Hyung Hee Cho

Nanowires for Enhanced Boiling Heat Transfer

Boiling is a common mechanism for liquid-vapor phase transition and is widely exploited in power generation and refrigeration devices and systems. The efficacy of boiling heat transfer is characterized by two parameters: (a) heat transfer coefficient (HTC) or the thermal conductance; (b) the critical heat flux (CHF) limit that demarcates the transition from high HTC to very low HTC. While increasing the CHF and the HTC has significant impact on system-level energy efficiency, safety, and cost, their values for water and other heat transfer fluids have essentially remained unchanged for many decades. Here we report that the high surface tension forces offered by liquids in nanowire arrays made of Si and Cu can be exploited to increase both the CHF and the HTC by more than 100%.

Local heat/mass transfer measurements in a rectangular duct with discrete ribs

The influence of arrangement and length of discrete ribs on heat/mass transfer and friction loss is investigated. Mass transfer experiments are conducted to obtain the detailed local heat/mass transfer information on the ribbed wall. The aspect ratio (width/height) of the duct is 2.04 and the rib height is one tenth of the duct height, such that the ratio of the rib height to hydraulic diameter is 0.0743. The ratio of rib-to-rib distance to rib height is 10. The discrete ribs were made by dividing each continuous rib into two, three, or five pieces, which were attached periodically to the top and bottom walls of the duct with a parallel orientation. The combined effects of rib angle and length of the discrete ribs on heat/mass transfer are considered for the rib angles (α) of 90 and 45 deg. As the number of the discrete ribs increases, the uniformity of the heat/mass transfer distributions increases. For α=90 deg, the heat/mass transfer enhancement with the discrete ribs is remarkable, while the heat/mass transfer performances are slightly higher than that of the transverse continuous ribs due to the accompanied high friction loss penalty. For α =45 deg, the average heat/mass transfer coefficients and the heat/mass transfer performances decrease slightly with the discrete ribs compared to the case of the angled continuous ribs.

Local heat/mass transfer and flow characteristics of array impinging jets with effusion holes ejecting spent air

Abstract The present study investigates the effects of spent air flows with and without effusion holes on heat/mass transfer on a target plate for array impinging jets. For a conventional type of array impinging jets without effusion holes, the spent air of the injected jets forms a cross-flow within the confined space and affects significantly the downstream jet flow. The injection plate of array impinging jets is modified having effusion holes to prevent the cross-flow of the spent air where the spent air is discharged through the effusion holes after impingement on the target plate. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate using a heat and mass transfer analogy. The flow patterns of the array impinging jets are calculated numerically and compared for the cases without and with the effusion holes. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to the strong effects of cross-flow and re-entrainments of spent air. However, uniform distributions and enhancements of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the effect of cross-flow is weak and the distributions and levels of heat/mass transfer coefficients are similar for both cases.

Local heat/mass transfer measurement on the effusion plate in impingement/effusion cooling systems

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged in two different configurations: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the midline of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). In general, higher heat/mass transfer is obtained with smaller gap distance between two perforated plates.

Control of Superhydrophilicity/Superhydrophobicity using Silicon Nanowires via Electroless Etching Method and Fluorine Carbon Coatings

Surface roughness is promotive of increasing their hydrophilicity or hydrophobicity to the extreme according to the intrinsic wettability determined by the surface free energy characteristics of a base substrate. Top-down etched silicon nanowires are used to create superhydrophilic surfaces based on the hemiwicking phenomenon. Using fluorine carbon coatings, surfaces are converted from superhydrophilic to superhydrophobic to maintain the Cassie-Baxter state stability by reducing the surface free energy to a quarter compared with intrinsic silicon. We present the robust criteria by controlling the height of the nanoscale structures as a design parameter and design guidelines for superhydrophilic and superhydrophobic conditions. The morphology of the silicon nanowires is used to demonstrate their critical height exceeds several hundred nanometers for superhydrophilicity, and surpasses a micrometer for superhydrophobicity. Especially, SiNWs fabricated with a height of more than a micrometer provide an effective means of maintaining superhydrophilic (<10°) long-term stability.

