Liquid Film Atomization Research Articles

Boiling induced atomization of liquid film produced by oblique jet impingement on superheated wall

For the thermal management of industrial devices, a reduction in the net coolant flow rate by droplet dispersion from a liquid film is important because it can cause unexpected thermal failure. To understand the process of droplet dispersion from a liquid film better, we experimentally and theoretically evaluated the characteristics of boiling-induced atomization in a liquid film formed by oblique jet impingement on a superheated wall. Atomization processes were visualized using magnified high-speed imaging using a backlight technique. In this study, two types of droplets were observed using high-speed-magnification imaging. These were large droplets that disintegrated from the ligament formed on a relatively high-temperature wall, and small droplets from the ligament formed via bubble bursting in the nucleate boiling regime. For the atomization induced by nucleate boiling, larger droplets were produced via bubble bursting further downstream from the impingement point because the bubble size and liquid film thickness increased. Finally, the total volume of the droplets produced by nucleate boiling was estimated from the frequency of bubble bursting and droplet size measured from the visualization results. The estimation results suggest that the ratio of the total volume flow rate of the ejected droplets to the injection flow rate of the liquid was negligible (2%). Thus, most of the injected liquid eventually reached the wetting front of the sheet, separating it from the wall before drying out.

Fouling mechanism in airblast atomizers and its suppression for water desalination

A detailed experimental study is presented on fouling behavior of the anti-clogging perforated plate atomizer designed for high salinity applications, and compared with a conventional (plain-jet) airblast atomizer. Low-pressure regions around fast moving air in the outer layer of spray (as in conventional atomizers: plain-jet and prefilming) due to Venturi suction were identified as the root cause of atomizer clogging, as they facilitate salt accumulation on the atomizer surface from spray. Accordingly, severe atomizer fouling, and fluctuations in spray cone angle were observed in the conventional airblast atomizer over 2 h at 100°C air and 50°C saline (44° to 76° at 35,000 ppm, and 44° to 91° at 100,000 ppm). In this regard, the perforated plate atomizer provides a novel liquid-film airblast atomization by maintaining a liquid-annulus film (around the air outlet) as the outer layer of spray. Doing so we achieved nearly complete suppression of fouling, and spray cone angle fluctuations (28° ± 1° at 35,000 ppm, and 30° ± 1° at 100,000 ppm). Later, novel liquid-film atomization was adopted in the conventional airblast atomizer. While, the conventional airblast atomization needed atomizer cleaning/maintenance after 35 min for 175°C air and 65°C saline at 100,000 ppm, the liquid-film atomization showed no sign of fouling over 14 h. Hence, current work establishes a benchmark liquid-film airblast atomization mechanism in the anti-clogging perforated plate atomizer for complete suppression of fouling in airblast atomization. This extends the application of airblast atomizers from high evaporation jet engines to ZLD-HDH desalination systems, spraying, powder metallurgy, pharmaceuticals and hospitals, and spray drying and cooling.

An experimental analysis of droplet charging in a wind tunnel

The aim of this paper is to introduce an experimental analysis of droplet charging that occurs due to liquid film atomization on two different surfaces. The atomization of the droplets from the liquid film at high speed is still not satisfactory understood. The phenomena can be found in steam turbines, wind turbines, aeroplanes and other technical applications. The liquid film atomization is associated with the electrostatic charge of the formed droplets. The paper describes the experiment when the coated and non-coated surfaces were tested for an estimation of the droplet size distribution function and the droplet charge. A new wind tunnel was developed for the study of liquid film atomization in similar conditions in steam turbines. The coarse droplets are formed in the steam turbine from the liquid films on the blades and inner casings. The coarse droplets formed on the non-moving blades don´t exactly follow the bulk flow and they collide with the moving blades. These collisions cause erosion and corrosion processes which have an unfavourable effect on the reliability and the efficiency of steam turbines. The tunnel is equipped with the standard instrumentation for the measurement of the flow properties and for the analysis of the size distribution function of the droplets. Two measurements methods were used to attain the size of the droplets, along with their photogrammetry and light scattering. The first tested surface is polished stainless steel and the second is the electrical isolating and hydrophobic coating. The results from the measurement suggest the possibility of the better understanding the erosion processes in steam turbines.

AN ANALYSIS OF THE LIQUID FILMS BREAK-UP USING THE COMPUTER VISION TECHNIQUE

The aim of this paper is to introduce a novel approach for analysing the droplet formation in a transonic flow. The method suggested in this work is based on the combination of the measured data from the wind tunnel by an optical measurement and image processing. A new wind tunnel was developed for the study of the liquid film atomization at high speed flows similar to ones that can be found in steam turbines. The coarse droplets in steam turbines are formed from the liquid films on the blades and inner casings. The coarse droplets formed on the stator blades don´t follow the bulk flow and collide with the following moving blades. These collisions cause erosion and corrosion processes, which have an unfavourable effect on the reliability and the efficiency of the low pressure stages of steam turbines. The tunnel is equipped with a standard instrumentation for the measurement of the flow properties and for the analysis of the size distribution of the droplets. Two measurement methods were used for the measurement of the size of the droplets, photogrammetry and light scattering. In this paper, the image processing of the captured images is discussed. The images were taken by a camera with a telecentric lens. The paper contains an assessment of three image processing methods used for the measurement of the droplet distribution by the light extinction. Moreover, the formation of the droplets is captured on the trailing edge of the profile in the flow. The results bring a new view on the formation of droplets at high speeds.

