Deborah V. Pence

Reduced pumping power and wall temperature in microchannel heat sinks with fractal-like branching channel networks

Comparisons are made of the maximum channel wall temperature along, and total pressure drop across, a heat sink with a fractal-like branching channel network with those in a heat sink having a straight channel array. The total channel lengths are identical between the heat sinks, as are the applied heat fluxes. The hydraulic diameter of the straight channel array is equal to that of the terminal branch of the branching channel network. The number of branches per level, number of branching levels, and channel dimensions in the fractal-like network remain fixed. Minor losses are neglected and both hydrodynamic and thermal boundary layers are assumed to reinitiate following each channel bifurcation in the branching flow network. With identical total convective surface areas for both configurations and maintaining a heat sink surface area equal to that of the convective surface area, the fractal-like channel network yielded a 60% lower pressure drop for the same total flow rate and a 30°C lower wall temperature under identical pumping power conditions. The two heat sinks were also compared under identical pressure drop conditions. Channel packing densities in which the convective surface area in the fractal-like channel network is 50% less than that in the straight channel array yield approximately the same pressure drop and maximum wall temperature for fixed-flow-rate conditions.

Fluid Flow Through Microscale Fractal-Like Branching Channel Networks

Flow through fractal-like branching networks is investigated using a three-dimensional computational fluid dynamics approach. Results are used to assess the validity of and provide insight for improving, assumptions imposed in a previously developed one-dimensional model. Assumptions in the one-dimensional model include (1) reinitiating boundary layers following each bifurcation, (2) constant thermophysical fluid properties, and (3) negligible minor losses at the bifurcations. No changes to the redevelopment of hydrodynamic boundary layers following a bifurcation are recommended

Thermal Characteristics of Microscale Fractal-Like Branching Channels

Heat transfer through a fractal-like branching flow network is investigated using a three-dimensional computational fluid dynamics approach. Results are used for the purpose of assessing the validity of and providing insight for improving, assumptions imposed in a previously developed one-dimensional model for predicting wall temperature distributions through fractal-like flow networks. As currently modeled, the one-dimensional code fairly well predicts the general wall temperature trend simulated by the three-dimensional model; hence, demonstrating its suitability as a tool for design of fractal-like flow networks. Due to the asymmetry in the branching flow network, wall temperature distributions for the proposed branching flow network are found to vary with flow path and between the various walls forming the channel network. Three-dimensional temperature distributions along the various walls in the branching channel network are compared to those along a straight channel. Surface temperature distributions on a heat sink with a branching flow network and a heat sink with a series of straight, parallel channels are also analyzed and compared

Adiabatic Flow Boiling in Fractal-Like Microchannels

Fractal-like branching channels are proposed for a number of microscale applications, including heat sinks, heat exchangers, absorbers, desorbers, and micro-mixers. Based on model predictions, the benefit of fractal-like channel designs is a lower pressure drop than parallel straight channels for a given flow rate, when compared to an equal channel surface area basis with the terminal channel cross-section of the fractal-like network used to define the parallel channel geometry. The fractal-like flow networks are a unique geometry that follows fractal bifurcation patterns, in this case mimicking the flow patterns found in nature. Two-phase flow applications require an understanding of how the geometric constraints impact the flow characteristics during multiphase flow. One-dimensional modeling predictions are used in this study to asses the relative impact of flow network designs on pressure drop and void fraction distributions for adiabatic flow boiling. The characterization of the flow networks includes a specified branching ratio of channel length and channel width (or diameter) and also the number of branching levels, or bifurcations, in a given length. The goal of the present study is to identify the adiabatic boiling characteristics within the fractal-like flow network and compare results to straight parallel channels. The model used is a compilation of two-phase flow models presented in the literature but modified to include a local two-phase flow parameter, flow re-development, as well as variable property effects. Results are compared with straight channels based on flow boiling conditions, pressure drop, and vapor quality distributions for a range of flow rates and subcooling.

Simulation of Compressible Micro-Scale Jet Impingement Heat Transfer

A computational study is presented of the heat transfer performance of a micro-scale, axisymmetric, confined jet impinging on a flat surface with an embedded uniform heat flux disk. The jet flow occurs at large, subsonic Mach numbers (0.2 to 0.8) and low Reynolds numbers (419 to 1782) at two impingement distances. The flow is characterized by a Knudsen number of 0.01, based on the viscous boundary layer thickness, which is large enough to warrant consideration of slip-flow boundary conditions along the impingement surface. The effects of Mach number, compressibility, and slip-flow on heat transfer are presented. The local Nusselt number distributions are shown along with the velocity, pressure, density and temperature fields near the impingement surface

