Fluid flow and heat transfer characteristics of cone orifice jet (effects of cone angle)

The use of a jet from an orifice nozzle with a saddle‐backed‐shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of its larger centerline velocity. However, it requires more power to operate than a common nozzle because of its higher flow resistance. We therefore initially considered the use of a cone orifice nozzle to obtain better heat transfer performance as well as to decrease the flow resistance. We examined the effects of the cone angle α on the cone orifice free jet flow and heat transfer characteristics of the impinging jet. We compared two nozzles: a pipe nozzle and a quadrant nozzle. The first one provides a velocity profile of a fully developed turbulent pipe flow, and the second has a uniform velocity profile at the nozzle exit. We observed a significant enhancement of the heat transfer characteristics of the cone orifice jets at Re=1.5×10⁴. Using the cone orifice impinging jets enhanced the heat transfer rates as compared to the quadrant jet, even when the jets were supplied with the same operational power as the pipe jet. For instance, a maximum enhancement up to approximately 22% at r/d_o?0.5 is observed for α=15^°. In addition, an increase of approximately 7% is attained as compared to when the pipe jet was used. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20243     

Experimental investigation of the flow and heat transfer of an impinging jet under acoustic excitation

Classic and high speed particle image velocimetry and infrared thermography are used to investigate the behavior of a round jet impinging on a flat plate for a Reynolds number 28,000, for orifice-to-plate distances of 3 or 5 nozzle diameters and for two different nozzles, a contraction and a long tube. The contraction nozzle reveals a different heat transfer distribution on the impinging plate compared to the long tube case. The jet can be excited by a loudspeaker at Strouhal numbers 0.26, 0.51 and 0.79. This acoustic forcing changes the jet structure, modifying annular vortex rings in the shear layer of the jet and increasing the turbulent values. The heat transfer is therefore modified, resulting in an increase of the Nusselt number near the jet axis and an alleviation or a shift of the secondary peak.     

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.     

Streamwise distribution of the recovery factor and the local heat transfer coefficient to an impinging circular air jet

An investigation of the radial distribution of the recovery factor and the local heat transfer for an axisymmetric impinging air jet formed by a smooth nozzle is described. The recovery factor is dependent on the jet nozzle to impingement plate spacing, but is independent of jet Reynolds number. The maximum stagnation region heat transfer occurs at a nozzle to impingement plate spacing of about eight jet diameters. A correlation is obtained for the average heat transfer from the surface.     

Flow characteristics and heat transfer performances of a semi-confined impinging array of jets: effect of nozzle geometry

The flow and heat transfer characteristics of confined jet array impingement with crossflow is investigated. Discrete impingement pressure measurements are used to obtain the jet orifice discharge flow coefficient. Digital particle image velocimetry (DPIV) and flow visualization are used to determine the flow characteristics. Two thermal boundary conditions at the impinging surface are presented: an isothermal surface, and a uniform heat flux, where thermocouple and thermochromic liquid crystal methods were used, respectively, to determine the local heat transfer coefficient. Two nozzle geometries are studied, circular and cusped ellipse. Based on the interaction with the jet impingement at the surface, the crossflow is shown to influence the heat transfer results. The two thermal boundary conditions differ in overall heat transfer correlation with the jet Reynolds number. Detailed velocity data show that the flow development from the cusped ellipse nozzle affects the wall region flow more than the circular nozzle, as influenced by the crossflow interactions. The overall heat transfer for the uniform heat flux boundary condition is found to increase for the cusped ellipse orifice.     

旋进射流冲击平板的传热实验研究

对旋进射流冲击平板时的传热进行了实验研究。通过在圆筒套管内设置一块孔板构成旋进射流喷嘴,得到了持续稳定的旋进射流。对旋进射流的流动特性作了研究,给出了旋进射流的频率与尺寸、Re的关系。用两种不同孔径的旋进射流冲击一块加热平板,并与普通的射流冲击传热作对比。结果表明,由于旋进射流与流体混合作用加剧而大大地降低了流速,使得强化传热的效果减弱,这种趋势在驻点附近尤为明显。     

Heat transfer enhancement of round impinging jet by orifice nozzle (Effects of contraction area ratio)

The effects of the nozzle contraction ratio on the flow and heat transfer characteristics of an orifice impinging jet were investigated in this experiment. The nozzle diameter was d_o=10.0 mm=const., and the contraction area ratio CR=(d_o/d_i)², where d_i is the inner pipe diameter was varied from CR=1.00 to 0.11 and the nozzle‐plate distance was varied from H/d_o=2.0 to 5.0. The nozzle Reynolds number was Re=1.5×10⁴=const. The flow characteristics were clarified by measuring the pressure and velocity distributions on the plate and flow visualization. The Nusselt number obtained from measuring the temperature distribution on the plate of an orifice impinging jet with a CR of 0.11 and 0.69 were respectively larger by 19% and 9% than those of a pipe impinging jet (CR=1.00), because the centerline velocity of the orifice jet was larger than that of the pipe jet. Under the same operation power, an orifice impinging jet has improved heat transfer characteristics. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20225     

