Jörg Meyer describes how a compact exhaust gas system has been developed for a test rig.
Whether mounted on aircraft or in stationary applications, gas turbines are not immune to the demand for CO2-neutral drives associated with climate change. Over recent years, gas turbine manufacturers have been working to reduce CO2 emissions from the combustion process, with the long-term aim of developing a CO2-neutral thermal turbomachine. Accordingly, development work on machines like these focuses on the combustion chamber. That in turn makes high-pressure combustion chamber test rigs a valuable resource for experimental investigations. These test rigs need to be designed with two particular considerations in mind. Firstly, they have to give users the opportunity to validate pressure and exhaust gas temperature values under full-load conditions. Secondly, they must satisfy statutory requirements for acoustic emissions at the installation site. This is where the test rig’s exhaust system plays a crucial role.
An Untenable Situation
The German Aerospace Center (DLR) operates a number of high-pressure combustion chamber test rigs for various application scenarios. Due to increased demands on gas turbine design, the existing exhaust system of high-pressure combustion chamber test rig 2 (HBK 2) was no longer able to withstand the mechanical and thermal stresses associated with the exhaust flow. After each test cycle, stress fractures would be found in the guide vanes of the exhaust system, requiring extensive repairs to enable the next-generation technology to be validated in time. On both technical and financial grounds, the situation was simply untenable in the long term.
In 2019, the DLR took the decision to rebuild the exhaust system of HBK 2. DLR awarded the contract for developing and installing the new system, complete with silencer and flow redirection mechanism, to G+H Schallschutz. The company had already demonstrated its capabilities in the same area back in 2014, and its experts also developed and supplied the exhaust gas stack and silencer for the larger test rig known as HBK 5. However, this latest order came with some special challenges. Firstly, the timeframe for the project was extremely tight. Work had to be completed by the end of the year, which meant taking care of everything in the space of five and a half months. Secondly, the new silencer was not to reach any higher than the mouth of the old model and the existing foundations had to be left as they were.
New Concept Developed With CFD Simulation
“The fundamental technical challenge for the new exhaust system concept was how to deal with the exceptionally inhomogeneous exhaust gas flow and convert it to a flow profile that would be compatible with the silencer,” recalls Paul Karle, the project manager at G+H. The concept was ironed out in close coordination with the customer and using computational fluid dynamics (CFD). DLR had made a number of stipulations for the design. Among these was a requirement that the exhaust system and silencer of HBK 2 must be designed for core exhaust streams at flow rates of up to 1,000m/s and temperatures in excess of 1,300°C at the outlet of the combustion chamber choke. Demanding acoustic requirements also had to be met.
A New Approach
One of the key problems when working with such a compact test rig was how to redirect the exhaust flow 90° from the horizontal duct into the stack. “From the outset, we knew we would have to develop an alternative concept for redirecting the flow 90° if we were to ensure an even flow against the silencer panels in the stack,” says Karle. G+H therefore worked with DLR to develop a prototype exhaust routing system tailored specifically to the test rig that was able to achieve an exceptionally homogeneous flow profile after redirection. Karle explains: “The flow rate, the flow distribution and the temperatures generated in the combustion chamber place exceptional demands on the materials used. The knowledge we have acquired by manufacturing this prototype will be a valuable asset for the development of future bespoke solutions for our customers.”
Outperforming The Noise Emission Requirements
Ultimately, Karle’s team overcame all the challenges. Besides satisfying the construction-based requirements of the order, the team also completed the project on time. In fact, the new plant even outperforms the operational parameters that were stipulated in the order, keeping noise emissions at the stack opening well below the maximum permissible limit.
Karle is particularly pleased his company has been able to play its part in the future of gas turbine technology. After all, DLR uses HBK 2 as a research platform for developing combustion chamber technologies to be used in next-generation gas turbines. Karle comments: “Lots of people have said that the days of gas turbines are over, but that’s just not true.” His reasoning is rooted in the “future fuels” that are coming online, such as hydrogen and synthetic fuels, which could be used to considerably reduce environmentally harmful emissions from gas turbines. “Thanks to low-emission and even entirely emission-free usage, and against the backdrop of renewable energy sources, the powerhouse that is the gas turbine is taking on a new meaning in power plant technology,” points out Karle. With renovations potentially having to be made to thousands of gas turbines in power plants around the world, he can see a whole new field of activity opening up for his noise control team.
Jörg Meyer is with G+H Noise Control