Droplet Evaporation in a Gas-Droplet Mist Dilute Turbulent Flow behind a Backward-Facing Step

The mean and fluctuation flow patterns and heat transfer in a turbulent droplet-laden dilute flow behind a two-dimensional single-side backward-facing step are numerically studied. Numerical simulations are performed for water droplets, with the inlet droplet diameters d1 = 1–100 μm; they have a mass fraction of ML1 = 0–0.1. There is almost no influence of a small number of droplets on the mean gas flow and coefficient of wall friction. A substantial heat transfer augmentation in a droplet-laden mist-separated flow is shown. Heat transfer increases both in the recirculating flow and flow relaxation zones for fine, dispersed droplets, and the largest droplets augment heat transfer after the reattachment point. The largest heat transfer enhancement in a droplet-laden flow is obtained for small particles.

Estimating the Reattachment Length by Realizing a Comparison between URANS k-Omega SST and LES WALE Models on a Symmetric Geometry

In this study, a water reattachment length was calculated by adopting two different models. The first was based on Unsteady Reynolds-Averaged Navier–Stokes (URANS) k-omega with Shear Stress Transport (SST); the second was a Large Eddy Simulation (LES) with Wall-Adapting Local Eddy-Viscosity (WALE). Both models used the same mesh and were checked with Taylor length-scale analysis. After the analysis, the mesh had 11,040,000 hexahedral cells. The geometry was a symmetrical expansion–contraction tube with a 4.28 expansion ratio that created mechanical energy losses, which were taken into account. Moreover, the reattachment length was estimated by analyzing the speed values; the change of speed value from negative to positive was used as the criterion to recognize the reattachment point.

Numerical study on mean flow field of turbulent dual offset jet

A numerical investigation on the mean flow and turbulence characteristics of dual offset jet for various separation distances between the two jets with a fixed offset height of the lower jet from the bottom wall is reported in this study. The numerical simulations have been performed by solving the Reynolds-averaged Navier-Stokes equations (RANS) with two-equation standard [Formula: see text] turbulence model. The Reynolds number based on the jet width and the inlet turbulence intensity are considered as 15,000 and 5%, respectively. The computational results for the mean flow reveal that after issuing from the nozzles, the adjacent shear layers of the offset jets meet together at the merging point and then the merged jets reattaches on the bottom wall at the reattachment point before they combine together at the combined point forming a single jet flow. In the far downstream, the flow field behaves like a classical single wall jet flow. The self-similarity of mean flow field is achieved at far down stream of combined point. An increase in separation distance between the two jets [Formula: see text] results in a decrease in magnitude of the streamwise maximum velocity of the combined jet but with same rate of decay. The converging region of the jets has depicted considerable growth of turbulence as the jet centrelines bend towards the merging point. According to the mean flow results, the distances of the reattachment point and the combined point from the nozzle exit gradually increase with the progressive increase in separation distance between the two jets within the range d/ w = 3–8.

Numerical Investigation of the Effect of Sudden Expansion Ratio of Solid Fuel Ramjet Combustor with Swirling Turbulent Reacting Flow

In this paper, the effect of sudden expansion ratio of solid fuel ramjet (SFRJ) combustor is numerically investigated with swirl flow. A computational fluid dynamics (CFD) code is written in FORTRAN to simulate the combustion and flow patterns in the combustion chamber. The connected-pipe facility is used to perform the experiment with swirl, and high-density Polyethylene (HDPE) is used as the solid fuel. The investigation is performed with different sudden expansion ratios, in which the port and inlet diameters are independently varied. The results indicated that the self-sustained combustion of the SFRJ occurs around the reattachment point at first, and then the heat released in reattachment point is used to achieve the self-sustained combustion in the redevelopment zone. The average regression rate is proportional to the sudden expansion ratio for the cases with a fixed port diameter, which is mainly dominated by the enhancement of heat transfer in backward-facing step. However, the average regression rate is inversely proportional to the sudden expansion ratio for the cases with fixed inlet diameter, which is influenced by the heat transfer mechanism of developed turbulent flow in the redevelopment zone.

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

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.

