• 설비공학논문집
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• 제16권9호
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• pp.845-854
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• 2004

The evaporation heat transfer coefficient for R-l34a, R-407C (a mixture of 23wt% R-32, 25 wt% R-125, and 52 wt% R-l34a) and R-410A (a mixture of 50 wt% R-32 and 50 wt% R-125) flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the exchanger by four plates of commercial geometry with a corrugated sinusoid shape of a chevron angle of 45 degree. The effects of the mean vapor quality, mass flux, heat flux, and saturation temperature of different refrigerants on the evaporation heat transfer were explored in detail. Similar to the case of a Plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. It is found that the evaporation heat transfer coefficient in the plates is much higher than that in circular pipes. The present data show that the evaporation heat transfer coefficients of all refrigerants increase with the vapor quality. At a higher mass flux h, is higher than for the entire range of the vapor quality. Raising the imposed wall heat flux was found to slightly improve h$_{r}$, while h$_{r}$ is found to be lower at a higher refrigerant saturation temperature. A comparison of the performance of the various refrigerants reveals that R-410A has the highest heat transfer performance followed by R-l34a, and R-407C had the lowest performance of the refrigerants tested. Based on the present data, empirical correlations of the evaporation heat transfer coefficient were proposed.sed.

• 열처리공학회지
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• 제36권6호
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• pp.402-411
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• 2023

In order to examine the effect of cementite precipitated on the steel surface on the carburizing rate, the carburizing process was carried out at various boost times to measure the mass gain and carbon flux, phase analysis and carbon concentration analysis were performed on the surface of the carburized specimen. In the case of the only boost type, the longer the boost time, the more the mass gain by the diffused carbon follows the parabolic law and tends to increase. In particular, as the boost time increased, the depth of cementite precipitation and the average size of cementite on the steel surface increased. At a boost time of 7 min, the fraction of cementite precipitated on the surface is 7.32 vol.%, and the carburizing rate of carbon into the surface (surface-carbon flux) is about 17.4% compared to the calculated value because the area of the chemical (catalyst) where the carburization reaction takes place is reduced. The measured carbon concentration profile of the carburized specimen tended to be generally lower than the carbon concentration calculated by the model without considering precipitated cementite. On the other hand, in the pulse type, the mass gain by the diffused carbon increased according to the boost time following a linear law. At a boost time of 7 min, the fraction of cementite precipitated on the surface was 3.62 vol.%, and the surface-carbon flux decreased by about 4.1% compared to the calculated value. As a result, a model for predicting the actual carbon flux was presented by applying the carburization resistace coefficient derived from the surface cementite fraction as a variable.

• 설비공학논문집
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• 제18권9호
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• pp.687-697
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• 2006

The air and water flow distribution are experimentally studied for a heat exchanger composed of round headers and 10 flat tubes. The effects of tube protrusion depth as well as mass flux, and quality are investigated, and the results are compared with the previous 30 channel results. The flow at the header inlet is annular. For the downward flow configuration, the water flow distribution is significantly affected by the tube protrusion depth. For flush-mounted geometry, significant portion of the water flows through frontal part of the header. As the protrusion depth increases, more water is forced to the rear part of the header. The effect of mass flux or quality is qualitatively the same as that of the protrusion depth. Increase of the mass flux or quality forces the water to rear part of the header. For the upward flow configuration, different from the downward configuration, significant portion of the water flows through the rear part of the header. The effect of the protrusion depth is the same as that of the downward flow. As the protrusion depth increases, more water is forced to the rear part of the header. However, the effect of mass flux or quality is opposite to the downward flow case. As the mass flux or quality increases, more water flows through the frontal part of the header. Compared with the previous thirty channel configuration, the present ten channel configuration yields better flow distribution. Possible explanation is provided from the flow visualization results.

• Journal of Mechanical Science and Technology
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• 제18권12호
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• pp.2284-2293
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• 2004

The evaporation heat transfer coefficient h$\_$r/ and frictional pressure drop Δp$\_$f/ of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45 chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R-134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficient h$\_$r/ and pressure drop Δp$\_$f/ increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in the h$\_$r/ and Δp$\_$f/. But the effect of the average heat flux does not show significant effect on the h$\_$r/ and Δp$\_$f/. Finally, at a higher saturation temperature, both the h$\_$r/ and Δp$\_$f/ are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.

• 대한기계학회논문집B
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• 제25권10호
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• pp.1318-1326
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• 2001

The evaporation heat transfer coefficient and pressure drop for refrigerant R-22 flowing in the plate and shell heat exchanger were investigated experimentally in this study. Two vertical counterflow channels were farmed in the exchanger by three plates of commercial geometry with a corrugated trapezoid shape of a chevron angel of 45 ° Upflow boiling of refrigerant R-22 in one channel receives heat from the hot downf1ow of water in the other channel. The effects of the mean vapor quality, mass flux, heat flux and pressure of R-22 on the evaporation heat transfer and pressure drop were explored. The quality change of R-22 between the inlet and outlet of the refrigerant channel ranges from 0.03 to 0.05. The present data showed that both the evaporation heat transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux, the evaporation heat transfer coefficient and pressure drop are higher for the entire range of the vapor quality Raising the imposed wall heat flux was found to slightly improve the heat transfer, while at a higher refrigerant pressure, both the heat transfer and pressure drop are slightly lower.

