• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.20 no.9
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• pp.571-580
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• 2008

In this work, pool boiling heat transfer coefficients(HTCs) of 5 refrigerants of differing vapor pressure are measured on horizontal smooth square surface of 9.52 mm length. Tested refrigerants are R123, R152a, R134a, R22, and R32 and HTCs are taken from $10\;kW/m^2$ to critical heat flux of each refrigerant. Wall and fluid temperatures are measured directly by thermocouples located underneath the test surface and by thermocouples in the liquid pool. Test results show that pool boiling HTCs of refrigerants increase as the heat flux and vapor pressure increase. This typical trend is maintained even at high heat fluxes above $200\;kW/m^2$. Zuber's prediction equation for critical heat flux is quite accurate showing a maximum deviation of 21% for all refrigerants tested. For all refrigerant data up to the critical heat flux, Stephan and Abdelsalam's well known correlation underpredicted the data with an average deviation of 21.3% while Cooper's correlation overpredicted the data with an average deviation of 14.2%. On the other hand, Gorenflo's and lung et al.'s correlation showed only 5.8% and 6.4% deviations respectively in the entire nucleate boiling range.

• Nuclear Engineering and Technology
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• v.50 no.7
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• pp.1068-1078
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• 2018

Experiments were performed to investigate bubble behaviors and pool boiling Critical Heat Flux (CHF) on a thin flat rectangular copper heater fabricated on Printed Circuit Board (PCB), at various inclination angles. The surface inclination angles were $0^{\circ}$, $45^{\circ}$, $90^{\circ}$, $135^{\circ}$, and $180^{\circ}$. Results showed the Onset of Nucleate Boiling (ONB) heat flux increased with increasing heater orientation from $0^{\circ}$ to $90^{\circ}$, while early ONB occurred when the heater faced downwards ($135^{\circ}$ and $180^{\circ}$). The nucleate boiling was observed to be unstable at low heat flux (1-21% of CHF) and changed into typical boiling when the heat flux was above 21% of CHF. The result shows the CHF decreased with increasing heater orientation from $0^{\circ}$ to $180^{\circ}$. In addition, the bubble departure diameter at the heater facing upwards ($0^{\circ}$, $45^{\circ}$, and $90^{\circ}$) was more prominent compared to that of the heater facing downward ($135^{\circ}$). The nucleation site density also observed increased with increasing heat flux. Moreover, the departed bubbles with larger size were observed to require a longer time to re-heat and activate new nucleation sites. These results proved that the ONB, CHF, and bubble dynamics were strongly dependent on the heater surface orientation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.7
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• pp.665-676
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• 2011

In this study, the nucleate pool boiling heat transfer coefficients (HTCs) and critical heat flux (CHF) for a smooth and square flat heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at $60^{\circ}C$ were measured. Tested aqueous nanofluids were prepared using CNTs with volume concentrations of 0.0001%, 0.001%, and 0.01%. The CNTs were dispersed by chemically treating them with an acid in the absence of any polymers. The results showed that the pool boiling HTCs of the nanofluids are higher than those of pure water in the entire nucleate boiling regime. The acid-treated CNTs led to the deposition of a small amount of CNTs on the surface, and the CNTs themselves acted as heat-transfer-enhancing particles, owing to their very high thermal conductivity. There was a significant increase in the CHF- up to 150%-when compared to that of pure water containing CNTs with a volume concentration of 0.001%. This is attributed to the change in surface characteristics due to the deposition of a very thin layer of CNTs on the surface. This layer delays nucleate boiling and causes a reduction in the size of the large vapor canopy around the CHF. This results in a significant increase in the CHF.

• Proceedings of the Korean Nuclear Society Conference
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• 2003.05a
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• pp.138.2-138.2
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• 2003

• Proceedings of the Korean Nuclear Society Conference
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• 2001.05a
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• pp.147.2-147
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• 2001

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.2 s.233
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• pp.169-179
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• 2005

