Flow and Heat Transfer Characteristics of Confined Noncircular Turbulent Impinging Jets

Abstract

A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layer κ–η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number.     

Flow and Heat Transfer Characteristics of Confined Noncircular Turbulent Impinging Jets

A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layer κ-η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number.     

Numerical Study on the Effect of Cross-Flow on Turbulent Flow and Heat Transfer Characteristics Under Normal and Oblique Semi-confined Impinging Slot Jets

Abstract

Impinging jets are commonly used in industrial dryers and electronics chip cooling. Since in industrial practice it is necessary to use multiple jets, the interaction between jets can have important effect on their heat transfer performance. Hence, the study of cross-flow caused by the spent flow of upstream jets is obviously significant. In this study, a computational fluid dynamics simulation was carried out of the flow and heat transfer characteristics for a single semi-confined turbulent slot jet of air impinging normally or obliquely into an imposed air cross-flow of the same or different temperature. The standard k?ε and the Reynolds stress models were used. Effects of the various flow parameter (e.g., jet-to-cross-flow mass ratio) and geometric parameters (e.g., nozzle-to-target spacing and jet angle) were evaluated at a fixed Reynolds number (11,000 and 12,000) for equal and unequal temperatures of the jet and cross-flow. Results indicate the significant degradation of the impingement heat transfer rates due to cross-flow and a relatively minor influence of the temperature difference between the jet and cross-flow over the ranges of parameters studied. Both the turbulence models produced comparable Nusselt number distributions along the impingement surface.     

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.     

Numerical Study on the Effect of Cross-Flow on Turbulent Flow and Heat Transfer Characteristics Under Normal and Oblique Semi-confined Impinging Slot Jets

Impinging jets are commonly used in industrial dryers and electronics chip cooling. Since in industrial practice it is necessary to use multiple jets, the interaction between jets can have important effect on their heat transfer performance. Hence, the study of cross-flow caused by the spent flow of upstream jets is obviously significant. In this study, a computational fluid dynamics simulation was carried out of the flow and heat transfer characteristics for a single semi-confined turbulent slot jet of air impinging normally or obliquely into an imposed air cross-flow of the same or different temperature. The standard k-ε and the Reynolds stress models were used. Effects of the various flow parameter (e.g., jet-to-cross-flow mass ratio) and geometric parameters (e.g., nozzle-to-target spacing and jet angle) were evaluated at a fixed Reynolds number (11,000 and 12,000) for equal and unequal temperatures of the jet and cross-flow. Results indicate the significant degradation of the impingement heat transfer rates due to cross-flow and a relatively minor influence of the temperature difference between the jet and cross-flow over the ranges of parameters studied. Both the turbulence models produced comparable Nusselt number distributions along the impingement surface.     

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.     

圆形自由水射流冲击换热及喷嘴布置

引言 水射流冲击冷却由于具有较高的换热能力,广泛应用于机械和化工行业,以实现工件的快速冷却和控制工件的温度变化.在大型轴类工件(工件直径D=1000～3000 mm)喷水冷却装置中,多喷嘴圆孔自由水射流以特定阵列布置冲击至工件表面,相对短暂的沸腾换热结束后,阵列自由水射流即以强制对流方式实现工件冷却.因此,自由水射流冲击换热特性及多喷嘴布置形式对喷水冷却装置的结构设计至关重要.     

Nanofluid impingement jet heat transfer

Experimental investigation to study the heat transfer between a vertical round alumina-water nanofluid jet and a horizontal circular round surface is carried out. Different jet flow rates, jet nozzle diameters, various circular disk diameters and three nanoparticles concentrations (0, 6.6 and 10%, respectively) are used. The experimental results indicate that using nanofluid as a heat transfer carrier can enhance the heat transfer process. For the same Reynolds number, the experimental data show an increase in the Nusselt numbers as the nanoparticle concentration increases. Size of heating disk diameters shows reverse effect on heat transfer. It is also found that presenting the data in terms of Reynolds number at impingement jet diameter can take into account on both effects of jet heights and nozzle diameter. Presenting the data in terms of Peclet numbers, at fixed impingement nozzle diameter, makes the data less sensitive to the percentage change of the nanoparticle concentrations. Finally, general heat transfer correlation is obtained verses Peclet numbers using nanoparticle concentrations and the nozzle diameter ratio as parameters.     

