Turbomachinery Evolution through Generative Design

As human-beings, our differences are what makes us unique (if I may quote the Seek Discomfort crew – “What makes you different is what makes you beautiful”). For turbomachines, this sentiment also rings true.  We design different turbomachines because we have varied roles, needs and constraints for them. To that effect, there is no universally best turbine, compressor, or pump. Therefore, figuring out which set of “skills” a turbomachine should have is the key role of a design engineer so that they may effectively capture and estimate performances of the machine they will work on early on while having the certitude this is the best that can be done.

Generative design is one of these recent buzzwords that characterizes an approach to the design of components (or systems) that has been around for quite some time already. Rather than producing one geometry for one value of each input (such as boundary conditions, flow coefficients, number of stages, etc.), generative design allows you to create thousands of designs within minutes that you can review, compare, and filter to select the one that best suits your needs. Let’s look at an example of an axial turbine design process comparing traditional preliminary design vs. generative design.

Approach 1 or what most companies call Traditional Preliminary Design,  is to look in textbooks and previous examples of what a given turbine for that application “should” look like. It may involve things like using Ns-Ds diagrams, load-to-flow diagrams, blade speed ratio vs. isentropic velocity ratio correlations, scaling/trimming existing designs, etc. These have served their purpose well enough, but they have their limitations which make them fairly challenging to really innovate. Such limitations include previous experience/data being restricted to a given fluid, relative clearance size, given configuration, lack of secondary flows, etc. A summary of a traditional preliminary design workflow (familiar to too many engineers) is presented below.

Summary of traditional preliminary design workflow
Figure 1 Summary of traditional preliminary design workflow

Now, we know that changing (ahem, improving) your workflow is not always easy. But growth happens through discomfort and switching to a generative design approach does NOT mean rebuilding everything your team has done in the past. What it effectively gives you is the confidence that the input parameters you finalized will provide not only the desired performance but the best ones that can be achieved (and it saves time too…a lot of time). From there, you can use these inputs in your current design software or you can continue the design process in our design platform, AxSTREAM® (meaning you can add generative design capabilities upstream of your existing workflow or replace parts/all of that workflow depending on what makes the most sense for you). You can pay your engineers to do engineering work, instead of visiting online libraries and guessing input parameters in hope they will find the needle in the haystack. Or, with generative design, you kind of look for haystacks and shake them until the needle falls off.

So, how does this work in AxSTREAM, you may ask? Very well, I may reply :D. Read More

Challenges in Aero Engine Performance Modeling

As is the case with every machinery, manufacturers want to improve their products. This is especially true for aero engines, where even a small improvement in fuel consumption can lead to an advantage on the market. But with any type of propulsion equipment, regulations also play an important role, specifically that certain noise levels or CO¬¬2 emissions should not be exceeded. These factors combine to make the process of developing and manufacturing an aero engine anything but simple. In today’s blog, we’ll take a look at these challenges in more detail and briefly touch upon development strategies to account for such challenges.

PW 1000G
Figure 1: BERLIN – APRIL 26, 2018: The stand of MTU Aero Engines and high-bypass geared turbofan engine family Pratt & Whitney PW1000G. Exhibition ILA Berlin Air Show 2018.

In general, engineers have two options to develop a better engine. The first is to create a completely new design, like implementing a geared turbofan, which takes a lot of time and research. An example of this is the PW1000G engine from Pratt and Whitney, which was in development during the late 1990s and had its first flight test in 2008 [1]. This approach is less common which is reflected by other manufacturers who are backing down from the idea of using geared turbofans due to weight and reliability concerns [2]. The second option and this is the common method, is to gradually improve existing engines. This however brings new challenges, because simply improving one engine component does not necessarily mean that part of the machine will work well together with the rest of the machine. Furthermore, the design process for aero engines is very time-consuming. A general overview is shown in Figure 1. The process starts with an assumption for certain performance characteristics, for example, efficiencies for a compressor. After that, a cycle analysis is performed where the design point and off-design behavior are determined. With the newly gained information, the design process of the single component takes place. Upon successful creation of the component which satisfies all requirements, the process moves to the test phase. In this phase, the designed machine will be evaluated through experimental testing or intensive CFD studies. Modifications will be made if necessary to reach the desired operating conditions. Since changes were made to the geometry, these changes need to be investigated in an additional cycle analysis to understand how they will affect the overall engine performance. This process repeats until a converged solution is found. Read More

The Evolution of Gas Turbines From the First Designs to the Latest Environmentally Friendly Development Trends: Part 2

Part 1

Gas Turbine Development Trends: Hydrogen Energy

Recent world trends related to the development of clean energy have led to an increased focus on the use of hydrogen as a cleaner fuel for gas turbines and with it, the need to develop gas turbine plants that can operate both on a mixture of hydrogen with natural gas and on pure hydrogen. The use of hydrogen as a fuel can significantly reduce COx emissions, but burning hydrogen with air increases the amount of nitrogen oxides NOx, therefore leading gas turbine manufacturers have made great efforts over the past decades to develop low NOx combustion technologies that can provide a high proportion of hydrogen content in the fuel, up to 100%.

