Gas-liquid visualization system for improving aeration efficiency

Abstract

A gas-liquid visualization system for improving aeration efficiency relates to the field of gas-liquid mixing and mass transfer intensification, and comprises a stepped pipeline, a liquid storage tank, a loop pipeline, a gas passage unit, a concentration monitoring unit, a three-stage pump pressurization unit and a high-speed camera unit, the stepped pipeline is used for inducing turbulent flow through multi-stage stepped flow to increase gas-liquid contact time; the liquid storage tank is used for accommodating a gas-liquid mixture and is internally provided with a stirring assembly; gas is injected into the liquid storage tank through the gas passage unit; the three-stage pump pressurization unit is used for maintaining system pressure in a segmented mode and promoting gas-liquid mixing. The stepped pipeline, liquid storage tank aeration, rotating wheel stirring and three-stage pump pressurization are integrated, high-speed camera shooting is combined to shoot the dispersion effect of bubbles of different sizes, the concentration monitoring unit is combined, and the aeration efficiency is improved through multiple pipeline circulation; and flow parameter cooperative matching and optimal flow control conditions of the maximum aeration efficiency of the current system are given.

Description

Technical Field

[0001] The present invention relates to the technical field of gas-liquid mixing and mass transfer enhancement, and in particular to a gas-liquid visualization system for improving aeration efficiency. The present invention is applicable to the fields of biological breeding, wastewater treatment, chemical reaction tanks, etc. Background Art

[0002] In natural water bodies, the respiration of various aquatic organisms and the decomposition of organic matter continuously consume oxygen. Maintaining an appropriate dissolved oxygen concentration is crucial for biological survival. Adequate oxygen prevents the development of anaerobic conditions in the water, helps inhibit water quality deterioration, reduces sulfate reduction, and weakens corrosion of metal structures in hydraulic structures. Air enters the water flow under conditions of high surface turbulence and is transferred into the water in the form of small bubbles, creating a whitewash effect. The formation of bubbles, strong turbulence, and a large surface area for gas transfer allow for the absorption of large amounts of oxygen. This process is called aeration.

[0003] Traditional aeration methods, such as diffusion aeration and mechanical stirring, have the following shortcomings:

[0004] 1. The gas-liquid contact time is short and the dissolved oxygen saturation is low;

[0005] 2. The bubble size distribution is uneven and the mass transfer efficiency is low;

[0006] 3. Lack of real-time monitoring and parameterization methods. Summary of the Invention

[0007] The present invention aims to solve the above-mentioned technical problems existing in the prior art and further proposes a gas-liquid visualization system for improving aeration efficiency.

[0008] The technical solution adopted by the present invention to solve the above problems is:

[0009] The present invention includes a stepped pipeline, a liquid storage tank, a loop pipeline, a gas passage unit, a concentration monitoring unit and a high-speed camera unit.

[0010] A stepped pipeline, one end of which is connected to the loop pipeline, and the other end is connected to the liquid storage tank,

[0011] The liquid storage tank is made of transparent material, and its liquid outlet is connected to the loop pipe; a stirring component is provided inside it;

[0012] A gas passage unit is located below the liquid reservoir and is used to inject gas into the liquid reservoir;

[0013] A concentration monitoring unit, the inlet of which is connected to the loop pipeline; and the outlet of which is connected to the inlet of the liquid storage tank;

[0014] High-speed camera unit used to capture the dynamic behavior of bubbles in the reservoir.

[0015] Furthermore, the system also includes a three-stage pump pressurizing unit, including a circulating pump, a first centrifugal pump and a second centrifugal pump, which are respectively located at the inlet of the stepped pipeline, the outlet of the liquid reservoir and the loop of the concentration monitoring unit. A fourth flow control valve is provided on the loop pipeline between the circulating pump and the stepped pipeline; a first flow control valve is provided on the loop pipeline between the first centrifugal pump and the liquid reservoir, and a third flow control valve is provided on the loop pipeline between the second centrifugal pump and the liquid reservoir.

