수직 채널내의 가열된 돌출 배열에서의 대류 열전달

趙秉守,白秉峻,朴福春 全北大學校 1996 論文集 Vol.41 No.-

An electronic circuit board was simulated by protrusions, which was heated by thin stainless foil attached on the surface, in a vertical channel. Natural and forced convection experiments were carried out in order to investigate the effects of channel spacing, gap between protrusions and number of rows of protrusion. In natural convection, the optimum channel spacing was found to be approximately 30mm regardless of the protrusion gap. For optimum channel spacing the heat transfer coefficients were converged to an asymptotic value after fourth row. The heat transfer coefficient for each row approaches to constant values for protrusion gaps larger than 10 mm. An experimental correlation has been suggested by using a modified Rayleigh number, based on the dimensionless characteristic length(G/L). In forced convection, the heat transfer coefficients were not merged to an asymptote until the fifth row and increased as channel spacing at the constant Reynolds number decreases.

A Turbulent Friction and Heat Transfer Study of Dispersed Fluids with Ultra-Micronized Metallic Particles

Bock Choon Pak,Seong Hoon Choi,Byung Joon Back,H. Masuda 대한기계학회 1994 KSME International Journal Vol.8 No.3

Separation and Reattachment of Non-Newtonian Fluid Flows Downstream of an Abrupt Circular Duct Expansion

Bock Choon Pak,Young I. Cho 대한기계학회 1990 대한기계학회 춘추학술대회 Vol.- No.-

Viscoelastic Fluid Flow in a Sudden Expansion Circular Channel as a Model for the Blood Flow Experiments

Pak, Bock-Choon,Kim, Cheol-Sang The Korea Society of Medical and Biological Engine 1990 의공학회지 Vol.11 No.2

In the current flow visualization studies, the role of non-Newtonian characteristics (such as shearra to dependent viscosity and viscoelasticity ) on flow behavior across the sudden ex- pansion step in a circular pipe as a model for blood flow experiments is investigated over a wide range of Reynolds numbers. The expansion ratios tested are 2.000 and 2.667 and the range of the Reynolds number covered in the current flow visualization tests are 10~35, 000 based on the inlet. diameter. The reattachment longuEs for the viscoelastic fluids in the lami- nar flow regime are found to be much shorter than those for the Newtonian fluid. In addition it decreases significantly with increasing concentration of viscoelastic fluids at the same Reynolds number. However, in the turbulent flow regime, the reattachment length for the viscoelastic fluids Is two or three times longer than those for water, and gradually increases with increasing concentration of viscoelastic solutions, resulting In 25 and 28 step-height dis- tances for 500 and 1, 000 lpm ployacrylamide solutions, respectively. This may be due to the fact that the elasticity in pobacrylamide solutions suppresses the eddy motion and controls separation and reattachment behavior in the sudden expansion pips flow.

Impacts of Fouling and Cleaning on the Performance of Plate Fin and Spine Fin Heat Exchangers

Bock Choon Pak,Byung Joon Baek,Eckhard A. Groll 대한기계학회 2003 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.17 No.11

An experimental study was conducted to investigate the effects of air-side fouling and cleaning on the performances of various condenser coils used in unitary air-conditioning systems. A total of six condenser coils with different fin geometry and row number were tested. Performance tests were performed at three different conditions: clean-as-received, after fouling, and after cleaning. In all cases, it was observed that the fouling was mostly confined to the frontal face of the heat exchanger as reported in the previous investigations. The amount of deposited dust was more dependent on fin geometry for the single-row heat exchangers than for the double-row heat exchangers. The predominant effect of fouling was to cause a more significant increase in air-side pressure drop than a degradation in heat transfer performance. For the single-row heat exchangers, the pressure drop increased by 28 to 31 %, while the heat transfer performance decreased by 7 to 12% at the standard air face velocity of 1.53 mls depending on fin shape. For the double-row heat exchangers, the pressure drop increased by 22 to 37%, and heat transfer performance decreased by only 4-5% at the same air face velocity. Once the contaminated coils were cleaned according to the given cleaning procedure the original performance of the heat exchangers could almost be recovered completely. The pressure drop could be restored within 1 to 7% and the heat transfer performance could be recovered to within 1 to 5% of the originally clean heat exchangers. Therefore, it is concluded that a periodic application of the specified cleaning technique will be effective in maintaining the thermal performance of the condenser coils.

