重力热管内部相变及传热传质过程的数值模拟

重力热管内部包含复杂的两相流动以及相变传热过程,传统理论分析及实验手段不能直观给出其内部流动、相变、热质传递的详细信息。采用VOF(volume of fluid)多相流模型对重力热管内气液两相流动及传热进行模拟,捕捉到蒸发段气泡产生、合并、长大、上升,以及冷凝段壁面附近液滴形成、合并、下滑、汇集到液池的全过程,得到的壁温分布与实验测量值对比体现良好一致性,表明数值模拟的正确性。同时,以热阻、传热量和热效率为评价标准,研究不同充液率和倾斜角度下对重力热管运行性能的影响。结果表明:在所研究的参数范围内,随着充液率的增加,热阻逐渐减小,冷凝段传热量逐渐增大。且工质初始充注量充满蒸发段时热管性能较好;倾角对热阻的影响不明显,冷凝段传热量和热效率均随倾角增加而增长。     

Transient response of thermosyphon solar collectors

The response of thermosyphon solar water heaters to step changes of insolation is investigated. Measurements of the transient flow development in a thermosyphon circuit were obtained using a laser doppler anemometer and a mathematical model was developed to simulate the transient performance. The results show that although there are long time delays associated with the development of the thermosyphon flow the energy collection capability is not affected by thermosyphon time delays.     

System modeling and operation characteristics of thermosyphon solar water heaters

An efficient numerical simulation model for thermosyphon solar water heaters has been developed and compared with test data from two locations. The model was used to study the characteristics of vertical and horizontal tank thermosyphon systems. The results indicate that thermosyphon systems have optimum performance when the daily collector volume flow is approximately equal to the daily load volume. Heat conduction in one tank horizontal system was found to significantly reduce solar contribution.     

Virtual prototyping of storage tanks by means of three-dimensional CFD and heat transfer numerical simulations

The aim of this work is to show the current computational possibilities of three-dimensional computational fluid dynamics (CFD) simulations, using loosely coupled parallel computers (Beowulf clusters) in the virtual prototyping of thermal storage tanks. The transient thermal behaviour of a storage tank forming part of a thermosyphon solar heating system is simulated numerically. The influence of the inlet mass flow rate on the degree of thermal stratification during an unloading process is analysed. Special attention is given to the validation of the assumed mathematical model, the verification of the numerical solutions, and the post-processing tasks carried out in order to quantify the level of thermal stratification. The computational possibilities and limitations of this kind of detailed numerical experiments are pointed out.     

倾斜状态下热虹吸管冷凝段流动传热研究

建立了太阳能热水装置用的热虹吸管在倾斜状态下冷凝段流动传热模型,模型考虑了倾斜角度和界面剪切力对冷凝液膜的影响.分析结果表明,在小管径情况下,特别是在小倾斜角度工况下,界面剪切力的作用不能忽略.界面剪切力对流动冷凝的作用主要表现在使周向的液膜厚度变得均匀.计算模型还考虑了过冷度对冷凝段有效传热长度的影响,计算结果与经典的经验公式和试验数据相比较,吻合良好.     

Simulation of the long term performance of thermosyphon solar water heaters

A finite element simulation model for predicting the long term performance of thermosyphon solar water heaters is presented, and the simulation results are compared with the measured performance of six systems supplying typical domestic hot water loads. To obtain consistent simulation results the storage tank temperature stratification had to be accurately simulated using 20 nodes with 5 min time steps for vertical tanks and 30 nodes with 2 min time steps for horizontal tanks. A distributed return model for the mixing of the collector return flow in the tank was also found to be necessary to accurately model tank temperature stratification. The performance of single tank thermosyphon systems is shown to improve as the flow through the collector is reduced to approximately 1 tank volume per day and thermosyphon systems are shown to be slightly more efficient than equivalent pumped circulation systems.     

Simulation study of flow and heat transfer in a thermosyphon

By establishing a mathematical model in the thermosyphon system, the numerical simulation method is used for the temperature field, flow field distribution in heat pipe simulated calculation in recent years. In this article, we combine the thermosyphon engineering actuality, build up the Nusselt model, write the visual program with the VB code, and make use of the computer to carry on imitating the calculation. Meanwhile, the calculation predicted temperature profile in the thermosyphon was compared with experimental measurements and a good agreement was observed. Through the research, it can provide theoretical basis for the optimization of heat pipe model in the future.     

