Computational Fluid Dynamics in Turbomachinery Design

The evolution of turbomachinery technology can be traced back several centuries and has resulted in the high efficiency turbomachines of today. Since the 1940s, turbomachinery development has been led mainly by gas turbine and aeroengine development, and the growth in power within the past 60 years has been dramatic. The development of numerical methods and the increasing computing capacity helped establish a strong design capability in the industry.

The first numerical methods related to turbomachinery were developed years before the use of digital computations. In 1951 Wu [1] introduced the blade-to-blade (S1) and hub-to-tip (S2) stream surfaces, which dominated the field until the 1980s when computer resources made it possible to account for 3D methods. The axisymmetric S2 calculations, also called “throughflow calculation” became the backbone of turbomachinery design, while the S1 calculation remains the basis for defining the detailed blade shape.

Fully 3D methods replaced the stream surface calculations by a single calculation for the whole blade row. This removed the modelling assumptions of the quasi three-dimensional approach but required far greater computer power and so was not usable as a routine design tool until the late 1980s. For similar reasons, early methods had to use coarser grids that introduced larger numerical errors than in the Q3D approach. Such limitations are now overcome with the rapid growth of computer technology.

Nowadays, the design of advanced turbomachinery components [2] is facing more demanding requirements. Higher performance must be achieved within shorter design cycles and at lower cost. Ambitious objectives in the reduction of weight, complexity and manufacturing cost lead to fewer compressor and turbine stages, and therefore to increased stage loading. For designers, this new situation implies the capability to control the very complex flow phenomena occurring in highly loaded stages, on the whole operating range of the engine, early in the design process. In addition to aerodynamic performance, the aggressive design of advanced, fully 3D blades also requires an early focus on all the aspects related to engine mechanical limitations such as blade flutter, forced response and thermal constraint.

The increased requirements on 3D CFD modelling lead to parallel processing of the flow phenomena. The majority of commercial CFD tools demands additional cost for parallel computing, which increase the total cost of the design process. With AxCFD, the users have the opportunity to use parallel calculation without the need to pay extra! AxCFD along with all design modules is fully integrated in the AxSTREAM Software Suite, the most complete engineering platform on the market. Try it now and enjoy the comfort of designing turbomachines from scratch to complete 3D CAD in a couple of hours.

References:

[1] Wu, C. H. A general through flow theory of fluid flow with subsonic or supersonic velocities in turbomachines of arbitrary hub and casing shapes. NACA paper TN2302, 1951
[2] H. Joubert, H. Quiniou, “Turbomachinery designed used intensive CFD”, Snecma http://www.icas.org/ICAS_ARCHIVE/ICAS2000/PAPERS/ICA6104.PDF 

The Future of Combined Cycle

In modern days, power generation planners are faced with the challenge of pushing out the most energy from fuel while at the same time minimizing cost and emission.However, finite fuel also generates mass concerns regarding the reserve left to be used in nature. Consequently, people are continuously looking for an economical and highly efficient solution.

To this date, combined cycle gas turbine applications are found to be the best solution to the problem. The application is known to be highly efficient, have favorable energy conversion rates, comparatively lower start up time compared to conventional steam cycles and able to squeeze more power from the same amount of fuel.

countriesOver the past decade, the use of combined cycles has taken over most of the power generation industry. Triggered in the 1990s by the higher costs and environmental concerns of coal power plants, people starting to look for an alternative to cover demands in energy. At the time natural gas seems to be the most logical substitute.

With the increase of renewable energy application, the demand for combined cycles also increases and helps offset the fluctuations of renewable technology. Combined cycle power plants are also found to emit significantly fewer greenhouse gasses compared to most traditional power plants. With this in mind, the use of combined cycle power plants has substantially reduced the amount of emission.

Due to all of the advantages of CCGT mentioned above and more–not to mention the low installed cost, fuel flexibility, flexible duty cycle, and short installation cycle,  investors find combined cycle implementation to be attractive. According to Black & Veatch, natural gas-fired generation is projected to add 348,000MW to U.S grid, where most (if not all) of it will be supplied by a combined cycle generation.

Interested in optimizing your combined cycle plant? AxCYCLE  should do the trick!