[0016] Furthermore, the gas passage unit includes at least one bubble generator, a gas collecting passage, a one-way valve, a gas flow valve, a pressure reducing valve, a gas switching valve and a detachable high-pressure gas cylinder. The outlet of the bubble generator is connected to the liquid reservoir, and the inlet is connected to the gas collecting passage. The inlet of the gas collecting passage is connected to the outlet of the detachable high-pressure gas cylinder through a loop pipeline. The loop pipeline between the gas collecting passage and the detachable high-pressure gas cylinder is provided with a one-way valve, a flow valve, a pressure reducing valve and a gas switching valve in sequence.

[0017] Furthermore, the number of the bubble generators is four, and the four bubble generators are equidistantly arranged and detachable, and the micropore sizes thereof are 0.5 mm, 1.0 mm, 1.5 mm and 2 mm respectively.

[0018] Furthermore, the concentration monitoring unit includes a three-way connecting valve, a switch valve, a second flow control valve, a concentration sensor, a glass measuring cylinder and a connecting pool; the three-way valve is fixed and connected to the loop pipe, and the branch outlet at the upper end is connected to the inlet of the glass measuring cylinder through the loop pipe; the switch valve and the second flow control valve are sequentially provided on the loop pipe between the three-way valve and the glass measuring cylinder; the outlet of the glass measuring cylinder is connected to the connecting pool, and the concentration sensor is inserted into the glass measuring cylinder.

[0019] Furthermore, the camera unit includes photo processing software, a signal converter and a computer, a high-speed camera and a light source. The high-speed camera and the light source are respectively located on both sides of the liquid reservoir, arranged relative to each other and highly synchronized.

[0020] Furthermore, the stirring assembly includes a frequency modulation motor, a connecting shaft and a rotor. The output shaft of the frequency modulation motor is connected to the rotor through the connecting shaft. The rotor adopts a three-blade arrangement and the speed is adjusted by the frequency modulation motor.

[0021] Furthermore, the liquid reservoir is also provided with a pressure sensor and a pressure relief valve. The pressure sensor is fixed and connected above the liquid reservoir, and the pressure relief valve is installed on the side wall of the liquid reservoir.

[0022] Furthermore, the stepped pipeline is composed of several sections of horizontal and vertical pipelines connected in series through elbows, which are used to generate longitudinal and transverse vortices during the flow process to enhance gas-liquid mixing.

[0023] The beneficial effects of the present invention are:

[0024] 1. The present invention integrates stepped piping, liquid storage tank aeration, rotor stirring, and three-stage pump pressurization. It uses a high-speed camera system to capture the dispersion effect of bubbles of different sizes and a real-time concentration monitoring system. It improves aeration efficiency through multiple pipeline cycles and provides coordinated matching of flow parameters and optimal flow control conditions for maximum aeration efficiency in the current system.

[0025] 2. By combining the four processes of step-by-step complex flow, stirring, bubbling and multiple cycles, the dissolved concentration can be increased multiple times. The concentration monitoring system ensures the accuracy of online concentration monitoring under complex turbulence, and the three-stage pump pressurization system avoids gas precipitation and concentration attenuation caused by pressure drop.

[0026] 3. The present invention improves aeration efficiency and achieves optimal flow control under multi-parameter coordinated matching by changing the impeller speed, bubble generator diameter, gas flow rate, liquid flow rate, and the number of stepped pipeline stages. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the overall structure of the present invention.

[0028] In the figure: 1-stepped pipeline; 2-liquid accumulator; 3-frequency modulation motor; 4-pressure sensor; 5-pressure relief valve; 6-light source; 7-connecting shaft; 8-three-blade impeller; 9-first flow control valve; 10-first centrifugal pump; 11-bubble generator; 12-gas collecting passage; 13-one-way valve; 14-gas flow valve; 15-pressure reducing valve; 16-gas switching valve; 17-electromagnetic flowmeter; 18-high-pressure gas cylinder; 19-three-way connecting valve; 20-switching valve; 21-second flow control valve; 22-concentration sensor; 23-glass measuring cylinder; 24-connecting pool; 25-second centrifugal pump; 26-third flow control valve; 27-high-speed camera; 28-circulating centrifugal pump; 29-fourth flow control valve; 30-loop pipeline. DETAILED DESCRIPTION

