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.     

Experimental study on CPU cooling system of closed‐loop two‐phase thermosyphon

This paper reports on an indirect cooling method of high‐power CPU of notebook computers using a closed‐loop two‐phase thermosyphon with Fluorinert (FC‐72) as the working fluid. The experimental setup consists of an evaporator with an electric heater, a condenser, and flexible tube connecting them. The heater and condenser act as a high‐power CPU and a cooling plate located behind the display of a notebook computer, respectively. The evaporator and the condenser have the outer dimensions of 50mm × 50mm × 20mm and 150mm × 200mm × 20mm, respectively. Four possible boiling surfaces of an evaporator were examined, i.e., a smooth surface (Type A), rough one, ones with smooth plate fins and rough plate fins (Type D). Type D evaporator shows the highest performance, i.e., it reduces the temperature at the evaporator/heater interface by about 18% in comparison with that of the smooth surface evaporator (Type A). Type D evaporator keeps the temperature difference between the evaporator/heater interface and the ambient to be around 55 K at the highest heat input Q = 30W. The effects of the heat input Q, the volumetric amount of Fluorinert liquid F in the thermosyphon, and the evaporator type on the heat transfer characteristics of the cooling system were examined experimentally. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(3): 147–159, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20057     

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.     

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.     

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.     

固体吸附式冷管循环工作过程的动态模拟与分析

针对余热驱动吸附式冷管单元(直径16mm,总长1020mm)制冷循环过程的变压吸/脱附特性,采用线性驱动力(LDF)模型,建立了动态的传热传质数学模型。对吸附式冷管的主要部件——吸附器和冷凝/蒸发器在加热解吸和冷却吸附不同阶段的工作过程,分别建立了耦合的动态方程。对所提出的模型进行分析和合理简化后,利用数值方法对模型进行了求解,获得了冷管单元吸附器、冷凝/蒸发器的循环工作参数的动态变化规律,计算结果与实验结果较好地吻合。为吸附式冷管单元及其组合式制冷系统进一步的优化设计和实用化的改进研究提供了重要参考。     

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.     

Integral-type solar-assisted heat pump water heater

An integral-type solar-assisted heat pump water heater (ISAHP) is designed and tested in the present study. The storage tank and the Rankine cycle unit are integrated together to make a more compact size. A thermosyphon loop is used to transfer the heat from the condenser to the water storage tank. The highest COP obtained in the tests is 3.83.     

Effect of adiabatic length on the performance of closed-loop thermosyphon system

This paper reports the effect of different adiabatic lengths on the thermal performance of loop thermosyphon with different filling ratios (FRs) and heat inputs. The carbon steel thermosyphon loop for three different adiabatic lengths is used in this analysis. The loop with plate-type, forced air-cooled condenser, along with two vertical inline evaporators, is filled and tested with distilled water. The transient and steady-state analyses are carried out to understand the thermal behavior of the loop. The thermal resistance is found to be lowest at 800-mm adiabatic length and 60% FR. The geyser boiling phenomenon is also noticed in the present thermosyphon loop. The period of oscillation and temperature fluctuations in the evaporator and condenser increase with the adiabatic length. The geyser boiling phenomenon may disappear at a very small adiabatic length of the loop thermosyphon with forced air-cooled condenser. This paper proposes a mathematical model for the loop thermosyphon in terms of the Nusselt number, the Reynolds number, the Prandtl number, and the adiabatic length-to-diameter ratio, and the comparative study shows that proposed model validates the experimental results. Also, it is found that the adiabatic length-to-diameter ratio inversely varies with the Nusselt number.     

A distributed model of a space heat pump under transient conditions

A prototype heat pump was designed and tested, as means of active thermal management for electronics packages to be used on stratospheric balloon missions. The evaporator worked as a cold plate to absorb heat dissipated by the electronics, while the condenser rejected heat primarily by radiation to the rarified environment. To predict the transient performance of the heat pump under varying environmental temperature and cooling load conditions, a dynamic model of the heat pump is created with a graphical user interface (GUI). The simulation of the evaporator and condenser are fully transient and the components are segmented, whereas the compressor and expansion device are lumped models and assumed to be at quasi-steady state. A detailed model for the mass and energy conservation in the two heat exchangers is presented. The spatial and temporal variation of temperature and mass flow rate in the heat exchangers are predicted. Several types of transient conditions such as step changes of the space temperature and cooling load, system start-up, shutdown, and cycling, are studied. The space temperature, cooling load, compressor power, mass flow rates of the compressor and expansion device, pressures and refrigerant charges of the condenser and evaporator, and temperature distribution in the heat exchangers are dynamically displayed on the GUI. The simulation results are compared with experimental data for step changes in the cooling load and show good agreement in terms of trends. Copyright © 2002 John Wiley & Sons, Ltd.     