Highly selective fluorescent chemosensor for Zn2+ derived from inorganic-organic hybrid magnetic core/shell Fe3O4@SiO2 nanoparticles

Experimental investigation to study the heat transfer between a vertical round alumina-water nanofluid jet and a horizontal circular round surface is carried out. Different jet flow rates, jet nozzle diameters, various circular disk diameters and three nanoparticles concentrations (0, 6.6 and 10%, respectively) are used. The experimental results indicate that using nanofluid as a heat transfer carrier can enhance the heat transfer process. For the same Reynolds number, the experimental data show an increase in the Nusselt numbers as the nanoparticle concentration increases. Size of heating disk diameters shows reverse effect on heat transfer. It is also found that presenting the data in terms of Reynolds number at impingement jet diameter can take into account on both effects of jet heights and nozzle diameter. Presenting the data in terms of Peclet numbers, at fixed impingement nozzle diameter, makes the data less sensitive to the percentage change of the nanoparticle concentrations. Finally, general heat transfer correlation is obtained verses Peclet numbers using nanoparticle concentrations and the nozzle diameter ratio as parameters.     

A Numerical Study on the Convective Heat Transfer Characteristics of Pulsed Impingement Drying

Impinging jets issuing from the tailpipe of pulse combustors have been evaluated in recent studies for possible applications in rapid drying of continuous sheets such as grades of paper, textiles, etc. In order to further understand the effect of pulsed flows on the heat and mass transfer rates of impinging jets, a numerical study was performed on a two-dimensional pulsating impinging jet array. A computational fluid dynamics approach was used to examine the effect of periodic sinusoidal pulsation on the local Nusselt number distribution of the wet target surface being dried. Because a high temperature and large temperature difference between the jet flow and impingement surface are used to obtain high heat transfer rates in impingement drying, the thermophysical properties of jet flows were taken into account in the present mathematical model. A parametric study including phase angle and frequency as well as amplitude of pulsating flows was conducted for optimization and design of pulsating jet arrays. Examination of the velocity and thermal fields showed that the instantaneous heat transfer rate on the target surface was highly dependent on the mass transfer characteristic and development of the hydrodynamic boundary layer with time.     

Experimental and Numerical Simulation Study of Heat Transfer Due to Confined Impinging Circular Jet

An experimental and numerical simulation study of heat transfer due to a confined impinging circular jet is presented. In this research, a stainless steel foil heated disk was used as the heat transfer surface of a simulated chip, and the thermocouples were mounted symmetrically along the diameter of the foil to measure the temperature distribution on the surface. Driven by a small pump, a circular air jet (1.5 mm and 1 mm in diameter) impinged on the heat‐transfer surface with middle and low Reynolds numbers. The parameters, such as Reynolds number and ratio of height‐to‐diameter, were changed to investigate the radial distribution of the Nusselt number and the characteristics of heat transfer in the stagnation region. Numerical computations were performed by using several different turbulence models. In wall bounded turbulent flows, near‐wall modeling is crucial. Therefore, the turbulence models enhanced wall treatment, such as the RNG κ‐? model, may be superior for modeling impingement flows. The numerical results showed reasonable agreement with the experimental data for local heat transfer coefficient distributions. The impinging jet may be an effective method to solve the cooling problem of high power density electronic packaging.     

Impingement Heat Transfer Effects on Baking of Flat Bread

An impingement oven designed for flat breads baking was instrumented to monitor the processing conditions and record variables responsible for changes in dough characteristics. Image processing was used to monitor the volume and surface color changes during baking. Fluctuations of oven temperature and jet velocity were minimized using electronic controllers, leading to better repeatability and accuracy. Impingement heat transfer distribution effects on flat bread baking was investigated by numerical simulation and related to the bread color. Considerable effect of local variations in impingement heat transfer and nonuniform nozzle exit velocity distribution on the color of baked bread was observed. At H/D > 4, jet temperatures < 200°C, and jet velocity of 10 m/s, local heat transfer distribution and surface darkening was negligible and a uniform surface color was obtained. Notably, velocity variations of ±20% inherent in the finger nozzles used in the oven design did not result in significant color change, which depends more on the local temperature attained by the bread surface.     

