Effects of activation energy on the instability of oblique detonation surfaces with a one-step chemistry model

A numerical study was performed to investigate the detailed effects of activation energy Ea on the oblique detonation wave surface instability. Numerical simulations were performed using an ideal reactive flow model given by the inviscid Euler equations with one-step irreversible Arrhenius reaction kinetics. The numerical results demonstrate two types of unstable structures following the initial smooth surface after detonation initiation. One exhibits by a “saw-tooth” reactive front and the other exhibits by a “keystone” feature. To quantify the destabilization processes, two characteristic length scales, L1 and L2, are defined statistically to be the length of the smooth detonation surface before the appearance of instabilities and the length of the unstable surface before the first cellular structure with the onset of right-running transverse waves, respectively. Their dependence on Ea was simulated and analyzed. In general, both lengths decrease with increasing Ea, making the surface more unstable. However, with increasing Ea, the high temperature sensitivity of the mixture causes an abrupt explosion in the initiation region, introducing a high overdriven surface and suppressing the instability. With the balance between the destabilizing effect of Ea and the stabilizing effect of increasing overdrive factor, both L1 and L2 are found to approach a near-constant value in the high Ea limit

Experimental Study on Pulse Detonation Engine with Two-Phase Inhomogeneous Mixture

In order to investigate the effects of fuel distribution on the operation of two-phase pulse detonation engine (PDE), a series of cold flow and multicycle PDE experiments was carried out with 9 mixing schemes. Homogeneity degree with fuel distribution considered in terms of space and time was proposed to quantitatively evaluate the mixing of liquid fuel and air by particle image velocimetry (PIV) in cold flow experiments. Operation stability of multicycle PDE was presented by statistical analysis of peak pressure at the outlet of a detonation tube. The relationship between operation stability and homogeneity degree was quantitatively elaborated. These experimental results indicated that not only using mixing reinforcement devices (such as pore plate and reed valve) was fuel distribution improved but also the effect of inlet ways on the homogeneity degree was weakened. The homogeneity degree of fuel distribution ζ=0.72 was a critical value for stable working of multicycle PDE. When homogeneity degree was lower than 0.72, stable state was not maintained and detonation wave in some cycles was not established due to poor fuel distribution. Therefore, it is necessary to hold homogeneity degree larger than 0.72 to achieve stable operation of PDE. These results contribute to enhancing the operation stability and offering guidelines for the design of PDE’s mixing scheme

A designed wall roughness approach to improve turbulent heat transfer to supercritical CO₂ flowing in horizontal tubes

Supercritical flow through a horizontal pipe leads to a non-uniform peripheral wall temperature distribution even when the wall heat flux is kept constant and uniform. This is attributed to lower heat transfer coefficient at the top section where the denser fluid tends to sink. Hence, to obtain a uniform wall temperature, a designed wall roughness is devised. Uniform sand-grain roughness is employed to only partly cover the top half of the pipe wall. Numerical simulations were conducted using the SST k−ω turbulence model. The simulation results indicate that our proposed design can lead to a more uniform heat transfer distribution over the wall periphery compared with the smooth pipe. An extreme case was also considered where the inner wall was completely covered with roughness elements. While heat transfer augmentation was observed for this case, the excess pressure drop was prohibitively higher compared with a pipe with designed wall roughness.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Process and Energ