• 동력기계공학회지
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• 제16권4호
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• pp.37-43
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• 2012

This paper performed experimental research in order to measure boiling heat transfer coefficient of water in microchannel with hydraulic diameter of $500{\mu}m$. Tests were conducted within the ranges of heat fluxes from 100 to 400 kW/$m^2$, vapor qualities from 0 to 0.2, and mass fluxes of 200, 400, and 600 kg/$m^2s$. From the experimental results, it was found that flow boiling heat transfer coefficient is not dependent on mass flux or vapor quality, but instead on heat flux to a certain degree. The measured data of heat transfer are compared to a few available correlations proposed for mini-channels. Among them, Sun and Mishima's correlation is found to predict the present data well, within the mean absolute error of 17.84%.

• 대한기계학회논문집B
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• 제32권4호
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• pp.290-299
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• 2008

Evaporation heat transfer characteristics of $CO_2$/propane mixtures in horizontal smooth and micro-fin tubes have been investigated by experiment. The experiments were carried out for several test conditions of mass fluxes, heat fluxes, compositions of $CO_2$/propane refrigerant mixtures and tube geometries. Direct heating method was used for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. Heat transfer coefficient data during evaporation process of $CO_2$/propane mixtures were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 mm, respectively. The tests were conducted at mass fluxes of 318 to 997 $kg/m^2s$, heat fluxes of 6 to 20 $kW/m^2$ and for several mixture compositions (100/0, 75/25, 50/50, 25/75, 100/0 by wt% of $CO_2$/propane). The differences of heat transfer characteristics between smooth and micro-fin tubes for various compositions of $CO_2$/propane refrigerant mixtures and the effect of mass flux, and heat flux on enhancement factor (EF) and penalty factor (PF) were presented.

• 대한기계학회논문집B
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• 제25권7호
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• pp.981-988
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• 2001

Characteristics of evaporation heat transfer in 6.2 and 5.1mm OD micro-fin tubes were investigated in the present study. The data were taken at evaporation temperatures of -5$^{\circ}C$ and 5$^{\circ}C$ and heat fluxes 5kW/$m^2$ to 10kW/$m^2$. Mass flux was consequently maintained at 210, 300 and 410kg/$m^2$s for the 6.2mm OD tube and 465, 500 and 600kg/$m^2$s for the 5.1mm OD tube. The effects of heat flux, mass flux, and outer diameter on the heat transfer coefficient are explored in the present study. The data showed that the evaporation heat transfer coefficient for the 6.2mm OD tube was averagly higher by 16% than that for a 7.0mm OD tube, while the 5.1mm OD tube had approximately 30% higher value than the 6.2mm OD tube.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2008년도 동계학술발표대회 논문집
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• pp.227-232
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• 2008

The present study was investigated the local heat transfer characteristics and temperature distribution on the louver fin by using the expansion model. Heat transfer rate, frost mass and temperature distribution of the louver fin under frosting condition were experimentally investigated. Local heat transfer rate and heat flux on the louver were analyzed by the conduction heat transfer between top and lower part of the louver. The experimental key parameter was brine inlet temperature(-5, -10, $-15^{\circ}C$). The heat transfer performance and frost mass at brine temperature of $-15^{\circ}C$ were increased by maximum 3 time than the brine temperature of $-5^{\circ}C$. At all experimental case, local heat transfer rate and heat flux of the louver were almost symmetry at the louver number of 6. Especially, local heat transfer rate and heat flux were maximum increased on the louver number of 4 and 8.

• Journal of Mechanical Science and Technology
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• 제14권9호
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• pp.997-1004
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• 2000

The experiments were conducted by using water and paraffin slurry to investigate the effect of a chip spacing in the multichip module on the cooling characteristics from an in-line $4{\times}3$ array of discrete heat sources which were flush mounted on the top wall of a channel. The experimental parameters were chip spacing in a multichip module, heat flux of simulated VLSI chip, mass fraction of paraffin slurry, and channel Reynolds number. The removable heat flux at the same chip surface temperature decreased as the chip spacing decreased at the first and fourth rows. The local heat transfer coefficients for the paraffin slurry were larger than those for water, and the chip spacing on the local heat transfer coefficients for paraffin slurry influenced less than that for water. The enhancement factor for paraffin slurry showed the largest value at a mass fraction of 5% regardless of the chip spacing, and the enhancement factors increased as the chip spacing decreased. This means that the paraffin slurry is more effective than water for cooling of the highly integrated multichip module.