Laser-assisted Smoke Flow Visualization Research Articles

Time-averaged flow characteristics and mixing properties of double-concentric jets

We examined the flow behaviors and mixing characteristics of double-concentric jets using laser-assisted smoke flow visualization method to analyze typical flow patterns and binary boundary detection technique to investigate jet spread width. Time-averaged velocity vectors, streamline patterns, velocity distributions, turbulence properties, and vorticity contours were analyzed using Particle Image Velocimetry (PIV). Topological flow patterns were analyzed to interpret the vortical flow structures. Mixing properties were investigated using a tracer-gas concentration detection method. Four characteristic modes were observed: annular flow dominated mode, transition mode, central jet dominated mode-low shear, and central jet dominated mode-high shear. The jets’ mixing properties were enhanced by two major phenomena: the merging of annular flow and central jet at the centerline and the large turbulence fluctuations produced in the flow field. The merging of the jets induced stagnation points on the central axis in the annular flow dominated mode, which caused reverse flow on the central axis and drastic turbulence fluctuations of the near field region. When the central jet penetrated the recirculation region in the other three modes, the stagnation points on the central axis and the reverse flow vanished. Therefore, the mixing behaviors were prominently enhanced in the annular flow dominated mode.

Dispersion and upwind-side shear-layer characteristics of forward-inclined transverse jet in crossflow

The effects of the forward inclination of a jet in crossflow on the jet fluid dispersion and upwind-side shear-layer characteristics were studied experimentally in an open-loop wind tunnel. The jet forward-inclination angle θ was varied from 0° to 50°, and the jet-to-crossflow momentum–flux ratio was adjusted between 1 and 2. Short- and long-exposure flow patterns were examined using the laser-assisted smoke flow visualization technique. The binary edge-detection technique was applied to the long-exposure flow images to obtain the jet penetration height. The instantaneous velocities in the upwind-side shear layer of the bent jet were captured by a hot-wire anemometer through a high-speed data acquisition system. The fast Fourier transform method was used to convert the instantaneous velocities from time to the frequency domain. Transverse dispersion of the jet fluids and jet trajectories described by the maximum jet-fluid concentrations were determined using the tracer-gas concentration detection method. The results revealed that at the same jet-to-crossflow momentum–flux ratio, a forward inclination did not significantly influence the flow characteristics and dispersion properties when the inclination angle was smaller than about 12°. At moderate forward inclinations (about 12°–42°), the upstream-side coherent flow structures appeared periodically. The penetration height, transverse width, and dispersion of jet fluids were greater than those of the elevated transverse jet and the slightly forward-inclined transverse jet. At forward inclinations larger than about 42°, the upstream-side coherent flow structures, penetration height, transverse width, and dispersion of jet fluids became the largest when compared with those of the elevated transverse jet as well as the slight and moderate inclination ranges. The jet trajectories in the moderate and large inclination ranges were located at transverse levels higher than those in the slight inclination range and elevated transverse jet due to the appearance of the large upwind-side shear-layer coherent flow structures. The effects of the forward inclinations on the time and length scales of the upwind-side coherent flow structures and the time and length scales of the turbulent eddies were presented and discussed.

Flow and mixing characteristics of a forward-inclined stack-issued jet in crossflow

