Planar Air Jet Research Articles

Numerical investigation of the thermal efficiency of a multi-nozzle air curtain doorway under different wind conditions

Loading gate openings cause thermal inefficiency due to the substitution of indoor–outdoor air in cold seasons, and as a consequence, escalating energy consumption of warehouse buildings or unloading docks. Research findings have demonstrated that air infiltration/exfiltration through open doors results in the loss of a quarter of annual heating loads in buildings. Air curtains have shown a bright and promising future to compensate for the infiltration rate through the door opening, however, the most prior investigations were allocated to the jet planar air curtains and as yet there is a gap in the literature for other modified air curtain devices, including multi-nozzle air curtains (MNAC) with higher induction and resistance against the strong frontal winds, especially for gates higher than 4.27 m of installation height. Hence, in this work, designed MNAC with various configurations (i.e. large nozzles, small nozzles and holes) and installed on a warehouse loading gate was first characterized using the validated numerical turbulence model, SST (k−ω), to solve governing equations. The validate model is then used to evaluate the MNAC thermal efficiency, using the sensible energy gained by the inside zone, under various outside temperatures (−5, −10, −20, and −30∘C), and two wind conditions (i.e. with and without a front wind as recommended by ASHRAE (2010)). While the air curtains demonstrated limitations in fully controlling the infiltration rate during extremely cold and windy conditions, the results indicate that the MNAC systems have obtained thermal efficiency values higher up to 10% when compared to conventional planar air jets.

Buoyancy distribution in a filling box segmented by a planar jet

Air curtains are produced by thin vertical planar air jets and are used to prevent exchange flows between two fluids with a horizontal density gradient. They have been shown to work effectively provided the deflection modulus (DM) is greater than a critical value (DM = 0.14). Herein, results are presented from an experimental and modeling study of the initial transient development of the buoyancy distribution within an enclosure containing a round buoyant turbulent plume that is segmented by a co-flowing planar turbulent jet. A theoretical model based on filling box theory is developed to predict the leakage of buoyant fluid across the planar jet and the time at which the planar jet is no longer able to contain the buoyant plume outflow. Results from a series of analog salt-bath experiments are also presented that show that the model accurately predicts the rate of transport of buoyant fluid across the planar jet up until DM falls below the critical value of 0.14. The experiments show that, following the breakdown of the model there is a transition period after which the horizontal distribution of buoyant fluid throughout the enclosure is the same as it would be in the absence of the planar jet.

Airflow characteristics under planar opposed ventilation jets in a controlled indoor environment

Healthy, comfortable and intelligent indoor environment is a key objective in comprehensive well-being. This is also a main target of advanced HVAC-technology. In indoor environments, air distribution plays a major role while providing clean air to occupants. Therefore, investigating ventilation jets is an essential matter. In this study, the main objective was to improve knowledge on numerical modeling and airflow characteristics. In addition, the reliability of modeling methods were investigated. The experiments were carried out in a test room by using omnidirectional anemometers. The planar air jets were supplied below the ceiling from the opposite long-side walls. The exhaust openings were correspondingly near the floor. Isothermal and incompressible viscous airflow was simulated by using RANS, URANS, DES (SST-k-ω - LES) and SBES (SST-k-ω - LES) methods. The results show that modeling method has considerable effects on the predicted airflow field. However, the study indicates that correctly implemented numerical modeling may predict well the averaged airflow characteristics. Furthermore, the unsteady simulation allows airflows to fluctuate reasonably. In addition, SBES and DES methods were more sensitive in generating the airflow fluctuations than URANS. A recommendation is to carefully test and choose a modeling method for indoor airflows.

Dry beneficiation of fine coal using planar air jets

In this study, a novel separation method deploying planar air jets was proposed for the dry beneficiation of −6+2mm fine coal and a laboratory separation equipment was established. The separation principle of this method was based on the separation of horizontal movement trajectories of coal particles depending on the density differences when passing through the flow field of a planar air jet. The effects of operational parameters, including air jet velocity, air jet angle, feeding height, and feeding rate, on the beneficiation performance were experimentally studied and the results showed that all of these parameters had significant effects to varying degrees. The results also showed that the separation accuracy, the probable error E, is 0.2, indicating that this method can effectively beneficiate fine coal in a dry way.

Self-excited oscillations of two opposing planar air jets

This paper investigates the self-excited oscillations of two co-planar air jets impinging on each other, referred to, hereafter, by opposing planar jets. The study is performed for a single jet thickness of h = 2 mm and a span-wise aspect ratio of 50. The effects of flow velocity of the jets and the distance between them are examined. The opposing planar jets are found to produce strong acoustic tones over a wide range of jet Mach number and separation distance. Phase-locked averaged particle image velocity measurements showed that the tone is generated by out-of-phase flapping oscillations of the planar jets. These oscillations are sustained by two effects. The first is the high stagnation pressure at the impingement region, which deflects the jets laterally in opposite directions away from their common centerline, while the second effect is the cross stream oscillating velocity in the entrainment regions around the jet exits, which forces the jets back towards their common centerline.

