Patent Publication Number: US-10315170-B2

Title: Fine bubble-containing liquid generating apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 § § 371 national phase conversion of PCT/JP2015/056185, filed Mar. 3, 2015, which claims priority to Japanese Patent Application No. 2014-058168, filed Mar. 20, 2014, the contents of both of which are incorporated herein by reference. The PCT International Application was published in the Japanese language. 
     TECHNICAL FIELD 
     The present invention relates to a fine bubble-containing liquid generating apparatus. 
     BACKGROUND ART 
     In recent years, liquids containing bubbles with diameters of 1 millimeter (mm) or less have been used in various fields. Also, liquids containing bubbles with diameters of 1 micrometer (μm) or less (ultrafine bubbles) have recently been gathering attention in various fields, and apparatuses for generating such liquids have been proposed. 
     For example, in a fine-bubble generating apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-272719 (Document 1), a gas-liquid mixed fluid sent from a pump is broken up into fine bubbles by a gas swirling shearing unit and then sent to a liquid storage tank and stored. In Document 1, the liquid in the liquid storage tank is repeatedly circulated to the gas swirling shearing unit in order to increase the density of fine bubbles in the liquid (i.e., the number of fine bubbles per unit volume). 
     Incidentally, Document 1 describes the liquid stored in the storage tank being extracted and used in various applications. However, the fine-bubble generating apparatus of Document 1 is a batch type apparatus that can generate an amount of liquid that can be stored in the storage tank, and cannot continuously generate and supply a liquid that contains a high density of fine bubbles. 
     SUMMARY OF INVENTION 
     The present invention is intended for a fine bubble-containing liquid generating apparatus, and it is an object of the present invention to continuously generate a fine-bubble containing liquid that contains a high density of fine bubbles. 
     The fine bubble-containing liquid generating apparatus according to the present invention includes a generator including a lead-in part for leading in gas and pressurized liquid, and a discharge part for discharging liquid that contains fine bubbles of the gas led in from the lead-in part, a circulation passage for returning liquid discharged from the discharge part to the lead-in part in a state in which the liquid is isolated from outside air, an extraction part for extracting, as a fine-bubble containing liquid, part of liquid circulating through the generator and the circulation passage, and a replenisher for replenishing the circulation passage with liquid to maintain an amount of liquid circulating through the generator and the circulation passage. 
     With this fine bubble-containing liquid generating apparatus, it is possible to continuously generate a fine-bubble containing liquid that contains a high density of fine bubbles. 
     In a preferred embodiment of the present invention, the fine bubble-containing liquid generating apparatus further includes a drain passage that branches off from the circulation passage and is connected to a drain port, and a switching mechanism for switching a delivery destination of liquid discharged from the discharge part between the lead-in part and the drain port. In a state prior to starting extraction of the fine-bubble containing liquid from the extraction part, the liquid led in from the replenisher to the lead-in part through the circulation passage is guided from the discharge part to the drain port by the switching mechanism. 
     In another preferred embodiment of the present invention, the fine bubble-containing liquid generating apparatus further includes a bypass passage that branches off from the circulation passage and is connected to the circulation passage on a downstream side of a branch point, an initial reservoir provided on the bypass passage and for storing liquid, and a switching mechanism provided between the circulation passage and the bypass passage. The switching mechanism performs switching such that prior to starting extraction of the fine-bubble containing liquid from the extraction part, the liquid discharged from the discharge part is guided to the initial reservoir through the bypass passage, temporally stored in the initial reservoir, and returned to the lead-in part through the bypass passage, and during the extraction of the fine-bubble containing liquid from the extraction part, the liquid discharged from the discharge part is returned to the lead-in part through the circulation passage. 
     In another preferred embodiment of the present invention, the replenisher includes a liquid supply passage for guiding liquid pumped from a liquid supply source to the circulation passage, and a pressure controller provided on the liquid supply passage and for controlling a pressure of liquid flowing through the liquid supply passage. 
     In another preferred embodiment of the present invention, the replenisher includes a liquid supply passage for guiding liquid from a liquid supply source to the circulation passage, and a pump provided on the liquid supply passage and for pumping liquid in the liquid supply passage toward the circulation passage. 
     In another preferred embodiment of the present invention, the fine bubble-containing liquid generating apparatus further includes a replenishment controller for controlling a pressure or flow rate of liquid supplied from the replenisher to the circulation passage, on the basis of an extraction flow rate of the fine-bubble containing liquid from the extraction part. 
     In another preferred embodiment of the present invention, the fine bubble-containing liquid generating apparatus further includes a bubble-density measuring part for measuring a density of fine bubbles in the fine-bubble containing liquid to be extracted from the extraction part, a storage for storing flow-rate/density information that indicates a relationship between an extraction flow rate of the fine-bubble containing liquid from the extraction part and a density of fine bubbles in the fine-bubble containing liquid to be extracted from the extraction part, and an extraction controller for controlling an extraction flow rate of the fine-bubble containing liquid from the extraction part, on the basis of a measurement result obtained by the bubble-density measuring part and the flow-rate/density information. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a fine bubble-containing liquid generating apparatus according to a first embodiment; 
         FIG. 2  is a cross-sectional view of a mixing nozzle; 
         FIG. 3  is a cross-sectional view of a fine-bubble generating nozzle; 
         FIG. 4  illustrates flow-rate/density information; 
         FIG. 5  illustrates a relationship between the elapsed time from the start of extraction and the concentration of fine bubbles in a fine-bubble containing liquid; 
         FIG. 6  is a cross-sectional view showing another example of the fine bubble-containing liquid generating apparatus; 
         FIG. 7  is a cross-sectional view of a fine bubble-containing liquid generating apparatus according to a second embodiment; and 
         FIG. 8  is a cross-sectional view of another fine bubble-containing liquid generating apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a cross-sectional view of a fine bubble-containing liquid generating apparatus  1  according to a first embodiment of the present invention. The fine bubble-containing liquid generating apparatus  1  is an apparatus for mixing gas and liquid to generate a liquid that contains fine bubbles of the liquid. In the following description, “fine bubbles” refers to bubbles with diameters of 100 μm or less, and “ultrafine bubbles” refers to fine bubbles with diameters of 1 μm or less. The “density” of fine bubbles refers to the number of fine bubbles per unit volume contained in the liquid. 
