An experimental and numerical study into turbulent condensing steam jets in air

 Temperatures, velocities, and droplet sizes are measured in turbulent condensing steam jets produced by a facial sauna, for varying nozzle diameters and varying initial velocities (Re=3,600–9,200). The release of latent heat due to droplet condensation causes the temperature in the two-phase jet to be significantly higher than in a single-phase jet. At some distance from the nozzle, droplets reach a maximum size and start to evaporate again, which results in a change in sign of latent heat release. The distance of maximum size is determined from droplet size measurements. The experimental results are compared with semi-analytical expressions and with a fully coupled numerical model of the turbulent condensing steam jet. The increase in centreline temperature due to droplet condensation is successfully predicted. Received: 5 April 2000 / Accepted: 15 November 2000     

水下欠膨胀高速气体射流的实验研究

采用实验途径研究了下水高速气体射流的动力学特性,研制了水下高速气体射流实验系统并发展了相应的测试手段。实验中,用插入式静压探针测量了射流轴线静压分布;用γ射线衰减法测量了径向空隙率分布,从而揭示了水下高速气体射流均压和掺混两个过程的基本规律。测量结果表明：水下高速气体射流在欠膨胀工况下运行时,近场将出现含有复杂波系结构的膨胀压缩区域,由于气水的掺混作用,水下欠膨胀气体射流均压化过程比空气中衰减得快。测量结果还表明,水下射流在近场区的混合层由气水两相占据,其流态从靠近气体侧的液滴流型过渡到靠近液体侧的气泡流型。     

Effects of the gas phase molecular weight and bubble size on effervescent atomization

The effervescent atomization from an industrial Coker feed nozzle is compared for two different gas densities (air and mixed gas of 81.4 vol.% helium/18.6 vol.% nitrogen) at equivalent operating temperatures. The application is to observe the similarity of lab tests using air at 20 °C to the industrial process using steam at 300-400 °C. The effects of operating conditions, such as gas to liquid mass ratio, mixing pressure and void fraction on the flow regime, bubble size, and droplet size distribution were also examined in this study. The experiments were performed using mixtures of water with air or mixed gas, which resulted in gas to liquid mass ratios ranging from 1% to 4%.Stroboscopic back scattered imagery (SBSI) indicates that the average bubble size inside the nozzle conduit is similar when air and water are used as the process fluids, when compared to the case when mixed gas and water are used as the process fluids. Under similar conditions, the Phase Doppler Particle Anemometer (PDPA) data indicate that the droplet size in the spray is similar when using either mixed gas or air as the atomization gas.Experimental results obtained by high-speed video shadowgraphy (HSVS) indicate that the flow pattern inside the nozzle feeding conduit was slug flow with a tendency to attain annular flow with increased air to liquid mass ratios. Thus, from the experimental results it is evident that the smaller molecular weight of the mixed gas versus air (8.4 versus 29) does not significantly reduce the bubble (<±10% difference) and droplet size (<±1.5% difference), indicating a weak dependence of the gas phase density on two-phase atomization. This confirms that laboratory experiments on effervescent nozzles using air have reliable similarity to systems that use high temperature steam for the gas phase.     

A detailed experimental investigation of well-defined,turbulent evaporating spray jets of acetone

This paper aims at investigating the detailed structure of turbulent non-reacting dilute spray flows using advanced laser diagnostics. A simple spray jet nozzle is designed to produce a two-phase slender shear flow in a co-flowing air stream with well-defined boundary conditions. The carrier flow is made intentionally simple and easy to model so that the focus can be placed on the important aspects of droplet dispersion and evaporation, as well as turbulence–droplet interactions. Phase Doppler interferometry is employed to record droplet quantities, while planar laser-induced fluorescence imaging is applied separately to obtain acetone vapour data. Measurements are conducted for four acetone spray jets in air at several axial stations starting from the nozzle exit. The combined liquid and vapour mass fluxes of acetone integrated across the jet at downstream locations agree satisfactorily with the total mass flow rate of acetone injected.     

