• 대한기계학회논문집B
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• 제28권1호
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• pp.118-125
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• 2004

The experimental study on flow and heat transfer characteristics was conducted to investigate and to compare the performance of swirl jet by a twisted tape as a swirl generator with the performance of impinging single circular jet in fully developed flow tube. The effects of jet Reynolds number(Re=8700, 13800, 20000, 26500), dimensionless distance of nozzle-to-plate(H/d=2, 4, 6, 8) and swirl ratio(S=0.11, 0.23, 0.30) of the jet on the local and average Nusselt number have been examined. Measurements of local heat transfer rate and flow patterns on the jet impinging plate were used naphthalene sublimation technique and flow visualization technique respectively. Mean velocity and turbulence intensity of the jet along the centerline were measured. With a twisted tape in the nozzle exit, average Nusselt number at the around area of stagnation point were higher than those without the twisted tape at H/d=2, 4 and with the increase of Reynolds number. With a twisted tape in the nozzle, in the case of H/d=2, Re=26500 and S=0.11, maximum local Nusselt number at the region of y/d=0 and x/d=0.44 was obtained.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.2211-2218
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• 2003

The present study investigates heat/mass transfer characteristics in a rotating two-pass duct for smooth and ribbed surfaces. The duct has an aspect ratio of 0.5 and a hydraulic diameter of 26.67 mm. 70-angled rib turbulators are attached on the leading and trailing sides of the duct in parallel and cross arrangements. The pitch-to-rib height ratio is 7.5 and the rib height-to-hydraulic diameter ratio is 0.075. The Reynolds number based on the hydraulic diameter is constant at 10,000 and the rotation number ranges from 0.0 to 0.2 Detailed local heat/mass transfer coefficients are measured using a naphthalene sublimation technique. The results show that the secondary flows generated by the $180^{\circ}-turn$, rib turbulators, and duct rotation affect the wall heat/mass transfer distribution significantly, As the duct rotates, the rotaion-induced Coriolis force deflects the main flow and results in differences on the heat/mass transfer distribution between the leading and trailing surfaces. Its effects become more dominant as the rotaion number increases. Discussions are presented describing how the rib configuration and the rotaion speed affect the flow patterns and local heat/mass transfer in the duct.

• Journal of Mechanical Science and Technology
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• 제16권1호
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• pp.109-115
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• 2002

Vortex generators are fabricated on the fin surface of a fin-tube heat exchanger to augment the convective heat transfer. In addition to horseshoe vortices formed naturally around the tube of the fin-tube heat exchanger, longitudinal vortices are artificially created on the fin surface by vortex generators. The purpose of this study is to investigate the local heat transfer phenomena in the fin-tube heat exchangers with and without vortex generators, and to evaluate the effect of vortices on the heat transfer enhancement. Naphthalene sublimation technique is employed to measure local mass transfer coefficients, then analogy equation between heat and mass transfer is used to calculate heat transfer coefficients. Experiments are performed for the model of fin -circular tube heat exchangers with and without vortex generators, and of fin-flat tube heat exchangers with and without vortex generators. Average heat transfer coefficients of finn-flat tube heat exchanger without vertex generator are much lower than those of fin-circular tube heat exchanger. On the other hand, fin-flat tube heat exchanger with vortex generators has much higher heat transfer value than conventional fin-circular tube heat exchanger At the same time, pressure losses for four types of heat exchanger is measured and compared.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.274-279
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• 2001

An experimental investigation of heat transfer characteristics at the surface of two-dimensional protruding heated blocks using confined impinging multiple slot jets has been performed. The effects of jet-to-jet distances(S=16B, 24B), dimensionless nozzle-to-block distances(H/B=2, 6) and jet Reynolds numbers(Re=2000, 3900, 5800, 7800) on the local and average heat transfer coefficients have been examined with five isothermally heated blocks at streamwise block spacing(p/w=1). To clarify local heat transfer characteristics, naphthalene sublimation technique was used. From the results, it was found that the local and average heat transfer of heated blocks increases with decreasing jet-to-jet distance and increasing jet Reynolds number. Measurements of local heat transfer coefficients have given an indication of the nature of the interaction between jets and of the uniformity of heat transfer obtainable with various arrangements. In the case of S/B=16, H/B=6 and Re=7800, maximum average Nusselt number of overall blocks was obtained.

• 대한기계학회논문집B
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• 제24권3호
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• pp.435-442
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• 2000

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged for the two different ways: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5,000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the mid-line of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). More uniform and higher heat/mass transfer characteristic is obtained in overall region with small gap between two perforated plates.

