Using 1D Models to Predict the Thermal Growth and Stresses During The Start up and Shutdown Phase of a Steam Turbine

Steam turbines are not just restricted to conventional or nuclear power plants, they are widely used in combined cycle power plants, concentrated solar thermal plants and also geothermal power plants. The operational requirements of a steam turbine in the combined cycle and CSP’s means that they operate under transient conditions. Even in conventional steam turbines, the market requirements are changing with requirements for faster and more frequent start-up which can result into faster deterioration of the equipment and reduced lifespan. During the startup phase, significant heat exchange takes place between the steam and the structural components that include the valves, rotor and casing. The accuracy of the life prediction is strongly affected and dependent on the accuracy of the transient thermal state prediction [1].

Though the expansion of steam takes place in the nozzles and blades, the influence of the leakage steam during the startup phase is significant with steam expanding through the labyrinths resulting in expansions, condensation, and increased velocities which may even reach supersonic levels. During cold start, the flow is minimal, the temperature of the metal is at room temperature and heat exchange happens between the steam and metal parts resulting in thermal stress.

Every designer is interested in making a prediction that is as accurate as possible. This requires modelling the entire flow path with all the intricate details which means generating a complex 3D model,use of extensive computational resources and so on, which ultimately results in more time and cost. Even if one has the luxury of using a complex 3D model with all the intricate details, the question is how to get the appropriate boundary conditions to be applied for the 3D simulation and how to reduce the number of iterations required between the flow analysis and structural analysis. The flow parameters and the area between sealing fins in the labyrinth (refer Fig.1) is varied, not only in each stage, but also within each component which means the heat transfer coefficient being applied to each of these locations is also different.

sealing-fins-in-a-turbine-stage
   Figure 1: Sealing fins in a turbine stage

 

As seen in Fig. 2, the entire flow through the seals and cavities can be modelled as a 1D model considering both the flow and heat transfer between the fluid and metal. During design phase of the flow path, a streamline analysis tool such as AxSTREAM® can give the leakage flow parameters which can be further detailed using AxSTREAM® NET, the 1D flow and heat transfer module. The results from the 1D module, which gives the thermodynamic parameters and heat transfer coefficients at different zones in the seal, can be used to apply the boundary conditions for performing thermos-structural analysis of the steam turbine.

1d-model
                    Figure 2: 1D model of the flow and heat transfer in sealing fins of a turbine stage

 

To learn more about how the AxSTREAM® platform can be used for designing steam turbines, from concept to optimizing operating curves, please contact info@SoftInWay.com

 

 

Understanding the Characteristics of Varying Centrifugal Blower Designs

Many people speculate about the confusion on what is considered a compressor, a blower, or simply a fan.  In essence, each of these turbo-machines achieve a pressure rise by adding velocity to a continuous flow of fluid.  The distinctions between fans, blowers, and compressors are quite simply defined by one parameter, the specific pressure ratio.  Each machine type, however, utilizes a number of different design techniques specific to lower and higher-pressure applications.  As per the American Society of Mechanical Engineers (ASME), the specific pressure is defined as the ratio of the discharge pressure over the suction pressure (or inlet pressure).  The table shown below defines the range at which fans, blowers, and compressors are categorized.
table

Similarities between the design of fans and blowers occur near the lower end of a blower’s range.  As well, many design parallels exist between high-pressure blowers and compressors.  For the article, we will be investigating the different design characteristics of centrifugal blowers. Blower selection depends on a number of factors including operating range, efficiency, space limitations, and material handled.   Figure 1 shows a number of different impeller blade designs that are available for centrifugal blowers.

dans-doc
Figure 1 – Impeller Blading Arrangements for Centrifugal Blowers

Each of these blading arrangements have unique operating ranges limited by overload and stall conditions.  Forward curve and radial blowers exhibit a performance curve for horsepower needed that increases with the volume flow rate.  This means that a motor can be overloaded if discrepancies occur which bring the system to flow rates higher than at the operating point.  On the other hand, any backward arrangement blading exhibits a horsepower curve that increases to a maximum as airflow increases and then drops off again.  This allows the engineer to specify a motor to accommodate the peak horsepower, which in turn ensures that the system cannot overload and is therefore considered “non-overloading”.  The stable range of a blower is defined as the condition under which enough air flows through the fan wheel to fill the spaces in between the blades.  Below this range, the instability of the machine will cause one or several section of blades to stall.

