• 대한기계학회논문집B
• /
• 제34권3호
• /
• pp.291-299
• /
• 2010

본 연구에서는 열의 수가 세 개인 원형관 및 종횡비가 고려된 납작관-평판휜 형상의 밀집형 열교환기 내 공기의 유동 및 열전달 특성을 RNG $k-{\varepsilon}$ 난류모델을 적용하여 수치적으로 해석하였다. 수치조건의 범위는 원형관인 경우 1,000<$Re_D_h$<10,000이며, 납작관인 경우 1,000<$Re_{D_h}$<10,000 및 3.06$f\;=\;CRe^m_{D_h}$의 형태로, Colburn j 계수는 $j\;=\;CRe^m_{D_h}$ 형태로 상관관계식을 제시하였다. 이러한 상관관계식은 향후 원형관 및 납작관-평판휜 형상의 열교환기 설계시에 기본적인 자료로 충분히 활용될 수 있을 것으로 판단된다.

• 대한조선학회논문집
• /
• 제50권6호
• /
• pp.399-406
• /
• 2013

In the present study, the flat plate model test method is developed to evaluate the skin friction of the marine coating in the cavitation tunnel. Six-component force balance is used to measure the profile drag of the flat plate and strut. LDV(laser Doppler velocimetry) technique is also employed to evaluate the drag and to figure out the reason of the drag reduction. The flow velocities above the surface can be used to assess the skin friction, combined with direct force measurement. Since the vortical structure in the coherent turbulence structure influences on the skin friction in the high Reynolds number regime, the interaction between the turbulence structure and the surface wall is paying more attention. This sort of thing is important in the passive control of the turbulent boundary layer because the skin friction can't be determined only by wall condition. As complicated flow phenomena exist around a paint film, systematic measurement and analysis are necessary to evaluate the skin friction appropriately.

• Journal of Ship and Ocean Technology
• /
• 제8권1호
• /
• pp.1-9
• /
• 2004

Experiments on friction drag reduction by injecting polymer (Polyethylene oxide) solution or micro-bubbles were carried out in the cavitation tunnel of KRISO. Two different drag reduction mechanisms were applied to a slender axi-symmetric body to measure the total drag reduction. And then the amount of friction drag reduction was estimated under the assumption that the reduction mechanisms were effective only to the friction drag component. As the result of the tests, polymer solution drag reduction up to 23％ of the total drag was observed and it corresponds to about 35％ of the estimated friction drag of the axi-symmetric body. This result matched reasonably well to that of the flat plate test "(Kim et al, 2003)". The normalization of the controlling parameters was tried at the end of this paper. Micro-bubble drag reduction was within 1％ of its total drag. This unexpected result was quite different from that of the flat plate case "(Kim et at, 2003)" The possible reasons were discussed in this paper.

• 한국분말야금학회:학술대회논문집
• /
• 한국분말야금학회 2002년도 제3회 최신 분말제품 응용기술 Workshop
• /
• pp.133-155
• /
• 2002

1. Quasicrystalline powders shows exotic physical and mechanical p properties 2. Applications: structural application: strengthening particles for composites C Coating application: wear resistance, low friction coefficient 3. For thermal spaying: material loss during process should be c considered to control chemical composition of deposit 4. Friction coefficient is strongly dependent on contact geometry F Friction coefficient from pin on plate: 0.1-0.2 Friction coe야icient from flat on plate: about 0.46. 5. Quasicrystalline materials show lower friction coefficient but higher w wear rate than corresponding values of $Cr_20_3$ coated layer. 6. Amorphous coating seems to be promising

• Journal of Advanced Marine Engineering and Technology
• /
• 제29권4호
• /
• pp.453-460
• /
• 2005

Experiments were conducted to investigate the effect of the surface roughness on the flat plate turbulent boundary layer. The square rods were installed at the leading edge to make surface roughness. The particle image velocimetry was used to measure the mean velocities and velocity fluctuation component. All measurements were made over a range of w/k=1. 2 5 and $Re_x=80.000{\sim}360,000$. Friction velocity was measured by using Clauser plot method. The level of turbulent intensities on roughness surface appears more strongly than that of turbulent intensities on flat plate. A correlation of boundary layer thickness in term of $Re_x$ and w/k are presented.

