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.     

Enhancement of Heat and Mass Transfer with Innovative Impinging Jets

Impinging jets are widely used throughout various industries. The nozzle design used most frequently for jet impingement is the in-line jet. There have been many attempts at altering a jet's flow to increase its transport characteristics. The transport characteristics of two recently developed innovative and practical submerged impinging jets are described. Specifically, the heat and mass transfer characteristics of the Radial Jet Reattachment (RJR) nozzle and Self-Oscillating Jet Impinging Nozzle (SOJIN) are presented and cornoared lo the in-line iet nozzle. Both nozzles orovide high transport Coeficients. The RJR nozzle is for applications where'it is crucial to control the normal force on the imoinaement surface and those aoolications where space above the surface is to be minimized. The SOJIN offers an inexpensive modification to in-line nozzles in order to increase their transport properties. The SOJIN is exceptionally beneficial when standard in-line nozzles are required to work closer to the impingement surfaces than their optimal spacing.     

Enhancement of Heat and Mass Transfer with Innovative Impinging Jets

ABSTRACT

Impinging jets are widely used throughout various industries. The nozzle design used most frequently for jet impingement is the in-line jet. There have been many attempts at altering a jet's flow to increase its transport characteristics. The transport characteristics of two recently developed innovative and practical submerged impinging jets are described. Specifically, the heat and mass transfer characteristics of the Radial Jet Reattachment (RJR) nozzle and Self-Oscillating Jet Impinging Nozzle (SOJIN) are presented and cornoared lo the in-line iet nozzle. Both nozzles orovide high transport Coeficients. The RJR nozzle is for applications where'it is crucial to control the normal force on the imoinaement surface and those aoolications where space above the surface is to be minimized. The SOJIN offers an inexpensive modification to in-line nozzles in order to increase their transport properties. The SOJIN is exceptionally beneficial when standard in-line nozzles are required to work closer to the impingement surfaces than their optimal spacing.     

EFFECT OF LARGE TEMPERATURE DIFFERENCES ON LOCAL NUSSELT NUMBER UNDER TURBULENT SLOT IMPINGEMENT JET

In heating, cooling or drying applications involving large temperature differences between the jet and the target surface, it is necessary to incorporate the temperature-dependence of fluid properties on the flow and temperature fields. Despite their frequent occurrence in industrial practice, there is little research reported in the literature on this subject. It is also necessary to distinguish between heating and cooling applications since the thermo-physical properties of the fluid in the vicinity of the target surface vary in different directions for the two cases. The objective of this work is to present computational fluid dynamic model results for heat transfer under a semi-confined slot turbulent jet under thermal boundary conditions such that the temperature-dependence of the fluid properties affects the flow and thermal fields. A comparative analysis in the turbulent flow regimes is made of the standard k-ε and Reynolds stress turbulence models for constant target surface temperature. Nusselt number distributions with different definitions of Nusselt number were compared. The results show that, under large temperature differences between the jet and the target surface, the Nusselt number calculated at jet temperature shows the least spread. Results are consistent with the very limited experimental results available in the literature.     

EFFECT OF LARGE TEMPERATURE DIFFERENCES ON LOCAL NUSSELT NUMBER UNDER TURBULENT SLOT IMPINGEMENT JET

ABSTRACT

In heating, cooling or drying applications involving large temperature differences between the jet and the target surface, it is necessary to incorporate the temperature-dependence of fluid properties on the flow and temperature fields. Despite their frequent occurrence in industrial practice, there is little research reported in the literature on this subject. It is also necessary to distinguish between heating and cooling applications since the thermo-physical properties of the fluid in the vicinity of the target surface vary in different directions for the two cases. The objective of this work is to present computational fluid dynamic model results for heat transfer under a semi-confined slot turbulent jet under thermal boundary conditions such that the temperature-dependence of the fluid properties affects the flow and thermal fields. A comparative analysis in the turbulent flow regimes is made of the standard k–ε and Reynolds stress turbulence models for constant target surface temperature. Nusselt number distributions with different definitions of Nusselt number were compared. The results show that, under large temperature differences between the jet and the target surface, the Nusselt number calculated at jet temperature shows the least spread. Results are consistent with the very limited experimental results available in the literature.     

