Two-phase Loop Thermosyphon Research Articles

Impact of working fluid properties on heat transfer and flow characteristics of two-phase loop thermosyphon with high filling ratios

The two-phase loop thermosyphon (TPLT), known for its excellent heat transfer performance, simple and compact structure, and lack of need for a pump, effectively addresses heat transfer challenges in confined spaces under high heat loads.Compared to TPLT with low filling ratio, high-filling-ratio TPLT not only exhibit a higher maximum heat transfer capacity but also has a more complex heat and mass transfer process, leading to increased sensitivity to the working fluid's properties. Therefore, studying the impact of the working fluid on the operational state of high-filling-ratio TPLTs is crucial for understanding their heat transfer mechanisms. In this paper, comprehensive experiments were conducted on TPLT filled with H2O and R134a as working fluids in a wide filling ratio range (30 % -90 %), and their heat transfer performance and flow characteristics were compared. Heat transfer diagram, two-phase flow pattern diagram, and the distribution of gas-liquid two-phase of the TPLT was established with different filling ratio and heat input. Due to differences in latent heat of vaporization, the maximum heat transfer capacity of the H2O-TPLT (390 W/cm2) is greater than that of the R134a-TPLT (270 W/cm2). In the H2O-TPLT, the predominant large-volume slug flow leads to significant flow resistance. Whereas in the R134a-TPLT, the flow pattern is primarily dominated by small-volume bubbly flow and churn flow, resulting in low resistance. High viscosity and flow pattern in the H2O-TPLT cause oscillation phenomena, leading to significant temperature and pressure fluctuations. Under high filling ratio, both types of TPLT experiences geyser boiling phenomena causing periodic temperature and pressure fluctuations and flow pattern changes. In summary, R134a is the preferred working fluid when heat transfer requirements are met, as it effectively reduces temperature fluctuations while dissipating heat. When exceeding the R134a-TPLT's maximum heat transfer capacity and less stringent temperature control is acceptable, H2O may serve as the working fluid.

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.

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.

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.

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.

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.

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.

Investigation of the thermal response characteristics of the loop thermosyphon under convective boundary conditions

The two-phase loop thermosyphon (TPLT) is a highly efficient passive heat transfer device that is widely applied in various fields. This paper experimentally and numerically investigates the thermal response performance of a large-scale TPLT under various convective boundary conditions. Experimental studies have been carried out to explore the effects of different working fluids, filling ratios, cold and heat source temperatures, and mass flow rates on the thermosyphon’s performance. Meanwhile, a two-dimensional Computational Fluid Dynamics (CFD) simulation model of the TPLT is established and validated. The results indicate that the start-up procedure of the TPLT is highly dependent on the heat source temperature and the working fluid. Normally, the TPLT can start when the heat source reaches a certain temperature. In general, using methanol as the working fluid is more favorable to start-up compared to the water and ethanol TPLTs. The experimental results also show that the heat transfer capacity of the TPLT increases with increasing heat source temperature while increasing cooling temperature decreases heat transfer capacity. The best heat transfer performance of a TPLT charging methanol is observed at a filling ratio of 70 % with a heating mass flow rate of 1 L/min. At that point, the convective heat transfer coefficient of outside the evaporator is 1036.7 W/(m2·K). Moreover, the simulation results reveal that the average wall heat flux of the TPLT’s evaporator initially decreases and then gradually stabilizes with time. During stable operation, the wall heat flux density of the evaporator exhibits an initial increase in the pool section followed by a subsequent decrease with increasing wall height. This work provides insight into the heat and fluid flow characteristics of the TPLT under convective boundary conditions.

