Category: Compressor

Common Design Challenges in Compressed Air Energy Storage Systems

Posted by Vladyslav Kotechenkov

Renewable energy is a topic which has gained significant traction in recent years. Unlike fossil fuels, which are finite and contribute to environmental degradation, renewable energy provides a cleaner, healthier, and more sustainable path forward for meeting our energy needs. Energy storage systems refer to technologies that store energy for later use, allowing for a more flexible and reliable energy supply from renewable sources such as solar and wind.

There are a wide variety of energy storage systems that enhance the power generation capabilities of renewable power plants. The most familiar system may be hydropower, with over 95% of today’s energy grid storage being held by pumped hydropower. When electric demand is low, “turbines pump water to an elevated reservoir using excess electricity. When electricity demand is high, the reservoir opens to allow the retained water to flow through turbines and produce electricity” [5]. Thanks to its performance, pumped hydropower has dominated the energy grid storage market for years. However, other emerging technologies are gaining notoriety, including compressed air energy storage, which will be the topic of today’s blog.

Have you ever wondered what happens to the air when you blow up a balloon? Well, some clever people have figured out how to use that air to store electricity. It’s called compressed air energy storage (CAES), and it’s basically like having a giant balloon underground that you can fill up with air when you have extra electricity and let it out when you need more. Sounds simple, right? Well, not quite. Some challenges are involved, like keeping the air from getting too hot or cold, and making sure it doesn’t leak or explode. But if done right, CAES systems can help us use more renewable energy sources like wind and solar, and reduce our dependence on fossil fuels.

Fig – 1. Green Energy Conservation
Fig – 1. Green Energy Conservation [4]
Compressed air energy storage is a technology that stores excess electricity as compressed air in underground reservoirs or containers. When electricity is needed, the compressed air is heated and expanded to drive a turbine and generate power. CAES can help balance the supply and demand of electricity, especially from intermittent renewable sources like wind and solar. There are two main types of CAES: diabatic and adiabatic. Diabatic CAES dissipates some of the heat generated during compression to the atmosphere and uses natural gas or other fuels to reheat the air before expansion. Adiabatic CAES stores the heat of compression in a thermal storage system and uses it to reheat the air without additional fuel. Adiabatic CAES has higher efficiency and lower emissions than diabatic CAES but also higher costs and technical challenges.
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Scaling Laws in Turbomachinery Design and Operational Optimization

Posted by Fernando Pelcastre

Turbomachines are undoubtedly complex. While designing them from scratch has the best potential to maximize performance, it is not always the best route.

With the help of similarity concepts and the associated nondimensional parameters, the preliminary design of a new machine can be based on features of an existing machine, even one which may have been designed for a different fluid, other flow conditions, or a different rotational speed.

Let’s say we have a turbomachine, in this case, a one-stage Centrifugal Compressor. It was designed for a specific mass flow rate and rotational speed value to achieve a certain pressure ratio at the best efficiency possible.

Table 1. Specification for Baseline Compressor
Table 1. Specification for Baseline Compressor
Figure 1. Baseline Centrifugal Compressor
Figure 1. Baseline Centrifugal Compressor

One would be able to get the same performance at any value of mass flow rate or rotational speed required just by scaling the machine (scaling is the process of changing a geometry while preserving similarity between the prototype and the model). And not just this specific point, but the whole performance map could be moved either to lower or higher mass flow rates. This is possible thanks to the concept of “Similarity”. Read More

Choosing the Best Environmental Control System When Designing an Aircraft

Posted by Vladyslav Kotechenkov

As human beings, we are very vulnerable to environmental conditions, especially those in the stratosphere. Unlike cockroaches (which seem oddly equipped for pretty much anything), humans cannot survive in extremely low or extremely high ambient pressures or temperatures. Perhaps the best minds of our generation didn’t immediately think “how can I be more like this indestructible insect”, but nevertheless technological advancements have helped us get one step closer to their tenacity…at least in stratospheric conditions. Technology does not stand still though and is constantly improving and with it, we’re given more choices and variety in environmental control systems.

Figure 1 – Environmental Control System in an aircraft [2]
When designing environmental control systems (ECS), it is very important to understand that the top priority of these systems is to provide safety and comfort under absolutely any conditions, whether flying over the Sahara Desert or Alaska. We are talking about a minimum of 75 kPa and 20-24 °C. Relative humidity should range from 15 to 60% [1].

The ECS is usually split in one air conditioning machine (ACM) pack per engine. The ACM size is dictated by the ventilation requirement of 6 (g/s)/pax minimum (e.g. 1.2 kg/s minimum for the 200 pax capacity of A320; some 2 kg/s is the typical design value). This air can be taken both from the engine and through separate air intakes (but that’s a completely different story). Read More

Tip Clearance Loss Models in Centrifugal Compressors

Posted by Evgen Rublevskiy

This blog discusses tip clearance loss models in centrifugal compressor impellers with large relative clearances

In the flow path of turbomachines, there is a clearance between the tip of the rotor blades and the housing parts of the machine. This clearance is necessary in order to prevent the rotor from touching the stator during rotation of the impeller. The tip clearance value depends on the following features:

  • Deformation of the rotor under the action of gas, thermal and centrifugal loads
  • Housing deformations under the influence of air pressure and uneven heating
  • Clearance in bearings
  • Design features

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There is a pressure gradient between the suction side and the pressure side, which results in a flow from one side of the blade to the other through the clearance. Studies of the flow in the tip clearance of the blades of turbomachines indicate its complex nature. The flow through the tip clearance affects the flow in the shroud section of the blade and has a significant impact on performance and efficiency. According to the results of the studies, an increase in the relative clearance by 1% reduces efficiency by 2%. Known methods for evaluating the effect of tip clearance on efficiency are most often reduced to a linear dependence of the reduction in efficiency on the relative clearance. This provides acceptable accuracy for engineering calculations with a relative clearance of no more than 3%.

