Nowadays, transonic axial flow compressors are very common for aircraft engines in order to obtain maximum pressure ratios per single-stage, which will lead to engine weight and size reduction and therefore less operational costs. Although the performance of these compressors is already high, a further increment in efficiency can result in huge savings in fuel costs and determine a key factor for product success. Therefore, the manufacturers put a lot of effort towards this aspect, while trying to broaden the operating range of the compressors at the same time.
The creation of shocks, strong secondary flows and other phenomena increases the complexity of the flow field inside a transonic compressor and challenges the designers who need to face many negative flow characteristics such as, high energy losses, efficiency decrease, flow blockage, separation and many more. As the compressor operates from peak to near-stall, the blade loading increases and flow structures become stronger and unsteady. Despite the presence of such flow unsteadiness, the compressor can still operate in a stable mode. Rotating stall arises when the loading is further increased, i.e. at a condition of lower mass flow rate. There are several possible techniques to limit the negative effect of the flow features mentioned above. Here we will present only two. The first one is related to the blade shape generation, while the second one is linked to flow control techniques.
This being my last post for 2017, I wanted to do a short review of what we have been discussing this year. During the beginning of the year, I decided to focus on the 3D analyses and capabilities that were implemented in our AxCFD and AxSTRESS modules for fluid and structural dynamics. With that in mind, my posts were tailored towards such, highlighting the importance of the right turbulence modelling for correct flow prediction. Among other topics, we studied the key factors that lead to resonance, the importance of not neglecting the energy transfer between fluid and structure, and the great advantage that increasing computing capacity offers to engineers in order to understand turbomachinery in depth. However, no matter how great the benefits are, the approximations and errors from CFD can still lead to high uncertainty. Together, we identified the most important factors, from boundary conditions all the way to mesh generation and simulation of cooling flows, and we put an emphasis on the necessary development of uncertainty quantification models. This 3D module related topic finished with an extensive article on fatigue in turbomachinery which plays a crucial role in the failure of the machine, and was the cause for many accidents in the past.
The second part of my posts focused on different industries that rely on turbomachinery as we tried to identify the challenges that they face. Being fascinated by the space industry along with the increasing interest of the global market for launching more rockets for different purposes, I started this chapter with the description of a liquid rocket propulsion system and how this can be designed or optimized using the AxSTREAM platform. Moving a step closer to earth, next I focused on the aerospace industry and the necessity for robust aircraft engines that are optimized, highly efficient, and absolutely safe. One of the articles that I enjoyed the most referred to helicopters and the constant threats that could affect the engine performance, the overall operation and the safety of the passengers. Dust, salt and ice are only a few of the elements that could affect the operation of the rotating components of the helicopter engine, which allows us understand how delicate this sophisticated and versatile aircraft is. Read More
The helicopter is a sophisticated, versatile and reliable aircraft of extraordinary capabilities. Its contribution to civil and military operations due to its high versatility is significant and is the reason for further research on the enhancement of its performance. The complexity of helicopter operations does not allow priority to be given for any of its components. However, the main engine is key for a successful flight. In case of engine failure, the helicopter can still land safely if it enters autorotation, but this is dictated by particular flight conditions. This article will focus on the possible threats that can cause engine failure or deteriorate its performance.
When a helicopter is operating at a desert or above coasts, the dust and the sand can challenge the performance of the engine by causing erosion of the rotating components, especially the compressor blades. Moreover, the cooling passages of the turbine blade can be blocked and the dust can be accumulated in the inner shaft causing imbalance and unwanted vibration. The most common threat of this kind is the brownout which is caused by the helicopter rotorwash as it kicks up a cloud of dust during landing.
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 .
