Real-time Imaging of in-flow microparticles using dispersive Fourier Transformation – RIFT

High-speed liquid jets and sprays are complex multiphase flow phenomena involved in many industrial applications such as energy production and transformation or aerosol science. Environmental constraints conjugated to energy scarcity now call for new monitoring instruments to optimize these industrial processes. In the frame of high-pressure automotive injection, the trend towards injection pressure increase in diesel engines leads to very high liquid velocities, making impossible the detailed characterization of the involved processes with standard diagnostic systems. In the past decade, significant efforts have been devoted to monitor high-speed jets using imaging techniques based on X-rays or ultrafast infrared lasers. Nevertheless, all these techniques are fundamentally limited by the intrinsic characteristics of 2D image detectors (CCD, CMOS) which hinder real-time measurements. Indeed, optical imaging systems are nowadays technology-limited by the classical trade-off between sensitivity (i.e. detection threshold) and speed (i.e. imaging frame rate). In the field of liquid jets imaging, best systems to date thus reach frame rates in the order of hundreds of kHz.

A promising solution to overcome this electronic bottleneck is to use an all-optical imaging technique based on the principle of space-time duality. This technique, known as serial time-encoded amplified imaging (STEAM), consists in encoding an image onto a broadband spectrum from an ultrafast pulsed laser, which is then mapped into the time domain, allowing record-high acquisition frame rates using high-speed time-domain digitizers. The frequency-to-time mapping is achieved by time-stretching the broadband pulses in a dispersive medium (such as an optical fiber), a process termed dispersive Fourier transform (DFT) which achieved great success e.g. among the nonlinear optics community.

The RIFT project aims at developing a DFT-based ultrafast all-optical imaging system dedicated to high-speed liquid jets and sprays. It will be the first time to the best of our knowledge that such an ultrafast imaging system with MHz frame rates would be used in the field of combustion. Its implementation will thus give new insights about the behaviors and ultrafast dynamics of liquid jets and sprays in combustion processes. DFT-based measurement would then allow to monitor previously inaccessible physical parameters such as speed of fuel droplets, formation of liquid ligaments or secondary atomizations. The precise knowledge of these parameters, among others, would allow to feed complex state-of-the-art numerical models that currently lack from experimental data. The realization of this innovative system will thus be a remarkable advance in the field, paving the way for a better understanding of combustion processes and towards the realization of cleaner engines with improved energetic efficiencies.

Our project is intrinsically multidisciplinary as it gathers specialists from the fields of optical diagnostics, ultrafast laser sources and fluid mechanics. The research works of the members involved in RIFT have indeed been highlighted by each community: real-time measurements, ultrafast fiber lasers and optical imaging in fluids. This assures a fast and efficient implementation of this project through several consecutive milestones, from the conception of the DFT technique to the realization of a functional imaging system usable in industrial conditions. Our almost all-fiber architecture is expected to be robust and stable against perturbations, which is of considerable interest when operating in harsh experimental conditions. It goes without saying that the development of such an ultrafast system dedicated to combustion processes would potentially strongly concern the industrial sector, specifically in the fields of automotive and aerospace.