixed to Rec = 313. Annulus pulsation was induced by employing a solenoid valve. Streak pictures of instantaneous smoke flow patterns and long-exposure images of jet flows were observed using the laser-light-sheet-assisted smoke flow visualization method. A binary edge-detection technique was used to compute the jet spread widths. Particle image velocimetry (PIV) was utilized to examine the time-averaged velocity vectors, streamline patterns, velocity properties, turbulence intensity distributions, and vorticity contours. A radial dispersion improvement index was defined to quantitatively estimate the mixing property using the tracer-gas concentration detection technique. When the annular flow was pulsated with high amplitudes, it merged toward the central axis owing to an increase in radial momentum. Stagnation points were intermittently witnessed on the central axis. Large jet spread width exhibited, especially at low annulus Reynolds numbers. Turbulence intensities increased drastically in the flow field by the axial elongation of the recirculation region due to the annulus pulsations. The axial elongation of the recirculation region and the intermittent stagnation points led to effective dispersion of the central jet into the annular flow. The radial dispersion improvement index revealed that the mixing capability between the jets could be enhanced up to 80% in the recirculation region by applying annulus pulsations.

Effects of Sinusoidal Excitation on Transverse Jet Dynamics, Structure, and Mixing

This experimental study explored the effects of axisymmetric sinusoidal excitation on structural and mixing characteristics in the equidensity jet in crossflow (JICF). Planar laser-induced fluorescence imaging of acetone seeded in the jet fluid was used in this quantification, where the jet Reynolds number was fixed at and crossflow velocity was varied. Jet-to-crossflow momentum-flux ratios ranged from , with a convectively unstable upstream shear layer (USL) in the absence of forcing, to , with a globally unstable USL. Sinusoidal excitation had a relatively significant influence on the convectively unstable JICF at larger values, especially when the USL was locked-in to the forcing frequency, causing the jet cross section to become more symmetric and causing molecular mixing, quantified in both centerplane and cross-sectional planes, to improve. For a globally unstable JICF, only sinusoidal forcing corresponding to clear lock-in conditions, close to the fundamental shear-layer frequency and/or at very high amplitudes, affected jet structure, with somewhat moderate improvements in molecular mixing. Typically, excitation conditions creating enhanced jet penetration and spread did not correlate directly with improved molecular mixing; there was a stronger correlation of improved mixing with creation of a more symmetric jet cross section, in some cases with a counter-rotating vortex pair structure.

Generalised Eddy Dissipation Concept for MILD combustion regime at low local Reynolds and Damköhler numbers. Part 2: Validation of the model

The generalised Eddy Dissipation Concept (EDC) developed in the first part of this article is thoroughly validated against twelve flames from the Delft and Adelaide jet-in-hot-coflow (JHC) burners. These flames emulate Moderate or Intense Low Oxygen Dilution (MILD) conditions. Modelling of turbulence-chemistry interactions in this regime is a non trivial problem and many standard combustion models may fail. Recent Direct Numerical Simulation studies revealed a distributed appearance of the reaction zone indicating non-flamelet regime, which justified the use of reactor type modelling approaches. Those kind of models are of empirical nature and are sometimes criticized for being dependent on a number of tunable parameters. Also, most of new concepts are validated against a limited number of experiments. In this study, using the same modelling setup, twelve flames with different jet Reynolds number, level of oxidizer dilution with various fuel mixture were simulated. It turned out that the generalised EDC model considerably improved predictions with respect to the standard model for all the considered flames. Even though the predictions of the other EDC extensions provided better results in some regions, only the proposed generalised approach could cover the broad tange of operating conditions, proving its “universality” and reliability.