Impingement Drying of Paper

The complete drying history of paper dried under an array of multiple round jets of air was determined. The drying rate - paper moisture content relations, determined as a continuous function by on-line measurements for about 80 sheets, permitted examination of the following parameters: basis weight 20 to 50 g/m2, which covers the tissue to communication paper range; initial moisture content 1.5 to 3 kg water/kg fibre; nozzle to paper spacing of 5, 7.6 & 8.5 nozzle diameters: nozzle plate open area ratio from 1.4 to 3.1%; jet Reynolds number in the range of 450 to 11.100.

Three methods of quantifying the complete drying rate curves were tested. Use of the common assumption of drying rate linear with moisture content over the falling rate period gave inadequate representation. Secondly, the general method of Churchill for any transport process that has a transition between two asymptotic relations was applied for the fmt time to the drymg of paper. This approach gives     

Impingement Drying of Paper

ABSTRACT

The complete drying history of paper dried under an array of multiple round jets of air was determined. The drying rate – paper moisture content relations, determined as a continuous function by on–line measurements for about 80 sheets, permitted examination of the following parameters: basis weight 20 to 50 g/m2, which covers the tissue to communication paper range; initial moisture content 1.5 to 3 kg water/kg fibre; nozzle to paper spacing of 5, 7.6 & 8.5 nozzle diameters: nozzle plate open area ratio from 1.4 to 3.1%; jet Reynolds number in the range of 450 to 11.100.

Three methods of quantifying the complete drying rate curves were tested. Use of the common assumption of drying rate linear with moisture content over the falling rate period gave inadequate representation. Secondly, the general method of Churchill for any transport process that has a transition between two asymptotic relations was applied for the fmt time to the drymg of paper. This approach gives     

AN EXPERIMENTAL STUDY OF AN INCLINED BUOYANT PLANE TURBULENT AIR JET

An experimental study of the effects of buoyancy forces resulting from temperature differences on the mean flow and turbulence characteristics of a plane turbulent jet discharged at an angle to the horizontal has been undertaken. The jets were generated by discharging air through a rounded nozzle which was 10 mm wide by 580 mm long. The discharged air could be heated and tests were undertaken with discharge temperatures of 20° and 40°C above the ambient air temperature at a single nozzle discharge velocity which gave a discharge Reynolds number of approximately 1700. Tests were undertaken with angles of jet discharge to the horizontal of 0°, 30°, 60° and 90°. Velocity and temperature measurements were undertaken using hot wire anemometers. Measurements of the mean velocity and temperature, the turbulence normal and shear stresses, the temperature fluctuation intensity and the two components of the turbulence heat flux were obtained. Profiles of these quantities were measured at six equally spaced positions to a distance of approximately 60 nozzle widths from the discharge plane. Measurements at much more closely spaced intervals were also taken along the jet “centre-line”, this being defined as the locus of the point of maximum velocity in the jet.     

BUBBLE MEASUREMENTS IN A GAS-LIQUID JET

Measurements have been carried out in the developing and fully developed regions of a free, axisymmetric, isothermal, air-water, bubbly jet. Three experiments have been conducted at a fixed jet-exit Reynolds number and gas superficial velocity using three different bubble injector assemblies producing bubbles of moderately different average sizes and size distributions. The volume fraction of the bubbly jet flow examined in this study is low and the resulting dispersed flow is dilute. A one-component Phase-Doppler Velocimetry system has been employed to measure bubble size and velocity non-intrusively. Visual data collected simultaneously with the light-scattering measurements were analyzed with the aid of image processing and used to verify the trends portrayed by the light-scattering measurements and to determine average bubble size. Our measurements show that, even in the dilute flow examined here, differences in initial bubble-size and size distribution can influence the RMS velocity fluctuations of the bubbles, particularly in the jet development region. The average bubble velocities are less sensitive. Evidence that the development pattern of the jet near the exit is affected by the presence of the bubbles is also presented. Near the exit of the jet, bubbles are shown to be ejected laterally outside the jet due to the significant lift force caused by the high velocity gradient in the axisymmetric shear layer. The observed sensitivity of the bubble flow to size-related parameters and initial conditions in this dilute case, indicates that discrepancies in previous measurements of dispersed, bubbly flows could be attributed to different size characteristics and/or initial conditions.     

圆形自由表面水射流冲击换热特性

对圆形自由表面水射流冲击换热特性进行了系统的实验研究,总结了喷距、射流出口速度以及相变等因素对换热的影响,并对其形成机理进行了分析,得到了驻点传热系数以及局部传热系数径向分布的关联式.     

