Effervescent Atomizer Research Articles

Comparison of an Effervescent Nozzle and a Proposed Air‐Core‐Liquid‐Ring (ACLR) Nozzle for Atomization of Viscous Food Liquids at Low Air Consumption

AbstractRecent results regarding the effervescent atomization of viscous food‐based liquids reveal that an unfavorable slug flow is formed inside the nozzle for low gas to liquid ratios (GLRs). As a result, a very unsteady spray drop size is formed, which leads the nozzle to be unsuitable for an implementation in spray drying. In this study, we propose a modified nozzle design named ACLR (Air‐Core‐Liquid‐Ring) to establish the favorable annular flow inside the exit orifice. High‐speed imaging proved that the desired flow pattern was present in the investigated GLR range of 0.015–0.414 and for liquid viscosities of 0.001–0.308 Pa·s. Comparison with a conventional effervescent nozzle showed similar values of the time averaged Sauter mean diameter of the spray drops. Considering the temporal unsteadiness of the spray drop size, a considerable reduction can be achieved for viscous liquids and low GLRs by the proposed ACLR nozzle.Practical ApplicationsFor an economic spray drying process, the highest possible dry matter content prior to drying is aspired. However, as feed viscosity increases rapidly with an increase in dry matter content, atomization is considerably hindered. The effervescent nozzle, originally invented in the field of combustion science, is reported to atomize viscous liquids into small spray drops at low gas consumption. Recent results of our group reveal that an unsuitable slug flow is formed inside the nozzle when food‐based liquids with elevated viscosities are atomized at low gas to liquid ratios. Consequently, the effervescent nozzle is not suitable for an implementation in spray drying. In this study, we propose a modified nozzle design that overcomes the drawbacks of a conventional effervescent nozzle. The Air‐Core‐Liquid‐Ring nozzle offers the possibility to save large amounts of drying energy of existing spray dryers as higher concentrated feeds might be used.

Twin-fluid atomization of viscous liquids: The effect of atomizer construction on breakup process, spray stability and droplet size

This study focuses on the low-pressure spraying of viscous liquids (μ=60, 147 and 308mPas) using four types of internal-mixing twin-fluid atomizers. We compare two well-known designs, namely the Y-jet and “outside in gas” (OIG) effervescent atomizers, with our new design (CFT) and an “outside in liquid” (OIL) configuration for the effervescent atomizer. The atomizers were operated by two gas inlet pressures (0.14 and 0.28MPa) and various gas-to-liquid ratios (GLR=2.5%, 5%, 10% and 20%). The comparison focused on internal liquid–gas flow, spray stability, primary breakup, and droplet size.The primary breakup was investigated using a high-speed camera. A near-nozzle spray pattern was related to the ratio of forces, which affects liquid deformation, by dimensionless numbers. The breakup was driven mainly by air resistance in the OIG, OIL, and CFT atomizers and by surface tension in the Y-jet atomizer.The OIL and Y-jet atomizers provided the most stable spray, regardless of the working regime or atomized liquid. The OIL atomizer produced the smallest droplets at low GLRs, while the droplet sizes for the Y-jet atomizer increased significantly at low GLRs. For the OIG atomizer, spray stability was influenced by the GLR, with the best stability being achieved at a GLR of 10% and 20%. The presence of large droplets at a low GLR caused an increase in droplet size. Switching the inlet ports of the effervescent atomizer (OIG–OIL) affected the internal flow, which differed under the same working regimes for these two configurations. The internal flow pattern of the OIL atomizer was estimated to be annular for all regimes, while for the OIG atomizer, it changed from a plug to slug flow with an increase in the GLR.

Influence of feed viscosity on the two-phase flow inside the exit orifice of an effervescent atomizer and on resulting spray characteristics

The effervescent atomizer, originally developed for combustion applications, is reported to atomize viscous liquids into small drops at low working costs. This offers the possibility to enhance the energy efficiency of spray drying foodstuffs by the atomization of a higher concentrated feedstock. For a stable drying process a steady formation of small drops must be ensured which depends mainly on the two-phase flow inside the exit orifice. In this study, the influence of the viscosity of a food based liquid on the flow pattern inside the exit orifice was investigated by measuring the time-dependent gas phase distribution using an optical sensor. Further, the spray in the near nozzle region as well as the shape and the size of the spray drops was characterized by means of shadowgraphy. The results indicate an atomization into small and spherical drops when an annular flow is formed inside the exit orifice. In contrast, very large and deformed spray drops are present for a slug flow, which are critical for spray drying purposes.

