Heat transfer characteristics of a two-phase closed thermosyphon to the fill charge ratio

In this study, the heat transfer characteristics of a two-phase closed thermosyphon were investigated. For the test, a two-phase closed thermosyphon (copper container, FC-72 (C₆F₁₄) working fluid) was fabricated with a reservoir which could change the fill charge ratio. The experiments were performed in the range of 50-600 W heat flow rate and 10-70% fill charge ratio. Some findings are as follows.The heat transfer coefficients of the evaporator to the fill charge ratio were nearly negligible. These presented about 1-5 kW/m² K with the increase of heat flux and compared with those of smooth surface, showed some enhancement by the grooved surface. However at the condenser, the heat transfer coefficients showed some enhancement with the increase of fill charge ratio by the expanded working fluid pool. And the heat transport limitations appeared in different ways to the fill charge ratio. For the relatively small fill charge ratio (Ψ<20%), it presented about 100 W (Ψ: 10%) by the dry-out limitation.For the large fill charge ratio, it occurred by the flooding limitation and the maximum heat flow rate was about 500-550 W (Bo: 26-28), 230 W (Bo: 18.3) respectively and the Kutateladze number was about 1.9-2.1.     

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闼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...     

Numerical investigations and engineering applications on freezing expansion of soil restrained two-phase closed thermosyphons

A numerical simulation has been carried out to describe the coupled heat transfer of water-saturated soil with a two-phase closed thermosyphon. This problem is characterized by the phase change occurring in the water-saturated soil, moving freezing front and heat transfer of a two-phase closed thermosyphon. According to the governing equations in the porous media and heat transfer characteristics of the two-phase closed thermosyphon, the temperature distributions in the water-saturated soil and the moving freezing front are solved numerically by the finite-difference method. The predictions of the present study are well agreed with the measured data. The mechanism of the freezing expansion restrained by the two-phase closed thermosyphon is exposed, based on which the effective radius can be determined for engineering applications.     

Near-Critical Natural Circulation Flows Inside an Experimental Loop: Stability Map and Heat Transfer

The near-critical CO₂-based natural circulation loop (NCL, or thermosyphon) has been proposed in many energy conversion systems, such as the solar heater, waste heat recovery, next-generation nuclear cooling, and so on. There is an increasing need to obtain detailed information about such systems, as it is less verified from a basic system operation viewpoint. This paper presents an experimental investigation of a near-critical CO₂ thermosyphon. The closed thermosyphon is specially designed for high-pressure (in the critical region, from 6.0 MPa to 15.0 MPa), natural circulation flows. The basic transient flow behaviors and parameter behaviors are found to be dependent on initial pressure. The system stability evolution from subcritical oscillating flow to supercritical stable operations is presented. From the experimental data analysis, the stability map for the current supercritical natural circulation loop system is given. It is found that the stability pressure lines will divide the operation into stable, transition, and unstable regions. It is found that the effectiveness of the cooler will greatly affect the system stability, while the heat transfer efficiency is mainly controlled by the heater conditions. Parameter evolutions of the fluid temperature, mass flow rate, and loop pressure are presented in this paper. The heat transfer dependency on operation pressure and evolution mechanisms are also discussed in detail in this paper.     

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.     

两相闭式热虹吸管换热特性的数值模拟

本文建立数学模型对两相闭式热虹吸管换热特性进行数值模拟。考察了几何尺寸、工质及加热条件对液膜流态、液膜厚度等的影响,从而提示了这三个部因素影响冷凝段换热特性的内在机理。     

Effects of Working Fluid,Tubeside Enhancement and Bundle Depth on the Optimized Fin Geometry of a Horizontal Condenser Tube

A theoretical study has been made to optimize the fin geometry of a horizontal finned tube which is to be usedfor condensers that handle the vapor load of a liquid phase change cooling module.Systematic numerical calcu-lations of the vapor to coolant heat transfer have been performed for parametric values of fin height,fin spacing,vertical bundle depth and tubeside heat transfer coefficient.Three dielectric fluids (R-113,FC-72,and FC-87)at atmospheric pressure were selected as the working fluids.For a single tube with optimized fin geometry,theaverage heat flux increased in the order of FC-87,R-113 and FC-72.Both the optimum fin height and optimumfin spacing increased with increasing vertical bundle depth.     

