Separate Heat Pipe Research Articles

The study of passive cooling system assisted with separate heat pipe for decay heat removal in spent fuel pool

The nuclear energy plays an important role in world energy production, and the safety of nuclear power plant has drawn increasing attention as well, where the decay heat removal system is an essential part of nuclear plants to ensure its passive safety. This paper aims to propose the way to achieve entirely passive cooling for spent fuel pool. A separate heat pipe assisted passive cooling system with a large cooling capability of 16MW is discussed, which focuses on passive cooling in the condenser section by mean of natural draft air cooling. After comparing several condenser section configurations, the optimization of the cooling system is discussed to meet the demands of both high-efficiency heat transfer and economic feasibility. In order to find an overall optimal solution for the passive cooling system with the minimum cost, the multi-objective optimization method is employed to investigate a set of system parameters.

Experimental study on transient characteristic of passive containment cooling system

Separated heat pipe is an efficient passive heat transfer device. It is widely used on chemical and aerospace industry. In this paper, the transient characteristics of the separated heat pipe which applied to a containment cooling system were studied. According to the experiment, the start-up transition characteristic of cooling system were mainly described. The influences of containment initial pressure and thermal load of separated heat pipe loop were analyzed. Based on the experimental data, it shows that the separated heat pipe is feasible for containment cooling system. It can remove the heat of containment vessel passively on design basis accident. And the separated type heat pipe cooling system has the working condition of flow instability region. The influence factors of which is relevant to cooling water tank temperature, containment pressure and system input power. The flow instability has little effects on the trend of containment pressure.

Boiling heat transfer characteristics of the R744 coolant in the evaporator of the separated heat pipe system

This paper studies boiling heat transfer characteristics of the R744 coolant. The heat transfer characteristics comprise the boiling heat transfer coefficient and the flowing pattern inside the vapor tube. The R744’s boiling heat transfer coefficients were measured for different filling ratios, and the results show that within the 107–138% filling ratio range, the heat transfer coefficient is almost the same for different filling ratios, but increases with increasing the heat flux. Next, the heat transfer coefficient at the high pressure region is measured. For the same filling ratio and heat transfer capacity, increasing the pressure increased the heat transfer coefficient. The boiling heat transfer coefficients and the flowing patterns for different coolants is compared, including R134a, R410A, and R744, and the boiling heat transfer coefficient of R744 was much higher than those of the other two coolants. In addition, for R744, the diameter of bubbles at the exit point of the evaporator was also smaller than those for R410A and R134a. Finally, we compared our experimental results with existing empirical results, and developed our own empirical formula based on our experimental results.

Simulation of a micro channel separate heat pipe (MCSHP) under low heat flux and low mass flux

A micro channel separate heat pipe (MCSHP) has many great features for cooling applications. The design and control of an MCSHP generally employs a numerical simulation. However, the majority of available simulation models were developed for conventional channels where the hydraulic diameter is greater than 3mm; as well, they generally have high heat flux and high mass flux. With an MCSHP, the channel size, heat flux and mass flow flux are much smaller, which might change the thermodynamics and simulation accuracy of the cycle, especially the two-phase flow section in the evaporator and the condenser. In this study, a distributed-parameter model with a combination of ε-NTU method and Nusselt laminar liquid film condensation theory was developed for the scenario. Six correlations were tested in the model under different filling ratio conditions. The results show that the model with the Gungor and Winterton correlation provides the best agreement, with an average relative error of about 5%. By using the selected model, the thermal performance of MCSHP under different refrigerant filling ratios, air flow rates and height differences were analyzed and findings were presented.

Flow pattern, pressure drop and heat transfer coefficient during two-phase flow boiling of R134a in pump-assisted separate heat pipe

The pump-assisted separate heat pipe (PASHP) has a broad application prospect in the field of saving energy. Two-phase flow boiling in PASHP is a multifarious physical process and the study of the characteristic of the flow boiling in PASHP is significant. The experimental investigation of two-phase flow boiling of R134a in PASHP was conducted. The results indicate that the flow patterns agree with the Wojtan-Ursenbacher-Thome map. The pressure drop has been compared with Muller-Steinhagen-Heck correlation, and the compared results indicate that the experimental data are inside the 10% confidence interval predictions. The heat transfer coefficients were compared with correlation given by Mohseni. It indicates that all the experimental data are inside the 10% confidence interval predictions at x>0.1. However, part of error is beyond 30% at x≤0.1. A new heat transfer coefficient correlation was developed, and most of the estimated values are within an error band of ±10% of the experimental data.