In general, forward curved blading arrangements (or sirocco blowers) are better suited for high volume with lower pressure applications.  These blowers and fans operate at relatively low speeds and pressures, which permit lightweight and cost-effective construction.  Restricted stability ranges, overloading at higher flow rates, and low static efficiencies all limit the capabilities of the forward curved centrifugal blower.  Radial centrifugal blowers have the same problem with overloading at high flow rates; however, they are quite beneficial for moving air in dirty environments due to their flat geometry that prohibits dust or sticky materials from rapidly accumulating.  Composed of 6 to 12 rugged blades extending radially from the hub, these types of blowers run at medium speeds and deliver low air volumes at medium to high pressure.

blower-for-blog
Figure 2 – Centrifugal Blower Design with Airfoil Impeller Arrangement in AxSTREAM™

Perhaps the most important centrifugal fan impeller orientation, the backward orientation, comes in three standard shapes: backward inclined, backward curved, and backward inclined aerofoil (or airfoil). All of these designs exhibit most of the same characteristics, with some discrepancies in their efficiencies.  Generally, the “flat-bladed” back inclined design achieves an efficiency of about 82%, while the backward curved and airfoil designs near 88% and 90%, respectively.  In addition to high efficiencies, backward oriented blade designs allow for the highest operating speeds of all centrifugal blowers and are considered non-overloading.  The only drawbacks to these types of blowers would be the high cost of manufacturing as well as the inability to handle flows with high particulates due to the close running clearances and complex geometries.

If you would like to learn more about the design of centrifugal fans, blowers, and compressors, please click here (http://www.softinway.com/machine-type/centrifugal-compressor/).

References:

http://www.wesellfans.com/pdf/underStanding-Centrifugal-Fans.pdf

http://www.saylor.org/site/wp-content/uploads/2011/09/Chapter-3.5-Fans-Blowers.pdf

http://www.clarage.com/docs/fan-engineering-letters/fan-performance-characteristics-of-centrifugal-fans—fe-2400.pdf?sfvrsn=2

Gaining Turbomachinery Insight Using a Fluid Structure Interaction Approach

Existing research studies for the corresponding flow-induced vibration analysis of centrifugal pumps are mainly carried out without considering the interaction between fluid and structure. The ignorance of fluid structure interaction (FSI) means that the energy transfer between fluid and structure is neglected. To some extent, the accuracy and reliability of unsteady flow and rotor deflection analysis should be affected by this interaction mechanism.

In recent years, more and more applications of FSI are found in the reliability research of turbomachinery. Most of them are about turbines, and a few of them address pumps. Kato [1] predicted the noise from a multi-stage centrifugal pump using one-way coupling method. This practical approach treats the fluid physics and the solid physics consecutively.

cent-pump
Figure 1: Multistage centrifugal pump [1].
In the CFD computations of the internal flows, Kato could successfully predict the pressure fluctuations despite turbulent boundary layer in the impeller passages was not resolved. The computed pressure fluctuations on the internal surface agreed well with the measured ones not only at the blade passing frequencies, (BPF) but also on the base level. By visualizing the distributions of the pressure fluctuations at the BPFs, it was found that the fluctuation was especially high at the second harmonics of the BPF. This was consistent with the vibration velocity measured on the outer surface. On the other hand, he overpredicted the total head by about 10%. This is because turbulent boundary layer in the impeller passage was not resolved, and therefore, the blockage effect was not taken into account appropriately at this stage of the research.

Vibration of the structure portion was then calculated by a dynamical structural analysis with the calculated pressure fluctuations on the internal surface as input data. It was clearly shown that the dominant vibrations of the pump originate from the rotor-stator interaction. The trivial vibrations were damped off over time. The vibration levels of the BPF on the outer surface of the pump structure agreed reasonably well with the  measured ones. The computations revealed the feasibility of the fluid-structure coupled simulation for flow-induced noise generated in turbomachinery.

