Building these systems requires knowing how reduced gravity affects boiling and condensation, which all heating, ventilation and air conditioning systems use to operate in Earth's gravity.

A Purdue University experiment launching Aug. 1 on Northrop Grumman's 19th commercial resupply services mission (NG-19) to the International Space Station for NASA aims to collect data scientists need to answer decades-old questions about how boiling and condensation work in reduced gravity.

"We have developed over a hundred years' worth of understanding of how heat and cooling systems work in Earth's gravity, but we haven't known how they work in weightlessness," said Issam Mudawar, Purdue's Betty Ruth and Milton B. Hollander Family Professor of Mechanical Engineering.

The NG-19 spacecraft is expected to launch at 8:31 p.m. on Aug. 1 from the Mid-Atlantic Regional Spaceport at NASA's Wallops Flight Facility in Virginia and arrive at the space station Aug. 4. A livestream of the launch is available via NASA Live.

Onboard this flight is a module for conducting the second experiment of a facility called the Flow Boiling and Condensation Experiment (FBCE), which has been collecting data on the space station since August 2021.

Last July, Mudawar and his students finished their first experiment gathering data from a module of FBCE on the space station that measures the effects of reduced gravity on boiling. When the facility's additional components arrive with the NG-19 spacecraft, the researchers will be able to conduct the second experiment, which will investigate how condensation works in a reduced-gravity environment.

Both experiments' modules for FBCE will remain in orbit through 2025, allowing the fluid physics community at large to take advantage of this hardware.

"We are ready to literally close the book on the whole science of flow and boiling in reduced gravity," Mudawar said.

To develop FBCE, Mudawar's lab worked with NASA's Glenn Research Center in Cleveland, which engineered and built the flight hardware funded by the agency's Biological and Physical Sciences Division at NASA Headquarters. The team spent 11 years developing FBCE hardware to fit into the Fluids Integrated Rack on the orbiting laboratory.

FBCE's answers on boiling and condensation will not only support exploration on the moon or Mars but also help spacecraft to travel longer distances. The farther missions are from Earth, the more likely that the spacecraft for those missions will need innovative power and propulsion systems, such as ones that are nuclear thermal or electric. Compared to other types of processes that enable heating and cooling in space, boiling and condensation would be much more effective at transferring heat for spacecraft with these systems.

In addition, FBCE data could help enable spacecraft to refuel in orbit by providing scientific understanding of how reduced gravity affects the flow boiling behavior of the cryogenic liquids spacecraft use as propellant.

FBCE is among NASA's largest and most complex experiments for fluid physics research. Mudawar's team is preparing a series of research papers unpacking data the FBCE has collected on the space station, adding to more than 60 papers they have published on reduced gravity and fluid flow since the project's inception.

"The papers we have published over the duration of this project are really almost like a textbook for how to use boiling and condensation in space," Mudawar said.

With more than 30,000 citations, Mudawar is one of the most highly cited researchers in the field of heat transfer. Google Scholar ranks him No. 1 in flow boiling, spray cooling, microchannels, and microgravity boiling. He also is the most cited author in the International Journal of Heat and Mass Transfer.

For more than a decade, Mudawar and his students have been developing three sets of predictive tools to be validated using FBCE data. One set of tools puts the data into the form of equations that engineers can use to design space systems. Another set identifies fundamental information about fluid physics from the data, and the third set is computational models of the fluid dynamics.

All together, these models would make it possible to predict which equipment designs could operate in lunar and Martian gravity.

"The amount of data coming out of the FBCE is just absolutely enormous, and that's exactly what we want," Mudawar said.

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