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Pipes in heat exchanger systems are usually made of metallic materials to achieve a high level of thermal conductivity and thus high efficiency. However, substitution with polymer materials has a great potential. Plastics save weight, can be produced at lower cost and emissions, and are ideal for use in corrosive or chemically aggressive environments, where most metals fail. The challenge, however, is their much lower thermal conductivity in comparison. To reduce this disadvantage, fillers are used to transport thermal energy through the less conductive polymer matrix. In fact, high filler ratios are required to achieve an acceptable level of thermal conductivity for heat exchanger systems. However, these high filler ratios have a significant impact on the material properties of the polymer, such as the flexibility, which is drastically reduced. The result is a brittle material with limited suitability for use in heat exchanger systems. This paper describes a novel approach to effectively increase the thermal conductivity of polymer materials by optimizing the orientation of anisotropic filler particles by die design. Standard pipe extrusion dies result in continuous shear forces in the direction of extrusion. Accordingly, the resulting filler network has a higher thermal conductivity in the axial direction of the pipe than radially through the pipe wall. However, radial heat transfer is essential in pipe heat exchanger systems. To improve this, various die designs for targeted radial alignment of filler particles are discussed in this paper. Furthermore, the potential for improvement of the die designs is investigated. So far, the most efficient die design achieves an increase in thermal conductivity of up to 74 % compared to a standard pipe extrusion die. [ABSTRACT FROM AUTHOR]

Heat exchangers are a common heat transfer mechanism used in a variety of applications ranging from home to industrial, aircraft, and vehicles. The aim of this research is to study the hydrothermal efficiency of a double pipe heat exchanger with helical fins on annular side of counter flow heat exchanger numerically and compare the results of a double tube with and without screw tape. The investigation begins by employing numerical modeling of helical tapes under different configurations, with different pitches (10, 15, and 20) mm and one height of 10 mm as passive heat transfer augmentation strategies, which are simulated using ANSYS 2022 R1 software. The findings reveal that the current work's numerical results and earlier research's experimental findings are in good accord. Furthermore, while using (Helical Tap Insert) (HTI), the thermal performance of the new typical is found to be better than the typical model. Furthermore, it was found that the Nusselt number achieves the highest growth when the pitch (p) is (20) mm and increases the pressure drop. Furthermore, it was discovered that when the pitch (p) is (20) mm and pressure drop is increased, the Nusselt number grows the most. Furthermore, as associated to the typical model, the total thermal effectiveness of heat exchanger for the new model has greater by 9.9%. This difference is due to the fact that in the theoretical study, ideal conditions are assumed. The experimental design, the assumptions, operating circumstances, and accuracy of the devices are taken into account. It was noted that the value of the specified percentage of hydrothermal improvement achieved the highest value in performance for the same case that was with spiral fin and pitch length (20) mm, which is 1.6% compared to the traditional model. [ABSTRACT FROM AUTHOR]

This paper aims to improve the thermal performance of Closed loop pulsating heat pipe (CLPHP) with nanofluid fused with refrigerants. In this paper, the thermal studies were carried out by SWCNT and MWCNT nanofluids fused with R-134a. Experimentation was conducted at 10W heating power and 90° inclination angle, temperature difference from evaporator and condenser were validated. Results indicate that using nanofluid fused with R-134a, could improve the thermal performance of heat pipes. And the readings were taken every two minutes. And the obtained result was plotted to and the result shows the addition of CNT nanofluid to the refrigerant improves the Thermal performance of the closed-loop heat pipe (CLPHP). Compare to SWCNT, MWCNT with R-134a shows better heat transfer and improve the cooling capacity. [ABSTRACT FROM AUTHOR]

Extracting clean Energy through different technologies and producing electricity has played a major role nowadays due to constant growing energy consumption because of increase in population as well as deteriorating effect on atmosphere due usage of fossil fuels. To have an efficient clean energy technology has become the necessity of time. Application of solar thermal energy as Solar cooker, Solar dryer, Solar Water Heater, Solar desalination etc, are widely used in industry and domestic applications. Among these different application of Solar Thermal Energy, Evacuated Tube Collectors are used extensively so far as it proposes less convection losses which arise in solar concentrators from the movement of air over hot receiver surfaces. It is found that use of nanofluid for the thermal performance enhancement of Evacuated tube collector using heat pipe significantly changed scenario of using ETC. With this paper various characteristics of using nano fluid with Evacuated tube collector has been critically reviewed for the thermal performance enhancement of ETC with effect of parameters like weight fraction, filling ratio, type of nano particle. [ABSTRACT FROM AUTHOR]

In order to transfer heat with minimal leakage to surroundings and to increase efficiency energy exchanger systems are used through various methods. the output from the twin-pipe energy exchanger system is essential in terms of both system economics and efficiency. as a result, boosting the rate of thermal gradient rate is important for long-term energy growth. the goal of this study was to review heat transmission and friction properties in the horizontal double pipe heat exchanger. after thorough literature review, it has been concluded that various author has investigated different types of twisted tape insert their experimental and modeling studies and a little work is observed using twisted tape inserts with dimple configurations in a double pipe heat exchanger. [ABSTRACT FROM AUTHOR]

In this paper research has been conducted by varying the internal design of the pipes that are used in the compact air to air heat exchanger to see if the addition of turbulizers in the internal section can result in an improvement in the ΔT at the exit of the pipe. We are also looking at the internal flow characteristics of the air inside the heat exchanger pipes. The drop in pressure at the exit of the pipes because of the addition of the turbulizers has also been investigated. All these analysis have been performed using the commercial software package ANSYS CFX and the results are presented in this paper. [ABSTRACT FROM AUTHOR]

