Two-phase Loop Thermosyphon Research Articles

An indirect heating solution to reduce CO2 emission and improve efficiency of gas distribution networks

The gas industry relies on indirect heating to prevent gas from freezing when it is transferred from high-pressure networks to lower pressure distribution systems. The main challenge in preheating natural gas is designing an indirect heating system capable of consistently maintaining a target temperature, despite large load diversity. The most common form of heating technology has traditionally been water bath heaters and boiler houses. In this paper, a novel technology is introduced, and its performance compared to existing installations. The Immersion Tube Thermosyphon Heater was developed specifically to address high load diversity; it combines a high-efficiency immersion burner with a sub-atmospheric two-phase loop thermosyphon. The use of low-temperature steam provides a flexible and precise solution for temperature control easily adapted to variable gas flows. The Immersion Tube Thermosyphon achieved an average thermal efficiency of 90%, considerably higher than the 46% efficient water bath, allowing an estimated annual saving of 7,660 tonnes CO2 for 1-megawatt gross heat capacity operating with a 50% load factor.

A performance evaluation index for two-phase thermosyphon loop used in HVAC systems

For energy conservation, two-phase thermosyphon loops (TPTLs) have been widely used in air conditioning systems. Unfortunately, the evaluation index and design criterion of the TPTL is currently lacking, and causes for poor performance are inconclusive. In this study, an “ideal cycle” (optimal state) of the TPTL was proposed, and an approaching degree ε was defined to evaluate the extent of approaching the “ideal cycle”. Furthermore, a visual experimental platform was developed, and real cycles under different conditions were measured and evaluated. The results indicate that: (1) the performance of the “ideal cycle” is the upper limit of the performances of all real cycles, and the approaching degree provides an effective evaluation index and design criterion; (2) causes for low approaching degree have been identified and illustrated visually, from the perspective of refrigerant distribution, including under-charged refrigerant, over-charged refrigerant, insufficient height difference, and considerable flow resistance; and (3) the “ideal cycle” can be approached, only when the filling ratio (100%) and the height difference (1.2 m) is optimal, and the flow resistance is negligible simultaneously.

Three typical operating states of an R744 two-phase thermosyphon loop

As the two-phase thermosyphon loop (TPTL) gets more extensive application in data centers, problems have been encountered. One of the most common problems is undesirable overheating or subcooling during the operation of the TPTL. The overheating or subcooling results from abnormal operating states. Herein, an experiment conducted on an R744-based TPTL is described. The TPTL experienced a prestart operating state, an oscillatory operating state, and a stable operating state successively as the heat transfer quantity increased. In the prestart operating state, no regular circulation was formed in the loop, and overheating was usually measured in the top part of the riser. In the oscillatory operating state, the mass flow rate, temperature, and pressure all exhibited periodical variation with an identical period. In the stable operating state, the TPTL functioned well until the heat transfer quantity exceeded the maximum heat transfer ability of the TPTL. The stable operating state was divided into the normal operating state and the overload operating state, and the normal operating state is the goal to strive for in practical application. In the experiment, at the same fill ratio of 100%, the reasonable heat load ranges for normal operating of the TPTL were 650–1400W, 1400–3300W, and 2000–5400W for diameters of 6, 9, and 12mm, respectively. Higher heat load demanded for larger diameter.

Performance analysis of a separator assisted two-phase thermosyphon based on an experimentally validated model

An experimentally validated mathematical model of two-phase separator assisted two-phase thermosyphon loop (STPTL) is developed to evaluate its cooling performance and compared with the basic two-phase thermosyphon loop (BTPTL) in this paper. The simulation results show that the thermal resistance and evaporator outside wall temperature of STPTL are lower than that of the BTPTL about 8.3–15.96°CkW-1 and 2.49–11.17°C for heat load ranges from 300W to 700W, respectively, under given operating conditions. At different working fluid charge, diameter of two-phase tube and mass flow rate of cooling water, the STPTL can always achieved a better performance than that of BTPTL under a large heat load. Besides, compared with the BTPTL, the STPTL has a better benefit in thermal resistance and evaporator outside wall temperature at higher charge, smaller diameter of two-phase tube or lower mass flow rate of water. Furthermore, the variation of the mass flow rate of working fluid, the pressure drop of working fluid at each component, the height of the liquid in the separator are also evaluated in this study. These simulation results indicate that the STPTL can achieve a better performance than BTPTL.

