Patent Publication Number: US-11649175-B2

Title: Heat dissipation apparatus and methods for UV-LED photoreactors

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Application No. 62/280,630 filed on 19 Jan. 2016 and entitled HEAT DISSIPATION APPARATUS AND METHODS FOR UV-LED PHOTOREACTORS. For purposes of the United States, this application claims the benefit under 35 U.S.C. § 119 of U.S. Application No. 62/280,630 filed on 19 Jan. 2016 and entitled HEAT DISSIPATION APPARATUS AND METHODS FOR UV-LED PHOTOREACTORS. U.S. application No. 62/280,630 is hereby incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates to thermal management for ultraviolet light (radiation) emitting diode (UV-LED) reactors used to irradiate fluids. Particular embodiments provide apparatus and methods for providing heat dissipation for UV-LEDs and/or other heat producing electronic devices used in UV-LED photoreactors. 
     BACKGROUND 
     Ultraviolet (UV) reactors—reactors that administer UV radiation—are applied to many photoreactions, photocatalytic reactions, and photo-initiated reactions. One application for UV reactors is for water and air purification. In particular, UV reactors have emerged in recent years as one of the most promising technologies for water treatment. Prior art UV reactor systems typically use low- and medium-pressure mercury lamps to generate UV radiation. 
     Light (radiation) emitting diodes (LEDs) typically emit radiation of such narrow bandwidth that radiation emitted by LEDs may be considered (for many applications) to be monochromatic (i.e. of a single wavelength). With recent advances in LED technology, LEDs may be designed to generate UV radiation at different wavelengths, which include a wavelength for DNA absorption as well as wavelengths that can be used for photocatalyst activation. UV-LEDs have many advantages compared to traditional mercury UV lamps, including, without limitation, compact and robust design, lower voltage and power requirements, and the ability to turn on and off with high frequency. These advantages of UV-LEDs make them an attractive alternative for replacing UV lamps in UV reactor systems. This replacement also makes possible the development of novel UV reactors with new applications. 
     UV-LED reactors may generally be used for irradiating fluids, with applications such as water disinfection. However, in a typical UV-LED reactor, there is considerable heating of the UV-LED (or other electronic devices) used in the reactor. Excessive heating of a UV-LED used in a UV-LED photoreactor may decrease radiation output, decrease the useful lifetime of the UV-LED and/or shift the peak wavelength of the emitted radiation. The radiation output of a UV-LED and/or its lifetime performance may significantly improve with appropriate thermal management (e.g. heat dissipation). The heat generated by UV-LEDS may adversely affect the performance of other electronic components electronically connected to the UV-LED (e.g. mounted on the same printed circuit board (PCB)) and/or vice versa. 
     The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. 
     Aspects of the invention provides apparatus and methods for thermal management of the heat generated by UV-LEDs, which involve dissipation of the heat generated by UV-LEDs. Thermal management may enhance the UV-LED radiation output and/or the operational lifetime of the UV-LEDs. Particular embodiments provide apparatus and methods for providing heat dissipation for the UV-LEDs and/or other electronic devices used in UV-LED photoreactors for the irradiation of a fluid flow. By way of non-limiting example, the UV-LED reactor may be a fluid treatment reactor, such as a water treatment reactor. 
     In accordance with some aspects of the invention, a fluid flowing through fluid flow channels of a UV-LED photoreactor is used to dissipate heat generated by the UV-LEDs and/or other electronic devices of the photoreactor. The UV reactor is configured so that part of the irradiated fluid is circulated in the proximity of the UV-LEDs or the UV-LED circuit board, and/or by incorporating thermally conductive materials in the walls of the fluid conduit. Heat dissipation may be achieved by thermally coupling the highly thermally conductive material of a LED printed circuit board (PCB) on which one or more UV-LEDs are operatively connected to at least one fluid conduit-defining wall of the photoreactor. Such fluid conduit-defining wall(s) of the photoreactor may also be made of highly thermally conductive material. With this thermal coupling, heat generated by the UV-LEDs spreads through the highly heat conductive PCB and the at least one highly heat conductive conduit-defining wall(s) of the photoreactor, such that as the fluid flows in the conduit of the photoreactor, it dissipates the heat generated by the UV-LED(s) away from the UV-LED(s) through the conduit-defining wall(s) of the photoreactor. In this configuration, the PCB on which the UV-LEDs are connected can be connected to the fluid conduit either directly or through other thermally conductive parts of the reactor or through other thermally conductive materials from the side of the PCB on which the UV-LED(s) is connected. The thermal coupling may be achieved, in some embodiments, through regions (e.g. edges) of a metal-core PCB that do not have the typical soldering mask coating (or have this solder mask coating removed) and as a result are highly thermally conductive. This configuration may improve thermal management and, consequently, the radiation power output and lifetime of the UV-LED(s) and the corresponding UV-LED photoreactor. 
     One aspect of the invention provides an ultraviolet (UV) reactor for irradiating a flow of fluid with UV radiation. The reactor comprises: a fluid conduit defined by a heat conducting conduit body comprising one or more heat conducting walls for permitting a flow of fluid therethrough; and a UV light emitting diode (UV-LED) operatively connected to a printed circuit board (PCB), the UV-LED oriented for directing radiation into the fluid conduit. The PCB comprises a heat conducting substrate having a first surface. The heat conducting conduit body is in thermal contact with the first surface of the heat conducting substrate of the PCB. Heat is dissipated from the UV-LED via the heat conducting substrate, the thermal contact between the first surface of the heat conducting substrate and the heat conducting conduit body, and from the one or more heat conducting walls of the heat conducting conduit body to the fluid flowing through the fluid conduit. 
     The UV-LED may be oriented for directing radiation to have a principal optical axis extending in a first direction from the UV-LED to the fluid conduit. The first surface of the heat conducting substrate may be planar with a normal vector oriented substantially in the first direction. In some embodiments, the orientation of normal vector of the first surface being substantially in the first direction means that in any plane, the angular difference between normal vector and the first direction is less than 25°. In some embodiments, this angular difference is less than 15°. In some embodiments, this angular difference is less than 5°. 
     The thermal contact between the heat conducting conduit body and the first surface of the heat conducting substrate of the PCB may comprise a thermal contact enhancing component interposed between the heat conducting conduit body and the first surface of the heat conducting substrate. The thermal contact enhancing component may reduce a thermal contact resistance (increasing the thermal contact conductivity) between heat conducting conduit body and the heat conducting substrate of the PCB. The thermal contact enhancing component may comprise a thermally conductive and deformable thermal pad. The thermal contact enhancing component may comprise a thermally conductive gel or paste. The thermal contact between the heat conducting conduit body and the first surface of the heat conducting substrate of the PCB may comprise a heat conducting intermediate component interposed between the heat conducting conduit body and the first surface of the heat conducting substrate. 
