Experimental investigation of the loop thermosyphon with different adiabatic lengths charged with different working fluids

This paper reports an experimental investigation of a closed-loop thermosyphon system charged with water and other low saturation fluids, such as ethanol, acetone, and methanol, for different adiabatic lengths, filling ratios, and heat loads. The closed-loop thermosyphon with two inline vertical heaters in the evaporator section and forced air-cooled plate-type heat exchanger in the condenser section, connected by a changeable adiabatic length, is investigated at different working conditions. Out of five filling ratios used in the analysis, at 0.6 filling ratio, the loop thermosyphon is seen to be operated at its best. The acetone-charged loop thermosyphon shows the lowest values (up to 72% reduction) of overall thermal resistance than that of other fluids and significantly higher effectiveness, due to the plate-type forced air-cooled condenser. For the acetone-filled thermosyphon, an almost 15% increase in the effectiveness is observed by changing the adiabatic length from 800 to 200 mm. This study suggests that the limitation of the loop thermosyphon with a water-cooled condenser to cool electronic components, computational clusters, and data centers is well fulfilled by the loop thermosyphon with plate-type forced air-cooled condenser. The nucleate pool boiling correlation is developed and validated for the loop thermosyphon system to determine the evaporator heat transfer coefficient.     

Effect of Inclination Angle and Filling Ratio on Thermal Performance of a Two-Phase Closed Thermosyphon under Normal Operating Conditions

In this paper, the effect of the inclination angle on the thermal performance of a two-phase closed thermosyphon with different filling ratios has been investigated experimentally under normal operating conditions. A series of experiments were carried out for inclination angle range of 5°–90° and filling ratios of 15%, 22%, and 30%. A copper thermosyphon with an outside diameter of 16 mm, an inside diameter of 14.5 mm, and a length of 1000 mm was employed. Distilled water was used as the working fluid. The results show that the two-phase closed thermosyphon has the highest thermal performance in the inclination angle range of 15°–60°. A good agreement was observed between the experimental results of this study and those available in the open literature. The interesting phenomenon of geyser boiling occurred in our experiments for filling ratios equal or greater than 30%. The geyser boiling puts no limitation on thermal performance of thermosyphon, but it should be avoided because it damages the condenser end cap due to the slug striking.     

Thermal performance of a closed advanced two-phase thermosyphon loop for cooling of radio base stations at different operating conditions

In this investigation an advanced thermosyphon loop with extended evaporator and condenser surfaces has been tested at high heat fluxes. The thermosyphon investigated is designed for the cooling of three parallel high heat flux electronic components. The tested evaporators were made from small blocks of copper in which five vertical channels with a diameter of 1.5 mm and length of 14.6 mm were drilled. The riser and downcomer connected the evaporators to the condenser, which is an air-cooled roll-bond type with a total surface area of 1.5 m² on the airside. Tests were done with Isobutane (R600a) at heat loads in the range of 10–90 W/cm² to each of the components with forced convection condenser cooling and with natural convection with heat loads of 10–70 W.     

Investigation of Geyser Boiling Phenomenon in a Two-Phase Closed Thermosyphon

In this paper, geyser boiling phenomenon (GBP) in a two-phase closed thermosyphon has been investigated experimentally. Here, the effects of the inclination angle, filling ratio, input heat rate, mass flowrate of coolant, and inside diameter of the tube on the GBP have been discussed. Three copper thermosyphons with inside diameters of 14 mm, 20 mm, and 24 mm and a length of 1000 mm were employed. Distilled water was used as the working fluid. A series of experiments was carried out to investigate the effect of the inclination angle range of 5° to 90°, the input heat rate range of 50 to 312.4 W, the coolant mass flow rate range of 0.00389 to 0.0164 kg/s, and the filling ratio range of 15 to 45%. The GBP has been investigated by analyzing the time variations of the evaporator and adiabatic wall temperature and outlet water temperature from condenser jacket. The results show that the period of GBP was longer for higher inclination angles and filling ratios. Furthermore, it was discovered that the GBP did not take place for inclination angles of less than 15°.     