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.     

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.     

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.     

Heat transfer characteristics of spray cooling in a closed loop

A closed loop spray cooling test setup is established for the cooling of high heat flux heat sources. Eight miniature nozzles in a multi-nozzle plate are used to generate a spray array targeting at a 1 × 2 cm² cooling surface. FC-87, FC-72, methanol and water are used as the working fluids. Thermal performance data for the multi-nozzle spray cooling in the confined and closed system are obtained at various operating temperatures, nozzle pressure drops (from 0.69 to 3.10 bar) and heat fluxes. It is exhibited that the spray cooler can reach the critical heat fluxes up to 90 W/cm² with fluorocarbon fluids and 490 W/cm² with methanol. For water, the critical heat flux is higher than 500 W/cm². Air purposely introduced in the spray cooling system with FC-72 fluid has a significant influence on heat transfer characteristics of the spray over the cooling surface.     

Experimental investigation on heat extraction using a two-phase closed thermosyphon for thermoelectric power generation

An experimental investigation on heat extraction using a two-phase closed thermosyphon charged with water (a filling ratio of 40%) for thermoelectric power generation was conducted to study the temperature gradients on the thermosyphon and the thermoelectric conversion characteristics. Results showed that the thermosyphon had a relatively stable working state at 100–300°C, and the maximum output power increased exponentially with temperature difference, being 20 W at a temperature difference of 210°C. The power generation efficiency increased in Hill function with increasing heating power input, the maximum value being approximately 0.01924.     

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.     

Nucleate boiling heat transfer enhancement for water and FC-72 on titanium oxide and silicon oxide surfaces

An experimental study was performed to investigate the nucleate boiling and critical heat flux (CHF) of water and FC-72 dielectric liquid on hydrophilic titanium oxide (TiO₂) nanoparticle modified surface. A 1 cm² copper heater with 1 μm thick TiO₂ coating was utilized in saturated pool boiling tests with water and highly-wetting FC-72, and its performance was compared to that of a smooth surface. Results showed that TiO₂ coated surface increased CHF by 50.4% and 38.2% for water and FC-72, respectively, and therefore indicated that boiling performance enhancement depends on the level of wettability improvement. A silicon oxide (SiO₂) coated surface, exhibiting similar surface topology, was tested to isolate the roughness related enhancement from the overall enhancement. Data confirmed that hydrophilicity of TiO₂ coated surface provides an additional mechanism for boiling enhancement.     

Study of trans-critical CO2 natural convective flow with unsteady heat input and its implications on system control

Natural convective flow of supercritical fluid has become hot topic both in scientific research and engineering applications. Natural circulation thermosyphon using supercritical/trans-critical CO₂ can be a potential substitute for effective transportation of heat and mass without valves/pumping devices. This paper presents numerical investigations into the effect of unsteady heat input on the trans-critical CO₂ thermosyphon, including sudden/quick increase of heat input, gradual/slow increase of heat input and sudden decrease of heat input. Those unsteady input situations are often seen in real applications and have become the core problem of efficiency and safety improvement. In the present study, two-dimensional rectangular natural circulation loop model is set up and numerically investigated. New heat transport model aiming at trans-critical thermosyphon heat input and system stability laws is proposed with supercritical/trans-critical turbulence model incorporated. It is found that when compared with supercritical CO₂ condition, trans-critical CO₂ thermosyphon has quite different behaviors. Natural convective thermosyphon stability is found to be of routinely dependent for different heat input change mode. Stability factors of natural convective trans-critical CO₂ flow and its implications on real system control are also discussed in this paper.     