     The PCB may comprise a thermal contact region where the first surface of the heat conducting substrate is exposed. The thermal contact between the heat conducting conduit body and the first surface of the heat conducting substrate may be made in the thermal contact region. A solder mask coating of the PCB is removed from the thermal contact region of the PCB. The PCB may comprise a solder mask covering the first surface of the heat conducting substrate in a circuit region adjacent to the thermal contact region, the UV-LED located in the circuit region. 
     The fluid flowing through the fluid conduit may contact the one or more heat conducting walls of the fluid conduit to dissipate heat from the one or more heat conducting walls of the fluid conduit into the fluid. The contact between the fluid flowing through the fluid conduit and the one or more heat conducting walls of the fluid conduit may occur, at least in part, inside a UV active region of the reactor. 
     The heat conducting conduit body may comprise: a plurality of fluid flow channels, each fluid flow channel defined by one or more heat conducting walls; and a manifold located at the ends of at least two of the plurality of fluid flow channels and shaped to provide fluid communication between the at least two fluid flow channels. The thermal contact between the heat conducting conduit body and the first surface of the heat conducting substrate may comprise thermal contact between the manifold and the first surface of the heat conducting substrate. The manifold may be integrally formed with the plurality of fluid flow channels. The manifold may be joined to, and in thermal contact with, the plurality of fluid flow channels. 
     The principal optical axis may be generally parallel with a direction of flow of the fluid through the fluid conduit. Where the heat conducting conduit body comprises a plurality of longitudinally extending fluid flow channels, the principal optical axis may be generally parallel with a longitudinal direction of fluid flow through the plurality of longitudinally extending fluid channels. The first direction in which the optical axis extends from the UV-LED to the fluid conduit may oppose the longitudinal direction of fluid flow in at least one of the plurality of fluid flow channels. The first direction in which the optical axis extends from the UV-LED to the fluid conduit may additionally or alternative be the same as the longitudinal direction of fluid flow in at least one of the plurality of fluid flow channels. 
     Another aspect of the invention provides a method for thermal management in an ultraviolet (UV) reactor for irradiating a flow of fluid with UV radiation. The method comprises: permitting a flow of fluid through a fluid conduit defined by a heat conducting conduit body comprising one or more heat conducting walls; operatively connecting a UV light emitting diode (UV-LED) to a printed circuit board (PCB), the PCB comprising a heat conducting substrate having a first surface; orienting the UV-LED for directing radiation into the fluid conduit; and making thermal contact between the heat conducting conduit body and the first surface of the heat conducting substrate of the PCB; wherein heat is dissipated from the UV-LED via the heat conducting substrate, the thermal contact between the first surface of the heat conducting substrate and the heat conducting conduit body, and from the one or more heat conducting walls of the heat conducting conduit body to the fluid flowing through the fluid conduit. 
     The method may comprise features similar to those of the reactors described herein and methods of fabricating, assembling, and/or using same. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
         FIG.  1 A  is a schematic perspective view of a UV-LED reactor according to an example embodiment of the present invention; 
         FIG.  1 B  is a schematic side cross-section view of the  FIG.  1 A  UV-LED reactor; 
         FIG.  1 C  is a schematic front view of the UV-LEDs operatively connected to the PCB of the  FIG.  1 A  UV-LED reactor; 
         FIG.  1 D  is a schematic top cross-section view of the  FIG.  1 A  UV-LED reactor; 
         FIG.  1 E  is a schematic exploded perspective view of the  FIG.  1 A  UV-LED reactor; 
         FIG.  2    is a perspective view of part of a UV-LED reactor according to an example embodiment of the present invention; 
         FIG.  3 A  is a schematic perspective view of a UV-LED reactor according to an example embodiment of the present invention; 
         FIG.  3 B  is a schematic side view of the  FIG.  3 A  UV-LED reactor; 
         FIG.  4    is a schematic top view of a UV-LED reactor according to an example embodiment of the present invention; 
         FIG.  5    is a schematic top view of a UV-LED reactor according to an example embodiment of the present invention; and 
         FIG.  6    is a schematic top view of a UV-LED reactor according to an example embodiment of the present invention. 
         FIG.  7    shows a water treatment system according to one embodiment. 
         FIG.  8    shows a refrigerator according to one embodiment. 
         FIG.  9    shows a hemodialysis machine according to one embodiment. 
     
    
    
     DESCRIPTION 
     Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     Unless the context dictates otherwise, “fluid” (as used herein) refers to a liquid (including, but not limited to water) and/or a gas (including, but not limited to air). 
     Unless the context dictates otherwise, “ultraviolet” (as used herein) refers to electromagnetic radiation having a wavelength shorter than that of the violet end of the visible spectrum but longer than that of X-rays. Typically, ultraviolet refers to electromagnetic radiation with a wavelength from about 10 nm to about 400 nm. 
     This application makes use of the phrase “thermal contact”. Unless the context dictates otherwise, thermal contact should be understood to comprise physical contact between two or more thermally conductive components, such as physical contact between metals or between components having thermal conductivities on the order of those of metals. For example, in some embodiments, materials having thermal conductivities on the order of those of metals and capable of making “thermal contact” may comprise materials having thermal conductivities of at least 60% of the thermal conductivity of typical stainless steel at room temperature and pressure. In some embodiments, such materials have thermal conductivities of greater than 10 W/(mK) at room temperature and pressure. In some embodiments, such materials have thermal conductivities of greater than 12 W/(mK) at room temperature and pressure. In some circumstances, thermal contact between components may be enhanced by thermal contact enhancing components. Such thermal contact enhancing components may comprise pastes, gels, deformable solids and/or the like which enhance the thermal conductivity between two or more components in thermal contact. 
     This application describes materials and components as being “thermally conductive” or “heat conducting”. Unless the context dictates otherwise, these phrases should be understood to refer to materials and components that have thermal conductivities on the order of those of metals. For example, in some embodiments, materials and/or components having thermal conductivities on the order of those of metals and described as being “thermally conductive” or “heat conducting” may comprise materials having thermal conductivities of at least 60% of the thermal conductivity of typical stainless steel at room temperature and pressure. In some embodiments, such materials have thermal conductivities of greater than 10 W/(mK) at room temperature and pressure. In some embodiments, such materials have thermal conductivities of greater than 12 W/(mK) at room temperature and pressure. 
