Thermo-Physical Properties of Fluids for Simulation of Turbomachinery

Computer simulation and use of CAE/CAD are well-established tools used to understand the critical aspects of energetics (various losses), kinematics (velocities, mach no. etc.) and thermodynamics (pressures, temperatures, enthalpy etc) in thermodynamic cycles and turbomachinery. Computational models are now enabling the design and manufacture of machines that are more economical, have higher efficiency and are more reliable. Accuracy of complex processes that are simulated depends on thermos-physical properties of the working fluid used as input data. The importance of such properties was recognized when it became evident that a steam turbine cycle can have efficiency variance by a few percentage points depending on the chosen set of fluid properties.

Today the thermo-physical properties data is represented in the form of a set of combined theoretical and empirical predictive algorithms that rest on evaluated data. These techniques have been tested and incorporated into interactive computer programs that generate a large variety of properties based upon the specified composition and the appropriate state variables. Equations of state, correlations, or empirical models are used to calculate thermos-physical properties of fluids or mixtures. Examples of this include Helmholtz energy based equations, cubic equation of state, BWR pressure explicit equations, corresponding states models, transport models, vapor pressure correlations, spline interpolations, estimation models or calculation methods for vapor-liquid equilibrium or solubility, and surface tension correlations. Further fitting techniques, and group contribution methods are incorporated. The following broad level properties are often used in simulation tools:

  1. Thermodynamic properties including equation of state, phase equilibria, p-V-T behavior, heat capacity, enthalpy, thermal expansion, sound speed, and critical phenomena.
  2. Transport properties including thermal and electrical conductivity, viscosity, mass diffusion, thermal diffusion, non-Newtonian behavior, and thermal, thermoacoustic, and other diffusion waves.
  3. Optical and thermal radiative properties including dielectric constant, refractive index, emissivity, reflectivity, and absorptivity.
  4. Interfacial properties including solid-solid interfaces, surface tension, interfacial profiles, interfacial transport, and wetting.



Databases are now available for hydrocarbon mixtures, including natural gas, as well as a number of pure and mixed fluids of industrial importance. IAPWS, NIST and Coolprop are a few examples of such resources that provide valuable tools for turbomachinery and refrigeration engineers, and chemical and equipment manufacturers. One example is the IAPWS-IF97 that divides water and steam properties into five distinct regions.

Another example is properties of R134a expressed as 32 term, modified Benedict-Webb-Rubin (MBWR) equation of state, the accuracy of equation of state is estimated to be ± 0.2 % in density, ± 1 % in constant volume heat capacity and ± 0.6 % in sound velocity. The thermos-physical property databases provide core information for process modeling and development. The completeness, correctness, currency and reliability of the data as well as the integrity and management of the database itself are important factors in the ultimate reliability of the modeled process.

Micro Gas Turbines – Addressing the Challenges with AxSTREAM

During the last decade the development and extensive use of unmanned air vehicles (UAV) has accelerated the need for high performing micro gas turbines. In fact, their large energy density (Whr/kg) makes them attractive not only for UAV application, but also for portable power units, as well as for distributed power generation in applications where heat and power generation can be combined.
Micro gas turbines have the same basic operation principle as open cycle gas turbines (Brayton open cycle). In this cycle, the air is compressed by the compressor, going through the combustion chamber, where it receives energy from the fuel and thus raises in temperature. Leaving the combustion chamber, the high temperature working fluid is directed to the turbine, where it is expanded by supplying power to the compressor and for the electric generator or other equipment available [1].

Regarding the design of micro gas turbines, there are several challenges that need to be overcome. First, scaling is a common technique to define larger or smaller geometries with similar characteristics. However, a simple scaling of a high performance large gas turbine is not the right way to go for a good micro gas turbine design. One of the main reasons is the big change of the Reynolds number, as well as the heat transfer between the hot and cold components, which is not negligible [2].

Moreover, the high rotational speed that is needed to obtain the enthalpy and pressure changes prescribed by the gas turbine cycle constitutes a major mechanical problem. As far as geometrical constraints are concerned, material and manufacturing technique selection is crucial in order to lower the cost of the production, since micro gas turbines need to compete with heavier but cheaper batteries in many cases (i.e. for UAV applications). Finally, another major issue in micro gas turbines is the decrease of compressor and turbine efficiency with decreasing dimensions.

