Minking Chyu - Profile on Academia.edu (original) (raw)

Papers by Minking Chyu

TechConnect Briefs, Jun 21, 2010

In this paper we report our new approach to synthesize cation-exchanged Laponite nanohybrids for ... more In this paper we report our new approach to synthesize cation-exchanged Laponite nanohybrids for use as gel-forming, viscosity and weighting material stabilizing additives for drilling fluid application. To synthesize the nanohybrids, we used a laser beam and ablated micron sized metal particles (Cu, Co, or Al) in an aqueous suspension containing 1 wt% laponite. As a result, the suspensions underwent a transition to a stable gel very quickly. Although the weight percent of the nanohybrids was negligibly small compared to that of Laponite, aqueous suspensions of these cation-exchanged Laponite crystals were highly viscous with excellent shear thinning and thixotropic behavior. The fluid gelled quickly when it was at rest nevertheless its structure was broken easily and it transformed into a low viscosity fluid quickly on shearing.

Volume 3: Heat Transfer; Electric Power; Industrial and Cogeneration, Jun 7, 1999

This study investigates the effect of gap atop an inline array of cubic fins on the heat transfer... more This study investigates the effect of gap atop an inline array of cubic fins on the heat transfer from various participating surfaces in the channel housing the array. Five different gap sizes of C/H = 0.25, 0.5, 1.0 and 2.0, as well as the baseline case without gap (C/H = 0.0) are examined and compared. The array consists of twelve rows of three columns. All tests use a unified Reynolds number at 16,000 based on the mean velocity in the channel and cube height. The heat transfer measurement uses a liquid crystal imaging technique combined with a onedimensional, transient conduction model and a lumped heatcapacity model. The results reveal that the heat transfer from the surfaces uncovered by the cubic fins in the test channel generally decreases with the size of the clearance atop the array. However, such a decreasing trend is insignificant for cases with relatively smaller gaps, i.e. C/H = 0.25 and 0.5. Under these conditions, the heat transfer from the surface of cubic fin, in fact, is higher than that of the baseline case. The vortex enriched shear layer separated from the sharp edges around the cube top is considered to be responsible for this phenomenon. A heat transfer area C gap clearance Cp constant pressure specific heat H height or width of cubic fin h local heat transfer coefficient = q/(T-T) k thermal conductivity IT man q local heat transfer rate per unit area Nu local Nusselt number = hH/k Re Reynolds number = UH/v T temperature t time U mainstream mean velocity x streamwise coordinate z spanwise coordinate Greek Symbols a thermal diffusivity T time parameter p fluid density Subscripts i initial m mainstream, mixed-mean p pin surface s smooth surface r reference t total wetted surface u uncovered endwall w wall

Comparison of Heat Transfer From Staggered Pin Fin Arrays With Circular, Cubic and Diamond Shaped Elements

This study investigates the effects of pin shape of staggered arrays on heat transfer enhancement... more This study investigates the effects of pin shape of staggered arrays on heat transfer enhancement. Three different pins: circular, cubic, and diamond, are studied. The arrays consists of twelve rows of five columns with geometry configuration of ST/D = 2.5 and SL/D = 2.5, H/D = 1. Tests were conducted at Reynolds number between 12,000 and 19,000. The heat transfer measurement uses a liquid crystal imaging technique combined with a one-dimensional, transient conduction model and a lumped heat-capacitance model. The results reveal that the heat transfer from the cubic pin arrays and diamond pin arrays is higher than that from the circular pin arrays at the same Reynolds number. However, the circular pin arrays provide the lowest pressure loss among the three arrays. Considering the trade-offs between heat transfer and pumping power, the circular pin arrays may still be the better choice as a heat exchanger.

Convective Heat Transfer from Finite Cylinders Mounted on a Plane Wall

High-performance compact heat sinks have been developed for the effective cooling of high-density... more High-performance compact heat sinks have been developed for the effective cooling of high-density LSI packaging. Heat transfer and pressure loss characteristics of the heat sinks in both air-cross-flow and air-jet cooling have been experimentally studied. The present heat sinks were of plate-fin and pin-fin arrays with a fin pitch of 0.7 mm. The plate-fin heat sinks had higher cooling performance than the pin-fin heat sinks in the range of large airflow rates both in air-cross-flow and air-jet cooling. The thermal conductance in cross-flow cooling was 20 or 40% larger than that in jet cooling. The correlation of Colburn j-factor/Fanning friction factor versus the Reynolds number for the present heat sinks was found to be very close to that of a conventional large-size heat exchanger.

Journal of materials science and nanotechnology, Aug 1, 2013

We have synthesized Au/CuO and Au/ZnO nanocomposites using the laser soldering technique. The pro... more We have synthesized Au/CuO and Au/ZnO nanocomposites using the laser soldering technique. The process was carried out by irradiating a solution containing Au-CuO and Au-ZnO nanoparticles using 532 nm laser pulses of 0.1 J/cm 2 continuously for 20 minutes. The beam was focused using a 75 mm focal lens and the laser power near the focal region was estimated to be about 2.4 x 10 12 W/m 2 . Their UV-VIS absorption and transmission were observed and the results indicated that the bandgap energies of the Au/CuO and Au/ZnO are significantly lower than those of pure CuO and ZnO. A theoretical model was developed and the calculation showed that the soldering process was due to the laser melting of the gold nanoparticles and the molten gold got soldered to the ZnO as well as CuO nanoparticles nearby.