The flow and mixing characteristics of a forward-inclined stack-issued jet at various inclination angles (θ) and jet-to-crossflow momentum flux ratios (R) were experimentally studied in an open-loop wind tunnel. Flow behaviors were examined using the laser-assisted smoke flow visualization technique. The instantaneous velocities of the upwind-side shear-layer were digitized by a hot-wire anemometer using a high-speed data acquisition system. The instability frequencies in the upwind-side shear-layer vortices were obtained by the fast Fourier transform method. Long-exposure flow images were processed using the binary edge-detection technique to obtain the jet spread width. Transverse dispersion of jet fluids was determined using tracer gas concentration detection. The upwind-side shear-layer vortices revealed four characteristic flow modes: the High impingement-crossflow dominated mode (about θ < 15° and low R), the High impingement-jet flow dominated mode (about θ < 25° and high R), the Low impingement-crossflow dominated mode (about θ > 15° and low R), and the Low impingement-jet flow dominated mode (about θ > 25° and high R). Increasing θ in the crossflow dominated regimes eliminated the upwind-side shear-layer vortices, while increasing θ in the jet flow dominated regimes emphasized the upwind-side shear-layer vortices. Increasing θ at a fixed value of R increased jet spread width in the far field in all modes. In the near field, at x/d < 5 in the High impingement-crossflow dominated regime, the jet spread width was greater than in the Low impingement-crossflow dominated regime. In the jet flow dominated regimes, higher θ values led to greater jet spread width. Transverse dispersion of the jet fluids approached the jet spread width results. In the Low impingement-jet flow dominated regime, transverse dispersion of the jet fluids was significantly increased compared to the other regimes. In addition, the maximum tracer gas concentration was severely reduced at all axial stages, which implied better dispersion of the jet fluids in this regime.

Flow and heat transfer characteristics of a pulsed jet impinging on a flat plate

The flow and heat transfer characteristics of a pulsed jet impinging on a flat plate under various excitation conditions were experimentally studied. The flow behaviors and time-averaged velocity fields of the characteristic flow structures were examined using a laser-assisted smoke flow visualization method and particle image velocimetry (PIV). The heat transfer of a pulsed impinging jet was measured by a thermal imaging camera. Two primary flow modes were observed in the domain of the jet pulsation intensity and excitation Strouhal number. The coherent vortices and vortex breakup modes appeared in the regimes of low and high jet pulsation intensities, respectively. In the coherent vortices mode, two submodes (contiguous vortices and discrete vortices) were additionally classified. The vortex ring maintained its coherency before impinging on the flat plate in the coherent vortices mode, while the vortex ring lost its coherency and broke up quickly before impinging on the flat plate in the vortex breakup mode. The increase rates of the axial traveling distance and axial convective velocity of the vortex rings in the vortex breakup mode were greater than those in the coherent vortices mode. Pulsating the impinging jet in vortex breakup mode, the effect of a large convective velocity of the vortex rings caused large mean axial velocities and axial velocity fluctuation intensities. The convective heat transfer was enhanced by increasing the convective velocity of the vortex ring and fluctuation intensity.

Influence of high heat load on flow and containment of an inclined air-curtain (IAC) fume hood

ABSTRACTThe inclined air-curtain (IAC) fume hood has been reported to have “almost null leakage”[1] at low suction flow rates when operated at regular temperatures. However, previous research has not investigated the performance or optimized operating parameters when a high heat load is used in the IAC fume hood. For the present work, the effects of a high heat load on the flow field and contaminant leakage characteristics of the IAC fume hood were examined. The heat load was supplied to an IAC hood according to the standard method of EN14175-7:2012. The laser-assisted smoke flow visualization technique was employed to identify the characteristic flow patterns. The standard tracer-gas concentration test method (EN14175-3:2003) was used to examine the leakage levels of the IAC fume hood under static conditions, sash movement, and simulated walk-by conditions. When the IAC fume hood was operated at a high heat load, the static test results showed negligibly small leakage levels at a face velocity greater than or equal to only 0.19 m/s (37.4 ft/min). The sash movement and simulated walk-by test results showed that to obtain negligibly small leakage levels at high heat load operation, the IAC fume hood required a face velocity greater than or equal to 0.32 m/s (63 ft/min). In addition, the IAC fume hood exhibited a superior hood containment performance with low energy consumption when compared with conventional fume hoods operated at a high heat load.

Flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations

The flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations were investigated experimentally. The transversely-oscillating plane jet was generated by a specially designed fluidic oscillator. Isothermal flow patterns were observed using the laser-assisted smoke flow visualization method. Meanwhile, the flame behaviour was studied using instantaneous and long-exposure photography techniques. Temperature distributions and combustion-product concentrations were measured using a fine-wire type R thermocouple and a gas analyzer, respectively. The results showed that the combusting transversely-oscillating plane jets had distributed turbulent blue flames with plaited-like edges, while the corresponding combusting non-oscillating plane jet had laminar blue-edged flames in the near field. At a high Reynolds number, the transversely-oscillating jet flames were significantly shorter and wider with shorter reaction-dominated zones than those of the non-oscillating plane jet flames. In addition, the transversely-oscillating combusting jets presented larger carbon dioxide and smaller unburned hydrocarbon concentrations, as well as portrayed characteristics of partially premixed flames. The non-oscillating combusting jets presented characteristics of diffusion flames, and the transversely-oscillating jet flame had a combustion performance superior to its non-oscillating plane jet flame counterpart. The high combustion performance of the transversely-oscillating jets was due to the enhanced entrainment, mixing, and lateral spreading of the jet flow, which were induced by the vortical flow structure generated by lateral periodic jet oscillations, as well as the high turbulence created by the breakup of the vortices.

Modulating flow and aerodynamic characteristics of a square cylinder in crossflow using a rear jet injection

The temporally evolved flow behaviors around a square cylinder subject to modulation of a planar jet issued from the cylinder’s downstream surface into the wake were studied using the laser-assisted smoke flow visualization method and synchronized hot-wire anemometers. The drag force asserted on the square cylinder was obtained by measuring the surface pressures. Four characteristic flow modes (wake-dominated, transitional, critical, and jet-dominated) were observed in different regimes of freestream Reynolds number and jet injection ratio. In the wake-dominated mode, the jet swung periodically back and forth on the downstream surface due to the wake vortex shedding. In the transitional mode, the vortex shedding in the wake vanished so that the flow around the cylinder presented no periodic oscillations. In the critical mode, the wake width became smaller and therefore made the vortex shedding frequency larger than that observed in the wake dominated mode. In the jet-dominated mode, the jet had a large momentum that entrained wake fluids and therefore stabilized the instabilities of the wake, separated boundary layers on lateral surfaces, and stagnation point on the upstream surface. Two standing vortices appeared in the near wake beside the high-momentum jet. The width of the wake was decreased substantially by jet entrainment. The drag coefficient decreased with an increase in the jet injection ratio. The downstream surface jet injection caused the pressure coefficients to decrease at the upstream surface and to increase at the downstream surface. Therefore, the drag coefficients were decreased significantly by 26%, 33%, and 38% at the injection ratios of 0.5, 1.5, and 2.5, respectively.

Swirling dual-disk double-concentric jets at low annulus Reynolds numbers

The flow characteristics and mixing properties of swirling dual-disk double-concentric jets at low annulus Reynolds numbers were studied. The flow patterns, velocity characteristics, and mixing properties were characterized using the laser-assisted smoke flow visualization method, particle image velocimetry (PIV), and tracer-gas concentration detection technique. Three characteristic flow modes,annular jet-dominated wake, central jet-dominated wake, and central jet-dominated radial flow, were observed within different ranges of central jet Reynolds numbers for the annulus Reynolds and the swirl numbers were less than 270 and 0.245. The flow structure of the central jet-dominated wake (a dual-ring recirculation zone in the wake region and two pairs of counter-rotating vortices in the gap between the up- and downstream disks) allowed the annular jet fluids to be transported into the gap between the upstream and downstream disks to premix with the central jet fluids before further being transported to the disk wake. The flow fluctuation intensities in the wake of the central jet-dominated wake were significantly larger than those of the annular jet-dominated wake and central jet-dominated radial flow because the large-scale fluctuation eddies that possess large turbulence kinetic energy existed in the wake. These properties prominently promoted the mixing in the central jet-dominated wake.

Effects of boundary-layer separation controllers on a desktop fume hood

ABSTRACTA desktop fume hood installed with an innovative design of flow boundary-layer separation controllers on the leading edges of the side plates, work surface, and corners was developed and characterized for its flow and containment leakage characteristics. The geometric features of the developed desktop fume hood included a rearward offset suction slot, two side plates, two side-plate boundary-layer separation controllers on the leading edges of the side plates, a slanted surface on the leading edge of the work surface, and two small triangular plates on the upper left and right corners of the hood face. The flow characteristics were examined using the laser-assisted smoke flow visualization technique. The containment leakages were measured by the tracer gas (sulphur hexafluoride) detection method on the hood face plane with a mannequin installed in front of the hood. The results of flow visualization showed that the smoke dispersions induced by the boundary-layer separations on the leading edges of the side plates and work surface, as well as the three-dimensional complex flows on the upper-left and -right corners of the hood face, were effectively alleviated by the boundary-layer separation controllers. The results of the tracer gas detection method with a mannequin standing in front of the hood showed that the leakage levels were negligibly small (≤0.003 ppm) at low face velocities (≥0.19 m/s).

