Influential cooling of the free surface by jet impingement of aqueous nanodispersion dominant with hybridized nanoparticles

Abstract

The sustenance of industries depends on implementing energy conservation and sustainability concepts. They partially or wholly rely on thermal energy to fabricate products that are necessary for the paced livelihood. Amidst them, jet impingement cooling is one of the fast-emerging techniques due to its capability of producing very efficient localised heat transfer. The impinging jet cooling technology has been extensively employed in various industrial systems, such as cooling of gas turbine system and components, cooling of the rocket launcher, and cooling of high power density electrical density machines, to remove a considerable quantity of heat. However, it is vital to remember that increasing the impingement cooling magnitude isn't always the best design decision when trying to control the engine's thermal conditions and reactions. The bulk metal temperature and local thermal stresses have the most significant impact on the life expectancy of the hot gas path component. The component will break sooner if the bulk temperature is too high, but cracks will form, spread, and eventually collapse (cycles) if the thermal stress is too high. A high bulk temperature may be reduced by increasing the cooling flow; however, this might increase the problem of thermal stress. In order to eliminate local gradients, surface roughness or orientation of the impingement jets may be used to increase the heat transfer coefficient around a lower starting magnitude. Hence, the current study investigates the influence of free surface cooling by jet impingement of hybrid nanofluids prepared by dispersing MWCNT (5 nm) and Al2O3 (<7 nm) in DI water in the ratio of 90:10. The fluid was characterised by TEM and DLS to understand the dispersion and hydrodynamic size. The fluid stability was evaluated and quantified by visual inspection, zeta potential and transient viscosity approach. The nanofluid properties such as viscosity, thermal conductivity and surface tension were measured at different volume concentrations (0.025, 0.05, 0.1, 0.15%) and temperatures (10 to 60°C). The heat transfer experiments were focused on cooling the targeted copper round surface (D=42 mm) using a jet nozzle (Dj=1.65mm inner diameter) to impinge the HNFs at a constant jet-surface distance at H/Dj=4 susceptible to a turbulent flow regime. Numerical studies by the Eulerian-Eulerian approach were also carried out by assuming that both gas and solid phases are at interpenetrating continua with the k-w SST model evaluating the changing velocity and turbulent viscosity. Furthermore, we investigated a transient cooling rate for varying particle vol% concentrations. The effect of flow rate, advection, and surface tension on the thermal performance of the nanofluid in cooling the surface is measured by relating the Nusselt number with Reynolds number (6000 ≤ Re ≤ 16500), Peclet number (80000 ≤ Pe ≤ 205000) and Weber number (1000 ≤ We ≤ 7000) respectively. The maximum augmentation in Nu number is at 0.05% HNF with a 17% increase compared to DI water. From the CFD study, a maximum improvement of 19.7% in Nu number is seen by the 0.15% HNF, and the improvement in the 0.05% particle concentration fluid is 13.7%. The impinging Nu number for HNF with 0.05% stands at 180, 18.7% augmentation compared to DI-water and has the maximum improvement than any other particle vol% concentration fluids. It was concluded that the 0.15% HNF is the worst performing fluid at the jet-surface domain and hurts the Nu number. However, this trend was not shown in the CFD analysis and could be caused by other factors of the experiment such as size and shape of nanoparticles, mixing methods, surfactant amount and heat loss. In terms of the transient cooling rate investigated, the best performing fluid is 0.15% particle concentration fluid with relaxations time 1 second and 1.75 seconds in CFD and experiment, respectively. The steady-time (the time the cooling curve is approaching the infinity line) is 23 seconds and 33 seconds in CFD and experiment, respectively. Correlation for Nusselt number as a function of Re and volume concentration was also proposed.