Results 1 - 10 of 10
Results 1 - 10 of 10. Search took: 0.013 seconds
|Sort by: date | relevance|
[en] The thermal design of counterflow heat exchanger which adopts incompressible flow as working fluid is performed by introducing the concept of entropy production. In contrast with the case of ideal gas of which the entropy generation is expressed by temperature differences and pressure differences explicitly, the entropy production of incompressible fluid is expressed by the temperature differences only, but the temperature differences contain irreversibilities both by heat transfer and by fluid fraction implicitly. From a relation between conventional heat exchanger design parameters and actual flow parameters, entropy production is expressed by flow and heat transfer parameters, and optimal parameter which minimized the entropy production exists for given contraints. And this method is compared with traditional one which neglects pressure drops. (Author)
[en] In this study, the effects of the leakage flow between the baffle and tube bundles on the performance of a shell and tube heat exchanger (STHE) were examined using the commercial software ANSYS FLUENT v.14. A computational fluid dynamics model was developed for a small STHE with five different cases for the ratio of the leakage cross-sectional area to the baffle cross-sectional area, ranging from 0 to 40%, in order to determine the optimum leakage flow corresponding to the maximum outlet temperature. Using fixed tube wall and inlet temperatures for the shell side of the STHE, the flow and temperature fields were calculated by increasing the Reynolds number from 4952 to 14858. The present results showed that the outlet temperature, pressure drop, and heat transfer coefficient were strongly affected by the leakage flow, as well as the Reynolds number. In contrast with a previous researchers finding that the leakage flow led to simultaneous decreases in the pressure drop and heat transfer rate, the present study found that the pertinent leakage flow provided momentum in the recirculation zone near the baffle plate and thus led to the maximum outlet temperature, a small pressure drop, and the highest heat transfer rate. The optimum leakage flow was shown in the case with a ratio of 20% among the five different cases.
[en] The hot mixing chamber (HMC) is a device that mixes streams of fluid to obtain gas with a pertinent operational temperature in the final outlet section that can meet the industry requirements. In this numerical simulation study, the flow and temperature characteristics in the hot mixing chamber in different cold air inlets conditions were obtained using the software FLUENT. The number of cold air inlets was 2, 3 and 4. For the two cold air inlets, there were four models with various inlet angles of 90°, 120°, 150° and 180°. Therefore, a total of six models including 3 air inlets angle of 120° and 4 air inlets angle of 90°, were simulated in this study. The simulation results showed the model with 3 cold air inlets had the best performance for the highest temperature uniformity at the HMC outlet and the lowest wall temperature over the surface of the HMC.
[en] Design and performance analysis of a steam turbine for variations of degree of reaction were performed by computer simulation. Design parameters such as blade angles, exit areas, and heights of the nozzle and moving blade were represented as functions of the degree of reaction. The main performance factors such as turbine power, diagram efficiency, and axial thrust were also expressed in terms of the degree of reaction. For further information about the design and performance, the blade angles and main performance factors were investigated as functions of the flow coefficient. The turbine power and diagram efficiency reached a maximum value for a given degree of reaction and flow coefficient, and the symmetric shape of the moving blade showed distortion as the degree of reaction was increased
[en] The use of ocean thermal energy conversion (OTEC) to generate electricity is one of the methods proposed to utilize renewable energy and to protect the environment. In this study, simulations were performed to investigate the effect of weather conditions in the Ulsan region, Korea, on the efficiency of a solar heating OTEC (SH OTEC) system. This system utilizes solar thermal energy as the secondary heat source. Various working fluids were also simulated to select one that is suitable for this system. The results showed that R152A, R600, and R600A, in that order, were the most suitable working fluids. The effective area of the solar collector for a 20 .deg. C increase in the collector outlet temperature fluctuated from 50 to 97m'2' owing to the change in the monthly average solar gain. The annual average efficiency of the SH OTEC increases to 6.23%, compared to that of a typical conventional OTEC, which is 2-4%
