Predicting Critical Heat Flux With Multiphase CFD: 4 Years in the Making

Advancement in the experimental techniques have brought new insights into the microscale boiling phenomena, and provide the base for a new physical interpretation of flow boiling heat transfer. A new modeling framework in Computational Fluid Dynamics has been assembled at MIT, and aims at introducing all necessary mechanisms, and explicitly tracks: (1) the size and dynamics of the bubbles on the surface; (2) the amount of microlayer and dry area under each bubble; (3) the amount of surface area influenced by sliding bubbles; (4) the quenching of the boiling surface following a bubble departure and (5) the statistical bubble interaction on the surface. The preliminary assessment of the new framework is used to further extend the portability of the model through an improved formulation of the force balance models for bubble departure and lift-off. Starting from this improved representation at the wall, the work concentrates on the bubble dynamics and dry spot quantification on the heated surface, which governs the Critical Heat Flux (CHF) limit. A new proposition is brought forward, where Critical Heat Flux is a natural limiting condition for the heat flux partitioning on the boiling surface. The first principle based CHF is qualitatively demonstrated, and has the potential to deliver a radically new simulation technique to support the design of advanced heat transfer systems.

Computational Modeling of Particulate Pollutant Transport in a Ventilated Room in the Presence of Two Heated Breathing and Rotating Manikins

Recently, attention has been given to indoor air quality due to its serious health concerns. Clearly the dispersion of pollutant is directly affected by the airflow patterns. The airflow in indoor environment is the results of a combination of several factors. In the present study, the effects of thermal plume and respiration on the indoor air quality in a ventilated cubicle were investigated using an unsteady computational modeling approach. The person-to-person contaminant transports in a ventilated room with mixing and displacement ventilation systems were studied. The effects of rotational motion of the heated manikins were also analyzed. Simulation results showed that in the cases which rotational motion was included, the human thermal plume and associated particle transport were significantly distorted. The distortion was more noticeable for the displacement ventilation system. Also it was found that the displacement ventilation system lowered the risk of person-to-person transmission in an office space in comparison with the mixing ventilation system. On the other hand the mixing system was shown to be more effective compared to the displacement ventilation in removing the particles and pollutant that entered the room through the inlet air diffuser.

Comparative Calculations of TROI TS-2 and TS-3 Steam Explosion Experiments With TEXAS-V

During a severe accident, contact of the molten corium with the coolant water may cause an energetic steam explosion which is a rapid increase of explosive vaporization by transfer to the water of a significant part of the energy in the corium melt. This steam explosion has been considered as an adverse effect when the water is used to cool the molten corium and could threaten reactor vessel, reactor cavity, containment integrity. In this study, TROI TS-2 and TS-3 experiments as part of the OECD/SERENA-2 project were analyzed with TEXAS-V. Input parameters were based on actual TROI experiment data. In mixing simulations, calculated results were compared to melt front behavior, void fraction in trigger time and other parameters in experiment results. In explosion simulations, corresponding to TROI experiments an external triggering was employed at the moment that melt front reached heights of 0.4 m. Calculated results of peak pressure and impulse at the bottom were compared with TROI experiment results. Melt front behaviors of the melt was different from the experimental results in both TS-2 and TS-3. Void fraction in triggering time in TS-2 was in good agreement with the experiment results and in TS-3 was slightly overestimated. The peak pressure and impulse at bottom were successfully predicted by TEXAS-V. These calculations will allow establishing whether the limitations and differences observed in the simulations of the experiments are important for the reactor case.

Characteristics of Dynamic Forces Generated by a Flapping Butterfly and its Wake Structure

A typical example of the flow field around a moving elastic body is that around butterfly wings. Butterflies fly by skillfully controlling this flow field, and vortices are generated around their bodies. The motion of their elastic wings produces dynamic fluid forces by manipulating the flow field. For this reason, there has been increased academic interest in the flow field and dynamic fluid forces produced by butterfly wings. A number of recent studies have qualitatively and quantitatively examined the flow field around insect wings. In some such previous studies, the vortex ring or vortex loop formed on the wing was visualized. However, the characteristics of dynamic forces generated by the flapping insect wing are not yet sufficiently understood. The purpose of the present study is to investigate the characteristics of dynamic lift and thrust produced by the flapping butterfly wing and the relationship between the dynamic lift and thrust and the flow field around the butterfly. We conducted the dynamic lift and thrust measurements of a fixed flapping butterfly, Idea leuconoe, using a six-axes sensor. Moreover, two-dimensional PIV measurement was conducted in the wake of the butterfly. The butterfly produced dynamic lift in downward flapping which became maximum at a flapping angle of approximately 0.0 deg. At the same time, the butterfly produced negative dynamic thrust during downward flapping. The negative dynamic thrust was not produced hydrodynamically by a flapping butterfly wing because a jet was not formed in front of the butterfly. The negative dynamic thrust was the kicking force for jumping and the maximum of this kicking force was about 6.0 times as large as the weight. On the other hand, the butterfly produced dynamic thrust in upward flapping which was approximately 6.0 times as large as the weight of the butterfly. However, the attacking force by the abdomen of the butterfly was included in the dynamic thrust and we have not yet clarified quantitatively the dynamic thrust produced by the butterfly wing.

