• Solar Energy
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• 제13권1호
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• pp.22-30
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• 1993

This Paper deals with the experimental study of the axisymmetric air jet impinging vertically on the flat heating surface with and without swirl. The purpose of this study is to investigate the characteristics of flow, augmentation of heat transfer rate, turbulent intensity, and the comparison of heat transfer rate, the optimal swirling condition about the swirl and nonswirl axisymmetric air jet. In order to augment the heat transfer on the flat heating surface without introducing any additional power, the technique used in the present work was placement of twisted tape inserted pipe in front of the nozzle exit in order to make a swirl. The effect of swirl degree is investigated in case of S=0., 0.056, 0.111, 0.222 and the velocity of the jet was 14, 20, 26, 32, 38, 44m/s. The distance between the nozle exit and the stagnation point on the impinging plate was the H/D=$1{\sim}14$. In order to analyze of the flow structure which increase heat transfer, the velocity and the turbulent intensity of the axisymmetric jet was measured by a hot wire anemometer according to the swirl number and H/D.

• Journal of Energy Engineering
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• 제13권1호
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• pp.28-33
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• 2004

The objective of the study was to investigate the characteristics of heat transfer and flow in 2-dimensional impinging air jet system, in which trapezoid rods have been set up in front of impinging plate in order to increase heat transfer. Experiments were carried out first using without the rods to establish the baseline heat transfer performance. And this result compared with the experimentation with rods. When rods are installed in front of the impinging plate, the acceleration of the flow and the eddies due to the rods seem to contribute to the heat transfer enhancement. Heat transfer performance was best under the condition of C=1 n and as the pitch is 30 mm. In this case, maximum rate of heat transfer augmentation is about 1.62 times greater compared to that without trapezoid rods.

• Journal of Energy Engineering
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• 제21권2호
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• pp.187-193
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• 2012

In this study, it was experimentally investigated the effect of the clearances distance between the rod and the impinging plate on characteristics of the thermal flow. For the heat transfer enhancement of wall jet region, the right triangle and the square rods were arranged in front of the impinging plate with various clearance distances. As results, the heat transfer enhancement rate of potential core region at H/B=2 was higher than that of transition region at H/B=10. In this experiment range, the maximum heat transfer enhancement rate was about 46 % higher at the square rod with H/B=2 and C=1mm compared with the flat plate. The heat transfer enhancement rate of the right triangle rod was on average about 3 to 8 % higher than that of the square rod, regardless of the clearance.

• Journal of Energy Engineering
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• 제4권1호
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• pp.59-66
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• 1995

본 연구에서는 액체 유동층 열교환기의 수직관내에 유리알 (직경 3.0mm, 비중 2.54) 이 물과 함께 흐를 때 압력 손실, 열전달 계수 및 화울링 계수를 측정하였다. 실험 결과 유리알은 1.0 m/s 이하의 낮은 유속에서 열전달을 증진시켰고 실험범위 내에서 열전달 계수는 유속 및 고체의 흐름양에 관계없이 거의 일정하였다. 압력 손실은 고체의 흐름양이 증가할수록 급격히 증가하였다. 실험 결과를 토대로 압력손실 및 열전달 계수를 예측할 수 있는 상관식을 개발하였다. 유리알 유동층 흐름은 화울링 조절에 효과적인 것으로 일어나지 않았고 이미 화울링이 심하게 일어난 때에도 유리알은 효과적으로 산화철을 제거할 수 있음을 알 수 있었다.

• Journal of Energy Engineering
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• 제23권2호
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• pp.217-222
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• 2014

In order to improve the heat efficiency of the general residential boiler, we performed an experiment of condensation heat transfer to air pre-heating exchanger adhered to the condensing boiler. In this study, surface roughness was imposed on the surface of stainless steel by etching. And in order to evaluate the heat transfer performance on each plate, the counter flow heat exchanger fabricated with polycarbonate in used. As a result, on etching treated plate's overall heat transfer coefficient is higher than the original plate. And etching treated plate during 60 seconds with etchant is the to average 15% compared to bare stainless steel. And we studied the heat transfer enhancement factor through the analysis of surface characteristics using AFM.

