Two-dimensional Air Jet Research Articles

Heat transfer and entropy generation in air jet impingement on a model rough surface

Air jet impingement is an effective enhanced heat transfer technique which has been widely applied in cooling of electronic components and drying of continuous sheets of materials, etc. Although steady and unsteady impingement systems have led to many studies on jet impingement flow and heat transfer, the vast majority of them focus on smooth surfaces. As the flow and thermal physics of jet impingement on a rough surface are significantly different from that on a smooth surface, a numerical study was performed on the two-dimensional air jet impingement with a model rough surface by computational fluid dynamics method. A sinusoidal wave was employed to model the rough impinging target surface subjected to a slot jet nozzle. A dimensionless heat transfer enhancement factor was introduced to quantify the effect of jet flow Reynolds number, jet impingement dimension, and surface roughness as well as temperature difference between jet flow and impinging target on the heat transfer of jet impingement. It is observed that the roughness effect is minimal in the impingement zone while it is prominent in the wall jet region, and the surface roughness plays a dominant role on the enhancement factor of heat transfer rate compared with jet geometrical dimension, Reynolds number of jet flow as well as temperature difference. Furthermore, entropy generation analysis was also performed on the heat and mass transfer in air jet impingement, and optimal designs were found with entropy generation minimization principle accordingly.

CFD analysis of free shear layer approximations for a pulsed air jet

We propose a numerical investigation on the validity of the free shear layer approximations for laminar 2D pulsed jets in both isothermal and heated configurations. More precisely, we determine the range of values for the Strouhal number St and amplitude of the perturbation A for which these approximations remains valid. For this, we have considered a two-dimensional air jet exhausting into a quiescent environment. At the nozzle exit, the flow is characterized by a uniform temperature T0 and submitted to a longitudinal and periodic velocity disturbance: u=u0+a∗sin(ωt). A finite volume method is performed to solve the general Navier–Stokes equations governing this flow type. The present study is evaluated by comparing with previous numerical results. Conclusions are drawn about the ability of a model that does not consider the shear layer approximations to handle properly the behavior of pulsed air jet. The discussion relates to the effect of the most significant parameters, such as the pulsation frequency and amplitude, on the flow characteristic fields. The effect of Reynolds and Grashof numbers was also examined. The results state that the free shear layer approximations are valid in different regions of the isothermal or the heated pulsed jet, and this, only for high Strouhal numbers (St⩾1) and for low pulsation amplitude (A<3%) Otherwise, these approximations remain valid for a free pulsed jet only in the fully developed flow region.

Pressure Measurements in Jet Diffusion Using a Developed Hot-Wire Static Pressure Sensor

The aim of this experimental study is to investigate the diffusion of jets, with particular attention focused on the relationship between the static pressure and the stream wise mean velocity on the development of a two-dimensional air jet. A jet with three injection Reynolds numbers (Re=1.0×104, 2.0×104, 3.0×104) was generated in a two-dimensional wind tunnel. The velocity distributions were measured by an X-type hot-wire anemometer. The static pressure distributions were measured by a static pressure probe developed in our laboratory, which incorporates a hot-wire anemometer. There are two important characteristics for the hot-wire static pressure probe. The one is that mean static pressure and the static pressure fluctuation can be measured at the same time. The other is that the probe is not affected by the flow and the changes of physical properties (thermal conductivity) of gases, because the sensing hot-wire is always in the air bias flow. The sensitivity of the fstatic pressure probe is 92.3 mV/Pa. The frequency response is flat from 16 Hz to 2.5 kHz. As a result of the experiment, it was found that negative static pressure exists in the turbulent shear layer. It is considered that the entrainment processes from the negative static pressure by the vortex structure motion of the turbulent shear layer.

21414 噴流拡散過程のエントレインメントと圧力変化に関する研究(せん断流の計測,OS.4 流れの計測と制御)

This experimental study aims at investigating the turbulent diffusion mechanism of jets, in particular the relationship between the static pressure and the lateral mean velocity on the development of a two-dimensional air jet. The dimensional jet with three injection velocities (11.7, 23.3 and 35.0m/s) was generated in a two-dimensional wind tunnel. The velocity distributions were measured by X-type hot-wire anemometer. The static pressure distributions were measured by the static pressure probe that was developed in ore laboratory, using a hot-wire anemometer. The result of experiment was that the negative static pressure exists in the turbulent shear layer. It is considered that the entrainment proceed from the negative static pressure by the eddy motion of the turbulent shear layer.

