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Fan failure of loop thermosyphon systems (LTS) is a major factor affecting thermal safety of data centers. This paper develops a distributed-parameter model to accurately predict the overall and local heat transfer performance of a loop thermosyphon (the terminal equipment of LTS) under fan failure conditions. The simulation results show that the positions of faulty fans have a significant influence on the evaporator's local airside heat transfer coefficient, but have slight influences on the evaporator's local refrigerant heat transfer coefficient and the condenser's overall heat transfer coefficient, which would not increase the risk of heat transfer performance. Thus, fan airflow rate backup can be used to handle fan failure. Then, the effect of operating parameters including backup airflow rate (Bfan), cold water temperature (Tcw) and increment of cold water flow rate (Im,cw) on the effectiveness and efficiency of the cooling capacity compensation under fan failure conditions is analyzed by using the Taguchi method with an L16 orthogonal array. The Bfan is the primary factor contributing 81.51% on cooling capacity compensation, with Tcw and Im,cw contributing 15.25% and 2.38%, respectively. Considering the energy efficiency, the order of adjustment should be Bfan first, Im,cw second, and Tcw last when fan failure occurs.

Comparison on the heat transfer performance (HTP) of copper screen mesh wick and metal foam inserted evaporator section of miniature loop thermosyphon is experimentally investigated. Porosity of both screen mesh wick and metal foams are fixed to be 63%. Experiments are conducted using DI water under vertical orientation for different heat loads (40 W–280 W) and filling ratios (FR = 20%, 30% and 40%). Results showed a decrease of 12.8% and 5.2% in thermal resistance and wall temperature respectively for the metal foam inserted evaporator. In addition 10.6% enhancement is noted for heat transfer coefficient at the evaporator region for 30% FR. Reduction of 4.6% in heat transfer irreversibility is also noted for metal foam inserted thermosyphon compared to screen mesh wick condition. Increase in interstitial heat transfer between the fibers of metal foam and larger pore size of metal foams resulted in higher permeability compared to screen mesh wick are responsible for the enhancement in the overall HTP of metal foam inserted thermosyphon. Based on the observed results, the use of metal foams inserted thermosyphon will be a better substitute for thermal management applications of electronic devices.

• Thermal characteristics in loop thermosyphon with serpentine tube heat exchanger are studied. • Wide filling ratios of 10.4%∼99% and input heat flux of 16.7∼266.7 W•cm−2 are examined. • Flow instability and transient response are studied. • The serpentine tube condenser provides a small thermal resistance. The loop thermosyphon, which has an excellent heat transfer performance, can effectively solve the problem of high heat flux heat transfer in outdoor high-power electronic equipment. In order to solve the problem of high heat flux heat transfer, the researchers focused on the new evaporator, but the impact of the condenser on the entire system cannot be ignored. This paper designed an air-cooling type loop thermosyphon with a condenser composed of a serpentine copper tube and corrugated aluminum fins. The experiment confirmed the serpentine tube condenser is a condenser with excellent performance. It can be used in conjunction with the loop thermosyphon to realize the 266.7 W•cm−2 heat flux transfer. Intriguingly, using a serpentine tube evaporator will bring about two kinds of flow instability, and the filling ratio is the key to restraining the flow instability. A high filling ratio is a suitable choice, which avoids flow instability and has a fast transient response. Moreover, the air temperature has little effect on the thermal resistance and the maximum temperature difference of the loop thermosyphon; therefore, this loop thermosyphon can be used in various weather conditions. However, extreme weather conditions (extremely high/ low temperature) may affect the performance of the loop thermosyphon. Consequently, the loop thermosyphon with serpentine finned tube condenser at an optimized high filling ratio is worthy of being employed in outdoor high-power electronic equipment. [ABSTRACT FROM AUTHOR]

• The LEE model with dynamic mass transfer time relaxation parameter (r c) is built. • Dynamic- r c model improves the accuracy of simulation of working fluid distribution. • The effect of the saturation temperature on the simulation result is analyzed. • The best performance of the loop thermosyphon is found at a filling ratio of 50 %. The loop thermosyphon is an efficient two-phase heat transfer device for heat recovery and energy saving. Generally, the Lee model is commonly applied in the Computational Fluid Dynamics (CFD) methods to predict the simultaneous evaporation and condensation in the loop thermosyphon. However, the Lee model has the non-conservation of mass problem due to the mismatch between the condensation mass transfer time relaxation parameter (r c) and the evaporation mass transfer time relaxation parameter (r e) in the two-phase closed system. This paper improves the Lee model with dynamic- r c by considering the mass balance in the loop thermosyphon. The value of the r c is related to the total mass of the liquid working fluid, the evaporation intensity, and the condensation intensity. The CFD simulated temperature distributions were in good agreement with the experimental temperature data. This study investigates the effect of different r c and saturation temperatures (T sat) on the simulation results. The results show that the refined model ensures the total working fluid mass remains stable during the calculation process, and the simulation of the gas-liquid two-phase distribution in the loop thermosyphon is more accurate. The value of the T sat will have a more significant effect on the wall temperature distribution of the loop thermosyphon but less on the working fluid distribution. Additionally, the impact of filling ratios is investigated while setting the T sat as constant in the CFD models. The results experimentally and numerically indicate that when the filling ratio is 50 %, the overall thermal resistance is the smallest, and the liquid working fluid distribution is optimal. This work provides a method and approach for analyzing the heat transfer procedure inside the loop thermosyphon and improving the accuracy of the numerical model. [ABSTRACT FROM AUTHOR]

