Experimental and numerical analysis of microstructured surfaces

Doctoral thesis OPEN
Diani, Andrea (2014)
  • Subject: ING-IND/10 Fisica tecnica industriale

Heat dissipation is one of the most important issues for the reliability of electronics equipment. Up today, air represents the most safe, cheap, and common working fluid for electronics thermal management applications. Due to its poor heat transfer characteristics, air always flow through enhanced surfaces, such as plain and louvered fins, pin fins, offset strip fins and wire screens, in order to increase the heat transfer area and to create turbulence. Recently, metal foams have been proposed as promising enhanced surfaces to improve the overall heat transfer performance of the cooling system. In several applications air might be not enough for high level of heat dissipation, thus two-phase systems can represent a viable solution. Boiling is the heat transfer mechanism with the highest heat transfer coefficients, thus it can be used to spread high heat fluxes to maintain the wall temperature at low values with compact heat sinks. Microstructured surfaces, such as metal foams and microfin tubes, can exploit positive benefits on the flow boiling mechanism, i.e. they can promote bubble nucleation, reduce onset of nucleate boiling, augment two-phase mixing, enhance critical heat flux. On the other hand, the environmental issues associated to the use of synthetic refrigerants call for a continuous improvement of the technical solutions. Recently, new low-GWP refrigerants, in particular R1234ze(E) and R1234yf, have been proposed as possible alternatives of the traditional R134a. This PhD thesis explores the use of microstructured surfaces for thermal management applications. Metal foams, plain finned and pin finned surfaces are experimentally and numerically investigated during air forced convection. In addition, single- and two-phase flow (vaporization) of refrigerants through a copper foam and in a microfin tube is experimentally studied. The first chapter is focused on the air forced convection through metal foams. Nine copper foams are experimentally tested, and the overall heat transfer coefficients and pressure drops are calculated from the experimental measurements. The effects of the geometrical parameters (foam core height, pore density, and porosity) on the thermal and hydraulic behaviour of such materials are discussed. The experimental data points, coupled with other measurements previously obtained on aluminum foams, have permitted the development of a new semi-empirical equation for the estimation of the foam finned surface efficiency and of the heat transfer coefficient. The air forced convection through plain finned and pin fin surfaces is discussed in the second chapter. Numerical simulations are performed on different geometrical configurations of fin thickness, pitch, and height for the plain finned surfaces, and different configurations of pin diameter, longitudinal and transverse pin pitch, and pin height for the pin fin surfaces. The effects of the geometrical characteristics on the thermal and hydraulic behaviour are reported. From the numerical results, four correlations have been developed for the estimation of the Colburn j-factor and friction factor for plain finned and pin fin surfaces. In the end, an optimization of a plain finned surface is reported. The third chapter proposes a numerical approach to study the air forced convection through metal foams. The real structure of four copper foams, whose experimental results are reported in the first chapter, is obtained by micro-computed tomography scanned images. Once reconstructed, the real foams are meshed and the air flow simulated with a commercial software. Numerical results of pressure drop and heat transfer coefficient are compared against the experimental values. The design and development of a new experimental facility to study the phenomenon of the flow boiling inside microstructured surfaces is reported in the fourth chapter. The numerical design of the test section, which hosts a 200 mm long metal foam, is presented. Every component of the set up is discussed in details. The results of the calibration tests are reported. The flow boiling of refrigerants inside a metal foam is shown in the fifth chapter. The tested copper foam is 200 mm long, 10 mm wide, and 5 mm high. Three different refrigerants are studied: R134a, R1234ze(E), and R1234yf. R1234ze(E) and R1234yf (GWP=6 and 4, respectively) are possible substitutes of R134a (GWP=1400). Tests are run at a saturation temperature of 30 °C, which can be considered suitable for the case of electronic cooling applications, at different working conditions, in order to highlight the effects of the vapour quality, mass velocity, and heat flux on the thermal and hydraulic performance. Finally, the sixth chapter reports some results about the flow boiling of refrigerants inside a 3.4 ID microfin tube. Three different refrigerants are studied: R134a, R1234ze(E), and R1234yf. As for the case of flow boiling inside a metal foam, tests are run at a saturation temperature of 30 °C under different working conditions, i.e. different vapour quality, mass velocity, and heat flux. The experimental results of heat transfer coefficient, vapour quality at the onset of the dryout, and pressure drop are compared against values predicted by correlations from the open literature
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