     The fine bubble-containing liquid generating apparatus  1  includes a generator  11 , a circulation passage  12 , an extraction part  13 , a replenisher  14 , a pump  15 , and a drain part  16 . The generator  11  includes a mixing nozzle  31 , a pressurized-liquid generating tank  32 , and a fine-bubble generating nozzle  2 . The mixing nozzle  31  mixes liquid pumped by the pump  15  and gas flowing from a gas inlet and ejects a resultant mixed fluid  72  into the pressurized-liquid generating tank  32 . The liquid and gas mixed in the mixing nozzle  31  are, for example, deionized water and a nitrogen gas. 
       FIG. 2  is an enlarged cross-sectional view of the mixing nozzle  31 . The mixing nozzle  31  includes a liquid inlet  311  for intake of the liquid pumped by the pump  15 , a gas inlet  319  for intake of the gas, and a mixed-fluid outlet  312  for ejection of the mixed fluid  72 . The mixed fluid  72  is generated by mixing the liquid flowing from the liquid inlet  311  and the gas flowing from the gas inlet  319 . The liquid inlet  311 , the gas inlet  319 , and the mixed-fluid outlet  312  have generally circular shapes. A nozzle flow passage  310  that extends from the liquid inlet  311  to the mixed-fluid outlet  312  and a gas flow passage  3191  that extends from the gas inlet  319  to the nozzle flow passage  310  also have generally circular flow passage cross-sectional shapes. Here, “flow passage cross-sections” refer to cross-sections perpendicular to central axes of flow passages such as the nozzle flow passage  310  and the gas flow passage  3191 , i.e., cross-sections perpendicular to the flow of fluid in the flow passages. In the following description, the area of a flow passage cross-section is referred to as a “flow passage area.” The nozzle flow passage  310  is in the shape of a venturi tube whose flow passage area decreases in the middle portion of the flow passage. 
     The mixing nozzle  31  includes a lead-in part  313 , a first tapered part  314 , a throat part  315 , a gas mixing part  316 , a second tapered part  317 , and a lead-out part  318  that are arranged sequentially in order from the liquid inlet  311  toward the mixed-fluid outlet  312 . The mixing nozzle  31  further includes a gas supply part  3192  that includes the gas flow passage  3191 . 
     The lead-in part  313  has a flow passage area that is approximately constant at each position in the direction of a central axis J 1  of the nozzle flow passage  310 . The first tapered part  314  has a flow passage area that gradually decreases in the direction of flow of the liquid (i.e., toward the downstream side). The throat part  315  has an approximately constant flow passage area. The throat part  315  has the smallest flow passage area in the nozzle flow passage  310 . Note that even if the throat part  315  has a flow passage area that changes slightly, the entire part of the nozzle flow passage  310  that has roughly the smallest flow passage area is regarded as the throat part  315 . The gas mixing part  316  has an approximately constant flow passage area that is slightly larger than the flow passage area of the throat part  315 . The second tapered part  317  has a flow passage area that gradually increases to the downstream side. The lead-out part  318  has an approximately constant flow passage area. The gas flow passage  3191  also has an approximately constant flow passage area, and is connected to the gas mixing part  316  of the nozzle flow passage  310 . 
     In the mixing nozzle  31 , the liquid flowing from the liquid inlet  311  into the nozzle flow passage  310  is caused to accelerate in the throat part  315  and thus has reduced static pressure, as a result of which the pressure in the throat part  315  and the gas mixing part  316  of the nozzle flow passage  310  falls to a value lower than atmospheric pressure. This causes the gas to be drawn in from the gas inlet  319  by suction, flow into the gas mixing part  316  through the gas flow passage  3191 , and be mixed with the liquid to generate the mixed fluid  72 . The mixed fluid  72  is caused to decelerate in the second tapered part  317  and the lead-out part  318  and thus has increased static pressure, as a result of which the mixed fluid  72  is ejected through the mixed-fluid outlet  312  into the pressurized-liquid generating tank  32  as described above. 
     The interior of the pressurized-liquid generating tank  32  illustrated in  FIG. 1  is pressurized to a state (hereinafter referred to as a “pressurized environment”) in which the pressure is higher than atmospheric pressure. In the pressurized-liquid generating tank  32 , the gas is dissolved in the liquid under pressure and a pressurized liquid is generated while the fluid (hereinafter, referred to as “mixed fluid  72 ”) obtained by mixing the liquid and gas ejected from the mixing nozzle  31  flows in the pressurized environment. 
     The pressurized-liquid generating tank  32  includes a first flow passage  321 , a second flow passage  322 , a third flow passage  323 , a fourth flow passage  324 , and a fifth flow passage  325  that are stacked in the up-down direction. In the following description, the first flow passage  321 , the second flow passage  322 , the third flow passage  323 , the fourth flow passage  324 , and the fifth flow passage  325  may be collectively referred to as “flow passages  321  to  325 .” The flow passages  321  to  325  extend in the horizontal direction and have generally rectangular cross-sectional shapes perpendicular to the lengths of the flow passages  321  to  325 . 
     The upstream end (i.e., the end on the left side in  FIG. 1 ) of the first flow passage  321  is attached to the aforementioned mixing nozzle  31 , and the mixed fluid  72  ejected from the mixing nozzle  31  flows to the right side in  FIG. 1  in the pressurized environment. In the present embodiment, the mixed fluid  72  is ejected from the mixing nozzle  31  upward of the liquid surface of the mixed fluid  72  flowing in the first flow passage  321 , and the mixed fluid  72  that has just been ejected collides directly with the liquid surface before colliding with the downstream wall surface (i.e., wall surface on the right side in  FIG. 1 ) of the first flow passage  321 . In order to cause the mixed fluid  72  ejected from the mixing nozzle  31  to collide directly with the liquid surface, the length of the first flow passage  321  is preferably 7.5 times greater than the distance in the up-down direction between the center of the mixed-fluid outlet  312  (see  FIG. 2 ) of the mixing nozzle  31  and the lower surface of the first flow passage  321 . 