Modelling the cleaning of viscoplastic layers by impinging coherent turbulent water jets

Impinging liquid jets are widely used to clean unwanted soil layers from the walls of structures and vessels. When the soil is a thin layer of an immiscible viscoplastic material, removal involves the growth of a cleared area (which is circular for a jet impinging normally) bounded by a berm of displaced material. Glover et al. [1] presented a semi-empirical model relating the rate of removal to the momentum flow rate in the liquid film. We present a first-order model for cleaning thin layers of these materials based on the rate of viscous dissipation in a shallow wedge of material at the cleaning front. This yields a result of the form of the model in [1], with expressions linking the kinetic parameters to measurable quantities including the rheology of the soil. The fully coupled problem is not solved: the wedge angle and residual layer thickness need to be specified and were obtained here by fitting to the data. The model is compared with experimental results obtained for three soft solids – two petroleum jellies and a soft paraffin – which exhibited Bingham plastic behaviour and creep, for jet Reynolds numbers between 10,000-37,000, and 0.1<h¯/δo<1.5, where h¯ is the average film depth and δo the layer thickness. The data show reasonable agreement with the model.

Low Frequency Pulsating Jet Impingement Boiling and Single Phase Heat Transfer

Experiments are carried out to characterize the influence of jet pulsations on the single phase and boiling heat transfer under a confined and submerged impinging jet, using water and FC-72. A novel jet pulsation mechanism is developed for introducing the mean flow pulsations at different jet flow rates. The effects of jet Reynolds number, pulsation frequency and amplitude are studied by comparison of the time-averaged Nusselt numbers and boiling curves against steady state data. The root-mean-squared dimensionless temperature oscillations in the heater are correlated to the jet Strouhal number, mean Reynolds number and the diffusion Fourier number of the heater that is based on the frequency of jet oscillations. Under both boiling and single phase cooling conditions, temperature oscillations are found to be inversely related to the Strouhal number with an exponent in the range 0.63-0.75. During pulsating jet impingement boiling, a periodic renewal of the boiling process is observed, where the bubbles on the heater surface are cyclically flushed downstream of the stagnation point in-phase with the pulsating flow. For the cases considered, the root-mean-squared dimensionless temperature is found to be negligible during fully developed nucleate boiling, indicating that the heat transfer process is pseudo-stationary. Here, vigorous ebullition is the dominant mechanism of heat transfer and suppresses any temperature oscillations that can arise due to the flow oscillations. For heat fluxes in the partial nucleate boiling regime and close to critical heat flux however, temperature oscillations decrease with an increase in jet Strouhal number. No marked influence of low frequency jet pulsations is found on the time averaged single phase or boiling heat transfer characteristics (including critical heat flux) for the range of parameters studied.

Impingement of coaxial jet on convex element for confined and unconfined flow

Jet impingement is most effective and active method for cooling and heating of any surface or system. The ability of jet impingement is greatly influenced by nozzle configuration and other dimensional and non-dimensional parameters. Impinging coaxial swirl jet generates interesting flow filed on any test surface and influences both pressure and heat distribution on impinging surfaces. In present study, an experimental investigation is carried to analyze the effects of turbulent coaxial swirl jet on the pressure distribution (PC &amp; PCO) on convex element. For better and acceptable results, the desirable parameters are identified from previous research works. The present experimental result highlights the independency of pressure coefficient (PC) for jet-Reynolds number (Re=70000 to 45000), effect of circumferential angle (θ) or inclination of test element, effect of jet exit to test element distance (Z/dh) and effect of confinement on PC &amp; PCO pattern on a convex test element. The higher pressure coefficient value are obtained at lower Z/dh = 1 &amp; at θ = 15° to 12°and significant drop in the values are seen with increase in the Z/dh &amp; θ. At θ = 20° to 30° the value of PC &amp; PCO reaches to negative magnitude. The use of confinement tube enhancementthe pressure distribution (PC &amp; PCO) by 61% to 64% is seen for the same flow conditions.

