Study of the thermal mixing between two non-isothermal sprays using combined three-color LIF thermometry and phase Doppler analyzer

The aim of this experimental work was to demonstrate the ability of three-color laser-induced fluorescence (3cLIF) thermometry to study the thermal mixing of two non-isothermal water sprays. Combined 3cLIF-phase Doppler analyzer measurements were also implemented to derive correlations between droplet size and temperature. Both sprays had different characteristics in terms of flow rate and droplet size distribution. The liquid spray was successively pre-heated, and the other spray was maintained and injected at ambient temperature. The thermal mixing will be discussed in light of a wide set of experimental results obtained under various experimental conditions, including different liquid flow rates, droplet size distributions and droplet concentrations. To analyze the potential effect of droplet coalescence on the mean local liquid temperature, both sprays were alternatively seeded with fluorescent dye. Main results show that significant heating of cold spray is possible when the hot spray is injected with the higher flow rate. Moreover, this heating affects only the smallest droplets.     

Noise radiated by a non-isothermal, temporal mixing layer

The ability of Lighthill's analogy to predict the sound radiated by a transitional mixing layer is evaluated by means of direct numerical simulation (DNS). The specific case of low Mach number flows with density variations is investigated. In order to limit the global computational cost, the acoustic source information is based on numerical results where the sound waves have been removed. It is shown that the low Mach number approximation coupled with the acoustic analogy can lead to very accurate predictions for the radiated sound if the acoustic sources in Lighthill's equation are taken into account carefully. Results for the acoustic intensity deduced from a repeated use of the Lighthill's analogy over a wide range of Mach numbers allow us to discuss the adequacy of scaling laws proposed by previous authors (J. Sound Vib. 28(3), 563–585, 1973; 31(4), 391–397, 1973; 48(1), 95–111, 1976) for the prediction of noise from hot jets.     

Characterization of fan spray atomizers through numerical simulation

The present paper focuses on the mathematical modeling of industrial fan spray atomizers. The two-phase flow taking place inside the nozzle’s tip and the exterior region near the outlet of three different industrial nozzle designs has been modeled and simulated. As a result, valuable information has been obtained regarding the influence of the inner geometry on the flow and also the formation and development of the liquid sheet. Characteristic magnitudes such as the discharge coefficient and the liquid sheet thickness factor have been obtained and validated through experimental measurements. The accumulation of liquid at the border of fan-shaped liquid sheets, also known as rim, has been studied in the analyzed designs, revealing the presence of a tangential velocity component in the liquid sheet and a relationship between the incoming flow rate of the rim and the angle of the liquid sheet. The dependence of the results on turbulence modeling has also been analyzed, drawing interesting conclusions regarding their influence on the liquid sheet mean flow characteristics and on the surrounding gas. Thus, the mathematical model developed has been proved to be a useful tool for nozzle manufacturers; it provides the most important characteristic parameters of the liquid sheet formed given certain nozzle geometry and, additionally, those data necessary to carry out studies of instability, breakup and atomization of the liquid sheet.     

Numerical study of global rainbow technique: sensitivity to non-sphericity of droplets

The measurement of droplet temperature and size distribution in sprays is a difficult task. To reach this aim, the global rainbow technique (GRT) has been developed on the assumption that the synthetic rainbow created by a large number of droplets is insensitive to the non-sphericity of droplets if the droplets’ orientations were sufficiently random. In order to test this assumption, numerical as well as experimental analyses of GRT are carried out by our team. As a companion to the work done in experiments, the objective of this work is to quantify the sensitivity of the GRT to the non-sphericity of droplets from a numerical aspect. Light scattering properties around the rainbow angle are investigated by using the Null-field method within a T-matrix formulation, both for a single spheroid in an arbitrary orientation and for an ensemble of spheroids in random orientations illuminated by a plane wave. Refractive index and size distribution of droplets are extracted from simulated global rainbow signals so as to quantify the sensitivity of the GRT to the non-sphericity. Exemplifying results are compiled and presented. Additionally, comparisons between the Null-field method and the generalized Lorenz-Mie theory for spheroids are also provided in this paper.     

