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?

Figure 1. 3D view of a mixed-flow (diagonal) compressor in AxSTREAM

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

Holistic Modeling of Energy Conversion & Propulsion Systems using 0D – 3D Approaches

The fields of energy conversion and propulsion systems stand at the forefront of technological innovation, driving advancements in transportation, aerospace, and sustainable energy solutions. In the quest for optimized performance and efficiency, researchers are increasingly turning to holistic modeling approaches that bridge the gap between 0D and 3D simulations. The integration of simulation levels offers a more comprehensive understanding of complex systems by combining simplified, yet accurate, 0D models with highly detailed 3D representations. In this exploration of holistic modeling, we’ll look into how combining 0D and 3D perspectives is essential for designing, analyzing, and optimizing various systems such as steam power plants and rocket propulsion.

Holistic modeling combines simplified 0D models with detailed 3D simulations, allowing engineers and scientists to get a complete picture of how different components interact within systems. This approach considers both the big picture and the finer details, providing a more faithful view of how systems work in the real world.

Figure 1 Holistic Modeling Framework in the AxSTREAM Platform

Energy conversion and propulsion systems operate as thermodynamic cycles, which engineers must design, evaluate, and optimize. The engineer’s process requires simulation of how a new or existing system and its components perform under different conditions. However, given the sheer multitude of components involved in such cycles, software tools are absolutely necessary for studying cycle operation and component interaction. AxSTREAM System Simulation is SoftInWay’s software tool that enables engineers to study and optimize cycles using methods such as Design of Experiment and Monte Carlo. The platform provides a virtual lab where one can tweak and test different setups to find the most desirable ones, both at steady states and in transient conditions. Read More

Gas Turbine Units and Their Impact on the Environment – Part 2

Part 1

As discussed in Part 1 of this blog, Part Two will delve into various development strategies aimed at reducing emissions and enhancing gas turbine performance. Several approaches are currently being explored or employed to mitigate exhaust gas toxicity, as outlined below:

  1. Injection of water or steam into the combustion chamber of a gas turbine unit to boost power and reduce NOx content.
  2. Creation of low-emission multi-zone combustion chambers with variable geometry, pneumatic nozzles, and special flame stabilization.
  3. The use of catalytic combustion chambers or coherent afterburning systems.
  4. Use of environmentally friendly fuel – hydrogen as the main and additional fuel.


As previously mentioned, the toxicity and composition of exhaust gases from a gas turbine plant depend on the type of fuel used. For instance, a critical factor in understanding the mechanism of NOx generation in fuels is the content of chemically bound nitrogen [N]. However, NOx and CO emissions exhibit opposite dependencies on most parameters in the combustion zone (temperature, residence time, volume of the combustion zone, air flow, etc.), prompting the search for a compromise solution to minimize them.

As an illustration, Figure 5a depicts the dependencies of mass emissions of NOx and CO, and the excess air factor when using burner devices with diffusion mixture formation in afterburning chambers [9]. At α = 1.7…2.0, NOx and CO emissions are minimal. Figure 5b illustrates the dependence of NOx and CO emissions on temperature.

Figure 6 - Effect of Air Excess Factor and Temperature on NOx and CO Emissions
Figure 6 – Effect of Air Excess Factor and Temperature on NOx and CO Emissions [9,10]
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Gas Turbine Units and Their Impact on the Environment – Part 1

Part 2

The earliest device for extracting rotary mechanical energy from a flowing gas stream was the windmill. It was followed by the smokejack, first sketched by Leonardo da Vinci and subsequently described in detail by John Wilkins, an English clergyman, in 1648. This device consisted of a number of horizontal sails that were mounted on a vertical shaft and driven by the hot air rising from a chimney. With the aid of a simple gearing system, the smokejack was used to turn a roasting spit. Various impulse and reaction air-turbine drives were developed during the 19th century, making use of air compressed externally by a reciprocating compressor to drive rotary drills, saws, and other devices. While many such units are still in use, they bear little resemblance to the modern gas turbine engine, which includes a compressor, combustion chamber, and turbine to make up a self-contained prime mover. The first patent approximating such a system was issued to John Barber of England in 1791, though no working model was ever built [1].

