Multiple Air Jets Research Articles

Numerical study on heat transfer characteristics of multiple air jet impinging at lower jet to plate spacing

For multi-nozzle jet impinging, the effects of impinging distance (Z/D), streamwise spacing (X/D) and spanwise spacing (Y/D) on heat transfer characteristics should be considered. The heat transfer characteristics for multi-nozzle jet impinging structure with different nozzle spacings were investigated by numerical method. When the impinging distance is constant (Z/D = 0.2), the effects of Reynolds number (Re = 12,000–28,000), streamwise spacing (X/D = 3, 4, 6, 8, 10), spanwise spacing (Y/D = 3, 4, 6, 8, 10) and impinging nozzle arrangement (in-line structure and staggered structure) are considered. The Standard k-ε model was chosen for simulating after comparison to the relevant experimental values. The influences of nozzle spacing and the Reynolds number on the heat transfer characteristics were examined. The Colburn factor j and friction factor f were investigated in relation to Re. The performance criteria η of the Reynolds number ranging from 12,000–28,000 was calculated. The calculations indicated that the jet impinging heat transfer coefficient is highest under the structure of X/D = 3 and Y/D = 3. Compared to the in-line arrangement, the staggered arrangement has a higher average Nusselt number. This is because the turbulent kinetic energy of the staggered structure on the impinging target surface is larger and the heat exchange efficiency is higher. However, the gap between the average Nusselt numbers of the staggered and in-line nozzle arrangements decreases with an increase in Reynolds number. Given the same heat exchange surface, the average Nusselt number is highest under the X/D = 8 and Y/D = 8 arrangements. This is because the larger the impinging nozzle spacing, the fewer the number of nozzles. Therefore, the greater the speed of the gas reaching the nozzle outlet, the greater the average Nu of the target surface.

Local heat transfer characteristics of circular multiple air jet impinging on a semicircular concave surface

ABSTRACT The local heat transfer characteristics of circular multiple air jet impingement on semicircular concave surface were studied experimentally. For constant Reynolds number (Re = 9,000), the effect of jet to concave surface spacing (z/d = 1, 4.2, 6), ratio of curvature (D/d = 4.28, 6, 8.6), jet-to-jet spacing (s/d = 2.4, 4, 5.6), and number of jets were considered. The local heat transfer characteristics were determined by using infrared thermal camera and thin foil method. Lower curvature value ratio (D/d = 4.28) resulted higher average heat transfer coefficient due to broader spread of heat transfer distribution, whereas higher curvature ratio (D/d = 8.6) resulted in lower average heat transfer coefficient due to steeper heat transfer distribution in the stagnation area. At lower D/d, z/d, and s/d with seven jet configuration, the heat transfer coefficients were found effective as compared to other configurations due to broad heat transfer distribution and less interaction between the jets. At given D/d and s/d configuration, the stagnation Nu at θ = 0° do not coincide with jet hole center axis and this shift is more pronounced at higher z/d value. This could be due to additional push before jet impinging on concave surface. The secondary peaks were observed between two neighboring stagnation regions in Nu distribution along the center longitudinal line for lower z/d and higher s/d values. This resulted due to upwash that was obtained from jet interactions related to collisions with the wall jets. The correlations of stagnation line average Nu and overall average Nu were obtained. The coefficient of variance of local heat transfer coefficient for each configuration was determined, and correlation was proposed to analyze the degree of non-uniformity.

Effect of jet plate thickness on the local heat transfer coefficient with multiple air jet impingement

For designing a multiple jet impingement cooling system, the thickness of jet plate (a passage through which jet comes out) is one of the important parameters. Considering the thermal point of view, the thickness of the plate should be chosen so that it would enhance the heat transfer rate. In this study, the effect of jet plate thickness on the local heat transfer coefficient on a flat plate with multiple impinging air jets is investigated. The thickness ratios (t/d) of 0.5, 1.0 and 2.67 are studied. Reynolds number is varied from 5000 to 20,000 and jet-to-plate distance (z/d) is varied from 1 to 5. Jet diameter (d) is chosen to be 6 mm. The jet-to-jet distance (p/d) is kept constant at 3.Steady-state thin metal foil technique, along with infrared thermography, is used to obtain the temperature distribution and thereby the heat transfer coefficient. Coefficient of discharge for each jet plate, central line velocity decay and pressure distribution on the target surface are measured. These measurements provided new insights to better understand the heat transfer results in this study. For lower jet-to-plate distances (t/d = 0.5 to 1), heat transfer decreases with the increase in the jet plate thickness ratio. However, heat transfer is not affected with the further increase in the jet plate thickness ratio i.e., t/d = 1 to 2.67. Therefore, smaller jet plate thickness ratio is suitable to obtained high heat transfer rate. This study presents a new correlation for the local Nusselt number distribution or different jet plate thickness.