To address the above challenges and ensure a robust design, powerful tools are needed. AxCYCLE allows the user to design, analyse and optimize the thermodynamic cycles of the micro gas turbines and export the boundary conditions to AxSTREAM software platform for design and optimization of the components. The integration of preliminary design, CFD, FEA, and rotordynamic modules along with the simulation of cooling and secondary system flows under one common platform gives the power of controlling the overall design process while decreasing significantly the engineering time. Request now for a demonstration of AxSTREAM and speak to our engineers for additional details on the design process.

References:

[1] http://dx.doi.org/10.5772/54444

[2] “Micro Gas Turbines – A Short Survey of Design Problems”, R.A. Van den Braembussche, von Kármán Institute for Fluid Dynamics

Upcoming Webinar: Design and Optimization of Axial and Mixed Flow Fans for High Efficiency and Low Noise

Thursday, May 18 | 10:00 – 11:00 AM EST

Axial Fan CAD Image
Registration is now open for our May webinar demonstrating best practices for the development of competitive, high efficiency, and low noise axial and mixed flow fans for different aerodynamic loadings.

Axial and mixed flow fans have been in high demand for a number of years. The application of these machines span many different industries including HVAC, automotive, appliance, military equipment, and much more. Like many other types of turbomachinery, changing industry standards and market trends have resulted in fierce rivalry to compete on lifespan, efficiency, environmental and user friendliness, and overall quality. With this in mind, it goes without saying that companies are looking for tools needed to develop highly efficient equipment while minimizing noise as quiet fans are typically more desirable which results in higher demand and marketability.

Over the course of the last few years our company has experienced a number of clients coming to us requesting help with axial fan redesign. This is due to many factors including flawed code during the initial design. Often what would happen is the original design code would have issues with matching meanline/streamline simulations and CFD. This resulted in an insufficient design which, given the increased competition in the market, is less than ideal. Currently, we support different clients in these industries through the use and application of AxSTREAM®. By utilizing our software platform clients can develop competitive, high efficiency, and low noise axial and mixed flow fans the first time around or redesign existing models to meet their full potential.

Due to our experience, we wanted to share our expertise in our upcoming complementary webinar. Topics covered include:

Axial Fan CAD Front

  1. An overview of axial and mixed flow fans and their practical application
  2. Requirements of axial and mixed flow fans
  3. Noise estimation techniques
  4. Axial fan design using the AxSTREAM® software platform
  5. Fan blade design and optimization
  6. Fan aerodynamic blade loadings and performance simulation

 

The webinar also consists of a live software demonstration and a Q&A session with the presenter at the end of the presentation. Register below or follow this link for more information!

Register Here!

Turbomachinery Software for Education

Turbomachinery design has significantly evolved over the last two decades, as supporting education and training methods and techniques remains a challenge. Diversity of technologies covered in the varying courses and extensive use of software by industry designers makes the task of delivering the course curriculum that meets expectations of industry and students difficult. Many educational institutes and business use generic CAE tools for the purpose of learning turbomachinery through student projects. While generic tools have proven their value in research and design, the comprehensiveness of these tools to tackle real life turbomachinery situations is far from desired. The inexperience of fresh graduates from universities and colleges in their inability to perceive a 4D machinAxSTREAM EDUe (3D plus time), traditionally taught using a 2D blackboard, is evident. A student is not only required to have a very good understanding of underlying fundamentals, but is also required to address multitude of design, analysis and optimization problems within the limited time available for education. Coupling of theoretical and computer aided design knowledge to augment the capability of students to contribute to the industrial endeavor is necessary. Such a solution provides students with implicit understanding of the level of detail required by final designs, such as mean line design to the specification of a blade profile varying from hub to tip of a blade, and further complexities of iteration due to an aerodynamically correct blade profile being unsuitable because of stress levels or excitation frequencies and much more. AxSTREAM® EDU introduces multiple dimensions of design required by turbomachinery very early in the instruction process which, by using,  the students are able to develop insights that traditionally are difficult to attain in the same time frame. The use of AxSTREAM® EDU as a design software has been proven to multiply the skills of the students, enabling broad 3-D design considerations and visualization seldom possible otherwise.