The first successful gas turbine, built in Paris between 1903 and 1906, consisted of a three-cylinder, multistage reciprocating compressor, a combustion chamber, and an impulse turbine. It operated by supplying air from the compressor, which was then burned in the combustion chamber with liquid fuel. The resulting gases were cooled somewhat by the injection of water and then fed to an impulse turbine. This system, with a thermal efficiency of about 3 percent, demonstrated for the first time the feasibility of a practical gas turbine engine [1]. More detailed information about the history of the development of gas turbine units can be found in [2].

Figure 1: The Armengaud-Lemale Early Experimental Gas Turbine. St Denis, Paris,1906.

Continuous engineering development has significantly increased the electrical efficiency, advancing from 18% in the first commercially operational gas turbine, the 1939 Neuchatel gas turbine, to current maximum levels of approximately 40% for simple cycle operation (Figure 2, a). Gas turbines find application in various fields, including powering aircraft, trains, ships, generating electricity in power plants, powering pumps, gas compressors, tanks, marine propulsion, locomotive propulsion, and automotive propulsion.

Improvements to the simple cycle and additions of steam turbine bottoming cycles offer the capability of further increases in efficiency. Today, a combined gas turbine and steam turbine cycle is capable of achieving an efficiency of almost 60% (Figure 2, b) [3]. Figure 3 shows a timeline of the development of power generation technology. 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.


Figure 1: Example of a Compressor Performance Map

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.

Figure 1 – Example of Heat Pump Installation. Source.

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

Turbomachinery Design Strategies & Tips: How to Choose Between an Axial or Radial Configuration

From the electricity that charges our phones to the jet engines that propel airplanes across the sky, turbines can be found powering our modern world in various forms and configurations.  These mighty machines are the silent heroes of our energy infrastructure, found in everything from locomotives and power plants to industrial machinery and rocket engines. But what distinguishes one turbine from another? How do engineers decide on the design and configuration of these mechanical marvels? This intricate task requires an understanding of turbomachinery design, including axial and radial configurations. So, let’s dive into the differences between an axial and radial configuration.

Fig.1 Example of an axial turbine used in a jet engine. Source

In an axial turbine, the fluid (such as steam, gas, or water) flows along the rotation axis, similar to a windmill where the fluid enters and exits in the same direction. The turbine blades are arranged in stages along the rotor, with each stage converting the fluid’s energy into mechanical energy. Read More

Bearing Optimization: Enhancing Reliability and Performance in High-Speed Machines

Introduction: Bearing Optimization Approach

Hydrodynamic plain bearings play a crucial role in the overall reliability, vibration, and performance of rotary bearing systems. High-performance rotating machines operate at high speeds and subject bearings to significant static and dynamic loads. Consequently, bearing modeling must accurately simulate physical effects. However, the increasing complexity and demanding applications of bearings pose challenges for engineers striving to develop reliable designs.

One critical aspect of bearing design is optimizing the bearing clearance to prevent metal-to-metal contact, especially under heavy loading. This optimization is closely linked to the selection of the minimum oil film thickness (MOFT), which becomes a limiting factor during the process. Another limitation to consider is the maximum allowable level of bearing loading (eccentricity).

