Ph.D. Thesis Defense by Samuel N. Heffington
Wednesday, December 20, 2000

(Dr. William Black, advisor)

"Development and Analysis of a Vibration-Induced Droplet Atomization Module for High Heat Flux Cooling Applications"

Abstract

 Two-phase cooling of a heated surface with water has been shown to provide high heat transfer rates while maintaining the heated surface temperature around 120ºC.  Heat pipes and thermosyphons are two examples of closed devices that utilize the efficient phase change process to dissipate heat from a heated surface with small areas.  The high heat transfer capability associated with two-phase cooling is a possible solution to many thermal management problems, especially with regards to cooling microelectronic packages.

 A new closed, two-phase heat transfer device similar to a thermosyphon is presented in this experimental investigation.  The basis of this heat transfer cell is vibration induced droplet atomization, or VIDA.  The VIDA process involves placement of a liquid on a thin metal diaphragm that is vibrated by an attached piezoelectric transducer.  The vibrating driver accelerates the liquid on its surface causing the liquid to atomize into smaller secondary droplets.  A VIDA driver is incorporated in the heat transfer cell in order to move the liquid phase to the evaporator section.  The condenser portion of the heat transfer cell is kept cool in order to condense the resulting vapor and return the liquid back to the VIDA driver.  This new device is called a VIDA heat transfer cell.

 In this experimental investigation, several small VIDA heat transfer cells that utilized water as the working fluid were designed and tested.  Prior to constructing a heat transfer cell, the VIDA process was investigated to determine operational characteristics.  The amount of water on the driver, the size of the driver, and the operating frequency of the driver were important parameters that governed the formation of secondary droplets.  In an effort to regulate the VIDA process, two new “VIDA” techniques were discovered and termed the orifice VIDA effect and the fountain effect.  Both new techniques involved generating secondary droplets with the aid of an orifice plate located directly above the driver.  Experiments determined that the hole diameter in the orifice and the thickness of the orifice plate as well as the vertical separation between the driver and the orifice affect the atomization process.

 Five generations of VIDA heat transfer cells were designed and tested.  Each new generation provided valuable experience and information.  The final design was a cell that utilized forced air to cool extended surfaces that were connected to the condensation region within the cell.  The VIDA heat transfer cell was capable of dissipating over 120W while keeping the heated surface temperature below 90ºC.  This level of heat dissipation was higher than an equivalent heat transfer cell that used a copper conductor instead of the VIDA process.  The self-contained, two-phase VIDA heat transfer cells that were tested provided an alternative and efficient means to cool microelectronics.