Supersonic liquid jets: Their generation and shock wave characteristics

The generation of high-speed liquid (water and diesel fuel) jets in the supersonic range using a vertical single-stage powder gun is described. The effect of projectile velocity and mass on the jet velocity is investigated experimentally. Jet exit velocities for a set of nozzle inner profiles (e.g. straight cone with different cone angles, exponential, hyperbolic etc.) are compared. The optimum condition to achieve the maximum jet velocity and hence better atomization and mixing is then determined. The visual images of supersonic diesel fuel jets (velocity about 2000 m/s) were obtained by the shadowgraph method. This provides better understanding of each stage of the generation of the jets and makes the study of their characteristics and the potential for auto-ignition possible. In the experiments, a pressure relief section has been used to minimize the compressed air wave ahead of the projectile. To clarify the processes inside the section, additional experiments have been performed with the use of the shadowgraph method, showing the projectile travelling inside and leaving the pressure relief section at a velocity of about 1100 m/s. Received 23 January 2001 / Accepted 2 July 2001     

PIV Measurements in the Near and Intermediate Field Regions of Jets Issuing from Eight Different Nozzle Geometries

An experimental study was conducted to investigate the effect of nozzle geometry on the mixing characteristics and turbulent transport phenomena in turbulent jets. The nozzle geometry examined were round, square, cross, eight-corner star, six-lobe daisy, equilateral triangle as well as ellipse and rectangle each with aspect ratio of 2. The jets were produced from sharp linear contoured nozzles which may be considered intermediate to the more widely studied smooth contraction and orifice nozzles. A high resolution particle image velocimetry was used to conduct detailed velocity measurements in the near and intermediate regions. It was observed that the lengths of the potential cores and the growth rates of turbulence intensities on the jet centerline are comparable with those of the orifice jets. The results indicate that the decay and spreading rates are lower than reported for orifice jets but higher than results for smooth contoured jets. The jets issuing from the elliptic and rectangular nozzles have the best mixing performance while the least effective mixing was observed in the star jet. The distributions of the Reynolds stresses and turbulent diffusion clearly showed that turbulent transport phenomena are quite sensitive to nozzle geometry. Due to the specific shape of triangular and daisy jets, the profiles of mean velocity and turbulent quantities are close to each other in their minor and major planes while in the elliptic and rectangular jets are considerably different. They also exhibit more isotropic behavior compared to the elliptic and rectangular jets. In spite of significant effects of nozzle geometry on mean velocity and turbulent quantities, the integral length scales are independent of changes in nozzle geometry.     

A numerical study of turbulent opposed impinging jets issuing from triangular nozzles with different geometries

In present research, two turbulent opposed impinging air jets issuing from triangular nozzles with fixed and variable exit velocity ratios and different nozzle-to-nozzle distances have been studied numerically and then compared with rectangular and circular nozzles. The finite volume method has been applied for solving mass and momentum equations. The turbulence model being used here is k-ε RNG. Distributions of pressure, turbulence, kinetic energy and its dissipation rate in various regions especially on the impingement regions have been obtained with high accuracy. Study of the nozzle geometries has shown the advantage of triangular nozzles over other geometries. First, the triangle’s base in nozzle geometry has an important role in our study case which, mixing two flows and regions with high turbulence intensity, directly depends on it. Second, our results show that circular and rectangular nozzles have less efficiency than triangular nozzles in mixing applications. Third and last, it was found that the radial jet being created by opposed jets has some similarities to free jets. In this investigation, air in standard atmospheric pressure has been applied as working fluid.     

CFD modelling of most probable bubble nucleation rate from binary mixture with estimation of components’ mole fraction in critical cluster

The employment of different mathematical models to address specifically for the bubble nucleation rates of water vapour and dissolved air molecules is essential as the physics for them to form bubble nuclei is different. The available methods to calculate bubble nucleation rate in binary mixture such as density functional theory are complicated to be coupled along with computational fluid dynamics (CFD) approach. In addition, effect of dissolved gas concentration was neglected in most study for the prediction of bubble nucleation rates. The most probable bubble nucleation rate for the water vapour and dissolved air mixture in a 2D quasi-stable flow across a cavitating nozzle in current work was estimated via the statistical mean of all possible bubble nucleation rates of the mixture (different mole fractions of water vapour and dissolved air) and the corresponding number of molecules in critical cluster. Theoretically, the bubble nucleation rate is greatly dependent on components’ mole fraction in a critical cluster. Hence, the dissolved gas concentration effect was included in current work. Besides, the possible bubble nucleation rates were predicted based on the calculated number of molecules required to form a critical cluster. The estimation of components’ mole fraction in critical cluster for water vapour and dissolved air mixture was obtained by coupling the enhanced classical nucleation theory and CFD approach. In addition, the distribution of bubble nuclei of water vapour and dissolved air mixture could be predicted via the utilisation of population balance model.     