Effects of Pin Detached Space on Heat Transfer and From Pin Fin Arrays

ABSTRACT Heat transfer and pressure characteristics in a rectangular channel with pin-fin arrays ... more ABSTRACT Heat transfer and pressure characteristics in a rectangular channel with pin-fin arrays of partial detachment from one of the endwalls have been experimentally studied. The overall channel geometry (W = 101.6 mm, E = 25.4 mm) simulates an internal cooling passage of wide aspect ratio (4:1) in a gas turbine airfoil. With a given pin diameter, D = 6.35 mm = 1/4 E, three different pin-fin height-to-diameter ratios, H/D = 4, 3, and 2, were examined. Each of these three cases corresponds to a specific pin array geometry of detachment spacing (C) between the pin-tip and one of the endwalls, i.e. C/D = 0, 1, 2, respectively. The Reynolds number, based on the hydraulic diameter of the un-obstructed cross-section and the mean bulk velocity, ranges from 10,000 to 25,000. The experiment employs a hybrid technique based on transient liquid crystal imaging to obtain distributions of the local heat transfer coefficient over all of the participating surfaces, including the endwalls and all the pin elements. Experimental results reveal that the presence of a detached space between the pin-tip and the endwall have a significant effect on the convective heat transfer and pressure loss in the channel. The presence of pin-to-endwall spacing promotes wall-flow interaction, generates additional separated shear layers, and augments turbulent transport. In general, an increase in detached spacing, or C/D leads to lower heat transfer enhancement and pressure drop. However, C/D = 1, i.e. H/D = 3, of a staggered array configuration exhibits the highest heat transfer enhancement, followed by the cases of C/D = 0 and C/D = 2, i.e. H/D = 4 or 2, respectively.

Effects of Perpendicular Flow Entry on Convective Heat/Mass Transfer From Pin-Fin Arrays

Journal of heat transfer, Aug 1, 1999

Convective heat transfer with pin-fin arrays have been studied extensively in laboratory experime... more Convective heat transfer with pin-fin arrays have been studied extensively in laboratory experiments where flow is introduced to the array uniformly over the channel span. However, the flow path in actual cooling designs is often serpentine-shaped with multiple turns, and the pin-fin array section is often located immediately downstream of a turn. The present study, using an analogous mass transfer technique based on naphthalene sublimation, investigates the effects of three different, nonaxial flow entries on array heat transfer for both an inline and a staggered arrangement of pins. The measurement acquires the mass transfer rate of each individual pin in a five row by seven column array for the Reynolds number varying from 8000 to 25,000. The mass transfer and associated flow visualization results indicate that the highly nonuniform flow distribution established at the array entrance and persisting through the entire array can have significant effects on the array heat transfer characteristics. Compared to the conventional case with axial-through flow entrance, the overall array heat transfer performance can be either enhanced or degraded, depending on the actual inlet arrangements and array configurations.

Heat Transfer on Convective Surfaces With Pin-Fins Mounted in Inclined Angles

Casting of pin fins at the trailing edge of the turbine blades often presents some difficulties d... more Casting of pin fins at the trailing edge of the turbine blades often presents some difficulties due to tight dimensional tolerances, leaving the pin fins inclined after the casting process. This study is to experimentally examine the effects of such an imperfect manufacturing phenomena on the heat transfer and friction characteristics over pin-fin arrays with different pin inclinations. The test model is a staggered short (H/D = 1) pin-fin array with an inter-pin spacing of 2.5 times the pin-diameter (S/D = 2.5) in both longitudinal and transverse directions. Detailed local heat transfer coefficients on both array endwalls and pin elements are determined using the transient liquid crystal technique, as the inclined angle θ varies from 40° to 90° and the Reynolds number ranges from 7.0 × 103 and 1.3 × 104. The measured data suggest that an increase in pin inclined angle relative to its normal orientation (90-degree) significantly reduces the level of heat transfer enhancement from the array. Such a reduction amounts to nearly 50% for the 40-degree case. The accompanied friction loss also decreases.

Volume 3: Heat Transfer; Electric Power; Industrial and Cogeneration, Jun 7, 1999

Computations were performed to investigate the three-dimensional flow and heat transfer in a high... more Computations were performed to investigate the three-dimensional flow and heat transfer in a high aspect ratio channel in which one or two wall are lined with four rows of hemispherical cavities arranged in a staggered fashion with two Reynolds numbers (23,000 and 46,000). The focus is on understanding the flow induced by cavities and how that flow affects surface heat transfer. Computed results were compared with available experimental data. This computational study is based on the ensemble-averaged conservation equations of mass, momentum (compressible Navier- Stokes), and energy closed by the low Reynolds number shear-stress transport k-oi turbulence model (wall functions were not used). Solutions were generated by a cell-centered finite-volume method that uses third-order accurate flux-difference splitting of Roe with limiters, multigrid acceleration of a diagonalized ADI scheme with local time stepping, and patched/overlapped structured grids. NOMENCLATURE D diameter of sphere used to generate cavity (Fig. ) depth of concavity (Fig. ) H height of channel (Fig. ) heat transfer coefficient (q../(T; -T w)) kinetic energy static pressure qw wall heat transfer rate per unit area Re Reynolds number (pilliFUp) • static temperature LI; average velocity at channel inlet x, y, z streamwise, spanwise, and normal coordinates Greek • dynamic viscosity • density to dissipation rate per unit k Subscripts and Superscripts conditions at channel inlet conditions on channel wall