An experiment study was conducted on the operational characteristics of the JEST-type loop thermosyphon when an initial liquid level of a working fluid was lowered. This thermosyphon was invented in 2012 by one of the authors with a jet explosion stream technology (JEST) for cooling high-heat-generation and high-heat-flux CPUs. The present experiment aims at lowering the height of the thermosyphon for rack-level thermal management in a datacenter. Hydrofluoroether (HFE)-7000 was used as the working fluid. In experiment, an evaporator section of the thermosyphon was heated with a heating block while a condenser section was water-cooled using a thermostatic bath. Temporal changes in temperatures of the thermosyphon were obtained with thermocouples. Moreover, the circulation flow rate of the working fluid in the thermosyphon was obtained with a simple measurement method. The initial liquid level of the working fluid was changed as 166, 268, 368 mm while the height of the thermosyphon was 1200 mm. Experimental results are shown regarding the effect of the initial liquid level on the circulation flow rates of the vapor and liquid phases of the working fluid as well as the heat transfer coefficient at the evaporator section. An additional experiment was also conducted when the height of the thermosyphon was lowered from 1200 mm to 480 mm. It was confirmed that the thermal performance of the thermosyphon decreased when the initial liquid level was lowered; however, the thermal performance was recovered by lowering the height of the thermosyphon. Therefore, the JEST-type loop thermosyphon can be applied to the rack-level thermal management in a datacenter.

Nanofluids (NFs) are an attractive alternative to traditional working fluids for thermosiphons, but the solid nanoparticles (NPs) within the NF can agglomerate and reduce the thermal performance. This study focused on clarifying the effect of a NF with surfactants on the heat transfer characteristics of an advanced loop thermosiphon with a check valve (ALT/CV). In an experiment, the ALT/CV was filled with different working fluids at filling ratios of 30%, 50%, and 80% with respect to the evaporator volume. Heat was supplied at 20%, 40%, 60%, 80%, and 100% of the heater output (2000 W). Five working fluids were considered: deionized (DI) water, a DI water-based NF with 0.5 wt% silver NPs, and the same NF containing 0.5, 1, and 1.5 wt% oleic acid (OA) and potassium oleate (OAK+) as surfactants. The results showed that the ALT/CV provided a better heat transfer performance than a normal thermosiphon. The maximum heat transfer rate was achieved with the NF containing 0.5 wt% silver NPs and 1 wt% OAK+. The NF containing OAK+ demonstrated a heat transfer rate approximately 80% higher than that of the DI water

• Enhancement loop thermosyphon with metal foam for solar energy was developed. • A numerical model of MF-HLTS was developed for predicting dynamic characteristics. • Fast response speed with time lag of 2∼4 s meets requirement of dynamic regulation. A metal foam horizontal loop thermosyphon (MF-HLTS) for a solar receiver was proposed and investigated under the variable conditions. Meanwhile, a dynamic response model for predicting the thermal response characteristics of MF-HLTS was established using Nusselt theory and lumped parameter method. Results of the dynamic model show good agreement with the experimental data, with a maximum deviation of 3.5 % for temperature response. Under sudden power conditions, the temperature variation curve of MF-HLTS shows a progressive monotonic change with two stages: both rising and decreasing rates of temperature show linear and rapidly changes at first stage, and them begins to decrease and gradually tends to 0 at second stage. The time required to reach steady-state increases with the increase of power step. For start-stop of power and cooling, the recovery time gradually increases with increasing stop time, but start-stop ratio firstly decreases rapidly and then tends to be flat. Dynamic response of MF-HLTS is relatively fast with a temperature time lag of 2 s to 4 s, which can meet the requirements of dynamic regulation of system temperature under actual conditions. [ABSTRACT FROM AUTHOR]

