Model Of Heat Pipe Research Articles

Finite time thermodynamics analysis and research of pulsating heat pipe cold storage device

Phase change energy storage technology can effectively solve the energy mismatch in space and time. There are many disadvantages of phase change materials (PCMs), such as high supercooling, phase separation and poor heat transfer performance. As a new type of heat exchange structure, pulsating heat pipe has the advantages of simple structure, high heat transfer coefficient and good economic behavior. The system efficiency can be greatly improved by using pulsating heat pipe combined with cold storage technology. A set of pulsating heat pipe type cold storage device is developed, the finite time thermodynamic analysis is carried out, and the correlation between heat pipe efficiency and power is established. The three-dimensional physical model of pulsating heat pipe is simulated and verified by experiments. The experimental results show that in the cold storage stage, with the increase of the filling rate, the greater the pressure is, the better the heat transfer effect is; in the cold release stage, with the decrease of the filling rate, the smaller the pressure is, the better the heat transfer effect is.

Proposed Modeling of Natural Convection Cooling Heat Pipe

Proposed Modeling of Natural Convection Cooling Heat Pipe

Design and verification of ultra-high temperature lithium heat pipe based experimental facility

Lithium heat pipe has broad applications in heat pipe cooled reactors and hypersonic vehicles due to its ultra-high working temperature which is around 1700 K. In this paper, a lithium heat pipe based experimental facility has been designed to test the heat transfer performance of the lithium heat pipe. A simplified mathematical model of heat pipe has been implemented into a CFD approach, which is used to verify the design of lithium heat pipe and its experimental facility. Results showed that the CFD approach is in good agreements with some well-known existing models and experimental data, and deviation between the results is within 5% range. The adjustment range of mixed gas thermal resistance and cooling water flow rate was obtained by analyzing the effects of different cooling conditions on the performance of the experimental facility. It is necessary to ensure the cooling water flow rate is above 0.11 Lph to prevent water boiling when the heating power is 10 kW around, and the optimal proportion of helium is 70-90%. The operation characteristics of the lithium heat pipe under unsteady-state with varying heating power were simulated numerically. The results show that the proportion of helium must be less than 60% for normal operation of the lithium heat pipe. This work provides a reference and numerical verification for the design of lithium heat pipe based experimental facility, which can be used to reveal the heat transfer mecha?nisms of the lithium heat pipe during the experiment.

Improving thermal performance of battery at high current rate by using embedded heat pipe system

This study establishes a resistance-based thermal model for the battery cooled by the heat pipe. Unlike the traditional thermal model, the proposed battery thermal model includes the influence of the battery temperature, state of charge (SOC) and current on the heat generation in the battery body. Moreover, it also integrates the heat generation inside the tabs to improve its prediction accuracy. The heat pipe model is developed based on the thermal resistance of its components, and this model is integrated with the battery thermal model to form the thermal model of the BTMS based on the embedded heat pipe system. Experiment are carried out to verify the prediction precision. At the level of the battery unit, the prediction error is within 1 °C. Then, the different heat pipe number, fin number and heat transfer coefficient at the condensation section of the heat pipe are used for the embedded heat pipe system in order to improve its cooling performance. According to the research results, increasing the heat pipe number can reduce the battery temperature and its difference. However, when the heat pipe number is over a certain threshold, both the battery temperature and temperature difference change little. Although the heat transfer coefficient has limited influence on the battery thermal behavior, increasing the heat transfer coefficient and fin number at the same time can reduces the battery temperature a lot. In addition, comparative studies show that the embedded heat pipe system significantly outperforms the bar-type heat pipe and flat heat pipe at an extreme high current rate.