Heat Transfer Contributions of Pins and Endwall in Pin-Fin Arrays: Effects of Thermal Boundary Condition Modeling

Journal of turbomachinery, Apr 1, 1999

Short pin-fin arrays are often used for cooling turbine airfoils, particularly near the trailing ... more Short pin-fin arrays are often used for cooling turbine airfoils, particularly near the trailing edge. An accurate heat transfer estimation from a pin-fin array should account for the total heat transfer over the entire wetted surface, which includes the pin surfaces and uncovered endwalls. One design question frequently raised is the actual magnitudes of heat transfer coefficients on both pins and endwalls. Results from earlier studies have led to different and often contradicting conclusions. This variation, in part, is caused by imperfect or unrealistic thermal boundary conditions prescribed in the individual test models. Either pins or endwalls, but generally not both, were heated in those previous studies. Using a mass transfer analogy based on the naphthalene sublimation technique, the present experiment is capable of revealing the individual heat transfer contributions from pins and endwalls with the entire wetted surface thermally active. The particular pin-fin geometry investigated, S/D = X/D = 2.5 and H/D = 1.0, is considered to be one of the optimal array arrangement for turbine airfoil cooling. Both inline and staggered arrays with the identical geometric parameters are studied for 5000 ≤ Re ≤ 25,000. The present results reveal that the general trends of the row-resolved heat transfer coefficients on either pins or endwalls are somewhat insensitive to the nature of thermal boundary conditions prescribed on the test surface. However, the actual magnitudes of heat transfer coefficients can be substantially different, due to variations in the flow bulk temperature. The present study also concludes that the pins have consistently 10 to 20 percent higher heat transfer coefficient than the endwalls. However, such a difference in heat transfer coefficient imposes very insignificant influence on the overall array-averaged heat transfer, since the wetted area of the uncovered endwalls is nearly four times greater than that of the pins.

The present study explores the heat transfer enhancement induced by arrays of cubic fins. The fin... more The present study explores the heat transfer enhancement induced by arrays of cubic fins. The fin element is either a cube or a diamond in shape. The array configurations studied include both inline and staggered arrays of seven rows and five columns. Both cubic arrays have the same geometric parameters' , i.e., 11/0=1, S/D=)UD=2.5, which are similar to those of earlier studies on circular pin-fm arrays. The present results indicate that the cube element in either array always yields the highest heat transfer, followed by diamond and circular pin-fm. Arrays with diamondshaped elements generally cause the greatest pressure loss than those with either cubes or pin fins. For a given element shape, a staggered array generally produces higher heat transfer enhancement and pressure loss than the corresponding inline array. Cubic Arrays can be viable alternatives for pedestal cooling near a blade trailing edge.

Heat Transfer in the Tip Region of Grooved Turbine Blades

Journal of turbomachinery, Apr 1, 1989

Local convective heat transfer at the tip region of grooved blades is experimentally investigated... more Local convective heat transfer at the tip region of grooved blades is experimentally investigated. The present study models the problem by flow over a shrouded, rectangular cavity, with the shroud moving opposite to the main flow direction. The naphthalene sublimation technique together with a computer-controlled measurement system provides detailed local transfer information on all the participating surfaces. The local heat transfer coefficient in the cavity is strongly influenced by the cavity aspect ratio, gap size, and leakage flow Reynolds number. Within the present study range, the effect of relative motion between the shroud and cavity on the heat transfer is found to be minor, particularly for the average heat transfer coefficient. With the same leakage flow rate, the average heat transfer coefficient over the entire tip area decreases with an increase in cavity depth. However, in terms of total heat transfer to the tip, an overly deep cavity is undesirable, because it provides larger surface area but only a small increase in flow resistance.

Influence of Topology on Heat Transfer in a Double Wall Cooling Channel: Potential of Series-Linked Jets

The use of lean burning flames stabilized by highly swirling flows represents the most effective ... more The use of lean burning flames stabilized by highly swirling flows represents the most effective technology to limit NOx emissions in modern aeroengine combustors. In these devices up to 70% of compressed air is admitted in the combustor through the injection system, which is usually designed to give strong swirling components to air flow. Complex fluidynamics is observed with large flow recirculations due to vortex breakdown and precessing vortex core, that may result in a not trivial interaction with liner cooling flows close to combustor walls. This interaction and its effects on the local cooling performance make the design of the cooling systems very challenging and time-consuming, considering design and commission of new test rigs for detailed analysis. Keeping in mind costs and complexities related to the investigation of swirl flow/wall cooling interaction by experimental approach, CFD can be considered an accurate and reliable alternative to understand the associated phenomena. The widely known overcomes of RANS formulation (e.g. underestimation of mixing and inability to properly describe swirling flows) and the more and more impressive increase in computational resources, pushed hybrid RANS-LES models as valuable and affordable approaches to accurately solve the main turbulent flow structures. This work describes the main findings of a CFD analysis intended to accurately investigate the flow field and wall heat transfer as a result of the mutual interaction between a highly swirling flow generated by a lean burn nozzle and a slot-effusion liner cooling system. In order to overcome some limitations of RANS approach, the simulations were performed with SST-SAS, a hybrid RANS-LES model. Moreover, the significant computational effort due to the presence of more than 600 effusion holes was limited exploiting two different modelling strategies: a homogeneous model based on the application of uniform boundary conditions on both aspiration and injection sides, and another solution that provides a coolant injection through point mass sources within a single cell. CFD findings were compared to experimental results coming from an investigation carried out on a three sector linear rig. The comparison pointed out that advanced modelling strategies, i.e. based on discrete mass sources, are able to reproduce the effects of mainstream-coolant interactions on convective heat loads. Validated the approach through a benchmark against time-averaged quantities, the transient data acquired were examined in order to better understand the unsteady behaviour of the thermal load through a statistical analysis, providing useful information with a design perspective.

