Submerged Air Jet Research Articles

Heat Transfer and Fluid Dynamics Study in Solar Air Heater Employing Impinging Circular Air Jet Array for Effective Jet Stability

Abstract This experimental and numerical investigation is intended to increase heat transfer, reduce pumping power, and present important guidelines on the use of confined submerged air jet impingement in solar air heater designs that employ an array of circular air jets for high thermohydraulic performance. In order to increase thermohydraulic performance, a novel approach is adopted, i.e., by improving jet stability and consequently jet deflection that gives origination to four solar air heater designs for this investigation. The obtained numerical results revealed that thermohydraulic performance improves significantly by stabilizing the air jet array, which means lowering the jet deflection. The thermohydraulic performance of design-I is high compared to design-II, design-III, and design-IV, both at equal mass flow rate and Reynolds number. Moreover, design-I presents 20%, 33%, and 52% relative improvement in deviation angle of the jet core compared to design-II, design-III, and design-IV, respectively, at a minimum mass flow rate of 0.01 kg/s and Reynolds number of Re=2000, respectively. In addition, correlations to define heat transfer and fluid dynamics characteristics are established. An important guideline upon the use of solar air heater with jet impingement is that a solar air heater of short length and height (jet and plate spacing), and large width should be used for high thermohydraulic performance.

Removal mechanism of submerged air jet polishing considering the state of abrasive particles

Removal mechanism of submerged air jet polishing considering the state of abrasive particles

Experimental validation of inviscid linear stability theory applied to an axisymmetric jet

We study the development of perturbations in a submerged air jet with a round cross-section and a long laminar region (five jet diameters) at a Reynolds number of 5400 by both inviscid linear stability theory and experiments. The theoretical analysis shows that there are two modes of growing axisymmetric perturbations, which are generated by three generalized inflection points of the jet's velocity profile. To validate the results of linear stability theory, we conduct experiments with controlled axisymmetric perturbations to the jet. The characteristics of growing waves are obtained by visualization, thermoanemometer measurements and correlation analysis. Experimentally measured wavelengths, growth rates and spatial distributions of velocity fluctuations for both growing modes are in good agreement with theoretical calculations. Therefore, it is demonstrated that small perturbations to the laminar jet closely follow the predictions of inviscid linear stability theory.

Characteristics of the circular subsonic laminar jets flow with an initial Poiseuille profile

In this work, the experimental data are compared with the version of the “strong” jet (Re ≫ 1) of the exact Landau-Squire solution. The experiments were performed for a submerged air jet flowing out of a tube with a diameter of D = 3.2 mm and a length of more than 100D at a Reynolds number equal to Re = 436. The initial conditions in the jet are the Poiseuille velocity profile, the level of velocity pulsations is less than 1%. Measurements were carried out using a hot-wire anemometer. It is shown that satisfactory agreement with theory is achieved at distances from the tube starting from x/D = 5.6 and up to the zone of transition to turbulence (x/D > 35). Turbulence along the jet axis will increase from 1% to 2.5%, while in the mixing layers it increases to 4.7%.

Investigation of Submerged Jets with an Extended Initial Laminar Region

The method of producing laminar submerged jets using a device, whose length is comparable with the jet diameter, is described. A submerged air jet, 0.12 m in diameter, produced by means of this technique is experimentally investigated in the Reynolds number range from 2000 to 13 000. Hot-wire anemometer measurements of the flow parameters and laser visualization of the flow are performed. It is shown that the device developed makes it possible to produce submerged jets with the laminar regions as long as 5.5 jet diameters. The initial regions of such jets can be used to study the development of disturbances in submerged jets, as well as used in medicine and engineering in organizing various gasdynamic curtains which produce zones with given properties with respect to purity and composition inside another gas media.

