Preventing Choke and Surge in a Compressor

Turbo Compressors are used to increase the pressure of a gas, which are required in propulsion systems like a gas turbine, as well as many production processes in the energy sectors, and various other important industries such as the oil and gas, chemical industries, and many more.

Such compressors are highly specific to the working fluid used (gas) and the specific operating conditions of the processes for which they are designed. This makes them very expensive. Thus, such turbo compressors should be designed and operate with high level of care and accuracy to avoid any failure and to extract the best performance possible from the machine.

Axial Compressor and Centrifugal Compressor in AxSTREAM
Figure 1 (A) Axial Compressor (B) Centrifugal Compressor in AxSTREAM®

Turbo Compressor Characteristic Curves

The characteristic curves of any turbo compressor define the operating zone for the compressor at different speed lines and is limited by the two phenomenon called choke and surge. These two opposing constraints can be seen in Figure 2.

Choke conditions occurs when a compressor operates at the maximum mass flow rate. Maximum flow happens as the Mach number reaches to unity at some part of the compressor, i.e. as it reaches sonic velocity, the flow is said to be choked. In other words, the maximum volume flow rate in compressor passage is limited by limited size of the throat region.  Generally, this calculation is important for applications where high molecular weight fluids are involved in the compression process.

Surge is the characteristic behavior of a turbo compressor at low flow rate conditions where a complete breakdown of steady flow occurs. Due to a surge, the outlet pressure of the compressor is reduced drastically, and results in flow reversal from discharge to suction. It is an undesirable phenomenon that can create high vibrations, damage the rotor bearings, rotor seals, compressor driver and affect the entire cycle operation.

Compressor Performance Curve
Figure 2 Compressor Performance Curve

Preventing Choke and Surge Conditions

Both choke conditions and surge conditions are undesirable for optimal operation of a turbo compressor.  Each condition must be considered during design to ensure these conditions are prevented. Read More

What is Rotor Dynamics? And Where is it Found?

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Welcome to this next edition of our “Introduction to Rotor Dynamics” series! In this edition we’ll be covering the definitions of rotor dynamics, and how it is an important factor in the lifetime of a rotating machine. So, for starters, what is rotor dynamics?

Well, if you read our preface which can be found here, you probably knew the answer already; or if you’ve been working in this field, you probably also have a good answer. For those of you new to rotor dynamics, however, it’s a branch of applied mechanics in mechanical engineering and is concerned with the behavior of all rotating equipment; considering phenomena like vibration, resonances, stability, and balancing. It accounts for many effects: from bearings, seals, supports, loads and other components that can act on the rotating system.

Is rotor dynamics vibration analysis?

Yes partially, but there is much more that needs to be considered as you can guess from the above definition. Vibration analysis simply isn’t enough, because the rotors in these machines spin at such high RPMs and are so heavily loaded. Something as simple as the bearing’s position and stiffness, or a slight asymmetry from blade creep can affect a rotor’s behavior.

Where can rotor dynamics be found and analyzed?

The short answer is, there are numerous machines where rotor dynamics can be considered.  In fact, it’s probably easier to list the numerous applications where rotor dynamics doesn’t exist.

A 3D model of a gas turbine rotor train
Figure 1: A 3D Model of a Gas Turbine Rotor Train.

Below is a very short list of some examples where rotor dynamics can be considered: Read More

The Role of Turbomachinery in Modern Hypersonic Cycles

In the coming age of hypersonics, a variety of engine types and cycles are being innovated and worked on. Yet turbomachinery remains unique in its ability to use a single airbreathing engine cycle to carry an aircraft from static conditions to high speeds. One of the largest limitations of turbomachinery at hypersonic speeds (Mach 5+) is the stagnation temperature, or the amount of heat in the air as it is brought to a standstill. While material improvements for turbomachinery are made over time which increases the effective range of temperatures steadily (Figure 1), this steady rate means that the ability of these materials to allow use at stagnation temperatures of more than 1600K remains unlikely any time soon.

