Two-phase Closed Loop Thermosyphon Research Articles

Experimental study on a helium cryogenic closed loop two-phase thermosyphon with a long heat transport distance

In this paper, the thermal performance of a cryogenic closed loop two-phase thermosyphon (CLTPT) with a long heat transport distance is investigated experimentally. The cryogenic CLTPT consists of an evaporator and a condenser connected by two stainless steel tubes. Helium serves as the working fluid in the cryogenic CLTPT, and there is a vertical distance of 0.9 m between the condenser and the evaporator. The experiments are carried out to investigate the effects of heat load, charging ratio, and condenser temperature on the thermal performance of the cryogenic CLTPT. According to the findings, for the cryogenic CLTPT with a liquid charging ratio of 32.5 % and the condenser temperature of 4.36 K the effective thermal conductivity (ETC) is between 30,000 and 60,000 W·m−1·K−1, and the ETC rises when the heat load on the evaporator is increased. The ETC of the cryogenic CLTPT at the condenser temperature of 4.2 K decreases with increasing liquid charging ratios for liquid charging ratios between 25 % and 42.5 %, Additionally, the maximum heat transport initially rises and subsequently falls with rising liquid charging ratio. In the end, it is discovered that raising the condenser temperature of the cryogenic CLTPT enhances its thermal performance.

Performance of Two-phase Closed Loop Thermosyphon in Cooling Down of Compact Superconducting Magnet

Performance of Two-phase Closed Loop Thermosyphon in Cooling Down of Compact Superconducting Magnet

Experimental investigation of R600a as a low GWP substitute to R134a in the closed-loop two-phase thermosyphon of the mini thermoelectric refrigerator

Various energy efficiency and environmental regulations around the world, such as the European Union's (EU) F-Gas regulation, are pushing refrigerator manufacturers to replace halogenated refrigerants with environmentally-friendly alternatives with low Global Warming Potential (GWP) without energy efficiency decrease. In this respect, increasing energy conversion performance by using a low GWP refrigerant in the mini thermoelectric refrigerators (MTERs) gains important for revealing sustainable and environmentally friendly refrigerators. To make this possible, the paper provides a comprehensive examination of the effect of climate-friendly refrigerant replacement in the closed-loop two-phase thermosyphon (CLTPT) on the performance of the MTER. This study treats the experimental investigation of the R600a as a low GWP substitute to R134a in the CLTPT of the MTER. Different from the other study in the literature, an extensive performance evaluation of the commercial MTER in terms of exergy as well as energy was performed and the various filling ratios (FR) of the R600a from 19% to 78% were analyzed for the best performance. The results showed that the MTER with a 46% and 32% FR of R600a had the highest exergy efficiency with 8.55% and 8.11%, respectively while reference MTER with R134a had 5.27%. It is concluded that 32% and 46% FR of the R600a enhanced the MTER energy conversion performance according to the 19% FR of the R134a, in line with the F-Gas regulation. The 32% and 46% FR of the R600a for MTER are the best refrigerant replacement, both from thermal performance and environmental points of view.

Experimental Study on the Stability and Transient Behavior of a Closed-Loop Two-Phase Thermosyphon (CLTPT) Charged with NOVEC 649

