rus
Issue #5/2020
R.V.Kleymanov, S.E.Alexandrov
High performance aerodynamic nebulizer for liquid reagents based on the Laval nozzle
High performance aerodynamic nebulizer for liquid reagents based on the Laval nozzle
DOI: 10.22184/1993-8578.2020.13.5.264.274
The authors propose a high-performance device for supplying a liquid reagent for the chemical vapor deposition of high-purity quartz. The liquid silicon-containing reagent is supplied by converting it into an aerosol using an aerodynamic nebulizer with a flat Laval nozzle incorporating an internal body. The developed design allows of achieving a high flow rate of the liquid reagent by changing the nozzle cross-sections so as to adjust the speed of the nebulizer gas. The proposed design features higher manufacturability and simple adjustment of the reagent flow rate over a wide range, as distinct from the common coaxial nozzle designs.
The authors propose a high-performance device for supplying a liquid reagent for the chemical vapor deposition of high-purity quartz. The liquid silicon-containing reagent is supplied by converting it into an aerosol using an aerodynamic nebulizer with a flat Laval nozzle incorporating an internal body. The developed design allows of achieving a high flow rate of the liquid reagent by changing the nozzle cross-sections so as to adjust the speed of the nebulizer gas. The proposed design features higher manufacturability and simple adjustment of the reagent flow rate over a wide range, as distinct from the common coaxial nozzle designs.
Теги: atomizer chemical equipment chemical vapor deposition laval nozzle распылитель сопло лаваля химическое оборудование химическое осаждение из газовой фазы
INTRODUCTION
The essence of the chemical vapor deposition processes, so called CVD-processes, is to prepare substances in a solid state as a result of chemical transformations of reagents supplied to the reactor volume simultaneously in gaseous or plasma state. Obviously, the system to supply the reagents is definitely simple in the case when reagents that are in gaseous state under normal conditions or are characterized by a high saturated vapor pressure at comparatively low temperatures (to 100 оС). However, interest to relatively inexpensive organometallic compounds with low volatility grew continuously for years. Still, in order to ensure the acceptable partial pressure process productivity, it is necessary to obtain temperatures higher than 300 оС and manufacture complicated evaporating devices wherein all transport lines are heated to prevent condensation.
Besides, there exist a number of CVD-processes, such as chemical deposition of ultrapure quartz glass in a hydrogen burner flame requiring significant expenditures of a silicon-containing reagent (up to 1 kg/h) which cannot be achieved using conventional bubble or mirror evaporators [1]. Hence, in order to solve similar problems wide use is made of the supply of reagents in kind of aerosols generated with nebulizers of different types wherein aerosol drops come to a heated zone and are transformed to a vapor state [2, 3]. Ultrasonic nebulizers are mostly used in the reagent supplying systems to provide for CVD-processes of films deposition [4, 5], but they are characterized by low efficiency and limited period of continuous operation.
The most interesting are the aerodynamic nebulizers [4] used in CVD-processes to synthesize materials at a high rate and in significant quantities.
Detachment of drops in aerodynamic nebulizers occurs due to the dynamic (high-speed) head of gas passing through the elements of the flow part and breaking of drops is a result of turbulent pulsations. Aerodynamic nebulizers can be divided into the following groups: jet, shock-jet, centrifugal, centrifugal-jet and impact jets [4]. Jet nozzles present tips with holes of various shapes and arrangement through which liquid flows pass. Spraying in shock-jets occurs due to impact of flow on the reflector placed in front of a nozzle. Centrifugal nozzles have tangential inlets and a swirler (or a spiral channel) supplying liquid as a film that rotates around the longitudinal axis. Centrifugal jet nozzles differ from centrifugal nozzles by two rotating flows supplied to the mixing camera. Operation of the impact-jet nozzles is based on collision of several flows coming from intersecting nozzles. Depending on the applied gas, it is possible to manufacture both nebulizers with a small and precisely metered reagent supply or high-efficient supply systems [3-5].
