Stephen A. Solovitz

Manifold Design for Micro-Channel Cooling With Uniform Flow Distribution

High-power electronic systems often require temperature uniformity for optimal performance. While many advanced cooling systems, such as micro-channels, result in significant heat removal, they are also susceptible to flow mal-distribution that can impact the local temperature variation on a device. By examining the pressure drops through each flow path in a multi-channel cooling system, an analytical model is predicted for the optimal manifold shape to produce uniform velocities. This is a simple power law, whose exponent depends on the flow regime in the manifold passages. The model is validated for laminar fully developed conditions using a series of computational simulations. With the power law design, the speeds in a parallel channel design are uniformly distributed at low Reynolds numbers, with a standard deviation of less than 3% of the overall mean channel speed. At higher Reynolds numbers, some mal-distribution is observed due to developing flow conditions, but it is not as significant as with typical untapered designs.

Integral micro-channel liquid cooling for power electronics

A novel integral micro-channel heat sink was developed, featuring an array of sub-millimeter channels fabricated directly in the back-metallization layer of the direct bond copper or active metal braze ceramic substrate, thus minimizing the material between the semiconductor junction and fluid and the overall junction-to-fluid thermal resistance. The ceramic substrate is bonded to a baseplate that includes a set of interleaved inlet and outlet manifolds for uniform fluid distribution across the actively cooled area of the heat sink. The interleaved manifolds greatly reduce the pressure drop and minimize temperature gradient across the heat sink surface. After performing detailed simulations and design optimization, a 200 A, 1200 V IGBT power module with the integral heat sink was fabricated and tested. The junction-to-fluid thermal resistivities for the IGBTs and diodes were 0.17°C⋆cm2/W and 0.14°C⋆cm2/W, respectively. The design is superior to all reported liquid cooled heat sinks with a comparable material system, including the micro-channel designs. It is also easily scaleable to larger heat sink surfaces without compromising the performance.

Micro-channel thermal management of high power devices

Heat fluxes in semiconductor power devices have been steadily increasing over the past two decades, now approaching 500 W/cm2 . This dissipation requires advanced thermal management in order to maintain device maximum junction temperatures below the Si limit of 150degC. Micro-channel cooling shows great promise for high heat flux removal, with the potential for greater than 750 W/cm2 performance. As flow passages decrease in size to sub-millimeter scales, the surface area-to-volume ratio increases, allowing greater potential heat transfer area. However, the correspondingly higher pressure losses across the channel can quickly exceed the maximum pump performance at these small dimensions. A novel micro-channel heat sink was developed, featuring micro-channel passages fabricated directly into the active metal braze (AMB) substrate, minimizing the junction-to-fluid thermal conduction resistance. The heat sink performance was simulated using computational fluid dynamics models and the results show that heat fluxes above 500 W/cm2 could be achieved for a 50degC device junction-to-coolant temperature rise. The heat sink was fabricated and tested using an array of power diodes, and infrared thermography measurements validated the simulation results. The demonstrated thermal performance is superior to any existing micro-channel heat sink with a comparable electrical assembly

Dynamic Flow Response Due to Motion of Partial-Span Gurney-Type Flaps

Uninhabited air vehicles are commonly designed with high-aspect-ratio wings, which can be susceptible to significant aeroelastic vibrations. These oscillations can result in a loss of control or structural failure, and new techniques are necessary to alleviate them. A multidisciplinary effort at Stanford developed a distributed flow control method that used small trailing-edge actuators, known as micro-trailing-edge effectors (MiTEs) to alter the aerodynamic loads at specific spanwise locations along an airplane wing. The actuators were based on a Gurney flap, which is a trailing-edge flap of small size and large deflection, allowing an increase in lift with a small drag penalty. The transient response caused by relatively rapid MiTE actuation was studied using particle image velocimetry in the near wake. The transient response was quasi-steady for dimensionless actuation times (tU∞/c) near unity. A shorter dimensionless actuation time of 0.2 produced a transient response with significant overshoot of the downwash velocity in the near wake. This indicated a nonmonotonic response of the aerodynamic loads for rapid actuation.

Spanwise response variation for partial-span Gurney-type flaps

Uninhabited air vehicles with high-aspect-ratio wings can experience substantial aeroelastic modes, which can adversely affect performance. An active, distributed control system is being developed to alleviate these vibrations, using a series of small span trailing-edge actuators to alter the aerodynamic loads at specific spanwise locations. Particle image velocimetry was used to determine the spanwise effects of these devices because it is necessary to understand whether their application will have nonlocal effects. The wake structure indicates that the primary influence is confined to within two actuator spans of the applied device, demonstrating that the response is quite localized.

Experimental Study of Near-Field Entrainment of Moderately Overpressured Jets

Particle image velocimetry (PIV) experiments have been conducted to study the velocity flow fields in the developing flow region of high-speed jets. These velocity distributions were examined to determine the entrained mass flow over a range of geometric and flow conditions, including overpressured cases up to an overpressure ratio of 2.83. In the region near the jet exit, all measured flows exhibited the same entrainment up until the location of the first shock when overpressured. Beyond this location, the entrainment was reduced with increasing overpressure ratio, falling to approximately 60% of the magnitudes seen when subsonic. Since entrainment ratios based on lower speed, subsonic results are typically used in one-dimensional volcanological models of plume development, the current analytical methods will underestimate the likelihood of column collapse. In addition, the concept of the entrainment ratio normalization is examined in detail, as several key assumptions in this methodology do not apply when overpressured.