Efficiency enhancement of a loop thermosyphon on a mixed-wettability evaporator surface

This study presents an experimental investigation of the heat transfer performance of a two-phase loop thermosyphon with an enhanced mixed-wettability evaporator surface at sub-atmospheric pressures. For central-processing-unit (CPU) cooling applications, a lowering of the saturation temperature (pressure) is essential when water is used as the working fluid. Compared with copper mirror surfaces, up to over 100% enhancement of high heat transfer coefficient (HTC) was observed using surfaces with spotted wettability patterns, which consists of hydrophobic spots with contact angle ranged from 145° to 150°. The results revealed that the boiling behaviors changed drastically with the application of hydrophobic spots coating by artificially increasing the nucleation site density. Parametric tests with a variety of operating conditions, including different filling ratios, condenser temperatures, and heat loads revealed the minimum thermal resistance (i.e., the optimum thermosyphon performance) to be 0.03K/W on the boiling side.

Thermodynamic calculations of a two-phase thermosyphon loop for cold neutron sources

A new method is described for thermodynamic calculations of a two-phase thermosyphon loop based on a one-dimensional finite element division, where each time-step is split up in a change of enthalpy and a change in entropy. The method enables the investigation of process responses for a cooling loop from room temperature down to cryogenic temperatures. The method is applied for the simulation of two distinct thermosyphon loops: a two-phase deuterium and a two-phase hydrogen thermosyphon loop. The simulated process responses are compared to measurements on these loops. The comparisons show that the method can be used to optimize the design of such loops with respect to performance and resulting void fractions.

Transient simulation of a two-phase loop thermosyphon with a model out of thermodynamic equilibrium

Numerical investigation of two-phase loop thermosyphon (2PLT) in steady and transient states is addressed. A one-dimensional two-phase flow model describing a liquid-gas mixture in both mechanical and thermal equilibrium but out of thermodynamic equilibrium is developed. The model considers subcooled liquid and over heated vapor as well as phase transition (evaporation and condensation). The flow model is solved with a specific hyperbolic solver based on Godunov method and Harten-Lax-van Leer-Contact (HLLC) Riemann solver. A parametric study on the thermal power at the evaporator is performed in steady and transient states, the aim being to determine the effects of thermal power increase at the evaporator on the loop behavior. The comparison between Goodwin and Stiffened Gas (SG) equation of state (EOS) models shows fair agreement for latent heat of vaporization, specific volume and enthalpy for both liquid and vapor phases. Simulation of four test cases, corresponding to different evaporator thermal loads, is also carried out in transient state showing that loop response is correctly reproduced by this numerical approach, novel in the context of thermosyphon loops.

Experimental study of the self-regulating performance of an R744 two-phase thermosyphon loop

The two-phase thermosyphon loop (TPTL) is an efficient solution for use in air-conditioning systems in data centers. The typical TPTL system pattern in data centers is the multi-evaporator TPTL. When the heat generated from the evaporators varies, the TPTL will self-regulate according to its own rules. In the current study, an experiment on a double-evaporator R744-based TPTL was conducted. Uniform, non-uniform, and variable heating power conditions were investigated, and a theoretical analysis was performed. The experimental results show that the mass flow rate of the high-power evaporator is higher than that of the low-power evaporator, but the self-regulating ability of the TPTL is very limited. The mass flow rate ratio (Mmax/Mmin) was between 1 and 1.5, with the heating power ratio (Qmax/Qmin) changing from 1 to 3; even when Qmax/Qmin is as high as 15, Mmax/Mmin is only 1.67. If the heating power of the two evaporators is different, the operating conditions of the evaporators will be affected. Therefore, when a TPTL is designed, more attention should be paid to the load of each single evaporator and the load of the other evaporators in parallel. In practical applications, significant differences between the heat transfer loads of parallel evaporators should be avoided as much as possible.