Figure 1 Material Improvements Over Time
Figure 1 Material Improvements Over Time

This is the limiting point for traditional turbojet cycles, as Mach 5+ speeds result in temperatures far exceeding these limitations, even for the compressor. However, improvements in cryogenic storage of liquid hydrogen has allowed the concept of precooling, using the extremely low liquid temperature of hydrogen to cool the air enough to push this Mach number range, as well as improve compressor efficiency. To drive the turbine, the exhaust gas and combustion chamber can used, heating the hydrogen and reducing the nozzle temperature for given combustion properties. This has the added effect of separating the turbine inlet temperature from the combustion temperature, reducing limitations on combustion temperatures.  This type of cycle can reduce the inlet temperatures underneath material limits. Read More

Optimization of the Closed Supercritical CO2 Brayton Cycle with the Detailed Simulation of Heat Exchangers

Recently scientists and engineers have turned their attention again to carbon dioxide as a working fluid to increase the efficiency of the Brayton cycle. But why has this become such a focus all of a sudden?

The first reason is the economical benefit. The higher the efficiency of the cycle is, the less fuel must be burned to obtain the same power generation. Additionally, the smaller the amount of fuel burned, the fewer emission. Therefore, the increase in efficiency also positively affects the environmental situation. Also, by lowering the temperature of the discharged gases, it is possible to install additional equipment to clean exhaust gases further reducing pollution.

So how does all of this come together? Figure 1 demonstrates a Supercritical CO2 power cycle with heating by flue gases modeled in AxCYCLE™. This installation is designed to utilize waste heat after some kind of technological process. The thermal potential of the exhaust gases is quite high (temperature 800° C). Therefore, at the exit from the technological installation, a Supercritical CO2 cycle was added to generate electrical energy. It should be noted: if the thermal potential of waste gases is much lower, HRSG can be used. More information on HRSG here:

Any cycle of a power turbine installation should consist of at least 4 elements : 2 elements for changing the pressure of the working fluid (turbine and compressor) and 2 elements for changing the temperature of the body (heater and cooler). The cycle demonstrated in Figure 1 has an additional regenerator, which makes it possible to use a part of the heat of the stream after the turbine (which should be removed in the cooler) to heat the stream after the compressor. Thus, part of the heat is returned to the cycle. This increases the efficiency of the cycle, but it requires the introduction of an additional heat exchanger.

The heat exchangers used in the sCO2 cycle are of three basic types: heaters, recuperators, and coolers. Typical closed Brayton cycles using sCO2 as the working fluid require a high degree of heat recuperation.

Supercritical CO2 Power Cycle with Heating by Flue Gases
Figure 1 – Supercritical CO2 Power Cycle with Heating by Flue Gases

Having examined this scheme and examined the process in detail, we can draw the following conclusions about the advantages of this cycle which is demonstrated in Figure 2: Read More

Digital Engineering Modern Turbomachinery Design Platforms

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

New Series on Rotordynamics: Preface

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Welcome to our latest blog series on rotor dynamics! In this series we’ll be covering fundamentals and a general overview of the engineering discipline that is rotor dynamics, including some basic definitions, why it’s important, the different calculations, and the overall objectives and purposes for these calculations.

Axial Compressor Rotor

In the months ahead, you can expect to learn more about: Read More

Industrial Refrigerator Modeling

Refrigerators are an integral part of everyday life to the point where it is almost impossible to image our day without them. As in our everyday life, refrigeration units are also widely used for industrial purposes, not only as stationary units but also for transporting cold goods over long distances. In this blog, we will focus on the simulation and modeling of such an industrial refrigeration unit.