     The present technology is directed to a reactor (photoreactor) operating with one or more solid-state ultraviolet (UV) emitters (e.g. UV light (radiation) emitting diodes (UV-LEDs), thin dielectric films that emit UV, and/or the like), to cause photoreactions in a fluid. One or more photocatalyst structures, activated by UV, may be used in the photoreactor for photocatalytic reactions. Chemical oxidants may also be added to the reactor to react with the UV radiation and generate highly active radicals such as hydroxyl radicals for photo-initiated oxidation reactions. Embodiments of the UV-LED reactors described herein may be efficient and compact, with integrated components, and may offer precise control of both their fluidic and optical environments. The UV-LED reactors comprise one or more specifically designed flow channels and at least one UV-LED configured for irradiating the fluid flowing through the flow channels. 
     Embodiments of the UV-LED reactor may be used for water purification by inactivating microorganisms (e.g. bacteria and viruses) and/or degrading micro-pollutants such as chemical contaminants (e.g. toxic organic compounds) by direct photoreaction and/or photocatalytic reactions and/or photo-initiated oxidation. The fluid (e.g. water) flows through the UV-LED reactor by forced convection, using, for example, electrical pumps. The UV-LED(s) may be powered by wall plug, solar cells, or battery. The UV-LED(s) may be turned on and off automatically as the water flows or stops flowing. A photocatalyst such as titanium dioxide or other suitable photocatalyst may be immobilized on a solid substrate (where the fluid passes over the substrate) or on a perforated substrate (where the fluid passes through the substrate). In some embodiments, a combination of photocatalystics, catalyst supports, and/or co-catalysts may be disposed in the substrate in the fluid flow channel. If applicable, chemical oxidants may be injected into the reactor. The chemical oxidant may be hydrogen peroxide or ozone or other chemicals. If applicable, chemical reducing agents may be injected into the reactor. 
     Reactors that operate with one or more UV-LEDs as a source of UV radiation have advantages over traditional mercury UV lamps, including, without limitation, their compact and robust design, lower voltage and power requirements, and the ability to turn on and off with high frequency. Unlike UV lamps, UV-LEDs are radiation sources with individual, small sizes. They may be positioned in a reactor with a higher degree of freedom compared to the arrangement of traditional mercury UV lamps. Further, the performance of UV-LED reactors may be improved with optimizations to the reactor geometry as described herein. In particular, embodiments of the UV-LED reactor described herein may be optimized to dissipate heat away from the one or more UV-LEDs (and/or electronic devices of the UV-LED reactor), thereby facilitating improved radiation output and useful lifetime of the UV-LEDs. 
     To increase or maintain the lifespan of the UV-LEDs, the fluid flowing through and being irradiated by the UV-LED reactor may be used for the thermal management of the UV-LEDs by transferring heat generated by the UV-LEDs to the irradiated fluid and thereby dissipating heat from the UV-LEDs via the fluid being treated. The UV-LED reactor may be configured so that part of the irradiated fluid is circulated in the proximity of the UV-LEDs or the UV-LED circuit board, and/or by incorporating thermally conductive material in the walls of the fluid conduit of the reactor. 
       FIGS.  1 A- 1 E  show a UV-LED reactor  10  according to an exemplary embodiment of the present invention. UV-LED reactor  10  comprises a fluid conduit  20  defined by a heat conducting conduit body  21 , at least one UV-LED  30  operatively connected to a printed circuit board (PCB)  40  and oriented for directing radiation into fluid conduit  20 . More specifically, UV-LEDs  30  are oriented to direct radiation into fluid conduit  20  by having a principal optical axis  31  that extends from UV LEDs  30  toward the fluid in conduit  20  along a first direction  33 . Heat conducting conduit body  21  comprises one or more heat conducting channel walls  24  which in turn define fluid flow channels  22  in reactor  10 . An inlet  26  and an outlet  28  are respectively provided for fluid (e.g. water) to enter and exit fluid conduit  20 . The main fluid flow directions are shown in  FIGS.  1 A and  1 D  by arrows  35 , which illustrate that the fluid flow enters reactor  10  from inlet  26 , flows through longitudinally extending flow channels  22  and turns at the ends of adjacent interior flow channels  22 , and exits from outlet  28 . A UV-transparent window  29 , such as a quartz or silica glass window, may be embedded in heat conducting body  21 , between UV-LEDs  30  and flow channels  22 . As will be appreciated by those skilled in the art, UV-LED reactor  10  may comprise driver circuits (e.g. LED drivers  32  shown in  FIG.  2   ), microcontrollers, a power port, an on/off switch, and/or the like for UV-LEDs  30 . To avoid obscuring the drawing, none of these common components are shown in  FIGS.  1 A- 1 E . One or more lenses, including collimating, converging, and/or other lenses (not shown), or a combination thereof, may be disposed in UV-LED reactor  10  between UV-LEDs  30  and fluid flow channels  22  to focus the UV-LED radiation pattern into longitudinally extending fluid flow channels  22  along principal optical axes  31  corresponding to each UV-LED  30 . 
     PCB  40  comprises a heat conducting substrate  41  having a first surface  41 A. First surface  41 A of heat conducting substrate  41  is generally planar and has a normal vector n. As shown in  FIG.  1 B , normal vector n may be oriented substantially in the first direction  33  (i.e. in the direction which radiation is directed from UV-LED  30  into fluid conduit  20 ). In some embodiments, the orientation of normal vector n of first surface  41 A being substantially in the first direction  33  means that in any plane, the angular difference between normal vector n and first direction  33  is less than 25°. In some embodiments, this angular difference is less than 15°. In some embodiments, this angular difference is less than 5°. Heat conducting conduit body  21  is in thermal contact with first surface  41 A of heat conducting substrate  41  of PCB  40 . In this manner, heat is dissipated from UV-LED  30  via heat conducting substrate  41 , the thermal contact between first surface  41 A of heat conducting substrate  41  and the heat conducting conduit body  21 , and from the one or more heat conducting walls  24  of heat conducting conduit body  21  to the fluid flowing through fluid conduit  20 . 
     Referring to UV-LED reactor in  FIGS.  1 A- 1 E , reactor  10  comprises an array of longitudinally extending fluid flow channels  22 , each such flow channel  22  irradiated by one corresponding UV-LED  30  optionally through a corresponding radiation-focusing element (not shown) and, in the illustrated embodiment, through UV transparent window  29 . In other embodiments, each flow channel  22  may be irradiated by more than one corresponding UV-LED and a plurality of radiation-focusing elements may be incorporated (one or more radiation focusing elements for each UV-LED  30  and/or one or more radiation focusing elements shared between UV LEDs  30 ) to focus the radiation from each UV-LED  30 . The corresponding UV-LEDs  30  and/or the corresponding radiation-focusing elements may be positioned at longitudinal ends of their corresponding longitudinally-extending flow channel  22 . Reactor  10  may comprise several UV-LEDs  30  that are oriented to direct radiation into one corresponding flow channel  22  (i.e. a many to one LED to flow channel ratio). Reactor  10  may comprise several UV-LEDs  30  that emit different UV wavelengths. This may result in a synergistic effect and increase the rate of photoreactions and photocatalytic reactions. Adjacent pairs of fluid flow channels  22  may be connected at one end, for example through a manifold (such as manifold  160  shown in  FIG.  2   ) or some other suitable port, to enable the fluid to flow in a serpentine path from one longitudinally extending channel  22  to another longitudinally extending channel  22 . As can be seen from the exemplary embodiment shown in  FIGS.  1 A- 1 E , fluid travels through multiple longitudinally extending channels  22  and makes multiple passes as the fluid travels through UV-LED reactor  10  between inlet  26  and outlet  28 . 