Secondary peak in the Nusselt number distribution of impinging jet flows: A phenomenological analysis

This paper focuses on a wall-resolved Large Eddy Simulation (LES) of an isothermal round submerged air jet impinging on a heated flat plate, at a Reynolds number of 23 000 (based on the nozzle diameter and the bulk velocity at the nozzle outlet) and for a nozzle to plate distance of two jet diameters. This specific configuration is known to lead to a non-monotonic variation of the temporal-mean Nusselt number as a function of the jet center distance, with the presence of two distinct peaks located on the jet axis and close to two nozzle diameters from the jet axis. The objectives are here twofold: first, validate the LES results against experimental data available in the literature and second to explore this validated numerical database by the use of high order statistics such as skewness and probability density functions of the temporal distribution of temperature and pressure to identify flow features at the origin of the second Nusselt peak. Skewness (Sk) of the pressure temporal distribution reveals the rebound of the primary vortices located near the location of the secondary peak and allows to identify the initiation of the unsteady separation linked to the local minimum in the mean heat transfer distribution. In the region of mean heat transfer enhancement, joint velocity-temperature analyses highlight that the most probable event is a cold fluid flux towards the plate produced by the passage of the vortical structures. In parallel, heat transfer distributions, analyzed using similar statistical tools, allow to connect the above mentioned events to the heat transfer on the plate. Thanks to such advanced analyses, the origin of the double peak is confirmed and connected to the flow dynamics.

Experimental study on gas-liquid flow characteristics of submerged air jets

The gas-liquid flow structure and interfacial behavior of submerged air jets were investigated experimentally using high speed digital video camera and image processing techniques. The jet pressure ratio varied from 1.8 to 4.8 in the experiment. And results from different jet nozzles were processed and compared. Statistical characteristics of the jet diameters along the axial distance were obtained and analyzed. Time series analysis was implemented to study the interface unsteadiness by calculating the gas-liquid interface deviation. The results showed that the jet diameters increase first linearly then nonlinearly and its growth rate decreases along the axial distance. The reason for the divergence between the result of this experiment and those done by other researchers was analyzed. Comparing the results of different pressure ratios and nozzle diameters, we found that larger jet pressure ratios have larger jet diameters and nozzle diameters nearly have no bearing on the distribution of dimensionless jet diameters. The interface unsteadiness in low and high pressure ratios exhibited totally distinct properties. And a minimum unsteady value was found along the axis of the air jets.

Influence of the shape of the orifice on the local heat transfer distribution between smooth flat surface and impinging incompressible air jet

An experimental investigation is performed to study the influence of the shape of the orifice (circular, square, triangular and elliptical), jet to plate distances and Reynolds number on the local heat transfer distribution to normally impinging submerged air jet on smooth and flat surface. The Reynolds numbers were varied from 5000 to 30,000 in the steps of 5000 and the jet to plate distances used were 0.5, 1, 2, 4, 6 and 8. The equivalent diameter (ratio of area to the perimeter) of all the orifices were maintained nearly constant (5.7mm). The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. For all the shapes, the area averaged Nusselt number increases with increase in Reynolds number. The area averaged Nusselt number at all Reynolds number is observed to be highest at a z/d of 4. Axis switching is observed for all the shapes except circular orifice. The square, triangular and elliptical orifice respectively undergoes a 45°, 180° and 90° axis switch. Pressure loss coefficients of various orifices are reported.

FORMATION AND DEVELOPMENT OF SUBMERGED AIR JETS

An experimental investigation on submerged air jets was carried out in a 14×14 cm2 perspex columnwith a liquid height of 25 cm. Jets were generated by flowing air at different velocities through nozzles immersedat the bottom of a stagnant liquid pool. The diameters of the nozzles used were 1, 4 and 6 mm. The jet formationwas observed in three different liquids; namely, water, ethanol and glycerol. The jet formation velocity and jetlengths at different liquids and with different nozzles were measured by video recording and image processing.Two distinct regime transitions; bubbling to jetting and jetting to bubble plume were observed. The jet lengthincreased steadily with jet velocity. The nozzle size was found to affect both jet formation and jet length. Theproperties of liquid also affect the formation of jet.