Heat transfer characteristics of an external-fin-assisted two-phase closed thermosyphon: An experimental study

In the current paper, the performance of an external-fin-assisted thermosyphon is investigated experimentally. The thermosyphon is produced with a copper tube and includes three parts—the evaporator, the adiabatic, and the condenser. The condenser part is enhanced with external longitudinal fins. In this study, different number of fins, filling ratios (FRs), coolant flow rates, a wide range of heat inputs, and initial absolute pressures are considered. The experiments are carried out by measurement of temperature distribution of the thermosyphon's wall and the temperature difference of the coolant. The results depict that increasing the heat input and FR reduces the thermal resistance, while raising the coolant flow rate augments the thermal resistance. Adding external fins to the condenser causes further condensation, which enhances the thermosyphon thermal performance by a reduction of 26.32% in thermal resistance and an increment of 28.55% in the thermosyphon efficiency.     

Long-term thermal performance of a two-phase thermosyphon solar water heater

This article investigates experimentally the long-term thermal performance of a two-phase thermosyphon solar water heater and compares the results with the conventional systems. Experimental investigations are conducted to obtain the system thermal efficiencies from the hourly, daily and long-term performance tests. Different heat transfer mechanisms, including natural convection, geyser boiling, nucleate boiling and film-wise condensation, are observed in the two-phase thermosyphon solar water heater while solar radiation varies. The thermal performance of the proposed system is compared with that of four conventional solar water heaters. Results show that the proposed system achieves system characteristic efficiency 18% higher than that of the conventional systems by reducing heat loss for the two-phase thermosyphon solar water heater.     

EFFECT OF CONDENSER LOCATION AND IMPOSED CIRCULATION ON THE PERFORMANCE OF A COMPACT TWO-PHASE THERMOSYPHON

This study investigates the issues involved in the design of a compact two-phase thermosyphon in which the locations of evaporator and condenser need a high degree of freedom. Enhancement of boiling heat transfer in the compact evaporator space was achieved by a microfabricated structure. Anticipating situations where gravity does not provide sufficient potential to drive the condensate, a pump-assisted circulation loop was studied. The relative height between the evaporator and condenser and the pumping rate were systematically varied by utilizing two thermosyphon loops. Close examination of the data suggests that there could be an optimum point in the parametric domain where the thermal resistance is minimized with least assistance from the pump.     

Performance of stationary and vibrated thermosyphon working with water and R134a

An experimental apparatus of a two-phase closed thermosyphon has been designed and constructed to predict its performance characteristics under stationary and vibrated conditions. Water and R134a are used as working fluids. Experiments are carried out over wide ranges of liquid fill ratio (0.4, 0.5, 0.6 and 0.8), length of adiabatic section (275, 325 and 350 mm), vibration frequency (0.0–4.33 Hz) and input heat flux (160–2800 kW/m²). The results showed that adiabatic length of 350 mm and liquid fill ratio of 0.5 provide the highest output heat flux. The effect of vibration is to deteriorate the water-copper thermosyphon performance below the boiling limit by 5–20% and enhance it at the onset of boiling limit by almost the same ratio over the examined range of input heat flux. Minor or no effect is experienced with R134a below the boiling limit and enhancement up to 250% existed above the boiling limit.     

Experimental studies on heat transfer and friction factor characteristics of thermosyphon solar water heater system fitted with spacer at the trailing edge of twisted tapes

Experimental investigation of heat transfer and friction factor characteristics of thermosyphon solar water heater system with full- length twist, twist fitted with rod and spacer fitted at the trailing edge for lengths of 100, 200 and 300 mm for twist ratio 3 and 5 has been carried out and compared with plain tube collector for the same operating conditions. The experimental data for Nusselt number and friction factor for plain tube collector is verified with fundamental equations. Empirical correlations are developed for Nusselt number and friction factor for twist ratio 3 and 5. Results conclude that the decrease in Nusselt number for full length helical twist compared to twist fitted with rod is minimum and is quite significant for twist with spacer. But the decrease in friction factor is maximum in twist fitted with spacer compared to twist fitted with rod. The over all performance for twist fitted with rod is found to be better than twist fitted with spacer.     