     In the pressurized-liquid generating tank  32 , the mixed-fluid outlet  312  of the mixing nozzle  31  may be located partially or entirely below the liquid surface of the mixed fluid  72  flowing in the first flow passage  321 . In this case, in the first flow passage  321 , the mixed fluid  72  that has just been ejected from the mixing nozzle  31  collides directly with the mixed fluid  72  flowing in the first flow passage  321  as described above. 
     The lower surface at the downstream end of the first flow passage  321  has a generally circular opening  321   a , and the mixed fluid  72  flowing in the first flow passage  321  drops through the opening  321   a  into the second flow passage  322  located below the first flow passage  321 . In the second flow passage  322 , the mixed fluid  72  dropping from the first flow passage  321  flows from the right side to the left side in  FIG. 1  in the pressurized environment and drops through a generally circular opening  322   a , which is formed in the lower surface at the downstream end of the second flow passage  322 , into the third flow passage  323  located below the second flow passage  322 . In the third flow passage  323 , the mixed fluid  72  dropping from the second flow passage  322  flows from the left side to the right side in  FIG. 1  in the pressurized environment and drops through a generally circular opening  323   a , which is formed in the lower surface at the downstream end of the third flow passage  323 , into the fourth flow passage  324  located below the third flow passage  323 . As illustrated in  FIG. 1 , the mixed fluid  72  flowing in the first to fourth flow passages  321  to  324  is divided into a liquid layer that contains bubbles and a gas layer that is located above the liquid layer. 
     In the fourth flow passage  324 , the mixed fluid  72  dropping from the third flow passage  323  flows from the right side to the left side in  FIG. 1  in the pressurized environment and flows (i.e., drops) through a generally circular opening  324   a , which is formed in the lower surface at the downstream end of the fourth flow passage  324 , into the fifth flow passage  325  located below the fourth flow passage  324 . Unlike in the first to fourth flow passages  321  to  324 , there is no gas layer in the fifth flow passage  325 , and the liquid that fills the fifth flow passage  325  contains few bubbles in the vicinity of the upper surface of the fifth flow passage  325 . In the fifth flow passage  325 , the mixed fluid  72  from the fourth flow passage  324  flows from the left side to the right side in  FIG. 1  in the pressurized environment. 
     In the pressurized-liquid generating tank  32 , the gas in the mixed fluid  72 , which flows from top to bottom in the flow passages  321  to  325  while accelerating and decelerating in stages (i.e., flows while repeatedly alternating between a horizontal flow and a downward flow), is gradually dissolved in the liquid under pressure. In the fifth flow passage  325 , the concentration of the gas dissolved in the liquid is approximately equal to 60 to 90% of the (saturated) solubility of the gas in the pressurized environment. Excess gas that was not dissolved in the liquid remains as visible bubbles in the fifth flow passage  325 . Since the directions of flow of the mixed fluid  72  are opposite in the horizontal flow passages  321  to  325  that are vertically adjacent to each other, the size of the pressurized-liquid generating tank  32  can be reduced. 
     The pressurized-liquid generating tank  32  further includes an excess-gas separating part  326  that extends upward from the downstream upper surface of the fifth flow passage  325 . The excess-gas separating part  326  is filled with the mixed fluid  72 . The excess-gas separating part  326  has a generally rectangular cross-sectional shape perpendicular to the up-down direction, and the upper end of the excess-gas separating part  326  is connected to the extraction part  13 . Bubbles in the mixed fluid  72  flowing in the fifth flow passage  325  travel upward toward the extraction part  13  within the excess-gas separating part  326 . The details of the extraction part  13  will be described later. 
     By separating the excess gas in the mixed fluid  72  along with part of the mixed fluid  72  in this way, a pressurized liquid that substantially does not contain at least readily visible bubbles is generated and supplied to the fine-bubble generating nozzle  2 , which is directly connected to the downstream end of the fifth flow passage  325 . In the present embodiment, the gas dissolved in the pressurized liquid  71  has a (saturated) solubility that is approximately two or more times that of the gas under atmospheric pressure. In the pressurized-liquid generating tank  32 , the liquid in the mixed fluid  72  flowing in the flow passages  321  to  325  can also be regarded as a pressurized liquid that is in the process of being generated. 
     An exhaust valve  61  is also provided above the first flow passage  321 . When the pump  15  is stopped, the exhaust valve  61  is opened to prevent the mixed fluid  72  from flowing back to the mixing nozzle  31 . 
       FIG. 3  is an enlarged cross-sectional view of the fine-bubble generating nozzle  2 . The fine-bubble generating nozzle  2  includes a pressurized-liquid inlet  21  for intake of the pressurized liquid from the fifth flow passage  325  of the pressurized-liquid generating tank  32 , and a pressurized-liquid outlet  22  that is open to the circulation passage  12 . The pressurized-liquid inlet  21  and the pressurized-liquid outlet  22  have generally circular shapes, and a nozzle flow passage  20  that extends from the pressurized-liquid inlet  21  to the pressurized-liquid outlet  22  also has a generally circular flow passage cross-sectional shape. 
     The fine-bubble generating nozzle  2  includes a lead-in part  23 , a tapered part  24 , and a throat part  25  that are arranged sequentially in order from the pressurized-liquid inlet  21  to the pressurized-liquid outlet  22 . The lead-in part  23  has a flow passage area that is approximately constant at each position in the direction of a central axis J 2  of the nozzle flow passage  20 . The tapered part  24  has a flow passage area that gradually decreases in the direction of flow of the pressurized liquid (i.e., to the downstream side). The inner surface of the tapered part  24  is part of a generally circular conical surface centered on the central axis J 2  of the nozzle flow passage  20 . In a cross-section including the central axis J 2 , an angle α formed by the inner surface of the tapered part  24  is preferably greater than or equal to 10° and less than or equal to 90°. 
