Pressure gain combustion

Stuck in a rut

Gas turbines are stuck in a rut. These reliable workhorses rose to power at the end of the second world war and never really looked back, but they are a victim of their own success. As every engineer knows, you don’t fix what isn’t broken, and so the industry hasn’t – for almost 100 years.

Sure, gas turbine technology has improved in that time: we’ve added in interstage cooling and waste heat recovery systems, we’ve used new materials to allow us to increase cycle temperatures, and advances in computational methods have helped us to refine our designs. However, one thing remains untouched – the fundamental cycle itself – and as a result performance has plateaued. Achieving a step change in gas turbine performance is going to need us to take a step back and re-think our approach, and we may well have to challenge some of the well-established norms of gas turbine design.

What is pressure gain combustion?

If you carry out an exergy analysis of a gas turbine you quickly find that the greatest losses in thermodynamic availability occur across the combustion chamber. It follows that, if we want to see a step change in gas turbine performance, the combustion chamber is the place to start, and pressure gain combustion is a technology doing just that. Pressure gain combustion does what is says on the tin – achieves a pressure gain across the combustion process, rather than the pressure loss realised in more traditional ‘constant pressure’ combustors. As a result, the entropy rise across the combustion process is reduced and the turbine is able to extract more work.

There are a number of ways that this can be achieved in practice, but all rely on constraining the gas during combustion – moving more towards a constant volume combustion process, rather than a constant pressure one. A major consequence of this is that the combustion must be unsteady, often pulsing like the exhaust from an internal combustion engine. This is a major challenge to gas turbine integration and one that we’ll discuss later. However, the potential benefits of pressure gain combustion on cycle performance are significant. Although the predicted gains vary, depending on the gas turbine cycle and modelling assumptions, most authors would agree that fuel reductions of 5 to 10 percentage points are (theoretically) feasible.

The history of pressure gain

You would be forgiven for thinking that pressure gain combustion is brand new technology – it isn’t, it has been around almost as long as the gas turbine itself. In the early 20^th century, when gas turbines were in their infancy, achieving a gas turbine cycle that actually produced net power was a significant challenge. This was mainly due to the fact that, at the time, compressors and turbines were horribly inefficient. Inventor Hans Holzwarth got around this problem by ditching the compressor entirely, instead using intermittent combustion in ‘explosion chambers’ to generate the pressure rise needed to drive the turbine.

Another classic pressure gain combustion gas turbine design surfaced in the form of the Napier Nomad I (pictured left). This aeroengine comprised an axial compressor, a centrifugal compressor, a two-stroke 12-cylinder diesel engine, an additional constant pressure combustion chamber and two separate axial turbines. Shaft power was extracted from both the reciprocating engine and the turbines, distinguishing it from a turbo-changed internal combustion engine. The engine achieved unprecedented performance, but was ultimately let down by reliability issues stemming from its mechanical complexity.

Impact on turbomachinery design

So why isn’t pressure gain combustion already an established technology? In short, this is due to the difficulty of integrating a pressure gain combustor into a gas turbine cycle. In particular, the unsteady nature of combustion poses a range of challenges, and in the early days of gas turbine development it was much easier to use a constant pressure combustor and simply improve the compressor and turbine efficiencies. The problem now is that it’s becoming more and more expensive to make even modest incremental gains. So, if we want to see a step change in gas turbine performance, maybe it’s time we started to tackle some of those challenges, to allow us to realise the potential benefits of pressure gain combustion.

Perhaps the most obvious challenge (to any fluid dynamicist at least) is the impact that the unsteady combustor exhaust flow can have on the turbine performance. However, while this is definitely a consideration, it’s actually one of the least urgent factors. Mainly because the potential benefits of pressure gain combustion are so significant, that we could happily accept a lower turbine performance and still realise a considerable net benefit to the cycle performance.

Pressure gain combustion may also have implications for the design of any passive cooling flows, where air is bled from the compressor to the turbine. If a pressure gain is introduced across the combustor, then passive cooling many no longer be possible and additional compressor stages may be required to deliver the cooling flows. However, it is the mechanical aspects of gas turbine design that perhaps pose the biggest challenge for pressure gain combustion. Having spent the last 100 years carefully trying to remove all sources of unsteadiness from the gas turbine, to minimise any mechanical vibration, pressure gain combustion looks to install a fairly significant source of unsteadiness right in the middle of the gas turbine. This vibration risk will need to be mitigated and this will undoubtedly have a significant impact on the mechanical design of the gas turbine.

Route to market

Integration issues also pose an interesting challenge to bringing pressure gain combustion technology to market. Most research tends to focus on the benefits that the technology could have for aeroengines. However, the additional space and weight restrictions for these engines are likely to put manufacturers off seriously considering pressure gain combustion technology until it has matured. Instead, we will have to look to other gas turbine applications for our early adopters. The land-based power generation gas turbine market may be the answer here. In some senses, this market has less to gain from pressure gain combustion (they can already realise efficiency gains through established waste heat recovery technologies for example); however, the relaxed space and weight requirements of this market should make it quicker and cheaper to address the integration challenges. The expectation is that, once these challenges have been addressed in this market, the technology will start to filter through to other gas turbine applications.

Starting the journey to a step change

In summary, pressure gain combustion is a technology with significant potential benefits that could give rise to a step change in gas turbine performance. However, to date, most research has focused on improving the performance of the pressure gain combustion designs themselves, and relatively little has been done to address the challenges of gas turbine integration. If we want to realise the potential benefits of pressure gain combustion technology, we will need to step back and question our established thinking to drive forward innovative changes for the future.