Radiation Diagnostics of High Temperature High Speed Flows

David L Blunck, Purdue University

Abstract

Radiation emissions from exhaust plumes are of interest for security reasons and for studying the turbulent nature of high temperature flows. Temporal and spatial turbulent statistics of high temperature radiating flows can be obtained from planar radiation intensity measurements. Comparisons between measured and calculated mean and fluctuating radiation intensity values provide insights into the validity of scalar value calculations. Motivated by this, planar infrared images of a thin filament stretched across a buoyancy driven unsteady laminar hydrogen diffusion flame were obtained using an infrared camera. Transient line measurements of temperature and water vapor mole fractions were achieved using thin filament pyrometry and inverse radiation intensity calculations. The time-dependent spatial distributions of water vapor mole fractions and temperatures were affected by preferential diffusion and the altered velocity field of the flame. The experimentally determined scalar values are consistent with detailed chemistry calculations of the flame and laser diagnostic measurements of similar flames. Experience gained studying this laboratory scale flame was used to enhance studies of exhaust plumes. Narrowband radiation intensity measurements are reported for exhaust plumes exiting from an axisymmetric converging nozzle with varying temperatures and species concentrations corresponding to low equivalence ratios (0.17 - 0.28), Mach numbers between 0.4 and 1.0, and Reynolds numbers between 2.4 and 6.1×105. The intensity was measured using an infrared camera fitted with a narrowband filter (4.34+/-0.1 µm). The intensity leaving a diametric path exhibited a power dependence of 2.8 on the equivalence ratio. This dependence results from changes in the scalar values in the flow. Smaller changes in the intensity were observed for plumes where the velocity was varied. The variations are attributed to changes in entrainment into the plume and the nozzle exit temperature. Calculations of the radiation intensity indicate that the outer portion of the shear layer attenuates the radiation emitted from the inner portion of the plume. Turbulent radiation statistics have been reported for reacting flows and used to assess turbulence models, provide insights into turbulence radiation interactions, and estimate integral time and length scales. Seeking similar insights, the mean, root mean square, probability density function, auto and spatial correlation coefficients, integral time and length scales, and power spectral density functions of the radiation intensity are reported for three exhaust plumes. Axial and radial variation in the normalized root mean square of the radiation intensity are similar to those reported for flames. Autocorrelation coefficients of the radiation intensity emitted downstream of the core region are approximated reasonably well by exponential curves. Integral time and length scales increase monotonically downstream of the core region. The break frequency and slope of the normalized power spectral density function are comparable to those reported for turbulent jet flames. Measured and computed mean and fluctuating radiation properties are reported for a subsonic exhaust plume. Unsteady three-dimensional calculations were used to estimate both time-dependent and mean temperature and partial pressure values of the exhaust species in the flow. From these scalar values the mean and root mean square of the radiation intensity were calculated using a narrowband radiation model. Axial distributions of the calculated intensities based on the time-dependent scalar values qualitatively show trends similar to measurements, and quantitatively over predict the intensity by 40 to 60%. Intensity distributions based on mean scalars were in better quantitative agreement, but decay more rapidly downstream than the measured values. The importance of turbulent fluctuations increased monotonically downstream and radially resulting in the mean intensity being larger by factors greater than two toward the edges of the plume. Comparisons of intensity values found from computed three- and two-dimensional scalar values emphasize the need for the former calculations. The techniques developed for acquiring radiation intensity measurements and estimating scalar values in flows were implemented to study the spatial development and temperature of spark kernels. Infrared images show that the kernels develop into a toroidal shape after exiting from the igniter. Regions of high and low radiation intensity are observed in the kernels, indicating temperature gradients within the gases. Average temperature values decrease by less than 30% over two centimeters of the spark trajectory. Over that same distance the internal energy of the kernel decreases by 80%.

Degree

Ph.D.

Advisors

Gore, Purdue University.

Subject Area

Aerospace engineering|Mechanical engineering

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