Jr-Ming Miao

Numerical Investigation of Confined Multiple-Jet Impingement Cooling Over A Flat Plate at Different Crossflow Orientaions

This study investigates the fluid flow and heat transfer characteristics of round jet arrays impinging orthogonally on a flat-plate with confined walls at different crossflow orientations. A computational fluid dynamic technique based on a control volume method is used to compute the detailed Nusselt number distributions on the flat plate. This is achieved by solving the steady-state three-dimensional incompressible Reynolds-averaged Navier-Stokes equations. The Reynolds stress turbulence quantities are determined by a realizable κ-ϵ turbulence model with an enhancement near-wall treatment. Numerical computations are performed for two types of arrangements in round jet arrays, both inline and staggered, and three different crossflow directions, parallel, hybrid, and counter. The jet Reynolds numbers ranging from 2,440 to 14,640 and three different jet-to-plate spacing ratios (Zn/dj) of 1, 3, and 6 are investigated in this study. Results show that the flow exit crossflow direction would significantly affect the developing jet flow fields and Nusselt number distributions on the target flat-plate. Area-averaged Nusselt number increases with an increase of jet Reynolds number. Of all the cases tested, the highest average Nusselt numbers were obtained for the case with inline jets and hybrid crossflow orientation. The thermal performance of impingement multiple jets is enhanced when the value of Zn/dj decreases from 6 to 3. Results show that further reducing the value of Zn/dj to 1 creates a significant nonuniform distribution in local Nusselt number over the target plate regardless of the crossflow orientations. This study also provides a correlation of the area-averaged Nusselt number with the jet Reynolds number for both inline and staggered jet arrays.

NOISE REDUCTION OF A CROSS-FLOW FAN

ABSTRACT This study attempts to reduce the aerodynamic sound noise from a cross-flow fan by varying the geometrical parameters of the cross- flow fan as the similar amount of air flow rate is delivered by the cross- flow fan. Two different rotors (fixed-pitch and staggered-pitch) and tongues (wedge- shape and saw-tooth shape), and four different tongue clearances (e=3mm, 5mm, 7mm, and 10mm) are also tested. At Ω≥ 1200 r/min, a screech tone produced from the peak sound pressure levels at the harmonics of blade passage frequency always occurs for the cross- flow fan with the fixed-pitch rotor, the wedge shape, and e=3 mm. At Ω≥1200 r/min, the sound pressure level at the first harmonic of blade passage frequency is significantly reduced by either replacing the fixed- pitch rotor with the staggered-pitch one as well as replacing the wedge shape with the saw-tooth tongue or enlarging the tongue clearance. However, the overall sound pressure level from the cross-flow fan to deliver the same air flow rate is sli...

Novel technique for investigating the temperature effect on the diffusion coefficient of naphthalene into air

This article describes a technique that uses a piezoelectric quartz crystal microbalance (QCM) for determining the diffusion coefficient of naphthalene into air. The QCM is placed onto the open top of a closed Stefan tube and its active surface is covered with a thin layer of solid naphthalene. Due to the markedly enhanced QCM resolution of 10−9 g/cm2 over that of the conventional digital electronic balance, the QCM in this study requires much less time than previous studies using digital electronic balances for measuring the diffusion coefficient in a binary gas system. At P=0.1013 MPa and in a temperature range from 278.25 to 315.15 K, the empirical correlations to evaluate both the diffusion coefficient of naphthalene into air at 1 atm pressure and the Schmidt number of naphthalene at various temperatures are presented. The measurement uncertainty of the diffusion coefficient of naphthalene into air with the present system is less than 3%. In addition, measured results verify that the measurement of th...

Conjugate Heat Transfer of a Disk-Shaped Miniature Heat Pipe

ABSTRACT This study employs a computational fluid dynamics (CFD) software package to obtain a two-phase three-dimensional flow field and conjugate heat transfer results for a disk-shape miniature heat pipe (DMHP) that is used to dissipate heat from a laser diode. A mixture model is introduced into the governing equations for calculating the quality of mixture in the two-phase flow field. Predicted thermal resistances are presented at three different fluid charge volumes of 18%, 55%, and 92% at different heating powers of the laser diode. The predicted thermal resistance of the DMHP agrees well with measured results given in our previous study [1].

Endwall effect on the mass transfer in a square open cavity

Abstract In a lid-driven flow over a square open cavity with confined endwalls, a naphthalene sublimation technique was employed to measure the local mass transfer rates over the bottom surface of the cavity for the Reynolds numbers ranging from 2,000 to 4,000. The tested cavity has a span-to-width ratio ( SAR ) of 8 and a width-to-depth ratio ( AR ) of 1. The measured results are compared with predictions based on a deterministic vortex method with the assumption of two-dimensional cavity flow. The numerical results indicate an increase in mass transfer rate on the bottom surface of the cavity with an increase in the Reynolds number. The experimental results show the similar trend in the mass transfer rate with the presence of endwalls. In addition, a large variation in the spanwise direction of the measured local mass transfer rates implies the existence of Taylor-Gortler-like ( TGL ) vortices in the cavity. Furthermore, a correlation is derived between the measured overall averaged Sherwood number and the Reynolds number.

Effect of Upstream Wake on Shower-Head Film Cooling

The present study aims to investigate the effect of an upstream wake on the convective transport phenomena over a turbine blade with shower-head film cooling. A naphthalene sublimation technique was implemented to obtain the detailed mass transfer distributions on both suction and pressure surfaces of the test blade. All mass transfer runs were conducted on a blowing-type wind tunnel with a six-blade linear cascade. The leading edge of the test blade was drilled with three rows of equally spaced injection holes. The upstream wake was simulated by a circular bar with the same diameter as that of the trailing edge of the test blade.