Thermal-electrical coupling characteristic analysis of the heat pipe cooled reactor with static thermoelectric conversion

Heat pipe cooled reactor with static conversion is gaining more and more attention because of its unique advantages such as mobility, high reliability, simple structure, long lifetime, and high-power density. A 100 kW heat pipe reactor with the three-segment type thermoelectric generators namely NUSTER-100 is designed by Xian Jiaotong University. Based on the finite element software COMSOL Multiphysics, a multi-physics coupling analysis platform is established for the static heat pipe reactor NUSTER-100. The heat transfer models of the heat pipe and thermoelectric models of the three-segment type TEGs are supplemented in the simulation model. The thermal-electrical coupling characteristics of the static heat pipe reactor are investigated under different operation conditions. For the steady state condition, the temperature difference of the heat pipe between evaporator and condenser stabilizes at around 110 K, the main temperature gradient about 850 K appears between the two side of TEGs. The total output power of the NUSTER system is 123 kW and the system thermoelectric conversion efficiency is 12.3%. For the three rows unloading condition, the maximum temperature increases to 1483 K which is 133 K higher than that on the steady state condition. The output electrical power of the reactor system is 114.5 kW and the system thermoelectric conversion still could reach 11.5%. For the whole reactor unloading condition, the reactor core temperature increases by 393 K compared with the steady state and the maximum temperature is 1743 K which is lower than the core melting temperature. The operating temperatures of the heat pipes are still within the normal operating temperature range of the sodium heat pipe. The heat transfer capacity of the heat pipe is still sufficient to ensure that the core temperature is kept within the safe limits. In summary, the unloading condition will cause the core temperature to rise significantly. Therefore, the system safety characteristics under accident transient conditions need to be focused coupling with the unloading condition in the further research.

Analytical steady-state model based on Fourier integral transforms for cylindrical heat pipes under axisymetric conditions

A thermohydraulic analytical model of a capillary cylindrical heat pipe in steady-state is proposed in this article. It is based on an original representation by thermal quadrupoles to describe heat transfer in the wall and in the porous wick, via the use of Fourier integral transforms. Thanks to a validation from literature results, this model provides two-dimensional axisymetric thermal fields and one-dimensional pressure and velocity profiles of both liquid and vapour flows. Different developments and solutions are introduced according to the kind of boundary conditions at evaporator and at condenser, and with a more or less strong thermohydraulic coupling at the liquid/vapour interface. For the simple case with imposed uniform heat fluxes, intrinsic properties of the heat pipe are originally defined. The introduced model offers a generalisation of analytical models of standard heat pipe as a design or optimisation tool. Wider developments of analytical models for more complex three-dimensional geometries of heat pipe and in transient regime can be expected.

Coupled neutronic, thermal-mechanical and heat pipe analysis of a heat pipe cooled reactor

Heat pipe cooled reactors are solid-state, high temperature reactors with significant thermal expansion which influences the neutronics and thermal analyses. The heat pipe operating conditions determine the heat transfer rates and temperature distribution which then affect the core neutronic and thermal–mechanical behavior, especially during heat pipe failure accidents where the radial heat flux is uneven. A two-phase two-dimensional heat pipe model was developed and coupled to a previous neutronic and thermal–mechanical (N/T-M) model. The coupled neutronic, thermal–mechanical and heat pipe heat transfer (N/T-M/HP) strategy is described here with a focus on the iteration schemes, physical mapping and geometry reconstruction. The coupled N/T-M/HP method was then applied to the KRUSTY heat pipe cooled reactor, an experimental solid-state reactor designed to power space missions. The power distribution and reactivity predicted by the coupled method are validated against previous simulations for the KRUSTY design. The heat pipe model predictions are compared with heat pipe experiments which show that the predicted heat pipe wall temperature is within 30 K of the measured temperatures. A steady-state normal operating case and a single heat pipe failure accident were then simulated to show the redundancy and reliability of the solid-state reactor. For the normal case, the thermal–mechanical feedback is ∼850 pcm from the cold state to the hot state. The heat transfer rate into each heat pipe is very stable during the iterations. For the single heat pipe failure accident, the heat pipes adjacent to the failure area are the most affected, with those heat loads increasing from 500 W to 640 W and an operating temperature rising by 16 K. In addition, the single heat pipe failure accident significantly increases the stress concentration in the fuel contacted with the failed heat pipe wall, with the peak stress increasing from 53 MPa to 80 MPa, which is very close to the yield stress. Therefore, the mechanical performance of the solid-state reactor should be carefully analyzed during design and operation.