One type of the jet nozzle is a supersonic nozzle (Laval nozzle) in which gas outflow exceeds a speed of sound. Such designs have a wide application in various atomic adsorption spectrometers, firstly, in coaxial systems formed by two opposite cones with a small annular gap of narrow width, which forms the flow path of the nozzle [6]. Coaxial design has a number of disadvantages such as complexity of changing the flow path geometry and necessity to adjust the nozzle (it is required to manufacture new elements for sub and supersonic confusors) and high requirements to production and assembly due to small clearances. Flat designs of aerodynamic nebulizers are less used; however they find their application due to relative simplicity and a possibility to be manufactured without labour-intensive technological operations.
The aim of this work was to develop a design of an aerodynamic nebulizer based on a flat Laval nozzle with an inner body that ensures high productivity and a possibility to adjust the sprayed liquid flow in a wide range. In addition, to confirm the design and verification calculations, it was planned to experimentally test a nebulizer prototype for supplying a silicon-containing substance at a high flow rate (up to 300 ml/hr) – decamethylcyclopentasiloxane, which is a promising reagent for synthesis of high-quality quartz glass in an oxygen-hydrogen burner. Analysis of literature data shows that usually, when using hydrogen-oxygen burners to synthesize silica glass from organosilicon reagents, the gas flow rates in the range of 10-30 l/min are used per each burner, depending on the design of the reactor. In this regard, when creating a prototype of the nebulizer, the nebulizer gas flow rate was set to 20 l/min, while the reagent flow rate was up to 300 ml/h.
DESIGN OF THE NEBULIZER
Figure 1 presents images of the developed design of the nebulizer which consists of base 1 with a channel, internal body 2, which forms, together with a rectangular channel, the flow part of the nozzle, adapter sleeve 3, and fixing nut 4. Presence of the internal body simplifies supply of the reagent and allows of changing the number of supply holes by overlapping some of them if necessary, which expands the possibilities of adjusting the flow rate of the sprayed reagent over a wide range. Base walls and sleeve protrusions form a flat channel preset geometry of the converging and diverging portions. The design allows of using changeable flow parts to control the flow rate both of the carrier gas and the reagent. The created laboratory model lacks an external sealed case, which is required to eliminate liquid leaks through the case connector (a seal is installed on the model). If the external case is used, it is possible to reduce the manufacturing accuracy without affecting the performance since the liquid leaks out of the nebulizer will be collected in the external case and will not affect the operation of the device. The nebulizer is made of stainless steel 08X18H10 and can be located in the immediate vicinity of the reactor and burners to supply aerosol to the reaction zone or evaporation chambers of the burners. To improve stability of operation and uniform supply of the reagent, a stable and extended rarefaction zone is created in the flow path of the nozzle for the reagent supply, the zone dimensions were estimated using numerical simulation.
CALCULATION METHODS AND EXPERIMENTAL RESEARCH
In this work we used a numerical simulation by the finite element method in the COMSOL Multiphysics package, which includes calculation of a viscous compressible gas flow with the SST turbulence model in a low Reynolds stationary setting. The spray gas (working medium) was oxygen with an inlet temperature of 300 K, the inlet pressure (excess) varied in the range from 1 to 3 atm. Oxygen was chosen due to its higher molecular weight and density as compared with hydrogen. Change in the properties of the working medium, depending on temperature and pressure, was taken into account using the Benedict-Webb-Rubin multiparameter equation of state. The data was preset using the Refprop NIST library tables. When calculating the liquid flow through the feed holes, the properties of the liquid (local speed of sound, viscosity, surface tension, density) were also preset from the Refprop NIST library to take into account thermal effects. The selected range of gas pressure is associated with the need to ensure supersonic flow in the nozzle channels, as well as with the ability to operate at variable gas inlet pressure.
