Hyung Hee Cho

Yonsei University ·  Department of Mechanical Engineering

This study focuses on investigating the heat transfer characteristics of boiling with liquid (subcritical) carbon dioxide impinging jet at identical enthalpy conditions, specifically examining the influence of varying operating pressures on its behavior. The experiments were conducted within a range of operating pressures from 6.37 MPa (0.86 PC) to...

Metascreens are remarkable optical materials that have attracted significant research interest owing to their ability to manipulate electromagnetic waves by selectively transmitting, reflecting, and absorbing specific wavelengths. The present study introduces a new metascreen concept called visibly transparent multifunctional metascreen (VTMM), whi...

Missile launches are not typically at the same altitude as aircraft. However, few studies have examined the infrared (IR) susceptibility of aircrafts in three-dimensional (3D) space. In this study, we propose conditions for minimizing signature contrast with the background (i.e., signature synchronization) based on 3D IR signature characteristics....

An internal structure that additively manufactured (AM) within a double-wall cooling system for gas turbine applications was proposed and evaluated for its ability to increase cooling efficiency. Polymer AM specimens were fabricated using a Biot number similarity approach with actual gas turbine materials. Three types of double wall specimens were...

The use of an inner pipe within internal passages of gas turbine vanes has been proposed as a solution to address the issue of secondary air supply to the vane hub. This study investigates the effect of the inner pipe on heat transfer of the internal passage. An experiment using the naphthalene sublimation method is performed to acquire local heat/...

Boiling heat transfer is a vital process that needs to be incorporated into next-generation cooling systems due to its high heat transfer performance. The critical heat flux (CHF), which defines boiling performance, needs to be enhanced to expand the operating limits of heat transfer applications. The crucial factor enhancing CHF is wicking perform...

Metamaterials have the powerful ability to freely control multiband electromagnetic (EM) waves through elaborately designed “artificial atoms” and are hence in the limelight in various fields. Typically, camouflage materials manipulate wave–matter interactions to achieve the desired optical properties, in particular, various techniques are used for...

The boiling performance, represented by the heat transfer coefficient (HTC) and critical heat flux (CHF), must be enhanced because the energy demand of industrial processes that generate a lot of heat increases under extreme conditions. Surface manipulations have been used to improve boiling performance by controlling interfacial characteristics. S...

For a further improvement of impingement jet array heat transfer in a corrugated channel, a wing-shaped structure design is proposed to weaken crossflow degradation. The naphthalene sublimation method was used to measure local heat/mass transfer distributions in the present study. Pressure drop characteristics were examined through pressure measure...

The blade tip is one of the most susceptible parts of gas turbine blade due to the thermally induced failure caused by a hot gas tip leakage flow through tip clearance. A highly sophisticated and efficient blade tip cooling scheme plays a crucial role in reliable operation of a gas turbine blade. In the present study, we adopted a special slot cool...

One of the leading causes of thermally induced failure in gas turbine blade tip is a tip leakage flow. To protect blade tip efficiently from strong tip leakage flow, slot cooling is adopted in blade tip considering the application of additive manufacturing. For slot cooling scheme in blade tip, slot location ranging from PS slot to SS slot is a cru...

Boiling heat transfer is a favorable method for cooling high heat flux devices, and its performance is evaluated using critical heat flux (CHF), which indicates the maximum heat dissipation capacity. CHF occurs when a surface is covered with a vapor film due to bubble coalescence. Here, we propose a new nucleation patterning surface using rGO-coate...

Local heat/mass transfer and flow characteristics data applicable to the cooling design of the wheel space with rim seal in the gas turbine engine are essential. Experimental facility was designed to obtain local heat/mass transfer coefficients on stator and rotor surface in rotor-stator cavity with hub injection flows using the naphthalene sublima...

Durable micro-nano structures are a crucial issue to protect their superior performance from external forces. Although several methods have been suggested to enhance the mechanical durability of micro-nano structures, they are not applicable for camouflage materials to obtain the wanted properties because these methods usually induce undesired reso...

The scramjet engine burns fuel under supersonic flow condition in a combustion chamber while minimizing total pressure such that it can operate under both supersonic and hypersonic flows. For combustion under supersonic flow field, a flame holder is essential to overcome short fuel residence time and poor mixing, and a backward facing step is the s...

Optical confusion refers to a camouflage technique assimilated with the surroundings through manipulating colors and patterns. With the advances in multispectral imagery detection systems, multispectral camouflage studies on simultaneous deceptions in the visible to infrared ranges remain a key challenge. Thus, creating pixelated patterns is essent...

Infrared stealth technology used in aircraft is applied to reduce the infrared signal by controlling surface temperature and emissivity using internal heat sink, low emissivity material or metamaterial. However, there is one part of the aircraft where the use of this technology is limited, and that is the radome. Especially, radome should have tran...

