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.     

Impingement Heat Transfer for a Cluster of Laminar Impinging Jets Issuing from Noncircular Nozzles

Abstract:

Results of numerical simulation of the flow and heat transfer characteristics for a semi-confined cluster of laminar air jets impinging normally on a plane wall are presented. A central jet is surrounded by four equally spaced jets of the same configuration. Both circular and noncircular nozzles are considered. The nozzle footprint is displayed in the static pressure, temperature, and local Nusselt number contours on the impingement surface only for relatively short nozzle-to-surface distances. The heat transfer characteristics and performance of circular and noncircular nozzles are compared. It is observed that the Nusselt number based on property values at the jet temperature is relatively insensitive to the temperature difference between the jet and the impingement surface. Also, the local Nusselt numbers are independent of the thermal boundary condition; i.e., the values are nearly the same for both isothermal and uniform heat flux conditions at the target surface. Finally, Nusselt numbers for a single equivalent jet viz. one with the same area as the five nozzles in the cluster combined, are compared for the case of the circular jet.     

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.     

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.     

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.     

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.     

Stagnation Region Heat Transfer of Axisymmetric Impinging Jets on Solid Surfaces

A theoretical model is proposed to evaluate the heat transfer characteristics of axisymmetric impinging fluid jets on the horizontal hot surface in the stagnation region using the energy integral method. A generalized expression involving various modeling parameters such as Nusselt number, nozzle‐to‐plate distance, Prandtl number, Reynolds number, and the modeling parameter k is obtained from the analysis. Present predictions are found to agree well with the test data involving a wide range of coolant type, Reynolds number, and nozzle‐to‐plate distance. In addition, a mechanistic correlation is suggested between the modeling parameter k and flow parameter, i.e., Reynolds number.     

A mathematical model for drum drying of black liquor slurry using superheated steam impinging jets

A mathematical model is developed and tested for drum drying of an aqueous slurry subjected to impinging jets of superheated steam. The heat and mass transfer within the slurry film was modeled as a one dimensional, pseudo unsteady diffusion problem while the external convective heat transfer rate was obtained by solving the steady full conservation equations of mass, momentum and energy numerically in two and three dimensions. A modified low Reynolds number version of the k-? turbulence model was selected after a careful evaluation of the predictive performance of various k-? models for impingement flow and heat transfer. The computed heat transfer one-dimensional diffusion model for the slurry film. Agreement between experimental data obtained using a black liquor slurry and results of the model was found to be satisfactory.     

Experimental and numerical characterization of viscous flow and mixing in an impinging jet contactor

The laminar flow in an impinging jet contactor is examined as a first step toward the development of new technology for fast mixing of viscous fluids. The flow, velocity, and stretching fields in an impinging jet contactor are quantified for low Reynolds number flow using three-dimensional numerical simulations and particle image velocimetry measurements. Computational and experimental velocity fields are in close agreement, as quantified by the velocity probability density functions. Two steady-state flow regimes are found to exist: for jet Reynolds numbers (Re_j) < 10, the jets do not impinge and the velocity field scales linearly with Reynolds number; for Re_j > 10, the jets begin to impinge and recirculation regions form above and below the impingement point. The magnitude of the rate-of-strain tensor is calculated as a function of Re_j. While areas of essentially zero stretching occupy most of the flow domain, very high rates of stretching occur at specific locations in the flow. The maximum and average rates of stretching in the contactor increase roughly linearly as a function of Reynolds number. Mixing simulations show that no mixing occurs for the steady flow in a symmetric-jet contactor. However, mixing is improved substantially by a slight modification of the impinging jet geometry that disrupts geometric symmetry.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Mist flow visualization for round jets in Aerosol Jet® printing

With the microdroplets of water serving as light scattering particles, the mist flow patterns of round micro-jets can be visualized using the Aerosol Jet^® direct-write system. The visualization images show that the laminar mist jet (with sheath-to-mist ratio Y?=?1:1) appears to extend to more than 20 times the diameter of nozzle orifice D for jet Reynolds number Re?<?600, especially with D?=?0.3?mm and less. For smaller jets (e.g., with D?=?0.15?mm), laminar collimated mist flow might be retained to 40×D for Re?<?600 and for Re ～1500 within 20×D from the nozzle. The laminar part of mist flow associated with larger jets (e.g., with D?=?1.0?mm for Re?<?600) tends to exhibit noticeable gradual widening due to viscous diffusion. For free jets, their breakdown length—the distance from nozzle where transition from laminar to turbulent mist flow takes place as signaled by a rapid widening of mist stream—is shown to decrease with increasing Re. The presence of impingement wall tends to prevent turbulence development, even when the wall is placed further downstream of the free-jet breakdown length for a given Re. The critical Re for an impinging jet to develop turbulence increases as the standoff S is reduced. The mist flow of impinging jet of D?=?1.0?mm seems to remain laminar even for Re?>?4000 at S?=?12?mm.

Copyright © 2018 American Association for Aerosol Research     

Influence of magnetic field on jet impingement heat transfer with molten salt

Experimental study of jet impingement heat transfer with molten salt under the influence of external constant magnetic field was generated by permanent magnets. Both stagnation correlation and radial distribution of Nusselt number under magnetic field were obtained. The results showed that the Nusselt number with magnetic field became higher than that without magnetic field at stagnation region and jet impingement heat transfer was comparatively enhanced, while in wall jet region, the enhancement of heat transfer was gradually weakened. In addition, when the Reynolds number was constant, the Nusselt number of molten salt increased with increasing of the intensity of magnetic field, and the most enhanced heat transfer existed at the stagnation point. Under the conditions of Reynolds number Re=6400 and the intensity of magnetic field B=2800 Gs, the stagnation Nusselt number of molten salt was about 6 % higher than that without magnetic field. It can be seen that the magnetic field may promote the jet impingement heat transfer of molten salt.     

Drying paper by impinging jets of superheated steam. Part 1: Constant drying rate in superheated steam

Over a wide range of operating conditions, the drying of paper by impinging jets of superheated steam proceeds by a constant rate period followed by a falling rate period. The constant drying rate, investigated here in the jet temperature range 150 ≤ T_j/ ≤ 465°C and over jet Reynolds numbers of 1000 ≤ Re_j ≤ 12000, is predicted within + 12% by a heat transfer expression using Martin's (1977) correlation for the heat transfer coefficient corrected for mass transfer by the Couette flow approximation factor, and a property ratio to account for the large temperature difference between the jet and the paper.     

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.