| Abstract | During the isokinetic probe testing in the M-7 icing wind tunnel under conditions of high ice concentrations and an air velocity near 80 m/s possible pooling of water within the evaporator was encountered. This problem was identified when ice and water were no longer being injected but the measured total water content reading remained elevated for some time. At these conditions the total water content reading was therefore not representative of the true ingested total water content. This behavior would be problematic with unknown abrupt changes in total water content seen during flight testing where precise real-time measurements are sought.
To investigate this problem and define an operational envelope where this behavior can be avoided, a previous version of the isokinetic probe evaporator was modified so that the pooling phenomenon could be observed using a borescope. Assuming that the same amount of pooling would take place within each helix due to symmetry, conclusions of pooling could be drawn without further intrusion into the apparatus. A test volume of 5 mL of water was used to determine that a 6.5 g/s (330 SLPM) flow rate of air was required to initiate the passage of water through the helix. The flow rate above which the flow path was completely cleared of water was found to be 8.3 g/s (420 SLPM), at sea level conditions and was set as the benchmark. Flow rates between 6.5 g/s (330 SLPM) and 8.3 g/s (420 SLPM) resulted in variable volumes of water remaining at the pooling location.
The pooling conditions formed a basis for the following two analyses:
• The particle drag analysis determined the largest droplet size that could be pushed over the apex of the helix at the specified conditions and was compared to the droplet size calculated from the benchmark condition. This analysis included both first and second order effects of propelling a hydrometeor in an air stream, and was applied at different points of interest within the helix.
• The dynamic pressure analysis matched the dynamic pressure at operating conditions to that calculated from the conditions in this experiment, in the mass flow rate range of 6.5 g/s (330 SLPM) to 8.3 g/s (420 SLPM). This analysis included first order effects in propelling a hydrometeor in an airstream only. Each analysis was used to acquire the aircraft velocity at various altitudes which would produce the same amount of pooling as observed in the experiment. The aircraft velocity corresponding to the benchmark case was less than the velocity stated in the flight plan, indicating that pooling would not occur during flight.
Although the absence of pooling was predicted during the flight campaign, the design of the evaporator was modified to increase the heat transfer to potentially pooled water. A 2.7 mm drainage hole was machined at the pooling location, linking the first and second helical loops, allowing through-flow of water. The rate of heat transfer was increased due to the increased surface area to volume ratio of water. In the event that ice forms at this location, the melted water can drain away and increase surface contact with the ice. |
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