Today, absorption chillers and other two-phase cooling systems are used both in science and in everyday life: in nuclear reactors, thermal power engineering, high-performance microelectronic devices and servers, and in desalination systems. Maintaining a stable operating temperature is one of the most important tasks for cooling systems and for the equipment where they are being used. Scientists at Novosibirsk State University (NSU) and the Kutateladze Institute of Thermal Physics (TPI) SB RAS, together with students of NSU`s Department of Mechanics and Mathematics (MMD), proposed a new way to increase the efficiency of heat transfer during boiling in a vacuum. This makes it possible to develop more efficient and reliable two-phase cooling systems.
These systems include the immersion cooling system that supports maintaining the necessary temperature conditions by removing excess heat from the heat-generating surface through liquid boiling. In this case, boiling as a result of a constant process of vaporization provides a much higher heat transfer rate and temperature stability as compared to traditional air cooling. However, it is not uncommon for a device to be cooled even more. Examples include to reduce the corrosion rate of heat exchange equipment in various types of evaporators, to increase the efficiency and environmental friendliness of cogeneration power plants, and to reduce energy consumption in desalination plant evaporators. In these cases, it is necessary to lower the pressure and thereby lower the boiling point of the liquid.
Anton Surtaev, Candidate of Physical and Mathematical Sciences, Senior Researcher at TPI and the NSU`s Department of Physics, Laboratory of Physical and Technical Fundamentals for Power Engineering, talked about this work,
One of the main factors hindering the introduction of technologies based on the coolant boiling in a vacuum is a noticeable decrease in the intensity of heat transfer and critical heat loads with a decrease in pressure. For example, when the pressure drops from atmospheric to the pressure at which water begins to boil at a temperature close to room temperature, the intensity of heat transfer during boiling can decrease several times. Moreover, the process of boiling in a vacuum is accompanied by significant fluctuations in the temperature of the heat exchange surface. For a number of tasks such as submersible cooling of microelectronic devices, this extremely undesirable factor negatively affects the reliability of the equipment.
In response to this problem, various ways to increase the efficiency of heat transfer during boiling in a vacuum are being actively developed and discussed today. Most of them are based on the modification of the heat release surface. Among them, there are two main approaches: changing the morphology of the working surface by micro- and nanostructuring and controlling its wetting properties. Novosibirsk scientist’s research is dedicated to the latter. They developed and created the bifilar surface, which combines the advantages of the hydrophobic and hydrophilic properties of the surface in relation to increasing the efficiency of boiling. The experiments were conducted using the modern high-speed techniques of thermographic shooting and video filming.
As a result, the researchers demonstrated that the fabricated surface provides a significant intensification of heat transfer during boiling in a vacuum up to 3.7 times compared to a conventional surface. This makes it possible to significantly stabilize the temperature regime for cooling.
Georgy Patrin, a 3rd year student at the NSU`s Engineering School (MMD), summarized,
In other words, the use of a bifilar surface allowed us to break one large bubble into several smaller bubbles and ensure the uniformity of the temperature field of the surface during boiling in a vacuum.