The condensers of loop thermosyphon PV/T systems (LT-PV/T) are usually integrated inside water tanks, which may bring some challenges during combination use. This research innovatively proposed a concentric copper tube heat exchanger as the condenser, which is combined with a copper tube evaporator beneath the absorber. The gaseous working fluid flows in the inner tube and the cooling water flows in the outer tube. Since ordinary water pipes are used for water circulating between the outer tube and water tank, this LT-PV/T collector can be used individually or combined with other collectors flexibly. To access its' performance, researches have been conducted: (1) Designing and fabricating the system prototypes; (2) Investigating system performance with different volume-filling ratios (26.5%, 34.8%, 43.2%); (3) Investigating the influences of working fluid (water, ethanol and R134A). (4) Evaluating the systems' performance with energy efficiency, exergy efficiency, and semi-empirical system efficiency models; (5) Conducting two case studies in South China (an individual collector & a 4 parallelly/serially-combined LT-PV/T collectors system). The system is first-of-its-kind and has obvious advantages in reliability, flexibility, space-saving and large-scale applications. The typical primary energy-saving efficiency of the LT-PV/T with R134a of 40% filling ratio can reach 78.0%, higher than the published LT-PV/T systems. 1. A novel concentric copper tube heat exchanger is used as the condenser of LT-PV/T. 2. It has obvious advantages in reliability, flexibility, and large-scale applications. 3. The prototypes of the heat exchanger and LT-PV/T are designed and fabricated. 4. System performance with several filling ratios and working fluid are explored. 5. Case studies for its independently/combined utilization in South China are conducted. [ABSTRACT FROM AUTHOR]

The loop thermosyphons has advantages of high heat transmission efficiency and needing no additional power. It can quickly draw out the heat in the motorized spindle with no coolant getting into the shaft, and has a good application prospect in the field of motorized spindle cooling. However, the evaporation and condensation sections of a loop thermosyphon will be in the same pipe and at the same height under the heat conduction of the shaft. These structural changes will directly affect the flow characteristecs and heat transfer performance of the loop thermosyphon. This paper experimental studied the vapor-liquid two phase flow characteristics in a pentane loop thermosyphon. The effects of the heat input power and fill ratio on the flow characteristics were analyzed. Results show that the two-phase flow pattern transitions from stratified flow to wavy flow and finally to plug flow as fill ratio increases. The velocity of the liquid flow in the upper adiabatic section of the loop thermosyphon is on the order of 0.1 m·s-1, and it decreases with increasing heat input when the fill ratio is between 30 and 70 %. [ABSTRACT FROM AUTHOR]

A thermosyphon loop, designed for the thermal management of a large Medium voltage power converter 5 MW overall, corresponding to a 2.4 kW thermal load per cooling unit) is presented. The device is mainly made of an evaporator, a condenser and a reservoir connected with plastic liquid and vapor lines. Novec 649 (3M) has been chosen as the working fluid due to environmental and electrical concerns. A model of the loop is described, and its predictions are compared with experiments. A first comparison yields a maximum mean deviation of 20 % between experimental results and numerical simulation at the maximum coolant temperature. The main sources of errors are identified, and improvements are proposed for better model accuracy.

Loop thermosyphon with multiple evaporators has unique startup features and interaction characteristics between evaporators which are worth studying. In this paper, the startup influencing factors of a loop thermosyphon with multiple evaporators are studied experimentally and visually. Meanwhile, the interaction between evaporators is evaluated quantitatively using a newly defined parameter called interaction ratio, which represents percentage change in temperature divided by percentage change in heating power. The results show that the temperature of evaporators often has a local peak value during the startup, at which moment the boiling becomes fully developed. Also, the effect of filling ratio, heating power value and distribution on the local peak temperature is analyzed. For each evaporator, interaction ratio on itself is always the largest. Among the evaporators, interaction ratios of the evaporator in the middle are the largest because the competition and crowding of its flow with the other evaporators are more intense. With the increase of filling ratio, interaction ratios on itself decrease while interaction ratios on the others increase.

The period operation of power electronic acts as switching element, where the power dissipated consists of pulses at certain duty cycle, the semiconductor temperature oscillates and varies as a waveform. In the present study, an experimental investigation was carried out for a loop thermosyphon order to evaluate the effect of pulsate surface heat flux on the single-phase buoyancy driven convection of ethylene glycol flow through a minichannels heat sink with hydraulic diameter 1.5 mm. An electric heater block is used to supply the heat flux to minichannels heat sink in a rectangle waveform. The study is done at different heat flux frequencies of 2.777x10-3 Hz, 8.333x10-4 Hz, 5.555x10-4 Hz and 4.166x10-4 Hz, while the heat flux amplitude (2 watt), Rayleigh number (1864) and duty cycle (50 %) are kept constant. The results revealed that for a range of the measured frequency for the complete power cycle and due to unsteady state operation conditions, the pulse heat flux pattern is close to a rectangle-wave, this generates the fluid outlet temperature pattern close to a triangle-wave. The fluid outlet temperature increases with the decreases of heat flux frequency and tends to reach to the fluid outlet temperature for a constant and continuous heat flux case. Due to closed-loop of thermosyphon, the fluid inlet temperature is changed in pattern like that the fluid outlet temperature change. [ABSTRACT FROM AUTHOR]

• A water-cooled loop thermosyphon system (LTS) is studied under fan failure conditions. • The effects of fan failures on the thermal performance are analyzed and evaluated. • The anti-failure capability at different filling ratios are compared. • It is better to choose a high optimal filling ratio to ensure the good operating performance and anti-failure capability of LTS. Fan failure of water-cooled loop thermosyphon system (LTS) is a major factor for thermal safety of data center, thus it is necessary to investigate the effects of fan failures on the thermal performance. In this paper, different operating conditions including normal and eight possible fan failure modes of a LTS are tested at various refrigerant filling ratios. The optimal refrigerant filling ratio is ranged from 55.8% to 74.4% under normal operating condition. Compared with the conditions of leakage (Filling ratio = 46.5%) or overcharge (Filling ratio=83.7%), the LTS has better anti-failure capability with an optimal filling ratio. The cooling capacity at 55.8% filling ratio decreases by 8.2–10.1% and 20.8–23.8% under mild and serious fan failure modes, respectively, and the decrements increase with the filling ratio increasing. This is because the additional refrigerant mass flow rate increases with the filling ratio increasing, which increases the total system thermal resistance. However, the cooling capacity at the minimum optimal filling ratio (55.8%) is only 9.6% larger than that at the maximum optimal filling ratio (74.4%) under the worst fan failure condition. Considering the universality of the slight refrigerant leakage in refrigeration systems, it is better to choose a high optimal filling ratio to ensure the good operating performance and anti-failure capability of LTS. [ABSTRACT FROM AUTHOR]

Thermal management system (TMS) of a power converter directly dictates the available power rating, power density, semiconductor module reliability, and its operating lifetime. For the latest-generation wide bandgap (WBG) semiconductor device-based converters, it is challenging to extract the generated heat from the devices due to smaller die area as compared to its silicon (Si) counterparts. In this paper, the thermal performance of a new loop thermosiphon-based TMS for silicon carbide (SiC) semiconductor device-based power conversion system is presented. The working principle and design of the TMS are shown, and the performance of the designed TMS in both transient and steady-state conditions of power dissipation is evaluated. Furthermore, an accurate thermoelectrical model of the TMS is presented, and the circuit parameters are quantified by experimental results. This analysis helps to estimate the device junction temperature in real time during converter operation. Moreover, detailed simulations are carried out with the derived TMS thermal model to evaluate its performance at low fundamental frequencies at rated currents. The experimental results and the simulation studies indicate that the TMS offers a low thermal resistance and can extract a large amount of heat without increasing the device junction temperatures beyond their rated values. Furthermore, the designed TMS is able to maintain the junction temperature ripples at low fundamental frequencies within small values, which helps to increase the lifetime of the power modules significantly, as compared to conventional heatsinks. [ABSTRACT FROM AUTHOR]

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%. • A prototype of LT-PV/T system was designed and constructed. • System performance under four volume-filling ratios was experimentally studied. • A more precise filling interval was suggested based on long-term outdoor tests. [ABSTRACT FROM AUTHOR]

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. • Loop thermosyphon with evaporative condenser (LTEC) is studied experimentally. • Temperature field and flow field of the water spray out of a single nozzle are analyzed. • The distance with the best evaporative cooling effect is 200–400 mm from the nozzle. • The heat transfer of LTEC is 7%–33% larger than LTCC. • The annual free cooling time of LTEC is 7%–14% larger than LTCC. [ABSTRACT FROM AUTHOR]

• A temperature control system with controllable loop thermosyphon is designed. • Heat transfer and temperature management behaviours are examined. • Controllable loop thermosyphon provides heat transfer capability above 40 W in the long-term test. • Temperature control accuracy is significantly improved to 0.4 °C by shortening start-stop cycle. 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. [ABSTRACT FROM AUTHOR]

• A novel household refrigerator with controllable loop thermosyphons (CLTs) is designed. • Heat transfer characteristics of the refrigerator are numerically investigated. • Operating ratio of the CLT affects its cold loss reduction perforamce. • Energy saving ratio of the refrigerator varies within 12.9−19.5% in different boundary conditions. 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. [ABSTRACT FROM AUTHOR]

Highlights • Nanoparticle distribution in the nanofluid, preparation methods and degradation processes (aging) were studied. • The nanoparticles/water nanofluid is employed as the working fluid of the loop thermosyphon. • Using nanofluid, heat transfer in a loop thermosyphon was improved. 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. [ABSTRACT FROM AUTHOR]

Highlights • Unique instability phenomenon called sudden temperature change is found in the startup. • Sudden temperature change occurs when Δ T between evaporators reaches a certain value. • Oscillation is due to the periodic variation of the flow regime inside the loop. • Frequency is lower and amplitude is larger if multi-evaporator is heated compared with one. • For every two adjacent evaporators, the phases of oscillation are almost opposite. Abstract 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• A novel passive residual heat removal system with loop thermosyphons is proposed. • A numerical model couples the primary loop, reactor pool and loop thermosyphons. • PRHR HX prevents the reactor core from overheating and burnout. • Loop thermosyphons prevents the pool water from the saturated boiling state. Advanced pool-vessel reactors require reliable, secure, and passive mechanisms to discharge residual heat from the reactor following shutdown. For this reason, this paper proposes a novel Passive Residual Heat Removal System (PRHRS) with loop thermosyphons. To evaluate the residual heat removal and heat transfer capabilities of the PRHRS, a numerical analysis tool is developed. To achieve this, A thermal hydraulic model is established by secondary development of ANSYS FLUENT, designed to couple the primary loop, reactor pool, and loop thermosyphons. A feasibility study is conducted utilizing the proposed model, and the results demonstrate that this system can efficiently and promptly transfer residual heat to the reactor pool and ultimately to the atmosphere, preventing the core from overheating as well as the pool water from saturated boiling. This model is highly effective in analyzing the thermal hydraulic and safety characteristics of pool-vessel reactors and providing technical support for the design and optimization of the PRHRS. [ABSTRACT FROM AUTHOR]

Loop thermosyphon with multiple evaporators is an ideal equipment to achieve multi-source heat transfer with low energy consumption. In this paper, the transient response and oscillation characteristics of a loop thermosyphon with dual evaporators under different heating conditions, as well as the internal flow and interaction between the evaporators, are tested experimentally and visually. The results show that when the heating power is uniform, oscillation is more significant under higher heating power, which is due to the conflict and bypass flow between the evaporators; when the heating power of one evaporator is higher, outlet flow of the other evaporator is impeded to some extent and bypass flow from this evaporator to the other exists periodically. As a special case, when only one evaporator has heating power input, this bypass flow remains relatively stable; when the thermosyphon works at steady state, the working state changes smoothly if the heating power changes as step function. [ABSTRACT FROM AUTHOR]

The experiments for a Two-phase Closed Loop Thermosyphon (TCLT) of which an adiabatic section was flexible hose, were conducted to describe the effect of bending positions at both joints of vapour and liquid line of TCLT. The evaporator and condenser of TCLT were fabricated from copper tubing, while the vapour and liquid lines were fabricated from Teflon hose. R-134a refrigerant was used as a working fluid. The TCLT was examined with a constant heat flux ranging from 14.2 to 32.6 kW/m 2 . The condenser was cooled by cold water at about 20 °C. The effects of tilt angle and bending position on the TCLT thermal performance characteristics were investigated. The results showed that bending and tilting TCLT would decrease thermal performance compared to the TCLT vertical orientation without bending, Q c / Q 90 . However, the effect of bending and tilting would be diminished as the TCLT tilted at 75 degrees at an optimum heat flux of 28.2 kW/m 2 . [ABSTRACT FROM AUTHOR]

Given that loop thermosyphons are widely used as simple and high-efficiency heat transfer devices, their passive heat transfer performance has attracted considerable research interest. The capability of loop thermosyphons for active heat transfer adjustment can potentially improve the precision and efficiency of heat and temperature management in many fields, but has not received much attention. A variable conductance loop thermosyphon (VCLT) is developed in this study. Heat transfer is precisely adjusted by actively regulating the internal flow resistance of the working fluid. The performance of VCLT under variable flow resistances is tested. The boundary conditions are set to those of a novel cool-storage refrigerator, wherein temperature control directly affects the freshness of stored food. VCLT behaviors under static and dynamic adjustment modes are also examined with the working fluid R134a. The maximum ratio of heat transfer adjustment is approximately 85%, when decreases the mass flow rate from 1.67 g/s to 0.21 g/s with increasing the equivalent thermal resistance from 0.0074 K/W to 0.54 K/W. Finally, the performance is verified with R404a, R407a and R410a. Results demonstrate that VCLT is an efficient and low-cost temperature adjustment device for refrigerators and other applications that require precise temperature control. [ABSTRACT FROM AUTHOR]

A traditional spray cooling system usually requires a pump to circulate the fluid, and the kinetic energy of high-temperature steam is wasted. A two-phase loop thermosyphon (TPLT) which can circulate its working fluid is thermally driven. However, heat leakage may occur for low heating power. This paper proposes a compacted non-pump self-circulation spray cooling system named active two-phase loop thermosyphon (ATPLT). Dual synthetic jet integrated with spray cooling (DSJS) is used to enhance the performance of evaporator of ATPLT. Without an external pump, the waste heat and evaporation of liquid spray drive the system internal pressure to increase, which pumps the water to the reservoir as well as avoid heat leakage. It only needs a little working fluid and a little energy consumption for actuator, but can maintain hundreds of Watts of heat dissipation capability for a long time, which can facilitate the development of the energy systems. The performance of ATPLT is studied through temperature, pressure, laser particle size and particle image velocimetry experimental researches. The experimental results show that the cooling capability of ATPLT is mainly influenced by Re , We and Ja , and finally a correlation for ATPLT cooling is established with relative errors within ±18%. • A closed non-pump spray cooling system was designed. • The flow characteristics under different micropore diameters were obtained. • The spray droplet diameter distributions were obtained. • The circulation modes under different heating power were summarized. • A correlation for ATPLT cooling was established. [ABSTRACT FROM AUTHOR]

The loop thermosyphon was used in the heat dissipation of concentrating solar cells, to solve the problem of the high temperature of solar cells and their low efficiency. In this paper, a loop thermosyphon heat exchanger was designed and the effect of working fluid on heat transfer performance of loop thermosyphon was studied through experiments and the best working fluid was selected. The results show that the heat transfer effect of acetone as the working medium is better than that of water and ethanol. Acetone loop thermosyphon can be used for 7 times concentrating photovoltaic cells cooling, while the ethanol loop thermosyphon can be used for 3 times and the water loop thermosyphon only used for 1 time. The heat transfer characteristics of the fin with natural convection were studied by numerical calculation, and the fin structure was optimized. The influencing factors were analyzed by orthogonal analysis. [ABSTRACT FROM AUTHOR]