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.     

INVESTIGATION OF IMPINGEMENT HEAT TRANSFER COEFFICIENT AT HIGH TEMPERATURES

ABSTRACT

Very little information exists for the impingement heat transfer coefficient at high temperatures. All available empirical correlations are mainly based on experiments conducted at relatively low impingement temperatures, and thus cannot describe the heat transfer characteristics of the impingement air at high temperatures with sufficient accuracy. A comprehensive study of the impingement heat transfer coefficient at high temperatures is carried out and presented in this paper. The aim of the study is to give a summary of the experimental results of the impingement heat transfer covering a large impingement air temperature range from 100 to 700°C. Heat transfer measurements were carried out on a laboratory-scale test rig. The main parts of the rig were a fan, a gas burner for air heating, a heavily insulated nozzle array with 300 × 500 mm impingement surface, a 40 mm thick and 300 × 500 mm sized aluminium plate for determination of heat transfer, and a data acquisition system. The heat transfer rate was determined from the heat-up rate of the aluminium plate due to the high temperature jet impingement.     

Influence of Impingement Temperature and Nozzle Geometry on Heat Transfer—Experimental and Theoretical Analysis

Abstract

Based on the earlier studies the most common empirical correlations in the literature predict the impingement heat transfer coefficients rather well at low (close to 100°C) temperatures, but get inaccurate at higher temperatures. Impingement temperatures used in paper drying applications are typically 300 to 700°C, and there has been a need to improve the existing heat transfer correlations at high temperature area. This study presents experimental results of impingement heat transfer measurements with a laboratory-scale heat transfer test rig. Five nozzle configurations were measured. The parameters varied in the investigation were nozzle-to-plate distance, nozzle open area, nozzle diameter, impingement velocity and impingement air temperature. Regression model of heat transfer was developed based on the measurement data. A correction factor including the impingement and heat receiving surface temperatures is proposed. The correction factor can be used to improve the existing correlations at large jet to heat receiving surface temperature differences.     

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…     

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.     

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.     

Numerical Study of Heat Transfer and Fluid Flow in Multiple turbulent Impinging Jets

ABSTRACT

Impingement flows have been studied extensively for various geometries and configurations, but because of the complexity of the turbulent flow and its strong dependence on the geometry of the flow, further investigation is required to identify the suitable model for specific cases. This paper presents a study of various k–E turbulence models in order to identify the best model for an array of multiple confined impinging slot jets, with exhaust ports in the confinement surface located symmetrically between adjacent jets. Such a configuration is used in a novel drum dryer for black liquor. The “High Reynolds number” turbulence models including the standard k–E model fail to predict heat transfer to impingement surface accurately although they do predict the flow field reasonably well. On the other hand, the “Low Reynolds number” models yield considerably better results for both fluid flow and heat transfer. All computed results are compared with experimental data reponed in the literature. This work was motivated by the need to select an optimal multiple impinging jet configuration for a novel drum dryer for Kraft black liquor. It is also pertinent to impingement dryers for paper, films, textiles etc.     

Hydrothermal flame impingement experiments. Combustion chamber design and impingement temperature profiles

The current work presents the hydrothermal flame impingement experiments conducted for the design of a hydrothermal spallation drilling nozzle. The products of hydrothermal flames of mixtures of ethanol, water and oxygen were injected as free jets in a high pressure water bath. The nozzle design was based on ideas stemming from underwater welding and cutting of metal sheets. Water entrainment in the flame-jets and the heat transfer capabilities of flames injected from various nozzles have been analyzed by measuring their impingement temperature profiles on a flat stainless steel plate. It was found that the thermal-to-kinetic energy ratio of the jet has a direct influence on the entrainment of water in it. Furthermore, the cooling water of the combustion chamber was injected in various angles to the axis of the jet resulting to different entrainment rates. It was found that higher water injection angles reduced the rate of entrainment. Finally, it was indicated that at certain operational points of the jet, its trans-critical properties had an important influence on the impingement temperatures.