     Referring to UV-LED reactor  10  in  FIGS.  1 A- 1 E , the fluid flows in and out of UV-LED reactor  10 , passing through longitudinally extending flow channels  22 , and is irradiated by UV radiation from UV-LEDs  30 . In the illustrated embodiment, UV-LEDs  30  are positioned at one end of fluid conduit  20  and each longitudinally extending fluid flow channel  22 . The principal optical axes  31  of the radiation from UV-LEDs  30  (after optionally being focused by the lenses discussed above) extends in first directions  33 . These first directions  33  may be generally parallel with the longitudinal direction of fluid flow in longitudinally extending channels  22  and may be generally parallel to the longitudinal extension of the longitudinally extending flow channels  22 .  FIGS.  1 A- 1 E  show the illustrated flow channels  22  of fluid conduit  20  being irradiated from one end of reactor  10 . In general, the fluid conduit flow channels of a UV-LED reactor may be irradiated from either or both longitudinal ends of the flow channels. In some embodiments, UV-LEDs may be located on opposing longitudinal ends of the UV-LED reactor, so that the principal optical axes of radiation from longitudinally opposing UV-LEDs may be in opposing but parallel longitudinal directions. 
     The fluid flowing through, and being irradiated by, UV-LED(s)  30  may be used to dissipate the heat generated by UV-LED(s)  30  and/or other heat producing electronic devices (not shown) of reactor  10  away from UV-LED(s)  30  (and/or the other electronic devices). In the exemplary embodiment shown in  FIGS.  1 A- 1 E , reactor  10  is configured so that the irradiated fluid is circulated in the proximity of UV-LEDs  30  and PCB  40 . Reactor  10  also incorporates a thermally conductive material (heat conducting) conduit body  21  comprising heat conducting channel walls  24  to dissipate heat away from UV-LEDs and PCB  40 . Specifically, heat conducting conduit body  21  is in thermal contact with first surface  41 A of heat conducting substrate  41  of PCB  40 . In this manner, heat is dissipated from UV-LED  30  via heat conducting substrate  41 , the thermal contact between first surface  41 A of heat conducting substrate  41  and the heat conducting conduit body  21 , and from the one or more heat conducting walls  24  of heat conducting conduit body  21  to the fluid flowing through fluid conduit  20 . Heat exchange occurs inside the UV active region of UV-LED reactor  10  (i.e. in the region of reactor  10  irradiated by UV radiation from UV-LED(s)  30 ). 
     In some embodiments, reactor  10  may optionally comprise a thermal contact enhancing component  50  (shown in  FIG.  1 E ) which may be interposed in the thermal contact between heat conducting conduit body  21  and first surface  41 A of heat conducting substrate  41  of PCB  40 . Thermal contact enhancing component  50  comprises a thermally conductive material, including but not limited to paraffin wax and/or a silicone-based material. In some embodiments, thermal contact enhancing component  50  is deformable (e.g. a deformable pad or deformable gel or paste) to fill small irregularities in the physical contact between heat conducting conduit body  21  and first surface  41 A. When interposed between heat conducting conduit body  21  and first surface  41 A, thermal contact enhancing component  50  may reduce the thermal contact resistance (increase the thermal contact conductivity) between heat conducting conduit body  21  and first surface  41 A. Thermal contact enhancing component  50  is optional and is not necessary in all embodiments. 
     UV-LEDs  30  are operatively connected to PCB  40  in a circuit region  42 . In circuit region  42 , PCB may be covered (at least for the most part) by a solder mask coating  42 A in circuit region  42 . To facilitate thermal contact between heat conducting conduit body  21  and first surface  41 A of heat conducting substrate  41  of PCB  40 , in some embodiments, an exposed thermal contact region  44  is provided on first surface  41 A. Thermal contact region  44  may be the portion of first surface  41 A that is in thermal contact with heat conducting conduit body  21 . Solder mask coating  42 A may be removed from first surface  41 A of head conducting PCB substrate  41  in thermal contact region  44 . For example, as best seen in  FIG.  1 C , thermal contact region  44  (e.g. which is devoid from solder mask  42 ) may be located around the edges of PCB  40 , although thermal contact region  44  may be located at some other suitable region of PCB  40 , with no electronic components attached thereto and with no electrical connections). Solder mask coating  42  of PCB  40  may be removed, by way of non-limiting example, by laser-cutting, etching and/or the like to provide thermal contact between heat conducting conduit body  21  and first surface  41 A of heat conducting substrate  41  of PCB  40  to facilitate heat transfer between LEDs  30 , PCB  40 , channel wall(s)  24  and the fluid flowing in reactor  10 . The widths of thermal contact region  44  at the edge of PCB  40  could be several millimeters, in some embodiments. Generally, the larger the dimensions of thermal contact region  44 , the higher the rate of heat transfer. However, there is a trade-off since increasing the dimensions of thermal contact region  44  can increase the size of the entire reactor  10 . Removing outer layers (e.g. solder mask  42 ) to expose heat conducting substrate layer  41  of PCB  40  yields a significant improvement in the thermal contact/coupling between PCB  40  and heat conductive channel wall(s)  24  of fluid conduit  20 , so heat from UV-LEDs  30  can be transferred to the fluid travelling through flow channels  22  (e.g. because of the large surface area of channel wall(s)  24  which define fluid flow channels  22 , because of the nature of the moving fluid within channels  22 , and because of the temperature of the fluid inside flow channels  22 , which is typically lower than that of PCB  40 ). 
       FIG.  2    shows a partial perspective view of a UV-LED reactor  100  according to an exemplary embodiment of the present invention. Many features and components of reactor  100  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “1” being used to indicate features and component of reactor  100  that are similar to those of reactor  10 . However, UV-LED reactor  100  differs from UV-LED reactor  10  in that heat conducting conduit body  121  of UV-LED reactor  100  comprises a heat conducting manifold  160  that directs fluid flow between flow channels  122  at one longitudinal end to permit a fluid to flow from one longitudinally extending flow channel  122  to another longitudinally extending flow channel  122 , as discussed above in connection with  FIGS.  1 A- 1 E . Although not shown in  FIG.  2   , reactor  100  may have another manifold  160  at its opposing longitudinal end. Heat conducting manifold  160  may be integrally formed with heat conducting channel walls  124  of conduit body  121  or may be joined to an in thermal contact with heat conducting walls  124  of conduit body  121 . 