The variable parameters that can be considered in the optimization process are as follows:

Figure 1: Plain Cylindrical Bearing
Figure 1: Plain Cylindrical Bearing
  • Bearing clearance (Cb)
  • Radius (R)
  • Bearing length (L)
  • Bearing grooves positions
  • Oil supply temperature (Tin)
  • Oil viscosity (Visc)
  • Load factor (Load_f)


­Objective functions and constraints:

  • Minimum Power loss (Nfr)
  • Minimal allowable oil film thickness (MOFT)
  • Maximum allowable level of bearing loading (E)


Since bearings are integral to the rotor system, the next optimization step involves analyzing the rotor dynamics simulation for a rotor-bearing system. This analysis ensures that resonances are avoided, and sufficient margins are provided to separate critical speeds from the operating speed. Continue reading “Bearing Optimization: Enhancing Reliability and Performance in High-Speed Machines”

Coupled Digital Engineering Solutions in Podracing: More Bang for your Republic Credits!

Authors note:  While I have the utmost respect for die-hard Star Wars fans, I must confess that growing up, Episode I was my favorite. Perhaps it was the allure of Darth Maul’s dual lightsaber, the adrenaline-pumping Podracing on Tatooine, or Natalie Portman (no elaboration needed) that captivated my young mind. Although May 4th has already passed this year, my love of Episode I combined with my upcoming presentation at JANNAF had me thinking that perhaps it’s time to revisit the engineering behind Podracers.

In a previous blog post, we explored the possibilities of redesigning Anakin’s Podracer. Back then, we discovered that the movie’s turbine and compressor design fell far below the mark. However, poor Ani didn’t have access to the advanced AxSTREAM® software platform for designing turbomachinery systems and components, nor did he possess the financial means to acquire top-of-the-line hardware (let’s be honest, Watto’s junkshop hardly exuded luxury).

Figure 1 Watto’s junkshop in Mos Espa, Tatooine
Figure 1 Watto’s junkshop in Mos Espa, Tatooine. Source

Today, we revisit the exhilarating world of Podracing, determined to avoid the disastrous fate that befell Ben Quadinaros’ craft.

Ben Quadinaros' unfortanate start
Figure 2 Ben Quadinaros’ unfortunate start. Source 

Now, many factors could have contributed to that unfortunate incident. For example, frustration-induced control slamming is never advisable… However, the most plausible explanation is a failure at one of the subsystem interfaces. Why do I think so? Simply because this is a common issue with modern systems, including vehicles. Everyone involved means well and possesses the necessary technical prowess to perform their individual tasks. However, problems often arise when these meticulously designed parts and subsystems attempt to interface seamlessly. This is precisely where a coupled digital engineering solution comes into play! Close your eyes (then reopen them to keep reading) and imagine a place where all critical propulsion and auxiliary components can be modeled together, accommodating any desired conditions (including those we cannot run with physical hardware on a test bench).

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Calculation Methods for Dynamic Parameters of Rolling Element Bearings and Their Prediction Accuracy

Rolling element bearings are essential components in a vast range of rotating mechanical systems, providing rotational support and a wide range of methods for friction control. The performance of these bearings is crucial to the overall functionality of the machine. Therefore, predicting the dynamic behavior of rolling bearings accurately is vital to ensure their reliable operation. Figure 1 shows a general view of angular contact ball bearings.

Figure 1 Angular contact ball bearings, [1]
Figure 1 Angular contact ball bearings, [1]
Dynamic parameter calculation is the process of determining the bearing’s dynamic response under specific operating conditions, including load, speed, and lubrication. The calculation of these parameters is crucial for designing and selecting bearings and predicting their operating life. The main dynamic parameters are:

  • Radial and axial stiffness, which is the ability of a bearing to resist deformation when subjected to radial and axial loads. This parameter is essential in ensuring that the bearing maintains its shape and does not deform excessively under the applied load.
  • Damping, which is the ability of a bearing to absorb energy and dissipate it in the form of heat. This parameter is crucial in ensuring that the bearing does not overheat, leading to premature failure.


Moreover, bearing stiffness and damping are used for the rotor dynamic calculations of the machine supported by rolling element bearings for the accurate prediction of the rotor response under static and dynamic loads, taking into account the correct bearing characteristics. Read More