Energies, Jun 27, 2022

To better understand the mechanism influencing the periodic lattice structures in gas turbine bla... more To better understand the mechanism influencing the periodic lattice structures in gas turbine blade cooling, these numerical simulations present a systematic comparison of the effects in cases involving pin-fin, Kagome, and BCC lattice arrays on film-cooling effectiveness under three blowing ratios (i.e., M = 0.5, 1.0, and 1.5). The results indicate that the introduction of lattice array structures improves film-cooling effectiveness within the whole streamwise range, especially downstream of the film hole. With an increase in the blowing ratio, the superiority of lattice array structures relative to those without a lattice becomes increasingly evident. The local film-cooling effectiveness can be increased, to a maximum of about 100%, under a blowing ratio of 1.5. The secondary flow induced by the lattice array structure at the internal flow channel increases the TKE and accelerates the development of vortices in the film cooling hole. Using the lattice array model, the improvement of the Kagome and BCC lattice arrays in terms of film cooling is better than those of pin-fins. In addition, the effect of lattice arrays on film-cooling effectiveness is different at various blowing ratios, and the lattice array structures have little impact on the film cooling at a relatively low blowing ratio.

Dimensional Characterizations Using SEM and Surface Improvement With Electrochemical Polishing of Additively Manufactured Microchannels

MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with num... more MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with numerous applications, including turbine blade cooling structures, compact microchannel heat exchangers, and electronic cooling devices. Recent development of metallic additive manufacturing based on direct metal laser sintering technology is capable of fabricating micro-scale structures with high complexity and design flexibility. However, powder bed laser sintering process produces rough surface characteristics caused by hatch overlaps and particle attachments, leading to channel size reductions and rough surfaces. In this paper, dimensional metrology of cross-sectional views of multi-row microchannels made by additive manufacturing was conducted by a scanning electron microscope at different locations along the printing direction. Channel size reduction, surface roughness and circularity tolerance of the as-printed channels were analyzed based on micrographs captured by SEM. Results showed that both channel sizes and hole pitches affected the printing qualities of microchannels. The as-printed channel sizes reduced by more than 15% compared to the designed values. Two approaches were made in this paper to improve printing qualities. The first one was to redesign channel size in CAD model to make the as-printed channel sizes closer to the objective values. Electrochemical polishing was then applied as a second way using sulfuric acid solutions. Surface roughness value was reduced by more than 40% after the ECP process.

Influence of Turning Geometry on Convective Transport in a Square Duct with a 180-Degree Sharp Turn

Propulsion and Power Research, Mar 1, 2013

Heat-transfer coefficients (HTC) on surfaces exposed to convection environments are often measure... more Heat-transfer coefficients (HTC) on surfaces exposed to convection environments are often measured by transient techniques such as thermochromic liquid crystal (TLC) or infrared thermography. In these techniques, the surface temperature is measured as a function of time, and that measurement is used with the exact solution for unsteady, zero-dimensional (0-D) or one-dimensional (1-D) heat conduction into a solid to calculate the local HTC. When using the 0-D or 1-D exact solutions, the transient techniques assume the HTC and the free-stream or bulk temperature characterizing the convection environment to be constants in addition to assuming the conduction into the solid to be 0-D or 1-D. In this study, computational fluid dynamics (CFD) conjugate analyses were performed to examine the errors that might be invoked by these assumptions for a problem, where the free-stream/bulk temperature and the heat-transfer coefficient vary appreciably along the surface and where conduction into the solid may not be 0-D or 1-D. The problem selected to assess these errors is flow and heat transfer in a channel lined with a staggered array of pin fins. This conjugate study uses three-dimensional (3-D) unsteady Reynolds-averaged Navier-Stokes (RANS) closed by the shear-stress transport (SST) turbulence model for the gas phase (wall functions http://ppr.buaa.edu.cn/ www.sciencedirect.com

Cfd Simulation on Hydrodynamic and Thermal Behavior of Elliptical Condensing Tubes with an Improved Mass Transfer Model

Social Science Research Network, 2022

Energies, Jun 27, 2022

To better understand the mechanism influencing the periodic lattice structures in gas turbine bla... more To better understand the mechanism influencing the periodic lattice structures in gas turbine blade cooling, these numerical simulations present a systematic comparison of the effects in cases involving pin-fin, Kagome, and BCC lattice arrays on film-cooling effectiveness under three blowing ratios (i.e., M = 0.5, 1.0, and 1.5). The results indicate that the introduction of lattice array structures improves film-cooling effectiveness within the whole streamwise range, especially downstream of the film hole. With an increase in the blowing ratio, the superiority of lattice array structures relative to those without a lattice becomes increasingly evident. The local film-cooling effectiveness can be increased, to a maximum of about 100%, under a blowing ratio of 1.5. The secondary flow induced by the lattice array structure at the internal flow channel increases the TKE and accelerates the development of vortices in the film cooling hole. Using the lattice array model, the improvement of the Kagome and BCC lattice arrays in terms of film cooling is better than those of pin-fins. In addition, the effect of lattice arrays on film-cooling effectiveness is different at various blowing ratios, and the lattice array structures have little impact on the film cooling at a relatively low blowing ratio.

