Effervescent Sprays Research Articles

Novel atomizer concept for CCS applications: Impinging effervescent atomizer

The concept design, testing of spray characteristics, and performance comparison of an original type of atomizer, the impinging effervescent atomizer (IEA), are introduced in this study. The liquid impinging is beneficially deployed in plain-orifice and external mixing twin fluid atomizers. IEA uses a collision of two or more effervescent liquid streams to improve the atomization and alter the spray characteristics. The Phase Doppler Anemometry (PDA) measurement technique and high-speed visualization of the liquid discharge and spray structure were used. The discharge coefficient (Cd), atomization efficiency (ηa), spray characteristics as a spray cone angle (SCA), integral Sauter mean diameter (ISMD), integral relative span factor (IRSF), and droplet size and velocity distribution inside the spray were investigated. The correlations for Cd, SCA, ISMD and IRSF are provided for a fast and reliable atomizer design to fit any application and operating conditions. The IEA provides a controllable SCA governed mainly by the exit orifice geometry. The ISMD is primarily controlled by the gas to liquid ratio (GLR) and the inlet pressure (pin). The impinging angle (β) has little effect on ISMD. A narrower droplet size distribution, mass redistribution within the spray, and a more uniform droplet size spatial distribution were observed for the IEA in comparison to other tested atomizers. The IEA spray covers 116% more area than single-orifice effervescent sprays, and the area coverage can be controlled by β. This coverage increases by 177% if β changes from 15 to 45°. Spray eccentricity (e) decreases with the increase of the number of the exit orifices (No) resulting in a more uniform droplet spatial distribution. The β, GLR and pin have no effect on e. The ηa of the IEA is equivalent to other internally mixing twin-fluid atomizers; the atomization air contains more than 90% of the delivered energy. The favourable IEA spray characteristics, such as IRSF, ISMD, spray coverage, SCA and ηa, enable the potential use of IEA in mass-transfer related processes and especially in CO2 capture columns.

Continuing Results for Effervescent Aerosol Salt Water Spray Nozzles Intended for Marine Cloud Brightening

Marine Cloud Brightening (MCB) by effervescent spray atomization of mixed sea water brine with air is a candidate for solar radiation management to compensate for global warming. We discovered that the flow from mixing tee nozzle described earlier had occasional unstable slug flow. A new design that adding rotational swirl to the salt brine as it is mixed into the air stabilized the nozzle flow and no longer showed slug flow in spray pictures. Flow equations were developed for the relatively low speed of sound of a choked flow mixed brine and air nozzle. Experimental mixed flow measurements with 300b pressure and a 200 μm diameter nozzle and calculations using perfect gas, and isotropic processes equations compared well with the chocked flow equations. Analysis in EXCEL of particle sizers measurements from both a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS) showed production of many nanometer sized particles estimated as usable for MCB. A small number of micron sized particles were also always present but with about 90% of the sprayed mass. This is a first report with good data over the complete size range. The micron sized particles measured were similar to the measurements of earlier reports which reported no nanometer sized particles. We hypothesize that many nano-particles are always produced by liquid-air effervescent sprays, but earlier, were not observed because SMPS instruments were not available. The presence of the large mass percentage of large particles in the spray may cause problems by evaporative cooling preventing the rise of the MCB particles. We suggest future systems design with an impactor filter to remove the large particles. Calculations combining increased brine concentration, lower pressure, and larger nozzle area showed that significant reductions in required power and number of nozzles could be realized. An EXCEL model is developed to calculate flow from experimental analysis equations and compare with mixed choked flow equations. Solving with the model predicted the power required and the number of nozzles required to produce 1015 particles/s. The model showed that increasing brine concentration strongly lowered total power. Lowering pressure decreased power and increased number of nozzles. Increasing nozzle area lowered the number of nozzles. This model predicted that, at 300b pressure and 200μm diameter nozzle as the experiment but using an increased brine concentration of 0.1 instead of 0.032 would require only 115 nozzles instead of 358 and power of 146 kw instead of 493 kw. Combining increased brine concentration, lower pressure, and larger nozzle area, the model predicted that with a 1 mm diameter nozzle at 30b pressure and salt concentration of 0.2, the nozzle count and power required would drop to only 24 nozzles and power of 28 kw. Whether extending the model to these conditions is valid is not known but suggests further development should be investigated. Filtering out and reusing the 90% or greater large particles mass sprayed combined with the lower power advantage of higher brine concentration is suggested for future systems.

