Enhancement of Microscale Energy Transport in DASC Systems Using Nanoparticles

Abstract

This study presents a Eulerian-Lagrangian modelling approach to study particle transport at nano- and micro-scales within microchannels and to simulate the efficiency of nanofluid-based Direct Absorption Solar Collector (DASC) systems, utilizing transient in-house MATLAB code. The numerical solution of the Langevin equation offers insights into particle dynamics, with an emphasis on mean square displacement, particle trajectories, and the timescales of Brownian motion, highlighting the transition from ballistic to diffusive behaviour as time steps decrease below the momentum relaxation time. The research also investigates a one-way coupling approach for suspended alumina nanoparticles in a water-based fluid, with detailed validation against previously published works and analytical solutions for three different cases of particle sedimentation in a microchannel, showing excellent agreement. The influence of two-way coupling on the average terminal velocity across various particle loadings and locations was examined, providing a deeper understanding of particle-fluid interactions.

In analysing DASC systems, the study systematically evaluates how solar radiation absorption changes with different Reynolds numbers (Re). At a higher Re, specifically Re=40, absorption shifts predominantly to the glass cover of the collector, whereas at lower Re values, the bulk fluid absorbs most of the solar radiation. This observation highlights the flow regime’s critical role in the solar energy absorption process within the collector. Additionally, the study assesses the impact of aluminium nanoparticle concentration on absorption efficiency, demonstrating a decrease with increased nanoparticle concentration; absorption efficiencies of 0.8712 for 0.001% and 0.7351 for 0.1% aluminium nanoparticle concentrations in a 1000 µm channel height were noted. Moreover, the extinction coefficient's role in enhancing solar radiation absorption is significant, especially in the UV and visible spectrum, where higher nanoparticle concentrations, such as 0.1% aluminium, lead to notable increases in the extinction coefficient in those regions. This enhancement contributes to a more efficient solar energy capture, suggesting that the nanoparticle concentration in the nanofluid plays a key role in the overall performance of DASC systems. The findings highlight the complex relationship between nanoparticle dispersion, fluid flow regime, and solar energy absorption, offering valuable insights for optimizing the design and operation of solar thermal collectors.

Date of Award	6 May 2024
Original language	American English
Supervisor	Eiyad Abu-Nada (Supervisor)