Getting Cooling Efficiency Down Cold
NASA Facility Built to Conduct Tests in Subzero Atmosphere
Sixty-six years ago the National Advisory Committee for Aeronautics (now known as NASA) created the largest closed-loop wind tunnel testing facility in the world to examine why so many of America’s military aircraft were icing up and crashing. In the decades that followed World War II, aircraft scientists, design engineers, and aircraft builders from all over the world used the building to test aircraft components to prevent icing at temperatures as low -25˚F.
Although NASA’s Icing Research Tunnel (IRT) remains the largest and busiest testing facility of its type, it remained largely unchanged over the years. Fast-forward to late 2011, when major changes in equipment and systems were completed enabling the facility to conduct tests at a jaw-clenching -40˚ F.
The centerpiece of the new upgrade is a wall of advanced heat exchanger coils as large as a two-story house. The upgrade also included a new 8,400-square-foot refrigeration-equipment building next to the IRT that houses a 1,800-ton primary refrigeration system and a secondary coolant system.
Located in Cleveland, the IRT’s massive 26-by-50-foot wall of new heat exchanger coils makes the facility one of the coldest research facilities in the world for simulating icing weather conditions. The $20 million project also replaced other aging equipment that enables more precise control over aero-thermal testing, and broadens the understanding of icing phenomena on aircraft components in more severe icing weather conditions.
The new wall of coils was formed by stacking six identical coil modules, each measuring 4 feet, 4 inches high by 11 feet wide by 50 feet long and weighing 17,000 pounds. Besides their mammoth size, their slanted design allows an air stream from the closed-loop tunnel to travel over more surface area on the coils and cool down much faster, than if the coils were perpendicular to oncoming airflow (see Figure 1).
The Cold Facts
The design of the coils was developed by the prime contractor for the project, Jacobs Technology (JT) in Tullahoma, Tenn., in collaboration with Super Radiator Coils (SRC) in Minneapolis, where the coils were manufactured. Even though the company had built coils for 15 previous wind tunnel projects, including NASA’s Ames Research Center in Mountain View, Calif., the IRT project was especially challenging, according to Jim DeWitt, vice president, SRC.
Changing the orientation of the vertical tubes to horizontal tubes also reduced air pressure drop and velocity enough to reduce ice shedding, DeWitt said. Shedding can ruin expensive testing procedures.
Another design innovation to prevent shedding was locating the headers (or manifolds) at the ends of the 50-foot-long modules, where they are out of the airstream and cannot impede airflow. Headers on the IRT’s previous coils were located in the airstream, which created air pressure drop and were prone to ice shedding. Headers distribute the refrigerant mixture through the copper tubes in each module.
Each coil module contains thousands of aluminum cooling fins and more than 500 copper tubes, each 50 feet long with the ends brazed to a header. If all 3,000 tubes were laid end to end they would stretch nearly 30 miles, DeWitt said.
Each module went through multiple stages of testing; including checking for leaks by pressurizing the coils to 200 psi and submerging it in a specially built tank filled with 16,000 gallons of deionized water. All told, it took four months to manufacture the six modules, or about two and a half weeks per unit.
“Every step of fabrication through final testing was a challenge, especially holding precise dimensional tolerances,” DeWitt continued. For example, the thousands of aluminum fins in each coil module had to be perfectly aligned so the 50-foot-long tubes could be pushed through the holes in the fins without binding.
Yet another critical objective for this project at the IRT was to complete it in the least amount of time to avoid costly downtime. Toward that end, the facility itself was out of service only five months, even though the whole project spanned 20 months.
This was accomplished by designing and constructing the new refrigeration-equipment building next to the IRT to house the primary refrigeration system. That allowed the old compressors and other equipment inside the IRT facility to remain operational until the switch-over in August 2011. The building was also designed to contain a secondary coolant system with an improved temperature control loop for delivering a brine solution to the new heat exchanger coils, once they were installed inside the IRT.
Jacobs developed a secondary coolant loop configuration using brine and R-507 refrigerant to replace the R-134a fluid that was used to cool the previous coils. The R-507 refrigerant cools down the brine solution, which then circulates through the heat exchanger tubes, providing better control and temperature distribution, according to Chris Porter, project director for Jacobs Technology.
This is the third time the refrigerant used in the IRT has been changed, he said. The original R-12 refrigerant was replaced in 1994 with the R-134a fluid, which was more environmentally friendly, but proved to be approximately 10 percent less efficient.
The IRT has helped improve air travel safety by determining how to prevent ice from forming on all types of aircraft. The facility is part of NASA’s Glenn Research Center in Cleveland.
The IRT’s 5,000-hp propeller fan has 14 blades that are 25 feet in diameter and circulates air at 350 knots per hour (400 mph) through the wind tunnel. Misters are located downstream to release supercooled water droplets into the frigid airflow to form an icing cloud that freezes on contact with equipment being tested.
The test chamber itself is 6-feet high by 9-feet wide by 20-feet long, and can accommodate full-sized aircraft components, as well as scale models. The IRT averages 20-25 unique testing programs annually, totaling 1,600-2,400 operational hours over 100-150 test days.
Richard Parrish is president of MindShare Communications, which specializes in developing timely, relevant feature stories about organizations that are making a difference in a broad range of industries and professions.
Publication date: 6/24/2013