Assessing the enhancements in thermal performance of two-phase thermosyphon loops through riser wettability optimization

The behavior of vapor-liquid two-phase transport within the riser of the two-phase thermosyphon loop significantly impacts the heat transfer process from the evaporation to the condensation, as well as the circulation velocity, thus playing a crucial role in the heat transfer performance. However, existing research has predominantly focused on optimizing the performance of the evaporator, overlooking the optimization of vapor-liquid two-phase flow patterns and transport characteristics within the loop. This paper aims to optimize the heat transfer performance of the two-phase thermosyphon loop by controlling the wall wettability of the loop. The strategy employed involves modifying the wettability of the riser to regulate the two-phase flow within the loop, while visualization techniques are utilized to investigate the influence of riser wettability on the vapor-liquid two-phase distribution and heat transfer performance of the two-phase thermosyphon loop. The findings reveal that the impact of wall wettability on flow behavior and heat transfer performance within the loop varies significantly with changes in the filling ratio. This paper sheds some light on the riser wettability of two-phase thermosyphon loops to optimize the thermal performance.

Performance analysis of a separator assisted dual-evaporators two-phase thermosyphon loop under heating power maldistribution condition based on an experimentally validated model

Multiple evaporators two-phase thermosyphon loop is typical pattern which is utilized in the data centers. Under heating power maldistribution condition, the system could suffer in the temperature oscillation and performance degradation which limit its application. To solve this problem, a separator assisted dual-evaporators thermosyphon loop (SDTPTL) is evaluated and compared with traditional dual-evaporators thermosyphon loop (DTPTL) by using experimentally validated model. The simulation results show that the SDTPTL can always achieve better performances than the DTPTL at any operating conditions. Evaporator outer wall temperatures of two evaporators in the SDTPTL are lower than that in the DTPTL about 2.2––5.9 °C and 3.8–8.2 °C when heat load of evaporator2 (high heating power) ranges from 0.8 kW to 1.4 kW, and lower about 1.6–4 °C and 1.7–4 °C when heat load of evaporator1 (low heating power) ranges from 0.1 kW to 0.5 kW, respectively. Besides, compared with the DTPTL, the SDTPTL has a better benefit in evaporator outer wall temperatures at higher heat load and working fluid charge. In addition, to further improve the performances of the SDTPTL, the sensitivity analyses about structure configurations of the system are conducted as well. Results show that the system can achieve better performances at lower separator height. The evaporator outer wall temperatures of two evaporators can be decreased about 57.4 °C and 3.6 °C when separator height reduces from 0.8 m to 0.2 m. It is expected that this new cycle will be beneficial to develop dual evaporators thermosyphon loop under heating power maldistribution condition.

Experimental study of a two-phase thermosyphon loop with a horizontal parallel tube evaporator under non-uniform heating conditions

Experimental study of a two-phase thermosyphon loop with a horizontal parallel tube evaporator under non-uniform heating conditions

Experimental study on the thermal performance of a novel two-phase loop thermosyphon under low-heat-flux conditions

Two-phase loop thermosyphons are efficient heat transfer components that do not require extra power and can effectively transfer heat over long distances. In this study, two kinds of novel ethanol loop thermosyphons with and without wicks inside their evaporators were fabricated to experimentally investigate the start-up characteristics and thermal performance of the systems under low-heat-flux conditions and with different filling ratios; in addition, there was no height difference between the evaporator outlet and condenser inlet. As a result, temperature overshoot and geyser boiling were evidenced as start-up phenomena specific situations. Both systems failed to transfer heat when the filling ratio was below 30 % because some temperature measurement points were higher than 100 °C, at which point the limits of the experiment were reached. When the heat input was higher than 10 W, the loop thermosyphon with a wick structure could run and stay stable, while the regular loop thermosyphon system stabilized when the heat input was higher than 25 W. The increase in the filling ratio worsened the uniformity of the temperature distribution in the evaporator. The thermal resistance of the system was at its minimum when the filling ratio was 70 %. Both systems had good heat transfer performance when the heat input was high enough.