This study investigates the condensation processes in the vertical tube condenser of a loop thermosyphon. For visualization, a glass condensation tube with an untreated (static contact angle of 44°) or a hydrophobically-treated (static contact angle of 86°) inner surface is tested. To avoid the interference from unstable boiling, the evaporator is designed to operate with stable thin-film evaporation. The condensation process in the untreated glass tube is in a composite dropwise-film condensation mode. It was essentially dropwise condensation in the upper section. In the middle and lower sections, the condensing drops were intermittently swept down by the liquid streams falling from the upper section. The lower section was mostly covered with liquid films with circumferentially non-uniform thickness. The condensation mode in the hydrophobically-treated tube is dropwise condensation, for which the condenser resistances are less than for untreated glass surface by 1–2 °C-cm 2 /W. No nucleate boiling is observed in the evaporator undergoing stable, quiescent thin-film evaporation. The water levels in the evaporator are nearly the same for all heat loads. The evaporator resistances range from 0.28–0.14 °C-cm 2 /W for heat loads of 35.6–92.5 W. [ABSTRACT FROM AUTHOR]

Currently, large amounts of thermal energy dissipated from automobiles are emitted through hot exhaust pipes. This has resulted in the need for a new efficient recycling method to recover energy from waste hot exhaust gas. The present experimental study investigated how to improve the power output of a thermoelectric generator (TEG) system assisted by a wickless loop heat pipe (loop thermosyphon) under the limited space of the exhaust gas pipeline. The present study shows a novel loop-type heat pipe-assisted TEG concept to be applied to hybrid vehicles. The operating temperature of a TEG's hot side surface should be as high as possible to maximize the Seebeck effect. The present study shows a novel TEG concept of transferring heat from the source to the sink. This technology can transfer waste heat to any local place with a loop-type heat pipe. The present TEG system with a heat pipe can transfer heat and generate an electromotive force power of around 1.3 V in the case of 170°C hot exhaust gas. Two thermoelectric modules (TEMs) for a conductive block model and four BiTe TEMs with a heat pipe-assisted model were installed in the condenser section. Heat flows to the condenser section from the evaporator section connected to the exhaust pipe. This novel TEG system with a heat pipe can be placed in any location on an automobile. [ABSTRACT FROM AUTHOR]

Data centers require a set of uninterruptible cooling systems to operate. For uninterruptible cooling, latent thermal energy storage (TES) systems offer an alternative solution in the event of a power outage. In this study, a novel integrated cooling system comprising a loop thermosyphon and latent TES is proposed for emergency cooling. A water-cooled loop thermosyphon was used to absorb the heat dissipated from the servers. In addition, a multi-tube TES unit containing the paraffin RT11HC with a phase transition temperature of 10–12 °C was designed as a supplementary cold storage system. To evaluate the thermal characteristics of the emergency cooling system, an experiment was conducted by shutting down the chiller. The results showed that the integrated system could maintain the servers operational for approximately 6 min. Moreover, parametric analysis was performed to extend the emergency cooling time to 15 min. The analysis showed that as the thermal conductivity of the phase change material (PCM) increased to 2 W/(m·°C), the cooling capacity met the emergency cooling demand within 15 min. The proposed cooling system can act as a reference for the optimal design of emergency cooling systems in data centers. • The cooling system integrated a loop thermosyphon and a latent heat storage unit. • The novel system could reduce the risk of servers overheating under emergency. • Effects of PCM thermal properties on the thermal performance were analyzed. • Increasing PCM thermal conductivity could obviously prolong emergency cooling time. [ABSTRACT FROM AUTHOR]

Abstract: Non-condensable gas (NCG) generated inside two-phase heat transfer devices can adversely affect the thermal performance and limit the lifetime of such devices. In this work, extensive experimental investigation of the effect of NCG on the steady-state operation of an ammonia-stainless steel loop thermosyphon was conducted. In the experiments, nitrogen was injected into the loop thermosyphon as NCG, and the thermal performance of the loop thermosyphon was tested at different NCG inventories, heat loads applied to the evaporator and condenser cooling conditions, i.e. natural air cooling or circulating ethanol cooling. Experimental results reveal that NCG elevates the steady-state operating temperature of the evaporator, especially when the loop thermosyphon is operating in the low temperature range; meanwhile, the more NCG exists in the loop thermosyphon, the higher the operating temperature of the evaporator, and the lower the reservoir temperature. In addition, the existence of NCG results in the decrease of the overall thermal conductance of the loop thermosyphon, and the overall thermal conductance under the ethanol cooling condition may be even lower than that under the air cooling condition when the heat load is smaller than a certain value. Finally, the experimental results are theoretically analysed and explained. [Copyright &y& Elsevier]

