Joon Sik Lee

Direct numerical simulation of turbulent thermal boundary layers

In this paper, a method of generating realistic turbulent temperature fluctuations at a computational inlet is proposed and direct numerical simulations of turbulent thermal boundary layers developing on a flat plate with isothermal and isoflux wall boundary conditions are carried out. Governing equations are integrated using a fully implicit fractional-step method with 352×64×128 grids for the Reynolds number of 300, based on the free-stream velocity and the inlet momentum thickness, and the Prandtl number of 0.71. The computed Stanton numbers for the isothermal and isoflux walls are in good agreement with power-law relations without transient region from the inlet. The mean statistical quantities including root-mean-square temperature fluctuations, turbulent heat fluxes, turbulent Prandtl number, and skewness and flatness of temperature fluctuations agree well with existing experimental and numerical data. A quadrant analysis is performed to investigate the coherence between the velocity and temperature...

Measurements of impinging jet flow and heat transfer on a semi-circular concave surface

Abstract An experimental study of fluid flow and heat transfer has been carried out for jet impingement cooling on a semi-circular concave surface. The distributions of mean velocity and velocity fluctuation on the concave surface have been measured in free, impinging and wall jet flow regions by using a Laser Doppler Anemometer. Local Nusselt numbers have also been measured. Variations of jet Reynolds numbers, the spacing between the nozzle and the target and the distance from the stagnation point in the circumferential direction have been considered. Emphasis has been placed on measuring turbulent jet flow characteristics including impinging and evolving wall jets and interpreting heat transfer data, particularly, the occurrence and its location of secondary peak in connection with data of measured mean velocity and velocity fluctuations on the concave surface.

An experimental study of slot jet impingement cooling on concave surface : effects of nozzle configuration and curvature

Abstract An experimental study has been carried out for jet impingement cooling on a semi-circular concave surface when jet flows were ejected from three different slot nozzles—round shaped nozzle, rectangular shaped nozzle and 2D contoured nozzle. Experiments have been conducted with variations of nozzle exit Reynolds number (Re2B) in the range of 5920 ⩽ Re2B ⩽ 25500 and nozzle-to-surface distance (zn) in the range of 1/2 ⩽ zn/B ⩽ 20 to determine the heat transfer coefficients under a constant heat flux condition. The developing structures of free jets were measured by hot-wire anemometer to understand the characteristics of heat transfer in conjunction with measured jet flows. Markedly different flow and heat transfer characteristics have been observed depending on different nozzle shapes. Average heat transfer rates for impingement on the concave surface are found to be more enhanced than the flat plate results due to the effect of curvature. Comparisons between the present results and the existing experimental results have also been made.

Charging and Collection of Submicron Particles in Two-Stage Parallel-Plate Electrostatic Precipitators

ABSTRACT A well-defined two-stage ESP experiment was carried out in the submicron particle size range. The experimental setup consisted of five components: a clean wind tunnel, a submicron particle generation system, an aerosol sampling and transport system, a submicron particle number concentration measurement system, and a pilot-scale two-stage parallel-plate ESP. Experimental collection efficiency data were obtained with a GMD of 0.03–0.2 μm and a GSD of about 1.67 for air velocities between 1.9 and 4.1 m/s under the nominal operation condition of two-stage ESP. The experimental data were then compared with the results of a numerical collection efficiency model which takes into consideration charging rate equations, particle equations of motion, and collection performance models. The comparison showed good agreement. It was confirmed from the comparison that the partial charging regime, where a portion of incoming particles is not charged, exists when the particle size range is below about 0.03 μm This...

Quantitative Measurement with Scanning Thermal Microscope by Preventing the Distortion Due to the Heat Transfer through the Air

Because of its high spatial resolution, scanning thermal microscopy (SThM) has been developed quite actively and applied in such diverse areas as microelectronics, optoelectronics, polymers, and carbon nanotubes for more than a decade since the 1990s. However, despite its long history and diverse areas of application, surprisingly, no quantitative profiling method has been established yet. This is mostly due to the nonlocal nature of measurement by conventional SThM: the signal measured by SThM is induced not only from the local heat flux through the tip-sample thermal contact but also (and mostly) from the heat flux through the air gap between the sample and the SThM probe. In this study, a rigorous but simple and practical theory for quantitative SThM for local measurement is established and verified experimentally using high-performance SThM probes. The development of quantitative SThM will make possible new breakthroughs in diverse fields of nanothermal science and engineering.

