The experiment of rotating detonating engine using an asymmetric vortex shape on an annulus chamber

Rotating detonating engine is new propulsion technology with promising advantages. Higher thermal efficiency and simple design with higher thrust attracted intention worldwide to make it realistic. The normal design uses a circular shape of an annulus chamber. The circular shape of the annulus chamber was well-known and tested with various conditions. The new interest area of changing a circular shape into an asymmetric shape is still unknown. Exploring the new shape of the annulus chamber was done by applying the new shape of the annulus chamber on RDE. The experiment was successfully done using methane at different conditions. Three modes can be observed from pressure product and thrust result: a mode of detonation, unstable detonation, and failure on transition. The highest thrust happens at an equivalence ratio of 1.2 with 18N. The new shape of the annulus offers the versatility of RDE.


Introduction
The rotating detonation engine (RDE) represents an exciting and novel advancement in propulsion technology, aiming to improve the efficiency and performance of traditional engines.It operates on the principle of propagating continuous detonation waves through an annular combustion chamber, promising higher thermal efficiency, increased power density, and simplified design [1].The primary advantage of RDEs lies in the speed and efficiency of the detonation process.Detonation combustion is significantly faster and more efficient than traditional deflagration combustion, leading to improved thermodynamic efficiency and higher thrust output [2].
Despite the promising advantages, RDEs face several challenges that require further exploration and resolution.One of the key challenges is achieving stable and continuous detonation waves throughout the engine's operation [3].Ensuring the structural integrity and thermal management of the engine under extreme conditions are also critical concerns.Moreover, the investigation of alternative shapes for the annular chamber, such as asymmetric vortex shape designs, is an emerging area of interest.Understanding how these new geometries impact combustion dynamics and engine performance could lead to further efficiency improvements.
While the circular shape of the annulus chamber has been well-established and tested, it's not uncommon for researchers and engineers to explore alternative geometries to see how they can influence the combustion process and overall engine performance.Asymmetric vortex shapes may offer unique advantages that circular chambers cannot provide, and they could potentially lead to improved combustion dynamics and efficiency.Asymmetric vortex combustion involves utilizing an uneven combustor shape to create vortex combustion, which differs from the standard vortex combustion approach.In the typical method, a cylindrical shape is employed, and air is introduced tangentially to produce a vortex flame.However, in 1998, Gabler [4] experimented with both axisymmetric and asymmetric chamber shapes to generate a vortex flame.Building on Gabler's work, Saqr [5] further improved the technique in 2011 by redesigning the combustion geometry.This redesign introduced asymmetry with separate air and fuel inlets (two each), as illustrated in Figure 1.Here, 'a' represents the asymmetric distance, 'b' indicates the fuel shift distance, and 'd' stands for the inner diameter of the combustor.In contrast to regular vortex combustion, asymmetric vortex combustion (AVC) introduces axial air components.This leads to a distinct process of mixing and flame stabilization.Fuel is rapidly injected from the bottom and mixes with fresh air from the side before combustion occurs along the vortex motion boundaries.This process generates a recirculation zone at the centre of the combustor, enhancing the mixing of exhaust gases and contributing to upstream fuel-air mixing.Heat transfer primarily occurs in the central flow region, while the reaction zone surrounds the recirculation zone in a circumferential manner.
Despite being a non-premixed mixing combustion, AVC exhibits a bluish premixed flame due to its distinctive flame stabilization mechanism.The AVC concept ensures flame stability by anchoring the reaction zone to the boundary of a forced vortex field.This configuration facilitates rapid mixing of air and fuel upstream of the reaction zone.This phenomenon of AVC holds true across different scales, from large-scale to meso-scale applications, and has been demonstrated in deflagration conventional combustion [5], meso-scale combustors [6], and flameless combustion [7].However, the impact of AVC on detonation combustion within this framework remains unexplored.So, the primary objective of this paper is to investigate the influence of asymmetric vortex shape design on a rotating detonation engine.

Methodology
The asymmetric shape was adopted from Saqr [4] and Raid [6] with the purpose to explore a new space of design for RDE.This type of chamber initially guides the combustion wave flow to be in one direction during the circling inside the annulus chamber.Basically, the shape of the combustion chamber is the main factor that differs from the common RDE.The schematic of experiment setup for the study shown in Figure 2. The RDE was connected to the pre-detonator for the initiation purpose at the top of the combustion chamber.The RDE was designed on a small scale due to several limitations, such as the low flow rate of reactant suppliers and space.The delivery of fuel and oxidizer into the combustion chamber of the RDE was control through the utilization of pneumatic valves.However for pre-detonator the combustible gas and oxidizer was control using solenoid valve injector.The front view of RDE is shown in Figure 3.The annulus space for an asymmetric shape was created by offsetting the half-round shape to the other half shape.Before the offset, the annulus had a radius of 25.5mm, which corresponded to the round form of the RDE combustion chamber.The offset was fixed at 5mm, resulting in a homogeneous asymmetric annular gap.As indicated in Figure 3, the minimum annulus gap was 4mm, while the maximum annulus gap was 9mm.The dimensions for the swirl feeding point are the same as for the injection of the main RDE, which is 1mm for the oxidizer slot and 0.7mm for each 80 of the fuel holes.The height of the swirl feeding point was adjusted by 15mm from the base of the combustion chamber.Pre-detonator was tangentially placed at combustion chamber body.Pre-detonator generates detonation wave at the end of the tube upon entering the combustion chamber.Pre-detonator was designed with 4mm internal diameter, 12mm outer diameter and 230mm long.The pre-detonator initiates the detonation inside the RDE chamber and repeatability of the ignition energy.

