Modern Approaches and Significance of Multiphase Flow Modeling

Introduction

Corresponding with the development of industrial technology in the middle of the nineteenth century, people dealt with multiphase flows but the decision to describe them in a rigorous mathematical form was first made only 70 years ago. As the years progressed, development of computers and computation technologies led to the revolution in mathematical modeling of mixing and multiphase flows. There are a few periods, which could describe the development of this computation:

«Empirical Period» (1950-1975)

There were a lot of experiments that were done during this period. All models were obtained from experimental or industrial facilities which is why using them was difficult for different cases.

«Awakening Period» (1975-1985)

Because of sophisticated, expensive, and not universal experiments, the researchers’ attention was directed to the physical processes in multiphase flows.

«Modeling Period» (1985-Present)

Today, the models for multi-flow calculation using the equations of continuity together with equations of energy conservation are obtained, which allow describing phase’s interaction for different flow regimes. (A.V. Babenko, L. B. Korelshtein – Hydraulic calculation two-phase gas-liquid course: modern approach // Calculations and modeling journal. – 2016. – TPА 2 (83) 2016. – P.38-42.)

Technology Development

Since the time of industrial development, installation designs have undergone great changes. For example, there are shell and tube evaporators for freeze systems where the heat transfer coefficient has increased 10 times over during the last 50 years. These results are a consequence of different innovation decisions. Developments led to research into mini-channels systems, which is the one of the methods to increase intensification of phase transition. Research has shown that heat exchange systems with micro and nano dimensions have a much greater effect than the macrosystems with channels dimensions ≤3-200 mm.

In order to organize fundamental research, it is very important to understand hydro, gas dynamics, and heat changes in two-phase systems with the phase transition. At present, the number of researchers using advanced CFD programs has increased. Our team is one of the lead developers of these program complexes.
Mathematical modeling of compressible multiphase fluid flows is interesting with a lot of scientific directions and has big potential for practical use in many different engineering fields. Today it is no secret that environmental issues are some of the most commonly discussed questions in the world. People are trying to reduce the emissions of combustion products. One of the methods to decrease emissions is the organization of an environmentally acceptable process of fuel-burning with reduced yields of nitrogen and sulfur. The last blog (https://blog.softinway.com/en/modern-approach-to-liquid-rocket-engine-development-for-microsatellite-launchers/) discussed numerical methods, which can calculate these tasks with minimal time and cost in CFD applications.

Waste Heat Boiler
Picture 1 – Waste heat boiler http://tesiaes.ru/?p=6291

For more effective use of energy resources and low-potential heat utilization, the choice of the Organic Rankine Cycle (ORC) is justified. Due to the fact that heat is used and converted to mechanical work, it is important to use a fluid with a boiling temperature lower than the boiling temperature of water at atmospheric pressure (with working flow-boiling temperature about 100⁰C). The usage of freons and hydrocarbons in these systems makes a solution impossible without taking into account the changes of working fluid phases. Read More

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

Saving the Planet, One Turbine Simulation at a Time

This is an excerpt from the Siemens Blog. You can read the full version here.

Originally written By Erik Munktell  on January 14, 2021

 

Listening to the turbine experts: a review of 2020

Can a turbine simulation save the planet? One simulation alone is not enough. But one simulation with that intent can inspire other engineers and researchers to do the same. This butterfly effect is happening today in turbine design.

I live in Sweden, the same country as Greta Thunberg. Her message is that we must act now to save the planet. For a long time, Sweden’s electricity has come mainly from Hydropower and Nuclear power. But lately a lot of focus has still been on building wind and solar power plants. Here in Sweden we also have more trees being grown than being cut down. As a result we are close to being carbon negative, transportation and industry included.

With this country-wide energy focus friends often ask me why I work with gas turbines. Surely, they are not needed?  I rarely get them to listen to my answer for more than a few minutes, as in my enthusiasm it quickly gets technical! But in short – I do it to save the planet. Or rather, we do it to save the planet. Because I am not alone in this effort. Many people work on making turbines run more efficiently, with fewer – or even no – polluting emissions. We can’t do everything by ourselves, but our work can inspire others and together we can create a clean energy future.

My small part in this plan is to make sure the actual makers of gas turbines have the best tools in the world for designing new and better electricity generating or flight enabling propulsion products. In this blog, I take a look at the many ways we saw in 2020 that turbine companies are using our simulation tools to do just that.

Read More

Hydrogen in Combined Cycles

Hydrogen is a clean and carbon-free fuel and is considered a key element for energy transition. Renewable power generation by solar and wind is increasing, which requires flexible operation to balance the load on the energy grid with the ability to rapidly adjust the output. Gas turbines with a combustion system for hydrogen operation offers a low carbon solution to support the stability of the energy grid. This provides a solution to the need for energy storage, in the form of hydrogen, and flexible power generation.

Discharging green-house gases and particulates into the atmosphere has an impact on the global climate. With this current trend of increasing awareness towards the environment, alternative fuels are again being examined to reduce the impact of emissions. Hydrogen is perceived as the only long-term solution to global warming concerns. It is also the only fuel that can create large reductions in carbon emissions. There are zero CO2 emissions produced in hydrogen combustion. Hence, NOx emissions are the only remaining concern. Micro-mix combustion is used to implement miniaturized diffusive combustion to combust hydrogen with low emissions. With miniaturized diffusive combustion, local flame hot spots, which are caused by arising stoichiometric conditions of hydrogen, are reduced substantially with an increase in the local mixing intensity. Improvements in the mixing quality provide reduced emissions of NOx with a more balanced flame profile. Micro-mix combustion was also studied with different mixtures of fuels including hydrogen, kerosene and methane establishing an adaptive combustors [1,2].