Thermal performance of two-phase thermosyphon loop in rotating thin pad

Thermal performances of a sub-atmospheric Rotating Two-Phase Loop Thermosyphon Pad (RTPLTP) of 1 mm channel height were experimentally studied. Flow structures of vapor-liquid circulations, boiling/condensation heat transfer rates and thermal resistances (Rth) for the RTPLTP were detected at rotor speeds of 0, 100, 300 and 500 rev/min. With each rotor speed tested, experiments were performed at four ascending boiling numbers (Bo) with four descending non-dimensional condenser thermal resistances (Rth,con) at each Bo. Parametric test conditions specified by Bo, Rth,con and relative centrifugal acceleration (Ω) were 0.16–0.82, 0.07–0.3 and 0–28.2, respectively. Acting by the rotational force effects, the churn flow boiling structures in present thin static TPLTP initially transited into intermittent boiling flow structures with the temporal drifts of vapor bubbles along the vortical flow pathways in the rotating pad at Ω = 1.1. Further increasing Ω, the boiling flow structure yielded to the swaying and then the stable continuous tiny-bubble streams centered at the nucleate sites on the rotating boiling surface. Both averaged Nusselt numbers over evaporator (Nueva) and condenser (Nucon) of present RTPLTP increased as Ω increased. While Nucon kept increasing as Bo increased at all Ω tested, increases of Bo initially reduced Nueva at low Ω but turned to elevate Nueva at high Ω due to the Ω-induced variations in boiling flow structures. With the combined Bo and Ω effects on Nueva and Nucon, the total thermal resistances (Rth) of present thin RTPLTP decreased as Bo and/or Ω increased; but were increased by raising Rth,con. Empirical correlations evaluating Nueva, Nucon and Rth with Bo, Ω and Rth,con as the determining variables were devised to assist the relevant applications.

Validation of the generalized model of two-phase thermosyphon loop based on experimental measurements of volumetric flow rate

AbstractThe current paper presents the experimental validation of the generalized model of the two-phase thermosyphon loop. The generalized model is based on mass, momentum, and energy balances in the evaporators, rising tube, condensers and the falling tube. The theoretical analysis and the experimental data have been obtained for a new designed variant. The variant refers to a thermosyphon loop with both minichannels and conventional tubes. The thermosyphon loop consists of an evaporator on the lower vertical section and a condenser on the upper vertical section. The one-dimensional homogeneous and separated two-phase flow models were used in calculations. The latest minichannel heat transfer correlations available in literature were applied. A numerical analysis of the volumetric flow rate in the steady-state has been done. The experiment was conducted on a specially designed test apparatus. Ultrapure water was used as a working fluid. The results show that the theoretical predictions are in good agreement with the measured volumetric flow rate at steady-state.

Analysis of Heat Transfer and Fluid Flow in Two-Phase Thermosyphon Loop with Minichannels

The present paper offers an analysis of heat transfer and fluid flow in two phase thermosyphon loop with minichannels. A one-dimensional model of two-phase flow and heat transfer in a closed thermosyphon loop with minichannels was examined. The created general model is based on mass, momentum, and energy balances in the evaporators, rising tube, condensers and the falling tube. The separate two-phase flow model is used in calculations. The numerical results obtained for the selected heater and cooler using the general model of thermosyphon loop indicate that the mass flux increases with increasing length of the heated section and decreases with increasing length of the cooled section of the loop. It was found that the heat transfer coefficient for flow boiling and flow condensation in the steady state increases with increasing heat flux in the heater and cooler with minichannels, respectively. The design and configuration of heaters and coolers has a considerable impact on the efficiency of thermosyphon loop. These factors make it possible to optimize the computer processor cooling.