Modeling and numerical investigation on the effects of filling ratio in a large separate heat pipe loop

Large separate heat pipe is a potential selection to establish the passive cooling system to remove decay heat from the Spent Fuel Pool. In this system, heat pipes are expected to work properly and effectively for a long time without human intervention. Due to the physical dimensions and structures, flow patterns, and fluid distribution in large-scale heat pipes differ from those in conventional ones. Based on the condensation flow patterns, a flow condensation model is presented. Then, a separate heat pipe model considering filling ratio effects is developed. Using the new model, the mean absolute errors of simulation are 5.5% (water), 5.1% (ammonia), and 4.3% (R134a). While in the conventional condensation model, these errors are 29.4%, 9.7%, and 10%. It is found that the influence of filling ratio varies with working fluids. For water heat pipe, the optimal filling ratio is about 17%. Higher or lower filling ratios lead to degradation of heat transfer capacity. For R134a or ammonia, the optimal filling ratio is a range, about 20% to 70%. The boundary of the range relates to both evaporator outlet vapor quality and heat pipe downcomer liquid column height.

Research on Optimization of collector module of new flat plate heat pipe PV/T heat pump system

The mathematical model of a new flat plate heat pipe PV/T heat pump system is established. The experimental data of the system under various working conditions are obtained through experimental measurement, and the accuracy and reliability of the model are verified. Based on the verified mathematical model, the thermal performance, electrical performance and the performance of the heat pump system are simulated. The results show that under winter conditions, the daily average thermal power, electrical power and COP of the system are 274.5 W, 93.5 W and 2.7 W respectively. Due to the low outdoor ambient temperature in winter, during winter operation, the heat collection system will lose a lot of heat to the surrounding environment through the photovoltaic panel surface, resulting in the heat collection of the system cannot meet the heat demand of the heat pump side, which is intuitively shown as the water temperature of the heat collection tank on the evaporation side shows a downward trend throughout the day. Therefore, the collector module of the system is optimized by adding a collector. After optimization, the daily average thermal power of the system is increased to 654.2 W and the COP is increased to 6.9.

Steady-state operating characteristics analysis of loop heat pipes with flat-plate evaporator

A mathematical model of gravity-assisted flat loop heat pipe has been established, based on mass conservation, energy balance, combining pressure-temperature and heat & mass transfer relations. The effects of gravity, heat sink and ambient temperature on steady-state performance of the loop heat pipe were comprehensively investigated. The modeling results show that there exist two driving modes: gravity-driven mode and capillary-gravity co-driven mode when plate loop heat pipes operate under gravity-assisted condition. Besides, a critical heat flux has been found between the gravity-driven mode and the co-driven mode. The condensate capacity of heat sink and ambient temperature to the heat regenerative ability of the liquid line affect the variable heat-conductance zone and constant heat-conductance zone. These findings are beneficial to the comprehensive understanding of the operating characteristics, which can guide the design and optimization of the flat loop heat pipes.

