Transformation needed in thermal management research

Researchers are recommending changes in how to study rapidly changing temperatures in complex systems such as aircraft and power plants, a transformation that could bring advances for applications ranging from fighter jets to energy production.

The researchers are calling for a change similar to the one that transformed the genre of very large-scale integration, or VLSI, leading to unprecedented advances in computing and electronics in recent decades.

"We are saying that this progress occurred precisely because the VLSI community embraced the need to transform itself, building from scratch the tools necessary to understand, optimize, and validate extremely complex, dynamic systems, ultimately creating the most transformative industry of the past century," said Timothy Fisher, Purdue University's James G. Dwyer Professor of Mechanical Engineering and director of the Center for Integrated Thermal Management of Aerospace Vehicles (CITMAV). "The thermal community today faces a similar challenge and opportunity. "

The center was launched in 2014 by the Aerospace Systems Directorate of the U.S. Air Force Research Laboratory and also is supported by an industry consortium.

Researchers involved in the center have authored a research paper they hope will provide the basic elements of a framework for change in areas including thermal storage, heat exchangers, thermal controls, "uncertainty quantification" and system-level modeling. The paper is available online and will be published in an upcoming issue of the Journal on Thermophysics and Heat Transfer.

Whereas previous research has focused on individual components within a complex system such as an aircraft, researchers are working to better control heating by learning precisely how it occurs in the whole system. Major challenges stem from the fact that the systems undergo rapid changes in heat loads, introducing extreme cooling demands and a level of uncertainty that must be better understood. New research is needed to probe the processes behind these changing, or "transient," heating events.

"Almost all of the studies done on high-flux cooling techniques have waited for them to reach steady state," Fisher said. "However, we are really focused on what happens in this transient process. We need to learn how random it is and how repeatable it is from one test to another. Those sorts of things haven't been really explored in depth before."

Future work will aim to quantify uncertainty in a dynamic event, such as the dramatic heating that takes place on the skin of aircraft traveling at hypersonic speeds, or faster than Mach 5.

"More generally, if you suddenly have a need even in commercial aircraft to put cooling in a certain place, for example systems for onboard air conditioning, we really need to understand how the systems work together under those circumstances," he said.

Another example is when aircraft take off from airports in hot desert climates, he said. Innovations are needed to improve thermal storage technologies designed to maintain a specific temperature range for systems to function properly.

"You want to tamp down temperature fluctuations, so you have to put the energy someplace temporarily," he said.

Advanced materials are needed to handle spiking fluctuations of heat, said Fisher, who is leading work to develop new thermal-management approaches with nanoscale carbon materials.

"Normally thermal technologies are heavy and they are slow. They may take minutes to take up and control heat, and we need something that's lightweight and fast, on the order of 1-to-10 seconds," he said. "We need new thermal storage technologies that are much faster than what we now have."

The modern aircraft is a collection of complicated and heterogeneous subsystems including propulsion, flight control and actuation, environmental control and pressurization, electrical power generation, and cooling and thermal management. It is important to understand how these separate subsystems interact and participate in the operation of the overall aircraft system.

Many of the subsystems require a certain range of operating temperatures to function effectively. To ensure that this remains the case during the entire mission, a thermal management system must be integrated in a system-level architecture. Recently, a shift has resulted in conceptual design through the use of integrated modeling and simulation.

"For example, the differences in subsystem interactions between the fourth-generation aircraft such as an F-16 and fifth-generation aircraft such as the F-22 and F-35 fighter aircraft are substantial," said Fisher, also a professor of aeronautics and astronautics. "The interactions are becoming larger and more frequent in number and, thus, more complex. As a result, it is not possible to examine the aircraft thermal management subsystem in isolation; instead, it must be viewed in the context of all the interactions between the subsystems that occurs onboard the aircraft."

One of the major goals of CITMAV is to create an integrated, cohesive research program involving several major tenets including the development of models that capture high-rates, and high-amounts of heat transfer, said John Doty, an associate professor in the Department of Engineering Management and Systems at the University of Dayton.

"The high rate of heat transfer necessitates that the models be posed and implemented dynamically," Doty said. "Common practices for model-based validation typically employ a steady-state, or quasi-steady state, approach for model development, which is inadequate to capture the important physics of the dynamic heat transfer."

Another major component is how uncertainty is adapted to models in a rigorous statistical manner to capture its magnitudes and sources and determine how it influences various responses. Models also must be validated with experiments, he said.

Model-based predictive controls must be developed to manage the high-rate thermal responses.

"Additionally, optimized analyses enable processes to be modeled in nearly real time so that we can adjust on the fly and potentially even include near-future predictions and anticipation," Doty said. "Taken in total, all of these major efforts are being developed and integrated to support advanced thermal management systems for aerospace vehicles."