Fluctuating flow in a liquid layer and secondary spray created by an impacting spray

A spray impacting onto a wall produces a flow of secondary droplets. For relatively sparse spray these secondary droplets are produced by the splashing of the impacting drops and their interactions. For dense sprays, like Diesel injection sprays, these secondary droplets are created by the fluctuating liquid film created on the wall. In the present paper hydrodynamic models are presented for these two extreme cases. The velocities of the secondary droplets produced by the crown splash in a sparse spray are described theoretically. Next, the fluctuations in the motion of the liquid film created by a dense impacting spray are analyzed statistically. This motion yields the formation of finger-like jets, as observed in experiments of a Diesel spray impacting onto a rigid wall. The characteristic size and velocity of the film fluctuations are estimated. These two theoretical models are validated by comparison with the experimental data.     

Numerical simulations of non-isothermal three-dimensional flows in an extruder by a finite-volume method

This study considers numerical applications of a finite-volume method to steady non-isothermal flows in geometries close to a single-screw extruder. Two geometrical configurations of the channel, with gap and zero gap, are investigated. The simulations concern incompressible fluids obeying different constitutive equations: Newtonian, generalized Newtonian with shear-thinning properties (Carreau–Yasuda law), and two viscoelastic differential models, the upper convected maxwell (UCM) and the Phan–Thien/Tanner (PTT). The temperature dependence is described by a Williams–Landel–Ferry (WLF) equation. For discretizing the equations and unknowns, we use a staggered grid with a QUICK scheme for the convective-type terms and solve the set of governing equations by a decoupled algorithm, stabilized by a pseudo-transient stress term and an elastic viscous stress splitting (EVSS) technique, in the viscoelastic case for the UCM model. The numerical results enable us to state the influence of temperature and rheological properties on the flow characteristics in the geometries investigated and underline the complex behaviour of the materials in such configurations.     

Fluctuations of a spray generated by an airblast atomizer

This paper is devoted to the study of the aerodynamic instability of the spray generated by an airblast atomizer. As a result of this instability the spray shape and its velocity fluctuate with a certain frequency, which depends on the operational parameters of the atomizer. The effect of three parameters, namely; chamber pressure, liquid phase flow rate and the gas phase flow rate on the spray fluctuating frequency are investigated. The velocity vector of the drops in the spray and the arrival times to the detection volume are measured using the laser Doppler instrument. The slotting technique is applied to the data of axial velocity and arrival times of the drops in order to estimate the dominating spray frequencies. Additionally, the shape of the spray has been observed using the high-speed video system. The frequencies of the shape fluctuations are estimated using proper orthogonal decomposition of the time-resolved images of the spray. We show that the frequencies of the spray velocity and those exhibited by spray shape coincide over a wide range of spray parameters. Finally, a simple scaling for the spray frequency is proposed and validated by the experimental data.     

基于SPH/FEM方法的浅水冲击喷溅及其抑制结构研究

物体浅水冲击会造成结构响应和自由液面的剧烈喷溅,以往的研究多集中在物体受水的冲击响应方面,而对浅水层受物体冲击引起的剧烈喷溅问题及其喷溅抑制结构的相关研究很少。本文基于SPH/FEM耦合方法对浅水冲击的液体喷溅特性开展研究。首先,计算了刚性长方体结构的冲击喷溅,与文献中准二维溅水试验结果进行了比较,将浅水喷溅分为初始喷溅、二次喷溅和底部射流三部分。分别研究了各部分喷溅的形成机理,指出二次喷溅由结构对水层的挤压和底部射流的能量传递共同引起,并分析了水深对二次喷溅的影响规律。其次,对喷溅抑制结构的机理做了数值研究,发现拱形翻边构型可以抑制初始喷溅,但不能有效降低二次喷溅。最后,提出了一种带凹槽和阻水边条的新型翻边构型,对二次喷溅具有更好的抑制效果。     

Numerical modeling and experimental measurements of water spray impact and transport over a cylinder

This study compares experimental measurements and numerical simulations of liquid droplets over heated (to a near surface temperature of 423 K) and unheated cylinders. The numerical model is based on an unsteady Reynolds-averaged Navier–Stokes (RANS) formulation using a stochastic separated flow (SSF) approach for the droplets that includes submodels for droplet dispersion, heat and mass transfer, and impact on a solid surface. The details of the droplet impact model are presented and the model is used to simulate water spray impingement on a cylinder. Computational results are compared with experimental measurements using phase Doppler interferometry (PDI). Overall, good agreement is observed between predictions and experimental measurements of droplet mean size and velocity downstream of the cylinder.     

