• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.3
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• pp.309-314
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• 2000

Experimental results for heat transfer characteristics of natural refrigerants R-290, R-600a and HCFC refrigerant R-22 during evaporating inside horizontal double pipe heat exchangers are presented. The experimental apparatus is basically a vapour heat pump system, composed of a compressor, a condenser, expansion devices, a evaporator, and some other peripheral devices. The test sections were horizontal double pipe heat exchangers, which were a pair of smoothed tube, having 10.07 mm ID, 12.07 mm OD, and grooved inner fin tube, having 12.70 mm OD, 0.25 mm fin height, and 75 fins. The local evaporating heat transfer coefficients of natural refrigerants were not much affected with the mass velocity than R-22 and it could be interpreted that the local evaporating heat transfer coefficients of R-22 were increased more than those of R-290, R-600a according to the increment of mass velocity. Moreover, the maximum increment of the heat transfer coefficient was found in R-290. The average heat transfer coefficient was obtained the maximum value in R-290 and the minimum value in R-22. It reveals that the natural refrigerant can be used as a substitute for R-22. In the grooved inner fin tube, 70% of the increment of the heat transfer coefficient was obtained compared to the smoothed tube.

• Journal of Korea Foundry Society
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• v.13 no.5
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• pp.432-440
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• 1993

Ductile cast iron has a good ductility and toughness than those of gray cast iron, because the shape of graphite is spheroidal. Also, it has been reported that, additional strengthening and toughening of the ductile cast iron can be obtainded from the proper combination of matrix structures by the heat treatment and addition of alloying elements. In this study the effect of special heat treatment and addition of alloying elements(Ni, Mo) on the multi-phase(ferrite-bainite-martensite) structures, strength and toughness of ductile cast iron were studied systematically. In water quenching from $770^{\circ}C$, the martensite volume(%) increased, but the ferrite volume(%) decreased with increment of Ni content. In as cast, pearlite volume(%) and hardness increased with increment of Mo and Ni contents. And with the increment of the destabilization austempering holding time, the bainite volume(%) increased but the martensite volume(%) decreased. As destabilization austempering holding time is same, bainite volume(%) decreased, martensite increased with the increment of Ni and Mo contents. The hardness and tensile strength decreased, but impact energy increased with the decrease of Ni and Mo contents, and increment of holding time of destabilization austempering treatment.

• Journal of the Korean Society of Industry Convergence
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• v.11 no.4
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• pp.157-163
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• 2008

The present study investigated the pressure drop and the total frost mass of the louvered fin type heat exchanger, which is widely used at the air-conditioning system. The pressure drop due to the frosting phenomenon and the total frost mass were investigated by changing the wet bulb temperature condition of the inlet air. Hence the brain of 55wt% was used as a cooling solution instead of a common refrigerant. The temperature difference between the brine and the tube outside wall at the outlet of heat exchanger was $10^{\circ}C$, at maximum, higher than that at the inlet of heat exchanger. As the wet bulb temperatures were increased, the pressure drop was linearly increased due to the increment of frost mass. And the increment of heat exchange rate was smaller than that of inlet air enthalpy due to the increment of frost mass. The pressure drop of air side was rapidly increased due to the progress of frosting phenomena. The run time that the pressure drop occurred rapidly was decreased by the growth of frost.

• KSTLE International Journal
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• v.1 no.1
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• pp.63-68
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• 2000

Angular contact ball bearings are mainly used in the spindle, which requires high speed and stiffness. The heat generation is studied by experiments and simulations using a pair of angular contact ball bearings. The temperature variation of inner and outer races and the temperature increment distribution are measured by using thermocouples for the rotational speed, preload, viscosity of lubricant. The measured values from experiments are used to estimate the heat conduction rate. The method of oil-air lubrication is used for the experiment. The amount of conduction heat transfer to the test spindle and the convection heat transfer coefficients long the spindle are computed by using inverse method with temperature increment distribution. Total heat generation rate is estimated with the heat partition rate which is calculated from temperatures of inner and outer races. In addition, the empirical factor of oil-air lubrication method for Palmgren's heat generation model is suggested. The empirical friction coefficients, which are obtained from the experiments, depend on the preload condition, and can give us more accurate estimation of the heat generation in ball bearings.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.18 no.1
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• pp.15-22
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• 1985