     Like reactor  10  described above, UV-LED reactor  100  comprises a fluid conduit  120  defined by a heat conducting conduit body  121 , at least one UV-LED  130  operatively connected to a printed circuit board (PCB)  140  and oriented for directing radiation into fluid conduit  120 . More specifically, UV-LEDs  130  are oriented to direct radiation into fluid conduit  120  by having a principal optical axis  131  that extends from UV LEDs  130  toward the fluid in conduit  120  along a first direction  133 . Heat conducting conduit body  121  comprises one or more heat conducting channel walls  124  which in turn define fluid flow channels  122  in reactor  110 . PCB  140  comprises a heat conducting substrate  141  having a first surface  141 A. First surface  141 A of heat conducting substrate  141  is generally planar and has a normal vector n. As shown in  FIG.  2   , normal vector n may be oriented substantially in the first direction  133  (i.e. in the direction which radiation is directed from UV-LEDs  130  into fluid conduit  120 ). In some embodiments, the orientation of normal vector n of first surface  141 A being substantially in the first direction  133  means that in any plane, the angular difference between normal vector n and first direction  133  is less than 25°. In some embodiments, this angular difference is less than 15°. In some embodiments, this angular difference is less than 5°. Heat conducting conduit body  121  is in thermal contact with first surface  141 A of heat conducting substrate  141  of PCB  140 . In this manner, heat is dissipated from UV-LED  130  via heat conducting substrate  141 , the thermal contact between first surface  141 A of heat conducting substrate  141  and the heat conducting conduit body  121  (via manifold  160 ), and from the one or more heat conducting walls  124  of heat conducting conduit body  121  to the fluid flowing through fluid conduit  120 . 
     Like reactor  10  described above, reactor  100  may comprise a thermal contact enhancing component  150  having features similar to those of thermal contact enhancing component  150  described above, except that thermal contact enhancing component  150  is interposed between manifold  160  and first surface  141 A (at thermal contact region  144 ) of heat conducting substrate  141  of PCB  140 . Reactor  100  of the illustrated  FIG.  2    embodiment also comprises a pressure plate  118  (e.g. fabricated from a rigid material including, but not limited to stainless steel) which may be coupled to manifold  160  by suitable fasteners (not shown) to hold manifold  160  of heat conducting body  121  and first surface  141  of PCB  140  in thermal contact with each other, thereby minimizing air gaps and enhancing thermal conductivity. 
     Like reactor  10  described above, PCB  140  may comprise a circuit region  142  on which LED  130  are located and circuit region  142  may be covered with solder mask  142 A. Like reactor  10  described above, solder mask  142 A may be removed from surface  141 A of heat conducting substrate  141  in thermal contact region  144  or thermal contact region  144  may otherwise be devoid of solder mask  142 A.  FIG.  2    shows UV-LED driver circuitry  132 , which, in the illustrated embodiment, is located on the same PCB  140  as LEDs  130 . This is not necessary and UV-LED driver circuitry  132  may be located in other locations if space becomes a design limitation. 
     While only one longitudinal end of reactor  100  is shown in  FIG.  2   , the same concept can be applied to the other longitudinal end of flow channels  122 . That is, the fluid in flow channels  122  may be irradiated by UV-LEDs from the other longitudinal end and the heat generated by such UV-LEDs (and/or other electronic components of reactor  100 ) may be removed by the same way described herein for one longitudinal end. 
     In the exemplary embodiments shown in  FIGS.  1 A- 1 E  and  FIG.  2   , the UV-LED reactors comprise a series of longitudinally extending flow channels through which fluid flows in corresponding longitudinal directions, which is irradiated, either with one UV-LED, or with an array of UV-LEDs. In a multi-channel reactor, such as the  FIGS.  1 A- 1 E  and  FIG.  2    embodiments, the fluid flow may go through the channels in parallel or in series (fluid flow going from one channel to another, where the flow channels are in fluid communication at their ends). In the exemplary embodiment shown in  FIGS.  3 A- 3 B , UV-LED reactor  200  comprises a fluid conduit  220  defined by a heat conducting conduit body  221  comprising a single longitudinally extending fluid flow channel  222  through which fluid flows in a corresponding longitudinal direction, which is irradiated with one or more UV-LEDs  230 . The main fluid flow directions are shown in  FIGS.  3 A- 3 B  by arrows  235 , showing fluid entering UV-LED reactor  200  from inlet  226 , flowing through longitudinally extending flow channel  222 , and exiting from outlet  228 . The UV-LED radiation is focused via a focusing element (not shown here), such as one or more converging and collimating lenses. The fluid flowing in the longitudinal direction in the reactor channel  222  is irradiated by the focused radiation from the UV-LED(s)  230  in the longitudinal directions of channel  222 . The UV-LED(s)  230  may be positioned at one or both ends of flow channel  122 . The total UV dose (UV fluence) delivered to the fluid may be controlled by adjusting the fluid flow rate and/or regulating UV-LED radiant power, and/or turning on/off the number of UV-LEDs. Many features and components of reactor  200  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “2” being used to indicate features and component of reactor  200  that are similar to those of reactor  10 . 
     In the exemplary embodiments shown in  FIGS.  1 A- 1 E  and  FIG.  2   , the UV-LED reactors comprise a series of longitudinally extending flow channels through which fluid flows in corresponding longitudinal directions, which is irradiated at one end with one UV-LED, or with an array of UV-LEDs. In a multi-channel reactor, such as the  FIGS.  1 A- 1 E  and  FIG.  2    embodiments, the main directions of the radiation from UV-LEDs  30  (after optionally being focused by the lenses discussed above) and of the fluid flow in longitudinally extending channels  22  are along longitudinal directions generally parallel to the longitudinal extension of the longitudinally extending flow channels  22 . In some embodiments, UV-LEDs may additionally or alternatively be positioned along flow channels so that the radiation from the UV-LEDs is generally orthogonal to the longitudinal extension of the longitudinally extending flow channels and the main fluid flow direction of fluid in the flow channels. For example,  FIG.  4    shows a top cross section view of a UV-LED reactor  300  according to an exemplary embodiment of the present invention. Many features and components of reactor  300  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “3” being used to indicate features and component of reactor  300  that are similar to those of reactor  10 . 
