The Research On The Heat Transfer Performance Of The Capillary Pumped Loop With Nanofluid

With the rapid development of the microelectronic technology, the miniaturization of electronic components in modern electronic devices has become the mainstream of the development trend. The decrease of the feature size of electronic components, the rise of the chip integration, packaging density and the working frequency make the chip produce more heat per unit volume than before. Therefore, how to cool the electronic equipment effectively becomes the key technology at moment. The heat pipe has been used to the cooling of the electronic devices because of its highly efficient heat transfer. The common heat pipe could not transport heat across long-distance, which causes its application to some certain restrictions. The capillary pumped loop (CPL) is a two-phase control system, which could transport heat over long distance with minimal temperature difference and need not pumping power. Because of these, the CPL is used as the cooling solution to high load/flux problem of advanced electronic packaging . Nanofluid, which is a colloidal suspension with nanoparticles dispersing uniformly in a base liquid, has anomalous heat transfer characteristics and has been used in the heat pipe. All researches, which carried out the investigations on the heat pipe using nanofluid as the working liquid, proved that the addition of nanoparticles in the working liquid could lead to a decrease in the wall temperature and an increase in the heat transport capability. Therefore, the research on the CPL with nanofluid as the working liquid is of great significance in the thermal design of the electronic equipment.This paper aimed at a fundamental understanding of the application of nanofluid in the CPL with a flat evaporator. The small capillary pumped performance analysis platform was built for the investigation. The influences of types of nanoparticles, the sizes of nanoparticles, nanoparticles mass concentrations and operational pressures on the heat transfer coefficient and the maximum heat flux of the CPL were investigated. At the same time, a theoretical study was carried out on the calculation of the maximum heat flux of the CPL. Main results and findings are summarized as following.1: The influence of surfactant on the nanoluid nucleate boiling characteristics was carried out in a vertical small heated tube with a closed bottom (thermosyphon). The results showed that the boiling heat transfer rates of nanofluids with surfactant are poorer than that of the base liquid. However, the boiling heat transfer rates of nanofluid without surfactant are better than that of the base liquid.2. The physical properties of three types of nanoluid were measured. These nanofluids include the 20 nm Cu-water nanofluid, 50 nm Cu-water nanofluid and 50 nm CuO-water nanofluid. The experimental results showed that the viscosity of nanofluid is slightly greater than that of the base fluid and the nanoparticles mass concentration is one of the factors that influence the viscosity of nanofluid. The thermal conductivity of the fluid is increased with the addition of nanoparticles. The nanoparticles mass concentration, the type of nanoparticles, the size of nanoparticles and the temperature are the key factors that influence the thermal conductivity of the nanofluid. The surface tension of the fluid is decreased to some extent with the addition of nanoparticles in the fluid. The smaller of the surface tension of nanofluid with the increasing of nanoparticles mass concentration. And the solid-liquid contact angles between the stainless steel and the fluid is also decreased with the addition of nanoparticles in the fluid.2. The experiments were carried out at a steady operational pressure, which was 5.81 kPa, 9.86 kPa and 16.14 kPa, respectively. The results showed that there exists an optimal filling ratio to the CPL and the thermal resistance is at the minimum when the optimum filling ratio is taken. The experimental results indicated that the thermal performance of the CPL is improved with the addition of the nanoparticles. There exists an optimal nanoparticles mass concentration corresponding to the maximum heat transfer enhancement. And the optimal nanoparticles mass concentrations are 1.0 % and 0.5 % for Cu nanofluid and CuO nanofluid, respectively. The nanoparticle type and the nanoparticle size influence the thermal performance of the CPL. The maximum heat flux is increased with the addition of nanoparticles in the working liquid. The influence of the operational pressure on both the heat transfer coefficient and the maximum heat flux of the evaporator are remarkable. The heat transfer coefficient and the maximum heat flux are increased with the increasing of the operational pressure.4. The startup and re-startup of the CPL with the nanofluid as the working liquid were investigated at the condition that the cooling condition unchanged. Two types of heating location were investigated, which included the evaporator was heated from the top and the evaporator was heated from the bottom. To the evaporator heated from the bottom, the results of experiments showed that the startup time and the evaporator wall temperature are reduced with the addition of nanoparticles in the working fluid. With the increasing of nanoparticles mass concentration, the smaller of the startup time f of the CPL. However, the startup time is increased with the addition of nanoparticles in the working fluid when the evaporator is heated from the top. The wall temperature of the evaporator is similar with that of evaporator with the pure water as the working liquid when the whole system is stable. Comparing the CPL with the pure water as the working liquid, the entire performance of the CPL with nanofluid as the working fluid has not changed. The whole system was stopped to work for 72 hours after the tests. Then the system was restarted. The results showed that the startup process for the CPL heated from the bottom with nanofluid as the working liquid is similar with that for the CPL heated from the bottom with nanofluid as the working liquid. However, the startup process for the CPL heated from the top with nanofluid as the working liquid is similar with that for the CPL heated form the top with pure water as the working liquid.5. An experiment was carried out to understand the heat exchanger performance of the CPL with nanofluid as the working liquid when the cooling condition was unchanged. The experimental results showed that the CPL with nanofluid as the working liquid is applicable in application.6. The maximum heat flux of the CPL with nanofluid as the working liquid is gotten through theoretical analysis. It is found that the trends are similar between the calculated results and the experimental results.