 

Reference:

http://www.power-eng.com/articles/2014/02/a-report-on-combined-cycle-projects-in-north-america.html

https://powergen.gepower.com/content/dam/gepower-pgdp/global/en_US/documents/technical/ger/ger-4206-combined-cycle-development-evolution-future.pdf

 

An Integrated Design System for Gas Turbines

In my earlier blog titled “Optimizing the Cooling Holes in Gas Turbine Blades, I wrote about how optimizing the cooling flow through turbine blades is important considering both performance and reliability. The design process differs between different designers and depends on a number of factors including expertise, availability of design tools, statistical or empirical data, corporate procedure and so on. That being said, the ultimate goal is to provide a design which is considered optimal. Though the designer is often satisfied on completion of a design and when the machine is put into operation, there is always the feeling  that we could have done better if there were more resources and time. Integrating the entire design process with multidisciplinary optimization provides a great opportunity to arrive at the optimal design rapidly with less manual intervention and effort.

axstream
Figure 1: Integrated AxSTREAM® Platform

Figure 1 shows the integrated approach to design a cooled gas turbine using multidisciplinary tools in an optimization environment. The flow path design starts from the conceptual stage to arrive at the optimal flow path geometry, accounting for a preliminary estimate of the cooling flow. Detailed design requires accurate estimation of the cooling flow considering the actual geometries and the material temperatures. Using ID head and flow simulation tools such as AxSTREAM® NET, the cooling flow can be modelled to produce the optimal geometric dimension in an iterative process to further fine tune the flow path performance. To meet the performance and reliability objectives, multidisciplinary optimization can be achieved via the integrated modules. The process when further integrated with a CAD package can help in generating the optimized geometry that can be taken for prototype development.

To learn more about how the AxSTREAM® platform can help you obtain an optimized gas turbine design quickly and accurately, please contact sales@softinway.com; info@softinway.com.

Fatigue in Turbomachinery

This post is based on DeLuca’s publication about fatigue phenomena in gas turbines [1]. One of the most significant characteristics of a gas turbine is its durability. Especially for the aerospace industry where engines must meet not only propulsion but also safety requirements, the failure of gas turbine blades is a major concern. The “cyclic” loading of the components associated with generator excursions is one of the principal sources of degradation in turbomachinery. In addition, fatigue can be caused during the manufacturing of the components. There are three commonly recognized forms of fatigue: high cycle fatigue (HCF), low cycle fatigue (LCF) and thermal mechanical fatigue (TMF).The principal distinction between HCF and LCF is the region of the stress strain curve (Figure 1) where the repetitive application of the load (and resultant deformation or strain) is taking place.

gas-turbine-alloy
Figure 1 – The stress vs. strain curve for a typical gas turbine alloy

HCF is metal fatigue that results from cracking or fracturing generally characterized by the failure of small cracks at stress levels substantially lower than stresses associated with steady loading. HCF occurs as a result from a combination of steady stress, vibratory stress and material imperfections [2].  It is initiated by the formation of a small, often microscopic, crack. HCF is characterized by low amplitude high frequency elastic strains. An example of this would be an aerofoil subjected to repeated bending. One source of this bending occurs as a compressor or turbine blade passes behind a stator vane. When the blade emerges into the gas path it is bent by high velocity gas pressure. Changes in rotor speed change the frequency of blade loading. The excitation will, at some point, match the blade’s resonant frequency which will cause the amplitude of vibration to increase significantly.

In contrast, LCF is characterized by high amplitude low frequency plastic strains. A good example of LCF damage is of the damage which is caused by local plastic strains at the attachment surfaces between a turbine blade and the turbine disk. Most turbine blades have a variety of features like holes, interior passages, curves and notches. These features raise the local stress level to the point where plastic strains occur. Turbine blades and vanes usually have a configuration at the base referred to as a dovetail or fir tree.
In the case of thermal mechanical fatigue (present in turbine blades, vanes and other hot section components) large temperature changes result in significant thermal expansion and contraction and therefore significant strain excursions. These strains are reinforced or countered by mechanical strains associated with centrifugal loads as the engine speed changes. The combination of these events causes material degradation due to TMF.

As you can see, it is important to take into account stresses on gas turbine blades in order to determine the viability of the component. AxCFD and AxSTRESS are both vital tools that can help you quantify the stresses on your blades and make the correct decision for the choice of materials and operation conditions of the machine.

Reference:

[1] D.P.DeLuca, “Understanding fatigue”, United Technologies Pratt & Whitney;
[2] Sanford Fleeter, Chenn Zhou, Elias N. Houstin, John R. Rice, “Fatigue life prediction of turbomachine blading”, Purdue University.