Numerical investigation of jet impingement flows with different nanofluids in a mini channel using Eulerian-Eulerian two-phase method

An in–house Fortran based two-phase Eulerian-Eulerian solver is used to numerically study an incompressible confined slot jet impinging with nanofluid on an iso-thermal hot flat surface of a mini channel in the laminar regime. The governing equations of liquid and solid phases are solved using finite difference method. Heat transfer performance of water-based nanofluid with different nanoparticles such as Al2O3, SiO2, Cu, Fe, diamond and Ag is studied. Heat transfer performance of nanofluids cannot be predicted solely from the thermal conductivity values of suspended nanoparticles as observed that Al2O3 water nanofluid shows better performance than iron-water nanofluid. Since the iron-water nanofluid has only 5% more average Nusselt number but 7% more pressure drop compared to Al2O3-water nanofluid for jet Reynolds number 300 and particle concentration 5%. Heat transfer performance of Cu-water nanofluid is studied and reported for different parameters such as jet Reynolds number, particle concentration and channel height to jet width ratios. Results show a significant change in heat transfer characteristics with respect to jet Reynolds number, particle concentration, and height to jet width ratio. The heat transfer enhancement is 25% for Al2O3 water nanofluid and 60% for Cu- water nanofluid compared to pure water at jet Reynolds number 300 and particle concentration 5%. An empirical correlation is developed to predict the average Nusselt number of the jet flow with Cu-water nanofluid impinging on a hot surface. Present results are validated with numerical and experimental results available in the literature.

Experimental investigation of jet-induced wall pressure fluctuations over a tangential flat plate at two Reynolds numbers

The wall pressure fluctuations induced by a subsonic circular jet on a rigid flat plate have been investigated considering two jets with different exit section diameters at the same Mach number. The analysis is aimed at completing the series of papers presented by the authors on the interaction between a subsonic jet and infinite tangential flat plate where the exit Mach number was the only parameter of the jet flow that was varied. In order to analyse other effects out of the Mach number, two configurations with different nozzle exhaust diameters were explored with the objective of isolating the Reynolds number effect keeping fixed the exit Mach number. The nozzle exhaust diameters are 12 mm and 25.4 mm and the instrumented flat plate, installed parallel to the jet flow, is moved at different radial distances from the jet axis. The pressure footprint on the plate has been measured in the stream-wise direction by means of a pair of flush-mounted pressure transducers, providing point-wise pressure signals. Wall pressure fluctuations have been characterised in terms of spectral and statistical quantities. The effect of Reynolds is evidenced and possible scaling relationships that account for the Reynolds dependence are proposed. Implications for modeling the spectral coherence have been considered by the application of the Corcos’ model and the effect of the jet Reynolds number on the model coefficients is analyzed.

Large Eddy Simulation of Heat Transfer Within a Multi-Perforation Synthetic Jets Configuration

Abstract Synthetic jets are produced by devices that enable a suction phase followed by an ejection phase. The resulting mean mass budget is hence null and no addition of mass in the system is required. These particular jets have especially been considered for some years for flow control applications. They also display features that can become of interest to enhance heat exchanges, for example, for wall cooling issues. Synthetic jets can be generated through different mechanisms, such as acoustics by making use of a Helmholtz resonator or through the motion of a piston as in an experience mounted at Institut Pprime in France. The objective of this specific experiment is to understand how synthetic jets can enhance heat transfer in a multi-perforated configuration. As a complement to this experimental setup, large-eddy simulations are produced and analyzed in the present document to investigate the flow behavior as well as the impact of the synthetic jets on wall heat transfer. The experimental system considered here consists in a perforated heated plate, each perforation being above a cavity where a piston is used to control the synthetic jets. Placed in a wind tunnel test section, the device can be studied with a grazing flow and multiple operating points are available. The one considered here implies a grazing flow velocity of 12.8 m s−1, corresponding to a Mach number around 0.04, and a piston displacement of 22 mm peak-to-peak at a frequency of 12.8 Hz. These two latter parameters lead to a jet Reynolds number of about 830. A good agreement is found between numerical results and experimental data. The simulations are then used to provide a detailed understanding of the flow. Two main behaviors are found, depending on the considered mid-period. During the ejection phase, the flow transitions to turbulence and the formation of characteristic structures are observed; the plate is efficiently cooled. During the suction phase, the main flow is stabilized; the heat enhancement is particularly efficient in the hole wakes but not between them, leading to a heterogeneous temperature field.