Computational Optimization of a Groove-Enhanced Minichannel

To meet the challenging thermal requirements of modern power electronics, surface enhancements are being considered to improve the performance of single-phase microchannels. A simple modification of a surface dimple—a two-dimensional groove—is considered due to its significant potential enhancement yet simple manufacture. A comprehensive series of simulations were performed to determine the optimal geometry and Reynolds numbers for thermal performance, which indicated that moderately deep features could result in nearly 75% greater average convective heat transfer coefficient with approximately 35% greater pressure penalty at turbulent conditions. This geometry promoted freestream separation at the leading edge, which resulted in downstream impingement and boundary layer development, producing an overall thermal benefit. This potential performance was demonstrated experimentally using a groove-enhanced minichannel design in a typical heat-sink application. Infrared thermography showed that deep grooves could indeed enhance the overall heat transfer coefficient of this module by the order of 20%, although shallower shapes had less impact.

Controls on the breach geometry and flood hydrograph during overtopping of noncohesive earthen dams

Overtopping failure of noncohesive earthen dams was investigated in 13 large-scale experiments with dams built of compacted, damp, fine-grained sand. Breaching was initiated by cutting a notch across the dam crest and allowing water escaping from a finite upstream reservoir to form its own channel. The channel developed a stepped profile, and upstream migration of the steps, which coalesced into a headcut, led to the establishment of hydraulic control (critical flow) at the channel head, or breach crest, an arcuate erosional feature that functions hydraulically as a weir. Novel photogrammetric methods, along with underwater videography, revealed that the retreating headcut maintained a slope near the angle of friction of the sand, while the cross section at the breach crest maintained a geometrically similar shape through time. That cross-sectional shape was nearly unaffected by slope failures, contrary to the assumption in many models of dam breaching. Flood hydrographs were quite reproducible—for sets of dams ranging in height from 0.55 m to 0.98 m—when the time datum was chosen as the time that the migrating headcut intersected the breach crest. Peak discharge increased almost linearly as a function of initial dam height. Early-time variability between flood hydrographs for nominally identical dams is probably a reflection of subtle experiment-to-experiment differences in groundwater hydrology and the interaction between surface water and groundwater.

Design of a microfluidic device with a non-traditional flow profile for on-chip damage to zebrafish sensory cells

Hearing loss affects millions of people worldwide and often results from the death of the sensory hair cells in the inner ear, and exposure to intense noise is one of the leading causes of hair cell damage. Recently, the zebrafish lateral line system has emerged as a powerful in vivo model for real-time studies of hair cell damage and protection. In this research, we designed a microfluidic device for inducing noise damage in hair cells of the zebrafish lateral line. As the first step, a 3D computational fluid dynamics (CFD) simulation was utilized to predict the flow pattern inside the device. An ideal flow pattern for our application should feature higher velocity near the sidewalls to over-stimulate the externally located hair cells, and minimum flow in the middle of the channel to protect the fish from high pressure on the head. Flow induced from ordinary channel geometry with a single inlet/outlet pair would not work because the parabolic velocity profile features the maximum flow speed in the middle of the channel. In order to achieve the desired flow pattern, sidewall inlet/outlet pairs were used to suppress the growth of boundary layers. CFD simulation was used to design parameters such as the dimensions of the microfluidic channel and the angle of the inlets and outlets. It was found that in the case of an empty 2.0?mm wide channel with the inlet/outlet pairs set to 45?, the flow velocity at the side of the channel would be 6.7?times faster than the velocity in the middle, approaching the optimal flow characteristics. In the case of a fish-loaded channel, simulation shows that a 1.0?mm wide channel with a 60? inlet/outlet angle creates the lowest pressure (0.3 Pa) on the fish head while maintaining a reasonably strong shear stress (1.9 Pa) on the lateral line hair cells.

Coupled fluid and solid evolution in analogue volcanic vents

Volcanic eruptions emit rock particulates and gases at high speed and pressure, which change the shape of the surrounding rock. Simplified analytical solutions, field studies, and numerical models suggest that this process plays an important role in the behavior and hazards associated with explosive volcanic eruptions. Here we present results from a newly developed laboratory-scale apparatus designed to study this coupled process. The experiments used compressed air jets expanding into the laboratory through fabricated rock analogue material, which evolves through time during the experiment. The compressed air was injected at approximately 2.5 times atmospheric pressure. We fabricated rock analogues from sand and steel powder samples with a three-dimensional printing process. We studied the fluid development using phase-locked particle image velocimetry, while simultaneously observing the solid development via a video camera. We found that the fluid response was much more rapid than that of the solid, permitting a quasi-steady approximation. In most cases, the solid vent flared out rapidly, increasing its diameter by 20 to 100%. After the initial expansion, the vent and flow field achieved a near-steady condition for a long duration. The new expanded vent shapes permitted lower vent exit pressures and larger jet radii. In one experiment, after an initial vent shape development and establishment of steady flow behavior, rock failure occurred a second time, resulting in a new exit diameter and new steady state. This second failure was not precipitated by changes in the nozzle flow condition, and it radically changed the downstream flow dynamics. This experiment suggests that the brittle nature of volcanic host rock enables sudden vent expansion in the middle of an eruption without requiring a change in the conduit flow.