Substrate Characteristics of Multiple Arrays of Turbulent Swirling Impinging Jets

This research investigates the flow aspects and thermal behavior of multiple air jet impingements on a flat plate through two different geometrical arrangements. A variety of flow conditions and geometrical parameters are considered including Reynolds number, swirl number, impingement distance, and jet-to-jet separation. A numerical approach is adopted to conduct the analysis using the SST k-ω turbulence model in the commercial CFD software package ANSYS Fluent v17. Results reveal that the static pressure is uniformly distributed on the impingement surface for the higher nozzle-to-surface distance at a strong swirl flow, while for no-swirl and medium-swirl configurations, the static pressure is accumulated at the stagnation regions. A lower impingement distance ameliorates the cooling performance while a higher impingement distance accelerates the consistency in temperature distribution. Moreover, the staggered nozzle arrangement is found superior in terms of generating integrity in thermal cooling. Overall, the pressure coefficient enhances up to 126% for medium swirl (S = 0.3) and 67% for high swirl (S = 0.75) cases, and in terms of Reynolds number (Re), the pressure coefficient decreases by 12% and 15% for Re = 24600 and 35000, respectively, compared to Re = 11600. Besides, the average Nusselt number amplifies by 8% and 17% for medium (S = 0.3) and high swirl (S = 0.75) flow, respectively, while compared to Re = 11600, the average Nusselt number raises by 58% and 98% for Re = 24600, and 35000, respectively. The eddy viscosity increases strongly with the growth of swirl intensity, while a monotonous vorticity distribution is achieved at high swirl conditions.

Convection from multiple air jet impingement - A review

• For S/D ≥ 3 and H/D = 2, the staggered pattern enhances Nu when compared with. • inline; • Wall motion is a topic of interest since even low velocities seem to enhance Nu; • Compressibility effects are important and Ma > 0.7 induces stronger instabilities; • SST k-ω model presents the best compromise between computational costs and. • accuracy. Multiple jet impingement is widely implemented in several industrial processes. The efficacy of this convective process depends on various parameters from the geometrical variables of the system, to the impinging surface roughness and the jet’s flow parameters. The study of this complex flow is challenging and increases with the confinement of the impinging domain. This work focus on a detailed analysis of confined and submerged multiple air jet impingement. To understand the physics under this process and the influence of the different variables on heat transfer performance, a review of the jet’s flow dynamics and heat transfer characteristics of submerged multiple air jet impingement systems is presented in this work. In this review, the parameters that affect the convective process are analyzed and special attention is given to less explored variables such as the surface configuration and motion, jet inclination and high jet temperature, and Mach number. The conclusions and relevant statements presented by several authors through their numerical and experimental works are presented and compared, to identify the values of each variable that enhance the heat transfer over the impinging surface. The numerical methods applied for the modeling of impinging jets are also analyzed. The advantages and disadvantages of each turbulence model discussed in the literature are presented and characterized in terms of their computational time and costs, in order to identify the most accurate for the multiple jet impingement process modeling. Finally, relevant correlations for the average Nusselt number, to characterize the heat transfer of single and multiple jet impingement are summarized.

