In Heat Transfer and Fluid Flow Laboratory, two state exams and two PhD thesis defences were taken on 1st February. Tereza Kroulíková and Ondřej Resl successfully defended their theses. Jiří Hvožďa and Kryštof Mráz passed the doctoral state exam. We wish all of them great success in the future. And what is the subject of their research? Take a look.

Tereza Kroulíková studied the effect of tension on the pressure drop of air crossing the flexible bank of polymeric hollow fibres. The thesis focused on polymeric hollow fibre heat exchangers with even more than thousands of fibres. Their usage is not only in applications that need clean and anticorrosion materials but also in applications where low weight is needed. Heat transfer and pressure drop are essential for the design of heat exchangers. Since the pump power required depends on the pressure drop. Common pressure drop relations for the bank of tubes in crossflow used in engineering are derived from big rigid metal tubes. When those relations are used for small flexible polymeric hollow fibres, sometimes there is no concordance with reality. Improved correlations found by Kroulíková will be used for innovative and complex polymeric hollow fibre heat exchanger design.

Hundreds of millions of tons of crude steel are produced worldwide every year. The crude steel is further processed mainly by hot rolling. It is necessary to control the cooling process during heat treatment to achieve the required mechanical and physical properties of final products. Ondřej Resl investigated the influence of surface quality (surface roughness, presence of oxides) on cooling intensity and the issue of remaining water (thin water layer that remains on the cooled surface after cooling). Broad types of water cooling and surfaces were considered by Resl. Revealed knowledge provides valuable and new information on the factors influencing the cooling process of hot steel surfaces. And further allows a better understanding of the entire cooling process, which is essential for successive optimization.

In recent years, energy and environmental topics have become more pronounced. Electric vehicles (EVs) with zero tailpipe emissions have shown great potential and advantages in alleviating these issues. One of the biggest current challenges is battery thermal management. A robust and reliable battery thermal management system (BTMS) must be designed to ensure EVs’ proper performance, long life span and high safety. Jiří Hvožďa deals with the applications of extruded polymeric heat exchangers in BTMS. He aims to design a new heat exchanger which is light, electrically non-conductive, and its production is easily scalable. Further, optimal operation conditions will be determined by Hvožďa for broader applications. 

The field of heat exchangers is generally characterized as a well-established industry where new changes and innovations must bring a considerable economical (or possibly environmental) benefit. However, new polymer extrusion technologies enabled the production of hollow polymer fibres with a diameter of about one millimetre and a wall thickness of about 10 % of the diameter. Fibres were identified as promising heat exchange surfaces in lightweight and compact heat exchangers that would be cheaper and less environmentally harmful than conventional heat exchangers. Since the idea of polymeric hollow fibre heat exchangers (PHFHE) is new and niche, it is not always possible to deploy useful shortcuts developed to facilitate the R&D of conventional exchangers. Here mainly widely respected correlation models for heat transfer and fluid flow are meant. It is impossible to apply them simply on PHFHEs because they are based on measurements with substantially larger tube diameters. Kryštof Mráz aims to create CFD models in strictly open-source software with an unconventional approach with the Lattice Boltzmann Method that will be used to fill the gap.