An experimental investigation of underexpanded jets from oval sonic nozzles

 Underexpanded jets from oval sonic nozzles were experimentally studied for various pressure ratios up to 20.3. The results revealed that the barrel type of shock structure was present only in the major axis plane except at low aspect ratios. The results also revealed that the jet spreading rate in the minor axis plane of the nozzle was much higher compared to that in the major axis plane, resulting in axis switching of jets. The cross sectional area of these jets were considerably higher compared to the axisymmetric jets indicating higher interface area for viscous mixing in the near field region and increased mixing in the far field region clear of shock structure. Received: 7 January 1997 / Accepted: 7 August 1997     

Comparison-speed liquid jets

The performance of a small high-speed liquid jet apparatus is described. Water jets with velocities from 200 to 700 m/s were obtained by firing a deformable lead slug from an air rifle into a stainless steel nozzle containing water sealed with a rubber diaphragm. Nozzle devices using the impact extrusion (IE) and cumulation (CU) methods were designed to generate the jets. The effect of the nozzle diameter and the downstream distance on the jet velocity is examined. The injection sequences are visualized using both shadowgraphy and schlieren photography. The difference between the IE and CU methods of jet generation is found.     

Effectiveness of secondary tabs for supersonic mixing

The effect of vortex generators, in the form of small tabs projecting into the flow at the nozzle exit, aided by secondary tabs on either side, on the mixing characteristics of an axisymmetric jet at Mach number 1.7 is investigated. Experimental studies on the basic features of the jet from a nozzle with secondary tabs are conducted to assess the free jet characteristics as well as the momentum and thermal mixing behavior. The secondary tabs were found to increase the jet spread and distort the jet cross-section and were found to cause substantial enhancement of mixing of supersonic jets. Jet structure is observed using flow-visualization techniques. LLS images are employed to obtain cross-sectional views of the jet with the introduction of secondary tabs. The ability of secondary tabs to eliminate the screech noise of the supersonic jet is also observed. Received: 3 February 2000/Accepted: 8 February 2001     

High-speed observation of ultrahigh-speed submerged water jets

Some peculiar phenomena occur around ultrahigh-speed submerged water jets accompanied by very severe cavitation erosion. Using the flow visualization technique with a xenon flash, the water jets were carefully observed, and the spatial distributions of highly erosive impulsive pressures around the jets were measured by means of a pressure-sensitive film technique. The effects of the injection pressure and the nozzle configuration are systematically clarified. Thus, the characteristics and structures of ultrahigh-speed submerged water jets are clearly shown.     

Flow field characterization of coaxial conical and serrated (chevron) nozzles

PIV measurements were performed to provide insight into the effect of serrated (chevron) nozzles on the flow field of a coaxial circular jet. The serrations were tested on the primary nozzle. Mean flow results showed that the chevron effectively redistributes momentum from the high velocity center stream outward to the lower velocity secondary stream by creating lateral jets. This leads to a more rapid decay of the peak jet velocity and a consequent reduction in the length of the jet potential core. Local increases of up to 65% in the outer stream velocity were measured. The interaction of the secondary jets with the lower velocity outer stream produces increases in turbulent kinetic energy (TKE) near the center nozzle lip. These flow field effects correlate with the jet’s acoustic emissions: Reduction of low-frequency noise due to large scale mixing and potential core shortening, and increased high-frequency noise due to increased near-field turbulence.     

Laboratory experiments of a jet impinging on a stratified interface

 The entrainment rates of vertical and inclined jets impinging on a stratified interface are measured in water tank experiments. At moderate Richardson number, the entrainment rate of the vertical jet is proportional to Ri ^-1/2, independent of Reynolds number. The inclined jets are tilted at 15° from the vertical. In one case, the jet nozzle is rotated about a vertical axis, so that the inclined jet precesses, while in the other, it is stationary. The inclined jets entrain at a rate proportional to Ri ^-3/2, whether precessing or not. This behavior is consistent with a new model of stratified entrainment which accounts for vortex persistence. Received: 15 October 1996/Accepted: 19 December 1996     