[0029] Specific implementation method 1: Combination Figure 1To illustrate this embodiment, a gas-liquid visualization system for improving aeration efficiency described in this embodiment includes a stepped pipeline 1, a liquid reservoir 2, a loop pipeline 30, a three-stage pump pressurization unit, a gas passage unit, a concentration monitoring unit and a high-speed camera unit. One end of the stepped pipeline 1 is connected to the loop pipeline 30, and the other end is connected to the liquid reservoir 2, and is used to induce turbulence through multi-stage stepped flow to increase the gas-liquid contact time; the liquid reservoir 2 is made of a transparent material and is used to accommodate a gas-liquid mixture, and its liquid outlet is connected to the loop pipeline 30; a stirring component is provided inside it; the gas passage unit is located below the liquid reservoir 2 and is used to inject gas into the liquid reservoir 2; the concentration monitoring unit, whose inlet is connected to the loop pipeline 30 and the outlet is connected to the inlet of the liquid reservoir 2, is used to monitor the dissolved oxygen concentration in real time; the high-speed camera unit is used to capture the dynamic behavior of bubbles in the liquid reservoir 2.

[0030] The three-stage pump pressurizing unit includes a circulation pump 28, a first centrifugal pump 10 and a second centrifugal pump 25, which are respectively located at the inlet of the stepped pipeline 1, the outlet of the liquid reservoir 2 and the loop of the concentration monitoring unit. A fourth flow control valve 29 is provided on the loop pipeline 30 between the circulation pump 28 and the stepped pipeline 1; a first flow control valve 9 is provided on the loop pipeline 30 between the first centrifugal pump 10 and the liquid reservoir 2, and a third flow control valve 26 is provided on the loop pipeline 30 between the second centrifugal pump 25 and the liquid reservoir 2.

[0031] The loop pipeline 30 connects the stepped pipeline 1, the liquid reservoir 2 and other flow-through components to form a circulation loop. The stepped pipeline 1 connects several sections of the loop pipeline 30 in series, and the transition points are connected by bends to form a whole. The stepped pipeline 1 is composed of several sections of horizontal and vertical pipelines connected in series by elbows, which are used to generate longitudinal and transverse vortices during the flow process to enhance gas-liquid mixing. The circulation pump 28 and the electromagnetic flowmeter 17 are placed upstream of the stepped pipeline 1, and the liquid reservoir 2 is placed downstream of the stepped pipeline 1. When the first flow control valve 9 and the fourth flow control valve 29 are opened and the first centrifugal pump 10 and the circulation pump 28 are operating normally, the fluid flows through the stepped pipeline 1 and the liquid reservoir 2 to achieve multiple circulations.

[0032] The gas passage unit includes at least one bubble generator 11, a gas collecting passage 12, a one-way valve 13, a gas flow valve 14, a pressure reducing valve 15, a gas switching valve 16 and a detachable high-pressure gas cylinder 18. The outlet of the bubble generator 11 is connected to the liquid reservoir 2, and the inlet is connected to the gas collecting passage 12. The inlet of the gas collecting passage 12 and the outlet of the detachable high-pressure gas cylinder 18 are connected through a loop pipeline 30. The loop pipeline 30 between the gas collecting passage 12 and the detachable high-pressure gas cylinder 18 is provided with a one-way valve 13, a flow valve 14, a pressure reducing valve 15 and a gas switching valve 16 in sequence.

[0033] Preferably, the number of the bubble generators 11 is four, and the four bubble generators are equidistantly arranged and detachable, with micropore sizes of 0.5 mm, 1.0 mm, 1.5 mm, and 2 mm, respectively. The gas collection passage 12 is in surface contact with the bubble generators 11, and the bubble generators of different sizes are detachable. When the gas flow stabilizes, a high-speed camera is used to record the gas flow.