Pool boiling on micro-fin array with wire mesh structures

Experimental investigations of pool boiling heat transfer on micro-fin arrays covered with porous structure were conducted. Experimental data were discussed for two kinds of enhanced surfaces: micro-fin structure without covering and micro-fins with the copper wire net (mesh structure). The experiments were carried out for water and FC-72 at atmospheric pressure. Micro-fins of 0.5 and 1 mm height were uniformly spaced on base surface. The wire mesh with aperture of 0.32, 0.4 and 0.5 mm (mesh pitch 0.52, 0.54 mm and 0.82 mm respectively), sintered with the fin tips, formed a system of connected perpendicular horizontal tunnels. Tunnel width was 0.6–1.0–1.5 mm and its depth was 0.5 or 1.0 mm. The effects of tunnel/micro-fin dimensions and wire mesh parameters on heat transfer coefficient in nucleate pool boiling was examined. Rather essential enhancement of heat transfer coefficient was observed for the surfaces with micro-fins of 0.5 mm with sintered wire net, especially at low and medium heat fluxes for boiling FC-72 and at the whole range of heat fluxes for boiling water. Structures with micro-fins of 1 mm height showed the best boiling heat transfer performance for water at all used heat fluxes and at the medium and highest heat fluxes for FC-72. For FC-72 at high heat fluxes (above 30 kW/m² for shorter micro-fins and 50 kW/m² for higher micro-fins) surfaces with plain micro-fins showed the highest heat transfer coefficients.     

Boiling of FC-72 on straight pin fin

This paper deals with an experimental investigation of pin fin boiling of saturated and subcooled FC-72 under atmospheric pressure. Fin base temperature and heat flux data are measured along with the fin tip temperature. The basic features of boiling stability of fluorocarbon boiling on pin fin had been reported for the first time. For a given liquid/heating surface combination there exist upper steady-state (USS) branch and lower steady-state (LSS) branch, and a large, unstable regime located in between. Zones with different stability characteristics are mapped according to boiling on fins with different aspect ratios. Liquid subcooling can largely enhance heat transfer performance. A longer fin can provide a safer operation.     

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.     

Experimental and numerical study of single and two-phase flow and heat transfer in aluminum foams

This paper investigates the flow boiling characteristics of water and FC-72 in aluminum foams. For the experiments, the heat transfer processes prior to nucleate boiling, the onset of nucleate boiling and the hysteresis effect were studied. The temperature jump, marking the transition from nucleate to film boiling was observed for FC-72. Numerical simulations were performed based on water to compare with the experiments. In the single-phase simulations, the Brinkman–Forchheimer and local thermal non-equilibrium models were adopted for the momentum and energy equations, respectively while in the two-phase simulations, the two-phase mixture model was used. Comparisons between the experimental and numerical results show reasonable agreement.     

Operating limit of heat transport in two-phase thermosyphon with connecting pipe (heated surface temperature fluctuation and flow pattern)

An experiment on heat transport phenomena has been carried out in a two-phase thermosyphon with an adiabatic connecting pipe using water as the working fluid at atmospheric pressure. The thermosyphon has an upper liquid chamber and a lower vapor chamber, which are connected with an adiabatic pipe. A horizontal upward-facing heated surface is installed in the bottom of the lower vapor chamber.The size of the connecting pipe is an inner diameter D_p = 2, 3, 4, 5, 6 and 8 mm and a length L = 250, 500 and 1000 mm. As the heat is supplied into the thermosyphon, the temperature of heated surface starts fluctuating at a heat flux at which unstable vapor–liquid counter current flow is generated in the connecting pipe. Bubbles at the upper end of the connecting pipe were photographed when the temperature fluctuation started. It was found that the heat flux at the onset of the temperature fluctuation increases with an increase in D_p and then can be predicted well by Eq. (1), which was derived based on the flooding velocity presented by Wallis [G.B. Wallis, One dimensional two-phase flow, McGraw Hill, New York, 1969], with C_w = 0.7 for D_p = 5, 6 and 8 mm. Furthermore, we clarified that the cause of this fluctuation comes from the inlet effect of the connecting pipe and we demonstrated this finding using a bell mouth, which was installed at either the bottom end or both ends of the connecting pipe.