Conceptual design and analysis of a combined passive cooling system for both reactor and spent fuel of the pool-vessel reactor system

A novel combined long-term passive cooling system is proposed for the pool-type reactor, aiming to enhance the safety and reliability of the entire system during extended station blackout accidents (SBO). This passive cooling system primarily consists of a natural circulation primary loop, a C-type heat exchanger submerged in the pool, the reactor pool (RP) and the spent fuel pool (SFP), and a two-phase loop thermosyphon (TPLT) cooling subsystem. The TPLT cooling subsystem comprises a separated evaporator and condenser, with the evaporation bundle immersed in the SFP and the condensation bundle placed within a dry air cooling tower. Three typical design configurations were selected for numerical simulation and performance evaluation, including C-type tube bundles placed inside the RP and TPLT evaporation bundles placed inside the SFP, C-type tube bundles and TPLT evaporation bundles both placed inside the SFP, and C-type tube bundles and TPLT evaporation bundles both placed inside the RP. The analysis results demonstrate that this combined passive cooling system can effectively achieve passive cooling of the reactor and spent fuel under accident conditions, mitigate the rise in reactor coolant temperature, and prevent saturation boiling in the RP and SFP. Furthermore, different system layout configurations have a significant impact on its operational characteristics, allowing for rational optimization and improvement based on specific safety requirements of the system.

Energy-saving potential analysis of a CO2 two-phase thermosyphon loop system used in data centers

Energy-saving potential analysis of a CO2 two-phase thermosyphon loop system used in data centers

Two-phase thermosyphon loop with different working fluids used in data centers

The two-phase thermosyphon loop (TPTL) is an effective cooling technology used in data centers. TPTLs filled with different working fluids typically have various operating characteristics that influence their practical applications. A comparative experiment was conducted on CO2, R134a, and R410A TPTLs, and differences in their operating states and thermal performances were analyzed. The optimal filling ratio of the CO2 TPTL was 45%, while that of the R134a and R410A TPTLs was 35%. At their optimal filling ratios, all the three TPTLs successively underwent the pre-start, oscillatory, and stable operation stages as the heat transfer quantity increased. The oscillations of the CO2 TPTL were periodic, whereas those of the R134a and R410A TPTLs were irregular. The normal working load ranges of the CO2, R134a, and R410A TPTLs were 550–1200, 760–1160, and 790–1300 W, respectively. When the heat transfer quantity was 1000 W, the driving temperature difference of CO2 TPTL was 2.4 °C lower than that of R410A TPTL and 6.0 °C lower than that of R134a TPTL.

Heat transfer characteristics and operational visualization of two-phase loop thermosyphon

The advantages of high-efficiency and pumpless long-distance heat transport make the two-phase loop thermosyphon (TPLT) a promising candidate for various energy- and thermal-related applications. In this work, the simultaneous temperature/pressure measurement and high-speed visualization observation were conducted to compare and evaluate the heat transfer performance and two-phase flow characteristics of a vertical-positioned TPLT having an inner diameter of 10 mm. R141b was used as the working fluid at volumetric filling ratios of 35–50 %. The TPLT functioned well and had high heat transfer capacity at relatively high filling ratios (⩾ 40 %), while it failed to operate at a low filling ratio of 35 % under the power inputs greater than 250 W due to the local dry-out. A maximum effective thermal conductivity of 2.0 × 105 W/(m·K) was achieved at the optimal filling ratio of 40 %. Further, a flow regime map was developed to identify the combination effect of filling ratio and heat power on the flow pattern evaluation and two-phase flow instability. The geyser boiling usually manifests itself as an alternation of two or even three flow patterns, and tends to occur at the combination conditions of relatively low filling ratios (35 % and 40 %) and low heat powers or a moderate filling ratio (50 %) and relatively high heat powers. Additionally, the white mist phenomena caused by the flash vaporization was observed in the riser at relatively higher power inputs. The liquid-vapor mixture flow in the riser would partially degrade the TPLT performance due to the inhibition of vapor condensation in the condenser region. This study provides new insight into the operation characteristics of TPLT and gives guidelines for proper design and applications.