An experimental study of two-phase closed thermosyphons for compact solar domestic hot-water systems

This paper focuses on the experimental analysis of the thermal behavior of two-phase closed thermosyphons with an unusual geometry characterized by a semicircular condenser and a straight evaporator. All the tests were done in an experimental indoor setup that uses electrical skin heaters to simulate the solar radiation. Different evaporator length, fill ratio of working fluid, cooling temperature and slope of the evaporator were tested for different heat fluxes, and the effects of these parameters on the overall thermal resistance were verified. An analysis of the transient results and the steady state performance was conducted in order to provide information for the design of a compact solar domestic hot-water system.     

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.     

Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

A thermoelectric domestic refrigerator has been developed, with a single compartment of 0.225 m³, for food preservation at 5 °C. The cooling system is made up of two equal thermoelectric devices, each composed of a Peltier module (50 W) with its hot side in contact with a two-phase and natural convection thermosyphon (TSV) and a two-phase and capillary lift thermosyphon (TPM), in contact with the cold side.The entire refrigerator has been simulated and designed using a computational model, based on the finite difference method. Subsequently an experimental optimization phase of the thermosyphons was carried out, until thermal resistance values of R_TSV = 0.256 K/W and R_TPM = 0.323 K/W were obtained. These values were lower than those obtained with finned heat sinks.Finally, a functional prototype of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal drop (Ambient Temperature–Inside Temperature) of 19 °C. The electric power consumption at nominal conditions was 45 W, reaching a COP value of 0.45. The study demonstrated that by incorporating these two-phase devices into thermoelectric refrigeration increases the COP by 66%, compared with those which use finned heat sinks.     

Numerical modeling of two-phase supersonic ejectors for work-recovery applications

An ejector is a fluid pumping device that uses the energy of a high pressure motive fluid to raise the pressure of a secondary lower-pressure fluid. Motive pressure is converted into momentum through a choked nozzle creating a high velocity jet which entrains the surrounding low-momentum suction flow. The two streams mix and finally pressure is recovered through a diffuser. There has been little progress on high fidelity modeling of the expanding supersonic two-phase flow in refrigerant expansion work recovery ejectors due to rather complex physics involving nonequilibrium thermodynamics, shear mixing, and void fraction-dependent speed of sound. However, this technology can be applied to significantly increase the efficiency of space cooling and refrigeration devices. The approach developed in this study integrates models for real-fluid properties, local mass and energy transfer between the phases, and two-phase sonic velocity in the presence of phase change into a commercial CFD code. The intent is to create a practical design tool with better fidelity than HEM CFD models yet with tractability lacking in current boundary tracking phase change CFD models. The developed model has been validated through comparison of key performance metrics against test data under certain operating conditions.     

Comparative heat transfer characteristics of a flat two-phase closed thermosyphon (FTPCT) and a conventional two-phase closed thermosyphon (CTPCT)

This study investigated the effects of cross-sectional geometries, filling ratio and aspect ratio on thermal performance of thermosyphon at different rates of heat input. Two cross-sectional geometries of thermosyphon (circular and flat) were used. Each cross-sectional geometry was charged with distilled water with different filling ratios, aspect ratios and heat input. The results indicated that the FTPCT had a higher average wall temperature in the evaporator section than that of the CTPCT. The maximum heat input had a significant influence on the heat fluxes for each filling ratio and evaporator length. Heat fluxes were increased with an increase of aspect ratio and heat input and decreased slightly at the maximum aspect ratios.     

