Wontae Hwang’s research while affiliated with Seoul National University and other places

The front opening unified pod (FOUP) is a carrier that transports multiple wafers as it moves between numerous processing facilities. It is inevitably exposed to air humidity coming from the equipment front end module (EFEM), which leads to the formation of harmful residual particles on the wafer surfaces due to the reaction of moisture with airborne molecular contamination (AMC). This can cause serious defects, and thus there is a need to understand the complex flow structure inside the EFEM and FOUP. Magnetic resonance velocimetry (MRV) is hereby employed to qualitatively and quantitatively measure the 3D flow when conventional load port purge (LPP) is utilized to protect the wafers. The front LPP forms a barrier between the FOUP and EFEM, blocking the EFEM flow from entering the FOUP. Additionally, at the rear of the FOUP, flow from the rear and front LPP collide and then travels between the wafers toward the FOUP entrance, thereby protecting the wafers. Using computational fluid dynamic (CFD) simulations, various combinations of flow rates from different purge ports were simulated, leading to an optimal flow condition. These findings suggest that independent control of the flow rates can be a practical way to protect the wafers from defects.

An electrostatic precipitator (ESP) is an air pollution reduction facility, which has a particle separation efficiency that varies with flow uniformity. Previous experimental research on ESPs has typically utilized simple models with uniform inlet flow and single-point measurement invasive instruments that affect the flow. Most of these studies were not able to properly analyze the complex three-dimensional internal flow structure. Prior numerical research typically modeled the perforated plates within the diffuser as porous media, which yielded inaccuracies in the flow structure. To address these limitations, this study employed a noninvasive flow measurement technique—magnetic resonance velocimetry (MRV)—to experimentally analyze the three-dimensional flow structure leading into and inside the diffuser of a realistic ESP model. The experimental results revealed non-uniform flow within the inlet ducts due to the wake of a guide vane in the upstream bifurcation region. As the flow passed through curved ducts, the non-uniform flow distribution was exacerbated before reaching the diffuser inlet and eventually led to the formation of a large recirculation zone within the diffuser. Inside the diffuser, mixing between the individual jets exiting the perforated plates and the recirculation zones occurred in a complicated manner. The non-uniform flow at the diffuser exit negatively affects the flow distribution entering the collection plates. Overall, this study highlights the significance of the upstream flow uniformity when designing ESPs. Graphical abstract

The nasal cavity has the function of conditioning the air inhaled into the lungs by heating, humidifying, and filtering dust and virus-borne aerosols. Analyzing the flow field in the nasal cavity is vital because its function is strongly related to flow dynamics. Due to experimental limitations posed by the complex internal geometry of the nasal cavity, most previous studies have utilized Reynolds averaged Navier–Stokes based computational fluid dynamics (CFD) simulations. In this study, the flow field in a post-operative nasal cavity was evaluated using not only CFD simulations but also four-dimensional magnetic resonance velocimetry. The study was conducted under resting breathing conditions in pre- and post-operative models of a patient who received septoplasty and turbinoplasty. The experimental results confirmed balanced flow rates in the left and right nasal cavities after septoplasty and a decrease in velocity after turbinoplasty with a reduction in regions with vortices and reverse flow. Upon comparison, CFD results using the laminar, k–ω, and shear stress transport models were deemed to be consistent with the experimental results. However, there was a relatively large deviation observed with the k–ε model. Using the validated laminar CFD model, it was shown that the pressure and wall shear stress decreased after surgery.

Turbine blade tip leakage flows occur between the unshrouded rotor and stationary casing. Such flow is one of the main sources of aerodynamic loss in a turbine. Many different methods have been proposed to reduce this loss, including the squealer tip and winglet design. In this study, numerical analysis showed that a large fraction of the leakage flow occurs after 50% of the axial chord. Thus, an additional rim was installed at the mid-point of the cavity, in an attempt to block the leakage flow to the suction side and redirect it back to the pressure side. The effect of an extended pressure-side winglet was also investigated and compared with the base squealer tip. The passage velocity field between the blades of a linear cascade was measured at 0, 25, 50, 75, and 100% of the axial chord using a 5-hole probe to assess the development of flow structures responsible for the tip leakage loss. The total pressure loss coefficient distribution was measured downstream of the cascade. Performance results from the different tip geometries were experimentally compared with each other, and also used to validate the numerical results. The winglet design showed the best performance. This design does not have a pressure-side rim, and thus the tip leakage flow has less hindrance passing over the suction-side rim, which creates a strong coherent tip leakage vortex compared to that of the squealer tip. This leads to an increase in loss beyond 90% of the span, but the counter-rotating tipwall passage vortex underneath interacts with the tip leakage vortex to reduce the loss in the 70–80% span region by a greater extent. Therefore, the pressure-side winglet design has an overall loss that is 28.3% smaller than that of the baseline squealer tip. The additional cavity rim did not show any noticeable improvements, possibly due to the angle at which it was placed, and thus needs further investigation.

Magnetic resonance velocimetry (MRV) provides capabilities to measure three velocity components across three-dimensional fields of view without the need for optical accessibility. Predominant usage of the technique in engineering applications centers on steady flows and the discussion of a single time-averaged set of data. In this second MRV Challenge comparison, phase-locked MRV measurements of a pulsatile flow are conducted by five teams from across the globe. A geometry is explored which consists of turbulent flow past six identical cubic elements placed in the center of a square water channel, with partial elements along each side of periodically varying heights. A pulsatile injection flow through the floor between the second and third cubic roughness elements creates a three-dimensional jet that interacts with the complex cube wakes. The details of each scan sequence are summarized for the teams. Results are compared for three different time windows—one pre-injection, a second while the injection flow accelerates, and a third during a quasi-steady condition with the injector fully on. Finally, the influence of the temporal resolution selection for the phase locked MRV is discussed. The results are remarkably similar despite the complex flow configuration selection and showcase a relatively underutilized capability to obtain time-dependent, volumetric data for periodic flows through three-dimensional geometries using MRI. Recommendations for best practices are also provided.

Proper internal cooling of the thin trailing edge of gas turbine blades is crucial for blade lifespan extension. In this study, numerical and experimental analyses are conducted to elucidate the relationship between the 3D flow structure and surface heat transfer in a realistic three-pass serpentine channel within the trailing edge of a blade. Magnetic resonance velocimetry is conducted to first experimentally validate the mean velocity field from the Reynolds-averaged Navier Stokes simulation. Heat transfer and friction coefficients are then obtained through numerical analysis. The serpentine channel is investigated at each section. The inlet S-duct contributes to heat-transfer promotion upstream of the first ribbed passage due to an intense vortex pair. This vortex pair generates high streamwise velocity along the pressure and suction side, as well as additional friction due to secondary flow. In the second passage, film cooling holes make the separation bubble attach only to the suction side and create a high streamwise velocity region around the holes by bringing the surrounding flow to the periphery of the holes. This effect also occurs near the ejection holes in the third passage. These flow features within the passages are analyzed using the momentum distortion parameter (D) and secondary flow energy parameter (E). We found that the distortion parameter is not sufficient in predicting the heat transfer trend due to in-plane wall shear. Thus, the estimator is revised as (D+E)/2 to additionally account for secondary flow. This composite parameter shows good performance in estimating heat transfer for all the passages, compared to the friction coefficient which is hard to directly measure.