Patent Publication Number: US-8988681-B2

Title: Spray droplet sizer

Description:
FIELD OF THE INVENTION 
     The present invention related to the field of spray droplet sizing, where the size of the liquid droplets which flow inside a gas are determined. 
     BACKGROUND OF THE INVENTION 
     Sprays are used in a wide range of industries with a variety of applications. For example, they are used to inject fuel inside engines, combustors, furnaces and boilers, to spray paint various vehicles, to spray pesticides, to spray water to cool hot surfaces, and many more. Other applications are cleaning/washing, coating, dust Control, fire protection, gas cooling and conditioning, humidification, NOx control, and tank cleaning. 
     In all these application, one of the most important parameters is the spray droplet sizes. For instance, in the fuel injectors, the size of the droplets in a spray dictates the efficiency of the combustion, as well as the pollution formation. In spray cooling, the cooling rate depends on the droplet size. In cleaning and dust removal, droplet size determines the efficiency of water consumption for cleaning, etc. 
     Prior art introduces several different droplet size measurement systems. The most common methods of non-contact measurement of particle size are: (1) Optical Particle Counter (2) laser diffraction analyzer, and (3) Laser or Phase Doppler Velocimetry. Most these systems have several lases and collection optics to increase the intensity of the scattered light detected. These systems are very complex and require an expert to operate them. Because of their complexity, they are also very expensive. Therefore, they are mainly used for research purposes and hardly use in the industry as a tool to characterize spray size. 
     Currently, there are no method of measuring droplet sizes in a simple and rapid manner. The present invention is developed to address this need. 
     SUMMARY OF THE INVENTION 
     The present invention introduces a novel spray droplet sizer to overcome the shortcoming of the presently available systems. The device is based on counting the number of drops passing through a control volume and measuring the volume of all such drops. By knowing the number of the drops and their volumes, an average droplet size for the spray is obtained. Other objectives, advantages and novel features of the present invention will become readily apparent from the following drawings and detailed description of preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments herein will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the scope of the claims, wherein like designations denote like elements, and in which: 
         FIG. 1  shows a perspective view of the first embodiment of the spray sizer; 
         FIG. 2  shows a cross sectional view and a top view of the first embodiment of the spray sizer; 
         FIG. 3  shows a cross sectional view of the first embodiment of the spray sizer with laser light and spray collection process; 
         FIG. 4  shows a perspective view of the second embodiment of the spray sizer; 
         FIG. 5  shows a shaded cross sectional view of the second embodiment of the spray sizer; 
         FIG. 6  shows a perspective of the main chamber of the second embodiment of the spray sizer; 
         FIG. 7  shows a cross sectional view of the main chamber of the second embodiment of the spray sizer; and 
         FIG. 8  shows a cross sectional view of the main chamber of the second embodiment of the spray sizer. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The underlying principal of the present spay sizer is to count a large number of the droplets and at the same time determine their volume. By knowing the total volume of the droplets and their number, an average droplet size can be determined. 
       FIG. 1  shows a perspective view of the present spray sizer. The spray sizer comprises of several main elements. (i) Spray separator  100 , (ii) an optical particle counter  200 , (iii) a liquid collector  300  and (iv) a microprocessor to analyze the data and provide droplet size  400 .  FIG. 2  shows a top view  150 , and a cross sectional view  160  of the spray sizer. 
     (i) Spray separator  100 : Sprays are usually very dense with millions of droplets flying at high speed. Since it is very difficult to count all the droplets in the spray, only a small fraction of the droplets are counted. This fraction has to be large enough so that an average droplet size representing the spray can be determined. Therefore, the device needs to collect a part of the spray with minimal disturbance to the spray. This is achieved by a set of cones  100 . A cone  100  is placed inside the spray, as illustrated in  FIG. 3 . Only the droplets that enter the cone  180  are counted. Cone separator unit may have one or more cones, preferably two cones  110 . Since spray droplets may be travelling at an angle, many of the droplets may hit the walls of the cone. A double cone separator can select a fraction of the droplets which travel mainly straight and separate the droplets that may hit the inner walls of the cone. The liquid collected in the region between the first cone  100  and the second cone  110  discharges through openings  115  at the bottom of the first cone  100 . The spray separator is attached to the main unit housing the optics  200 .
         Optical particle counter  200 : Optical particle counters are known art and are commonly used for the air quality measurements. The particle counters use a laser light and a light detector. The droplets passing through the laser light, scatter the laser light. A photo detector located in the system measures the scattered light. Each time a droplet passes by the laser, the light detector senses it and counts it as one droplet. The intensity of the scattered light is a function of the droplet size, shape, and index of refraction. Since the scattered light contains information on the droplet size, droplet properties and even droplet number density, several instruments are developed to gain more information on the spray as they pass through the laser beam. However, the present system only counts the number of droplets, and therefore, it is very simple system and readily available. A laser light is used since it provides a source of coherent, intense light of fixed wavelength, therefore, it is easier to analyze the scattered light. However, other lights can be used in the present sizer, since particle counting only requires on-off signals to detect droplets.       