Reacting flow characteristics based on the axis-switching phenomenon in a baffled micro combustor with rotated noncircular holes for micro-thermophotovoltaic system

• The geometric features related to the axis-switching phenomenon are numerically investigated for a multihole baffled micro combustor. • In the combustor of noncircular holes, secondary flows are more developed. • The flow mixing and combustion efficiency are improved by noncircular air holes. • The best thermal field for a combustion-based TPV system is obtained for baffles with 30°-rotated square holes and 15°-rotated triangular holes. • The emitter efficiency for TPV applications is better than the swirling flow combustor. For a thermophotovoltaic (TPV) generator, a combustor design for strengthening the axis-switching phenomenon is explored. Various simulations based on a Reynolds stress model and detailed chemical mechanism of GRI 3.0 are performed for methane (CH 4 )–air flames in a multihole baffled combustor. The reacting flow fields are investigated with the baffle plates of rotated square and triangular holes. The spreading and merging of multiple air jets past the baffle plate are significantly affected by rotating noncircular air holes. Considering the crossover location changes of the jet half-width, the air jets have a flow structure that quickly generates the axis-switching phenomenon. Compared with the baffled combustor of circular air holes, the center recirculation zone in the combustor of noncircular holes is enlarged by approximately 200% with a hole rotation. As the hole rotation increases, secondary flows become more complicated and the merging of air jets is quickly finished. Based on these flow structures, the region with the stoichiometric condition is observed in various forms of hexagonal, star, circular, and flower shapes. As a result, the flammable region and combustion efficiency are considerably increased. The flame lengths are roughly evaluated as three times the mixing lengths. From the viewpoint of a heat emitter for the TPV application, the heat transfer on the combustion chamber wall is discussed. Compared with the combustor with circular air holes, the combustor with noncircular air holes shows a 5% increase in flammable region and a 5.9% improvement in combustion efficiency. For a combustion-based TPV system, the best uniformity of wall temperature is observed for baffles with 30°-rotated square holes and 15°-rotated triangular holes and the highest mean wall temperatures are obtained when the flame length is approximately 1/3 of the combustor length.

PIV analysis of confined multiple jets impinging on a step surface

Forced convection by submerged multiple jet impingement for cooling applications is widely implemented in several industrial sectors. However, the complexity of the flow makes this process difficult to control. Therefore, it is important to conduct an in-depth study in order to characterize the flow field and this is possible through Particle Image Velocimetry (PIV) technique. PIV allows the measurement of the velocity field and information regarding flow vorticity and turbulence intensity can be obtained. To conduct the PIV measurements, an experimental setup was specially built to analyze the flow dynamics of multiple air jets impinging on a surface with a step. In PIV, the selection of the most appropriate tracer particles is crucial to ensure accurate measurements. In that sense, this work comprises an analysis of different seeding particles produced by a Concept Smoke Aerotech system. The measurement region consists of an array of multiple air jets spaced 3 D (jet diameters) between them according to a staggered configuration. The jets are confined and a distance between the nozzle plate and the target surface of 2 D is applied. The central, back, and front rows are analyzed in order to determine the effect of the jet’s interactions on the velocity fields and heat transfer. The results show that the complexity of the flow is increased by the implementation of a non-flat surface. The step induces a strong flow reversal that affects the jet flow development over the surface. Moreover, the measured data highlights the complex interactions between jets and confirm that PIV is able to capture the large-scale structures induced in the vicinity of the target plate, as well as the strong fountain flows generated between the adjacent jets. Furthermore, the results show that the combination of PIV and heat flux sensors is appropriate to characterize the jet’s flow dynamics and the heat transfer of multiple jets impinging on a surface, providing relevant insights for several engineering applications. This study underlines the advantages of the PIV system for a detailed characterization of a multiple jet impingement flow and the importance of a proper definition of the experimental condition to ensure the accuracy of the measurements.

FlowHaptics: Mid-Air Haptic Representation of Liquid Flow

Water is an essential substance for humans in their daily lives. There are many opportunities for us to come in contact with water, such as cooking, bathing, and swimming. However, few studies have reproduced the sensation of water touching the skin. This study aims to propose a novel midair haptic device, named FlowHaptics, that reproduces the feeling of the force of flowing water over human fingers using multiple air jets. We first estimated the temporal pressure distribution change of water in two-dimensional space using machine-learning-accelerated fluid simulation. We controlled the airflow based on the pressure distribution change obtained from the fluid simulation to reproduce the feeling of flowing water over the fingers using our proposed device, which can control multiple air jets in real time. We performed a psycho-physical evaluation of different flow velocities and a subjective evaluation of different velocity profiles. We found that FlowHaptics reliably created the illusion of the pressure distribution of flowing water on the fingers where the flow velocity could be distinguished within the range of 8.42% to 13.05%, and our estimated flow velocity profile with the configuration of three air jets felt more similar to flowing water when compared to a constant velocity profile according to the users.