An experimental study of the flow of subsonic flat mini and micro air jets

We have experimentally studied subsonic submerged air jets emitted from flat mini and micro nozzles with characteristic dimensions from 22 to 600 μm in a range of Reynolds numbers 70–2600. The point of laminar-turbulent transition (jet penetration range) was determined using the flow visualization technique. It is established that the penetration range of micro jets can reach 100–300 nozzle calibers. The Reynolds number for the transition to turbulence in flat mini and micro jets reaches high values (1000–2600), which are two to three orders of magnitude greater than the Reynolds numbers for the loss of stability (3–10). Available experimental data are summarized and generalized based on the Reynolds number determined for the jet penetration range.

Doubly spiraled supersonic jet

The instability of supersonic underexpanded submerged air jet outflow from a nozzle with a central cone in the form of two convergent interlaced spirals is described. A technique for controlling this structure by means of extending the central cone streamwise is proposed. It is shown that the central cone position makes it possible to control the gas outflow and to go over from the Mach reflection regime to the regime of wave spiral and gaseous phase condensation, which before were not observable. The results of the measurements of the dynamic head of jets issuing from the nozzle with and without the central cone are presented.

Time resolved investigations on flow field and quasi wall shear stress of an impingement configuration with pulsating jets by means of high speed PIV and a surface hot wire array

The effects of jet pulsation on flow field and quasi wall shear stress of an impingement configuration were investigated experimentally. The excitation Strouhal number and amplitude were varied as the most influential parameters. A line-array with three submerged air jets, and a confining plate were used. The flow field analysis by means of time resolved particle image velocimetry shows that the controlled excitation can considerably affect the near-field flow of an impinging jet array. These effects are visualized as organization of the coherent flow structures. Augmentation of the Kelvin–Helmholtz vortices in the jet shear layer depends on the Strouhal number and pulsation magnitude and can be associated with pairing of small scale vortices in the jet. A total maximum of vortex strength was observed when exciting with Sr = 0.82 and coincident high amplitudes. Time resolved interaction between impinging vortices and impingement plate boundary layer due to jet excitation was verified by using an array of 5 μm surface hot wires. Corresponding to the global flow field modification due to periodic jet pulsation, the impact of the vortex rings on the wall boundary layer is highly influenced by the above mentioned excitation parameters and reaches a maximum at Sr = 0.82.

Influence of the shape of the nozzle on local heat transfer distribution between smooth flat surface and impinging air jet

An experimental investigation is performed to study the effects of the shape of the nozzle, jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged air jet on smooth and flat surface. Three different nozzle cross-sections, viz. circular, square, and rectangular, each with an equivalent diameter of around 20 mm are used during this study. Reynolds number based on hydraulic diameter ( d) is varied between 5000 and 15000 and jet-to-plate spacing from 0.5 to 12 nozzle diameters. Length-to-equivalent diameter ratio ( l / de ) of 50 is chosen for each nozzle configuration. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Local and average Nusselt number on the impinged surface are presented for all the nozzle configurations investigated. Pressure drop measurements across nozzles are made and pressure loss coefficient for all nozzle configurations is reported. Average Nusselt numbers are found to be insensitive to the shape of the nozzle.

Temperature and flow-velocity measurements by use of laser-induced electrostrictive gratings

Light scattering from laser-induced electrostrictive gratings has been used for simultaneous, instantaneous, nonintrusive, and remote measurements of temperature and velocity in a submerged air jet. We accomplished phase-sensitive detection of scattered light by superimposing two signal beams whose frequencies were Doppler shifted by the movement of the grating. Temperatures in the range 295-600 K and flow velocities in the range 10-100 m/s were measured.