Numerical Study of Convective Heat Transfer From Expanding Annular Pipe Flows

Convective heat transfer from suddenly expanding annular pipe flows are numerically investigated within the steady laminar flow regime. A parametric study is performed to reveal the influence of the annular diameter ratio, k, the Prandtl number, Pr, and the Reynolds number, Re, over the following range of parameters: k = {0, 0.5, 0.7}, Pr = {0.7, 1, 7, 100}, and Re = {25, 50, 100}. Heat transfer enhancement downstream of the expansion plane is only observed for Pr > 1. Peak wall-heat-transfer-rates always appear downstream of the flow reattachment point, in the case of suddenly expanding round pipe flows, i.e. k = 0. However, for suddenly expanding annular pipe flows, i.e., k = 0.5 and 0.7, peak wall-heat-transfer-rates always appear upstream of the flow reattachment point. The observed heat transfer augmentation is more dramatic for suddenly expanding annular flows, in comparison with the one observed for suddenly expanding pipe flows. For a given annular diameter ratio and Reynolds number, increasing the Prandtl number, always results in higher wall-heat-transfer-rates downstream the expansion plane.

Features of a Laminar Separated Boundary Layer Near the Leading-Edge of a Model Airfoil for Different Angles of Attack: An Experimental Study

The evolution of a separated boundary layer over a model airfoil with semicircular leading-edge has been illustrated for angles of attack (α) varying from −3 deg to 10 deg, where the Reynolds number (Rec) based on chord is 1.6 × 105 and the inlet freestream turbulence (fst) being 1.2%. The features of boundary layer are described through measurements of velocity and surface pressure besides the flow visualization using a planar particle image velocimetry (PIV). Freestream perturbations are amplified because of enhanced receptivity of the separated boundary layer resulting in pockets of disturbances, which then propagate downstream attributing to random fluctuations near the reattachment. The separation and reattachment locations including the onset and end of transition are identified for changing α. The reattachment point changes from 18.8% to 47.7% of chord with the onset of separation at almost 7%, whereas the onset of transition moves upstream from 13.2% to 9% with increasing α. The bubble bursting occurs at α = 10 deg. The transition in the separated boundary layer occurs through Kelvin–Helmholtz (K–H) instability for α = 0 deg and 3 deg, whereas the K–H mechanism is bypassed for higher α with significant viscous effect.

Numerical Study of External Flow over Ducts with Various Cross-Sections

In this article a comprehensive numerical study is performed to compare the effect of fluid flow across a duct with various cross sectional shapes and with different velocities of the flow. Circular, elliptical and rectangular cross sections have been chosen for the ducts and air flows across them with four values of low Reynolds numbers in the range of Re = 1 to Re = 1000. Continuity and momentum equations with proper boundary conditions are solved in two dimensions. Streamlines, pressure distribution and Velocity profiles are obtained and creation of vortices, boundary layers, separation region, wake region, reattachment point and stagnation points are studied in detail and the results are compared for various cases. The value of the Reynolds number which the flow transits from steady to unsteady has been compared for the different cross sectional shapes.

Shear Cavitation in an Annular Jet Pump Under Recirculation Conditions

Recirculation accompanied by shear cavitation is a key flow feature in annular jet pumps (AJPs). In this study, a high-speed camera was used to capture the recirculation region and various types of cavity clouds. By monitoring the trajectories of the small bubbles, the main recirculation regions under each flow rate ratio were obtained. As the flow rate ratio decreases, the recirculation region continued expanding with the separation point moving upstream, while the reattachment point remained nearly stationary regardless of the decreasing flow rate ratio. Hill's spherical vortex theory was adopted to evaluate the variations of the recirculation regions. Moreover, the minimum local static wall pressure in the recirculation region decreases as well, which can promote the inception and development of shear cavitation. There are numerous vortices simultaneously induced by the large velocity gradient in the shear layer, the core of which becomes a potential site for cavitation. Consequently, with the growth of the recirculation region, three types of cavity clouds, viz., the ribbonlike, annular, and merged cavity clouds, appear in turn. The movement characteristics of these cavity clouds, including their inception, distortion, and collapse, are illustrated based on the high-speed imaging results. The ribbonlike and annular cavity clouds are both induced by the small vortices in the shear layer because of the low local pressure in the vortex core. However, the merged cavity clouds are caused by a combination of several ribbonlike and annular cavity clouds, which provides a larger scale and a longer life span. Hence, the collapse of the merged cavity clouds can cause a large pressure pulsation near the reattachment point of the recirculation region. The corresponding frequency spectra were also demonstrated based on the fast Fourier transform (FFT) method.