Kirk L. Yerkes

Micro-Cooler for Chip-Level Temperature Control

The objective of this paper is to design and fabricate a micro-cooler to provide integral cooling to electronics or Microelectromechanical Systems (MEMS) type components utilizing current MEMS technologies. A three-port capillary pumped loop (CPL) was analyzed and fabricated from silicon and quartz for this purpose. An analytical study of the device is presented in support of this design. This proves the feasibility of such a device, and thus the rationale for continuing its development.

Fully developed laminar flow in trapezoidal grooves with shear stress at the liquid–vapor interface

Abstract This paper discusses the behavior of liquid flowing in a groove with a trapezoidal cross-section. For fully developed laminar flow, the conservation of mass and momentum equations reduce to the classic Poisson equation in terms of the liquid velocity. A finite difference solution was employed to determine the mean velocity, volumetric flow rate, and Poiseuille number ( Po = fRe ) as functions of the groove aspect ratio, groove-half angle, meniscus contact angle and imposed shear stress at the liquid–vapor interface. Comparisons with existing solutions for fully developed flow in rectangular ducts and rectangular and triangular grooves are provided. The volumetric flow rate in a groove in which the fill amount varies is discussed. A semi-analytical solution and a two-point numerical solution for the mean velocity in a groove are presented and used to determine the capillary limit for a revolving helically grooved heat pipe. The effects of interfacial shear stress and groove fill ratio on heat pipe performance are investigated.

Design of A Microgravity Spray Cooling Experiment

Abstract : An analytical and experimental study was conducted for the application of spray cooling in a micro gravity and high-g environment. Experiments were carried out aboard the NASA KC-135 reduced gravity aircraft, which provided both the microgravity and high-g environments. In reduced gravity, surface tension flow was observed around the spray nozzle, due to unconstrained liquid in the test chamber and flow reversal at the heat source. A transient analytical model was developed to predict the temperature and the spray heat transfer coefficient within the heated region. Comparison of the experimental transient temperature variation with analytical results showed good agreement for low heat input values. The transient analysis also verified that thermal equilibrium within the heated region could be reached during the 20-25s reduced gravity portion of the flight profile.

LARGE AREA SPRAY COOLING

A multi -nozzle plate with 48 miniature nozzles is designed to generate an array of 4×12 spray cones for the cooling of high power directed energy source components. A closed loop spray cooling test setup with a large cooling area of 19.3 cm 2 is established. The initial thermal performance test is carried out at a spray distance of 10 mm using FC-72 as the working fluid. Spray cooling tests are performed in two orientations of the spray target surface - (a) vertical and (b) horizontal facing downward. In both cases, the spray cone axis remains normal to the cooling surface. Critical heat flux (CHF) is investigated at various spray saturation temperatures and nozzle pressure drops. It is found that the thermal performance of the spray cooling is higher for the horizontal facing downward surface than for the vertical surface. Compared with the previous data for a small cooling surface area of 2.0 cm 2 , the maximum heat transfer coefficient and CHF of the large area (19.3 cm 2 ) spray cooling are lower by around 30% and 34% respectively.

The Effects of Transverse Acceleration-Induced Body Forces on the Capillary Limit of Helically Grooved Heat Pipes

A helically grooved copper heat pipe with ethanol as the working fluid has been fabricated and tested on a centrifuge table. The heat pipe was bent to match the radius of curvature of the table so that uniform transverse (perpendicular to the axis of the heat pipe) body force fields could be applied along the entire length of the pipe. By varying the heat input (Q in = 25 to 250 W) and centrifuge table velocity (radial acceleration |a, = 0 to 10g), information on dryout phenomena, circumferential temperature uniformity, heat lost to the environment, thermal resistance, and the capillary limit to heat transport was obtained. Due to the geometry of the helical grooves, the capillary limit increased by a factor of five when the radial acceleration increased from |a r | = 0 to 6.0g. This important result was verified by a mathematical model of the heat pipe system, wherein the capillary limit to heat transport of each groove was calculated in terms of centrifuge table angular velocity, the geometry of the heat pipe and the grooves (including helix pitch), and temperature-dependent working fluid properties. In addition, a qualitative study was executed with a copper-ethanol heat pipe with straight axial grooves. This experimental study showed that the performance of the heat pipe with straight grooves was not improved when the radial acceleration was increased from |a r | = 0 to 10.0g.