TechConnect Briefs, Jun 21, 2010

In this paper we report our new approach to synthesize cation-exchanged Laponite nanohybrids for ... more In this paper we report our new approach to synthesize cation-exchanged Laponite nanohybrids for use as gel-forming, viscosity and weighting material stabilizing additives for drilling fluid application. To synthesize the nanohybrids, we used a laser beam and ablated micron sized metal particles (Cu, Co, or Al) in an aqueous suspension containing 1 wt% laponite. As a result, the suspensions underwent a transition to a stable gel very quickly. Although the weight percent of the nanohybrids was negligibly small compared to that of Laponite, aqueous suspensions of these cation-exchanged Laponite crystals were highly viscous with excellent shear thinning and thixotropic behavior. The fluid gelled quickly when it was at rest nevertheless its structure was broken easily and it transformed into a low viscosity fluid quickly on shearing.

Volume 3: Heat Transfer; Electric Power; Industrial and Cogeneration, Jun 7, 1999

This study investigates the effect of gap atop an inline array of cubic fins on the heat transfer... more This study investigates the effect of gap atop an inline array of cubic fins on the heat transfer from various participating surfaces in the channel housing the array. Five different gap sizes of C/H = 0.25, 0.5, 1.0 and 2.0, as well as the baseline case without gap (C/H = 0.0) are examined and compared. The array consists of twelve rows of three columns. All tests use a unified Reynolds number at 16,000 based on the mean velocity in the channel and cube height. The heat transfer measurement uses a liquid crystal imaging technique combined with a onedimensional, transient conduction model and a lumped heatcapacity model. The results reveal that the heat transfer from the surfaces uncovered by the cubic fins in the test channel generally decreases with the size of the clearance atop the array. However, such a decreasing trend is insignificant for cases with relatively smaller gaps, i.e. C/H = 0.25 and 0.5. Under these conditions, the heat transfer from the surface of cubic fin, in fact, is higher than that of the baseline case. The vortex enriched shear layer separated from the sharp edges around the cube top is considered to be responsible for this phenomenon. A heat transfer area C gap clearance Cp constant pressure specific heat H height or width of cubic fin h local heat transfer coefficient = q/(T-T) k thermal conductivity IT man q local heat transfer rate per unit area Nu local Nusselt number = hH/k Re Reynolds number = UH/v T temperature t time U mainstream mean velocity x streamwise coordinate z spanwise coordinate Greek Symbols a thermal diffusivity T time parameter p fluid density Subscripts i initial m mainstream, mixed-mean p pin surface s smooth surface r reference t total wetted surface u uncovered endwall w wall

Comparison of Heat Transfer From Staggered Pin Fin Arrays With Circular, Cubic and Diamond Shaped Elements

This study investigates the effects of pin shape of staggered arrays on heat transfer enhancement... more This study investigates the effects of pin shape of staggered arrays on heat transfer enhancement. Three different pins: circular, cubic, and diamond, are studied. The arrays consists of twelve rows of five columns with geometry configuration of ST/D = 2.5 and SL/D = 2.5, H/D = 1. Tests were conducted at Reynolds number between 12,000 and 19,000. The heat transfer measurement uses a liquid crystal imaging technique combined with a one-dimensional, transient conduction model and a lumped heat-capacitance model. The results reveal that the heat transfer from the cubic pin arrays and diamond pin arrays is higher than that from the circular pin arrays at the same Reynolds number. However, the circular pin arrays provide the lowest pressure loss among the three arrays. Considering the trade-offs between heat transfer and pumping power, the circular pin arrays may still be the better choice as a heat exchanger.

Convective Heat Transfer from Finite Cylinders Mounted on a Plane Wall

High-performance compact heat sinks have been developed for the effective cooling of high-density... more High-performance compact heat sinks have been developed for the effective cooling of high-density LSI packaging. Heat transfer and pressure loss characteristics of the heat sinks in both air-cross-flow and air-jet cooling have been experimentally studied. The present heat sinks were of plate-fin and pin-fin arrays with a fin pitch of 0.7 mm. The plate-fin heat sinks had higher cooling performance than the pin-fin heat sinks in the range of large airflow rates both in air-cross-flow and air-jet cooling. The thermal conductance in cross-flow cooling was 20 or 40% larger than that in jet cooling. The correlation of Colburn j-factor/Fanning friction factor versus the Reynolds number for the present heat sinks was found to be very close to that of a conventional large-size heat exchanger.

Journal of materials science and nanotechnology, Aug 1, 2013

We have synthesized Au/CuO and Au/ZnO nanocomposites using the laser soldering technique. The pro... more We have synthesized Au/CuO and Au/ZnO nanocomposites using the laser soldering technique. The process was carried out by irradiating a solution containing Au-CuO and Au-ZnO nanoparticles using 532 nm laser pulses of 0.1 J/cm 2 continuously for 20 minutes. The beam was focused using a 75 mm focal lens and the laser power near the focal region was estimated to be about 2.4 x 10 12 W/m 2 . Their UV-VIS absorption and transmission were observed and the results indicated that the bandgap energies of the Au/CuO and Au/ZnO are significantly lower than those of pure CuO and ZnO. A theoretical model was developed and the calculation showed that the soldering process was due to the laser melting of the gold nanoparticles and the molten gold got soldered to the ZnO as well as CuO nanoparticles nearby.

