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
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
Quite surprisingly, rockets in their primal form were invented before turbomachinery, even though turbines and pumps are both present in modern launcher engines. However, it is interesting to note that both can be traced to the same ancestor. In this post we will discuss some of the history and technical evolution of rockets and turbomachinery – and this all starts with an old pigeon.
Circa 400BCE, a Greek philosopher and mathematician named Archytas designed a pigeon-like shape made out of wood that was suspended with wires and propelled along these guides using steam demonstrating the action-reaction principle long before Newton formalized it as a rule in Physics. As we know today, the faster and the more steam escapes the pigeon, the faster it goes. Turn this 90 degrees to have the bird face upward, and you have a very basic rocket concept. However, rockets are a lot more complex than this, and do not typically use steam (except in the case of liquid hydrogen + liquid oxygen propellants) as the propelling fluid. Read More
High bypass ratio (BPR) fans are of heightened interest in the area of civil air vehicle propulsion. It increases the air inhaling and improves both the thrust and the propulsive efficiency. The specific fuel consumption is also reduced in today’s turbofan engines.
The inlet fan designs and optimizations are very important as the fan can be subjected to different inlet conditions. As a matter of fact, a modern high bypass fan system provides over 85% of the engine’s net thrust. Hence, a well-designed bypass fan system is crucial for the overall propulsion characteristics of a turbofan engine. A tool which can perform both inverse tasks and direct tasks on bypass fan system is a necessity for turbofan design.
AxSTREAM ® Streamline Solver
The AxSTREAM® streamline solver is a throughflow solver, the specificity of the outcome one should expect from this solver is up the meridional flow field. Hence, when we develop the model, we shall take Acarer and Özkol’s work  as a reference example. Read More
Axial fans have become indispensable in everyday applications starting from ceiling fans to industrial applications and aerospace fans. The fan has become a part of every application where ventilation and cooling is required, like in a condenser, radiator, electronics etc., and they are available in the wide range of sizes from few millimeters to several meters. Fans generate pressure to move air/gases against the resistance caused by ducts, dampers, or other components in a fan system. Axial-flow fans are better suited for low-resistance, high-flow applications and can have widely varied operating characteristics depending on blade width and shape, number of blades, and tip speed.
The major types of axial flow fans are: propeller, tube axial, and vane axial.
– Propellers usually run at low speeds and handle large volumes of gas at low pressure. Often used as exhaust fans these have an efficiency of around 50% or less.
– Tube-axial fans turn faster than propeller fans, enabling operation under high-pressures 2500 – 4000 Pa with an efficiency of up to 65%.
– Vane-axial fans have guide vanes that improve the efficiency and operate at pressures up to 5000 Pa. Efficiency is up to 85%.
Aerodynamic Design of an Axial Fan
The aerodynamic design of an axial fan depends on its applications. For example, axial fans for industrial cooling applications operate at low speeds and require simple profile shapes. When it comes to aircraft applications however, the fan must operate at very high speeds, and the aerodynamic design requirements become significantly different from more traditional fan designs. Read More
Ever since circa 100 BBY, Podracing in its modern version has drawn crowds from far far away to watch pilots compete in races like the Boonta Eve Classic which made Anakin Skywalker famous and won him his freedom. By beating Sebulba, the Dug, and the other Podracers, Anakin became the first human to be successful at this very dangerous sport. The Force helped him in his victory by sharpening his reflexes, but his repulsorcraft was also superior due to its size and the modifications made to its twin Radon-Ulzer 620C engines, especially the fuel atomizer and distribution system with its multiple igniters which makes them run similarly to afterburners seen on some military planes on Earth.
Let’s take a deeper look at what repulsorcrafts are and how we can help Anakin redesign his to gain an even better advantage against the competition, provided that Watto has the correct equipment in his junk yard. Read More
Aircraft fuel pumps are one of the most important elements of a fuel system. The operating characteristics and reliability of it are critical for the performance and safety of the aircraft.
Usually, the inlet pressure of the aircraft fuel pump is very low, for example, the aircraft fuel pump of a commercial aircraft needs to operate at altitudes up to 45,000 feet, where the standard atmospheric pressure is about 2.14 psi (about 0.146 atm). What’s more, because fuel is the only consumable fluid carried by the aircraft, it needs to provide all of the cooling necessary for the proper function of the airframe and engine systems. As a result, the temperature of the fuel in the pump increases significantly. The vapor pressure of common fuel used in aircraft gas turbine engines, like Jet A, Jet B, JP-4 etc., gets higher as the temperature increases. Cavitation may occur when the local static pressure in the fluid drops below the vapor pressure of the fuel.
It is very important to avoid the cavitation problem when designing the aircraft fuel pump, because it will cause serious wear, tear, damage of the impeller and performance penalty, which reduces the pumps’ lifetime dramatically. In order to prevent cavitation and have a better suction performance, aircraft fuel pumps use inducers either alone or in conjunction with radial or mixed-flow impeller depending upon the flow and pressure requirements. Figure 1 shows an assortment of fuel pump impellers including radial, mixed flow and inducer types. 
Radial turbines are quite popular for turbochargers and micro-gas turbines. They can also be found in compact power sources like in auxiliary power units of aircrafts. In short, they are suitable in power generation applications where expansion ratios are high and mass flow rates are relatively small. In a radial turbine, the flow enters radially and exits either axially or radially depending on whether it is an inflow or outflow type radial turbine. The most commonly used type of radial turbine is a radial-inflow turbine, in which the working fluid flows from a larger radius to a smaller radius. A centripetal turbine is very similar in appearance to the centrifugal compressor, but the flow direction is reverse. Figure 1 shows the radial-inflow turbine on the left and radial-outflow turbine on the right.
Nowadays, the popularity of radial-outflow turbines, in which the flow moves in the opposite direction (from the center to the periphery), is growing. With recent advancement in waste heat recovery applications, there has been a renewed interest in this type of turbines. These radial-outflow turbines are most commonly used in applications based on organic Rankine cycles (ORC).
The radial-outflow turbine design was first invented by the Ljungström brothers in 1912, however it was rarely used for a number of reasons. One of which was related to the decrease of turbine-specific work due to the increase of the peripheral velocity from inlet to outlet while expanding the vapor. Another reason was the usage of steam as a working fluid. It is known from thermodynamics that the expansion of steam is characterized by high enthalpy drops, high volumetric flows and high volumetric ratios. Thus, a significant number of stages are needed to convert the enthalpy drop of the fluid into mechanical energy.
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 which 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.)
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 (http://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.
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