Experimental study of closed‐loop thermosyphon with a different evaporator geometry

In this study, the effect of evaporator geometry on the loop thermosyphon's heat transfer coefficient is experimentally verified by using water as a working fluid with three filling ratios (50%, 70%, 90%), constant heat input (185 W), and condenser cooling water flow rate remaining constant at 2 Lpm. Three evaporator pipes are used (I: straight; II: helical coil evaporator with a diameter of 100‐mm coil and two turns; III: helical coil evaporator with a diameter of 50‐mm coil and four turns). From the experimental results, it can be observed that the performance of evaporator III is higher than the two other forms. A greater heat transfer coefficient value is found in case of type III evaporator and is equivalent to 2456 W/m²·°C. The maximum thermal resistance reduction occurs in the type III evaporator (37.32%), and the highest effective thermal conductivity for the same type is 6.123e + 05 W/m·°C. The experimental results demonstrate good agreement with the empirical equations.     

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.     

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.     

Influence of fan setting height on the cooling performance of a plate‐fin‐type heat sink with microprocessor

The cooling performance of a plate‐fin‐type heat sink equipped with a cooling fan was investigated experimentally. The heat sink was 80 mm long, 43 mm wide, and 24 mm in height (including the 4‐mm‐thick base). The cooling fan was 40 × 40 × 15 mm and was set to direct the air flow vertically in the downstream half of the heat sink. We focused on the influence of the height (which varied from 5 to 20 mm) that the fan was set at, on the heat transfer coefficient of the heat sink. The maximum value of the heat transfer coefficient was achieved at a setting height of 5 mm. At this height, the volumetric heat transfer coefficient was 1.8 times as high as that in a parallel flow under the same fan power. This result indicates that the cooling performance of heat sinks with a cooling fan can be improved by using this kind of compact structure. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 512–520, 2001     

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.     

Enhancement of natural convective heat transfer from tall,vertical heated plate to water

An enhancement technique was developed for natural convection heat transfer from a tall, vertical heated plate to water. Rectangular grid fins attached to the base plate were utilized as a heat transfer promoter. These grid fins redirect the high‐temperature fluid ascending along the base plate toward the outside of the boundary layer and introduce the low‐temperature ambient fluid toward the base plate instead. The heat transfer coefficients of thus‐treated surfaces were measured and compared with a nontreated surface and a surface with conventional vertical plate‐fins. The highest performance was achieved for the experimental surfaces. In particular, the experimental surfaces with 5‐mm‐high, nonconducting grids and with 10‐mm‐high, conducting grid fins show 27% and 80% higher heat transfer coefficients compared to the turbulent heat transfer coefficients of the nontreated surface, respectively. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 178–190, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10083     

Study on a twin‐bed adsorption refrigeration system using R134a‐activated carbon pair

A low capacity twin‐bed adsorption refrigeration system has been built with R134a as a refrigerant and activated carbon as the adsorbent. Simple tube‐in‐tube heat exchangers have been fabricated and have been used as the adsorber beds. Activated carbon (granular type) has been filled in the annular space of the inner tube and outer tube. A plate heat exchanger has been used as the condenser and the temperature of cooling water has been maintained between 25°C and 30°C, also the evaporator has been custom designed as per requirements. A mathematical model has also been developed and the results obtained have been found to be comparable. While operating the system in the single‐bed mode a cooling power of 250.4 W has been obtained with a coefficient of performance (COP) of 0.38 with an average evaporator temperature of 18.4°C against a predicted value of 263.7 W with a COP of 0.41. While operating in the twin‐bed mode a cooling power of 281.3 W with a COP of 0.47 with an average evaporator temperature of 17.6°C has been obtained against a predicted value of 294.5 W with a COP of 0.52.     

Instability phenomena in a two-phase microchannel thermosyphon

The behavior of a two-phase thermosyphon, consisting of a microchannel evaporator plate and a condenser, is investigated to gain insight into the system limiting instability. A microchannel plate has been fabricated with 56 square channels that have a 1 × 1 mm cross section and a length of 115 mm. Experiments have been conducted for various condenser heights with the heat flux as the control variable. A step increase in heat flux is used to quantify the response of the system, including variations in mass flow rate, temperature, and pressure drop. Results show that small fluctuations about the steady state give rise to the instability for situations with a uniform heat load. A predictive model based on the momentum equation is introduced to estimate the onset of instability, and the threshold heat flux is predicted to within ±10% uncertainty.     