新型传热工质纳米流体的研究与应用

介绍了一种在强化传热领域具有广阔应用前景的新型传热（冷却）工质——纳米流体,分析了纳米流体的导热机理、导热性能以及影响其导热系数的各种因素,阐述了纳米流体对流换热性能的研究、纳米流体的制备及其稳定性和应用前景。     

Numerical investigations on thermal performance and flame stability of hydrogen-fueled micro tube combustor with injector for thermophotovoltaic applications

In order to design a micro tube combustor with good thermal performance and flame stability for thermophotovoltaic applications, in this work, thermal performance and flame stability of hydrogen-fueled micro tube combustors with backward facing step and with injector are compared. It is found that the decrease of diameter ratio d₂/d₁ leading to expansion of the symmetrical recirculation zone, which is helpful for fluid and heat circulation, and higher flame locations, which is not helpful for flame stabilization. Furthermore, effects of diameter ratio d₂/d₁ on thermal performance and flame stability are analyzed and discussed. Results suggest that when the diameter ratio d₂/d₁ is decreased from 0.9 to 0.8, positive effects of injector on thermal performance are enhanced and flame stability is improved under lower hydrogen/air equivalence ratio. Finally, the applications conditions of the micro tube combustor with injector are achieved. This work will provides us significant reference for designing micro tube combustor with injector.     

两相闭式热虹吸管混沌特性及其传热性能研究

探究两相闭式热虹吸管的传热混沌,以及操作参数对其混沌性和传热性能的影响,建立混沌特征参数与传热性能间的联系。通过搭建实验台测量两相闭式热虹吸管稳定运行过程中不同工况下的管壁温度信号,基于非线性分析的混沌理论研究处理测量的温度脉动信号,绘制吸引子轨迹图,建立最大Lyapunov指数与传热特征参数的联系,揭示传热性能与混沌特性的关系。结果表明：两相闭式热虹吸管具有确定性混沌行为,操作参数对混沌性和传热性能具有显著影响,混沌性与传热性能呈正相关关系。     

CFD and ERT investigations on two-phase flow regimes in vertical and horizontal tubes

In the present study, different two-phase flow regimes in horizontal and vertical tubes have been studied experimentally and theoretically. A 3-D Computational Fluid Dynamics (CFD) modeling was carried out in order to model gas–liquid two-phase flow using volume of fluid (VOF) model. An Electrical Resistance Tomography (ERT) system was used to visualize these flow regimes, which were produced by change in the gas to liquid flow rate ratio. The reconstructed images from the ERT measurement and corresponding captured photographs for different flow regimes have been compared with the CFD predictions and a good qualitative agreement was observed between them.     

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.     

Theoretical and experimental investigations of a two-phase thermosyphon solar water heater

This article experimentally and theoretically investigates a two-phase thermosyphon solar water heater. The performance of this innovative solar water heater at different solar radiation intensities and tilt angles are experimentally discussed. The results show the best charge efficiency of the system is 82%, which is higher than the conventional solar water heaters. The theoretical model is also developed using the thermal resistance-capacitor method. The simulation predictions agree well with the experimental data within an average error deviation of ±6%. Two methods for improving the performance of this heater, double fin tubes and nano particle, are proposed. The results show that charge efficiencies can increase 3% and 4%, respectively, according to the theoretical model.     

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.     

Experimental investigations on operating characteristics of a closed loop pulsating heat pipe

The operating mechanism of the pulsating heat pipe (PHP) is not well understood and the present technology cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand the operation regimes of the PHP through experimental investigations. A series of experiments were conducted on a closed loop PHP with 5 turns made of copper capillary tube of 2 mm in inner diameter. Two different working fluids viz. ethanol and acetone were employed. The operating characteristics were studied for the variation of heat input, filling ratio (FR) and inclination angle of the tested device. The results strongly demonstrate the effect of the filling ratio of the working fluid on the operational stability and heat transfer capability of the device. Important insight into the operational characteristics of PHP has been obtained.     

Numerical analysis of two-phase flow in condensers and evaporators with special emphasis on single-phase/two-phase transition zones

A numerical study of the thermal and fluid-dynamic behaviour of the two-phase flow in ducts under condensation or evaporation phenomena is presented. The numerical simulation has been developed by means of the finite volume technique based on a one-dimensional and transient integration of the conservative equations (continuity, momentum and energy). The discretized governing equations are solved using the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) which allows back flow phenomena. Special emphasis is performed on the treatment of the transition zones between the single-phase and two-phase flow. The empirical inputs of single-phase and two-phase flow, including sub-cooled boiling and dry-out, have been adapted by means of adequate splines in the transition zones where the heat transfer correlations available in the literature are not suitable. Different numerical aspects have been evaluated with the aim of verifying the quality of the numerical solution. The mathematical model has been validated by comparison with experimental data obtained from literature considering condensation and evaporation processes. This comparison shown the improvements in the numerical solution not only in the transition zone but also in all condenser and evaporator ducts, when the special treatment for transitions is used. Illustrative results on double-pipe heat exchanger are also presented.     

