• Nuclear Engineering and Technology
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• v.54 no.3
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• pp.867-876
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• 2022

An experimental study has been conducted to investigate the heat transfer characteristics of supercritical carbon dioxide (sCO₂) uniformly heated in the horizontal circular smooth tube. The results illustrated that there was a significant difference in heat transfer between the top wall and bottom wall due to the buoyancy. Bulk flow acceleration cannot be negligible in the high heat flux region, which leads to heat transfer deterioration. A new heat transfer correlation is proposed, in which the buoyancy parameter and bulk flow acceleration have been taken into account. The new correlation and six classic correlations for sCO₂ are examined in horizontal tubes. The comparison indicates that the new correlation has a better performance for sCO₂ flowing through a horizontal heating tube under natural circulation conditions. For example, 94.9% of the calculated results using the new heat transfer correlation were within ±30% of the experimental results while only 87.9% of that using the Jackson correlation (the best of the six) were within the same error bands.

• Steel and Composite Structures
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• v.47 no.2
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• pp.289-296
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• 2023

The failure of a thin-walled tube was studied in this paper based on three failure models. Both proportional and non-proportional loading paths were applied. Proportional loading consisted of combined tension-torsion. Cyclic non-proportional loading was also applied. It was a circular out-of-phase axial-shear stress loading path. The third loading path was a combination of a constant internal pressure and a bending moment. The failure models under study were equivalent plastic strain, modified Mohr-Coulomb (Bai-Wierzbicki) and Tearing parameter models. The elasto-plastic analysis was conducted using J2 criterion and nonlinear kinematic hardening. The return mapping algorithm was employed to numerically solve the plastic flow relations. The effects of the hydrostatic stress on the plastic flow and the stress triaxiality parameter on the failure were discussed. Each failure model under study was utilized to predict failure. The failure loads obtained from each model were compared with each other. The equivalent plastic strain model was independent from the stress triaxiality parameter, and it predicted the highest failure load in the bending problem. The modified Mohr-Coulomb failure model predicted the lowest failure load for the range of the stress triaxiality parameter and Lode's angle.

• Computers and Concrete
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• v.33 no.6
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• pp.643-658
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• 2024

To date, shell-solid and fibre element model analysis are the most commonly used methods to investigate the seismic performance of concrete-filled steel tube (CFST) bridge piers. However, most existing research does not consider the loss of bearing capacity caused by the fracture of the outer steel tube. To fill this knowledge gap, a refined finite element (FE) model considering the ductile damage of steel tubes and the behaviour of infilled concrete with cracks is established and verified against experimental results of unidirectional, bidirectional cyclic loading tests and pseudo-dynamic loading tests. In addition, a parametric study is conducted to investigate the seismic performance of CFST bridge piers with different concrete strength, steel strength, axial compression ratio, slenderness ratio and infilled concrete height using the proposed model. The validation shows that the proposed refined FE model can effectively simulate the residual displacement of CFST bridge piers subjected to highintensity earthquakes. The parametric analysis indicates that CFST piers hold sufficient strength reserves and sound deformation capacity and, thus, possess excellent application prospects for bridge construction in high-intensity areas.

• Journal of the Korean Society for Advanced Composite Structures
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• v.2 no.4
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• pp.19-27
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• 2011

In this paper, we present a part of results to develop new type hybrid FRP-concrete composite pile (i.e., concrete filled fiber reinforced plastic circular tubes, hybrid CFFT, HCFFT). The purpose of this paper is to evaluate compressive loading capacity through compressive strength test. Before compressive strength test of HCFFT, we investigated mechanical properties of pultruded fiber reinforced plastic (PFRP) and filament winding fiber reinforced plastic (FFRP). For estimating the compressive strength of HCFFT, uni-axial compression strength tests of HCFFT compression members were conducted. The test variables are compressive strengths of concrete and thickness of FFRP. In addition, uni-axial compression strength tests of concrete filled fiber reinforced plastic circular tube (CFFT) except PFRP members were conducted. The test variable in the test is thickness of FFRP. From the test result, the compressive strength of the HCFFT in larger than compressive strength of CFFT as much as 47%. It can be observed that the uni-axial compressive strength of the HCFFT increased if the concrete strength and the thickness of exterior filament winding FRP tube increased. In addition, the finite element analysis result is compared with the experimental result. The difference between the experimental and FEM results is in the range of 0.14% to 17.95%.

• Journal of Energy Engineering
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• v.22 no.3
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• pp.287-293
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• 2013

This study was carried out to diminish the formation of scale in boilers which is one of the defect elements when they are operating. The defect relating to scale can cause a fatal impact on the explosion of boilers due to the overheating of their tubes, or it can affect the flow of water inside boilers with its bad circulation and result in a disparity of water inside the equipment. Heat transfer in the scale is low comparing to the boiler material, so it can lead to energy losses and has also impact on the global warming. In 2005, the Korean government introduced a system which requires boiler users to install the equipment which can prevent or eliminate the formation of scale to improve the management of water quality in boilers. The study on the techniques for preventing or eliminating the formation of scale started in 1821 and since then subsequently there have been lots of similar studies. The first one was about the scale reduction using potato starch. Since an ultrasonic scale preventer developed by a scientist from a Russian acoustic institute was introduced in1993, a variety of equipment of this kind have been disseminated in Korea. There has been a need to demonstrate the condition for the best performances of such equipment. Boilers are mostly composed of the main body and 288 the tube with a circular curved surface. I carried out a demonstration study on a circular tube which affects the scale defect the most among the boiler components. As a result of it, I found out the fact that the ultrasonic wave needs to reach a certain level of sound pressure and frequency to affect the formation of scale.