Component Matching of Industrial Gas Turbines

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.

two-shaft-gas-turbine
Figure 2 – Simplified Two-Shaft Gas Turbine Arrangement Modeled in AxCYCLE

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.

gas-turbine-arrangement-in-axcycle
Figure 1- Single-Shaft Gas Turbine Arrangement in AxCYCLE (Power Turbine and Gas Generator Turbine Considered One Turbine)

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.

axmap
Figure 3 – Off-Design Analysis for an Axial Turbine using AxSTREAM’s AxMAP Module

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.

 

References:

http://turbolab.tamu.edu/proc/turboproc/T29/t29pg247.pdf

Concentrated Solar Power

As time goes by, the demand for energy rises while finite resources gradually diminish. The concept of going ‘green’ or using infinite resources has become more and more common in the marketplace. With this in mind, the abundance and reliability of solar energy makes for an attractive alternative. This is because solar power is different. This statement, of course, begs the question of HOW solar power differs.

Common traditional power plants still utilizes finite fuel. Steam power plants, for example, use the fuel as an energy source to boil water which, in turn, allows the the steam to turn the turbine and drive the generator to produce electricity. Concentrated solar power systems, however, use heat energy from the sun as a heat source – which is renewable. This system works by using utilizing mirrors or mirror-like materials to concentrate energy from the sun and then takes that energy to produce steam. The system can also store the energy that is absorbed during the day, to be used at night when the sun is not present. There are a few different types of concentrated solar power systems which one can choose from.

solar
Source
  1. Parabolic Trough: This type of solar power uses a curved mirror to focus the sun’s energy to a receiver tube with high temperature heat transfer fluid which absorbs the sun’s energy and passes it through  a heat exchanger to heat water which produces steam.
  2. Compact Linear Fresnel Reflector: The working principle of this solar power type is rather similar to parabolic trough, though instead of using a curved mirror, this application utilizes flat mirrors which are more economical. These mirrors act as reflectors to focus the solar energy into the tubes to generate high-pressure steam.
  3. Power Tower: The power tower uses heliostats to track the sun movement and focus the solar energy to a receiver in the middle which is installed into an elevated tower. This application has been found to have better efficiencies compared to other types of solar power and can run on a higher temperature. The use of molten salt as a transfer fluid for the power tower applications is relatively common and helps improve efficiency.
  4. Dish-Engine: This type of solar power utilizes mirrors that are designed to be distributed over a dish surface to concentrate solar power to a receiver in the middle. The application runs on a very high temperature and uses transfer fluid with a very high boiling point to power a high requirement engine.

 

Newer applications tend to lead to the installation and use of power tower design, since this design allows technology storage implementation which can be seen as a reliable option for the future of concentrated solar power application, not to mention the economic benefit it has compared to other technology storage implementation.

References:
http://www.seia.org/policy/solar-technology/concentrating-solar-power
https://cleantechnica.com/2016/10/31/how-csp-works/

Heat Recovery Steam Generator Design

Heat recovery steam generators (HRSGs) are used in power generation to recover heat from hot flue gases (500-600 °C), usually originating from a gas turbine or diesel engine. The HRSG consists of the same heat transfer surfaces as other boilers, except for the furnace. Since no fuel is combusted in a HRSG, the HRSG have convention based evaporator surfaces, where water evaporates into steam. A HRSG can have a horizontal or vertical layout, depending on the available space. When designing a HRSG, the following issues should be considered:

hrsg-boiler
Figure 1: Schematic of a HRSG boiler
  • The pinch-point of the evaporator and the approach temperature of the economizer
  • The pressure drop of the flue gas side of the boiler
  • Optimization of the heating surfaces

The pinch-point (the smallest temperature difference between the two streams in a system of heat exchangers) is found in the evaporator, and is usually 6-10 °C, which can be seen in Figure 2. To maximize the steam power of the boiler, the pinch-point must be chosen as small as possible. The approach temperature is the temperature difference of the input temperature in the evaporator and the output of the economizer. This is often 0-5 °C. The pressure

hrsg-boiler-2
Figure 2: Example of a heat load graph for HRSG boiler

drop (usually 25-40 mbar) of the flue gas side also has an effect on the efficiency of power plant. The heat transfer of the HRSG is primarily convective. The flow velocity of the flue gas has an influence on the heat transfer coefficient. The evaporator of heat recovery boiler can be of natural or forced circulation type. The heat exchanger type of the evaporator can be any of parallel-flow, counter-flow or cross-flow. In parallel-flow arrangement the hot and cold fluids move in the same direction and in counter-flow heat exchanger fluids move in opposite direction.