Another example of fluid-structure interaction was presented by Pei et. Al [2] when an axial-flow pump device with a two-way passage was studied. A coupled solution of the flow field and structural response of the impeller was established using a two-way coupling method to study the distribution of stress and deformation in the impeller and quantitatively analyze that on the blade along the wireframe paths had different flow rates. This studied showed that the maximum equivalent stress and maximum total deformation in the impeller are greatly influenced by flow rate, and its values drops with an increasing flow rate and a decreasing head. In addition, the total deformation in the impeller is greater near the blade rim, where the maximum value can be found. The equivalent stress is greater near the blade hub, where the maximum value can be obtained.

The above studies are the best proof that by using the right methods, tools and expertise you can get an insight for any kind of turbomachinery. Try AxSTREAM using the CFD and FEA integrated modules to design your machine and understand the fundamentals of its operation in depth.

References:

[1] Prediction of the Noise From a Multi-Stage Centrifugal Pump, Chisachi Kato, Shinobu Yoshimura, Yoshinobu Yamade, Yu Yan Jiang, Hong Wang, Ryuta Imai, Hiroyuki Katsura, Tetsuya Yoshida and Yashushi Takano , ASME 2005 Fluids Engineering Division Summer Meeting, Volume 1: Symposia, Parts A and B, Houston, Texas, USA, June 19–23, 2005

[2] Fluid–structure coupling analysis of deformation and stress in impeller of an axial-flow pump with two-way passage, Ji Pei, Fan Meng, Yanjun Li, Shouqi Yuan, Jia Chen, National Research Center of Pumps, Jiangsu University, Zhenjiang, China

Minimizing Environmental Impacts of Geothermal Energy

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Geothermal energy is categorized as a “green energy”, with low emission of approximately 5% of carbon dioxide, 1% H2S, 1% sulfur dioxide and less than 1% of the nitrous oxide of an equal sized fossil or coal power plant. Concentrations of each environmentally disruptive gases are controlled by temperature, composition of fluid, and geological setting. Although most of the geothermal emissions commonly come from existing geothermal resource gas, some percentage of the emission also comes from various processes of the energy conversion process. Non-condensable gases are also emitted as a part of high temperature process of geothermal energy conversion.

According to various studies, the type of geothermal power plant design would really impact the production rate of the mentioned gasses. The selection between open-loop and closed (binary)-loop system is essential while taking into consideration air emission. Geothermal plants to this date are commonly separated into three main cycle design: dry-steam, flash-steam or binary –the first two extensively generate more greenhouse gasses (GHGs) compared to the last. In a binary loop system, gases which are removed from the system will not be transferred to the open atmosphere, instead, after transferring the heat gasses will be run through back to the ground, and result in minimal air pollution. In contrary, open-loop system emits all of the emission gas contained such as hydrogen sulfide, carbon dioxide and many more. There are also different factors which cause the technology to emits gases that are naturally present in the fluid such as fluid chemistry/composition, fluid phase, and geological setting to temperature.

The main types of air emission or contamination within the application of geothermal energy are commonly found to be carbon dioxide and hydrogen sulfide. Hydrogen sulfide reacts to produce SO2 once touched with the atmosphere. SO2 is known for its hazardous nature to health and environment, causing acid rain and respiration problem. Even though the concentration of this gas emission is significantly smaller than a conventional fuel power plant, reduction of hydrogen sulfide emission is still desirable for any conditions. Types of condensers installed to the design determines the ratio between the condensable and non-condensable gas. Consequently, with the right selection of condenser as well as implementing other reduction plans such as installation of adsorption tower, etc, hydrogen sulfide emission could be minimized.

