An experimental study on optimum concentration of silver-water microfluid for enhancing heat transfer performance of a plate heat exchanger

Abstract

Abstract The present work investigates the effects of different concentrations of silver-water microfluid on the thermophysical properties, Brownian motion and heat transfer performance of a plate heat exchanger. For this purpose, an experimental system which included a CR14-45 COMER plate heat exchanger, five pt100 platinum resistance temperature sensors with temperature control system, two rotameters for flow rate indication and control, an insulated reservoir tank with two immersed heaters in it and a stainless steel centrifugal pump for circulating fluid was prepared. In the next step, silver microparticles were synthesized from AgNO3 and then 8 L of silver-water microfluid was prepared with different concentrations in the range of 0–0.125 wt% and their thermal conductivity, heat capacity and temperature difference rate were experimentally gauged. The results indicated that all the concentrations of silver-water microfluid (in the range of 0–0.125 wt%) enhance thermal conductivity and heat transfer rate in comparison with base fluid (pure water). However, there is an optimum concentration for microfluid (0.03 wt%) in which the rate of heat transfer reaches its maximum value. In this case, the heat transfer rate of microfluid is 9% higher than the base fluid. At high microparticles concentrations, due to coagulation effect, some of the silver particles quickly stick together and build deposits on the surface of the plate heat exchanger. Consequently, the concentration of the particles in the microfluid decreases and the enhancement of thermal conductivity (1.25% for 0.125 wt%) lowers in comparison with the optimum state. Furthermore, at concentrations beyond the optimum, the slope of temperature difference rate in microfluid is negative and, therefore, the Brownian motion and particles movement diminish and the silver particles tend to sediment. Microparticles deposited on the plate heat exchanger surface contribute to additional heat transfer resistance and decrease the heat transfer coefficients in comparison with the optimum state.