Experimental investigation of small diameter two-phase closed thermosyphons charged with water,FC-84, FC-77 and FC-3283

An experimental investigation of the performance of thermosyphons charged with water as well as the dielectric heat transfer liquids FC-84, FC-77 and FC-3283 has been carried out. The copper thermosyphon was 200 mm long with an inner diameter of 6 mm, which can be considered quite small compared with the vast majority of thermosyphons reported in the open literature. The evaporator length was 40 mm and the condenser length was 60 mm which corresponds with what might be expected in compact heat exchangers. With water as the working fluid two fluid loadings were investigated, that being 0.6 ml and 1.8 ml, corresponding to approximately half filled and overfilled evaporator section in order to ensure combined pool boiling and thin film evaporation/boiling and pool boiling only conditions, respectively. For the Fluorinert? liquids, only the higher fill volume was tested as the aim was to investigate pool boiling opposed to thin film evaporation. Generally, the water-charged thermosyphon evaporator and condenser heat transfer characteristics compared well with available predictive correlations and theories. The thermal performance of the water-charged thermosyphon also outperformed the other three working fluids in both the effective thermal resistance as well as maximum heat transport capabilities. Even so, FC-84, the lowest saturation temperature fluid tested, shows marginal improvement in the heat transfer at low operating temperatures. All of the tested Fluorinert? liquids offer the advantage of being dielectric fluids, which may be better suited for sensitive electronics cooling applications and were all found to provide adequate thermal performance up to approximately 30–50 W after which liquid entrainment compromised their performance.     

重力热管内部相变及传热传质过程的数值模拟

重力热管内部包含复杂的两相流动以及相变传热过程,传统理论分析及实验手段不能直观给出其内部流动、相变、热质传递的详细信息。采用VOF(volume of fluid)多相流模型对重力热管内气液两相流动及传热进行模拟,捕捉到蒸发段气泡产生、合并、长大、上升,以及冷凝段壁面附近液滴形成、合并、下滑、汇集到液池的全过程,得到的壁温分布与实验测量值对比体现良好一致性,表明数值模拟的正确性。同时,以热阻、传热量和热效率为评价标准,研究不同充液率和倾斜角度下对重力热管运行性能的影响。结果表明:在所研究的参数范围内,随着充液率的增加,热阻逐渐减小,冷凝段传热量逐渐增大。且工质初始充注量充满蒸发段时热管性能较好;倾角对热阻的影响不明显,冷凝段传热量和热效率均随倾角增加而增长。     

空冷凝汽器椭圆翅片椭圆管束外空气的流动与传热特性

研究空冷凝汽器椭圆翅片椭圆管管束外空气的流动与传热特性,对火电站空冷岛的设计与运行具有重要意义.通过CFD模拟,获得了椭圆翅片椭圆管管束外冷却空气的流场和温度场,计算得到了空冷凝汽器冷却空气对流换热平均Nu和摩擦系数f随Re的变化规律,并采用最小二乘法拟合得到了相应的关联式.结果表明:随冷却空气流动Re的增大,Nu增大,f减小.     