Performance of Twin-Fluid Atomizers for Atomization of Viscous Solutions

Presented paper deals with a comparison of two internally mixing twin fluid atomizers. The well - known Y- jet atomizer and so called outside-in-liquid effervescent atomizer (OUIL) were investigated. The working regimes were defined by the pressure drop (Dp) and the gas to the liquid ratio (GLR). The internal and the external two-phase flows of both atomizers were studied. The influence of the mixing mechanism on the internal flow was evaluated by the gas to the liquid momentum ratio (F). In advance, the stability of the separated flow (liquid film) was examined in term of the critical wavelength of the surface disturbances (lc). The external flow was observed by the high - speed camera. The influence of the basic forces on the deformation of the liquid was determined by a dimensionless criterion w·m / s. The values of F 3, where the liquid momentum overcomes the gas momentum. The values of w·m / s > 20 for both atomizers indicates the dominant influence of the viscosity and the drag force on the breakup process.

Investigation on the Applicability of the Effervescent Atomizer in Spray Drying of Foods: Influence of Liquid Viscosity on Nozzle Internal Two‐Phase Flow and Spray Characteristics

AbstractThe influence of viscosity of a food‐based model liquid on the flow patterns inside an effervescent atomizer as well as on the spray was investigated. Viscosity ranged from 0.001 to 0.308 Pa·s. The flow pattern inside the atomizer changes from annular to an unfavorable slug flow for liquids with increased viscosity with increasing liquid volume flow rate. We used the time averaged Sauter mean diameter (SMD) as well as the fluctuation κt of the time‐discrete drop size around its mean to characterize the spray. For an annular flow, small values of SMD and κt are observed regardless of viscosity; but as the flow pattern transforms to slug flow, a considerable increase of SMD and κt is observed. The results indicate that the application of the effervescent atomizing principle to the tasks of food spray drying requires a specific flow pattern inside the atomizer.Practical ApplicationThe effervescent atomizer, originally invented in the field of combustion science, is reported to be capable of atomizing liquids with elevated viscosity into small drops at low gas consumption. These attributes make the atomizing principle also interesting for other techniques such as the spray drying of foods. If an implementation is possible, the atomization technique offers the potential of drying feeds with high viscosity at low gas consumption. This, in turn, opens two new possibilities in spray drying of foods: first, to save drying energy via a higher concentrated feed and, second, to prevent heat‐sensitive ingredients from damage because a lower feed temperature prior to atomization could be chosen compared with conventional atomizers. Unfortunately, an unsteady spray is reported for certain operation conditions. In this case, an application is likely to be limited.

Unsteadiness of the internal flow in an effervescent atomizer nozzle

In order to better understand the mechanisms that effect spray unsteadiness of effervescent atomizers and therefore to efficiently eliminate it, the two-phase flow pattern within the nozzle has been studied by pressure measurement and flow visualization. We found that the speed of sound in the two-phase flow can be predicted by the model suggested by Nguyen et al. (Int J Multiphase Flow 7:311–320, 1981) using an averaged void fraction value. Dominant peak frequencies of pressure fluctuations both inside and outside the mixing chamber become synchronized in the slug flow regime. We show that the slug passage frequency is linearly proportional to the speed of sound and almost equal to the fundamental frequency of the mixing chamber. At a constant liquid flow rate and as the gas flow rate is increased, pressure fluctuations reach a maximum value with the appearance of a synchronized peak frequency. The amplitude of pressure fluctuation is then insensitive to further increases in the gas flow rate. For a constant gas flow rate, the pressure fluctuation increases with the increment of the liquid flow rate, reaching the maximum value at the moment of transition from slug flow to the bubbly flow.

Experimental Study and Mechanism Analysis of Effervescent Atomization

The flowand spraycharacteristic of effervescent atomization nozzle has been studied experimentallyandthe inner flow fieldwasanalyzedwithnumerical method. Experiments show that the injection pressure and air/liquid ratio (ALR) are the major factors impacting the atomization stability, and the flow coefficientof the nozzleranged in 0.1-0.15. The VOF simulation of the inner flow field of the nozzle reveals the generaltransformationprocess of the bubble, and the region near the exit orifice plays the dominant roleduringtheatomizationprocesswhere miniature bubbles are generated, expanded in size and accelerated and interacting with the local flow field.

