Heat and fluid flow in microscale from micro and nano structured surfaces

Official URL: http://risc01.sabanciuniv.edu/record=b1489684 (Table of Contents)

The use of enhanced surfaces became one of the most popular studies in order to increase heat transfer performances of microsystems. There are various techniques/processes applied to surfaces to enhance excess heat removal from microsystems. In parallel to these research efforts, various micro and nano structured surfaces were evaluated in channel flow, jet impingement and pool boiling applications. In the first study, single micro pin-fins having the same chord thickness/diameter but different shapes are numerically modeled to assess their heat transfer and hydraulic performances for Reynolds number values changing between 20 and 140. The pin-fins are three dimensionally modeled based on a one-to-one scale and their heat transfer performances are evaluated using commercially available software COMSOL Multiphysics 3.5a. Navier-Stokes equations along with continuity and energy equations are solved under steady state conditions for weakly compressible and single-phase water flows. To increase the computational efficiency, half of the domain consisting of a micro pin-fin located inside a micro channel, is modeled using a symmetry plane. To validate the model, experimental data available in the literature are compared to simulation results obtained from the model of the same geometrical configuration as the experimental one. Accordingly, the numerical and experimental results show a good agreement. Furthermore, performance evaluation study is performed using 3D numerical models in the light of flow morphologies around micro pin-fins of various shapes. According to the results obtained from this study, the rectangular-shaped micro pin fin configuration has the highest Nusselt number and friction factor over the whole Reynolds number range. However, the cone-shaped micro pin-fin configuration has the best thermal performance index indicating that it could be more preferable to use micro pin fins of non conventional shapes in micro pin fin heat sinks. In the second study, the results of a series of heat transfer experiments conducted on a compact electronics cooling device based on single and two phase jet impingement technique are reported. Deionized and degassed water is propelled into four microchannels of inner diameter 500 μm, which are used as nozzles and located at a nozzle to surface distance of 1.5mm. The generated jet impingement is targeted through these channels towards the surface of two nanostructured plates with different surface morphologies placed inside a liquid pool filled with deionized-water. The size of these nanostructured plates is 35mm x 30mm and they are composed of copper nanorods grown on top of a silicon wafer substrate of thickness 350 μm coated with a 50 nm thick copper thin film layer (i.e. Cu-nanorod/Cu-film/Silicon-wafer). Nanorods were grown using the sputter glancing angle deposition (GLAD) technique. First type of nanostructured plates incorporates 600 nm long vertically aligned copper nanorod arrays grown with nanorod diameters and spacing varying between 50-100 and 20-100 nm, respectively. The second type incorporates 600 nm long tilted copper nanorod arrays grown with diameter values varying between 50-100nm and spacing in the range of 20-50 nm. Heat removal characteristics induced through jet impingement are investigated using the nanostructured plates and compared to the results obtained from a plain surface plate of copper thin film coated on silicon wafer surface. Heat generated by small scale electronic devices is simulated using four cylindrical aluminum cartridge heaters of 6.25 mm diameter and 31.75 mm length placed inside an aluminum base. Surface temperatures are recorded by a data acquisition system with four thermocouples integrated on the surface at various prescribed locations. Constant heat flux provided by the heaters is delivered to the nanostructured plate placed on top of the base. Volumetric flow rate and heat flux values are varied between 107.5-181.5 ml/min and 1-40 W/cm2 , respectively, in order to characterize the potential enhancement in heat transfer by nanostructured surfaces thoroughly. A single phase average heat transfer enhancement of 22.4% and a two phase average heat transfer enhancement of 85.3% has been realized using the nanostructured plate with vertical nanorods compared to flat plate. This enhancement is attributed to the increased heat transfer surface area and the single crystal property of the vertical Cu nanorods. On the other hand, nanostructured plate with tilted nanorods has shown poorer heat transfer performance compared to both the nanostructured plate with vertical nanorods and plain surface plate in the experiments performed. The lower heat transfer rate of the tilted Cu nanorods is believed to be due to the decreased supply of liquid jets to the base of the plate caused by their tilted orientation and closely spaced dense array structure. This leads to formation of air gaps that ultimately become trapped among the tilted nanorods, which results in reduced heat transfer surface area and increased resistance to heat transfer. In addition, non-single crystal structure of the tilted nanorods and resulting enhanced surface oxidation could further decrease their heat transfer performance. In the third study, a nanostructure based compact pool boiler cooling system consisting of an aluminum base housing the heaters, a pool and four different plates to change the surface texture of the pool is designed. Effects of nanostructured plates of different surface morphologies on boiling heat transfer performance of the system are studied. Three nanostructured plates featuring Si nanowires of diameter 850 nm and of three different lengths, 900 nm, 1800 nm and 3200 nm respectively, which are etched through single crystal p-type silicon wafers using metal assisted chemical etching (MaCE), are utilized to enhance the pool boiling heat transfer. A plain surface Si plate is used as the control sample. Constant heat flux is provided to the liquid within the pool on the surface of the aluminum base through the plate by boiling heat transfer. Existence of wall superheat gave rise to forming of vapor bubbles near the boiling temperature of the fluid, namely DI-Water. Bubbles emerged from the nanostructured plate along with the phase change. Nucleate boiling on the surface of the plate, bubble formation and bubble motion inside the pool created an effective heat removal mechanism from the heated surface to the liquid pool. Along with the enhancement in both boiling and single-phase region heat transfer coefficients, this study proves the ability of nanostructured plates in improving the performance of the cooling system.