The study and optimization of combustion processes requires precise knowledge of the temperature and species’ concentration fields. In such applications, measurements are often taken locally by intrusive techniques that affect the aero-thermal field of interest and lack spatial resolution. Thus, for high fidelity analysis and for physically inaccessible regions, there is much value in developing non-intrusive measurement alternatives.
In order to probe combustion processes, the most prominent species is CO2, which radiates within ~4.3µm wavelength band where the absorption of other atmospheric gases is negligible. This emission of CO2 is considered in context of an axisymmetric free jet, a common geometry of interest – mimicking the exhaust plume of many thermodynamic cycles. By establishing a validated model for CO2 emissivity and developing a method which extends radiation thermometry to gases, the research effort focuses on a measurement technique to reconstruct the temperature distribution of a steady jet with known source CO2 and air mixture concentrations in the upstream supply line.
The experimental setup consists of a free jet produced by an orifice. CO2 – air gasses flow into a mixing chamber, the flow rates of which are adjusted independently. The mixture temperature is regulated by an electric heater. The resulting jet flow temperature is monitored by traverses of a thermocouple probe. The emitted radiation flux is measured by an IR camera.
The HITRAN/HITEMP database, which includes line-by-line spectroscopic parameters for high resolution molecular absorption and radiance calculations, is used to predict and simulate the spectral emission and transmission of the jet as well as the atmospheric path. The species’ diffusion in the jet is modeled via correlation with the local temperature gradients. Then, all the unknowns can be related to geometric variables obtained from integral radiation flux measurements, and to local temperature distributions which are optimized to yield minimum deviation from the integral observations.
Given an approximation of atmospheric temperature, pressure and composition (typically known in most sensing applications), as well as an estimate of upstream CO2 concentration, the jet temperature distribution is reconstructed from integral IR camera radiation flux measurements, absent of per-scenario calibration. Finally, the assumption of known supply CO2 concentration may be relaxed to improve precision and utility, by applying additional camera observations with multiple optical filters.