Flow characteristics and drag force of a square cylinder in crossflow modulated by a slot jet injected from upstream surface

The flow characteristics and drag force of a square cylinder with a front jet injection were experimentally investigated in a wind tunnel. The evolution process of the characteristic flow patterns was recorded by the laser-assisted smoke flow visualization method. The time-averaged velocity fields measured by particle image velocimetry (PIV) were applied to analyze the velocity vectors, streamline patterns, vorticity contours, velocity distributions, and time histories of instantaneous velocities around the upstream region of the square cylinder. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the front and rear faces. The results show that the jet emitted from the upstream surface of the square cylinder periodically swings left-and-right in the experimental range of injection ratio ⩽0.9. In the time-averaged velocity field, the jet flow impinges the freestream at a four-way saddle and subsequently bifurcates into two streams; one stream goes toward the left edge of the upstream surface, while the other stream directs toward the right edge of the upstream surface. Two recirculation regions formed above the upstream surface of the square cylinder are enclosed by those two streams. The vorticity contours around the upstream surface of the square cylinder are characterized by two adjacent vorticity-concentrated areas of opposite signs. The time histories of the instantaneous velocities around the four-way saddle, jet exit, and shear layer of the swinging jet represent periodic oscillation. This characteristic oscillation frequency is dominated by the wake instability. The recirculation regions formed above the cylinder’s upstream surface prevent impingement from the freestream. Consequently, the surface pressure coefficients on the upstream surface of the square cylinder are reduced. This reduction in the surface pressure coefficient decreases the drag force acting on the square cylinder.

Flow Characteristics and Spillage Mechanisms of an Inclined Quad-Vortex Range Hood Subject to Influence from Draft

The flow and spillage characteristics of an inclined quad-vortex (IQV) range hood subject to the influence of drafts from various directions were studied. The laser-assisted smoke flow visualization technique was used to reveal the flow characteristics, and the tracer-gas (sulfur hexafluoride) concentration detection method was used to indicate the quantitative values of the capture efficiency of the hood. It was found that the leakage mechanisms of the IQV range hood are closely related to the flow characteristics. A critical draft velocity of about 0.5 m/s and a critical face velocity of about 0.25 m/s for the IQV range hood were found. When the IQV range hood was influenced by a draft with a velocity larger than the critical draft velocity, the spillage of pollutants became significant and the pollutant spillage rate increased with increasing draft velocity. At draft velocities less than or equal to the critical value, no containment leakages induced by the turbulence diffusion, reverse flow, or boundary-layer separation were observed, and the capture efficiency was about 100%. The IQV range hood exhibited a high ability to resist the influences of lateral and frontal drafts. The capture efficiency of the IQV range hood operated at the suction flow rate 5 to 9 m3/min is higher than that of the conventional range hood operated at 11 to 15 m3/min.

Development and characterization of an inclined air-curtain (IAC) fume hood.

An inclined air-curtain (IAC) fume hood was developed and characterized using the laser-assisted smoke flow visualization technique and tracer-gas (sulphur hexafluoride) concentration detection method. The IAC fume hood features four innovative design elements: (i) an elongated suction slot installed at the hood roof with an offset towards the rear wall, (ii) an elongated up-blowing planar jet issued from the work surface near the hood inlet, (iii) two deflection plates installed at the left and right side walls, and (iv) a boundary-layer separation controller installed at the sash bottom. Baffles employed in conventional hoods were not used. The suction slot and the up-blowing planar jet formed a rearward-inclined push-pull air curtain. The deflection plates worked with the inclined air curtain to induce four rearward-inclined counter-rotating 'tornados.' The fumes generated in the hood were isolated behind the rearward-inclined air curtain, entrained by the low pressure within the vortical flows, moved up spirally, and finally exhausted through the suction slot. The risk of containment leakage due to the large recirculation vortex that usually exists behind the sash of conventional hoods was reduced by the boundary-layer separation controller. The results of the tracer-gas concentration detection method based on the EN-14175 method showed that the flow field created by the geometric configurations of the IAC hood presented characteristics of low leakage and high resistance to dynamic disturbances at low face velocities. The leakage levels measured by the static, sash movement, and walk-by tests were negligible at a face velocity of 0.26 m s(-1).

