Within the realm of turbocharging, there are a number of different design challenges that influence the design process on both large-scale marine applications and smaller-scale commercial automobile applications. From aerodynamic loads to dynamic control systems to rotor dynamics and bearing challenges, turbochargers represent a special subset of turbomachinery that requires complex and integrated solutions. Turbocharger rotors specifically, have unique characteristics due to the dynamics of having a heavy turbine and compressor wheel located at the overhang ends of the rotor. The majority of turbocharger rotors are supported within a couple floating-ring oil film bearings. In general, these bearings provide the damping necessary to support the high gyroscopic moments of the impeller wheels. However, there are several disadvantages of working with these oil systems that have allowed different technologies to start to surface for these turbomachines. With the floating-ring oil models, varying ring speed ratios and oil viscosity changes significantly influence the performance of the rotor dynamic model.
The application of oil-free bearings have started to emanate due to the overall consistency of their performance and the minimized heat loss associated with air as the damping fluid. Studies on these bearing types for turbomachinery applications are neither trivial nor unique, as they have seen plenty of exposure within the commercial and military aircraft industries within turbo compressors and turboexpanders. However, the success of these specific applications are due to the fact that these turbomachines operate with light loads and relatively low temperatures. The main design challenges with foil air bearings are a result of poor rotor dynamic performance, material capabilities, and inadequate load capacities at high temperature/high load applications.
Foil air bearings operate based on a self-acting hydrodynamic air film layer during normal operation, but they exhibit serious wear on start up and shut down if not properly attended to. Prior to developing a gas film on start up, these bearings must handle the sliding that occurs between the rotor and the inner surface of the bearings. For this reason, solid lubricants like polymer foil coatings were considered for these bearings. Polymer coatings have a serious temperature restriction which do not allow them to be considered for high-temperature applications above 300 °C. Different chrome oxide based coatings have shown greater performance at higher temperatures. Initial testing of these coatings showed significantly poor performance at lower temperatures of 25 °C and difficulties with adhesion through repeated thermal cycles. However, NASA has developed a new high temperature PS400 formulation of this coating that performs well under different load conditions and between the temperature range of 25 °C and 650 °C. Essentially, the viability of these bearings within the automotive market has become a reality with individualized bearing designs. The question now becomes whether the foil gas bearing manufacturers can penetrate the market from a larger-scale and create a standard for these turbocharger setups to run free of oil altogether. To learn more about the simulation of both floating-ring oil film bearings and foil air bearings using the SoftInWay platform, please visit: http://www.softinway.com/software-applications/bearing-design/
Increasing regulation for reducing emissions has forced the automotive industry to accept different technologies over the years in order to stay ahead of the market. In an industry that is so accustomed to internal combustion engines, new solutions such as electric motors and turbocharger systems have allowed experts in other industries to cultivate an influence in the automotive market. Specifically in the realm of turbomachinery, increased development has gone into designing turbochargers in order to minimize the effect and size of internal combustion engines. Design challenges are inherent in the fact that an engine is a positive displacement device whereas the turbocharger falls under aerodynamic turbomachinery. The two separate machine types have distinctly different flow characteristics, and the proper sizing of a turbocharger for its parent engine requires proper modeling of the engineering system as a whole.
In general, initial turbocharger sizing becomes a matter of obtaining the necessary boundary conditions required for a preliminary design. A thermodynamic cycle analysis of an ICE-Turbocharger system will allow the designer to obtain an initial idea of the bounds
necessary for the compressor and turbine design. Given the engine information, necessary inlet conditions of the compressor such as temperature and pressure, efficiencies required, and heat transfer of the system, the user can then obtain the boundary conditions for the turbocharger compressor and turbine wheels.
From this point, the process becomes an exercise in turbomachinery design and analysis. With SoftInWay’s turbomachinery design and analysis platform, a boundary condition realization of the system eventually manifests into a full 3D design of the turbine/compressor wheel. Once the engineer designs both the turbine and compressor wheels, they will be left with two discrete physical systems. However, these two designs must eventually coincide into a harmonious system that accurately represents the “turbocharger”. In order to facilitate this representation, the user can overlay the different compressor and turbine maps based on a number of varying parameters. Given the Power and Pressure Ratio curves for a number of varying shaft speeds and temperatures, an off-design performance of the turbocharger system can be analyzed via AxSTREAM’s matching module (Figure 2). Another simultaneous analysis of the turbine and compressor wheels must be made on the component that connects them, the rotor. Rotor design, rotor dynamics, and bearings analysis are crucial to a legitimate turbocharger design and will be a topic of a next week’s blog post. If you would like to learn more about turbocharger design and analysis methods, please follow this link
Back when the California high-speed rail project was announced, Elon Musk (CEO of SpaceX and Tesla Inc. and perhaps the most admired tech leader of present day) was not only disappointed with this project, but also introduced an alternative to this system called the Hyperloop in 2012. Since the abstract of this project was introduced, many engineers around the world have started to evaluate the feasibility of this “5th Mode of Transportation”.