     The throat part  25  connects the tapered part  24  with the pressurized-liquid outlet  22 . The inner surface of the throat part  25  is a generally cylindrical surface, and the flow passage area of the throat part  25  is approximately constant. The flow passage cross-section of the throat part  25  has the smallest diameter in the nozzle flow passage  20 , and the flow passage area of the throat part  25  is the smallest in the nozzle flow passage  20 . The length of the throat part  25  is preferably greater than or equal to 1.1 times the diameter of the throat part  25  and less than or equal to 10 times the diameter thereof, and more preferably greater than or equal to 1.5 times the diameter of the throat part  25  and less than or equal to 2 times the diameter thereof. Note that even if the throat part  25  has a flow passage area that changes slightly, the entire part of the nozzle flow passage  20  that has roughly the smallest flow passage area is regarded as the throat part  25 . 
     The fine-bubble generating nozzle  2  further includes an enlarged part  27  that communicates with the throat part  25  and surrounds the pressurized-liquid outlet  22  while being spaced from the pressurized-liquid outlet  22 , and an enlarged-part opening  28  provided at the end of the enlarged part  27 . A flow passage  29  is provided between the pressurized-liquid outlet  22  and the enlarged-part opening  28  outside the pressurized-liquid outlet  22 , and is hereinafter referred to as an “external flow passage  29 .” The external flow passage  29  and the enlarged-part opening  28  have generally circular flow passage cross-sectional shapes, and the external flow passage  29  has an approximately constant flow passage area. The diameter of the external flow passage  29  is greater than the diameter of the throat part  25  (i.e., the diameter of the pressurized-liquid outlet  22 ). 
     In the following description, an annular surface between the edge of the inner peripheral surface of the enlarged part  27  on the pressurized-liquid outlet  22  side and the edge of the pressurized-liquid outlet  22  is referred to as an “outlet end surface  221 .” In the present embodiment, an angle formed by the outlet end surface  221  and the central axis J 2  of both the nozzle flow passage  20  and the external flow passage  29  is approximately 90°. The diameter of the external flow passage  29  is in the range of 10 to 20 mm, and the length of the external flow passage  29  is approximately equal to the diameter of the external flow passage  29 . In the fine-bubble generating nozzle  2 , the external flow passage  29 , which is a recessed part, can be regarded as being formed at the end on the side opposite to the pressurized-liquid inlet  21 , and the pressurized-liquid outlet  22 , which is an opening smaller than the bottom of the recessed part, can be regarded as being formed at the bottom of the recessed part. The enlarged part  27  has an enlarged flow passage area for the pressurized liquid between the pressurized-liquid outlet  22  and the circulation passage  12 . 
     In the fine-bubble generating nozzle  2 , the pressurized liquid flowing from the pressurized-liquid inlet  21  into the nozzle flow passage  20  flows toward the throat part  25  while gradually accelerating in the tapered part  24 , passes through the throat part  25 , and is ejected as a jet from the pressurized-liquid outlet  22 . The flow velocity of the pressurized liquid in the throat part  25  is preferably in the range of 10 to 30 meters per second. Since the static pressure of the pressurized liquid decreases in the throat part  25 , the gas in the pressurized liquid becomes supersaturated and is precipitated as fine bubbles into the liquid. The fine bubbles pass through the external flow passage  29  of the enlarged part  27 , along with the pressurized liquid. In the fine bubble generation nozzle  2 , the precipitation of fine bubbles occurs even while the pressurized liquid is passing through the external flow passage  29 . Thus, a liquid containing fine bubbles is generated and supplied to the circulation passage  12 . The fine bubbles generated by the fine-bubble generating nozzle  2  primarily include ultrafine bubbles. 
     In the generator  11  illustrated in  FIG. 1 , the mixing nozzle  31  is a lead-in part for leading in the gas and the liquid pressurized by the pump  15  to the pressurized-liquid generating tank  32 . The fine-bubble generating nozzle  2  is a discharge part for discharging a liquid that contains fine bubbles of the gas led in from the mixing nozzle  31 , to the circulation passage  12 . 
     One end of the circulation passage  12  is connected to the enlarged-part opening  28  (see  FIG. 3 ) of the fine-bubble generating nozzle  2 , and the other end is connected to the liquid inlet  311  (see  FIG. 2 ) of the mixing nozzle  31 . The aforementioned pump  15  is provided on the circulation passage  12 . The liquid containing fine bubbles, discharged from the fine-bubble generating nozzle  2 , is pumped into the circulation passage  12  by the pump  15  and returned to the mixing nozzle  31 . The circulation passage  12  is a sealed pipeline, and the liquid discharged from the fine-bubble generating nozzle  2  is returned to the mixing nozzle  31  in a state of being isolated from the outside air. The liquid returned to the mixing nozzle  31  is passed through the pressurized-liquid generating tank  32 , the fine-bubble generating nozzle  2 , and the circulation passage  12  and return again to the mixing nozzle  31 . In the fine bubble-containing liquid generating apparatus  1 , the liquid containing fine bubbles circulates through the generator  11  and the circulation passage  12  in a state of being isolated from the outside air. The density of fine bubbles in the liquid is increased by repetition of this circulation. 
     In the fine bubble-containing liquid generating apparatus  1 , part of the liquid circulating through the generator  11  and the circulation passage  12  is extracted as a fine-bubble containing liquid by the extraction part  13 . The extraction part  13  includes an extraction passage  131  and a bubble removing part  132 . The extraction passage  131  is connected to the upper end of the excess-gas separating part  326 . The bubble removing part  132  is provided on the extraction passage  131  to remove bubbles (i.e., readily visible bubbles) other than fine bubbles from the liquid flowing from the excess-gas separating part  326  into the extraction passage  131 . For example, the bubble removing part  132  may be a vent valve. The liquid passing through the bubble removing part  132  is a fine-bubble containing liquid that substantially does not contain readily visible bubbles and that contains a high density of fine bubbles. The fine-bubble containing liquid is extracted from an output port  133  at the tip end of the extraction passage  131 . 