[en] Film cooling characteristics has been investigated numerically with the aid of FLUENT software for the sunk or the lifted upstream wall from the slot injection exit. In this study, with the fixed blowing ratio of 1 and the fixed coolant injection angle of 30 .deg., the downstream flow field and the downstream temperature field were examined in terms of velocity vector, turbulent kinetic energy, temperature contours, and downstream wall temperature. Upstream wall was sunk or lifted from 1d to 5d(d=slot width). The result shows that the up-1d upstream wall has the best film cooling performance. This is due to the fact that the up-1d upstream wall configuration reduces velocity gradient just enough to minimize the turbulent mixing between the mainstream and the coolant just off the slot exit
[en] In this study, we investigate the performance variations of an automotive turbocharger compressor with respect to the height variation of the recirculating casing treatment (RCT). We use three RCT heights, namely 1.2 mm, 1.5 mm, and 1.8 mm. We vary the compressor speed from 90,000 to 150,000 rpm, and the flow rate from 0.015 kg/s to 0.08 kg/s. The calculation results of the total pressure ratio and isentropic efficiency showed good agreement with the performance data provided by the manufacturer within a 0.7 percent error. The results showed that the RCT heights of 1.2 mm, 1.8 mm, and 1.5 mm, in that order, exhibited a more uniform pressure distribution, higher pressure ratio, and wider operational range. As the number of revolutions per minute increased, we obtained typical characteristics of a compressor map having a narrower operational range in the region of higher pressure ratio
[en] Based on fully self-consistent dynamical mean field theory (DMFT) method, we investigate electronic structure and Fermi surface nesting property of LiFeAs and NaFeAs, focusing on the correlation effect of iron 3d orbital. For LiFeAs, good nesting property by density functional theory (DFT) method is much suppressed by DFT+DMFT method due to the orbital-dependent renormalization magnitude. NaFeAs shows a similar behavior, but a better nesting is obtained than LiFeAs from DFT+DMFT Fermi surfaces. Our result is consistent with the observed superconducting (spin density wave) ground state of LiFeAs (NaFeAs).
[en] The radioactivity of 14C of the graphite samples from the dismantled Korea Research Reactor 1 and 2 (the KRR-1 and 2) site was analyzed and proposed to be disposed of as a low level radioactive waste rather than self-disposed of. The graphite wastes, with a weight of seven tons, have been generated during the dismantling of a research reactor with a capacity of one MW from 1995 to 2006. The graphite was used as a moderator for the research reactor and so has been radio-activated by thermal neutron. It was thought that the graphite wastes mainly included a radioisotope of stable carbon, 14C, a pure beta emitter with a half life of 5,730 years and with a maximum decay energy of 156 keV. Therefore, it has been requested to sec whether the dismantled graphite radioactive wastes including 14C can be self-disposed of or not. In the present study, the radioactivity of 14C in the graphite sample used in the research reactor was analyzed by using a commercialized high temperature furnace and a Liquid Scintillation Counter (LSC). The combustion temperature of the furnace was five hundred degrees centigrade and especially the temperature in the catalyst region was eight hundred degrees centigrade. The recovery from the furnace was 95% for 14C and the LSC had a quenching efficiency of approximately 66%. Carbosorb was used as a trapping solution for 14C. The radioactivity of 14C was measured by a LSC through the procedure of a pre-treatment such as the combustion of a sample in the temperature range of 500-800 degrees centigrade by a high temperature furnace, trapping of 14C into Carbosorb and cocktailing it with a scintillator. The radioactivity was analyzed to have a concentration with a value of much more than a domestic legal limit for a self-disposal. And an individual effective dose rate estimation was also carried out. Finally, it is suggested that the graphite wastes from the dismantled research reactor should be disposed of at a low level radioactive waste disposal site and monitored. (authors)
[en] 210Po in the daily diet in Korea was analyzed and the ingestion dose from an intake of 210Po was estimated by considering the dietary habit of a Korean person. The 210Po concentrations of a leafy vegetable (0.36 Bq x kg-1 for lettuce) in the terrestrial food were higher than those of grain, whereas Chinese cabbage had a lesser 210Po concentration (0.019 Bq x kg-1). The 210Po concentration of the animal product was similar to those detected in the grain and vegetable. The 210Po concentrations in the shell and crustaceous were high from 19.1 to 33.0 Bq x kg-1, however, its value fell in the overall range of the reported values. The effective dose from 210Po for an adult from the Korean population was about 269.4 μSv x y-1. Nearly 80% of the ingestion dose from the intake of 210Po was attributed to the consumption of seafood. It suggests that the marine food ingestion is a critical pathway for natural 210Po to the Korean population. (author)