Impinging Air Jet Research Articles

Investigation for Convective Heat Transfer on Grinding Work-Piece Surface Subjected to a Mist/Air Impinging Jet

Two objectives were outlined in the current study. The first objective aimed to assess the detailed flow and heat transfer features in the vicinity of a rotating grinding wheel with jet impingement directed at grinding zone by numerical investigation. The second objective aimed to assess the quantitative evaluation for heat transfer enhancement on a grinding work-piece surface subjected to the mist/air jet impingement by experimental investigation. The results show that the coupled action of swirl air entrainment and jet impingement is benefit somewhat for overall convective heat transfer in relative to stationary disk case whether the disk rotates in clockwise or contrary clockwise. When the jet impinging direction is consistent with the rotational direction of rotating disk, convective heat transfer enhancement is achieved near grinding region, especially at higher rotating speed. Furthermore, the increasing of water droplet in mist/air jet impingement showed significant enhancement of the cooling effect.

Heat and mass transfer during evaporation of thin liquid films confined by nanoporous membranes subjected to air jet impingement

Thin liquid films can provide efficient means for heat and mass transfer by supporting a large rate of evaporation, while maintaining temperatures under saturation conditions. The use of nanoporous membranes establishes thin liquid films minimizing the possibility of dryout by exploiting capillary confinement of the fluid. In combination with gas jet impingement, this yields record high heat and mass fluxes. Consequently, for applications relying on efficient heat and mass transfer, such as, phase change cooling of electronics and membrane distillation of water, it is essential to identify and characterize all the underlying transport mechanisms affecting overall performance. A detailed computational analysis of heat and mass transfer is therefore carried out to predict the performance of thin film evaporation. Heat conduction and convection are both found crucial in determining the temperature distribution within the thin film region, which ultimately controls the rate of evaporation from the liquid–vapor interface. Furthermore, the effect of vapor transport from the liquid–vapor interface on overall performance is also assessed. The relative effects of these performance-governing parameters are discussed and quantitatively compared at different operating conditions. The results of the computational analysis are supported by comparison with experiments.

Surface roughness effects on the heat transfer due to turbulent round jet impingement on convex hemispherical surfaces

The effect of surface roughness on the convective heat transfer from a heated convex hemispherical surface due to a turbulent round cold air jet impingement has been investigated numerically using the ANSYS FLUENT CFD code. Initially, the performance of a few turbulence models, namely, the RNG k–ɛ model, the realizable k–ɛ model, the standard k–ω model, and the SST k–ω model, in the prediction of the convective heat transfer for such a flow configuration, is evaluated against experimental data. Based on this evaluation, the SST k–ω model is chosen and employed to further investigate the surface roughness effect on the jet impingement heat transfer process for the above mentioned configuration. The flow and geometric parameters for this study are the jet exit Reynolds number (Re), the jet diameter (d), the diameter of the hemispherical surface (D), the distance of the impingement surface from the jet exit (L), and the equivalent average sand grain roughness height of the hemispherical surface (ks). Computations have been performed for various combinations of these parameters for a range of their values. It is observed from the results that, the local and average Nusselt number at the hemisphere surface is enhanced with increasing surface roughness height. Thus it is concluded that the impinging jet heat transfer will be augmented if roughness is added to the hemispherical surface.

Investigation for convective heat transfer on grinding work-piece surface subjected to an impinging jet

Investigation was carried out to illustrate some of the flow and heat transfer characteristics that occur when a single-air jet or mist/air jet impinges on a flat plate with a rotating disk mounted above the surface, representative of the work-piece in a grinding process. The first objective aimed to assess the detailed flow and heat transfer features in the vicinity of a rotating grinding wheel with single-air jet impingement directed at grinding zone by numerical investigation. The second objective aimed to assess the quantitative evaluation for heat transfer enhancement on a grinding work-piece surface subjected to the mist/air jet impingement by experimental investigation. The results show that the coupled action of swirl air entrainment and jet impingement is benefit somewhat for overall convective heat transfer in relative to stationary disk case whether the disk rotates in clockwise or contrary clockwise. When the jet impinging direction is consistent with the rotational direction of rotating disk, convective heat transfer enhancement is achieved near grinding region, especially at higher rotating speed. Furthermore, the increasing of water droplet in mist/air jet impingement showed significant enhancement of the cooling effect.

