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.

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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.

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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.

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Optimizing the Cooling Holes in Gas Turbine Blades

To increase the overall performance of the engine and reduce the specific fuel consumption, modern gas turbines operate at very high temperatures. However, the high temperature level of the cycle is limited by the melting point of the materials. Therefore, turbine blade cooling is necessary to reduce the blade metal temperature to increasing the thermal capability of the engine. Due to the contribution and development of turbine cooling systems, the turbine inlet temperature has doubled over the last 60 years.

thermal-effiency
Figure 1: Variations of Thermal Efficiency with TIT [1]
The cooling flow has a significant effect on the efficiency of the gas turbine. It has been found that the thermal efficiency of the cooled gas turbine is less than the uncooled gas turbine for the same input conditions (see figure 1). The reason for this is that the temperature at the inlet of turbine is decreased due to cooling and therefore, work produced by the turbine is slightly decreased. It is also known that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of  the gas turbine.

With this in mind, during  the design phase of gas turbine it is very important to optimize the cooling flow if you are considering both the performance and reliability. Cooled Gas turbine design is quite complicated and requires not only the right methodology, but also the most appropriate design tools, powerful enough to predict the results accurately from thermodynamics cycle to aerothermal design, ultimately generating the 3D blade.

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Exotic Turbomachinery – Viscous Disc Pumps

Turbomachinery can be divided into two main groups. Group one consists of machines that perform work on the fluid, requiring energy and increasing its pressure, such as compressors, pumps, and fans. Group two consists of those that extracts energy from the fluid flowing through it – for example, wind, hydro, steam, and gas turbines.

Pumps specifically are devices whose purpose is to move fluid at a constant density, increasing its kinetic energy and its pressure while consuming energy in the process. We are quite used to seeing centrifugal and axial pumps, as they are the most common configurations.  However, more exotic designs have been tested and developed throughout the history of fluid machinery.

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The Origin of Rotordynamics

Rotordynamics, the study of vibrational energy in rotors, has a rich history dating back to North America during the 1750’s. This branch of applied mechanics began with theories, but advanced quickly due to practices – starting with Mr. W.J.M Rankine in 1869 and his spinning shaft experiment. Now, decades later, we have strengthened our understanding of rotordynamics and created leading software tools, including AxSTREAM, that are able to simulate analyses to stabilize and increase the reliability of a turbomachinery.

W.J.M Rankine

Not only was W.J.M Rankine a prestigious theoretical scientist and educator, he was a main contributor in the development of rotordynamics and he contributed to thermodynamics and the development of heat engines throughout his lifetime. During his spinning shaft experiment, he concluded that beyond the shaft’s first critical speed, the shaft would be unstable simply because its shape had been bent. By not taking into consideration support damping and Coriolis force in his analysis, many engineers were left confused for almost two centuries, until Gustaf de Laval, a Swedish engineer, ran a steam turbine to supercritical speeds in the late 1880’s. Laval also introduced the use of bearings to oppose absolute motion in his machinery. As the years went by, many other engineers discovered and investigated additional phenomenons (FEM for example) that have an influence in today’s practices.
It is because of these previous innovators that companies like SoftInWay have been able to develop the advanced  rotodynamics modules that we use today.

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Working with Geothermal Heat Pumps

A geothermal heat pump utilizes earth’s thermal energy as a way to manipulate temperature. This is seemingly attractive toward HVAC utilization due to the relatively high efficiency as well as economic benefit. Temperature fluctuations below ground are relatively low as earth absorbs solar energy all year round and insulates the heat underground. Taking advantage of this event, geothermal energy heat pump application for residential and commercial building uses the “underground” as a heat source/sink.

geothermal heat pumps
Source: http://tidewatermechanical.com/geothermal-heat-pumps/

How does geothermal heat pump work?

A heat pump system mainly consists of a heat-pump unit, a pipeline loop functioning as a heat exchanger for a desired area (it can be horizontal, vertical or installed to an aquatic medium), and a duct – to deliver the controlled temperature flow to the consumer.