Stereoscopic particle image velocimetry study of the single side entrainment behaviour of multiple air jets with different nozzle cross-sections

Achieving energy savings in the building sector while maintaining high air quality of indoor environments requires efficient ventilation systems. Active chilled beams (ACBs) are a promising solution for the ventilation in non-residential buildings. Their functionality is often based on the single side entrainment of multiple air jets. In order to enhance this entrainment and thus increase the efficiency of ACBs, this study investigates the specific flow situation of a single side entrainment experimentally. Additionally to the reference circular type, three different cross-sections (elliptic, cross-like, lobed) of an orifice nozzle are investigated with different experimental methods. The unique flow situation in ACBs with multiple interacting jets entraining air from one side is extracted to a test bench. First, the general nozzle efficiency and total entrainment is investigated. Then, the detailed understanding of the entrainment process is achieved with stereoscopic particle image velocimetry (stereo-PIV). The results show that a minimum distance between the jets is necessary for the development of individual flow behaviour affecting the entrainment. For a larger nozzle distance we measured an increased overall entrainment for the non-circular nozzle geometries, with the lobed geometry showing the greatest improvement. The PIV results show how the jets from the different nozzle geometries pass through different stages of entrainment behaviour and profile transformation.

Application of Taguchi Method for the Analysis of a Multiple Air Jet Impingement System with and without Target Plate Motion

Multiple air jet impingement is a complex heat transfer process involving several parameters which interfere with the cooling and heating performance. Due to its wide applicability in engineering applications, the influence of these parameters in jet impingement efficiency has been extensively analyzed by several researchers. However, the majority of the studies developed experiments and numerical simulations to determine the effect of each parameter individually. Considering that the heat transfer performance depends on the correct combination of the multiple jet impingement parameters, the implementation of a Design of Experiments (DoE) seems to be more effective since this method allows a comparative parameter analysis as well as an optimization of the experiments and numerical simulations. In this study, a DoE based on the Taguchi method and an Analysis of Variance (ANOVA) is applied to conduct an experimental study on multiple air jets impinging a surface. This work focuses on jet-to-jet spacing (S), nozzle-to-plate distance (H), Reynolds number, and target plate geometry. Taking into consideration these four parameters, the experiments are conducted with the target plate static and in motion and the results obtained in both cases are compared. Even if the static target plate is the most studied case, the surface motion is identified in several engineering applications, such as reflow soldering and drying processes. The results show that the heat transfer of a multiple jet impingement configuration is enhanced by S=3D, H=2D for a staggered configuration, and a higher Reynolds number for both static and moving plates.

Heat Transfer Investigation with Multiple Jet Impingement

An experimental study was carried out to study the effect of multiple jet impingement on a virtual electronic component using water and air as working fluids. It consists of an electrically heated test plate of size 20mm×20mm. Heat flux is varied between 25 to 250W/cm2 was dissipated using 0.25 and 0.5mm diameter jets placed in a 7×7 array with a pitch of 3mm. The difference in temperature between test surface and fluid inlet is within 30 degC for water jets and within 75 degC for air jet experiments. Experiments were conducted by changing the heat flux, flow rate and distance between the test surface and jet exit and [iv] horizontal and vertical positioning of the jets. It was found that heat flux, jet diameter and Reynolds number are important factors in determining the heat transfer. The effects of distance between test surface and jet exit [Z] and positioning of the jets were insignificant. Though the multiple jet impingement heat transfer problem is complex, the heat transfer results could be correlated using a simple relationship in the form of Nu = AqmRen. The constant (m) which indicates the effect of heat flux has the value of 0.8 and 0.9 depending upon the jet diameter and the coolant. The constant (n) which indicates the influence of Reynolds number has the value of 0.25 for both water and air jets. The value of constant (A) is different for water and air jets. The correlation developed in this research work can be effectively used to design multiple water and air jet cooling system for electronic components.

