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Rim-Rotor Rotary Ramjet Engines (video)
Industry needs gas turbines with low carbon and NOx emissions and that can operate on renewable fuels. This is necessary both for the aeronautic industry as well as the ground transportation and power generation industry.
The aeronautic industry is struggling with stringent policies, mostly in Europe regarding NOx emissions . Developing clean gas turbines based on conventional gas turbine designs is challenging. On a short term perspective, using biofuels such as methane or biodiesel in current gas turbines is rather straight forward but, with current combustor design, these fuels still produces relatively high NOx emissions . On a longer term, hydrogen could become a clean fuel solution, but current metallic turbine designs face hydrogen embrittlement problems .
The ground transportation industry is entering an electrification phase, which begins with hybrid systems due to the present costs and durability of current batteries and fuel cell systems. Gas turbines are envisioned as range extenders for electric vehicles, see figure 1 and 2. Compared to aeronautical gas turbines, ground transportation turbines have been facing a major technology roadblock for years: they must be produced at low costs.
The R4E (Rim-Rotor Rotary Ramjet Engine) is a gas turbine concept using a single disk containing ramjet thrusters. Compared to current gas turbines which can contain up to 4000 parts , making for complex and expensive systems, the R4E uses a single rotating part and has potential to offer twice the power density, see table 1. Further, the R4E can use hydrogen as a fuel and has potential to achieve comparable efficiencies (for power levels below 1 MW). Its characteristics or high power density, good efficiency, low emissions, and low cost make the R4E a potential breakthrough technology for future small gas turbines, both for aeronautical and ground use.
Figure 1 : The 780bhp Jaguar C-X75 electric concept-car
Figure 2 : Micro axial gas turbines
Table 1 : Comparison of the R4E technology to state-of-the-art gas turbine in the 500 kW range
What is the R4E ?
The R4E mounts multiple high speed supersonic thrusters, ramjets, on a high speed rotor. Ramjets produce thrust by their particular geometry that realize the complete Brayton cycle (compression, combustion, expansion) with the sole “ram” inlet compression effect available at high rotating speeds, see figure 3. The ram compression is done by shock waves at the thrusters’ inlet. Shock waves are very compact physical phenomena occurring on a distance of about 10-6 m . Shock wave compression enables ramjets to achieve the complete Brayton cycle in a short distance and with a single rotating part. Hence rotary ramjet engines are simple, compact, and have high specific power. A 1D generalized flow model of rotary ramjets including loss factors suggest that power density can ultimately climb to 7.6 kW/kg, which is about twice that of current gas turbine, see figure 4 [ 8].
Figure 3 : R4E configuration
Figure 4 : R4E performance
As seen from figure 4, the key for rotary ramjet to achieve high power density is to operate at high inlet Mach numbers, in the order of 2 to 3, to get enough ram compression . Such Mach numbers corresponds to tip speeds in the 600 to 1000m/s range that brings important centrifugal loads. Hence the R4E integrates ramjet thrusters to one of the best rotating flywheels known to date: the rim-rotor carbon fiber flywheel . These flywheels have shown possible tip speeds of 1400m/s, about twice the speeds of conventional rotor based flywheels (~700m/s) [6,7].
The rim-rotor configuration proposed in the R4E is a breakthrough technology to reach the high inlet speeds of M=2-3 or 600-1000m/s necessary for rotary ramjet engines to work properly . The rim-rotor also has the following advantages: (1) the reversed configuration of the rim-rotor allows to stabilize the flame from the outer surface. (2) The high strength of the unique rim-rotor of the R4E engine allows using hydrogen safe combustor materials such as ceramics. This can potentially allow higher combustion temperatures, and thus higher power density and efficiency. (3) The rim-rotor configuration sends the blades interface with non-rotating parts to the blade's inside radius rather than the outside radius like traditional turbomachinery. Because the inside pressure is lower due to centrifugal forces, bleeding losses are reduced compared to conventional gas turbines. (4) Power losses to aerodynamic friction can be reduced by decreasing the ambient pressure on the rim’s outer surface (i.e. by using a vaccum pump to decrease air pressure in the rim-rotor enclosure) or by filling it with a light gas (e.g. H2). (5) The rim-rotor is made from a 1D composite material having a crack propagation mode that is transversal to the fibers orientation. This mode is favorable, because the rupture is less catastrophic than the rupture of isotropic rotors . With respect to demonstrating the R4E feasibility, three key scientific accomplishments were achieved in the course of the first Strategic Grant:
- A physical configuration integrating a spark ignition system has been proposed and successfully tested up to 560 m/s without any signs of failure (see figure 5). For simplicity, the prototype was designed for transient operation only, with combustion events under 1 s. Models predict a maximum tip speed of 860 m/s with current materials.
- The engine was ignited without its onboard ignition system, by directly igniting the incoming air-fuel mixture. Inlet ignition is key for high speed operation by avoiding the need for an ignition system spinning with the engine.
- Conception d’algorithmes d’amortissement actif des torsions dans les arbres;
- Combustion in high g-fields prior to our work has only been verified experimentally up to 104 g’s. Nothing proved that stable and efficient combustion was possible for rotary ramjets. A 1D combustion model developed in-house showed good agreement with Fluent CFD results up to 2x105 g’s (see figure 6). Both the model and Fluent were used to design an optimal combustion chamber that demonstrated stable combustion under g-fields of 4x105 g’s, with combustion efficiencies of up to 85% leading to positive blade power (see a video). The design shows low combustion times (less than 2 ms) that can significantly reduce NOx emissions.
Figure 5 : Rim-rotor rotary ramjet engine.
Figure 6 : R4E combustion under high g-field.
Current Research Objective and Approach
Current main objective is to demonstrate net brake power under continuous operation. A priori, this will require an engine design that can operate around Mach numbers of 2.0 (tip speeds in the 700m/s range) with continuous combustion. The research addresses 5 sub-objectives:
- heat management;
- optimal aerothermodynamic design (inlet starting);
- windage loss reductions;
- system integration;
- prototype testing.
Significance of the Research
The proposed research is a high risk / high reward type. The R4E concept studied in this research has the potential to become a new breed of low cost high power density turbomachinery for lightweight applications (7.6 vs 3.1 kW/kg for gas turbines). The R4E could have a significant impact in many applications such as electric and fuel cell ground transportation vehicles, aeronautical gas turbines, portable electric power generation, and ground based power applications.
The proposed research will benefit the Canadian economy in 3 general ways: (1) Experts suggest that Canada’s economy must diversify from the traditional exploitation of natural resources to the development of product/processes with high R&D content . The R4E and its derived technologies will do just that. They are extremely high R&D content and they would be designed and manufactured in Canada. (2) The potentially lower NOx emission of this engine concept could make the R4E a new solution for turboshaft power generation. (3) Derived technologies from this research could beneficiate existing world-leading turbomachinery specialists such as GE, Pratt, Rolls Royce.
Undergrad – Experimental demonstration of the SRGT (supersonic rim-rotor gas turbine) :
Francois Teasdale : Francois.Bolduc-Teasdale@USherbrooke.ca
Master Student – Combustion :
Hugo Fortier-Topping : Hugo.Fortier-Topping@USherbrooke.ca
Master Student – Fluid dynamics:
Gabriel Vézina : Gabriel.Vezina@USherbrooke.ca
Supervision et Direction :
Prof. Jean-Sébastien Plante : firstname.lastname@example.org
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