A comparison of mass transfer between a plate and submerged conventional and multichannel impinging jets

This study presents a work on the comparison of mass transfer distribution on a plate subjected to impingement by a conventional impinging jet (CIJ) and multichannel conventional impinging jet (MCIJ). The local mass transfer coefficients were measured by the electrochemical limiting diffusion current technique (ELDCT). The jet Reynolds number varied in the range of 10,950–50,800 and jet-to-plate distance 2d–10d, where d is the nozzle diameter. The local mass transfer values were recorded beginning from the impingement point to a radial distance of approximately 15d. The distribution of the local mass transfer coefficients (LMTC) was compared for both jet systems.     

The role of jet inlet geometry in impinging jet heat transfer,modeling and experiments

Effects of jet inlet geometry and aspect ratio on local and average heat transfer characteristics of totally nine confined impinging jets have been investigated experimentally using thermochromic liquid crystals and numerically by using a 3-D low Reynolds number k–? model. Experimental study by using liquid crystals for temperature measurement was conducted for three different jet exit geometries (circular, elliptic, rectangular). In addition, simulations were performed at the same mass flow rate for totally nine jet exit geometries including circular, elliptic and rectangular jets with different aspect ratios for dimensionless jet to plate distances 2, 6, and 12.As the aspect ratio of equal cross-sectional area elliptic and rectangular jets increases, heat transfer enhancement in the stagnation region was obtained. As a result higher aspect ratio jets can be used as a passive enhancement technique for localized heating or cooling especially at small jet to plate distances. Wall jet region comprises very large portion of the impinging plate under study and generally lower heat transfer rates were attained for higher aspect ratio jets in this region especially at small jet to plate distances. Therefore as the aspect ratio increases, lower average heat transfer rates were acquired. The effect of aspect ratio on local and average heat transfer decreases with increasing jet to plate distance. Even though the mass flow rate is the same, heat transfer rate of rectangular jets were reduced with increasing the cross-sectional area. With increasing jet to plate distance very similar heat transfer characteristics were observed along the major and minor axis directions.     

Experimental Investigation on Thermal Protection of High Temperature and High Velocity Jet Impinging a Cross-Shaped plate

Abstract

This paper investigated the heat transfer of high temperature jet impinging a cross-shaped plate. An experimental mockup was designed and used to produce high temperature (～500?°C) impinging gas jet with stable high speed (～56?m/s) at the nozzle exit. A one-dimensional theoretical model was proposed to predict the temperature increment of the back-side surface(s) that opposed to the impinging surface, by simplifying and assuming the heat transfer process can be governed by convection. Three materials were selected due to its potential high heat transfer resistance, speedy installation and long duration under severe impinging conditions. For each material, four types of thermal insulation structures were designed and subjected for testing. The following conclusions from comparison of the results of the model predictions with experimental data can be drawn. The proposed simplified one-dimensional model can be used to predict heat transfer at the back-side under high temperature and high-speed impinging. The optimal case among the 12 thermal protecting designs was obtained, majorly due to low back temperature and less material consumption.     

A Study of Under-expanded Moist Air Jet Impinging on a Flat Plate

When a gas expands through a convergent nozzle in which the ratio of the ambient to the stagnation pressures is higher than that of the critical one, the issuing jet from the nozzle is under-expanded. If a flat plate is placed normal to the jet at a certain distance from the nozzle, a detached shock wave is formed at a region between the nozzle exit and the plate. In general, supersonic moist air jet technologies with non-equilibrium condensation are very often applied to industrial manufacturing processes. In spite of the importance in major characteristics of the supersonic moist air jets impinging to a solid body, its qualitative characteristics are not known satisfactorily. In the present study, the effect of the non-equilibrium condensation on the under-expanded air jet impinging on a vertical flat plate is investigated numerically in the case with non-equilibrium condensation, frequency of oscillation for the flow field becomes larger than that without the non-equilibrium condensation, and amplitudes of static pressure become small compared with those of dry air. Furthermore, the numerical results are compared with experimental ones.     