Formulation and analysis of the heat pipe turbine for production of power from renewable sources

The heat pipe turbine or thermosyphon Rankine engine is a new concept for power generation using solar, geothermal or other available low grade heat sources. The basis of the engine is the thermosyphon cycle, with its excellent heat and mass transfer characteristics, modified to incorporate a turbine in the adiabatic region. The basic configuration is a closed vertical cylinder functioning as an evaporator, an insulated section and a condenser. The turbine is placed in the upper end between the insulated section and condenser section, and a plate is installed to separate the high pressure region from the low pressure region in the condenser. Conversion of enthalpy to kinetic energy is achieved through the nozzles. The mechanical energy developed by the turbine can be converted to electrical energy by direct coupling to an electrical generator.This paper describes the development of the heat pipe turbine from concept to reality, a series of development steps taken to optimise the design and manufacture. Also in this paper, attempts have been made to provide relationships for the developed power in terms of the geometric and thermodynamic parameters and to discuss limitations on the efficiencies of these turbines.     

Theoretical and experimental investigation of a heat pipe heat exchanger for energy recovery of exhaust air

Heat pipes and two-phase thermosyphon systems are passive heat transfer systems that employ a two-phase cycle of a working fluid within a completely sealed system. Consequently, heat exchangers based on heat pipes have low thermal resistance and high effective thermal conductivity, which can reach up to the order of (10⁵ W/(m K)). In energy recovery systems where the two streams should be unmixed, such as air-conditioning systems of biological laboratories and operating rooms in hospitals, heat pipe heat exchangers (HPHEs) are recommended. In this study, an experimental and theoretical study was carried out on the thermal performance of an air-to-air HPHE filled with two refrigerants as working fluids, R22 and R407c. The heat pipe heat exchanger used was composed of two rows of copper heat pipes in a staggered manner, with 11 pipes per row. Tests were conducted at different airflow rates of 0.14, 0.18, and 0.22 m³/h, evaporator inlet-air temperatures of 40, 44, and 50°C, filling ratios of 45%, 70%, and 100%, and ratios of heat capacity rate of the evaporator to condenser sections (C_e/C_c) of 1 and 1.5. For HPHE's steady-state operation, a mathematical model for heat-transfer performance was set and solved using MATLAB. Results illustrated that the heat transfer rate was in direct proportion with the evaporator inlet-air temperature and flow rate. The highest HPHE's effectiveness was obtained at a 100% filling ratio and (C_e/C_c) of 1.5. The predicted and experimental values of condenser outlet-air temperature were in good agreement, with a maximum difference of 3%. HPHE's effectiveness was found to increase with the increase in evaporator inlet-air temperature and number of transfer units (NTU) and with the decrease in airflow rate, up to 33% and 20% for refrigerants R22 and R407c, respectively. Refrigerant R22 was the superior of the two refrigerants investigated.     

Study of a Variable Conductance Two-Phase Closed Thermosyphon With Two-Component Working Fluid

In this article a numerical investigation is performed on a two-phase closed thermosyphon. Particular attention is given to the condenser section. The working fluid is a binary mixture that consists of two components (R ₁₁ + R ₁₁₃) causing a variable conductance behavior in the thermosyphon. The mass, energy, and species conservation equations in conjunction with the overall mass conservation and continuity of momentum at liquid–vapor interface constraints and the thermodynamic equilibrium condition are solved numerically by use of the integral method. The effects of diffusion at the interface of effective and noneffective (shut-off) lengths in condenser are taken into account. The results of the present model are compared with available experimental data and it is found that there is a good agreement with experimental data at low evaporator power levels.     

Heat transfer characteristics in a condenser of closed two‐phase thermosyphon: Effect of entrainment on heat transfer deterioration

Condensation heat transfer in a closed two‐phase thermosyphon is experimentally examined using two different types of test section. Test Section 1 is a straight‐pipe‐type thermosyphon, whereas Test Section 2 has a large‐diameter evaporator compared with a condenser to minimize entrainment at the evaporator. Condensation heat transfer in Test Section 1 shows much lower heat transfer coefficients than those estimated by a Nusselt theory. This low condensation heat transfer occurs due to a working fluid entrainment. It is confirmed from a result of Test Section 2 that the condensation heat transfer is similar to the values predicted by the Nusselt theory as far as the effect of the working fluid entrainment is negligible and flooding does not occur. A new correlation for the heat transfer coefficient considering the effect of entrainment is proposed. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 212–225, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10030     

PERFORMANCE OF A HEAT PIPE THERMOSYPHON RADIATOR

A heat pipe thermosyphon radiator for use in domestic and industrial heating applications is presented. A test cell for the radiator is described and various experimental tests have been performed to determine the feasibility and performance of a heat pipe thermosyphon radiator. The thermosyphon radiator has been tested with freon 11, acetone, methanol and water as working fluids, and was compared with a conventional radiator. Best performance was obtained using methanol and acetone, and compares well with the conventional radiator. In addition, with these working fluids the thermosyphon radiator, by design, has desirable isothermal surfaces. The worst performance was with water, where local hot and cold spots formed on the radiator surface and the performance was poor. A natural convection/radiation model is presented for the thermosyphon radiator, and good agreement between measured and calculated heat transfer is obtained. The model reveals that typically 60% of the heat is transferred by natural convection and the remaining 40% by radiation. Advantages and further development of the thermosyphon radiator are discussed. © 1997 by John Wiley & Sons, Ltd.     