Geometry of the channel is adjusted in such a way that in the middle plane of the channel section the shock wave is reflected behind the throat section of the nozzle, therefore maximizing the discharge zone. The longer rarefaction zone is necessary so that the inlet holes for the sprayed liquid remain in the low pressure region at all times during flow pulsations, and the outflow of the reagent is more uniform.
RESULTS OF CALCULATIONS
During simulation, half of the channel was chosen as the computational domain due to the symmetry of the device. In the section, where the holes for the sprayed liquid are brought out, a plane is drawn on which the following distributions of the flow parameters are plotted. In the course of the calculations, the angle of the sub- and supersonic confuser was changed to displace the shock wave beyond the critical section of the nozzle (the critical section is understood as the section with the minimum area [6]). Visualization of the Mach number distribution in the flow path of the nozzle is shown in Fig.2. The calculation results indicate the presence of multiple reflection of the shock waves from the channel walls and an extended vacuum zone, which contributes to a stable supply of the reagent, since the flow pulsations will not lead to the shock going beyond the zone of the supply holes. A nozzle with overexpansion or re-reflections of shock waves is less efficient in terms of aerodynamic losses and final flow velocity at the outlet [6], however, these features improve the degree of liquid atomization due to presence of strong local pressure gradients in the shock zones, thereby improving droplet fragmentation [1]. A rather extended rarefaction zone can be noted on the pressure distribution in the flow path of the nebulizer (Fig.3). Acceleration of the flow leads to an increase in its kinetic energy, and, according to the law of conservation of the mechanical energy of the flow, its static pressure (potential energy) decreases. From the aerodynamic point of view, presence of multiple reflection of shock waves leads to an increase in aerodynamic losses. Indeed, oblique shock waves, reflecting from the channel walls, move downstream, stretching the transonic transition zone and increasing the total wave resistance of the channel; however, in contrast to the direct shock, in this case there is a smoother decrease in pressure and flow temperature, and losses also decrease internal energy for conversion into kinetic energy of the flow in each individual jump. Thus, replacing a single direct shock wave with a series of oblique shock waves will not lead to a significant increase in the total wave resistance of the nozzle channel.
In aerodynamic nebulizers, droplet breakdown occurs under the action of inertial forces in the flow. An increase in the degree of turbulence and inhomogeneity of the flow with its strong mixing or shock waves will increase the effect of inertial forces on the flow of liquid droplets and lead to their breakdown. The increase in the expansion zone behind the shock leads to the fact that the reagent supply holes will always be in the area of reduced pressure, therefore, it becomes necessary to use a device for injecting liquid into the nebulizer.
During simulation of the gas flow, the vacuum value in the nozzle channel was determined and was used to calculate the reagent flow rate. With the known parameters of the reagent (viscosity, density, surface tension) available at different modes of carrier gas flow, pressure drops and required productivity, the diameters of the holes supplying the sprayed liquid in the inner body of the Laval nozzle were selected. The holes are located on the side surface of the nozzle in the low pressure zone (Fig.3), the reagent is supplied to the holes through a channel made in the inner body of the nozzle.
At a channel height of 4 mm and a critical section of the Laval nozzle of 0.3 mm (the parameters were determined based on the flow rate, pressure and type of gas), the inlet holes present four 0.25 mm dia. drills in the wall of the inner body (2 drills in each of the symmetrical sides of the channel). Such a configuration of the nebulizer provide for the required flow rates of liquid and gas at minimal dimensions and a possibility of changing the flow path configuration and, accordingly, the flow rate of gas or liquid without significant alteration of the structure. Since the simulation of the processes of detachment and breakdown of droplets during operation of the nebulizer at various pressure levels and use of various liquids is quite costly in terms of computing resources, evaluation of the atomization efficiency was carried out during an experimental study of the nebulizer prototype.
