Abstract:
A simple, passive and rugged device for measuring the flow rate of liquid. A variable area obstruction valve, a differential pressure sensor and a densitometer are combined in a single housing to provide for a highly accurate and precise measure of mass flow.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application No. 60/979,476, filed Oct. 12, 2007, incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention is generally directed to flow meters and more particularly pertains to a mass flow meter. The device measures both volumetric flow as well as density to yield mass flow. 
         [0003]    Aircraft engine and airframe designers seek highly accurate, wide flow range, fast response, and rugged flow meters to measure the flow rate of hydrocarbon based fuel (jet fuel) for the purposes of engine control. Heretofore used volumetric flow meters have a limited range and are incapable of providing the accuracy that can be exploited by modern engine control systems. What is needed is a simple, passive, accurate and reliable method of measuring mass flow. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a volumetric flow meter in combination with a densitometer to provide a mass flow measurement. The volumetric flow meter relies on an variable area obstruction meter to yield an extended turn-down ratio without active controlling mechanisms and a differential pressure sensor to measure the pressure difference across the obstruction. The densitometer relies on capacitance tubes to yield a density measurement of the fluid passing there through. A fuel temperature sensor serves to correct both the pressure bridge signal as well as the density signal to enable a highly accurate and precise mass flow computation to be performed. 
         [0005]    These and other features of the present invention will become apparent from the following detailed description of the preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a block diagram generally illustrating the system of the present invention; 
           [0007]      FIG. 2  is a cross-sectional view of a preferred embodiment of the flow meter of the present invention; 
           [0008]      FIG. 3  is a cross-sectional view of another preferred embodiment; 
           [0009]      FIG. 4  is a perspective view of the embodiment shown in  FIG. 3 ; and 
           [0010]      FIG. 5  is an enlarged perspective view of a portion of  FIG. 4  showing details of the obstruction valve. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]      FIG. 1  is a block diagram providing an overall illustration of the system  12  of the present invention. The pressure differential across an obstruction positioned in the flow path of a liquid is measured via pressure bridge  13  and processed at  14  to yield a pressure signal which is proportional to volumetric flow rate while the capacitance of the flowing liquid is measured at  15  and processed at  16  to yield a density signal. The temperature of the liquid is measured at  18  and is used in the correction of both the pressure bridge signal as well as the densitometer signal. An additional correction  20  of the pressure bridge signal is made as a function of the temperature of the pressure differential sensor itself. The temperature sensor is integrated into the diaphragm of the sensor as the diaphragm temperature may be different than the liquid or environmental temperatures. The final computation of mass flow is made at  22 . 
         [0012]      FIG. 2  is a cross-sectional view of a preferred embodiment of the flow meter  24  of present invention. The flow meter housing  26  has a lumen  28  defined therein that extends therethrough from its proximal end  30  to its distal end  32 . The housing accommodates a volumetric flow meter  34  and a densitometer  36 . The volumetric flow meter includes a variable area obstruction valve  38  that is positioned within the flow path. Pressure measurement passages  40 ,  42  extending from either side of the obstruction valve allow a differential pressure sensor  44  to measure the pressure drop across the obstruction. The area of the obstruction is variable to the extent that flexible petal valves  46  extending therefrom deflect as flow rate increases to thereby gradually decrease the area of obstruction and increase the flow area. The petal valve stiffness is selected to so as to accommodate the anticipated range of flow rates. The resulting reduced change in differential pressure over a given flow range allows the flow meter to be useful over a wider flow range. The variable venturi throat design is self regulating so its flow rate versus flow area relationship will be repeatable and therefore flow rate versus pressure drop is predictable. As a result, a rangeability of 30 to 1 is achievable and can readily accommodate for example, a flow range of 200 to 6000 pph. The obstruction valve additionally includes a support structure  48  that is disposed on its downstream side which serves to prevent the petal valves from creasing or taking a set and functions as a stop. By preventing the petal valves from being damaged both the repeatability of flow measurement is enhanced and service life is extended. 
         [0013]    The densitometer  36  consists of a capacitance probe in the form of a set of concentric tubes  50 ,  52 ,  54 ,  56  through which the fuel flows. The dielectric constant of the fuel is measured as it passes through the tubes. Since the fuel density can be correlated to the dielectric constant, it can be used as a density sensor when the signal is corrected by fuel temperature. The fuel temperature measurement is achieved by bonding a film temperature sensor  58  to the densitometer surface. This temperature signal is used to correct the density reading and the mass flow computation which is dependent on fuel properties, which vary predictably with temperature. Since the fuel type is a variable but known to be within limits, the fuel density is a random variable within known limits. The density sensor allows a significant reduction of system error due to this random variable, since it is related directly to the mass flow measurement error. The product of the volumetric flow rate and the density, corrected by fuel temperature, allows for a precise mass flow measurement. 
         [0014]      FIG. 3  is a cross-sectional view of another preferred embodiment of a flow meter  60  of the present invention for measuring flow through orifice  69 . Some of the modifications included the incorporation of a temperature sensor  62  within the pressure differential sensor, the incorporation of an orifice valve regulator  64  to serve as a petal valve support structure and the location of a low thermal mass temperature probe  66  within the fuel stream. The orifice valve regulator serves to vary the valve length and stiffness as the valve deflects open. The regulator thereby controls the orifice effective area as a function of fluid flow rate. Such feature also prevents excessive deflection that would result in valve deformation. 
         [0015]      FIG. 4  is a perspective view of the flow meter shown in  FIG. 3 . 
         [0016]      FIG. 5  is a close up view of the obstruction valve  68  of the flow meter depicted in  FIGS. 3 and 4  showing the configuration and orientation of the petal valves  70 . Flow is directed through multiple orifices  64  to create a pressure drop that is proportional to volumetric flow rate. The flexible petal valves seal the orifices at zero flow rate and gradually deflect to increase flow area as flow rate increase. 
         [0017]    While a particular form of the invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except by the appended claims.