A comparison of the heat transfer performance of thermosyphon using a straight groove and a helical groove

This study is focused on the comparison of heat transfer performance of two thermosyphons having 60 straight and helical internal grooves. Distilled water has been used as working fluid. Liquid fill charge ratio defined by the ratio of working fluid volume to total internal volume of thermosyphon, the inclination angle and operating temperature were used as experimental parameters. The heat flux and heat transfer coefficient are estimated from experimental results. The conclusions of this study may be summarized as follows; Liquid fill charge ratio, inclination angle and geometric shape of grooves were very important factors for the operation of thermosyphon. The optimum liquid fill charge ratio for the best heat flux were 30%. The heat transfer performance of helically grooved tube was higher than that of straight grooved tube in low inclination angle (less than 30°), but the results were opposite in high inclination angle (more than 30°). As far as optimum inclination angle concerns, range of 25°-30° for a helically grooved tube and about 40° for a straight grooved tube are suggested angles for the best results.     

Influence of the inclination angle and liquid charge ratio on the condensation in closed two-phase thermosyphons with axial internal low-fins

This study concerns the performance of the heat transfer of the thermosyphons having 60, 70, 80, 90 axial internal low-fins in which boiling and condensation occurr. Water, HCFC-141b and CFC-11 have been used as the working fluids. The operating temperature, the liquid charge ratio and the inclination angle of thermosyphons have been used as the experimental parameters. The heat flux and heat transfer coefficient at the condenser are estimated from experimental results. The experimental results have been assessed and compared with existing theories. As a result of the experimental investigation, it was found that the maximum heat flow rate in the thermosyphons is dependent upon the liquid charge ratio and inclination angle. A relatively high rate of heat transfer has been achieved by the thermosyphon with axial internal low-fins. The inclination of a thermosyphon has a notable influence on the condensation. In addition, the overall heat transfer coefficients and the characteristics at the operating temperature are obtained for the practical applications.     

Investigation of boiling heat transfer characteristics of two-phase closed thermosyphons with various internal grooves

The boiling heat transfer characteristics of two-phase closed thermosyphons with internal grooves are studied experimentally and a simple mathematical model is developed to predict the performance of such thermosyphons. The study focuses on the boiling heat transfer characteristics of a two-phase closed thermosyphons with copper tubes having 50, 60, 70, 80, 90 internal grooves. A two-phase closed thermosyphon with plain copper tube having the same inner and outer diameter as those of grooved tube is also tested for comparison. Methanol is used as working fluid. The effects of the number of grooves, the operating temperature, the heat flux are investigated experimentally. From these experimental results, a simple mathematical model is developed. In the present model, boiling of liquid pool in the evaporator is considered for the heat transfer mechanism of the thermosyphon. And also the effects of the number of grooves, the operating temperature, the heat flux are brought into consideration. A good agreement between the boiling heat transfer coefficient of the thermosyphon estimated from experimental results and the predictions from the present mathematical model is obtained. The experimental results show that the number of grooves and the amount of the working fluid are very important factors for the operation of thermosyphons. The two-phase closed thermosyphon with copper tubes having 60 internal grooves shows the best boiling heat transfer performance.     

Theoretical and experimental studies on boiling heat transfer for the Thermosyphons with various helical grooves

Boiling heat transfer characteristics of a two-phase closed thermosyphons with various helical grooves are studied experimentally and a mathematical correlation is developed to predict the performance of such thermosyphons. The study focuses on the boiling heat transfer characteristics of two-phase closed thermosyphons with copper tubes having 50, 60, 70, 80, 90 internal helical grooves. A two-phase closed thermosyphon with plain copper tube having the same inner and outer diameter as those of grooved tubes is also tested for comparison. Water, methanol and ethanol are used as working fluid. The effects of the number of grooves, various working fluids, operating temperature and heat flux are investigated experimentally. From these experimental results, a mathematical model is developed. In the present model, boiling of liquid pool in the evaporator is considered for the heat transfer mechanism of the thermosyphons. And also the effects of the number of grooves, the various working fluids, the operating temperature and the heat flux are brought into consideration. A good agreement between the boiling heat transfer coefficient of the thermosyphon estimated from experimental results and the predictions from the present mathematical correlation is obtained. The experimental results show that the number of grooves, the amount of the working fluid and the various working fluids are very important factors for the operation of thermosyphons. Also, the thermosyphons with internal helical grooves can be used to achieve some inexpensive and compact heat exchangers in low temperature.     

