Effect of Trapped Air on Pressure Measurements in Immersed Fuel Pipes in Aeronautical Applications 2016-01-2050

The effect of air pockets in capillaries in terms of pressure variations is investigated experimentally. Pressure sensors in aircraft are often installed separate to the pipes and connected with capillaries to minimise ignition sources within fuel tanks. Trapped air within these capillaries might distort the measurement. These effects are characterised in this paper. Extensive tests with different capillary configurations, trapped air volumes and pressure transients are studied. The data obtained shows that the main effect of trapped air during pressure transients is a delay in the pressure response against its excitation, causing local pressure oscillations around its source value until the pressure is equal in both places again. These oscillations can turn into high pressure peaks under critical conditions. Greater amounts of air can cause greater delays and viceversa. When the amount of air is such that capillary pressurisation synchronises with the maximum pressure value in the main gallery, pressure peaks can reach their highest. Conversely, when the pressure rise in the capillary occurs much before the maximum in the main gallery or when it is already descending, pressure peaks are much softer. However, if the pressure transient is not abrupt enough (>200kPa/s), pressure oscillations in the capillary are not significant, independently of the amount of air. Transient origin and trapped air volume are then the two main parameters affecting the pressure reading in the sensor installed at the end of the capillary. If both parameters align, the measured pressure peak can become extremely high.