AxSTREAM® EDU provides the user with the ability to design many different types of turbomachinery from scratch, such as axial turbines and compressors, radial compressors and turbines, axial fans, integrally geared compressors, mixed flow turbines and compressors and more. The moot question is how important is preliminary design? The efficiency gain possible to achieve in the preliminary design is of the order of 5-10 %, as compared to 0.5 % using 3D optimization (blade profiling, stress and CFD). One has an option of spending several weeks running  full 3D CFD calculations in generic software to try to optimize 0.5% of design, or spending much less time and resources using AxSTREAM® to figure out the best flow path design, followed by use integrated stress, CFD and rotor dynamic solvers!

Air Conditioning in Automotive

Car AC
                          Source

While the term of air conditioning in relation to automotive might instantly correlate to a system which provides passenger with a comfortable air temperature/environment, HVAC systems also are used for heating and cooling of batteries in such application as well as cooling of the vehicle fuel systems. Thermal management for automotive application isn’t easy though. Many factors have to be accounted for in order to build a dependable cooling system.

While talking about HVAC concerns and challenges which arise in automotive application, the biggest inconvenience commonly comes down to the lack of cold air produces. Mobile refrigeration/air conditioning systems come with quite a few concerns from two sides: the refrigeration side, where it removes heat and injects cold air, and from the electrical side which provides control. From the system, the most common challenges are found in moisture –which would fail the cooling system if present in the air, soiled condenser which would block air flow, and various other mechanical complications which might occurs.

While diagnosing an air conditioning issue, especially if environment temperature seems higher than it should be, there are few conditions that can be looked into including freon leak, failed blower, damaged or failed motor, damaged condenser to the most common problem usually arises from the compressor. Compressor, compressor clutch switch, fuses, wires, fan belt and seal are at the top of the list to be check for functional adequacy. Consequently, with many concerns arising from the compressor side of the system, a good and reliable compressor design must be implemented to avoid unwanted challenges during operation. Design your automotive turbomachinery with SoftInWay! Ask us about the projects that we’ve done in this field and how our turbomachinery development code will be helpful for your automotive and HVAC design, analysis and optimization activities.

References:

http://www.doityourself.com/stry/5-common-car-air-conditioning-system-problems

http://www.aa1car.com/library/ac98.htm 

http://www.agcoauto.com/content/news/p2_articleid/256

http://blog.firestonecompleteautocare.com/ 

HVAC Design for Humid Climates

Blog for HVAC system Humid climates commonly come with the challenge of moisture standards. When HVAC (heating, ventilations, and air-conditioning) systems do not maintain proper moisture conditions/humidity control, it causes damages and defects to the building.

A humid climate is defined as a condition where the average monthly latent load (energy required to remove moisture from the air) of environment’s air is the same or higher than the average monthly energy needed to cool the air during the cooling season. Using air with high latent load easily brings moisture in and accumulates it in building materials.

Maintaining humidity control isn’t an easy task. The HVAC unit has to be able to support a proper pressurization system using dehumidified air to entire the building. In order to provide the right dehumidification, a HVAC system must be able to dehumidify the air that flows across the cooling oil (which means the precise sizing of cooling coil must be selected to meet the load of both outside and return air). That is not the only criteria that an HVAC system needs to fulfill though. The system must also meet the sufficient run time to remove moisture from the interior air. In a humid condition, temperature control is not enough. Moisture control comes second on the priority list ( though this has to be fulfilled without scarifying the main goal of giving comfortable temperature to users).

In geographical areas with humid weather, such as in the southeast, public housing generally uses chilled water and direct expansion for the cooling system. This requires an outdoor condenser unit to exchange heat to the outdoor air.

Reference:

http://www.greenseal.org/Portals/0/Documents/IG/PHA%20Manuals/Chapter2_Southeast_Green_Building_OM_Manual_PHA.pdf
http://www.tandfonline.com/doi/abs/10.1080/01998590309509232?journalCode=uene20

Foil Air Bearings for High-Temperature Turbocharger Applications

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.

