Loop Thermosyphon Research Articles

Heat transfer performance of a multiport mini-channel loop thermosyphon for cooling miniaturized electronic devices

Multiport mini-channel thermosyphon with hydraulic diameters less than 2 mm undergoes a severe entrainment problem both in vapor and liquid regions, impeding the flow and affecting the performance characteristics. To address this issue, a novel multiport mini-channel loop thermosyphon (MPMC-LT) design is proposed. This design separates the vapor and liquid flow paths, reducing entrainment and enhancing the performance characteristics. Tests are conducted with heat loads ranging from 5 to 45W at four inclinations (0°, 30°, 60°, and 90°) relative to the horizontal plane. Results revealed that the MPMC-LT design extends the entrainment limit by 10.6 times compared to conventional designs, reducing the interaction between the vapor and condensed working fluid. Additionally, the total entropy and thermal resistance drops by 20.7 % and 19.9 %, respectively at an optimum inclination angle of 30°. It is also noted that the effective thermal conductivity at the optimal inclination is 5.8 times higher compared to the conventional multiport thermosyphon designs. The design effectivity uses the buoyancy, inertial and surface tension forces to reduce resistance in the fluid flow, thereby enhancing the heat transfer characteristics. These results highlight that MPMC-LT is a suitable thermal management device for power electronic systems.

Experimental study on a pump-driven CO2 two-phase thermosyphon loop

In order to better understanding the internal operating principles of liquid pump-driven two-phase thermosyphon loop (LPTPTL) and promote the application of environmentally friendly working fluids, a LPTPTL using CO2 as the working fluid was studied through experiment. Both the operating mechanisms and performance of the CO2 LPTPTL was investigated. It was found that the impact of the pump on the two-phase thermosyphon loop (TPTL) varies significantly across different operation stages. During the oscillatory operation stage, activating the liquid pump eliminated the geyser boiling in the loop and made the TPTL to transition from oscillatory to stable operation. During the normal operation stage, the pump power had little effect on the total thermal resistance of the TPTL. While the system’s EER (Energy Efficiency Ratio) significantly decreased with the increasing pump power. During the overload operation stage, activating the liquid pump helped alleviate overheating or subcooling in the loop. With the pump power increasing from 3.0 W to 18.8 W, the heat transfer limit of the TPTL increased from 1800 W to 3300 W, while the system’s EER decreased from 600 to 176. This study proves the feasibility of CO2 LPTPTL, and provides theoretical guidance for its real application.

Experimental investigation on the thermal performance of high temperature two-phase loop thermosyphon

High-temperature two-phase loop thermosyphons (TPLTs) have the potential to be applied in high-temperature thermochemical conversions, concentrated solar energy, and nuclear energy. However, there has been limited investigation on the thermal performance of TPLTs so far. This work experimentally examines the thermal performance of TPLTs using alkali metals (NaK alloy and pure Na) as working fluids, aiming for internal energy recovery of a double-riser reactor. The effects of filling ratios (FRs), heating temperature, and altitude difference between the evaporator and condenser are investigated regarding start-up characteristics and steady-state performance. The experimental results show that the TPLT with NaK as the working fluid can successfully complete the start-up when the heating temperature is set to above 650 °C under the test conditions. However, the TPLT using pure Na as the working fluid can't successfully complete the start-up from a frozen state without a preheating operation. According to the variations in thermal resistance, an FR of 38% in the TPLT allows for the best heat-transfer performance under steady state when compared to the conditions that FRs are equal to 25% and 50%. Additionally, placing the condenser slightly higher than the evaporator is beneficial for improving the steady-state performance. However, if the condenser is lower than the evaporator, its steady-state performance is remarkably degraded.