Experimental Investigation of a Drafting Cyclist in Cross-Wind

Drafting is commonly utilized in cycling, particularly during competitions. According to the literature, in a racing environment, the riders can expend 90% of their energy on overcoming drag, and can save about 30% of their energy by riding behind another rider in the absence of cross-wind. In the presence of a strong cross-wind, competitive cyclists form echelons by placing themselves about halfway behind each other, while being slightly offset sideways. Although forming an echelon is a common practice, the formation has not been sufficiently studied in the literature. To address this, the drag and side forces on a model cyclist were studied experimentally. A simplified 3D model was built based on the outline of a competitive cyclist. Two 1:11 scale models were rapid-prototyped and tested in a wind tunnel. The drafting effects on a cyclist were investigated for different yaw angles — the angles of the apparent wind with respect to the direction of cyclist motion. The effects of wind-stream-wise position and wind-off-stream-wise position were studied for each angle by measuring the drag and side-force on a model placed in the wake of another identical model. The results suggest that there is a significant decrease in both drag and side force when a cyclist is riding in the wake of another cyclist. Although a smaller wind-stream-wise offset generally results in smaller forces, this effect is not significant for most configurations. The offset in the wind-off-stream-wise direction has a noticeable effect on the forces — no off-stream-wise offset results in the lowest drag and side force, except for low yaw angles at which it may be beneficial for the drafting cyclist to be slightly forward with respect to the in-line (no offset) position.

Experimental and Computational Investigation of an Air Core Inside a Milling Circuit Hydroclone

Hydrocyclone separators are widely used in various industrial applications in the oil and mining industries to sort, classify and separate solid particles or liquid droplets within liquid suspensions. Often, studies in the literature have investigated idealized and simplified geometries, which are also typically scaled down to very small sizes. In this study, the two phase flow system inside a transparent acyclic model with actual milling circuit cyclone hydraulics was investigated computationally and experimentally. The diameter and height of the hydrocyclone are 12.7 cm and 94 cm, respectively. In many industrial applications, a single phase flow system in a hydrocyclone is a rarity, since nearly all cyclones have an underflow which is open to atmosphere, and therefore an air core is present along the central axis. In this study, the flow field with an air core present has been investigated. The computational modelling was conducted using Star CCM+, a commercial Computational Fluid Dynamics (CFD) software package. Large Eddy Simulation (LES) and the Volume of Fluid multiphase model was used. Additionally, the computational studies also focused on the prediction of the dimensions of the air core, which were measured experimentally. The tests were conducted in the Reynolds number range of 20,000–150,000 and 9000–67,800 for the water and NaI solution respectively. The model hydrocyclone was made of optically transparent acrylic plastic with flat, smooth outer surfaces so that there were no reflections, distortions, or obstructions. Refractive index matching, to minimize refraction effects, between the test fluid and acrylic test piece was achieved using a test liquid of sodium iodide aqueous solution (63.3% NaI by weight). Images of the flow field with the air core were taken using a Canon DSLR camera. A comparison between the experimental data and the computational results were made in the r-z plane. The experimental results and the computational results will be discussed in this paper.