RESULTS OF AN EXPERIMENTAL RESEARCH
Since the design of the flow path allows a change in the passage area and partial blocking of the channel, full-scale tests of the nebulizer were carried out with various versions of its operation:
with fully open gas supply channels and at maximum gas flow rate;
when one of the two gas supply channels is closed and the flow rates of liquid and gas are reduced;
with a decrease in the flow area in both gas supply channels at the maximum liquid flow rate and a decrease in gas flow rate.
In the course of the experimental study, the gas flow rate at the outlet of the nebulizer and the gas pressure at its inlet were measured using a GSB-400 flow meter of accuracy class 1.0 and a MO-11202 reference pressure gauge (accuracy class 0.4), respectively. Reagent consumption was measured using a 1-1-2-50-0.1 burette (grade 2, graduation 0.1 ml) and a stopwatch.
Compressed air was used as a spray gas. At the first stage, distilled water was used as the sprayed liquid. Spraying of the model liquid took place when one of the two nozzle channels was shut-off to test performance of the developed nebulizer at low spray gas flow rates. The experimental results showed that there is an almost linear nature of the dependence of the gas flow rate versus its pressure, which greatly simplifies the gas flow rate adjustment (Fig.4). Fig.5 shows the dependence of the flow rate of sprayed water versus the spray gas pressure, which is also close to linear. In addition, it was found that, due to the high surface tension of water at the outlet from the nebulizer, droplets of a sufficiently large size (up to 1 mm) are formed, which cannot be broken by a relatively low-velocity gas flow; however, it can be expected that when changing water to a reagent of a lower surface tension the quality of the spray will be better at such spray configuration.
To verify this assumption, as well as to test the design for spraying a silicon-containing reagent under the same conditions (one of the gas channels was plugged), experiments on spraying decamethylcyclopentasiloxane were carried out. This liquid has a significantly lower surface tension, which influences is reflected in the atomization quality: the jet spray of the sprayed liquid did not contain large droplets. However, the high viscosity of the reagent influenced reflected in a decrease in the liquid flow rate at the parameters of the flow rate and gas pressure that were similar to the case with water spray (Fig.6). At low air pressures the flow rate of the reagent practically coincides with the flow rate of water, and with an increase in the pressure drop across the nebulizer, due to the higher viscosity of the sprayed reagent, the increase in the flow rate of the liquid occurs more slowly than for water. In the course of the experiment, the water was replaced with decamethylcyclopentasiloxane while maintaining the settings for the spray gas supply, while the sprayer continued to work, despite a significant difference in the properties of the liquid (at room temperature): the surface tension of water 73 ∙ 10–3 N/m, viscosity 0.894 mPa ∙ s, decamethylcyclopentasiloxane under the same conditions has a surface tension of about 17 ∙ 10–3 N/m and a viscosity of 1.7 Pa ∙ s). Within the range of pressures and flow rates of the nebulizer gas, the gas flow rate can be adjusted for better liquid atomization for which the nebulizer design provides for the use of spacers that change the critical section of the Laval nozzle.
If required, spraying performance can be doubled by simply opening the plugged port. However, in this case the spray gas consumption will also double. If it is necessary to ensure the maximum liquid flow rate at a limited gas flow rate, it is necessary to reduce the flow area of both channels (without complete shut-off); this is done by adding flat thin sheet gaskets between the channel wall at the base of the nozzle (item 1 of Fig.1) and an insert with an internal body (item 2, Fig.1). The main task of the experiments was to experimentally verify a possibility of achieving the required flow rate of decamethylcyclopentasiloxane at a limited flow rate and pressure of the spray gas. Operation of the nebulizer with a reduced critical section of the Laval nozzle and at the maximum supply of decamethylcyclopentasiloxane is shown in Figs.7, 8.
A decrease in the critical section of the nozzle made it possible to increase the vacuum in the area of the feed openings and thereby increase the amount of reagent entering the nebulizer due to the pressure difference. As can be seen from the data presented, the required reagent flow rate was achieved within the specified pressure and spray gas flow ranges.
