INCREASING THE ENERGY EFFICIENCY OF COOLING SYSTEMS FOR ENERGY-SATURATED EQUIPMENT BY SHIFTING THE SECOND KIND HEAT TRANSFER CRISIS TO THE HIGHER TEMPERATURE REGION

Abstract

The relevance. The development of fundamentally new strategies and technical solutions leading to an increase in energy efficiency and resource saving of cooling systems for energy-saturated equipment is impossible without the creation of new structural materials with targeted functional properties. Promising cooling systems of heat-loaded equipment, for example, 5G communication devices emitting high-density heat fluxes (up to 1000 W/cm2), include systems based on drip irrigation. Such cooling makes it possible to increase the intensity and uniformity of heat removal, and significantly reduce the coolant consumption. However, the results obtained so far show that the use of traditional approaches (the use of surfaces made of steel, copper, aluminum and their alloys, processed by grinding or polishing) does not allow solving the problem of intensive cooling of surfaces that release high-density heat fluxes. Laser strategies for processing heat transfer surfaces are one of the most effective ways to intensify evaporation and boiling. In relation to the development of laser systems in the last decade, financially sound technologies for creating targeted, functional surface properties of metals have become available. The practical use of laser-modified heat-transfer surfaces of cooling systems can solve a number of problems associated with meeting the growing global demand for energy resources, in particular, when intensifying the removal of high-density heat fluxes from elements of energy-saturated equipment by shifting the heat transfer crisis of the 2nd kind to higher temperatures. The purpose is to assess the possibility of shifting the heat transfer crisis (Leidenfrost effect) to higher temperatures by modifying heat transfer heating surfaces with nanosecond laser radiation. Methods. Using nanosecond laser processing, unique textures with specified geometric characteristics were created on typical materials used in practice in the design of heat transfer surfaces for heating cooling systems. Texture characteristics were determined using confocal and electron microscopy equipment. The Leidenfrost effect was recorded using a specially made setup with equipment for the shadow optical method and high-speed video recording of fast physical processes. Results. The modes of a single nanosecond laser pulse action on the stainless steel and aluminum surfaces were detected. These modes guarantee the formation of an ablation crater. We showed the possibility to apply the graphic-analytical technique for the formation of a given microtexture during the treatment of metal surfaces with nanosecond laser radiation. Laser treatment of metal surfaces with pulse energy up to 0,6 mJ makes it possible to control the boiling crisis characteristics (2nd kind), in particular, the Leidenfrost temperature, over a wide range of values by more than 110 °C on aluminum surface and by more than 45 °C on stainless steel surface in atmospheric conditions when using distilled deaerated water as a heat carrier due to the formed texture and superhydrophilic properties.

Keywords

• heat transfer crisis
• drop
• surface
• texture
• roughness
• laser texturing