Leidenfrost Driven Waste-Water Separator

The research pursued in the NSTRF grant, Leidenfrost Driven Waste-Water Separator, is conducted to support NASA’s TA 6.1.2: Water Recovery and Management through the development of a non-contact, passive distillation method compatible with microgravity and partial gravity (Lunar and Martian surfaces) environments. At least 98% water recovery from waste-water streams is required for deep space missions to Mars, a performance goal that is not yet attainable with current state of the art technology. Non-contact Leidenfrost distillation is a heat driven processes, capable of achieving such metrics.

Heat driven distillation is routinely employed to distill liquid-liquid and liquid-solute solutions. Such processes typically employ pool boiling. In the pool boiling regime, liquid droplets deposited on substrates slightly above the liquid’s boiling temperature will undergo nucleate boiling and will boil away in seconds. However, if the substrate temperature is significantly higher than the liquid’s boiling temperature, the droplet will not undergo nucleate boiling, but will instead levitate on its own vapor layer completely out of contact from the surface. This vapor layer insulates the droplet from the heated substrate, reducing heat transfer to the liquid droplet by orders of magnitude. These combined effects increase droplet lifetimes by orders of magnitude, from seconds to minutes. This phenomenon is called the Leidenfrost effect and is exploited in this work to perform non-contact distillation of waste-water droplet streams.

The activities of this NSTRF grant involved three key stages including experimentation, analytical modelling, and Leidenfrost distiller development, which were conducted over the course of 2. In the experimentation stage, terrestrial (1g) Leidenfrost experiments were conducted including distillation of aqueous salt solutions and Leidenfrost droplet lifetime experiments. The comprehensive suite of experiments provided scientific results on Leidenfrost droplet evaporation rates, total lifetimes, and distances travelled for dynamically rolling droplets. Microgravity Leidenfrost experiments, the first of their kind, were conducted in the unique Dryden Drop Tower facility at Portland State University (PSU). Dynamic Leidenfrost droplet impact experiments and Leidenfrost droplet transport experiments through tortuous conduits resulted in unique scientific results as well as insights on transport methods of Leidenfrost droplets in microgravity.

Stage 2 of this NSTRF grant included analytically describing experimental results. We provided a set of analytical expressions that can be utilized as engineering guidelines for sizing and designing Leidenfrost distillers for any gravity level (µg – 1g). The analytical results matched comprehensive experimental data and were therefore verified. In stage 3, experimental results and insights from stage 1 and analytical models from stage 2 were utilized to develop a Leidenfrost distiller. A Leidenfrost distiller was successfully designed, fabricated, and tested. The distiller achieved no-touch distillation of salt-water solutions without experiencing fouling of system surfaces. The efficacy of the Leidenfrost distillation method and validity of analytical and experimental results were therefore proven. Further development of this technology is encouraged for its utility in a variety of life support environments including deep space transit and planetary outposts. Further study of microgravity Leidenfrost phenomenon is also encouraged for the prevalence of the phenomenon in cryogenic systems, as an example.