Result and discussion
The experiment was successfully done using methane at different equivalence ratios and flow rates.In order to investigate the operation range of RDE, a series of experiments was done by fixing oxygen flow rates at 5.91E-3kg/s, 6.33E-3kg/s, 6.76E-3kg/s, 7.18E-3kg/s, and 802E-3kg/s.Methane as a main fuel was changed to produce different equivalence ratios using a flow meter.Figure 4 shows the result of operation RDE.There are three modes that happened on RDE which are a failure, unstable and continuous detonation.Failure refers to unable to transition from a deflagration wave to a detonation wave.RDE was run with a deflagration wave rather than a detonation wave.Unstable detonation refers to a detonation wave that happens inside the RDE combustion chamber however there is an interruption happen to produce multiple explosions and the wave was not stable.Continuous detonation is the successful transition from deflagration wave to detonation wave combustion then it runs without any interruption on combustion.These three modes are observed from the pressure product and thrust result.
Failure and unstable detonation may be due to the asymmetric geometries that can create complex shock wave patterns that interact with the detonation wave.These interactions can lead to pressure variations and affect the stability and propagation of the detonation front may cause failure and unstable detonation.The experimental evaluation of the impact of the equivalence ratio on the operability of the Asymmetric Rotating Detonation Engine (RDE) using methane with oxygen has revealed an interesting and significant observation.The tests, conducted at various equivalence ratios ranging from 0.6 to 1.2 with increments of 0.1, have shown a clear trend in thrust generation for the Asymmetric RDE.From the graph plotting thrust against the equivalence ratio in Figure 5, it is evident that the Asymmetric RDE experiences an increasing trend in thrust production as the equivalence ratio varies.This means that as the fuel-to-air mixture becomes progressively richer (equivalence ratio approaches 1.0 and beyond), the engine's thrust output also increases.This observation highlights the potential advantage of the Asymmetric RDE design in terms of generating higher thrust at certain equivalence ratios, particularly in the richer mixture conditions.It suggests that the engine's combustion process may be more favourable and efficient in the fuel-rich regime.
However, it is essential to note that while the increasing trend indicates the potential for higher thrust, the tests might have also revealed an upper limit or a critical threshold in the equivalence ratio beyond which the engine experiences instability or failure.The engine achieved its highest thrust (18N) when operating at an equivalence ratio of 1.2.This means that the fuel-air mixture was slightly fuel-rich, as an equivalence ratio of 1.0 corresponds to a stoichiometric mixture.Detonation initiation failure occurred in just three mixing conditions, while an unstable detonation was produced in only one specific mixing condition.In general, mixing with the chosen equivalence ratio resulted in successful detonation across various thrust levels.

Conclusion
The exploration of new shapes for the annulus chamber in RDEs is an exciting avenue for research and development.The result of this experiment showed a promising outcome on applying a new shape combustion chamber on RDE.RDE with asymmetric shape design was successfully operated at different equivalence ratio and different flowrate.Asymmetric RDE was able to operate at different equivalence ratio from 0.5 up to 1.2.Result showed the highest thrust happens at an equivalence ratio of 1.2 with 18N.Stable detonation was occurred in this asymmetric vortex shape RDE.
However, it's essential to approach this research with thorough further analysis and testing using computational modelling, and experimental testing to understand the full potential and limitations of these new designs.As technology progresses, it will be interesting to see how asymmetric annulus chambers and other innovations contribute to the advancement of rotating detonation engines.

Figure 1 .
Figure 1.Asymmetric vortex combustor (AVC) design.In contrast to regular vortex combustion, asymmetric vortex combustion (AVC) introduces axial air components.This leads to a distinct process of mixing and flame stabilization.Fuel is rapidly injected from the bottom and mixes with fresh air from the side before combustion occurs along the vortex motion boundaries.This process generates a recirculation zone at the centre of the combustor, enhancing the mixing of exhaust gases and contributing to upstream fuel-air mixing.Heat transfer primarily occurs in the central flow region, while the reaction zone surrounds the recirculation zone in a circumferential manner.Despite being a non-premixed mixing combustion, AVC exhibits a bluish premixed flame due to its distinctive flame stabilization mechanism.The AVC concept ensures flame stability by anchoring the reaction zone to the boundary of a forced vortex field.This configuration facilitates rapid mixing of air and fuel upstream of the reaction zone.This phenomenon of AVC holds true across different scales, from large-scale to meso-scale applications, and has been demonstrated in deflagration conventional combustion[5], meso-scale combustors[6], and flameless combustion[7].However, the impact of AVC on detonation combustion within this framework remains unexplored.So, the primary objective of this paper is to investigate the influence of asymmetric vortex shape design on a rotating detonation engine.

Figure 2 .
Figure 2. Schematic of the experiment setup.

Figure 3 .
Figure 3. Top view of RDE for the asymmetric shape of the combustion chamber.Value is on mm.

Figure 4 .
Figure 4. Operational space of the Asymmetric RDE fuelled by methane.

Figure 5 .
Figure 5.The equivalence ratio produces multiple explosions with unstable detonation.