Simplified Transient Numerical Model of a Supersonic Jet Impacting a Substrate

The transient flow field near the surface of a substrate impacted by a pulsating supersonic jet emerging from a long tube is investigated using a simplified axially symmetric numerical approach. In the system being modeled, the pulses are created using a rotary valve located at the tube entrance. This flow situation approximates the conditions existing in the Shock-Induced Cold Spray process for coating surfaces with metallic particles. Previous numerical studies of transient supersonic jets either focused on jets emerging from orifices or did not give details of the complex supersonic flow field in the jet impact region. The current approximate numerical method considers the flow within the long tube and in the jet impact region. The procedure involves two stages. The upstream pressure variation with time is first determined using a one-dimensional compressible flow approximation of the entire tube and rotary valve arrangement. The resulting pressure versus time curve serves as the transient inlet boundary condition for an axially symmetric computational fluid dynamic solution of the flow through the tube and region of jet impact on the substrate. The numerical solutions of substrate pressure on the jet centerline versus time are compared with available experimental results and predict certain general features of the substrate pressure traces. Although the simplified model is only in fair agreement with some aspects of the experimental curves, it is shown to be useful in explaining certain peculiar flow features. With the aid of the numerical solution, an explanation for the movement and instability of the bow shock wave which forms ahead of the substrate is described.

A Numerical Investigation to Study Roughness Effects in Oscillatory Flows

Effects of roughness on the near-bed turbulence characteristics in oscillatory flows are studied by means of particle-resolved direct numerical simulations (DNS). Two particle sizes of diameter 375 and 125 in wall units corresponding to the large size gravel and the small size sand particle, respectively, in a very rough turbulent flow regime are reported. A double-averaging technique is employed to study the nature of the wake field, i.e., the spatial inhomogeneities at the roughness length scale. This introduced additional production and transport terms in double-averaged Reynolds stress budget, indicating alternate pathways of turbulent energy transfer mechanisms. Budgets of normal components of Reynolds stress reveal redistribution of energy from streamwise component to other two components and is attributed to the work of pressure in both flow cases. It is shown that the large diameter gravel particles significantly modulate the near-bed flow structures, leading to pronounced isotropization of the near-bed flow; while in the sand case, elongated horseshoe structures are found as a result of high shear rate. Effect of mean shear rate on the near-bed anisotropy is significant in the sand case, while it is minimal for the gravel-bed. Redistribution of energy in the gravel case showed reduced dependence on the flow oscillations, while for the sand particle it is more pronounced towards the end of an acceleration cycle.

Comparison Between NACA 65 Profile and Circular Arc Blade Based on Numerical Investigation

For the axial flow fans NACA profiles have been well explored. However, the development and production of NACA profiles are also very expensive. Due to their lower cost of production circular arc blades are also applied to axial flow fans. But there is few information in the open literature focusing on flow loss and behavior of circular arc blades. Therefore, one question remains: how much is the difference of flow loss and behavior between NACA profiles and circular arc blades. In this paper NACA 65 profile and circular arc blade are examined by numerical method. The paper shows the flow loss of both blades in dependence of incidence, Reynolds number and spacing ratio. The occurrence of flow behavior, such as separation bubbles on the leading edge and flow structure on the sidewall is examined and discussed. The flow structure is given on basis of numerical flow picture. Additionally, the flow loss in the sidewall region of both investigated blades are worked out and compared.

Experimental Investigation on the Influence of Particle Size in a Submerged Slurry Jet on Erosion Rates and Patterns

Sand particles entrained in fluids can cause erosive wear and damage to piping materials by impacting their surfaces which could result in failure of the piping system. Several parameters have been determined to affect the erosion behavior and mechanism of solid particle erosion. Some of these parameters include surface material, particle impact speed and angle, and particle size, shape and hardness. However, the effect of particle size on the total erosion rate and local erosion pattern has not been thoroughly investigated. It has been observed that sand particles with various sizes cause different slurry erosion patterns. Changing the particle size alters the Stokes number and consequently produces different erosion patterns and magnitudes. Thus, the effects of particle size on total erosion rate and erosion pattern in a submerged slurry jet are investigated for different impingement angles. Experiments are performed on 316 stainless steel specimens for average particles sizes of 25, 75, 150, and 300 μm. The jet angle is varied to 45, 75 and 90 degrees, and the slurry jet velocity is set to 14 m/s. The erosion pattern of the specimen is examined by obtaining the 3D microscopic profile of the eroded specimen by means of an optical profiler. It is found that the erosion profile changes as the jet angle varies. It is also observed that erosion profile is significantly different for smaller particles as compared to the larger particles. Moreover, these differences become more pronounced as the jet angle decreases. The present work discusses the differences of erosion patterns produced by both large and small particles. Computational Fluid Dynamics (CFD) is also used to study the effect of particle size on particle trajectories, impact speed, and impact angle. Also, CFD results help in explaining the differences observed in the erosion profiles caused by different particle sizes.