     The fine bubble-containing liquid generating apparatus  1  further includes an extraction controller  134 , a bubble-density measuring part  135 , and a storage  136 . The extraction controller  134  is provided between the bubble removing part  132  and the output port  133  on the extraction passage  131 . For example, the extraction controller  134  may be a flow control valve for controlling the flow rate of the fine-bubble containing liquid flowing through the extraction passage  131 , and be a valve controller for controlling the degree of opening of the flow control valve. The bubble-density measuring part  135  is connected to the extraction passage  131  between the bubble removing part  132  and the output port  133 . The bubble-density measuring part  135  measures the density of fine bubbles in the fine-bubble containing liquid to be extracted from the extraction part  13 . The bubble-density measuring part  135  may be implemented by, for example, a technology such as NanoSight Limited&#39;s NS500. 
     The extraction controller  134  is connected to the storage  136 . The storage  136  stores flow-rate/density information in advance. The flow-rate/density information indicates a relationship between the extraction flow rate of the fine-bubble containing liquid from the extraction part  13  and the density of fine bubbles in the fine-bubble containing liquid to be extracted from the extraction part  13 . 
       FIG. 4  illustrates the flow-rate/density information. In  FIG. 4 , the horizontal axis represents the extraction flow rate of the fine-bubble containing liquid, and the vertical axis represents the density of fine bubbles in the fine-bubble containing liquid. A plurality of circles in  FIG. 4  indicate the results of measurement of the density of fine bubbles in the fine-bubble containing liquid, extracted at each extraction flow rate of the fine-bubble containing liquid. This measurement is conducted under approximately the same conditions, except for the extraction flow rate. The solid line  81  in  FIG. 4  indicates the flow-rate/density information obtained from the circles. As illustrated in  FIG. 4 , the density of fine bubbles in the fine-bubble containing liquid decreases as the extraction flow rate of the fine-bubble containing liquid increases. 
     The measurement results obtained by the bubble-density measuring part  135  (i.e., the measured densities of fine bubbles) are transmitted to the extraction controller  134 . The extraction controller  134  controls the extraction flow rate of the fine-bubble containing liquid from the extraction part  13  on the basis of a target density that is input in advance, the measurement result obtained by the bubble-density measuring part  135 , and the flow-rate/density information stored in the storage  136 . As a result, the density of fine bubbles in the fine-bubble containing liquid to be extracted from the extraction part  13  becomes approximately equal to the target density. 
       FIG. 5  illustrates a relationship between the elapsed time from the start of extraction and the density of fine bubbles in the fine-bubble containing liquid to be extracted, when the fine bubble-containing liquid generating apparatus  1  continuously extracts the fine-bubble containing liquid. In  FIG. 5 , the horizontal axis represents the elapsed time from the start of extraction of the fine-bubble containing liquid, and the vertical axis represents the density of fine bubbles in the fine-bubble containing liquid. In the fine bubble-containing liquid generating apparatus  1 , as a result of control by the extraction controller  134 , the fine-bubble containing liquid containing an approximately desired density of fine bubbles can be continuously extracted over a long period of time as illustrated in  FIG. 5 . 
     The replenisher  14  is connected to the circulation passage  12  and replenishes the circulation passage  12  with the same type of liquid (in the present embodiment, deionized water) as the liquid circulating through the generator  11  and the circulation passage  12 . The replenisher  14  maintains the amount of liquid circulating through the generator  11  and the circulation passage  12  by replenishing the circulation passage  12  with the approximately same amount of liquid as the amount of fine-bubble containing liquid to be extracted from the extraction part  13 . 
     The replenisher  14  includes a liquid supply passage  141 , a pressure controller  142 , and a replenishment controller  143 . One end of the liquid supply passage  141  is connected to the circulation passage  12  between a switching mechanism  162  and the pump  15 , and the other end is connected to a liquid supply source  91  that is provided outside the fine bubble-containing liquid generating apparatus  1 . The liquid supply source  91  is, for example, a deionized-water supply line that is installed in, for example, a facility to pump deionized water into various apparatuses. The liquid supply passage  141  guides the liquid pumped from the liquid supply source  91  to the circulation passage  12 . The liquid supply passage  141  is a sealed pipeline, and the liquid from the liquid supply source  91  is guided to the circulation passage  12  in a state of being isolated from the outside air within the liquid supply passage  141 . The pressure controller  142  is provided on the liquid supply passage  141  and controls the pressure of the liquid pumped from the liquid supply source  91  and flowing through the liquid supply passage  141 . The pressure controller  142  is, for example, a pressure control valve. 
     The replenishment controller  143  is connected to the pressure controller  142 . When the pressure controller  142  is a pressure control valve, the replenishment controller  143  is, for example, a valve controller for controlling the degree of opening of the pressure control valve. The replenishment controller  143  controls the pressure controller  142  on the basis of the extraction flow rate of the fine-bubble containing liquid from the extraction part  13 . More specifically, the replenishment controller  143  controls the pressure or flow rate of the liquid supplied from the replenisher  14  to the circulation passage  12  so that the flow rate (hereinafter, referred to as “replenishment flow rate”) of the liquid supplied from the liquid supply passage  141  of the replenisher  14  to the circulation passage  12  is approximately equal to the extraction flow rate of the fine-bubble containing liquid from the extraction part  13 . As a result, an approximately constant amount of liquid circulating through the generator  11  and the circulation passage  12  (hereinafter, referred to as “circulation amount”) is maintained. 