Dan blog bearing for turbochargers
Figure 1 – Floating-Ring Bearing Model for a Turbocharger

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 Bearing
Figure 2 – Foil Air Bearing

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/

References:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000004303.pdf

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090033769.pdf

 

Performance Simulation and Optimization of CCPP with Turbine Inlet Air Cooling

It is well established that the performance of combustion air turbines (gas turbines) is sensitive to ambient air temperature. As the ambient air temperature increases beyond standard design point  (ISASLS), the power output and exhaust gas flow rate reduces while the heat rate and exhaust gas temperature increases. While the trends are similar for heavy duty and aeroderivative gas turbines, due to the inherent nature of design the results are steeper for aeroderivatives.  Various types of turbine inlet cooling technologies such as evaporative cooling, refrigerated inlet cooling and thermal energy storage systems have been practiced with varying degree of success, each having its potential advantages and limitations.  Simplicity and cost advantage gained in evaporative cooling is offset by limitation of cooling along web bulb depression line (and is a function of site relative humidity). Refrigerated inlet cooling (direct and indirect) offer advantage of higher cooling and lesser sensitivity to site conditions, and result in greater power output with an impact on relative cost and complexity. Selection of optimum technology of turbine air inlet cooling is hence a tradeoff between competing factors.

Combined Cycle
              Combined Cycle Power Plant

The complexity of combined cycles, without any turbine inlet air cooling, poses significant challenge in design of steam system and HRSG due to competing factors such as pinch point, heat and mass flows optimization etc. Knowledge of fluid viz properties of standard air (psychrometrics), standard gas for Joule Brayton cycle, steam for bottoming Rankine cycle and refrigerant for cooling system( for refrigerated inlet air cooling) as applied to complete cycle makes the process complete as well as complex. AxCYCLE™ is one such unique tool to simulate such combined cycle processes with multi fluid-multi phase flows including refrigeration. The standard HVAC features can easily be used for inlet air cooling refrigeration and integrated into the CCPP. Once a digital representation of the complex process is replicated and successfully ‘converged’ at design point, the challenge of optimization emerges. To facilitate optimization various tools namely AxCYCLE™ Map, Quest, Plan and Case are embedded integrally. As a first cut, users based on their experience apply AxCYCLE™ Map and vary one or two parameters to see the effect of operational parameters on cycle performance. AxCYCLE™ Quest opens the gates by allowing users to vary unlimited parameters, according to quasi-random Sobol sequences. mutli-Parameter optimization tasks are possible using AxCYCLE™ Plan – it uses design of experiments concepts. Once optimized the AxCYCLE™ Case tools allows off design simulation tasks. Exhibit below represents complexity of a combined cycle plant represented conveniently:

To learn more please check out the following demos:

Cost Estimation & Economic Analysis http://learn.softinway.com/Webinar/Watch/51

Vapor Compression Refrigeration System http://learn.softinway.com/Webinar/Watch/86

Gas for Power

Gas turbines are one of the most widely-used power generating technologies, getting their name by the production of hot gas during fuel combustion, rather than the fuel itself. Today, the industry is clearly driven by the need of fast and demand-oriented power generation, thus additional effort is put in extremely short installation times, low investment costs and an enormously growing volatility in the electrical distribution in order to achieve higher levels of reliability in the power grid [2].

The majority of land based gas turbines can be assigned in two groups [3]: (1) heavy frame engines and (2) aeroderivative engines. The first ones are characterized by lower pressure ratios that do not exceed 20 and tend to be physically large. By pressure ratio, we define the ratio of the compressor discharge pressure and the inlet air pressure. On the other hand, aeroderivative engines are derived from jet engines, as the name implies, and operate at very high compression ratios that usually exceed 30. In comparison to heavy frame engines, aeroderivative engines tend to be very compact and are useful where smaller power outputs are needed. Gas turbine image

Nowadays, The increase of energy demand along with the growth of transportation market led to requirements for machines of highest efficiency (i.e. minimal fuel consumption), ability to operate in some certain range of conditions, and weight restrictions. In addition, to maintain competitiveness, it is essential to decrease the amount of time needed to complete the design cycle [4]. Most of machine’s geometrical properties are selected during preliminary design phase and remain almost unchangeable throughout next design phases, predefining its layout significantly. Therefore, the preliminary design task is the basis and the effort must be put in developing complete engineering tools to cover this task taking into account all possible aspects of a successful gas turbine design. In particular, a key advancement to the future of turbine technology is the turbine cooling of components in gas turbine engines to achieve higher turbine inlet temperatures, as increased inlet temperatures lead to better performance and higher lifespan of the turbine [5].

