Introduction

Background

Objectives


Test facilities

Diagnostic techniques







 

 
Boiling flow in microchannels

micro nano engineering
   

  Background

Micro-channel heat sinks with phase change are particularly promising because they allow achieve very low thermal resistance with a minimal volume. However, the design and optimization of two-phase micro-channel heat sinks are complicated by the distributions of pressure and saturation temperature along the channel, particularly for the case of no uniform heat generation in the chip. One of the biggest fundamental challenges is the huge temperature and pressure oscillations: temperature fluctuations must be controlled to consider two-phase micro-channels as a serious technology; poor flow distribution in two-phase micro-channels might lead to less flow in the regions of hot spots, leading to localized dry out, which will result in a large and rapid increase in the temperature of the hot spot.

Another big challenge for micro-channel cooling technology is that the coolant is also used as the lubricant for pump bearings, because the pump has to be hermetically sealed. While from a lubrication point of view, a liquid with high viscosity is preferable, from a pressure drop point of view, a liquid with low viscosity is desired. These are opposing requirements.

The topography of the surface and properties of the tertiary system liquid-surface-vapor modify the wetting characteristics and determine the heat transfer mechanism. Moreover, nanotechnology works at the atomic and molecular and supra-molecular levels in order to create materials, devices and systems with fundamentally new properties and functions because of their small structure. Liquids containing nanometer sized metallic or non-metallic solid particles, show an increase in thermal conductivity compared to that of the base liquid. Compared to ordinary fluids containing micro-sized particles, nanofluids are more stable, do not clog flow channels and also show a considerable increase in thermal conductivity for very small volume fraction of solid particle.

Also, nanotechnologies can play a major role in improving heat transfer rates at micro-scale, causing the formation of cold spots, which are able to transmit a significant percentage of the total heat transfer. Nanopatterning of the surface becomes a tool to overcome operational challenges required to consider two-phase micro-channels as a serious technology: the need for low pressure to match the saturation temperature necessary for cooling; unstable operation due to rapid bubble expansion and occasional flow reversal; low critical heat fluxes caused by flow instability.