CONCLUSIONS
The reagent supply device designed, manufactured and tested in the course of this work allows of atomization of viscous high-boiling reagents to be fed into the evaporators or directly into the reactor chambers at the reagent flow rates up to 300 ml/h (when scaling the device, it is possible to increase the reagent supply up to 500 ml/h and more for large burners). Versatility of the device, achieved due to the easily adaptable geometry of the flow path for the required gas and reagent flow rate, allows atomization of high-boiling viscous liquids (for example, decamethylcyclopentasiloxane) while ensuring the required gas and liquid flow rates. An nebulizer based on a Laval nozzle with an internal body is able to change the throat section of the nozzle without reworking it, and is also distinguished by easy injection of the reagent into the flow. The overexpanded nozzle design provides for sufficient droplet breakdown in the flow behind the reflected shock wave. ■
The essence of the chemical vapor deposition processes, so called CVD-processes, is to prepare substances in a solid state as a result of chemical transformations of reagents supplied to the reactor volume simultaneously in gaseous or plasma state. Obviously, the system to supply the reagents is definitely simple in the case when reagents that are in gaseous state under normal conditions or are characterized by a high saturated vapor pressure at comparatively low temperatures (to 100 оС). However, interest to relatively inexpensive organometallic compounds with low volatility grew continuously for years. Still, in order to ensure the acceptable partial pressure process productivity, it is necessary to obtain temperatures higher than 300 оС and manufacture complicated evaporating devices wherein all transport lines are heated to prevent condensation.
Besides, there exist a number of CVD-processes, such as chemical deposition of ultrapure quartz glass in a hydrogen burner flame requiring significant expenditures of a silicon-containing reagent (up to 1 kg/h) which cannot be achieved using conventional bubble or mirror evaporators [1]. Hence, in order to solve similar problems wide use is made of the supply of reagents in kind of aerosols generated with nebulizers of different types wherein aerosol drops come to a heated zone and are transformed to a vapor state [2, 3]. Ultrasonic nebulizers are mostly used in the reagent supplying systems to provide for CVD-processes of films deposition [4, 5], but they are characterized by low efficiency and limited period of continuous operation.
The most interesting are the aerodynamic nebulizers [4] used in CVD-processes to synthesize materials at a high rate and in significant quantities.
Detachment of drops in aerodynamic nebulizers occurs due to the dynamic (high-speed) head of gas passing through the elements of the flow part and breaking of drops is a result of turbulent pulsations. Aerodynamic nebulizers can be divided into the following groups: jet, shock-jet, centrifugal, centrifugal-jet and impact jets [4]. Jet nozzles present tips with holes of various shapes and arrangement through which liquid flows pass. Spraying in shock-jets occurs due to impact of flow on the reflector placed in front of a nozzle. Centrifugal nozzles have tangential inlets and a swirler (or a spiral channel) supplying liquid as a film that rotates around the longitudinal axis. Centrifugal jet nozzles differ from centrifugal nozzles by two rotating flows supplied to the mixing camera. Operation of the impact-jet nozzles is based on collision of several flows coming from intersecting nozzles. Depending on the applied gas, it is possible to manufacture both nebulizers with a small and precisely metered reagent supply or high-efficient supply systems [3-5].
One type of the jet nozzle is a supersonic nozzle (Laval nozzle) in which gas outflow exceeds a speed of sound. Such designs have a wide application in various atomic adsorption spectrometers, firstly, in coaxial systems formed by two opposite cones with a small annular gap of narrow width, which forms the flow path of the nozzle [6]. Coaxial design has a number of disadvantages such as complexity of changing the flow path geometry and necessity to adjust the nozzle (it is required to manufacture new elements for sub and supersonic confusors) and high requirements to production and assembly due to small clearances. Flat designs of aerodynamic nebulizers are less used; however they find their application due to relative simplicity and a possibility to be manufactured without labour-intensive technological operations.
