Confined Air Jet Research Articles

Heat Transfer Intensification of a Confined Impinging Air Jet Via a Guiding Baffle

Abstract The heat transfer intensification of a confined impinging jet was achieved using a rough surface or pin fins as well as using modified nozzles such as chamfering, chevron, sweeping, swirling, etc. In this work, an enhanced cooling process utilizing a single confined air jet impinged on a flat plate using a guiding baffle is implemented. The impacts of Reynolds number (Re) ranged from 500 to 5000, guiding baffle diameter-to-nozzle diameter (D/d) of 2, 4, and 6, and guiding baffle height-to-nozzle to impinging plate distance (h/H) of 1/3, 1/2, and 3/4 on the cooling process are studied. The distributions of surface temperature are acquired experimentally using a thermal infrared camera. As well, the local Nusselt number (Nu), stagnation point Nusselt number (Nust), average Nusselt number (Nu¯), and average Nusselt number ratio (Nu¯r) are evaluated. The results reveal that the enhancement of heat transfer is achieved due to installing a guiding baffle with a D/d of 2 for all values of baffle height and Reynolds number. In addition, the Nu¯r is increased with increasing the Re in the range from 500 to 2500, then it is decreased by a further increase in Re. Moreover, based on the experimental results, an empirical correlation is proposed to compute the Nu¯ depending on Re, D/d, and h/H with a ±2.65% standard deviation.

Periodic jetting and monodisperse jet drops from oblique gas injection

When air is blown in a straw or tube near an air-liquid interface, typically one of two behaviors is observed: a dimple in the liquid's surface, or a frenzy of sputtering bubbles, waves, and spray. Here we report and characterize an intermediate regime that can develop when a confined air jet enters the interface at an angle. This regime is oscillatory with a distinct characteristic frequency and can develop periodic angled jets that can break up into monodisperse aerosols. The underlying mechanisms responsible for this highly periodic regime are not well understood. Here we flow a continuous stream of gas through a tube near a liquid surface, observing both optically and acoustically the deformation of the liquid-air interface as various parameters are systematically adjusted. We show that the Kelvin-Helmholtz instability is responsible for the inception of waves within a cavity formed by the gas. Inertia, gravity, and capillary forces both shape the cavity and govern the frequency and amplitude of these gas-induced cavity waves. The flapping cavity focuses the waves into a series of periodic jets that can break up into droplets following the Rayleigh-Plateau instability. We present scaling arguments to rationalize the fundamental frequencies driving this system, as well as the conditions that bound the periodic regime. These frequencies and conditions compare well with our experimental results.

Effects of mist and jet cross-section on heat transfer for a confined air jet impinging on a flat plate

Convective heat transfer to an impinging air jet is known to yield high local and area-average Nusselt numbers. We simulate this heat transfer over a wide range of study parameters, including Reynolds number and mist mass fraction, and three jet shapes: circular, half circuit, and quarter circuit. Simulations show that when compared with air only, mist provides higher heat-transfer enhancements for the first two shapes but is insignificant for the third. The area-average Nusselt number is higher by ∼100% for the half-circuit jet than for the circular jet for both single-phase air and mist. With 0.5% mist, the area-average Nusselt number of the circular jet is enhanced by 41% at Reynolds number 10000.

WALL STATIC PRESSURE DISTRIBUTION DUE TO CONFINED IMPINGING CIRCULAR AIR JET

The wall static pressure of confined air jet, impinging normally onto a flat plate has been experimentally investigated. The mean velocities, and wall static pressures were measured for Reynolds numbers ranging from 18,000 to 40,000 and the nozzle-to-plate spacing’s in range of 0.25–4. The effects of Reynolds number, nozzle-to-plate spacing and flow confinement on the pressure coefficient are reported. The pressure distributions are independent of the Reynolds number and depend on nozzle-to-plate spacing only. The maximum pressure occurs at the stagnation point (r/d=0) for the all nozzle-to-plate spacing’s studied and as r/d increases, the pressure decreases. At very low spacing (z/d=0.25) stagnation pressure is rather larger than those of the other spacing’s due to rapidly decreasing of axial velocity in the deflection zone. It is concluded that a sub atmospheric region occurs on the impingement plate for the nozzle-to-plate spacing’s up to one for all the Reynolds number; however, there is no evidence of the sub atmospheric region in confined jet For the nozzle-to-plate spacing’s greater than one.

