Neighboring Jets Research Articles

Influence of novel equilaterally staggered jet impingement over a concave surface at fixed pumping power

A new cooling configuration is proposed for the leading edge cooling of turbine vane by using five equilaterally staggered air jets. Two configurations, having one central and four neighboring jet impingement pipes (JIP), are tested. One configuration has all JIP of circular section and in the second, all four neighboring JIP are provided with 45° chamfer. The chamfered exit of JIP faces the concave target surface. Experimental data of Nusselt number and thermal performance factors (TPF) for chamfered configuration is compared with the non-chamfered configuration, both operated at a constant pumping power. It is found that for the fixed pumping power condition the chamfered configuration performs better than the non-chamfered configuration showing: (i) significant increase in the cooling coverage area (ii) increase in heat transfer and (iii) reduction in coolant requirement. In the range of experimental condition used in the present work, the thermal performance factor of the chamfered configuration is 8–24% better than that of the non-chamfered configuration. The effect of jet-to-plate spacing is found to be mild. The average Nusselt number correlations developed for both the chamfered and non-chamfered configurations are given by-Nu¯chamfered=0.409×Re0.6×(H/d)0.053Nu¯non-chamfered=0.351×Re0.6×(H/d)0.06

An experimental study of interacting swirl flows in a model gas turbine combustor

In this experimental work, we analyze the flow structures emerging from the mutual interaction between adjacent swirling flows at variable degrees of swirl, issued into a semi-confined chamber, as it could happen in a three cup sector of an annular premixed combustor of a modern gas turbine engine. Stereoscopic particle image velocimetry (sPIV) is used to characterize both the non-reacting and reacting flow fields in the central diametrical (vertical) plane of the swirlers and the corresponding transverse (horizontal) planes at different heights above the swirlers. A central swirling flow with a fixed swirl vane angle is allowed to interact with its neighboring flows of varied swirl levels, with constant inlet bulk flow velocity through the central port. It is found that the presence of straight jets with zero swirl or co-rotating swirling jets with increasing swirl on both sides of the central swirling jet, significantly alters its structures. As such, an increase in the amount of swirl in the neighboring flows increases the recirculation levels in central swirling flow leading to a bubble-type vortex breakdown, not formed otherwise. It is shown with the aid of Helmholtz decomposition that the transition from conical to bubble-type breakdown is captured well by the radial momentum induced by the azimuthal vorticity. Simultaneous sPIV and OH-planar laser-induced fluorescence (PLIF) are employed to identify the influence of the neighboring jets on the reacting vortex breakdown states. Significant changes in the vortex breakdown size and structure are observed due to variation in swirl levels of the neighboring jets alongside reaction and concomitant flow dilatation.

Effects of Plume Impingements of Clustered Nozzles on the Surface Skin Friction

Skin-friction values on the surfaces of clustered nozzles were measured using a pointwise local skin-friction sensor. This sensor has a floating head that can detect the local shear force using the flow passing over the head surface. Cold-flow wind-tunnel experiments were carried out with clustered nozzle models. The test models had three or four clustered cell nozzles, or a nonclustered two-dimensional nozzle with a designed Mach number of 3.5, followed by a straight planar section. The experimental conditions were chosen according to the cell nozzle jet expansions. Hence, some conditions involved an oblique shock wave impinging on the bottom surface and others did not. The measured local skin-friction coefficients were compared to those predicted by using an existing prediction model. The results showed that the local skin-friction value significantly increased when the cell nozzle expansion was under the overexpansion condition, whereas the measured values were identical to those predicted by the existing model under the underexpansion condition. The physics behind this observation showed that the expansion waves hardly affected the skin friction, but the oblique shock wave and crossing neighboring jet boundaries could increase the skin friction.

