RESUMEN

Graded foam-filled channels are a very promising solution for improving the thermal performance of heat sinks because of their customized structures that leave large amounts of room for heat transfer enhancement. Accordingly, this paper proposes a comprehensive optimization framework to address the design of such components, which are subjected to a uniform heat flux boundary condition. The graded foam is achieved by parameterizing the spatial distributions of porosity and/or Pores Per Inch (PPI). Mono- and multi-objective optimizations are implemented to find the best combination of the foam's fluid-dynamic, geometrical and morphological design variables. The mono-objective approach addresses the Performance Evaluation Criterion (PEC) as an objective function to maximize the thermal efficiency of graded foams. The multi-objective approach addresses different objective functions by means of Pareto optimization to identify the optimal tradeoff solutions between heat transfer enhancement and pressure drop reduction. Optimizations are performed by assuming a local thermal non-equilibrium in the foam. They allowed us to achieve a 1.51 PEC value with H* = 0.50, ReH = 15000, iε = iPPI = 0.50, ε(0) = 0.85, ε(1) = 0.97, PPI(0) = 5, PPI(1) = 40, and ks→f = 104 as the design variables. For the three multi-objective functions investigated, one can extrapolate the optimum from the Pareto front via the utopia criterion, obtaining h¯ = 502 W/m2 K and Δp = 80 Pa, NuH,unif¯ = 2790 and f = 42, ⟨Ts*⟩s¯= 0.011, and Δp* = 91. The optimal solutions provide original insights and guidelines for the thermal design of graded foam-filled channels.

RESUMEN

Diesel exhaust aerosols (DEAs) can absorb and accumulate toxic metal particulates and bacteria suspended in the atmospheric environment, which impact human health and the environment. The use of acoustic standing waves (ASWs) to aggregate DEA is currently considered to be an efficient particle removal method; however, study of the effect of different temperatures on the acoustic aggregation process is scarce. To explore the method and technology to regulate and optimize the aerosol aggregation process through temperature tuning, an acoustic apparatus integrated with a temperature regulation function was constructed. Using this apparatus, the effect of different characteristic temperatures (CTs) on the aerosol aggregation process was investigated experimentally in the ASW environment. Under constant conditions of acoustic frequency 1.286kHz, voltage amplitude 17V and input electric power 16.7W, the study concentrated on temperature effects on the aggregation process in the CT range of 58-72°C. The DEA opacity was used. The results demonstrate that the aggregation process is quite sensitive to the CT, and that the optimal DEA aggregation can be achieved at 66°C. The aggregated particles of 68.17µm are composed of small nanoparticles of 13.34-62.15nm. At CTs higher and lower than 66°C, the apparatus in non-resonance mode reduces the DEA aggregation level. For other instruments, the method for obtaining the optimum temperature for acoustic agglomeration is universal. This preliminary demonstration shows that the use of acoustic technology to regulate the aerosol aggregation process through tuning the operating temperature is feasible and convenient.

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RESUMEN

In order to obtain an acceptable rate of survival of the frozen cells, they must be first cooled in programmable freezers, while controlling the cooling rate, and then stored in maintenance freezers. Different solutions are already used to preserve cells at cryogenic temperatures without liquid nitrogen, while this is not true for controlling the cooling rate. A Pulse Tube (PT) cryorefrigeration system type, can be used in the automatic freezing if combined with a system that monitors and "drives" the temperature generated on the cold part (cold head). To make the Pulse Tube system a suitable one for freezing processes, the cooling curve must be "corrected" in a linear one with a slope given by requested cooling rate. The temperature regulation is obtained with the use of a power dissipator based on Joule effect. In this study, the power to be dissipated is calculated individualizing the trend of temperature on the Pulse Tube cold head and inside the cell test tubes. The control system is not based on an historical series of data so it can be used in different operative conditions. The obtained temperature curve is in good agreement with the theoretical values, with errors within those accepted by commercial systems. The Pulse Tube cryorefrigerator may represent a valid alternative solution to programmable liquid nitrogen freezer, especially where nitrogen's supply is difficult or extremely expensive.

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