Influence of the initial conditions on axisymmetric jets in a parallel flow

An experimental investigation was made of the initial-section flow of axisymmetric helium, air, and freon-12 jets in a parallel air flow for two different velocity profiles at the nozzle exit near the boundary of the jet. In one case, the velocity profile was determined by boundary layers on the nozzle walls; in the other case, it was produced artificially by means of a honeycomb of tubes of variable length. Measurements were made of the profiles of the mean and the pulsation velocity and the temperature. The flow was also photographed. The investigations showed that, depending on the initial conditions, the intensity of mixing of the jets in the initial section at Reynolds numbers Re 10⁴ (calculated using the jet diameter) can change from the level determined by molecular diffusion to the level characteristic of developed turbulent flow. The flow structure in the annular mixing layer also depends strongly on the initial conditions. The observed ordered structures in the mixing layer are related to a section of development of perturbations near the nozzle. The ordered structures are strongly influenced by the effect on the jet of acoustic vibrations from an external source. When the initial velocity profile is produced by the honeycomb, the transition to developed turbulence may be due to the development of long-wavelength perturbations or to the development of small-scale turbulence generated by the flow over the end of the honeycomb.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 18–24, July–August, 1980.We thank V. M. levlev and K. I. Artamonov for assistance and for discussing the work.     

Special characteristics of the propagation of three-dimensional viscous jets

Numerical investigations were made of the propagation, in a supersonic wake, of uncalculated jets, flowing out of nozzles of square and rectangular cross section, and of lumped jets, made up of from two to nine individual jets; the special characteristics of their flow were investigated in the initial, transitional, and main sections. Specifically, for lumped jets, the possibility of replacing them by a single axisymmetric jet, equivalent in mass-flow rate, is discussed. To calculate a three-dimensional unexpanded supersonic jet, flowing out into a wake, in [1] it was proposed to use a numerical method for solving a simplified system of Navier-Stokes equations for steady-state flow, and numerical investigations were made of the three-dimensional interaction of four jets in a supersonic wake, at small distances from the outlet cross section of the nozzle, i.e., mainly in the initial sections of the jets, where the mixing layers along the boundaries of the jets are still not closed. Here the method of [1] is used to study the special characteristics of three-dimensional viscous jets at large distances from the outlet cross section of the nozzle in the region of the main section, where the mixing layers have come together and a single three-dimensional jet has been formed. The system of equations, the boundary conditions, the numerical method, the system of coordinates, and the nomenclature used are the same as in [1].     

An experimental study of circular sand–water wall jets

Laboratory experiments were carried out to study the effects of sand particles on circular sand–water wall jets. Mean and turbulence characteristics of sand particles in the sand–water wall jets were measured for different sand concentrations c_o ranging from 0.5% to 2.5%. Effects of sand particle size on the centerline sand velocity of the jets were evaluated for sand size ranging from 0.21 mm to 0.54 mm. Interesting results with the range of measurements are presented in this paper. It was found that the centerline sand velocity of the wall jets with larger particle size were 15% higher than the jets with smaller particle size. Concentration profiles in the vertical direction showed a peak value at x/d = 5 (where x is the longitudinal distance from the nozzle and d is the nozzle diameter) and the sand concentration decreased linearly for x/d > 5. Experimental results showed that the turbulence level enhanced from the nozzle to x/d = 10. For sand–water wall jets with a higher concentration (c_o = 1.5–2.5%), the turbulence intensity became smaller than the corresponding single-phase wall jets by 34% due to turbulent modulation. A modified logarithmic formulation was introduced to model the longitudinal turbulent intensity at the centerline and along the axis of the jet.     

Characteristics of impact-driven high-speed liquid jets in water

This paper describes a preliminary investigation of the characteristics of high-speed water jets injected into water from an orifice. The high-speed jets were generated by the impact of a projectile launched by a horizontal single-stage powder gun and submerged in a water test chamber. The ensuing impact-driven high-speed water jets in the water were visualized by the shadowgraph technique, and the images were recorded by a high-speed digital video camera. The processes following such jet injection into water, the jet-induced shock waves, shock wave propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud re-generation were observed. Peak over-pressures of about 24 and 35 GPa measured by a Polyvinylidence difluoride (PVDF) piezoelectric film pressure sensor were generated by the jet impingement and the bubble impingement, respectively. The peak over-pressure was found to decrease exponentially as the stand-off distance between the PVDF pressure sensor and the nozzle exit increases.     

Performance studies of energy consumption for single and multiple nozzle systems under impinging air jets

Impinging air jets of various shapes, sizes and configurations are commonly used in heating, cooling and drying industrial processes. An analytical study has been carried out to optimise the thermal performance of single and multiple nozzle systems using impinging air jets. The optimisation of the nozzle array was given for practical purposes. The results show that within practical limits, a narrower nozzle size results in a greater heat and mass transfer coefficient. An economical analysis of the drying processes is also given for slot nozzles.