Flow Mechanism of a Novel Active Micro-Rotor Mixer

The purpose of this study is to investigate the complex vortex flow patterns within a novel active micro-rotor mixer under various Reynolds numbers and rotating speeds by employing of CFD technique. The concept of present micro-rotor mixer is inspired from the Wankel-type combustor which is widely used in the power machines. The configuration of present micro-mixer is consisted of a rotor with shape of triangle column, a blending chamber and individual inlet and outlet ports. The blending chamber is served as the mixing chamber since the separated three sub-regions will change their volumes as the rotor undergoing the rotating motion with a fixed eccentricity. The dynamic flow patterns and mixing process of two species within the mixing chamber were simulated and visualized with streak lines. The governing equations are unsteady, two-dimensional incompressible Navier-Stokes equation and the two working fluids are pure water and alcohol. The concentration equation for species is also solved to reveal the mass transfer process in various sub-regions then being calculated on the outlet port to evaluate the mixing efficiency. The dynamic mesh technique was applied to re-distribute the computational meshes when the rotor finished a complete rotation cycle. Inspection on the flow developing stages within the mixing chamber over one complete cycle, it seems that multi-vortex flow field was generated due to the interaction of the shear force from the rotor, viscous force and inertial force of working fluids. The Coanda flow appeared in some conditions. When the Reynolds number is below of 10, the rotating speed of rotor has less influence on the mixing efficiency. An obvious enhancement in the mixing efficiency can be found in cases of the rotating speed of rotor changed from 30 rpm to 150 rpm when the Reynolds number in range of 25 to 100. Generally, the maximum mixing efficiency of 85% can be achieved for 1

Experimental Study on Performance of Vertical Axis Wind Turbine with NACA 4-Digital Series of Blades

The experimental studies of output power performances of a vertical-axis-wind-turbine (VAWT) had been conducted in suction-type low speed wind tunnel with various free stream velocity. Torque and rotation speed of blades were measured by using torque meter and optical detector to analyze the effect of blade-section shape on the performance of wind turbine. The test model of experiments in the research was H-rotor VAWT. Three shapes of the NACA 4-digital series blade-section, NACA0022, NACA6404, and NACA6422 were taken in this work. Effects of thickness and camber of blade-section, blade numbers, and blade setting angles on the performance of VAWT have been analyzed in detail. The results show that NACA6422 blade-section has rotation speed of 42% higher than that of NACA0022 when the free stream velocity is below 12 m/s and the blade numbers are 4-blade type. Wind turbines with NACA6422 blades also showed that about 10% higher output power than that of NACA0022 blades among the tested range of free stream velocity. Results indicated that wind turbine with blades of anti-symmetric and thick blade-section was generally more suitable for applying to VAWT. All results of this study can be used the optimization design of VAWT blades in further.

Three-dimensional simulations on the formation of droplets in a T-type microchannel

Three-dimensional simulations on the formation of droplets in a T-type microchannel Jr-Ming Miao1,2, Fuh-Lin Lih3, Yi-Chun Liou4, Hsiu-Kai Chen1 Summary To date, miniaturization of fluid handling and fluid analysis devices in the medicine engineering has been emerging in the interdisciplinary research field of micro-fluidics, as a result of miniaturization of the detective device to allow parallelization as well as to reduce analysis time and sample volume. Micro-totalanalysis-system (μ-TAS) researches aimed at developing miniaturized and integrated “lab-on-a-chip” devices for biochemical analysis applications. Dropletbased micro-mixer is the one of the key components in the developing of μ-TAS. Numerical approach on the dynamic formation of water droplets in a T-type microchannel with a 200μm×50μm rectangular cross section and 1000μm long was performed in present study. The transient three-dimensional two-phase flow is solved using computational fluid dynamics in conjunction with a volume of fluid (VOF) method. Simulations of the processes of individual droplet emergence, growth, deformation, detachment and movement in the straight microchannel are performed to explicitly track the evaluation of liquid-liquid interface. The effects of contact angle values, volume flow ratio and Ca number on the departure diameter and frequency of droplets were systematically conducted to build empirical correlations. Parametric studies highlight the importance of various factors affecting the formation of liquid droplet from the junction of microchannels. Besides, inspection on the flow pattern within single droplet reveals the existence of counter-rotating vortices.

Numerical study of Transient Flow in a Full-Size Reflected Shock Tunnel

The aim of this paper is to develop a CFD solver that used to simulate the transient flow phenomena in a reflected shock tunnel. The transient flow phenomena in a shock tunnel include the reflected shock/boundary layer interaction and the starting process of nozzle flow that can affect the duration of test flow in actual conditions. To numerically simulate these transient flow features, a full-size, axisymmetric reflected shock tunnel model is used. The governing equations are a full Navier-Stokes equation, a species equation and a simplified polynomial correlation to simulate the real gas effects. The numerical code is developed based on the finite volume method coupled with the upwind Roes scheme and the total variation diminishing (TVD) method. To increase the calculation efficiency, a multi-block and multi-mesh grid generation technique is employed in a huge computational domain. The present computational results have not only confirmed the theoretical characteristics of a shock tube, but have also qualitatively presented the phenomena of reflected shock/boundary layer interaction and the starting process of nozzle flow. This numerical code is a useful tool to demonstrate the actual flow phenomena and to assist the design of experiments.