Picture 1 - Industrial Refrigerator
Industrial Refrigerator

Like any stationary refrigeration unit, a unit used for cooled transportation includes an intermediate heat exchanger, a pump, an evaporator, a compressor, a condenser, and a throttle. The most common refrigeration scheme uses three heat fluids in the industrial refrigeration cycle. There is Water, which is used for heat removal from Refrigerant- R134A and Propylene glycol 55%. These other fluids are used as intermediate fluids between the refrigerator chamber and refrigerant loop. The working principle of all fridge systems are based on the phase transition process that occurs during the refrigerator cycle shown in Figure 1. The propylene glycol is pumped into the evaporator from the heat exchanger, in which it cools and transfers heat to the refrigerant. In the evaporator, the refrigerant boils and gasifies during the heat transfer process and takes heat from the refrigerator. The gaseous refrigerant enters the condenser due to the compressor working, where its phase transition occurs to the liquid state and cycle repeats.
Read More

Considerations when Designing Turbomachinery with sCO2 as a Working Fluid

Supercritical CO2 (sCO2) power cycles offer higher efficiency for power generation than conventional steam Rankine cycles and gas Brayton cycles over a wide range of applications, including waste heat recovery, concentrated solar power, nuclear, and fossil energy. sCO2 cycles operate at high pressures throughout the cycle, resulting in a working fluid with a higher density, which will lead to smaller equipment sizes, smaller carbon footprint, and therefore lower cost. However, the combinations of pressure, temperature, and density in sCO2 power cycles are outside the experience of many designers. Challenges in designing sCO2 cycles include turbomachinery aerodynamic and structural design, bearings, seals, thermal management and rotordynamics. According to the report from Sandia National Lab, compressors operating near critical point and turbines have received only TRL (technical readiness level) 4 and 5 out of 9. This blog discusses the impact on turbomachinery design.

Radial or Axial

The selection of radial or axial for turbomachinery is typically performed based on the operating conditions (adiabatic head H and inlet volumetric flow Q). Non-dimensional turbomachinery parameters of specific speed Ns and specific diameter Ds can be selected from NsDs charts to estimate size, speed, and type of turbomachinery. Turbomachinery types for a sCO2 recompression cycle with scales ranging from 100 kW to over 300 MW have been studied and concluded that systems below 10 MW will likely feature only radial turbines and compressors with a single-stage or low stage counts. Such recompression cycle can be simulated in AxCYCLE™ tool which is shown in Figure 1. As size increases, the most efficient configuration for the turbine and recompressor transitions from radial to axial at approximately 30 MW and 100 MW, respectively. Suitable types of turbomachinery and its components for different power range can be reviewed in Figure 2. A radial configuration for the main compressor was expected at all scales due to its lower volume flow and wider range to facilitate variation in gas properties due to operation near the critical point.

Recompression Cycle simulated in AxCYCLE
Figure 1 – Recompression Cycle Simulated in AxCYCLE

Read More

Introduction to Heat Recovery Steam Generated (HRSG) Technology

[:en]The acronym HRSG (Heat Recovery Steam Generated) is in different sources describing the operation of cogeneration and heating plants, but what does it mean? Heat Recovery Steam Generated (HRSG) technology is a recycling steam generator which uses the heat of exhaust from a gas turbine to generate steam for a steam turbine generating electricity.

The simplest scheme of a Combined Cycle Gas Turbine (CCGT) is presented in Figure 1.

The simplest scheme of CCGT
Figure 1: The simplest scheme of CCGT.

In Figure 1, the exhaust flue gases temperature on the outlet of the turbine is equal to 551.709 ℃. This is a too high a temperature to release the gasses into the environment. The excess heat is able to be disposed of while receiving additional electric power which is approximately equivalent to 30% of the capacity of a gas turbine.

To reach the maximum economical and eco-friendly criteria possible for the installation, many pieces of equipment are used including: a waste heat boiler (HRSG); turbines with a selection for a deaerator (Turbine With Extraction, Deaerator); feed and condensate pumps (PUMP2, PUMP); a condenser (Condenser); and a generator (Generator 2). Exhaust gases entering into the HRSG transfer heat to water which is supplied by the condensate pump from the steam turbine condenser to the deaerator and further by the feed pump to the HRSG. Here boiling of water and overheating of the steam occurs. Moving further, the steam enters the turbine where it performs useful work.

