• Proceedings of the KSME Conference
• /
• 2004.04a
• /
• pp.1540-1545
• /
• 2004

This paper presents a semi-empirical model to predict the frost growth formed on the cold cylinder surface. The model is composed of the correlations for frost properties including the various frosting parameters and local heat transfer coefficient. The effects of varying the correlations for local heat transfer coefficient on the frost growth are examined to establish the model. The numerical results are compared with experimental data obtained by the previous researchers. The results agree well with the experimental data within a maximum error of 13%. As the results, the frost thickness decreases with changing angular position from front stagnation to separation point. Also the effects of air velocity on the frost growth are negligible, as compared to the other frosting parameters.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.18 no.5
• /
• pp.402-409
• /
• 2006

For the design of a liquid direct contact cooling system, thermal and hydraulic analysis has been carried out. Well-known Zukauskas correlations are used to estimate the Nusselt number between the liquid refrigerant columns and the inlet airflow. The inlet air velocity is set at a typical value used in an actual showcase. For a constant column number, the best nozzle arrangement is determined for the maximum heat transfer. Heat transfer increases as the transverse pitch of the refrigerant column decreases. Among all the cases dealt with in the present study, the staggered arrangement with 140-columns of $14{\times}10$ shows the best thermal peformance and the expected temperature drop is $27.8^{\circ}C$. The effect of downstream refrigerant columns on the overall thermal performance is investigated as well.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.18 no.12
• /
• pp.1007-1016
• /
• 2006

Although the advance in electronic technology enables a large number of circuity to be packed in a small volume, it is simultaneously required to remove the high heat load produced by them. In this study, the heat transfer and pressure drop characteristics of a mini-channel heat exchanger, which is designed for liquid cooling of electronic components, are investigated by varying operating conditions. Water and FC-72 were used as working fluids. The mini-channel heat exchanger was made with circular shape channels having din-meters of 2, 3, and 4 mm in regular intervals, and the channel length was 100 mm. The header and inlet guide pathway to provide uniform inflow were attached at the inlet of the test section. Copper block including the heaters was attached at the sidewall of the test section as a heat source, which provided the heat flux from 5 to $15W/cm^2$. The entrance effects enhanced the heat transfer coefficient in the mini-channel significantly. In addition, the single-phase pressure drop in the mini-channel was very similar to that predicted by the laminar flow correlation except that the transition Re decreased due to flow instability in the entrance region.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.37 no.1
• /
• pp.27-33
• /
• 2013

To ensure proper functioning of electrical and mechanical systems, cooling devices are of great importance. A heat sink is the most common cooling device used in many industries such as the semiconductor, electronic instrument, LED lighting, and automotive industries. To design an optimal heat sink, the required surface area for heat radiation should be calculated based on an accurate expectation of the heat flow rate in the target environment. In this study, the convective heat flow characteristics were numerically investigated for a vertically installed typical heat sink and a horizontally installed one in free convection using ANSYS CFX. Comparative experiments were carried out to reveal the quantitative effect of the installation direction on the cooling performance. Moreover, the result was analyzed using the dimensionless correlation with the Nusselt number and Rayleigh number and compared with well-known theories. Finally, it was observed that the cooling performance of the vertically installed heat sink is approximately 10~15% better than that of the one in natural convection.

• Journal of the Korean Society of Combustion
• /
• v.12 no.1
• /
• pp.1-10
• /
• 2007

Analytical investigation of acoustic wave scattering from turbulent premixed flames was conducted to evaluate the acoustic energy amplification/damping. Such acoustic energy change is attributed to the acoustic velocity jump due to flame's heat release. Small perturbation method up to second order and stochastic analysis were utilized to formulate net acoustic energy and the energy transfer from coherent to incoherent energy. Randomly wrinkled flame surface is responsible for the energy transfer from coherent to incoherent field. Nondimensional parameters that govern net acoustic energy were determined: rms height and correlation length of flame front, incident wave frequency, incidence angle, and temperature ratio. The dependence of net acoustic energy upon these parameters is illustrated by numerical simulations in case of Gaussian statistics of flame front. Total net energy was amplified and the major factors that affect such energy amplification are incidence angle and temperature ratio. Coherent (incoherent) energy is damped (amplified) with rms height and correlation length of flame front.

