An improved three-dimensional concentration measurement technique using magnetic resonance imaging

Abstract

Magnetic resonance concentration (MRC) was introduced by Benson et al. (Exp Fluids 49:43–55, 2010) to obtain three-dimensional, time-averaged concentration fields in complex turbulent flows without the need for optical access using magnetic resonance imaging. It has since been applied to a wide variety of flows including jet engine film cooling configurations, mixing layers, and urban dispersion cases. However, the measurement uncertainty is currently limited to about 5% of the injected concentration, irrespective of the local concentration. This work presents an advanced MRC technique to greatly reduce the uncertainty at low concentration. Best practices for conducting MRC experiments are described to establish a baseline methodology. These include the choice of scan settings and fluids, calibration procedure, mixing experiment details, and method for computing concentration fields from scan data. An advanced technique is developed to reduce the uncertainty at low concentration by combining data from multiple experiments at increasing molarity of injected fluid, using Fourier-space averaging to reduce noise, and by minimizing fluid property differences using a low flip angle to reduce the maximum injected molarity without degrading the signal-to-noise ratio. The method is flexible and can be optimized to meet the uncertainty requirements of specific applications. Experiments are performed on the turbulent mixing downstream of an isolated, rectangular building as a test case. The advanced technique is validated against the baseline method and maps of spatially dependent experimental uncertainty are presented. Less than 1% uncertainty based on a 95% confidence interval is achieved near the plume boundaries. Results from the new technique reveal dilute but non-zero concentration regions near the wall which could not be resolved using the baseline method.