This article focuses on a fuel-efficient gas turbine engine featuring intercooling and heat recuperation, which is being developed to power a new generation of warships. Modern warships are often powered by gas turbine engines so they can take advantage of the turbine’s rapid response capabilities, solid operational reliability, high power density, and compact dimensions. For medium-size surface combatants such as destroyers, aircraft-derivative gas turbines have become the dominant propulsion engine type, having largely replaced traditional steam or diesel power plants. Though the all-electric concept is far from new, having been applied previously to merchant vessels, the technology is looking better of late. The NRC panel stated that gas turbine propulsion units, modular rare-earth permanent magnetic motors, and power control module technologies have matured to the point that all-electric ships appear feasible. The technology cited “unique advantages” in reduced volume, modular flexible propulsion, lower acoustic signature, enhanced survivability, high propeller torque at low speed, and inherent reversing capability. The result would be a submarine-type propulsion design with diesel-like fuel consumption.
Modern warships are often powered by gas turbine engines so they can take advantage of the turbine's rapid response capabilities, solid operational reliability, high power density, and compact dimensions. For medium-size surface ' combatants such as destroyers, aircraft-derivative gas turbines have become the dominant propulsion engine type, having largely re placed traditional steam or diesel powerplants. Some navies, though, still pair diesels and gas turbines, retaining diesel drives to boost their vessels' fuel efficiency at low speed. Despite their many positives, simple-cycle marine gas turbines are comparatively fuel-thirsty.
In the early 1990s, the navies of the United States, Britain, and France combined to support an international effort to develop a next-generation gas turbine propulsion plant for medium-sized surface vessels. Now nearing commercialization, the WR-21 Intercooled Recuperated (ICR) gas turbine is the first aeroderivative gas turbine to incorporate compressor intercooling and exhaust heat recuperation systems to cut consumption of marine distillate oil. Engineers expect that the advanced-cycle drive technology will lead to a 27- to 30-percent reduction in overall fuel usage compared to existing U.S. Navy surface propulsion gas turbine units, according to John Chiprich, manager of the ICR program for Northrop Grumman Marine Systems in Sunnyvale, Calif., which leads the WR- 21 development team.
The Naval Sea Systems Command in Crystal City, Va., selected the Northrop Grumman-led team for the nine-year, $400 million contract in 1991, choosing it over another team headed by General Electric Co. Northrop Grumman Marine (formerly the Marine Division of Westinghouse Electric Corp.) is responsible for systems engineering and integration of the WR-21 system. Design and development of the unit's gas generator and power turbine are being handled by Rolls-Royce Industrial & Marine Gas Turbines of Ansty, England, while AlliedSignal Aerospace Systems & Equipment Group in Los. Angeles is providing the intercooler and recuperator heat-exchanger cores. CAE Electronics of Saint-Laurent, Quebec, designed and developed the engine/ system control based on control laws supplied by Rolls-Royce. As noted, Britain and France provided significant financial and material support for the ICR program.
The new powerplant could find application in hundreds of naval ships, said John Ulliman, director of business development and strategic planning at Northrop Grumman Marine. The nearest-term application is the 22-ship British, French, and Italian Horizon Frigate Program. The propulsion system. proposed for this class of vessel incorporates two WR -21 gas turbines, each driving a propeller through a main reduction gear system. A small electric motor drives each shaft for low-speed, quiet operation. The first US. application will likely be the US. Navy's new DD-21 land-attack destroyer program, he explain ed. Depending on the final size of the ship, the DD- 21 propulsion system will incorporate two to four WR-21 engines in an electric drive, mechanical drive, or hybrid drive system. Other navies expressing interest in the WR-21 include those of Japan, South Korea, the Netherlands, Spain, Germany, and Norway. After that, civilian market niches, such as propulsion drives for cruise ships, fast ferries, and even land-based distributed power generation applications, might open up, Ulliman said.
The WR-21 is being developed to replace the simple-cycle LM2500 turbine engine-the existing main propulsion engine for U S. Navy surface combatants, such as the DD-963 destroyer, the CG-47 guided missile cruiser, and the DDG-51 guided missile destroyer. The LM2500, which has since largely superseded diesels, was first produced in the 1960s.
A further evolution of this simple-cycle powerplant design is an intercooled regenerative-cycle unit, which offers higher thermal efficiency. " ICR s have been done previously," Chiprich noted. "For the Navy, the issue with gas turbine engines is fuel economy. Single-cycle gas turbines tend to burn a lot of fuel, particularly at partial power. Since the Navy typically runs gas turbines at 30 percent of maximum power about 60 to 70 percent of the time, it wants better fuel efficiency at low-power settings."