Effect of nozzle geometry on heat transfer characteristics from a single circular air jet

The paper reports results on the effects of nozzle geometry on local and average heat transfer distribution in unconfined air jet impingement on a flat plate. An infrared thermography camera is used to record the temperature distribution from isotherms on a uniformly heated impingement surface. Experiments have been conducted with variation of exit Reynolds number, Re, in the range of 6000 ≤ Re ≤ 40,000 and plate surface spacing to nozzle diameter, H/d, in the range of 1 ≤ H/d ≤ 6 for single nozzle with square edge (non-chamfered) and chamfered nozzles of the same diameter, 5 mm. The chamfered length, Lc is varied from 1 mm to 3.65 mm with constant chamfered angle, θ = 60 and length to diameter ratio of 50 is chosen for each nozzle configuration. It is observed that the local and average Nusselt numbers have the highest value for square edge inlet nozzle when compared with other nozzle configurations. The average Nusselt number was correlated for the nozzle Reynolds number as Nu¯∝Re0.75(H/d)−0.016 for square edge. For chamfered edge nozzles, the value of average Nusselt number is depend on jet Reynolds number, separation distance and chamfered length, Lc, Nu¯∝Re0.76(H/d)−0.015(Lc/d)−0.01.

Hysteresis in annular impinging jets

Annular impinging air jets were investigated experimentally using the techniques of flow visualization, measurement of the stagnation pressure, and of the mass transfer by the naphthalene sublimation method. The bistability and hysteresis lead to two different alternative flow field patterns (A or B) under identical boundary conditions. The patterns differ in the size of the separated-flow recirculation regions: the pattern A is characterized by recirculation region (bubble) of separated flow quite small and located immediately under the nozzle centerbody. The heat/mass transfer distribution on the impingement wall is bell-shaped, with the maximum in the central stagnation point. On the other hand, the flowfield of the pattern B exhibits a large recirculation region of the separated flow, reaching up to the impingement wall, on which there is a stagnation circle. The entire space between the inner lip of the nozzle exit and the stagnation circle is filled with the recirculating fluid. The heat/mass transfer rate distributions reach the maximum on the stagnation circle. The bistability and hysteresis were found only at small Reynolds numbers, and at small annular nozzle slot widths. An extension of the nozzle centerbody promotes the tendency to the hysteretic behavior. An enhancement effect for the area-average heat/mass transfer coefficients for the pattern B over A has been quantified. The highest achieved increase was quite significant 32%.

Second-moment closure simulation of flow and heat transfer in a gas-droplets turbulent impinging jet

The numerical model for description of flow dynamics and heat transfer in an impinging axisymmetric gas-droplets jet is presented. The Eulerian model uses for computations of the impinging gas-droplets jet. In this paper the two-phase turbulent jet is numerically predicted by the set of axisymmetic Reynolds averaged Navier–Stokes equations. The flow structure and heat transfer in the gas-droplets impinging spray with low mass fraction of droplets (ML1≤1%) is studied numerically. Gas phase turbulence is modeled with the use of Reynolds stress transport model for two-phase flow. Droplets addition causes a significant increase in heat transfer intensity (almost twice) in comparison with a single-phase impinging air jet in the stagnation zone. In the region of wall jet the heat transfer intensity in the two-phase impinging jet decreases and approaches the value of a single-phase impinging jet.

Experimental investigation for enhancement of heat transfer from cooling of electronic components by circular air jet impingement

An experimental investigation is carried out to study the enhancement of heat transfer from surface of the electronic components by impingement of a circular air jet. Local and stagnation Nusselt number on the impinged surface of the electronic components are presented for different nozzle configurations. Reynolds number, based on nozzle diameter (d) is varied between 5,500 and 28,500 and nozzle-to-electronic component spacing from 2 to 10 nozzle diameters. The measured data were correlated into a simple equation and compared to the predictions of several other correlations proposed by other researchers. The heat transfer mechanisms involved in the enhanced performance are discussed. The study provides a lot of useful information for the application of impinging jet heat transfer in electronic industry.