Nucleate boiling experiments on heating surface of constant wall temperature were performed using R113 for almost saturated pool boiling conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain a constant wall temperature condition of heating surface and to measure the heat flow rate with high temporal and spatial resolutions. Bubble images during the bubble growth were taken as 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble growth behavior was analyzed using the new dimensionless parameters for each growth regions to permit comparisons with previous experimental results at the same scale. We found that the new dimensionless parameters can describe the whole growth region as initial and later (thermal) respectively. The comparisons showed good agreement in the initial and thermal growth regions. In the initial growth region including surface tension controlled, transition and inertia controlled regions as divided by Robinson and Judd, the bubble growth rate showed that the bubble radius was proportional to $t^{2/3}$ regardless of working fluids and heating conditions. And in the thermal growth region as also called asymptotic region, the bubble showed a growth rate that was proportional to $t^{1/5}$, also. Those growth rates were slower than the growth rates proposed in previous analytical analyses. The required heat flow rate for the volume change of the observed bubble was estimated to be larger than the heat flow rate measured at the wall. Heat, which is different from the instantaneous heat supplied through the heating wall, can be estimated as being transferred through the interface between bubble and liquid even with saturated pool condition. This phenomenon under a saturated pool condition needs to be analyzed and the data from this study can supply the good experimental data with the precise boundary condition (constant wall temperature).

• International Journal of Air-Conditioning and Refrigeration
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• v.8 no.2
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• pp.23-28
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• 2000

In this study, pool boiling performance of a structured enhanced tube for a flooded refrigerant evaporator was experimentally investigated. Tests were performed for three different refrigerants(R-11, R-123, R-l34a). Compared with the heat transfer coefficients of the smooth tube, the heat transfer coefficients of the enhanced tube were 6.6 times larger for R-11, 6.0 times larger for R-123 and 3.5 times larger for R-l34a, which are comparable with the performance of foreign products. The heat transfer coefficients of R-l34a was higher than those of R-11 or R-123, both for the enhanced tube and for the smooth tube. At 4.4$^\circ$C saturation temperature, however, the heat transfer coefficients of R-l34a was approximately the same as those of R-11. The effect of the saturation pressure on the boiling performance was similar to that of the smooth tube-the heat transfer coefficient increased as the saturation pressure increased.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.9
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• pp.484-492
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• 2013

Nucleate pool boiling heat transfer coefficients (HTCs) are measured with HFC32/HFC152a mixture at several compositions. All data are taken at the liquid pool temperature of $7^{\circ}C$, on a horizontal plain square surface of $9.53{\times}9.53$ mm, with heat fluxes of 10 $kW/m^2$ to 100 $kW/m^2$ with an interval of 10 $kW/m^2$, in the increasing order of heat flux. Test results show that the HTCs of these mixtures are up to 45% lower than those of the ideal HTCs calculated by a linear mixing rule with pure fluids' HTCs, due to the mass transfer resistance associated with non-azeotropic refrigerant mixtures. Pool boiling data show the deduction in HTCs with an increase in GTD of the mixture. The present mixture data agree well with five well known correlations, within 20% deviation.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.11a
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• pp.230-235
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• 1996

The new concept of dry area formation based on Poisson distribution of active nucleation sites and the concept of the critical active site density is presented. A simple statistical model is developed to predict the change of slope of the boiling curve up to critical heat flux (CHF) quantitatively. The predictions by the present model are in good agreement with the experimental data. Also it turns out that the present model well explains the mechanism on how the surface wettability influences CHF.

• Nuclear Engineering and Technology
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• v.28 no.1
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• pp.1-16
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• 1996

In an effort to determine the combined effects of major parameters of heat exchanger tubes on the nucleate pool boiling heat transfer in the scaled in-containment refueling water storage tank (IRWST), a total of 1,966 data for q'quot; versus ${\Delta}T$ has been obtained using various combinations of tube diameters, surface roughness, and tube orientations. The experimental results show that (1) increased surface roughness enhances heat transfer for both horizontal and vertical tubes, (2) the two heat transfer mechanisms, i.e.,enhanced heat transfer for both horizontal and vertical tubes, (2) the two heat transfer mechanisms, i.e., enhanced heat transfer due to liquid agitation by bubbles generated and reduced heat transfer by the formation of large vapor slugs and bubble coalescence are different in two regions of low heat fluxes (q'quot; $\leq$50kW/$m^2)$ and high heat fluxes (q'quot; $\geq$50kW/$m^2)$ depending on the orientation of tubes and the degree of surface roughness, and (3) the heat transfer rate decreases as the tube diameter is increased for both horizontal and vertical tubes, but the effect of tube diameter on the nucleate pool boiling heat transfer for vertical tubes is greater than that for horizontal tubes. Two empirical heat transfer correlations for q'quot;, one for horizontal tubes and the other for vertical tubes, are obtained in terms of surface roughness $({\varepsilon})$ and tube diameter (D). In addition, a simple empirical correlation for nucleate pool boiling heat transfer coefficient $(h_b)$ is obtained as a function of heat flux (q'quot;) only.ucleate pool boiling heat transfer coefficient $(h_b)$ is obtained as a function of heat flux (q'quot;) only.