Flow characteristics in median plane of a backward-inclined elevated transverse jet

The time-averaged flow patterns and instantaneous upwind-side shear-layer vortex flow modes of a backward-inclined elevated transverse jet were studied in an open-loop wind tunnel by laser-assisted smoke flow visualization method and hot-wire anemometry. The characteristic flow modes at various backward inclination angles θ and jet-to-crossflow momentum flux ratios R were identified from long- and short-exposure pictures of flow visualization. The jet width, which may denote the transverse dispersion characteristics of the jet, in the near and far fields were determined from the long-exposure picture by using a binary edge-detection method. The frequency and Strouhal number of the upwind-side shear-layer vortices were measured by the hot-wire anemometer. At θ<25°, large downwash recirculation bubble always appeared at small R’s. Increasing θ would quickly decrease the size of the downwash recirculation bubble. At θ>25°, no downwash recirculation bubble was observed in the near wake of the tube tip region. The jet width decreased with increasing θ. The Strouhal number of the upwind-side vortices increased with increasing θ. At a fixed θ, the Strouhal number of the upwind-side vortices decreased with increasing R at R<1. At R>1, the decrease rate became small. The ultimate Strouhal numbers at large R at different backward inclination angles θ did not converge to a single value—the larger the θ, the larger the Strouhal number.

Flow behavior around a square cylinder subject to modulation of a planar jet issued from upstream surface

The flow behavior in the up- and downstream regions of a square cylinder subject to the modulation of a planar jet issued from the cylinder׳s front surface was studied using the laser-assisted smoke flow visualization method and hot-wire anemometer measurement. Reynolds numbers were from 1628 to 13000. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the up- and downstream faces. The temporally evolving smoke flow patterns in the up- and downstream regions were synchronously revealed through the smoke flow visualization. The frequency characteristics of the instability waves in the up- and downstream regions were synchronously detected by the two hot-wire anemometers. Four characteristic flow modes were observed within the different ranges of the injection ratios. At the low injection ratios (IR<1), the ‘swinging jet’ appeared. The jet swung periodically leftward and rightward and formed a fluid bubble on the front surface. The fluid bubble contained a pair of counter-rotating vortices and presented a periodic variation in its height. At moderately low injection ratios (1<IR<4.3), the ‘deflected oscillating jet’ appeared. The jet was deflected in either the left or the right direction and wrapped around one of the edges of the square cylinder. Both the swinging and oscillating motions of the jet in the swinging jet and deflected oscillating jet modes were induced by the periodic feedback pressure signals generated by the vortex shedding in the wake. At the moderately high (4.3<IR<8.3) and high (IR>8.3) injection ratios, the ‘deflection jet’ and ‘penetrating jet’ appeared. The jet detached from the cylinder׳s front surface and penetrated a long distance into the upstream region due to large jet momentum. Neither periodic jet oscillation in the upstream region nor vortex shedding in the wake was observed. The drag coefficient was found to be decreasing quickly with increasing the injection ratio.