Jet Emerging from a Nozzle and Impinging on a Conical Cavity: Influence of Nozzle and Cavity Geometric Configurations

A jet emerging from a nozzle with different cone angles and impinging onto a conical cavity with different depths and diameters is considered. The flow simulation is extended to include a jet emerging from a pipe and impinging onto the cavity for the comparison. The Reynolds stress turbulence model is incorporated to account for the turbulence. The control volume approach is used to discretize the governing equations of flow and heat transfer. It is found that the flow structure above the cavity differs significantly due to radial expansion of the flow emerging from the nozzle. This modifies the flow structure in the cavity, particularly for a large diameter. The influence of the nozzle cone angle on the heat transfer coefficient and the shear stress along the cavity wall is more pronounced for a large diameter cavity.     

Wall shear stress measurements and parametric analysis of impinging wall jets

This paper investigates the influence of some governing parameters on the near wall characteristics of a circular impinging jet onto a smooth flat plate. Laser Doppler anemometry (LDA) is used to characterize the mean and turbulent fields including the wall shear stress. The experiments were conducted at one nozzle-to-plate space (H/D = 2) and Reynolds number of 47,100. The work makes a parametric analysis of impinging jets based on (i) conventional parameters that include the nozzle diameter, the nozzle-to-plate distance and the bulk velocity of the jet and (ii) gross parameters like the jet momentum flux. Parametrization schemes based on conventional quantities are shown to be very sensitive to the particular choice of reference quantity, resulting in functional behaviours that can be represented through either power law or linear expressions. On the other hand, it is shown that the jet momentum flux and the kinematic viscosity suffice to determine the mean and fluctuating flow parameters, even in the initial region of wall jet development (1 < r/D < 5). With the latter choice, the streamwise variation of the maximum mean velocity and maximum Reynolds longitudinal stress are shown to decay according to power law expressions. A particular near wall parametrization scheme for the mean velocity profile that resorts to a scaling procedure based on the stream-wise evolution of the flow characterized by its maximum velocity is also presented. Higher-order moments of the velocity fluctuations are discussed.     

A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface

A numerical finite-difference approach was used to compute the steady and unsteady flow and heat transfer due to a confined two-dimensional slot jet impinging on an isothermal plate. The jet Reynolds number was varied from Re=250 to 750 for a Prandtl number of 0.7 and a fixed jet-to-plate spacing of H/W=5. The flow was found to become unsteady at a Reynolds number between 585 and 610. In the steady regime, the stagnation Nusselt number increased monotonically with Reynolds number, and the distribution of heat transfer in the wall jet region was influenced by flow separation caused by re-entrainment of the spent flow back into the jet. At a supercritical Reynolds number of 750 the flow was unsteady and the net effect in the time mean was that the area-averaged heat transfer coefficient was higher compared to what it would have been in the absence of jet unsteady effects. The unsteady jet exhibited a dominant frequency that corresponded to the formation of shear layer vortices at the jet exit. Asymmetry in the formation of the vortex sheets caused deformation or buckling of the jet that induced a low-frequency lateral jet “flapping” instability. The heat transfer responds to both effects and leads to a broadening of the cooled area.     

LES of a Turbulent Slot Impinging Jet to Predict Fluid Flow and Heat Transfer

In this article, large eddy simulation (LES) is performed for a turbulent slot jet impingement heat transfer at a Reynolds number of 13,500 and a nozzle to plate spacing of 10. Various aspects of predicting a turbulent jet impinging flow in an optimum domain size and grid resolution for LES have been assessed. Two inflow conditions, one without any fluctuations and the other with fluctuations generated by the spectral synthesizer, were tested and comparisons of various mean flow, turbulence, and heat transfer data showed that LES without any inflow fluctuations provides good agreement with the corresponding experimental and numerical results reported in the literature. Further, various important dynamical flow structures have been visualized from the instantaneous computed data. Finally, mean flow and turbulence statistics have been presented in the wall jet region close to the stagnation point, which could be useful as data for validation of RANS-based turbulence models.     

Annular Impinging Jet Controlled by Radial Synthetic Jets

This experimental study focuses on generation and control of annular impinging jets. The annular nozzle used in the investigations was designed with an active flow control system using 12 synthetic jets issuing radially from the central nozzle body. Measurements of the control effects were made on the impingement wall. The data acquisition involved wall pressure and wall mass transfer (by the naphthalene sublimation technique) using air as the working fluid. Also measured was time-mean flow velocity (by a Pitot probe) in the jet flow field. Moreover, flow visualization was carried out. Two main flow-field patterns (A and B) were identified. The patterns differ in the size of the separated-flow recirculation regions that develop attached to the nozzle central body: While pattern A is characterized by a quite small recirculation region (bubble) extending not far from the nozzle exit, pattern B exhibits a large recirculation region, reaching up to the impingement wall, on which it forms a stagnation circle. The control action modifies the flow field, resulting in changes of the corresponding heat/mass transfer distributions. The convective transfer rate on the stagnation circle can be demonstrably enhanced by 20% at a moderate nozzle-to-wall distance, equal to 0.6 of the nozzle outer diameter.     