References:

http://www.ucsusa.org/clean_energy/our-energy-choices/renewable-energy/environmental-impacts-geothermal-energy.html#.WBO61fkrLb0

http://georg.hi.is/files/Bjarni%20Mar%20Juliusson%20From%20Wast%20to%20Value.pdf

http://geo-energy.org/reports/GeothermalGreenhouseEmissionsNov2012GEA_web.pdf

http://www.merichem.com/company/overview/technical-lit/tech-papers/geothermal-power-plants

A Reasonable Approach to Pump Design While Avoiding Resonance

For the majority of pump application, the growing use of variable speed operation has increased the likelihood of resonance conditions that can cause excessive vibration levels, which can negatively impact pump performance and reliability. Mechanical resonance is the tendency of a mechanical system to absorb more energy when the frequency of its oscillations (external excitation source) matches the system’s natural frequency of vibration more than it does at other frequencies. To avoid vibration issues, potential complications must be properly addressed and mitigated during the design phase.

Some of the factors that may cause excitation of a natural frequency include rotational balance, impeller exit pressure pulsations, and gear couplings misalignment. The effect of the resonance can be determined by evaluating the pumping machinery construction. All aspects of the installation such as the discharge head, mounting structure, piping and drive system will affect lateral, torsional and structural frequencies of the pumping system. It is advised that the analysis be conducted during the initial design phase to reduce the probability of reliability problems and the time and expense associated.

Natural frequencies of a pump and motor can be calculated by performing a modal analysis using the Finite Element Method Analysis (FEA). The finite element modelling and analysis techniques provide an understanding of the mechanical system behaviour, including the natural frequency values during design phase.

Understanding the predicted natural frequency values allows an evaluation of the expected separation between the pump natural frequency and excitation frequencies, such as pump operation speed. The separation is established by the pump manufacturer to avoid mechanical resonance.

The boundary conditions assumed during FEA are essential to the accuracy of predicted results. In some cases, the final as-built conditions (such as foundation stiffness) significantly affect the analysis accuracy if they differ from those conditions assumed during the analysis. In such case a pump test is recommended. Tests like that indicate that increasing the natural frequency of the system is the best solution. This increase in natural frequency could be accomplished by modifying two of the pump system’s physical characteristics, reducing mass or increasing stiffness of the system.

It is therefore important to know the type of acceptable solution that will provide the best pump operation. And this is where AxSTREAM adds significant value at the design process. Using SoftInWay fully integrated engineering platform the customers are able to optimize the pumping machine, and next to perform all the necessary structural analysis using AxSTRESS, our express structural, modal and harmonic analysis FEM solver with a customizable, automatic turbomachinery-specific mesh generation.

Source:

http://empoweringpumps.com/lateral-torsional-structural-analyses-pumps-can-avoid-vibration-problems-pump-issues-related-resonance-conditions/

http://www.pumpsandsystems.com/topics/pumps/vertical-turbine-pumps/structural-resonance-problems-vertical-pumps

Development of Molten Salt Energy Storage

Over the past couple of years, energy storage technology has significantly evolved to meet engineering demand and political regulations. This wasn’t initially looked as a desirable investment due to the high production cost, however over time, exploration of such technology by bigger companies has driven down the manufacturing cost and generated more demand. With occurrences such as rapid capital raise of smaller start-up companies, to the acquisition of Solar City by Tesla, the market of energy storage is predicted to continue growing. The technology allows for collection of energy produced to be used at a later time. Energy storage systems have wide technology variation to manage power supply – from thermal, compressed air to everyday batteries.

blog-post-2-image-1Molten Salt Usage

The usage of molten salt in thermal energy storage applications has become more common. In commercial solar energy storage, molten salt (from potassium nitrate, lithium nitrate and more) is used in conjunction with concentrated solar energy for power generation. Molten salts are able to absorb and keep heat energy transferred from the fluid mediator, then to transfer it again when it’s needed. In the liquid state, molten salt has a similar state to water. It also has the capacity to retain temperatures of  1000 Fahrenheit. Though efficiency is known to be lower than other storage media such as batteries, (70% vs 90%), the main advantage of the usage of molten salt is lower costs which allows the technology to be implemented in a higher volume production.