Fluid elastic instability of a small circular cylinder in the shear layer of a two-dimensional jet

Fluid elastic instability of a long, small-diameter circular cylinder in the shear layer of a two-dimensional air jet is investigated experimentally. A metal wire 0.23 mm in diameter, used as a small circular cylinder, was flexibly mounted along the span-wise direction of the two-dimensional jet, and in the shear layer region where the shear layer thickness is much larger than the wire diameter. The motion of the metal wire was monitored by a two-channel vibration measurement system, and the flow data of the jet were documented by a hot-wire anemometer. Stability of the wire was examined in the experimental ranges of Reynolds number from 8 to 120, reduced velocity from 100 to 1400, and shear parameter from 0 to 0.14. The local flow velocity at the wire location and the wire diameter were chosen as the characteristic velocity and length, respectively, for the three non-dimensional parameters. It is illustrated that in the experimental ranges the wire is unstable and induced to vibrate if initially mounted in a shear layer with sufficient high flow velocity and velocity gradient, while the wire is stable in uniform cross flow. Fluid elastic instability is identified to be responsible for the flow induced vibration. The critical condition for the onset of the instability is determined from amplitude diagrams. It is found that on the critical condition the reduced velocity R and the shear parameter K approximately satisfy the relation RK1.44=5.0 for the small circular cylinder in the airflow with a mass ratio of 5112. Past the critical condition, the amplitude of the wire vibration increases significantly to the scale of the shear layer thickness. By analyses of the vibration amplitudes, spectral data, and orbits of the wire, it is illustrated that the wire vibration in the shear layer of the two-dimensional jet is self-excited through a Hopf bifurcation to a limit cycle state.

The Shear Layers of a Two-Dimensional Jet Excited by Flexible Wires

A two-dimensional air jet, heated at a density ratio of 0.8, under external forcing by flexible wires is investigated experimentally. In each shear layer of the hot jet flow, a wire of diameter 0.23 mm (0.015 jet width) is flexibly mounted along the spanwise direction. By flow visualization, temperature measurements, and spectral analysis, the study demonstrates that the wires have quite different effects on the jet flow depending on that the wires are motionless or vibrating in the flow, and the shear layers of the heated plane jet can be manipulated by means of flexible wires.

2430 二次元衝突噴流自励発振系のせん断層音響励起による制御

The shear layer of two-dimensional air jet impinging on a plate with a rectangular hole under the condition of a self-sustained oscillation is subjected to pure-tone acoustic excitation at the exit, and the unsteady jet response is characterized in terms of the excitation frequency and the excitation voltage. A locked-in response at excitation frequency (fe) or 1/n (n=1&acd;12) oscillation of a self-sustained frequency (fo) appears. A sound pressure level (SPL) has the minimum at fe/fo=0.54 and 0.9 in 2 Volts of excitation voltage, and in those cases the component fo of SPL spectrum decreases.

C207 高温空気二次元衝突噴流の局所熱伝達特性(オーガナイズドセッション6 : 自由噴流,衝突噴流およびはく離を伴う熱・物質移動)

C207 高温空気二次元衝突噴流の局所熱伝達特性(オーガナイズドセッション6 : 自由噴流,衝突噴流およびはく離を伴う熱・物質移動)