Instability characteristics and heat transfer of a separator assisted dual-evaporators loop thermosyphon under heating power maldistribution condition

Thermosyphon loop with multiple evaporators has been widely used in the data centers thanks to its compact structure and local hot spot avoiding. However, under the heating power maldistribution condition, on account of the separation and convergence of flow steams from different evaporators, the instability of the system can be exacerbated which leads to premature failure and damage of servers. Thus, a separator assisted loop thermosyphon with dual evaporators had been proposed to improve its heat transfer performances and stability. An experimental study of the instability characteristics and heat transfer of the system are carried out under different filling ratios, cooling water temperatures and heating powers in this paper. To fully understand the interaction mechanism of two evaporators behind the oscillation, the visualization of flow patterns in the liquid tube and two-phase tube are observed as well. Results showed that during the oscillatory operation, since the evaporator2 (high heat load) circuit will circulate first, the variation of fluid temperature at evaporator1(low heat load) outlet lags others. Besides, when the inlet water temperature increases, the operating states of the system transfer from oscillatory operation to stable operation. When the filling ratio varies from 50 %-70 %, instability occurs in the system, the oscillation period increases from 115 s to 911 s and amplitude increases from 2.5 °C to 11.1 °C with an increase of the filling ratio. It should be noted that in our new system, it still can operate stably even when the heating power difference between two evaporators are as large as 350 W. In addition, alternatively oscillation in the two evaporators, maximum heat transfer coefficient of evaporator2 and minimum heat transfer coefficient of evaporator1 can be found when the heat load varies due to the mass competition between two circuits. According to above results, it is expected that the system could be beneficial to the application in the data center cooling.

Numerical study and assessment of a novel combined passive cooling system for tank-in-pool reactors

To evaluate the safety and reliability of a novel combined passive cooling system for tank-in-pool reactors during Station Black-Out (SBO) accidents, a numerical analysis employing ANSYS FLUENT is conducted. A model including the Reactor Pool (RP) and the Spent Fuel Pool (SFP) is established. The porous media approach and User Defined Function (UDF) are utilized to simulate the thermal–hydraulic processes of the coupling system of the primary loop, Passive Residual Heat Removal Heat Exchanger (PRHR HX), Two-Phase Loop Thermosyphon (TPLT), and spent fuel. The principal thermal–hydraulic parameters such as thermal power, heat transfer coefficients, temperature, fluid velocity, and mass flow rate are obtained via numerical computations. The results demonstrate that the system can effectively cool the reactor and spent fuel, preventing saturation boiling within pools. Consequently, this study proves its feasibility and potential advantages for tank-in-pool reactors, thereby providing important theoretical substantiation for reactor design and cooling system optimization.

Simulation of a two-phase loop thermosyphon using a new interface-resolved phase change model

The remarkable progress made in power electronics has been coupled with the miniaturization of systems, requiring high-performance cooling. A solution that has garnered significant interest involves the use of two-phase loops, in which heat transfer is associated with the phase change of the working-fluid. This paper aims at modeling two-phase flow with separated phases in a gravity-driven two-phase loop. The Volume-of-Fluid method is employed to capture the moving interface. Amid the variety of the phase change models available in the literature, we have chosen to employ a hybrid model: a subgrid-scale model to generate phase change at the wall in the absence of an interface, and a interface-resolved model for phase change at pre-existing liquid-vapor interfaces. These models have been implemented in the OpenFOAM computational code. The developed solver makes possible the simulation of compressible laminar two-phase flow with diabatic liquid-vapor phase change. Conjugate heat transfer between the wall and the fluid is also taken into account. Two-dimensional numerical simulations demonstrate that the developed solver is capable of reproducing flow regimes obtained from experiments, including the formation of Taylor bubbles and the propulsion of liquid from the evaporator to the condenser during geyser boiling. Furthermore, this study examines how the filling ratio affects flow regimes and performance.

Cooling of Electronic Components by an Aluminum Loop Thermosyphon Containing Two Evaporators and One Condenser

Cooling of Electronic Components by an Aluminum Loop Thermosyphon Containing Two Evaporators and One Condenser