Solder-assembled large mems flaps for fluid mixing

We describe surface-micromachined thermal actuator-based micro-electro-mechanical systems (MEMS) flaps with a length scale of 1,000 /spl mu/m. These flaps were developed for the enhancement of fluid mixing in an experimental study of a planar air jet. The scales of the flow physics required the actuators to be much larger than the typical MEMS scales, and the experiment required an array of 10 flaps (1 cm in length) to be soldered onto a ceramic substrate with high precision. These unusual requirements made it difficult to achieve good assembly yields that could provide large deflections. To improve the yields and deflections, we modified the initial flap design by reducing the size of the solder pads, removing sharp corners, changing the number and the width of the actuator's hot arms, and strengthening the support beams. In addition, we developed a controllable assembly process to reflow tin/lead solder on gold pads, and we used numerical simulations extensively in the design process to improve the performance. The experimental measurements corroborated these simulations: the deflection was increased from 6.4 to 11.1 /spl mu/m under a 5 V applied voltage. Flow velocity measurements showed that these MEMS flaps amplified the natural instabilities of the planar jet.

Measurement of gas jet flow velocities using laser-induced electrostrictive gratings

We used time-resolved light scattering of cw probe laser radiation from laser-induced electrostrictive gratings for the determination of flow velocities in air at room temperature. Some possibilities of the technique have been experimentally demonstrated with submerged planar air jets in atmosphere, both for accumulated and single-shot measurements. The range of investigated flow velocities was 5–200 m/s. The method of data treatment and of the estimate of the experimental parameters is described.

Alterations to coherent flow structures and heat transfer due to pulsations in an impinging air-jet

An investigation was carried out to study the effect of flow pulsation on the characteristics of a planar air jet impinging normally on a heated surface. Such information was further utilized to determine the influence of flow characteristics in the plane of impingement on Nusselt number distribution. Time-resolved system properties were investigated with modern instrumentation that allowed instantaneous heat transfer and flow velocity measurements to be performed simultaneously. Based on good coherence function estimates between the signals, heat transfer measurements were used in return to infer flow dynamics near the impingement surface. Experiments were performed for steady and pulsating jets at jet Reynolds numbers of 1 000, 5 500, and 11 000, pulse frequencies up to 82 Hz (corresponding to Strouhal numbers below 0.13), and pulse amplitude at the nozzle exit up to 50 % of the mean flow velocity. Special techniques commonly used for periodically disturbed flow fields elucidated the dynamics of the pulse and associated coherent flow structures. Results indicated the parametric conditions for which alterations are expected in time-averaged heat transfer from the surface. Engineering applications include cooling of electronic packages and heat transfer to gas turbine blades.

Sinuous instability of a planar air jet: propagation parameters and acoustic excitation.

The sinuous instability wave of a planar air jet is excited by localized acoustic flow across the nozzle. Phase velocity and the growth exponent are found from synchronous hot-wire measurements made beyond the excited region, where the profile is approximately sech-squared. In the observed range of scaled radian frequency, 0.02-1.33 (the stability limit), results agree with real-frequency (spatially growing) analysis but not with complex-frequency (temporally growing) analysis. The latter predicts smaller phase velocity at low frequencies and has been questioned in edgetone analysis. In further tests, the acoustic driving signal is made independent of downstream distance, as in an organ pipe. The jet deflection is then the sum of acoustic convection and of the instability wave, summing to zero at the nozzle, as proposed by Fletcher, Elder, and others. The instability-wave theory applies to linear behavior in the inviscid limit and therefore to a hypothetical nonspreading jet. The local velocity profile width must be considered in relating to a physical jet. In a flue organ pipe oscillating at equilibrium amplitude the stability-wave theory is not applicable near the lip, where the laminar flow assumed in the theory disappears and the jet deflection exceeds the range of linear behavior. Direct sound generation by the jet is investigated briefly.

Acoustically driven sinuous instability of a planar air jet

The sinuous instability wave of a planar air jet is excited by localized acoustic flow across the nozzle. Phase velocity and the growth exponent are found from synchronous hot-wire measurements made beyond the excited region, where the profile is approximately sech squared. In the observed range of scaled radian frequency, 0.02–1.33 (the stability limit), results agree with real-frequency (spatially growing) analysis but not with complex-frequency (temporally growing) analysis, which predicts smaller phase velocity at low frequencies. In further tests, the acoustic driving signal is independent of downstream distance, as in an organ pipe. The jet deflection is then the sum of acoustic convection and of the instability wave, summing to zero at the nozzle, as proposed by Fletcher, Elder, and others. The instability-wave theory applies to linear behavior in the inviscid limit and therefore to a hypothetical nonspreading jet. The local velocity profile width must be considered in relating to a physical jet. In a flue organ pipe oscillating at equilibrium amplitude the jet deflection at the lip exceeds the range of validity. Direct sound generation by the jet is investigated briefly.

Local convective heat transfer to submerged pulsating jets

An experimental investigation was performed to study the effect of flow pulsations on the local heat transfer characteristics of a planar air jet. A microsensor provided localized and essentially instantaneous heat flux and temperature signals. Heat transfer and thermal anemometry flow measurements were synchronized to associate influences of periodic flow structures and pulsations on heat transfer. Pulsation frequencies corresponded to Strouhal numbers below 0.106, and pulse amplitude was varied from 0 to 50% of the mean flow velocity. Nusselt numbers in the nozzle mid-plane increased by up to 12%, and by up to 80% at distances downstream.

Heat transfer for a planar air jet striking a concave surface

Results are presented from an experimental study of the local and mean heat transfer rates when a rectangular jet enters a relatively short concave body of parabolic profile.