Effects of jet-to-jet spacing and jet height on heat transfer characteristics of an impinging jet array

The Nusselt number distributions for five confined circular air jets vertically impinging on a flat surface were measured. The jets were arranged as an equilaterally staggered array (a middle jet and four neighboring jets). The jet-to-jet spacing to jet diameter ratio (s/d) and the jet height to jet diameter ratio (H/d) were in the range of 2.0–8.0 and 0.5–3.0 respectively. The jet Reynolds number was 20,000. At small s/d and H/d values (s/d=2.0 and H/d=0.5), a maximum Nu, attributed to a strong flow impact (jet interaction) on the impingement plate, was clearly observed between the middle jet and every neighboring jet. The jet interaction decreases with an increase of the s/d and H/d. At small s/d and large H/d values (s/d=2.0 and H/d⩾2.0), jet interference before impingement causes a decrease in heat transfer. However, it achieves a uniform Nusselt number distribution in the region directly covered by the jet array. At intermediate s/d and large H/d values (s/d=4.0 and H/d⩾2.0), both the jet interaction and jet interference are weak, but the expelled air of the middle jet (cross flow) reduces the heat transfer of the neighboring jets. At large s/d values (s/d⩾6.0), regardless of the H/d value (in the range of 0.5–3.0), every jet possesses an independent cooling area on the impingement plate. The result also shows that the maximum Nu variation in the region under the jet array almost linearly increases with the s/d.

Direct printing of copper conductive micro-tracks by multi-nozzle electrohydrodynamic inkjet printing process

To overcome the limitation of low production speed of electrohydrodynamic (EHD) inkjet printing technology and to fabricate low cost electronic micro-structures, this paper presents the direct deposition of copper colloidal solution through the multi-nozzle EHD inkjet printing process. Multi-nozzle EHD inkjet printing head, consisting of five nozzles, was used for simultaneous printing of electrically conductive micro-tracks onto the glass substrate. Nozzle-to-nozzle distance and experimental conditions were optimized to prevent the interaction i.e., cross-talk between electrically charged neighboring jets. After sintering in inert (N 2 gas) atmosphere, printed tracks exhibited approximately five times larger resistivity (9.20 μΩ cm) than that of the bulk copper which is a remarkable achievement. These electrically conductive micro-tracks show the feasibility of multi-nozzle EHD inkjet printing technique using the copper nano-colloidal ink for low cost fabrication of microelectronic structures and devices.

Effects of nozzles array configuration on cross-talk in multi-nozzle electrohydrodynamic inkjet printing head

Abstract This paper presents the analysis of a triangular array of nozzles in a multi-nozzle electrohydrodynamic (EHD) inkjet printing head. A methodology has been proposed to minimize the “end effect” by changing the traditional linear array of nozzles to triangular array of nozzles in multi-nozzle EHD printing head. Interaction (cross-talk) between the electrically charged neighboring jets is investigated and analyzed both numerically and experimentally using three glass nozzles with independent voltage connections and independent ink supply to each nozzle. In order to scrutinize the performance of triangular array multi-nozzle EHD inkjet printing head, comparative study of triangular array multi-nozzle EHD inkjet printing head to the linear array multi-nozzle EHD inkjet printing head has been made on the basis of electric field simulations and experiments. Experimental results illustrate that in triangular array multi-nozzle EHD inkjet printing head, the over potential requirement is low and individual capillaries are operating with more independence as compared to linear array multi-nozzle EHD inkjet printing head.

Effects of spray targeting on mixture development and emissions formation in late-injection low-temperature heavy-duty diesel combustion

The effects of fuel-spray targeting on mixture development, combustion, and pollutant formation were investigated for a late-injection low-temperature combustion (LTC) operating condition in an optically accessible heavy-duty diesel engine. Equivalence-ratio maps, derived from toluene fuel-tracer fluorescence measurements, quantify the effects of fuel-jet targeting on mixture preparation processes under non-combusting conditions. Planar laser-induced fluorescence of formaldehyde (H 2CO), hydroxyl radical (OH), and polycyclic aromatic hydrocarbons (PAH) provide complementary measurements during the combustion process from fuel-lean to fuel-rich reaction zones. Three different injector nozzles with included angles of 124°, 152° and 160° yield unique jet–wall and jet–jet interactions. The baseline 152° nozzle directs the fuel jet toward the vertical center of the piston bowl-wall, where the jets impinge on the wall and merge with neighboring jets prior to the peak heat-release rate. Fuel-rich jet–jet interaction regions develop between the jets near the floor of the bowl, which is where the majority of soot and/or PAH formation occurs. These fuel-rich jet–jet interactions can be reduced by either a more narrow injection angle (124°) or a wider angle injection (160°), both of which lead to decreased soot formation. However, jet–wall interactions may play an important role for late-cycle bulk flows. With the 160° injection, less bulk-fluid motion in the piston bowl occurs, leading to extensive quenched regions throughout the center of the combustion chamber. By contrast, the 124° injection enhances large-scale fluid motion, transporting hot second-stage ignition regions into the fuel-lean quenched mixtures at the center of the chamber, potentially improving late-cycle oxidation of unburned hydrocarbons (UHC) and carbon monoxide (CO).