     The droplet counter in the present system comprises of a laser source  210  and a light sensor  220  like a photo detector. Both the laser source and the detector are housed just below the spray separator  100 . The separated droplets pass directly in front of the laser light  215  and scatter the light onto the detector  220 . The detector counts the number of droplets. The detectors can be any type of photo detector, like photodiodes, that convert light to electric currents. 
     If during the operation small droplets land on any of the optics, the intensity of the light emitted and detected may change as compared to the original calibrated system. Therefore, in one embodiment of the present droplet sizer, the laser source and the photo detectors are installed at two ends of a relatively long tube  250 . This provides a distance between the flow zone and the location of the optics, reducing the chance of any droplets impacting on the optics. 
     Liquid Collector  300 : The droplets that are passed through the laser enter into a container at the bottom of the system. This container can be any container to collect liquid. Depending on the type of liquid, a container suitable for that liquid has to be chosen. In addition, the size of the container has to be such that it can collect enough mass to represent the spray. A spray with large droplets requires a larger container, whereas a fine spray requires a small container. It is preferred to have a transparent container so that the user can decide when the container is full. Sensors can be installed in the container to automatically determine when the container is full. 
     Microprocessor  400 : The signals provided by the detector are converted into electrical current. The electric currents can be amplified to provide voltage signals. The voltage signals are then supplied through a multiplexer to arithmetic operating unit and the result of the arithmetic operation is displayed on a display. All these elements are included in the microprocessor. The processor can be connected to a computer or a keyboard to input the required information. The microprocessor analyzes the data and determines the droplet counts. The volume to be collected can be pre-stored on the microprocessor. 
     Only the droplets that pass through the laser beam are measured and the other droplets which pass outside of the beam are not accounted for. Therefore, the number of the droplets measured relates to the number of the droplets that pass through the area covered by the laser light. If the area of the spray that is covered by the laser light is referred to as A L  and the total area of the spray in the detection zone is referred to as A s , then the number of droplets in the spray that enter the collector N s  is related to the number of the measured droplets, N L  according to:
 
 N   s   =N   L    A   s   /A   L  
 
     The area of the spray in the collection zone and the area of the laser in the collection zone are predetermined and stored in the microprocessor. Therefore, the microprocessor will first determine the number of the droplets in the spray that have entered into the measuring container and perform the following analysis to determine the average droplet size. First the mean droplet volume is determined by dividing the volume of the liquid collected by the number of the droplets counted:
 
 V   mean =Mean droplet volume=Volume/ N   s   =V A   L   /N   L    A   s  
 
     Then the droplet diameter is determined by using the equation for the volume of a spherical droplet:
 
 D   mean =[6  V   mean /π] 1/3  
 
     This diameter is the average spray droplet diameter that is needed for any spray. The diameter is displaced on the microprocessor system. 
       FIGS. 4-7  show another embodiment of the same device. This system has a double cone spray separator,  510  and  520 . In this case the laser  530  and the detector  540  have a certain angle with respect to each other. In the first embodiment shown in  FIG. 1 , the laser is aligned with the detector. Each time a particle passes by the light, the light is interrupted and the light intensity measured by the detector reduces. In the embodiment shown in  FIG. 4  the light is detected what particles scatter it. For this purpose the detector is position at certain angle, preferable about 70 degrees from the laser light. Any other angle is possible. The scattered light is detected and analyzed for the droplet. The scattering light has more information about the droplet. However, the software to analyze it is more complex. The microprocessor  580  in this system has software that can analyze the scattered light and determine the information on the size range of the droplets. 
     In another embodiment of the same device (not shown), more than one light source and one light detector can be used. Each light source having its own corresponding light detector located right across it. In order to make sure that the light scattering from each light source does not interfere with the light scattering from the light sources, the detectors are located inside tubes, and towards the end of the tubes. The inner part of the tubes are painted back, therefore, only directly forward scattered light can penetrate into the tubes and reach the detector. This will allow counting of the droplets of each light path separately. The advantage of this method is that a larger part of the spray cross section is measured. 
     The light scattering depends on the droplet size. Smaller droplets scatter light at a larger angle. Also, the light scattering intensity depends on the droplet sizes. Generally, larger droplets scatter light at narrower angles with higher intensity, whereas smaller droplets scatter at wider angles but with lower intensity. Therefore, by using plurality of detectors at different scattering angles, the drops can be divided into several size bins. This type of sizer can provide a more detailed size measurement. 
     Another embodiment of the spray sizer is shown in  FIG. 8 . This sizer is made more compact by locating the laser system  710  outside of the sizer main body. This is achieved by using a set of mirrors  720  and  730  to guide the laser to the detection zone. The sizer is held under a spray, and the first cone  750  collects part of the spray. The second cone  760  collects a part of this spray to pass through the laser beam  735 . The remaining part of the liquid that does not enter the second cone is drained through the drain holes  770  at the sides of the container. Knowing the length of the laser beam and its thickness in the measuring zone, the detection area of the laser A L  is obtained. The droplets are then collected inside a container  780 . 
     The droplet sizer can be equipped with more than one laser and detector system to cover a larger area of the measurement zone. Each laser light covers only the area it passes through. By putting two laser beam next to each other, the area of the measurement is doubled and, therefore, the error in extrapolating the number of particles to the whole flow area is reduced. 
     The present system can also measure the average droplet velocity, by using two laser beams one on top of the other. The time of travel of each droplet between the two beams can be determined by inspecting the signal received by the light detectors. Since the distance between the two beams is known, the particle velocities can also be determined. 
     The container may be equipped with sensors to indicate when the container is full and has collected the desired volume. The sensors are in particular useful when testing sprays which require a nontransparent container or for more accuracy. The sensor are also connected to the microprocessor letting the microprocessor determine when to stop counting the droplets. 
     The droplet sizer can be equipped with memory unit to store all information inside its memory. The data can be extracted once the tests are completed. The system may also have a Bluetooth system to transfer data directly to a remote microprocessor or computer or small phone device. 
       FIG. 4  also shows a handle  600 . The spray sizer has a handle to hold the system inside a spray. Any type of handle known in the art can be used. The main features of the handle are that it has to withstand the force of the spray, and preferable removable. Even a tripod can be used holding the sizer underneath a spray. 
     The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. 
     With respect to the above description, it is to be realized that the optimum relationships for the parts of the invention in regard to size, shape, form, materials, function and manner of operation, assembly and use are deemed readily apparent and obvious to those skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention.