Loop Thermosyphon Research Articles

Thermal-hydraulic simulation and evaluation of a natural circulation thermosyphon loop for a reactor cavity cooling system of a high-temperature reactor

Abstract The investigation into a full-scale 27 m high, by 6 m wide, thermosyphon loop. The simulation model is based on a one-dimensional axially-symmetrical control volume approach, where the loop is divided into a series of discreet control volumes. The three conservation equations, namely, mass, momentum and energy, were applied to these control volumes and solved with an explicit numerical method. The flow is assumed to be quasi-static, implying that the mass-flow rate changes over time. However, at any instant in time the mass-flow rate is constant around the loop. The boussinesq approximation was invoked, and a reasonable correlation between the experimental and theoretical results was obtained. Experimental results are presented and the flow regimes of the working fluid inside the loop identified. The results indicate that a series of such thermosyphon loops can be used as a cavity cooling system and that the one-dimensional theoretical model can predict the internal temperature and mass-flow rate of the thermosyphon loop.

Study of a Loop Thermosyphon Evaporator for Thermal Control of Aircrafts

This work presents experimental studies of a two-phase loop thermosyphon, designed to use the aircraft fuselage low temperatures at high altitude to promote passive thermal management of electronics components in avionics. The main purpose of this work is to develop the evaporator of a loop thermosyphon composed of one evaporator coupled to two condensers presented in the literature. The proposed evaporator comprises ten square stacked copper plates bonded by diffusion. Channels were manufactured in the inner plates so that internal grooves are obtained after the stack is bonded. Studies with concentrated and distributed heat sources over the evaporator external surface are under performance. Parameters such as start-up and operation temperatures will be compared, according to the condition submitted.

Experimental investigation on a loop thermosyphon with evaporative condenser for free cooling of data centers

Integrating evaporative cooling with loop thermosyphon can significantly improve the free cooling ability. In this paper, a loop thermosyphon with evaporative condenser is investigated experimentally. The mist water flow out of a single nozzle is observed and analyzed. The performance and annual free cooling potential of the system are investigated, compared with conventional loop thermosyphon. The results show that the evaporative cooling effect of the locations below the horizontal level of the nozzle is more significant due to gravity. With the increase of horizontal distance from the nozzle, the temperature decreases and then increases and the optimal distance is 200–400 mm. The heat capacity of LTEC increases with the increase of indoor and outdoor temperature difference while it decreases with the increase of humidity. Evaporative cooling can enhance the heat transfer of LTEC by 7%–33% compared with loop thermosyphon with conventional condenser, and this value is larger for smaller indoor and outdoor temperature difference and higher indoor temperature. LTEC can expand the annual free cooling time by 7%–14% compared with LTCC and the effect is more significant in regions with drier weather.

Experimental study and analytical modeling of thermosyphon loop for cooling data center racks

This paper presents experimental study and analytical model for a passive two-phase thermosyphon loop designed to remove directly the hot air flow inside a data center. The cooling loop with separated lines working without electricity is tested under different conditions. A mini-channel parallel-flow evaporator and a finned-tubes condenser are used to transfer heat from the data center to outside environment. Experimental tests are conducted with different n-pentane fill ratios. According to these tests, the cooling performance is showed and the refrigerant distribution is specified. The proposed steady-state model is based on the combination of the thermal and hydraulic models of the two-phase flow inside the loop. It well predicts the experimental results such as the mass flow rate and working fluid pressure drops. As investigated by the model, the most parameters influencing the cooling capacity are: the working fluid, the condenser tubes number and the outside temperature.

Modeling of Passive Safety Thermosyphon Loops Cooling System for Nuclear Applications

Abstract: Heat pipes are passive heat transfer devices, with very long lives. Material and testing reactors (MTR) have residual heat after shutdown. Also they have an auxiliary spent fuel tank that also have heat generated need to removed, also heat should be removed in case of emergency to keep fuel temperature within safe limit. A gravity assisted two-phase closed heat-pipe loop (GTPHL) with evaporator and condenser lengths each 100m helical coil shape with outer diameter 15 cm and 3 mm thickness as a passive cooling system for a nuclear spent fuel storage pool is used to remove this heat. This paper presents and proposes a completely passive cooling system using loop thermosyphon for cooling and dissipation the residual heat of wet spent fuel storage by running as main or alternative cooling system. The design is focus on heat removal from the auxiliary spent fuel tank of the research reactor to be in safe mode. The model considers natural convection by air for the condenser part of the heat-pipe loop to confine the residual heat. A numerical simulation using special design of gravity assisted two-Phase closed heat-pipe loops were used to investigate the (GTPHL) thermal performance. The effects of heat loads were analyzed. Demineralized water was used as the (GTPHL) working fluid. The atmospheric air was circulated around the condenser as a cooling system. The effect of heat input (25kW≤Q≤150kW) working fluid filling ratio (100%) The results show that the best thermal performance was obtained at high evaporator heat load.

Two-phase refrigerant flow in the evaporator of a stirling cooling system with a thermosyphon loop

ABSTRACTIn this study, implementation of a thermosyphon heat exchanger into a Stirling refrigeration system is demonstrated by means of both theoretical and experimental analyses. R-134a was employed as the working fluid which formed a liquid column at the condenser exit, enabling a two-phase flow through the evaporator. Experiments were conducted in a controlled chamber. Cooling capacity, coefficient of performance, and two-phase pressure drop of the cooler were compared at two different voltages. The flow regime for the refrigerant in the evaporator was found to be stratified based on the calculated wavy flow and Lockhart–Martinelli parameters.

Experimental study on the temperature management behaviours of a controllable loop thermosyphon

The loop thermosyphon with active heat transfer control capability is a promising solution for the temperature management of the fresh food compartment of cool-storage refrigerators or other precise temperature control devices. However, the frequently start-stop operating mode of the loop thermosyphon has not received enough attention, its active temperature management behaviours still requires further investigation. In this study, a controllable loop thermosyphon used in the temperature control of a fresh food compartment is developed and its start-stop is controlled by the temperature controller with the assistance of two magnetic valves. The performances of the controllable loop thermosyphon in long-term and start-stop operating modes are tested. The controllable loop thermosyphon with 35% filling ratio present highest heat transfer rate, reliable heat transfer capability, and good stability for the varying temperatures in the fresh food compartment. Based on this filling ratio, the heat transfer rate remains higher than 40 W during the long-term operating test. When the controllable loop thermosyphon operates in the start-stop mode, the start-stop cycle decreases from 39.4 min to 27.2 min and the operating ratio increases from 12.6% to 53.7% as the temperature of the fresh food compartment rises. Particularly, when the start-stop cycle reduces from 42.6 min to 18.1 min, the temperature control accuracy is improved from 2.0 °C to 0.4 °C. The results demonstrate that the controllable loop thermosyphon is promising as a novel precise temperature management device and is qualified for the use in cool-storage refrigerators.

Start-up in microgravity and local thermodynamic states of a hybrid loop thermosyphon/pulsating heat pipe

A wickless passive two phase closed loop heat transfer device especially designed for a future implementation on the heat transfer host module of the International Space Station is tested in relevant environment on board a parabolic flight. The tube internal diameter (3 mm) is larger than the static capillary threshold evaluated in normal gravity for this working fluid (FC-72), leading the device to work as a loop thermosyphon on ground and in hyper-gravity conditions, and as a Pulsating Heat Pipe when micro-gravity occurs. Novel start up tests, where the heat load has been provided after the occurrence of microgravity, show that the 20 s microgravity period is enough for the device activation and, most important, that the device activation is purely thermally induced and not affected by the previous acceleration field. Two miniaturized pressure transducers and direct fluid temperature measurement via two micro-thermocouples, allow to provide a detailed insight on the fluid local thermodynamics states both in the evaporator and in the condenser zone during microgravity. It is shown that the two-phase fluid close to the evaporator and the condenser is subjected to several degrees (up to 5 K) of superheating or subcooling. The level of subcooling seems to increase with the heat input level both in terms of temperature difference and in terms of percentage time with respect to the whole microgravity period.

Experimental optimization on the volume-filling ratio of a loop thermosyphon photovoltaic/thermal system

Loop thermosyphon (LT) is a two-phase closed device that provides an alternative solution to overcome the freezing problem associated with the traditional photovoltaic/thermal (PV/T) system. The overall performance of a LT-PV/T system is significantly affected by volume-filling ratio. To reveal this effect, a prototype of the LT-PV/T system was designed and constructed in this paper; and long-term outdoor tests under 24%, 32%, 40% and 48% volume-filling ratios were carried out. The results show that the daily average photothermal performance of the LT-PV/T system presented a trend of increased first and then decreased with the volume-filling ratio increasing on the selected sample days. The highest performance was generated under the 32% filling condition. The solar evaporator had an even temperature distribution when the volume-filling ratio was no less than 32%; the temperature difference of the solar evaporator decreased with the volume-filling ratio increased, the values were 1.5 °C, 1.2 °C and 0.6 °C, respectively. Performances of the collector and system were linear fitted based on the outdoor tests, the fitting results showed that both of them shared the same variation trend, the optimum volume-filling ratio lied in between 32% and 40% of the whole system volume, but nearer to 32%.

Numerical analysis of a novel household refrigerator with controllable loop thermosyphons

Efficiency improvement of household refrigerators exerts extensive influences on the reduction in residential energy consumption and alleviation of environmental pollution. Controllable loop thermosyphons (CLTs) are low-cost, high-precision heat transfer control devices that can be potentially applied in refrigerators as crucial energy saving and temperature management components. A novel refrigerator with CLTs is proposed in this study. The evaporator in fresh food compartment is removed, the cold loss of the topside freezer is partly reused by the CLTs to cool the fresh food compartment in intermittent mode, and the overall cold loss of the refrigerator is efficiently reduced. The energy saving behaviour of the refrigerator is numerically studied based on tested temperature boundary conditions. The energy saving mechanism is comparatively analyzed, and the impacts of the operating condition, installation position of the CLTs, and ambient temperature are investigated. The energy saving ratio of the novel refrigerator increases from 12.9% to 19.5% when the ambient temperature rises from 17.0 °C to 31.0 °C, the operating condition varies from 1 to 4, and the locations of CLTs from the inner wall of the freezer are changed from 2.1 to 4.9 mm. Results preliminarily prove the energy saving capability of the novel refrigerator.

Experimental investigation of a novel type of two-phase closed thermosyphon filled with functionalized carbon nanotubes/water nanofluids for electronic cooling application

Two-phase closed thermosyphon as a gravity-assisted heat pipe is an efficient heat transfer device in which heat transmits by evaporation and condensation of a working fluid. Nanofluids are suspensions of nano-sized solid particles in a base fluid which can be used as heat transfer media in the thermosyphon. In the present study, thermal performance of functionalized multi-walled carbon nanotubes/water nanofluids was investigated in a novel type of thermosyphon which was made of an evaporator with square frustum shape, and a condenser composed of two connected tubes. To prepare nanofluids, multi-walled carbon nanotubes were first functionalized using two oxidant agents (potassium persulfate and sodium persulfate) to improve the cooling performance of nanofluids. The prepared nanofluids were characterized by Fourier transform infrared spectroscopy, inductively coupled plasma optical emission spectroscopy, zeta potential, and thermal conductivity analyzer. The results of characterization showed that the nanofluid with sodium functional group had higher stability as well as higher thermal conductivity. Thermal efficiency and thermal resistance were calculated to evaluate the thermal performance of thermosyphon. Using nanofluid with sodium functional group in the thermosyphon showed better performance in terms of higher thermal efficiency and lower thermal resistance. Experimental results also showed that the thermosyphon could perform at higher input powers (up to 90 W) using the nanofluid compared to water (up to 70 W). Furthermore, the results of the temperature distribution of thermosyphon during the test confirmed that two connected tubes in the condenser section could provide pathways for both condensate liquid and vapor. In other words, this behavior indicated that this type of thermosyphon can be considered between a tubular thermosyphon and a loop thermosyphon.

Experimental study of a two-phase thermosyphon loop in conditions of high fluidic and thermal loads

Experimental study of a two-phase thermosyphon loop in conditions of high fluidic and thermal loads

Start-Up and Operation of a 3D Hybrid Pulsating Heat Pipe on Board a Sounding Rocket

A large tube may still behave, to a certain extent, as a capillary in a micro-gravity environment. This very basic concept is here applied to a two-phase passive heat transfer device to obtain a new family of hybrid wickless heat pipes. Indeed, a Loop Thermosyphon, which usually consists of a large tube, closed end to end in a loop, evacuated and partially filled with a working fluid and intrinsically gravity assisted, may become a capillary tube in space condition and turn its thermo-fluidic behavior into a Pulsating Heat Pipe. This work presents the results obtained on such a hybrid device heated at 200 W both on board a sounding rocket (ESA REXUS 22, microgravity period ~120 s), and on ground in vertical and anti-gravity orientation. Since no steady state occurred in microgravity conditions, the comparison between flight and ground data focuses on the startup phenomenon, whereas the thorough ground test campaign describes the limits and performances of the device working in thermosyphon mode. The expected thermal behavior in microgravity conditions is between that of a purely conductive tube in anti-gravity conditions on ground and that of a gravity assisted thermosyphon. Since a microgravity period of approximately 120 s is not enough to reach a pseudo steady state regime, further investigation on a longer-term weightless condition is mandatory.

Heat transfer and instability characteristics of a loop thermosyphon with wide range of filling ratios

A series of experiment using flow visualization technique and simultaneous measurement of pressure and temperature signals, are conducted to thoroughly investigate the heat transfer and instability characteristics in a loop thermosyphon with filling ratios ranging widely from 38% to 87% over a broad range of input heat flux from 35 to 395 W cm−2. The filling ratios are classified into three groups based on their distinctive heat transfer and instability behavior. The effects of filling ratio, input heat flux and inclination angle on the temperature nonuniformity, thermal resistance and instability features are examined in detail. The results show that, under the high filling ratios, the heat transfer capacity is seriously limited by the phase-change suppression and the consequent pressure soaring, while is advantageous in avoiding the evaporator dryout. Under the moderate filling ratios, the heat transfer capacity could be significantly higher, but the geyser boiling instability is obvious for a relatively large range of input heat flux, inducing the fluctuation of pressure and temperature. Under the low filling ratios, the heat transfer capacity is the highest; however, the transient local “Near-dryout” is easier to occur, causing large temperature overshoot and oscillation. The different variation trends of thermal resistance and temperature nonuniformity with filling ratios and input heat flux can generally be explained by the variation of the portions of latent and sensible heat transfer. Under the moderate and high filling ratios, the loop thermal resistance decreases somewhat with the decrease of inclination angle due to the elongation of the phase-change surfaces. A thermodynamic state diagram is drawn, indicating the measured average pressure - temperature of the working fluid at the evaporator for the three groups of filling ratios, with distinctly different instability issues, along with the saturation line to aid in the mechanism analysis and trouble diagnosis. A stability map in terms of the phase change number, Npch, and the filling ratio is presented, in which the boundaries dividing the different types of instability and the stable conditions are marked as a guidance for practical design and operation.

Performance Investigation on the Thermosyphon with Evaporative Condenser for Free Cooling of Data Centers

The microchannel heat exchanger is applied as the heat exchanger of the thermosyphon for data center cooling, and the evaporative cooling experiment is carried out on the side of the microchannel condenser. The influence of evaporative cooling on the microchannel condenser is measured by using the enthalpy different laboratory in the experiment, and the temperature difference between the inlet and outlet air of the microchannel condenser and with different indoor and outdoor temperature and humidity was analyzed. The experimental results show that evaporative cooling can make the condenser inlet air temperature achieve a large temperature drop and increase the temperature drop of the inlet and outlet air, while the spraying can cool the air through the thermosyphon about 0.3-1.9 degrees and increase about 0.1-0.6kW heat transfer of the system. Moreover, the evaporative cooling can improve the heat exchange efficiency of condensers, expand the application area of the loop thermosyphon and extend the annual utilization time.

Preliminary evaluation of the energy-saving behavior of a novel household refrigerator

The refrigerator is one of the most essential domestic appliances and causes considerable energy consumption every year. The efficiency of a normal household refrigerator is significantly affected by the heat-insulating property of the thermal barrier of the freezer due to its low inner temperature. In this study, a novel refrigerator with a loop thermosyphon is put forward to decrease the heat transfer between the freezer and ambient air. It has great potential to be popularized as a sustainable energy technology or applied in the renewable energy field considering its significant energy-saving effect, simple structure, and low cost. The energy-saving behavior of the refrigerator is preliminarily evaluated based on the theoretical calculation. The temperatures of ambient, the fresh food compartment, and the location of the heat transport component are varied; the impacts are analyzed. The total cold loss of the topside and side freezer walls is reduced from 16 W to 14 W in the normal test conditions, and the energy-saving ratio varies from 6.3% to 28.5% when the ambient temperature, the location of the heat transport component, and the fresh food compartment temperature vary. The results certificate the energy-saving capability of the novel refrigerator and its reliability in hot climate.