Currently, the growing need for efficient refrigeration resources in the industrial sector has led to an increasing interest in finding technologies with a higher heat removal potential and better cooling performance. Along these lines, two-phase liquid cooling appears to be a very interesting solution, with the CLTPT (closed-loop two-phase thermosyphon) being one of the leading alternatives. Most works in the scientific literature study loop thermosyphons that work in flow boiling conditions in steady state. The present paper analyzes the transient thermal behavior of a pool boiling CLTPT gravitational channel as a passive cooling system using NOVEC 649 as working fluid. The evaporator works with two submerged cylindrical heaters that represent different heat sources located in different positions. The initial transient behavior and consequent instabilities of a laboratory-scale facility were studied, followed by a stability analysis for various power inputs. Parameters such as temperature and pressure along the experimental setup were monitored, and the effects of internal pressure and room conditions were also tested. The results show some instabilities in the process to start the flow circulation and a relative stability and quick adaptation to changes when circulation is reached. The temperature in the evaporator chamber was highly homogeneous during the whole process; however, the temperature changes in the riser and the loop top were delayed with respect to the evaporator zone. The analysis shows several pressure and temperature raises before the vapor flux reaches the condenser. When the flow circulation is established, the system becomes highly stable and thermally homogeneous, decreasing the thermal resistance when increasing the power input. The stability analysis also showed that, when the system reaches the steady state, the changes in the power input produce a transient increase in the pressure and temperature of the fluid, followed by a quick decrease of the previous steady state values. The heat transfer analysis in the evaporator shows a higher heat flux on the upper heater caused by the buoyancy flow that rises from the lower heater. It was also observed that the lower heater reaches the CHF point with a lower heat flux.

Heat Transfer Characteristics of a Closed-Loop Two-Phase Thermosyphon System With a Structured Heating Surface

Abstract The present work reports an experimental investigation of heat transfer characteristics of a closed-loop two-phase thermosyphon (CLTPT) system with a structured heating surface. A novel structured heating surface, i.e., segmented finned microchannels, has been used to promote heat transfer rate, and its performance has been compared with that of plane heating surface. Compared with the plane heating surface, the segmented finned structured heating surface produces an enhancement of 157% in heat transfer coefficient (HTC), whereas it produces a 145% enhancement in the critical heat flux (CHF). The present structured surface also delivers better heat transfer performance compared with the available data in the literature.

Characteristic map of working mediums in closed loop two-phase thermosyphon: Thermal resistance and pressure

As an effective heat transfer unit, closed loop two-phase thermosyphon is widely applied due to simple structure and natural circulation. The type of fluid filled in the loop determines the heat transfer performance. To study the effect of fluid type on the closed loop two-phase thermosyphon, nine fluids are analyzed numerically, namely R-116, CO2, R-125, R-134a, R-1234ze, R-600a, R-600, R-123 and ethanol. The color maps of thermal resistance and pressure are drawn relative to changes in evaporator temperature and condenser temperature. The figure illustrating the optimum operating temperature range of those working fluids is obtained. It is concluded that the larger the critical temperature of the fluid is, the larger the optimum operating temperature is. Then the criterion of fluid selection in terms of thermal resistance is achieved with dimensionless calculation of evaporator temperature and condenser temperature.

Numerical analysis of a closed loop two-phase thermosyphon under states of single-phase, two-phase and supercritical

Closed loop two-phase thermosyphon is an effective heat transfer device, of which the research is necessary to clarify the heat transfer and fluid flow mechanism. Numerical analysis was carried out in the article with one-dimensional model. The model was based on mass, momentum and energy conversation equations, solved by iterative method. Single-phase and two-phase flow were simulated, besides supercritical flow. Experimental data was used in the validation of the model with the error less than ±10%. The axial evolution of several parameters, namely pressure, enthalpy, void fraction, vapor quality and heat transfer coefficient in the loop, were presented firstly. Then the effects of pipe diameter, filling ratio, height difference, heat load, cold bath temperature and fluid type on the closed loop two-phase thermosyphon were investigated. Large pipe diameter and medium filling ratio led to low thermal resistance. The thermal resistance decreased with the increase of height difference firstly, and then increased at large height differences. Larger heat load led to higher mass flux, as well as larger subcooled or superheated region. Vapor quality at the outlet of evaporator increased with the increase in cold bath temperature, leading to low thermal resistance. Fluids with low viscosity and large density difference between liquid and vapor phase were suggested for favorable heat transfer performance. It was also concluded that thermal resistance increased sharply when supercritical phenomenon occurred due to the disappearance of two-phase region.

