Performance Testing of Axial Compressors

Performance testing is a key part of the design and development process of advanced axial compressors.  These are widely used in the modern world and can be found in nearly every industry, and include the core compressor for aeropropulsion turbofan engines, as well as aeroderivative gas turbine engines for power generation.  An example of this are the turbine engines shown in Figure 1 and 2, which feature an industrial gas turbine and a high bypass ratio turbofan engine with a multistage high-pressure core compressor. The development time of these machines can involve numerous expensive design-build-test iterations before they can become an efficient and competitive product. This places a great importance on the accuracy of the data taken during the performance tests during the development of the compressor since the test data taken is often used to anchor the loss models within the design tools. Modern axial compressors typically have high aerodynamic loadings per stage for improved system efficiency and requires precise aerodynamic matching of the stages to achieve the required pressure ratio with high efficiency. Variable geometry inlet guide vanes and stators in the first few stages are typically required to provide acceptable operability while maintaining high efficiency and adequate stall margin.

Industrial gas turbine for power generation.
Figure 1. Industrial gas turbine for power generation. Source
Figure 2. Turbofan engine for aeropropulsion.
Figure 2. Turbofan engine for aeropropulsion. Source

Performance Testing of Axial Compressors

Axial compressors all undergo a thorough design and development phase in which performance testing is vital to their ultimate success as a product. Performance testing during the development phase of these high-power density machines can ensure that the design meets the specified requirements or can identify a component within the turbomachine which falls short of its expected performance, and may require further development, and possible redesign. Performance testing can also ensure that the unit can meet all the conditions specified and not merely the guaranteed condition. Aerodynamic performance testing multistage axial compressors during the early part of development is often done in phases. The development test program is planned and executed with a design of experiments approach and includes varying the air flow and shaft rotational speed as well as the variable geometry schedule in order to fully characterize the compressor. In the first phase, the front block of the compressor is built and tested at corrected (referenced) air flow rate, inlet pressure, temperature and shaft rotational speed. Instrumentation includes utilizing traditional rakes and surveys at the exit, to obtain spanwise distributions of pressure, temperature, and flow angles. Testing in phases is typically done for two reasons. Read More

Willis Carrier, Air Conditioning, and His Contribution to Mechanical Engineering and HVAC Systems

Welcome to this special edition of the SoftInWay blog! While we at SoftInWay are known for helpful articles about designing various machines, retrofitting, and rotor dynamics, we believe it is also important to examine the lives of some of the men and women behind these great machines.

Commonly listed among the greatest mechanical engineering inventions of the 20th Century, the air conditioning system has gone from basic use in refrigeration to a staple of living in many countries. Locales that were previously borderline uninhabitable for people sensitive to heat or poorer air quality, became available, thanks to this device that could be installed in homes and businesses. But who invented the air conditioning system?

A portrait photograph of Willis Carrier in 1915

Willis Haviland Carrier (1876-1950) was born on November 26th, 1876 in Angola, a small town in Upstate New York just outside of Buffalo. Carrier was the inventor of modern air conditioning as we know it. While other forms of air conditioning had been around for millennia, what Carrier invented was utterly life-changing for those who were able to use it, and work/live in air-conditioned environments.  His work has been so influential on modern HVAC engineering and the world in general, that his legacy company has a website in his honor.

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Rotor Dynamics Challenges in High-Speed Turbomachinery for HVAC Applications

In comparison to large steam and gas turbines, the rotating equipment found in heat ventilation and air conditioning (HVAC) applications is often seen as more simplistic in design. However, sometimes a simpler model of a rotating machine does not mean a simpler approach can be used to accurately investigate its rotor dynamics behavior. For example, a large number of effects should be taken into account for single-stage compressors used in HVAC applications. Three important ones include:

  1. High values of rotational speeds above the first critical speed;
  2. Rigid rolling element bearing used in the design and therefore a relatively flexible foundation which should be modeled properly;
  3. Aerodynamic cross-coupling adding additional destabilizing forces to the structure.

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All these effects should be modeled properly when performing lateral rotor dynamics analyses of HVAC machines. And, in some cases, this simpler model can prove a much more challenging task than building the complex model of a steam turbine rotor.