High Schmidt mass transfer in a laminar impinging slot jet flow

In this paper, high Schmidt-number mass transfer to a line electrode in laminar impinging slot-jet flows is investigated experimentally and numerically. Slot-based Reynolds numbers from 220 to 690 are considered. The mass-transfer measurements, made by the electrochemical method on 100 μm electrodes, are compared to the computed wall shear via an established analytical relationship. The local shear is obtained from steady, two-dimensional flow-field simulations of the Navier–Stokes equations. The use of an isolated line electrode and a small electrode size (100 μm) makes it possible to resolve the sharp variations in shear and mass transfer in the stagnation region. Both the experimental and theoretical results show that the peak values in Nusselt number occur at a point one-half to one jet width away from the stagnation point. Earlier experimental studies of heat transfer (Prandtl number of 0.72) reported by Gardon and Akfirat [R. Gardon, J.C. Akfirat, Heat transfer characterstics of impinging two-dimensional air jets, ASME Journal of heat Transfer 101 (1966) 101–108] (referred to in the figures as GA66) and mass transfer by Alkire and Ju [R., Alkire, J. Ju, High speed selective electroplating with impinging two-dimensional slot jet flow, J. Electrochemical Society 134 (2) (1987) 294–299] (referred to in the figures as AJ87) were not able to observe this behavior near the stagnation point.

Spray Chamber Evaluation of Air‐Assisted Spraying on Broccoli

Conventional over‐the‐row sprayers achieve very little deposit on leaves near the ground and on the underside surfaces of leaves throughout the canopy. Air assistance has the potential to improve deposition of droplets on these leaf surfaces. To gain some insight on the effect of air assistance, the effects of airspeed, airflow rate, and air jet orientation were isolated. The study was carried out in a spray chamber with a standard spray boom over micro‐plots of greenhouse grown broccoli (Brassica oleracea var. botrytis L.) plants. Air was delivered slightly behind the nozzles from a variable width slot producing a uniform two‐dimensional air jet. The orientation of the air jet with respect to the vertical could be adjusted from −10 to 40°. The ranges of the independent variables were airspeed, 0 to 36 m s−1; airflow rate, 0 to 1.3 m3 s−1 m−1, and air jet angle, −10.2 to 40.2°. Two sets of flat fan nozzles (Volume Median Diameter = 230 and 400 μm, both delivering 250 L ha−1 at 6 km h−1) were used to carry out two full sets of experiments. Results showed that airspeed had the larger impact on leaf coverage. Higher airspeeds (&gt;25 m s−1) and airflow coupled with finer sprays increased the coverage of the underside of the leaves at all levels within the canopy and of the top side of the leaves in the lower third of the canopy. However, lower airspeeds (&lt;20 m s−1) are desirable for a better coverage of the upper side of the leaves in the higher two‐thirds of the canopy. In all cases, angling the air jet forward at 20 to 25° is recommended.

The effect of inclination on the heat transfer between a flat surface and an impinging two-dimensional air jet

Abstract An experimental study was performed to determine the effect of the inclination of an impinging two-dimensional air jet on the heat transfer from a uniformly heated flat plate. The impingement surface was a stainless steel plate of the same width as the jet nozzle. Local Nusselt numbers were determined as a function of three parameters: (a) inclination angle of the air jet relative to the plate in the range of 90–40°, (b) nozzle exit-to-plate spacing ( z / D ) in the range of 4–12 and (c) Reynolds number based on the hydraulic diameter of the slot nozzle in the range of 4000–12 000 (corresponding to an exit jet velocity from 6.3 to 18.7 m/s). The results are presented in the form of graphs showing the variation of the local Nusselt number as a function of these parameters. The region of maximum heat transfer shifts towards the uphill side of the plate and the maximum Nusselt number decreases as the inclination angle decreases. The location of the maximum heat transfer region appears to fall between 0 and 3 D uphill from the geometrical impingement point, and was found to be insensitive to the Reynolds number in the range used in this study. For low values of inclination angle, the local Nusselt number on the uphill side from the maximum heat transfer point was insensitive to jet exit-to-plate spacing. Correlations are proposed to predict the local Nusselt number as a function of x / D , z / D , θ and Re .