How Molten Salt Energy Storage Works

Using solar energy as the main source of energy, heliostats (mirrors used to track sun/solar heat) are used to reflect the solar radiation into an energy receiver at the power plant. Molten salt then is used to collect this heat energy from the concentrated pool. The molten salt will later be stored. When power is needed, hot molten salt is transferred to a HX (or steam generator) to produce steam at a high pressure and temperature. The steam then will be used for electricity generation as the live steam in a conventional steam power plant. After exiting the generator, molten salt will then be transferred back to the thermal storage tank to again absorb energy.

The Benefits of Molten Salt Energy

There are three main benefits of molten salt energy storage – reliability, economic savings and environmental friendliness. While in a liquid state, molten salt improves long term reliability as well as reduces operation and maintenance cost. The capital cost of the material itself is also relatively cheap and easily accessible. Molten salt is also known as a non-toxic compound, thus completely green and comparable to fertilizer.

Reference:

http://www.renewableenergyworld.com/articles/2008/06/storing-the-sun-molten-salt-provides-highly-efficient-thermal-storage-52873.html

Interested in designing and optimizing your molten salt energy storage? Check out AxCYCLE!

Impeller Design Challenges on Integrally Geared Centrifugal Compressors

The integrally geared compressor, also known as a multi-shaft compressor, is a technology that has been around since the 1960s, but remains underdeveloped.  Usually seen in applications in the industrial gases industry, integrally geared compressors (IGCs) can range in size from small product machines to steam turbine driven high-horsepower, high-flow compressors for air separation plants.  These compressors modular construction principle, consisting of as many as eight different stages, allows for implementation in a large number of varied customer processes.  The main advantages of IGCs in the industrial gases industry is the compact design and smaller installation footprint, efficiency increases due to the use of multiple speeds for separate impellers, and overall lower operational and installation costs.

semi-open-impeller
Figure 1 – Semi-Open Impeller

One of the key design differences between the standard inline compressors and the IGCs is that the integrally geared compressor makes use of both closed AND semi-open impellers.  The reason for the use of open impellers in IGCs are the higher strengths due to better manufacturing techniques, speed of manufacture, and the inherent lower costs.  However, the main drawback to having an open impeller in your system is that in the event of impeller rub, the damage to the compressor would be significantly worse than with a closed impeller.

Engineers have introduced several techniques in order to minimize this risk.  Axial position monitoring of the pinions as well as the use of high-speed thrust bearings can result in an improvement against this rub phenomenon.  However, even with these innovative solutions, open impellers still have a much larger tip clearance (clearance / blade height) then its closed impeller counterparts.  It is very important to realize the efficiency decrease present in compressors with varying tip clearances.

In SoftInWay’s latest update for its turbomachinery design and analysis software, AxSTREAM™, calculating models were improved for centrifugal compressors with large tip clearances.  By taking into account the additional flow deviation from the direction of the blade’s tail as well as the flow recirculation in the meridional direction, better estimates for the flow models can be identified on compressors with tip clearances greater than 0.1.

large-tip-clearance
Figure 2 – Flow Characteristic on a Compressor with Large Tip Clearance

Although integrally geared compressors present several elegant solutions in the industrial gases industry and overall show higher efficiencies than single rotor compressors, many different challenges arise when working with them.  Among them are the impeller blade design challenges mentioned above as well as numerous additional challenges to rotor dynamics that these intricate gearing designs introduce.  SoftInWay has worked alongside MAN Diesel & Turbo since 2005 to solve these problems and many others. To learn more about designing integrally geared compressors or for an in-depth look into our rotor dynamics software please explore the links below.

Integrally Geared Compressors 

AxSTREAM for Rotor Dynamics

 

 

References:

http://turbolab.tamu.edu/proc/turboproc/T20/T20131-144.pdf

http://turbolab.tamu.edu/proc/turboproc/T32/t32-21.pdf

http://turbomachinery.man.eu/products/compressors/integrally-geared

The Balancing Act – Rotor Stability

When designing rotating equipment, it is extremely important to take into account the types of unbalance that can occur. Forgetting this step can result in vibrations that lead to damage of the rotating parts, increasing maintenance costs and lowering efficiency. Currently, if a rotating part already vibrates or makes any noises, maintenance engineers rely on OEMs (Original Equipment Manufacturer) or third parties services companies to conduct balancing services.

