A Streamlined Approach to Venturi Sizing

Abstract

Many spacecraft propulsion systems use orifices or cavitating venturis to mitigate water hammer pressure spikes, but the current process for sizing and validating these devices is time-consuming. This paper discusses the effort to find a relationship between peak water hammer pressure and venturi throat diameter that can be used to shorten the amount of time required to size venturis for flight. The venturi throat diameter must be chosen carefully to minimize pressure drop at steady-state flight flow rates for thruster burns but still provide components enough protection from damaging pressure spikes. At NASA Goddard Space Flight Center, cavitating venturis are sized through testing alone. Depending on the complexity of the system and the number of venturis being tested, water hammer testing can take months of engineer and technician time. Standard sets of water hammer tests were performed using venturis with throat diameters ranging from 0.035" to 0.080", located in both the simplest representative system that could be created -a straight line setup, as well as a system that is slightly more complex -a single branch was added to the straight line setup. Both setups included a flight-like latch valve used to mimic flight scenarios, such as system priming, and dynamic pressure transducers to record pressure surges in locations of concern, chosen by investigating test data from previous missions. Each setup was also tested without a venturi or other dedicated pressure surge mitigation device. The experimental data from this testing resulted in a simple scaling relationship between peak water hammer pressure and venturi throat diameter: the peak waterhammer pressure is proportional to the diameter of the venturi throat to the 1.5 power. This scaling relationship has the potential to significantly shorten the time and testing required to size cavitating venturis for flight propulsion systems by allowing engineers to determine an approximate venturi throat diameter based on very few water hammer tests. The testing data was compared with test data from previous GSFC missions, and the scaling relationship was found to provide conservative estimates for venturi throat size. NomenclatureAFT = Applied Flow Technology AIAA = American Institute of Aeronautics and Astronautics c = Speed of sound DR# = Dual gauge pressure regulator d vt = Venturi throat diameter Eq = Equation FO = Flow orifice FT = Forward thruster G# = Pressure gauge GHe = Gaseous Helium GPM = Global Precipitation Measurement mission GSE = Ground support equipment In = Inches k = Constant ln = Natural logarithm LPP = Low pressure panel LRO = Lunar Reconnaissance Orbiter LV = Latch valve mL/s = Milliliters per second 2 NV = Needle valve OD = Outer diameter P# = Pressure transducer psi = Pounds per square inch P wh = Peak water hammer pressure R# = Pressure regulator R 2 = Correlation coefficient RV# = Relief valve s = Seconds SDO = Solar Dynamics Observatory u = Velocity V# = Valve VP = Vacuum pump ρ = Density

Cavitating Venturi Model Using Standard Element and Options in Commercially Available Lumped-Parameter Software

“…Selecting the appropriate venturi configuration for the vehicle propellant system often requires multiple experimental iterations 1,2 in which venturis of different throat diameters are installed in the propellant system and both steady and transient flow tests are conducted. This experimental process is effective but can also be timeconsuming, requiring at least one engineer and one technician full-time for several months for the sizing process, followed by an additional few months' time to characterize the flight venturis.…”

Experimental study of water hammer pressure surge

“…To prevent this potential hazard, the solution is to slow down the §ow through the use of a §ow restriction device (venturi [3,4] or ori¦ce [5,6]) or by using the gas cushion e¨ect of a pre¦lled inert gas in the line.…”

Surge Pressure Mitigation in the Global Precipitation Measurement Mission Core Propulsion System

“…A research testing program was completed to better understand the relationship that exists between peak surge pressure and venturi throat diameter. 2 The goal of this testing program was to provide a quick estimate of a baseline venturi throat diameter for a flight surge pressure testing program.…”