Demonstration of a micro-CPL based on MEMS fabrication technologies

Utilizing current micro electro mechanical systems (MEMS) technologies, a three-port microcapillary pumped loop (micro-CPL) was designed, fabricated and tested to provide integral cooling to electronics or MEMS type devices. The two wafer design consists of one silicon and one borofloat glass wafer. An analytical study was used in determining the geometry of the device, including the evaporator dimensions (1000 /spl mu/m/spl times/2000 /spl mu/m) and the length of the liquid and vapor lines (35 mm). Using laser spot heating, the finished device was run near steady-state. It was determined that a laser delivering 8.5 W (+/-0.2 W) with a spotsize diameter of 3.5 mm caused dry-out of the micro-CPL.

Influence of the Coulomb Force on Spray Cooling

Effects of the Coulomb electrical body force on heat transfer performance of an instrumented spray cooling experiment are reported. Heat transfer performance is documented for a range of spray volume flow rates and heater power levels using the dielectric liquids, FC‐72 and HFE‐7000, sprayed onto a Thick Film Resistor (TFR) heater; along with flow visualization results using a transparent Indium‐Tin Oxide (ITO) heater. Two Coulomb force electrode geometries show modest but consistent improvements in heat transfer (order of 5–15%), but only at heat fluxes where boiling of the liquid film occurs. Flow visualization shows a highly contorted liquid film forming on the heater surface. These flow visualization results are used to aid in the estimation of characteristic time scales governing the effects of surface tension, gravity, heating of the liquid film, and vaporization of the film. For the present dense liquid sprays, it is concluded that none of these time scales are as short as the average time between ...

Actively pumped two-phase loop for spray cooling

A new closed two-phase loop that combines with a large area spray cooling unit for the cooling of high heat-flux power sources is developed. The fluid circulation is sustained by a magnetic gear pump operating with an ejector unit. The motive flow of the ejector shares the pumping liquid flow with the multinozzle spray. With the assistance of the ejector, the maximum spray pressure drop across the nozzle can be enhanced by at least 0.56 bar at critical heat fluxes (CHF). This increases CHF of the spray cooling by up to 16%. More importantly, the use of the ejector prevents the uncondensed vapor from entering the magnetic gear pump and stabilizes the circulation of the two-phase flow. During the experiment, a multinozzle assembly with 48 miniature nozzles is employed. The target spray cooling area is 19.3 cm 2 . FC-72 and water are used as the working fluid. The present design concept can be applied to cooling systems operating in the aerospace environment.

Fully-Developed Laminar Flow in Sinusoidal Grooves

The flow of a constant property fluid through a sinusoidal groove has been analyzed. A numerical solution of the conservation of mass and momentum equations for fully developed flow is presented. The mean velocity, volumetric flow rate, and Poiseuille number are presented as functions of the groove geometry, meniscus contact angle, and shear stress at the liquid-vapor interface

Energy Transport in Closed Quantum Systems

We examine energy transport in an ensemble of closed quantum systems driven by stochastic perturbations. One can show that the probability and energy fluxes can be described in terms of quantum advection modes (QAMs) associated with the off-diagonal elements of the density matrix. These QAMs play the role of Landauer channels in a system with discrete energy spectrum and the eigenfunctions that cannot be described as plane waves. In order to determine the type of correlations that exist between the direction and magnitudes of each QAM and the average direction of energy and probability fluxes we have numerically solved the time-dependent Schrödinger equation describing a single particle trapped in a parabolic potential well which is perturbed by stochastic ripples. The ripples serve as a localized energy source and are offset to one side of the potential well. As the result a nonzero net energy flux flows from one part of the potential well to another across the symmetry center of the potential. We find that some modes exhibit positive correlation with the direction of the energy flow. Other modes, that carry a smaller energy per unit of the probability flux, anticorrelate with the energy flow and thus provide a backflow of the probability. The overall picture of energy transport that emerges from our results is very different from the conventional one based on a system with continuous energy spectrum.