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[en] In the present paper, we provided a partial explanation of this abrupt change of CHF enhancement departed from the phenomena based on the flow instability of Lienhard and Dhir. As noted by Lienhard and Dhir, the finite size of heater affect bubble departure characteristics during boiling and enhance CHF comparing to the heater with sufficiently large heat transfer area. There are competitive theories to explain it: Lienhard and Dhir's model based on the instability following Zuber and the macro film model of Haramura and Katto. Lee et al. recently produced CHF data for the heaters with thinner width than the data used to evaluate those models which were scattered and covered the estimation of both models. However, the CHF data of Lee et al. clearly showed that the CHF model of Lienhard and Dhir is better than the model of Haramura and Katto in the more thinner width than the conventional data set. Furthermore, Lee et al found that the CHF model of Lienhard and Dhir even underpredicted CHF data obtained from their experiments. It was found that the empirical correlation from the data fitting has the value of exponent from -3/8 to -1/2 which is bigger than -1/4 of Lienhard et al
[en] Viscosity, shear ratio, bubble injection method, and surface contamination were considered as possible reasons on the discrepancy. Therefore, on the universal understanding of lift force acting on bubble, now it is very unclear state. To solve the problem, experimental basis are crucial due to computational limitations of current CFD method. In this paper, experimental results on bubble shift in shear flow similar to Li et al. are presented. Although similar test condition is tried, this kind of experiment is valuable to do, due to rareness of the experimental data. Shift of air-water bubble in linear shear flow is experimentally investigated. It is observed that similar trend to results of Li et al.(2016) can be well reconstructed from the current experimental setup. Based on the experimental results, it seems that critical bubble diameter for air-water bubble is lower than 2mm like as transitional diameter for wake instability of free rising air-water bubble.
[en] The pebble bed reactor (PBR) is a candidate reactor type for the very high temperature reactor (VHTR), which is one of the Generation-IV reactor types. The HTGR design concept exhibits excellent safety features due to the low power density and the large amount of graphite present in the core which gives a large thermal inertia in an accident such as loss of coolant. The conclusions are made and may contribute to a better design of a PBR core and a closer inspection of the local hot spots to avoid destruction of pebbles from happening. Thermal field of a PBR core is investigated in this study. Specifically, experiments on measuring the pebbles' surface temperature are performed. It is found that the upper pebble has an overall higher temperature profile than the other pebbles and the stagnation zone under does not increase its surface's temperature. In addition, the temperature profile of the side pebble shows a concave form and it keeps decreasing from the contact point to the vertex in the lower pebble. Lastly, the maximum temperature difference among these points is 5.83 deg. C. These findings above are validated by CFX simulations under two different turbulence models (k-e, SST) and two contact areas (diameter of 6mm and 3.5mm). By contrasting the temperature variation trends of all simulation cases, it is concluded that SST turbulence model with 20% intensity shows a better agreement with the experiment result, nevertheless, slightly deviation is also found in terms of total temperature difference and the peak appears in position 17-19 in experiments
[en] A laser machining process has been applied in many manufacturing fields and it provides an excellent energy control for treating materials. However, a heat effect during laser machining can deteriorate material properties. Specifically, a thermally induced stress can be a problem in laser-machined structures on a metal surface. In this study, temperature and stress on cold-rolled carbon steel sheet machined with laser hole drilling were explored in an experimental approach and a numerical method. Stresses by temperature gradients inside the materials were generated in fast cooling. The stresses were measured by using a hole-drilling method and the material properties of carbon steel (Scp-S) were obtained in the experiment. It was found that the stress predicted from the numerical analysis was in agreement with the stresses measured by using the hole-drilling method. The analysis can be applied for evaluating structure characteristics machined with a laser
[en] We study the steady-state behavior of a damped, driven nonlinear LRC oscillator, where the nonlinearity arises due to voltage-dependent capacitance. The driving or input signal is assumed to be a pure tone. Using an iterative, perturbative solution technique combined with an energy conservation argument, we show that the oscillator transfers energy from the fundamental to higher harmonics. We determine a series expansion of the two-norm of the steady-state output signal and show that in a large region of parameter space, the two-norm depends superlinearly on the input amplitude. We also use the two-norm calculation to devise a performance goal that the infinity-norm of the steady-state output signal should satisfy, in order for the nonlinear system to have a genuine boost over the corresponding linear system. Taken together, these results are a step toward the automatic design of nonlinear systems that have an optimal boost over corresponding linear systems.
[en] Highlights: • Heat transfer for PCHE in TEG was investigated in detail by 3D CFD analysis. • Experimental data for a 200-W TEG implemented with PCHEs are newly presented. • Power density of the TEG was sufficiently high at low temperature. • Reduction of TEG flow rate requirements from use of PCHEs is estimated. - Abstract: Printed circuit heat exchangers (PCHEs) are employed to improve the compactness of a thermoelectric generator (TEG). PCHEs allows miniaturization of the heat exchanger without excessive additional cost, and permit high temperature and pressure (up to 1100 K and 600 bar) of working fluid, which enable high thermoelectric conversion efficiency. To investigate the pressure loss and thermal resistance of a PCHE in detail, three-dimensional computational fluid dynamic (CFD) analysis is conducted. Experimental results of the proposed TEG with PCHEs are newly presented. The TEG provides power density of 233.1 kW/m3 at inlet temperatures of 448.15 K (hot side) and 293.15 K (cold side), which is the highest value in literature for a low-temperature TEG (<505.15 K hot side). Based on the models of friction and heat transfer in a PCHE validated by the experiment, it is noted that the flow rate required for the heat exchangers in a TEG producing a given amount of electrical power can be reduced by adaption of PCHEs. Such novel results on the TEG with PCHEs might be helpful for more compact design and expands the applicability of TEGs for waste heat recovery.