Engineers Jeff Mackey (left) and Monica Guzik (right) perform engineering checks on the Fluid Module 2 prior to final hardware assembly. FBCE seeks to validate a model for flow boiling critical heat flux (CHF) and develop an integrated two-phase flow boiling and condensation facility for the International Space Station.

In space, weight matters. So do size and energy efficiency. Which is why scientists will be conducting a series of experiments to see if they can reduce the size and weight of the temperature control systems that will be needed to support space travel to the Moon, Mars and beyond.

In August 2021, the Northrop Grumman’s 16th commercial resupply services mission will deliver the hardware needed to conduct the first in a series of experiments on board the International Space Station. Known as the Flow Boiling and Condensation Experiments, or FBCE, this is a joint effort between the Purdue University Boiling and Two-Phase Flow Laboratory in West Lafayette, Indiana, and NASA's Glenn Research Center in Cleveland, Ohio. This facility, the largest fluid temperature control science facility deployed by NASA to date, will allow scientists to compare data from the ground with the microgravity found on the Space Station – all in an effort to determine the influence of various forces needed to control fluid temperature in different gravity environments.

“We need more robust fluid temperature control systems than we currently have to travel long distances in space,” says Issam Mudawar, the principal investigator of FBCE at Purdue. “These systems will provide precise and efficient fluid temperature control for a variety of important applications, including maintaining temperature and humidity inside space vehicles and both Lunar and Martian habitats, and providing accurate design methods for future cryogenic in-space transfer depots and nuclear thermal propulsion systems.”

One of the current challenges we have with these systems is how fluids act when heated and cooled in microgravity conditions. On the ground, if you boil water in a container, the bubbles -- whose formation is essential to removing heat from the heating surface -- rise by buoyancy and pop when they reach the free liquid surface. In space however, the bubbles don’t rise to the top. Instead, contrary to what we’re used to seeing on the ground, the bubbles simply remain close to the hot surface that the liquid is designed to cool down, growing appreciably in size. Because in microgravity they’re not appreciably lighter than the liquid, the bubbles don’t naturally move upward, which precludes the necessary replenishment of liquid at the surface, which is needed for new bubbles to form.

So, instead, these experiments use two-phase fluid flow. The two “phases” refer to a liquid and a vapor, with what’s known as flow boiling used to remove the heat and flow condensation to return the fluid into liquid state, all in a single closed system. The flow – motion – of the fluid flushes the bubbles away. This allows the liquid to continuously replenish and cool the hot surface, and new bubbles to be generated.

The experiments will be performed in an interchangeable test module that fits into the Fluids Integrated Rack, a standard facility for space experiments on the Space Station. All will use perfluorohexane as the test liquid which has a boiling point of 57.14 °C (134.9 °F), and employ a series of high-speed cameras. Cameras will watch to see what happens as the vapor and liquid mix – something we know is complex and currently difficult to predict.

How will this data be used? Again, it’s all about weight, size, and temperature control. Ultimately, Mudawar would like to see the data help designers develop new temperature control systems that are lighter in weight than what we have now, which is why the transition from current single-phase liquid to two-phase thermal systems is necessary. They are also more energy efficient.

Joining FBCE on the NG-16 delivery to the space station are the following Biological and Physical Sciences investigations:

●      Advanced Colloids Experiment-Temperature control-1 (ACE-T1),

●      Advanced Imaging, Folding, and Assembly of Colloidal Molecules (ACE-T9),

●      and Ring Sheared Drop (RSD).

These investigations are supported by NASA’s Biological and Physical Sciences, or BPS, Division as part of its mission to pioneer scientific discovery and enable exploration. To learn more about BPS research initiatives, go to: https://science.nasa.gov/biological-physical.

To learn about more experiments launching aboard NG-16, go to: https://www.nasa.gov/mission_pages/station/research/news/ng-16-science-highlights.

Read an interview with FBCE creators.