Micro Fluidic Jets for Thermal Management of Electronics

Micro fluidics devices are conventionally used for boundary layer control in many aerospace applications. Synthetic Jets are intense small scale turbulent jets formed from entrainment and expulsion of the fluid in which they are embedded. The idea of using synthetic jets in confined electronic cooling applications started in late 1990s. These micro fluidic devices offer very efficient, high magnitude direct air-cooling on the heated surface. A proprietary synthetic jet designed in General Electric Company was able to provide a maximum air velocity of 90 m/s from a 1.2 mm hydraulic diameter rectangular orifice. An experimental study for determining the thermal performance of a meso scale synthetic jet was carried out. The synthetic jets are driven by a time harmonic signal. During the experiments, the operating frequency for jets was set between 3 and 4.5 kHz. The resonance frequency for a particular jet was determined through the effect on the exit velocity magnitude. An infrared thermal imaging technique was used to acquire fine scale temperature measurements. A square heater with a surface area of 156 mm2 was used to mimic the hot component and extensive temperature maps were obtained. The parameters varied during the experiments were jet location, driving jet voltage, driving jet frequency and heater power. The output parameters were point wise temperatures (pixel size = 30 μm), and heat transfer enhancement over natural convection. A maximum of approximately 8 times enhancement over natural convection heat transfer was measured. The maximum coefficient of cooling performance obtained was approximately 6.6 due to the low power consumption of the synthetic jets.

Calculation of Flow in Mixing Vessels With Various Turbulence Models

The paper presents calculations of flow in a mixing vessel stirred by a six-blade Rushton impeller. Mathematical model used in computations is based on the ensemble averaged conservation equations. An efficient finite-volume method based on unstructured grids with rotating sliding parts composed of arbitrary polyhedral elements is used together with various turbulence models. Besides the standard k-ε model which served as a reference, k-ε-v2 model (Durbin, 1995) and the recently proposed hybrid EVM/RSM turbulence model (Basara & Jakirlic, 2003) were used in the calculations. The main aim of the paper is to investigate if more advanced turbulence models are needed for this type of CFD applications. The results are compared with the available experimental data.

Experimental Study on Water Mist Suppression of Poly(Methyl Methacrylate) PMMA Fires

Suppression of Poly(methyl methacrylate) PMMA fires by water mist will be studied in this paper. A simple test is developed to study the critical water flow rate under different radiant heat flux. The test is found to be suitable for studying the extinguishment effects of fine water droplets involving oxygen displacement, gas phase and fuel surface cooling. Water mist is generated by a single pressure nozzle, with the water mist characteristics measured by the Laser Doppler Velocimetry or the Adaptive Phase Doppler Velocimetry System (LDV/APV system). The interaction between water mist and the PMMA flame will be studied in a confined space with ventilation control in a cone calorimeter. The heat release rate, oxygen, carbon dioxide and carbon monoxide concentrations, and other important parameters of the interaction under various conditions are measured. It is found that discharging adequate amount of water mist would suppress the diffusion flame in the confined space. Reignition might occur once water mist stopped discharging to the fuel surface. Higher heat release rate and more smoke and toxic gases were produced than from those in first ignition.

Effects of Ambient Temperature and Oxygen Concentration on Soot Evolution in Diesel Spray Combustion

A newly developed Forward Illumination Light Extinction (FILE) soot measurement technique was applied in a constant volume spray chamber to study the effects of ambient temperature and oxygen concentration on soot evolution in diesel combustion. The FILE technique with the capability of two-dimensional time-resolved quantitative soot measurement provides the much-needed information to investigate the soot formation mechanism. The ambient temperatures of 1200K, 1000K and 800K were tested to study the temperature effects on soot formation. A decrease of ambient temperature results in a longer ignition delay, which promotes a larger premixed combustion zone combining with higher heat release rates. The change of ambient temperature from 1200K to 800K increases the fuel portion burnt in the premixed combustion period. At 800K, combustion is dominated by the premixed combustion and much less soot is formed. Diesel combustion with 21% and 15% ambient oxygen concentration was also studied. With lower ambient oxygen concentration, the combustion process is basically not changed, but expands into a longer time span with a lower heat release rate. The lower heat release rate results in a lower flame temperature, which benefits the NOx emission control. However, with about the same amount of soot within the flame, and much longer soot life, soot has more chance to escape to the exhaust.

Integrated Thermomechanical Design and Optimization of BGA Packages Using Genetic Algorithm

A new multidisciplinary design and optimization methodology in electronics packaging is presented. A genetic algorithm combined with multi-disciplinary design and multi-physics analysis tools are used to optimize key design parameters. This methodology is developed to improve the electronic package design process by performing multidisciplinary design and optimization at an early design stage. To demonstrate its capability, the methodology is applied to a Ball Grid Array (BGA) package design. Multidisciplinary criteria including thermal, thermal strain, electromagnetic leakage, and cost are optimized simultaneously. A simplified routability analysis criterion is treated as a constraint. The genetic algorithm is used for systematic design optimization while reducing the total computational time. The present methodology can be applied to any electronics product design at any packaging level from the chip level to the system level.