STAGNATION REGION HEAT TRANSFER OF A TURBULENT AXISYMMETRIC JET IMPINGEMENT

The turbulent heat transfer characteristics in a stagnation region were investigated experimentally for an axisymmetric submerged air jet impinging normal to a heated flat plate. The temperature distribution on the heated flat surface was measured by a thermochromic liquid crystal (TLC) with digital image processing technique. To get precise heat transfer data inside the stagnation region, a fully developed straight pipe nozzle was used in this study. The stagnation Nusselt number was correlated for the jet Reynolds number and the nozzle-to-plate spacing as Nu theta alpha Re 0.565(L/D)0.0384. For the nozzle-to-plate spacing of L/D=2, the stagnation Nusselt number varies according to Nu theta alpha Re 0.050 and the local heat transfer rates exhibit two maxima. The primary and secondary maxima are attributed to the accelerated radial flow and the transition from a laminar to a turbulent boundary layer, respectively. For larger nozzle-to-plate spacings (L/D<6), the local heat transfer decreases monotonically with radial distance. The Reynolds number dependence is enhanced as L/D increases (Nu theta alpha Re0.58 for L/D=6 and Nu theta alpha Re0.65 for L/D=10). This results from the increase of the centerline turbulent intensity of approaching jet due to strong momentum exchange with ambient fluids.

Local heat transfer measurements from an elliptic jet impinging on a flat plate using liquid crystal

Local heat transfer measurements are reported for a turbulent submerged air jet issuing, normal to a heated flat plate, from an elliptical nozzle with an aspect ratio of 2.14. The Reynolds numbers used were 5000, 10 000, and 20 000 and the nozzle-to-plate distances were L/D e = 2,4,6 and 10 . The temperature on the heated plate surface was measured using a thermochromic liquid crystal and a digital image processing system that yielded an unbiased means for the quantitative color determination of the liquid crystal. The isothermal contour on the uniformly heated plate changed in shape from elliptic to near circle to elliptic again with an increasing nozzle-to-plate distance as the elliptical cross-section switched its orientation. For L/D e from 4 to 10 and for all Reynolds numbers, the local Nusselt number decreased monotonically with increasing radial distance. However, for L/D e = 2 and Re = 10 000 and 20 000, the Nusselt number distributions exhibited second and third maxima. This may be attributed by the induced toroidal vorticities that were formed at the boundary between the impingement region and the wall jet region. For L/D e from 2 to 6, the stagnation point Nusselt number varied according to Nu s ∝ Re 0.5 . For a larger nozzle-to-plate distance of L/D e = 10 , the Reynolds number dependence was stronger ( Nu s ∝ Re 0.74 ) due to an increase of turbulence in the approaching jet as a result of the stronger exchange of momentum with the surrounding air. The Nusselt number for the elliptic jet in the impingement region was larger than that for a circular jet. This may be caused by the large entrainment rate and large scale coherent structure of the elliptic jet.

Heat transfer to a row of impinging circular air jets including the effect of entrainment

An experimental investigation is performed to characterize the convective heat transfer from a flat surface to a row of impinging, circular, submerged air jets formed by square-edged orifices having a length/diameter ratio of unity. Distributions of recovery factor, effectiveness, and local heat transfer coefficient are determined. Spanwise-average and surface-average heat transfer coefficients are calculated from the local heat transfer coefficients. The heat transfer coefficient is independent of the temperature difference between the jet and the ambient, if it is defined with the difference between the (heated) wall temperature and the adiabatic wall temperature. The effectiveness is independent not only of the temperature difference between the jet and the ambient but also of the jet Reynolds number in the range studied. Spanwise-average and surface-average heat transfer coefficients have the largest values on the impingement line and decrease with increasing distance away from it.

