[:en]An unsteady flow is one where the parameters change with respect to time. In general, any liquid flow is unsteady. But if a hydraulic system is working at constant boundary conditions, then the parameters of the fluid flow change slowly; thus this flow is considered steady. At the same time, if the parameters of the fluid flow oscillate over time relative to some constant value, then it called quasi-steady flow 1.
In practice, most fluid flows are steady or quasi-steady. Examples of the three flows are presented in Figure 1. Steady flow is presented by a simple pipe. The quasi-steady flow is represented by a sharpened edge channel. The unsteady flow is presented by an outflow from a reservoir.
Different Cases of Unsteady Flow
During operations, hydraulic systems act for long intervals at steady conditions which are called operating modes. Change between two different operating modes occurs over a short time interval (called a transient mode). If any hydraulic system works more than 95% of the time at these operating modes though, why is the unsteady flow is so important? Because the loads depend on time intervals. If the load is less, then the maximum system pressure is higher. Read More
[:en]In just about every corner of the globe, machines are used and needed for the modern world and its people to function in normal everyday life. But what if these machines were to just…disappear? How badly would it disrupt modern society? Our young protagonist is about to find himself in the midst of such a scenario as he comes to realize he’s taken residence in an apartment building located inside a little neighborhood known as…the twilight zone.
I open my eyes slowly and rejoin the world, in all of its silence. Hold on, it’s Thursday, the alarm was supposed to wake me up! I roll over to check the time, and the screen is just a blank, black façade. That’s odd. The alarm clock isn’t working. The fan isn’t on either. The power must be out. What could have happened to shut off the power? Read More
Reduction in CO2 emissions is driving the development of different electric, turbo-electric and hybrid electric propulsion systems for various applications and industries including space, aviation, automotive and marine. Electric propulsion (EP) is not a new concept, having been studied in parallel with chemical propulsion for many years. EP is a generic name encompassing all the ways of accelerating a propellant using electric power by different possible electric and/or magnetic means. The simplest way to achieve electric propulsion is to replace the heat generated by combustion in conventional chemical engines with electrical heating.
Electric propulsion systems offer several advantages compared to other conventional propulsion systems. It not only helps reduce the environmental emissions but also helps reduce fuel consumption and increases safety levels. Electric propulsion has become a cost effective and sound engineering solutions for many applications. Electric propulsion engines are also more efficient than others. It is proven to be one of the most energy saving technologies as we can use more renewable sources of energy (due to the versatility of electricity generation) instead of non-renewable sources of energy like gasoline. The major limitation of electric propulsion, when compared with conventional propulsion is limited by the available electric power capacity on board, this may be the reason, it is not the default propulsion system.
Generally, electric propulsion architectures vary depending on the application. Figure 1, above, shows the EP architectures for an aviation application. These architectures rely on different electric technologies (batteries, motors, generators, and so on). Typical aircrafts use gas turbine engines as the source of propulsion power, but all electric aircraft systems use batteries as the only source of propulsion power as shown in Figure 1 on the right. The hybrid systems use gas turbine engines for propulsion and to charge batteries which also provide energy for propulsion and accessories during one or more phases of flight as shown in Figure 1 on the left. Read More
Gas turbines have had a presence in many industries for more than a century. They are a unique technology for either producing an energy or propelling a vehicle and the efficiency of modern gas turbines is being improved continuously. One of them, a cooling system, has been described in earlier blogs. Another is the lubrication system of a gas turbine which we will cover in this blog. This system, similar to that of a piston engine or a steam turbine, provides lubrication to decrease mechanical losses and prevent of wear on friction surfaces. Another function is the removal of heat released during friction by high rotational part and transmitted from the hot part of a turbine. The basic units which need lubrication are the bearings supporting a shaft of a gas turbine 2.
Elements for lubrication
In a common case, gas turbine installation contains three main journal bearings used to support the gas turbine rotor 3. Additionally, thrust bearings are also maintained at the rotor-to-stator axial position 4. Click here for additional information about optimization of journal bearings. The bearing has important elements in its construction to prevent leakages from a lubrication system. The work, design and analysis of labyrinth seals is describe here.
Turbo Compressors are used to increase the pressure of a gas, which are required in propulsion systems like a gas turbine, as well as many production processes in the energy sectors, and various other important industries such as the oil and gas, chemical industries, and many more.
Such compressors are highly specific to the working fluid used (gas) and the specific operating conditions of the processes for which they are designed. This makes them very expensive. Thus, such turbo compressors should be designed and operate with high level of care and accuracy to avoid any failure and to extract the best performance possible from the machine.
Turbo Compressor Characteristic Curves
The characteristic curves of any turbo compressor define the operating zone for the compressor at different speed lines and is limited by the two phenomenon called choke and surge. These two opposing constraints can be seen in Figure 2.
