@article{wahls_ekkad_2022, title={A new technique using background oriented schlieren for temperature reconstruction of an axisymmetric open reactive flow}, volume={33}, ISSN={["1361-6501"]}, DOI={10.1088/1361-6501/ac51a5}, abstractNote={A new technique, called 3D ray tracing, for refractive index field reconstruction of axisymmetric flows from displacement fields measured from background oriented schlieren (BOS) experiments is developed and applied to a lean premixed methane/air reactive flow at Reynolds number of 4000 on a 12 mm diameter circular burner. The temperature distribution is then calculated using a species independent direct relationship between refractive index, temperature, and ambient conditions. The error introduced by the approximation to reach this relationship is quantified using simulated flow fields and is found to be 8% within the inner unburnt region of the flow field, decreasing to 2% through the reaction zone, and then quickly reducing to 0% outside the flow field. The effect of random noise and reconstruction resolution on the accuracy of the method is assessed via application to synthetically generated data sets that mimic the characteristics of a heated air jet expelled into ambient. The novel 3D ray tracing allows for accurate temperature reconstructions of open axisymmetric reactive flows where 2D displacement fields are measured, which is shown to be a shortcoming of current direct methods in literature. Additionally, this is done without the need for any prior knowledge of flow field parameters; only ambient conditions to the system must be known. The simple experimental setup and low computational cost make this approach with BOS a good option for application into existing experimental combustion systems with minimal effort.}, number={5}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Wahls, Benjamin H. and Ekkad, Srinath V}, year={2022}, month={May} }
 @article{ahmed_wahls_ekkad_lee_ho_2022, title={Effect of Spanwise Hole-to-Hole Spacing on Overall Cooling Effectiveness of Effusion Cooled Combustor Liners for a Swirl-Stabilized Can Combustor}, volume={144}, ISSN={0889-504X 1528-8900}, url={http://dx.doi.org/10.1115/1.4054442}, DOI={10.1115/1.4054442}, abstractNote={Abstract One of the most effective ways to cool the combustor liner is through effusion cooling. Effusion cooling (also known as full-coverage effusion cooling) involves uniformly spaced holes distributed throughout the combustor liner wall. Effusion cooling configurations are preferred for their high effectiveness, low-pressure penalty, and ease of manufacturing. In this article, experimental results are presented for effusion cooling configurations for a realistic swirl driven can combustor under reacting (flame) conditions. The can combustor was equipped with an industrial engine swirler and gaseous fuel (methane), subjecting the liner walls to engine representative flow and combustion conditions. In this study, three different effusion cooling liners with spanwise spacings of r/d = 6, 8, and 10 and streamwise spacing of z/d = 10 were tested for four coolant-to-main airflow ratios. The experiments were carried out at a constant main flow Reynolds number (based on combustor diameter) of 12,500 at a total equivalence ratio of 0.65. Infrared thermography (IRT) was used to measure the liner outer surface temperature, and detailed overall effectiveness values were determined under steady-state conditions. The results indicate that decreasing the spanwise hole-to-hole spacing (r/d) from ten to eight increased the overall cooling effectiveness by 2–5%. It was found that reducing the spanwise hole-to-hole spacing further to r/d = 6 does not affect the cooling effectiveness implying the existence of an optimum spanwise hole-to-hole spacing. Also, the minimum liner cooling effectiveness on the liner wall was found to be downstream of the impingement location, which is not observed in the existing literature for experiments done under nonreacting conditions.}, number={7}, journal={Journal of Turbomachinery}, publisher={ASME International}, author={Ahmed, Shoaib and Wahls, Benjamin H. and Ekkad, Srinath V. and Lee, Hanjie and Ho, Yin-Hsiang}, year={2022}, month={May} }
 @article{wahls_ekkad_2022, title={Temperature reconstruction of an axisymmetric enclosed reactive flow using simultaneous background oriented schlieren and infrared thermography}, volume={33}, ISSN={["1361-6501"]}, DOI={10.1088/1361-6501/ac83e2}, abstractNote={The temperature distribution of a premixed methane air flame running at a Reynolds number of 1300 on a circular burner, 12.7 mm diameter, enclosed in a fused silica cylindrical liner has been experimentally reconstructed using a non-invasive approach combining background oriented schlieren (BOS) and infrared (IR) thermography. BOS is used to characterize both the air ambient to the system, using an existing technique called 3D ray tracing, and the reactive flow inside the enclosure, with a novel modified version of 3D ray tracing. IR thermography is used to characterize the thermal/optical characteristics of the quartz glass enclosure itself, since the information is required as BOS is a line of sight imaging technique. Out of necessity, an approximated species independent relationship is used to calculate flow temperature from refractive index. A simulation is used to show this error is in the range of 5.8%–7%. Additionally, it is found that drastically simplifying the approach by removing the IR thermography system entirely and using the near outer wall air temperature from BOS/3D ray tracing to characterize the internal temperature of the quartz liner itself only causes a 1.5%–3.8% degradation in the accuracy of the reconstructed temperature field. The technique as presented is a relatively inexpensive, experimentally simple approach capable of determining the steady state temperature characteristics of optically accessible axisymmetric reactive flows.}, number={11}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Wahls, Benjamin H. and Ekkad, Srinath V}, year={2022}, month={Nov} }