Experimental Performance of R-134a-Filled and Water-Filled Loop Heat Pipe Heat Exchangers

Experimental investigations were conducted to determine the thermal performances of an R-134a-filled thermosyphon heat pipe heat exchanger (THPHE) and a water-filled loop heat pipe heat exchanger (LHPHE) for hot and cold energy recovery for air conditioning purposes. For such applications, the heat pipe heat exchangers are operated at low temperatures. Both exchangers were operated in the countercurrent flow mode. This article presents the experimental results obtained. The results showed that heat transfer rate increased as evaporator inlet temperature increased and as both evaporator and condenser velocities increased. The overall effectiveness for the THPHE ranged from 0.8 to a minimum of about 0.5, while for the LHPHE it ranged from 0.9 to 0.3. Overall effectiveness was found to approach a minimum when both air streams have equal velocities.     

Cooling load reduction of building by seasonal nocturnal cooling water from thermosyphon heat pipe radiator

In this study, a concept of using thermosyphon heat pipe to extract heat from water in a storage tank to generate cooling water was proposed. Heat pipe condenser was attached with an aluminum plate and acted as a thermal radiator while its evaporator was dipped in the water storage tank. Cooling water in the tank could be produced during the nighttime and used to serve the cooling load in a room during the daytime. A heat transfer model to calculate the water temperature and the room temperature during both the nighttime and daytime was developed. The input data were ambient temperature, dew point temperature, area of the radiator, volume of cooling water and room cooling load. The experiment was setup to verify the heat transfer model. A 9.0 m² tested room with six cooling coils, each of 0.87 m² was installed at the ceiling, was constructed along with the 1.0 m³ water storage tank. A 500–2000 W adjustable heater was taken as an artificial load inside the room. A 6.36 m² radiator is installed on a 45° tilting roof of the tested room. The simulated results agreed very well with those of the experimental data. With the developed model, a simulation to find the sizing of the radiator area and the volume of cooling water for cooling water production during winter of Chiang Mai, Thailand was carried out. The cooling water was used for cooling during summer in an air‐conditioned room with different cooling loads. The parameters in terms of room temperature, radiator area, volume of cooling water, cooling load and UA of cooling coil were considered to carry out the percent of cooling load reduction. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance characteristics of integral type solar-assisted heat pump

The characteristic of an integral type solar-assisted heat pump water heater (ISAHP) is investigated in the present study. The ISAHP consists of a Rankine refrigeration cycle and a thermosyphon loop that are integrated together to form a package heater. Both solar and ambient air energies are absorbed at the collector/evaporator and pumped to the storage tank via a Rankine refrigeration cycle and a thermosyphon heat exchanger. The condenser releases condensing heat of the refrigerant to the water side of the thermosyphon heat exchanger for producing a natural-circulation flow in the thermosyphon loop. A 105-liter ISAHP using a bare collector and a small R134a reciprocating-type compressor with rated input power 250 W was built and tested in the present study. The ISAHP was designed to operate at an evaporating temperature lower than the ambient temperature and a matched condition (near saturated vapor compression cycle and compressor exhaust temperature <100°C). A performance model is derived and found to be able to fit the experimental data very well for the ISAHP. The COP for the ISAHP built in the present study lies in the range 2.5–3.7 at water temperature between 61 and 25°C.     

Startup characteristics of a vertically placed high‐temperature heat pipe fin

In order to observe startup characteristics, a vertically installed high‐temperature heat pipe fin was tested. The temperature curves during the startup process are given. It was found that the evaporator bottom temperature in the high‐temperature heat pipe fin with a constant heat input increased very quickly over time. The temperature at the evaporator top and the condenser temperature lagged behind the temperature of the evaporator bottom. The evaporator outlet temperature coincided with the condenser middle temperature. The temperature at the end of the condenser exhibited a phenomenon of temperature pulsation. If the high‐temperature heat pipe fin was placed horizontally for a certain period of time and then tested in its vertical position, the temperature pulsation phenomenon at the condenser disappeared and a good isothermal condition emerged. Further analysis showed that larger heat inputs yielded faster startups and weaker pulsation during the startup period. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 411–416, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20022     

A new conception of an adsorptive solar-powered ice maker

An experimental data base obtained from specific tests carried out on an adsorptive solar-powered ice maker using methanol/carbon pair and equipped with the collector–condenser technology is analysed and the part of the evaporator in the condensation of the methanol vapour during the desorption phase is highlighted. Consequently, a new adsorptive solar-powered ice maker equipped with a single heat exchanger playing alternatively the role of condenser and evaporator is conceived. When working as a condenser, this heat exchanger is simply cooled by natural convection by means of fins. The estimated heat transfer area of the fins needed to reach the performance of the tested machine (COPs≈12%) is 1.3 m². This new conception of the machine leads to a 30 kg lightening of the metallic mass and enables to equip the collector with airing shutters, in order to improve its nocturnal cooling, which is the limiting factor of the collector–condenser technology. A notable reduction of the manufacturing cost and more than 10% improvement of performance are expected.