INVESTIGATION OF TAYLOR BUBBLE PROPERTIES AND THEIR REACTION ON THE CO-CURRENT UPWARD SLUG FLOW FROM EXPERIMENTAL DATABASE OF ULTRAFAST X-RAY TOMOGRAPHY

AKMAL IRFAN MAJID, Dr. Deendarlianto

2015 | Tesis | S2 Teknik Mesin

Aliran slug adalah pola aliran dua fasa yang cukup bersifat kompleks. Pola aliran ini ditandai adanya Taylor bubble dan liquid slug yang berisi bubble kecil yang terdispersi di dalamnya. Karena sifat intermiten dan tidak teraturnya, pola aliran ini mungkin menyebabkan dampak mekanis yang tinggi terhadap pipa, korosi, dan tingginya fluktuasi tekanan. Oleh karena itu, aliran slug umum dihindari dalam berbagai aplikasi keteknikan. Di sisi lain, perkembangan pesat model-model simulasi Computational Fluid Dynamics (CFD) perlu didukung dengan database eksperimen yang berkualitas tinggi untuk validasi dan peningkatan model simulasi. Investigasi mendetail mengenai sifat-sifat Taylor bubble diperlukan untuk mendapatkan mekanisme fisik dari fenomena slugging. Percobaan mengenai aliran air-udara searah ke atas dalam berbagai kecepatan superfisial gas dan cairan di dalam pipa vertikal dengan diameter 54,8 mm dilakukan di Helmholtz-Zentrum Dresden-Rossendorf. Metode pengukuran berupa ultrafast dual-layers X-ray tomography dikembangkan untuk memenuhi kebutuhan akan pengukuran akurat dengan resolusi spasial dan temporal yang tinggi. Hasil pemindaian tomografi dilakukan rekonstruksi dan segmentasi untuk mendapatkan ukuran dan parameter individu bubble. Bubble pair algorithm dipakai untuk memperkirakan pergerakan individual dari setiap bubble. Metode ini mampu menentukan pasangan bubble dari titik pengukuran atas dan bawah dengan mempertimbangkan probabilitas tertinggi dari posisi, volume, dan kecepatan bubble. Oleh karena itu, karakteristik individu setiap bubble dapat ditentukan. Analisis statistika juga digunakan dalam mendukung analisis data Karakteristik umum dari aliran slug dijelaskan dalam bentuk pengamatan kualitatif dan penjelasan kuantitatif dari karakteristik fraksi hampa. Sifat-sifat Taylor bubble seperti kecepatan, frekuensi, dan panjang diamati dan alasan distribusi kecepatannya juga didaatkan. Dengan adanya Taylor bubble menghasilkan perilaku aliran yang berbeda pada bubble kecil di sekitarnya, seperti bubble yang berada di area falling film dan di area belakang Taylor bubble. Hubungan antara sifat-sifat Taylor bubble dan liquid slug terhadap kecepatan juga diungkap. Peningkatan stasiun aksial sepanjang pipa mempengaruhi pola aliran fraksi hampa, dan sifat-sifat Taylor bubble dan liquid slug. Metode bubble pair mampu mengetahui sifat-sifat Taylor bubble dan karakteristik aliran slug. Hasil penelitian ini relevan dengan penelitian yang telah dilakukan sebelumnya.

Slug flow is a complex two-phase flow pattern that characterized by the presence of Taylor bubbles and liquid slugs with small bubbles inside. Due to the intermittent and irregular natures, the pattern might cause high mechanical impact to the pipeline, corrosion and high pressure fluctuation. Therefore, it is common avoided in engineering applications. On the other hand, the rapid developments of new Computational Fluid Dynamics (CFD) models need to be supported by high quality experimental database to validate and improve the simulation models. Investigations about detail properties of Taylor bubbles are necessary to obtain physical mechanisms of slugging phenomena. At the Helmholtz-Zentrum Dresden-Rossendorf, experiments on co-current upward air-water flow in 54.8 mm diameter vertical pipe with various gas-liquid superficial velocities were performed. As measurement technique, an ultrafast dual-layers X-ray tomography was developed to fulfil the requirement of an accurate measurement with high spatial and temporal resolutions. Results of tomography scanning were reconstructed and segmented to carry out each gas bubble size and parameters. A bubble pair algorithm is used to estimate the instantaneous movement of each bubble. This method is able to assign the correct paired bubbles from both measurement layers by considering the highest probability of position, volume, and velocity. Therefore, each gas-bubble individual characteristics can be revealed. A statistical analysis was also used to support the data analysis. General characteristics of slug flow were explained in the form of qualitative observation and quantitative explanation of void fraction characteristics. Taylor bubble properties such as the velocity, frequency, and length were observed and reasons of velocity distribution were carried out. The presence of Taylor bubble generates different flow behaviors for small bubbles around it such as bubbles in falling film region and behind area of the Taylor bubble. Reasons of the Taylor bubble velocity distributions were explained. Relationship between Taylor bubble properties and as well liquid slug to the velocities are also revealed. The increase of axial station along the pipe influences flow pattern, void fraction, and properties of Taylor bubble and liquid slug as well. Bubble pair method was able to reveal the Taylor bubble properties and slug flow characteristics. A relevant agreement between the previous studies was carried-out.

Kata Kunci : Bubble, Aliran slug, Taylor bubble, Bubble pair algorithm, Ultrafast X-ray tomography