Experimental investigation on the effects of the geometry of microchannels based heat sinks on the flow boiling of HFE-7100

In this paper, nine Microchannel Heat sinks (MCHs) all with channel lengths of 20 mm and heights of 1 mm but with different aspect ratios 1.33; 2 and 4, wall thicknesses 0.25-0.75 were tested in single and two-phase flows using the HFE-7100 as the working fluid. Three different scenarios were established, changing only the inlet temperature of the fluid: ambient (18-25 degrees C), intermediate (38-42 degrees C) and near saturation (55-58 degrees C). In all experiments the flow at the inlet was in the laminar region with Reynolds (Re) number values ranging from 50 to 130. A thermographic camera was placed below the microchannel heat sink, to observe the cooling process during the flows. Thus, the main objective of this study is to evaluate the effects of channel geometry on both the overall heat transfer coefficient and the pressure losses. The results demonstrate that, for both single and two phase flow, narrower channels exhibit better thermal performance but higher-pressure losses than wider channels. When maintaining channel width constant and varying thicknesses, channels with wider walls presented higher overall heat transfer coefficients and lower pressure losses

Fluid flow and heat transfer in microchannel devices for cooling applications: experimental and numerical approaches

Microchannel heat sinks are pointed to have a great potential in cooling systems. This paper presents a systematic study to develop a microchannel heat sink to be used in PV panels cooling. A systematic experimental approach is used to optimize the heat sink geometry. Then the potential advantage of using flow boiling conditions is explored in both numerical and experimental approaches. The results show that a heat exchanger with thin walls and wide channels can dissipate a greater amount of heat. Comparing the results obtained for one and two-phase flow conditions, one must conclude that although in the boiling tests the heat transfer coefficient was higher, the cooling method with single-phase flow using water dissipated a greater amount of heat, which was mainly due to flow instabilities. In this context, the numerical work clearly evidences that boiling can be an advantage in microchannel heat sinks, as long as the flow is controlled. The work also shows that the considered numerical simulation tool is sensitive enough to quantify the heat transfer enhancement due to boiling within the examined microchannel paths

Multiple Relationships of Nursing Care: the Emergence of Care "of the us"

The aim of this qualitative study was to comprehend the relationships of the care of the self, of care of the other, and of care "of the us" in the different dimensions of care, through an educational/reflexive/interpretative process with nursing professionals in a University Hospital, using the complexity perspective. The data were collected through workshops and submitted to content analysis. The following categories emerged: reflecting upon the meaning of care of the self, care of the other, and "of the us" for the "I - human being", and for the "I - nursing professional"; and reflecting and (re)constructing the meanings of the relationships of care for the self, care for the other, and care "for the us". The care "for the us" is an emerging theme, in construction, and impels a concern for the collective, as well as remits to the comprehension of the multiple and unending phenomenon of constant movement among the beings and between them and their environment, modifying, altering, and causing to be altered the networks of existent relationships.Se trata de un estudio de abordaje cualitativo con el objetivo de comprender las relaciones de cuidado de sí, del otro y "de nosotros" en las diferentes dimensiones del cuidado, a través de un proceso educativo/reflexivo/interpretativo con profesionales de enfermería de un Hospital Escuela, bajo la perspectiva de la complejidad. Los datos fueron recolectados mediante talleres y sometidos al análisis de contenido. Surgieron las categorías: reflexionado sobre el significado del cuidado de sí, del otro, y "de nosotros" para el "yo - ser humano" y para el "yo - profesional de enfermería"; y, reflexionando y (re)construyendo los significados de las relaciones del cuidado de sí, del otro y "de nosotros". El cuidado "de nosotros" es un tema nuevo, en construcción, incentiva la preocupación con el colectivo y se relaciona con la comprensión de los fenómenos múltiples e inagotables del constante movimiento entre los seres y de estos con su ambiente, modificando, alterando y haciendo alterar las redes de relaciones existentes.Este é estudo de abordagem qualitativa, com o objetivo de compreender as relações de cuidado de si, do outro e "do nós", nas diferentes dimensões do cuidado, através de um processo educativo/reflexivo/interpretativo com profissionais de enfermagem de um hospital escola, sob a perspectiva da complexidade. Os dados foram coletados mediante oficinas e submetidos à análise de conteúdo. Emergiram as categorias: refletindo o significado do cuidado de si, do outro e "do nós" para o "eu - ser humano" e para o "eu - profissional de enfermagem" e, refletindo e (re)construindo os significados das relações do cuidado de si, do outro e "do nós". O cuidado "do nós" é tema emergente, em construção, impele a preocupação com o coletivo e remete à compreensão dos fenômenos múltiplos e inesgotáveis do constante movimento entre os seres e, desses, com seu ambiente, modificando, alterando e fazendo alterar as redes de relações existentes