Experimental Study on Skin Friction Reduction With Micro-Blowing

Reducing skin friction, such as friction on a car hood or a plane wing, can significantly reduce the drag force and decrease specific fuel consumption. Many techniques and methods have been tried. The Micro-blowing Technique (MBT) is an innovative way to reduce skin friction. Suggested by early research in boundary layer injection in 1950s, MBT was actually brought to effective use in 1994 by Hwang [1]. The basic idea is that by blowing fluid, same as or different from the mainstream flow, at an angle with that of the main flow, a decrease in the velocity gradient at the wall can be achieved, and thus the shear stress on the surface is reduced. Although the experimental data on boundary layer with micro blowing show a significant friction reduction, the mechanism of MBT is still not well understood and thus its full range of application is not yet established. In this paper, we further the understanding of the MBT mechanism. An experimental system is set up to visualize the flow structure on a plate with and without micro blowing in a tunnel. A long distance microscope is combined with a Full Field Flow Tracking visualization method in order to elucidate the nature of the flow interaction and mixing between the blowing flow and the main flow. The flow above the porous plates is visualized and velocities both in the blowing layer immediately adjacent to the plate and in the main flow are quantified using the PIV procedure. The flow and shear stress analysis shows that MTB has significantly different effects on a flow with a boundary layer and fully developed internal flows.

A Study of an Oscillating Corner Meniscus With Phase Change Using Image Analyzing Interferometry

The thickness and curvature profiles in the contact line region of a moving evaporating thin liquid film of pentane on a quartz substrate were measured for the thickness region, δ < 2.5 microns. The critical region, δ < 0.1 microns, was emphasized. The profiles were obtained using image analyzing interferometry and an improved data analysis procedure. The precursor adsorbed film, the thickness, the curvature, and interfacial slope (variation of the local “apparent contact angle”) profiles were consistent with previous models based on interfacial concepts. Isothermal equilibrium conditions were used to evaluate the Hamaker constant in-situ and to verify the accuracy of the procedures. The profiles give fundamental insights into the phenomena of phase change, pressure gradient, fluid flow, spreading, and the physics of interfacial phenomena in the contact line region. The experimental results demonstrate explicitly for the first time, with microscopic detail, that the disjoining pressure controls fluid flow within an evaporating completely wetting thin curved film and the stability of the thin film. The change in the thickness of the adsorbed film with time is demonstrated for the first time.

Numerical Simulation of Heat Transfer Mechanisms During Femtosecond Laser Heating of Nano-Films Using 3-D Dual Phase Lag Model

In the present work we investigate femtosecond laser heating of nanoscale metal films irradiated by a pulsating laser in three dimensions using the Dual Phase Lag (DPL) model and consider laser heating at different locations on the metal film. A numerical solution based on an explicit finite-difference method has been employed to solve the DPL heat conduction equation. The stability criterion for selecting a time step size is obtained using von Neumann eigenmode analysis, and grid function convergence tests have been performed. The energy absorption rate, which is used to model femtosecond laser heating, has been modified to accommodate for the three-dimensional laser heating. We compare our results with classical diffusion and hyperbolic heat conduction models and demonstrate significant differences among these three approaches. The present research enables us to study ultrafast laser heating mechanisms of nano-films in 3D.

Development of Microfluidic Flow Sensor in a Polymeric Microchip

A microfluidic flow sensor has been developed to precisely measure the flow rate in a micro/nanofluidic channel for lab-on-a-chip applications. Mixed electroosmotic and pressure driven microflows are investigated using this sensor. Our microflow sensor consists of two components: fluidic circuit and electronic circuit. The fluidic circuit is embedded into the microfluidic chip, which is formed during the microfabrication sequences. On the other hand, the electronic circuit is a microelectronic chip that works as a logical switch. We have tested the microflow sensor in a hybrid poly di-methyl-siloxane (PDMS)-glass microchip using de-ionized (DI) water. Softlithography techniques are used to form the basic microflow structure on a PDMS layer, and all sensing electrodes are deposited on a glass plate using sputtering technique. In this investigation, the microchannel thickness is varied between 3.5 and 10 microns, and the externally applied electric field is ranged between 100V/mm and 200V/mm. The thickness of the gold electrodes is kept below 100nm, and hence the flow disturbance due to the electrodes is very minimal. Fairly repeatable flow results are obtained for all the channel dimensions and electric fields. Moreover, for a particular electric field strength, there is an appreciable change in the flow velocity with the change of the channel thickness.

Experimental Thermal Behavior of Electronic Chips in a Channel During Pump-On Transient Period

Experiments are performed to study the heat transfer characteristics during the pump-on transient period from an array of 4 × 1 flush mounted discrete heat sources in a vertical rectangular channel using water as the working fluid. The experimental data covers the flow regime with Reynolds number based on heat source length ranging from 1050 to 2625. The applied uniform heat fluxes to the chips are 1, 3, 5 and 7 W/cm2. The effects of heat flux, flow rates and chip numbers are investigated and empirical correlations are developed for investigated and empirical correlations are developed for individual chips as well as for overall data in the transient regime. The transient correlation recommended is Nul/(Pel)1/3 = 3.5(Fo)1/4.