SoftInWay has extensive experience with gas turbine design and optimization. From our flagship software platform AxSTREAM® to AxCYCLE™ , designed for the thermodynamic simulation and heat balance calculations of heat production and electric energy cycles, to our extensive engineering consultant services, you can rest assured that all your project needs will be met by our engineering experts. The use of gas turbines for generating electricity dates back to 1939, where a simple-cycle gas turbine was designed and constructed by A. B. Brown Boveri in Baden, Switzerland, and installed in the municipal power station in Neuchâtel, Switzerland [6]. Today, SoftInWay Switzerland GmbH is located not far from Baden and allows the support of our European clients by offering consulting services, software and training for all engineers tastes. Visit our website and find out how you can take advantage of SoftInWay turbomachinery expertise.

References

[1]http://www.wartsila.com/energy/learning-center/technical-comparisons/gas-turbine-for-power-generation-introduction

[2]https://library.e.abb.com/public/ccb152e5e798b1cdc1257c5f004d64c1/DEABB%201733%2012%20en_Gas%20Turbine%20Power%20Plants.pdf

[3]https://energy.gov/fe/how-gas-turbine-power-plants-work

[4]http://softinway.com/wp-content/uploads/2013/10/Integrated-Environment-for-Gas-Turbine-Preliminary-Design.pdf

[5]Joel Bretheim and Erik Bardy, “A Review of Power-Generating Turbomachines”, Grove City College, Grove City, Pennsylvania 16127

[6]https://www.asme.org/about-asme/who-we-are/engineering-history/landmarks/135-neuchatel-gas-turbine

Analytical Tools for Determination of Damped Unbalanced Rotor Response

Blog 4 image 2
Representative Rotor Response Plot (API Standards) & Analytical Simulation

Lateral rotor-dynamic behavior is the most critical aspect in determining the reliability and operability of rotating equipment in the oil and gas industry – be it a centrifugal pump , compressor, steam or gas turbine, motor or generator. One way to evaluate operating reliability is identifying lateral rotor response to unbalance, i.e. by analytically determining damped unbalanced rotor response. Torsional response is sought only for train units comprising three or more coupled machines (excluding any gears). Experience suggests that the effect of other equipment in the train is normally not included in the lateral damped unbalanced response. Hence brief summary of various characteristics and a technique for analytical predictions of lateral behavior deserves attention by all.

The purpose of damped unbalanced rotor response is to identify critical speeds, associated amplification factors-AF (as per API standards AF greater than or equal to 2.5 is considered critical) and ability of rotor dynamics system to meet the separation requirements (margin of operating speed away from critical speed/s). The first step is ‘undamped’ unbalance response analysis for identifying mode shapes and critical speed-support stiffness map.  ‘Damped’ unbalanced response analysis then follows for each critical speed within the speed range of 0 % to 125 % of trip speed. Unbalance or side load is placed at the locations that have been determined by the undamped analysis to affect a particular mode most adversely. The magnitude of the unbalances is four times the value of U as calculated by U = 6350 W/N. The aim then is to identify the frequency of each critical speed, frequency-phase and response amplitude data, deflected rotor shape due to unbalance and bode plots to compare absolute shaft motion with shaft motion relative to the bearing housing (support stiffness <  3.5 times the oil-film stiffness).

Blog 4 image

To verify the analytical model, subject to various practical requirements, an unbalanced rotor response test could be performed as part of the mechanical running test. The actual response of the rotor on the test stand to the same arrangement of unbalance as used in the analysis is the criterion for determining the validity of the damped unbalanced response analysis. Sample summary results generated using SoftInWay’s integrated tool for rotor dynamics AxSTREAM®, as shown below indicate a very good agreement between test results and analytical predictions, for both amplitude and frequency.

For further understanding the analytical procedure, testing to validate damped unbalanced rotor response and implications please view http://learn.softinway.com/Webinar/Watch/90 or contact us to learn more.