The aim of this work was to develop a design of an aerodynamic nebulizer based on a flat Laval nozzle with an inner body that ensures high productivity and a possibility to adjust the sprayed liquid flow in a wide range. In addition, to confirm the design and verification calculations, it was planned to experimentally test a nebulizer prototype for supplying a silicon-containing substance at a high flow rate (up to 300 ml/hr) – decamethylcyclopentasiloxane, which is a promising reagent for synthesis of high-quality quartz glass in an oxygen-hydrogen burner. Analysis of literature data shows that usually, when using hydrogen-oxygen burners to synthesize silica glass from organosilicon reagents, the gas flow rates in the range of 10-30 l/min are used per each burner, depending on the design of the reactor. In this regard, when creating a prototype of the nebulizer, the nebulizer gas flow rate was set to 20 l/min, while the reagent flow rate was up to 300 ml/h.
DESIGN OF THE NEBULIZER
Figure 1 presents images of the developed design of the nebulizer which consists of base 1 with a channel, internal body 2, which forms, together with a rectangular channel, the flow part of the nozzle, adapter sleeve 3, and fixing nut 4. Presence of the internal body simplifies supply of the reagent and allows of changing the number of supply holes by overlapping some of them if necessary, which expands the possibilities of adjusting the flow rate of the sprayed reagent over a wide range. Base walls and sleeve protrusions form a flat channel preset geometry of the converging and diverging portions. The design allows of using changeable flow parts to control the flow rate both of the carrier gas and the reagent. The created laboratory model lacks an external sealed case, which is required to eliminate liquid leaks through the case connector (a seal is installed on the model). If the external case is used, it is possible to reduce the manufacturing accuracy without affecting the performance since the liquid leaks out of the nebulizer will be collected in the external case and will not affect the operation of the device. The nebulizer is made of stainless steel 08X18H10 and can be located in the immediate vicinity of the reactor and burners to supply aerosol to the reaction zone or evaporation chambers of the burners. To improve stability of operation and uniform supply of the reagent, a stable and extended rarefaction zone is created in the flow path of the nozzle for the reagent supply, the zone dimensions were estimated using numerical simulation.
CALCULATION METHODS AND EXPERIMENTAL RESEARCH
In this work we used a numerical simulation by the finite element method in the COMSOL Multiphysics package, which includes calculation of a viscous compressible gas flow with the SST turbulence model in a low Reynolds stationary setting. The spray gas (working medium) was oxygen with an inlet temperature of 300 K, the inlet pressure (excess) varied in the range from 1 to 3 atm. Oxygen was chosen due to its higher molecular weight and density as compared with hydrogen. Change in the properties of the working medium, depending on temperature and pressure, was taken into account using the Benedict-Webb-Rubin multiparameter equation of state. The data was preset using the Refprop NIST library tables. When calculating the liquid flow through the feed holes, the properties of the liquid (local speed of sound, viscosity, surface tension, density) were also preset from the Refprop NIST library to take into account thermal effects. The selected range of gas pressure is associated with the need to ensure supersonic flow in the nozzle channels, as well as with the ability to operate at variable gas inlet pressure.
Geometry of the channel is adjusted in such a way that in the middle plane of the channel section the shock wave is reflected behind the throat section of the nozzle, therefore maximizing the discharge zone. The longer rarefaction zone is necessary so that the inlet holes for the sprayed liquid remain in the low pressure region at all times during flow pulsations, and the outflow of the reagent is more uniform.