An experimental evaluation of the performance of a remote 2U loop thermosyphon

In recent years, the increasing application and diversity of electronic systems have resulted in a significant demand for research and development in electronic cooling. Designing and evaluating thermal modules is crucial, especially for systems generating high heat flux levels. This study proposes a novel remote loop thermosyphons (RLTS) for high heat flux applications and cooling 2U servers, which has been assessed experimentally. Additionally, several critical factors are examined, including the filling ratio (FR), air flow rate, and heat load. The experimental study evaluated air flow rates range from 130 to 170 cubic feet per minute (CFM), and the filling ratios span from 40 % to 70 %. The heat load varied from 100 W to 700 W. Compared to the conventional loop thermosyphon, the remote loop thermosyphon exhibited superior performance and sustained higher heat flux. Specifically, this thermal module can effectively manage 640 W at an inlet air temperature of 28 °C with an airflow rate of 170 CFM. Moreover, compared to other available loop thermosyphons, the proposed thermal module demonstrated enhanced heat flux up to 94 W/cm2. These results suggest that the new remote loop thermosyphon is a promising solution for cooling high heat flux applications, such as 2U servers. Furthermore, among all the investigated FR effects the 50 % FR performed better than the other filling ratios. At this filling ratio, the thermal resistance reached a minimum value of 0.078 K/W. The findings indicated that increasing the heat load decreased thermal resistance to a certain threshold, beyond which thermal resistance started to increase. This increase in thermal resistance was caused by restricted space above the evaporator. The confined space effect in a loop thermosyphon system significantly influences the performance and should be considered as a crucial factor in future design endeavors.

Heat transfer enhancement in a loop thermosyphon with microencapsulated phase change material suspension

The microencapsulated phase change material suspension (MPCMS) is a novel functional thermal fluid. The application of MPCMS to loop thermosyphon is an innovative attempt, which can effectively improve the heat transfer efficiency of loop thermosyphon. In this paper, a three-phase loop thermosyphon using water-based MPCMS as the medium is developed. A comprehensive experimental investigation was conducted to study the heat transfer performance by varying the filling ratio (50 %, 60 %, and 70 %), heat flux (25 − 125 W/cm2), and mass concentration (0 %, 0.1 wt%, and 1 wt%). The findings of this study indicate that the loop thermosyphon incorporating MPCMS exhibits improved heat transfer performance compared to a two-phase loop thermosyphon utilizing pure water. The addition of MPCMS has been observed to reduce the wall temperature by up to 6.6 °C and decrease the loop thermal resistance by 13.5 %. Furthermore, the loop thermosyphon with MPCMS demonstrates superior start-up characteristics and thermal homogeneity when compared to water-based systems.

Dynamic thermal response characteristics of a metal foam enhanced horizontal loop thermosyphon

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.

Simulation on the performance and cooling capacity compensation solution of a loop thermosyphon system under fan failure conditions

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.

Experimental research and energy effectiveness analysis on chip-level two-stage loop thermosyphon system for data centre free cooling

The development of 5G technology has put forward new requirements for computing the abilities of chips, and chip power has also significantly increased. Consequently, the heat load of the rack will increase rapidly in the future. Traditional computer room air conditioning (CRAC) systems cannot satisfy the rack’s high heat flux dissipation requirements, and new cooling forms such as chip-level cooling should be adopted. In this study, a two-stage chip-level cooling system, which is pumpless and waterless, offers flexible and detachable installation based on practical requirements, was proposed. The cooling system removes heat from the chips to the outdoors via two thermosiphon loops, where the heat is first transferred to cold plates tightly attached to the chip surfaces, then transferred through heat exchangers attached to the rack side, and finally dissipated outdoors through air cooling. In this study, a two-stage chip-cooling system was constructed and tested in an enthalpy-difference laboratory. The effects of the liquid filling ratio on the thermal resistance, total temperature difference, and fluid flow pattern under different heat loads were investigated. Second, a heat transfer model for analysing thermal resistance networks was constructed, and the proportions of different types of thermal resistance were determined. Third, an energy-saving analysis of the system is established. The experimental results showed that at single chip heat load 200 W and total heat load 2 kW, the heat transfer temperature difference between chips and outdoors is less than 20 °C at extremely high energy efficiency (cooling load factor reaches 0.05 at outdoor temperature 40 °C), which makes it feasible for free cooling throughout the year in most Chinese areas. The annual PUE in Beijing using the proposed system is 1.15, which indicates a 30 % reduction of the total energy consumption of data centres.

