Translation. Region: Russian Federal
Source: Novosibirsk State University –
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Scientists have made a significant breakthrough in understanding fundamental boiling processes Faculty of Physics of Novosibirsk State University and the S.S. Kutateladze Institute of Thermophysics of the Siberian Branch of the Russian Academy of Sciences, working as part of one of the research teams of the large-scale international project RUBI (Reference mUltiscale Boiling Investigation). For the first time, they observed the growth of an individual bubble during liquid boiling in zero-gravity conditions on the ISS, described it, and created numerical models of its growth. In doing so, the researchers made significant advances in understanding fundamental boiling processes. Two articles presenting a detailed analysis of these unique experiments have been published in leading international journals: first article, second articleThis research was supported by the Russian Science Foundation under grants No. 21-79-10357 and 19-19-00695.
This large-scale international project was implemented aboard the ISS by an international scientific team under the auspices of the European Space Agency. To study individual vapor bubbles nucleating on a superheated substrate, the Reference Multiscale Boiling Investigation (RUBI) facility was built and delivered to the ISS. Conducting this experiment on Earth was impossible because gravity on our planet masks key physical mechanisms—bubbles quickly break away and are carried away by the Archimedes force, and natural convection significantly influences temperature distribution in liquids. Thanks to zero gravity, the ISS became an ideal "laboratory," allowing the bubbles to remain on the heater and grow to sizes unusual for terrestrial conditions. It provides a particularly suitable environment for studying individual vapor bubbles nucleating on a superheated substrate and the mechanisms involved. This was the first such experiment with a single vapor bubble on an artificial vapor center under carefully controlled conditions on the ISS, where the bubble grows to large sizes without detachment and in the absence of natural convection.
The boiling process is used in many industrial applications for matter and energy conversion devices. We can also observe it in nature—for example, in geothermal geysers or during volcanic eruptions. While a vast amount of scientific research has been conducted on boiling, scientists have focused on integral boiling parameters, which are crucial for engineering problems. The growth of an individual bubble can also be considered an elementary boiling process, so for a detailed study of boiling mechanisms, it is advisable to focus specifically on individual bubbles. This has never been done before in zero gravity due to the complexity of the process itself. The difficulty lies in the fact that the physics of boiling depends on many factors, and despite numerous long-term studies, a complete understanding of all multi-scale phenomena remains. Experiments in zero gravity conditions can shed light on these phenomena. In zero gravity, bubbles can grow in size without premature detachment. Thus, boiling phenomena can be observed on larger spatial and temporal scales with better resolution. At the same time, boiling in zero-gravity conditions is itself a subject of research that is important for space missions, explained Fyodor Ronshin, a senior lecturer at the NSU Physics Department.
Conditions close to weightlessness can also be achieved on Earth using short-term zero-gravity platforms. Initially, scientists used ground-based structures such as drop towers, then parabolic flights, and sounding rockets. However, these capabilities were clearly insufficient for studying bubble formation during liquid boiling, as zero-gravity conditions were created only for a few seconds or minutes. In this case, longer periods of time were required, achievable only on the International Space Station (ISS). It is here, thanks to the stable conditions of zero-gravity, that long-term experiments can be conducted. Zero-gravity provides a particularly suitable environment for studying individual vapor bubbles nucleating on a superheated substrate and the mechanisms involved.
"The specially designed RUBI setup was delivered to the ISS six years ago. The experiment continued until 2021, when it was returned to Earth. During this time, scientists from five international research teams were able to observe its progress from Earth, monitor instrument readings, and access data online. The results were discussed and analyzed weekly. The setup was a sealed cell. The working fluid was FC-72, a dielectric fluid used to cool electronics. It was housed inside the cell. The bubble growth dynamics were visualized using a high-speed black-and-white camera on the side and a high-speed infrared camera underneath. The setup was also equipped with a fluid circulation loop that generated the flow. It was possible to set the fluid temperature, pressure, heat flux on the heater, and the time between heater activation and the laser pulse that initiates bubble formation. All of this was necessary to cover the entire range of parameters for constructing models of the observed processes," explained Fyodor Ronshin.
A short (20 millisecond) laser pulse was used to form a single vapor bubble on an artificial nucleation site. The bubble then grows under the influence of Joule heating. This process occurs inside the cell. The setup was also equipped with microthermocouples, which could be placed at various locations within the chamber to determine the temperature distribution within the liquid. It was also possible to study the effect of shear flow, which could be used to remove bubbles. Furthermore, the chamber contained an electrode that generated an electric field, which could cause the bubble to detach from the substrate (analogous to Archimedes' force on Earth).
Our research currently focuses on the results of a single-bubble growth experiment, with particular attention to the effect of liquid subcooling (the difference between the saturation temperature and the liquid temperature). This allows us to better understand the dynamics of single vapor bubble growth in zero-gravity conditions, with particular attention to the role of dissolved (non-condensable) gases. The experimental results are confirmed by numerical simulations based on the developed model. Some observed phenomena, such as the absence of bubble collapse and the subsequent resumption of bubble growth, proved difficult to explain without the assumption of the presence of non-condensable gases, despite careful degassing of the working fluid. The model was appropriately modified to test this picture of the phenomenon, which included Marangoni thermocapillary convection induced by dissolved gases in the liquid. "We found that in our case, the presence of even a small amount of dissolved gases (~1%) after thorough degassing has a positive effect on heat transfer because the superheated liquid is distributed along the bubble, moving away from the heater toward the top of the bubble, and the bubble doesn't condense, but continues to evaporate and grow faster. This allows for more efficient heat transfer," explained Fyodor Ronshin.
As a result of experiments conducted aboard the International Space Station using the RUBI facility in conjunction with advanced numerical modeling, scientists modified the numerical model to account for noncondensable gases and thermocapillary effects, which was in good agreement with experimental observations. Accounting for these factors eliminated discrepancies between subcooling conditions. The researchers also concluded that the presence of noncondensable gases within a bubble significantly affects its survival and growth dynamics, ensuring bubble survival even under conditions of relatively high subcooling that would otherwise collapse pure vapor bubbles. They noted that thermocapillary convection, driven by temperature gradients along the bubble surface caused by the presence of noncondensable gases, enhances heat and mass transfer near the interface. This phenomenon promotes intensified evaporation at the base of the bubble and reduces the intensity of condensation at its apex, facilitating its stable growth.
"Under terrestrial conditions, the influence of dissolved gases in a liquid can be suppressed by natural convection. In zero gravity, this does not occur, and their manifestation generally has a positive effect on bubble growth. We have discovered that by varying the concentration of dissolved gases in a liquid, we can influence the processes of bubble formation and growth. Using this data, we will be able to predict bubble growth in liquids with any concentration of dissolved gases, including in space," concluded Fyodor Ronshin.
Studying bubble growth in zero-gravity conditions without external forces is only part of the research, which is now complete. However, the RUBI experiment was not limited to this. Now, scientists will explore it under more complex conditions—for example, under the influence of an electric field, using the bubble removal method, and under varying electric field intensities. According to Fyodor Ronshin, the data received from the ISS will be sufficient for at least another five years of work. The results obtained will have both fundamental significance for the physics of heat and mass transfer and boiling, as well as practical applications—they will enable the development of more efficient cooling systems for spacecraft and orbital stations, where boiling is a promising method for removing high heat fluxes in zero-gravity conditions.
Material prepared by: Elena Panfilo, NSU press service
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