Scramjet Combustion Chamber Radiation
The combustion processes in a scramjet generate significant heat release leading to high gas temperatures, thermal radiation emission, and significant heat loads on the walls of the combustor chamber and nozzle of the engine. The wall heating and flow cooling may play an important role in the combustion processes and the wall heat fluxes need to be characterized accurately in order to assess thermal protection requirements for the scramjet engine.
The objective of the task is to analyze the radiative heat fluxes for of the HIFiRE-2 Scramjet combustor. A diagram of the HIFiRE-2 Direct Connect Rig (HDCR) experimental setup is given in Figure 1. The uncertainties in radiative heat flux predictions may significantly affect the ability to properly model the HDRC. Uncertainties can arise from the spectral modeling processes or uncertainties in the flowfield modeling.
Figure 1: NASA Langley HDCR diagram. Flow is from right to left.
A Discrete Ordinates Method (DOM) provides a computationally efficient method of calculating total chamber radiative intensities. In these analyses, DOM uses banded spectral model a spectral model in which the primary thermal radiators are water, carbon dioxide, carbon monoxide and the hydroxyl radical. The spectrum is divided into 25 cm-1 wide spectral bands ranging from 25 to 10000 cm-1, spectral modeling information is calculated from the HITEMT 2010 database.
Radiative Heat Transfer
The DOM simulations post-process three dimensional flowfield results from RANS CFD turbulent reacting combustion simulations. The results for the Mach 6.5 RANS Simulation of the HIFiRE-2 scramjet combustor with an equivalence ratio of 1.0 are given in Figure 2. Figure 2A below gives the spectrally integrated radiative heat flux, and a relative comparison to the convective wall heat flux given in Figure 2B. The radiative heat flux can be as high as 56 kW/m2, and as much as 10% of the convective heat flux.
Figure 2: Quarter section of HIFiRE 2 combustion chamber: (A) Radiative wall heat flux in the HIFiRE-2 combustion chamber. (B) Ratio of radiative to convective wall heat flux. Flow from right to left.
Uncertainty and V&V
Spectral modeling and database uncertainties are predicted with from a spectral-perturbation analysis. Predicted relative spectral modeling errors are given in Figure 3. The relative uncertainty ranges as high as 20% in low flux regions, and the uncertainty remains around 10% for high flux regions. These methods characterize the uncertainties stemming from spectral modeling, but the uncertainties stemming from the flowfield variations and accuracy of the CFD model remain.
Figure 3: Quarter section of HIFiRE 2 combustion chamber: Predicted spectral modeling errors for radiative wall heat flux.
Additional verification of the system occurs with experimental measurements taken on the HDCR at NASA Langley. A series of 16 photodetectors are placed at the exit nozzle of the ground test rig, and physical measurements are taken over the spectral range of 5550 to 9100 cm-1. The experimental measurements with associated uncertainty bars are compared to radiative simulations with associated spectral uncertainty in Figure 4. The experiments and simulations agree well for sensors with wide fields of view, but not with sensors with narrow fields of view, indicating that the overall radiative heat flux predictions are accurate, but the exact locations of radiative heating are still uncertain. These discrepancies indicate that the overall flowfield variation must be more rigorously investigated in order to properly simulate the radiate heat wall flux.
Figure 4: Radiative wall heat flux at 16 locations around the combustion chamber exit plane. Experimental measurements, and simulations with uncertainty bars given.
With spectral uncertainty characterized but the spacial variations in the system still indicating a large uncertainty, exploration of the spatial and temporal flowfield variations and CFD modeling uncertainty are required. Temporally varying CFD Large Eddy Simulations are being investigated for their effects on thermal radiation in addition to the steady state RANS simulations currently given. Temporally/spatially-varying thermal radiation simulations are ongoing.
This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] under Award Number NA28614.
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