Flow characteristics of a laminar swirling impinging jet: A numerical study

The primitive Navier-Stokes equations were solved to predict the flow field induced by a partially confined swirling laminar jet impinging normally on a flat surface. A study is made of the influence of the jet Reynolds number, nozzle-to-plate spacing, axial and swirl velocity profiles at nozzle exit plane, size of the confining and impingement plate and uniform suction applied at the plate. An interesting feature of the flow configuration is the predicted development of a recirculation bubble in the stagnation region which influences the heat and mass transfer characteristics. Axial and swirl velocity profiles at entry have dominant influence on the development of the flow field.     

DRYING CHARACTERISTICS OF SLOT JET REATTACHMENT NOZZLE AND COMPARISON WITH A SLOT JET NOZZLE

ABSTRACT

Slot Jet Reattachment (SJR) nozzle is an extension of the Radial Jet Reattachment (RJR) concept used to provide high heat and mass transfer while minimizing flow exerted forces on the reattachment surface. The SJR is a slot jet nozzle with a bottom plate attached to it, which is machined to direct impinging flow at different angles to the surface. The drying characteristics of the SJR nozzle with four exit angles on a paper sample were studied for three Reynolds numbers, three temperatures and two initial moisture contents. Dry air was used as the jet fluid. Correlations to predict drying rates and moisture content for the SJR nozzle as a function of exit angle, temperature, Reynolds number and drying time, for a given initial moisture content, were developed. A comparison of the drying characteristics and net forces of the slot jet and SJR nozzles was also performed under the same flow power and surface peak pressure.     

DRYING CHARACTERISTICS OF SLOT JET REATTACHMENT NOZZLE AND COMPARISON WITH A SLOT JET NOZZLE

Slot Jet Reattachment (SJR) nozzle is an extension of the Radial Jet Reattachment (RJR) concept used to provide high heat and mass transfer while minimizing flow exerted forces on the reattachment surface. The SJR is a slot jet nozzle with a bottom plate attached to it, which is machined to direct impinging flow at different angles to the surface. The drying characteristics of the SJR nozzle with four exit angles on a paper sample were studied for three Reynolds numbers, three temperatures and two initial moisture contents. Dry air was used as the jet fluid. Correlations to predict drying rates and moisture content for the SJR nozzle as a function of exit angle, temperature, Reynolds number and drying time, for a given initial moisture content, were developed. A comparison of the drying characteristics and net forces of the slot jet and SJR nozzles was also performed under the same flow power and surface peak pressure.     

加齿被动控制自由湍射流多尺度涡结构的实验研究

1 INTRODUCTION Noncircular jets were identified as an efficient tech- nique of passive control that allows significant im- provements of performance on various practical sys- tems at a relatively low cost because noncircular jets rely solely on the change in the geometry of the noz- zle. In both aerospace industry and chemical industry noncircular jets are widely applied to promote frag- mentation of the large eddy structures in the flow, enhance mixing of various fluid components, speed u…     

小间距两喷嘴对置撞击流流场的数值模拟与实验研究

运用热线风速仪和CFD软件对小喷嘴间距下两喷嘴对置撞击流时均流场进行了实验研究和数值模拟,并和文献中的实验结果和近似解析式进行了比较。研究结果表明：由于边界层存在,单股喷嘴出口速度分布为“礼帽”形状分布;在L<2D(L为喷嘴间距,D为喷嘴直径)时,喷嘴出口速度剖面出现中间低、两边高的“双峰”形状, L=2D时,“双峰”形状消失。随着喷嘴间距的增大,相同气速比导致的撞击面驻点的偏移量增大。相同气速比下,喷嘴出口为“礼帽”分布时驻点的偏移量比均匀分布时大。文献中的撞击流流场的近似解析式对喷嘴出口速度分布为均匀分布有很好的精度,当喷嘴出口速度为“礼帽”分布时,文献中近似解析式的预报精度变差。     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

WALL JET DEVELOPMENT IN A TURBULENT RECIRCULATING CAVITY FLOW

A turbulent wall jet within a confining recirculating cavity was examined using flow visualization, particle image velocimetry (PIV), and laser Doppler velocimetry (LDV). The PIV technique was used macroscopically for flow visualization as well as at a much finer resolution to determine detailed velocities within the developing wall jet. Comparison data collected using laser Doppler velocimetry (LDV) was used for validation purposes. The wall jet was well characterized using conventional unbounded wall jet parameters. Wall jet velocity profiles collapsed onto a single profile with the application of the appropriate parameters, indicating some degree of similarity after the initial jet region and up to the mid wall distance. Distances past this region were characterized by a rapid decay in the jet caused by the adverse pressure gradient created by the bounding wall. It was found that the wall jet growth rate within the recirculating cavity was greater than that of an unbounded wall jet in a stagnant fluid. This phenomenon is attributed to the recirculating flow, confining boundaries, and the initial jet exit velocity conditions.     