On the Interaction of Jupiter’s Great Red Spot and Zonal Jet Streams

Abstract In this paper, Jupiter’s Great Red Spot (GRS) is used to determine properties of the Jovian atmosphere that cannot otherwise be found. These properties include the potential vorticity of the GRS and its neighboring jet streams, the shear imposed on the GRS by the jet streams, and the vertical entropy gradient (i.e., Rossby deformation radius). The cloud cover of the GRS, which is often used to define the GRS’s area and aspect ratio, is found to differ significantly from the region of the GRS’s potential vorticity anomaly. The westward-going jet stream to the north of the GRS and the eastward-going jet stream to its south are each found to have a large potential vorticity “jump.” The jumps have opposite signs, and as a consequence of their interaction with the GRS, the shear imposed on the GRS is reduced. The aspect ratio of the GRS’s potential vorticity anomaly depends on the ratio of the imposed shear to the strength of the anomaly. The east–west to north–south aspect ratio is found to be ∼2:1, but without the opposing jumps it would be much greater. The GRS’s high-speed collar and quiescent interior require that the potential vorticity in the interior be approximately half that in the collar. No other persistent geophysical vortex has a significant local minimum of potential vorticity in its interior, and laboratory vortices with such a minimum are unstable.

Effect of a neighboring sonic jet on the shock structure of a sonic jet

Effect of a neighboring sonic jet on the shock structure of a sonic jet

Impingement heat transfer of staggered arrays of air jets confined in a channel

Impingement heat transfer of circular air jets confined in a channel was experimentally investigated. The impingement plate was exerted with a constant surface heat flux. Five jets, including one center jet and four neighboring jets, in staggered arrays were considered. The considered jet Reynolds number ( Re) was in the range 5000–15,000; the jet height-to-jet diameter ratio ( H/ d) was in the range 1.0–4.0; the jet spacing-to-jet diameter ratio ( S/ d) was in the range 4.0–8.0; the jet plate width-to-jet diameter ratio ( W/ d) was in the range 6.25–18.75. Two jet plate length-to-jet diameter ratio ( L/ d), 31.7 and 83.3, were individually arranged. For the center jet with a specific Reynolds number, its stagnation Nusselt number was found to linearly increase with the jet Reynolds number of the four neighboring jets. For all the five jets with the same Reynolds number, the correlation result shows that the stagnation Nusselt number of the center jet is proportional to the 0.7 power of the Re and the −0.49 power of the W/ d. A weak dependence of the stagnation Nusselt number on H/ d, S/ d and L/ d was found.

Large-eddy simulations of excitation effects on a VTOL upwash fountain

The responses of an upwash fountain to various azimuthal and axisymmetric excitations, applied at the exits of its neighboring jets, are investigated. Kinematic arguments are used to determine the interactions of large-scale structures in the fountain and the effects of these interactions on the fountain’s characteristics. Numerical simulations of a row of impinging jets, which contain the essential features of twin jets impinging on the ground, are used to simulate the hovering configuration of a vertical takeoff and landing (VTOL) aircraft. The flow is assumed to be governed by the time-dependent, incompressible Navier–Stokes equations. The large-eddy simulation approach is followed. Distinct fountain characteristics are shared among the cases in which azimuthal perturbations are applied at both jet exits in the same direction. These include a high fountain spreading rate, a strong lateral interaction with the neighboring jets, and a weak upload at the aircraft’s undersurface. A strong similarity in fountain characteristics also exists for the cases of axisymmetric forcing and azimuthal forcing in opposite directions at the two jet exits. The similar characteristics here include a low fountain spreading rate and a strong upload at the aircraft’s undersurface.