Acoustic performance of effervescent sprays by time–frequency method with different atomizer structures under different operating conditions

The acoustic performance of effervescent sprays was experimentally investigated at various liquid flow rates and gas to liquid ratios (GLRs) by mass under pressure<0.5MPa. A transparent effervescent atomizer with different mixing chambers and exit orifices was used in the study. An unconventional description of spray characteristics under various internal flow regimes was obtained using the adaptive optimal kernel (AOK) spectrogram of the spray acoustic in the time–frequency domain. Acoustic energy was calculated using the Hilbert Huang transform (HHT) method, which provides a quantitative comparison of vibration intensity under different structures and operating conditions. A new evaluation of spray unsteadiness was established based on acoustic energy analysis. Results showed that gas flowing out of the orifice leads to an increase in the amplitude of high frequency components. The spray characteristics under different internal flow regimes could be reflected from the AOK spectrograms, through which a new perspective on effervescent sprays in the time domain is presented. Acoustic energy was mostly influenced by both the air flow rate and geometry of the exit orifice and showed reduced sensitivity to the structure of the mixing chamber. The structure of the atomizer affected the internal gas–liquid two-phase flow structures and further influenced spray steadiness. A study of the present condition revealed that a structure with a farther location of aerator holes and a small exit orifice diameter strengthens the stability of pray performance.

Spray characteristics and liquid distribution of multi-hole effervescent atomisers for industrial burners

The present paper provides an experimental study and optimisation of multi-hole effervescent atomisers for industrial burners using oil-based fossil, bio- or waste fuels with prospects of emission reduction. Several multi-hole nozzles were designed based on our previous work. We probed the spray quality by phase-Doppler anemometry. 3-D plots of Sauter mean diameter and mean droplet velocity demonstrate their spatial distribution within the spray. The effect of geometrical and operational factors on the spray is discussed. Droplet size–velocity correlations as well as the size and velocity distributions are presented, and differences are found against other investigations. A spray macrostructure is photographically observed and spray cone angles of the multi-hole nozzles are analysed. An internal two-phase flow is estimated using the Baker's map for horizontal two-phase flow. Our previous two-phase flow visualisations suggested a liquid–gas gravitational separation when the multi-hole atomiser operated horizontally. This issue is addressed here; the results of spray heterogeneity measurements document that fuel flow rates through individual exit holes differ significantly. This difference spans between 0 and 70% depending on the nozzle design and flow regime. Effervescent sprays are unsteady under some operating conditions; spray unsteadiness was detected at low pressure and low gas-to-liquid-ratios.

Analysis of effervescent spray quality for oil-fired furnace application

The quality of liquid fuel atomization is a key factor in the formation of gaseous pollutants and particulate emissions. This paper investigates the quality of effervescent spray, which has established itself as a promising alternative to conventional spray formation mechanisms. The reported results were obtained using a dual-phase Doppler analyser. Droplet data were collected in multiple locations varying both axially and radially, in intermediate spray region. The results display spatial development of Sauter mean diameter (SMD) in effervescent sprays at different operating conditions, but they also show that SMD may be insufficient to characterize the spray, especially near to the dense spray region. This conclusion is based on the observed large variance of drop size and velocity and on their mutual correlation. It is concluded that for modelling purposes, joint distributions of drop size and velocity would be more appropriate to fully characterize the primary atomization in an effervescent spray.

Time–frequency analysis of acoustic and unsteadiness evaluation in effervescent sprays

The acoustic signals from effervescent sprays were experimentally investigated across a range of injection pressures, flow rates, and gas to liquid ratios by mass (GLR). An unconventional description of spray features under various operational conditions was obtained through the time–frequency–amplitude distributions of spray acoustic by adaptive optimal kernel (AOK). The acoustic energy was calculated through the Hilbert Huang transfer (HHT) method and was confirmed to be affected by both the GLR and air flow rate. A new evaluation of spray unsteadiness was established based on this finding. The results show that the gas flowing out of the orifice leads to an increase in the amplitude of the high frequency component. Discrete phenomenon in effervescent sprays was exactly observed by AOK spectrogram as the amplitude distribution was discontinuous with time and the amplitude fluctuations showed variation in different conditions. The factors influencing the acoustic energy include both air and liquid flow rate. The unsteadiness levels under different conditions were calculated after the new spray unsteadiness was evaluated through a simple atomizer. The unsteadiness levels had a low value when the GLR was higher than 5% and varied across a wide range when the GLR was below 4%. The flow regime had a significant effect on the spray unsteadiness as the bubbly and churn flow regimes produce a relatively stable spray, whereas the slug flow regime induces unsteadiness.

Characteristics of droplet motion in effervescent sprays

Time resolved droplet size and velocity measurement was made using Phase-Doppler anemometry in an effervescent spray at GLR of 6 % and operation pressure drops 21 – 52 kPa. The spray shows a size dependent variation of mean as well as fluctuating axial and radial velocities of droplets similarly for all operation regimes. Particles under 13 μm follow the gas flow, axially decelerated due to gas expansion. Velocity of medium sized particles is positively size correlated and larger particles keep high velocity, given them during discharge. Fluctuating radial velocity of small particles is larger than that of large particles while fluctuating axial velocity increases with size. Small particles thus reach a ratio of radial to axial velocity fluctuations ~ 0.6 but large particles only ~ 0.1, which indicates large transverse dispersion of small particles. Overall fluctuating velocity ratios smaller than 0.5 document an anisotropic character of the liquid mass fluctuations. Power spectral density (PSD) of axial velocity fluctuations of large droplets is uniform up to 1 kHz, while PSD of smaller particles drops down with frequency for frequencies > 100 Hz. Large particles thus preserve the fluctuations imposed during discharge while the gas turbulence drops with frequency. Turbulence intensity reaches 14 to 21 % depending on pressure. Such high-turbulence character of the flow probably results from a heterogeneous gas–liquid mixture at the discharge.