Experimental investigation of geyser boiling in a two-phase closed loop thermosyphon with high filling ratios

The geyser boiling instability in a two-phase closed loop thermosyphon (TPCLT) is experimentally investigated through flow visualization and simultaneous measurement of pressure and temperature fluctuations. Wide ranges of filling ratios of R134a fluid from 90% to 53%, and heat flux from 20 W cm−2 to 220 W cm−2 are examined. The Power Spectrum Density (PSD) method is applied to analyze the periodicity of geyser boiling, and the parameter Standard Deviation (SD) is used to characterize the oscillating amplitude. The effects of heat flux and filling ratio on the geyser boiling occurrence and the oscillation characteristics are discussed in detail. The results show that, the flow regimes experience the consecutive variation of single-phase flow, bubbly flow, churn flow, bubbly flow, and single-phase flow within each geyser boiling cycle, leading to the fluctuation of flow and heat transfer characteristics. The geyser boiling is more liable to occur in the conditions of higher filling ratio and moderate heat flux. The initiating heat flux for the onset of geyser boiling decreases with the increase of filling ratio, but the range of heat flux for the geyser boiling occurrence is narrow under the high filling ratio conditions. The frequency of geyser boiling increases with the increase of heat flux for a certain filling ratio. With the increase of filling ratio, the oscillating frequency firstly decreases and then increases. The minimum oscillating frequency occurs at the combination of filling ratio of 76% and heat flux of 90 W cm−2 under the experimental conditions in this work. Both the fluctuation amplitude of pressure and temperature increase with the increase of heat flux, while decrease with the increase of filling ratio. Compared to R134a under the same filling ratio of 76%, the water has higher heat flux for geyser boiling occurrence, smaller oscillating amplitude of pressure, and lower oscillating frequency due to the difference in thermophysical properties.

Effect of bending position on heat transfer performance of R-134a two-phase close loop thermosyphon with an adiabatic section using flexible hoses

The experiments for a Two-phase Closed Loop Thermosyphon (TCLT) of which an adiabatic section was flexible hose, were conducted to describe the effect of bending positions at both joints of vapour and liquid line of TCLT. The evaporator and condenser of TCLT were fabricated from copper tubing, while the vapour and liquid lines were fabricated from Teflon hose. R-134a refrigerant was used as a working fluid. The TCLT was examined with a constant heat flux ranging from 14.2 to 32.6kW/m2. The condenser was cooled by cold water at about 20°C. The effects of tilt angle and bending position on the TCLT thermal performance characteristics were investigated. The results showed that bending and tilting TCLT would decrease thermal performance compared to the TCLT vertical orientation without bending, Qc/Q90. However, the effect of bending and tilting would be diminished as the TCLT tilted at 75 degrees at an optimum heat flux of 28.2kW/m2.

An insert-type two-phase closed loop thermosyphon for split-type solar water heaters

The requirement for improving the capacity, efficiency and reliability of a solar water heater typically increases its cost and installation difficulty in most situations. In this work, a novel insert-type two-phase closed loop thermosyphon (or called gravity return loop heat pipe) is introduced for reducing the cost and the installation/maintenance difficulty, especially for a split-type solar water heater. A series of systematic visualization experiments were conducted to explore the operation characteristics of the proposed loop thermosyphon at different filling ratios, different heat loads and different cooling conditions. The results show the filling ratio has a critical impact on the thermal resistance, flow patterns and start-up behavior of the loop thermosyphon. The practical comparative study indicates that the proposed insert-type loop thermosyphon is superior to a conventional thermosyphon in terms of thermal transportation limit and start-up speed through a prototype demonstration. Meanwhile, the transition conditions including stable operation, interval operation and dry-out operation were also observed and discussed accordingly.

