• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.7
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• pp.980-991
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• 1998

In this study, nucleate pool boiling heat transfer coefficients of alternative refrigerants on a plain, low fin, and two enhanced tubes were measured and compared against each other. To obtain data at conditions similar to the actual evaporator, a fluid heating method was employed instead of an electric heating method in the experiments. R123, R134a, R22 and R410a were used as working fluids and data were taken at 7 deg.C ar heat fluxes of 20 ~ 100 kW/m$\^$2/. Comparison of the plain tube data against some correlations showed that the simplest correlation of Cooper based on reduced pressure predicted the data for all fluids tested with a 10% deviation. For all refrigerants, enhanced tubes composed of subsurface and subtunnels, especially Thermoexcel-E tube, showed the highest heat transfer coefficients among the tubes tested with one exception that the low fin tube's performance was better than those of enhanced tubes for high vapor pressure fluid such as R410a at high heat flux. Finally, a low fin and enhanced tubes showed higher heat transfer enhancement for low vapor pressure of R123 than for high vapor pressure fluisd. For R123, the enhancement factors for Turbo-B and Thermoexcel-E tubes were 2.8 ~ 4.8 and 4.6 ~ 8.1 respectively.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.9
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• pp.761-772
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• 2015

Microfin tubes having an outside diameter (O.D.) of 7.0 mm are widely used in residential air conditioning systems and heat pumps. It is known that the mass fluxes for air conditioners and heat pumps under partial load conditions are several tens of $kg/m^2s$. However, literature surveys reveal that previous investigations were limited to mass flux over $100kg/m^2s$. In this study, we conduct R-410A evaporation heat-transfer tests at low mass fluxes ($50-250kg/m^2s$) using a 7.0 mm O.D. microfin tube. During the test, the saturation temperature was maintained at $8^{\circ}C$, and the heat flux was maintained at $4.0kW/m^2$. For comparison purposes, we also test a smooth tube with a 7.0 mm O.D. The results showed that the heat-transfer enhancement factor of the microfin tube increased as the mass flux decreased up to $150kg/m^2s$, which decreased as the mass flux further decreased. The reason for this was attributed to the change of the flow pattern from an annular flow to a stratified flow. Within the test range, the frictional pressure drops of the microfin tube were approximately the same as those of the smooth tube. We then compare experimental data obtained with the predictions obtained for the existing correlations.

• Aerospace Engineering and Technology
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• v.4 no.1
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• pp.196-202
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• 2005

In cryogenic feeding system of turbo pump fed liquid-propulsion rocket, rise of cryogenic propellant temperature can bring into geysering in pipe or cavitation in turbo pump. In this paper, performance analysis of recirculation line which is one of the method to inhibit these phenomenon is carried out based on the configuration of KSLV-I 1st stage LOX feeding system, and parametric study to find design parameter. Diameter and re-entrance height, initial LOX temperature, ullage pressure, and natural convection heat transfer coefficient are varied to see the effects on performance. Additional He is injected into recirculation line to promote LOX recirculation. 1-dimensional analysis using network-solver, SINDA/FLUINT is carried out.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.7
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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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.8
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• pp.1097-1103
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• 2000

This research was concerned with the enhancement of heat transfer by surfactant added to the aqueous solution of LiBr. Different horizontal tubes were tested with and without an additive of normal octyl alcohol. The test tubes were a bare tube, floral tube and hydrophilic tube. The additive mass concentration was about 0.05${\sim}$5.5%. The heat transfer coefficient was measured as a function of solution flow rate for the range of 0.01${\sim}$0.034 kg/ms. The experimental results were compared with cases without surfactant. The enhancement of heat transfer by Marangoni convection effect generated by the addition of the surfactant is observed in each test tube. The increase of heat transfer coefficient by surfactant addition is about 35${\sim}$90% for bare tube, 40${\sim}$70% for the floral tube, 30${\sim}$50% for the hydrophilic tube and was higher for the cases with smaller a little solution flow rates.