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.     

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     

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     

Experimental investigation on thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger

The thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger was investigated to show its applicability in China. The effect on the performance of the collector of using a heat exchanger between the collector and the tank was analyzed. A “heat exchanger penalty factor” for the system was determined and energy balance equation in the system was presented. Outdoor tests of thermal performance of the thermosyphon flat-plate solar water heater with a mantle heat exchanger were taken in Kunming, China. Experimental results show that mean daily efficiency of the thermosyphon flat plate solar water heater with a mantle heat exchanger with 10 mm gap can reach up to 50%, which is lower than that of a thermosyphon flat-plate solar water heater without heat exchanger, but higher than that of a all-glass evacuated tubular solar water heater.     

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     

One‐zone simulation model of an oil‐injected screw chiller

This paper presents a one‐zone steady‐state system model of an oil‐injected screw chiller. The model can be used as a design and optimization tool for system performance of multiple‐chiller plant in process industries. All major components of the system are modelled in a modular format including the oil‐injected screw compressor, shell and tube condenser, flooded evaporator and a high side‐float value. The model results are validated with the experimental data from a multiple‐chiller plant at a process industry. The validated results show that the part‐load ratio and the glycol–water temperature at the evaporator inlet significantly affect the system performance as compared to the temperature of cooling water entering the condenser. Copyright © 2004 John Wiley & Sons, Ltd.     

Investigation on thermal management performance of wedge‐shaped microchannels for rectangular Li‐ion batteries

The performance of power battery is a significant factor affecting the overall quality of electric vehicles. To optimize the thermal management effect of battery pack, cold plate with wedge‐shaped microchannels was proposed in this paper. On the basis of the models of the independent cold plate and the battery‐cooling module, the effects of outlet aspect ratio, flow rate, and branching structure on the heat dissipation performance of the cold plate were studied at first. Afterwards, the effects of cooling surface, flow rate, and branching structure on the temperature distribution of the battery module were simulated. The results showed that the wedge‐shaped channels provided a good cooling efficiency and surface temperature uniformity. When the wedge‐shaped channel was used in thermal management of the battery module, the side‐cooling method reduced the temperature difference of batteries by more than 35.71% compared with front cooling under the mass flow rate of 2 × 10^?5 kg/s. At a discharge rate of 3.5 C, the flow rate of 1 × 10^?4 kg/s controlled the battery temperature to within 45°C, and the branching structure designed for the module successfully decreased the maximum temperature difference from 7.27°C to 4.67°C, which has been reduced by approximately 35.78%.     

Neural networks—a new approach to model vapour‐compression heat pumps

The aim of this paper is to model the steady‐state performance of a vapour‐compression liquid heat pump with the use of neural networks. The model uses a generalized radial basis function (GRBF) neural network. Its input vector consists only of parameters that are easily measurable, i.e. the chilled water outlet temperature from the evaporator, the cooling water inlet temperature to the condenser and the evaporator capacity. The model then predicts relevant performance parameters of the heat pump, especially the coefficient of performance (COP). Models are developed for three different refrigerants, namely LPG, R22 and R290. It is found that not every model achieves the same accuracy. Predicted COP values, when LPG or R22 are used as refrigerant, are usually accurate to within 2 per cent, whereas many predictions for R290 deviate more than ±10 per cent. Copyright © 2001 John Wiley & Sons, Ltd.     

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 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.     

Thermo‐economic optimization of superheating and sub‐cooling heat exchangers in vapor‐compressed refrigeration system

In this study, superheating and sub‐cooling heat exchangers in vapor‐compressed refrigeration system are analyzed from thermodynamics and economical (refrigeration system operation cost, investment cost) viewpoints. Using four different refrigerants (R22, R502, R134a and R404a), the temperature of condenser at the interval of (35–55°C) and temperature of evaporator at the interval of (?10 to 10°C) have been obtained from the calculation process. The second law analysis (analysis of irreversibility) of a refrigeration system is carried out and then the whole system is optimized thermo‐economically. As a result of calculations, optimum superheating and sub‐cooling temperatures of heat exchanger (superheating, sub‐cooling) areas corresponding to these temperatures are obtained. Copyright © 2007 John Wiley & Sons, Ltd.