Theoretical modeling and numerical solution of stratified condensation in inclined tubes

The heat transfer phenomenon occurring during stratified condensation inside an inclined tube is investigated theoretically and numerically. Differential equations governing the kinematic, dynamic, and thermal aspects for vapor condensation inside inclined tubes, which are derived from a thin film flow modeling, are solved simultaneously. These solutions are achieved by applying an explicit finite difference numerical method to predict the condensation heat transfer coefficient variations along the tangential and axial coordinates. The inclination angle is found to have a significant effect on condensation heat transfer coefficient inside inclined tubes. In addition, in accordance with the given physical and thermal condition of working fluids, there is a specific optimum inclination angle. In this study, the 30°–50° range from the horizontal position is found to be the range of the optimum inclination angle for achieving the maximum condensation heat transfer coefficient, with R134a, R141b, and R11 as the working fluids. The results of the present study are compared with experimental data, and a good agreement is observed between them.     

The study on pressure oscillation and heat transfer characteristics of oscillating capillary tube heat pipe

In the present study, the characteristics of pressure oscillation and heat transfer performance in an oscillating capillary tube heat pipe were experimentally investigated with respect to the heat flux, the charging ratio of working fluid, and the inclination angle to the horizontal orientation. The experimental results showed that the frequency of pressure oscillation was between 0.1 Hz and 1.5 Hz at the charging ratio of 40 vol.%. The saturation pressure of working fluid in the oscillating capillary tube heat pipe increased as the heat flux was increased. Also, as the charging ratio of working fluid was increased, the amplitude of pressure oscillation increased. When the pressure waves were symmetric sinusoidal waves at the charging ratios of 40 vol.% and 60 vol.%, the heat transfer performance was improved. At the charging ratios of 20 vol.% and 80 vol.%, the waveforms of pressure oscillation were more complicated, and the heat transfer performance reduced. At the charging ratio of 40 vol.%, the heat transfer performance of the OCHP was at the best when the inclination angle was 90°. the pressure wave was a sinusoidal waveform, the pressure difference was at the least, the oscillation amplitude was at the least, and the frequency of pressure oscillation was the highest.     

横管蒸发纵管冷凝式热管充液量、倾角对传热的影响

通过搭建试验台对横管蒸发纵管冷凝式热管进行试验研究,分析该型热管在不同充液量和不同的倾角的情况下传热性能的变化。经分析发现,充液量和倾角两个因素都对该型热管的传热性能有很大影响。当倾角变化时,其热阻随倾角增加而不断增加且存在临界角。倾角在到达临界角之前热阻上升缓慢,倾角大于临界角后其热阻急剧增大。当充液量变化时,随充液量增大热管热阻先减小后增大;且在60%充液量时热阻最小,传热性能最佳。     

Comparative study on heat transfer characteristics of nanofluidic thermosyphon and grooved heat pipe

The present study used TiO₂-nanofluid with different volume ratios as the working fluids of a therrmosyphon and grooved heat pipe and investigated various parameters such as volume concentration of nanoparticles, orientation, heat flux, and cooling media. Further, the present study used nanofluids and dispersed TiO₂-nanoparticles into pure water with each cross-blended concentration of 0.05%, 0.1%, 0.5%, and 1%. The authors observed the best heat transfer performance in the 0.05% concentration with thermosyphon. The present study presents the enhancement of heat transfer performance with TiO₂-nanofluids, and fabricated a heat pipe from a straight stainless steel tube with an outer diameter and length of 10 and 500 mm, respectively. At the optimum condition for the pure refrigerant, the thermosyphon with 0.05% TiO₂-nanoparticle concentration gave 1.40 times higher efficiency than that of pure water.     

