Revestimentos superhidrofóbicos resistentes à ebulição em meio quiescente

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Experimental evaluation of green nanofluids in heat exchanger made oF PDMS

Publication . Nobrega, Glauco; Souza, Reinaldo Rodrigues de; Cardoso, Beatriz D.; Afonso, Inês Santos; Pereira, José Eduardo; Cardoso, Elaine; Moita, Ana S.; Ribeiro, J.E.; Lima, Rui A.

Conventional methods for synthesizing metallic nanoparticles face challenges such as instability and environmental concerns. Therefore, new, simpler, and more eco-friendly methods are being explored. In this context, the study reports a green synthesis process to produce magnetic iron oxide nanoparticles using an aqueous extract of the alga Chlorella vulgaris. This process leverages natural resources to create a sustainable nanofluid known as green nanofluid. To evaluate the characteristics of this nanofluid, experimental measurements of wettability, viscosity, thermal conductivity, and qualitative stability analysis were conducted. An experimental setup consisting of a heat exchanger made of polydimethylsiloxane (PDMS) was used to assess the thermal performance and the results were compared to theoretical equations and numerical simulation. Additionally, thermographic imaging of temperature gradients as the fluids passed over the heated surface of the serpentine channel were made. The main findings confirmed that the nanofluid was more stable than that obtained by traditional methods and had a more uniform temperature distribution over the heat exchanger. The higher concentration exhibited superior thermal performance compared to DI-Water. Moreover, the green nanofluid was used at a weight concentration of 0.1 wt%, provided thermal performance results of nearly 4.5% superior to those estimated by the numerical model and 6.4% higher than those experimentally obtained with the base fluid, respectively. Finally, the results obtained for the nanofluid also showed an average increase of around 5% in the viscosity of the base fluid, with a more significant sedimentation at a concentration of 0.1 wt%.

2024Journal article

Open access

Thermal performance evaluation of pure PDMS and PDMS composites heat exchangers

Publication . Souza, Reinaldo; Nobrega, Glauco; Afonso, Inês Santos ; Pereira, José; Cardoso, Elaine; Marques, Filipe; Vilarinho, Cândida; Moita, Ana; Lima, Rui A.

This study investigates the heat transfer performance of three types of heat exchangers: one made of pure polydimethylsiloxane (PDMS), another incorporating recycled graphite (PDMS + Graphite 30 mass%), and a third using commercial aluminium nanoparticles (PDMS + Aluminium 30 mass%). Thermal performance was evaluated by measuring the thermal conductivity of the materials, analysing experimental convection tests with deionized water in a single-phase regime and using a thermal camera to obtain temperature profiles of the different surfaces. The results revealed that the composites formed with PDMS matrix and recycled graphite showed elevated thermal conductivity, approximately 2.7 times higher than pure PDMS. The heat transfer coefficient performance was 2.5 times superior to that of the heat exchanger made with commercial aluminium nanoparticles and up to 5 times higher compared to pure PDMS. The thermal analysis highlighted the benefits of the composites, showing a more uniform temperature distribution both in the serpentine channel and along the sides of the PDMS. The study aimed to provide an economical alternative that also contributes to waste valorisation. These findings validate the effectiveness of recycled particles in improving heat transfer performance in heat exchangers made from a combination of PDMS matrix and recycled, economical materials.

2025Journal article

Open access