Effects of Pin Detached Space on Heat Transfer and From Pin Fin Arrays

ABSTRACT Heat transfer and pressure characteristics in a rectangular channel with pin-fin arrays ... more ABSTRACT Heat transfer and pressure characteristics in a rectangular channel with pin-fin arrays of partial detachment from one of the endwalls have been experimentally studied. The overall channel geometry (W = 101.6 mm, E = 25.4 mm) simulates an internal cooling passage of wide aspect ratio (4:1) in a gas turbine airfoil. With a given pin diameter, D = 6.35 mm = 1/4 E, three different pin-fin height-to-diameter ratios, H/D = 4, 3, and 2, were examined. Each of these three cases corresponds to a specific pin array geometry of detachment spacing (C) between the pin-tip and one of the endwalls, i.e. C/D = 0, 1, 2, respectively. The Reynolds number, based on the hydraulic diameter of the un-obstructed cross-section and the mean bulk velocity, ranges from 10,000 to 25,000. The experiment employs a hybrid technique based on transient liquid crystal imaging to obtain distributions of the local heat transfer coefficient over all of the participating surfaces, including the endwalls and all the pin elements. Experimental results reveal that the presence of a detached space between the pin-tip and the endwall have a significant effect on the convective heat transfer and pressure loss in the channel. The presence of pin-to-endwall spacing promotes wall-flow interaction, generates additional separated shear layers, and augments turbulent transport. In general, an increase in detached spacing, or C/D leads to lower heat transfer enhancement and pressure drop. However, C/D = 1, i.e. H/D = 3, of a staggered array configuration exhibits the highest heat transfer enhancement, followed by the cases of C/D = 0 and C/D = 2, i.e. H/D = 4 or 2, respectively.

Effects of Perpendicular Flow Entry on Convective Heat/Mass Transfer From Pin-Fin Arrays

Journal of heat transfer, Aug 1, 1999

Convective heat transfer with pin-fin arrays have been studied extensively in laboratory experime... more Convective heat transfer with pin-fin arrays have been studied extensively in laboratory experiments where flow is introduced to the array uniformly over the channel span. However, the flow path in actual cooling designs is often serpentine-shaped with multiple turns, and the pin-fin array section is often located immediately downstream of a turn. The present study, using an analogous mass transfer technique based on naphthalene sublimation, investigates the effects of three different, nonaxial flow entries on array heat transfer for both an inline and a staggered arrangement of pins. The measurement acquires the mass transfer rate of each individual pin in a five row by seven column array for the Reynolds number varying from 8000 to 25,000. The mass transfer and associated flow visualization results indicate that the highly nonuniform flow distribution established at the array entrance and persisting through the entire array can have significant effects on the array heat transfer characteristics. Compared to the conventional case with axial-through flow entrance, the overall array heat transfer performance can be either enhanced or degraded, depending on the actual inlet arrangements and array configurations.

Heat Transfer on Convective Surfaces With Pin-Fins Mounted in Inclined Angles

Casting of pin fins at the trailing edge of the turbine blades often presents some difficulties d... more Casting of pin fins at the trailing edge of the turbine blades often presents some difficulties due to tight dimensional tolerances, leaving the pin fins inclined after the casting process. This study is to experimentally examine the effects of such an imperfect manufacturing phenomena on the heat transfer and friction characteristics over pin-fin arrays with different pin inclinations. The test model is a staggered short (H/D = 1) pin-fin array with an inter-pin spacing of 2.5 times the pin-diameter (S/D = 2.5) in both longitudinal and transverse directions. Detailed local heat transfer coefficients on both array endwalls and pin elements are determined using the transient liquid crystal technique, as the inclined angle θ varies from 40° to 90° and the Reynolds number ranges from 7.0 × 103 and 1.3 × 104. The measured data suggest that an increase in pin inclined angle relative to its normal orientation (90-degree) significantly reduces the level of heat transfer enhancement from the array. Such a reduction amounts to nearly 50% for the 40-degree case. The accompanied friction loss also decreases.

Volume 3: Heat Transfer; Electric Power; Industrial and Cogeneration, Jun 7, 1999

Computations were performed to investigate the three-dimensional flow and heat transfer in a high... more Computations were performed to investigate the three-dimensional flow and heat transfer in a high aspect ratio channel in which one or two wall are lined with four rows of hemispherical cavities arranged in a staggered fashion with two Reynolds numbers (23,000 and 46,000). The focus is on understanding the flow induced by cavities and how that flow affects surface heat transfer. Computed results were compared with available experimental data. This computational study is based on the ensemble-averaged conservation equations of mass, momentum (compressible Navier- Stokes), and energy closed by the low Reynolds number shear-stress transport k-oi turbulence model (wall functions were not used). Solutions were generated by a cell-centered finite-volume method that uses third-order accurate flux-difference splitting of Roe with limiters, multigrid acceleration of a diagonalized ADI scheme with local time stepping, and patched/overlapped structured grids. NOMENCLATURE D diameter of sphere used to generate cavity (Fig. ) depth of concavity (Fig. ) H height of channel (Fig. ) heat transfer coefficient (q../(T; -T w)) kinetic energy static pressure qw wall heat transfer rate per unit area Re Reynolds number (pilliFUp) • static temperature LI; average velocity at channel inlet x, y, z streamwise, spanwise, and normal coordinates Greek • dynamic viscosity • density to dissipation rate per unit k Subscripts and Superscripts conditions at channel inlet conditions on channel wall

Heat Transfer Contributions of Pins and Endwall in Pin-Fin Arrays: Effects of Thermal Boundary Condition Modeling