Transient heat pipe failure accident analysis of a megawatt heat pipe cooled reactor

Heat pipe cooled reactors, which use heat pipes rather than fluid flow for passive cooling, are an excellent candidate for micro nuclear power sources. Heat pipes provide excellent built-in redundancy and safety margin. However, a heat pipe failure is quite likely over the reactor lifetime, which is one of the essential design basis accidents in the reactor design. This work analyzes heat pipe failure accidents to investigate the system transient performance and the response of a heat pipe cooled reactor. Single heat pipe failure and cascading heat pipe failure are simulated and analyzed by establishing a transient analysis code, including a point reactor kinetics model, a core thermal-mechanical model, and a heat pipe model. The simulation shows that in the single heat pipe failure accident, the peak monolith temperature at the failed channel increases and is only 8 K lower than the fuel center temperature. In the cascading heat pipe failure accident, the temperatures in each channel suddenly increase over a short period and rise to the maximum at 22,900 s as the heat losses increase and the core power decreases. The sensitivity of the external convection for residual heat removal is also analyzed. For small external convection coefficients less than 100 W/m2K, increasing the external convection coefficient significantly reduces the maximum temperatures. By contrast, for external convection coefficient greater than 500 W/m2K, increasing external convection coefficient still reduces the core peak temperature, but the effect is weak.

Mathematical model of bent-flattened sintered-grooved heat pipe with concept of receding-and-excessing fluid

A mathematical model that uses the concept of evaporation and condensation to explain the process of heat and mass transfer within the bent, flattened heat pipe has been established to predict its wall temperature and thermal resistance. When the heat pipe was bent and flattened in the application, the wick was damaged, resulting in less liquid being returned. It caused the wick to become unsaturated at the evaporator section. It can be determined that the liquid-vapor interface will gradually recede into the wick. This is defined as the “Receding” model (R). In the condenser section, the remaining condensate was collected as excess fluid. This is referred to as the “Excess Fluid” model (EF). It was observed that the low thermal conductivity of the unsaturated wick decreased the effective thermal conductivity at the evaporator section. The effective thermal conductivity at the condenser section also decreased due to the low thermal conductivity of the liquid film. The low effective thermal conductivity resulted in higher evaporator temperatures and lower condenser temperatures, resulting in higher thermal resistance. The established “REF” model has been validated in the same conditions by the experimental results and provides accurate predictions for the heat transfer performance of heat pipes.

Preliminary design and assessment of a heat pipe residual heat removal system for the reactor driven subcritical facility

A heat pipe residual heat removal system is proposed to be incorporated into the reactor driven subcritical (RDS) facility, which has been proposed by MIT Nuclear Reactor Laboratory for testing and demonstrating the Fluoride-salt-cooled High-temperature Reactor (FHR). It aims to reduce the risk of the system operation after the shutdown of the facility. One of the main components of the system is an air-cooled heat pipe heat exchanger. The alkali-metal high-temperature heat pipe was designed to meet the operation temperature and residual heat removal requirement of the facility. The heat pipe model developed in the previous work was adopted to simulate the designed heat pipe and assess the heat transport capability. 3D numerical simulation of the subcritical facility active zone was performed by the commercial CFD software STAR CCM + to investigate the operation characteristics of this proposed system. The thermal resistance network of the heat pipe was built and incorporated into the CFD model. The nominal condition, partial loss of air flow accident and partial heat pipe failure accident were simulated and analyzed. The results show that the residual heat removal system can provide sufficient cooling of the subcritical facility with a remarkable safety margin. The heat pipe can work under the recommended operation temperature range and the heat flux is below all thermal limits. The facility peak temperature is also lower than the safety limits.

Physical principles and state-of-the-art of modeling of the pulsating heat pipe: A review

