Parallel Jets Research Articles

Experimental study on free and submerged multi-jets

The flow characteristics of the hydraulic jump due to parallel jets are different from the classical jump emerging from a single gate. Due to the highly complex flow field at the downstream pool, deciding about the tailwater measuring location is a challenging issue affecting the flow measuring accuracy. Experiments are conducted herein, on different parallel jets’ configurations for both free and submerged flow conditions. To quantify the flow uniformity, for any downstream cross section, the associated momentum correction factors, β2, were estimated for the free-flow condition. It is found that β2-values depend significantly on the measuring location, and consequently the available conjugated depths relationship results in poor estimation when measuring location moves downstream. Employing Buckingham analysis, a general formula is proposed to calculate the momentum correction factors associated with the free hydraulic jump at different downstream measuring locations. The experimental results of this study indicated that such a formula enhances distinguishing between free and submerged flow conditions of the gates installed in parallel. Finally, a dimensionless stage-discharge formula is presented to predict the submerged flow rate through parallel gates of different gate openings and widths.

Influence of Non-Structural Parameters on Dual Parallel Jet Characteristics of Porous Nozzles.

As an important actuator of the dual parallel jet, the porous nozzle has some non-structural parameters (such as inlet pressure, nozzle spacing ratio, etc.) which have a significant influence on energy transport, chemical combustion and pollutant generation. The research on the microfluidic state of the porous nozzle dual parallel jet, however, remains insufficient because of its microjet pattern and complex intersection process. In this paper, the authors used numerical simulation and an experimental method to clarify the influence of porous nozzles’ non-structural parameters on dual parallel jet characteristics. The results show that the inlet pressure only changes the pressure peak value on the parallel jet axis; the starting point (SP) and peak point (PP) on the parallel jet axis, which are located at Xsp = 22 mm and Xpp = 75 mm, respectively, are not changed; and with the increase in the nozzle spacing ratio, the merging points (MPs) on the parallel jet axis are Xmp = 25 mm, 32 mm and 59 mm, respectively. The merging point and the combined point move to a farther distance and the inner deflection angle of the jet is weakened.

Optimal perturbations in viscous round jets subject to Kelvin–Helmholtz instability

Abstract

Effect of fuel type on the MILD combustion of syngas

The MILD (Moderate or Intense Low-oxygen Dilution) combustion of four typical syngas fuels was examined in CRN (chemical reaction network) model and in parallel jet forward flow combustor. The results were presented on the thermodynamics, ignition kinetics and NO chemistry using CRN calculation. The flow field mixing was obtained from numerical simulation, the global flame signatures and exhaust emissions from experiments. The syngas fuel with higher content of inert gas (N2 and CO2) and lower concentration of active components (H2 and CO) exhibited higher mixture ignition temperature under MILD condition, as a result, greater gas recirculation ratio was required to establish the thermodynamic condition of MILD scheme. H2-enriched syngas and CO-enriched syngas showed different chemical kinetics in the ignition process of MILD mixture. In the NO chemistry under MILD condition, it was revealed that the H2 containing syngas fuels produce the NO emission mainly through the N2O-intermediate and NO-reburning mechanisms, whereas for pure H2 MILD combustion, the NNH route plays a more important role. The N2 component in the syngas fuel had physical effect in delaying the interaction between the air and fuel stream and in favoring the turbulent mixing as well as the gas recirculation. The CO2 component in the syngas fuel had chemical effect in lowering the reaction rate. The critical equivalence ratio above which MILD combustion occurs (with the reaction region covering the entire combustor) was identified for the four syngas fuels. It was observed that the syngas fuel with higher N2 and CO2 content can achieve MILD combustion at leaner condition and with lower NOx emissions. Reduced CO emission and single-digit NOx emissions were achieved for the four syngas fuels.