Read More

“I have spent months running CFD but still my design is far from optimal! Why?”

[:en]There has been a tremendous development in Computational Fluid Dynamics (CFD) in the last few decades along with the continuous enhancement of computing resources. CFD is now a very popular tool for all designers. However, if not used wisely, it can lead to the waste of significant engineering time as well as high costs. CFD not reached the state of replacing traditional analytical methods in the design process despite its rapid growth.

CFD Analysis Results using AxSTREAM

Let’s assume that you have been tasked with designing a new component from scratch. Would you be able to use CFD straight away? The answer is no, simply because there is no geometry available at this step. At the very beginning of designing a new component, a user needs a preliminary design tool which can quickly generate the design space based on specific requirements, boundary conditions and geometric constraints.

Preliminary Design in AxSTREAM

At this early stage, there is no point employing CFD because it could take months to generate the basics of the design space in this tool.  Using CFD at this stage would be a waste of time and money not just for the designer, but also hardware. Assuming ownership of a cluster, the hourly rate of a CPU can be as low as 0.06$ and it can increase up to 0.2$ as the computing performance deteriorates within 5 years [1].

Once the preliminary design has been completed and a geometry is selected, the designer employs 1D/2D solvers to calculate the performance of the component under different operating conditions and to generate off-design performance maps. At this stage, CFD can be used to validate the solution against the 1D/2D methods first for the design point and then for few off-design conditions. Depending on the agreement between the results, the CFD may or may not be selected to be used to further evaluate the designs.

Another reason to use CFD is to study complex flowfields and get an in-depth understanding of the phenomena taking place in the flowpath. These results can be useful for further investigation of fluid-structure interactions in order to avoid unwanted vibrations and stability problems. Optimization of an existing turbomachine may also require the use of CFD coupled with Design of Experiment (DoE) approaches to generate more accurate macromodels and response surface which defines the characteristics of the given machine for the provided range of values and parameters.

Figure 1 Response Surface Generated during DoE Optimization
Response Surface Generated during DoE Optimization

Moreover, designers should exploit CFD as a tool to drive innovation when they deal with flow phenomena like separations, cavitation developments etc. For instance, flow control devices to suppress such phenomena have been studied. These phenomena can vary from trailing edge blowing for blade wake manipulation [2] to phased plasma actuators [3] and boundary layer suction technique to increase operating ranges of a turbomachine. However, in order to study such devices, complex geometries need to be generated. CFD is necessary to understand these geometries, which in turn need to be supported by experiments

Stator Blade and Schematic
Stator Blade with Actuators as presented at [2] (left) &  Schematic Illustrating of the Linear Plasma Field Model as presented at [3] (right)

To conclude, CFD is a powerful tool, but it needs to be used with great care because of time and cost implications. It can definitely help optimize existing machines and understand the flow physics of new designs, but designer cannot rely exclusively to CFD to create new machines. This could change in the next years along with further development of computing resources. Till then a combination of preliminary design tools, 1D/2D solvers and even experimental setups is essential. If you need some help to optimize your engineering activities and resources our experts are here to assist you. Feel free to drop a line at for a short follow up chat, or meet our team at one of the following events: Turbo Expo, Paris Air Show, EUCASS.

[1] Walker E, The Real Cost of a CPU Hour, IEEE Computer Society, 2009, 0018-9162/09

[2] Kiesner M, King R. Closed-Loop Active Flow Control of the Wake of a Compressor Blade by Trailing-Edge Blowing. ASME. Turbo Expo: Power for Land, Sea, and Air, Volume 2A: Turbomachinery ():V02AT37A004. doi:10.1115/GT2015-42026.

[3] De Giorgi M. G, Traficante S, Ficarella A, Performance improvement in turbomachinery using plasma actuators, Proceedings of ASME Turbo Expo 2011[:]