No ’Holes’ Barred from Twin-Rod Nd:YAG Solution

Daniel C. McCarthy

Air cools most of today's non-flight industrial gas turbines using the effusion-cooled design approach. This process requires thousands of tiny holes drilled through the combustor skin to allow in-flowing air to help control temperature.

Solar Turbines Inc. was an early adopter of laser technology for its turbine components, applying lasers in a trepan approach to drill up to 5000 holes perpendicular to the surface. But the demand for combustors with greater fuel efficiency and lower emissions led to designs requiring more holes with smaller diameters to be drilled at various angles. Besides improving efficiency and emissions, this would reduce the number of detail parts added to combustors and improve overall durability.

Percussion drilling would have provided Solar Turbines with the speed, hole diameters and control that it needed to manufacture the 30-odd parts of its new combustor design, but the technique required laser pulses with much greater power density. The lasers it had been using for trepan drilling integrated a single rod in a ceramic cavity with two flashlamps. These instruments would have required 8 to 10 seconds to drill a hole angled at 20°.

Therefore, it would not be cost-effective for the company to manufacture combustor designs that require drilling 30,000 holes in some parts.

Both combustion and cooling in today's gas turbines rely on air flowing through tiny holes drilled in the combustor skin. Twin-rod Nd:YAG laser technology delivered the power and speed to create 30,000 small-diameter holes required in one manufacturer's combustor parts. Courtesy of GSI Lumonics Inc.

"The new system had to at least be able to produce combustors at an equal cost," said Jim Sperling, the company's manager of manufacturing technology. "It became a manufacturing challenge to economically produce the combustor that our customers needed."

Solar Turbines teamed up with GSI Lumonics Inc. to develop a system that incorporated twin-rod Nd:YAG lasers producing 400 W of average power. The instruments delivered 60 J of peak power -- three times more than earlier lasers. This significantly increased the pace of production, drilling one hole per second, and actually reduced the cost per hole.

The smaller, denser hole pattern also reduced the carbon monoxide and hydrocarbon emissions.