Comparison of background-oriented Schlieren and retroreflective shadowgraph visualizations of sand burner fire

• Comparing simultaneous Background-Oriented Schlieren and Retroreflective Shadowgraph images of the sand burner flame. • Introducing a novel method for quantifying convective heat flux of the sand burner. • One of the main findings of the paper is that RS captures eddies with much higher resolution compared to BOS. • The paper studies the impact of the side glass on both visualization methods performance. Simultaneous records of background-oriented schlieren (BOS) images and retroreflective shadowgrams (RS) of a stationary sand burner flame are compared to demonstrate the capabilities of each method. The schlieren images from the BOS method are post-processed in the PIVlab software via the reference image sequencing style method. The shadowgrams obtained in the RS setup are enhanced by adjusting the levels and contrast of the images. The range of length scales, the spectra at each length scale, and the effect of a glass sidewall window on the resolution of the images are determined. It is found that adding glass as a sidewall reduces the resolution and thus the number of detected eddies, but dominant eddies with their energy remain unaffected. In addition, the convective heat transfer is calculated and analyzed for both visualization methods. The results show that, in the described system, the resolution of the BOS images is lower than that of the shadowgrams, so smaller eddies are not as clearly visible as in corresponding RS results. However, BOS has the advantage of more easily quantifying the parameters studied in the present work. The paper describes a novel BOS-calibrated RS technique to extract the heat transfer contours from shadowgrams, which complements a similar approach in which heat transfer is obtained from BOS images.