Numerical Study of Impinging Cooling of a Porous Block Under a Straightening or Non-Straightening Jet Flow

This work numerically investigated the thermal characteristics of the aluminum-foam block under the confined impinging air jet flow with straightening (model A) or non-straightening (model B) jet flow models. The dimensions of the aluminum-foam block were 60 mm in length (L) and 60 mm in width (W). Two kinds of aluminum foams with different porosities (ϵ)/pore densities (PPI) were applied: ϵ = 0.93/20 PPI and ϵ = 0.97/5 PPI. Two computational domains were employed to build the jet flow models: (1) model A (excluding the jet-nozzle region) whose jet velocity profile was uniform; and (2) mode B (including a sufficiently long jet-nozzle region) whose velocity profile was uniform at the inlet of the jet nozzle and changed at the exit of the jet nozzle via numerical results. For model B jet flow, a quasi-parabolic velocity profile would be formed at the nozzle exit when the w j /L was small enough; however, a saddle-shape velocity profile gradually appeared when the w j /L increased. Relevant parameters were the jet Reynolds number (Re = 250 ∼ 1000), the relative jet nozzle width (w j /L = 0.125 ∼ 1), the relative impinging distances (C/L = 0 ∼ 0.5), and the relative porous block heights (H/L = 0.125 ∼ 1). Generally, a quasi-parabolic velocity profile will have a bigger momentum to make more fluid to penetrate the porous block and reach near the heated wall, promoting the total heat transfer. On the contrary, the saddle-shape velocity profile leads much more fluid to bypass the porous block, reducing the total heat transfer. The results indicate that the present results agreed with those of others studies reasonably. At C/L = 0, the 0.93-porosity Al-foam block generally had a bigger than the 0.97-porosity one; but the contrary result was found at C/L = 0.5. Besides, increasing C/L and w j /L would decrease the . The sharply fell first and then changed smoothly as increasing w j /L. Moreover, model A was generally better than model B in heat transfer, particularly for those cases with bigger w j /L. However, at C/L = 0.5 and small-w j /L condition, the of model B was higher than that of model A. Furthermore, the generally declined with increasing the H/L, but might first raise and then drop when increasing the H/L when the C/L or w j /L is bigger. Finally, the correlations of (H/L) OPT having the maximum and the corresponding parametric ranges were provided.

Direct and Inverse Methods for an Air Jet Impingement

The reconstruction of the distribution of the heat transfer coefficient (HTC) for the air jet impinging of a solid object is discussed. After an optimal turbulence model is selected, computational fluid dynamics is used to obtain detailed information about the HTC produced by a jet and to validate its invariability in time. Two inverse models of the temperature field in the solid impinged by the jet are derived as a linear combination of known, constant, or linear auxiliary fields. Simultaneous minimization of the discrepancies between the temperatures measured by an infrared camera and both inverse models produces the distribution of the HTC for various jets configurations.

Use of Wigner-Ville transformations for fluid particles in laser Doppler flow accelerometry

Flow acceleration with Lagrangian description is crucial to understanding particle movements in turbulent jet flows or dissipation statistics in isotropic turbulence. Laser Doppler anemometry is regarded as a suitable experimental tool for measuring flow acceleration, because scattering particles generate trajectories in the measurement volume, which process gives rise to flow acceleration at a fixed measuring point with the Lagrangian description. The most useful algorithm for processing Doppler signals is either the quadrature demodulation technique (QDT) or the iterative parametric method (alternatively, the minimization of least squares, LSM) as in the literature. In the present study, another algorithm using the Wigner-Ville transform (W-V) is introduced to give more accurate estimation of flow acceleration than the QDT or the LSM. Five signal-processing algorithms, including the QDT, the LSM, the MC (maximization of correlation), and the W-V, were compared with each other in experiments with an impinging air jet flow with a cylindrical rod and a round free-air jet flow. Mean flow acceleration distribution in the streamwise direction was mainly investigated. Processing speeds for the above-mentioned signal-processing algorithms were checked to find the best algorithm, which has best performance with short processing time. Although QDT was found to be an accurate algorithm with short processing time, it has limited applications to flows with large acceleration and high SNR. The MC was also found to be a good algorithm with moderate processing speed, which can be useful in flows with low SNR because the MC is an iterative parametric method. The W-V gave the most accurate values for flow acceleration; however, the processing time for this method was the slowest among the signal-processing algorithms.

Removal Rates of Explosive Particles From a Surface by Impingement of a Gas Jet

The rate of particle removal from a surface by air jet impingement has been evaluated for 3 different types of trace explosives. Samples of research development explosive (cyclotrimethylenetrinitramine), trinitrotoluene, and C-4 were each transferred to glass surfaces and then subjected to a short burst of air from a jet with varying diameter, standoff distance, and backpressure to achieve a range of shear stresses at the surface. TNT was observed to be easiest to remove, while C-4 required the greatest shear force to resuspend. An analytical model has been developed to predict removal of spherical particles as a function of particle diameter and nondimensionalized downstream distance from a gas jet. This model was fitted to experimental data from the removal of ceramic microspheres of various sizes. The removal rate of these ceramic microspheres was observed to be much greater than that of the 3 types of explosive particles, despite the particles’ similar sizes. Copyright 2012 American Association for Ae...