Let’s consider a seemingly simple example of a high-speed single-shaft compressor for HVAC application (Figure 1). It consists of the compressor and motor rotors, the flexible coupling connecting them, the ball bearings connecting the rotors to the bearing housing joined with the compressor volute, and the structural support.

Fig. 1 - Single stage compressor model
Fig. 1 – Single-stage compressor model [1]
The compressor rotor is connected with the motor through a flexible coupling. Its lateral vibrations can be considered uncoupled from the motor rotor vibrations, and the lateral rotor dynamics model appears pretty straightforward (Figure 2).

Fig. 2 - Rotor dynamics model of the single stage compressor rotor
Fig. 2 – Rotor dynamics model of the single-stage compressor rotor

However, additional factors are discovered if you include the mechanical properties of the supporting structure when considering the lateral rotor dynamics calculations. These factors are very important to an accurate model. Read More

An Introduction to Accurate HVAC System Modeling

HVAC (Heat, Ventilation and Air Conditioning) is all about comfort, and comfort is a subjective feeling associated with many parameters like air quality, air temperature, surrounding surface temperature, air flow and relative humidity. For example, while it is easy to understand how the temperature of the air in your living impacts how good you feel, the surfaces with which you are in contact also strongly affect your comfort. For example, last night I got out of bed to clean up after my dog who thought it would be a good idea to swallow (and give back) her chew toy. If I was wearing my slippers, it would have been much easier to go back to sleep between the warm bed sheets without the discomfort of waiting my cold feet warm up to normal temperature.

Speaking of sleep discomfort, many stem from HVAC imbalances.  If you wake up in the middle of the night quite thirsty, then you should probably check how dry your bedroom is. The recommended range is 40-60% relative humidity. A higher humidity puts you at risk for mold while lower humidity can lead to respiratory infections, asthma, etc.

Now that we know how HVAC contributes to our comfort, let’s look at the HVAC unit as a system and see its role, functioning and simulation at a high level. The following examples provided are for a house, but similar concepts apply to residential buildings, offices, and so on.

Controlling Temperature

The easiest parameter to control is the air temperature. It can be set by a thermostat and regulated according to a heating or cooling flow distributed from the HVAC unit to the different rooms through ducting. Without the introduction of thermally-different-than-ambient air, the house will heat or cool itself based on a combination of outside conditions and how well the building is insulated. Therefore, to keep a constant temperature a certain amount of energy must be used to provide heating (or cooling) at the same rate the house is losing (or gaining) heat.  This is a match of the house load and heating/cooling capacity. Figure 1 provides a graph of the energy needed.

Illustration of dependency of house load and heating capacity on outside temperature
Figure 1 Illustration of dependency of house load and heating capacity on outside temperature

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A Century of Chiller Technology

A convergence of technologies had to occur to make the modern, high-efficiency centrifugal chiller a reality. To appreciate the technology fully, we must go back in history and understand the origins of the air conditioning and refrigeration industry. Along the way, we will find an important diversion in aerospace and the critically important centrifugal compressor. Ultimately, we will find that the modern chiller is a testament to advanced technology that was developed in multiple fields.

Some of the first advances in and applications of modern industrial refrigeration were in the United States. In May 1922, Willis Carrier revealed the “Centrifugal Refrigeration Machine” – a very early incarnation of what we now call a chiller [1]. The first installation went to a Philadelphia candy manufacturer; it’s interesting to know that the birth of modern refrigeration and air conditioning started on a large scale. Back in those days, economy of scale enabled the technology to be developed. It was not until a decade later that the core technology began to be adopted into compact units that could be used in smaller businesses such as boutique shops. It took several more decades for smaller residential air conditioners to take off commercially.

Shown in the photograph below is Carrier’s first centrifugal chiller in his New Jersey factory [1].

First Centrifugal Chiller
Photo from [1]
The size of this machine is evident, as is the fact that its design, at the time, necessitated components be spread out in space for assembly and maintenance. By modern standards, the same footprint space could be used to accommodate a modern chiller with over 500 refrigeration tons in capacity. By comparison the original design has less than 100 refrigeration tons of capacity.

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Design of Waste Heat Recovery System based on ORC for a Locomotive Gas Turbine

This is an excerpt from a technical paper, presented at the Asian Congress on Gas Turbines (ACGT) and written by Abdul Nassar, Nishit Mehta, Oleksii Rudenko, Leonid Moroz, and Gaurav Giri. Follow the link at the end of the post to read the full study!