Types of Unbalances

ubalance
Figure 1: Static and Couple Forms of Unbalance

The three types of unbalances to consider are static, couple and dynamic. Static unbalance (Figure 1) occurs when a mass at a certain radius from the axis of rotation causes a shift in the inertia axis. Couple unbalance, usually found in cylindrical shapes, occurs when two equal masses positioned at 180 degrees from each other cause a shift in the inertia axis, leading to vibration effects on the bearings. Lastly and most common, dynamic unbalance occurs when you have a combination of both static and couple unbalance.

Balancing Methods used Today

high-speed-balance
Figure 2 High-Speed Balance

Two of the most popular balancing methods that are performed on existing turbine rotors are low-speed balancing and high-speed balancing methods. A low-speed balance helps determine wear and tear on rotor under minimum speed conditions. This method is generally used by companies who lack access to a high-speed balance cell. A high-speed balance (Figure 2) allows the test be run at or passed operating speed, and even though it is more expensive, it tends to be a more accurate method of testing. The primary reason why  more end users are opting for this type of balance, for both turbine rotors and power generator fields, is that all major components can be tested under rigorous conditions. By doing this, the full operating lives of these components can accurately be determined.

Want to make sure you’re taking into account unbalances when designing rotors in rotating equipment? Look into our rotordynamics software capabilities 

References:

Reciprocating Machinery Dynamics by Abdulla S. Rangwala – Chapter 9

http://www.powermag.com/high-speed-turbine-rotor-balancing-lowers-costs-improves-operation/?printmode=1

The Importance of Turbulence Modelling

What is the importance of turbulence modelling in capturing accurate 3D secondary flow and mixing losses in turbomachinery? An investigation on the effect of return channel (RCH) dimensions of a centrifugal compressor stage on the aerodynamic performance was studied to answer this question by A. Hildebrandt and F. Schilling as an effort to push turbomachinery one step further.

W. Fister was among the first to investigate the return channel flow using 3D-CFD. At that time the capability of commercial software was not extended and any computational effort was limited by the CPU-capacity. Therefore, only simplified calculations that included constant density without a turbulence model (based on the Prandtl mixing length hypothesis) embedded in in-house code, were performed.

Although separated flow without a predominant flow direction could not have been calculated, the method indicated separated flow regions with relatively accurate precision, and it predicted the magnitude of loss coefficients to a higher degree than experimental data. The study was further
simplified using incompressible flow, and an axial U-turn inlet flow.AxCFD

The biggest drawback of using inverse methods for return channel design refers to the question of appropriate flow distribution across the RCH surface. Furthermore, flow separation cannot be predicted with the help of singularity methods. In order to circumvent the problem of predicting flow separation, nowadays compressible viscous 3D-CFD applied with different highly complex turbulence modeling is the state of the art even at the conceptual stage of the design.

Hildebrandt and F. Schilling analyzed three different centrifugal stages regarding the return channel system performance. All three stages featured the same impeller type, two of them being applied with a 3D-RCH at different flow coefficient and one impeller being applied with a 2D-RCH system. The 3D-RCH stage featured both CFD calculated and measured superior aerodynamics over the 2D-RCH stage regarding the overall performance as well as regarding the outlet flow angle. The comparison between the measured and the CFD-predicted performance showed agreement both when it comes to overall performance (efficiency, pressure rise coefficient) and also regarding detailed flow field (outlet flow field). The 3D secondary flow and mixing losses of the entire domain downstream the vaneless diffuser were either underestimated or overestimated by the CFD-calculations, depending on the turbulence modeling and the impeller fillet radii-modeling which affects the RCH-inlet flow conditions. The effect of fillet radii-modeling on the RCH-exit flow angle spanwise distribution was found to be significant in order to better match the experimental results.

It is worth noting that the rather simple Spalart–Allmaras turbulence model provided better agreement with the measured RCH-exit flow angle distribution than the more sophisticated k-epsilon model, which on the other hand, outputted a closer fit with the measured surface vane pressure distribution. Regarding the RCH total pressure loss distribution, none of the models showed a perfect agreement with the measurement data.