Journal of turbomachinery, Apr 1, 1999

Short pin-fin arrays are often used for cooling turbine airfoils, particularly near the trailing ... more Short pin-fin arrays are often used for cooling turbine airfoils, particularly near the trailing edge. An accurate heat transfer estimation from a pin-fin array should account for the total heat transfer over the entire wetted surface, which includes the pin surfaces and uncovered endwalls. One design question frequently raised is the actual magnitudes of heat transfer coefficients on both pins and endwalls. Results from earlier studies have led to different and often contradicting conclusions. This variation, in part, is caused by imperfect or unrealistic thermal boundary conditions prescribed in the individual test models. Either pins or endwalls, but generally not both, were heated in those previous studies. Using a mass transfer analogy based on the naphthalene sublimation technique, the present experiment is capable of revealing the individual heat transfer contributions from pins and endwalls with the entire wetted surface thermally active. The particular pin-fin geometry investigated, S/D = X/D = 2.5 and H/D = 1.0, is considered to be one of the optimal array arrangement for turbine airfoil cooling. Both inline and staggered arrays with the identical geometric parameters are studied for 5000 ≤ Re ≤ 25,000. The present results reveal that the general trends of the row-resolved heat transfer coefficients on either pins or endwalls are somewhat insensitive to the nature of thermal boundary conditions prescribed on the test surface. However, the actual magnitudes of heat transfer coefficients can be substantially different, due to variations in the flow bulk temperature. The present study also concludes that the pins have consistently 10 to 20 percent higher heat transfer coefficient than the endwalls. However, such a difference in heat transfer coefficient imposes very insignificant influence on the overall array-averaged heat transfer, since the wetted area of the uncovered endwalls is nearly four times greater than that of the pins.

The present study explores the heat transfer enhancement induced by arrays of cubic fins. The fin... more The present study explores the heat transfer enhancement induced by arrays of cubic fins. The fin element is either a cube or a diamond in shape. The array configurations studied include both inline and staggered arrays of seven rows and five columns. Both cubic arrays have the same geometric parameters' , i.e., 11/0=1, S/D=)UD=2.5, which are similar to those of earlier studies on circular pin-fm arrays. The present results indicate that the cube element in either array always yields the highest heat transfer, followed by diamond and circular pin-fm. Arrays with diamondshaped elements generally cause the greatest pressure loss than those with either cubes or pin fins. For a given element shape, a staggered array generally produces higher heat transfer enhancement and pressure loss than the corresponding inline array. Cubic Arrays can be viable alternatives for pedestal cooling near a blade trailing edge.

Heat Transfer in the Tip Region of Grooved Turbine Blades

Journal of turbomachinery, Apr 1, 1989

Local convective heat transfer at the tip region of grooved blades is experimentally investigated... more Local convective heat transfer at the tip region of grooved blades is experimentally investigated. The present study models the problem by flow over a shrouded, rectangular cavity, with the shroud moving opposite to the main flow direction. The naphthalene sublimation technique together with a computer-controlled measurement system provides detailed local transfer information on all the participating surfaces. The local heat transfer coefficient in the cavity is strongly influenced by the cavity aspect ratio, gap size, and leakage flow Reynolds number. Within the present study range, the effect of relative motion between the shroud and cavity on the heat transfer is found to be minor, particularly for the average heat transfer coefficient. With the same leakage flow rate, the average heat transfer coefficient over the entire tip area decreases with an increase in cavity depth. However, in terms of total heat transfer to the tip, an overly deep cavity is undesirable, because it provides larger surface area but only a small increase in flow resistance.

Influence of Topology on Heat Transfer in a Double Wall Cooling Channel: Potential of Series-Linked Jets

The use of lean burning flames stabilized by highly swirling flows represents the most effective ... more The use of lean burning flames stabilized by highly swirling flows represents the most effective technology to limit NOx emissions in modern aeroengine combustors. In these devices up to 70% of compressed air is admitted in the combustor through the injection system, which is usually designed to give strong swirling components to air flow. Complex fluidynamics is observed with large flow recirculations due to vortex breakdown and precessing vortex core, that may result in a not trivial interaction with liner cooling flows close to combustor walls. This interaction and its effects on the local cooling performance make the design of the cooling systems very challenging and time-consuming, considering design and commission of new test rigs for detailed analysis. Keeping in mind costs and complexities related to the investigation of swirl flow/wall cooling interaction by experimental approach, CFD can be considered an accurate and reliable alternative to understand the associated phenomena. The widely known overcomes of RANS formulation (e.g. underestimation of mixing and inability to properly describe swirling flows) and the more and more impressive increase in computational resources, pushed hybrid RANS-LES models as valuable and affordable approaches to accurately solve the main turbulent flow structures. This work describes the main findings of a CFD analysis intended to accurately investigate the flow field and wall heat transfer as a result of the mutual interaction between a highly swirling flow generated by a lean burn nozzle and a slot-effusion liner cooling system. In order to overcome some limitations of RANS approach, the simulations were performed with SST-SAS, a hybrid RANS-LES model. Moreover, the significant computational effort due to the presence of more than 600 effusion holes was limited exploiting two different modelling strategies: a homogeneous model based on the application of uniform boundary conditions on both aspiration and injection sides, and another solution that provides a coolant injection through point mass sources within a single cell. CFD findings were compared to experimental results coming from an investigation carried out on a three sector linear rig. The comparison pointed out that advanced modelling strategies, i.e. based on discrete mass sources, are able to reproduce the effects of mainstream-coolant interactions on convective heat loads. Validated the approach through a benchmark against time-averaged quantities, the transient data acquired were examined in order to better understand the unsteady behaviour of the thermal load through a statistical analysis, providing useful information with a design perspective.