The typical value of the tip clearance for the centrifugal compressor impeller is 0.2-0.5 mm. However, in some cases, the clearance is significantly higher and reaches 1-3 mm. An example would be the impellers of low-pressure compressors, which are made of plastic. Plastic is not a sufficiently rigid material, which requires the designer to significantly increase the tip clearance in order to avoid the impeller touching the housing part of the compressor in operation.

A feature of centrifugal compressors is the low blade height at the outlet of the impeller. Figure 1 shows the impeller of the compressor designed for pressure ratio ptr=2.4 with the diameter and height of the blades at the outlet, respectively, 220 mm and 15.1 mm. For such an impeller, with an absolute clearance of 0.5 mm, the relative clearance will be 3.3%. This means that simple clearance loss estimation methods will have a large margin of error for such an impeller. It should be taken into account that an impeller designed for the same outlet diameter, but at pressure ratio ptr=5, will have approximately half the blade height, respectively, and the relative clearance is twice as large.

Figure 1 - Scheme of a Centrifugal Compressor
Figure 1 – Scheme of a Centrifugal Compressor

Recently, there has been strong interest in small turbomachines. The impeller diameter of such compressors ranges between 50-70mm. The real estimation of clearance losses for this kind of compressor is a problem due to the large relative tip clearance. Read More

An Introduction to Pipe Diffusers in Centrifugal Compressors

Posted by Taras Hryshchenko

A pipe diffuser is a special type of radial vane diffuser and is widely used in centrifugal compressors of gas turbine engines. Employing pipe diffusers can lead to increased efficiency in centrifugal compressors by an average of 2-4% compared to other vaned diffusers. The efficiency gains are especially prominent at high-pressure ratios (over 5.0), toward which we are seeing a growing trend in gas turbine engines.

Pipe Diffuser and assembly
Figure 1 (left): Single flow path of pipe diffuser & Figure 2 (right): Pipe diffuser assembly

Figure Source: Stanislaw Antas, Exhaust System for Radial and Axial-Centrifugal Compressor with Pipe Diffuser, Int J Turbo Jet Eng 2016;

Pipe Diffuser Geometry

The initial part of a pipe diffuser is a cylindrical section (throat), followed by the conical diffuser (Figure 3). The axis of the pipe diffuser must be tangent to the circle created by the tip of the centrifugal compressor, i.e., the circle defined by the impeller tip radius R2. The leading edge of the pipe diffuser channel is elliptical due to the oblique intersection of the cylindrical throat with the cylindrical surface of the diffuser radius R3. This intersection design results in gradually adapting the effluent flow from the impeller to the flow through the cylindrical and conical sections of the diffuser. The downstream duct of a compressor with a pipe diffuser is a diffusing trumpet, also called a “fishtail” diffuser. Read More

How AI Improves Axial Compressor Map Generation

Posted by Sascha Podlech

A performance map is a key step in the design process of axial compressors. Performance maps represent the compressor characteristics and are used for compressor turbine matching and stall margin evaluation. Maps can also be used to compare different compressors, in order to determine which design would be most suitable for a given application. To accomplish these goals, maps usually plot the pressure ratio against corrected mass flow rate and corrected rotational speed. The map has a left bound limit called the surge line, and a right bound limit called the choke line.

Compressor Map
Figure 1: Compressor Map [1]
Now, how exactly do we generate these maps? The traditional approach is through physical experiments. The early prototype or finalized design is integrated into a test rig, which has components like pressure sensors, mass flow gauges, throttles, and others. Such a machine is then run at different operating points, which in turn allows for the plotting of pressure ratios. Unfortunately, this approach is highly time-consuming and requires expensive equipment. Moreover, if the operator over-throttles the mass flow rate, the compressor may pass its surge line. This can lead to an explosive discharge at the inlet, and thus to severe damage. 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

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.

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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.

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

Digital Engineering Modern Turbomachinery Design Platforms

Posted by Valentyn Barannik
Automatic Turbocharger Compressor Design 

In the age of green energy and increased efforts to minimize our carbon footprint,  the design of a turbocharger plays an important role in reducing engine fuel consumption and emissions while increasing the performance.  When developing an engine with a turbocharger, the general approach is to select a turbocharger design from a product list. The primary issue with this approach is that it does not cover 100% of the requirements of engine characteristics, i.e. it has non-optimal construction for the engine being developed. The operational characteristics of an engine directly depends on the interactions between the system components. This non-optimal construction will always lead to a decrease in the engine’s performance. In addition, the iteration process of turbocharger selection is time and resource consuming.

That is why the most optimal way to develop an engine with turbocharging is to design a turbocharger from scratch; wherein the operational points of compressor needed to satisfy the engine’s optimal operation are known, i.e. compressor map (Figure 1). But how do we quickly get a compressor map? Even at the preliminary design level, the design of turbocharger flow path requires dozens of hours for high-level engineers. And what about less experienced engineers?

Figure 1 Compressor Map Generated in AxSTREAM
Figure 1 Compressor Map Generated in AxSTREAM®

Incorporating a digital engineering approach with a turbomachinery design platform such as  AxSTREAM® allows designers to find the compressor design with all the required constraints which correspond to the specified compressor map needed. The design process is presented in Figure 2. Read More