Abstract
This paper presents the results of CFD investigations of siphoning between tanks of an aircraft fuel system under high-g conditions typical of an extreme pull-up maneuver. Negative gauge pressures in the siphon tank can occur, potentially breaking the siphon and decreasing the available fuel flow to the engine. A representative configuration of a system has been modeled, consisting of two fuel tanks plus a vent tank, fuel transfer lines, and gas lines for venting and supply of inert gas to the fuel tank ullage spaces. The fuel pressures and dynamics are simulated using a time-dependent Volume-of-Fluid modeling with normal force increasing rapidly from 1 to 9 g corresponding to an extended period of rapid pull-up during which the main supply tank becomes empty. Realistic values for engine fuel flow rate and fuel properties (JP-8) are used. This work is part of a broad effort to investigate fuel system performance issues that are difficult to test. Four simulations were performed in total, comparing 1 g flight and a 9 g pull-up both with and without a low-capacity pump assisting the siphon transfer. We find that the pump influences the siphon flow rate, the siphon break characteristics as the supply tank empties, and the magnitude of negative gauge pressures that occur in the siphon tank.
Highlights
This paper presents the results of CFD simulations aimed at understanding potential worst-case negative pressures occurring in an aircraft fuel tank during a rapid pull-up maneuver
We are interested in the pressures that arise in a supply tank that delivers fuel to the engine but is itself supplied with fuel from another tank via a siphon line
A systematic mesh-resolution study was performed to assess the accuracy of this resolution for predicting the pressure drops in the pump and siphon transfer lines, which are the main factors in the transient evolution of siphon tank pressures and the siphon break event
Summary
The red line is the engine defueling line It has two pumps, labeled forward and backward, and both are sometimes needed to meet the fuel demand, e.g. during usage of the afterburner. Out of the siphon/Tank 2, low pressure conditions are created, and this draws fuel from Tank 1 via the siphon line to augment the pump transfer. This is a conservative approximation that results in less than 1% error in modeling a dive and pull-out maneuver in which the gravity vector is canted 8 degrees forward of the body-Z axis at the beginning and 8 degrees backward at the end This is sufficient for understanding the effect of the pull-up maneuver on the pressures in the tanks
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