WALL JET DEVELOPMENT IN A TURBULENT RECIRCULATING CAVITY FLOW

A turbulent wall jet within a confining recirculating cavity was examined using flow visualization, particle image velocimetry (PIV), and laser Doppler velocimetry (LDV). The PIV technique was used macroscopically for flow visualization as well as at a much finer resolution to determine detailed velocities within the developing wall jet. Comparison data collected using laser Doppler velocimetry (LDV) was used for validation purposes. The wall jet was well characterized using conventional unbounded wall jet parameters. Wall jet velocity profiles collapsed onto a single profile with the application of the appropriate parameters, indicating some degree of similarity after the initial jet region and up to the mid wall distance. Distances past this region were characterized by a rapid decay in the jet caused by the adverse pressure gradient created by the bounding wall. It was found that the wall jet growth rate within the recirculating cavity was greater than that of an unbounded wall jet in a stagnant fluid. This phenomenon is attributed to the recirculating flow, confining boundaries, and the initial jet exit velocity conditions.     

圆排波瓣喷管引射器高效掺混流场数值计算

采用贴体曲线坐标、SIMPLEC算法和完全压力修正,在非正交性非常大的结构型同位网格上求解了圆排波瓣喷管引射器内流场的不可压Navier-Stokes方程,湍流采用标准k-ε涡黏模型.计算得到了引射器中热混合效率沿混合管轴向的变化规律和流向涡的演变规律.混合管中的流向涡阵列大大强化了主次流的掺混.但该型引射器的总体热混合效率不高,实验的3种主流温度下最大热混合效率为0.874.将混合管出口截面速度分布和温度分布的数值模拟结果与实验值比较,两者比较一致;模拟结果与混合管沿轴向的静压系数分布规律实验值也基本相同.     

Heat transfer from a turbulent hot air jet impinging normally on a flat plate

The heat flux distribution on a flat plate resulting from a normally impinging free jet of hot air was measured for a variety of experimental conditions. A method has been developed for calculating local heat flux in terms of nozzle exit parameters, nozzle to plate spacing and surface temperature distribution. The applicability of this method has been thoroughly checked by comparing computed and measured heat flux distributions for nozzle to plate spacings of 3.41 to 31.05 nozzle diameters; nozzle exit temperatures of up to 1149°F; and nozzle exit Mach numbers of up to 0.837.     

不凝气体对蒸汽射流冷凝的影响

对含不凝气体蒸汽射流在冷水中直接接触冷凝现象进行了实验研究,通过测量流场中的温度分布确定汽羽长度,进而推导其传热系数。实验使用直径为1.6 mm的圆形喷嘴,出口混合气体质量流量密度在100~330 kg·m^-2·s^-1之间,不凝气体的含量在0~15%之间,冷水温度在300~340 K之间。实验结果表明:不凝气体的加入,使喷嘴出口附近的温度下降减慢;汽羽长度随不凝气体含量的增加而变长,其受喷嘴出口质流密度和过冷度的影响规律与纯蒸汽射流一致;冷凝传热系数在0.7~2 MW·m^-2·K^-1之间,随过冷度的增大和不凝气体含量的增加而减小,受气体流量的影响较小。对实验数据进行拟合,获得了汽羽长度的关联式,并由此得到了冷凝传热系数关联式。     

CFD simulation of flow pattern and plume dimensions in submerged condensation and reactive gas jets into a liquid bath

Submerged gas jets into a liquid bath are widely used in metal processing and thermal processes. These systems are classified as (a) condensation jet and (b) reaction jet systems. This paper presents the CFD simulation of both the types of jets. The CFD model considers phase change, gas-liquid and gas-gas reactions and the accompanied rates of mass transfer. Mass transfer coefficient was estimated using small eddy model where the value of mass transfer coefficient is calculated based on the local values of turbulent kinetic energy (k) and the dissipation rate (ε). A good agreement with the available experimental data of plume length validates the CFD model. The CFD simulations have also been compared with the available experimental data on velocity and temperature profiles which shows excellent agreement. A comparison between the condensation and the reaction jets has been presented in terms of plume dimensions, flow and temperature patterns. The relative predictions of the present model and the rational correlations have been presented for the estimation of plume length for both the types of jet systems.     

矩形喷嘴射流近喷口流场的大涡模拟

采用Smagorinsky提出的亚网格应力模型对三维矩形喷嘴射流近喷口处流场进行了大涡模拟,着重研究了较高Reynolds数下小宽高比矩形射流近喷口处流场的流速、脉动量等特征量的分布特性,并与实验结果进行了对比.气相运动控制方程的离散采用有限容积法,时间步进格式采用二阶精度的隐式Crank-Nicolson差分格式.模拟结果与实验结果具有较好的一致性.     