Development of transient thermal-hydraulic analysis model for the two-phase loop thermosyphon

To comprehensively and accurately simulate and predict the flow and heat transfer characteristics of the two-phase loop thermosyphon (TPLT), a two-fluid two-phase model was developed. A six-equation set was formulated to account for two-phase flow, encompassing mass equations, momentum equations, and energy equations. The interactions between the fluids in terms of mass, momentum, and energy were also taken into account. To obtain numerical solutions for the partial differential equations within a staggered mesh framework, we employed the 2nd-order Lax-Wendroff scheme to obtain the discrete linear system, complemented by the inclusion of a flux limiter. The Semi-Implicit Method for Pressure-Linked Equation (SIMPLE) algorithm was employed to address the solution of the linear equation system. Data from a small-scale TPLT test was used to assess the accuracy and reliability of both the physical models and the numerical method proposed in this study. Simulations were performed under two working conditions of the thermosyphon with filling ratio of 45 % and 64 %. The simulation results exhibited good agreement with the experimental data, demonstrating the accuracy of the model. Furthermore, the model is capable of providing information on the complex two-phase flow inside the TPLT that cannot be measured experimentally.

Experimental study on operating characteristics of a controllable CO2 two-phase thermosyphon loop

To improve the performance and applicability of the two-phase thermosyphon loops (TPTL) used in data centers, TPTLs with auxiliary devices were widely studied. In the existing literature, the regulating valve on TPTL is mainly used for heat transfer rate regulation or start-stop control. This paper proposes a controllable CO2 TPTL with a valve on its downcomer to improve the practicability of the TPTL under low-heat-load conditions. It was found that valve regulation could widen the normal working load range (NWLR) of a TPTL. When the valve was fully open, the NWLR of the CO2 TPTL was 700–1300 W. By regulating the valve, the NWLR was expanded to 340–1300 W without changing the structure of the TPTL. Increasing the flow resistance could change the operating state of the TPTL by affecting the vapor–liquid distribution. During the oscillatory operating stage, when the valve opening decreased from 90° to 30°, the TPTL changed from an oscillatory to a stable operating state. In practical applications, the valve regulation is required under low load conditions, where the changes in valve opening have minimal impact on the thermal resistance of the TPTL.

Experimental study on a loop thermosyphon with microencapsulated phase change material suspension

Microencapsulated phase change material suspension (MPCMS) represents an innovative category of functional thermal fluids. This novel working fluid not only preserves the substantial energy density and high latent heat of phase change materials (PCM), but also mitigates the issues related to PCM, including susceptibility to aggregation and low thermal conductivity. This article selects phase change microcapsules with a phase change temperature of 70 °C, and uses pure water as the base liquid to prepare MPCMS as the working fluid for the loop thermosyphon. A series of heat transfer experiments are conducted, and the results are compared with those of pure water experiments. A 135mm*650mm copper loop thermosyphon, is designed and constructed to investigate the effect of various input power on the heat transfer performance. The results show that the addition of MPCMS can reduce the wall temperature by up to 2.9°C and the loop thermal resistance by 6.3%. Compared with water, the loop thermosyphon with MPCMS has better start-up characteristics. The performance of the MPCMS is affected by various parameters, which are interconnected. Particles in close proximity to the wall display erratic movement, fluctuating across different temperature zones, thereby undergoing a continuous cycle of melting and solidification. This study establishes a basis for further investigation into the practical implementation of MPCMS in industries.

Visualized-experimental assessment on a two-phase thermosyphon loop downcomer state with wide range of filling ratios