The general idea for the Hyperloop consists of a passenger pod operating within a low-pressure environment suspended by air bearings. At the realistic speeds estimated by NASA of 620 mph, the pod will be operating in the transonic region. While Japan’s mag-lev bullet train has succeeded at achieving speeds of up to 374 mph, the scale and complexity of a ground transportation system rising above 600 mph bring to surface an unusual number of engineering challenges. As well, brand new designs such as the one proposed by Musk have a certain amount of risk involved due to this technology inherently having no previous run history on a large scale.
Of the many concerns with his original design, perhaps the largest resides on how to design and operate the axial compressor in front of these pods. The supposed function of the compressor is two-fold. The first function would be to overcome the Kantrowitz limit. Musk uses an analogy between the pod and tube and a syringe:
“Whenever you have a capsule or pod (I am using the words interchangeably) moving at high speed through a tube containing air, there is a minimum tube to pod area ratio below which you will choke the flow. What this means is that if the walls of the tube and the capsule are too close together, the capsule will behave like a syringe and eventually be forced to push the entire column of air in the system. Not good.”
An onboard compressor in front of the pod will allow the collected column of air traveling in front of the pod to flow through the system without compromising the increasing velocities of the pod. A second function of the compressor would be to supply air to the air bearings that support the weight of the capsule throughout the passage.
Traditionally, axial compressors are coupled with a complimentary turbine at the exhaust that provides mechanical power to the compressor. In the hyperloop, the proposed compressor arrangement will be driven by electric motors instead of turbines. This is a relatively new design that has only been tested on a handful of electric powered jet aircrafts for research purposes. Furthermore, Musk proposed a compression ratio of about 20:1, which would require several compression stages for an axial compressor arrangement and an intercooler system. The temperature increases resulting from this high order compression require a complex cooling method or a traditional steam pressure vessel for the proper dumping of hot air. A final challenge on the compressor end would be the fact that it will be operating at a very low pressure. Only a handful of companies like Safran Aero Boosters have the necessary experience with low-pressure compression.
In general, while this new proposed mode of transportation is very exciting and innovative from an engineering standpoint, the following challenges specific to the on-board compressor will require serious collaborations amongst the leaders in the compressor design industry:
- Electric Motor Driven Compressor
- High Compression Ratio – 20:1
- Complex intercooler system
- Low-Pressure Compression Environment
If you would like to learn more about SoftInWay’s integrated platform for axial compressors, please visit our axial compressor page
Centrifugal and axial compressors must operate within certain parameters dictated by both the constraints of the given application as well as a number of mechanical factors. In general, integrated control systems allow compressors to navigate dynamically within their stable operating range. Typical operating ranges for compressors are represented on a plot of volumetric flow rate versus compression ratio. Compressors have a wide number of applications, from household vacuum cleaners to large 500 MW gas turbine units. Compression ratios found in refrigeration applications are typically around 10:1, while in air conditioners they are usually between 3:1 and 4:1. Of course, multiple compressors can be arranged in series to increase the ratio dramatically to upwards of 40:1 in gas turbine engines. While compressors in different applications range dramatically in their pressure ratios, similar incidents require engineers to carefully evaluate what is the proper operating range for the particular compressor design.
For intensive applications of centrifugal and axial compressors, the phenomenon of surge resides as one of the limiting boundary conditions for the operation of the turbomachine. Essentially, surge is regarded as the phenomena when the energy contained in the gas being compressed exceeds the energy imparted by the rotating blades of the compressor. As a result of the energetic gas overcoming the backpressure, a rapid flow reversal will occur as the gas expands back through the compressor. Once this gas expands back through to the suction of the compressor, the operation of the compressor returns back to normal. However, if preventative measures are not taken by the appropriate controls system or any implemented mechanical interruptions, the compressor will return to a state of surge. This cyclic event is referred to as surge cycling and can result in serious damage to the rotor seals, rotor bearings, driver mechanisms, and overall cycle operation.
Because of surge and other phenomena such as stall, engineers must embed proper control systems that effectively handle different off-design conditions seen in particular compressor arrangements. Depending on the application, certain compressors will rarely operate away from their design point, and such control systems are not necessary. However, in advanced applications such as large gas turbine unit compressors, controls systems allow the compressor to navigate within a range between the choke, stall, minimum speed, and maximum speed limits. The chart seen in Figure 1 describes the operating range of a compressor using a Rc—Qs map. In many cases, an antisurge valve (ASV) working in conjunction with an antisurge PI controller will action open or closed based on varying transient conditions seen on the compressor. For design purposes, it is vital to understand compressor limits in order to properly develop or outsource a compressor based on the performance metrics needed for the application.