Effect of the Slot Number on the Breakup and Atomization of Liquid Film in Swirl Nozzle

Effect of the Slot Number on the Breakup and Atomization of Liquid Film in Swirl Nozzle

Simulation of Primary Film Atomization Due to Kelvin–Helmholtz Instability

Liquid film atomization under a high-speed air flow (water was considered as the liquid) due to the Kelvin–Helmholtz instability is studied using the volume of fluid (VOF) method. We develop an approach for modeling the primary breakup and use it to investigate the grid convergence, choose the optimal size of the grid cell, and calculate the primary breakup of the film in the channel. The dependences of the mean break-off angle, the velocity modulus, and the Sauter droplet diameter on the longitudinal coordinate of the channel are obtained. The step-by-step averaging over the ensemble of droplets and over time allows us to get smooth coordinate dependences of the characteristics of the ensemble of the droplet. The value of the most useful parameter for engineering applications, the mean Sauter diameter D32 (equal to the ratio of the mean droplet volume to its mean area) is close to that obtained using a semiempirical formula from the literature, based on the experiment where hot wax is atomized by a high-speed airflow. The dependence of the Sauter mean diameter on the thickness of the liquid layer agrees qualitatively with the experimental dependence. The study of the grid’s convergence showed that the number of the smallest droplets increases rapidly with decreasing cell size. Their contribution to the average characteristics of the droplet’s ensemble, however, remains insignificant; nonetheless, their input to the mean characteristics remains insignificant; thus, there is no reason to decrease the grid cell size to account for small droplets.

Condensation in Radial Turbines—Part II: Application of the Mathematical Model to a Radial Turbine Series

In the second part of this two part paper, the condensation process and the movement of the liquid phase near the impeller blades of a radial turbine are studied. The investigation methodology presented in part 1 is applied to a radial turbine type series used for waste heat recovery. First, the subcooling necessary for the beginning of the condensation process is examined and a relationship between the location of maximum subcooling and the onset of droplet deposition at the surfaces of the turbine impeller is determined. Thereafter, the movement of liquid films on the impeller blades is analysed and characterized. Correlations determining the movement of droplets originating from liquid film atomization on the edge of the impeller blade along the casing are derived. Finally, conclusions are drawn depicting the most important findings of condensing flows in radial turbines.

FROM HIGH-FIDELITY NUMERICAL SIMULATIONS OF A LIQUID-FILM ATOMIZATION TO A REGIME CLASSIFICATION

High-fidelity numerical simulations of spray formation were conducted with the aim of improving fundamental understanding of airblast liquid-film atomization. The gas/liquid interaction in the near-nozzle region is investigated for a multitude of operating conditions in order to extrapolate phenomenological and breakup predictions. To reach this goal, the robust conservative level-set (RCLS) method was used. For a fixed prefilmer geometry, we performed a parametric study on the impact of various liquid and gas velocities on the topological evolution of the liquid interface. The behavior and development of the liquid film is found to be influenced mainly by the relative inertia of the gas and the liquid, the liquid surface tension, and interfacial shear stresses. Preliminary regime maps predicting the prefilming liquid-sheet atomization behavior are constructed based on our numerical results. Three distinct types of “regime” are reported: accumulation, ligament-merging, and three-dimensional wave mode. In addition, these results also show the influence of vortex action and rim-driven dynamics on the breakup mechanism at the atomizer edge. An increase in liquid injection speed leads to the generation of smaller droplets; whereas, an increase in air velocity does not point to one simple conclusion.

Geometric Effects on Liquid Oxygen/Kerosene Bi-Swirl Injector Flow Dynamics at Supercritical Conditions

A systematic study is carried out to investigate the flow dynamics and mixing of liquid oxygen/kerosene bi-swirl injectors at supercritical conditions. The theoretical framework is based on the full conservation laws and accommodates real fluid thermodynamics and transport theories over the entire range of fluid states. Turbulence closure is achieved using large-eddy simulation. A grid independence study was conducted to ensure appropriate numerical resolution and accurate flow physics. The liquid-film atomization and breakup processes at subcritical conditions are replaced by the turbulent mixing and diffusion processes typical at supercritical conditions. Various injector geometries, with differences in the recess region, post thickness, and kerosene annulus width, are examined to explore the influence of geometry on mixing efficiency and flow dynamics. A critical mixing line is defined to measure the mixedness of propellants. The presence of a recess region is found to advance the interaction of liquid oxygen and kerosene and improve mixing efficiency. A thicker post of the inner swirler or a wider annulus leads to a larger spreading angle of the inner liquid oxygen film and intercepts the outer kerosene film in a broader area, thereby enhancing mixing in the recess region. The flow structures in the recess region are complex, and the kerosene mass fraction decreases significantly near the post surface, which might increase the thermal load on the surface in the case of reacting flows. Appropriate selection of the post thickness, recess length, and annulus width must be carefully determined for optimum injector performance. The present study offers useful information in bi-swirl injector design and in studies of the underlying flow physics of swirl injection under supercritical conditions.

