Knowing the pure component Cp0 or mixture Cp0 as computed by a flexible code such as NIST-STRAPP or McBride-Gordon, one can, within reasonable accuracy, determine the thermophysical properties necessary to predict the combustion characteristics when there are no tabulated or computed data for those fluid mixtures or limited results for lower temperatures. (Note: Cp0 is molar heat capacity at constant pressure.) The method can be used in the determination of synthetic and biological fuels and blends using the NIST code to compute the Cp0 of the mixture.
In this work, the values of the heat capacity were set at “zero” pressure, which provided the basis for integration to determine the required combustor properties from the injector to the combustor exit plane. The McBride-Gordon code was used to determine the heat capacity at zero pressure over a wide range of temperatures (room to 6,000 K). The selected fluids were Jet-A, 224TMP (octane), and C12. It was found that each heat capacity loci were form-similar. It was then determined that the results [near 400 to 3,000 K] could be represented to within acceptable engineering accuracy with the simplified equation Cp0 = A/T + B, where A and B are fluid-dependent constants and T is temperature (K).
With this information, a model for JP8 was established using NIST Code STRAPP with a 12-component mixture. Selected pure components such as C12 and 224TMP have representations in both the McBride-Gordon and NIST codes, and were calculated and compared. A 12-component mixture was defined for JP8 and Cp0 computed using the NIST code to 1,000 K. The simplified representation of the Cp0 for JP8 was form-similar to Jet-A, C12, and 224TMP over the range of 400 to 3,000 K. This defined the ability to predict the Cp0 for a variety of hydrocarbon mixtures using the NIST code to 1,000 K, and representing these data by the simplified Cp0, which can then be extrapolated to 3,000 K within reasonable engineering accuracy. Knowing Cp0(T) results for enthalpy, entropy, and free energy can be determined and input into the combustion code.
The simplified form of the gas phase caloric equations generated using the NIST STRAPP code, the NASA McBride code, and a systematic curve-fitting methodology, work well within an established computational fluid dynamics (CFD) flow solver. Computed flow structure for the four fuels, using a trapped vortex combustor experimental rig as a test case, show strong similarities. This is true for the temperature as well as the CO and CO2 mass fraction contours. Inspection of the mass-averaged combustor exit quantities, however, indicates that temperature differences may be sufficient to require reconsideration of turbine fueling schemes.
This work was done by Robert C. Hendricks of Glenn Research Center; Andreja Brankovic and Robert C. Ryder of Flow Parametrics; and Marcia Huber of the National Institute of Standards and Technology.
Inquiries concerning rights for the commercial use of this invention should be addressed to:
NASA Glenn Research Center
Innovative Partnerships Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Ohio 44135. Refer to