Theoretical and experimental validation study on automotive air-conditioning based on heat pipe and LNG cold energy for LNG-fueled heavy vehicles

As a clean fuel, LNG has been used in heavy vehicles widely in China. Before reaching the engine for combustion, LNG store in a high vacuum multi-layer thermal insulation tank and need to be evaporated from its cryogenic state to natural gas. During the evaporation, the available cold energy of LNG has been calculated. The concept has been proposed that the separated type heat pipe technology is employed to utilize the available cold energy for automotive air-conditioning. The experiment has been conducted to validate the proposal. It is found that it is feasible to use the separated type heat pipe to convey the cold energy from LNG to automotive air-conditioning. And the cooling capacity of the automotive air-conditioning increase with the LNG consumption and air flow rate increasing.

Experiment research on influence factors of the separated heat pipe system, especially the filling ratio and Freon types

This paper mainly studied the influence factors of separated heat pipe system. Such factors include the type of Freon coolant and filling ratio. R134a, R22 and R410A are tested. Separated heat pipe system can be used in data center cooling. In order to be more similar to the real data center cooling system, there are two evaporators which are connected in parallel and the system uses one plate heat exchanger as condenser. A room in constant temperature controlled by enthalpy difference laboratory is used as heat source while chilled water is heat sink. The paper analysis the relationship between filling ratio and heat transfer capacity and make a comparison between the heat transfer capacity of each coolant. The experiment results find that with the increasing of filling ratio, the heat transfer capacity first increases, then almost remains constant. Finally, when filling ratio is high enough, heat transfer capacity decreases. The reason of this phenomenon is given in the paper. The results of the experiment are useful to engineering.

열교환기 형상에 따른 분리형 히트파이프 성능 비교

열교환기 형상에 따른 분리형 히트파이프 성능 비교

Application of separated heat pipe system in data center cooling

This paper mainly studies the application of separated heat pipe system on data center cooling. Separated heat pipe system mainly consists of four parts: evaporator, condenser, vapor pipe and liquid pipe. Compared with traditional computer room air conditioning (CRAC) system, which is a kind of room level cooling, heat pipe cooling system is rack level cooling. There are mainly three advantages for separated heat pipe system, firstly, air flow organization is better for there is nearly no hot and cold air mixing, secondly, such system can use cold ambient air as heat sink in winter, thirdly, as the evaporator is placed directly inside the rack, heat generated from IT equipment will be soon absorbed by heat pipe system, thus there is nearly no local hot spots inside the rack. To describe the working condition of the system, there are mainly three parts in this paper. Firstly, operation performance in summer, winter and trans-season is tested. Secondly, as the system can use free cold ambient air to produce chilled water, the time of using free cold ambient air is tested. Finally, T-Q diagram is used to compare the heat transfer process in heat pipe system and traditional CRAC system. The results show that, IT equipment can work in a safe and reliable environment. Besides, the heat sink temperature in heat pipe system is much higher than CRAC system and there is nearly no cold and hot air mixing in heat pipe system. In summary, heat pipe system can save more electrical energy than CRAC system.

Feasibility study on novel room air conditioner with natural cooling capability

In order to improve the energy efficiency of room air conditioners, this paper proposed a new air conditioner that combined evaporative cooling technology, separate type heat pipe technology, and vapour compression refrigeration technology (called “combined air conditioner”). The mathematical model of the air conditioner was established and its reliability was verified by experiments. Based on the model, the simulation of the operating performance of the combined air conditioner and a conventional air conditioner was studied in typical climate regions during the cooling period, with the following results: In cold and dry areas like Shenyang, compared with the conventional air conditioner, the average cooling coefficient of performance (COP) of the combined air conditioner was increased by 27.40%. As the climate gradually became warmer and humidity gradually increased, the running time of the heat pipe cooling mode was gradually reduced, and then the energy-saving effect of the combined air conditioner became worse. For example, in the hot and humid Guangzhou, the energy saving rate was only 11.81%. Therefore, it was found that the combined air conditioner had good energy-saving potential in cold and dry areas.

Experimental study on the thermal characteristics of micro channel separate heat pipe respect to different filling ratio

A micro channel separate heat pipe (MCSHP) that can use natural cold energy was developed to reduce the cooling energy consumptions of telecommunications stations (TSs). Experimental investigations of exploring the optimal refrigerant filling ratio of MCSHP under different outdoor conditions and flow rates were presented. R22 was used as the working fluid. Various refrigerant filling ratios were tested in an enthalpy difference laboratory (EDL) in order to determine the optimal thermal performance for the exterior space temperature range from 8°C to 23°C and the air volume flow rate range from 1712m3/h to 2980m3/h. In addition, transient and steady-state performance under the optimal refrigerant filling ratio of the MCSHP was studied. As a result of experiments, the optimal refrigerant filling ratio was found in the range from 88% to 101% for different exterior conditions and airflow rates. For the optimal refrigerant filling ratio, the startup time was about 1100s and then the refrigerant pressures entered a steady state. The cooling capacity and energy efficiency ratio (EER) increased with increasing temperature difference between the indoor and the outdoor; these values were high to 7440W and 23, respectively, at temperature difference 20°C. The results are useful for the design and operational control of the MCSHP for TSs.