Fluid is pumped through an installed pipeline loop which transfers heat based on the season. During the hotter season (summer), heat will be absorbed from the air in the building, transferred into the ground and then cooler air will be circulated to the designated area. The contrary happens during the winter. In colder months, heat will be transferred into the fluid from the ground and collected heat will be distributed.

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Performance Effects of Axial Turbines & Compressors Due to Roughness Variations

As turbomachinery technology continues to advance in efficiency as well as overall power, many engineers want an estimate on how long these manufactured machines will operate.  Specifically, in high-temperature and high-flow turbomachinery applications, one of the main sources of performance degradation can be attributed to increases in surface roughness.  Gas turbine and compressor blades in particular experience a substantial amount of surface degradation over their lifetime.

gas turbine blade
Figure 1 – Gas Turbine Blade and Annulus Surface Wear (Source PowerMag)

There are many mechanisms that contribute to surface degradation in airfoils and annulus surfaces.  Foreign particles adhering to the material surface (or fouling) is generally caused by any increase in contaminants such as oils, salts, carbon, and dirt in the airflow.  Corrosion occurs when there is a chemical reaction between the material surface and the environment that causes further imperfections on the machine surfaces.  Additional mechanical factors such as erosion and abrasion will play a part in a machine’s surface degradation as well.

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Discussion – Alien Signal or Radio Noise: Leveraging Turbomachinery

The Internet practically exploded early yesterday morning with talk of an extraterrestrial discovery after a signal was detected by a Russian telescope. The star in question, HD 164595 located a vast 95 light years away, sent out a strong radio spike that was picked up and sparked a boom of excitement. According to an article published by National Geographic, however, this signal may not be what it was first interpreted as.

Astronomers have pointed their radio telescopes towards the stars for over half a century, hoping to catch a glimmer of life beyond this planet. Short of a futuristic rocket ship, these telescopes seem to be the best bet for catching a peak of something out of this world. That is a main causStarse as to why this discovery is so tantalizing to both scientists and the rest of us earthlings. However, after further investigation, neither the Allen Telescope Array, commanded by the SETI (the Search for Extra-Terrestrial Intelligence) Institute, nor the Green Bank Telescope, used by the Breakthrough Listen project, turned up additional signals or observations.

Another issue that has risen according to this article is that the signal did not repeat and could have been caused by something else. A source on Earth, such as a faulty power supply, military transmission, or arcing electrical fence for example. Another possible explanation could be that gravity from another object in space amplified a weaker signal. That being said, it would appear that HD 164595 is similar in many ways to our sun. It is composed of the same ingredients, is approximately the same age and has at least one planet in its orbit. This would suggest that theoretically, it would be plausible for life to exist within this system.

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Rotor Dynamics – Importance of Fundamental Understanding & Software tools

Rotor and bearings are the most critical components of any rotating machinery. Rotor lifetime and reliability depend, first of all, on the level of rotor vibrations. In order to meet highest requirements of reliability each step of the rotor design should be based on accurate Rotor Dynamics prediction.

Rotor dynamics is the branch of engineering that studies the lateral and torsional vibrations of rotating shafts, with the objective of predicting the rotor excessive vibrations. Rotor Dynamics is different from structural vibrations analysis because of gyroscopic moments, cross-coupled forces, critical speeds, whirling effect, etc. These difference makers are all due to the rotation of the rotor assembly.

Rotor-Dynamics

Understanding of basic rotor dynamics phenomena and the various types of problems is absolutely mandatory when designing and developing rotor-bearing systems for various applications. Fundamental approach for Rotor Dynamics analysis generally is based on the following steps:

  1.  Predict critical speeds.
  2. Determine design modifications to change critical speeds.
  3. Predict natural frequencies of torsional vibration.
  4. Predict amplitudes of synchronous vibration caused by rotor unbalance.
  5. Predict threshold speeds and vibration frequencies for dynamic instability.
  6. Determine design modifications to avoid dynamic instabilities.
  7. Calculate balance correction masses and locations from measured vibration data.

Another factor that determines accuracy of Rotor Dynamics calculation is rotor system simplification and the adequate modelling for rotor parts such as Impeller/disks, Sleeves, Balance pistons, Seals, Thrust collars, Couplings, Addition of Stiffening Due to Shrink Fits and Irregular Sections etc. (more…)