Effects of the jet pulsation intensity on flame behavior, thermal structure and combustion product concentration of a gas turbine model with swirl combustor

Effects of jet pulsation intensity on flame behavior, thermal structure and combustion products concentration in a swirling dual-disk double-concentric jets are experimentally investigated at different swirling strengths, central fuel jet Reynolds number and excitation Strouhal number. Close-up and full length images of the flames were captured for different flame modes by a digital camera. A hot-wire anemometer was used for measuring the central jet velocities at different jet pulsation intensities. The flame temperatures were measured by a fine-wire thermocouple, at radial and axial directions. The toxic combustion products of different flame modes were measured with a gas analyzer. The flame lifted from blockage disk and stabilized over the control disk at large values of the jet pulsation intensity. The lifted flame was very turbulent, blue, short and stable. Low values of toxic combustion products NO, CO and UHCs were observed from the lifted flame. The lifted flame also had high temperatures at radial and axial direction. High values of the jet pulsation intensity can significantly improve combustion efficiency at flames of swirling dual-disk double-concentric jets. The acoustic excitation can significantly improve combustion efficiency at gas turbine and swirl combustors. As the swirl combustors are widely used at propulsion and power, efficiency of energy transmission systems can significantly improve at high jet pulsation intensities.

Thermal Characterization of a Turbulent Free Jet With Planar Laser-Induced Fluorescence

Abstract Two-color, toluene-based, planar laser-induced fluorescence (PLIF) is utilized to characterize the thermal structure of a turbulent, free jet. The PLIF technique has been used to measure concentration gradients for combustion applications, but its use to quantify thermal gradients is limited. To validate the method, compressed air is seeded with toluene particles. The seeded airflow is heated to temperatures varying from 300 to 375 K, and the heated jet exits a 1.27-cm diameter orifice into quiescent, room temperature air. The jet Reynolds number is varied from 5000 to 15,000. As the jet exits the orifice, the toluene particles fluorescence across a 266 nm laser light sheet which ultimately provides a two-dimensional temperature distribution of the free jet. The rigorous calibration procedure for the PLIF technique is described along with the seeding nuances needed to quantify the thermal structure of the jets. The PLIF technique has been demonstrated for this fundamental flow field, and it has proven to be applicable to more complex heat transfer and cooling applications. Furthermore, the time-averaged temperature distributions obtained in this investigation can be used in the validation of turbulent computational fluid dynamics (CFD) solvers.

Influence of spent air confinement on pressure distribution over a flat plate impinged by an array of jets

The paper deals with an experimental study carried out to determine the effect of spent air confinement on the wall-static pressure distribution (Cp ) over a flat plate normal to a single row of impinging air jets. The jet after impingement is constrained to exit from one side, two opposite sides and all sides. The parameters used in the study are jet Reynolds number (Re = 10000), dimensionless nozzle-exit to target-plate spacing (z/d = 1, 2, and 3), stream-wise jet-to-jet spacing (s/d = 2 and 4), and dimensionless length of the jets (l/d = 1.0). The flat target plate is mounted on a 2D traverse table which also facilitates setting up the nozzle-exit to target-plate spacing. The pressure distribution is measured in the stream-wise direction by using a single column manometer. The cross-flow effect is found to be maximum when the spent jet is free to exit from one side only.