Effects of Orientation Angles on Film Cooling Over a Flat Plate: Boundary Layer Temperature Distributions and Adiabatic Film Cooling Effectiveness

Presented are experimental results describing the effects of orientation angle of film cooling holes on boundary layer temperature distributions and film cooling effectiveness. Film flow data were obtained from a row of five film cooling holes on a flat test plate. The inclination angle of the hole was fixed at 35 deg and four orientation angles of 0, 30, 60, and 90 deg were investigated. The velocity ratios surveyed were 0.5, 1.0, and 2.0. The boundary layer temperature distributions were measured at three downstream locations using 1 mm platinum wire. Detailed adiabatic film cooling effectiveness distributions were measured using thermochromic liquid crystal. Results show that the increased lateral momentum in the case of large orientation angle injection strongly affects boundary layer temperature distributions. Temperature distribution characteristics are, in general, explained in the context of the interactions between injectant and free-stream fluid and between injectants issuing from adjacent holes. The adiabatic film cooling effectiveness distributions are discussed in connection with the boundary layer temperature distributions. Spanwise-averaged effectiveness distributions and space-averaged effectiveness distributions are also presented with respect to the velocity ratios and the orientation angles.

Experimental Study on the Flow Characteristics of Streamwise Inclined Jets in Crossflow on Flat Plate

Experimental study has been conducted to investigate the flow characteristics of streamwise 35 deg inclined jets, injected into a turbulent crossflow boundary layer on a flat plate. Flow is visualized by schlieren photographs for both normal and inclined jets to determine the overall flow structure with the variation of the velocity ratio. A three-dimensional velocity field is measured for two velocity ratios of 1.0 and 2.0 by using a five-hole directional probe. The visualization study shows that the variation of the injection angle produces a significant change in the flow structure. It is recognized that the jet flow is mainly dominated by the turbulence for a small velocity ratio, but it is likely to be influenced by an inviscid vorticity dynamics for a large velocity ratio

How to Reliably Determine the Complex Refractive Index (RI) of Graphene by Using Two Independent Measurement Constraints

Reliable determination of the complex refractive index (RI) of graphene inherently requires two independent measurement realizations for two independent unknowns of the real (nG) and imaginary (kG) components, i.e., RI = nG + i kG. Thus, any single set of measurement realization provides only one constraint that is insufficient to uniquely determine the complex RI of graphene. Tandem uses of two independent measurement techniques, namely the surface plasmon resonance (SPR) angle detection and the attenuated total reflection (ATR) intensity measurement, allow for the unique determination of the complex RI of CVD-synthesized graphene. The presently measured graphene RI is determined to be 2.65 + 1.27i for the E-field oscillating parallel to graphene at 634 nm wavelength, with variations for different numbers of L (1, 3 and 5) remaining within ±3%. Thus, our demonstration results for the specified wavelength serve as an impetus to suggest the need for two independent measurement techniques in determining both the real and imaginary RI values for graphene. Additional efforts have been made to characterize graphene layers using the density function theory (DFT): this calculation provides RIG = 2.71 + 1.41i.

Quantitative scanning thermal microscopy using double scan technique

Although scanning thermal microscope has shown the highest spatial resolution in local temperature and thermophysical property measurement, its usefulness has been severely limited due to difficulties in quantitative measurement. We propose a double scan technique that measures temperature only from the heat transfer through the tip-sample contact by the subtraction of the signal due to the heat transfer through the air. A rigorous theoretical model for this technique is derived. The effectiveness of the double scan technique in quantitative temperature measurement is demonstrated experimentally.

A microfluidic chaotic mixer using ferrofluid

A novel micromixer which utilizes chaotic mixing induced by ferrofluid actuation is developed. The micromixer is fabricated by a polydimethylsiloxane micromolding technique. The micromixer consists of a T-shaped main mixing channel and two parallel sub-channels that intersect the main channel. Oscillation of a couple of ferrofluid slugs in the sub-channels, induced by external permanent magnet actuation, generates chaotic advection in the main channel flow. To visualize the mixing, red fluorescent particles are supplied to one of the inlets of a T-shaped channel, and mixing flow is observed by a fluorescence microscope and an intensified CCD camera. The mixing experiments are performed with various ferrofluid perturbation frequencies and main stream flow velocities. The optimal mixing conditions determining the upper and lower limits of the most effective Strouhal numbers are found from the experimental results.