An Analysis Of The Effective Thermal Conductivity For Nanofluids Based On Micro-Mixing Convective Conduction

"Nanofluids" with excellent heat transfer performance is attached much attention to by scientists and engineers all over the world since the 1990s.Compared with traditional micro-fluids in two phase, nanofluids with less size particles not only does better in heat transferring but also avoids jamming the pipe with rare friction with the surface of pipe and less deposition. So nanofluids own more potential to be the next generation refrigerant from the point of energy-saving.This paper refers to systemic information about nanofluids and analyzes former and up to date development of research home and aboard. On the basis of those, we stress the analysis of primary facts which may affect thermal conduction of nanofluids, and propose our model.On the basis of the role of interfacial thickness and kapitza resistance, we further analyze the effect of micro-mixing convection-conduction caused by Brownian movement of nanoparticles on the adjacent liquid area, and rationally introduce the characteristic ratio of size of two components of nanofluids in order to exactly depict the limited range of the effect of micro-mixing convection-conduction. The effective thermal conductivity of naofluids in our proposed model is expressed as a function of average size,volume fraction and thermal conductivity of nanoparticles, the size,thermal conductivity,dynamic viscosity,mean free path,specific heat in constant pressure of liquid, the kapitza resistance and thickness of interfacial layer, and temperature of system. This model comprehensively describes all primary factors which may affect the effective thermal conductivity of nanofluids.With the analysis of our model, conclusions are gotten as fellows:First, the effective thermal conductivity of nanofluids apparently increases with the increase of volume fraction of nanoparticles, but this trend is nonlinear;Second, the effective thermal conductivity of nanofluids notably increases with the decrease of size of nanoparticles, however, according to the graph of the effective thermal conductivity of nanofluids versus size of nanoparticles, we find a distinct critical dimension of nanoparticles. When the size of nanofluids is much less than the critical size, the effective thermal conductivity of nanofluids notably increases with the decrease of size of nanoparticles; however, when the size of nanofluids is much more than the critical size, the effective thermal conductivity of nanofluids nearly keeps unaltered with the increase of size of nanoparticles and can be described by Maxwell model and H-C model which can well describe the thermal conductivity of micro-fluids in two phase.Third, when the size of nanoparticles is less than the range of characteristic size, the effective thermal conductivity of nanofluids decreases with increase of thickness of interfacial layer. This abnormal conclusion looks like to be beyond our mind, but it confirms our existing knowledge in reality. This conclusion reveals both that the thermal conductivity of material decreases with the rapidly decrease of size of material and that the Kapitza resistance will play a more important role in the improvement of thermal conductivity of nanofluids with the decrease of size of nanoparticles because the dispersion of phonon will intensify at interfacial layer.Fourth, the effective thermal conductivity of nanofluids evidently increases with increase of temperature of nanofluids, and this trend is nonlinear.Good agreement between predictions from our model and experimental data confirms both the truth of former conclusions and the validity of our model in interpreting anomalously improved thermal conductivity of nanofluids.