The study on computation time, accuracy, and convergency characteristic of the implicit finite difference method is presented with the variation of the space increment and time step in a two-dimensional transient heat conduction problem with a dirichlet boundary condition. Numerical analysis were conducted by the model having the conditions of the solution domain from 0 to 3m, thermal diffusivity of 1.26 $m^2/h$, initial condition of 272 K, and boundary condition of 255.4 K. The results obtained are summarized as follows : 1) The degree of influence with respect to the accuracy of the time step and space increment in the alternating-direction implicit method and Crank-Nicholson implicit method were relatively small, but in case of the fully implicit method showed opposite tendency. 2) To prescribe near the zero for the space increment and tine step in a two dimensional transient problem were good in a accuracy aspect but unreasonable in a computational time aspect. 3) The reasonable condition of the space increment and the time step considering accuracy and computation time could be generalized with the Fourier modulus increment, F, ana dimensionless space increment, X, irrespective of the solution domain.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.6
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• pp.82-91
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• 2021

We compared the heat transfer characteristics of the parallel and the counterflow flow in the concentric double tube of the Al₂O₃/water nanofluids using numerical methods. The high- and low-temperature fluids flow through the inner circular tube and the annular tube, respectively. The heat transfer characteristics according to the flow direction were compared by changing the volume flow rate and the volume concentration of the nanoparticles. The results showed that the heat transfer rate and overall heat transfer coefficient improved compared to those of basic fluid with increasing the volume and flow rate of nanoparticles. When the inflow rate was small, the heat transfer performance of the counterflow was about 22% better than the parallel flow. As the inflow rate was increased, the parallel flow and the counterflow had similar heat transfer rates. In addition, the effectiveness of the counterflow increased from 10% to 22% rather than the parallel flow. However, we verified that the increment in the friction factor of the counterflow is not large compared to the increment in the heat transfer rate.

• 사료
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• s.51
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• pp.46-50
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• 2011

• Journal of Advanced Marine Engineering and Technology
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• v.24 no.4
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• pp.414-420
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• 2000

In this paper, evaporation heat transfer characteristics at a inner grooved tube were studied using a new natural refrigerants R-290, R-600a and HCFC refrigerant R-22. Experiments were performed in the inner tube with outside diameter of 12.70mm, having 75 fins with a fin height of 0.25mm. The following results were obtained from this research. On the evaporating heat transfer characteristics, the maximum increment of heat transfer coefficient was found in R-290. Average heat transfer coefficient was obtained the maximum value in R-290 and the minimum value in R-22. It reveals that the natural refrigerant can be used as a substitute for R-22. In the grooved inner tube, 70% of the increment of the heat transfer coefficient was obtained compared to the smooth tube. Comparing the heat transfer coefficient between experimental results and simulation data of other's, the Kandlikar's correlated equation was closely approximated to the author's experimental results in the smooth tube or grooved inner one.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.54 no.11
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• pp.477-482
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• 2005

We have investigated volume resistivity and thermal properties showed by changing the content of carbon black which is the component parts of semiconducting shield in underground power transmission cable. Specimens were made of sheet form with the nine of specimens for measurement. Volume resistivity of specimens was measured by volume resistivity meter after 10 minutes in the preheated oven of both 25$\pm$1[$^{\circ}C$] and 90$\pm$1[$^{\circ}C$]. And specific heat (Cp) and thermal conductivity were measured by Nano Flash Diffusivity and DSC (Differential Scanning Calorimetry). The measurement temperature ranges of specific heat using the BSC was from 20[$^{\circ}C$] to 60[$^{\circ}C$], and the heating rate was 1[$^{\circ}C$/min]. And the measurement temperatures of thermal conductivity using Nano Flash Diffusivity were both 25[$^{\circ}C$] and 55[$^{\circ}C$]. Volume resistivity was high according to an increment of the content of carbon black from these experimental results. And specific heat was decreased, while thermal conductivity was increased by an increment of the content of carbon black. And both specific heat and thermal conductivity were increased by heating rate because volume of materials was expanded according to rise in temperature.

• Journal of Biosystems Engineering
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• v.36 no.4
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• pp.267-272
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• 2011

The latent heat of vaporization in rough rice, brown rice, white rice and rice hull was calculated by Clausius-Clapeyron equation, which does not require complex constraints as in Othmer method. Equilibrium relative humidity and ratio of the latent heat of vaporization with ln$P_{\upsilon}$ and ln$P_S$ were estimated with moisture contents ranging from 10% (d.b.) to 36% (d.b.) with 2% (d.b.) increment and temperatures ranging from $10^{\circ}C$ to $50^{\circ}C$ with $2.5^{\circ}C$ increment. An empirical equation for calculating the latent heat of vaporization in rice was developed as a function of moisture content and temperature. The equation agreed well with the calculated results. The ratio for latent heat of vaporization were the greatest for white rice while they were similar among rough rice, brown rice and rice hull.