An important first step in understanding the gas turbine design process is the knowledge of how individual components act given their particular boundary conditions. However, in order to effectively leverage these individual design processes, a basic knowledge of how these components interact with each other is essential to the overall performance of a gas turbine unit. The power and efficiency outputs of a gas turbine are the result of a complex interaction between different turbomachines and a combustion system. Therefore, performance metrics for a gas turbine are not only based on the respective performances of each turbine, compressor, and combustion system, but also on their interactions. The concept of component matching becomes crucial in understanding how to deal with these systems simultaneously.
In general, gas turbines for industrial applications consist of a compressor, a power turbine, and a gas generator turbine designed into one of two arrangements. The first arrangement invokes the use of the gas generator turbine to drive the air compressor, and a power turbine to load the generator on a separate shaft. This two-shaft arrangement allows the speed of the gas generator turbine to only depend on the load applied to the engine. On a single-shaft arrangement, the system obviously cannot exist at varied speeds and the power turbine coupled with the gas generator turbine would be responsible for driving both the generator and the compressor. A simplified diagram of each arrangement is displayed in Figures 1 and 2.
The efficiency of gas turbine engines can be improved substantially by increasing the firing temperature of the turbine, however, it is important to remember that the surface of the components exposed to the hot gas must remain below a safe working temperature consistent with the mechanical strength and corrosion resistance of the employed materials. Along with this firing temperature limit, obvious upper bounds exist on the speed of the gas generator due to mechanical failures and reduced lifetimes at high RPMs. These two limits help construct a particular range at which the engine can perform. There is a certain “match” temperature that controls whether the engine will be operating at its maximum gas generator speed (speed toping) or its maximum firing temperature (temperature topping). At ambient temperatures above the match temperature, the engine will operate at its max firing temperature and below its max generator speed. In a similar vein, the engine will operate at its max generator speed and below its max firing temperature at ambient conditions below the match temperature. The match temperature is the ambient temperature at which the engine reaches both limits, and it represents the highest efficiency of that engine.
This match temperature is not a trivial or fixed value. Several auxiliary factors cause changes in the gas engine’s match temperature, which must be appropriately accounted for in the gas turbine design. The following factors alter the match point of any gas engine
- – Changes in the fuel properties
- – Reduction in compressor or turbine efficiency due to fouling, increased leakage, tip clearance, and material roughness variations
- – Accessory loads imparted by pumps and other secondary systems
- – Inlet and Exhaust losses
These auxiliary factors along with the routine changes described by varying ambient temperature, ambient pressure, humidity, load, and power turbine speed all contribute to the complexity involved in properly designing a gas turbine. Correctly analyzing off-design conditions becomes an art of variable manipulation and generally requires the use of cohesive design and analysis platforms for proper evaluation. SoftInWay’s integrated software platform allows for streamlined manipulation of your gas turbine design together with immediate off-design analysis based on any prescribed changes. If you would like to learn about how our AxSTREAM platform assists with off-design analysis in gas turbines and other turbomachinery, please visit our software page.
In the ever-expanding market for waste-heat recovery methods, different approaches have been established in order to combat the latest environmental restrictions while achieving more attractive power plant efficiencies. As gas turbine cycles continue to expand within the energy market, one particular technology has seen a significant upsurge due to a number of its beneficial contributions. Supercritical CO2 (S-CO2) bottoming cycles have allowed low power units to utilize waste heat recovery economically. For many years, the standard for increasing the efficiency level of a GTU (Gas Turbine Unit) was to set up a steam turbine Rankine cycle to recycle the gas turbine exhaust heat. However, the scalability constraints of the steam system restrict its application to only units above 120MW.
HRSGs (Heat Recovery Steam Generators) are water-to-steam boilers which capture the waste heat exhaust of GTUs and convert this heat into energy in the form of high-pressure, high-temperature steam. These systems can exist in a single or modular fashion depending on the scope of the project. Modular HRSGs consist of any number of low pressure, intermediate pressure, and high pressure sections. Each section allows for the extraction of gas turbine exhaust heat using separate steam drum and evaporator sections. Even in a single pressure HRSG combined cycle, the immense amount of auxiliary equipment, the high installation costs, and the frequent maintenance necessary for such a system prevent them from providing viable heat recovery for low power GTUs.
With the introduction of a different fluid, gas turbines of small and medium size are able to utilize waste heat recovery. Unlike steam, a supercritical CO2 system is designed to lie in the simply in the gaseous phase. This single-phase fluid design removes the boiling process necessary for a steam system and therefore results in higher fluid temperatures and cycle efficiencies. As well, the high energy density reduces the system component’s size and cost, and offers higher system efficiencies, reduced footprints, and significantly easier installation methods. While all these advantages do exist within a supercritical CO2 system, working with a relatively new fluid presents different challenges that have not had the time and exposure with engineering experts as steam and gas systems have. In particular, developing a turbine that will most efficiently run under this new fluid presents perhaps the tallest demand within the supercritical cycle. The task becomes to embrace these challenges for the benefit of higher efficiencies, lower O&M costs, and reduced greenhouse emissions.