The 'Eckert number phenomenon' in pipeline decompression

The Eckert number emerges as an important non-dimensional parameter, in addition to the Reynolds number and the Prandtl number, in problems involving heat transfer in compressible flows. The Eckert number is considered to represent a ratio of the flow kinetic energy at the wall and the specific enthalpy difference between the wall and the fluid, and is important when viscous dissipation is significant. This paper investigates the role played by the Eckert number during rapid decompression of high pressure gas pipelines. During such processes, the gas temperature attains very low values corresponding to sonic flow at the vent location, and it is often assumed that the pipeline material is also cooled to a comparable degree ("Low Temperature Excursions"). This has often led to over-specification of the properties required of the pipeline material. In this paper, it is shown using Computational Fluid Dynamics (CFD) simulations that the highly complex flow during rapid decompression of high-pressure gas pipelines leads to significant frictional dissipation (heating) adjacent to the pipe wall. This prevents the pipeline from attaining excessively low temperatures. It is shown that frictional heating may sometimes lead to a heat transfer reversal. This finding may help pipeline designers in making appropriate recommendations regarding the properties required of the pipeline materials. The paper also describes a preliminary experiment designed carried out to validate the CFD simulations. More detailed experiments are under way.