Category: Aerospace & Space Exploration

Turbomachinery Evolution through Generative Design

Posted by Clement Joly

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

Posted by Sascha Podlech

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

Posted by Alexander Yudin

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

Posted by Alexander Yudin

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

How Work Distribution Between the Stages of an Axial Multistage Compressor Affect Compressor Characteristics

Posted by Oleh Dehtyarov

One of the important factors affecting compressor performance is the distribution of work between compressor stages.

There can be many different axial compressors with the same design point value for efficiency, airflow and pressure ratio and all of these compressors will meet the required parameters at the design point. Compressors operate in a variety of modes. Many compressors are equipped with a turning inlet guide vane (IGV) or guide vane (GV). At the same time, the question of the number of stages with a turning IGV or GV, at what angle to turn them, and whether the maximum efficiency will be the same remains unresolved. To answer this, designers need to compare the impact of the distribution of work between compressor stages at the design point on the compressor characteristic. This allows you to choose the form of work distribution to achieve the specified parameters of the compressor in off-design modes of operation. This also allows you to obtain the required degree of pressure ratio and airflow at a given frequency of rotation of the compressor rotor. This analysis is necessary for jet engines since the compressor works in conjunction with the combustion chamber, turbine, and nozzle. This can also be carried out for other applications of multistage compressors.

With the help of AxSTREAM and ION, studies were carried out on multistage compressors with various shapes of the flow path, a different number of stages, and a different distribution of work among the stages.

Figure 1. Calculated typical characteristics of a multi-stage, variable compressor.

The problem is considered in the following formulation:

Influencing factor:

  • Distribution of work on compressor stages.

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Variable parameter:

  • Compressor operating mode.

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Axial compressor type:

  • With fixed IGV or GV.
  • With turn of IGV or GV.

­

Additional task for the compressor with turn IGV or GV:

  • Develop a criterion for the required number of compressor stages with restagger angle of the blades IGV or GV.

­

The compressor is taken from the article sourced at the end of this post Read More

Rocket Nozzle Cooling

Posted by Viktor Yevlakhov

It is impossible to imagine the design of any rocket engine without a nozzle – a technical device that serves to accelerate gas flow passing through it to speeds exceeding the speed of sound. The main types of nozzle profiles are shown in Figure 1.

 

Figure 1 – Nozzle Types
Figure 1 – Nozzle Types. Source 

The most widely used are Laval nozzles due to their high efficiency in accelerating gas flow. Read More

Rotor Dynamics Study of 4-Stage Compressor – from Theory to Application

Posted by Volodymyr Martynenko

Rotating machines have huge and important roles in our daily life although we may rarely think about them. Steam turbines at electrical power plants rotate the electrical generator shafts which produce electricity coming into our homes and offices. Driving to or from work, the reciprocating cycle in your vehicle’s internal combustion engine results in rotation of the transmission and the wheels of vehicles, while the electric car wheel operation is a result of induction motor rotation. If you get on an airplane, rotation of the turbo reactive gas turbine engine produces the effective thrust to sustain flight by moving, compressing and throwing the gas behind the plane. We can even find the useful effects of rotation in our kitchens when we are blending the food or washing our closes.

Although these rotating machines are different, the approaches to modelling their rotor dynamics are pretty much the same, since similar processes occur in rotating parts which differ in their vibrations from the non-rotating machines.

Do you remember the example of rotating washing machine? Have you ever seen it jumping on the floor trying to squeeze out your closet? We bet you have. This is the simplest example of the increased unbalance affecting the amplitudes of machine vibrations. Washing machines are designed to experience these noticeable vibrations during their operation without breaking. But the steam turbine or compressor rotors which have the tight clearances between the impellers and the casing can not boast of that leeway. In addition to that, the excessive vibrations significantly influence the machine’s useful life due to the increased fatigue.
This is why the rotor dynamics predictions are one of the most important parts of rotating machine analyses. And although they may seem easier than comprehensive stress-strain investigations of machine components, in some cases the rotor dynamics analysis can be trickiest part.