Heat transfer from a grid jet in a large fluidized bed

Heat transfer from a vertical grid jet within a 2 ft diameter and 4 ft deep fluidized bed of cracking catalyst was studied. The test nozzle diameter was varied from ¼ to 1 in. and the nozzle velocity from 50 to 250 ft /sec which is within the range of industrial practice. The axial temperature data have been related to a Froude, a Reynolds and a Nozzle number: In (δT/δT_o) = –58.1 Fr^?0.562 No^1.08 Re^?0.112 A simple jet quenching model yielded heat transfer coefficients between the fluid bed and grid jet which ranged from 300 to 1200 Btu/ft² hr.^o F.     

HEAT TRANSFER IN A LAMINAR CYLINDRICAL WALL JET WITH UNIFORM SURFACE HEAT FLUX

The steady state flow and heat transfer characteristics of a laminar cylindrical wall jet are obtained for uniform surface heat flux conditions. Local nonsimilarity solutions as well as series solutions are presented for the velocity and thermal fields. Numerical results are given for the wall shear stress, surface temperature variation and temperature field for a Prandtl number of 0.73.     

Modeling the mixing of a gas-liquid spray jet injected in a gas-solid fluidized bed: The effect of the draft tube

In several important industrial processes such as fluid coking, fluid catalytic cracking and gas-phase polymerization, a gas-liquid spray jet is injected into a fluidized bed. Recent findings [Chan et al., 2004. US Patent, US 2004065590] have shown that for the fluid coking operation, locating a cylindrical draft tube near the exit of the injector can enhance the contacting efficiency of the droplets and the particles.The main objective of this paper was to model the liquid-solids mixing in the draft tube. A two-dimensional model that characterizes the radial mixing by means of an eddy diffusivity coefficient is presented. The model predictions are validated by experimentally measuring the liquid-solids mixing by a technique based on temperature measurements in the spray region. The simulation results for a commercial fluid coking nozzle confirm the potential benefits of the mixing chamber. Moreover, the model allows ascertaining the effect of some important parameters such as nozzle size, gas and liquid properties, and the air-to-liquid mass ratio to the nozzle.     

MEASUREMENT OF LAMINAR JET VELOCITY DISTRIBUTIONS IN LIQUID-LIQUID SYSTEMS USING FLASH PHOTOLYSIS

The feasibility of using Rash photolysis to measure jet velocity profiles was investigated in the region near the nozzle exit for xylene jets injected vertically into a mutually saturated stationary immiscible water phase. Dye trace measurements obtained from high speed motion pictures were optically corrected, and velocity profiles were obtained by a streamline generation mass balance technique. The standard deviation in axial velocities was less than 8%. Profiles close to the nozzle exit showed good agreement with the fully developed parabolic profile within the nozzle, and downstream profiles were similar to those predicted by existing approximate theories.     

LOCAL OVERALL GAS-LIQUID VOLUMETRIC MASS TRANSFER COEFFICIENTS IN A GAS-LIQUID TWO-PHASE REVERSED FLOW JET LOOP REACTOR

Local overall gas-liquid volumetric mass transfer coefficient profiles at the specified point were experimentally investigated in a gas-liquid two-phase reversed flow jet loop reactor with Newtonian and non-Newtonian systems. It was observed that the local overall gas-liquid volumetric mass transfer coefficient profiles of this reactor with Newtonian and non-Newtonian systems increase with increase in gas jet flow rates and liquid jet flow rates, and with decrease in nozzle diameter and CMC concentration.     

THE EFFECTS OF THE SOLUBILITY LAW ON MASS TRANSFER IN ANNULAR LIQUID JETS

Underpressurized, annular liquid jets have been studied under steady and transient conditions in order to assess the effects of Henry's and Sievert's solubility laws on mass transfer. It is shown that, in general, Henry's solubility law predicts a higher, steady state mass absorption rate than Sievert's law. The rate at which the under-pressurized, annular liquid jet tends towards an equilibrium, non-pressurized configuration is higher for Henry's than for Sievert's solubility law. Both this rate and the steady state mass absorption rate increase as the Weber number, the ratio of the solubility at the inner interface to that at the outer one, the product of the gas constant times the temperature and the solubility of the gases surrounding the jet, the ratio of the product of the pressure of the gases surrounding the annular jet times the annular jet's mean radius at the nozzle exit to the liquid surface tension, and the nozzle exit angle are increased, and as the Froude and mass Peclet numbers, the annular jet's thickness-to-mean radius ratio at the nozzle exit and the initial pressure of the gases enclosed by the annular jet are decreased.