[0034] The concentration monitoring unit includes a three-way connecting valve 19, an on-off valve 20, a second flow control valve 21, a concentration sensor 22, a glass measuring cylinder 23, and a connecting tank 24. The three-way valve 19 is fixed to and connected to a loop pipe 30, with its upper branch outlet connected to the inlet of the glass measuring cylinder 23 via the loop pipe 30. The on-off valve 20 and the second flow control valve 21 are sequentially installed on the loop pipe 30 between the three-way valve 19 and the glass measuring cylinder 23. The outlet of the glass measuring cylinder 23 is connected to the connecting tank 24, and the concentration sensor 22 is inserted into the glass measuring cylinder 23. The outlet of the connecting tank 24 is connected to a second centrifugal pump 25, and the outlet of the second centrifugal pump 25 is connected to the liquid reservoir 2 via a third flow control valve 26. The three-way valve 19 connects the main loop pipe 30 to the concentration monitoring unit. The concentration monitoring unit performs online dynamic measurements. After pressurization by the second centrifugal pump 25, some of the fluid returns to the liquid reservoir 2. When measurement stops, the on-off valve 20 and the second flow control valve 21 are closed.

[0035] The imaging unit includes image processing software, a signal converter and computer, a high-speed camera 27, and a light source 6. The high-speed camera 27 and light source 6 are located on either side of the liquid reservoir 2, arranged opposite each other and highly synchronized. The high-speed camera 27 generates images on the computer after processing through the signal converter and image processing software. The high-speed camera 27 can achieve high frame rates to capture the rupture, convergence, and merging of bubbles.

[0036] Specific implementation method 2: Combination Figure 1 This embodiment describes a gas-liquid visualization system for improving aeration efficiency. The liquid reservoir 2 is made of transparent acrylic and contains a stirring assembly. The stirring assembly includes a frequency-modulated motor 3, a connecting shaft 7, and a rotor 8. The output shaft of the frequency-modulated motor 3 is connected to the rotor 8 via the connecting shaft 7. The rotor 8 has a three-blade arrangement, and its speed is adjusted by the frequency-modulated motor 3. The connecting shaft 7 is made of stainless steel and rigidly connected to the rotor shaft 8. The frequency-modulated motor 3 adjusts the rotor speed from 1000 rpm to 4000 rpm.

[0037] The other components and connection methods of this embodiment are the same as those of the first embodiment.

[0038] Specific implementation method three: Combination Figure 1This embodiment describes a gas-liquid visualization system for improving aeration efficiency. The liquid reservoir 2 is further equipped with a pressure sensor 4 and a pressure relief valve 5. The pressure sensor 4 is fixed and connected to the top of the liquid reservoir 2, while the pressure relief valve 5 is installed on the side wall of the liquid reservoir 2. The pressure sensor 4 is used to monitor and regulate the internal pressure. When the pressure exceeds the rated pressure of the liquid reservoir 2, the pressure relief valve 5 automatically releases air to reduce the pressure.

[0039] The other components and connection methods of this embodiment are the same as those of the first or second embodiment.

[0040] Example

[0041] First, open the first and fourth flow control valves 9 and 29, as well as the first centrifugal pump 10 and circulating pump 28, to a given rotational speed. When the system is operating normally and stably, further open the on-off valve 20, the second and third flow control valves 21 and 26. The measuring end of the dissolved oxygen concentration sensor 22 is placed at the bottom of the glass measuring cylinder 23 to reduce turbulence and achieve stable concentration measurement. When the solution flows into the connecting tank, the second centrifugal pump 25 is turned on to transport the solution back to the liquid reservoir 2. The various flow-through components are connected via a loop pipe. The high-speed camera 27 and light source 6 are adjusted and placed on either side of the transparent liquid reservoir 2 at relatively equal heights. The frequency-modulated motor 3 mounted on the liquid reservoir 2 is turned on, driving the three-blade impeller 8 via the drive shaft 7. The speed is adjustable, and a low speed is initially used to check the operation of the experimental platform. Finally, the gas on-off valve 16 and pressure reducing valve 15 connected to the high-pressure gas cylinder 18 are opened. The depressurized gas is measured by flowmeter 14. The gas is then injected into the liquid reservoir 2 as a cluster of bubbles through the one-way valve 13, gas collection passage 12, and bubble generator 11. After stirring by the impeller 8, the bubbles are fully dispersed throughout the liquid phase. This forced convection mass transfer increases the gas-liquid mass transfer interface and enhances interfacial turbulence, promoting aeration efficiency. The bubbles and liquid exchange momentum and energy, achieving concentration accumulation and diffusion. When the system is operating stably, a high-speed camera 27 is activated for filming. The bubble size and distribution are statistically analyzed using a resolution function and compared with the oxygen concentration monitoring system to determine the optimal gas injection flow rate and size. The gas-liquid mixture in reservoir 2 is transported to stepped pipeline 1 via first centrifugal pump 10 and circulating pump 28. Stepped pipeline 1 slows fluid movement and induces large-scale backflow vortices at bends, promoting further accumulation of concentration. After multiple steps of flow, the fluid is continuously mixed, and the fluid entrained with tiny bubbles is further transported to reservoir tank 2 for repeated aeration. After multiple cycles, the solution is discharged when the concentration monitoring system indicates that the concentration is no longer increasing. This implementation system shows a schematic diagram of the principle of increasing aeration effect. In practice, low dissolved oxygen concentration liquid can be injected into the system according to the application environment and discharged after reaching the target dissolved oxygen concentration.

