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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.
홍봉환(Hong Bong Hwan),황상동(Sang Dong Hwang),조형희(Hyung Hee Cho) 대한기계학회 2003 대한기계학회 춘추학술대회 Vol.2003 No.4
Experimental study is conducted to investigate the cooling performance of impinging jet from mictotube using Joule-Thomson effect to apply practical applications. And also the heat transfer characteristics of a impinging jet itself and the impinging jet with Joule-Thomson effect are tested to make a comparative study of the two different general ideas. For this propose, two kinds of copper microtubes which have 200㎛ and 300㎛ in inner diameter respectively were tested and N₂was used as a working fulid. In case of impinging jet without Joule-Thomson effect, heat transfer coefficients distributions were similar to those of normal impinging jet. But in impinging jet with Joule-Thomson effect, the heat transfer coefficients decrease as jet-to-surface increases contrary to the case of the normal jet. As a result, much higher heat transfer coeffcients are obtained with Joule-Thomson effect than those of the normal jet without J-T effect.
가스터빈용 열교환기의 주름진 덕트에서 종횡비 변화가 열전달 특성에 미치는 영향
김한호(Han Ho Kim),황상동(Sang Dong Hwang),조형희(Hyung Hee Cho),최재호(Jae Ho Cho),전승배(Seung Bae Jeon) 한국유체기계학회 2001 유체기계 연구개발 발표회 논문집 Vol.- No.-
The present study investigates convective heat/mass transfer and flow characteristics in wavy ducts of primary surface heat exchanger. Experiments using a naphthalene technique are carried out to determine the local transfer characteristics for flow in the corrugated wall duct. The aspect ratios of the rectangular duct cross-section are 7.3, 4.7 and 1.8 with a corrugation angle of 145°. The Reynolds numbers, based on the duct hydraulic diameter, are ranged from 1000 to 5000. The local heat/mass transfer measurement is conducted in the spanwise directions. The results show that Tayler-Gortler vortices exist on the pressure surface. Flow separation on the suction surface appears at a high Reynolds number resulting in a sharp decrease in the local transfer rates, but relatively high transfer rates are obtained in the reattachment region.
2차 유동 영역에서 꺽임각 변화에 따른 주름진 사각 덕트에서의 열/물질전달 및 유동 특성
권현구(Hyun Goo Kwon),황상동(Sang Dong Hwang),조형희(Hyung Hee Cho) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
The present study investigates the two dimensional flow and heat/mass transfer characteristics of wavy duct with various corrugation angles. For the heat/mass transfer coefficients, a naphthalene sublimation technique is used. Numerical analysis and wall pressure measurement show detailed two dimensional flow features. The corrugation angles change from 145° to 100°. The operating Reynolds numbers based on the duct hydraulic diameter vary from 700 to 3,000. The duct aspect ratio maintains 7.3. On the pressure wall, strong flow mixing enhances heat/mass transfer coefficients at the front position. In addition, the rear side of pressure wall, the near of peak, is affected by the acceleration and the shedding of main flow. On the suction wall, however, flow separation and reattachment lead to the valley and the peak of heat/mass transfer coefficient. Also, highly increasing boundary layer at the suction wall affects the decrease of heat/masst transfer. As decreasing corrugation angles, the spanwise average Sherwood number increases and the peak or the valley positions of the local Sherwood number are varied.
채널유동성분이 존재하는 오목 충돌면에서 배열충돌제트에 의한 국소 열전달 특성 고찰
이원희(Won Hee Lee),황상동(Sang Dong Hwang),조형희(Hyung Hee Cho) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.11
In this study, the effect of channel flow in the concave surface on local heat transfer characteristics of array jets was investigated experimentally. A TLC method is employed to determine local heat transfer coefficients on the target plate and also flow visualization has been conducted to investigate the behavior of a row of impinging jets and array of impinging jets. Two different array patterns of impinging array jets devices are tested for Reynolds number(Re=10,000). In a row of impinging jets, secondary vortex is strongly maintained by main vortex at nozzle-to-plate distance of H/d=2. Therefore, the Nusselt number slowly decreased at the mid-way region between adjacent jets. In array jets, the local maximum Nusselt number region move further in the downstream direction due to the increase of channel flow velocity.
델타윙에 의한 이차유동이 딤플의 열/물질전달 특성에 미치는 영향에 대한 연구
권현구(Hyun Goo Kwon),황상동(Sang Dong Hwang),조형희(Hyung Hee Cho) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.11
In this study, we have investigated the effects of upstream rectangular winglet pair (RWP) on the heat/mass transfer coefficients in a dimple. Dimple print diameter was fixed at 20mm and the dimple depth was 4.0mm (0.4H). The dimple surface was coated with naphthalene for mass transfer experiment and the test plate was positioned at a rectangular straight duct whose aspect ratio (W/H) was 20. A rectangular winglet pair was positioned at y/d=-2.5. The RWP angle (β) was varied from 15° to 60°. The Reynolds number, based on the duct height (H), was 5,000. with changing the RWP angle (β), the induced vortices had different flow characteristics; longitudinal or transversal vortices. These variation of induced vortices affected on the heat/mass transfer characteristics in the dimple.