Abstract:
A digital flow control system for testing ramjet engines that permits  conl of fuel in steps, or percentages, of full flow rate. Four cavitating venturis connected to four valves control the flow rate through the valves such that the fuel flow rate is regulated in 10% increments of full flow rate depending upon the combination of valves which are open.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to flow rate control, and more particularly to flow rate control of a fluid in increment steps. 
     During ramjet testing it often is desirable to bring the engine to a certain operating condition, i.e., fuel flow to engine, hold for a short time to take necessary data, then change to a new point. In the past the fuel has been controlled by a hand operated needle valve. A typical ramjet engine combustor test lasts about 30 seconds; usually because the engine is in a developmental stage and has little or no thermal protection. Having only 30 seconds, the operator tends to move the needle too fast, never stopping. The engine stays in a transient state during the entire operation, thus giving invalid data. 
     Also, it is often desirable to compare different engine configurations operating with the same fuel flow. With the needle valve fuel control and an automated data acquisition system, the operator seldom settles on the same fuel flow point twice. 
     SUMMARY OF THE INVENTION 
     The digital flow control system provides a supply of fluid in steps, or percentages, or full flow rate. Four cavitating venturis, which for a given upstream pressure allow a flow rate directly proportional to their respective throat areas, with area ratios 1:2:2:5 are controlled by individual valves to give flow rates from 0% to 100% of full flow rate in steps of 10%. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the present invention; 
     FIG. 2 is a cross-section of a cavitating venturi; and 
     FIG. 3 is a schematic of the valve control switch/position indicator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Consider the fluid flow through a venturi as illustrated in FIG. 2. The flow rate between the entrance and the throat is given by 
     
         m = C.sub. d A.sub.t √2(P.sub.ent - P.sub.t) ρg 1. 
    
     where 
     M = mass flow rate, 
     P ent , 
     P t  =  pressure at the entrance and throat, respectively, 
     A t  = area of cross-section of the throat, 
     ρ = density of fluid, 
     G = gravity constant, and 
     C d  = discharge coefficient. 
     If A t  is very small so that P t  is equal to the vapor pressure of the fluid, and P ent  is very large compared to P t , then 
     
         m = C.sub.d A.sub.t √2 P.sub.ent ρg.            2. 
    
     If P ent  is maintained at a constant value, then 
     
         m ˜ A.sub.t.                                         3. 
    
     Referring now to FIG. 1, fluid enters the digital flow control system at an inlet port 10. A pressure regulator 12 connects the incoming fluid to each of four cavitating venturis 14, 16, 18, 20. Electrically operated flow control valves 15, 17, 19, 21 control the flow fluid through respective venturis 14, 16, 18, 20. The fluid then exits the digital flow control system through an exit port 22 to the engine under test. The valves 15, 17, 19, 21 operate in either an open or a closed configuration. 
     The four venturis 14, 16, 18, 20 have throat areas in the ratio of 1:2:2:5 to control the exit flow rate in steps of 10% of full flow rate. The pressure regulator 12 maintains a constant pressure at the entrance of each of the four venturis 14, 16, 18, 20. Referring to Table 1 the exit flow rate is dependent upon which of the valves 15, 17, 19, 21 are open. 
     
                       TABLE 1______________________________________Percent  Valve 1   Valve 2   Valve 3 Valve 4Total Flow    (A = 1)   (A = 2)   (A = 2) (A = 5)______________________________________ 0       0         0         0       010       1         0         0       020       0         1         0       030       1         1         0       040       0         1         1       050       0         0         0       160       1         0         0       170       0         1         0       180       1         1         0       190       0         1         1       1100      1         1         1       1______________________________________ where 0 = closed, 1 = open. 
    
     A rotary switch 30 with a plurality of wafers 30A, 30B, 30C, 30D, 30E is wired as shown in FIG. 3 to switch the valves 15, 17, 19, 21 according to the &#34;truth table&#34; illustrated in Table 1, and to indicate the position of the rotary switch. A dry cell battery 32 in series with a combination of series resistors 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, one between each pair of consecutive switch positions of wafer 30E for 0 to 10, and a potentiometer 44, provides an output voltage between the switch position and one side of the battery that is equal to the position of the rotary switch 30, or the percent of total flow. For example, with the battery 32 being 12 volts a 2 volt output might be equal to switch position 2, or 20% of total flow, where the resistors 34, 35, 36, 37, 38, 39, 40, 41, 42, 43 around the switch wafer 30E are of the same value and potentiometer 44 is provided to adjust for a 10 volt drop across the series resistors. 
     The flow control valves 15, 17, 19, 21 are electrically operated and opened by 110 VAC. Each position of the rotary switch 30 supplies power to the correct valves, thus opening them, and shuts off power to the other valves, thus closing them, depending upon the flow rate selected. 
     The pressure regulator 12 sets and maintains the supply pressure, P ent , to the venturis 14, 16, 18, 20, which venturis are machined of stainless steel and are configured to fit inside a tubing connector. 
     The use of this digital flow control system gives discrete stable points of engine operation, thus eliminating data taken during transient conditions of ramjet engine testing. It allows repeatability of the engine test conditions from test to test, and is an accurate means of determining fuel flow rate to the engine. 
     Obviously, any device requiring step control of a fluid flow rate can use this digital flow control system, coupling a digital code with the throat areas of cavitating venturis.