     As can be seen from the exemplary embodiment shown in  FIG.  4   , fluid travels through multiple longitudinally extending channels  322  (defined by heat conducting conduit body  321  and its heat conducting walls  324 ) and makes multiple passes as the fluid travels through UV-LED reactor  300  between an inlet and an outlet (not shown here). The fluid flowing through channels  322  and being irradiated by UV-LEDs  330  may be used to dissipate the heat generated by UV-LEDs  330  and/or other heat producing electronic devices (not shown) of reactor  300  away from UV-LEDs  330  (and/or the other electronic devices). In the exemplary embodiment shown in  FIG.  4   , reactor  300  is configured so that the irradiated fluid is circulated in the UV active region of UV-LEDs  330 . More specifically, UV-LEDs  330  are oriented to direct radiation into fluid conduit  320  by having a principal optical axis that extends from UV LEDs  330  toward the fluid in conduit  320  along a first direction. Heat conducting conduit body  321  comprises one or more heat conducting channel walls  324  which in turn define fluid flow channels  322  in reactor  310 . PCB  340  comprises a heat conducting substrate having a first surface. The first surface of the heat conducting substrate is generally planar and has a normal vector n which may be oriented substantially in the first direction (i.e. in the first direction which radiation is directed from UV-LEDs  330  into fluid conduit  320 ). The meaning of substantially in the first direction may have the meaning described elsewhere herein. Heat conducting conduit body  321  is in thermal contact with the first surface of the heat conducting substrate of PCB  340 . In this manner, heat is dissipated from UV-LED  330  via the heat conducting substrate, the thermal contact between the first surface of the heat conducting substrate and the heat conducting conduit body  321 , and from the one or more heat conducting walls  324  of heat conducting conduit body  321  to the fluid flowing through fluid conduit  320 . Reactor  300  shown in  FIG.  4    may comprise other characteristics and components similar to the reactors  10 ,  110  described above. 
       FIG.  5    shows a top cross section view of a UV-LED reactor  400  according to an exemplary embodiment of the present invention. UV-LED Reactor  400  comprises a series of longitudinally stacked reactors  300 . Fluid travels through UV-LED reactor  400  between an inlet and an outlet (not shown here) as described above in relation to  FIG.  4   . The thermal management technique employed by reactor  400  is similar to the thermal management technique described above in relation to  FIG.  4   . Many features and components of reactor  400  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “4” being used to indicate features and component of reactor  400  that are similar to those of reactor  10 . 
     The longitudinally extending fluid flow channels described herein have a cross section which may take any suitable shape, including, without limitation, a circle, a semi-circle, a square, a rectangle, a triangle, a trapezoid, a hexagon, and the like. These cross sections may enhance the reactor performance by improving thermal management. For example, a fluid flow channel having a circular cross section may provide optimal thermal management to the UV-LEDs (and/or other electronic devices) of the reactor. In the exemplary embodiment shown in  FIG.  6   , UV-LED reactor  500  comprises longitudinally extending fluid flow channels  522  having triangular cross section. Fluid travels through UV-LED reactor  500  between an inlet and an outlet (not shown here) as described above in relation to  FIG.  4   . The thermal management technique employed by reactor  500  is similar to the thermal management technique described above in relation to  FIG.  4   . Many features and components of reactor  500  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “5” being used to indicate features and component of reactor  500  that are similar to those of reactor  10 . 
     The thermal management techniques described herein takes advantage of fluid (typically water) to dissipate heat from electronics, including UV-LEDs, connected on a PCB. This is accomplished by maximizing the thermal contact between a heat conducting substrate of the PCB and the fluid conduit heat conducting walls, which are cooled continuously with fluid moving in the flow channels (and/or manifold). The thermal contact resistance of the thermal contact between the heat conducting conduit body and the heat conducting substrate of the PCB may be reduced significantly by interposing deformable and thermally conducting thermal contact enhancing components (e.g. thermal contact component  50 ,  150 ) between the heat conducting conduit body and the first surface of the heat conducting substrate of the PCB to fil the thermal gaps and/or by removing the solder mask coating from the edges (or other region(s)) of the PCB, as described elsewhere herein. Such thermal contact enhancing components are optional. 
     Other techniques of active or passive heat removal and thermal management, such as the use of a heat sink(s) or passing a fluid flow at the back of the PCB (i.e. the side opposite that on which the UV-LEDs are connected) may also be used in combination with the heat dissipation apparatus and methods described herein. 
     Some embodiments of a UV-LED reactor (not shown here) comprise a plurality of UV-LEDs irradiating the fluid through a longitudinally extending fluid flow channel. In some embodiments (not shown here), a plurality of radiation-focusing elements is incorporated (one for each UV-LED), and the radiation from each UV-LED is focused by its corresponding focusing element. In some embodiments, groups of one or more LEDs may share groups of one or more corresponding focusing elements (or one or more corresponding lenses from within one or more corresponding focusing elements) in any suitable matter. For example, there may be a total of 9 LEDs and 3 lenses, where the LEDs are grouped into three groups of 3 LEDs, and the radiation from each group of 3 LEDs passes through a single lens corresponding to the LED group. A UV-LED reactor incorporating multiple UV-LEDs may be particularly suitable for fluid flow channels which have a bore having relatively large cross-section. The multiple UV-LEDs may help to maximize irradiance coverage by increasing irradiance in such fluid flow channels, as compared to an embodiment operated with a single UV-LED for irradiating the fluid flow channel. 
     The UV-LED reactor of the present invention may be used for many photoreactions, photocatalytic reactors, and photo-initiated reactions. One particular application is the purification of water or purification of other UV-transparent fluids. Water treatment may be achieved by the inactivation of microorganisms (e.g. bacteria and viruses) and the degradation of micro-pollutants, such as chemical contaminants (e.g. toxic organic compounds), by direct photoreactions, photocatalytic reactions, and/or photo-initiated oxidation reactions. Water may flow through the UV-LED reactor by the use of a fluid-moving device, such as an electrical pump. The UV-LEDs may be powered by a wall plug, solar cells, or a battery. If applicable, a photocatalyst may be immobilized on a solid substrate, where the fluid passes over, and/or on a perforated substrate where the fluid passes through, including for example a mesh, screen, metal foam, cloth or combination thereof. The photocatalysts that are supported on the solid and/or perforated substrates may be positioned in the longitudinally extending fluid flow channels. The photocatalyst may also be positioned in the cross section of the fluid flow channel, to cover the cross section partially or entirely. If the photocatalyst covers the entire cross section of the flow channel, a perforated substrate may be used to allow for the fluid to pass through the photocatalyst substrate. The photocatalyst is irradiated with focused UV radiation from UV-LED, providing a UV-LED photocatalytic reactor. The photocatalyst may comprise titanium dioxide, or any other photocatalyst. In certain embodiments, a combination of one or more photocatalysts, catalyst supports and co-catalysts are provided on the solid and/or perforated substrate(s). If applicable, chemical reagents, such as chemical oxidants may be injected in the UV reactor. The chemical oxidant may comprise hydrogen peroxide, ozone, or other chemicals. The UV-LED may be turned on and off automatically by an external signal. The reactor may contain one or more components to restrain the fluid flow in the conduit, such as static mixers, vortex generators, baffles, and/or the like. 