Each DD-21 vessel, for example, would save about $1.5 million a year in fuel and operating costs, Chip rich said. The savings provided by the new technology could pay back the premium on the original purchase ofWR-21 in two to six years.
Improved fuel economy can translate into a range of enhanced mission capabilities as well. These benefits could include a 30-percent increase in weapons payload for the DD-21, a 27- to 30-percent reduction in fuel tankage, increased speed, additional days on station, or greater range.
"The other engineering challenge here," Chiprich explained, "was to get everything into a small package... to get the necessary power density for a warship." Sized at 318 inches long and 190 inches high, the new unit was required to share the same footprint as the LM2500 turbine. The ICR engine, with a 29,000-horsepower nominal power output, will weigh in at about 120,000 pounds.
The compression process in the WR-21 engine is split approximately 30:70 between the intermediate-pressure (IC) compressor and the high-pressure (HP) compressor, with intercooling in between. The intercooler reduces the temperature of air entering the high-pressure compressor, reducing the work necessary to compress the air, which improves the HP spool efficiency and raises the net output power. The intercooler also reduces the HP compressor discharge temperature, which increases the effectiveness of the recuperator due to an increased air to-gas temperature differential.
The recuperator preheats the combustion air by recovering waste energy from the exhaust, thereby reducing the amount of fuel required to reach the desired turbine inlet temperature, which in turn cuts the fuel needed to achieve the required cycle temperatures. The result is improved fuel efficiency over the entire range, with a dramatic improvement at low power.
Chiprich said that the use of these advanced-cycle technologies is expected to allow the WR-21 to cut fuel use by 17 percent at maximum power, by 25 to 30 percent at 40 percent full power, by 30 percent at 30 percent full power, and by 40 to 60 percent in the 10-percent power range.
Intake air enters the WR-21 ICR engine's air inlet, which is a low-loss radial design of composite construction. The air then moves into the first compressor (called the intermediate pressure compressor), which is derived from a Rolls-Royce 211-535 flight engine but features certain modifications to the• casing to improve inspection and maintenance access. Downstream is the on-engine portion of the intercooler, whose counter flow plate-fin heat exchanger works something like a car intercooler. Next comes the high-pressure compressor, which is adapted from a Rolls-Royce 211-535 engine, a compact six-stage fixed-geometry axial flow compressor with a compression ratio of 4.9. Air from the HP compressor is ducted to the recuperator.
The system's combustor section is based on Rolls-Royce Spey engine technology. "It operates in a manner similar to the dry, 10w-NOx section of an industrial Rolls-Royce 211 ," Chiprich noted. The can combustor design is oriented perpendicular to the center line of the shaft to provide sufficient volume for maintaining the standard bearing spacing, he said.
The high-pressure power turbine (single-stage) is derived from the Rolls-Royce 211-524 engine. The intermediate-pressure power turbine (single-stage) was modified from a Rolls-Royce 211-535 engine.
At the point, a single stage of variable- area nozzles closes progressively with diminishing power to reduce the power turbine mass flow rate. Thus, for a given partial load, high cycle temperatures are maintained. This technique increases the recuperator air-to-gas temperature differential and the associated recuperator heat transfer, which results in an improvement in partial-load specific fuel consumption.
The exhaust is dumped into a low-loss exhaust collector system that provides uniform flow to the recuperator, which is based on counterflow platefin heat exchanger technology.
The WR-21 features a full-authority digital control system that is fully redundant. The system's enclosure includes thermal- and noise-containment measures as well as removable panels for better maintainability.
Successful light-off of the first engine occurred in July 1994. The ICR turbine passed a key 500-hour endurance test in September 1997. "Right now we're at the eighth development build of an 11-build plan," Chiprich reported. "All the enabling technologies have been proofed out." Initial orders could be taken in 1999, said Ulliman.
Chiprich said the WR-21 "could be a power module for a future electric drive system." The design is suited to modern integrated full-electric-propulsion concepts, which are expected to replace traditional warship mechanical transmissions in the relatively near future. Just last February, the National Research Council's advisory Committee on Technology for Future Naval Forces recommended development of all-electric ships and associated drives, power conditioning, and distribution systems.
Though the all-electric concept is far from new, having been applied previously to merchant vessels, the technology is looking better of late. The NRC panel stated that gas turbine propulsion units, modular rare-earth permanent magnetic motors, and power control module technologies have matured to the point that all-electric ships appear feasible. The technology cited "unique advantages" in reduced volume, modular flexible propulsion, lower acoustic signature (elimination of gear noise), enhanced survivability, high propeller torque at low speed, and inherent reversing capability. The result would be a submarine-type propulsion design with diesel-like fuel consumption.