Choke conditions occurs when a compressor operates at the maximum mass flow rate. Maximum flow happens as the Mach number reaches to unity at some part of the compressor, i.e. as it reaches sonic velocity, the flow is said to be choked. In other words, the maximum volume flow rate in compressor passage is limited by limited size of the throat region. Generally, this calculation is important for applications where high molecular weight fluids are involved in the compression process.
Surge is the characteristic behavior of a turbo compressor at low flow rate conditions where a complete breakdown of steady flow occurs. Due to a surge, the outlet pressure of the compressor is reduced drastically, and results in flow reversal from discharge to suction. It is an undesirable phenomenon that can create high vibrations, damage the rotor bearings, rotor seals, compressor driver and affect the entire cycle operation.
Preventing Choke and Surge Conditions
Both choke conditions and surge conditions are undesirable for optimal operation of a turbo compressor. Each condition must be considered during design to ensure these conditions are prevented. Read More
Welcome to this next edition of our “Introduction to Rotor Dynamics” series! In this edition we’ll be covering the definitions of rotor dynamics, and how it is an important factor in the lifetime of a rotating machine. So, for starters, what is rotor dynamics?
Well, if you read our preface which can be found here, you probably knew the answer already; or if you’ve been working in this field, you probably also have a good answer. For those of you new to rotor dynamics, however, it’s a branch of applied mechanics in mechanical engineering and is concerned with the behavior of all rotating equipment; considering phenomena like vibration, resonances, stability, and balancing. It accounts for many effects: from bearings, seals, supports, loads and other components that can act on the rotating system.
Is rotor dynamics vibration analysis?
Yes partially, but there is much more that needs to be considered as you can guess from the above definition. Vibration analysis simply isn’t enough, because the rotors in these machines spin at such high RPMs and are so heavily loaded. Something as simple as the bearing’s position and stiffness, or a slight asymmetry from blade creep can affect a rotor’s behavior.
Where can rotor dynamics be found and analyzed?
The short answer is, there are numerous machines where rotor dynamics can be considered. In fact, it’s probably easier to list the numerous applications where rotor dynamics doesn’t exist.
Below is a very short list of some examples where rotor dynamics can be considered: Read More
In the coming age of hypersonics, a variety of engine types and cycles are being innovated and worked on. Yet turbomachinery remains unique in its ability to use a single airbreathing engine cycle to carry an aircraft from static conditions to high speeds. One of the largest limitations of turbomachinery at hypersonic speeds (Mach 5+) is the stagnation temperature, or the amount of heat in the air as it is brought to a standstill. While material improvements for turbomachinery are made over time which increases the effective range of temperatures steadily (Figure 1), this steady rate means that the ability of these materials to allow use at stagnation temperatures of more than 1600K remains unlikely any time soon.
This is the limiting point for traditional turbojet cycles, as Mach 5+ speeds result in temperatures far exceeding these limitations, even for the compressor. However, improvements in cryogenic storage of liquid hydrogen has allowed the concept of precooling, using the extremely low liquid temperature of hydrogen to cool the air enough to push this Mach number range, as well as improve compressor efficiency. To drive the turbine, the exhaust gas and combustion chamber can used, heating the hydrogen and reducing the nozzle temperature for given combustion properties. This has the added effect of separating the turbine inlet temperature from the combustion temperature, reducing limitations on combustion temperatures. This type of cycle can reduce the inlet temperatures underneath material limits. Read More
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.
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
In the age of green energy and increased efforts to minimize our carbon footprint, the design of a turbocharger plays an important role in reducing engine fuel consumption and emissions while increasing the performance. When developing an engine with a turbocharger, the general approach is to select a turbocharger design from a product list. The primary issue with this approach is that it does not cover 100% of the requirements of engine characteristics, i.e. it has non-optimal construction for the engine being developed. The operational characteristics of an engine directly depends on the interactions between the system components. This non-optimal construction will always lead to a decrease in the engine’s performance. In addition, the iteration process of turbocharger selection is time and resource consuming.
That is why the most optimal way to develop an engine with turbocharging is to design a turbocharger from scratch; wherein the operational points of compressor needed to satisfy the engine’s optimal operation are known, i.e. compressor map (Figure 1). But how do we quickly get a compressor map? Even at the preliminary design level, the design of turbocharger flow path requires dozens of hours for high-level engineers. And what about less experienced engineers?
Incorporating a digital engineering approach with a turbomachinery design platform such as AxSTREAM® allows designers to find the compressor design with all the required constraints which correspond to the specified compressor map needed. The design process is presented in Figure 2. Read More
Welcome to our latest blog series on rotor dynamics! In this series we’ll be covering fundamentals and a general overview of the engineering discipline that is rotor dynamics, including some basic definitions, why it’s important, the different calculations, and the overall objectives and purposes for these calculations.
In the months ahead, you can expect to learn more about: Read More