[0042] Principle of improving aeration efficiency: Compared with the traditional stirring reaction model, the combination of four processes—stepped pipeline 1, aeration in reservoir 2, agitation by impeller 8, and pressurization by three-stage pumps—can achieve a forced convection mass transfer model, significantly improving gas absorption efficiency. The purity of the gas-water solution is ensured without the need for additional chemical oxygenation. The stepped, stacked pipeline design not only increases the transport pressure but also increases the water-gas contact time. Multiple cycles of three-stage pump pressure can achieve faster results, improving gas dissolution efficiency. This system is suitable not only for improving the aeration efficiency of air in water, but also for increasing the dissolution rate of a range of insoluble gases in solution.

[0043] This system aims to improve aeration efficiency. It induces strong turbulence through multi-stage stepped flow and multiple cycles of pressurization to promote concentration accumulation, increase the contact time of gas-liquid two-phase fluid, and realize forced convection mass transfer process to improve aeration efficiency. It can be applied to biological breeding systems, reaction tanks and wastewater treatment.

[0044] Compared to traditional aeration methods, the combination of aeration, multi-stage stepped flow, and cyclic pressurization can significantly increase the dissolved oxygen concentration. This experimental device can control the size of the bubble clusters in the bubble generator to match the dynamic changes in impeller speed, achieving a rapid increase in dissolved oxygen concentration through multiple cyclic pressurization.

[0045] As the gas-liquid mixture flows through the stepped channel, it is restricted by the solid boundaries and subjected to sudden flow changes, generating longitudinal and transverse vortices. These vortices increase the energy and momentum exchange between the water and the gas. Compared to a constant-diameter pipeline, the stepped distribution increases the contact time between the water and the gas. Furthermore, multiple sets of bubble generators force the gas through numerous tiny pores under high pressure, creating a strong shearing effect with the surrounding liquid, breaking the gas into small plumes. The impeller then breaks up the bubble clusters. The combined effects of the impeller's centrifugal force and surface tension disperse the bubbles throughout the reservoir 2. The centrifugal pump then pumps the solution back into the stepped channel, repeatedly circulating it. After multiple cycles, the solution concentration nearly reaches equilibrium saturation. The experimental system's control parameters include gas flow rate, liquid flow rate, valve opening, bubble generator size, impeller speed, and the speeds of the centrifugal and circulating pumps. These multiple parameters are synergistically matched to achieve flow control conditions that maximize dissolved oxygen efficiency. The system structure of the present invention is stable, no additional chemical agents are needed to promote oxygen increase, and it can operate for a long time, providing a theoretical basis for improving the aeration performance in biological breeding systems, reaction tanks and wastewater treatment.