Application of a two-phase thermosyphon loop with minichannels and a minipump in computer cooling

AbstractThis paper focuses on the computer cooling capacity using the thermosyphon loop with minichannels and minipump. The one-dimensional separate model of two-phase flow and heat transfer in a closed thermosyphon loop with minichannels and minipump has been used in calculations. The latest correlations for minichannels available in literature have been applied. This model is based on mass, momentum, and energy balances in the evaporator, rising tube, condenser and the falling tube. A numerical analysis of the mass flux and heat transfer coefficient in the steady state has been presented.

Experimental study on the effect of fill ratio on an R744 two-phase thermosyphon loop

As a natural, environmentally friendly fluid with excellent thermodynamic and transport properties, carbon dioxide is an effective alternative refrigerant. This paper describes an experiment conducted on an R744-based two-phase thermosyphon loop (TPTL). With different fill ratios of 45~151%, the effect of fill ratio on the working performance of the R744 TPTL is investigated. To maintain the conservation of momentum, part of the fluid circulates in the loop without changing phase; this part of the fluid may be liquid, vapor, or both liquid and vapor depending on the fill ratio. This is how the R744 TPTL self-adjusts among different heat loads. The experimental results show that the working state of the R744 TPTL has a lot to do with the fill ratio. With a low fill ratio, the TPTL is more likely to fluctuate under small heat loads. When the fill ratio is around 100%, the TPTL reaches its maximum heat transfer ability, and when the fill ratio is around 62%, the lowest driving temperature difference is achieved. Considering that the fill ratio's effect on the driving temperature difference is not very significant and that pursuing maximum heat transfer ability is more meaningful, a fill ratio of around 100% is recommended.

Experimental investigation on two-phase thermosyphon loop with partially liquid-filled downcomer

Two-phase thermosyphon loops (TPTLs) are beginning to be extensively used in the field of air conditioning and heat recovery, where they have quite different flow characteristics compared with the traditional TPTLs used in cooling of electronics. However, in the existing studies, the flow features in the downcomer were ignored, and most researchers simply thought the downcomer was always full of liquid. In this study, a visual experimental setup was established, the flow features in the downcomer were observed and measured. And the influencing factors including temperature difference, liquid charge, height difference, and circulation flow resistance on the liquid head have been identified and investigated experimentally. The results show that, different from the conventional understandings, the downcomer can be partially liquid filled. At this time, the upper part of downcomer is a static saturation gas blockage, surrounded by a layer of liquid film, which does not provide the driving force. The liquid head in the downcomer, which provides the driving force, shows great self-regulation ability with different working conditions. Increasing the refrigerant charge, temperature difference, circulation flow resistance, and decreasing the height difference drives the liquid head to rise, and the downcomer tends to be fully liquid filled.

An experimental investigation of an R744 two-phase thermosyphon loop used to cool a data center

As an environmentally friendly natural fluid with excellent thermodynamic and transport properties, carbon dioxide is an effective alternative refrigerant. This study presents an experimental investigation of an R744-based two-phase thermosyphon loop (LTS) used to cool a data center. With a heat transfer rate change of 1–4 kW, the thermal resistance and driving temperature difference of the LTS are examined. The total thermal resistance decreases with increasing heat transfer rate due to the reinforced flow boiling heat transfer of R744. The R744 LTS can work under a small driving temperature difference of 5 °C. According to a contrast experiment, the R744 LTS has superior heat transfer performance, with a driving temperature difference that is 2 °C smaller than that of an R22 LTS when the heat transfer rate is 2 kW. Besides, a theoretical model is also used to analyze the performance of the LTS. The calculated results show that the air-side thermal resistance accounts for the majority (68–82%) of the total thermal resistance.

Modeling and performance analysis of a two-phase thermosyphon loop with partially/fully liquid-filled downcomer

Two-phase thermosyphon loops (TPTLs) are extensively used in industries. Numerical simulation is always applied for their configuration design and performance rating. The conventional models are usually developed under large temperature difference, and the fully liquid-filled downcomer are taken for granted. In this study, a generalized model for TPTL is developed, in which the downcomer can be partially liquid filled or fully liquid filled under different conditions. The model is verified by experiments on an air-to-air finned-tube TPTL. Then, it is used to analyze the flow and heat transfer features of a TPTL, under different refrigerant charge and temperature difference. The results show that the downcomer is partially liquid filled in the case of small temperature difference and refrigerant charge. Additionally, the heat transfer rate first increases then decreases with the increase of refrigerant charge, and first increases linearly then increases relatively slowly with the increase of temperature difference.