RESULTS OF CALCULATIONS
During simulation, half of the channel was chosen as the computational domain due to the symmetry of the device. In the section, where the holes for the sprayed liquid are brought out, a plane is drawn on which the following distributions of the flow parameters are plotted. In the course of the calculations, the angle of the sub- and supersonic confuser was changed to displace the shock wave beyond the critical section of the nozzle (the critical section is understood as the section with the minimum area [6]). Visualization of the Mach number distribution in the flow path of the nozzle is shown in Fig.2. The calculation results indicate the presence of multiple reflection of the shock waves from the channel walls and an extended vacuum zone, which contributes to a stable supply of the reagent, since the flow pulsations will not lead to the shock going beyond the zone of the supply holes. A nozzle with overexpansion or re-reflections of shock waves is less efficient in terms of aerodynamic losses and final flow velocity at the outlet [6], however, these features improve the degree of liquid atomization due to presence of strong local pressure gradients in the shock zones, thereby improving droplet fragmentation [1]. A rather extended rarefaction zone can be noted on the pressure distribution in the flow path of the nebulizer (Fig.3). Acceleration of the flow leads to an increase in its kinetic energy, and, according to the law of conservation of the mechanical energy of the flow, its static pressure (potential energy) decreases. From the aerodynamic point of view, presence of multiple reflection of shock waves leads to an increase in aerodynamic losses. Indeed, oblique shock waves, reflecting from the channel walls, move downstream, stretching the transonic transition zone and increasing the total wave resistance of the channel; however, in contrast to the direct shock, in this case there is a smoother decrease in pressure and flow temperature, and losses also decrease internal energy for conversion into kinetic energy of the flow in each individual jump. Thus, replacing a single direct shock wave with a series of oblique shock waves will not lead to a significant increase in the total wave resistance of the nozzle channel.
In aerodynamic nebulizers, droplet breakdown occurs under the action of inertial forces in the flow. An increase in the degree of turbulence and inhomogeneity of the flow with its strong mixing or shock waves will increase the effect of inertial forces on the flow of liquid droplets and lead to their breakdown. The increase in the expansion zone behind the shock leads to the fact that the reagent supply holes will always be in the area of reduced pressure, therefore, it becomes necessary to use a device for injecting liquid into the nebulizer.
During simulation of the gas flow, the vacuum value in the nozzle channel was determined and was used to calculate the reagent flow rate. With the known parameters of the reagent (viscosity, density, surface tension) available at different modes of carrier gas flow, pressure drops and required productivity, the diameters of the holes supplying the sprayed liquid in the inner body of the Laval nozzle were selected. The holes are located on the side surface of the nozzle in the low pressure zone (Fig.3), the reagent is supplied to the holes through a channel made in the inner body of the nozzle.
At a channel height of 4 mm and a critical section of the Laval nozzle of 0.3 mm (the parameters were determined based on the flow rate, pressure and type of gas), the inlet holes present four 0.25 mm dia. drills in the wall of the inner body (2 drills in each of the symmetrical sides of the channel). Such a configuration of the nebulizer provide for the required flow rates of liquid and gas at minimal dimensions and a possibility of changing the flow path configuration and, accordingly, the flow rate of gas or liquid without significant alteration of the structure. Since the simulation of the processes of detachment and breakdown of droplets during operation of the nebulizer at various pressure levels and use of various liquids is quite costly in terms of computing resources, evaluation of the atomization efficiency was carried out during an experimental study of the nebulizer prototype.
RESULTS OF AN EXPERIMENTAL RESEARCH
Since the design of the flow path allows a change in the passage area and partial blocking of the channel, full-scale tests of the nebulizer were carried out with various versions of its operation:
with fully open gas supply channels and at maximum gas flow rate;
when one of the two gas supply channels is closed and the flow rates of liquid and gas are reduced;
with a decrease in the flow area in both gas supply channels at the maximum liquid flow rate and a decrease in gas flow rate.
In the course of the experimental study, the gas flow rate at the outlet of the nebulizer and the gas pressure at its inlet were measured using a GSB-400 flow meter of accuracy class 1.0 and a MO-11202 reference pressure gauge (accuracy class 0.4), respectively. Reagent consumption was measured using a 1-1-2-50-0.1 burette (grade 2, graduation 0.1 ml) and a stopwatch.