A new hydrate deposition prediction model for gas-dominated systems with free water

Hydrate deposition is an important issue for flow assurance in subsea pipelines. Current models for hydrate deposition in gas-dominated systems with free water mainly consider hydrate formation in liquid films on pipe walls. However, hydrate particles formed from water droplets in the gas phase may also play a significant role in hydrate deposition. In this work, a new model for predicting hydrate deposition is proposed. This model considers hydrate formation from both liquid film and liquid droplets. In the model, an effective deposition ratio is introduced to calculate the deposition of hydrate particles from the gas phase by considering the influence of liquid film atomization. The simulation results agree well with the experimental data. It is indicated that the deposition of hydrate particles formed by liquid droplets in the gas phase has a significant influence on the reduction of the flow passage in the pipeline. By using the new model, the non-uniform distribution of the flow passage at different times and locations can be obtained, and the most vulnerable position for hydrate deposition can be predicted. The model predicts the risk of hydrate deposition more reliably than current methods and provides helpful advice for the prevention of hydrate deposition in the field.

Prediction of the entrained liquid fraction in vertical annular gas–liquid two-phase flow

The study considers the prediction of the entrained liquid fraction in adiabatic gas–liquid annular two-phase flow in vertical pipes. Nine empirical correlations have been tested against an experimental data bank drawn together in this study containing 1504 points for 8 different gas–liquid combinations and 19 different tube diameters from 5.00 mm to 57.1 mm. The correlation of Sawant, Ishii and Mishima and the one of Oliemans, Pots and Trompé were found to best reproduce the available data. A new correlating approach, derived from both physical intuition and dimensional analysis and capable of providing further physical insight into the liquid film atomization process, was proposed and worked better than any of the existing methods. This new correlation is based on the core flow Weber number that is also a controlling dimensionless group in determining the wall shear stress and associated frictional pressure gradient of annular flows.

Comments on “Liquid Film Atomization on Wall Edges-Separation Criterion and Droplets Formation Model”

Comments on “Liquid Film Atomization on Wall Edges-Separation Criterion and Droplets Formation Model”

Parametric study on the fuel film breakup of a cold start PFI engine

In order to provide more insight on improving the cold start fuel atomization for reducing unburned hydrocarbon emissions, the liquid fuel film breakup phenomenon in the intake valve/port region was investigated in depth for port-fuel-injected engines. Experiments were conducted using high-speed high-resolution imaging techniques to visualize the liquid film atomization and airflow patterns in an axisymmetric steady flow apparatus. The impact of valve/port seat geometry, surface roughness, and fuel properties on airflow separation and fuel film breakup were determined through a parametric study. CFD simulations were also performed with FLUENT to help understand the airflow behavior inside the intake port and valve gap region and its potential impact on fuel film atomization.

Liquid Film Atomization on Wall Edges—Separation Criterion and Droplets Formation Model

In order to predict the fuel mixture preparation inside the cylinder of port fuel injection engines, a model for the aerodynamic stripping of the fuel film deposited on the manifold walls is discussed, and a model for the fuel film separation and atomization near the sharp edges is developed. A separation criterion is set up using an analogy with Rayleigh-Taylor instabilities driven by the inertial forces of the liquid film. To determine the physical parameters of the resulting droplets, a liquid sheet atomization scheme is used. The critical value for the separation criterion is adjusted using experimental data obtained in 2D wind tunnel equipped with different steps shaped as a valve seat, and reproducing the main characteristics of the intake of spark ignition engine. CFD simulations are performed using the KMB code, a modified version of KIVA-2 already including a stochastic Lagrangian description of the spray, and an Eulerian liquid film model. Computations results for different operating conditions are in good agreement with the images of film separation and measured droplet size distributions.

Generalized liquid film atomization theory

The increase of the fuel burning area required by most practical combustion processes in order to guarantee the minimum energy density rate release for their start up and operation is normally achieved by the proper choice among several existing types of atomizers. For instance, impinging and multi-impinging jets atomizers are used in rocket combustion chambers, while splash-plate atomizers find their use when wall film cooling is required. Pressure swirl atomizers, either of simplex or duplex kind, along with Y-Jet or Spider Jet atomizers are used in industrial applications and in turbine combustion chambers. Notice, however, that all the types of atomizing devices listed above have one point in common: they are of pre-filming kind, i.e., before the droplet spray is generated, a liquid film is formed. This liquid film is broken into unstable ligaments which contract under the action of surface tension forming the droplets. Once the film thickness is estimated, the droplets’ SMD (Sauter Mean Diameter) can be calculated, yielding a crucial parameter for the combustion chamber design. However, although this mechanism of droplet formation has been under study for several decades, most of the available results, are based upon experimental data, valid for a special type of atomizer under the given specific conditions only. This work offers a generalized theory for theoretically estimating the SMD of sprays generated by liquid pre-filming atomizers in general.