高熱流束二相閉ループサーモサイフォンの熱輸送特性

高熱流束二相閉ループサーモサイフォンの熱輸送特性

Experimental analysis of Closed Loop Two Phase Thermosyphon (CLTPT) for energy systems

The authors have designed and realized an experimental test rig for the analysis of Closed Loop Two Phase Thermosyphons (CLTPT) of small dimensions where heat flow rate up to 1.7kW can be furnished. The experimental test rig consists of an evaporator and a water cooled horizontal condenser placed about 1m over the evaporator. The main characteristic of this apparatus is the presence of three different ways of measuring the mass flow rate: a continuous mode, an integral mode and an indirect mode.The purpose of this analysis is to investigate the correlation between mass flow rate and heat flow rate. The results of an experimental analysis by using water and ethanol as tested fluids at different operating conditions are shown discussed and analyzed.The influence of several parameters on the performances was studied experimentally: in particular heat load, operating pressure and fluid filling. The limits in the heat and mass transport are evidenced together with the unstable behavior at the high heat input.From this work, understanding and useful information are provided for designing and building a two-phase thermosyphon for systems like solar heating.

Numerical investigation on effect of riser diameter and inclination on system parameters in a two-phase closed loop thermosyphon solar water heater

In this work, the effect of riser diameter and its inclination angle on system parameters in a two-phase closed loop thermosyphon solar water heater has been numerically investigated. Here, receivable heat flux by the collector, circulating mass flow rate, driving pressure, total pressure drop, heat transfer coefficient in risers and collector efficiency are defined as system parameters. For this aim, a model of two-phase thermosyphon solar water heater that is acceptable for various inclinations is presented and variations of riser diameter and inclination are considered. The riser tube size is varied from 1.25cm to 2.5cm with inclination range 2–75°. The system absolute pressure is set as 3567Pa and water is chosen as working fluid. The results show that higher inclination angle is required for higher latitude location to obtain maximum solar heat flux. At local solar noon of 21.996 north latitude, the optimum inclination angle increases in the range of 24–44° with increasing of riser diameter giving maximum circulating mass flow rate from 0.02288kg/s to 0.03876kg/s. The longer two-phase heat transfer characteristics can be obtained at smaller inclination angles and mass flow rate for all riser tube sizes. Therefore, it is observed that the optimum inclination angles and diameters for solar heat flux, circulating mass flow rate and heat transfer coefficient in two-phase thermosyphon systemdo not coincide. From this work, better understanding and useful information are provided for constructing two-phase thermosyphon solar heaters.

Closed Loop Two-Phase Thermosyphon of Small Dimensions: a Review of the Experimental Results

A bibliographical review on the heat and mass transfer in gravity assisted Closed Loop Two Phase Thermosyphons (CLTPT) with channels having a hydraulic diameter of the order of some millimetres and input power below 1 kW is proposed. The available experimental works in the literature are critically analysed in order to highlight the main results and the correlation between mass flow rate and heat input in natural circulation loops. A comparison of different experimental apparatuses and results is made. It is observed that the results are very different among them and in many cases the experimental data disagree with the conventional theory developed for an imposed flow rate. The paper analyses the main differences among the experimental devices and try to understand these disagreements. From the present analysis it is evident that further systematic studies are required to generate a meaningful body of knowledge of the heat and mass transport mechanism in these devices for practical applications in cooling devices or energy systems.

354 閉ループ型二相サーモサイフォンによる舶用機関機器の冷却に関する実験的研究(GS 熱工学)

354 閉ループ型二相サーモサイフォンによる舶用機関機器の冷却に関する実験的研究(GS 熱工学)

A Flow Boiling Microchannel Evaporator Plate for Fuel Cell Thermal Management

In order to provide a high-power density thermal management system for PEM fuel cell applications, a flow boiling microchannel evaporator plate has been developed that operates in a closed loop two-phase thermosyphon. The flow is passively driven by gravity, and the flow rate initially increases with increasing evaporation rate and then decreases after reaching a peak flow rate. A microchannel plate has been fabricated with 56 square channels that have a 1 mm × 1 mm cross-section and are 115 mm long. The working fluid, HFE-7100, has been chosen due to its favorable saturation temperature at one atmosphere. Experiments have been conducted with the heat flux as the control variable. Measurements of mass flow rate, temperature field, and pressure drop have been made. The flow regimes are predominately bubbly and slug. The maximum heat flux observed, 32 kW/m2, is an order of magnitude greater than that required in current fuel cells and is limited by a Ledinegg instability. Two-phase thermal hydraulic models give a reasonable prediction for the mass flow rates and wall temperatures using standard flow boiling correlations. This paper will thoroughly describe the performance of the two-phase thermal management system over a wide range of operating conditions.