• Journal of the Korean Institute of Gas
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• v.25 no.1
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• pp.14-19
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• 2021

The change of the vorticity and the temperature distribution in heat exchanger tube bank were analyzed for the flows with the constant inlet velocity and the sinusoidal inlet velocity. The flow frequency characteristics were examined by analyzing power spectral density of lift and drag at a typical circular tube in the tube bank. Karman vortex street could be seen at the upstream region of tube bank for the case of constant inlet velocity. It could be seen that the Karman vortex street was affected by the change of inlet velocity near the circular tubes for the case with the sinusoidal inlet velocity. It was observed that the unsteady temperature distributions for both inlet velocity conditions had almost the same motion as the flow vorticity behavior. The flow frequency for the case with the constant inlet velocity is 37.25Hz, and that with the sinusoidal inlet velocity, the flow frequency is 18.63Hz, which is equal to the sinusoidal inlet velocity. The mean surface Nusselt number(Nu) for overall heat exchanger tube bank was 1051 for the case with the constant inlet velocity and 1117 for the case with the sinusoidal inlet velocity. From the result of heat transfer analysis, it could be seen that Nu with the sinusoidal inlet velocity showed 6.3% increase than that with the constant inlet velocity.

• 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.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2402-2407
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• 2007

Heat transfer distributions and friction factors in square channels (3.5 ${\times}$ 3.5 cm) with twisted tape inserts and with twisted tape inserts plus interrupted ribs are respectively investigated. Tests are performed for Reynolds numbers ranging from 8,900 to 29,000. The rib height-to-channel hydraulic diameter, e/Dh, is kept at 0.057 and test section length-to-hydraulic diameter, L/Dh is 30. The twisted tape is 0.1 mm thick carbon steel sheet with diameter of 3.3cm, length of 90cm, and 2.5 turns. The square ribs are arranged to follow the trace of the twisted tape and along the flow direction defined as axial interrupted ribs. Each wall of the square channel is composed of isolated aluminum sections. The following conclusions from the experimental study were drawn as: 1) In the 4 heating wall channel with twisted tape inserts, Nusselt number based on bottom wall temperature is enhanced by 1.2 - 1.6 times if adding the axial interrupted ribs on the bottom wall only. 2) The twisted tape with interrupted ribs under the two-sided heating condition produces the highest heat transfer performance. 3) Friction factor data obtained for the square channel with twisted tape inserts plus axial interrupted ribs are less than those in the past publications for circular tubes with axial interrupted ribs and twisted tape inserts.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.34 no.2
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• pp.355-365
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• 2014

Concrete-filled tubes (CFTs) have been used in civil engineering practices as a column of buildings and a bridge pier. CFTs have several advantages over the conventional reinforced concrete columns, such as rapid construction, enhanced buckling resistance, and inherited confinement effect. However, CFT component have not been widely used in civil engineering practice, since the design provisions among codes significantly vary each other. It leads to conservative design of CFT component. In this study, the design provisions of AISC and EC4 for CFT component were examined, based on the extensive test results conducted by previous researchers and finite element analysis results obtained in this study. Especially, the focus was made on the validation of P-M interaction curves proposed by AISC and EC4. From the results, it was found that the current design codes considerably underestimated the strength of CFT component under general combined axial load and bending. Finally, the modified P-M interaction curve was proposed and successfully verified.

• Steel and Composite Structures
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• v.18 no.6
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• pp.1569-1582
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• 2015

This study explores the lateral deflections of diagonal braces in concentrically-braced earthquake-resisting frames. The performance of this widely-used system is often compromised by the flexural buckling of slender braces in compression. In addition to reducing the compressive resistance, buckling may also cause these members to undergo sizeable lateral deflections which could damage surrounding structural components. Different approaches have been used in the past to predict the mid-length lateral deflections of cyclically loaded steel braces based on their theoretical deformed geometry or by using experimental data. Expressions have been proposed relating the mid-length lateral deflection to the axial displacement ductility of the member. Recent experiments were conducted on hollow and concrete-filled circular hollow section (CHS) braces of different lengths under cyclic loading. Very slender, concrete-filled tubular braces exhibited a highly ductile response, undergoing large axial displacements prior to failure. The presence of concrete infill did not influence the magnitude of lateral deflection in relation to the axial displacement, but did increase the number of cycles endured and the maximum axial displacement achieved. The corresponding lateral deflections exceeded the deflections observed in the majority of the previous experiments that were considered. Consequently, predictive expressions from previous research did not accurately predict the mid-height lateral deflections of these CHS members. Mid-length lateral deflections were found to be influenced by the member non-dimensional slenderness (${\bar{\lambda}}$) and hence a new expression was proposed for the lateral deflection in terms of member slenderness and axial displacement ductility.