Energy management issues of data center cooling systems have been aggravated because of the miniaturization of electronic components. The cooling systems consume considerable energy, requiring more electric power supplied by power plants to prevent system failure from overheating. Among cooling methods applied in industrial fields, two-phase immers...

Via temperature swing adsorption, a dual circulating fluidized bed reactor can capture CO2 through a continuous process, but controlling the temperature of the reactor at a large scale is difficult due to the non-linearity hydrodynamics and heat transfer in the scaled-up two-phase flow. In the present study, an energy-efficient design of a circulat...

The efficiency of the gas turbine engine has been enhanced by improving aerodynamic performance and reducing heat load through control of secondary vortices according to the first-stage vane endwall configurations. In this study, the heat transfer features of flat and profiled endwalls were investigated under different turbulence intensities. Exper...

Light, heat, and waves in electromagnetic energy are the foundation for the advancement of human being. Camouflage materials based on metamaterials are used to excel the performance limits by manipulating the electromagnetic energy. However, multispectral camouflage materials with flexibility are difficult to fabricate because required radiative pr...

Solar energy is widely advocated by many countries which is the most popular renewable energy. To get the higher efficiency of the photovoltaic (PV) cell, enhancing cooling performance of the coolant channel is important. In this study, the effect of acoustic excitation was analyzed to enhance the cooling performance of internal channel imprinted w...

Anisotropic wicking surfaces have attracted the attention of numerous research groups in recent years. Wicking characteristics can be improved by generating unidirectional flow behavior within surfaces composed of micropillars that are bent in the direction of interest. In the present work, we created surfaces composed of bent polymerized pillar ar...

Aircraft's infrared (IR) signature modeling considering thermal-flow characteristics and emissivity control for IR camouflage have been recently studied. However, there are few studies trying to analyze the camouflage effect by applying IR camouflage material to the aircraft surface based on the surface temperature distributions or to find suitable...

Infrared guided weapons act as threats that greatly degrade the survivability of combat aircraft. Infrared weapons detect and track the target aircraft by sensing the infrared signature radiated from the aircraft fuselage. Therefore, in this study, we analyzed the infrared signature and susceptibility of supersonic aircraft according to omni-direct...

The heat transfer characteristics in 180-degree turn region of two-pass channel with three different aspect ratios were investigated as coolant being extracted through tip-holes. The naphthalene sublimation technique was used for the heat/mass transfer measurements from the internal ceiling surface of the 180-degree turn region. The channel aspect-...

This study considered effects from thermoelectric property changes due to mesoporous thin film ZnO lattice deformation through doping with various group III elements. The distorted hexagonal wurtzite structure occurred in the ZnO thin film due to ion size differences between Zn and other doping elements. These strains cause distortion, resulting in...

Recently, domestic gas turbines are primarily operated under partial loads. Hence, thermal expansion of components decreases, leading to a discontinuity between the 1st blade and 2nd vane. In this study, the local heat transfer coefficients are derived using the naphthalene sublimation method to analyze thermal characteristics based on step height,...

The tip leakage flow between rotating turbine blades and the stationary turbine casing causes aerodynamic losses and high thermal loads. Numerous novel tip geometries have been proposed for reducing both of these effects, one of them is a shelf squealer tip. The objective of this study is to investigate the aerothermal performance of three squealer...

Improving the cooling performance of internal channels is important to extend the lifespan of hot components exposed to heat in many types of machinery. This study was performed to examine the effects of placing a delta-winglet-pair (DWP) upstream of a single dimple in an internal cooling channel. The local heat/mass transfer coefficients (H/MTCs)...

The accurate information of the thermal stresses and temperature in isotropic elastic solids is the key for many engineering applications. At present the classical linear coupled theory of thermoelasticity deduced with the assumptions of small temperature changes is widely used to solve the thermoelastic problems in engineering. In this paper, to d...

Rim-seals are exposed to one of the most complex flows in modern high-efficiency gas turbines. The flow physics in the vicinity of rim-seals should be understood before predicting the hot gas ingestion that may cause a rise in disc temperature and a catastrophic failure. To investigate the seal performance of rim-seals, experiments were performed u...

This paper investigates the surface heat transfer caused by interaction of a jet and a supersonic crossflow near the jet injection hole. A sonic jet with different momentum ratios (J=0.510, 1.018, 1.477) was injected perpendicularly into a crossflow with a Mach number of 3.0 in a supersonic wind tunnel. Surface temperature through time measured by...

Impingement cooling is one of the powerful cooling methods in high-temperature devices. For gas turbine applications, impingement cooling is commonly applied in the transition piece of a combustor and in the leading edge, suction, and pressure sides of a turbine blade/vane. In the suction side and pressure side, impingement cooling is applied as a...

The present study is to investigate the effect of gap height on heat/mass transfer for impingement/effusion cooling of gas turbine hot components, using a hollow cylinder structure created by three perforated plates with two channels. The detailed local heat/mass transfer coefficients on all surfaces of the structure are measured in the test sectio...