A Eulerian/Lagrangian model to calculate the evolution of a water droplet spray

We introduce a Eulerian/Lagrangian model to compute the evolution of a spray of water droplets inside a complex geometry. To take into account the complex geometry we define a rectangular mesh and we relate each mesh node to a node function which depends on the location of the node. The time-dependent incompressible and turbulent Navier-Stokes equations are solved using a projection method. The droplets are regarded as individual entities and we use a Lagrangian approach to compute the evolution of the spray. We establish the exchange laws related to mass and heat transfer for a droplet by introducing a mass transfer coefficient and a heat transfer coefficient. The numerical results from our model are compared with those from the literature in the case of a falling droplet in the atmosphere and from experimental investigation in a wind tunnel in the case of a polydisperse spray. The comparison is fairly good. We present the computation of a water droplet spray inside a complex and realistic geometry and determine the characteristics of the spray in the vicinity of obstacles.     

Characterization of a water mist based on digital particle images

The characteristics of a water mist, such as the droplet size and velocity distribution, the water mist atomization angle and the breakup length were determined with a newly developed nonintrusive pattern method, particle image velocimetry and sizing (PIVS), which is mainly based on the manipulation of digital particle images. A series of experiments were conducted using a semiconductor laser (with wavelength 650 nm and about 35-50-mW power), a plano-convex cylindrical lens and some other lenses to produce a light sheet approximately 1-mm thick. The typical results of a water mist under different conditions are shown in this paper.     

The mechanics and deformation of high temperature steel frame rapidly cooled by spray water in fire fighting

A finite element is established for analyzing the dynamical mechanics and deformation of steel frame at high temperature when it is rapidly cooled down by spray water in fire fighting. The simulation result shows that remarkable mechanical coupling effects are produced in the process, and the sectional stress in rapid cooling down is found considerably larger than that in heating-up. Meanwhile, the stress and deformation of a beam mainly related to cooling rate and location are much larger than those of a column in rapid cooling. In fire fighting, the structure on the first or second floor was more dangerous than those on other floors in rapid cooling. These results could provide a theoretical reference for the design of steel structure and fire fighting.     

An experimental study of non-isothermal miscible displacements in a Hele-Shaw cell

Non-isothermal miscible displacements in a radial Hele-Shaw cell were experimentally investigated using a scheme in which room temperature liquids of relatively high viscosity were displaced by high-temperature (80 °C), less-viscous liquids. Fundamental characteristics have been presented regarding how the effect of a non-isothermal field on miscible displacement patterns varies in terms of factors such as the viscosity ratio of the more- and less-viscous liquids at 20 °C, M₂₀, the rate of an increase in the pattern’s area, R, and the gap width of the cell, b. The concept of area density was used to quantitatively evaluate the effect of the non-isothermal fields on the patterns. We have found that the effect of the non-isothermal field on the patterns does not monotonically vary with M₂₀ and b. In contrast, it increases with R in the present experimental condition. The experimental results can be explained by introducing an assumption in which heat is transferred mainly to the plates of the cell, in other words, the temperature of the more-viscous liquid remains constant, whereas that of the less-viscous liquid spatiotemporally decreases and the viscosity of it increases along with the temperature decrease. Visualization of non-isothermal field in the cell has been done by means of a thermo sheet and the results support the assumption mentioned above.     

Numerical modeling and experimental measurements of a high speed solid-cone water spray for use in fire suppression applications

Experimental measurements and numerical simulations of a high-speed water spray are presented. The numerical model is based on a stochastic separated flow technique that includes submodels for droplet dynamics, heat and mass transfer, and droplet–droplet collisions. Because the spray characteristics near the nozzle are difficult to ascertain, a new method for initialization of particle diameter size is developed that assumes a Rosin–Rammler distribution for droplet size, which correctly reproduces experimentally measured Sauter and arithmetic mean diameters. By relating the particle initialization to lower moments of the droplet statistics, it is possible to take advantage of measurements without substantial penalties associated with the greater experimental uncertainty of individual droplet measurements. Overall, very good agreement is observed in the comparisons of experimental measurements to computational predictions for the streamwise development of mean drop size and velocity. In addition, the importance of modeling droplet–droplet collisions is highlighted with comparison of selected droplet–droplet collision models.     