Energies, Jun 27, 2022

To better understand the mechanism influencing the periodic lattice structures in gas turbine bla... more To better understand the mechanism influencing the periodic lattice structures in gas turbine blade cooling, these numerical simulations present a systematic comparison of the effects in cases involving pin-fin, Kagome, and BCC lattice arrays on film-cooling effectiveness under three blowing ratios (i.e., M = 0.5, 1.0, and 1.5). The results indicate that the introduction of lattice array structures improves film-cooling effectiveness within the whole streamwise range, especially downstream of the film hole. With an increase in the blowing ratio, the superiority of lattice array structures relative to those without a lattice becomes increasingly evident. The local film-cooling effectiveness can be increased, to a maximum of about 100%, under a blowing ratio of 1.5. The secondary flow induced by the lattice array structure at the internal flow channel increases the TKE and accelerates the development of vortices in the film cooling hole. Using the lattice array model, the improvement of the Kagome and BCC lattice arrays in terms of film cooling is better than those of pin-fins. In addition, the effect of lattice arrays on film-cooling effectiveness is different at various blowing ratios, and the lattice array structures have little impact on the film cooling at a relatively low blowing ratio.

Dimensional Characterizations Using SEM and Surface Improvement With Electrochemical Polishing of Additively Manufactured Microchannels

MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with num... more MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with numerous applications, including turbine blade cooling structures, compact microchannel heat exchangers, and electronic cooling devices. Recent development of metallic additive manufacturing based on direct metal laser sintering technology is capable of fabricating micro-scale structures with high complexity and design flexibility. However, powder bed laser sintering process produces rough surface characteristics caused by hatch overlaps and particle attachments, leading to channel size reductions and rough surfaces. In this paper, dimensional metrology of cross-sectional views of multi-row microchannels made by additive manufacturing was conducted by a scanning electron microscope at different locations along the printing direction. Channel size reduction, surface roughness and circularity tolerance of the as-printed channels were analyzed based on micrographs captured by SEM. Results showed that both channel sizes and hole pitches affected the printing qualities of microchannels. The as-printed channel sizes reduced by more than 15% compared to the designed values. Two approaches were made in this paper to improve printing qualities. The first one was to redesign channel size in CAD model to make the as-printed channel sizes closer to the objective values. Electrochemical polishing was then applied as a second way using sulfuric acid solutions. Surface roughness value was reduced by more than 40% after the ECP process.

Influence of Turning Geometry on Convective Transport in a Square Duct with a 180-Degree Sharp Turn

Propulsion and Power Research, Mar 1, 2013

Heat-transfer coefficients (HTC) on surfaces exposed to convection environments are often measure... more Heat-transfer coefficients (HTC) on surfaces exposed to convection environments are often measured by transient techniques such as thermochromic liquid crystal (TLC) or infrared thermography. In these techniques, the surface temperature is measured as a function of time, and that measurement is used with the exact solution for unsteady, zero-dimensional (0-D) or one-dimensional (1-D) heat conduction into a solid to calculate the local HTC. When using the 0-D or 1-D exact solutions, the transient techniques assume the HTC and the free-stream or bulk temperature characterizing the convection environment to be constants in addition to assuming the conduction into the solid to be 0-D or 1-D. In this study, computational fluid dynamics (CFD) conjugate analyses were performed to examine the errors that might be invoked by these assumptions for a problem, where the free-stream/bulk temperature and the heat-transfer coefficient vary appreciably along the surface and where conduction into the solid may not be 0-D or 1-D. The problem selected to assess these errors is flow and heat transfer in a channel lined with a staggered array of pin fins. This conjugate study uses three-dimensional (3-D) unsteady Reynolds-averaged Navier-Stokes (RANS) closed by the shear-stress transport (SST) turbulence model for the gas phase (wall functions http://ppr.buaa.edu.cn/ www.sciencedirect.com

Cfd Simulation on Hydrodynamic and Thermal Behavior of Elliptical Condensing Tubes with an Improved Mass Transfer Model

Social Science Research Network, 2022

Energies, Jun 27, 2022

To better understand the mechanism influencing the periodic lattice structures in gas turbine bla... more To better understand the mechanism influencing the periodic lattice structures in gas turbine blade cooling, these numerical simulations present a systematic comparison of the effects in cases involving pin-fin, Kagome, and BCC lattice arrays on film-cooling effectiveness under three blowing ratios (i.e., M = 0.5, 1.0, and 1.5). The results indicate that the introduction of lattice array structures improves film-cooling effectiveness within the whole streamwise range, especially downstream of the film hole. With an increase in the blowing ratio, the superiority of lattice array structures relative to those without a lattice becomes increasingly evident. The local film-cooling effectiveness can be increased, to a maximum of about 100%, under a blowing ratio of 1.5. The secondary flow induced by the lattice array structure at the internal flow channel increases the TKE and accelerates the development of vortices in the film cooling hole. Using the lattice array model, the improvement of the Kagome and BCC lattice arrays in terms of film cooling is better than those of pin-fins. In addition, the effect of lattice arrays on film-cooling effectiveness is different at various blowing ratios, and the lattice array structures have little impact on the film cooling at a relatively low blowing ratio.