     The fine bubble-containing liquid generating apparatus  1  may be configured such that a relationship between the extraction flow rate from the extraction part  13  and the pressure of the liquid supplied from the replenisher  14  when the circulation amount is maintained is stored in advance, and the pressure of the liquid supplied from the replenisher  14  is controlled on the basis of this relationship and the extraction flow rate. Alternatively, a configuration is also possible in which the replenisher  14  is provided with a flowmeter for measuring the replenishment flow rate, and the replenishment controller  143  performs feedback control of the pressure controller  142  so that the measurement result of the flowmeter is equal to the extraction flow rate of the fine-bubble containing liquid from the extraction part  13 . 
     The drain part  16  includes a drain passage  161  and the switching mechanism  162  (e.g., a switching valve such as a three-way valve). One end of the drain passage  161  is connected to the circulation passage  12  between the fine-bubble generating nozzle  2  and the pump  15 , and the other end is connected to a drain port  92  that is provided outside the fine bubble-containing liquid generating apparatus  1 . In other words, the drain passage  161  branches off from the circulation passage  12  and is connected to the drain port  92 . The switching mechanism  162  is provided at the connection (i.e., branch point) between the circulation passage  12  and the drain passage  161  and switches a delivery destination of the liquid received from the fine-bubble generating nozzle  2 , between the drain port  92  and the mixing nozzle  31 . 
     The pressure in the generator  11  fluctuates immediately after startup of the fine bubble-containing liquid generating apparatus  1 , i.e., immediately after the liquid starts flowing through the generator  11 . In view of this, the operation of supplying liquid from the replenisher  14  to the generator  11  through the circulation passage  12  and guiding the liquid passing through the generator  11  to the drain port  92  via the switching mechanism  162  is performed for a predetermined period of time (e.g., several tens of seconds) immediately after startup of the fine bubble-containing liquid generating apparatus  1 . At this time, the fine-bubble containing liquid is not extracted from the extraction part  13 . In other words, in the state prior to starting the extraction of the fine-bubble containing liquid from the extraction part  13 , the liquid led in from the replenisher  14  to the mixing nozzle  31  of the generator  11  through the circulation passage  12  is guided from the fine-bubble generating nozzle  2  to the drain port  92  via the switching mechanism  162 , without circulating through the generator  11  and the circulation passage  12 . This allows approximately constant pressure to be maintained in the generator  11  and stabilizes the startup of the fine bubble-containing liquid generating apparatus  1 . 
     In the fine bubble-containing liquid generating apparatus  1 , when the pressure in the generator  11  becomes approximately constant, the delivery destination of the fine-bubble containing liquid discharged from the fine-bubble generating nozzle  2  is switched by the switching mechanism  162 , and the liquid is returned to the mixing nozzle  31  through the circulation passage  12 . The fine-bubble containing liquid then circulates through the generator  11  and the circulation passage  12 , so that the density of fine bubbles in the liquid is increased to a desired density. The fine-bubble containing liquid is not extracted from the extraction part  13  until the density of fine bubbles in the liquid reaches the desired density, and the replenishment with the liquid from the replenisher  14  is also stopped. When the density of fine bubbles in the liquid circulating through the generator  11  and the circulation passage  12  reaches the desired density, the extraction part  13  starts extracting the fine-bubble containing liquid, and the replenisher  14  also starts replenishment with liquid. 
     As described above, the fine bubble-containing liquid generating apparatus  1  includes the generator  11  including the mixing nozzle  31  and the fine-bubble generating nozzle  2 , the circulation passage  12  for returning the liquid discharged from the fine-bubble generating nozzle  2  to the mixing nozzle  31  in a state in which the liquid is isolated from the outside air, the extraction part  13  for extracting part of the liquid circulating through the generator  11  and the circulation passage  12  as a fine-bubble containing liquid, and the replenisher  14  for replenishing the circulation passage  12  with liquid to maintain the amount of liquid circulating through the generator  11  and the circulation passage  12 . With this configuration, it is possible to continuously generate a fine-bubble containing liquid that contains a high density of fine bubbles. As a result, the fine-bubble containing liquid can be continuously supplied in various applications. 
     Incidentally, apparatuses such as semiconductor manufacturing apparatuses are required to avoid a situation in which processing liquids used in the processing of semiconductor substrates accumulate within the apparatuses before being supplied to the semiconductor substrates. In the fine bubble-containing liquid generating apparatus  1 , the fine-bubble containing liquid circulates through the generator  11  and the circulation passage  12  without accumulating within the apparatus, as described above. This makes the fine bubble-containing liquid generating apparatus  1  particularly suitable for the supply of the fine-bubble containing liquid to apparatuses such as semiconductor manufacturing apparatuses. Moreover, in the fine bubble-containing liquid generating apparatus  1 , the liquid flowing through the generator  11  at the time of startup of the apparatus is discharged to the drain port  92  without circulating through the generator  11  and the circulation passage  12 . This prevents the liquid from accumulating in the apparatus at the time of startup of the fine bubble-containing liquid generating apparatus  1 . Accordingly, the fine bubble-containing liquid generating apparatus  1  is even more suitable for the supply of the fine-bubble containing liquid to apparatuses such as semiconductor manufacturing apparatuses. 
     The fine bubble-containing liquid generating apparatus  1  includes the bubble-density measuring part  135  for measuring the density of fine bubbles in the fine-bubble containing liquid to be extracted from the extraction part  13 , the storage  136  for storing the flow-rate/density information, and the extraction controller  134  for controlling the extraction flow rate of the fine-bubble containing liquid from the extraction part  13 , on the basis of the measurement result obtained by the bubble-density measuring part  135  and the flow-rate/density information. Thus, it is possible to readily generate a fine-bubble containing liquid that contains a desired density of fine bubbles. 
     As described above, the replenisher  14  includes the liquid supply passage  141  for guiding the liquid pumped from the liquid supply source  91  to the circulation passage  12 , and the pressure controller  142  for controlling the pressure of the liquid flowing through the liquid supply passage  141 . Thus, the amount of liquid circulating through the generator  11  and the circulation passage  12  can be readily maintained. Moreover, the replenishment controller  143  controls the pressure or flow rate of the liquid that is supplied from the replenisher  14  to the circulation passage  12 , on the basis of the extraction flow rate of the fine-bubble containing liquid from the extraction part  13 . This allows the circulation amount to be automatically maintained by replenishment with the liquid from the replenisher  14 . 
