• Journal of Advanced Marine Engineering and Technology
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• v.18 no.3
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• pp.26-33
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• 1994

A new reliable method to prediet the axial vapor fraction distribution from the measured probability density of the liquid bulk temperature is suggested in this paper. And also the actual quality of the subcooled boiling flow is easily calculated from the liquid bulk temperature. When the heat generating rate is reached to the CHF value, the sharp wall temperature increasing by the wall temperature fluctuation is occurred under the CHF condition. This paper presents the simple wall temperature fluctuation model of transition boiling by the repeating process of overheating and quenching, when the coalescent bubble passes slowly near the wall. Experiments for the subcooled R-113 flow are carride-out in the range of(0.9399~4.461)${\times}10^6$kg/$m^2$hr mass velocity and 10~3$0^{\circ}C$ intel subcooling condition.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2297-2302
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• 2007

Numerical simulations are conducted to analyze conjugate heat transfer characteristics in a ribbed channel. In this simulation, the effects of Reynolds number and heat capacity of the solid channel wall on convective heat transfer are observed in the turbulent flow regime. In the case of the conducting wall against isothermal wall, the relative ratio of the thermal resistance between the solid wall and the flow field varies with Reynolds number. Thus the characteristics of the conjugate heat transfer are changed with the Reynolds number. Heat capacity ratio affects the temperature fluctuation inside solid wall. The temperature fluctuation inside the solid wall decreases with increasing the heat capacity of the solid wall so that the convective heat transfer increases. When the thermal conductivity ratio is smaller than 10, the effects of flow characteristics on heat transfer are changed.

• International Journal of Advanced Culture Technology
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• v.3 no.2
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• pp.171-178
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• 2015

In this work, mixing processes of hot and cold fluids of three different jet types are predicted by large-eddy simulation (LES) on FLUENT platform. Temperature at different positions of internal wall and mixing conditions of T-junctions at different times are obtained, then the simulated normalized mean and root-mean square (RMS) temperature, temperature contour and velocity vector of every case are compared. The results indicate that, the mixing regions in the tee junction is related to the jet type, and temperature fluctuations on the pipe wall in the type of the deflecting jet is the least.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.9
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• pp.1272-1280
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• 2001

An experimental study has been conducted to elucidate the resonance of natural convection in a side-heated square enclosure having a mechanically oscillating bottom wall. Under consideration is the impact of the imposed oscillating frequency, amplitude and the system Rayleigh number on the fluctuation of air temperatures. The experimental results show that the magnitude of the fluctuation of air temperature is substantially augmented at a specific forcing frequency of the oscillating bottom wall. The resonant frequency is increased with the increase of the Rayleigh number and it is little affected by the amplitude of the oscillating wall. It is also found that the resonant frequency is relevant to the Brunt- V$\"{a}$iS$\"{a}$l$\"{a}$ frequency which represents the stratification degree of the system.

• Nuclear Engineering and Technology
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• v.54 no.10
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• pp.3928-3942
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• 2022

Thermal striping is a complex thermal-hydraulic phenomenon caused by fluid temperature fluctuations that can also cause high-cycle thermal fatigue to the structural wall of sodium-cooled fast reactors (SFRs). Numerical simulations using large-eddy simulation (LES) were performed to predict and evaluate the characteristics of the temperature fluctuations related to thermal striping in the upper internal structure (UIS) of the prototype generation-IV sodium-cooled fast reactor (PGSFR). Specific monitoring points were established for the fluid region near the control rod driving mechanism (CRDM) guide tubes, CRDM guide tube walls, and UIS support plates, and the normalized mean and fluctuating temperatures were investigated at these points. It was found that the location of the maximum amplitude of the temperature fluctuations in the UIS was the lowest end of the inner wall of the CRDM guide tube, and the maximum value of the normalized fluctuating temperatures was 17.2%. The frequency of the maximum temperature fluctuation on the CRDM guide tube walls, which is an important factor in thermal striping, was also analyzed using the fast Fourier transform analysis. These results can be used for the structural integrity evaluation of the UIS in SFR.