     In some embodiments, static mixers, vortex generators, baffles, or the like may be deployed in the longitudinally extending fluid flow channels to increase mixing and/or to rotate the fluid flow as it goes through the fluid flow channels. This may enhance the UV-LED reactor performance by delivering a more uniform UV dose or by improving mass transfer near the photocatalyst surface where photocatalysts are present in the reactor. The static mixers, vortex generators, baffles, or the like may also serve as flow-restraining elements which may be adjusted dynamically to accommodate various incoming flow regimes to match the UV radiation fluence rate profile in the fluid flow channel. 
     The heat conducting conduit body of the embodiments of the UV-LED reactors described herein may be made of aluminum, stainless steel, or of any other sufficient and strong material, such as metal, alloy, high-strength plastic, or the like. The internal walls of the fluid conduit, which define the fluid flow channels, may (but need not necessarily) be made of or be coated with material with high UV reflectivity to reflect to the fluid any part of the radiation that is incident on the internal walls. 
     While the embodiments described herein are presented with particular features and fluid flow channel configurations or lens configurations and the like, it is to be understood that any other suitable combination of the features or configurations described herein may be present in a UV-LED reactor. 
     Further, the UV-LED reactor may incorporate UV-LEDs of different peak wavelengths to cause synergistic effects to enhance the photoreaction efficiency. 
     In some embodiments, the UV-LED reactor comprises a planar flow channel covered with a quartz or a silica glass window, which is irradiated with an array of UV-LEDs. This configuration may have two distinct forms: 
     a. The fluid flowing in the channel(s) (including parallel channels) is irradiated by UV-LEDs mainly in a direction that is perpendicular to the axis of the flow channel length (or main flow direction). In this case, the LED(s) are positioned along the length of the flow channel(s). The flow is mainly moving under/over UV-LEDs and is irradiated.
 
b. The fluid flowing in the channel(s) is irradiated by UV-LEDs mainly in a direction that is parallel to the axis of the flow channel length (or main flow direction). In this case, the LED(s) are positioned at one end or both ends of the flow channel(s). The flow is mainly moving towards or away from UV-LEDs and is irradiated.
 
     In either of these configurations, the exposure of fluid to UV radiation may be controlled. The flow channels and UV-LED arrays can be arranged in a way that the flow is exposed to the desired number of LEDs. The design may be a single flow channel, a series of parallel flow channels, or a stack of multiple flow channels. The total UV dose delivered to a fluid may be controlled by adjusting the flow rate and/or regulating UV-LED power, and/or turning on/off the number of UV-LEDs. This design enables the manufacture of thin planar UV-LED reactors. For example, in some embodiments the UV-LD reactor may be approximately the size of a smart phone, in terms of geometry and dimensions, with inlet and outlet ports for a fluid. 
     In some embodiments, a plurality of LED(s) are positioned along the length of a longitudinally extending fluid flow channel so that the main direction of irradiation is perpendicular to the main direction of the flow. The LED(s) may be positioned along one side or along opposite sides of a longitudinally extending fluid flow channel. The flow may mainly move under (or over) the UV-LEDs and may be irradiated as it travels in longitudinal directions through the longitudinally extending fluid flow channel. The internal wall of the channels may be made of or be coated with material with high UV reflectivity for facilitating radiation transfer to the fluid. Two adjacent fluid flow channels may be connected at one end, for the fluid to go from one channel to another channel (fluid experiences multi-pass through the reactor). Different lenses including collimating, diverging, converging, and other lenses may be installed in the UV-LED reactor to adjust the UV-LED radiation pattern. 
     Particular applications of the UV-LED reactor include processing and treating water of low to moderate flow rates, for example, in point-of-use applications. Further, due to its compact configuration and high efficiency, the UV-LED reactor in accordance with the embodiments described herein may be incorporated in appliances (e.g. refrigerators, freezers, water coolers, coffee machines, water dispensers, icemakers, etc.), health care or medical devices or facilities, dental equipment, and any other devices which require the use of clean water. The UV-LED reactor may be either incorporated into the device or be applied as an add-on into the existing device. For example, the UV-LED reactor may be positioned somewhere through the waterline so that the UV-LED reactor treats the water that is used in (e.g. passing through the waterline of) the device. This may be of particular interest where the fluid has to be irradiated/treated while passing through a pipe, or where there is a need to prevent the formation of potential microorganism biofilm inside a pipe, or where the flow needs to be treated at the end of a pipeline before being used. The UV-LED reactor may be integrated in the device along with one or more other forms of water purification methods (such as filtration). Exemplary point-of-use fluid treatment applications of the UV-LED reactor are next described with references to  FIGS.  7  to  9   . 
       FIG.  7    shows a water treatment system  600 , comprising an inlet pipe  626 , an outlet pipe  628 , and a water tap  605 , and incorporating a UV-LED reactor  610  operated with UV-LEDs  630  for the treatment of water. The water enters the reactor  610  via inlet  626 , passes through the UV-LED reactor  610 , and is irradiated by UV radiation emitted from the UV-LEDs  630 , prior to exiting from outlet pipe  638  and going to the tap  605  for general use. The general fluid flow directions are shown by the arrows. Many features and components of reactor  610  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “6” being used to indicate features and component of reactor  600  that are similar to those of reactor  10 . 
     In some embodiments, the UV-LED reactor may be incorporated in appliances that dispense or use water (or water-based fluids) for human consumption, such as freezers, water coolers, coffee makers, vending machines, and the like. The water used for human consumption needs a high degree of purification. The main water supply for refrigerators, freezers, and water coolers, for example, may contain harmful pathogens. This is of particular concern in developing countries and remote areas where water may not be treated properly before distribution in the water network. In addition, due to its particular structure, a refrigerator/freezer waterline may be prone to biofilm and microbial contamination. Polymeric tubing typically transfers water from the main water supply to refrigerators to be used in through-the-door ice and drinking water. Bacterial biofilm can form in the waterline, in particular when the water is not in use (e.g., biofilm can form within 8 hours). Intermittent patterns of water use lead to stagnation of the entire water column within the waterlines for extended periods during the day. The susceptibility of water supply tubes to colonization of bacteria on surfaces and formation of biofilm is a well-recognized problem. 
     The UV-LEDs of the reactor may be turned on and off automatically in response to the water starting and stopping flowing. Sensors may be used to detect the flow of fluid and send a signal to the reactor to turn the UV-LEDs on or off. The UV-LED reactor may reduce the microbial contamination in the water leaving the waterline (for consumption) and reduce the risk for infection. This is facilitated by the operating conditions of UV-LEDs. For example, a UV-LED can operate at a range of temperatures and can be turned on and off with high frequency, which is particularly important for refrigerator and water cooler applications. 