     The structure of the replenisher  14  in the fine bubble-containing liquid generating apparatus  1  is not limited to the above example, and may be modified in various ways. For example, the fine bubble-containing liquid generating apparatus  1  may include a replenisher  14   a  illustrated in  FIG. 6 , instead of the replenisher  14  illustrated in  FIG. 1 . The replenisher  14   a  includes a liquid supply passage  141 , a replenishment controller  143 , and a pump  144 . One end of the liquid supply passage  141  is connected to the circulation passage  12  between the switching mechanism  162  and the pump  15 , and the other end is connected to a liquid supply source  91   a  that is provided outside the fine bubble-containing liquid generating apparatus  1 . The liquid supply source  91   a  is, for example, a reservoir for storing deionized water. The liquid supply passage  141  guides the liquid from the liquid supply source  91   a  to the circulation passage  12 . The liquid supply passage  141  is a sealed pipeline, and the liquid from the liquid supply source  91   a  is guided to the circulation passage  12  in a state of being isolated from the outside air within the liquid supply passage  141 . The pump  144  is provided on the liquid supply passage  141  and pumps the liquid flowing through the liquid supply passage  141  toward the circulation passage  12 . Thus, the amount of liquid circulating through the generator  11  and the circulation passage  12  (i.e., circulation amount) can be readily maintained as in the case where the replenisher  14  illustrated in  FIG. 1  is provided. 
     The replenishment controller  143  is connected to the pump  144  and controls driving of the pump  144 . As a result of the replenishment controller  143  controlling the pump  144 , the pressure or flow rate of the liquid supplied from the replenisher  14   a  to the circulation passage  12  is controlled so that the replenishment flow rate from the replenisher  14   a  is approximately equal to the extraction flow rate of the fine-bubble containing liquid from the extraction part  13 . Thus, the circulation amount can be automatically maintained by replenishment with the liquid from the replenisher  14   a , as described above. The replenisher  14   a  may be provided with a flow controller such as a throttle valve in the liquid supply passage  141 . In this case, the pump  144  is driven by a given output, and as a result of the replenishment controller  143  controlling this throttle valve, the flow rate of the liquid supplied from the replenisher  14   a  to the circulation passage  12  is controlled so that the replenishment flow rate from the replenisher  14   a  is approximately equal to the extraction flow rate of the fine-bubble containing liquid from the extraction part  13 . 
       FIG. 7  is a cross-sectional view of a fine bubble-containing liquid generating apparatus  1   a  according to a second embodiment of the present invention. The fine bubble-containing liquid generating apparatus  1   a  includes an initial circulation part  17 , instead of the drain part  16  illustrated in  FIG. 1 . The other constituent elements are identical to those of the fine bubble-containing liquid generating apparatus  1  illustrated in  FIG. 1 , and the same constituent elements are given the same reference numerals in the following description. 
     The initial circulation part  17  includes a bypass passage  171 , switching mechanisms  172   a ,  172   b , and  172   c  such as valves, and an initial reservoir  173 . One end of the bypass passage  171  is connected to the circulation passage  12  between the fine-bubble generating nozzle  2  and the switching mechanism  172   c . The other end of the bypass passage  171  is connected to the circulation passage  12  between the switching mechanism  172   c  and the pump  15  on the downstream side of the above one end (i.e., on the forward side in the direction of flow of the liquid in the circulation passage  12 ). In other words, the bypass passage  171  branches off from the circulation passage  12  at a branch point on the circulation passage  12  and is connected to the circulation passage  12  on the downstream side of the branch point on the circulation passage  12 . 
     The initial reservoir  173  is provided between the switching mechanisms  172   a  and  172   b  on the bypass passage  171  and stores the liquid flowing through the bypass passage  171 . The initial reservoir  173  is, for example, a reserve tank capable of storing a certain amount of liquid. Each of the switching mechanisms  172   a  and  172   b  is provided between the circulation passage  12  and the bypass passage  171 . The switching mechanisms  172   a ,  172   b , and  172   c  switch the delivery destination of the liquid from the fine-bubble generating nozzle  2  between the circulation passage  12  and the bypass passage  171 . 
     The pressure in the generator  11  fluctuates immediately after startup of the fine bubble-containing liquid generating apparatus  1   a , i.e., immediately after the liquid starts flowing through the generator  11 . In view of this, the liquid (e.g., deionized water) stored in the initial reservoir  173  is supplied through the bypass passage  171  and the circulation passage  12  to the generator  11  for a predetermined period of time (e.g., several tens of seconds) immediately after startup of the fine bubble-containing liquid generating apparatus  1   a . The liquid passing through the generator  11  is guided to the bypass passage  171  and to the initial reservoir  173  through the bypass passage  171  by the switching mechanisms  172   a ,  172   b , and  172   c , without being guided to the generator  11  via the switching mechanism  172   c . The liquid is temporarily stored in the initial reservoir  173  and then supplied to the generator  11  through the bypass passage  171 . At this time, the fine-bubble containing liquid is not extracted from the extraction part  13 . 
     In other words, in the state prior to starting the extraction of the fine-bubble containing liquid from the extraction part  13 , the liquid discharged from the fine-bubble generating nozzle  2  is guided through the bypass passage  171  to the initial reservoir  173 , temporarily stored in the initial reservoir  173 , and then returned to the mixing nozzle  31  through the bypass passage  171 . This allows approximately constant pressure to be maintained in the generator  11  and stabilizes the startup of the fine bubble-containing liquid generating apparatus  1   a . In addition, the amount of liquid consumed at the time of startup of the apparatus can be reduced because the liquid is not discharged to the outside of the apparatus at the time of startup of the fine bubble-containing liquid generating apparatus  1   a.    