Compressed air was used as a spray gas. At the first stage, distilled water was used as the sprayed liquid. Spraying of the model liquid took place when one of the two nozzle channels was shut-off to test performance of the developed nebulizer at low spray gas flow rates. The experimental results showed that there is an almost linear nature of the dependence of the gas flow rate versus its pressure, which greatly simplifies the gas flow rate adjustment (Fig.4). Fig.5 shows the dependence of the flow rate of sprayed water versus the spray gas pressure, which is also close to linear. In addition, it was found that, due to the high surface tension of water at the outlet from the nebulizer, droplets of a sufficiently large size (up to 1 mm) are formed, which cannot be broken by a relatively low-velocity gas flow; however, it can be expected that when changing water to a reagent of a lower surface tension the quality of the spray will be better at such spray configuration.
To verify this assumption, as well as to test the design for spraying a silicon-containing reagent under the same conditions (one of the gas channels was plugged), experiments on spraying decamethylcyclopentasiloxane were carried out. This liquid has a significantly lower surface tension, which influences is reflected in the atomization quality: the jet spray of the sprayed liquid did not contain large droplets. However, the high viscosity of the reagent influenced reflected in a decrease in the liquid flow rate at the parameters of the flow rate and gas pressure that were similar to the case with water spray (Fig.6). At low air pressures the flow rate of the reagent practically coincides with the flow rate of water, and with an increase in the pressure drop across the nebulizer, due to the higher viscosity of the sprayed reagent, the increase in the flow rate of the liquid occurs more slowly than for water. In the course of the experiment, the water was replaced with decamethylcyclopentasiloxane while maintaining the settings for the spray gas supply, while the sprayer continued to work, despite a significant difference in the properties of the liquid (at room temperature): the surface tension of water 73 ∙ 10–3 N/m, viscosity 0.894 mPa ∙ s, decamethylcyclopentasiloxane under the same conditions has a surface tension of about 17 ∙ 10–3 N/m and a viscosity of 1.7 Pa ∙ s). Within the range of pressures and flow rates of the nebulizer gas, the gas flow rate can be adjusted for better liquid atomization for which the nebulizer design provides for the use of spacers that change the critical section of the Laval nozzle.
If required, spraying performance can be doubled by simply opening the plugged port. However, in this case the spray gas consumption will also double. If it is necessary to ensure the maximum liquid flow rate at a limited gas flow rate, it is necessary to reduce the flow area of both channels (without complete shut-off); this is done by adding flat thin sheet gaskets between the channel wall at the base of the nozzle (item 1 of Fig.1) and an insert with an internal body (item 2, Fig.1). The main task of the experiments was to experimentally verify a possibility of achieving the required flow rate of decamethylcyclopentasiloxane at a limited flow rate and pressure of the spray gas. Operation of the nebulizer with a reduced critical section of the Laval nozzle and at the maximum supply of decamethylcyclopentasiloxane is shown in Figs.7, 8.
A decrease in the critical section of the nozzle made it possible to increase the vacuum in the area of the feed openings and thereby increase the amount of reagent entering the nebulizer due to the pressure difference. As can be seen from the data presented, the required reagent flow rate was achieved within the specified pressure and spray gas flow ranges.
CONCLUSIONS
The reagent supply device designed, manufactured and tested in the course of this work allows of atomization of viscous high-boiling reagents to be fed into the evaporators or directly into the reactor chambers at the reagent flow rates up to 300 ml/h (when scaling the device, it is possible to increase the reagent supply up to 500 ml/h and more for large burners). Versatility of the device, achieved due to the easily adaptable geometry of the flow path for the required gas and reagent flow rate, allows atomization of high-boiling viscous liquids (for example, decamethylcyclopentasiloxane) while ensuring the required gas and liquid flow rates. An nebulizer based on a Laval nozzle with an internal body is able to change the throat section of the nozzle without reworking it, and is also distinguished by easy injection of the reagent into the flow. The overexpanded nozzle design provides for sufficient droplet breakdown in the flow behind the reflected shock wave. ■
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