The necessity for a robust aircraft engine design is strongly associated with not only flight performance, but also to passengers’ safety. The fatigue on the blade of CFM56 engine did not prove to be fatal in last August’s incident. None of the 99 passengers was hurt, but parts of the engine broke apart damaging the fuselage, wing and tail, and forcing the Boeing Co. 737-700 to an emergency landing. However, that was not the case in July 6, 1996, when the left power plant on a Boeing MD-88 broke apart while accelerating for take-off and the shrapnel was propelled into the fuselage killing a mother and a child seated in the Delta Air Lines Inc. aircraft . A few years earlier, in January 8, 1989, a CFM56-3 blade failure proved to be fatal for 47 out of 118 passengers of the British Midlands Airways (BMA) Ltd Flight 92 departed from London Heathrow Airport en route to Belfast International Airport. Based on Federal Aviation Administration’s accident overview  post-accident investigation determined that the fan blade failed due to an aero-elastic vibratory instability caused by a coupled torsional-flexural transient non-synchronous oscillation which occurs under particular operating conditions. An animation describing this process is available at the following link: (Fan Blade Failure).
The last example  of this not so cheerful post took place on July 29, 2006, when a plane chartered for skydiving experienced jet engine failure and crashed. Tragically, there were no survivors. The failure was attributed to aftermarket replacement parts. The aircraft was originally equipped with Pratt & Whitney jet engines, specifically made with pack-aluminide coated turbine blades to prevent oxidation of the base metal. However, during the plane’s lifetime, the turbine blades were replaced with different blades that had a different coating and base metal. As a result of the replaced turbine blade not meeting specification, it corroded, cracked and caused engine failure.
Back when the California high-speed rail project was announced, Elon Musk (CEO of SpaceX and Tesla Inc. and perhaps the most admired tech leader of present day) was not only disappointed with this project, but also introduced an alternative to this system called the Hyperloop in 2012. Since the abstract of this project was introduced, many engineers around the world have started to evaluate the feasibility of this “5th Mode of Transportation”.
The general idea for the Hyperloop consists of a passenger pod operating within a low-pressure environment suspended by air bearings. At the realistic speeds estimated by NASA of 620 mph, the pod will be operating in the transonic region. While Japan’s mag-lev bullet train has succeeded at achieving speeds of up to 374 mph, the scale and complexity of a ground transportation system rising above 600 mph bring to surface an unusual number of engineering challenges. As well, brand new designs such as the one proposed by Musk have a certain amount of risk involved due to this technology inherently having no previous run history on a large scale.
Operation of most liquid-propellant rocket engines, first introduced by Robert Goddard in 1926- is simple. Initially, a fuel and an oxidizer are pumped into a combustion chamber, where they burn to create hot gases of high pressure and high speed. Next, the gases are further accelerated through a nozzle before leaving the engine. Nowadays, liquid propellant propulsion systems still form the back-bone of the majority of space rockets allowing humanity to expand its presence into space. However, one of the big problems in a liquid-propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to bring the temperature down.
The term, “mixed flow compressor”, refers to a type of compressor that combines axial and radial flow paths. This phenomenon produces a fluid outflow angle somewhere between 0 and 90 degrees with respect to the inlet path. Referred to as the meridional exit angle, the angled outflow of this mixed flow configuration possesses the advantages of both axial and centrifugal compressors. Axial compressors can produce higher order efficiencies for gas engines, but they have relatively low-pressure ratios unless compounded into several stages. Centrifugal compressors can produce high-pressure ratios in a single stage, but they suffer from a drop in efficiency. The geometrical distinction of mixed flow compressors allows for higher efficiencies while maintaining a limited cross-sectional area. The trade-off for a mixed flow compressor when introduced to aero gas turbines is that there is an associated weight increase due to the longer impellers needed to cover this diagonal surface. However, when related to smaller gas turbines, the weight increase becomes less significant to the overall performance of the engine.
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 cause 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.
We can all agree that airplanes are cool, and rockets are awesome, but when combined, the result is even better! Besides getting engineers to jump up and down for this revolutionary concept, Reaction Engines Ltd applied it to an actual SABRE engine concept.
SABRE stands for Synergistic Air-Breathing Rocket Engine and one typically does not associate “Air-Breathing” with “Rocket.” which makes this engine a one of a kind to reach new heights (literally). Let’s dig into the geeky technical specs of the engine while going through some quick history of this revolutionary single stage to orbit propulsion system.