Investigation of supersonic isobaric submerged turbulent jets

Turbulent supersonic submerged air jets have been investigated on the Mach number interval Ma = 1.5–3.4 and on the interval of ratios of the total enthalpies in the external medium and the jet i0 = 0.01 – 1. Oxyhydrogen jets with oxidizer ratios α = 0.3–5 flowing from a nozzle at Mach numbers Ma = 1 and 2.4 have also been investigated. When α < 1 the excess hydrogen in the jet burns up on mixing with the air. Special attention has been paid to obtaining experimental data free of the influence on the level of turbulence in the jet of the initial turbulence in the nozzle forechamber, shock waves occurring in the nozzle or in the jet at the nozzle exit, and the external acoustic field. The jet can be divided into two parts: an initial part and a main part. The initial part extends from the nozzle exit from the section x′, in which the dimensionless velocity on the jet axis um = ux/ud = 0.75. Here, ux is the velocity on the jet axis at distance x from the nozzle exit, and ua is the nozzle exit velocity. The main part of the jet extends downstream from the section x′. For the dimensionless length of the initial part xm′ = x′/da, where da is the diameter of the nozzle outlet section, empirical dependences on Ma and i0 are obtained. It is shown, that in the main part of the jet the parameters on the flow axis — the dimensionless velocity and temperature — vary in inverse proportion to the distance, measured in units of length x′, and do not depend on the flow characteristics which determine the length of the initial part of the jet. The angles of expansion of the viscous turbulent mixing layer in the submerged heated or burning jet increase with decrease in i0 and Ma and are practically independent of the afterburning process.

The physical behavior of a gas jet injected horizontally into liquid metal

The gas fraction and bubble frequency distributions in a submerged air jet, injected horizontally into mercury, have been measured under isothermal, nonreactive conditions for nozzle diameters of 0.325 and 0.476 cm and jet Froude numbers ranging from 20.5 to 288. The measurements reveal that the jets expand extremely rapidly upon discharge from the nozzle with an initial expansion angle of 150 to 155 deg. This value, which is over seven times greater than is found with air jets in water, indicates that the physical properties of the liquid exert considerable influence on the jet behavior. In conjunction with the rapid expansion, the air jets in mercury were also found to penetrate extensively behind the nozzle, and in many respects resembled a vertically injected jet. The extent of backward penetration of the jets was constant for all blowing conditions studied while the forward penetration increased with both increasing jet Froude number and nozzle diameter. The measured jet penetration in both the forward and backward directions were considerably larger than expected from model predictions. The core of the jets consists of a high concentration of gas bubbles. Both the gas volume fraction and bubble frequency in the core increase with increasing jet Froude number and nozzle diameter. The gas concentration and bubble frequency decrease with increasing distance along the jet trajectory due presumably to entrainment of liquid metal and bubble coalescence. On the basis of these findings, it is likely that process jets, such as are injected into copper converters, also expand rapidly and penetrate only a short distance into the bath. Thus rather than reacting in the middle of the bath, the jets may be impinging on the backwall refractory and contributing to the erosion observed there.

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY RESEARCH IN HIGH-SPEED DEVICES FOR THE RECOVERY OF THIN-FILM OIL SPILLS

ABSTRACT The United States Environmental Protection Agency has sponsored two separate research contracts in the field of high-speed devices for the recovery of thin-film oil spills. Both projects involve deflecting oil that is moving relative to the device by means of an air jet so that the resulting oil and water spray can be captured and separated. One project has been conducted by Tetradyne Corporation of Richardson, Texas, and involves the use of air jets to concentrate and subsequently deflect oil into a sump for gravity separation. This device has achieved up to 86% (vol/vol) recovery efficiency at a speed of six FPS in towing tank tests of a full-scale prototype; it is relatively insensitive to waves, small debris, and changes in oil viscosity. The second project has been conducted by Science Applications, Inc., of McLean, Virginia, and involves use of a slightly submerged air jet to deflect oncoming oil in the form of a spray lofted into a rotating polyurethane foam belt used for oil-water separation. Collection efficiencies of up to 83% (wt/wt) have been achieved in towing tank tests of a section of a full-scale prototype at a speed of 5.5 FPS. This device operates satisfactorily in waves and is somewhat sensitive to oil viscosity. Material is presented on the rationale for developing the devices, the testing technology that has been used for these efforts, and the test results for each device.