Model for Predicting the Mean Drop Size in Effervescent Sprays

This paper is focused on the development of a model for predicting the mean drop size in effervescent sprays. A combinatorial approach is followed in this modeling scheme, which is based on energy and entropy principles. The model is implemented in cascade in order to take primary breakup (due to exploding gas bubbles) and secondary breakup (due to shearing action of surrounding medium) into account. The approach in this methodology is to obtain the most probable drop size distribution by maximizing the entropy while satisfying the constraints of mass and energy balance. The comparison of the model predictions with the past experimental data is presented for validation. A careful experimental study is conducted over a wide range of gas-to-liquid ratios, which shows a good agreement with the model predictions: It is observed that the model gives accurate results in bubbly and annular flow regimes. However, discrepancies are observed in the transitional slug flow regime of the atomizer.

Model for Predicting the Mean Drop Size in Effervescent Sprays

Model for Predicting the Mean Drop Size in Effervescent Sprays

Effect of Air Injection Configuration on the Characteristics of Effervescent Sprays

Effect of Air Injection Configuration on the Characteristics of Effervescent Sprays

Effect of Air Injection Configuration on the Characteristics of Effervescent Sprays

Effect of Air Injection Configuration on the Characteristics of Effervescent Sprays

MASS DISTRIBUTION STUDIES IN EFFERVESCENT SPRAYS

An experimental study on liquid mass distribution in effervescent sprays using water and air as working fluids is presented in this paper. Optical patternation and techniques of image processing are employed for analyzing the spray. The flow regime inside the effervescent atomizer largely dictates the mass distribution patterns. The patterns are seen to vary from concentrated, poorly atomized liquid lumps to uniformly distributed, fine droplets as the flow regime changes from bubbly flow to annular flow. A large variety of instantaneous spray patterns are observed in bubbly flow regime indicating a highly unsteady atomization process. However, relatively better consistency in spray patterns is observed at higher gas flow rates. Thus, the degree of unsteadiness gradually diminishes as gas flow rate is increased. The axial evolution of the spray in annular mode shows good mixing of liquid and gas across the interface.

Evaluation of unsteadiness in effervescent sprays by analysis of droplet arrival statistics – The influence of fluids properties and atomizer internal design

The ideal spray theory of Edwards and Marx was used to investigate the dependence of effervescent spray unsteadiness on fluid properties and atomizer internal design. Results demonstrate that fluid properties and internal design of atomizer directly affect the two-phase flow pattern inside the atomizer which consequently affects the spray unsteadiness of the atomizer. Water sprays are more unsteady when the air to liquid ratio ( ALR) increases, whereas, more unsteady is observed for using glycerol/water mixture (high-viscosity Newtonian fluid) or glycerol/water/xanthan (non-Newtonian fluid) mixture as ALR reduces. In addition, sprays using low-viscosity or strong non-Newtonian fluids usually are more unsteady, regardless of ALR. A short mixing chamber results in less unsteady for water but has no effect on spray unsteadiness for high-viscosity or non-Newtonian fluids at ALR of 0.15. Otherwise, the influence of mixing chamber distance on the spray quality is weak at ALR of 0.15. Large diameter of inclined aeration holes shows the low spray unsteadiness and good spray quality for water but causes more unsteady for glycerol/water/xanthan mixture at ALR of 0.15. Furthermore, the diameter of the inclined aeration holes has little influence on spray unsteadiness for glycerol/water mixture. Spray unsteadiness and quality are not affected by the angle of aeration holes for three fluids at ALR of 0.15.

Ballistic imaging in the near-field of an effervescent spray

We have investigated liquid breakup mechanisms in the near nozzle region of a high-pressure effervescent atomizer using ballistic imaging. This technique has revealed various breakup regimes depending upon total flow rate and the gas-to-liquid ratio (GLR). At low total speeds, the jet does not exhibit the wide spread angle and rapid breakup for which effervescent sprays are known, even at high GLR. Above a distinct threshold value for total flow rate, the jet passes through several recognizable flow regimes depending on GLR and it does achieve the expected wide spread angle and rapid breakup. Intermediate GLR’s produce interesting flow patterns that seem to be generated by surging at the nozzle exit, and this surging can probably be attributed to the flow pattern just at the nozzle exit. Indeed, specific interior flows seem to generate the most rapid breakup and should be investigated further.