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Combustion is often accompanied by environmental pollution due to incomplete reaction. A non-excited flame usually exhibits poor combustion performance with features of diffusion flame such as dual-peak temperature distributions and large combustion product concentrations. Efficient fuel-air mixing has been identified as a key factor in mitigating this issue. The present study investigated the effects of the acoustic excitation Strouhal number on the combustion performance of a backward-inclined jet flame in crossflow. A loudspeaker was used to induce jet pulsations with excitation Strouhal numbers ranging from 0.47 to 1.54 at a pulsation intensity of 0.90. Time-averaged and instantaneous flame images were captured using digital cameras. A customized R-type thermocouple and a commercial gas analyzer were employed to analyze the thermochemical structures. A critical excitation Strouhal number of approximately 0.9, distinguishing two characteristic flame modes: strongly affected flame (SA flame) and weakly affected flame (WA flame), was found. At excitation Strouhal numbers lower than the critical value, the SA flame featured single-peak temperature profiles and low unburned hydrocarbon, carbon monoxide, and nitric oxide emissions. The combustion performance was significantly improved due to the strong mixing effect induced by the acoustic excitation. The WA flame appeared at excitation Strouhal numbers higher than the critical value. It exhibited dual-peak temperature distributions and slightly improved combustion performance. The dynamic behavior and emissions of acoustically excited jet flames offered a fascinating insight into the influence of excitation Strouhal numbers on combustion characteristics. More excitation conditions were expected for future research. [ABSTRACT FROM AUTHOR]

The wake flow features and aerodynamic characteristics of a circular cylinder in an oscillating crossflow were experimentally investigated. The study focused on crossflow oscillation Strouhal numbers that were smaller than the natural wake vortex-shedding Strouhal numbers. Crossflow oscillations were generated using a downstream rotating plate method. The flow evolution processes were observed using laser-light sheet assisted smoke flow visualization technique. Wake instability and time-averaged velocities were assessed using a one-component hot-wire anemometer and particle image velocimetry. Pressure distributions on the cylinder's surface were quantified using a linear pressure scanner, obtaining pressure coefficient distributions and drag forces. The wake instability Strouhal number, velocity vectors, streamline patterns, and recirculation bubble geometries in the wake region of the circular cylinder were determined. The wake turbulence properties were analyzed using the triple-decomposition method, including turbulence intensities and Lagrangian integral length and time scales. The results of natural and oscillating crossflows were compared. The wake vortex-shedding Strouhal number was lower than the natural Strouhal number but higher than the crossflow oscillation Strouhal number. It was primarily influenced by the oscillation Strouhal number and the Reynolds number of the crossflow. Critical crossflow Reynolds and oscillation Strouhal numbers were identified, beyond which the wake vortex-shedding Strouhal number reached a constant value. The crossflow oscillation intensity did not significantly affect the wake vortex-shedding behavior. The study provided quantitative descriptions and discussions of recirculation bubble geometries and statistical turbulence properties. Furthermore, the crossflow oscillations led to a substantial reduction in the drag coefficient experienced by the circular cylinder. [ABSTRACT FROM AUTHOR]

The evolution process and turbulence properties of a transversely oscillating flow induced by a fluidic oscillator are studied in a gravity-driven water tunnel. A planar jet is guided to impinge a specially designed crescent surface of a target blockage that is enclosed in a cavity of a fluidic oscillator. The geometric configuration of the cavity transforms the inherent stability characteristics of the jet from convective instability to absolute instability, so that the jet precedes the persistent back and forth swinging in the cavity. The swinging jet is subsequently directed through two passages and issued alternatively out of the fluidic oscillator. Two short plates are installed near the exits of the alternatively issuing pulsatile jets to deflect the jets toward the central axis. The deflected jets impinge with each other and form a pair of counter-rotating vortices in the near wake of the oscillator with a stagnation point at the impingement point. The stagnation point of the counter-rotating vortex pair moves back and forth transversely because of the phase difference existing between the two issued jets. The merged flow evolving from the counter-rotating vortices formed by the impingement of the two pulsatile jets therefore presents complex behavior of transverse oscillation. The topological models corresponding to the flow evolution are constructed to illustrate the oscillation process of the oscillating flow. Significant momentum dispersion and large turbulence intensity are induced by the transverse oscillation of the merged flow. The statistical turbulence properties show that the Lagrangian integral time and length scales of the turbulence eddies (the fine-scale structure) produced in the oscillating flow are drastically reduced. [DOI: 10.1115/1.2746905]

