Analisis CFD pada Geometri Backward-facing Step dengan variasi Bilangan Reynolds

Steven Darmawan; Joshua Nove Octavian

doi:10.24843/JEM.2018.v11.i02.p07

Steven Darmawan Program Studi Teknik Mesin, Universitas Tarumanagara, Jakarta, Indonesia
Joshua Nove Octavian Program Studi Teknik Mesin, Universitas Tarumanagara, Jakarta, Indonesia

DOI: https://doi.org/10.24843/JEM.2018.v11.i02.p07

Abstract

Sistem tata udara pada bidang perhotelan merupakan salah satu aspek penting untuk menunjang kenyamanan, yang pada daerah pariwisata seperti Bali dengan iklim tropis dilakukan dengan aplikasi pendingin udara (AC). Aliran udara pada hotel bertingkat biasa dibuat terpusat dan dialirkan ke tempat tujuan dengan menggunakan ducting dengan penampang segi empat. Menyesuaikan dengan denah yang ada, seringkali terdapat bagian ducting yang harus dibuat bertingkat pada titik datum yang sama (zona ekspansi). Geometri bertingkat ini dapat menghasilkan kerugian aliran karena pada zona ekspansi tersebut timbul aliran berputar (aliran resirkulasi). Pemahaman terhadap aliran berputar dapat dilakukan dengan lebih mudah, menarik dan berbiaya rendah dengan menggunakan geometri Backward-facing step (BFS). Pada penelitian ini, dilakukan analisis numerik melalui simulasi CFD terhadap aliran resirkulasi pada sebuah geometri backward-facing step, dengan panjang total (L) = 4050 mm, step height (h) = 41 mm, upstream height (H) = 81 mm, rasio ekspansi = 1.5, dan lebar (t) = 20h. Eksperimen dilakukan dengan fluida kerja asap. Untuk mendapatkan hasil yang lebih luas, eksperimen juga dilakukan pada 3 (tiga) variasi bilangan Reynolds: Re = 7.315,79; Re = 21.947,37; dan Re = 29.263,16. Simulasi CFD dilakukan secara 3 dimensi dengan menggunakan model turbulen RNG k-?, mesh jenis triangular dengan jumlah nodal sebanyak 36.806 nodal. Vorteks resirkulasi ditunjukkan melalui zona resirkulasi yang direpresentasikan oleh vektor kecepatan arah-x dan titik penyatuan (reattachment point) yang diukur dari zona resirkulasi. Hasil simulasi CFD menunjukkan bahwa vorteks resirkulasi timbul pada X/h = 29.2 hingga X/h = 35 untuk seluruh bilangan Reynolds uji yang direpresentasikan oleh vektor kecepatan arah-x, dimana Reynolds Re = 7.315,79 menghasilkan titik penyatuan pada X/h > 35 dari zona resirkulasi, lebih jauh dibandingkan dengan yang dicapai oleh bilangan Reynolds yang semakin besar

Air conditioning system is an important aspect in tourism industry that has become a priority sector in Bali, Indonesia. It’s tropical climate make the air conditioning system use for cooling system only (AC). The centralized air conditioning system in high rise building uses ducting system with rectangular cross-sectional area which often depends on a given blue print. Some specific area of the blue print may lead to adjustment of the ducting system, e.g. the gradual step of the ducting system. This gradual base of the ducting may cause additional consequences, a recirculation flow at the gradation area (the expansion zone). As already known, the recirculation flow leads to flow losses. This paper discuss an numerical analysis with CFD simulation of a backward-facing step (BFS) geometry. The BFS geometry used in this has total length (L) = 4050 mm, step height (h) = 41 mm, upstream height (H) = 81 mm, expansion ratio = 1.5, and width (t) = 20h. The CFD simulation conducted three dimensionaly with RNG k-? turbulence model, and 36.806 triangular nodes. Recirculation vortices represented by recirculation zone with x-velocity and reattachment point. The result of CFD simulation that is conducted to Reynolds number Re = 7.315,79; Re = 21.947,37; dan Re = 29.263,16 shows that recirculation vortices occur at X/h = 29,2 to X/h = 35 for tested Reynolds number. Reattachment point at Reynolds number of 7.315.79 occur at X/h > 35, which farther than that is achieved at larger Reynolds number.

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