EFFECT OF NEAR-WALL MODELLING ON PREDICTION OF IMPINGEMENT HEAT TRANSFER

Due to enhanced transport characteristics, impinging jets are widely used in industry to dry large surface area products such as paper and textiles. The present numerical study concerns the modelling of convective heat transfer for impingement drying. Flow and heat transfer under a confined two dimensional turbulent air jet impinging on a flat surface were modelled by solution of two-dimensional Navier-Stokes and energy equations. The turbulence model used was the high-Re number version of the well known two-equation (κ-?) model and numerical solution was by the upwind finite difference scheme. The specific objective was to evaluate the accuracy of schemes for modelling the near-wall turbulent flow.

The mean flow properties such as centerline velocity decay and the pressure distribution at the impingement surface show no dependence on the near-wall model used. Heat transfer predictions were found to be quite sensitive to the choice of near-wall model. Best agreement between predictions and experiments was obtained for a Chieng-Launder type model with a new modification, use of k_p instead of k_v in the calculation of τ_w.     

Effect of Surface Motion on Transport Processes Due to Circular Impinging Jets—A Numerical Study

This article presents a numerical study of transport phenomena under impinging circular jet banks over a moving surface by solving three-dimensional Navier-Stokes equation in both the laminar and the turbulent regime. A periodic element of the jet bank was used with jet pitch of 10d, span of target surface as 10d, and jet height of 2d, where d is the jet diameter. For the turbulent closure, a realizable k-ε model was used. The distributions of the Nusselt number and the skin friction coefficients were computed from the analyzed data. The surface velocity was found to influence strongly the flow structure over the impinging surface, leading to reduction in heat transfer.     

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.     

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.     

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.     

ANALYSIS OF EXPERIMENTAL AND NUMERICAL RESULTS OF A TURBULENT SWIRLING FLOW IN A TUBE

In many applications swirling flow is used to enhance heat and mass transfer. One of the problems of modelling a turbulent swirling flow is the choice of the turbulent closure model which is acceptable for engineering purposes. To evaluate which model performs best, numerical results are compared with experimental data. Local velocity measurements are carried out on a swirling flow in a circular tube. The measurement method is hot-wire anemometry combined with visualization techniques. The numerical analysis is carried out using the k-ε model and the Algebraic Stress Model. In the experiment a symmetrical swirl was observed, Comparing the experimental data with the numerical results shows that the Algebraic Stress Model represents the experimental data quite well, whereas the k-ε model fails.     

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.     

Simulation of Superheated Steam Turbulent Flows and Heat Transfer in an Impingement Dryer

Numerical simulations are presented for two-dimensional flow field and heat transfer characteristics due to a turbulent single slot jet of superheated steam discharging tangentially into a confined cylinder. A finite volumes method was used to solve the equations that describe the problem. Calculations were performed for steady state turbulent flow and unsteady state heat transfer. Constant velocities and superheated steam temperature are imposed at the inflow. Particle properties were assumed to be the same of a cubic particle of carrot. Numerical tests were performed to ensure that the model solutions were “grid independent” and also independent of the turbulence intensity of stated as boundary condition at the inlet slot. The solution procedure developed is fast and that convergence is reached after a few iterations. The results obtained are relevant to flow and heat transfer behaviors of the impingement dryer and it will be useful future studies considering particles interactions.     

Effects of pressure and impingement angle in flaming processes

This work investigates the effects of varying the pressure of the mixture reactants and the angle of impingement on the performance characteristics of a turbulent premixed jet flame impinging on a solid surface. These effects are important for the design of torches and flaming machines used for material and metal cutting and forming. The combustion and flow characteristics are modelled using a finite volume computational approach. Based on the simulation results, it is shown that, by increasing the pressure of the mixture reactants, the flame‐surface interaction mechanisms are modified. Changing the impingement angle increases the role of chemical kinetics and reduces maximum temperature values due to increased local flame extinction. The heat released and temperature predictions are compared to experimental data and the agreement is satisfactory.     

MATHEMATICAL MODELLING OF A ROTARY SWIRL CYCLONE SCRUBBER

A Rotary Swirl Cyclone Scrubber (RSCS) is a device designed to remove SO₂ and ash from the combustion products of an entrained-flow gasifier. It uses a combination of highly swirling flow and water sprays to produce high heat and mass transfer rates. In order to develop a better understanding of the operation of this device, modelling has been performed using an extended version of CFX-F3D. Simulations have been performed to examine the flowfield in the device and to determine SO₂ and ash capture efficiencies. The simulations show that the water jets used to remove the SO₂ from the gas completely change the direction or swirl in the device, resulting in a highly turbulent flow. The experimental SO₂ capture efficiency and the outlet temperature are well reproduced using a water droplet size of 60 μm, with this value being determined via fitting to a particular experiment. Predictions of ash particle capture are found to be in good agreement with the experimental data.     

Effect of Surface Motion on Transport Processes Due to Circular Impinging Jets—A Numerical Study

This article presents a numerical study of transport phenomena under impinging circular jet banks over a moving surface by solving three-dimensional Navier-Stokes equation in both the laminar and the turbulent regime. A periodic element of the jet bank was used with jet pitch of 10d, span of target surface as 10d, and jet height of 2d, where d is the jet diameter. For the turbulent closure, a realizable k-ε model was used. The distributions of the Nusselt number and the skin friction coefficients were computed from the analyzed data. The surface velocity was found to influence strongly the flow structure over the impinging surface, leading to reduction in heat transfer.     

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.     

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.     