Pumping characteristics of a thermosyphon applied for absorption refrigerators with working pair of LiBr/water

A thermosyphon with a separate heating chamber was taken into consideration as a model device for a high-temperature generator in a small capacity absorption refrigerator with LiBr/H₂O solution. Quantitative measurements of operating characteristics of the thermosyphon were made as functions of pipe diameter, pumping height and heating power. It was found that vapor production rate is mainly affected by the heating power, and the flow rate of pumped liquid is limited by the pumping height. It was also observed that, as the pipe diameter increases, dependency of liquid flow rate on the pumping height becomes stronger. For a LiBr/H₂O solution system, ratio of vapor production rate to liquid flow rate was found to be well correlated as a function of the power input. Present experimental data are expected to provide useful information for determination of design parameters of a thermosyphon.     

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.     

Experimental investigation of natural convection heat exchange within a physical model of the manifold chamber of a thermosyphon heat-pipe evacuated tube solar water heater

The high capital costs associated with heat-pipe evacuated tube solar water heating systems can be reduced by replacing forced circulation with thermosyphon circulation. Currently research on thermosyphon heat-pipe evacuated tube solar water heaters is limited. An experimental investigation of the natural convective heat exchange regime that exists within the manifold chamber of a proprietary heat-pipe evacuated tube solar water was undertaken. This paper presents experimental data from a heat-pipe Evacuated Tube Solar Water Heater (ETSWH) subjected to the Northern Maritime Climate at the University of Ulster’s outdoor solar testing facility located at the Jordanstown campus. The thermal performance of this across solar noon (±30 min) was experimentally determined to be comparable to two physical laboratory 10 pin-fin model manifolds constructed to the same dimensions and geometry as the manifold chamber of the heat-pipe ETSWH when operated under steady laboratory conditions. When the surface temperatures of the pin-fins (simulated condensers) in the model manifold were normalised with respect to the lowest most pin-fin in the array the influence of buoyant flow was observed. Similarly to related studies in this field it was found that normalised surface temperatures on downstream pin-fins do not increase monotonically as would be expected if no interactions occur. It was found that at the pin-fin diameter to pitch used in the model manifold that normalised surface temperatures decrease at certain points in the array due to the action of buoyant flow generated from upstream pin-fins which increased heat transfer. Two-dimensional Particle Imaging Velocimetry (2D-PIV) was used to visualise the thermosyphon fluid flow regime. It was observed that the fluid flow regime varied across the model due to interactions between the fluid, chamber walls and pin-fins.     

Application of Heat Transfer Enhancement on Vertical Thermosyphon Reboilers Using Tube Inserts

Vertical thermosyphon reboilers and evaporators are widely used in the process industries. However, increasing the thermal efficiency of these units is very difficult. They are commonly used for about 70% of all evaporation duties in chemical industries. The flow in these units depends on the amount of buoyancy created by vaporization. The flow rate is therefore related not only to heat transfer rate, but also to evaporation, friction, and static pressure loss. The hydrostatic heat present at the base of a vertical thermosyphon reboiler suppresses boiling, creating a sub-cooled region. At the base of the tube bundle, this region length sometimes approaches a significant percent of tube length. Because single-phase convective heat transfer is the dominant heat transfer mechanism in this region, tube inserts can be used to promote heat transfer without blocking flow. In this article, using a simulation model that has been validated against the result of HTFS software, the effect of using different types of tube inserts in a sub-cooled zone of a vertical thermosyphon reboiler on the thermal performance of this unit is discussed.     

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.     

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.     

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.     

Enhancement of heat transport in thermosyphon air preheater at high temperature with binary working fluid: A case study of TEG–water

In this research, the critical heat flux (CHF) due to flooding limit of thermosyphon heat pipe using triethylene glycol (TEG)–water mixture has been investigated. From the experiment it is found that, use of TEG–water mixture can extend the heat transport limitation compared with pure water and higher heat transfer is obtained compared with pure TEG at high temperature applications. Moreover it is found that ESDU equation is appropriate to predict the CHF of the thermosyphon in case of TEG–water mixture.For thermosyphon air preheater at high temperature applications, it is found that with selected mixture content of TEG–water in each row of the thermosyphon the performance of the system could be increased approximately 30–80% compared with pure TEG for parallel flow and 60–115% for counter flow configurations. The performances also increase approximately 80–160% for parallel flow and 140–220% for counter flow compared with those of pure dowtherm A which is the common working fluid at high temperature applications.     

