Forced Convective Heat Transfer of Nanofluids in Minichannels

Abstract

Nanofluids are a new class of heat transfer fluids which are engineered by dispersing nanometer-sized metallic or non-metallic solid particles or tubes in conventional heat transfer fluids such as water, ethylene glycol, and engine oil. This is a rapidly emerging interdisciplinary field where nanoscience, nanotechnology, and thermal engineering meet. Since this novel concept of nanofluids was innovated in the mid-last decade (Choi, 1995), this research topic has attracted tremendous interest from researchers worldwide due to their fascinating thermal characteristics and potential applications in numerous important fields such as microelectronics, transportation, and biomedical. With an ever-increasing thermal load due to smaller features of microelectronic devices and more power output, cooling for maintaining desirable performance and durability of such devices is one of the most important technical issues in many high-tech industries. Although increased heat transfer can be achieved creating turbulence, increasing heat transfer surface area and other way, the heat transfer performance will ultimately be limited due to the low thermal properties of these conventional fluids. If extended heating surface is used to obtain high heat transfer, it also undesirably increases the size of the thermal management system. Thus, these conventional cooling techniques are not suitable to meet the cooling demand of these high-tech industries. There was therefore a need for new and efficient heat transfer liquids that can meet the cooling challenges for advanced devices as well as energy conversion-based applications and the innovation of nanofluids has opened the door to meet those cooling challenges. In the field of heat transfer, all liquid coolants currently used at low and moderate temperatures exhibit very poor thermal conductivity and heat storage capacity resulting in their poor convective heat transfer performance. Although thermal conductivity of a fluid plays a vital role in the development of energy-efficient heat transfer equipments and other cooling technologies, the traditional heat transfer fluids possess orders-of-magnitude smaller thermal conductivity than metallic or nonmetallic particles. For example, thermal conductivities of water and engine oil are about 5000 times and 21000 times, respectively smaller than that of multi-walled carbon nanotubes (MWCNT) as shown in Table 1 which provides values of thermal conductivities of various commonly used liquids and nanoparticle materials at room temperature. Therefore, the thermal conductivities of fluids