HeadBlaster

We present HeadBlaster, a novel wearable technology that creates motion perception by applying ungrounded force to the head to stimulate the vestibular and proprioception sensory systems. Compared to motion platforms that tilt the body, HeadBlaster more closely approximates how lateral inertial and centrifugal forces are felt during real motion to provide more persistent motion perception. In addition, because HeadBlaster only actuates the head rather than the entire body, it eliminates the mechanical motion platforms that users must be constrained to, which improves user mobility and enables room-scale VR experiences. We designed a wearable HeadBlaster system with 6 air nozzles integrated into a VR headset, using compressed air jets to provide persistent, lateral propulsion forces. By controlling multiple air jets, it is able to create the perception of lateral acceleration in 360 degrees. We conducted a series of perception and human-factor studies to quantify the head movement, the persistence of perceived acceleration, and the minimal level of detectable forces. We then explored the user experience of HeadBlaster through two VR applications: a custom surfing game, and a commercial driving simulator together with a commercial motion platform. Study results showed that HeadBlaster provided significantly longer perceived duration of acceleration than motion platforms. It also significantly improved realism and immersion, and was preferred by users compared to using VR alone. In addition, it can be used in conjunction with motion platforms to further augment the user experience.

Optimization of turbulent air jet impingement for energy efficient commercial cooking

Thermally balancing and optimizing performance of conveyor ovens is a complicated problem requiring a thorough understanding of multiple turbulent air jet impingement. A 3D model of a conveyor oven is created. Number of high impact parameters are varied through a systematic approach. Velocity distribution and air leakage from the oven to surrounding air is analyzed to optimize energy consumption. Surface heat flux and heat transfer coefficient are studied. Through process of optimization, an oven configuration is selected which is expected to perform ~20% better than the starting configuration.

Multiple DOF platform with multiple air jets

Multiple DOF platform with multiple air jets

Experimental and Numerical Analyses of Multiple Jets Impingement Cooling for High-Power Electronics

Effective cooling of electronic equipment has emerged as a challenging and constraining problem of the 21st century. Jet impingement is a high-performance cooling technology for enhancing the heat transfer in thermal devices. This paper reports the experimental and numerical investigations carried out to study the heat transfer characteristics of multiple air jets impinging on a stationary flat copper plate. The software used for the numerical studies is a finite-volume-based software, ANSYS Fluent. An experimental setup was designed and fabricated. The results obtained from numerical studies were validated with that obtained from experimental studies. A copper plate, which is heated from the bottom, was used as the heat transfer surface of a simulated chip. The jets were produced by three sharp-edged circular orifices arranged in an in-line configuration. The nozzle-to-plate distance was varied as 1, 2, 4, 6, and 7 times the nozzle diameter ( $d$ ). The effects of Reynolds number (4000–8000) and jet-to-jet spacing on heat transfer characteristics were examined numerically. From the studies, it was found that for a Reynolds number of 5000, optimum cooling is obtained when nozzle-to-plate spacing ( $H$ ) is 6 times the orifice diameter. A correlation was proposed for Nusselt number in terms of Reynolds number, and it is valid for air as the cooling medium.

Evaluation of aerosolization characteristics of biocontaminated particles from flood-damaged housing materials

The purpose of this study was to examine the aerosolization characteristics of biocontaminated particles, including particle release behavior and the concentrations of culturable fungi and (1→3)-β-D-glucan from four flood-affected floor and bedding materials. We applied an aerosolization process using multiple air jets, then measured the particles aerosolized from biocontaminated materials in real time using an optical particle counter, while simultaneously collecting particles in a BioSampler. The total count of particles released over 10min was highest from linoleum (25,503 particles/cm2), followed by rugs (1562 particles/cm2), carpets (508 particles/cm2), and pillows (24 particles/cm2). Linoleum, which has a hard surface, released particles the fastest (<6s) among the test materials. During fractional particle concentration analysis, the portion of submicron particles between 0.3 and 1.0μm was 66.1% (linoleum) – 77% (carpet) of the total particle concentration. Additionally, based on biological assays of the collected particles, the levels of culturable fungi and (1→3)-β-D-glucan in the four materials ranged from undetectable (linoleum; lower limit=0.2 CFU/cm2) to 0.83 CFU/cm2 (rug) and from 0.84 (carpet) to 3.26ng/cm2 (rug), respectively. We suggest that these results are helpful for further understanding of the aerosolization characteristics of biocontaminated particles from flood-affected materials, with benefits for the safe restoration of flood-damaged homes.