An experimental study of a two-phase closed loop thermosyphon with dual evaporator in parallel arrangement

A Two-phase Closed Loop Thermosyphon (TCLT) has been applied to many industrial fields as a waste heat recovery system, a telecommunication cooling system and other heat transport devices for the last 2 decades. It is common to design a TCLT to consist of one evaporator, one condenser and 2 separated lines of condensate liquid and vapor of working fluid. The present study was conducted in search of the new possibility of connecting multiple evaporators in parallel to the evaporator section. There has been little information about this so far. It is expected that a TCLT with multi-evaporator would provide a new solution to designing a compact telecommunication cooling system where lots of heat generating circuit boards are connected in parallel arrangement. In the present study a TCLT with two evaporators was prepared and given to a series of operational performance tests. The study was focused on investigating its operational problems and collecting information about its performances. Some important findings are as follows. The main source of operational instability was an occurrence of a hot dried patch on the heated surface when the rate of heat input was raised to a certain limit. For a single evaporator mode of the present TCLT model, the maximum allowable heat transport rate was about 650W. For dual evaporator mode of operation, this instability problem was found to be closely related not only to the quantity Of heat flux but also the difference in heat input rate to each evaporator. For all those problems, it was found that a TCLT with multi-evaporator could operate in a proper way when the thermal loads were evenly distributed to all the evaporator or the maximum power gap were contained under about ±5~7% of the average power input.

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 performance of a two-phase thermosyphon energy storage system

This article presents an energy storage system, which can be readily integrated with the building structure. It stores heat supplied by solar energy via the two-phase closed loop thermosyphon to storage tank and releases stored heat in energy storage material via two-phase closed thermosyphon to the heat exchanger through the flow of transport fluid. The functions of such energy storage system have three operating modes, i.e., heat charge, heat discharge, and simultaneous charge and discharge. The thermal performance of the system with alcohol and water as working fluid is experimentally investigated. The results show that the storage system employing alcohol as working fluid in the loop thermosyphon provides better performance; the system gives optimum heat charge and discharge performance under 35–40% fill ratio, regardless whether the working fluid is water or alcohol. The system displays optimum charge efficiency of 73% and optimum discharge efficiency of 85% with alcohol as working fluid.

A Natural Circulation Model of the Closed Loop, Two-Phase Thermosyphon for Electronics Cooling

This study presents a model for the two-phase flow and heat transfer in the closed loop, two-phase thermosyphon (CLTPT) involving co-current natural circulation. The focus is on CLTPTs for electronics cooling that exhibit complex two-phase flow patterns due to the closed loop geometry and small tube size. The present model is based on mass, momentum, and energy balances in the evaporator, rising tube, condenser, and the falling tube. The homogeneous two-phase flow model is used to evaluate the friction pressure drop of the two-phase flow imposed by the available gravitational head through the loop. The saturation temperature dictates both the heat source (chip) temperature and the condenser heat rejection capacity. Thermodynamic constraints are applied to model the saturation temperature, which also depends upon the local heat transfer coefficient and the two-phase flow patterns inside the condenser. The boiling characteristics of the enhanced structure are used to predict the chip temperature. The model is compared with experimental data for dielectric working fluid PF-5060 and is in general agreement with the observed trends. The degradation of condensation heat transfer coefficient due to diminished vapor convective effects, and the presence of subcooled liquid in the condenser are expected to cause higher thermal resistance at low heat fluxes. The local condensation heat transfer coefficient is a major area of uncertainty.