引射喷嘴流量系数的计算方法

引　言引射雾化喷嘴是在螺旋导水芯雾化喷嘴基础上发展起来的一种新型的二次雾化型喷嘴 .由于它雾化效果好 ,目前被广泛应用于煤炭、冶金、化工行业 .引射喷嘴的结构如图 1所示 ,它是由带引射风罩的喷嘴外壳与双头 (三头 )螺旋导水芯组成 .导水芯有直通孔 ,当压力水通过导水芯时 ,形成三股(四股 )水流 .其中两股 (三股 )沿螺旋沟槽螺旋前进 ,另一股沿中心直线前进 ,三股 (四股 )水在喷嘴出口处汇合 .在出口处形成实心圆锥形水雾 ,这是水的第一次雾化 .当水雾以一定锥角经过喷嘴引射罩时 ,形成密封区 ,水雾将喷嘴出口后的空气带走 ,形成负压 …     

IMPINGING JET DRYER

In coating and gravure printing, an impinging jet nozzle with high thermal efficiency for drying of coated film was developed.

Trial production 0f 40 kinds of nozzle enables to develop a high-performance impinging jet nozzle with heat transfer coefficient 1.5 times larger than that of current slit nozzle, through measurement of heat transfer coefficient, visualizations of air flow and heat transfer, and measuremenu of jet velocity and turbulence distribution. The purpose of the trial production was to expand a range of high heat transfer and promote turbulence compared with the current nozzle.

Paying attention to mass transfer within gravure ink coated film, drying characteristic of the film was analyzed by numerical solution of a set of equations governing the drying process in which concentration dependencies 0f the diffusion coefficient and the equilibrium vapor pressure were considered.

Applying these analyses. an industrial scale dryer with excellent drying efficiency has finally been developed.     

Batch and continuous spouted bed drying of cereal grains: The thermal equilibrium model

A complete set of equations with no adjustable parameters are written for numerically predicting the variation of grain temperature and moisture content with time for batch drying of well mixed deep beds of three cereal grains, as well as the exit grain and air moisture contents and temperatures for continuous drying. Unlike previous studies, the surface moisture content of the grains is not assumed constant. Agreement with both batch and continuous spouted bed drying data from the literature is good, in the continuous case especially when the assumed exit age distribution of the isothermal bed solids is specific to spouting.     

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.     

IMPINGING JET DRYER

ABSTRACT

In coating and gravure printing, an impinging jet nozzle with high thermal efficiency for drying of coated film was developed.

Trial production 0f 40 kinds of nozzle enables to develop a high-performance impinging jet nozzle with heat transfer coefficient 1.5 times larger than that of current slit nozzle, through measurement of heat transfer coefficient, visualizations of air flow and heat transfer, and measuremenu of jet velocity and turbulence distribution. The purpose of the trial production was to expand a range of high heat transfer and promote turbulence compared with the current nozzle.

Paying attention to mass transfer within gravure ink coated film, drying characteristic of the film was analyzed by numerical solution of a set of equations governing the drying process in which concentration dependencies 0f the diffusion coefficient and the equilibrium vapor pressure were considered.

Applying these analyses. an industrial scale dryer with excellent drying efficiency has finally been developed.     

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.     

液体中气—液射流特性的研究

在气-液两相喷射式装置中气体或/和液体是经由单个(或多个)喷咀射入塔内的。本研究用频闪摄影技术对气、液向上共同喷射时液体中的气-液射流特性进行了实验研究,并建立了气泡直径分布函教及射流特征尺寸的关联式。同时,对射流两相区内两相的流动用一两相模型进行模拟,考察了喷咀直径、初始条件、液体压头和气泡直径对器内两相流动的影响。     

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.     

HEAT AND MASS TRANSFER IN IMPINGEMENT DRYING

In industrial drying applications, efficient transfer of heat and mass between a drying medium and the material being dried is very critical for the overall economics of the operation. Impinging jets are therefore widely used for their enhanced tmnsport characteristics, especially for drying of continuous sheets of materials such as paper and textiles. In such applications, a thin sheet of material, as wide as 6m in cross machine direction, speeds at velocities as high as 90 km/hr under high velocity jets emerging from a confining surface parallel to the material surface. Many variables and effects need to k considered for proper design of such impinging jet systems: nozzle geometry and size, nozzle configuration, location of exhaust pons, nozzle-to-surface separation, jet-to-jet separation, cross flow, jet exit velocity and surface motion. For permeable materials, additional enhancement of heat and mass transfer that occur when some of the impinging gas is removed through the material makes this option an atmctive one.

Here, we review the above effects and offer predictive correlations from literature which may be used in the design of high velocity impinging jet systems.