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원형제트출구 전단류 조절에 따른 제트충돌면에서의 열전달 특성 ( 2 ) - 음향여기된 제트 -
황상동,이창호,조형희,Hwang, Sang-Dong,Lee, Chang-Ho,Cho, Hyung-Hee 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.3
The flow and heat transfer characteristics on the impingement surface can be controlled by the change of vortex with the acoustic excitation, because the flow characteristics of an impinging jet are affected strongly by the vortices formed at the jet exit. To investigate the effects of acoustic excitation, we measured the velocity, turbulent intensity distributions for the free jet and local heat transfer coefficients on a impingement surface. As the acoustic excitation, subharmonic frequency of excited frequency plays an important role to the control of the jet flow. If the vortex pairings are promoted by the acoustic excitation, turbulence intensity of the jet flow is increased quickly. On the other hand if the vortex pairings are suppressed, the jet flow has low turbulence intensity at the center of the jet. Therefore, the low heat transfer rates are obtained on the impingement plate for a small nozzle-to-plate distance. However, it has high heat transfer rates at a large distance between the nozzle and plate due to the increasing of potential-core length.
딤플/돌출이 설치된 사각채널에서의 열전달 및 압력강하 특성에 관한 실험적 연구
황상동(Sang Dong Hwang),권현구(Hyun Goo Kwon),조형희(Hyung Hee Cho) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
An experimental study was conducted to investigate the heat transfer and pressure drop of a rectangular channel with the dimpled and/or protruded walls. And the effects of complex geometries of dimple and protrusion on heat transfer and pressure drop were investigated. In the present study, three different roughened surfaces of dimpled, protruded and complex(dimple-protrusion) surface were tested. The dimples/protrusions were installed at both top and bottom walls of the rectangular duct. The dimple depth is 0.25 times dimple diameter(D) and the protrusion height is also 0.25D. The dimple or protrusion has staggered array pattern, and the complex case has the pattern of repeat of the rows of dimples and protrusions. The local heat transfer coefficients on the dimpled/protruded walls were measured using a transient TLC(Thermocromic Liquid Crystal) technique. The friction factors of the rectangular channel with dimples/protrusions were obtained using pressure taps installed at the channel side wall. And the performance factors, which indicate the enhancement levels by both considerations of heat transfer enhancements and pressure loss increases were evaluated. As a result, high heat transfer region appeared at the rear side of the dimple due to the increased flow mixing on the dimpled surface. For the protruded surface, heat transfer was enhanced on the front side of the protrusion by the impingement effects of the flow induced by the horseshoe vortices. In case of complex geometry, dimple-protrusion, the compound effects of the dimple and protrusion occurred. The protrusion case showed the highest heat transfer enhancement among test cases. However, pressure loss increased. For the performance factor, the dimple case shows the highest performance factor among tested cases due to the slightly low pressure drop increase.
Reynolds 수에 따른 꺾어진 덕트에서 열/물질전달 특성 고찰
장인혁,황상동,조형희 대한설비공학회 2003 설비공학 논문집 Vol.15 No.10
The present study investigates effects of flow velocity on the convective heat/mass transfer characteristics in wavy ducts of a primary surface heat exchanger application. Local heat/mass transfer coefficients on the wavy duct sidewall are determined by using a naphthalene sublimation technique. The flow visualization technique is used to understand the overall flow structures inside the duct. The aspect ratio and corrugation angle of the wavy duct is fixed at 7.3 and 145$^{\circ}$ respectively, and the Reynolds numbers, based on the duct hydraulic diameter, vary from 100 to 5,000. The results show that there exist complex secondary flows and transfer processes resulting in non-uniform distributions of the heat/mass transfer coefficients on the duct side walls. At low Re (Re<1000), relatively high heat/mass transfer regions like cell shape appear on both pressure and suction side wall due to the secondary vortex flows called Taylor-Gortler vortices perpendicular to the main flow direction. However, at high Re (Re>1000), these secondary flow cells disappear and boundary layer type flow characteristics are observed on pressure side wall and high heat/mass transfer region by the flow reattachment appears on the suction side wall. The average heat/mass transfer coefficients are higher than those of the smooth circular duct due to the secondary flows inside wavy duct. And also friction factors are about two times greater than those of the smooth circular duct.
열교환기 내부 유로 종횡비 변화에 따른 국소 열/물질전달 특성 고찰
장인혁,황상동,조형희 대한설비공학회 2005 설비공학 논문집 Vol.17 No.6
The present study investigates the convective heat/mass transfer characteristics in wavy ducts of a primary surface heat exchanger. The effects of duct aspect ratio and flow velocity on the heat/mass transfer are investigated. Local heat/mass transfer coefficients on the corrugated duct sidewall are determined using a naphthalene sublimation technique. The aspect ratios of the wavy duct are 7.3, 4.7 and 1.8 with the corrugation angle of 145 . The Reynolds numbers, based on the duct hydraulic diameter, vary from 300 to 3,000. The results show that at the low Re(Re 1000) the secondary vortices called Taylor-Gortler vortices perpendicular to the main flow direction are generated due to effect of duct curvature. By these secondary vortices, non-uniform heat/mass transfer coefficients distributions appear. As the aspect ratio decreases, the number of cells formed by secondary vortices are reduced and secondary vortices and corner vortices mix due to decreased aspect ratio at Re 1000. At Re>1000, the effects of corner vortices become stronger. The average Sh for the aspect ratio of 7.3 and 4.7 are almost same. But at the small aspect ratio of 1.8, the average Sh decreases due to decreased aspect ratio. More pumping power (pressure loss) is required for the larger aspect ratio due to the higher flow instability.