The two-phase thermosyphon loop (TPTL), which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the heat transfer problem under high heat flux. It is necessary to optimize the TPTL to operate in good conditions, especially the state of the downcomer in which the liquid head is one of the critical parameters to determine the driving force of the circulation. However, in the existing studies, the flow features under high filling ratio and high heat flux were ignored in the downcomer. Since a high filling ratio has been proved to have good performance at high heat flux. Herein, the flow characteristics of the TPTL downcomer were seen and quantified using an experimental visual setup under different filling ratios in this study. Two flow features under high heat flux, for the first time, are reported: 1. two-phase flow in the downcomer at high filling ratios. 2. liquid column height oscillation under a partially liquid-filled state. According to the results, the filling ratio significantly impacts the flow characteristics of the downcomer. The TPTL downcomer state, contrary to the traditional understanding, can be partially filled with liquid but not totally filled with fluid all the time, implying that the driving force may be less than the typical assumption. TPTL with a high filling ratio can also provide a maximum gravity pressure head driving force due to the full downcomer. In addition, the bubble pump, which influences by the heat flux, also accelerates the mass flow in the TPTL. Consequently, TPTLs with a high filling ratio showed a significant thermal performance. A partially filled with the liquid phenomenon is demonstrated in moderate filling ratios, and the height of the liquid column will change as the heat flux increases, which indicates the self-regulation of TPTL. When the filling ratio is low, the liquid column height is greatly influenced by the filling ratio. The high liquid column will also give the TPTL excellent thermal performance. The ideal state of a TPTL downcomer is the downcomer filled with two-phase liquid under a high void fraction, which is realized by a high filling ratio TPTL. This paper sheds some light on the downcomer state of TPTLs to optimize the TPTL thermal performance.

Experimental study on an integrated system of thermosyphon loop and vapor compression assisted with phase change material for pulse heat load

In order to safely and energy-efficient handle the pulse heat load of avionics, the paper proposes an integrated system of thermosyphon loop and vapor compression cycle, especially assisted with phase change material (PCM). Under low heat load and heat sink temperature, the system operates as a thermosyphon loop (TL) for energy saving; otherwise as a vapor compression cycle (VCC) for safety. A prototype of the integrated system was developed, where a whole three-dimensional (3D) printed structure worked as the evaporator of the system, consisting of a mini-channel cold plate and a high thermal-conductivity matrix containing PCM. The dynamic performance of the proposed system has been experimented under different pulse heat loads and heat sink temperatures, and compared to the integrated system without PCM and a traditional VCC. The proposed system can handle a peak heat load of 1000 W for 120 s without running the compressor, saving 100 % energy compared to traditional VCC. The PCM enhances both the safety and energy efficiency of the integrated system. On the one hand, it reduces wall temperature during the transition from TL to VCC, avoiding overheating issue. On the other hand, it reduces the operating time of VCC, which is more significant with lower peak heat load and heat sink temperature. Overall, the integrated system assisted with PCM is a promising solution for solving pulsating heat loads.

ANALYSIS OF THERMAL PERFORMANCE IN A TWO-PHASE THERMOSYPHON LOOP BASED ON FLOW VISUALIZATION AND AN IMAGE PROCESSING TECHNIQUE

Thermal performance was analyzed based on flow visualization and an image processing technique in a two-phase thermosyphon loop (TPTL) with boiling water as the working fluid at low pressure. The bubble geometries, bubble frequency, and dynamic void fraction were estimated using direct image analysis combined with the power spectrum and statistical analysis. At a heating rate of 400 W and a filling ratio of 0.88, the thermal performance of the TPTL was enhanced with a thermal efficiency of 91% and effective thermal conductivity of 11,336 Wm<sup>-1</sup> °C<sup>-1</sup>. The enhancement was due to the higher frequency cap bubble flow of 11.2 Hz obtained by the direct flow visualization. The bigger Taylor bubbles with slug flow were examined to contribute to the negative effect on the heat transfer rate due to their film boiling regimes. The detailed analysis reveals the mechanism of bubble flow interacting with thermal performance.

Experimental Study on the Small Two-Phase Thermosyphon Loop With Minichannel Evaporator

Abstract This study experimentally investigates the small two-phase thermosyphon loop with a minichannel evaporator. The influences of heating power, fan power, and inclination angle on the heat transfer performance are presented, and the related mechanisms are revealed. Results show that the thermal resistance of the thermosyphon loop tends to decrease as the heating power is increased. It is found that there exists an optimal value of fan power. Interestingly, the invalidity behavior of thermosyphon under the inclination condition prefers to occur at higher heating power. To sum up, the small two-phase thermosyphon loop with a minichannel evaporator could produce excellent heat transfer performance with energy-saving characteristics, indicating its wide potential applications in the field of electronic chip cooling.

Loop Thermosyphon for Cooling Heat-Loaded Electronics Components

Loop Thermosyphon for Cooling Heat-Loaded Electronics Components