Moreover, the incident losses of the 3D-RCH system seemed to play a minor role within the overall RCH-loss which is significantly dominated by the 3D-secondary losses.

Interested in learning more? Check out AxSTREAM and AxCFD!

[1] A. Hildebrandt and F. Schilling, 2017 “Numerical and Experimental Investigation of Return Channel Vane Aerodynamics With Two-Dimensional and Three-Dimensional Vanes”, Journal of Turbomachinery Vol. 139 / 011010-1

[2] Fister, W., Zahn, G., and Tasche, J., 1982, “Theoretical and Experimental Investigations About Vaneless Return Channels of Multi-Stage Radial Flow Turbomachines,” ASME Paper No. 82-GT-209.

The Future of Turbocharger Technology

­One of the main setbacks in scaling different turbochargers for diesel, petrol, and gas engines is the inherit variability that different turbochargers would exhibit at low or high RPMs. In order to understand this further, a common term used to describe a flow characteristic of these machines is the A/R ratio.  Technically, this ratio is defined as the inlet cross-sectional area divided by the radius from the turbo center to the centroid of that area (Figure 1).  This ratio is essentially a metric for the amount of air that is allowed into the turbine section of the turbocharger.

Ratio visualization
Figure 1 – A/R Ratio Visualization

For smaller turbochargers, lower A/R ratios allow the fast exhaust velocities to drive the turbine at lower speeds.  This results in a more responsive engine and overall higher boosts at lower RPMs.  However, once a vehicle starts to navigate at a higher RPM, smaller turbochargers experience a significant reduction in performance due to the high backpressure present in the system.  This occurs because of the low A/R ratio limits the flow capacity and does not allow a sufficient amount of air to feed into the turbine.  The same effect is present for larger turbochargers, only in reverse.  They will perform most efficiently at higher RPMs, but in turn exhibit a significant reduction in performance at lower RPMs.

In order to overcome this phenomenon, many engineers have developed more complex turbocharger systems over the years, which attempt to leverage the benefits of each type of turbo.  One of the first solutions to this dilemma was the twin turbo: simply comprised of two separate turbochargers operating in the system in parallel or in series.  The problem with this system is that it disproportionately increases the cost, complexity, and space necessary for implementation.

Other technologies are very common for overcoming this problem without the use of two separate turbos.  One design is the twin-scroll turbocharger, which works like a pair of turbochargers connected in parallel.  Instead of using two separately sized turbochargers, the twin-scroll design has two exhaust-gas inlets and two nozzles feeding into a single turbocharger.  Another design, the variable geometry turbocharger, uses internal vanes within the turbocharger to dynamically alter the A/R ratio of the system at different rev levels.  The variable geometry turbocharger (VGT) displays exceptional performance at the low and high RPM ranges, however, it requires the use several moving parts and exotic materials in order to handle the high exhaust temperatures present in petroleum engines.  For this reason, the variable geometry turbocharger is generally only used in diesel engines due to the relatively low exhaust temperatures in comparison.  Twin-scroll turbochargers, while more commonly used in gasoline applications, do not have the variability of the VGT and are not as efficient over the full rev range of the vehicle.

turbocharger
Figure 2 – BorgWarner Twin-Scroll Variable Geometry Turbocharger Concept

BorgWarner has recently showcased a new turbocharger concept that makes use of both technologies discussed above.  The “Twin-Scroll Variable Geometry Turbocharger” has a valve that can redirect airflow from just a single scroll to an airflow completely split between two scrolls.  It allows for the any range in between these extremes much like the VGT; however, it only uses only one valve as opposed to the several vanes needed in the VGT.  It derives the same exhaust separation idea as the twin-scroll while allowing for the variability of the VGT.  With all the new emissions regulations calling for increases in efficiency, it will be interesting to see if this new technology can find its way into the market.

References:

http://www.economist.com/news/science-and-technology/21638917-why-turbocharged-four-cylinder-engines-now-rule-road-little-engine-could

https://www.carthrottle.com/post/engineering-explained-6-different-types-of-turbocharger-and-the-advantages-of-each-setup/