Impingement cooling is an effective method for removing heat from a high temperature target surface using sprayed jets oriented to impinge on the surface. Detailed local heat-/mass-transfer distributions are measured using the naphthalene sublimation method. The main parameters of this study are jet hole spacing ( $s/d = 4$ , 8) and crossflow rate...

Artificial camouflage surfaces, such as metamaterials, that interact with the electromagnetic (EM) wave represent an emerging field in which performance breakthroughs are occurring. In particular, the visible and infrared (IR) waves are of interest, because these waves are desirable for energy, military and space applications. However, needs of add...

Dimples are widely known to increase heat transfer coefficients in internal passages with a minimal pressure drop. The naphthalene sublimation method was used to measure local heat transfer coefficient variations of single dimple imprint channel with the analogy between heat and mass transfers. The effect of a boundary layer trip-wire with differen...

The wakes that occur periodically owing to blade rotation, affect the HTCDs (heat transfer coefficient distributions) on the turbine blade tip and shroud. We investigated the wake effects on the HTCDs from the turbine blade tip with different configurations and the corresponding shroud. The NSM (naphthalene sublimation method) was used for measurin...

Impingement cooling is one of the powerful cooling methods in high-temperature devices. For the gas turbine applications, impingement cooling is commonly applied in the transition piece of a combustor and in the leading edge, suction and pressure sides of a turbine blade/vane. In the suction side and pressure side, impingement cooling is applied as...

Since the domestic power plants are typically operated at a partial load rather than the design point, the thermal expansion decreases, resulting in misalignment between first stage rotor and second stage stator. In this study, we investigate the heat transfer characteristics of a misalignment under purge flow at the second stage vane endwall throu...

CO2 capture technology using temperature-swing adsorption faces many obstacles for commercialization. Circulating fluidized bed systems have high capture performance, but an energy-efficient design is essential to reduce the CO2 capture cost. In this study, thermal-fluid characteristics of circulating fluidized bed with two-phase flow are analyzed,...

Each turbine rotor assembly is composed of several blades, and there is a mid-passage gap between blade platforms. Purge air is supplied through the mid-passage gap to prevent the ingress of hot combustion gas which could lead to thermal damage to the blade platforms. The leakage flow (coolant) egested through the mid-passage gap affects the heat t...

Hot combustion gas in gas turbines can be ingested into the mid-passage gap between the blade platforms, resulting potentially a catastrophic damage. To prevent the ingress of hot gas, the blade platform underneath cavity is pressurized with purge air. Seals are typically installed in the mid-passage gap to minimize the purge air requirement. Under...

This study experimentally investigated the influence of heterogeneous wettability-patterned surfaces with varying inter-spacing on flow boiling heat transfer characteristics. The test surfaces consisted of three fluorooctyltrichlorosilane hydrophobic-patterned array structures having a triangle, inverted triangle, and circular shape on a SiO2- hydr...

The appropriate coolant mass flow of turbine blade rim seal has become an important issue as turbine blades are exposed to increasingly higher thermal load owing to increased turbine inlet temperature. If the coolant is deficient, hot gas ingresses to the rim seal, or if sufficient, the efficiency of turbine decreases. Therefore, we analyzed sealin...

The aim of this study is to investigate heat transfer characteristics in new laminated plates having impingement/effusion cooling with a hollow cylinder structure. Three perforated plates are set up in parallel position to model impingement/effusion cooling system with a hollow cylinder structure. Local heat/mass transfer coefficients on all surfac...

Emerging issues in boiling heat transfer include enhancing heat transfer uniformity/stability, and critical heat flux (CHF). Microcavity structures improve heat transfer uniformity/stability by departing bubbles with arranged formation due to pinning effects and regular pitch. Ultrasonic actuation induces an acoustic field on departed bubbles, and...

A dual circulating fluidized bed reactor has been studied for carbon dioxide (CO2) capture. It is important to maintain the reaction temperature of the adsorbent in the carbonation–regeneration cycle. The dual circulating fluidized bed reactor will be used ultimately in the large reactors of industrial-sized power plants to control CO2 emissions. H...

Artificial camouflage surfaces for assimilating with environment have been utilized for controlling optical properties. Especially, the optical properties of IR camouflage materials should be satisfied with two requirements: deception of IR signature in detected band through reduced emissive energy and dissipation of reduced emissive energy for pre...

Critical heat flux (CHF) enhancement is necessary in order to ensure a high operating limit for two-phase cooling applications. As the boiling is developed, formation of vapor film layer becomes vigorous, which causes CHF. Here, a graphene-coated micropillar structure (GMS) is proposed in order to enhance boiling heat transfer by suppressing vapor...

Detailed heat transfer measurements are necessary to protect the blades under harsh and complex flow conditions. Therefore, this study investigated the heat transfer characteristics on the blade endwall under flow conditions that simulate high turbulence intensity of the main flow and the generation of wakes by the trailing edge of the vane. The en...