#### What Exactly is a Mixed-Flow Turbomachine?

A review of the literature on mixed-flow turbomachinery reveals a surprising variation in the ways engineers use the term ‘mixed-flow turbomachine’. In particular, two main conceptions of the term stand out. On the first conception, ‘mixed-flow’ refers to the union of axial and radial flow directions into one diagonal flow, resulting in what some call diagonal turbomachines. In the second conception, ‘mixed-flow’ refers to the combination of distinct elements where the flow is axial in some and radial in others. This kind of machine is sometimes called an axial-radial combined turbomachine.

Diagonal turbomachines utilize a flow angle that is between axial and radial and may be considered mere variants of radial machines. The diagonal flow angle allows these machines to enjoy some benefits from both axial and radial flows. In contrast, axial-radial combined turbomachines represent a strategic integration of both axial and centrifugal designs.

In light of the ambiguous use of the term, one might reasonably wonder what truly defines a mixed-flow turbomachine. Is it the convergence of flow directions, the strategic blending of different machine types, or a fusion of design elements?

Diagonal turbomachines are characterized by their flow’s meridional exit angle, which ranges between 0 and 90 degrees. This geometry enables the flow to exit closer to an axial direction, with the exit mean radius greater than that of the inlet. In mixed-flow compressors, this design facilitates higher efficiencies within a constrained cross-sectional area. This advantageous setup addresses a critical need in various applications like unmanned aerial vehicles (UAVs), where the integration of gas turbines demands superior performance within limited spatial constraints as well as a high thrust-to-weight ratio. Read More

#### Automated 1D Analysis of Stall Inception in Multistage Compressors

During the design process of a multistage compressor, engineers need to consider a wide variety of geometric and performance parameters. If a particular compressor design exhibits poor performance, the engineer can make geometric changes to compensate. One crucial parameter in this regard is the stall inception point and the corresponding surge margin of a specific design. The stall points for several speedlines make up the Stall Line of a compressor performance map (as in Figure 1).  This line designates the lowest flow rates a compressor can stably operate at. If there is an issue with the stall point of a given design, wouldn’t it be advantageous for the engineer to identify it as early as possible in the design process? SoftInWay has developed a methodology that can quickly predict the onset of stall using a 1D-solver approach.

While there are several definitions of stall that one could use, we employed a definition based on the total-static pressure ratio. According to this criterion, a compressor stage is said to be stalling if its total-static pressure ratio decreases as the mass flow rate through the compressor decreases. This type of definition of stall has been used previously in other studies [1]. Another way to state this is when the slope of the total-static pressure ratio speedline reaches zero, the stall point is reached. Read More

#### Heat Pump Applications and Modern Design Strategies

A heat pump serves as an alternative to gas or electric boilers, relying on the production of heat. Unlike boilers, a heat pump doesn’t generate heat but extracts energy from the air, water, or ground.

Heat pumps and electric boilers both draw power from the mains electricity supply, yet heat pumps exhibit higher efficiency. This efficiency is contingent upon the conversion efficiency, measured by the Coefficient of Performance (COP), of a specific heat pump. The COP represents the ratio of heat energy received to the electricity consumed, particularly in the operation of the pump’s compressor unit. Notably, a heat pump consumes 3-6 times less electricity than an electric boiler with the same output.

Even in challenging conditions, such as an outside air temperature of -25°C, heat pumps excel in providing heating. Simultaneously, they achieve a high COP – generating 2-5 kW of heat or cold (depending on the type of heat pump) per 1 kW of electricity. This starkly contrasts the lower efficiency of gas and electric boilers.

Heat Pump Use Potential

The economic (rising energy costs) and environmental (effects of climate change) aspects of heat pumps should also be noted when discussing heat pumps. Heat pumps make it possible to utilize renewable heat resources such as geothermal, solar thermal energy and recovered heat from the urban environment. In addition, heat pumps maximize the decarbonization potential of renewable electricity sources (such as wind and solar) by converting them into renewable heat. In combination with thermal storage and electric boilers, heat pumps provide flexibility and security to the building life-support system, offering daily, weekly, and seasonal flexibility. Read More

#### Common Design Challenges in Compressed Air Energy Storage Systems

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.

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.

#### Scaling Laws in Turbomachinery Design and Operational Optimization

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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