Month: August 2021

A vital piece of gas engines, combustors—the chambers in which the combustion powering the engine occurs—have the problem of breaking down due to fatal high-frequency oscillations during the combustion process. Now, through advanced time-series analyses based on complex systems, researchers from Tokyo University of Science and Japan Aerospace Exploration Agency have found what causes them, opening up novel paths to solving the problem.

In a breakthrough, published in Physics of Fluids, a team including Prof. Hiroshi Gotoda, Ms. Satomi Shima, and Mr. Kosuke Nakamura from Tokyo University of Science (TUS), in collaboration with Dr. Shingo Matsuyama and Dr. Yuya Ohmichi from the Japan Aerospace Exploration Agency (JAXA), have used advanced time-series analyses based on complex systems to find out.

Explaining their work, Prof. Gotoda says, “Our main purpose was to reveal the physical mechanism behind the formation and sustenance of high-frequency combustion oscillations in a cylindrical combustor using sophisticated analytical methods inspired by symbolic dynamics and complex networks.” These findings have also been covered by the American Society of Physics in their news section, and by the Institute of Physics on their news platform Physics World.

The combustor the scientists picked to simulate is one of model rocket engines. They were able to pinpoint the moment of transition from the stable combustion state to combustion oscillations and visualize it. They found that significant periodic flow velocity fluctuations in fuel injector affect the ignition process, resulting in changes to the heat release rate. The heat release rate fluctuations synchronize with the pressure fluctuations inside the combustor, and the whole cycle continues in a series of feedback loops that sustain combustion oscillations.

Additionally, by considering a spatial network of pressure and heat release rate fluctuations, the researchers found that clusters of acoustic power sources periodically form and collapse in the shear layer of the combustor near the injection pipe’s rim, further helping drive the combustion oscillations.

These findings provide reasonable answers for why combustion oscillations occur, albeit specific to liquid rocket engines. Prof. Gotoda explains, “Combustion oscillations can cause fatal damage to combustors in rocket engines, aero engines, and gas turbines for power generation. Therefore, understanding the formation mechanism of combustion oscillations is an important research subject. Our results will greatly contribute to our understanding of the mechanism of combustion oscillations generated in liquid rocket engines.”

Indeed, these findings are significant and can be expected to open doors to novel routes of
exploration to prevent combustion oscillations in critical engines.

Two-dimensional “nanosheets” made of bonds between metal atoms and organic molecules are attractive candidates for photoelectric conversion, but get corroded easily. In a new study, scientists from Japan and Taiwan present a new nanosheet design using iron and benzene hexathiol that exhibits record stability to air exposure for 60 days, signalling the commercial optoelectronic applications of these 2D materials in the future.

Converting light to electricity effectively has been one of the persistent goals of scientists in the field of optoelectronics. While improving the conversion efficiency is a challenge, several other requirements also need to be met. For instance, the material must conduct electricity well, have a short response time to changes in input (light intensity), and, most importantly, be stable under long-term exposure.

Lately, scientists have been fascinated with “coordination nanosheets” (CONASHs), that
are organic-inorganic hybrid nanomaterials in which organic molecules are bonded to metal atoms in a 2D network. The interest in CONASHs stems mainly from their ability to absorb light at multiple wavelength ranges and convert them into electrons with greater efficiency than other types of nanosheets. This feat was observed in a CONASH comprising a zinc atom bonded with a porphyrin-dipyrrin molecule. Unfortunately, the CONASH quickly became corroded due to the low stability of organic molecules in liquid electrolytes (a medium commonly used for current conduction).

“The durability issue needs to be solved to realize the practical applications of CONASH-based photoelectric conversion systems,” says Professor Hiroshi Nishihara from Tokyo University of Science (TUS), Japan, who conducts research on CONASH and has been trying to solve the CONASH stability problem.

Now, in a recent study published in Advanced Science as a result of a collaborative research between National Institute for Materials Science (NIMS), Japan and TUS, Prof. Nishihara and his colleagues, Dr. Hiroaki Maeda and Dr. Naoya Fukui from TUS, Dr. Ying-Chiao Wang and Dr. Kazuhito Tsukagoshi from NIMS, Mr. Chun-Hao Chiang and Prof. Chun-Wei Chen from National Taiwan University, Taiwan, and Dr. Chi-Ming Chang and Prof. Wen-Bin Jian from National Chiao-Tung University, Taiwan, have designed a CONASH comprising an iron (Fe) ion bonded to a benzene hexathiol (BHT) molecule that has demonstrated the highest stability under air exposure reported so far. The new FeBHT CONASH-based photodetector can retain over 94% of its photocurrent after 60 days of exposure! Moreover, the device requires no external power source.

What made such a feat possible? Put simply, the scientists made some smart choices. Firstly, they went for an all-solid architecture by replacing the liquid electrolyte with a solid-state layer of Spiro-OMeTAD, a material known to be an efficient transporter of “holes” (vacancies left behind by electrons). Secondly, they synthesized the FeBHT network from a reaction between iron ammonium sulfate and BHT, which accomplished two things: one, the reaction was slow enough to keep the sulfur group protected from being oxidized, and two, it helped the resultant FeBHT network become resilient to oxidation, as the scientists confirmed using density functional theory calculations.

In addition, the FeBHT CONASH favoured high electrical conductivity, showed an enhanced
photoresponse with a conversion efficiency of 6% (the highest efficiency previously reported was 2%), and a response time < 40 milliseconds for UV light illumination.

With these results, the scientists are thrilled about the prospects of CONASH in commercialized optoelectronic applications. “The high performance of the CONASH-based photodetectors coupled with the fact that they are self-powered can pave the way for their practical applications such as in light-receiving sensors that can be used for mobile applications and recording the light exposure history of objects,” says Prof. Nishihara excitedly.