Experimental and Numerical Analysis on Noise Characteristics of Parallel Multiple Jets Obliquely Impinging on a Flat Surface in the Steel Slab Scarfing

Scarfing applications cause severe noise pollution in metallurgical engineering. To determine the noise characteristics generated by parallel multiple impinging jets in the slab scarfing process, a physical experiment is performed, and a 3D mathematical model is established. Mutual attraction of the parallel multiple jets is recorded, and its influence on the acoustic properties is revealed. The effects of impact height, impact angle, and Reynolds number on the multi‐jet impingement are analyzed. The results indicate that the parallel multiple jets combine in pairs in the upstream. The multi‐jet impingement increases the magnitude of dipole acoustic source, and the noise directivity presents two peaks above the impacted surface. As the impact height increases, the noise intensity first increases to later decrease, and the noise propagation is deflected along the axial direction. As the impact angle decreases, the noise intensity gradually increases, and the spectrum shifts toward higher frequencies. The jet Reynolds number only affects the noise intensity, but has little influence on the spectrum and directivity. The results of this research may provide a theoretical basis for the formulation of guidelines to reduce noise pollution in slab scarfing processes.

Application of lattice Boltzmann method for studying interaction dynamics of parallel plane minijets

In this work, the method of lattice Boltzmann equations was applied to simulate the systems of two and three parallel laminar plane jets. Simulation was carried out for different Reynolds numbers (Re = 20-200). For the case of two jets, the character of jet interaction has been determined to be, in general, non-stationary, but for small Re values, the process becomes stable due to the action of viscosity forces. For the case of three jets, the character of jets interaction is found to differ significantly from the case of two jets and is non-stationary even for small Re numbers.

Experimental study on the flow field and economic characteristics of parallel push-pull ventilation system

Abstract Push-pull ventilation systems provide excellent control of contaminants and harmful gases. However, since both a push inlet and a pull outlet are used in the push-pull ventilation system, the flow rate required by the system is large. In that case, the energy consumption of the system is large. The purpose of this paper is to study the flow field and economic characteristics of a parallel push-pull ventilation system by reducing the flow rate of the exhaust outlet, which will be achieved by reducing the size of the exhaust hood. The three commonly used push-pull ventilation systems were analyzed: a high velocity push-pull system with high air supply velocity, a low velocity push-pull system with wide airflow and small velocity, and a parallel push-pull system with wide airflow and uniform air supply velocity. Results showed that the parallel push-pull ventilation system was the only one in which the flow rate of the exhaust outlet could be reduced, reducing the overall energy consumption. Under conditions of the parallel air supply jet, the diffusion range of contaminants in the push-pull flow field was the smallest and reducing the exhaust air flow rate did not affect the capture efficiency of pollutants. These results may be useful in guiding the design of push-pull ventilation system and optimize economic constraints.

The levitation of a sphere by two parallel turbulent jets

Two vertical turbulent round jets are used to levitate a spherical particle. First, a symmetric configuration is investigated with the two jets of equal flow rates. A structure reminiscent of a pitchfork bifurcation is reported and characterized between a double trap configuration for low flow rates and a single trap regime at higher flow rates. A second configuration is then studied with the flow rates that differ between the left and right jets. In this case, a preferential transition toward the jet of a higher flow rate is reported and quantified. A model is presented predicting the force field for the symmetric and asymmetric configurations. The model is compared to the force field measurements realized with a particle on a flexible cantilever. Finally, a particle dynamics simulation is proposed to analyze in more detail the transition for the levitation in the symmetric and asymmetric configurations.

Hydrodynamics and Local Turbulent Mixing of Submerged, Parallel Liquid Jets: Experiments and CFD Simulations

The hydrodynamics and local turbulent mixing of parallel multiple liquid jets, submerged in liquid, were investigated by means of experiments and computational fluid dynamics (CFD). A renormalization group (RNG) k-ε turbulence model was used to simulate the flow field. The model was validated experimentally by particle image velocimetry (PIV) measurements. In the converging region adjacent to the nozzle exits, the recirculation region disappears, and there is only ambient fluid entrainment. Different jet arrays were compared to evaluate the effects of the jet spatial arrangement on the hydrodynamics and mixing performance. A shorter mixing length in the merging region suggests that mixing is more efficient in the triple-jet system than in other jet systems. Compared with the jet Reynolds number, the jet spacing plays a more significant role in determining the critical mixing regions, while the linear relationship between them is more sensitive than that for multiple parallel plane jets.

