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[en] Heat transfer is one of the most important processes in many industrial and consumer products. The inherently poor thermal conductivity of conventional fluids puts a fundamental limit on heat transfer. Therefore, for more than a century since Maxwell, scientists and engineers have made great efforts to break this fundamental limit by dispersing micrometer or nano sized particles in liquids. The nano fluid terminology, which describes fluid combined nanoparticles, was introduced by Choi of the Argonne National Laboratory in the U.S Department of Energy. The carbon particles with metal lattice or graphite structures generally exhibit thermal conductivities that are hundreds of times greater than pure fluids. Especially due to their outstanding electric and thermal conductivities, carbon nano tubes and graph ene have become an important entity in the scientific field. Therefore, in this work, experiments are carried out to measure the thermal conductivity via transient hot wire method and the viscosity using a rotary type digital viscometer of carbon nano tubes and graph ene
[en] The results of this experiment were that the CHF of the two nanofluids increased along with the volumetric fraction until 0.001 vol%, and the two types of nanofluids are the highest CHF at 0.001 vol%. Also, the results show clearly that the rate of CHF increase of the CM-100 MWCNT nanofluid with longer-length nanoparticles is higher than that of the CM-95 MWNCT nanofluid. These results indicate that the length of carbon nanotube influences the pool boiling CHF of carbon nanotube nanofluid and that long-length MWCNT, as above-noted, offers a superior effect in this regard. Boiling heat transfer is used in a variety of industrial processes and applications, such as refrigeration, power generation, heat exchangers, cooling of high-power electronics components and cooling of nuclear reactors. The critical heat flux (CHF) phenomenon is the thermal limit during a boiling heat transfer phase change; at the CHF point the heat transfer is maximised, followed by a drastic degradation after the CHF point. The consequence is a substantial increase in wall temperature which may result in physical failure phenomenon of heat transfer systems. Therefore, the CHF is important being considered in the cooling device design, such as nuclear reactor and nuclear fuels, steam generators, high-density electronic component, etc. And, CHF enhancement is essential for safety of heat transfer system. Recently, CHF reported increased when applied to the nanofluids, with its high (higher-than-base-fluid) thermal characteristic in the nuclear power plant system. Therefore, in this study, carried out the pool boiling CHF experiments by the particle length using carbon nanotube nanofluids, and the results are compared and analyzed for the CHF enhancement. The pool boiling CHF of experiments of carbon nanotube nanofluids carried out by the length of particles and the various concentrations
[en] The energy is the basis for almost all industrial activities and domestic needs. But recently there are increasing concerns internationally over environmental problems and consequent climate changes caused by the excessive use of fossil fuels. Furthermore the price of crude oil is increasing steadily with unstable supplies. In order to solve these national energy problems, the utilization of ocean energy is introduced as one of the best alternative technologies for the future. OTEC power plant has been installed at the west Inchon power plant site. Temperature differences of 20∼25 deg. C have been utilized for plant operations, where R22 is used as a working fluid. The system is composed of low pressure turbine, plate type heat exchanger, and pumps. In the present investigation the experimental results, such as gross power, net power and objective function, are analysed when temperature differences change from the reference design point
[en] Combined convection and radiation heat transfer in a circular tube with circumferential fins and circular disks is investigated for various operating conditions. Using a finite volume technique for steady laminar flow, the governing equations are solved in order to study the flow and temperature fields. The P-I approximation and the Weighted Sum of Gray Gases Model (WSGGM) are used for solving the radiation transport equation. The results show that the total Nusselt number of combined convection and radiation is higher than that of pure convection. If the temperatures of the combustion gas and the wall in a tube are high, radiation becomes dominant. Therefore, it is necessary to evaluate the effect of radiation on the total heat transfer