     In the fine bubble-containing liquid generating apparatus  1   a , when the pressure in the generator  11  becomes approximately constant, the delivery destination of the fine bubble-containing liquid discharged from the fine-bubble generating nozzle  2  is switched by the switching mechanisms  172   a ,  172   b , and  172   c  so that the liquid is returned to the mixing nozzle  31  via the switching mechanism  172   c  in the circulation passage  12  without passing through the bypass passage  171  and the initial reservoir  173 . Then, the fine bubble-containing liquid circulates through the generator  11  and the circulation passage  12 , and therefore the density of fine bubbles in the liquid is increased to the desired density. The fine-bubble containing liquid is not extracted from the extraction part  13  until the density of fine bubbles in the liquid reaches the desired density, and the supply of liquid from the replenisher  14  is also stopped. 
     When the density of fine bubbles in the liquid circulating through the generator  11  and the circulation passage  12  reaches the desired density, the extraction of the fine-bubble containing liquid from the extraction part  13  is started, and the supply of liquid from the replenisher  14  is also started. In this way, in the fine bubble-containing liquid generating apparatus  1   a , the liquid discharged from the fine-bubble generating nozzle  2  is returned through the circulation passage  12  to the mixing nozzle  31  while the fine-bubble containing liquid is being extracted from the extraction part  13 . Accordingly, it is possible to continuously generate the fine-bubble containing liquid that contains a high density of fine bubbles, as in the fine bubble-containing liquid generating apparatus  1  illustrated in  FIG. 1 . 
     The fine bubble-containing liquid generating apparatus  1   a  may further include another initial circulation part  18  as illustrated in  FIG. 8 . The initial circulation part includes a bypass passage  181  and a switching mechanism  182  such as a valve. One end of the bypass passage  181  is connected to the extraction part  13  between the bubble removing part  132  and the extraction controller  134 . The other end of the bypass passage  181  is connected to a predetermined part (in  FIG. 8 , the initial reservoir  173 ) out of the bypass passage  171  between the switching mechanisms  172   a  and  172   b  and the initial reservoir  173  of the initial circulation part  17 . The switching mechanism  182  is provided on the bypass passage  181  and operates in synchronization with the switching mechanisms  172   a ,  172   b , and  172   c . That is, when the switching mechanisms  172   a ,  172   b , and  172   c  supply the liquid stored in the initial reservoir  173  to the generator  11  through the bypass passage  171  and the circulation passage  12  without supplying the liquid to the generator  11  via the switching mechanism  172   c , the switching mechanism  182  guides the liquid from which bubbles other than fine bubbles have been removed, from the bubble removing part  132  to the initial circulation part  17 . The switching mechanism  182  does not guide the liquid from the bubble removing part  132  to the initial circulation part  17  when the switching mechanisms  172   a ,  172   b , and  172   c  return the liquid from the fine-bubble generating nozzle  2  to the mixing nozzle  31  via the switching mechanism  172   c  in the circulation passage  12  without passing the liquid through the bypass passage  171  and the initial reservoir  173 . As described above, the addition of the initial circulation part  18  increases the efficiency of circulation of the liquid in the generator  11 . 
     The fine bubble-containing liquid generating apparatuses  1  and  1   a  described above may be modified in various ways. 
     For example, the liquid that is mixed with the gas in the mixing nozzle  31  is not limited to pure water, and may be a liquid consisting primarily of water. For example, the above liquid may be water with additives or a nonvolatile liquid. The liquid may also be ethyl alcohol. The gas that forms fine bubbles is not limited to nitrogen, and may be air or other gas. However, it is necessary for the gas to be insoluble or poorly soluble in the liquid. 
     In the fine bubble-containing liquid generating apparatuses  1  and  1   a , the extraction part  13  does not necessarily have to be connected to the excess-gas separating part  326  of the pressurized-liquid generating tank  32  as long as it is possible to extract part of the liquid circulating through the generator  11  and the circulation passage  12  as a fine-bubble containing liquid. For example, the extraction part  13  may be connected to a part other than the excess-gas separating part  326  of the generator  11 , and may be connected to the circulation passage  12  between the fine-bubble generating nozzle  2  and the pump  15 . 
     The structure of the generator  11  may be modified in various ways, and the generator  11  may have a different structure. For example, the fine-bubble generating nozzle  2  may include a plurality of pressurized-liquid outlets  22 . The fine-bubble generating nozzle  2  does not necessarily have to be directly connected to the fifth flow passage  325  of the pressurized-liquid generating tank  32 , and the downstream end of the fifth flow passage  325  and the fine-bubble generating nozzle  2  may be connected by a sealed connection passage. The passages in the pressurized-liquid generating tank  32  may have circular cross-sectional shapes. The mixture of gas and liquid may be implemented by other methods such as mechanical agitation. 
     The fine-bubble containing liquid generated by the fine bubble-containing liquid generating apparatuses  1  and  1   a  may be used in various applications that have heretofore been proposed for conventional fine-bubble containing liquid. The fine-bubble containing liquid may be used in novel fields, and conceivable fields of application span a diverse range. Examples include food products, beverages, cosmetics, drugs, medical treatment, plant cultivation, semiconductor devices, flat panel displays, electronic equipment, solar cells, secondary batteries, new functional materials, and radioactive material removal. 
     The configurations of the above-described preferred embodiments and variations may be appropriately combined as long as there are no mutual inconsistencies. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore to be understood that numerous modifications and variations can be devised without departing from the scope of the invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  1   a  Fine bubble-containing liquid generating apparatus 
               2  Fine-bubble generating nozzle 
               11  Generator 
               12  Circulation passage 
               13  Extraction part 
               14 ,  14   a  Replenisher 
               31  Mixing nozzle 
               91 ,  91   a  Liquid supply source 
               92  Drain port 
               134  Extraction controller 
               135  Bubble-density measuring part 
               136  Storage 
               141  Liquid supply passage 
               142  Pressure controller 
               143  Replenishment controller 
               144  Pump 
               161  Drain passage 
               162  Switching mechanism 
               171  Bypass passage 
               172   a ,  172   b ,  172   c  Switching mechanism 
               173  Initial reservoir