Assessment of heat transfer and flow characteristics of a two-phase closed thermosiphon

In this study, comprehensive modeling and simulations were developed and carried out to perform the investigation of the thermal performance of the enclosed thermosiphon through pool boiling in the evaporator sector and the condensation of the liquid film in the condenser part. To simulate these phenomena, the volume of fluid model was utilized. The simulation modeling using the computational fluid dynamics (CFD) technique was validated with existing experimental results, and a good agreement was reached. The simulation results were presented and evaluated in terms of temperature profiles and contours, the volume of fraction contours, and velocity vector distribution. Moreover, the thermal performance (ie, the heat transfer coefficient and thermal resistance) through the thermosiphon operation was analyzed. From the simulation results, it is found that the thermosiphon performance can be improved by the tilt angle and fill ratio. The results indicated that the optimal performance (ie, a high heat transfer coefficient and a low thermal resistance) was attained at a power input of 250 W, tilt angle of 90°, and fill ratio of 0.5. The established CFD simulations effectively predicted the formation of two-phase flow pattern and boiling and condensation zones with water at a low power input, termed as geyser boiling.     

A numerical study on the effect of a heated hollow cylinder on mixed convection in a ventilated cavity

The present work is aimed to study mixed convection heat transfer characteristics within a ventilated square cavity having a heated hollow cylinder. The heated hollow cylinder is placed at the center of the cavity. In addition, the wall of the cavity is assumed to be adiabatic. Flows are imposed through the inlet at the bottom of the left wall and exited at the top of the right wall of the cavity. The present study simulates a practical system such as air-cooled electronic equipment with a heat component or an oven with heater. Emphasis is sited on the influences of the cylinder diameter and the thermal conductivity of the cylinder in the cavity. The consequent mathematical model is governed by the coupled equations of mass, momentum and energy and solved by employing Galerkin weighted residual method of finite element formulation. A wide range of pertinent parameters such as Reynolds number, Richardson number, cylinder diameter and the solid-fluid thermal conductivity ratio are considered in the present study. Various results such as the streamlines, isotherms, heat transfer rates in terms of the average Nusselt number and average fluid temperature in the cavity are presented for different aforesaid parameters. It is observed that the cylinder diameter has significant effect on both the flow and thermal fields but the solid-fluid thermal conductivity ratio has significant effect only on the thermal field.     

Thermal characteristics of a resurfaced condenser and evaporator closed two-phase thermosyphon

This paper reports a study on the effect of the condenser and evaporator resurfacing on overall performance of a 1 m height closed two-phase thermosyphon. Water was used as working fluid with a fill ratio and operating pressure was 0.75 and 160 mbar, respectively. The thermosyphon performances for plain and modified thermosyphon were studied at 44 power inputs from 43 W to 668 W. The results show that by making the evaporator more hydrophilic and the condenser more hydrophobic, it will be possible to increase the average thermal performance by15.27% and decrease the thermal resistance by 2.35 times compared with the plain one.     

Natural convection in partially divided square enclosures: Effects of thermal boundary conditions and thermal conductivity of the partitions

Natural convection in partitioned square enclosures filled with air is numerically studied, trying to characterize these enclosures mainly in what concerns its overall heat transfer performance. Two partitions of finite thickness are considered, placed in the enclosure following an ordered arrangement, which position, length and thermal conductivity are varied for some values of Rayleigh number and for different thermal boundary conditions. Study starts considering the simplest enclosures with two adiabatic partitions, after the more realistic enclosures of heat conductive walls and partitions are considered, and finally the even more realistic situation of enclosures with heat conductive partitions and walls subjected to cyclic thermal boundary conditions in the vertical direction is also considered. Position and length of the enclosures’ effects depend on the thermal boundary conditions prescribed for the enclosure, and different thermal boundary conditions (corresponding to the heating or cooling operations or seasons) are considered to capture this effect. Fluid flow field, thermal field and heat transfer are analyzed for some particular situations through the streamlines, isotherms, and heatlines. The overall thermal performance of the enclosure is analyzed through the overall Nusselt number, and many data are compactly presented for different placements and lengths of the partitions, for different thermal conductivity of the walls and partitions of the enclosure, for different Rayleigh numbers and for different thermal boundary conditions imposed to the enclosure. Considered boundary conditions and the enclosure walls and partitions of finite thickness and finite thermal conductivity are much more realistic conditions than simply the single cavity without walls and with perfectly adiabatic partitions usually considered in many studies of this kind.     