Improving combustion performance of swirling double-concentric jets flames with rich equivalence ratios

The acoustic excitation method was used to improve the combustion performance of rich-combustion flames of swirling dual-disk double-concentric jets. The unexcited and excited conditions were compared. The central jet was propane gas and the swirling annular jet was air. The central fuel jet was acoustically excited by a loudspeaker that was installed by the “downstream longitudinal irradiation” method. A home-made, one-component hot-wire anemometer was used to measure the central jet velocities to reveal the jet pulsation behavior. Long and short exposure flame images were captured for identification of flame behavior. A home-made L-shaped fine-wire thermocouple was used to measure the flame temperatures. A gas analyzer was used to measure the concentrations of main combustion products, including carbon dioxide (CO2), unburned hydrocarbons (UHCs), carbon monoxide (CO) and nitric oxide (NO). The process parameters (Rec, Rea, Stexc, and Ip, identified in the nomenclature) able to provide higher combustion efficiency were introduced. The best operating zones of the unexcited and excited flames were observed. At large central fuel jet Reynolds numbers, the unexcited rich-combustion flames exhibited a long and soot-radiating yellow flame that had low combustion performance. By applying appropriate acoustic excitation, the rich-combustion flames presenting a soot-radiating yellow appearance could be changed to short, blue, high-turbulence, high-stability lifted flames and the combustion performance was improved. The acoustically excited lifted flame had a higher temperature and lower toxic emissions when compared with the unexcited flame. According to the presented results, two strategies of operating the burners of swirling dual-disk double-concentric jets at low and high central fuel jet Reynolds numbers were suggested.

Liquid Impingement Flow and Heat Transfer of a Cone Heat Sink With Filet Profiles

Abstract Jet impingement is a technique for removing heat efficiently. A liquid jet impingement on a cone heat sink was investigated numerically to explore the effect of filet profiles at the top and bottom edge of conical protuberances on fluid flow and heat transfer. An adopted turbulence model was validated through an experiment as described in the literature. Numerical results of pressure coefficient and Nusselt number were obtained for cases with and without filet profiles for variable jet Reynolds numbers and conical angles. Results showed that the flow and heat transfer of conical protuberances with small tip filet profiles are similar to that of the original cone. Pressure coefficient curves are similar to that of convex surfaces, and the average heat transfer slightly increases when the radius of the tip filet profiles exceeds 1 mm. A small filet profile of a conical bottom edge can improve the average Nusselt number. A secondary jet that enhanced the overall heat transfer was demonstrated, and the heat transfers of convex surfaces, as the comparison, with small angles were enhanced in most cases.

Experimental and numerical study of flow and heat transfer from a pulsed jet impinging on a pinned surface

ABSTRACT The present study aims to investigate the effects of the pin on flow and heat transfer from pulsating jet impinging on a heated flat surface. A row of cylindrical pins has been applied on a flat plate with a constant heat flux of 2000 W/m2. The experiments have been performed for two jet to target surface distances, frequency range from 50 Hz to 100 Hz and jet Reynolds number from 7000 to 15000. The effect of presence of pin, pulse frequency, Reynolds number, and jet to target surface distance have been investigated. Results of the numerical simulations indicate that the RNG k-ε model can predict the distribution of Nusselt number in good agreement with the experimental data. Results of the present study showed that the presence of pin changes the coherent vortical of the flow structure at both upstream and downstream of the pins. It is shown that at Re = 10000 and H/d = 5 the pulsed jet with frequencies of 50 Hz, 80 Hz, and 100 Hz increases the heat transfer from the pinned surface by 21%, 30%, and 36%, respectively, in comparison the steady jet. Finally, experimental observation indicated that the Nusselt number increases with increasing the pulse frequency and Reynolds number and decreases with increasing the jet to target surface distance.

Heat transfer characteristics of an inverse diffusion flame with induced swirl

The flame shapes and heat transfer characteristics of methane swirling inverse diffusion flame jet are studied experimentally in an inverse diffusion flame burner. The twisted tape of 15 mm pitch (corresponding to the twist ratio of 2.5 and swirl number of 0.62) is used to create the swirl in the flame jet. The influence of twisted tape at an air jet Reynolds number varying from 1500 to 3000, equivalence ratio varying from 0.4 to 1.4 and burner surface to impingement plate distance of 10–100 mm, is studied. The heat transfer characteristics from an IDF burner are studied through an evaluation of the average heat flux distribution and its coefficient of variance for the four distinct areas on an impingement plate. Results show that the swirl augments the heat flux distribution by up to 480% for slightly richer than stoichiometric flames. Furthermore, the impact of swirling is observed to boost the uniformity of heat flux distribution.