Void Fraction Variations in a Fractal-Like Branching Microchannel Network

Based on predictions of lower pressure drop penalties in fractal-like branching channels compared to parallel channels, an experimental investigation of two-phase void fraction variations was performed. The flow network, mimicking flow networks found in nature, was designed with a self-similar bifurcating channel configuration and etched 150 μ m into a 38.1 mm diameter silicon disk. A Pyrex® cover was anodically bonded to the silicon disk to allow for flow visualization. The length and width scale ratios between channels on either side of a bifurcation are fixed. The channel widths range in size from 100 μ m to 400 μm over a total channel length of approximately 17 mm. Experimental results of flow boiling are presented for a heater energy input power of 66 W and an inlet water flow rate of 45 g/min at a fixed inlet fluid temperature of 88°C. High-speed, high-resolution imaging was used to visualize the flow and quantify void fraction values in several channels within a branching structure. Both time-averaged and instantaneous two-dimensional void fraction data are presented, showing a correlation between channels at the same bifurcation level and between channels at different bifurcation levels.

Performance of a Smart Direct Fire Projectile Using a Ram Air Control Mechanism

The effectiveness of a direct fire penetrator projectile equipped with an actively controlled ram air actuation mechanism is investigated through dynamic simulation. The ram air control mechanism consists of a rotary sleeve valve which directs airflow from an inlet in the center of the nose to side ports. The coupled dynamics of the projectile, inertial measurement unit, and flight control system are included in the system model. This work shows that a ram air control mechanism provides sufficient control authority to significantly reduce dispersion of a direct fire penetrator, even in the presence of moderate levels of sensor bias and noise.

Droplet impingement dynamics: effect of surface temperature during boiling and non-boiling conditions

This study investigates the hydrodynamic characteristics of droplet impingement on heated surfaces and compares the effect of surface temperature when using water and a nanofluid on a polished and nanostructured surface. Results are obtained for an impact Reynolds number and Weber number of approximately 1700 and 25, respectively. Three discs are used: polished silicon, nanostructured porous silicon and gold-coated polished silicon. Seven surface temperatures, including single-phase (non-boiling) and two-phase (boiling) conditions, are included. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. Results of water and a water-based single-wall carbon-nanotube nanofluid impinging on a polished silicon surface are compared to determine the effects of nanoparticles on impinging dynamics. The nanofluid results in larger spreading velocities, larger spreading diameters and an increase in early-stage dynamic contact angle. Results of water impinging on both polished silicon and nanostructured silicon show that the nanostructured surface enhances the heat transfer for evaporative cooling at lower surface temperatures, which is indicated by a shorter evaporation time. Using a nanofluid or a nanostructured surface can reduce the total evaporation time up to 20% and 37%, respectively. Experimental data are compared with models that predict dynamic contact angle and non-dimensional maximum spreading diameter. Results show that the molecular-kinetic theorys dynamic contact angle model agrees well with current experimental data for later times, but over-predicts at early times. Predictions of maximum spreading diameter based on surface energy analyses indicate that these models over-predict unless empirical coefficients are adjusted to fit the test conditions. This is a consequence of underestimates of the dissipative energy for the conditions studied.

LAMINATE MIXING IN MICROSCALE FRACTAL-LIKE MERGING CHANNEL NETWORKS

A two-dimensional model was developed to predict concentration profiles and degree of mixing resulting from diffusion across laminate fluid layers within a fractal-like merging flow network. Numerical results were compared with concentration profiles experimentally acquired using laser-induced fluorescence. Although the numerical model well predicts DoM for Reynolds number less than or equal to 5, for higher Reynolds numbers the three-dimensionality of the experimental flow yields a higher degree of mixing than predicted by the model. The degree of mixing was studied for various geometries, and a nondimensional design parameter was developed to find an optimum number of branching levels for a given channel depth, final width, and total length.

Flow development of co-flowing streams in rectangular micro-channels

The diffusion and flow development characteristics of two co-flowing, laminar streams in a high aspect ratio rectangular micro-channel have been examined. A long, thin splitter plate initially separates the two streams such that fully developed flow in each of the two channels is established prior to merging. The co-flowing micro-channel has an aspect ratio of 16 with a width of 1006 μm and a height of 63 μm. Micro-Particle Image Velocimetry (μPIV) was utilized to observe the interaction between the streams for a range of flow rate ratios ranging from one to nine, for Reynolds numbers of one and ten. For flow rate ratios greater than one, a cross-stream pressure gradient exists immediately downstream of the splitter plate, which results in a strong lateral flow of the faster moving fluid into the slower moving fluid. Despite this rapid expansion, the fluids in the two streams do not mix. The two streams eventually recover a fully developed velocity profile across the entire channel. A model is presented to predict this development length based on the pressure imbalance between the two streams. The model is expressed in terms of the flow rate ratio between the streams, which is shown to be a function of channel aspect ratio. An asymptotic condition for the development length is found for high flow rate ratios and high aspect ratio channels. It is shown that existing entrance length relationships greatly underpredict this development length.