• Tribology and Lubricants
• /
• 제40권4호
• /
• pp.118-132
• /
• 2024

The triboelectric nanogenerator (TENG) has emerged as a groundbreaking technology for harvesting clean and sustainable energy cost effectively. For reliable TENG design, minimizing wear damage at the friction layers is crucial. This review provides a comprehensive overview of tribometer-integrated TENG testing configurations used in the simultaneous investigation of both tribological and electrical performance. It considers configurations such as plate-on-plate, ball-on-disc, and ball-on-flat tribometers designed for linear reciprocating or rotating sliding friction tests. These tribometers are either specifically designed or adapted for TENG testing. Triboelectric material holders facilitate friction tests by establishing electrical connections from the triboelectric materials or electrodes, thereby enabling accurate measurement of electrical signals. Electrometers and oscilloscopes record electrical outputs such as short-circuit current and open-circuit voltage. This integration enables the simultaneous measurement of both friction and electrical outputs, providing a thorough understanding of TENG performance. The review also summarizes how factors such as normal force, sliding frequency, and rotating speed affect friction coefficients and TENG performance. It also examines the relationship between the coefficient of friction and tribocharges under various loads and frequencies. The review emphasizes the importance of these testing configurations for evaluating both friction and electrical performance, which are crucial for optimizing TENG efficiency. Finally, the review explores future prospects for developing innovative tribometer designs suited for both tribology and TENG testing.

• International Journal of Air-Conditioning and Refrigeration
• /
• 제17권2호
• /
• pp.52-60
• /
• 2009

The purpose of this research is to develop high efficiency plastic plate heat exchangers which can be substituted for conventional aluminum plate heat exchangers. Four simulation models of plastic plate heat exchangers are designed and simulated: that is, flat plate type, turbulent promoter type, corrugate type and dimple type heat exchanger. The flat plate type is designed as the reference model in order to evaluate how much thermal performance increases. The turbulent promoter type is fabricated with cylindrical-type vortex generators and rib-type turbulent promoters. The corrugate type is obtained from the conventional stainless steel compact heat exchangers, which are called the herringbone-type compact heat exchangers. The dimple type has a number of dimples on its surface. In this study, the flow and heat transfer characteristics of the plastic plate heat exchanger are investigated using numerical simulation and compared with experimental results. Numerical simulation is carried out using the FLUENT code. The flows are assumed as a three-dimensional, incompressible and turbulent model. The computational analysis and experimental results both show that the friction coefficient and Nu number is highest in the corrugate type. The tendency of numerical simulation results is in good agreement with that of the experimental results.

• 대한기계학회논문집
• /
• 제19권1호
• /
• pp.240-250
• /
• 1995

A CPM (computational preston tube method) was developed to measure wall shear stress in a transitional boundary layer on a flat plate using Preston tubes. Correlation for the displacement factor of Preston tubes was improved for a CPM to be used in the transitional boundary layer. The distribution of skin-friction coefficient was reasonably predicted in the uniform free stream of 3.1% turbulence intensity. Reasonable and accurate estimation of displacement factor of Preston tubes was found to be of crucial importance for the CPM, especially in the laminar boundary layer. The mean velocity profiles of the boundary layer on the plate were also measured and presented.

• 대한기계학회논문집B
• /
• 제33권5호
• /
• pp.318-325
• /
• 2009

Numerical analysis has been carried out to investigate air-side convective heat transfer characteristics in compact heat exchanger with flat tubes and continuous plate fins having a symmetric and non-symm etric staggered arrangements. Simulation results such as air velocity and temperature distributions are presented, and heat transfer coefficients are compared with previous experimental results. In order to investigate the flow and heat transfer features by periodic boundary conditions, the three blocks were used. Predicted heat transfer coefficients between the three blocks are similar while there are relatively differences, compared with the experimental data. From the calculated results a correlation for Colburn j factor in the compact heat exchanger system is suggested. The predicted results in this study can be applied to the optimal design of air conditioning system.

• 대한기계학회논문집B
• /
• 제32권9호
• /
• pp.695-703
• /
• 2008

Numerical analysis has been carried out to investigate air-side convective heat transfer characteristics in a compact heat exchanger with flat tubes and continuous plate fins according to the aspect ratio. RNG k-$\varepsilon$ model is applied for turbulence analysis. Simulation results such as air velocity and temperature distributions are presented, and heat transfer coefficients are compared with previous correlations for circular tubes. The numerical conditions are considered for the aspect ratios ranging from 3.06 to 5.44 and Reynolds number ranging from 1000 to 10,000. The results showed that heat transfer coefficients decreased with the increase of aspect ratio. From the calculated results a correlation of Colburn j factor for the considered aspect ratio in the compact heat exchanger system is suggested. The predicted results in this study can be applied to the optimal design of air conditioning system.