Heavy Duty Gas Turbine, GT 26
Figure 7 Heavy Duty Gas Turbine, GT 26

In a modern gas turbine in a premixed combustor, operating conditions close to the lean-burn flammability limit are chosen to reduce oxides of nitrogen (NOx), where the lean-burn flammability limit is determined by whether or not a flame is ejected. The flame is blown off under the condition that the speed of the combustible mixture entering the combustion chamber is greater than the speed of the flame. The flame speed is highly dependent on the composition of the fuel, and in the case of hydrogen, the turbulent flame speed is known to be at least 10 times higher than that of a methane flame under gas turbine combustion chamber conditions due to its high diffusion and chemical reaction rate. In the case of gas turbine combustion chambers for power generation using natural gas, lean-burn combustion technology is mainly applied to reduce NOx (since NOx is exponentially dependent on the temperature in the combustion region), while gas turbines using fuel containing hydrogen (syngas ), are prone to flashback (flame speed is much higher than the speed of the incoming fuel mixture so that the flame moves back towards the entrance to the combustion chamber and nozzles). Previously, in such cases, combustion chambers without premixing were used to avoid the risk of damage and destruction of the nozzles and the entire system. In this case, a technique is applied that involves the injection of a large amount of steam or nitrogen to minimize the increase in NOx, but this, in turn, leads to a decrease in the temperature at the turbine inlet. Thus, for the latest hydrogen-fuelled gas turbines, leading manufacturers around the world have begun to develop special combustion technologies with pre-mixing or with special micro-mixers. Read More

The Evolution of Gas Turbines From the First Designs to the Latest Environmentally Friendly Development Trends: Part 1

Part 2

Gas turbines have a rich history and play a key role in many of the modern-day technology we rely on. Welcome to part 1 of this blog where we’ll look at the history and evolution of gas turbines and don’t forget to join us for part 2 (next week) which will take a deeper look into hydrogen energy and where these machines are headed. 

The First Industrial Gas Turbines

Gas turbines are unique in many respects. First, they are among the most ancient turbomachines in their idea (approximately the 15th century) and at the same time, quite young in terms of practical implementation (the turn of the 19th–20th centuries).

Prototypes of gas turbines, which included the so-called smoke machines, began surfacing as early as the 17th century. However, the starting point in the development of gas turbines can be considered to have taken place in 1791 when Englishman John Barber filed an application for a heat engine patent.[1].  The turbine was equipped with a chain-driven reciprocating compressor and had a combustor and turbine. Barber proposed the use of charcoal, gas, or other suitable fuels to produce inflammable gas. The gas from the producer went into a common receiver and then into the combustion chamber where it mixed with compressor air and was ignited. The resulting hot gasses were allowed to impose on a turbine wheel.  To prevent overheating of the turbine parts, provisions to cool the gas by means of water injection were incorporated. There is no record of this engine being built but, in any event, it is unlikely that it would have self-sustained because of the large power requirements of the reciprocating compressor.  A patent drawing of Barber’s device is shown in Figure 1 [2].

Figure 1 John Barber’s Gas Turbine. English Patent 1791 [2]
In 1872, Franz Stolze designed an engine with an axial compressor, an axial turbine on the same shaft, a heat exchanger, a gas producer, and a combustion chamber. The gas turbine unit (Figure 2) was created and designed to produce 200 hp at a speed of 2000 rpm. However, the tests were not successful and instead only produced 20 hp. [1] Read More

Waste Heat Recovery

During industrial processes, an estimated 20 to 50% of the supplied energy is lost, i.e., by dumping the exhaust gas into the environment [1]. The waste heat losses and the potential work output based on different processes including but not limited to the ones shown in Figure 1. Does it REALLY have to be thrown away? Sometimes yes, other times no. In this blog post, we will focus on the “no” through a process  called “Waste Heat Recovery”.

Waste heat losses and work potential of different process exhaust gases - Image 1
Figure 1: Waste heat losses and work potential of different process exhaust gases [US Department of Energy [2]]
Some well-known examples of waste heat recovery processes are found in turbochargers in cars or a heat recovery steam generator. One simple structure of application is when a heat exchanger is fed with the exhaust gas of a turbine, therefore being cooled down before being released into the air. This heat exchanger is part of a secondary (bottoming) cycle where another turbine provides additional power output without having to burn additional fuel. This heat exchanger is part of a secondary cycle where another turbine provides additional power output. Read More

Evolution of Reverse Engineering

Introduction

In today’s intensely competitive global market, product enterprises are constantly seeking new ways to shorten lead times for new product developments that meet all customer expectations. In general, product enterprise has invested in CAD/CAM, rapid prototyping, and a range of new technologies that provide business benefits. Nowadays, reverse engineering (RE) is considered one of the technologies that provide business benefits by shortening the product development cycle [1]. Figure 1, shows how reverse engineering can close the gap between what is “as designed” and what is “actually manufactured” [1].

