Recent advances and applications of digital holography in multiphase reactive/non-reactive flows: a review

In various multiphase flows, the characterization of particle dynamics is of great significance to understand the interaction between particles and the surrounding flows. Digital holography (DH) is a versatile 3D imaging technique, which has shown great advantages in quantitative analysis and non-intrusive diagnosis of various particle fields. This review focuses on the advances and applications of DH in multiphase reactive/non-reactive flows in the last two decades. The basic principles of DH are introduced firstly, including its mathematical background and representative experimental configurations. Then, the image processing algorithms for hologram reconstruction and automatic focusing are summarized, along with the methods for separating overlapping particles and tracking moving particles. As a prevailing and powerful tool, the recent applications of deep learning in processing holographic images is also included in this review. Furthermore, the applications of DH in the characterization of particle dynamics in multiphase reactive/non-reactive flows are surveyed in detail. Lastly, the review concludes with the discussion of the technical limits of DH and provides insights into its promising future research directions.

A Numerical Investigation of Mixing Models in LES-FMDF for Compressible Reactive Flows

The filtered mass density function (FMDF) model has been employed for large-eddy simulations (LES) of compressible high-speed turbulent mixing and reacting flows. However, the mixing model remains a pressing challenge for FMDF methods, especially for compressible reactive flows. In this work, a temporal development mixing layer with two different convective Mach numbers, and , is used to investigate the mixing models. A simplified one-step reaction and a real hydrogen/air reaction are employed to study the mixing and turbulence-chemistry interaction. Two widely used mixing models, interaction by exchange with the mean (IEM) and Euclidean minimum spanning tree (EMST), are studied. Numerical results indicate that no difference is observed between the IEM and EMST models in simple reaction flows. However, for hydrogen/air reactions, the EMST model can predict the reaction more accurately in high-speed flow. For mixing models in compressible reactive flows, the requirement of localness preservation tends to be more essential as the convective Mach number increases. With the increase of compressibility, the sensitivity of the mixing model coefficient is reduced significantly. Therefore, the appropriate mixing model coefficient has a wider range. Results also indicate that a large error may result when using a fixed mixing model coefficient in compressible flows.

Reconstruction of Temperature Distribution for a Turbulent Free Jet Using Background Oriented Schlieren

Background Oriented Schlieren (BOS) has been shown to be an excellent tool for qualitative flow visualization, and more recently, literature has shown that the technique can be expanded to yield quantitative measurements as well. In this study, a BOS setup was built to construct the temperature distribution of a heated turbulent free 12mm diameter jet near the nozzle. A 1080p DSLR camera was used to view a black and white speckled background plane through the heated free jet in question. Comparing images of the background with and without flow present using a cross correlation algorithm gave the apparent displacement of all points on the background viewed through the flow. Once this displacement field was obtained, a ray-tracing algorithm was implemented to reconstruct the refractive index of the center plane of the jet. Then, the Gladstone-Dale and ideal gas relations were combined and used to calculate the temperature of the center plane. Reynolds number, based on the jet diameter, was held constant at 6,000 for all cases, and steady state nozzle temperature was varied from 57°C to 135°C. Reconstructed temperature distributions were validated using K-type thermocouple measurements by allowing the system to reach steady state before acquiring data. Average agreement of 4–6% was observed between thermocouple and BOS measurements for axial locations of at least 30 mm downstream. Due to experimental error, accuracy decreases as axial location moves towards the nozzle, and as nozzle temperature increases. Improvements to the setup are being considered to improve the agreement in low accuracy regions. Further, this technique has the potential to be used to determine the temperatures in open and optically accessible closed reactive flows. Having information about near wall temperature in closed reactive flows will give insight into wall convective heat transfer characterization and will also help benchmark combustion based numerical models in applications such as gas turbines.