     Any appliances which dispense or use water or water-based fluids (e.g. coffee or other beverages) intended for human consumption may incorporate a UV-LED reactor according to the embodiments described herein to treat the water. For example,  FIG.  8    shows a refrigerator  700  comprising a body  711  and a pipe  713  for delivering water to a water/ice dispenser  714 . Refrigerator  700  incorporates a UV-LED reactor  710 . Many features and components of reactor  710  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “7” being used to indicate features and component of reactor  710  that are similar to those of reactor  10 . The water flowing in the pipe  713  passes through the UV-LED reactor  710  where it is irradiated by UV radiation prior to entering the water/ice dispenser  714 . The general fluid flow directions are shown by the arrows. Similarly, other appliances which may benefit from incorporating a UV-LED reactor include, without limitation, freezers, ice machines, frozen beverage machines, water coolers, coffee makers, vending machines and the like. 
     Other applications of the UV-LED reactor according to the embodiments described herein include the treatment of water or other fluids used in or by healthcare or dental-related or medical devices or facilities, either for operation, cleaning or another purpose which requires clean water. In particular, many healthcare applications require water quality to be of a higher standard than drinking water. The efficiency and compactness of the UV-LED reactors described herein may make them more attractive than conventional UV-lamp reactors for implementation in healthcare devices. 
     For example,  FIG.  9    shows a hemodialysis machine  800  comprising a body  821  and a pipe  823  containing a UV-LED reactor  810 . The water flowing in the pipe  823  passes through the UV-LED reactor  810  for treatment prior to use in the hemodialysis machine. Many features and components of reactor  810  are similar to feature and components of reactor  10 , with the same reference numerals preceded by the digit “8” being used to indicate features and component of reactor  810  that are similar to those of reactor  10 . Similarly, other appliances which may benefit from incorporating the UV-LED reactor include, without limitation, colon hydrotherapy equipment, and dental equipment which dispenses water for cleaning or operation, or the like. 
     With respect to applications in dental equipment, surveys of dental unit waterlines (DUWLs) indicate that biofilm formation is a problem and a great majority of bacteria that have been identified in DUWL are ubiquitous. Although such bacteria may be present in only low numbers in domestic water distribution systems, they can flourish as biofilms on the lumen surfaces of narrow-bore waterlines in dental units. Microorganisms from contaminated DUWL are transmitted with aerosol and splatter, generated by working unit hand-pieces. Various studies emphasize the need for reducing the microbial contamination in DUWL. 
     In some embodiments, a UV-LED reactor may be incorporated in a dental unit to treat the water used in the unit. The UV-LED reactor may be integrated in the dental units (such as a dental chairs) or the UV-LED reactor may be placed within the tray of the dental chair (assistant tray) holding the water spry, or within the water spray handle, or somewhere else through the waterline, for the treatment of the water prior to use. Features including instant on and off may be included in the UV-LED reactor integrated in a dental unit. 
     Some embodiments comprise UV-LEDs which are operated in a pulsed mode. For example, the LEDs may be pulsed at high frequencies. This mode of operation may affect the photoreaction rate as well as the photocatalyst&#39;s electron-hole recombination so as to increase photocatalytic efficiencies. 
     The UV-LEDs may be programmed to turn on and off automatically in some embodiments. For example it may be desirable to turn on/off the UV-LEDs as the fluid flow starts or stops moving in the reactor (which may be useful for water purification in point-of-use applications), or at specific time intervals. To control the UV-LEDs&#39; on/off status, a sensor may be used to detect the fluid motion in the fluid flow channels. Alternatively, a user may activate a sensor physically, either directly (for example, by turning a switch on and off), or as an indirect action (for example, through turning a tap on and off). This feature may advantageously save energy used by the reactor. As another example, it may be desirable to turn on/off the UV-LEDs at specific time intervals for cleaning of the UV reactor chamber when it is not in operation for some time, in order to prevent any potential growth of microorganisms, diffusion of microorganisms fro untreated upstream fluid, and/or to prevent any biofilm formation. To control the UV-LEDs&#39; on/off status, a microcontroller may be applied and programmed to turn the UV-LEDs on for a period of time (for example, a few seconds), at specific time intervals (for example, once every few hours). 
     In some embodiments, at least some of the UV-LEDs may be programmed to adjust their power output or to turn on or off automatically, in response to receiving a signal. The signal may be generated, for example, as the flow rate (or other measurable characteristic) of the fluid passing through the UV-LED reactor changes. In embodiments where the fluid is water, the measurable characteristic may be one that is indicative of the water quality or concentration of contaminants. Examples of water quality indicators include UV transmittance and turbidity. This configuration may facilitate appropriate radiation energy being directed to the fluid based on the particular operating conditions. 
     In some embodiments, a visual indicator, such as for example a liquid crystal display (LCD) or a radiation signal (such as a colored LED) may be provided on the UV-LED reactor, or in another visible place (for example, on the tap if the application is water treatment) to inform the user of the status of the reactor and UV-LEDs. As an example, when the UV-LEDs are on, a sign on the LCD can be displayed or a colored LED can be turned on which indicates the “on” status of the UV-LEDs to the user. 
     Further example embodiments of UV-LED based photoreactors which could incorporate the heat dissipation and thermal management methods and apparatus described herein are described in U.S. Application No. 62/280,630 filed on 19 Jan. 2016 and entitled HEAT DISSIPATION APPARATUS AND METHODS FOR UV-LED PHOTOREACTORS, which is incorporated herein by reference. 
     Interpretation of Terms 
     Unless the context clearly requires otherwise, throughout the description and the claims:
         “comprise”, “comprising”, and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”;   “connected”, “coupled”, or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof;   “herein”, “above”, “below”, and words of similar import, when used to describe this specification, shall refer to this specification as a whole, and not to any particular portions of this specification;   “or”, in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list;   the singular forms “a”, “an”, and “the” also include the meaning of any appropriate plural forms.       

     Words that indicate directions such as “longitudinal”, “transverse”, “horizontal”, “front”, “back”, “top”, “bottom”, “below”, “above”, “under”, and the like, used in this description and any accompanying claims (where present), depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly. 
     Where a component (e.g. a substrate, assembly, device, manifold, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments described herein. 
     Specific examples of systems, methods, and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions, and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and/or acts with equivalent features, elements and/or acts; mixing and matching of features, elements and/or acts from different embodiments; combining features, elements and/or acts from embodiments as described herein with features, elements and/or acts of other technology; and/or omitting combining features, elements and/or acts from described embodiments.