The Pulsating (called also oscillating) Heat Pipe (PHP) is a high performance passive heat transfer device. It consists in a closed capillary channel folded into several meanders, evacuated, and partially filled with a liquid and its vapor. One side is in thermal contact with a hot spot, the other with a cold spot. The oscillation of the liquid plugs and vapor bubbles spontaneously occurs after the heating beginning. The heat exchange takes place not only by latent heat transfer, but also by liquid convection. Both this advantage and the PHP structural simplicity make it competitive with respect to other kinds of heat pipes. However, the PHP operation is non-stationary and depends on many physical and material parameters. As a result, application of empirical correlations is quite unsuccessful. More sophisticated direct modeling is thus required. In this review, we present the past, present, and future perspectives of the theoretical PHP studies. The physical phenomena on the level of a single bubble and single liquid plug is addressed first. In this context, the modeling of the simplest, single branch PHP is described as a necessary first step toward the PHP understanding and model validation. The modeling of interaction between the bubbles and plugs (bubble generation and disappearance, plug evaporation) is discussed next. Finally, the state of the art of the physical modeling and simulation of the multi-turn PHP is reviewed and the difference between existing approaches is explained. • Pulsating heat pipe (PHP) numerical simulations are reviewed. • 1D PHP simulations begin to emerge as a main PHP design tool. • 2D and 3D simulations are not mature enough for PHP designing but can be used to improve the flow models for 1D simulations. • Improvement of the background models (in particular, of the liquid film model) is necessary. • The components of the 1D PHP model should be validated against experiment for simple PHP geometries, in particular single branch and U-turn PHP. More U-turn PHP experiments are necessary.

Code development and analysis on the operation of liquid metal high temperature heat pipes under full condition

Heat pipe cooled reactor (HPR) is an innovative solid-core reactor which utilizes heat pipes to extract heat from the core to the energy conversion system. In this paper, a mathematical model for high-temperature heat pipes from a frozen state to a steady state is developed based on experimental data. The model is simplified and solved numerically. The two-dimensional transient heat conduction model of the wall and wick and the one-dimensional quasi-steady compressible vapor flow model are coupled to simulate the operation of a heat pipe when vaporization and condensation occur at the liquid–vapor interface. And the equivalent heat capacity model and the rarefied vapor model are introduced to simulate the process of heat pipe startup from a frozen state. Numerical results indicate that the model of heat pipe can predict the operation of high-temperature heat pipe with an error of less than 1.04% at steady state and 23.4% during the startup. Based on the simulation results, the velocity of vapor decreases with the heat transfer power in the steady state and the velocity of potassium vapor is 58.4% of sodium vapor velocity under the same operation condition. During the startup, when the rarefied-continuum interface located in the end of the evaporator section, the maximum vapor velocity occurs which is 1.3 times of steady velocity. This work makes it possible to predict the operation of a high-temperature heat pipe and provides a reference for the design and application of HPR.

An advanced conduction based heat pipe model accounting for vapor pressure drop

Heat pipes are passive heat transfer devices which play an increasingly important role in state-of-the-art cooling solutions for various technologies like consumer electronics or electric machines. During design phase, reliable simulation results are the key for optimal performance and efficiency. In system level simulations, heat conduction models, characterized by low computational effort, are widely used to predict heat pipe thermal resistance. In order to accurately calculate overall thermal characteristics, a precise prediction of the vapor core’s temperature drop is crucial. In this study, a novel conduction-based method for calculating heat pipe performance is presented. Taking velocity components perpendicular to the main vapor flow into account, an analytic expression for calculating the axial pressure gradient is developed and validated through detailed CFD simulations. Assuming saturation conditions, results are used to determine the vapor core’s local effective thermal conductivity. Parameters needed for calculation are provided for both circular and flat heat pipes under various operating conditions. The final model accounts for phase change thermal resistance and temperature dependent fluid properties. The presented method yields significantly different results compared to state-of-the-art approaches in which thermal vapor resistance is either assumed to be zero or calculated assuming (Hagen-)Poiseuille flow. Simulation results suggest a strong dependence of vapor thermal resistance on evaporation and condensation mass fluxes, thermophysical fluid properties and local vapor temperature.