颗粒填充平行平板间流动与换热的边壁效应试验研究

考察了平行平板间流动与换热的边壁效应影响,测量了颗粒填充通道壁面温度分布,从而明确了板面开槽对流动和传热的作用机制。相同雷诺数的槽板平均表面换热系数是平板的2~3倍,同时减小了流动阻力。试验结果说明,开槽宽度与颗粒直径之间存在最佳组合。这为开发新型高效填充结构换热器提供了基础性的试验数据。     

Application of thermosyphon air-preheater for energy thrift from a furnace in a hot forging process

This research studied the heat transfer of the thermosyphon air-preheater. An empirical model was developed to predict heat transfer and applied to compute the thermosyphon air-preheater in a hot brass forging process. The thermosyphon air-preheater was designed, constructed and tested under medium temperature operating conditions with inlet hot gases, ranging between 390–440 °C in terms of using water as the working fluid with a 50 % fill by volume of the evaporator section. The experimental setup was comprised of a circular fin with 0.013 and 0.020 m internal diameter bare stainless steel tubes, with 0.015 m long evaporator and condenser sections. The thermosyphon air-preheater model had 6 rows, each composed of 4 columns. Experimental results found that the hot gas temperature was increased from 390 to 440 °C, the heat transfer rate increased. If the internal diameter changed from 0.013 to 0.020 m, the heat transfer rate slightly increased. The predicted results agreed well with the experimental data. This thermosyphon air-preheater has been designed, manufactured and tested for heat recovery in industry using medium temperatures ranging between 300-500 °C to recover flue gas energy from the furnace in a hot brass forging process.     

Flow topology,heat transfer characteristic and thermal performance in a circular tube heat exchanger inserted with punched delta winglet vortex generators

To improve the heat transfer rate and thermal performance, the punched delta winglet vortex generators, DWVGs, were inserted in the middle of the circular tube heat exchanger. The effects of the flow attack angles and the flow directions were investigated numerically for the Reynolds number Re = 100–2000. The finite volume method and the SIMPLE algorithm were used to study. The results are reported in terms of the flow structure, heat transfer behavior and thermal performance evaluation and also compared with the smooth tube with no vortex generators. As the numerical results, the use of the DWVGs in the tube can improve the heat transfer rate and thermal performance by creating the vortex flow through the tested section. The rise of the flow attack angle results in the increasing strength of the vortex flows. The flow attack angle of 25° performs the highest heat transfer rate and thermal performance, while the flow attack angle of 0o gives the reversed results. The computational results reveal that the optimum thermal enhancement factor is around 2.80 at Re = 2000, α = 25°, with the winglet tip pointing downstream. The correlations on both the Nusselt number ratio and friction factor ratio for the DWVG in the tube heat exchanger are presented.     

Thermal characteristics of an ammonia-charged closed-loop pulsating heat pipe

At this age, engineering applications are demanding effective ways of heat recovery and energy savings for their optimum performance. Among other cooling techniques, pulsating heat pipes have emerged as a convenient and cost effective thermal design solution due to its excellent heat transfer capability, high thermal efficiency and structural simplicity. The paper presents an experimental study on the operational limit of an aluminum closed-loop pulsating heat pipe (CLPHP) charged with ammonia. It consists of total 14 turns of aluminum pipe with 3 mm inner and 4 mm outer diameter. Ammonia was used as working fluid with 3 different filling ratios such as 0.4, 0.6 and 0.8. Operation orientations were vertical, 30°, 45°, 60°, 90° and 180° inclinations. Constant electric heat input of 36 W was applied to the heating block and temperature rise in various sections was monitored till steady state was reached. Temperature was measured at different locations of the CLPHP by using thermocouples. The effects of operational orientations and filling ratios were investigated on heat transfer by working fluid Q̇_php (Watt), overall heat transfer coefficient U (W/m² °C) and thermal resistance R (°C/W) considering the measured temperature. The result shows that, 0.4 and 0.6 fill ratios and inclination angle of 30º give better result than any other arrangements for CLPHP.