Adaptive control of bi-directionally coupled Lorenz systems with uncertainties

The bi-directionally coupled Lorenz systems are linked to the modeling of a coupled double loop thermosyphon system where the mass momentum and heat exchange are both considered. As the key parameters of the system, known as Rayleigh numbers, increase, the system of differential equations predicts typical flow dynamics in a thermosyphon from heat conduction to time-dependent chaos. In many applications including the thermosyphon systems, there are uncertainties associated with mathematical models such as unmodeled dynamics and parameter variations. Also, under the high heat environment for a thermosyphon, there exist external disturbances quantitatively linked to the Rayleigh numbers. All these sources constitute uncertainties to the dynamical system. Our objective is to design adaptive controllers to stabilize the chaotic flow in each thermosyphon loop with unknown system parameters and existence of uncertainties. The controllers consist of a proportional controller with an adaptive gain and a wavelet network that reconstructs the unknown functions representing the uncertainties. Explicit stability bounds and adaptive laws for the control parameters are obtained so that the coupled Lorenz systems are globally stabilized.

Experimental investigation on a loop thermosyphon with three evaporators: Unique startup and oscillation phenomena

Loop thermosyphon with multiple evaporators has tremendous potential in applications that involve multiple heat sources. In this paper, a loop thermosyphon with three evaporators is investigated experimentally. Unique phenomena of startup and oscillation compared with common loop thermosyphons are reported. In the startup process, a unique phenomenon called “sudden temperature change” is found. This phenomenon occurs in a temporary state in the startup, when the temperature differences between the evaporators without heating power and with heating power reach certain range (−2.6 °C to 0.8 °C in this experiment). In the quasi-steady state, an oscillation map which shows the range when oscillation occurs is obtained. Compared with only one evaporator with heating power, simultaneous heating on multiple evaporators reduces the oscillation frequency, enhances the oscillation amplitude and sometimes makes it irregular, due to the interaction between the evaporators. Also, for every two adjacent evaporators, the phases of oscillation are almost opposite.

Heat transfer enhancement in a loop thermosyphon using nanoparticles/water nanofluid

Abstract This research focuses on two-phase thermal control systems, namely loop thermosyphons (LTS) filled with nanofluids and their use as LED cooling devices. The behavior of the fluid in the thermosyphons and the mechanisms explaining the possible impact of nanoparticles on the thermal properties of the working fluid, as well as the processes in the LTS, are addressed. Nanoparticle distribution in the nanofluid, methods of preparing nanofluids and the nanofluid degradation processes (aging) are studied. The results are obtained from a set of experiments on thermosyphon characteristics depending on the thermophysical properties of the working fluid, filling volume, geometry and nanoparticle mass concentrations. The impact of nanofluids on the heat-transfer process occurring inside the thermosyphon is also studied. The results indicate the strong influence of nanoparticles on the thermal properties of the thermosyphons, with up to a 20–25% increase in the heat transfer coefficient. It is shown that this effect is due to the aggregation of nanoparticles and the formation of a micro/nano relief on the vaporization surface. Additionally, a method of calculating the hydrodynamic limit of the LTS is proposed, which allows for estimation of the maximum heat that can be transferred by means of the LTS. The nanofluids are shown to be effective means for enhancing heat transfer in two-phase thermal management systems.

Numerical study on the two-phase flow pattern and temperature distribution in a loop thermosyphon as a defrost device at the evaporator in the refrigerator

This paper discusses the two-phase flow pattern and temperature distribution in a loop thermosyphon as a defrost device at the surface of the evaporator in the refrigerator with different heater locations and different heating power. A computational fluid dynamics (CFD) study was carried out using ANSYS FLUENT 15.0. The volume of fluid (VOF) model was considered to simulate evaporation and condensation at the heater surface using user-defined functions (UDFs). 2D geometries were developed with a heater inserted in the loop thermosyphon. The simulation results were verified using Fadhl’s experimental and numerical temperature data [2]. The maximum difference is 2.4 % between the calculated data and Fadhl’s data. The two-phase flow pattern and the temperature field varied with the different heater locations and heating power values. The thermal performance was evaluated based on the average temperature and temperature uniformity inside the loop thermosyphon.