Characteristics of Swirl Effervescent Atomizer Spray Angle

In the application of sprays produced by an atomizer, spray angle is one of key performance parameters. A larger spray angle is often required in providing a better spray dispersion. Swirl effervescent atomizer is one of the existing atomizers with the capability to produce a large spray angle. The formation of spray angle from this atomizer however is hardly understood. A newly-designed swirl effervescent atomizer was developed and tested with different swirl-generating vane angle in order to understand the swirl intensity effect on the spray angle. Experiments were carried out based on a cold flow test approach using water as the working fluid and nitrogen gas as the atomizing agent. High-speed shadowgraph technique was deployed to record the resultant sprays produced. Video recordings, acquired using a high-speed video camera, were converted to a sequence of images for further analysis using an image processing software. It was found that the spray angle increases with the swirl-generating vane angle. Specifically, the spray angle shows an abrupt increase for the case of swirl-generating vane angle changing from 30° to 45° but visualizes only a gradual increase in the case of swirl-generating vane angle changing from 45° to 60°.

Bubble formation and flow instability in an effervescent atomizer

Bubble formation from a gas jet in a liquid cross flow inside the mixing conduit of an effervescent atomizer is studied using flow visualization. Air injection rates were varied from 0.99 × 10−5 to 4.94 × 10−5 kg/s, while maintaining a constant liquid cross-flow rate of 0.298 kg/s. Though gas–liquid ratio (GLR) and flow velocities dominate bubble formation, intermittent choking at the exit nozzle is shown to have a morphological impact on the bubbly flow. This impact is visible as a pressure wave that propagates upstream from the nozzle exit. This pulse is found to have a strong influence on bubble formation at the gas injection location leading to bubble distortion and breakup. In most cases, a liquid jet is observed to propagate through the centreline of larger bubbles as the pressure pulse passes.

Predicting the Liquid Film Thickness and Droplet–Gas Flow in Effervescent Atomization: Influence of Operating Conditions and Fluid Viscosity

Abstract The droplet–gas flow in effervescent atomization was simulated using a comprehensive numerical model. Liquid film thicknesses in the nozzle exit orifice and droplet size distribution at the downstream of spray were calculated. The thickness of liquid film in the nozzle exit orifice increased and approached the droplet size in the primary atomization, as the air–liquid ratio increased. The primary breakup model can accurately predict the Sauter mean diameter in the primary atomization when gas–liquid two-phase flow belongs to the annular flow in the nozzle exit orifice. The viscosity of fluid had minimal influence on the liquid film thickness for spray with fluids, the viscosity of which is significantly greater than that of water. Droplet size initially decreased and then increased along the axial distance because of the secondary atomization and droplet coalescence at the downstream of spray.

A Study on the New Portable Effervescent Atomized Water Mist Equipment

A new effervescent atomized water mist fire extinguishing equipment was developed by combining the characters of water mist and effervescent atomization. A series of wood crib and gasoline pool fire suppression experiments were performed using the equipment. The fire extinguishment time, temperature and radiation heat flux of the flame were measured, and then the process of putting out different type fire was analyzed. A wood crib fire of 0.6A can be put out in 180 seconds using water mist produced by effervescent atomization equipment. Only 38 seconds was taken to suppress a gasoline fire.

The experimental study on the viscosity effect on the discharge coefficient for effervescent atomizers

This paper draws a link between the liquid viscosity and the discharge coefficient for effervescent atomization. The experimental data shows that CD is a non-linear function of GLR, with value decreasing rapidly as GLR is increased from zero to around 0.07 and thereafter decreasing at a slower rate with further increase in GLR. Discharge coefficient depends on liquid viscosity. Mean drop size strongly depends on liquid viscosity and increases markedly with an increase in viscosity. The results obtained for CD and SMD are connected with character of flow and atomization mechanism. Mathematical analysis shows that CD depends on the shear viscosity in the power of −0.0475 and GLR in power of −0.80.

