Two-phase cooling meets the challenges of modern radar applications
As radar systems’ heat flux and thermal loads continue to increase alongside the pace of technology, two-phase liquid cooling is winning the race, beating more traditional methodologies (such as air-based and single-phase liquid cooling) to the finish line.
Customers today are seeking increased capability and performance from advanced military systems such as radar and directed energy weapons. The ever increasing thermal load and heat flux (heat rate per unit area) of such systems are challenging the practical limits of conventional air-based and single-phase liquid cooling solutions. Thermal performance as well as Size, Weight, and Power (SWaP) constraints are also causing engineers to turn to more advanced cooling methods. Active Electronically Scanned Array (AESA) radar using high-density Transmit/Receive (T/R) modules require special attention. In fact, single-phase liquid cooling solutions are projected to be insufficient for future programs such as the Navy’s Next Generation Jammer (NGJ) and Integrated Topside (InTop), both of which require AESA technology. For these reasons, two-phase liquid cooling technologies are gaining increased focus and attention, and will allow engineers to overcome the design challenges of advanced radar systems.
Radar and its thermal management evolve
In the ’70s, radar systems had low heat flux and could generally be managed by blowing air over the heated surfaces. By the ’90s, heat fluxes had increased enough to where air cooling was oftentimes no longer sufficient, prompting engineers to turn to closed-loop single-phase cooling systems using fluids such as Coolanol and mixtures of Ethylene Glycol and Water (EGW) or Propylene Glycol and Water (PGW). Although more complex and expensive from a system perspective, single-phase cooling systems have orders of magnitude greater capacity to remove waste heat as compared to air cooling. In aerospace applications, the advantage of liquid cooling over air cooling is amplified because of the decreasing density of air with altitude. For instance, a constant speed cooling fan at a 35,000 foot altitude delivers an air mass flow rate that is approximately 40 percent of the flow rate it can provide at sea level, greatly limiting the cooling capacity of the system. Today, single-phase liquid cooling is well understood and is already in use on many military platforms including the F-22 and F-35. These cooling solutions have met the thermal demands of the presently fielded radar and tactical environment and have proven to be reliable and robust.
However, next-generation radar and jamming systems have thermal load and heat flux requirements that are starting to exceed the practical cooling capacity of single-phase liquid cooling solutions, necessitating the deployment of two-phase cooling systems. Two phase-cooling systems outperform single-phase cooling systems for two key reasons. First, two-phase cooling takes advantage of not only the higher heat absorption capacity of the liquid (that is, sensible heat), but also the heat absorbed when the liquid vaporizes (that is, the heat of vaporization). Second, two-phase flow in cooling channels can support orders of magnitude higher heat flux than single-phase liquid flow at comparable coolant flow rates and thermal conditions.
Two-phase cooling advantages, considerations
For some applications, a two-phase cooling system can operate with less than 50 percent of the flow rate required in single-phase systems. This directly translates into smaller pumps, fluid lines, and quick disconnects, and less pumping power, resulting in a much smaller thermal management system. The resulting higher energy dissipation capacity of a two-phase system is especially important for aircraft and pod applications where SWaP is critical.
Additional benefits of two-phase cooling for radar systems are performance characteristics such as improved range and target detection as well as reliability gains from surface temperature uniformity of cooled components. Surface temperature uniformity can be achieved by ensuring that the phase change from liquid to vapor takes place at near constant pressure (relative to system pressure), as the two-phase mixture flows over the heated surface or through cooling channels. As a result, the bulk temperature of the two-phase mixture stays nearly constant even as the mixture absorbs substantially more amounts of heat from adjacent electronic components. In contrast, in single-phase liquid cooling systems, the coolant temperature rises as it absorbs sensible heat, easily rising between 10 °C and 20 °C in practical applications, which can have significant impact on radar performance.
To understand better the advantages of two-phase cooling systems for radar applications, it is useful to consider Newton’s law of cooling. Newton’s law of cooling states that heat removed from a surface is proportional to the area of the surface and the temperature difference between the surface and the bulk fluid that cools the surface, or Q=h*A*(Twall-Tfluid). The proportionality constant, h, is the well-known heat transfer coefficient, which has units of W/m2C. In radar cooling applications, an order of magnitude higher heat transfer coefficient can be achieved by employing two-phase flow than can be achieved with any practical single-phase cooling solution. For a desired heat removal rate through a fixed surface area, the higher heat transfer coefficient allows for a smaller temperature difference (ΔT) between the fluid and the surface being cooled. A smaller ΔT, in turn, means that the coolant flowing through a cold plate can have a higher temperature while maintaining the same surface temperature. Radar system requirements have a worst case ambient temperature and maximum radar temperature (fixed system temperature difference); delivering hotter fluid to the cold plates allows thermal engineers significant system-level SWaP benefits such as designing smaller heat exchangers or eliminating vapor compression cycles to condition the cold plate supply fluid. (For a detailed case study, see Sidebar 1.)
Ultimately, the decision on whether to adopt a two-phase system or stay with a single-phase solution will depend on a complete evaluation of system requirements along with its capabilities and operational limitations. On one hand, a well-designed and efficient two-phase system will usually require more time to design and implement, and much of the design time comes from the increased complexity of thermal analyses required because of the nature of two-phase flow. In addition, two-phase systems, specifically systems using refrigerants, will have slightly different operating and servicing requirements compared to single-phase systems. These differences can, however, be managed and are similar to servicing an air conditioner for a home or automobile.
On the other hand, the benefits of a smaller thermal management system have far reaching impact on the overall radar system. Consider, for example, the NGJ Broad Area Announcement (BAA), which states that 60 kW to 90 kW of power must be dissipated by the cooling system. Allowing a single-phase system to operate with a (typical) 12 °C ΔT at the cold plate, a Polyalphaolefin (PAO) system would require more than 3 kW of pumping power (Figure 1) compared to about 0.5 kW for an R134a two-phase system. Figure 1 also shows that a 60 percent EGW single-phase system requires more than 6 kW of pumping power under the same system requirements. The two-phase system, operating at a lower flow rate and pressure drop, can therefore use a smaller pump and heat rejection heat exchanger because less pump waste heat must be rejected.
Matching the thermal management system
As previously discussed, the demand for two-phase cooling systems, especially in advanced radar systems, continues to increase. Matching the right thermal management system to a particular application will yield the best solution for the warfighter. Understanding differences as well as advantages and disadvantages of the various types of thermal management solutions is essential. Fortunately, Parker Aerospace has been designing, building, and fielding robust single- and two-phase liquid-cooled systems for more than 20 years. Some of these systems have been developed for fixed wing aircraft including manned and unmanned vehicles, rotorcraft, ground mobile vehicles, shipboard applications, and even missiles. Many of the next-generation systems, where heat loads and heat flux are increasing so dramatically, stand to benefit the most from efficiencies gained by two-phase liquid cooling.
 Navy SBIR FY2008.1, Topic N08-035, Pod Mechanical Power Production, SBIR/STTR interactive topic information system,
Parker Hannifin Corp. www.parker.com