Effect of ZnO Nanoparticles on the Thermo-Physical Properties and Heat Transfer of Nano- Fluid Flows Asmaa H. Dhiaa * , Marwa A. Salih, Hayder A. Al-Yousefi Chemical Engineering Department, Faculty of Engineering, University of Kufa, Najaf 54002, Iraq Corresponding Author Email: asmaah.alhusseini@uokufa.edu.iq https://doi.org/10.18280/ijht.380316 ABSTRACT Received: 22 February 2020 Accepted: 30 August 2020 In this research, an improvement in alternative heat transfer and thermal properties of ZnO nanoparticles (NPs)/ distilled water, DW nanofluid are experimentally studied and investigated. The two-step approach is utilized to prepare the ZnO NPs/water nanofluid using different concentrations and different inlet temperatures. Scanning Electron Microscopy (SEM) and UV–visible spectrophotometer is employed to characterize the nanofluid. Varying flow rates and the temperatures are examined in the heat exchanger's horizontial tube and shell pipe as well as the impacts of ZnO nanoparticles and the temperatures are studied on nanofluid's thermal conductivity and viscosity. The findings show improvements in the thermal conductivity of ZnO NPs / DW nanofluid. A maximum value of 6.67% of the thermal conductivity is achieved at a temperature of 343K using 1 % ZnO NPs. Nusselt Number also shown an improvement of 38% at a temperature of 343K using 0.2% wt of ZnO NPs under the turbulent condition (Reynolds Number range of 8000 to 20000). Experimental results are compared with previous correlations and an acceptable agreement is observed. Keywords: nanofluid, heat transfer, Nusselt number, ZnO nanoparticles, flow rate, thermal conductivity, viscosity 1. INTRODUCTION To boost the heat transfer in heat exchangers, it is necessary of finding ways to enhance the properties of the fluids. Nanoparticles are used to improve heat transfer by influence the characteristics of traditional fluid such as water and the resulting mixture is called nanofluid. These types of fluids are usually having the combined merits of both base fluid and nanoparticles [1-3]. Therefore, it is very vital to prepare the nanofluids and study its thermal and physical properties due to the limitation of measurement techniques of heat transfer [4]. Several studies were employed with various types of nanoparticles as a passive technique to enhance heat transfer. Kumar et al. [5] investigated the properties of heat transfer and pressure drop of the CuO- Distilled water nanofluid in a horizontal circular pipe were conducted. Their results showed an increase in the characteristics of base fluid after addition to CuO nanoparticles such as thermal conductivity, viscosity, density, and specific heat. The heat transfer coefficient increased to 46.1 % at 0.5 vol. %, 20 LPH (liter per hour) flow rate, and the Friction factor were drops with the rising inflow rate. Xie et al. [6] worked on a range of nanofluid forms including TiO2, ZnO, Al2O3, and MgO in a fluid of 55%water and 45%ethylene glycol. At a steady wall temperature, the laminar stream was flowing into a circular copper pipe. An increase in convective heat transfer at a Reynolds number of 1000 to 252% of MgO has been achieved. Ajeel et al. [7-9] utilized SiO2 nanoparticles to analyze the thermal performance of corrugated channels. The findings highlighted new correlations for used SiO2-water nanofluid in terms of Nusselt number and friction factor. Many researchers utilized ZnONPs in their work with different boundary conditions and applications. Nemade [10] noticed that ZnO nanoparticles with water nanofluid representing a rise in heat transfer at different nanoparticles concentration. in addition, the concluded study that the ratio of thermal conductivity is more significant than unity which made it preferred in heat exchangers applications. An increase to about 136% at 50°C in thermal conductivity was observed and in Nusselt number development for ZnO volume concentration of 0.25% in the nanofluid [11]. Yu et al. [12] prepared nanofluid EG (Ethylene Glycol) / ZnO and recorded an increase in nanofluid thermal conductivity of 26.5% at 5% concentration. Mourgues et al. [13] stated that due to the effect of ZnO nanofluid, the critical heat flux increased by 54%. Jeong et al. [14] analyzed the impact of the shape of ZnO nanoparticles in terms of thermal conductivity and their findings showed that the thermal conductivity increase was 18% and 12% for rectangular and spherical shaped ZnO nanoparticles, respectively. Suganthi et al. [15] prepared and compared two types of nanofluids depend on using ZnO nanoparticles in terms of thermal conductivity. The outcomes uncovered that increase in thermal conductivity was 33.4% and 17.26% when used ZnO particles with Ethylene Glycol (EG) and water-EG as base fluid, respectively. Suganthi and Rajan [16] employed ZnO-propylene glycol nanofluids to examine the heat transfer characteristics. the study said that the heat transfer improvement was 4.24% due to effect 2 vol% of tested nanoparticles. Esfe et al. [17-19] studied rheological behavior of hybrid nanofluid and suggested a new experimental correlation to predict the viscosity of the nanofluid at different temperatures and the solid volume fraction. Recently, Thermo-physical properties measurements, such as thermal conductivity and viscosity, of ZnO in two type propylene glycol (PG) and ethylene glycol (EG) mixed with International Journal of Heat and Technology Vol. 38, No. 3, September, 2020, pp. 715-721 Journal homepage: http://iieta.org/journals/ijht 715