Survivability Investigation of Cylindrical Shaped Water Cooled Probe in High Heat Flux Conditions

Diagnostics and data collection downstream of turbine engine augmentors and scramjet combustors provide critical information for turbine engine and scramjet engine developers. One important diagnostic tool is the use of imaging measurement instruments, which are primarily used by the aeropropulsion ground testing community to assess flame holder stability and uniformity in turbine engines. The survivability limitations of these imaging probes are important to understand for use in applications behind ramjet or scramjet engines, where mass flows and exhaust temperatures exceed typical engine augmentor exhaust streams. At a minimum, imaging data acquired behind ramjet and scramjet engines would be used to adjust fuel splits to optimize performance of ramjet and scramjet engines.

The survivability of these metallic imaging probes is due, in part, by an internal water cooling process that is tailored to optimize energy exchange within the metallic structures. A lack of internal cooling would allow temperatures to exceed the melt temperature of the metallic probes, thus resulting in thermostructural failure. At these higher heat fluxes, the cooling flow can transition to nucleate boiling and still remove the required heat away from the metal. When nucleate boiling transitions to film boiling, typically this is referred to as the critical heat flux (CHF). After CHF is the unsteady transition to film boiling which is the situation that occurs where excess vapor blankets inner cooling channel walls, retarding transfer of energy from the heated probe structure to the coolant. For this reason, the prediction of the critical heat flux for expected cooling configurations is necessary to determine survivability and thermostructural margins of safety.

Modeling and simulation efforts to predict critical heat flux in subcooled forced convection flow boiling has made progress, but still leaves a lot to be desired. In particular, an Arnold Engineering Development Complex (AEDC) developed computer code (COOLWL) has been used to analyze nucleate boiling in several backside water cooling configurations. The code includes several theoretical correlations from literature, in addition to several CHF correlations formulated from experimental data.

This thesis documents work done in the attempt to predict the survivability limits of a cylindrical water cooled device in high enthalpy flows outside the bounds of which the probe has been subjected. Heat flux generated on the outside surface of the probe was predicted using Computational Fluid Dynamics (CFD) with relatively high enthalpy flow conditions and temperatures in excess of 5000 degrees Fahrenheit. Next, the COOLWL code was used to determine if the cooling water was able to avoid reaching critical heat flux while still removing the amount of heat required. The inlet parameters for the COOLWL code were varied including inlet cooling fluid properties and probe insertion depth. A key deliverable from the COOLWL parametric study was to determine the max heat flux conditions the probe could be subjected to before reaching the CHF point. Another key piece of information gained was a realistic convective heat transfer coefficient, in which the process described in this investigation allowed for an analytical way to converge on a realistic value. Lastly, a three dimensional model of the probe was generated and imported into a finite element analysis software (ANSYS) to compute the thermostructural limits, using a realistic heat transfer coefficient gained from the COOLWL analysis. This investigative research delivers a process which systematically acquires two important heat transfer values (critical heat flux and heat transfer coefficient) for thermostructural analysis and survivability of a given backside water cooled configuration. Results of this analysis reduce technical risk and qualifies the cylindrical probe for entry into more extreme temperature and mass flow environments (higher heat fluxes).