Optimising pool boiling performance of hybrid nanofluids through desirability function analysis

Abstract

Enhancements in critical heat flux (CHF), heat transfer coefficient (HTC), and other thermal parameters of nanofluids produced during pool boiling of Ag/ZnO hybrid nanoparticles are examined in the current work. We also present a study comparing performance outputs obtained after optimising the conflicting response variables. Copper rods having a cylindrical geometry with 20 mm outer diameter and 35 mm length and their upper face being modified over the die-sink electro-discharge machine (EDM) at various parameter settings were used as a test piece. This process produced test pieces of varying roughness upon which the experimental trials were undertaken. In order to disseminate the hybrid nanoparticles in the base fluid, i.e., DI (de-ionized) water, volume concentrations (ϕ) of 0.02, 0.06, and 0.1% were used to make stable nanofluids utilising a magnetic stirrer and an ultrasonicator. We carried out pool boiling experiments by adapting the Taguchi methodology to design the experiments. Our study demonstrates that the enhancements are a function of surface roughness and nanofluid concentration. The increased concentration of nanoparticles during pool boiling studies raised important metrics like CHF and HTC. Compared to DI water, Ag/ZnO hybrid nanofluids showed excellent gains in HTC (∼250%) and CHF (∼80%) at surface characteristics corresponding to EDM settings of I = 35 A and Ton = 100 µs. The degree of superheat required for boiling incipience contradicted the experimental surface and fluid combination that showed maximum enhancements. As a result, an attempt to optimise the overall boiling performance was undertaken by employing the desirability function analysis (DFA). Considerable improvements were seen in CHF, HTC, and ΔT (Wall superheat) after experimenting with the optimised settings obtained after DFA implementation and determination of the mean of means. No improvements in heat transfer were seen for nanofluid concentrations above ϕ > 0.1% due to the instability of nanofluids above this range.