• A dual-source passive building cooling system is innovatively designed for efficient cooling of industrial building. • The feasibility of coupling sky radiative cooling and direct evaporative cooling with loop thermosyphon is proved. • A cooling capacity of 398.3 ∼ 505.6 W is achieved at various outdoor boundary conditions. • The proposed dual-source cooling can efficiently mitigate the performance decline in air convective cooling caused by high ambient temperature. Passive cooling technique is an efficient way to avoid the high energy consumption of conventional air conditioner in heat dissipation of industrial buildings, in which loop thermosyphon has shown an attractive application prospect. But the normal loop thermosyphon cooler only relies on the air source heat exchange, and an obvious performance degradation will occur when the ambient temperature is high. In this paper, a new scheme combining sky radiative cooling and direct evaporative cooling for the passive condenser heat dissipation of loop thermosyphon is put forward. The characteristics of this dual-source passive building cooling system is simulative investigated. The effects of ambient temperature, ambient relative humidity, indoor air temperature, evaporator air velocity, and solar radiation intensity are discussed in detail to determine its thermal characteristics. The system exhibits a relatively stable heat exchange capacity of 398.3 ∼ 505.6 W at various outdoor boundary conditions and a preferable cooling performance under high ambient temperature in comparison with conventional air source loop thermosyphon cooler. The thermal analyses show that coupling of sky radiative cooling and direct evaporative cooling with a loop thermosyphon enables high cooling capability in normal working conditions to accomplish indoor thermal comfort. Results demonstrate effectiveness of the proposed system on achieving reliable, sustainable, and efficient passive cooling for industrial buildings. [ABSTRACT FROM AUTHOR]

The mini-diameter loop thermosyphon, which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the problem of heat transfer under high heat load in a confined space. The mini-diameter loop thermosyphon exhibits different gas-liquid transport and heat transfer patterns than the conventional-diameter one due to various gas-liquid phase transformation properties and a strong friction force in the mini-diameter channel. Furthermore, data on the visual analysis of gas-liquid transport and heat transfer properties of the mini-diameter loop thermosyphon under various filling ratios is still lacking. In this paper, a visible experimental research on a mini-diameter loop thermosyphon (3 mm) in a wide filling ratio range(44% -97%) is undertaken. The maximal heat transfer capacity increases initially and subsequently falls with the filling ratio, according to the testing data. At a low filling ratio, the flow pattern changes as slug flow - churn flow - annular flow with the heat load increase, leading to a low loop thermal resistance, and dry out occurs at 185 W heat load under 50% filling ratio. The gas-liquid flow shifts to slug flow-geyser boiling-bubble flow at high filling ratios (73% and 82%), and the bubble departure diameter steadily decreases with increasing heat load. Consequently, the heat transfer performance (thermal resistance) has been enhanced, and the maximum heat transfer capacity has been increased to 280 W. Single-phase flow is plainly seen in the loop when the filling ratio reaches 91% or even 97%, thus reduced heat transfer capacity. Therefore, the high-filling-ratio mini-diameter loop thermosyphon is better for high-load heat dissipation. However, a low thermal resistance shows at the low filling ratio, but the heat load is also low. As a result, a loop thermosyphon with a low filling ratio should be selected for the low heat load situation. [ABSTRACT FROM AUTHOR]

Abstract: Non-condensable gas (NCG) generated inside two-phase heat transfer devices can adversely affect the thermal performance and limit the lifetime of such devices. In this work, experimental investigation of the effect of NCG on the startup of an ammonia–stainless steel loop thermosyphon was conducted. In the experiment, nitrogen was injected into the loop thermosyphon as NCG. The effect of NCG inventory on the startup behavior was investigated by adjusting the injected amount of nitrogen. The experimental results reveal that NCG prolongs the startup time and increases the startup liquid superheat and temperature overshoot; the more NCG exists in the loop thermosyphon, the higher the liquid superheat and temperature overshoot. When NCG is present in the system, boiling usually occurs in the evaporator before startup, but it does not mean the system will start up instantly, which differs from the conditions without NCG. Under all the conditions, increasing the heat load can effectively shorten the startup time but leads to a large temperature overshoot; forced convection cooling of the condenser has almost no effect on shortening the startup time especially for large NCG inventory situations, but it can effectively limit the temperature overshoot. For large NCG inventory situations, the loop thermosyphon can start up at a small heat load (5 W) or even without a heat load when the condenser is cooled by forced convection of ethanol. No failed startups occurred during any of the tests. [Copyright &y& Elsevier]

Abstract: A new water-based SiO2 functionalized nanofluid was used in a loop thermosyphon as the working fluid. Functionalized nanofluid has unique dispersing ability and has no deposition on heated surfaces. The flow evaporating heat transfer characteristics of functionalized nanofluid in a loop thermosyphon were investigated to understand the sole effect of thermal properties of nanofluid on the heat transfer characteristics of the loop thermosyphon, regardless of the effects of heated surface characteristics. Results show that: functionalized nanofluid deteriorates both the evaporating heat transfer coefficient and the maximum heat flux of the loop thermosyphon. The changes in the thermal properties of functionalized nanofluid mainly result in the heat transfer deterioration. No especial nano-scale effect is found in the present study. [Copyright &y& Elsevier]

