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.     

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.     

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.     

Integration of Miniature Heat Pipes into a Proton Exchange Membrane Fuel Cell for Cooling Applications

In proton exchange membrane fuel cell (PEMFC) operations, the electrochemical reactions produce a rise in temperature. A fuel cell stack therefore requires an effective cooling system for optimum performance. In this study, miniature heat pipes were applied for cooling in PEMFC. Three alternatives were considered in tests: free convection, forced convection cooling with air, and also water. An analytical model was developed to show the possibility of evoking heat from inside a fuel cell stack with different numbers of miniature heat pipes. An experiment setup was designed and then used for further analysis. The proposed experiment setup consisted of a simulated fuel cell that produced heat and a number of thermosyphon miniature heat pipes to evoke heat from the simulated fuel cell. The experiment results reported in this paper present advantages and disadvantages of each tested cooling scenario. Results show that each cooling scenario, using a different number of heat pipes, provided different heat removal rates for PEMFC cooling.     

Performance investigation of nanofluids as working fluid in a thermosyphon air preheater

In recent years, there has been a substantial increase in energy demand due to industrialization development. This raises concern on issues such as depletion of fossil based energy and emission of green house gasses. Hence, optimization of energy use through the thermosyphon air preheater is one of the possible approaches to address this problem. It can be used to recover and transmit the heat from the hot air (flue gas) to the cold air used for combustion process in a boiler. This study focuses on the analytical analysis of the thermal performance of a thermosyphon operated with water and nanofluids. The thermo physical properties of the selected nanofluids and relevant formulations are taken from the literatures to perform the analysis. Study found that change of nanofluid properties such as thermal conductivity only plays minor role in enhancing the thermal performance of the thermosyphon. The study implied that the hot air velocity is capable of increasing the efficiency of a thermosyphon. It is found that 23% overall heat transfer enhancement is observed when the hot air velocity increases from 2.0 m/s to 4.75 m/s for water based (7%) alumina and (4%) titanium dioxide nanofluids.     

Energy and exergy analysis of internal reforming solid oxide fuel cell–gas turbine hybrid system

The aim of this work is to analyze methane-fed internal reforming solid oxide fuel cell–gas turbine (IRSOFC—GT) power generation system based on the first and second law of thermodynamics. Exergy analysis is used to indicate the thermodynamic losses in each unit and to assess the work potentials of the streams of matter and of heat interactions. The system consists of a prereformer, a SOFC stack, a combustor, a turbine, a fuel compressor and air compressor, recuperators and a heat recovery steam generator (HRSG). A parametric study is also performed to evaluate the effect of various parameters such as fuel flow rate, air flow rate, temperature and pressure on system performance.     

A Proper Orthogonal Decomposition Based System-Level Thermal Modeling Methodology for Shipboard Power Electronics Cabinets

A system-level thermal modeling methodology for shipboard power-electronics cabinets is presented and demonstrated for a PCM-1 cabinet, a complex air-to-water-cooled cabinet design of interest to naval applications. The cabinet is completely sealed and the heat dissipation in the power electronics bays is removed from the cabinet by re-circulating the hot air through an air-to-water-cooled packaged heat exchanger that is served by an external fresh water loop. A detailed unit-wise analytical model is developed for the packaged heat exchanger used in the cabinet. Under the prescribed design parameters, the PCM-1 cabinet operating-point air circulation rate was established to be 0.434 m³/s (920 CFM). A compact model is developed for the air convection within the cabinet using 3-D Computational Fluid Dynamics/Heat Transfer (CFD/HT) simulations in conjunction with Proper Orthogonal Decomposition (POD)-based reduced order modeling techniques. The compact model runs about 350 times faster with a mean prediction error within 3.6% for the velocity field, 0.04% for the temperature field, and 0.15% for the pressure field. The resulting overall cabinet model can be integrated into a system-level modeling platform to simulate the thermal response of multiple cabinets. The CFD/HT simulations of the PCM-1 cabinet architecture suggest that its two uppermost bays would experience high air temperatures due to insufficient local air flow.     