Product Development
Figure 1. Product Development Cycle. SOURCE: : [1]
Reverse engineering (RE) is now recognized as an important factor in the product design process which highlights inverse methods, deduction and discovery in design. In mechanical engineering, RE has evolved from capturing technical product data, and initiating the manual redesign procedure while enabling efficient concurrency benchmarking into a more elaborated process based on advanced computational models and modern digitizing technologies [2]. Today the application of RE is used to produce 3D digital models of various mechanical worn or broken parts. The main steps in any reverse engineering procedure are: sensing the geometry of the existing object; creating a 3D model; and manufacturing by using an appropriate CAD/CAM system [2]. Read More

Drilled Nozzle Application in Supersonic Turbines

Most designers associate drilled nozzles in turbomachinery with something exotic, uncharted, and specific only to a minuscule amount of high-loaded turbines operating with a high-pressure drop. Meanwhile, many engineers are not aware that this nozzle design has been applied since the very first turbomachines.

Karl Gustaf Patrik de Laval patented a turbine with asymmetric convergent-divergent nozzles in 1888. At that time the shape of the nozzle allowed him to reach more effective kinetic energy transformation and have an entirely new level of turbine performance.

Figure 1 - Laval Turbine with Drilled Nozzles
Figure 1 – Laval Turbine with Drilled Nozzles. Source

Over a hundred years later, drilled nozzles (or asymmetric nozzles, Laval’s nozzles) have been extensively used in rocket engines, flying vehicles, driving turbines, ORC turbines, and other units for which low cost and weight-dimension constraints play an important role.

Despite the wide application range of turbines with these nozzles, each has its own specific features.

Drilled Nozzles

The main characteristics of drilled nozzles in a turbine (Fig. 2) are the partial admission input, high heat drop per first stage, low reaction, and a low number of stages.

Fig. 2 - Turbine with Drilled Nozzles Flow Path in AxSTREAM
Fig. 2 – Turbine (with Drilled Nozzles) Flow Path in AxSTREAM®

For these turbines, the most critical point during the design process is the first nozzle design. The first supersonic nozzle provides the throughput of the turbine. The main kinetic energy transformation and the main portion of the available isentropic heat drop relates to the first nozzles. As a result, the Mach number at the outlet section of nozzles can reach 3.0 and even be higher. To operate in such regimes, the convergent-divergent vane channels are preferable. Read more

Enjoy Tuning Your Simulations? LOOK AWAY!

This is an excerpt from the Siemens Blog. You can read the full version here.

Originally written By Chad Custer  on March 11, 2020

Turbomachinery CFD in STAR-CCM+

We live in the day of automatic. It seems like every day there is a new task that can be handled automatically. Smart thermostats make sure the house is comfortable when returning from work. Home lighting themes customize an ambiance based on who is home and the time of day. Automatic delivery of your household goods ensures you’re never out of toothpaste. The automatic potato peeler saves you from having to peel potatoes by hand…ok, well maybe that’s going too far. The point is that technology is great for taking care of tasks based on rules or schedules so that you can focus on more important things.

The pervasiveness of automation makes it even more annoying if once you get to work and sit down to run a simulation, you need to fine-tune settings and fiddle with parameters. Or, even worse, if it’s necessary to babysit a challenging case to make sure it runs as expected. Now, there likely was a time in your professional life when diving head-first into the details of flow solvers was interesting and, dare I say, fun. I know it was for me and if you’re reading this blog, then I bet it was for you too 😊. In fact, when it’s possible to set aside time to dig into some new and different case, it’s still fun to research the best methods to use and how to apply them.

Daily work is different, though. Timelines are always shorter. More data is always needed. Your attention is always split between half-a-dozen urgent tasks, not to mention the ever-growing list of work that you keep meaning to get to once you have time. Forget tuning simulation methods for each case, you need to set up the case once knowing that it will run quickly, reliably and give accurate answers. Read More

An Introduction to Shock Waves

When you think of shock waves, I would wager that you picture a supersonic jet zooming past overhead. Or maybe you have experienced the famous (or infamous) “sonic boom” that accompanies shock waves attached to airplane engines. The engineering challenges associated with the often-troublesome behavior of shock waves is present in all scales, from carefully designing the bodywork of the aforementioned fighter jets, to the equally intricate details of flow passages and blade design in turbomachinery. The first step in taking into account the effect of shock waves is to understand what they are. In this post we will be reviewing a short introduction into what shock waves are and a few applications where they might be relevant.