ファジィ制御を用いた複数の空気噴流による平面上複数物体の非接触操作

We have been studying a noncontact object manipulation on a surface plane by multiple air jets. We could manipulate 3 DOF (X, Y, θ) of a single object, and also 2 DOF (X, Y) of multiple objects in both scattered and collected case. In this report, we try to introduce a fuzzy algorithm as a first step toward the robust control design against the nonlinear effect of the aerodynamics and the change in the plant parameter such as number, size and weight of the controlled objects.

English

Jet impingement systems provide an effective means for the enhancement of convective processes due to the high heat and mass transfer rates that can be achieved. The range of industrial applications that impinging jets are being used in is wide. In the annealing and tempering of materials, impinging jet systems are finding use in the cooling of hot metal, plastic, or glass sheets as well as in the drying of paper and fabric. Compact heat exchangers, with applications in the aeronautical or the automotive sector, often use multiple impinging jets in dense arrangements. In gas turbine applications, jet impingement has been routinely used for a long time. Because of increased thermal efficiency at high turbine inlet temperatures, many gas turbine components, such as rotor disks, turbine vanes and blades, or combustion chamber walls, are operated at temperatures well above highest allowable material limits. In order to assure durability and long operating intervals, effective cooling concepts are required for these highly loaded components.The current work involves studying of effect of spanwise jet-to-jet spacing on local heat transfer distribution due to an in-line rectangular array of confined multiple circular air jets impinging on a surface parallel to the jet plate computationally. Shear Stress Transport (SST) k-ω turbulence model is used. The flow, after impingement, is constrained to exit in two opposite directions from the confined passage formed between jet plate and target plate. Mean jet Reynolds numbers based on the nozzle exit diameter (d) covered is 10,000 and jet-to-plate spacings studied are d, 2d and 3d. Spanwise pitches considered are 2d, 4d and 6d in steps of 2d keeping the streamwise pitch at 5d. Length-to-diameter ratio of nozzles of the jet plate is 1.0. For all the configurations, the jet-plates have ten spanwise rows in streamwise direction and six jets in each spanwise row. The results obtained are validated against the experimental data available in the open literature.

Experimental and numerical investigation of geometry effects on multiple impinging air jets

The effect of jet geometry on the flow and the heat transfer characteristics was investigated experimentally and numerically for elliptic and rectangular impinging jet arrays. Detailed heat transfer measurements over a smooth surface by elliptic and rectangular impinging jet arrays were obtained using thermal infrared camera. Velocity measurements were performed with a Laser-Doppler Anemometry (LDA) system. The aspect ratios (AR) of elliptic and rectangular jets for 1.0, 2.0 and 0.5, jet Reynolds numbers ranging from 2000 to 10,000, and jet-to-target spacings ranging from 2 to 10 were considered to investigate impingement heat transfer performance. The Nusselt numbers for the elliptic jets in the impingement regions were larger than that for a circular jet. This may be caused by the large entrainment rate and large scale coherent structure of the elliptic jet. Best heat transfer performance was obtained with the elliptic jet arrangements. Correlations have been developed for the average Nusselt numbers for elliptic and rectangular jet geometries. Additionally, simulations have been performed and velocity distributions were obtained. Axial, radial velocity distributions and turbulent kinetic energy distributions have been calculated.

Contactless object manipulation using multiple air jets on planar surface (Experimental case studies for small control range with continuous air jets)

We propose a technology for manipulating the position and attitude of an object on a planar surface in a contactless manner. This technology involves multiple ejectors that stream jets of compressed air onto an object from various directions. By regulating air jet directions and volumes, the object can be actively "force closured" and its position and attitude can be manipulated freely. As the first step of this research, we discuss the fundamental ideas and the technological challenges as a manipulation method. Next we provide a preliminary investigation with one degree-of-freedom experimental system. Based on this, we examine two DOF system with three air jets and finally three DOF system with four air jets.