Multi-jet gas cooling of in-beam foils or specimens: CFD predictions of the convective heat-transfer coefficient

An experiment was designed to investigate the possible enhancement of the convective heat-transfer coefficient by utilizing multiple, parallel jets in the cooling of a small heated surface, such as typically induced by an accelerated ion beam on a thin foil or specimen. The hot spot was provided using a small electrically heated plate. It was found that heat-transfer calculations by means of simple empirical methods based on dimensional analysis are not useful in this case and that advanced computational fluid dynamics (CFD) mod-elling is essential to interpret the results. It is shown that enhanced convective cooling can indeed be obtained with a multi-jet configuration as compared to a single-jet configuration but only under very selective conditions. An improperly designed multi-jet configuration can also provide significantly reduced cooling relative to the single-jet case and the estimation of the behavior of any particular jet geometry is eminently non-intuitive. CFD provides acceptable quantitative results and seem to be the only tool available to gain an understanding of these complex flows where simple models and “rules of thumb” cannot be relied upon. – LA-UR-18-29455

Large eddy simulation of two parallel round jets

The results of large eddy simulation of flow dynamics in a system of two parallel round turbulent jets are presented. Twin jets are investigated at Reynolds numbers Re = 5500 and 11000; the cases with different distances between the jet axes s/d = 1.8 and 2.4 are examined. The data are compared with the classical case of a single round submerged jet.

Experimental study of interaction of two parallel circular jets

The results of experimental study of interaction in a system of two turbulent circular parallel jets are presented. The studies were conducted using a two-component laser-Doppler anemometer with an adaptive time selection of the velocity vector. The distance between the jet axes in experiments was varied within s/D = 1.8-3.0 with Reynolds numbers Re = 5500 and 11000. Distributions of average and pulsating components of the velocity vector at distances from the nozzle H/D = 0-10 were obtained. These distributions were analyzed in the region closest to the nozzles and the region of the greatest jet interaction.

Large Eddy Simulation of Compressible Parallel Jet Flow and Comparison of Four Subgrid-Scale Models

Large Eddy Simulation of Compressible Parallel Jet Flow and Comparison of Four Subgrid-Scale Models

Verification and validation of LES of a triple parallel jet flow in the context of a thermal striping investigation

Large Eddy Simulation (LES) consists in explicitly resolving the largest turbulent scales while the small-scale motions are taken into account by means of a subgrid-scale (SGS) model. Although it remains computationally expensive, LES seems to constitute an increasingly employed tool for engineering applications in fluid mechanics. In a nuclear context e.g., LES is a relevant approach for the numerical study and prediction of the thermal striping, which can occur during the functioning of sodium-cooled nuclear reactors and may cause some damage to the structures. Numerous experiments where three differentially heated jets mix inside a cavity were designed and conducted by the Japan Atomic Energy Agency (JAEA) to study these phenomena. In order to evaluate the quality of LES using the CEA in-house TrioCFD code, a Verification and Validation study is proposed. Several LES of triple parallel jets are conducted with various grid sizes and SGS models. Following the idea of Celik et al. (2009), the effective SGS kinetic energy is written as a polynomial function of the grid size Δ:keff,sgs=amodΔpmod+anumΔpnum, where the two terms in the right-hand side are respectively the contributions of the numerical scheme and of the subgrid-scale model. In the Verification part, the orders of convergence of these two contributions are assessed for the Smagorinsky and WALE models. In the Validation part, an evaluation of the results is proposed by comparing computed velocity profiles to experimental measurements. The effects of grid resolution, SGS model and boundary conditions are discussed.

Experimental study on sudden contraction and split into the inlets of two parallel rectangular jets

The inlets to parallel rectangular jets are unique and a critical component of its design. A sudden contraction (contraction ratio = 4.2) of a vertically upward entry flow into two inlets has been investigated experimentally. Changes in velocity profile close to the inlets were measured, while the corner vortices, fluid separation and the development of axial and lateral turbulent intensities were observed from the result of particle image velocimetry analysis. The velocity profiles in the fluid adjacent to the inlets are independent of the Reynolds number, generally the same for similar geometries, concave but changes rapidly before finally adapting to the positions of the inlets. The radii of the corner vortices decreased and became elliptical with an increase in Re. Also, the detachment length shrunk by 60% when the Re was doubled. Lateral and axial turbulence increased and more fluid flowed parallel to the wall as Re increased. The trend and magnitude of the lateral to axial velocity agree with those of similar works.