Model validation and application of the coupled system of pipe-encapsulated PCM wall and nocturnal sky radiator

• A coupled system of the pipe-encapsulated PCM wall and nocturnal sky radiator is presented. • The thermal models for the coupled wall system are integrated. • The system experimental platform is setup and the model validation is presented. • Thermal and energy performances of the coupled system in the cooling season are analyzed. With the increasing of the global energy consumption and carbon emissions, the energy decarbonization and sustainable developments are of great urgency. To achieve the carbon peak and carbon neutral in energy utilization of the world, the efficient energy systems by using the natural energy are the popular and important methods. The coupled system of pipe-encapsulated PCM wall and nocturnal sky radiator is a new wall system for improving the performance of the building envelope. It can effectively reduce the building energy consumption with the PCM and the nocturnal natural cooling energy. In this study, the system thermal model coupled by the simplified PCM wall model, the heat-pipe model and the nocturnal radiation model is developed and validated by comparing the model prediction with the coincident experiment measurements. The average temperature difference of the conventional wall internal surface and the wall surface by using the pipe-encapsulated PCM wall is 0.4 °C. The average heat flux difference of the wall internal surface is 2.3 W/m 2 . In addition, the thermal and energy performances of a typical room with the coupled wall system in the cooling season are evaluated. Comparing to the conventional wall system, the internal surface average temperature of the coupled wall system can reduce 0.7 °C. The internal surface heat flow can reduce 53.0% in the cooling season. The required cooling energy consumption of the room in the cooling season can reduce 16.1%. The coupled wall system of pipe-encapsulated PCM wall and nocturnal sky radiator has good application prospects in low energy buildings.

Steady-state modelling of dual-evaporator loop heat pipe

To promote the thermal management of loop heat pipe in aerospace and electronics, a steady-state heat transfer model of the asymmetric dual-evaporator loop heat pipe (DE-LHP) is established, in which the mass flow distribution at the junction of the liquid line is modified. The control of operating temperature between two compensation chambers (CCs), heat sharing characteristics and influence of heat sink and ambient temperature on operating temperature are analyzed. The control of operating temperature is influenced by the CCs position and heat load distribution between two evaporators. Two domination zones, namely the position domination zone and heat load distribution domination zone, are observed when the heat load of the distal evaporator is larger than that of the proximal evaporator. A formula for determining the influencing range of the position domination zone is also proposed. The operating temperature reaches the minimum when the heat loads applied to the two evaporators are equal. The ambient temperature influences the operating temperature at low system heat load while the heat sink temperature prevails at high heat load. The heat sharing is affected by the total heat load and the loop pressure distribution, and proportional to the flow distribution at the junction of vapor line.

Modeling and simulation of the temperature field of selective laser sintering

Selective Laser Sintering (SLS) is a kind of powder-bed additive manufacturing technology, which can directly shape parts with complex geometry. The heat transfer process has an important influence on the forming quality of the SLS process. Computational methods are useful to study the heat transfer in the process since the temporal and spatial scale is extremely tiny. This paper presented a model, coupling the discrete element method and heat transfer equations, to calculate the powder-applying and heat-transfer phenomenon. Firstly, the discrete element method and the basic framework of the heat pipe model were introduced, and then the process of manufacturing a mini propeller blade by SLS was simulated.

Modeling for Fluid Flow and Heat Transfer in Closed Loop Pulsating Heat Pipe

Abstract Numerical modeling of multi-turn closed-loop pulsating heat pipe (CLPHP) is presented in this paper for ethanol as a working fluid. Modeling is carried out for 1-mm and 2-mm ID PHP for different numbers of turns, different orientations, and at constant wall temperature boundary conditions. Momentum and heat transfer variations with time are investigated numerically solving the one-dimensional governing equations for vapor bubble and liquid plugs. Evaporation and condensation take place by heat transfer through liquid film present around the vapor bubble. The code takes into account the realistic phenomena such as vapor bubble generation, liquid plug merging, and superheating of vapor bubbles above its saturation temperature. During the merging of liquid plugs, a time-step adaptive scheme is implemented and this minimum time-step was found to be 10−7 s. Nature of flow is investigated by momentum variation plot. Model results are compared with the experimental results from literature for nine different cases. Maximum variation in heat transfer for all these cases is found to be below ±34%.