Loop thermosyphon system is an efficient cooling system for energy saving in data center. It is necessary to investigate the performance of the loop thermosyphon system filled with alternative refrigerants to prompt its application. In this paper, a R32 loop thermosyphon system is experimentally investigated in an enthalpy difference laboratory for its performance. R22 and R134a are also used for comparison. The results show that the optimal filling ratio of R22, R134a and R32 is the same (37.3%). At the optimal filling ratio, the flow stability of R32 is obviously better than those of R22 and R134a. When cold water temperatures are 10–22 °C, the cooling capacity of R32 is 5.4–15.6% higher than those of R22 and R134a. After controlling extractor fans, the energy efficiency ratio of R32 is 4.9–206.6% higher than those of R22 and R134a. Moreover, the free cooling potentials of the three refrigerants in five typical climate cities across China are analyzed and compared. Compared with R22 and R134a, R32 has 3.6–11.0% and 18.5–26.7% higher free cooling percentages and annual energy efficiency ratio, respectively; and its annual average partial power usage effectiveness is 0.0012–0.0017 lower than that of other two refrigerants. [ABSTRACT FROM AUTHOR]

With the development of big data and cloud computing technologies, data centers have grown rapidly worldwide. Although the commonly used data center cooling system (computer room air conditioning system, CRAC) is safe and reliable, it has a high energy consumption. Loop thermosyphon consumes less energy than the CRAC system and can be used in data center cooling system. This review presents the operating principles of the loop thermosyphon and its application in a data center cooling system. First, this review introduces the types of heat pipes and the operating principles of the loop thermosyphon. Second, the application of the loop thermosyphon in the room-, rack- and on-chip-level cooling of data center is reviewed. Finally, a combined compressor and loop thermosyphon system is introduced, which can be used to cool telecommunications base station. As the system can use cold ambient air directly as heat sink, it is more energy-efficient in comparison with using air conditioning alone as the cooling method. After reviewed and compared recent articles, we have indicated the imperfect points in recent researches and the future's development trend based on the results of researches and our experience. We think the points of this review is useful to the industrial applications. • The operation principle of the loop thermosyphon and the flow boiling heat transfer mechanism have been reviewed. • The application of the loop thermosyphon in data center cooling has been introduced, and it has better energy saving effect. • A combined compressor and loop thermosyphon systems by different researchers has been introduced and compared. • Choosing better coolants, using cold ambient air and on-chip level cooling will become the future development trend. [ABSTRACT FROM AUTHOR]

• Thermodynamic model of novel variable conductance loop thermosyphon was set up. • Parametric heat transfer characteristics are reported for the first time. • Technology is proven feasible for energy-efficient space conditioning in buildings. • Detailed thermal responses obtained are essential for precise valve regulation. Variable conductance loop thermosyphon (VCLT) manipulates natural phase-change cycle to regulate the heat transfer. Its primary advantages include high sustainability, simple design and low cost. One of the potential applications of variable conductance loop thermosyphon is thermal control in buildings for achieving high energy efficiency. In this study, a distributed steady-state model was implemented to determine the heat transfer control characteristics of variable conductance loop thermosyphon for the first time and evaluate its effectiveness on precise air-conditioning for buildings. The internal flow resistance rises from 0.002 K/W to 0.305 K/W and the heat transfer rate decreases from 468.5 W to 71.9 W when the relative opening degree of the regulating valve reduces from 1.00 to 0.17 under normal boundary conditions. The thermodynamic analyses show that the regulating valve of the variable conductance loop thermosyphon can enable effective thermal control over a wide range of heat transfer rate to accomplish indoor thermal comfort. The study also reveals that variable conductance loop thermosyphon can be effectively adopted with various working fluids and over wide ranges of heat source and heat sink temperatures. [ABSTRACT FROM AUTHOR]

• A new design of a loop thermosyphon was presented and tested. • Geyser Boiling occurrence was eliminating in the loop thermosyphons with flat evaporators. • The used of wick structures on a vertical internal wall of flat evaporator eliminates Geyser Boiling. • Mathematical model was developed to predict loop thermosyphon thermal behavior. This work presents theoretical and experimental analysis of a new loop thermosyphon, which evaporator has a flat box shape, manufactured by diffusion bonding, designed to promote passive thermal control of avionics. Transient start-up and steady state conditions were considered in this study. The thermal performance of the device was experimentally accessed, with special attention to the Geyser Boiling phenomenon, which is undesirable for aeronautics applications. The phenomenon was eliminated by covering one of the evaporator internal walls by a wick structure. An electrical circuit analogy model was proposed to estimate the overall thermal resistance and the temperature distribution, which depends of the thermal load position in the evaporator external wall. Theoretical predictions were compared favorably with wicked evaporator. [ABSTRACT FROM AUTHOR]