The effects of Reynolds number on flow and mixing characteristics of a merged single swinging jet induced by a V-shaped fluidic oscillator were studied experimentally. The jet Reynolds number was varied from 80 to 3000. A high-speed digital camera was used to capture the instantaneous flow evolution processes using the laser light-sheet-assisted flow visualisation method. Long-exposure images together with the binary edge detection technique were used to identify the jet spread width. The velocities, turbulence intensities, as well as turbulence macro time and length scales in the central axis were measured using a one-component hot-wire anemometer. Measurement results using the tracer-gas concentration detection method provided information on the dispersion characteristics of the merged swinging and non-swinging jets. Four characteristic flow modes (non-swinging, subcritical swinging, critical swinging, and supercritical swinging jets) were identified within different ranges of jet Reynolds number. At Reynolds numbers <150 ± 10, no jet swinging happened. At Reynolds numbers between 150 ± 10 and 1200 ± 23, the instantaneous velocities of the merged jet presented periodic oscillations of small amplitudes and was denoted as the subcritical swinging jet. At Reynolds numbers between 1200 ± 23 and 2350 ± 50, the merged jet proceeded irregular swinging motion and was called the critical jet. At Reynolds numbers higher than 2350 ± 50, the time-evolving velocities of the merged jet presented periodic oscillations of large amplitudes imposed by large turbulent fluctuations and was termed the supercritical swinging jet. The measured results of the axial velocities and turbulence intensities in the central axis revealed that in the regime of the supercritical swinging jet, the momentum conversion from the axial to transverse direction was rapidly performed in the near field at the axial distance <2.4 times of the jet exit width due to the induced transverse jet oscillation. The transverse oscillations of the swinging jets induced large turbulence intensities and small turbulence macro time and length scales, and led to large jet mass dispersion. Therefore, the mixing characteristics of the flow was significantly improved due to large jet mass dispersion. [ABSTRACT FROM AUTHOR]

The 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. [ABSTRACT FROM AUTHOR]

A 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). [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

The stabilization of non-premixed flame is often done by coupling an annular swirling flow. Acoustic excitation, however, can improve combustion through fuel jet oscillation but can sometimes blow out the flame. The writers aimed to experimentally investigate the effects of annular swirl strength on acoustically excited non-premixed flame. Visual observations coupled with flame length measurements showed that the flame can be particularly shortened and transformed to blue at a low swirl strength. Flame and flow images obtained through the Mie scattering technique showed that the recirculation bubble enlarged proportionally in accordance with the swirl strength. Jet oscillations were diagnosed by using a laser Doppler anemometer at the central jet. Turbulence and mixing properties obtained from the central jet velocity were statistically compared to identify the best combination of swirl strength and acoustic excitation level. Combustion performance was finely validated through flame length and temperature measurements. [ABSTRACT FROM AUTHOR]

Abstract: A newly developed computational approach different from the conventional methods which are popularly adopted by the fan designers is employed to study the flow field and performance characteristics of an axial flow fan by using the commercial code (STAR-CD) of computational fluid dynamics. The new approach, which is termed the “downstream flow resistance” (DFR) method, engages the flow resistance of the fan test bench in the downstream area of the computational domain. Both the static and moving grids are employed in the computation. The results show at least three emphases. Firstly, the deviations of the calculated fan performance curves from the experimental results can attain levels less than 3% and 1.5% by using the DFR method with static-grid and moving-grid schemes, respectively. These results present dramatic improvement over the maximum deviation of about 26% obtained by using the conventional method with the static-grid scheme. Secondly, usage of the static or moving grids for the purpose of fan performance prediction is no longer an important issue when the DFR method is employed because both the static and moving grids show almost identical results. Thirdly, the stall occurs when the aerodynamic contour of the fan blade is inappropriately designed for the mid and high back-pressure operations. Under this situation, recirculation bubble would appear around the trailing edge of the suction surface of the fan blade, and therefore obstructs the development of the flow. At deep-stall, large recirculation flow structures may even appear around the pressure surface and block the passage between the neighboring fan blades to cause the reduction of flow rate. [Copyright &y& Elsevier]

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 10(3)-10(4) 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 10(0) ppb across the whole hood face.

A novel design of range hood, which was termed the inclined quad-vortex (IQV) range hood, was examined for its flow and containment leakage characteristics under the influence of a plate sweeping across the hood face. A flow visualization technique was used to unveil the flow behavior. Three characteristic flow modes were observed: convex, straight, and concave modes. A tracer gas detection method using sulfur hexafluoride (SF6) was employed to measure the containment leakage levels. The results were compared with the test data reported previously in the literature for a conventional range hood and an inclined air curtain (IAC) range hood. The leakage SF6 concentration of the IQV range hood under the influence of the plate sweeping was 0.039 ppm at a suction flow rate of 9.4 m(3)/min. The leakage concentration of the conventional range hood was 0.768 ppm at a suction flow rate of 15.0 m(3)/min. For the IAC range hood, the leakage concentration was 0.326 ppm at a suction flow rate of 10.9 m(3)/min. The IQV range hood presented a significantly lower leakage level at a smaller suction flow rate than the conventional and IAC range hoods due to its aerodynamic design for flow behavior.

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.