KARAKTERISTIK HIDRODINAMIK DAN PERPINDAHAN KALOR PADA FENOMENA FLOODING DALAM PIPA PANAS VERTIKAL

Mahmuddin, Ir.,MT., Ir. Samsul Kamal, MSc., Ph.D.,

2011 | Tesis | S3 Teknik Mesin

Penelitian tentang aliran berlawanan arah udara dan air di dalam pipa vertikal telah dilakukan. Meliputi karakteristik aliran film, permulaan flooding (onset of flooding, OF) yang dipetakan terhadap waktu dan jarak pengukuran serta pengaruh karakteristik ketebalan film dan OF terhadap perpindahan kalor. Penelitian dilakukan dengan dua tahapan, pertama adalah proses adiabatik (Riset 1) dan kedua dengan proses non-adiabatik (Riset 2). Air dialirkan dari atas pipa melalui media berpori sebelum masuk ke seksi uji dan udara diinjeksikan dari sisi bagian bawah secara aksial. Variasi angka Reynolds air (ReL) antara 322 sampai dengan 2446 dan angka Reynolds udara (ReG) antara 4446 sampai dengan 13941. Seksi uji proses adiabatik adalah pipa transparan dari bahan fleksiglas yang dilengkapi dengan sensor ketebalan film. Sensor tersebut terdiri dua kawat paralel yang dipasang pada jarak (Z) 400 mm, 1600 mm dan 2200 mm dari injektor air. Pengukuran ketebalan film dengan metode konduktansi tipe dua kawat paralel. Seksi uji proses non adiabatik adalah pipa panas dari bahan tembaga. Seksi uji dilengkapi dengan sensor suhu aliran film, sensor suhu permukaan dinding pipa dan sensor ketebalan film. Sensor-sensor tersebut dipasang secara tangensial dengan jarak 120 o pada jarak (Z) 1450 mm, 1600 mm dan 1750 mm dari injektor air. Sinyal dari masing-masing sensor akan direalisasikan dengan komponen LM324 kemudian didigitalkan dengan LabJack U12 PH OEM. Rangkain dari sensor tersebut dilengkapi transduser yang dapat mengubah inputan data berupa suhu dan tahanan air ke dalam bentuk tegangan listrik. Data pengukuran akan direkam secara simultan dengan alat data akuisisi. Peningkatan kecepatan udara dapat menyebabkan lapisan film yang bersinggungan dengan aliran udara terdorong ke atas, dan lapisan film di dekat permukaan dinding pipa tetap mengalir ke bawah. Fenomena ini merupakan permulaan flooding. Pada ReL rendah OF terjadi lebih awal dari sisi bagian bawah pipa ditandai ketebalan film meningkat secara drastis. Peningkatan ketebalan film tersebut diindikasikan sebagai hydraulic jump look like. Selain itu dapat pula mengakibatkan suhu permukaan dinding pipa menurun dan koefisien perpindahan kalor konveksi meningkat. Sebaliknya, pada ReL tinggi OF diawali dari sisi bagian atas pipa ditandai ketebalan film menurun secara drastis. Penurunan ketebalan film dapat mengakibatkan suhu permukaan dinding pipa meningkat serta koefisien perpindahan kalor konveksi menurun.

Experiments have been conducted on countercurrent flow and water in a vertical pipe. These covered the characteristics of film flow and onset of flooding (OF) which were mapped against time and distance measurement and the influence of film thickness and the OF characteristics of heat transfer. The study was conducted in two stages, The first was the adiabatic process (Research 1) and the second was non-adiabatic processes (Research 2). Water flowed from the top side of the pipe through porous media prior entering the test section and the air was injected axially from the bottom side. Variations in water Reynolds numbers (ReL) was 322 to 2446 and air Reynolds number (ReG) was 4446 to 13941. Test section of adiabatic process is made of transparent flexiglass tube equipped with a sensor film thickness. The sensor consists of two parallel wires mounted at a distance (Z) of 400 mm, 1600 mm and 2200 mm below the water injector. The variation of the film thickness was measured with conductance method using two pin prallel type . Test Section of non-adiabatic process is a hot pipe from copper material. Test section is equipped with a temperature sensor film flow, temperature sensors surface pipe wall and the sensor film thickness. The sensors are mounted circumferally at 120 o each at the the distance of 1450 mm, 1600 mm and 1750 mm in front of water injector. The signal from each sensor will be acquired ith the LM324 component then digitized with a LabJack U12 PH OEM. Electrical circuit of the sensor is equipped with transducers that can change the input data of temperature and water resistance in the form of electrical voltage. The data measurement will be recorded simultaneously by means of data acquisition. Increased air velocity can cause the layer of film that is tangent to the flow of air pushed upward but, the film in near surface of the pipe wall was not affected. This phenomenon is konown as the onset of flooding. At low ReL OF initialized at the botton of the pipe and was indicated by a sudden increased of the film thickness. The sudden increase of film thickness is also known as hydraulic jump look like. The OF may decrease the wall temperature and inceease the heat transfer coeficient. In contrary, at high ReL Of started at the upperside of the pipe and was indicated by sudden decrease of the film thickness. Hence, the wall temperatures increased and the convestion heat transfer coefficient decreased simultaneously.

Kata Kunci : characteristic film thickness, onset of flooding, hydraulic jump look like, and convection heat transfer coefficient.