[0046] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the present profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical content disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent replacement and improvement of the above embodiments made according to the technical essence of the present invention, within the spirit and principles of the present invention, without departing from the content of the technical solution of the present invention, shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A gas-liquid visualization system for improving aeration efficiency, characterized by: The system comprises a stepped pipeline (1), a liquid storage tank (2), a loop pipeline (30), a gas passage unit, a concentration monitoring unit, a three-stage pump pressurizing unit and a high-speed camera unit. The stepped pipeline (1) has one end connected to the loop pipeline (30) and the other end connected to the liquid reservoir (2). The liquid storage tank (2) is made of a transparent material, and its liquid outlet is connected to the loop pipe (30); a stirring component is provided inside the liquid storage tank; A gas passage unit is located below the liquid reservoir (2) and is used to inject gas into the liquid reservoir (2); A concentration monitoring unit, the inlet of which is connected to the loop pipeline (30); and the outlet of which is connected to the inlet of the liquid storage tank (2); A high-speed camera unit is used to capture the dynamic behavior of bubbles in the liquid reservoir (2). The three-stage pump pressurizing unit comprises a circulation pump (28), a first centrifugal pump (10) and a second centrifugal pump (25), which are respectively located at the inlet of the stepped pipeline (1), the outlet of the liquid reservoir (2) and the loop of the concentration monitoring unit. A fourth flow control valve (29) is provided on the loop pipeline (30) between the circulation pump (28) and the stepped pipeline (1); a first flow control valve (9) is provided on the loop pipeline (30) between the first centrifugal pump (10) and the liquid reservoir (2); and a third flow control valve (26) is provided on the loop pipeline (30) between the second centrifugal pump (25) and the liquid reservoir (2).

2. A gas-liquid visualization system for improving aeration efficiency according to claim 1, characterized in that: The gas passage unit comprises at least one bubble generator (11), a gas collecting passage (12), a one-way valve (13), a gas flow valve (14), a pressure reducing valve (15), a gas switch valve (16) and a detachable high-pressure gas cylinder (18); the outlet of the bubble generator (11) is connected to the liquid reservoir (2), and the inlet is connected to the gas collecting passage (12); the inlet of the gas collecting passage (12) and the outlet of the detachable high-pressure gas cylinder (18) are connected via a loop pipeline (30); the one-way valve (13), the flow valve (14), the pressure reducing valve (15) and the gas switch valve (16) are sequentially provided on the loop pipeline (30) between the gas collecting passage (12) and the detachable high-pressure gas cylinder (18).

3. The gas-liquid visualization system for improving aeration efficiency according to claim 2, characterized in that: The number of the bubble generators (11) is four, and the four bubble generators (11) are equidistantly arranged and detachably arranged, and the micropore sizes thereof are 0.5 mm, 1.0 mm, 1.5 mm and 2 mm respectively.

4. The gas-liquid visualization system for improving aeration efficiency according to claim 1, characterized in that: The concentration monitoring unit comprises a three-way connecting valve (19), an on-off valve (20), a second flow control valve (21), a concentration sensor (22), a glass measuring cylinder (23) and a communication pool (24); the three-way valve (19) is fixed and connected to a loop pipe (30), and a branch outlet at the upper end is connected to the inlet of the glass measuring cylinder (23) through the loop pipe (30); the on-off valve (20) and the second flow control valve (21) are sequentially provided on the loop pipe (30) between the three-way valve (19) and the glass measuring cylinder (23); the outlet of the glass measuring cylinder (23) is connected to the communication pool (24), and the concentration sensor (22) is inserted into the glass measuring cylinder (23).

5. The gas-liquid visualization system for improving aeration efficiency according to claim 1, characterized in that: The camera unit comprises a high-speed camera (27) and a light source (6). The high-speed camera (27) and the light source (6) are respectively located on both sides of the liquid reservoir (2), arranged relative to each other and highly synchronized.

6. The gas-liquid visualization system for improving aeration efficiency according to claim 1, characterized in that: The stirring assembly comprises a frequency modulation motor (3), a connecting shaft (7) and a rotating wheel (8); the output shaft of the frequency modulation motor (3) is connected to the rotating wheel (8) via the connecting shaft (7); the rotating wheel (8) adopts a three-blade arrangement, and the rotating speed is adjusted by the frequency modulation motor (3).

7. The gas-liquid visualization system for improving aeration efficiency according to claim 1, characterized in that: The liquid reservoir (2) is further provided with a pressure sensor (4) and a pressure relief valve (5); the pressure sensor (4) is fixed and connected above the liquid reservoir (2); and the pressure relief valve (5) is installed on the side wall of the liquid reservoir (2).

8. The gas-liquid visualization system for improving aeration efficiency according to claim 1, characterized in that: The stepped pipeline (1) is composed of several sections of horizontal and vertical pipelines connected in series through elbows, and is used to generate longitudinal and transverse vortices during the flow process to enhance gas-liquid mixing.

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