SPRAY CHAMBER EVALUATION OF AIR-ASSISTED SPRAYING ON POTATO PLANTS

This article reports on a study of air-assisted spraying where the effects of air speed, airflow rate, and air jet orientation were isolated. The study was carried out in a spray chamber. A carriage supporting a standard spray boom was moved at 6 km/h over micro-plots of greenhouse grown potato plants. Air was delivered slightly behind the nozzles from a variable width slot producing a uniform two-dimensional air jet that was adjustable in tilt in the direction of travel. Experiments were carried out using a central composite design (CCD). Compared to a full factorial design, this design required fewer experiments and allowed one to find the optimum operating conditions. The range of the independent variables was: air speed, 0 to 36 m/s; airflow, 0 to 1.3 m 3 /s/m, and air jet angle, –10.2 to 40.2°. Two sets of flat fan nozzles (VMD = 230 and 400 µm, both delivering 250 L/ha at 6 km/h) were used to carry out two full sets of experiments. Results showed that air speed had the larger impact on leaf coverage. Higher air speeds (> 25 m/s) increase the coverage on the underside of the leaves at all levels within the canopy and on the top side of the leaves in the lower third of the canopy. However, lower air speeds (< 20 m/s) are desirable for a better coverage on the top side of the leaves in the higher two-thirds of the canopy. Based on data presented here, the use of an air speed higher than 25 m/s coupled with an air jet angle of 20° forward and a fine spray increase both the coverage of hidden parts of the plants and the spray penetration towards the bottom of the canopy. Airflow rate affected coverage significantly only in conjunction with finer spray.

220 マルチパルスレーザーホログラフィーによる非等温二次元衝突噴流の熱流動場の可視化

Local heat transfer characteristics of high temperature impinging two-dimensional air jets were investigated by using a novel multi-pulsed laser holographic interferometry. Thermal flow fields at nozzle-to-plate space were visualized real-time for various jet velocities and nozzle-to-plate distances, which were recorded by using a high-speed streak camera(5000fps). Instantaneous development and growth of streamwise vortices were clearly visualized by this novel method. The results indicate that the local heat transfer coefficients are greatly affected by the large-scale vortices which appear in the wall jet region. Furthermore, it could be demonstrated that this multi-pulsed laser holographic technique is effectively applicable to the high-speed observation of the impinging jets.

Analysis of and Experimental Measurements made on a Moving Air-Assisted Sprayer with Two-Dimensional Air-Jets Penetrating a Uniform Crop Canopy

Air-assisted crop sprayers use air-jets to enhance the transport and deposition of agricultural pesticides in crops that are difficult to spray using conventional equipment. However, the use of air-jets can cause excessive environmental contamination or give an ineffective treatment if the flow characteristics are poorly matched to the target crop. To establish a scientific basis for improving the design and control of air-assisted sprayers, this paper presents an analysis of the momentum and turbulent kinetic energy conservation equations for a two-dimensional air-jet penetrating a uniform crop canopy from a moving sprayer. From the analysis of these equations the velocity and turbulent kinetic energy along the jet centre-line are shown to decay exponentially with penetration distance. The decay exponents are shown to be proportional to the inverse of the jet width, the square of the ratio of sprayer speed to initial air-jet velocity and the crop density. Velocity and turbulence kinetic energy measurements are presented for a two-dimensional air-jet penetrating an artificial crop canopy. The canopy was constructed from a regular array of flow blockage planes which were adjusted to give an experimental range for area density (i.e. the cross-sectional area of blockage normal to the initial air-jet flow direction per unit volume) between 0·7 m - 1and 3·0 m - 1for two different values of plane spacing of 0·23 m and 0·46 m. The experimental results verify the exponential decay form for the distributions of air flow properties along the jet centre-line. The values derived for the exponential decay coefficient for both jet centre-line velocity and turbulent kinetic energy were found to give a poor correlation with the canopy area density over the full range of experiments. Instead, this correlation exhibited an asymptotic characteristic as the exponential decay coefficients for mean centre-line velocity and turbulent kinetic energy become very large at finite area density during high planar blockage. A rationale for this additional effect of crop structure is presented in terms of the additional losses produced by local flow channelling within dense canopies. The asymptotic characteristic was successfully modelled by redefining the crop density as the weighted sum of the inverse of the two orthogonal mean flow gaps in the artificial crop canopy. This new form of crop density replaces area density in an otherwise conventional model for momentum and turbulent kinetic energy losses due to small-scale volume averaging of air-jet and crop interactions.