• Nuclear Engineering and Technology
• /
• v.23 no.2
• /
• pp.200-209
• /
• 1991

A new semi-empirical average heat transfer correlation applicable for both laminar and turbulent film-wise condensation on a vertical surface has been presented. Re functional form of the present correlation is based on the representative existing correlations for laminar and turbulent film flows, whereas the numerical coefficients of the present correlation have been determined by the least squares method using experimental data obtained from the open literatures. In addition, the performance of the present as well as the seven existing correlations (four for laminar and three for turbulent film flow regimes) were evaluated for their accuracy and the range of application. The result shows that for laminar film filow regimes Zazuli's and the present correlations give the samllest values of mean error, whereas for turbulent film How regimes Kirkbride and Badger's and the present correlations produce the smallest values of mean error.

• Proceedings of the Korean Nuclear Society Conference
• /
• 1997.05a
• /
• pp.409-414
• /
• 1997

Reflood model in RELAP5/MOD3.1 are modified to improve the unrealistic prediction results of the model. In the new method, the modified Zuber pool boiling critical heat flux (CHF) correlation is adopted. The reflood drop size is characterized by the use of We=1.5 and the minimum drop size of 0.0007 m for $p^{*}\;{\leq}\;0.025$. To describe the wall to vapor heat transfer at low pressure and low flow condition, the Webb-Chen correlation is utilized . The suggested method has been verified through the simulations of the Lehigh University rod bundle reflood tests. Through sensitivity study it is shown that the effect of drag coefficients is dominant in the reflood model. It is proved that the present modifications result in much more improved quench behavior and accurate wan and vapor temperature predictions.

• Journal of Mechanical Science and Technology
• /
• v.15 no.3
• /
• pp.357-365
• /
• 2001

The aim of this paper is to perform the experiment and the numerical simulation for investigating the heat transfer in a regenerator system with ceramic honeycomb and to suggest a useful correlation for optimization of the regenerator system. For achieving this, the effects of some parameters were investigated, e. g., switching time, cell size and length of honeycomb on the mean temperature efficiency. The measured temperatures by R-type thermocouples were compared with the predictions by means of the commercial package, STAR-CD. A useful correlation for thermal efficiency was newly proposed as a function of the normalized switching time, defined in terms of switching time, cell size and length of honeycomb. The results showed that the thermal efficiency is above 90% and the normalized heat exchange rate is higher than 80% when the normalized switching time is less than 1000.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.10 no.7
• /
• pp.1465-1472
• /
• 2009

The effect of heat loss rate on NOx formation of $CH_4/air$premixed flame were examined numerically in a perfectly stirred reactor. The following conclusions were drawn. Under the adiabatic wall condition, an increase in the residence time causes a remarkable increases in NOx emission. Under the heat loss conditions, however, NOx decreases significantly as the heat transfer coefficient and residence time increase. As the heat loss rate increases, Thermal NO mechanism and Re-burning NO mechanism play an important role in the NOx reduction, but Prompt NO mechanism and $N_2O$-intermediate NO mechanism lead to the increase in NOx production. Although the NOx formation is actually related to complex NOx mechanism with the changes in the heat transfer coefficient and residence time, it was found that NOx concentration can be represented by independent Thermal NO mechanism. From these results, new NOx correlation combined with the heat loss rate and residence time was suggested for predicting the NOx concentration in a practical $CH_4/air$premixed combustor.

• Journal of Environmental Science International
• /
• v.3 no.2
• /
• pp.89-100
• /
• 1994

It is very important to assess accurately the terms which are included in the heat budget equation of soil surface because they are used in the UM and miso-scale circulation modeling as well as in the micrometeorological studies. Each terms in the heat budget equation change according to the soil moisture content. So, it is necessary to specify clearly the relations between soil moisture content and these terms. Special experiment with micrometeorological measurements was executed to study these relations at Environmental Research Center of Tsukuba University, Japan. The results are as follow: 1. The soil moisture contents of 1 cm and 4 cm depth are oscillated with one day Period in drying process and the amplitude of variation of 1 cm depth is greater than that of 4 cm. 2. Increase in soil moisture contents due to precipitation result in decrease of albedo with step function. 3. Sensible heat is in reverse proportion to the soil moisture content and latent heat is in direct proportion to it. Latent heat is more sensitive than sensible heat according to the soil moisture variation. Net long wave radiation have high correlation with soil moisture. 4. Comparing with the radiative term with the flux term in wetting process due to precipitation, the energy transfer of the aero and thermodynamic flux is greater than that of the radiative heat flux.