INTRODUCTION

Gas turbines find applications in aerospace, marine, power generation and many other fields. Recently there has been a renewed interest in gas turbines for locomotives. (Herbst et al., 2003) Though gas turbines were first used in locomotives in 1950 – 1960’s, the rising fuel cost made them uneconomical for commercial operation and almost all of them were taken out of service. The diesel locomotives gained popularity and presently locomotives are operated by diesel engines and electric motors. The emission levels in diesel locomotives have raised concerns among the environmentalists, leading to stringent emission norms in recent years. One of the solutions to reduce emission for these locomotives is to switch to LNG fuel which requires huge investment in upgrading the engines to operate with LNG. The other alternative is Gas Turbine based locomotives and this has gained renewed interest with RZD and Sinara Group of Russia successfully operating LNG based Gas Turbine-electric locomotives. Fig. 1 shows the GT1-001 freight GTEL from Russia, introduced in 2007. It runs on liquefied natural gas and has a maximum power output of 8,300 kW (11,100 hp). Presently, this locomotive holds the Guinness record for being the largest gas turbine electric locomotive (Source: http://www.guinnessworldrecords.com). Though there have been a lot of improvements in gas turbines, the thermal efficiency is still not very high unless the exhaust heat is efficiently utilized by a bottoming cycle.

Fig. 1 Russian GT1_001 gas turbine locomotive

Converting the gas turbine into a combined cycle unit, with a bottoming steam cycle, is employed in case of several land-based and marine applications; however, such an option is not practical in a locomotive gas turbine due to the requirements of steam generators, steam turbines and other auxiliaries. The next best alternatives are to utilize either an organic Rankine cycle (ORC) or a supercritical carbon dioxide cycle (sCO2) to extract heat from the exhaust of the gas turbine and convert it into useable energy in the bottoming cycle (Rudenko et al., 2015; Moroz et al., 2015a; Moroz et al., 2015b; Nassar et al., 2014; Moroz et al., 2014). Supercritical carbon dioxide cycles, operating in a closed-loop Brayton cycle, are still in research phase. There is not much practical experience in deploying an sCO2 unit for propulsion gas turbines even though there is considerable research currently in progress. Hence, the obvious choice is to incorporate an ORC based system which is compact, modular and easy to operate. The same concept can also be implemented in any gas turbine application, be it a land-based, power generation, or marine application. Read More

Combined Power Cycles: What Are They and How Are They Pushing the Efficiency Envelope?

Combined cycle power plants have introduced a significant increase in efficiency compared to simple cycle power plants. But what is a combined cycle power plant and how does it work?

What is a Combined Cycle Power Plant?

In simple terms, a combined cycle power plant is a combination of more than one type of cycle to produce energy. A combined cycle plant consists of a topping and bottoming cycle with the objective to maximize the energy utilization of the fuel. The topping cycle normally is a Brayton cycle based gas turbine while the bottoming cycle is a Rankine cycle based steam turbine.

Gas turbines are used because this equipment can very efficiently convert gas fuels to electricity with the choice of using different fuels. Recently, the simple cycle efficiencies of gas turbines have improved considerably. As an example, standard fossil fired Rankine cycles with conventional boilers have an efficiency in the range 40–47% depending on whether they are based on supercritical or ultra-supercritical technology. By utilizing waste heat from the heat recovery of the steam generators to produce additional electricity, the combined efficiency of the example power plant would increase to 60% or more. Combined cycles are the first choice if the goal is to generate maximal energy for a unit amount of fuel that is burnt.

Why Don’t all Power Plants Use Combined Cycles

You might be wondering why not all plants are based on the combined cycle. The primary reason is fuel availability. Not all regions are blessed with the availability of gas that can be easily utilized in a gas turbine. Transporting gas from one location to another, or converting a fuel to gas specifically for operating a gas turbine, may not be the best economic decision. The technological expertise required in maintaining a gas turbine is another challenge faced by gas turbine operators. A typical combined cycle plant is presented in Figure 1.