A hybrid dynamic modeling of active chilled beam terminal unit

This paper proposes a hybrid dynamic model of active chilled beam (ACB) terminal unit. The model encapsulates mechanical and thermal aspects of the confined air jet and the cooling coil contained in the terminal unit and could be divided into two sub-models respectively. The models for the primary air, secondary air and mixing of them are together taken as the confined air jet sub-model. Another sub-model is the heat transfer description of the cooling coil. The model is kept simple and practical, avoiding sophisticated jet flow theories as well as heat transfer theories. Thus, in deriving the model using first principles and estimating it experimentally, a reasonable compromise is made between capturing exact underlying physics and suitability for engineering applications. Supported by experimental results from a pilot plant, unknown model parameters are identified by either a linear or nonlinear least-squares method. It is shown that static and dynamic performances of the model are satisfied, which reflect the effectiveness of this hybrid modeling technique as well. The model developed in this work is expected to have wide control and optimization applications.

A Digital Synthesis Model of Double-Reed Wind Instruments

We present a real-time synthesis model for double-reed wind instruments based on a nonlinear physical model. One specificity of double-reed instruments, namely, the presence of a confined air jet in the embouchure, for which a physical model has been proposed recently, is included in the synthesis model. The synthesis procedure involves the use of the physical variables via a digital scheme giving the impedance relationship between pressure and flow in the time domain. Comparisons are made between the behavior of the model with and without the confined air jet in the case of a simple cylindrical bore and that of a more realistic bore, the geometry of which is an approximation of an oboe bore.

Numerical investigation of combined impingement and convection heat transfer.

This paper concerns development of a prediction method for combined turbulent impingement and convection heat transfer. Firstly, a prediction of a single round unconfined impinging air jet without crossflow is investigated to assess the performance of linear and non-linear two-equation turbulence models. The results show that realizable and/or non-linear two-equation models may successfully be used for impinging jet heat transfer predictions but there are significant differences between the formulations. Among the models tested, a non-linear k-omega showed superior performance. Secondly, the influence of crossflow is considered and a plane confined air jet is investigated. For this case the models based on a frequency-equation (omega) show quite different and more reasonable predictions compared to the dissipation-equation (epsilon) based models.

Enhancement of air jet impingement heat transfer using pin-fin heat sinks

The enhancement of heat transfer from a discrete heat source in confined air jet impingement was experimentally investigated. A variety of pin-fin heat sinks were mounted on the heat source and the resulting enhancement studied, Average heat transfer coefficients are presented for a range of jet Reynolds numbers (8000/spl les/Re/spl les/45000) and orifice diameters (12.7/spl les/d/spl les/38.1 mm). A total fin effectiveness was computed for the pinned heat sinks relative to the unpinned ones, and was in the range of 2.4 to 9.2; the highest value was obtained for the largest nozzle diameter. Heat transfer rates from the bare heat source were increased by a factor of 7.5 to 72 due to the introduction of the heat sinks. Results for the average heat transfer coefficient were correlated in terms of Reynolds number, fluid properties and geometric parameters of the heat sinks.

Effects of nozzle-inlet chamfering on pressure drop and heat transfer in confined air jet impingement

The effect of changing the nozzle geometry on the pressure drop and local heat transfer distribution in confined air jet impingement on a small heat source was experimentally investigated. Heat transfer and pressure drop measurements obtained with chamfered nozzles were compared to those obtained with a square-edged (non-chamfered) nozzle of the same diameter for different turbulent Reynolds numbers in the range of 5000–20,000, nozzle-to-target spacings, chamfer angles and chamfer lengths. The ratio of average heat transfer coefficient to pressure drop was enhanced by as much as 30.8% as a result of chamfering the nozzle, with narrow chamfering providing the better performance.

Measurements of Confined, Coaxial Jet Mixing With Pressure Gradient

The mixing of a subsonic confined air jet with a coaxial secondary air stream is investigated experimentally. This work differs from previous studies in that emphasis is placed on the effects of an imposed adverse pressure gradient on mixing characteristics in the initial mixing (potential core) and transition regions. The results indicate that the presence of an adverse pressure gradient promotes more rapid mixing and spreading of the shear layer in the initial mixing region, as well as elevated turbulent normal stress and shear stress levels in the outer portion of the mixing layer after the primary and secondary streams have merged. Significant radial static pressure variations occur in both the initial mixing and transition regions as a result of turbulent normal stress gradients and streamline curvature effects induced by potential core entrainment.