• The Journal of the Convergence on Culture Technology
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• v.7 no.2
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• pp.443-451
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• 2021

This study compared and analyzed the thermal characteristics based on the ratio of moisture content of infected and non-infected Bursaphelenchus Xylophilus by heat of combustion experiment. The experiment results are as follows. The analysisresult of the ratio of water content shows that 19.92% on average for infected Bursaphelenchus Xylophilus and 26.27% for non-infected which decreases water content as the size of conduction, convection, and radiant heat increases in case of fire. As the vaporization of the moisture content accelerates, the average moisture content is reached, and the result is contrary to the thermal characteristics of the wood that the moisture content is not ignited by a heat source when the moisture content is under 15%. The combustion heat analysis result showed that infected and non-infected Bursaphelenchus Xylophilus had no significant difference in combustion heat at about 3%. However, it shows that combustion heat is higher than other species. In conclusion, decreasing in moisture content based on the increasing conduction, convection, and radiant heat is one of the direct causes of ignition, and the lower the moisture content, the faster the fire spreads.

• Journal of the Korean Wood Science and Technology
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• v.44 no.3
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• pp.370-380
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• 2016

The black pine logs damaged by pine wilt disease in Jeju-do were heat-treated to extend the utilization of domestic trees damaged by pine wilt disease. The heat-treatment of wood requires wood to be heated to $56^{\circ}C$ for 30 min at the core. The average moisture content and top-diameter of the black pine logs were ranged from 46% to 141% and from 180 mm to 500 mm, respectively. And the basic specific gravity and oven-dry specific gravity of the black pine logs were 0.47 and 0.52, respectively. The time required for heat-treatment at $105^{\circ}C$ temperature was ranged from 7.7 h to 44.2 h, depending on moisture content and top-diameter. The temperature distribution was used to predict the time required for heat-treatment of black pine log with various moisture contents and top-diameters using finite difference method. The thermal properties of wood including the thermal conductivity and specific heat in accordance with moisture content were calculated. Heat transfer coefficient for mixed convection in form of adding natural convection and forced convection was used for heat transfer analysis. The error between the measured and predicted values ranged from 3% to 45%. The predicted times required for heat-treatment of black pine log with 50% moisture content and 200 mm, 300 mm, and 400 mm top-diameter were 10.9 h, 18.3 h, and 27.0 h, respectively. If the initial moisture content of black pine log is 75%, heat treatment times of 13.6 h, 22.5 h, and 32.8 h were predicted in accordance with top-diameter. And if the initial moisture content of black pine log is 100%, heat treatment times of 16.2 h, 26.5 h, and 38.2 h were predicted in accordance with top-diameter. When the physical properties of logs damaged by pine wilt disease are presented, these results can be applicable to the heat-treatment of red pine and Korean pine logs as well.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.12
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• pp.1383-1389
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• 2011

The current study focuses on the consistent analysis of heat transfer in multichannel volumetric solar receivers used for concentrating solar power. Changes in the properties of the absorbing material and channel dimensions are considered in an optical model based on the Monte Carlo ray-tracing method and in a one-dimensional heat transfer model that includes conduction, convection, and radiation. The optical model results show that most of the solar radiation energy is absorbed within a very small channel length of around 15 mm because of the large length-to-radius ratio. Classification of radiation losses reveals that at low absorptivity, increased reflection losses cause reduction of the receiver efficiency, notwithstanding the decrease in the emission loss. As the average temperature increases because of the large channel radius or small mass flow rate, both emission and reflection losses increase but the effect of emission losses prevails.

• Solar Energy
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• v.10 no.3
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• pp.13-18
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• 1990

When nonuniform thermal boundary conditions are imposed on the surface of a circular cylinder in crossflow, the heat transfer characteristics can be quite different compared to what is found for isothermal or constant heat flux boundary conditions. In the present analysis, two kinds of nonuniform boundary conditions along the circumference of the cylinder are considered in a uniform stream of air: step changes and linear profiles. Step changes in temperature can arise on the surface of an external, cylindrical, solar central receiver. As the working fluid(water) flows through the vertical tubes that ring the circumference of Solar One(a solar central receiver in Barstow, California), the solar flux on the receiver heats the water from a liquid to a superheated state. In this process, portions of the receiver panels, and thus portions of the circumference of the cylinder, function as a preheater, boiler, or superheater. Hence the surface temperature can vary significantly around the cylinder. Common engineering practice has been to use an average wall temperature with an isothermal cylinder heat transfer coefficient when estimating the convective loss in these kinds of situations.

• Solar Energy
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• v.15 no.1
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• pp.3-11
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• 1995

Heat transfer phenomena during inward freezing process of the water in a horizontal cylinder were experimentally studied. The cooling temperature of a wall more significantly affects the timewise average temperature than the initial superheating temperature of the water. In addition, it was absolved that the timewise average temperature was influenced by the initial volume ratio of the water($V_l/V_{tot}$) at the same temperature conditons. One the other hand, the freezing speed of the upper part in the water-ice interface was quickly progressed due to natural convection. Furthermore, experimental observation showed that the frozen mass fraction($M_s/M_{tot}$) was influenced by the initial volume ratio of the water($V_l/V_{tot}$). It was noted that the frozen mass fraction for each $V_l/V_{tot}$ represented by $Ste^*$ and Fo.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.9
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• pp.605-612
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• 2016

In this study, numerical simulations were conducted for conjugate heat transfer around ice balls in an encapsulated ice thermal storage system. Four shapes of ice balls were modeled; the default one was a sphere, and the other three shapes were designed to enhance convective heat transfer through the ball surface. The flow around the ball was laminar, for which the Reynolds number was 300, and both forced and natural convections inside and outside the balls were considered. The simulations revealed that the magnitude of convective heat transfer for the different shapes decreased in the following order: bone, dimple, hole, and sphere. For the entire simulation, the maximum difference in the average temperatures of water inside the capsules was found to be $0.9^{\circ}C$. Therefore, it can be said that the effect of ice-ball shape on the performance of the ice thermal storage system is significant, considering that more than 0.3 million balls are used in this system.

