Application of Background Oriented Schlieren (BOS) in Underground Mine Ventilation

The schlieren technique describes an optical analysis method designed to enhance light distortions caused by air movement. The ability to visualize gas flows has significant implications for analyzing underground mine ventilation systems. Currently, the widely utilized traditional schlieren methods are impractical underground due to complex equipment and design requirements. Background oriented schlieren (BOS) provides a solution to this problem. BOS requires two primary components, a professional quality digital camera and a schlieren background. A schlieren background is composed of a varying contrast repetitive pattern, such as black and white stripes or dots. This background allows the camera's sensor to capture the minor light diffractions that are caused by transparent inhomogeneous gases through image correlation. This paper investigates a possible means of mitigating some of the major problems associated with surveying underground mine ventilation systems with the BOS method.

BOS is an imaging technique first introduced in 1999 that allows the visualization of flowing inhomogeneous transparent media. In ventilation surveys, BOS can be used to attain qualitative data about airflows in complex areas and methane emissions from coal. The acquisition of such data would not only enhance the understanding of mine ventilation but also improve the accuracy of ventilation surveys. As an example, surveys can benefit from small scale BOS investigations around fans, regulators, overcasts, and critical junctions to identify effective data gathering positions. Regular inspections of controls and methane monitoring points could also be improved by the systematic nature of BOS.

Computer programs could process images of each location identically regardless of quantity. BOS can then serve as a check to identify items that were overlooked during the routine inspection. Despite the potential of BOS for ventilation analysis, several limitations still exist. These issues are sensitivity threshold and quantification of flow data. This paper specifically examines the qualitative potential of the BOS technique for imaging various underground ventilation flows and outlines initial experimental efforts used for the evaluation.

Three primary experiments were conducted to evaluate BOS as a potential qualitative analysis technique for underground mine ventilation. The first experiment used BOS to image of flow induced by an axial vane fan and an axial flow fan using an artificial background and an imitation rock background. This experiment showed that the BOS system was unable to image isothermal airflow from either fan. Heated airflow could be visualized with both fans using the artificial striped background but not with the imitation rock background. The BOS system lacked the sensitivity necessary to image isothermal airflow from the two fans. The focus of the overall BOS study was changed to explore higher pressure airflows through a regulator.

The second experiment used BOS to image flow through a regulator induced by an axial flow fan using an artificial striped background. The BOS images were compared to ones produced by a traditional schlieren single mirror systems for validation of the BOS experimental design. This experiment was unable to image isothermal airflow through the regulator from either system. However, heated airflow could be visualized by both systems. The BOS and traditional schlieren systems used in this experiment lacked the sensitivity necessary to image isothermal airflow through a regulator. However, the BOS procedures were successfully validated by the ability of both the BOS and traditional schlieren systems to image heated airflows. The focus of the study was changed to explore methane gas emissions.

Numerous mining industry techniques already exist to quantify methane content. However, methane content is different from the actual methane emission rate of exposed coal. Emission rates have been modeled using numerical simulation techniques, but the complexity of the methane migration mechanism still requires physical data to achieve higher accuracy. The third experiment investigated the feasibility of using the BOS technique for imaging methane flow by imaging methane emission from a porous medium. Laboratory grade methane was directly injected into a Brea sandstone core sample using a flexible tube.

The BOS system was successfully able to image methane desorption in this study. A repeating pattern consisting of alternating black and white stripes served as the schlieren background for the Nikon D700 camera. The ability to image methane emission even at low injection pressures (i.e. 20 psi) demonstrates that actual methane desorption from coal can potentially be imaged. This result can only be conjectured because of a lack of research in the area of methane emission. Despite this issue, the experimental results suggest that BOS can be feasibly utilized to image methane emissions from coal in an underground mine.

The results of the three experiment demonstrated that the potential for large scale implementation of BOS in underground mines does exist. Qualitative BOS information has the potential in the practical sense to optimize the procedures of ventilation surveys and design of ventilation monitoring equipment. For example, images of methane flow in active mining areas can be used to optimize the positioning of auxiliary ventilation equipment to dilute known areas of high methane concentration. BOS images could also be used to re-evaluate the placement of methane monitors on mining equipment to better facilitate the detection of dangerous methane concentrations in active mining areas. For these reasons, further investigation into the BOS technique for use in imaging underground airflows with differential temperatures and methane emissions in underground coal mines is suggested as an addendum to this study.