Power generation systems based on hydrogen could be an important alternative to conventional power systems based on the combustion of fossil fuels. The main effort in the field is oriented towards the use of hydrogen in fuel cells and combustion with gas turbines. Consider the main options for combined cycles based on a hydrogen-fueled gas turbine unit shown in Figure 1.

Basic Simple and Combined Co-generation Cycles

Figure 1. Brayton - Rankine combined cycle
Figure 1. Brayton – Rankine combined cycle

One of the most widely used combined cogeneration cycles is the Brayton – Rankine cycle. This cycle is a symbiosis of the Brayton (simple cycle gas turbine) cycle and the Rankine (steam turbine) cycle.

Figure 2. Typical efficiencies of various types of plants
Figure 2. Typical efficiencies of various types of plants [3]
Figure 2, above, shows the efficiency of the power plant depending on the type of cycle. The power plants referenced are: the simple cycle gas turbine (SCGT) plants with firing temperatures of 2400°F (1315°C); recuperative gas turbine (RGT) plants, where the exhaust gases from the turbine are used to heat the incoming air to the combustion chamber; the steam turbine plants; the CCPPs; and the advanced combined cycle power plants (ACCPs), such as CCPPs using advanced gas turbine cycles. Read More

Gas Turbine CFD – Driving Innovation with Data and Insight

This is an excerpt from the Siemens Blog. You can read the full version here.

Originally Written By Chad Custer – February 2, 2021

Once upon a time in a world without gas turbine CFD simulation.

Manager: The design team came up with a new blade concept, but they need to know the maximum possible temperature in the machine.

Test engineer: Anywhere in the whole machine?

Manager: Yes. And for any operating condition the machine might get used for. How long until you can have those results to the team?

Test engineer: Uhh

Terms like “virtual prototype”, “simulation testbed” and “digital twin” have become so common that you may dismiss them as buzzwords. However, to me, these terms not only still have meaning. These words do drive how I look at simulation. Read More

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

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

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

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

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

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

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

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

Turbomachinery CFD Simulation: Art in Motion

This is an excerpt from the Siemens Blog. You can read the full version here.

Originally Written By Justin Hodges - July 14, 2020  

Turbine blade simulation juxtaposed with turbine blade art. The resemblance is uncanny! 

Believe it or not, there is some true art in turbomachinery CFD simulation. From the creamer in your coffee to the tumbling of flow through a small waterway. There is something palpable with intrigue when observing fluid flows in our everyday routines. As computational fluid dynamics practitioners, we are fortunate to have a unique opportunity. That is to simulate and observe these same curious fundamentals of turbulence and fluid flow until our heart is content.

Read More

Improving Turbine and Compressor Design Matching

Compressor-Turbine-MatchingOne of the most prominent steps of complete gas turbine design is turbine-compressor matching. There are three major components to a gas turbine: compressor, combustor, and turbine. Although all of the components are designed individually, each of the components needs to correspond within the same operating condition range since all are integrated into one cycle. Consequently, an optimal design of each component must fit the requirement of other component’s optimal parameters. Corresponding operating points for each component must be found at equilibrium with the engine, thus the overall performance of gas turbine can be reached within the defined range of parameters.

The idea behind component “matching” process is to find flow and work compatibility between corresponding components. Based on the mechanical constraints, gas generator speed and firing temperature of a gas turbine have limitations depending on: ambient temperature, accessory load and engine geometry. The match temperature chosen should be the ambient temperature which reach both upper limits at the same time.  Pressure ratio needed to allow a certain gas flow is also one of the most prominent parameters that has to be taken into consideration. Designers need to make sure that the gas flow through the power turbine from gas generator satisfy the pressure ratio needed for compressor power requirements. Gas generator can easily show an altered match temperature due to some conditions i.e: reduction in compressor efficiency (due to fouling, etc), change of thermodynamic properties of combustion product, gas fuel with lower or higher hearing value, etc. Match parameters of an engine could also be altered by changing the flow characteristics on the first turbine nozzle.Turbine-Compressor

Using characteristic map/curve as well as thermodynamic relationships of turbine and compressor, calculations can be performed to identify the permitted operating range. It must be taken into consideration that all calculated value must match the value from map data.

Trying to find the fastest solution for this step? SoftInWay’s turbine-compressor matching feature in AxSTREAM could help you cut engineering time and simplify the process. Combining performance maps of turbine and compressor, making it easier for the user to determine points of joints operations.

Take a look into AxSTREAM’s to learn more about this.

Reference:

https://www.grc.nasa.gov/www/k-12/airplane/ctmatch.html
http://cset.mnsu.edu/engagethermo/components_gasturbine.html
http://turbolab.tamu.edu/proc/turboproc/T29/t29pg247.pdf
http://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=3255&context=etd
Turbine Compressor Matching Compatibility Mode Document

What’s Going to Happen to the Service Market?

http://www.businessweek.com/news/2014-04-29/ge-said-to-covet-alstom-business-servicing-electric-power-plants
Source: Business Week, “GE Said to Covet Alstom Business Servicing Power Plants”

GE considers buying Alstom but so is Siemens. What’s going to happen to the service market?

Currently Alstom covers 25% of the world’s service market and is the world’s third largest provider of equipment and services for power generation. (Source: Alstom)

Power plants are aging as we speak, so the service market is attracting the attention from different service providers.
Continue reading “What’s Going to Happen to the Service Market?”

Shortening Start-Up Time and Life Prediction of Critical Components

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steamturbine
Steam Turbine

This month we’re hosting the third segment to our Steam Turbine Webinar Series.

Shortening Start-Up Time and Life Prediction of Critical Components

Shortening  turbine start-up is a main concern for power machinery operators and manufacturers – is it a concern of yours? Continue reading “Shortening Start-Up Time and Life Prediction of Critical Components”