Fluid flow and heat transfer in microchannel devices for cooling applications: Experimental and numerical approaches

Microchannel heat sinks are pointed to have a great potential in cooling systems. This paper presents a systematic study to develop a microchannel heat sink to be used in cooling applications. Particular emphasis is given to PV panels cooling. A systematic experimental approach is used to optimize the heat sink geometry. Then the potential advantage of using flow boiling conditions is explored in both numerical and experimental approaches. The two-phase flow is characterized in two different sets of conditions. In the experimental approach, a constrained bubble flow was observed with a stable pattern and bubble frequency in the narrower channel. In the wider channel a bubbly flow was observed with increased bubble diameters. Numerical simulations were also performed in order to examine the first transient stages of the two-phase flow development close to the inlet of the considered microchannels assuming an initial arbitrary distribution of nucleation sites. For this purpose, a previously developed and validated numerical simulation framework was utilised. The proposed customized tool has been developed in the general context of OpenFOAM CFD Toolbox and it accounts for phasechange (boiling/condensation) as well as for Conjugate Heat Transfer between solid and two-phase flow domains. The numerical predictions reveal that the proposed tool is sensitive enough to capture the effects of channel aspect ratio, applied heat flux and applied mass flux on the generated transient bubble dynamics and the associated heat transfer characteristics and it can constitute an important tool for quantifying the underpinned complex physical mechanisms, providing further insight into the experimental observations and measurements

Enhancement of microchannel heat sink heat transfer: Comparison between different heat transfer enhancement strategies

This paper investigates the advantages and challenges associated with two-phase flows, specifically flow boiling of pure liquids and nanofluids, for cooling applications in microchannel heat sinks. The study explores various two-phase flow patterns, their related issues, and examines the potential of nanoparticles to enhance heat transfer. Alumina (Al2O3), gold (Au), and silver (Ag) nanoparticles at different concentrations were tested. Experimental tests were conducted under different working conditions using various working fluids, including water, Al2O3 1 wt%, Ag 1 wt%, Au 1 wt%, Au 0.75 wt%, Au 0.5 wt%. The heat fluxes used were 1.026 kW/m2, 1.696 W/m2 and 2.403 kW/m2, while the volumetric flows ranged between 0.5 mL/min and 1.5 mL/min. The observed results indicate that even for the lowest particle concentration tested, the water-Au nanofluid exhibits superior cooling performance compared to the other examined fluids. The findings suggest that although two-phase flow conditions may not yield significant benefits, even small concentrations of nanoparticles (φ ≪ 1%) can significantly impact heat transfer mechanisms. This approach provides a cost-effective and efficient alternative for cooling microchannel heat sinks without necessitating the use of two-phase flow conditions

Experimental investigation on the effects of the geometry of microchannels based heat sinks on the flow boiling of HFE-7100

In this paper, nine Microchannel Heat sinks (MCHs) all with channel lengths of 20 mm and heights of 1 mm but with different aspect ratios 1.33; 2 and 4, wall thicknesses 0.25–0.75 were tested in single and two-phase flows using the HFE-7100 as the working fluid. Three different scenarios were established, changing only the inlet temperature of the fluid: ambient (18–25 °C), intermediate (38–42 °C) and near saturation (55–58 °C). In all experiments the flow at the inlet was in the laminar region with Reynolds (Re) number values ranging from 50 to 130. A thermographic camera was placed below the microchannel heat sink, to observe the cooling process during the flows. Thus, the main objective of this study is to evaluate the effects of channel geometry on both the overall heat transfer coefficient and the pressure losses. The results demonstrate that, for both single and two-phase flow, narrower channels exhibit better thermal performance but higher-pressure losses than wider channels. When maintaining channel width constant and varying thicknesses, channels with wider walls presented higher overall heat transfer coefficients and lower pressure losse