Numerical investigations on the fast filling of hydrogen tanks

High-pressure storage of hydrogen in tanks is a promising option to provide the necessary fuel for transportation purposes. The fill process of a high-pressure tank should be reasonably short but must be designed to avoid too high temperatures in the tank. The shorter the fill should be the higher the maximum temperature in the tank climbs. For safety reasons an upper temperature limit is included in the requirements for refillable hydrogen tanks (ISO 15869) which sets the limit for any fill optimization. It is crucial to understand the phenomena during a tank fill to stay within the safety margins.The paper describes the fast filling process of hydrogen tanks by simulations based on the Computational Fluid Dynamics (CFD) code CFX. The major result of the simulations is the local temperature distribution in the tank depending on the materials of liner and outer thermal insulation. Different material combinations (type III and IV) are investigated.Some measurements from literature are available and are used to validate the approach followed in CFX to simulate the fast filling of tanks. Validation has to be continued in the future to further improve the predictability of the calculations for arbitrary geometries and material combinations.     

小型离心血液泵内流动特性的数值研究

采用数值求解Reynolds—Averaged Navier—Stokes方程技术,研究分析了小型离心血液泵内三维流动特性和水力性能。计算得到的不同流量条件下血液泵的水力性能与试验数据吻合良好,验证了计算方法的可靠性。分析了在设计流量、小流量和大流量条件下血液为工质的离心泵内流动形态。研究结果表明：在3种流量条件下,离心血液泵的出口流场均匀,满足设计要求。对血液泵内涡流产生及其容易产生溶血的位置进行了初步探讨,为改进血液泵结构设计提供参考。     

热管蓄冰过程的数值模拟研究．

通过建立热管的简化热阻模型,采用数值计算的方法模拟了热管的动态蓄冰。模拟结果与实验结果吻合。同时,对热管冷凝段和蒸发段的长度比为1：2和1：4两种形式进行了数值模拟比较。结果表明,1：4的布置形式更优。     

An experimental and theoretical investigation of the transient behavior of a two-phase closed thermosyphon

This paper presents an experimental and theoretical investigation of the two-phase closed thermosyphon (TPCT) behavior in transient regimes. Experimental results show two kinds of TPCT response. We focus on regular variations of operating system variables, where a mathematical model has been developed in order to obtain an analytical expression of the system response time. The dependence of this response time according to the various parameters is linked to geometry and heat transfer laws. The model can be considered as a simple and efficient tool for designing TPCTs in both transient and steady regimes.     

Numerical investigations on flashback dynamics of premixed methane-hydrogen-air laminar flames

Injecting hydrogen into the natural gas network to reduce CO₂ emissions in the EU residential sector is considered a critical element of the zero CO₂ emissions target for 2050. Burning natural gas and hydrogen mixtures has potential risks, the main one being the flame flashback phenomenon that could occur in home appliances using premixed laminar burners. In the present study, two-dimensional transient computations of laminar CH₄ + air and CH₄ + H₂ + air flames are performed with the open-source CFD code OpenFOAM. A finite rate chemistry based solver is used to compute reaction rates and the laminar reacting flow. Starting from a flame stabilized at the rim of a cylindrical tube burner, the inlet bulk velocity of the premixture is gradually reduced to observe flashback. The results of the present work concern the effects of wall temperature and hydrogen addition on the flashback propensity of laminar premixed methane-hydrogen-air flames. Complete sequences of flame dynamics with gradual increases of premixture velocity are investigated. At the flame flashback velocities, strong oscillations at the flame leading edge emerge, causing broken flame symmetry and finally flame flashback. The numerical results reveal that flashback tendency increase with increasing wall temperature and hydrogen addition rate.