Energy conversion during effervescent atomization

The paper qualitatively describes processes during the internal flow, discharge of two-phase mixture as well as spray formation with a particular focus on the energy transfer during effervescent atomization of light heating oil. The near nozzle spray visualization elucidates the liquid breakup process at different operation modes. Numerical results illustrate the forms of energy involved in the atomization process, their values and the influence of operational conditions on relations between the energy forms. The main part of the paper focuses on the atomization efficiency. A simple method for estimation of the atomization efficiency of pneumatic atomizers is proposed; surface tension energy of created droplets, obtained from phase-Doppler anemometry data, is compared with the energy required for atomization. The atomization efficiency of effervescent atomizers is found to be in fragments of per cents for atomizing pressures ranging between 0.1 and 0.5MPa, and gas-to-liquid ratios (GLRs) between 0.02 and 0.1, and it is inferior, by about one order of magnitude to the efficiency of simple pressure and pressure-swirl atomizers for a comparable droplet size. The efficiency declines with both the pressure and GLR with approximately logarithmic tendency.

Biodiesel (EN 14213) heating oil substitution potential for petroleum based light heating oil in a 1 MW stationary combustion facility

The aim of the present work was to experimentally investigate the combustion properties of methyl-ester of rapeseed oil (RME) that was preheated for three distinct temperatures ((30, 70, 110) °C) in the study. The experiments were carried out in a water-cooled horizontal combustion chamber with a nominal thermal load of 1500 kW. The experimental results were compared to the results obtained from the combustion tests with extra light heating oil (ELHO) that was used as a comparative fuel. The fuel was atomized by means of the twin-fluid effervescent atomizer with a maximal output of 100 kg h−1 of oil at gas to liquid mass flow rate ratio (GLR) equals to 20%.Primarily the experiments were focused on the investigation of the flame characteristics and the quality of combustion at distinct heat outputs ((475, 700, 900) kW) and distinct GLR ratios ((10, 15, 20) %). Second, experiments focused on the evaluation and comparison of local wall heat fluxes along the flame length and were performed only for one setting of operating conditions, namely for the heat output of 900 kW and the GLR 15%.The results revealed that substitution of ELHO requires preheating of RME to the temperature of about 70 °C, if pneumatic atomization is to be used. The preheating of RME to higher temperature did not show any significant improvement in terms of quality of combustion. Moreover, it causes the increase in nitrogen oxides formation up to 60% in comparison with the values measured during ELHO combustion.

A phenomenological model of two-phase (air/fuel) droplet developing and breakup

Effervescent atomization namely the air-filled liquid atomization comprehends certain complex two-phase phenomenon that are difficult to be modeled. Just a few researchers have found the mathematical expressions for description of the complex atomization model of the two-phase mixture air/diesel fuel. In the following review, developing model of twophase (air/fuel) droplet of Cummins spray pump-injector is shown. The assumption of the same diameters of the droplet and the opening of the atomizer is made, while the air/fuel mass ratio inside the droplet varies.

Atomization of petroleum-coke sludge slurry using effervescent atomizer

Experimental investigations were carried out on a bench-scale atomization test facility to explore the atomization performance of petroleum coke sludge slurry (PCSS) which comprises pulverized petroleum coke, sewage sludge, water and a certain amount of chemical additives. In the experiments, effervescent atomizer was employed to spray the petroleum coke sludge slurry. And effects of air injection pressure, air-to-liquid mass flow rate ratio (ALR), exit orifice diameter and rheological properties of petroleum coke sludge slurry with different amount of sewage sludge on the Sauter Mean Diameter (SMD or D32) of the atomized droplets were investigated. Results suggest that petroleum coke particles have strong hydrophobic surface. Solid–liquid separation occurs between petroleum coke particles and water film during atomization. Sewage sludge is viscoelastic non-Newtonian fluids, which would influence the solid–liquid separation process when pulverized petroleum coke and sewage sludge are blended to prepare petroleum coke sludge slurry for atomization. SMD of atomized petroleum coke sludge slurry droplets decreases with ALR increase. However, SMD increases with spray axial distance downstream from the nozzle tip primarily and then decreases. Reduction of exit orifice diameter and increase of air injection pressure could improve the atomization performance, while sewage sludge would weaken the influence on variation of the atomized droplets diameters with increase of the spray axial distance and ALR.