Numerical solution of natural convection flow past a non-isothermal vertical flat plate

The problem of natural convection over a semi-infinite flat plate with non-uniform wall temperature is studied by using a numerical method. The local rates of heat transfer as a function of the distance along the plate are tabulated for a range of Prandtl numbers (0.01 to 100) and for a few cases of wall temperature distributions. Such tabulations serve as a reference against which other approximate solutions can be compared in the future.     

Interaction of a polydisperse spray with vortices

The objective of the present work is to provide, through the association of optical diagnostics on a well-chosen experimental configuration, new insights into the coupling of a vortical gaseous flow with a polydisperse evaporating spray representative of practical injections. A cloud of droplets is injected in an inert laminar round jet, axisymmetric and pulsated, enabling the study of the interaction of strong-vorticity structures with a polydisperse spray. The experiment is a laboratory-scale representation of realistic injection configurations such as in engine combustion chambers or industrial burners. The chosen set-up leads to a well-controlled configuration and allows the coupling of two optical diagnostics, particle imaging velocimetry (PIV) and interferometric particle imaging (IPI), which leads to the study of both the flow dynamic and the droplet size distribution. The behaviour of droplets is analysed regarding their relaxing and evaporating properties. Size-conditioned preferential concentration of both weakly evaporating and strongly evaporating sprays is investigated. Droplet trajectories are also analysed by means of high-rate tomographic visualizations. The time history between their ejection from the nozzle and their interaction with the vortex is strongly related to the droplet preferential concentration and the observed heterogeneous repartition in the gas flow.     

A semi-analytic solution of a wellbore in a non-isothermal low-permeability porous medium under non-hydrostatic stresses

The transient stress, displacement, pore pressure and temperature fields around a wellbore in a thermo-poro-elastic (THM) medium subject to non-hydrostatic remote stresses are analyzed under non-isothermal plane-strain conditions. The linear THM model proposed by Coussy (1989) is adopted in the analysis with a focus on thermal effects in low-permeability saturated rocks, characterized by a latent heat associated with local changes of fluid mass content. Non-dimensionalized parameters are identified by reformulating the fully-coupled governing equations and boundary conditions. The wellbore problem is simplified by decomposing it into axisymmetric and deviatoric loading cases. The corresponding analytical solutions are obtained in Laplace space. The inverse Laplace transforms are performed numerically to find the time-dependent distributions of field variables in the rock mass around the wellbore. These numerical results show that although the pore pressure diffusion has little influence on temperature and stress, temperature changes can strongly affect the pore pressure and stress around the wellbore. The temperature change can lead to changes in near-well stresses and the resulting significant change in wellbore breakdown pressure illustrates the importance of considering the THM coupling.     

FPIV study of gas entrainment by a hollow cone spray submitted to variable density

The gas entrainment in a hollow cone spray submitted to variable density is studied experimentally in order to better understand the effect on mixture formation. Particle image velocimetry on fluorescent tracers, associated with a specific processing of the instantaneous velocity fields have been applied to obtain measurement in the close vicinity of the spray edge. In the “quasi-steady” region of the spray, important effect of the ambient density on the mass flow rate of entrained gas have been pointed out. The axial evolution of is in good agreement with an integral model that takes the momentum exchange between phases into account.     

Heat transfer of spray cooling using alumina/water nanofluids with full cone nozzles

The transient temperatures of a metal testing plate during spray cooling using alumina/water nanofluids were measured. The heat transfer coefficient (HTC) was calculated by an inverse heat-conduction technique using the measured temperatures. The results show a decrease of approximately 20?% of the HTC of spray cooling with the nanoparticle suspension changing from 0 to 16.45?%. The nature and the reason of the HTC deduction were investigated and the HTC correlations with the mass fluxes and nanoparticle fraction were specifically reported.     

Optimization of a global seventh-order dissipative compact finite-difference scheme by a genetic algorithm

A global seventh-order dissipative compact finite-difference scheme is optimized in terms of time stability. The dissipative parameters appearing in the boundary closures are assumed to be different, resulting in an optimization problem with several parameters determined by applying a generic algorithm. The optimized schemes are analyzed carefully from the aspects of the eigenvalue distribution, the ε-pseudospectra, the short time behavior, and the Fourier analysis. Numerical experiments for the Euler equations are used to show the effectiveness of the final recommended scheme.