Performance evaluation of controllable separate heat pipes

A controllable separate heat pipe (CSHP) combined with cool storage can be an efficient and low-cost solution to release the necessary cool power and to improve the precision in the temperature control of cool-storage refrigerators powered by solar energy or electricity with time-of-use price policy. However, the precise temperature control performance of separate heat pipes under active control depends on system design and thus should be confirmed. In this study, a CSHP was developed and an active control method was achieved by interrupting its two-phase natural circulation flow. A one-dimensional steady-state model was used to calculate the critical fill ratio, heat transfer rate, and average working temperature of CSHP. A series of experiments was conducted to analyze the start–stop and heat transfer performances of CSHP at various R134a fill ratios and heat sink temperatures. Under different test conditions, the optimum working points were obtained through fitting. The most suitable control mode was determined and the start–stop performance of CSHP was examined in detail. CSHP starts up quickly in approximately 15 seconds and the quickest stopping time of CSHP approximates to 80 seconds. Therefore, the performance of CSHP under active control can be applied to cool-storage refrigerators.

Simulation study on the operating characteristics of the heat pipe for combined evaporative cooling of computer room air-conditioning system

In order to improve the energy efficiency of air conditioning systems in computer rooms, this paper proposed a new concept of integrating evaporative cooling air-conditioning system with heat pipes. Based on a computer room in Shenyang, China, a mathematical model was built to perform transient simulations of the new system. The annual dynamical performance of the new system was then compared with a typical conventional computer room air-conditioning system. The result showed that the new integrated air-conditioning system had better energy efficiency, i.e. 31.31% reduction in energy consumption and 29.49% increase in COP (coefficient of performance), due to the adoption of evaporative condenser and the separate type heat pipe technology. Further study also revealed that the incorporated heat pipes enabled a 36.88% of decrease in the operation duration of the vapor compressor, and a 53.86% of reduction for the activation times of the compressor, which could lead to a longer lifespan of the compressor. The new integrated evaporative cooling air-conditioning system was also tested in different climate regions. It showed that the energy saving of the new system was greatly affected by climate, and it had the best effect in cold and dry regions like Shenyang with up to 31.31% energy saving. In some warm and humid climate regions like Guangzhou, the energy saving could be achieved up to 13.66%.

Application of pump-assisted separate heat pipe on dehumidifying enhancement in air conditioning system

A study was carried out to investigate the effect of pump-assisted separate heat pipe (PASHP) on energy consumption and dehumidifying enhancement in the air conditioning system. In the experiment, four sets of PASHP filled into R134a as the working fluid were constructed. when the number of PASHP employed in the system changes from 0 to 4 sets at the constant inlet air state of 28.5 °C and 60% relative humidity, the apparatus dew point falls from 11.7 °C to 8.2 °C, the supply air temperature rises from 11.7 °C to 24.1 °C, while dehumidifying capacity of the system has ultimately increased by 29.5%. The index β (ratio between the cooling energy saved by PASHP and the total cooling capacity of the air cooling system with PASHP) is 40.4%, and the index δ (ratio between the cooling energy saved by PASHP and the cooling capacity of the air conditioner's evaporator) is 66.0%. The study indicates that the air conditioning system employed with PASHP can significantly reduce its energy consumption and improve its dehumidifying capacity.

Study on thermal performance of micro-channel separate heat pipe for telecommunication stations: Experiment and simulation

A micro-channel separate heat pipe (MCSHP) as a cooling device for telecommunication stations (TSs) was experimentally investigated in this work. A steady-state mathematical model was built and validated with experimental data. The cooling capacity, inlet and outlet of refrigerant temperatures and refrigerant pressure of evaporator section were measured in an enthalpy difference laboratory (EDL). The average relative errors between simulation and experiment were less than 10%. The effects of geometrical design and environment conditions on thermal performance were analyzed using the validated model. As a result of the simulations, the refrigerant side pressure drop decreased by 96.61% at the flat tube height 3.0 mm compared to 1.4 mm. And the air side pressure drop decreased by 94.49%, when fin height ranged from 4 mm to 20 mm. The cooling capacity increased by 50.65% at the fin pitch 3.0 mm compared to 1.0 mm. These factors were of practical engineering importance in optimum design of MCSHP.