Mantle heat exchangers for horizontal tank thermosyphon solar water heaters

This paper describes the characteristics of horizontal mantle heat exchangers for application in thermosyphon solar water heaters. A new correlation for heat transfer in horizontal mantle heat exchangers with bottom entry and exit ports was used to predict the overall heat transfer and stratification conditions in horizontal tanks with mantle heat exchangers. The model of a mantle heat exchanger tank was combined with the thermosyphon solar collector loop model in TRNSYS to develop a model of a thermosyphon solar water heater with collector loop heat exchanger. Predictions of stratification conditions in a horizontal mantle tank are compared with transient charging tests in a laboratory test rig. Predictions of daily energy gain in solar preheaters and in systems with in-tank auxiliary boosters are compared with extensive outdoor measurements and the model is found to give reliable results for both daily and long-term performance analysis.     

Analysis of horizontal mantle heat exchangers in solar water heating systems

The characteristics of horizontal mantle heat exchangers are investigated for application in thermosyphon solar water heaters. An experimental model of a horizontal mantle heat exchanger was used to evaluate the flow patterns in the annular passageways and the heat transfer into the inner tank. Flow visualisation was used to investigate the flow structure, and the heat transfer was measured for isothermal inner tank conditions. A numerical model of the flow and heat transfer in the annular passageway was developed and used to evaluate the heat flux distribution over the surface of the inner tank. The numerical results indicate that configurations of mantle heat exchangers used in current solar water heater applications degrade thermal stratification in the inner tank. The effects of inlet flow rate, temperature and connecting port location are quantified.     

Enhanced Heat Transfer of Shaker-Bored Piston Cooling Channel with Twisted Tape Insert

This experimental study investigates the heat transfer augmentation in a reciprocating anti-gravity open thermosyphon using a twisted tape insert with relevance to the “shaker-bored” piston cooling system for marine propulsive diesel engine. A selection of experimental data illustrates the interactive effects of inertial, reciprocating, and buoyancy forces on heat transfer in the anti-gravity open thermosyphon with and without a twisted tape insert for subcooled and superheated conditions. The impacts of gravitational buoyancy on heat transfer in the static plain thermosyphon tube are reversed from impairing to improving heat transfer when the flow condition yields from subcooled to superheated condition. In the static thermosyphon tube fitted with twisted tape insert and in the reciprocating thermosyphon tubes with and without twisted tape insert, the buoyancy interactions enhance heat transfer coefficients. Due to the isolated reciprocating force effect, heat transfer coefficients are initially impaired from the static levels at low pulsating numbers but recovered to be enhanced at high pulsating numbers in the reciprocating plain thermosyphon tube. For the reciprocating thermosyphon tube fitted with a twisted tape insert, the isolated reciprocating force effect consistently improves heat transfer. The impacts of isolated reciprocating force and buoyancy interaction on heat transfer are Reynolds number-dependent. Heat transfer coefficients in the reciprocating thermosyphon tube fitted with the twisted tape insert could be augmented to the range of 1.2–6 times of plain tube levels. A set of empirical heat transfer correlations that considers the synergistic effects of inertial force, reciprocating force, and buoyancy interaction in the reciprocating anti-gravity open thermosyphon tube fitted with a twisted tape insert is developed to assist the design activity of the piston-cooling system.     

Performance of a R-134a-filled thermosyphon

Heat pipes are low cost and efficient heat exchange equipment. They are suitable for low temperature heat or cold recovery systems. The latter could be employed to cool incoming warm fresh air in air-conditioned ventilation systems. R-134a is an environmentally friendly refrigerant and has been generally accepted as a substitute for R-12 and R-22. The thermal performance of a thermosyphon filled with R-134a was investigated. The effects of temperature difference between bath and condenser section, fill ratio and coolant mass flow rates on the performance of the thermosyphon were determined. The experimental results indicate that the heat flux transferred increased with increasing coolant mass flow rate, fill ratio and temperature difference between bath and condenser section.     

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。