Spray Characteristics of Swirl Effervescent Injector in Rocket Application: A Review

Injector is one of the vital devices in liquid rocket engine (LRE) as small changes in its configurations and design can result in significantly different LRE performance. Characteristics of spray such as spray cone angle, breakup length and Sauter mean diameter (SMD) are examples of crucial parameters that play the important role in the performance of liquid propellant rocket engine. Wider spray cone angle is beneficial for widespread of fuel in the combustion chamber for fast quiet ignition and a shorter breakup length provides shorter combustion chamber to be utilized and small SMD will result in fast and clean combustion. There are several mechanisms of liquid atomization such as swirling, e.g. jet swirl atomization or introducing bubbles into the liquid and effervescent atomization. Introducing a swirl component in the flow can enhance the propellant atomization and mixing whereas introducing bubbling gas directly into the liquid stream inside the injector leads to finer sprays even at lower injection pressures. This paper reviews the influence of both operating conditions and injector internal geometries towards the spray characteristics of swirl effervescent injectors. Operating conditions reviewed are injection pressure and gas-to-liquid ratio (GLR), while the injector internal geometries reviewed are limited to swirler geometry, mixing chamber diameter (dc), mixing chamber length (lc), aeration hole diameter (da), discharge orifice diameter (do) and discharge orifice length (lo).

The effect of extensional viscosity on the effervescent atomization of polyacrylamide solutions

The paper contains the results of experimental investigation of the rheological characteristics and atomization process of solutions of polymer. The solutions had the polyacrylamide (PAA) mass fraction in the range from 0.0025 to 0.001. The solutions of polymer have non-Newtonian characteristics. It has been shown that shear viscosity increases with polymer concentration in the solutions. Extensional viscosity increases with increasing extensional rates. The Trouton ratio for glycerol solution is about 5.4. The Trouton ratios observed for the polymer solutions increase with increasing shear (extensional) rate and polymer concentration in solution.This paper draws a link between the extensional viscosity and the effervescent atomization. The polymer addition to cause the solution viscosity to increase, thereby different disintegration of liquid could be obtained. During two-phase atomization process of polymer solutions the characteristic stages in spray development have been observed. The phenomenon of the formation of “beads on a string” for solutions with low concentrations of polymer has been observed. The thin filament between droplets and/or bubbles was observed for all polymer solutions studied. Analysis of photos on forming the droplets showed that droplets diameter increases with increasing of polymer concentration. In all two-phase systems studied the Sauter mean diameter decreases with increase of gas to liquid flow rates ratio value. Sauter mean diameter is dependent on gas and liquid flow rates and the liquid properties. It is particularly important in processes in which the value of the average droplet diameter plays a key role.High molecular polymers added to a solvent involve the changes in the rheological properties of liquid and the disintegration of liquid jets and/or sheets. The effect of extensional viscosity on the droplets formation has been shown. The data obtained in the present study have shown that the decreasing of the extensional viscosity of the solution will reduce the spray's Sauter mean diameter. The results may be used, for example, to verify numerical models or in comparison with similar atomization processes.

Experimental Study of Droplet Size Distribution Produced by a Multi-Orifice Effervescent Atomizer

A new multi-orifice effervescent atomizer used for fire fighting was developed. The droplet size distribution of the spray produced by the nozzle was measured by laser Doppler particle analyzer. And the influence of liquid injection pressure, water flow rate and atomizer internal geometry was studied.

Viscosity ratio: A key factor for control of oil drop size distribution in effervescent atomization of oil-in-water emulsions

The present study focused on the effervescent atomization of oil-in-water emulsions and its influence on the internal emulsion structure. The impact of atomization conditions and nozzle geometry on an oil-in-water emulsion with varied viscosity ratio of disperse phase to emulsion was determined. The viscosity ratio is known as a key factor for the drop breakup in emulsification processes. The results depict that oil drops are broken up according to emulsification mechanisms during atomization. A maximum breakup was found for a viscosity ratio between 0.5 and 1. The expected stress dependency of the drop size change was observed for high viscosity ratios only. Relevance of elongational stress on the drop breakup was proven with significant drop size change at high viscosity ratios. The viscosity ratio thus allows for an efficient control of oil drop size change in effervescent atomization as the spray characteristics are invariant to viscosity changes within certain limits.