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

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

Droplet impingement and evaporation on a solid surface

An efficient spray injection leads to better vaporization and better air–fuel mixing, resulting in the stable combustion and reduced emissions in the internal combustion (IC) engines. The impingement of liquid fuels on chamber wall or piston surface in IC engines is a common phenomenon, and fuel film formed during the impingement plays a critical role in engine performance and emissions, particularly under cold start conditions. Therefore, the study on the characteristics of spray impingement on the chamber wall or piston surface is necessary. However, first, due to the complexity of the practical fuel injection systems, it is difficult to attain the detailed specific information of the spray impingement from the experiments such as droplet size, mass, number, and velocity distributions in the vicinity of wall region. Second, because of the Lagrangian particle/parcel concept (a particle representing a number of droplets in simulations), the spray–wall interaction model under Eulerian–Lagrangian approach is often developed based on the individual droplet. Therefore, the individual droplet’s impingement on wall and the droplet-to-droplet collision have been extensively studied to assist in a profound perception on the spray–wall impingement. In this chapter, the encouraging experimental observations of applying optical diagnostics technology to study droplet–wall impingement are extensively discussed. Single droplet impingement on a solid surface with various conditions was examined to understand the detailed impinging dynamic process. The droplet–wall interaction outcomes, in particular focusing on the splashing criteria, were inspected, and a new correlation of deposition–splashing is developed. Post-impingement characterizations including spreading factor, height ratio, contact line velocity, and dynamic contact angle were then analyzed based on the experimental data at various test conditions. Further, the non-evaporation volume of fluid (VOF) method based on Eulerian approach was used to characterize single droplet impinging on the wall and provide a better understanding of the dynamic impact process. The simulation results of the spreading factor and height ratio matched well with the experimental results during the droplet impingement process. In addition, due to the evaporation drawing more attention during the engine combustion process, an evaporation VOF (e-VOF) sub-model was developed and applied to multi-droplet impingement on a heated surface to qualitatively and quantitatively analyze the vaporizing process as droplets impacting onto the hot surface. The information obtained from VOF simulations can be applied to improve the spray–wall interaction models in the liquid spray Eulerian–Lagrangian method