Effects of acoustic excitation positions on heat transfer and flow in axisymmetric impinging jet: main jet excitation and shear layer excitation

An experimental study is conducted to investigate the flow and heat transfer characteristics of an impinging jet with acoustic excitations. Two different acoustic excitation methods based on the locations of the actuator are tested and compared: one is a main jet excitation and the other is a shear layer excitation. Effects of excitation level on the heat transfer and flow characteristics are also investigated. Local Nusselt number distributions are measured on the impingement surface and velocity and turbulence intensity distributions are also measured. The forcing Strouhal numbers (excitation frequency, St) are 1.2, 2.4, 3.0 and 4.0 and the excitation level varies from 80 to 100 dB. When the vortex pairing is promoted by the excitation St of 1.2 with the main jet excitation, low heat transfer rates are obtained at the large nozzle-to-plate distances. For the excitation St of 2.4 with the main jet excitation, high heat transfer rates are obtained at the large gap distances due to the extended potential core length. The main jet excitation method shows similar heat and flow characteristics to the shear layer excitation although the basic flow schemes and the location of the forcing actuator are different. The effects of the acoustic excitation to the jet increase as the excitation level increases. Therefore, the excitation frequency and the excitation level are both important factors in the acoustic excitation.

Heat transfer and flow structures in axisymmetric impinging jet controlled by vortex pairing

Abstract An experimental study is conducted to investigate the flow and heat transfer characteristics of an impinging jet controlled by vortex pairing. Two kinds of vortex control methods of secondary shear flow and acoustic excitation are applied. Local Nusselt numbers are measured on the impingement surface. Flow visualization, measurements of velocity and turbulence intensity and FFT analysis are used to understand the flow structures. The velocity ratio is changed from 0.45 to 1.75 for the shear flow control and the tested Strouhal number (excitation frequency, St D ) is 1.2, 2.4, 3.0 and 4.0 for the acoustic excitation. Enhancement or reduction in heat transfer is obtained by the control of vortex pairing due to the change of flow structures. When the vortex pairing is promoted by the secondary counter-flowing (suction flow) and St D =1.2, low heat transfer rates are obtained at large nozzle-to-plate distances. Conversely, the jet flow has an extended potential core length with the secondary co-flowing (blowing flow) and St D =2.4 and 3.0. Thus high heat transfer rates are obtained at large gap distances.

Heat (Mass) Transfer and Film Cooling Effectiveness With Injection Through Discrete Holes: Part I—Within Holes and on the Back Surface

A jet stream entering a crossflow is investigated for injection through a single hole and an array of holes for blowing rates of 0.2 to 2.2. The naphthalene sublimation technique has been employed to study the local mass (heat) transfer in the injection hole and in the vicinity of the hole entrance. The Sherwood number is fairly uniform along the circumference of the inside hole surface even at the low blowing rate considered. This is quite different from the case without injection (zero blowing rate), when the Sherwood number is highly nonuniform. The transfer rate in the hole is weakly influenced by the crossflow and the zone, which is directly affected, is confined close to the hole exit (about 0.15 hole diameter in depth). The average Sherwood number is similar to that in the absence of crossflow except at low blowing rates. The Sherwood numbers on the hole entrance surface (backside) are the same as when there is no crossflow. Thus, the Sherwood numbers inside the hole and on the back surface can be closely approximated from experiments without crossflow.

Interfacial wicking dynamics and its impact on critical heat flux of boiling heat transfer

Morphologically driven dynamic wickability is essential for determining the hydrodynamic status of solid-liquid interface. We demonstrate that the dynamic wicking can play an integral role in supplying and propagating liquid through the interface, and govern the critical heat flux (CHF) against surface dry-out during boiling heat transfer. For the quantitative control of wicking, we manipulate the characteristic lengths of hexagonally arranged nanopillars within sub-micron range through nanosphere lithography combined with top-down metal-assisted chemical etching. Strong hemi-wicking over the manipulated interface (i.e., wicking coefficients) of 1.28 mm/s0.5 leads to 164% improvement of CHF compared to no wicking. As a theoretical guideline, our wickability-CHF model can make a perfect agreement with improved CHF, which cannot be predicted by the classic models pertaining to just wettability and roughness effects, independently.

Local Heat/Mass Transfer Measurement on the Effusion Plate in Impingment/Effusion Cooling System

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged in two different configurations: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5,000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the mid-line of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). In general, higher heat/mass transfer is obtained with smaller gap distance between two perforated plates.Copyright