Schematic of a combined cycle power plant created in AxCYCLE
Figure 1. Schematic of a Combined Cycle Power Plant Created in AxCYCLE

The key component of the combined cycle power plant apart from the turbines is heat recovery steam generator (HRSG). The major objective is to convert maximal heat from the exhaust gas of the gas turbine into steam for the steam turbine. The HRSG, unlike the conventional boiler, will operate at a lower temperature and is not subject to the same temperature as the boiler furnace. The exhaust gas from the gas turbine is directed through the tubes of the HRSG wherein water flowing through these tubes, observes heat and converts into steam. The temperature of the live steam is in the range of 420 to 580 C with exhaust gas temperatures from the gas turbine in the range of 450 to 650 C. A supplementary burner could be included in the HRSG, but adding a supplementary burner reduces the overall cycle efficiency. Read More

The Lovable Underdog of Turbomachinery

Everyone knows that APUs need love too…..

For Valentine’s Day, we want to look at an underdog of turbomachinery. A machine that is often overlooked, and not really in the limelight the way some of its larger cousins are, nor is it given the trendy position of being the “technology of the future” like its smaller cousins. Without this technology, airplanes would be entirely reliant on external power plants to maintain an electric power supply on the ground, and to start the main engines. So, what is this underappreciated machine?

APU plane
Okay one last hint – you can see its exhaust port.

If you haven’t been able to guess it, our Valentine this year is the aircraft auxiliary power unit, or APU for short. Although these are not present on all aircraft, they are typically used in larger airplanes such as commercial airliners. This allows aircraft to rely less on ground services when the main engines are not running. As a result, less equipment, manpower, and time are required to keep the plane in standby mode, and the aircraft can also service airports with less available resources in remote locations.

Where this Underdog Started

The aircraft auxiliary power unit can be traced back to the First World War, as they were used to provide electric power onboard airships and zeppelins. In the Second World War, American bombers and cargo aircraft had these systems as well. APUs were small piston engines, as the gas turbine had yet to be developed. These engines were typically V-twin or flat configuration engines, similar to what you might find on a motorcycle, and they were called putt-putts. These two-stroke engines usually put out less than 10-horsepower, but that was all that was required to provide DC power during low-level flight.

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Centrifugal Compressor Reverse Engineering and Digital Twin Development

Centrifugal Compressors are the turbomachines also known as turbo-compressors, and belong to the roto-dynamic class of compressors. In these compressors the required pressure rise takes place due to the continuous conversion of angular momentum imparted to the working fluid by a high-speed impeller into pressure. These compressors are used in small gas-turbines, turbochargers, chiller units, in the process and paper industries, oil & gas industries and others.

The design and manufacturing of such compressors are always challenging because of its 3-dimensional shapes, high rotational speeds that interact with different loss mechanisms, and stringent working environments. In many circumstances, it is necessary to analyze an existing compressor, with the end goal being to redesign it, enhance its performance, or to use it in completely different applications. In order to meet such requirements, reverse engineering is a viable option. With reverse engineering, one can review competitor’s design to remain in market competition.

Reverse Engineering

Reverse engineering allows us to collect incomplete or non-existing design data and manufacture an accurate recreation, safely, of the original product or component.

Sometimes, it is also referred to as back engineering, in which centrifugal compressors or any other product are deconstructed to extract design information from them. Oftentimes, reverse engineering involves deconstructing individual components like the impeller or diffuser of larger compressors. End-users often use this approach when purchasing a replacement impeller or any other compressor part from an OEM is not an option. In some cases, where older impellers that have not been manufactured for 20 years or more, the original 2D drawings are no longer available.  When this is the case, the only way to obtain the design of an original compressor is through reverse engineering.

Reverse engineering requires a series of steps to gather precise information on a product’s dimensions. Once collected, the data can be stored in digital archives. Figure 1 (left) shows the typical process of reverse engineering. In figure 1 (right), one can see the scanning process of the centrifugal impeller using a laser scanner.

Figure 1 (left) Reverse Engineering Process (right) Scanning of impeller
Figure 1 (left) Reverse Engineering Process (right) Scanning of Impeller. Source

To reverse engineer an impeller or any other part of compressor, an organization will typically acquire the component and take it apart to examine its internal mechanisms. This way, engineers can unveil information about the original design and construction of the product. One can start by analyzing the dimensions and attributes of the impeller and make measurements of the blade widths, diameters and angles, as these dimensions often relate to the compressor’s performance. Read More

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

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