• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.7
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• pp.435-440
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• 2016

Effects of the increase rate of Reynold number on near-wall turbulent structures are investigated by performing direct numerical simulations of transient turbulent channel flows. The simulations were started with the fully-developed turbulent channel flow at $Re_{\tau}=180$, then temporal accelerations were applied. During the acceleration, the Reynolds number, based on the channel width and the bulk mean velocity, increased almost linearly from 5600 to 13600. To elucidate the effects of flow acceleration rates on near-wall turbulence, a wide range of durations for acceleration were selected. Various turbulent statistics and instantaneous flow fields revealed that the rapid increase of flow rate invoked bypass-transition like phenomena in the transient flow. By contrast, the flow evolved progressively and the bypass transition did not clearly occur during mild flow acceleration. The present study suggests that the transition to the new turbulent regime in transient channel flow is mainly affected by the flow acceleration rate, not by the ratio of the final and initial Reynolds numbers.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.11
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• pp.1289-1301
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• 2004

Direct numerical simulation (DNS) of strongly-heated air flows moving upward in a vertical tube has been conducted to investigate the effect of gas property variations on turbulence modification. Three heating conditions(q$_1$$^{+}$=0.0045, 0.0035 and 0.0018) are selected to reflect the experiment of Shehata and McEligot (1998) at the inlet bulk Reynolds numbers of 4300 and 6000. At these conditions, the flow inside the heated tube remains turbululent or undergoes a transition to subturbulent or laminarizing flow. Consequently, a significant impairment of heat transfer occurs due to the reduction of flow turbulence. The predictions of integral parameters and mean profiles such as velocity and temperature distributions are in excellent agreement with the experiment. The computed turbulence data indicate that a reduction of flow turbulence occurs mainly due to strong flow acceleration effects for strongly-heated internal gas flows. Thus, buoyancy influences are secondary but not negligible especially for turbulent flow at low heating condition. Digital flow visualization also shows that vortical structures rapidly decay as the heating increases.s.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.24 no.3
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• pp.265-270
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• 2012

A finned tube type heat exchange module has been proposed for a multi-burner water tube boiler. Fin density and length increase in streamwise direction to equalize the evaporation for each module, which makes it difficult to apply conventional bulk design procedure. The design program has been improved by updating data for every row of tubes along the flow. A numerical simulation has been also conducted to evaluate the effect of inlet conditions and validated with experiment. The heat transfer of the first row has been underpredicted by the conventional Zhukauskas correlation, since the acceleration of the flow due to the blockage is not fully inflected. The fin tip temperature is also underpredicted by Bessel solution, because of the interaction with neighboring fins.

• Proceedings of the SAREK Conference
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• 2007.11a
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• pp.528-533
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• 2007

A finned tube type heat transfer module has been proposed for a multi-burner water tube boiler. Fins change their geometry along the streamwise direction to maximize the performance, which makes it difficult to apply conventional bulk analysis. The design program has been improved by updating data for every row of tubes along the flow. A numerical simulation has been also performed to evaluate the effect of inlet conditions and validated with experiment. The heat transfer of the first row has been underpredicted by the conventional Zhukauskas correlation, where the acceleration of the flow due to the blockage is not fully inflected. The fin tip temperature is also underpredicted by Bessel solution, because of the interaction with neighboring fins.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.7
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• pp.607-618
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• 2014

An experimental investigation of turbulent heat transfer during the vertical upward flow of supercritical $CO_2$ was conducted in a circular tube with inner diameter of 4.5 mm. The experiments were conducted at bulk fluid temperatures ranging from 29 to $115^{\circ}C$, pressures from 74.6 to 102.6 bar, local wall heat fluxes from 38 to $234kW/m^2$, and mass fluxes from 208 to $874kg/m^2s$. At moderate wall heat and low mass fluxes, the wall temperature had a noticeable peak value. For observing the buoyancy and flow acceleration effects on heat transfer, the ratios of Nusselt numbers from the experimental data and a reference correlation were compared with the $Bo^*$ and $q^+$ distributions. The flow acceleration parameter $q^+$ appropriately represented the heat transfer phenomena in the experiments. A new heat transfer correlation for the vertical upward flow of the supercritical pressure fluid was developed, and was found to agree with the experimental data with an error margin of ${\pm}30%$.