Figure 1: Schlieren image showing the shock waves of a supersonic jet
Figure 1: Schlieren image showing the shock waves of a supersonic jet. Source

What are shock waves?

Shockwaves are non-isentropic pressure perturbations of finite amplitude and from the second law of thermodynamics we can say that shockwaves only form when the Mach number of the flow is larger than 1. We can distinguish between normal shocks and oblique shocks. In normal shocks, total temperature is constant across the shock, total pressure decreases and static temperature and pressure both increase. Across oblique shocks, flow direction changes in addition to pressure rise and velocity decrease. Read More

APPLICATION OF DIGITAL TWIN CONCEPT FOR SUPERCRITICAL CO2 OFF-DESIGN PERFORMANCE AND OPERATION ANALYSES

This is an excerpt from a technical paper, presented at the ASME Turbo Expo 2020 online conference and written by Leonid Moroz, Maksym Burlaka, Tishun Zhang, and Olga Altukhova. Follow the link at the end of the post to read the full study! 

Introduction

The attempts to simulate transient and steady-state sCO2 cycles off-design performance were performed by numerous authors [1], [2], [3], [4], and [5]. Some of them studied the dynamic behavior of regulators, some studied different control strategies or off-design behavior in different scenarios, which definitely has certain utility in the development of the reliable technology of sCO2 cycle simulation. Nevertheless, they used rather simplified models of components, especially turbomachinery and heat exchangers, which are of crucial importance to correctly simulate cycle performance.

The authors of this paper attempted to apply the digital twin concept to a simulation of off-design and part-load modes of the sCO2 bottoming cycle considering real machine characteristics and performance, which nobody tried to apply in this area.

On IGTC Japan 2015, SoftInWay Inc. has published a paper “Evaluation of Gas Turbine Exhaust Heat Recovery Utilizing Composite Supercritical CO2 Cycle”. The paper considered combinations of different bottoming sCO2 cycles for a specific middle power gas turbine. It mainly studied the advantages of different types of sCO2 cycles to increase the power production utilizing GTU waste heat.

The present paper is a further study based on that so the Cycle 2 [6] from that previous paper was selected as the sCO2 bottoming PGU layout in the present paper for subsequent analysis. The cycle is a combination of recompression cycle and simple cycle which offers 16.13 MW as output. GE LM6000-PH DLE gas turbine, was used as the heat source for bottoming PGU. According to GE official brochure [7], the GE LM6000 offers 40 MW to over 50 MW with up to 42% efficiency and 99% fleet reliability in a flexible, compact package design for utility, industrial and oil and gas applications. GE LM6000-PH DLE provides 53.26 MW output with exhaust temperature at 471 ℃ and exhaust flow at 138.8 kg/s. (This information came from GE products specification from 2015. It appears that GE continuously modifying the parameters of its turbines along with the naming of different modifications. Therefore, today’s parameters and configuration names might be slightly different comparing to 2015) Exhaust gas pressure was assumed to be 0.15 MPa. These parameters were taken to analyze the bottoming PGU and are presented below in TABLE 1.

SELECTED SET OF GE LM6000-PH DLE PARAMETERS
TABLE 1: SELECTED SET OF GE LM6000-PH DLE PARAMETERS

The digital twin (DT) concept is the developing technology that allows simulation of object behavior during its life cycle or in specified time due to changing ambient conditions, for example. The DT is applicable for performance tuning, digital machine building, healthcare, smart cities, etc [8] that allows decreasing the time and costs of development and optimize the object on the developing stage. GE has raised DT concepts for power plants to continually improves its ability to model and track the state of the plants [9].

In the context of this paper, DT is a simulation system comprised of physicist-based models organized in a special algorithmic structure that allows simulating the behavior of sCO2 PGU under alternating ambient conditions and grid demands.

The DT in this study was created utilizing AxSTREAM® Platform, which includes multiple software tools. The following software tools were utilized in this study: AxCYCLE™ was used to perform cycle thermodynamic calculation; solution generator in AxSTREAM® helped with finding possible machine geometry with given boundary conditions when performing preliminary design for compressors and turbines at design point; parameters and performance of turbomachinery including mass flow rate, pressure, power, efficiencies, etc. were calculated by Meanline/Streamline solver in AxSTREAM® for design and off-design conditions; AxSTREAM NET™ is a 1D system modeling solver and it was introduced here to simulate performance of heat exchangers (HEX) and pressure drop in the pipes involved in the cycle; AxSTREAM ION™ was used to integrate all modules and tools together in one simulation system. Read More