Improving Flow Patterns and Spillage Characteristics of a Box-Type Commercial Kitchen Hood

A conventional box-type commercial kitchen hood and its improved version (termed the “IQV commercial kitchen hood”) were studied using the laser-assisted smoke flow visualization technique and tracer-gas (sulfur hexafluoride) detection methods. The laser-assisted smoke flow visualization technique qualitatively revealed the flow field of the hood and the areas apt for leakages of hood containment. The tracer-gas concentration detection method measured the quantitative leakage levels of the hood containment. The oil mists that were generated in the conventional box-type commercial kitchen hood leaked significantly into the environment from the areas near the front edges of ceiling and side walls. Around these areas, the boundary-layer separation occurred, inducing highly unsteady and turbulent recirculating flow, and leading to spillages of hood containment due to inappropriate aerodynamic design at the front edges of the ceiling and side walls. The tracer-gas concentration measurements on the conventional box-type commercial kitchen hood showed that the sulfur hexafluoride concentrations detected at the hood face attained very large values on an order of magnitude about 103–104 ppb. By combining the backward-offset narrow suction slot, deflection plates, and quarter-circular arcs at the hood entrance, the IQV commercial kitchen hood presented a flow field containing four backward-inclined cyclone flow structures. The oil mists generated by cooking were coherently confined in these upward-rising cyclone flow structures and finally exhausted through the narrow suction slot. The tracer-gas concentration measurements on the IQV commercial kitchen hood showed that the order of magnitude of the sulfur hexafluoride concentrations detected at the hood face is negligibly small—only about 100 ppb across the whole hood face.

Flow and Leakage Characteristics of a Sashless Inclined Air-Curtain (sIAC) Fume Hood Containing Tall Pollutant-Generation Tanks

In many fume hood applications, pollutant-generation devices are tall. Human operators of a fume hood must stand close to the front of the hood and lift up their hands to reach the top opening of the tall tank. In this situation, it is inconvenient to access the conventional hood because the sash acts as a barrier. Also, the bluff-body wake in front of the operator's chest causes a problem. By using laser-assisted smoke flow visualization and tracer-gas test methods, the present study examines a sashless inclined air-curtain (sIAC) fume hood for tall pollutant-generation tanks, with a mannequin standing in front of the hood face. The configuration of the sIAC fume hood, which had the important element of a backward-inclined push-pull air curtain, was different from conventional configurations. Depending on suction velocity, the backward-inclined air curtain had three characteristic modes: straight, concave, and attachment. A large recirculation bubble covering the area—from the hood ceiling to the work surface—was formed behind the inclined air curtain in the straight and concave modes. In the attachment mode, the inclined air curtain was attached to the rear wall of the hood, about 50 cm from the hood ceiling, and bifurcated into up and down streams. Releasing the pollutants at an altitude above where the inclined air curtain was attached caused the suction slot to directly draw up the pollutants. Releasing pollutants in the rear recirculation bubble created a risk of pollutants’ leaking from the hood face. The tracer-gas (SF6) test results showed that operating the sIAC hood in the attachment mode, with the pollutants being released high above the critical altitude, could guarantee almost no leakage, even though a mannequin was standing in front of the sashless hood face.

Flow and Performance of an Air-curtain Biological Safety Cabinet

Using laser-assisted smoke flow visualization and tracer gas concentration detection techniques, this study examines aerodynamic flow properties and the characteristics of escape from containment, inward dispersion, and cross-cabinet contamination of a biological safety cabinet installed with an air curtain across the front aperture. The experimental method partially simulates the NSF/ANSI 49 standards with the difference that the biological tracer recommended by these standards is replaced by a mixture of 10% SF(6) in N(2). The air curtain is set up across the cabinet aperture plane by means of a narrow planar jet issued from the lower edge of the sash and a suction flow going through a suction slot installed at the front edge of the work surface. Varying the combination of jet velocity, suction flow velocity, and descending flow velocity reveals three types of characteristic flow modes: 'straight curtain', 'slightly concave curtain', and 'severely concave curtain'. Operating the cabinet in the straight curtain mode causes the air curtain to impinge on the doorsill and therefore induces serious escape from containment. In the severely concave curtain mode, drastically large inward dispersion and cross-cabinet contamination were observed because environmental air entered into the cabinet and a three-dimensional vortical flow structure formed in the cabinet. The slightly concave curtain mode presents a smooth and two-dimensional flow pattern with an air curtain separating the outside atmosphere from the inside space of the cabinet, and therefore exhibited negligibly small escape from containment, inward dispersion, and cross-cabinet contamination.