Computational fluid dynamics modeling and validation of swirling flow in a pipe with classic twisted tape

The twisted tape is a passive equipment enhancing the heat transfer capability of a heat exchanger. This technique was adopted in various industries including nuclear engineering to improve economic efficiency. Previous researchers used computational fluid dynamics (CFD) method to predict the performance of heat exchanger with twisted tape. Existing literatures insisted that simulation result with k-omega model most corresponds with the experimental data on friction factor, Nusselt number. However, fluid behavior in twisted tape was not validated due to the lack of experimental data. As the optical access to the inner space of a pipe with a twisted tape was limited, the detailed flow field data were not obtainable so far. The fluid behavior is important for optimization of shape of twisted tape in terms of size, pitch and distance between tapes. In this study, 2-Dimensional fluid velocity data was measured using particle image velocimetry (PIV) in wide Reynolds number range of 1370 ~ 9591. The test section was made by 3D printing technique of stereolithography. It enabled the production of transparent test section for optical access. A distortion due to difference of refractive indices of fluid and test section was diminished by matching the refractive index. A noble working fluid, which is mixture of herb oil and mineral oil, was used. The experimental data was compared with simulation result of various turbulent models. The PIV data showed characteristic two-peak motion in axial direction. The CFD simulation used standard k-epsilon and standard k-omega turbulence model. The geometry of fluid region in CFD was perfectly depicted as real flow path. The result showed that k-e turbulence model is proper to use for prediction of flow behavior in flow around twisted tape.