Novel adsorption refrigerators with separate type two phase closed thermosyphon designs

Two adsorption refrigerators with separate type two-phase closed thermosyphon heating processes are designed to improve the heat transfer performance and to reduce the system's moving parts number and system's size. One adsorption refrigerator uses the design of three-effect separate type two-phase closed thermosyphon heating/cooling; the other one uses the design of separated separate type two-phase closed thermosyphon heating and force convection cooling. The results show that the coefficient of performance (COP) and specific of cooling power (SCP) can be improved because of the pressure difference mass recovery process. The system with the separate type two-phase closed thermosyphon heating and force convection cooling design is more attractive because fewer moving parts are used and the system's size can be reduced for building. Moreover, the thermal transfer of the adsorption beds is designed as separate type heat pipe, which can avoid the problem of corrosion from heat or sink sources, such as heavy-duty exhaust gases or seawater. Under the nominal working condition, the evaporating temperature, COP and SCP can reach −21 °C, 0.26 and 474 W kg−1. Copyright © 2015 John Wiley & Sons, Ltd.

A seasonal cold storage system based on separate type heat pipe for sustainable building cooling

Seasonal cold storage is a high-efficient and environmental-friendly technique that uses the stored natural cold energy in winter (e.g., snow, ice or cold ambient air) for free-cooling in summer. This paper presents a seasonal cold storage system that uses separate type heat pipes to charge the cold energy from ambient air in winter automatically, without consuming any energy. The charged cold energy is stored in the form of ice in an insulated tank and is extracted as chilled water for cooling supply in summer, which help to reduce the chiller running time and reduce the associated electricity consumption and greenhouse gas emission significantly. A quasi-steady two-dimensional mathematical model of the system is developed for characterizing the dynamic performance of ice growth (i.e., cold charging). The model is validated using the field measurement data from an ice charging experiment conducted in Beijing. The impacts of various affecting factors, including the weather data and the key parameters of heat pipes, on the charging performance of the cold storage system are analyzed. The effectiveness and sustainability of the proposed system for cooling are demonstrated through a case study of a kindergarten building in Beijing.

Simulation of boiling flow in evaporator of separate type heat pipe with low heat flux

The separate type heat pipe heat exchanger is considered to be a potential selection for developing passive cooling spent fuel pool – for the passive pressurized water reactor. This paper simulates the boiling flow behavior in the evaporator of separate type heat pipe, consisting of a bundle of tubes of inner diameter 65mm. It displays two-phase characteristic in the evaporation section of the heat pipe working in low heat flux. In this study, the two-phase flow model in the evaporation section of the separate type heat pipe is presented. The volume of fluid (VOF) model is used to consider the interaction between the ammonia gas and liquid. The flow patterns and flow behaviors are studied and the agitated bubbly flow, churn bubbly flow are obtained, the slug bubble is likely to break into churn slug or churn froth flow. In addition, study on the heat transfer coefficients indicates that the nucleate boiling is the dominant mechanism in large pipes at low mass and heat flux, with the heat transfer coefficient being less sensitive to the total mass flux.

SIMULATION ON OPERATION CHARACTERISTICS OF HYBRID GROUND-AIR SOURCE HEAT PUMP SYSTEM WITH NATURAL COLD STORAGE

To solve the problem of the coefficient of performance (COP) decrease and power crunch in cooling dominated South China associated with the long-term usage of ground-source heat pump system (GSHPS), a hybrid ground-air source heat pump system (HGASHPS) with natural cold storage is presented in this paper. The system consists of a GSHP system and compound air-cooled chillers (CACC), which can be operated according to the vapor compression refrigeration cycle or separate type heat pipe natural cycle. The mathematical models of each part of the system were set up and the conversion conditions between operation modes for the system were determined. The transient simulation for HGASHPS with natural cold storage in Nanjing was carried out. The operation characteristics of the systems in operational life were comparatively analyzed. The simulation results indicated that the HGASHPS with natural cold storage could retain the thermal balance of soil temperature field in one year cycle and increase the COP and reliability of the system.

A study on a flat-plate type of solar heat collector with an integrated heat pipe

An improved structure of flat-plate solar heat collector applied in construction of the solar water heater system is proposed in this paper. The collector uses one large integrated wickless heat pipe instead of side-by-side separate heat pipes. High stability and leak avoidance between the water cooling side and the solar heating side are the main advantages. The present work includes three aspects. Firstly, an experiment was conducted to test thermal performance of the new collector on July 9, 2011. After a daily work from 09:00 to 16:00, the maximum collector efficiency could reach 66%, while the temperature of water of 200kg in the storage tank increased by about 25.0°C in the end. Secondly, a theoretical analysis on the energy balance for each component of the collector was made to understand the heat transfer process inside the collector. Thirdly, a transient heat transfer model has been developed to calculate the collector efficiency, temperatures of water, the glass cover and the absorber plate, of which the simulation results are qualitatively in agreement with the experimental data.