Highly heated steam could become a more environmentally friendly alternative to conventional methods in dishwashers in the future. Two German researchers simulated dishwashing with steam at 180 degrees on the computer and found that even heat-resistant bacteria are killed after 25 seconds. Natalie Germann from the Technical University of Dortmund and Laila Abu-Farah from the Technical University of Munich have published their study in the journal Physics of Fluids.
Superheated steam occurs when 100 degree hot steam continues to be heated but no water is added. “180 degrees hot steam cleans surfaces more thoroughly, faster and therefore more efficiently,” Germann is quoted as saying in a statement from her university. Because superheated steam reliably kills bacteria, it is also used in medical technology. In the case of dishwashers, it is particularly important in hospitals and restaurants that the dishes and cutlery are as germ-free as possible. The researchers see potential for dishwashers with superheated steam in these facilities.
Germann and Abu-Farah combined several simulation programs, including one for the effect on bacteria. The aim of their experiments was to kill off the problematic bacterial species Bacillus thermophiles and Geobacillus stearothermophilus, which tolerate heat particularly well. “The finished simulation shows the complex dynamics between flow, temperature and bacterial inactivation in a simplified dishwasher geometry and confirms that the hot steam actually cleans reliably and in a short time,” says Germann.
In the simulation, the superheated vapor spreads so quickly that it causes heat shock waves. Because superheated steam is better at dissipating heat than water-saturated steam, the surfaces of the dishes are quickly heated to such a high temperature that even heat-resistant bacteria die. The researchers found out how the steam spreads and where and when it forms vortices.
For the simulation, Germann and Abu-Farah greatly simplified the dishwasher, using only one steam nozzle and one plate. Further simulations are to follow to clarify where the steam nozzles should be located in order to clean the dishes in a full machine and make them as germ-free as possible. Simulations are also to be combined with experiments to find out whether superheated steam dishwashers still need detergents at all.