Stability and emission characteristics of nonpremixed MILD combustion from a parallel-jet burner in a cylindrical furnace

This experimental study investigates the stability and emission characteristics of the nonpremixed MILD combustion from a parallel jet burner firing propane (C3H8) in a cylindrical furnace without preheating and dilution. Forty cases are examined by varying the fuel-air nozzle separation S (= 8 mm–105 mm), global equivalence ratio Φ (= 0.6–1.0) and thermal input Pin (= 10 and 15 kW) to highlight the impact of S. The investigation is carried out based on the measurements of in-furnace temperature and exhaust species concentrations, as well as the visual inspection into the furnace. It is demonstrated that the NO emission (ENO) decreases generally with increasing either S (at S > 8 mm) or Φ (at least for Φ ≤ 1.0). Interestingly, the combustion for S = 8 mm behaves like a premixed counterpart and so is distinct from the other cases: e.g., this combustion is fully flameless or stable in the MILD regime over a wider range of Φ and its ENO has a weaker dependence on the furnace temperature. By contrast, for S = 45 mm, the flameless mode takes place only at Φ = 1.0 due to the small mixing zone under this air-fuel arrangement, which is revealed by aerodynamic analysis. Hence, the corresponding experimental ENO-S relationship appears to have a turning point around S = 45 mm. For the present study, all the measured NO emissions for S = 8–105 mm fall as the overall furnace temperature rises, which seemingly differs from the existing knowledge. To explain this and other results, the numerical analysis from computational fluid dynamics (CFD) is performed. It is found that the variations of NO emissions depends on the maximum temperature within the furnace rather than the average furnace temperature. Finally, the present study approves that the fuel-air jet separation should be taken as a critical operational parameter when designing the parallel multiple jet burner for nonpremixed MILD combustion.

Effect of inclination of twin impinging turbulent jets on flow and heat transfer characteristics

This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets.

The influence of the jets configuration on the intensity of their mixing

Experimental study results of two parallel turbulent jets and a set of discrete jets evenly distributed over a circle are presented. Distributions of averaged velocities and their pulsations were obtained and analysed. It was shown that in the case of a discrete annular jet, the flow structure changes significantly, and the distribution of average velocities indicates the presence of the large-scale vortex structure on the jet axis.

Large eddy simulations of the turbulent flows of twin parallel jets

Turbulent jet interactions play a significant role in terms of momentum and heat transfer. For example, in next generation nuclear reactors, high temperature flow mixing in the lower plenum and mixing fluctuations in the coolant may influence power oscillations. Thus, the estimation of the mixing condition needs to be accurate. Recent benchmark experiments using particle image velocimetry (PIV) technique provided high-fidelity experimental data that could be used in comparative studies. In the present study, large eddy simulation (LES) numerical methodology is used on a parallel jet system and validated with benchmark PIV experiments. The results indicate good agreement in terms of the merging point and time averaged velocity profile. Spectral analyses via Welch’s power spectral density functions are used to analyze frequency information in turbulence jets. The proper orthogonal decomposition (POD) analysis method is applied using a snapshot method. The POD analysis indicates vortex structures similar to those in the benchmark PIV experiment.

New technology and practice of dust pollution control with foam jet in underground mines

Foam is used as an efficient means of dust suppression in underground coal mines. The poor performance of conventional adding device of foaming agent restricts its wide application. The objective of this study is to propose and investigate a new parallel jet adding device (PJAD). Experimental results show that PJAD requires a greater water flow to produce negative pressure than the single stage jet adding device (SJAD) and is harder to generate cavitation. PJAD consumes a less pressure loss than SJAD and realizes any adding proportion below 1%, which is especially suitable for precision addition of foaming agent. A foaming system used for dust suppression is put forward with PJAD adding foaming agent. Field application indicates that foam achieves a far better dust suppression effect than the roadheader water spraying, and the foam cost is significantly reduced due to the low adding proportion of foaming agent. The marked dust suppression effect makes us believe that the proposed PJAD will greatly promote the large-scale application of foam technology used for dust suppression in underground coal mines.