The Effect of Header Shape on the Flow Distribution in a Liquid Cooling Module Manifold for Electronic Packaging

Pak, Bock-Choon,Cho, Young-I 全北大學校 1995 論文集 Vol.39 No.-

본 연구는 전자 패키징에서 단위체로 사용되는 평행 유동 메니폴들에서 유동분포에 대한 레이놀즈 수와 헤더형상의 영향에 대해서 고찰하였다. 메니폴드내에서 유동 분포는 레이놀즈 수와 헤더 형상에 의해서 많은 영향이 있음을 알 수 있다. 설계측면에서 여러 가지 다른 형상의 헤더(직사각형, 삼각형, 경사형)를 고려한 결과, 삼각형 헤더가 레이놀즈수에 관계없이 가장 좋은 유동분포를 보이고 있다. 따라서, 통상적인 사각형헤더를 삼각형으로 변형할 경우 메니폴드내의 유동분포를 상당히 개선시킬 수 있음을 알 수 있다.

Evidence for Erythrocyte Aggregational Pathology

Sanjay Jayavanth,Bock Choon Pak 한국유체기계학회 2006 유체기계 연구개발 발표회 논문집 Vol.- No.-

Erythrocyte aggregation is a physiological assemblage of red blood cells into neat stack like structure as though cells were placed one over / beside another like coins. Such stack of RBC interconnects to form three-dimensional networks often referred to as rouleaux in presence of plasma proteins aided by low flow. These neatly looking erythrocyte aggregates can turn into ugly, haphazard clump of cells under pathological conditions. Aggregational pathology can thus hamper blood flow, increase flow resistance and some times cause occlusion of blood vessels afflicting the microcirculatory rheology. This paper presents recent research report evidencing aggregational pathology in disease condition obtained through an in-vitro microhemofludic visualization system (μHVS). In a study conducted on normal and malaria infected subjects, erythrocyte aggregation showed irregular shapes and compactness aggregates when compared to the normal ones. Specific erythrocyte aggregates were rolling about themselves showing little or no deformation. While the aggregates of normal samples showed porous rouleaux network changing shape and orientation. Such deformation of rouleaux network was not seen among the clumped cells of parasitized samples. Erythrocytes were found adhering to each other and formed clumps tending toward agglutination. These results showed the extent to which microrheologic dysfunction could occur in severe cases of a specific disease.

MULTI-SHAPE ERYTHROCYTE IMAGING AND THEIR DEFORMABILITY ANALYSIS IN A MICROFLUIDIC ENVIRONMENT

Sanjay Jayavanth,Bock Choon Pak 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5

In their long journey through the cardiovascular circuit, erythrocytes are coerced to shape transform and assume different shapes on account of varying flow conditions in different blood vessels. The present work was aimed to visualize these erythrocyte shape transformations by an invitro microcirculatory model, and assess multi-shape erythrocyte deformability. The model uses an in-house fabricated, inexpensive disposable micro flow channel to mimic certain invivo conditions and a fast frame video microscopic system for imaging the shape changes in erythrocytes. Results show the multi-shape transformation of erythrocyte christened as discoidal shape, the asymmetrically deformed ‘hat’ and ‘bullet-like’ shapes, and the axially deformed ‘slipper’ and ‘spindle-like’ shapes. Specific erythrocytes showed the shape transition and transformation while passing through the observed window. The obtained erythrocyte shapes very analyzed for deformability index using image processing techniques that varied significantly (p<0.001) for different shapes as compared with the resting shape.

자동차용 고밀도 응축기(SCC)에서 다중채널 튜브의 부분적 폐쇄가 열적성능에 미치는 영향

김용일,박복춘,백병준,조병수 전북대학교 공업기술연구소 2000 工學硏究 Vol.31 No.-

It is one of important problems in the product quality whether the multi-channels of tube in an automotive super-compact condenser (SCC) are partially closed or not due to solder in manufacturing process. The present study primarily aimed to investigate the effects of partial closing of channels on the pressure drop and thermal performance of a SCC. In the single specimen tests of a SCC, the pressure drop increased with increasing closing rate. While the heat transfer rate decreased with increasing closing rate. In the system tests. The pressure drop and inlet temperature of refrigerant increased with increasing closing rate. On the other hand, the heat transfer rate was not decreased significantly regardless of the closing rate. Therefore, it is concluded that the air-conditioning system with the moderate partial closings of tubes in a SCC would be operated without any significant problems. However, as the velocity of a vehicle increases, the closings of tubes might influence significantly to the performance of a system, which should be further investigated in the future.

A Numerical Study of Impingement Heat Transfer in a Confined Circular Jet

In Gyu Park,Bock Choon Pak,Young I. Cho 대한기계학회 1997 KSME International Journal Vol.11 No.3