• Journal of Advanced Marine Engineering and Technology
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• v.32 no.1
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• pp.50-56
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• 2008

Experimental results for forced convection condensation of Refrigerant-22 and ternary Refrigerant-407C(HFC-32/125/134a 23/25/52 wt%) which is being considered as a substitute R-22 inside a horizontal micro-fin tube are presented. The test section was horizontal double-tube counterflow condenser with a length 4,000 mm micro-fin tube, having 8.53 mm ID, 0.2 mm fin height and 60 fins. The range of parameters of mass velocity were varied from 102.1 to 301.0 kg/(m2.s) and inlet quality 1.0. At the given experimental conditions. the average heat transfer coefficients for R-407C were lower than that for R-22 at a micro-fin tube. Over the mass velocity range tested. the PF(penalty factor) for R-22, R-407C were lower than the increasing ratio of heat transfer area by fins, and the EF(enhancement factor) for R-22, R-407C were higher than the increasing ratio of heat transfer area by fins.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.5
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• pp.1275-1287
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• 1994

Heat transfer performance improvement by fin and grooves is studied for condensation of R-11 on integral-fin tubes. Eight tubes with trapezoidal shaped integral-fins having fin densities from 748 to 1654 fpm and 10, 30 grooves are tested. A plain tube having the same diameter as the finned tubes is also tested for comparison. R-11 condenses at saturation state of $32^{\circ}C$ on the outside tube surface cooled by inside water flow. All of test data ate taken at steady state. Beatty and Katz's, Rudy's and Webb's theoretical models are used to predict the R-11 condensation coefficient of tubes having 748, 1024 and 1299 fpm. The predicted value by Betty and Katz's model is within 10% of experimental values in this study at fpm<1024 and Rudy's model predicted the experimental data at fpm>1024 within 15%. The tube having fin density of 1299 fpm and 30 grooves has the best overall heat transfer performance. This tube shows the overall heat transfer coefficient of 11500 $W/m^{2}K$,/TEX> at coolant velocity of 3.0m/s.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.5
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• pp.744-752
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• 2000

The influence of arrangement and length of discrete ribs on heat/mass transfer and friction loss is investigated. Mass transfer experiments are conducted to obtain the detailed local heat/mass transfer information on the ribbed wall. The aspect ratio (width/height) of the duct is 2.04 and the rib height is one tenth of the duct height, such that the ratio of the rib height to hydraulic diameter is 0.0743. The ratio of rib-to-rib distance to rib height is 10. The discrete ribs were made by dividing each continuous rib into 2, 3 or 5 pieces and attached periodically to the top and the bottom walls of the duct with a parallel orientation The combined effects of rib angle and length of the discrete ribs on heat/mass transfer ae considered for the rib angles $({\alpha})\;of\;90^{\circ}\;and\;45^{\circ}$. As the number of the discrete ribs increases, the uniformity of the heat/mass transfer distributions increases. For $(\alpha})=90^{\circ}$, the heat/mass transfer enhancement with the discrete ribs is remarkable, while the heat/mass transfer performances are slightly higher than that of the transverse continuous ribs due to the accompanied high friction loss penalty. For $(\alpha})=90^{\circ}$, the average heat/mass transfer coefficients and the heat/mass transfer performances decrease slightly with the discrete ribs compared to the case of the angled continuous ribs.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.9
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• pp.69-76
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• 2018

The present study analyzed the effect of film hole position of 45 degree ribbed cooling channel on film cooling performance of gas turbine blades. We also investigated the influence of the ribs under the fixed blowing ratio. Three-dimensional numerical model was constructed and extensive simulation was conducted using the commercial code (Fluent ver. 17.0) under steady-state condition. Base on the simulation results, We investigated the cooling effectiveness, flow velocity, streamline, and pressure coefficient. Moreover, We analyzed the effect of cooling hole position on ejection of the secondary flow caused by the rib structure. From the results, It was found that internal flow of the cooling channel forms a vortex pair in the counterclockwise from the top side, and clockwise from the bottom side. For the channels with ribs, the vortex flow generated by the ribs caused a higher pressure difference near the hole outlet, resulting in at least 12% higher cooling effectiveness than the channel without ribs. Additionally, when the hole is located on the left side of the ribbed channel (Rib-Left), it can be found that the secondary flow generated by the ribs hits against wall surface near the hole to form a flow in the direction of the hole inclination angle. Therefore, It is considered that the region where the cooling gas discharged to the blade surface stays in the main flow boundary layer is wider than the other cases. In this case, The largest pressure coefficient difference was observed near the outlet of the hole, and as a result, the discharge of the cooling gas was accelerated and the cooling efficiency was slightly increased.