• The KSFM Journal of Fluid Machinery
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• v.10 no.5
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• pp.9-19
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• 2007

Fluid temperature fluctuations in a mixing tee pipe were numerically analyzed by LES model in order to clarify internal turbulent flows and to develope an evaluation method for high-cycle thermal fatigue. Hot and cold water with an temperature difference $40^{\circ}C$ were supplied to the mixing tee. Fluid temperature fluctuations in a mixing tee pipe is analysed by using the computational fluid dynamics code, FLUENT, Temperature fluctuations of the fluid and pipe wall measured as the velocity ratio of the flow in the branch pipe to that in the main pipe was varied from 0.05 to 5.0. The power spectrum method was used to evaluate the heat transfer coefficient. The fluid temperature characteristics were dependent on the velocity ratio, rather than the absolute value of the flow velocity. Large fluid temperature fluctuations were occurred near the mixing tee, and the fluctuation temperature frequency was random. The ratios of the measured heat transfer coefficient to that evaluated by Dittus-Boelter's empirical equation were independent of the velocity ratio, The multiplier ratios were about from 4 to 6.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.148-153
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• 2000

An experimental study was conducted to investigate an isothermal characteristics of the condenser surface of disk type heat pipe using PF 5060 and FC 40 as working fluids. Desired temperature range at the condenser surface was $90^{\circ}C{\sim}130^{\circ}C$, which was cooled by natural convection. The operating temperature, the height of vapor space, the wall thickness of condenser, and the existence of a wick structure were considered as experimental factors. The temperature difference and the temperature fluctuation at the condenser surface were analysed to describe the performance of the heat pipe.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.9
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• pp.2953-2964
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• 1996

Heat Transfer with periodic fluctuation of fluid temperature caused by oscillatory flow or compression expansion can be out of phase with balk fluid-wall temperature difference. Newton's law of convection is inadequate to describe this phenomenon. In order to solve this problem the concept of the complex Nusselt number has been introduced by severla researchers. The complex Nusselt number expresses out of phase excellently while the first harmonic is dominant in the variations of both fluid-wall temperature difference and heat flux. However, in the case of oscillatory flow with non-linear wall temperature distribution, the complex Nusselt number is not appropriate to predict the heat transfer phenomena since the higher order harmonic components appear in periodic temperature variation. Analytic solutions to the heat transfer with an sinusoidal well temperature distribution were obtained to investagate the effect of non-linear wall temperature distribution. A new formula considering the thermal boundary layer was suggested based on the solutions. A comparison was also made with the complex Nusselt number. It was verified that the new formula describes well the heat transfer of oscillating flow even if the first harmonic component is not dominant in the fluid-wall temperature difference.

• Journal of Advanced Marine Engineering and Technology
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• v.13 no.4
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• pp.40-53
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• 1989

Boiling heat transfer phenomena is widely applied to BWR and electrical heating system because of its high heat transfer coefficient. In these systems, steady state heat transfer is dependent on nucleate boiling. When the heat generating rate is sharply increased or the cooling capacity of coolant is sharply decreased, sharp wall temperature rise is occurred under the critical heat flux(CHF) condition. This paper presents the simple wall temperature fluctuation model of transition mechanism in the repeating process of overheating and quenching, when coalescent bubble passes relatively slowly on the wall and simultaneously the transition from nucleate boiling to film boiling is carried at especially onset of the CHF state. The values calculated by the present model are resulted comparatively good with the measured.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.4
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• pp.15-22
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• 2002

As the emission regulations on the automobiles have been increasingly stringent, precise control of air/fuel ration is one of the most important issues on the gasoline engines. Although many researches have been carried out to identify the fuel transport phenomena in the port fuel injection gasolines, mixture preparation in the cylinder has not been fully understood due to the complexity of fuel film behavior, In this paper, the mixture preparation during cold engine start is studied by using a Fast Response Flame ionization Detector.(FRFID) In order to estimate the transportation of injected fuel from the intake port into cylinder, the wall wetting fuel model was used. The two coefficient($\alpha$,$\beta$) of the wall-wetting fuel model was determined from the measured fuel mass that was inducted into the cylinder at the first cycle after injection cut-in. $\alpha$( ratio of directly inducted fuel mass into cylinder from injected fuel mass) and $\beta$ (ratio of indirectly inducted fuel mass into cylinder from wall wetted fuel film on the wall) was increased with increasing cooling water temperature. To reduce a air/fuel ratio fluctuation during cold engine start, the appropriate fuel injection rate was obtained from the wall wetting fuel model. Result of air/fuel ratio control, air/fuel excursion was reduced.