Parametric evaluation of refrigerated air curtains for thermal insulation

The refrigerated air curtain used for cavity insulation in the present study is an idealization of the refrigerated air curtain used in supermarket food display cabinets. The thermal insulation performance of refrigerated air curtains is very important for perishable food storage and energy saving for refrigerated display cabinets. Thermal insulation ability of recirculated refrigerated air curtains was numerically studied in the present work. The results show that the refrigerated air curtains are negatively buoyant jets and tend to flow toward the inside cabinet due to stack pressure, so the initial momentum must be sufficiently large to sustain the pressure difference across the air curtain and assure the thermal insulation. The length–width ratio and the discharge angle of the air curtains, the height–depth ratio of the cavity and the dimension and position of the inside shelves would greatly influence the thermal insulation performance of air curtains, therefore were extensively discussed. The maximum Richardson numbers or the optimum parameter selections in each situation were presented for practical design of refrigerated air curtains used in multi-deck display cabinets.     

The effect of wind speed at the top of the tower on the performance and energy generated from _thermosyphon solar turbine

Energy generated from wind turbine depends to a great extent on the wind speed at its inlet. The use of thermosyphon solar tower is an attempt to increase the air velocity at inlet of the wind turbine and of course to increase its power. The wind speed in a certain location changes always with time and with the height above ground surface. In this work, the effect of wind speed at the top of the tower on the performance as well as on the energy generated from thermosyphon solar turbine was studied theoretically. One location in Egypt was chosen for this study. The calculations were achieved mainly with the solar turbine located at tower bottom. For the purpose of comparison, the energy generated from the solar turbine was compared with that generated from free wind turbine at tower height with the absence of solar tower. It was found that, the wind speed at the top of the tower results in a pressure drop which affects the performance of the thermosyphon solar turbine. This pressure drop increases with the rise in wind speed and will be zero only when the wind speed at the top of the tower reaches zero. It was found also that, there is an increase in friction losses through the tower and a decrease in both temperature difference between inlet and outlet of the tower and in heat losses from tower walls with the rise in wind speed in location. The inlet air velocity to the solar turbine and consequently its specific power were found to be increased with the increase in wind speed at the top of the tower. Therefore, the effect of wind speed at the top of the tower must be taken into account during thermosyphon solar tower calculations. By comparing the performance of solar turbine and the free wind turbine located at tower height with the absence of thermosyphon solar tower, it was found that the mean inlet air velocity to the solar turbine located at tower bottom and consequently its specific power are higher than these values for free wind turbine. The mean inlet air velocity to the solar turbine is found to be 117% of its value for a free wind turbine. The yearly specific energy generated from solar turbine is expected to be 157% of its value for free wind turbine.     

Dynamic defrosting characteristics of air source heat pump and effects of outdoor air parameters on defrost cycle performance

The reverse-cycle defrosting characteristics of a 11.2 kW split-type air source heat pump (ASHP) were experimentally investigated under the defrosting conditions. Based on the experimental results, the effects of outdoor air parameters on defrosting cycle performance as well as the dynamic defrosting characteristics of the ASHP unit were analyzed. The experimental results indicate that with the increase of the outdoor air relative humidity at a constant air temperature and velocity, the total power consumption, defrosting time and endotherm from indoor room during defrosting period decreased linearly, and they also decreased as the outdoor air temperature increased, but the trends of the curves presented the concave-up. The duration of the defrosting period mainly depends on the wall temperature of outdoor heat exchanger, which is corresponding to the condensing pressure during the defrosting cycle. In this paper, the concept of total coefficient of performance (COP) is used to evaluate the performance of ASHP unit, and as the air temperature increased under the conditions of a fixed air relative humidity and air velocity, the total COP increased linearly, but it decreased linearly as the air relative humidity increased.     

Experimental Investigation of the Thermal Performance of Graphite Foam for Evaporator Enhancement in Both Pool Boiling and an FC-72 Thermosyphon

High-conductivity graphite foam is investigated for use as a surface enhancement for improved thermal performance in both pool boiling and an FC-72 thermosyphon. The influences of heat load and fluid level on the overall system thermal performance including surface superheat, effective heat transfer coefficient, and thermal resistance are examined. The thermal resistance of the foam heat sink is found to be extremely low at a minimum of 0.024 K/W, well below that of many other methods. The featured low thermal resistance is the primary benefit of this system. The thermal resistance is found to rise with increasing heat flux, but still remains advantageously low and exhibits excellent potential for high heat flux dissipation with low surface superheat, making it suitable for thermal management of advanced electronics.     

Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors: Part 1: Indoor experiment

An especial open thermosyphon device used in high-temperature evacuated tubular solar collectors was designed. The indoor experimental research was carried out to investigate the thermal performance of the open thermosyphon using respectively the deionized water and water-based CuO nanofluids as the working liquid. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and the operating temperature on the evaporating heat transfer characteristics in the open thermosyphon were investigated and discussed. Experiment results show the optimal filling ratio to the evaporator is 60% and the thermal performance of the open thermosyphon increase generally with the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly enhance the thermal performance of the evaporator and evaporating heat transfer coefficients may increase by about 30% compared with those of deionized water. The CuO nanoparticles mass concentration has remarkable influence on the heat transfer coefficient in the evaporation section and the mass concentration of 1.2% corresponds to the optimal heat transfer enhancement.     

Thermal management of solid oxide fuel cells with liquid metal

Effective thermal management of SOFCs is necessary for their long life and highly efficient operation, while the conventional method through excess air cooling is limited due to the inherently low thermal conductivity and capacity of air. In this study, a novel temperature control strategy is proposed by using liquid metal as a new kind of coolant that can work in both the stable operation stage and start-stop stage of an SOFC stack. A three-dimensional model is developed considering chemical/electrochemical reactions, mass, momentum and heat transfer processes to assess the effect of liquid metal cooling. The simulation results show that liquid metal has an excellent ability to improve the temperature uniformity and electric performance of the cell unit. The temperature difference of the cell unit cooled by air cooling is 60 K, which can be decreased to 15 K with liquid tin cooling. Furthermore, inlet air has little effect on the performance of the cell unit when liquid metal is chosen as coolant. The pumping powers of the air and liquid metal are compared at different excess air ratios and inlet velocities of liquid metal. The total pumping power consumption could be dramatically decreased when liquid metal is utilized as the coolant. Furthermore, the variations in the conductivity, heat capacity and convective resistance at different liquid metal inlet velocities are discussed.     

The modeling of a standalone solid-oxide fuel cell auxiliary power unit

In this research, a Simulink model of a standalone vehicular solid-oxide fuel cell (SOFC) auxiliary power unit (APU) is developed. The SOFC APU model consists of three major components: a controller model; a power electronics system model; and an SOFC plant model, including an SOFC stack module, two heat exchanger modules, and a combustor module. This paper discusses the development of the nonlinear dynamic models for the SOFC stacks, the heat exchangers and the combustors. When coupling with a controller model and a power electronic circuit model, the developed SOFC plant model is able to model the thermal dynamics and the electrochemical dynamics inside the SOFC APU components, as well as the transient responses to the electric loading changes. It has been shown that having such a model for the SOFC APU will help design engineers to adjust design parameters to optimize the performance. The modeling results of the SOFC APU heat-up stage and the output voltage response to a sudden load change are presented in this paper. The fuel flow regulation based on fuel utilization is also briefly discussed.     

Experimental Study on Thermosyphon for Shipboard High-Power Electronics Cooling System

The closed-loop thermosyphon (CLT) has advantages of simple structure and reliability for transporting heat in long distances with small decrease in temperature. It is considered a promising cooling device for power electronics onboard ships. In this research, CLT for cooling of power electronics onboard ship was developed, and the performance was experimentally examined using a CLT apparatus. The performance was investigated for steady-state heat transfer under a wide range of pressures and heat loads from 18.3 kPa to 35.3 kPa and from 88.9 W to 616.2 W, respectively. The fill charge rates were 27% and 45%. The circulation coolant temperature at the condenser was set to 15°C. The measured data for each rated heat input were registered by a data logger in every 5-s increment of sampling data for a 30-min period. During the steady-state operation, CLT could maintain the system pressure and produced the vapor bulk temperature at around saturation boiling regime. The temperature distributions of the system were measured from each probed thermocouple along the loop. It is understood that higher heat inputs around above 349 W could keep the bulk vapor in an almost constant temperature from evaporation process up to the inlet position of the condenser. The condenser of the direct hull cooling method could also maintain the condensation process with a temperature decrease of around 30°C from the inlet vapor temperature of the condenser. It was clarified that the CLT has good thermal performance in the higher heat loads with low thermal resistance and provides a steady circulation loop from each two-phase process of heating in the evaporator and cooling during condensation.     

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.     

矩形钢翅片椭圆管簇的试验研究

本文着重阐述了电站凝汽器空冷系统的空冷式换热器所用的矩形钢翅片椭圆管簇小样的试验研究。通过对小样放热及阻力的试验，得出不同迎面风速下的放热性能及空气侧阻力性能的关系。     

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.