     

Prediction of forced convective boiling heat transfer coefficient of pure refrigerants and binary refrigerant mixtures inside a horizontal tube

Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect.     

Thermal performance of a spirally coiled finned tube heat exchanger under wet-Surface conditions

This paper is a continuation of the authors’ previous work on spiral coil heat exchangers. In the present study, the heat transfer characteristics and the performance of a spirally coiled finned tube heat exchanger under wet-surface conditions are theoretically and experimentally investigated. The test section is a spiral-coil heat exchanger which consists of a steel shell and a spirally coiled tube unit. The spiral-coil unit consists of six layers of concentric spirally coiled finned tubes. Each tube is fabricated by bending a 9.6 mm diameter straight copper tube into a spiral-coil of four turns. The innermost and outermost diameters of each spiral-coil are 145.0 and 350.4 mm, respectively. Aluminium crimped spiral fins with thickness of 0.6 mm and outer diameter of 28.4 mm are placed around the tube. The edge of fin at the inner diameter is corrugated. Air and water are used as working fluids in shell side and tube side, respectively. The experiments are done under dehumidifying conditions. A mathematical model based on the conservation of mass and energy is developed to simulate the flow and heat transfer characteristics of working fluids flowing through the heat exchanger. The results obtained from the present model show reasonable agreement with the experimental data.     

Wet surface heat transfer and pressure drop of aluminum parallel flow heat exchangers at different inclination angles

The effect of inclination angle on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers was experimentally investigated under wet conditions. Three samples having different fin pitches (1.25, 1.5 and 2.0 mm) were tested. Results show that heat transfer coefficients are not affected by the inclination angle. However, friction factors increase as the inclination angle increases with negligible difference between the forward and backward inclination. The effect of fin pitch on the heat transfer coefficient and on the pressure drop is also discussed. Comparison of the dry and wet surface heat transfer coefficients reveals that dry surface heat transfer coefficients are significantly larger than wet surface heat transfer coefficients. Possible explanation is provided by considering the condensate drainage pattern. The data are also compared with the existing correlation. This paper was recommended for publication in revised form by Associate Editor Man-Yeong Ha Nae-Hyun Kim is a Professor of Mechanical Engineering, University of Incheon. His area of interest spans boiling and condensation, heat transfer enhancement and heat exchanger design. He has been active in heat transfer community, and was a Chairman of Thermal Engineering Division of KSME. He holds several editorial position including Journal of Enhanced Heat Transfer. He is a recipient of Asian Academic Award awarded by SAREK and JSRAE.     

An analytic study on laminar film condensation along the interior surface of a cave-shaped cavity of a flat plate heat pipe

An analytic approach has been employed to study condensate film thickness distribution inside cave-shaped cavity of a flat plate heat pipe. The results indicate that the condensate film thickness largely depends on mass flow rate and local velocity of condensate. The increasing rate of condensate film for circular region reveals about 50% higher value than that of vertical region. The physical properties of working fluid affect significantly the condensate film thickness, such as the condensate film thickness for the case of FC-40 are 5 times larger than that of water. In comparison with condensation on a vertical wall, the average heat transfer coefficient in the cave-shaped cavity presented 10-15% lower values due to the fact that the average film thickness formed inside the cave-shaped cavity was larger than that of the vertical wall with an equivalent flow length. A correlation formula which is based on the condensate film analysis for the cave-shaped cavity to predict average heat transfer coefficient is presented. Also, the critical minimum fill charge ratio of working fluid based on condensate film analysis has been predicted, and the minimum fill charge ratios for FC-40 and water are about Ψ_crit= 3-7%, Ψ_crit=0.5-1.3%, respectively, in the range of heat fluxq” = 5-90kW/²     