TURBULENT MIXING IN THE IMPINGEMENT ZONE OF DUAL OPPOSED FREE JETS AND OF THE NORMAL WALL-IMPINGING JET

Marker nephelometry has been used to study the concentration fields of two jet-mixing systems: (i) equal opposed turbulent round free jets impinging upon each other, and (ii) the turbulent round free jet impinging upon a plane wall normal to its axis. Attention is focussed upon the impingement or deflection zone. The fields of mean concentration, concentration fluctuation intensity, and concentration intermittency were measured. Two-point correlations and frequency spectra were determined at selected points. Integral spatial scales were estimated. In the case of the opposed jets, the extent of mixing between the two jet source fluids was mapped. The results are of interest in relation to chemical reactors and combustors and for applications in heat and mass transfer.     

LIMITATIONS AND EMPIRICAL EXTENSIONS OF THE k-ε MODEL AS APPLIED TO TURBULENT CONFINED SWIRLING FLOWS

Shortcomings and recommended corrections to the standard two-equation k-ε turbulence model suggested by previous investigators are presented. They are assessed regarding their applicability to turbulent swirling recirculating flow. Recent experimental data on swirling confined flows, obtained with a five-hole pilot probe and a six-orientation hot-wire probe, are used to obtain optimum values of the turbulence parameters C_μ, C₂, and σ_ε, for swirling flows. General predictions of moderately and strongly swirling flows with these values are more accurate than predictions with the standard or previous simple extensions of the k-ε turbulence model.     

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.     

Numerical study on non-uniform heat transfer deterioration of supercritical RP-3 aviation kerosene in a horizontal tube

The convective heat transfer of supercritical-pressure RP-3(Rocket Propellant 3) aviation kerosene in a horizontal circular tube has been numerically studied, focusing mainly on the non-uniform heat transfer deterioration along the circumferential direction. The governing equations of mass, momentum and energy have been solved using the pressure-based segregated solver based on the finite volume method. The re-normalization group(RNG) k-ε turbulence model with an enhanced wall treatment was selected. Considering the heat conduction in the solid wall, the mechanism of heat transfer deterioration and the buoyancy effect on deteriorated heat transfer were discussed. The evolution of secondary flow was analyzed. Effects of the outer-wall heat flux,mass flux, pressure and tube thermal conductivity on heat transfer were investigated. Moreover, the buoyancy criterion and the heat transfer correlation were obtained. Results indicate that the poor flow performance of near-wall fluid causes the pseudo-film boiling, further leads to the heat transfer deterioration. The strong buoyancy has an effect of enhancing the heat transfer at the bottom of tube, and weakening the heat transfer at the top of tube, which results in the non-uniform inner-wall temperature and heat flux distributions. Decreasing the ratio of outer-wall heat flux and mass flux, increasing the pressure could weaken the heat transfer difference along the circumferential direction, while the effect of thermal conductivity of tube on the circumferential parameters distributions is more complicated. When the buoyancy criterion of(Gr_q/Gr_(th))_(max)≤ 0.8 is satisfied, the effect of buoyancy could be ignored. The new correlations work well for non-uniform heat transfer predictions.     

TRANSITION OF FLOW REGIME IN FLUIDIZED BEDS WITH SLANTING BLADE BAFFLES

A flow model is proposed to investigate the transition of flow regime from bubbling to turbulent fluidization postulating that the flow in the emulsion phase follows the Richardson-Zaki equation.

Void fraction of the whole bed ε_f and the mean velocity of bubbles U_b were measured in fluidized beds of 0.3 and 0.5 m ID, in which slanting blade baffles were positioned. Mo-catalyst, silica gel, sand and glass beads with size between 135-443 μm were fluidized by air.

Void fraction of the emulsion phase ε _e was calculated on the basis of the above model. Correlating ε _e with superficial gas velocity U_ƒ, we found that ε _e was very close to ε _mƒ in the bubbling regime and that e, increased with increasing U_ƒ in the turbulent regime.

Calculated values of the volume fraction of bubble phase δ were correlated with U_ƒ, from which apparent transition point from bubbling to turbulent regime was estimated. Combining information obtained, transition of flow regime in the above type of fluidized beds is discussed     

Convective Heat Transfer Enhancement of a Rectangular Flat Plate by an Impinging Jet in Cross Flow

Experiments were carried out to study the heat transfer performance of an impinging jet in a cross flow. Several parameters including the jet-to-cross-flow mass ratio （X=2%-8%）, the Reynolds number （Red=1434-5735） and the jet diameter （d=2-4 mm） were explored. The heat transfer enhancement factor was found to increase with the jet-to-cross-flow mass ratio and the Reynolds number, but decrease with the jet diameter when other parameters maintain fixed. The presence of a cross flow was observed to degrade the heat transfer performance in respect to the effect of impinging jet to the target surface only. In addition, an impinging jet was confirmed to be capable of en-hancing the heat transfer process in considerable amplitude even though the jet was not designed to impinge on the target surface.     

NUMERICAL ANALYSIS OF NON-ISOTHERMAL HELICAL FLOW OF HERSCHEL-BULKLEY FLUIDS

In this article a numerical solution of non-isothermal helical flow for the thermodependent Herschel-Bulkley fluids has been presented. The consistency term (μ₀) in comparison to other properties is more temperature dependent; therefore an exponential function in the form of μ₀ = exp(-bT) is considered. The governing equations are solved for two different physical situations; constant temperature walls and for a case of constant heat flux using the implicit finite difference method. The results from numerical solution are compared with results of first law of thermodynamics and a good agreement is noticed.