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Abstract :

To improve the efficiency of thermal-cracking processes, and to reduce the coking phenomena due to high wall temperature, the use of ribbed tubes is an interesting technique as it allows better heat transfer and mixing. However it also induces significant increase of the pressure loss, leading to higher power consumption to push the flow. The complexity of the turbulent flow, the chemical system, and their coupling makes it difficult to estimate a priori the real gain of ribbed tubes in terms of selectivity. Experiments combining turbulence, heat transfer and chemistry are very rare in laboratories and too costly at the industrial scale. In the present work, Wall-Resolved Large Eddy Simulation (WRLES) is used to study firstly non-reacting and then reacting flow in both smooth and ribbed tubes, to reveal the impact of the ribs on the turbulence and chemistry. Simulations were performed with the code AVBP, which solves the compressible Navier-Stokes equations for turbulent flows, using reduced chemistry scheme of ethane and butane cracking for reacting cases.

Special effort was devoted to the wall flow, which is analyzed in detail and compared for both geometries, providing useful information for further development of roughness-type wall modeling. The impact of grid resolution and numerical scheme is also discussed, to find the best trade-off between computational cost and accuracy for industrial application. Results investigate and analyze the turbulent flow structures, as well as the effect of heat transfer efficiency and mixing on the chemical process in both smooth and ribbed tubes. Pressure loss, heat transfer and chemical conversion are finally compared.

Jury :

Kevin VANGEEM                              Gent University                                 Referee

Franck NICOUD                                Université Montpellier 2                   Referee

Laurent JOLY                                    ISAE                                                 Invited member

André NICOLLE                                IFPEN                                              Invited member

Pierre-Alexandre GLAUDE               Université de Lorraine                      Invited member

Philippe RICOUX                              TOTAL                                              Invited member

David JOHN-BROWN                       TOTAL                                              Invited member

Dominique VIENNET                        TOTAL                                              Invited member

Bénédicte CUENOT                          CERFACS                                        Advisor