Investigation of a long term passive cooling system using two-phase thermosyphon loops for the nuclear reactor spent fuel pool

Given postulated accidents that go beyond the plant design basis, long term passive cooling for the nuclear reactor spent fuel pool is important to demonstrate. Large amount of heat transport with small temperature difference is required. Two-phase thermosyphon loops are suitable for these cases. Flow and heat transfer characteristics of a two-phase thermosyphon loop were numerically investigated using RELAP5 code. Effects of fill charge ratio on the heat transfer were analyzed. The fill charge ratio should be chosen between 30% and 80%, smaller than 30% or larger than 80% will reduce the heat transfer. Based on the analysis of two-phase thermosyphon loops, a complete design of a passive cooling system for the spent fuel pool was proposed. An analysis method for decoupling of the thermo-hydraulics of the water natural circulation in spent fuel pool, two-phase flow of ammonia in thermosyphon loop and air natural circulation in cooling tower was presented. The results indicate that 1528 two-phase thermosyphon loops can passively remove 16MW decay heat from the spent fuel pool successfully. Finally, the velocity and temperature distributions in the spent fuel pool were numerically simulated using ANSYS FLUENT software. It was proved that natural circulation of water in spent fuel pool can be realized with the present design. The temperature uniformity was fairly well and the maximum water temperature was 81°C.

Experimental study on the performance characteristics of an enhanced two-phase loop thermosyphon

A two-phase loop thermosyphon (TPLT) is an apparatus for heat transmission from the hot section of system (evaporator) to the cold part (condenser), with relatively small temperature differences. The setup used in this study consists of a TPLT, including evaporator, riser, an advanced condenser and downcomer. The condenser inlet has a nozzle. The steam rises from evaporator, flows through the nozzle and sprays on a vertical copper surface inside the condenser. To cool the copper sheet, then a cooler system was installed rear of it. The experimental tests were performed for presence of the nozzle and without it. The results showed that the TPLT efficiency increases for the modified condenser using nozzle. Also, the overall heat transfer coefficient of TPLT is enhanced by the nozzle. In the case of the nozzle with distance of 13 mm from copper sheet, and T e,sat > 55 °C, the value of $$\bar{h}_{c}$$ suddenly increased.

루우프형 2상 유동 열사이폰의 응축열전달 특성에 관한 연구

This study concerns the performance of condensation heat transfer in two-phase loop thermosyphons. In the present work, R134a has been used as the working fluid. Liquid fill charge ratio defined by the ratio of working fluid volume to total internal volume of thermosyphon, heat flux and wind speed of condensation have been used as the experimental parameters. The results show that the filling rate of working fluid and heat flux are very important factors for the operation of two-phase loop thermosyphons. The optimum liquid fill charge ratio for the best condensation heat transfer rate was 80%.

Two-phase loop thermosyphon using carbon dioxide applied to the cold end of a Stirling cooler

This work focuses on an experimental investigation of a carbon dioxide thermosyphon loop designed to fulfill the geometric and temperature requirements of a specific FPSC (Free Piston Stirling Cooler). Experiments were carried out varying the temperature difference between the heat source, i.e. air at the entrance of the evaporator and heat sink, i.e. internal surface of the condenser, refrigerant charge and evaporator airflow rate. The experimental results were explored using the thermal conductance concept applied to each heat exchanger and also to the whole loop. It was found that the loop was able to carry a maximum heat transfer rate of 514 W with a heat source-sink temperature difference of 11 °C. A first principles model was also developed and 85% of the calculations fell within ±20% of measurements. Further exploration of the model showed a good capability to capture trends and be an aid in the thermosyphon design phase.