Minimum momentum flux ratio required to prevent air curtain breakthrough in case of cross-curtain pressure gradients: CFD versus analytical equation

This paper presents a numerical study on the required momentum flux ratio to prevent air curtain breakthrough in case of cross-curtain (i.e. cross-jet) pressure gradients. 2D steady Reynolds-averaged Navier-Stokes (RANS) CFD simulations with the RNG k-ε turbulence model are employed for jet Reynolds numbers ranging from 5,000 to 30,000. First, the computational model is validated based on particle image velocimetry (PIV) measurements. Second, the influence of several jet parameters on the separation efficiency is evaluated for a moderate cross-jet pressure difference of 10 Pa. These are the ratio of the jet discharge momentum flux to the jet cross-flow momentum flux (momentum flux ratio), the jet height-to-width ratio and the jet discharge angle. Finally, the minimum deflection modulus to prevent jet breakthrough and the corresponding momentum flux ratio by an analytical equation and by CFD are compared. The results show that, for the configuration under study: (1) jets with the smallest height-to-width ratios (β = 18) provide the highest separation efficiency; (2) inclined jets with discharge angles α0 = 5° and 10° provide slightly higher separation efficiency than straight jets (α0 = 0°) and jets with α0 = 20°; (3) the maximum modified separation efficiency is reached at lower momentum flux ratios for jets with smaller height-to-width ratios and for inclined jets; (4) the analytical and CFD values of the optimal momentum flux ratio differ with up to 31.2%. This study shows how the separation efficiency of air curtains can be improved by adjusting certain jet parameters.

Momentum Diffusion Near Jet Exit in a Round Jet Controlled by Half Delta-Wing Tabs

Direct numerical simulations for a round jet controlled by half delta-wing tabs are performed to elucidate the transition of turbulent diffusion near the jet exit. The aim also involves clarifying the function of the tabs in round jet control. The jet Reynolds number is set as 10,000. The number of the tab is set to 3 and 6. The results reveal that the installation of the tabs generally decreases the turbulent diffusion coefficient except immediately after the jet exit of $$x/d<1.5$$ , where x is the streamwise direction and d is the diameter of the jet exit. The magnitude relation of the turbulent diffusion coefficient between the behind-the-tab area and between-the-tabs area changes along the streamwise direction in the upstream region of $$x/d<4$$ . Also turbulent diffusion does not always follow the conventional gradient-diffusion model there. On the other hand, in the downstream region of $$x/d \ge 4$$ , the difference in terms of the turbulent diffusion coefficient disappears between the two areas. However, the jet shape is triangular in the case of three tabs due to the remaining streamwise vortices generated by the tabs. This fact leads to the larger mean velocity gradient and momentum transport in the behind-the-tab area than those in the between-the-tabs area. Conversely, in the case with six tabs, the turbulent diffusion coefficients at the two areas become almost identical in the downstream region because the jet shape approaches a round shape.

Investigation of the Horizontal Motion of Particle-Laden Jets

Particle-laden jet flows can be observed in many industrial applications. In this investigation, the horizontal motion of particle laden jets is simulated using the Eulerian–Lagrangian framework. The two-way coupling is applied to the model to simulate the interaction between discrete and continuum phase. In order to track the continuum phase, a passive scalar equation is added to the solver. Eddy Life Time (ELT) is employed as a dispersion model. The influences of different non-dimensional parameters, such as Stokes number, Jet Reynolds number and mass loading ratio on the flow characteristics, are studied. The results of the simulations are verified with the available experimental data. It is revealed that more gravitational force is exerted on the jet as a result of the increase in mass loading, which deflects it more. Moreover, with an increase in the Reynolds number, the speed of the jet rises, and consequently, the gravitational force becomes less capable of deviating the jet. In addition, it is observed that by increasing the Stokes number, the particles leave the jet at higher speed, which causes a lower deviation of the jet.