| Résumé | An isokinetic total water content probe is under development in the Gas Turbine Laboratory at the National Research Council Canada. The purpose of the probe is to measure the total water content of clouds during atmospheric flight testing and of the air stream in wind tunnels. No reliable probe exists to measure the water content of clouds containing ice particles, which have been recognised as a threat to commercial aircraft. The probe will be incorporated into the High Ice Water Campaign to quantify the ice content of convective systems in tropical regions. In addition to ice, it measures liquid water content including super cooled conditions. The principle of operation is to sample the air stream isokinetically, to avoid biasing smaller particle sizes, evaporate the solid and liquid water content and measure the humidity of the resulting air stream.
An initial aerodynamic prototype was developed and tested. This provided the basis for designing the evaporation system to allow isokinetic flow driven only by the dynamic head recovered in the diffuser near the inlet. The test results also allowed a numerical model of the probe to be developed verifying isokinetic function across the range of speeds and altitudes it was required to operate in. Based on the experiments with the aerodynamic prototype, and resulting models, a full prototype was developed. This included the evaporator, heated intake to allow operation under super cooled liquid conditions, mass flow measurement to determine when isokinetic operation is achieved and a control valve to maintain isokinetic flow.
Heat transfer models were produced for the evaporator and intake. The models were validated with sea level testing and used to determine performance at altitude. Due to the reduced air density at altitude the heat transfer is reduced. In the evaporator this is a disadvantage for transferring heat to the hydrometeors, but causes the walls to be hotter thus increasing the heat transfer for particles that impact. The reduced heat transfer is an advantage for maintaining the intake ice free, as less power is required to maintain the same surface temperature. This is offset, however, by the higher speed and lower ambient temperature experienced at altitude.
The prototype was tested during the winter of 2009. Problems with the hygrometer required it be calibrated for each day of running based on data gathered in the test cell. This reduced the accuracy of the results but the error seems to be limited to 5%. The experimental results indicate the evaporator is performing well. However, the uncertainty in the results, due to the hygrometer and the limited calibrated range of the tunnel, does not allow for a definitive conclusion on the evaporator effectiveness.
Future work includes modifying the evaporator to improve the heat transfer to the hydrometeors, redesigning the intake to reduce the high temperatures required to maintain ice free conditions, servicing of the hygrometer, installation of a back-up commercial humidity sensor, procurement of a data acquisition and control system that can interface with the research aircraft and testing of the complete system. |
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