Abstract:

Energy-efficient humidity management as part of heating, ventilation and air conditioning systems is becoming increasingly important as the need for dehumidification rises due to global warming. The study provides new insights into improving the condensation rate under the influence of turbulence generated by fractal grids. The condensation rate is measured on a cross-flow tube bundle consisting of 16 tubes, each with an outer diameter of 8 mm and a length of 400 mm. The tube bundle is placed in an air stream with velocities of 3, 6.4 and 9.5 m/s. At each of these velocities, the relative inlet humidity is varied between 35 %, 47 %, and 60 %. The grids used are a regular structure, a fractal square pattern grid and a fractal double-T grid. The novelty of the study lies in the fact that, for the first time, a mass transfer problem under turbulence generated by fractal grids is investigated. The study goes beyond similar work in that it examines not just a single tube, but a tube bundle heat exchanger, which is more relevant to applied thermal engineering. The lower parts of the tubes are embedded in a Plexiglas® block. The condensate running down on the outer surface of a specific tube enters a cavity and is directed into a drainage channel that leads to collecting containers. Electronic balances with a reproducibility of 0.01 g are employed to measure the condensation rate over time. The regular grid and the square grid show similar pressure loss coefficients of 0.58 and 0.54 due to their almost identical blockage ratios. The lower blockage ratio of the double T-grid results in a pressure loss coefficient of 0.14. The experiments indicate an increase in the condensation rate of 0.9 % for the regular grid, of 1.3 % for the double T-grid, and of 4.6 % for the square grid compared to the case without turbulence. However, not all tubes benefit equally from the turbulence. Tube row effects are best mitigated by the fractal double T-grid. In order to better understand the relation between pressure loss and condensate rate the ratio of these two parameters is analysed. The two fractal grids reach with ratios of 8.4 and 9.3 very similar values but are significantly better than the regular grid with 1.6. These results indicate the beneficial influence of turbulence generated by fractal grids on condensation. However, the selection of a specific fractal grid to improve, for example, the dehumidification of air conditioning systems depends on the respective technical and economic conditions.

@Article{BUSCHMANN_2025_ATE_128418,
  author    = {Buschmann, M.H.},
  journal   = {Applied Thermal Engineering},
  title     = {An experimental study on the condensation under the influence of turbulence generated by fractal grids},
  year      = {2025},
  issn      = {1359-4311},
  pages     = {128418},
  volume    = {281},
  abstract  = {Energy-efficient humidity management as part of heating, ventilation and air conditioning systems is becoming increasingly important as the need for dehumidification rises due to global warming. The study provides new insights into improving the condensation rate under the influence of turbulence generated by fractal grids. The condensation rate is measured on a cross-flow tube bundle consisting of 16 tubes, each with an outer diameter of 8 mm and a length of 400 mm. The tube bundle is placed in an air stream with velocities of 3, 6.4 and 9.5 m/s. At each of these velocities, the relative inlet humidity is varied between 35 %, 47 %, and 60 %. The grids used are a regular structure, a fractal square pattern grid and a fractal double-T grid. The novelty of the study lies in the fact that, for the first time, a mass transfer problem under turbulence generated by fractal grids is investigated. The study goes beyond similar work in that it examines not just a single tube, but a tube bundle heat exchanger, which is more relevant to applied thermal engineering. The lower parts of the tubes are embedded in a Plexiglas® block. The condensate running down on the outer surface of a specific tube enters a cavity and is directed into a drainage channel that leads to collecting containers. Electronic balances with a reproducibility of 0.01 g are employed to measure the condensation rate over time. The regular grid and the square grid show similar pressure loss coefficients of 0.58 and 0.54 due to their almost identical blockage ratios. The lower blockage ratio of the double T-grid results in a pressure loss coefficient of 0.14. The experiments indicate an increase in the condensation rate of 0.9 % for the regular grid, of 1.3 % for the double T-grid, and of 4.6 % for the square grid compared to the case without turbulence. However, not all tubes benefit equally from the turbulence. Tube row effects are best mitigated by the fractal double T-grid. In order to better understand the relation between pressure loss and condensate rate the ratio of these two parameters is analysed. The two fractal grids reach with ratios of 8.4 and 9.3 very similar values but are significantly better than the regular grid with 1.6. These results indicate the beneficial influence of turbulence generated by fractal grids on condensation. However, the selection of a specific fractal grid to improve, for example, the dehumidification of air conditioning systems depends on the respective technical and economic conditions.},
  doi       = {https://doi.org/10.1016/j.applthermaleng.2025.128418},
  keywords  = {Tube bundle heat exchanger, Condensation rate, Fractal grids},
  owner     = {B3},
  ranking   = {rank5},
  timestamp = {2026-06-08},
  url       = {https://www.sciencedirect.com/science/article/pii/S1359431125030108},
}