• International Journal of Air-Conditioning and Refrigeration
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• 제11권3호
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• pp.150-157
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• 2003

Local and overall heat transfer characteristics of fin-flat tube heat exchangers with and without vortex generators were investigated. Local heat transfer coefficients were measured with the heat exchanger model using naphthalene sublimation technique. In case of a fin-flat tube heat exchanger without vortex generators, only the horseshoe vortices formed around tubes augment the heat transfer. On the other hand, longitudinal vortices created artificially by vortex generators additionally enhance heat transfer in case of a fin-flat tube heat exchanger with vortex generators. Overall heat transfer coefficients were measured with the prototypes of the fin-flat tube heat exchanger with and without vortex generators in a wind tunnel and results were compared with those of a fin-circular tube heat exchanger with wavy fin. Friction losses for heat exchangers were also measured and compared. The fin-flat tube heat exchanger with vortex generators is found to be more effective than the fin-circular tube heat exchanger with wavy fin.

• Journal of Advanced Marine Engineering and Technology
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• 제28권4호
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• pp.624-631
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• 2004

The experimental study on heat transfer characteristics of protruding heated block array as conducted to investigate and to compare the performance of impinging single circular jet in fully developed tube with a twisted tape as a swirl generator. The effects of jet Reynolds number(Re=8700, 13800, 20000. 26500), dimensionless jet-to-block distance(H/d=1. 3, 5. 7) and swirl number(S=0.11, 0.23, 0.30) of the swirl jet on the average Nusselt number for each block and all blocks have been examined. Measurements of heat transfer rate on block surfaces were used naphthalene sublimation technique. Mean velocity and turbulence intensity of the jet along the axis were measured. Potential core length of the jet was 5 times of nozzle diameter because it was fully developed and initially turbulent. With the twisted tape in the nozzle, heat transfer coefficients were higher than those without the twisted tape. which are mainly caused with increasing the jet Reynolds number and swirl number.

• 대한기계학회논문집B
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• 제25권11호
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• pp.1606-1615
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• 2001

The present study has been conducted to investigate heat/mass transfer characteristics on a target plate fur arrays of circular impingement jets with and without effusion holes. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate. The effusion holes are located at the center of four injection holes in the injection plate where the spent air is discharged through the effusion hole after impingement on the target plate. For the array jet impingement without effusion holes, the array jets are injected into the crossflow formed by upstream spent air because the impinged jets must flow to the open exit. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to crossflow effects and re-entrainments of spent air. However, uniform distributions and enhanced values of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the crossflow has little influence on heat/mass transfer characteristics on the target palate due to the large cross-sectional open area between the injection and target plates. Therefore, the distributions and levels of heat/mass transfer coefficients are almost the same for both cases.

• 대한기계학회논문집B
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• 제26권7호
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• pp.927-936
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• 2002

An experimental study has been conducted to investigate the heat/mass transfer characteristics within a film cooling hole of square cross-section for various blowing ratios and Reynolds numbers. The experiments have been performed using a naphthalene sublimation method and the flow field has been analyzed by numerical calculation using a commercial code. A duct flow enters into a film cooling hole in a cross-direction. For the film cooling hole with square cross-section, it is observed that the reattachment of separated flow and the vortices within the hole enhance considerably the heat/mass transfer around the hole entrance region. The heat/mass transfer on the leading edge side of hole exit region increases as the blowing ratios decrease because the main flow induces a secondary vortex. Heat/mass transfer patterns within the square film cooling hole are changed little with the various Reynolds numbers.

• 대한기계학회논문집B
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• 제24권12호
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• pp.1670-1677
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• 2000

An experimental investigation on heat transfer characteristics of two-dimensional heated blocks using a confined impinging slot jet has been performed. The effect of jet Reynolds number(Re=3900, 5800, 9700), streamwise block spacing(p/w=0.5, 1, 1.5) and dimensionless nozzle to block distance(H/B=1, 2, 4, 6) have been examined with five isothermally heated blocks. With the measurement of jet mean velocity and turbulence intensity distributions at nozzle exit, initially turbulent regimes, are classified. To clarify local heat transfer characteristics, naphthalene sublimation technique as used. The maximum Nusselt number at the stagnation point for the jet Reynolds number is occurred at H/B=4. Besides, the local and a average heat transfer of heated blocks increase with decreasing streamwise block spacing and increasing jet Reynolds number.