مطالعه آزمایشی درباره افزایش انتقال حرارت با استفاده از نانوسیال ها در مبدل گرمایی دو لوله ای Experimental study of heat transfer enhancement using nanofluid in double tube heat exchanger

1. Introduction It is well known fact that solids have orders of magnitude higher thermal conductivity at room temperature than those of fluids. Thermal conductivity of copper is 700 times greater than that of water at room temperature. So it is well understood that by dispersing these solid particles in fluid thermal conductivity of fluid can be expected to increase significantly. Furthermore compared with microsize particles, nanoparticles have much larger surface area. The surface to volume ration is much higher thus enhances the thermal conductivity at much higher scale. Measurement of thermal conductivity started with oxide nanoparticles (Masuda et al[1],a Lee [2]) but nanofluids did not gain much attention until Eastman et al [3]showed for the first time that the copper nanoparticles have more significant effect compared with oxide based nanofluids. Many different nanomaterial particles are used in making nanofluids such as Al2O3, CuO, TiO2, Cu, Fe and now even carbon nanotubes and graphenes based structures. Over the previous decade many experimental studied have been conducted to investigate the possible effect and underlying mechanism of increasing thermal conductivity in convective heat transfer due to the use of nanoparticles in different base fluids. Pak and Cho[4] investigated Al2O3 and TiO2 nanoparticles having average size of 30nm in their studies to find out that Nu is 30% larger than base fluid and higher than predicted by Dittus Boelter equation. Li and Xuan [5] conducted there experiment using Cu nanoparticles with diameter less then 100nm to investigate the effect of nanoparticles in turbulent regime heat transfer. They showed that average increase of Nu is much larger than predicted by DB equation. Wen and Ding[6] used Al2O3 nanoparticles in laminar flow in a tube to study the effect of convective heat transfer with average particle size of 30 to 50 nm. They found out that Nu is greater than 4.36 for fully developed flow with constant wall heat flux. Ding [7] again in his study showed that using carbon nanotubes with rod like structure having average size greater than 100nm has a profound effect on convective heat transfer. Nu increase by more than 300% at Re =8000 that is in laminar flow regime. Heris et al[8] used 20nm size Al2O3 nanoparticles in water to experimentally investigate the convective heat transfer in laminar flow. His study showed that Nu measured is larger than that of pure water. Williams [9] experimentally studied the effect of increasing heat transfer due to use of Al2O3 and ZrO2 nanoparticles of size 46nm and 60nm respectively. In his studies he found no significant improvement in heat transfer and concluded that any traditional correlation can predict nanofluids heat transfer. Kolade et al [10] used 40nm to 50nm Al2O3 nanoparticles in his studies to investigate the effect of heat transfer and found an apparent increase in Nu compared to basefluid.