The removal of particles from flat surfaces using a high-speed air jet

The removal of fine particles adhering to a wall has been studied using a high-speed two-dimensional air jet paying particular attention to the geometric constitutions of the removal equipment and the duration of air jet blowing. The experimental results showed that the stable high efficiency region was confined to the vicinity of the impinging point of the air jet and depended on the impinging angle. The high efficiency region was well estimated by a new model where the impinging region was introduced based on the two dimensional free jet theory. The effect of the nozzle-wall distance and the particle diameter were also estimated. The change in the removal efficiency with time was also well represented by a model assuming that the efficiency was proportional to the number of removable adhered particles. In addition, the removal characteristics of the sequential pulse jet were the same as those obtained by a one-pulse jet as long as the total duration of the air jet was kept constant. The change in the efficiency with pulse number was also represented by the same model.

高速気流による壁面付着微粒子の除去

The removal of fine particles adhering to a wall has been studied using a high speed two-dimensional air jet and paying particular attention to the geometric constitutions of the removal equipment and the duration of air-jet blowing. The experimental results showed that the stable high efficiency region was confined to the vicinity of the impinging point of the air-jet depending on the impinging angle. The high efficiency region was well estimated by a new model where the impinging region was introduced based on the two-dimensional free jet theory. The effect of the nozzle-wall distance and the particle diameter were also estimated.The change in the removal efficiency with time was also well represented by a model assuming that the efficiency was proportional to the number of removable adhered particles. In addition, the removal characteristics of the sequential pulse jet were the same as those obtained by the one-pulse jet as long as the total duration of the air-jet was kept constant. The change in the efficiency with pulse number was also represented by the same model.

Impinging cooling of a high temperature flat surface by multiple two-dimensional air jets.

Two-dimensional impinging jets in line have been used for many industrial applications for cooling and heating, but the effectiveness of the heat transfer augmentation has not been well established. The purpose of this study is to clarify the augmented heat transfer performance of impinging air jets in line by means of providing roughnesses on a heat transfer surface. The optimum position of the roughnesses are studied experimentally for the jet Reynolds number of 1×104 when the jet-to-plate spacing is equal to the jet slot width. It is clarified that, even though of no roughness the two jets increases the total heat transfer performance by about 1.4 times that of one jet without roughness regardless of the jet spacing, under the optimum condition the average surface heat transfer coefficient in a semi-enclosed environment increases by about 2.30 times that of one jet without roughness.

Fundamental study on the flattening of temperature distribution of a high temperature steel slab. 3rd report Heat transfer characteristics due to a confined turbulent impinging jet flow.

Concerning the flattening of temperature distribution of a steel slab, the characteristics of local heat transfer due to impingement of a confined turbulent two-dimensional air jet on a heated surface have been conducted experimentally for a jet Reynolds number up to 20 000, for 6 heated surface-slot nozzle spacings and for 3 heated side surface-porous plate spacings. It was found that the spanwise heat transfer exhibits two or three local maxima. Nusselt number at the stagnation point was constant for the dimensionless spacing 1<h/2d<3 and also increased with decrease in h/2d(<1).

Simplified Analysis of Air-Bubble Plumes in Moderately Stratified Environments

This paper is concerned with the behavior of buoyant plumes driven by rising gas bubbles through a moderately stratified liquid environment. A two-phase mathematical model is proposed for calculating the induced turbulent vertical liquid flow and its temperature. Simultaneous numerical solutions of the continuity, momentum, and energy equations are obtained for the two cases of an axisymmetric and a two-dimensional air jet submerged in water. Water pumping rates, liquid velocities, plume radii (widths), and temperatures are obtained as a function of air flow rates, depth of submergence, and the assumed temperature profile of the water in the region far away from the jet. Effects of temperature stratification in the liquid region far away from the jet resulted in increased liquid flow rates as well as increased plume cross-section areas. Comparisons are made with the experimental information available in the literature.

Mass transfer due to a confined laminar impinging two-dimensional jet

Local mass transfer due to impingement of a confined laminar two-dimensional air jet on a flat surface has been studied both experimentally and theoretically for a jet Reynolds number up to 400 and for two different jet-to-plate spacings. The experimental study was carried out via double exposure and real-time holography and the theoretical study was made via the numerical solution of the transport equations. It was found that the span-wise variation of the local Sherwood number exhibits a local minimum and maximum.