• Journal of the Korean Society of Safety
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• v.35 no.3
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• pp.96-104
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• 2020

The numerical simulations were conducted to investigate the thermal-fluid phenomena occurred inside the experimental apparatus during a PCCS, used to remove heat released in accidents from a containment of light water nuclear power plant, operation. Numerical simulations of the flow and heat transfer caused by wall condensation inside the containment simulation vessel (CSV), which equipped with 18 vertical heat exchanger tubes, were conducted using the commercial computational fluid dynamics (CFD) software ANSYS-CFX. Shear stress transport (SST) and the wall condensation model were used for turbulence closure and wall condensation, respectively. The simulation using the actual size of the apparatus. However, rather than simulating the whole experimental apparatus in consideration of the experimental cases, calculation resources, and calculation time, the simulation model was prepared only in CSV. Selective simulation was conducted to verify the effects of non-condensable gas(NC gas) concentration, CSV internal pressure, and wall sub-cooling conditions. First, as a result of the internal flow of CSV, it was observed that downward flow due to condensation occurred surface of the vertical tube and upward flow occurred in the distant place. Natural convection occurred actively around the heat exchanger tube. Due to this rising and falling internal flow, natural circulation occurred actively around the heat exchanger tubes. Next, in order to check the performance of built-in condensation model using according to the non-condensable gas concentration, CSV internal flow and wall sub-cooling, the heat flux values were compared with the experimental results. On average, the results were underestimated with and error of about 25%. In addition, the influence of CSV internal pressure and wall sub-cooling was small, but when the condensate was highly generated due to the low non-condensable gas concentration, the error was large compared to the experimental values. This is considered to be due to the nature of the condensation model of the CFX code. However, in spite of the limitations of CFD, it is valid to use the built-in condensation model of CFD for PCCS performance prediction from a conservative perspective.

• Nuclear Engineering and Technology
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• v.23 no.2
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• pp.200-209
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• 1991

A new semi-empirical average heat transfer correlation applicable for both laminar and turbulent film-wise condensation on a vertical surface has been presented. Re functional form of the present correlation is based on the representative existing correlations for laminar and turbulent film flows, whereas the numerical coefficients of the present correlation have been determined by the least squares method using experimental data obtained from the open literatures. In addition, the performance of the present as well as the seven existing correlations (four for laminar and three for turbulent film flow regimes) were evaluated for their accuracy and the range of application. The result shows that for laminar film filow regimes Zazuli's and the present correlations give the samllest values of mean error, whereas for turbulent film How regimes Kirkbride and Badger's and the present correlations produce the smallest values of mean error.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.9 no.3
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• pp.312-322
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• 1997

The natural convective flow in a two-dimensional square enclosure with horizontal partitions is investigated numerically. The enclosure was composed of the lower hot and the upper cold horizontal walls and the adiabatic vertical walls, and two identical partitions were positioned perpendicularly at the mid-height of the right and left walls, respectively. The governing equations are solved by using the finite element method with Galerkin method. Calculations are made for different partition lengths, partition conductivites, and Rayleigh numbers based on the temperature difference between two horizontal walls and the enclosure height with water(Pr=4.95). An oscillatory motion of the natural convective flow is affected significantly by the variation of the gap width and Rayleigh number. When the gap width is comparatively short, the heat transfer rate is raised with the increase of the thermal conductivity of partitions. However, for sufficiently large gap widths at higher Rayleigh numbers, the average Nusselt numbers of the conductive partitions are smaller than those of the adiabatic partitions.

• Structural Engineering and Mechanics
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• v.88 no.6
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• pp.521-533
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• 2023

In the present study, the finite volume method is applied for the thermal performance prediction of the natural ventilation system using vertical solar chimney whereas, design parameters are optimized through the response surface methodology (RSM). The computational simulations are performed for various parameters of the solar chimney such as absorber temperature (40≤T_abs≤70℃), inlet temperature (20≤T₀≤30℃), inlet height of (0.1≤h≤0.2 m) and chimney width (0.1≤d≤0.2 m). Analysis of variance (ANOVA) was carried out to identify the design parameters that influence the average Nusselt number (Nu) and mass flow rate (ṁ). Then, quadratic polynomial regression models were developed to predict of all the response parameters. Consequently, numerical and graphical optimizations were performed to achieve multi-objective optimization for the desired criteria. According to the desirability function approach, it can be seen that the optimum objective functions are Nu=25.67 and ṁ=24.68 kg/h·m, corresponding to design parameters h=0.18 m, d=0.2 m, T_abs=46.81℃ and T₀=20℃. The optimal ventilation flow rate is enhanced by about 96.65% compared to the minimum ventilation rate, while solar energy consumption is reduced by 49.54% compared to the maximum ventilation rate.