[en] Boiling heat transfer is one of the most important processes in the various industries such as power generation, heat exchangers, cooling of high-power electronics components and cooling of nuclear reactors. The critical heat flux (CHF) phenomenon is signified the thermal limit during a boiling heat transfer. The heat transfer coefficient before the CHF is high enough to attain a high heat flux at a relatively low surface heat. However, the heat transfer coefficient remarkably decreases after the CHF occurs therefore the heating surface temperature of heat-transfer apparatus should be greatly increased. This induces risk of physical failure of heat transfer apparatus. Therefore, enhancement of CHF is essential for safety and economic efficiency of heat transfer system. In this study, the CHF characteristics of oxidized carbon nanotube and graphene nanofluids under the pool boiling state were comparative analysis. The pool boiling CHF experiments of oxidized carbon nanotube and graphene nanofluids carried out by the various concentrations. All of the two types of nanofluids showed higher CHF than the pure water. The result shows that the CHF of oxidized graphene nanofluids is higher than the oxidized carbon nanotube nanfluids
[en] This study deals with the effects of major geometric parameters on the sound wave output of a thermal acoustic system. The output power of the acoustic wave was dependent on the stack position, stack length, resonator tube length, and input power. In experiments, the maximum SPL was generated when the stack was placed between one fourth to half, resonator and stack length were longer, and input power was increased. The frequency was recorded to be 437 and 885 Hz when the resonator tube length was 200 and 100mm, respectively. Therefore, when the resonator tube length was shorter, a higher frequency was recorded
[en] Steam generator level control at low power is difficult due to its adverse thermal hydraulic properties, and is usually conducted by an operator. The basic model predictive control (MPC) is similar to the action of an operator in that the operator knows the desired reference trajectory for a finite period of time and takes the necessary control actions needed to ensure the desired trajectory. An MPC is based on a model; the performance as well as the efficiency of the MPC depends heavily on the exactness of the model. In this study, steam generator models that can describe in detail its thermal hydraulic behaviors, particularly at low power, are used in the MPC design. The design scope is divided into two parts. First, the MPC feedwater controller of the feedwater station is determined, and then the MPC level controller for the overall system is designed. Because the dynamic properties of a steam generator change with the power levels, a realistic situation is simulated by changing the transfer functions of the steam generator at every time step. The resulting MPC controller shows good performance.
[en] In this study, we hypothesize that the temperature range for crystallization (dT) has two meanings: the presence of a critical energy for crystallization (ECR) and the presence of a crystallization energy (ECK) at each temperature, and we investigated this hypothesis. In order to investigate the validity of the hypothesis, we need a differential thermal analysis (DTA), and we measured the change in the crystallization starting temperature (TX) caused by an electric field in the glass ceramic 50SiO2-40Na2O-10CaO (wt%). From the DTA results and the Kissinger equation, the calculated ECR for crystallization was 55 ±4 kcal/mol. The TX decreased at temperatures under 450 degrees in the presence of an electric field while it was 495 ± 8 degrees in the absence of an electric field. The electric field decreased the activation energy needed for ion movement, so the glass met the condition for crystallization and stoichiometry at an even lower temperature; hence the TX was decreased. From this, we concluded that the ECK was proportional to the activation energy needed for ion movement. From this conclusion, we verified the hypothesis.
[en] In this study, the optical and structural properties of an Fe-TiO2 sol were investigated. TEOT[Ti(OC2H5)4] and ferric nitrate[Fe(NO3)3·9H2O] were used as precursors to make the Fe-TiO2 sol. The samples for measurement were prepared on a glass substrate using the sol-gel dip coating method. The annealing temperature (Ta) was varied in the range of 673.15-873.15 K in intervals of 50 K. The cut-off points of the samples were increasingly red-shifted with increasing Fe content. Simultaneously, the optical band gap decreased from 4.03 to 3.64 eV. A change of the crystalline phase as the Fe content increased was observed from the X-ray diffraction measurements. Combined with the X-ray diffraction analysis, the observed variations in both the cut-off points and optical band gap were directly correlated with the structural change of the composite films.