Experiment-based thermal model for permeable clothing systems under hot air jet impingement conditions

An experiment-based, two-medium thermal model was developed to evaluate the thermal response and protective performance of permeable clothing systems under hot air jet impingement conditions. The clothing systems consisted of single- or double permeable fabric(s) and air volume between the fabrics and an enclosure wall (solid surface with an embedded heat sink of constant-temperature) to mimic human skin. Two types of permeable fabrics were considered for this study: (i) NOMEX III A as a thermal protective fabric and (ii) Cotton/Spandex as a commodity fabric. The predicted results from the model were well compared with the experimental results from a previous work. A parametric study was conducted using various jet impingement conditions and design variables of the protective clothing such as the number of fabric layers, air leakage rate, air gap thickness, and fabric type. It was concluded from the parametric study that the permeable clothing with multiple layers of fabric, less air leakage, and thicker air gap had better thermal protective performance under hot air jet impingement conditions.

High-resolution hot-film measurement of surface heat flux to an impinging jet

To investigate the complex coupling between surface heat transfer and local fluid velocity in convective heat transfer, advanced techniques are required to measure the surface heat flux at high spatial and temporal resolution. Several established flow velocity techniques such as laser Doppler anemometry, particle image velocimetry and hot wire anemometry can measure fluid velocities at high spatial resolution (µm) and have a high-frequency response (up to 100 kHz) characteristic. Equivalent advanced surface heat transfer measurement techniques, however, are not available; even the latest advances in high speed thermal imaging do not offer equivalent data capture rates. The current research presents a method of measuring point surface heat flux with a hot film that is flush mounted on a heated flat surface. The film works in conjunction with a constant temperature anemometer which has a bandwidth of 100 kHz. The bandwidth of this technique therefore is likely to be in excess of more established surface heat flux measurement techniques. Although the frequency response of the sensor is not reported here, it is expected to be significantly less than 100 kHz due to its physical size and capacitance. To demonstrate the efficacy of the technique, a cooling impinging air jet is directed at the heated surface, and the power required to maintain the hot-film temperature is related to the local heat flux to the fluid air flow. The technique is validated experimentally using a more established surface heat flux measurement technique. The thermal performance of the sensor is also investigated numerically. It has been shown that, with some limitations, the measurement technique accurately measures the surface heat transfer to an impinging air jet with improved spatial resolution for a wide range of experimental parameters.

New Use Heat Transfer Theories for the Design of Heat Setting Machines for Precise Post-Treatment of Dyed Fabrics

Fabrics are needed further treatment after dyeing to restore their original mechanical properties by suitable drying/shrinkage process because of wetted and elongated fabrics cannot be used for clothes making. Heating up the dyed fabrics at suitable temperature can restore their original shapes and geometries by releasing the internal stress introduced by dyeing process. Thus, heat setting is a commonly used post-treatment process to stabilize fabric geometrical dimensions and prevent further shrinkage. Hot air jet impingement [1] and moist heat are conventional drying methods for different applications. Despite the well establishments of these drying technologies, most of the applications are for materials like clay and paper, and few on the study of textile materials. In fact that most of the developed heat setting machines used in textile industry are only designed by empirical models and lack of theoretical bases. This situation will obstruct further improvement of the drying technology. In this paper, a theoretical basis heat transfer model is developed for a precise description of a heated air flowing process for heat setting machine design. In the machine design, a better airflow circulation strategy for an efficient drying is addressed. Equations for heat and mass transfer in moist porous materials and theories on thermo- and fluid-dynamics are used to support the machine design. Outcomes from the research are to develop a heat transfer model that provides more precise and effective calculation for heat setting machine design that unavailable from the developed machine prototypes.