Axisymmetric impinging jet excited by a synthetic jet system

A controlled impinging jet is a promising tool for various heat/mass transfer applications, such as drying technologies or cooling of highly loaded electronic devices or gas turbine blades. An axisymmetric air jet was excited using a system of four synthetic jets distributed around the circumference of the primary nozzle. First, the control synthetic jets were measured alone. After an adjustment, the primary axisymmetric jet was excited to the helical or bifurcating modes, and its behavior was studied experimentally including an impingement effect to the wall. For comparison purposes, a reference steady (unforced) jet from the same nozzle was also measured. The flow visualization, hot-wire anemometry, PIV, and naphthalene sublimation techniques were used. The main purpose was to investigate the influence of the actuation on the impingement heat transfer at the Reynolds numbers 1600 and 5000.The effects of the Strouhal number and nozzle-to-wall spacing on a distribution of the local heat transfer were evaluated. The most significant effects were found at the Strouhal numbers 0.14–0.32 at the ratio of the control to primary jet momentum rates only 0.24–2.4%. Under small nozzle-to-wall spacing H/D = 2, the excitation led to heat transfer increase in the stagnation area – the most prominent enhancement 40% was found at the stagnation point. Under moderate nozzle-to-wall spacing H/D = 6, the excitation made more uniform the Nusselt number distribution by means of a substantial reduction of the stagnation heat transfer rate.     

Impinging premixed butane/air circular laminar flame jet--influence of impingement plate on heat transfer characteristics

Experimental studies were performed to study the heat transfer characteristics of an impingement flame jet system consisting of a premixed butane/air circular flame jet impinging vertically upward upon a horizontal rectangular plate at laminar flow condition. There were two impingement plates manufactured with brass and stainless steel respectively used in the present study. The integrated effects of Reynolds number and equivalence ratio of the air/fuel jet, and distance between the nozzle and the plate (i.e. nozzle-to-plate distance) on heat transfer characteristics of the flame jet system had been investigated. The influence in using impingement plate with different thermal conductivities, surface emissivities and roughnesses on heat flux received by the plate was examined via comparison, which had not been reported in previous literatures. A higher resistance to heat transfer had been encountered when the stainless steel impingement plate of lower thermal conductivity was used, which led to a significantly lower heat flux at the stagnation region. However, the heat flux distribution in the wall-jet region of the plate was only slightly affected by using different impingement plates. Because of the significantly lower heat transfer, more fuel was not required to consume and existed at the stagnation region of the stainless steel impingement plate, which would be burned latter in the wall-jet region to release its chemical energy and enhance the local heat flux there.     

Parametric Study and Optimization of Staggered Inclined Impinging Jets on a Concave Surface for Heat Transfer Augmentation

The characteristics of the fluid flow and heat transfer of staggered inclined impinging jets on a concave surface have been investigated numerically using three-dimensional Reynolds-averaged Navier-Stokes analysis using the shear stress transport turbulence model. Shape optimization of the impinging jet has been performed with a weighted-average surrogate model. A constant temperature condition has been applied to the concave surface. The inclination angle of the staggered jet nozzles and the distance between the jet nozzles are chosen as the design variables, and their effects on the heat transfer performance have been evaluated. It is found that the overall heat transfer increases with the pitch of vertical jet nozzles, and the staggered inclination of jet nozzles improves the heat transfer on the concave surface. For the optimization of the impinging jet, the area-averaged Nusselt number on the concave surface is set as the objective function. Latin hypercube sampling is used to determine the training points as a design of experiment, and the surrogate model is constructed using the objective function values at the training points. Sequential quadratic programming is used to search for the optimal point from the constructed surrogate model. Through the optimization, the heat transfer performance has been improved by nearly 60% compared to the reference design.     

倾斜射流对移动平板表面紊动和传热特性的影响

采用雷诺应力湍流模型和Simplic算法对半封闭槽道内倾斜射流冲击移动平板的流动和传热特性进行了数值模拟,研究了不同射流角度和不同平板移动速度下平板近壁湍动能和板面努塞尔数的变化.结果表明：射流角度和平板运动速度对平板近壁湍动能和表面努塞尔数值分布影响显著;当入射角与平板运动方向相同时,板速的升高提高了近壁面的湍动能,但是降低了冲击区域的局部努塞尔数值;平板表面的平均努塞尔数值随板速的提高先降低后大幅升高,高速下角度对平板表面的平均传热效果影响较小;当入射角为80°,平板运动方向与入射方向相反且板速和射流速度相同时,在移动平板表面能够获得较佳的紊动和传热效果.