SABRE is an evolution of Alan Bond’s series of liquid air cycle engine (LACE) and LACE-like designs that started in the early/mid-1980s under the HOTOL project. Upon termination of HOTOL funding, Bond formed Reaction Engines Ltd. SABRE is currently being developed for hypersonic flights and runs on a combined cycle; the precooled jet engine configuration is used in the air-breathing phase of the flight until air becomes scarce and speed critical. From this moment on the engine switches to its close cycle rocket mode to bring the Skylon airplane to orbit (2 engines are mounted on the aerospace plane).
The air-breathing mode (below Mach ~5 and about 25 km altitude which is about 20% of the orbital velocity and altitude, respectively) works almost like a regular jet with one major difference being the apparition of a new component, first discussed in 1955; the air precooler which is placed behind the translating axisymmetric shock inlet cone that slows the air to subsonic speeds inside the air-breathing engine using 2 shock reflections. The precooler is “capable of cooling incoming air (without liquefying it, from around 1000°C) to −150°C (−238°F), to provide liquid oxygen (LOX) for mixing with hydrogen to provide jet thrust during atmospheric flight before switching to tanked LOX when in space.” This precooler also allows a considerable reduction of the thermal constraints of the engine which then requires “weaker” and much lighter materials that are a necessity when reaching orbital velocities and altitudes. With compressors working more efficiently with a colder fluid, and the incoming air already highly compressed from the flight speed and shock waves, the fed pressure in the combustion chamber is around 140 atm. When in rocket mode, the inlet cone is closed and liquid oxygen and liquid hydrogen are burned from on-board fuel tanks for the remaining 80% of velocity and climb required to reach orbit.
On a very recent note, feasibility studies conducted by the U.S. Air Force Research Laboratory were successfully passed in 2015.
Although the application of the SABRE engine is destined for orbital use, its cousin (Scimitar) has been designed for the environmental-friendly A2 hypersonic (top speed higher than Mach 5) passenger jet for 300 rushed passengers (about 3 times more than the Concorde) under the LAPCAT (Long-Term Advanced Propulsion Concepts and Technologies) study founded by the European Union.
When dealing with such high speeds, noise becomes a real constraint and flying above inhabited areas is restricted, which is why specific aerial routes are designed. According to Alan Bond, the A2 design could fly subsonically from Brussels International Airport into the North Atlantic, reaching Mach 5 across the North Pole and over the Pacific to Australia in about 4.6 hours, with a price tag similar to what you would pay for business class these days. This speed would heat the body of the craft so that windows are not an option because the appropriate thickness would represent a considerable weight. It is therefore thanks to flat panel displays showing images that you would be able to enjoy the scenery.
When one talks about high-velocity flight it is difficult not to think of the French Concorde that operated between 1976 and 2003 and could travel at Mach 2.04 (limited by thermal constraints due to the material used) using the Scramjet technology; scramjet standing for “supersonic combustion ramjet”. This allowed a New York City to Paris flight in less than 3.5 hours instead of 8 hours with a conventional jet.
The principle of this technology is to compress air with shock waves under the body of the aircraft before injecting the fuel (the Concorde’s intake ramp system can be seen on the figure on the right).
Due to the high inefficiency of this technology at low speeds, afterburners are used from take-off until reaching the upper transonic regime.
Keeping in mind that the heating of the Concorde’s body due to friction could make it expand by as much as close to a foot, it becomes easy to understand one of the reasons why high altitudes (scarcer air and therefore lesser aerodynamic resistance) are chosen for such high flight velocities; the Concorde cruising altitude was around 56,000 ft and would be decreased when sun radiation levels were becoming too high. On a side note you can keep an eye out at Charles de Gaulle airport in Paris (France) for a Concorde displayed outside.
Oh and did I forget to mention that the turbomachinery parts on the SABRE engine are currently being designed in the AxSTREAM suite??