Effect of Orifice Shape on Flow Behavior and Impingement Heat Transfer

Impingement jets are widely used in industries because they provide a high heat transfer coefficient near the stagnation region. However, few methods exist for controlling impingement heat transfer. Recently, a peculiar diffusion process called for three-dimensional free jet has begun to attract attention, and there is a novel possibility of control diffusion and mixture process using this phenomenon. In this paper, we report on the effect of non-circular polygonal orifice shapes on impingement heat transfer. In addition, we demonstrate axis-switching phenomenon by using flow visualization with hydrogen bubbles. Orifice configurations are the regular polygons with 3 to 6 sides. Heat transfer experiments covered the distance between the orifice-to-target plate is 4 to 8 and Reynolds number is 5 × 10 4 and the heat flux is 600 W/m 2 . The flow was visualized in Reynolds number 1,500. For a free jet emerging from a regular polygonal orifice, the location of axis-switching phenomenon shifts toward the orifice exit as the number of sides on the orifice is increased. The iso-Nusselt number profile tends to take the shape of a concentric circle farther upstream. However, with a decrease in the number of sides of the orifice, the iso-Nusselt number profile after axis switching remains downstream. ing the flow using hydrogen bubbles. We also investigated the heat transfer characteristics of an impinging air jet from a non-circular orifice shapes including triangle, square, penta- gon, and hexagon. Moreover, we examined the possibility of controlling impinging heat transfer by changing the orifice configuration.

Enhancement of turbulent heat transfer during interaction of an impinging axisymmetric mist jet with a target

The flow structure and heat transfer of a mist jet with a low mass concentration of droplets (within 1%) impinging onto a flat surface aligned normal to the jet are studied numerically. The mathematical model is based on solving a system of Reynolds-averaged Navier-Stokes equations for a two-phase flow with the kinetic equation of the probability density function for coordinates, velocity, and temperature of particles. Addition of droplets is demonstrated to enhance heat transfer substantially, as compared with an impinging single-phase air jet in the region directly adjacent to the stagnation point of the jet.

A numerical study of turbulent opposed impinging jets issuing from triangular nozzles with different geometries

In present research, two turbulent opposed impinging air jets issuing from triangular nozzles with fixed and variable exit velocity ratios and different nozzle-to-nozzle distances have been studied numerically and then compared with rectangular and circular nozzles. The finite volume method has been applied for solving mass and momentum equations. The turbulence model being used here is k-e RNG. Distributions of pressure, turbulence, kinetic energy and its dissipation rate in various regions especially on the impingement regions have been obtained with high accuracy. Study of the nozzle geometries has shown the advantage of triangular nozzles over other geometries. First, the triangle’s base in nozzle geometry has an important role in our study case which, mixing two flows and regions with high turbulence intensity, directly depends on it. Second, our results show that circular and rectangular nozzles have less efficiency than triangular nozzles in mixing applications. Third and last, it was found that the radial jet being created by opposed jets has some similarities to free jets. In this investigation, air in standard atmospheric pressure has been applied as working fluid.

Heat Transfer Characteristic Analysis of Confined Impinging Jet Cooling System with Micro-Scale Round Nozzles

In order to investigate the heat transfer of confined impinging jet with tiny size round nozzle, a bakelite laminate was used as the heat transfer surface of simulated chip. The thermocouples were mounted symmetrically along the diagonal of the laminate to measure the temperature distribution of the surface. The parameters such as Reynolds number (Re) and ratio of height-to-diameter were changed to investigate the radial distribution of Nu and the characteristics of heat transfer in stagnant section. The results show that hear transfer coefficient at stagnation point is maximal. It is decreased with the increases of the radial jet distance, but increased with Re and impinging height. Moreover, the effect of single-nozzle type is stronger than that of multi-nozzle type in the cases of same air flow. These studies will give a way for the application of air jet impingement in the electronics chip cooling.

Modeling Wall Film Formation and Breakup Using an Integrated Interface-Tracking/Discrete-Phase Approach

We propose a computationally tractable model for film formation and breakup based on data from experiments and direct numerical simulations. This work is a natural continuation of previous studies where primary atomization was modeled based on local flow information from a relatively low-resolution tracking of the liquid interface [Arienti and Soteriou, 2007, “Dynamics of Pulsed Jet in Crossflow,” ASME Paper No. GT2007-27816]. The submodels for film formation proposed here are supported by direct numerical simulations obtained with the refined level set grid method [Herrmann, 2008, “A Balanced Force Refined Level Set Grid Method for Two-Phase Flows on Unstructured Flow Solver Grids,” J. Comput. Phys., 227, pp. 2674–2706]. The overall approach is validated by a carefully designed experiment [Shedd et. al., 2009, “Liquid Jet Breakup by an Impinging Air Jet,” Forty-Seventh AIAA Aerospace Sciences Meeting. Paper No. AIAA-2009-0998], where the liquid jet is crossflow-atomized in a rectangular channel so that a film forms on the wall opposite to the injection orifice. The film eventually breaks up at the downstream exit of the channel. Comparisons with phase Doppler particle analyzer data and with nonintrusive film thickness point measurements complete this study.