• Solar Energy
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• 제17권1호
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• pp.47-58
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• 1997

The objective of this research was to investigate the enhancement of heat transfer by wire mesh in impinging air jet system at the potential core region. The first experiment was carried out without mesh between nozzle exit and flat plate and the second experiment was done with mesh between them. When mesh was installed in front of the plate, heat transfer has been Increased due to the acceleration between rectangular halls and divided small jet In case clearances are changed, heat transfer comes to maximum under the condition of C=1mm, irrespective of nozzle exit velocity and H/B. Also the average heat transfer enhancement rate of a flat plate with mesh has been increased about 44% at maximum under the condition of U=18m/s, H/B=2 and C=1mm, compared to the result of a flat plate without mesh.

• Journal of Energy Engineering
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• 제20권2호
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• pp.96-102
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• 2011

This research has been proceeded over the transition region(H/B=10) of two-dimensional impinging air jet system, in which square rods has been set up in front of heating surface in order to increase heat transfer. The objective of this research was to investigate the characteristics of heat transfer and air flow, in cases the clearance from rods to heating surface(C=1, 2, 4, 6 mm) and the width of rods(W=4, 6, 8 mm) changed. And this research compared the above with the experimentation without rods. As result, heat transfer performance was best under the condition of C=1 mm, and as the width is 8 mm, it is largely influenced by eddies and acceleration in case width of rods changed.

• Journal of Energy Engineering
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• 제13권4호
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• pp.275-280
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• 2004

Augmentation of heat transfer by ultrasonic vibration in pipes are investigated. Measurements of convective heat transfer coefficients on circular pipe walls are made with and without ultrasonic vibration applied to water. These data are compared with each other to quantify the effects of ultrasonic vibration on heat transfer enhancement. Numerical analysis has been also performed in order to extend the ranges of examined temperature and flow rate. FLUENT Ver.6.1 is used to simulate velocity and temperature fields and evaluate heat transfer coefficient with and without ultrasonic vibration. The results show that the ultra- sonic vibration enhances the Nusselt number of forced convection flow and the increase rate strongly depends on flow rate.

• Transactions of the Korean Society of Mechanical Engineers
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• 제16권10호
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• pp.1955-1962
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• 1992

The purpose of this study is to identify the radiative heat transfer augmentation by a coaxial cylinder introduced in the infinite cylindrical pipe enclosing a participating gas. The gas is either a mixture of water vapor and carbon dioxide or gray. The gas is assumed to be homogeneous at a constant temperature, and has a refractive index of unity. All of the surfaces are opaque and gray, diffusely emitting and reflecting at a constant temperature, The effect of system diameter, diameter ratio, wall emittances, gas and surface temperatures, mixture component on heat transfer augmentation are studied by using the zone method with participating gas radiative properties evaluated from the weighted sum of gray gases model. From the radiative equilibrium condition, the installed wall temperature is formulated and calculated by the iteration method. If the medium is a gray gas, the augmentation observed are negligible. For the range of values studied for a real gas, if the system diameter is larger than about 0.1m the augmentation parameter increases up to about 1.2 as the system diameter increases. The augmentation parameter have a maximum value at a certain diameter ratio. The augmentation parameters decreases as the emittance of the installed wall decreases. If the gas temperature is higher than about 1273 k, the augmentation parameter decreases as the gas temperature increases.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제38권7호
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• pp.619-624
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• 2014

The critical heat flux (CHF) on a heat transfer surface with nanostructures is known to be significantly better than that on flat surfaces. Several physical mechanisms have been proposed to explain this phenomenon. However, almost all studies conducted so far have been qualitative, and a generalized theory has not yet been established. In this study, we developed a quantitative mechanism for CHF enhancement on a surface with nanostructures, based on vapor recoil and surface adhesion forces. We focused on the increase in the length of the triple contact line owing to the formation of nanostructures and the adhesion force between them and the liquid.