Experimental Investigation of Free Convection Heat Transfer from Horizontal Cylinder to Nanofluids

Abstract

The results of free convection heat transfer investigation from a horizontal, uniformly heated tube immersed in a nanofluid are presented. Experiments were performed with five base fluids, i.e., ethylene glycol (EG), distilled water (W) and the mixtures of EG and water with the ratios of 60/40, 50/50, 40/60 by volume, so the Rayleigh (Ra) number range was 3 × 104 ≤ Ra ≤ 1.3 × 106 and the Prandtl (Pr) number varied from 4.4 to 176. Alumina (Al2O3) nanoparticles were tested at the mass concentrations of 0.01, 0.1 and 1%. Enhancement as well as deterioration of heat transfer performance compared to the base fluids were detected depending on the composition of the nanofluid. Based on the experimental results obtained, a correlation equation that describes the dependence of the average Nusselt (Nu) number on the Ra number, Pr number and concentration of nanoparticles is proposed.

Effect of Temperature and Nanoparticle Concentration on Free Convective Heat Transfer of Nanofluids

“…which is valid for 10 ≤ Ra ≤ 10 6 and (0% < ϕ v < 20%). Sawicka et al [36] proposed an experimentally developed correlation equation for free convection heat transfer of water-Al 2 O 3 , EG-Al 2 O 3 , and water/EG-Al 2 O 3 nanofluids from an isolated, horizontal cylinder in the form:…”

“…In this theoretical study, the same base fluids and nanoparticles were considered as used in [36], where distilled, deionized water and EG served as base fluids, and Al 2 O 3 were used as nanoparticles. Alumina nanoparticles were examined at the concentration of 0.01%, 0.1%, and 1% by weight.…”

“…Figure 8 illustrates the change of Ra number versus temperature for the tested nanofluids. In this case the temperature difference and the diameter of the horizontal cylinder are held constant at the values taken from the experiment presented in [36]-∆T = 10 K, D = 10 mm. The Ra number was a function of all discussed thermophysical properties; however, the most significant parameters were viscosity and thermal expansion coefficient in the case of water based nanofluids (Figure 5a).…”

“…As seen in Figure 10, the influence of nanoparticles on the Nu number for the tested mass concentration range 0.01%≤ ϕ m ≤ 1% was negligible for both tested Ra numbers. Using Equation ( 4) and the experimental data presented in [36] (D = 10 mm, ∆T = 10 K), the convective heat transfer coefficient was calculated for the water-Al 2 O 3 and EG-Al 2 O 3 nanofluids. The base fluid heat transfer coefficient was obtained from the Churchill and Chu correlation (Equation ( 1)).…”

“…In the present study, free convection heat transfer from a horizontal tube immersed in an unbounded nanofluid was analyzed. Alumina nanoparticles (Al 2 O 3 ) were considered at the concentration of 0.01%, 0.1%, and 1% by weight within the temperature range 20-40 • C. The range of the considered parameters was selected the same as in experimental study presented in [36]; therefore, a comparison of the calculated values of Nu numbers and heat transfer coefficients with experimental results was conducted.…”