Usually, the rotor dynamics analyses are divided into lateral and torsional stages depending on the nature of rotor response to be used. They are discussed in different types of standards (API [1], ISO [2], etc.). Let’s consider the example of the lateral vibrations of a 4 stage compressor rotor with an operational speed of 8856 rpm.

Fig. 1 - 4 Stage Compressor Rotor
Fig. 1 – 4 Stage Compressor Rotor

This rotor rotates in the 4 pad tilting, pad oil film journal bearings. The characteristics of these bearings should be determined carefully to ensure that there will not be an excessive wear, heat generation or friction in them. Read More

Inducer Design Considerations and its Effect on Turbopump Cavitation

Posted by Ben Conser

Why Use an Inducer?

Suppose you want to build a turbopump to increase the pressure of your working fluid. However, you find that the fluid that you are working with keeps vaporizing in the impellers you design, causing all sorts of performance issues. What can you do in this case? One solution is to design an inducer for your turbopump.

Axial inducers are used in turbopumps upstream of the impeller to avoid cavitation, reduce the inlet pressure requirement, and/or allow for operation at higher turbopump rotational speeds for a given inlet pressure [1]. For a turbopump to function properly, the inlet pressure of the pump must be high enough to avoid cavitation. Cavitation is a phenomenon where vapor bubbles which form in the flowing fluid collapse suddenly – potentially causing surface damage of the impeller, performance degradation, as well as catastrophic failure.

The cavitation phenomenon can be visualized in the below image. The inlet flow (flowing from the left side of the image) hits a blunt body in the fluid channel. This causes the pressure to locally drop and vapor bubbles to form. As the fluid continues to flow (towards the right side of the image), the vapor bubbles collapse once the fluid pressure has sufficiently increased.

Formation of vapor bubbles in cavitating fluid flow
Figure 1: Formation of vapor bubbles in cavitating fluid flow

Now that we understand the problem, how can we make sure these cavitation effects won’t happen in our pump? To predict when cavitation will occur, two parameters are commonly used. The available net positive suction head (NPSHa) describes how much greater the local inlet static pressure is relative to the local inlet vapor pressure. Essentially, NPSHa indicates whether the conditions for cavitation to occur are met. The required net positive suction head (NPSHr) describes the inlet head corresponding to a certain drop in performance capability. A typical NPSHr parameter is the standard 3% NPSH (NPSH3) which describes the inlet pressure corresponding to a 3% drop in head rise capability of the pump at a particular flow rate. Generally, NPSHr is measuring whether there is enough cavitation present to cause a noticeable decrease in the pump’s performance. If the NPSHr is much greater than NPSHa, then significant performance decreases due to cavitation may occur. That is to say, when the available net positive suction head is insufficient, bad things can happen, ranging from performance degradation to outright damage and failure.

Read More

Drilled Nozzle Application in Supersonic Turbines

Posted by Artem Babaiev

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

Turbo-compressor Technologies for Aviation Fuel Cell Systems: Operational Requirements and Development Trends

Posted by Eugenio Rossi

Introduction

Fuel cells are an important driver in the current energy system landscape with significant impact on the technology base and economic growth. Global fuel cell system shipments saw a 10% increase in 2020, totaling 1.3GW. The transport sector continues to lead with a growth of 25% on the number of units shipped globally.

The recent years have seen the launch of many projects aimed at the development of fuel cell systems for aviation powerplants. In this context, the effective integration of turbomachinery components is key in driving the overall performance and the economic viability of this technology. These aspects are the topic of this blog.

Fuel Cell Technology

Fuel cells are devices which convert the chemical energy of a fuel directly into electricity by electrochemical reactions. A fuel cell element has a matching pair of electrodes (anode and cathode) separated by an electrolyte. An appropriate flow of fuel (e.g. hydrogen) and oxidizer (frequently oxygen) is delivered to the electrodes: the resulting reaction produces electricity and water plus an amount of heat. The simplicity of this process is shown in Figure 1.

Fuel Cell Conceptual Scheme
Figure 1. Fuel Cell Conceptual Scheme (Source).

There are many advantages: efficiency, reliability, low noise, and compactness, all while implementing an environmentally progressive solution. The application potential is also very diversified, sometimes in very critical fields.

Read More