Multi-scale energy injection: a new tool to generate intense homogeneous and isotropic turbulence for premixed combustion

Abstract

Highly turbulent flow, generated by an original multi-scale device and nearly homogeneous and isotropic, is experimentally investigated. This multi-scale device is made of three perforated plates shifted in space such that the diameter of their holes and their blockage ratio increase with the downstream distance. The multi-scale turbulence injection (MuSTI) is compared with a mono-scale turbulence injection (MoSTI), the latter being only constituted of the last plate of the MuSTI. This comparison is done for both cold and reactive flows. For the cold flow the following are shown, in comparison with the classical mono-scale device, for the MuSTI device in the near-field: (i) The turbulent kinetic energy is larger, and the kinetic energy supply is distributed over the whole range of scales. This is emphasised by second- and third-order structure functions. (ii) The shear-stresses are enhanced. (iii) The homogeneity and isotropy are reached earlier (≈50%). (iv) The jet-merging distance is the relevant scaling length scale of the turbulent flow. (v) High turbulence intensity (≈15%) is achieved in the homogeneous and isotropic region, although the Reynolds number based on the Taylor microscale remains moderate (Re _λ≈80). In the second part, the interaction between the multi-scale-generated turbulence and the premixed flame front is investigated by laser tomography. A lean V-shaped methane/air flame is stabilised on a heated rod in the homogeneous and isotropic region of the turbulent flow. The main observation is that the flame wrinkling is amplified with the multi-scale device, as testified by the increase of the flame brush thickness.

Keywords:

• homogeneous and isotropic turbulence
• multi-scale injection
• premixed combustion

Acknowledgements

The authors are grateful to Dr. G. Godard for his very useful help with the LDV measurement system, to Dr. F. Corbin for his assistance with the LDV acquisition system. The authors are also thankful to Ms. A. Taquet for the time she took to help us improve the writing. This work has been supported by a grant of the Région Haute-Normandie.

Notes

^a Estimated from [36].

^b Estimated from [37].