Flow visualization of oscillation characteristics of liquid and vapor flow in the oscillating capillary tube heat pipe

The two-phase How patterns for both non-loop and loop type oscillating capillary tube heat pipes (OCHPs) were presented in this study. The detailed flow patterns were recorded by a high-speed digital camera for each experimental condition to understand exactly the operation mechanism of the OCHP. The design and operation conditions of the OCHP such as turn number, working fluid, and heat flux were varied. The experimental results showed that the representative flow pattern in the evaporating section of the OCHP was the oscillation of liquid slugs and vapor plugs based on the generation and growth of bubbles by nucleate boiling. As the oscillation of liquid slugs and vapor plugs was very speedy, the How pattern changed from the capillary slug flow to a pseudo slug flow near the annular flow. The flow of short vapor-liquid slug-train units was the flow pattern in the adiabatic section. In the condensing section, it was the oscillation of liquid slugs and vapor plugs and the circulation of working fluid. The oscillation flow in the loop type OCHP was more active than that in the non-loop type OCHP due to the circulation of working fluid in the OCHP. When the turn number of the OCHP was increased, the oscillation and circulation of working fluid was more active as well as forming the oscillation wave of long liquid slugs and vapor plugs in the OCHP. The oscillation flow of R-142b as the working fluid was more active than that of ethanol and the high efficiency of the heat transfer performance of R-I42b was achieved.     

An experimental apparatus measuring convective heat transfer coefficient from a heated fine wire traversing in nanofluids

Most of the previous convection experiments for nanofluids have been performed for internal tube flow with constant heat flux boundary condition. In contrast, a simple experimental apparatus measuring convective heat transfer coefficient from a heated wire to external nanofluids is proposed and its working principles are explained in detail. The convective heat transfer coefficient provided by the present system might be used as a useful indication justifying the adoption of prepared nanofluids as new efficient heat transfer fluids. Validation experiments by comparing convective heat transfer coefficients between the conventional correlation and measured values are carried out for base fluids. Also the effect of increased thermal conductivity of nano lubrication oil on the enhancement of convective heat transfer coefficient is investigated.     

Thermo-hydrodynamics of closed loop pulsating heat pipe: an experimental study

The experimental result on the thermal performance of closed loop pulsating heat pipe (CLPHP) is presented. The CLPHP is made of copper capillary tubes, having inner and outer diameters of 2.0 mm and 3.6 mm respectively. The working fluids employed are water, ethanol, methanol and acetone also binary mixture (1:1 by volume) of water-ethanol, water-methanol and water-acetone. For all experimentations, filling ratio (FR) 50%, two-turns and vertical bottom heat mode position was maintained. The lengths of evaporator, condenser and adiabatic section are selected as 42 mm, 50 mm and 170 mm, respectively. The transparent adiabatic section is partially made of glass tube having length 80 mm, for flow visualization. The CFD analysis by VOF model in Star CCM+ simulation is carried out to validate the experimental results. The result shows that the thermal resistance decreases smoothly up to 40W heat input, thereafter reasonably steady. In comparison with all working fluids, water-acetone binary working fluid has shown the best thermal performance over other working fluids used in CLPHPs.     

声空化强化沸腾换热的试验观察与分析

以乙醇为工作液体,对声空化作用下直径为20 mm的水平铜管的沸腾传热进行试验研究.试验中对空化强度及超声换能棒和试件间的距离进行测定.试验研究发现,声空化对过冷沸腾有显著强化作用;在核态沸腾区,声空化对沸腾起始区域的强化率最大,但是随热流密度的增加,强化率逐渐减小;当热流密度相等时,强化率随空化强度的增加而增大.声空化引起的液体的宏观湍动及由于声冲流的存在导致的边界层厚度的减薄,直接或间接地影响加热表面上气泡胚胎的生成、长大和脱离.