@InProceedings{Safarik_2025_CLIMA_15th_788,
  author       = {Safarik, Mathias and Honke, Marcus and Schreiber, Christian and Steffan, Christoph},
  booktitle    = {Proceedings of the 15th REHVA HVAC World Congress - CLIMA 2025},
  title        = {Vacuum Ice Slurry: Efficient, Powerful and Flexible Cold Thermal Energy Storage},
  year         = {2025},
  organization = {15th REHVA HVAC World Congress - CLIMA 2025},
  pages        = {236–245},
  publisher    = {Springer, Cham.},
  volume       = {788},
  comment      = {Lecture Notes in Civil Engineering},
  doi          = {10.1007/978-3-032-10546-2_22},
  keywords     = {vacuum ice slurry; cold thermal energy storage; triple point process; renewable energy integration;},
  owner        = {martinez},
  timestamp    = {2026-06-09},
  url          = {https://doi.org/10.1007/978-3-032-10546-2_22},
}

Abstract:

The tightness of cryovials is a quality feature for the long term storage of biological samples. Because of detected leakages on the cryovials after simulating the transport on dry ice and storage in LN2, the necessity to improve the tightness was requested. This led to the development of a new cryovial, which is tight from −196 °C to +55 °C. To achieve this, cryotube and cap should be welded together. A prototype of a welding tool was designed, built and tested. The welding tool melts the thermoplastic material of tube and cap reliably and reproducibly. Loosening of the cap is possible with a tool. The three different test methods gravimetric leakage test, CO2-test and LN2-test were utilized to evaluate the tightness of the new welded cryovial. The results show that all welded cryovials pass the gravimetric leakage test and CO2-test. The leakage after contact with LN2 can be drastically reduced.

@InProceedings{Miksche_2025_C-18th-IIR_0005,
  author       = {Miksche, R. and Kretschmer, R. and Kade, A.},
  booktitle    = {Cryogenics - IIR Conference on Cryogenics},
  title        = {Welded Cryovials - a Contribution to Minimize Contamination},
  year         = {2025},
  note         = {Record ID:30033868},
  number       = {0005},
  organization = {Cryogenics 2025. Proceedings of the 18th IIR International Conference on Cryogenics, Prague, Czech Republic, 7-11 April 2025.},
  pages        = {6 p.},
  abstract     = {The tightness of cryovials is a quality feature for the long term storage of biological samples. Because of detected leakages on the cryovials after simulating the transport on dry ice and storage in LN2, the necessity to improve the tightness was requested. This led to the development of a new cryovial, which is tight from −196 °C to +55 °C. To achieve this, cryotube and cap should be welded together. A prototype of a welding tool was designed, built and tested. The welding tool melts the thermoplastic material of tube and cap reliably and reproducibly. Loosening of the cap is possible with a tool. The three different test methods gravimetric leakage test, CO2-test and LN2-test were utilized to evaluate the tightness of the new welded cryovial. The results show that all welded cryovials pass the gravimetric leakage test and CO2-test. The leakage after contact with LN2 can be drastically reduced.},
  doi          = {10.18462/iir.cryo.2025.0005},
  owner        = {martinez},
  timestamp    = {2026-06-10},
  url          = {http://dx.doi.org/10.18462/iir.cryo.2025.0005},
}