Experimental Study on Thermosyphon for Shipboard High-Power Electronics Cooling System

The closed-loop thermosyphon (CLT) has advantages of simple structure and reliability for transporting heat in long distances with small decrease in temperature. It is considered a promising cooling device for power electronics onboard ships. In this research, CLT for cooling of power electronics onboard ship was developed, and the performance was experimentally examined using a CLT apparatus. The performance was investigated for steady-state heat transfer under a wide range of pressures and heat loads from 18.3 kPa to 35.3 kPa and from 88.9 W to 616.2 W, respectively. The fill charge rates were 27% and 45%. The circulation coolant temperature at the condenser was set to 15°C. The measured data for each rated heat input were registered by a data logger in every 5-s increment of sampling data for a 30-min period. During the steady-state operation, CLT could maintain the system pressure and produced the vapor bulk temperature at around saturation boiling regime. The temperature distributions of the system were measured from each probed thermocouple along the loop. It is understood that higher heat inputs around above 349 W could keep the bulk vapor in an almost constant temperature from evaporation process up to the inlet position of the condenser. The condenser of the direct hull cooling method could also maintain the condensation process with a temperature decrease of around 30°C from the inlet vapor temperature of the condenser. It was clarified that the CLT has good thermal performance in the higher heat loads with low thermal resistance and provides a steady circulation loop from each two-phase process of heating in the evaporator and cooling during condensation.     

Analyzing the pressure-difference phenomenon between the condensing and boiling sections in a heat pipe cooling system

The heat pipe cooling system in this study consists of a flat evaporator, a condenser, and rising and falling tubes with water as working fluid. The working fluid has different water levels inside the two components. This is due to the vapor pressure deficits of the evaporation section and condenser section. This paper utilizes condensing and boiling pressure-difference theory and measures the temperature of the condenser wall to develop a theoretic model for the water level deficit inside the thermal module. Results indicate that the working fluid infiltrates the condenser and indirectly verifies the phenomenon leading to the different water levels inside the cooling system. Moreover, the water level height difference theory presented in this study may reduce the length of the condenser by 3.14 cm.     

空冷凝汽器局部翅片温度低于大气环境温度现象的讨论

实际运行中,存在空冷凝汽器局部翅片表面温度低于大气环境温度的现象,运用管道内绝热流动的简化能量方程不能客观地解释此现象;经分析,指出此现象是温度测点显示环境冷空气干球温度下的湿球温度值所致。不凝气体的聚积是空冷凝汽器管束冬季结冻的主要原因之一,由此,针对防冻工作提出相关建议。     

Study on heat transfer characteristics of reservoir embedded loop heat pipe (Influence of condenser cooling method on heat transfer characteristics)

As heat generation in satellites increases, securing sufficient radiator panel area is an important problem. Deployable radiators, whose radiator panels are deployed post‐launch in space to increase the effective radiator panel area of the satellite, is becoming an important thermal control technology. A reservoir embedded loop heat pipe (RELHP) is applied to the deployable radiator for a thermal transport device. This paper presents the heat transport dynamic characteristics of a RELHP using a radiant cooling condenser and liquid forced convection cooling condenser by an experimental study. It was found that heat leak into the liquid line, flexible line, and reservoir increases the length of the sub‐cooling region in the condenser. In the case of the radiant cooling condenser, the sub‐cooling region length is shorter than that of a liquid forced convection cooling condenser. Furthermore, vapor temperature is mainly decided by the radiation capacity of the radiator panel, because liquid temperature returned into the evaporator rises with an increase in radiator panel temperature. In addition, time length from start‐up until steady state is greater than the liquid forced convection cooling condenser case, because the radiator panel has a large heat capacity. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20229