Recently heat transfer enhancement utilizing double pipe heat exchangers had been a key of engineering research topic in different field. The present work illustrates how magnesium oxide nanofluid (MgO) affects the enhancement of heat energy exchange in a double pipe heat exchanger at different concentrations of nanoparticles. The governing equations of heat and fluid flow had been solved using ANSYS FLUENT based upon finite volume scheme. The temperatures for hot and cold streams at the inlet are 50 oC & 25 oC respectively. The volumetric flowrates for hot fluid (8-20) LPM and for cold fluid 8 LPM. The magnesium oxide nanoparticles MgO have concentrations range (0.125 % - 2%) by volume were implemented. According to the findings of the study implemented in sustainable applications and different industries fields, heat energy exchange rises when the volume concentration of nanoparticles increases also. Currently, there are several studies have been conducted to assess the consequence of nanofluid that enhancement heat energy exchange in the double pipe heat exchanger. The type and concentration of nanoparticles in the base fluid play important role in the enhancement of heat energy exchange. This paper illustrated the effect of increased magnesium oxide nanofluid MgO nanoparticles concentration on the heat energy exchange of a double pipe heat exchanger. [ABSTRACT FROM AUTHOR]

This study investigates heat transmission and thermal performance factor characteristics in a tube with coil wire and twisted tape inserts using air as the working fluid. This study investigates heat transmission and thermal performance factor characteristics in a tube with coil wire and twisted tape inserts using air as the working fluid. The goal of this research is to improve forced convection heat transfer in tubes with and without swirl inserts and coil wire inserts for air Reynolds numbers ranging from 10.328×103 to 11.769×103. The impacts of different parameters of heat transfer are considered in this work with heat flux are constant conditions of 600 W/m2 and 800 W/m2 with using the coil wire and twisted tapes with three ratios (y/w = 3, 4 and 5). The test section is a horizontal annular passage with outer diameter 10 cm. The results showed that using coil wire inserts to improve heat transfer coefficient factors in tubes resulted in local heat transfer coefficient percentages of (5.1%) to (7.8%) with heat flux are constant conditions of 600 W/m2 and 800 W/m2, respectively. While tube with swirl inserts provide local heat transfer coefficient percentages of (3.6% to 6.1%) for heat fluxes (600) W/m2 and (800) W/m2, respectively. The obtained results demonstrate that as the twisted ratio (y/w) decreases, the mean Nusselt number and heat transfer coefficient in the tube with twisted tape and coil wire rise. Experimental results is shown, heat transfer coefficient for the coil wire when compared with that of swirl inserts tube were ranged between (10.6%) to (16.5%) and (11.5%) to (17.3%) for the heat flux (600) W/m2. [ABSTRACT FROM AUTHOR]

Investigation still on board for finding the best techniques to develop the thermal and hydraulic performance of the heat exchanger. One of the useful, practical methods for finding the modification is the numerical analysis. This numerical analysis was obtained by modelling and simulation using ANSYS FLUENT software. In this present work five elements were modelled and simulated of three different configuration models, helical tape, rectangular and triangular straight strip inserts were inserted inside the inner pipe. This simulation was employed according to the working operation conditions of the real practical double pipe heat exchanger. The results show that, the helical tape configuration demonstrated the effective enhancing technique of the heat transfer, which is improving the thermo-hydraulic performance of this type of heat exchanger in this study. According to this work, the technique of studying numerically the design enhancement of the heat exchanger can produce an excellent result and gives a good recommendation to transfer it into practical work. [ABSTRACT FROM AUTHOR]

Energy crisis and carbon emission are increasingly prominent issues in our society. As one of the clean energy sources, thermoelectric power generation is a promising alternative energy technology to convert heat into electricity. If there is a heat source, thermoelectric generators can provide electricity for watches, sensors, electronics, spacecraft, etc., and can also be used to recover waste heat, such as automobile exhaust heat, industrial waste heat, ship waste heat, etc. This paper begins with the basic principles of thermoelectric generators and an outlook of thermoelectric materials in different application scenarios. Then, the thermoelectric generator systems, which can be classified into different groups according to their different power generation levels, also the corresponding progress, challenges, and future development are presented. In addition to exploring high-performance thermoelectric materials, the system efficiency of thermoelectric generators is much dependent upon advanced structural design and thermodynamic optimizations. In the previous and present works, some new optimization methods are applied to improve the performance of thermoelectric devices and systems, these include such as the asymmetric design, phase change heat transfer, optimization for thermoelectric devices based on temperature distributions, also heat pipe and converging design for thermoelectric generator systems. These works help breakthrough in thermoelectric power generation from low-level power generation to relatively higher-level power generation. The current progress helps provide a comprehensive insight into thermoelectric power generation technology from micro power supplies to kilowatt systems. [ABSTRACT FROM AUTHOR]

Evaporative cooling has been around since ancient times. As a cooling device, evaporative cooling has several advantages and disadvantages. This paper discusses some of the results of investigations on three types of evaporative cooling, namely direct evaporative cooling, indirect evaporative cooling, and combined evaporative cooling. Cooling media is part of the evaporative cooling that is widely studied. Some researchers also made several innovations to improve the efficiency of evaporative cooling. One of the innovations that will be discussed in this paper is to add heat pipes to evaporative coolers. As a passive heat transfer, heat pipes are expected to increase the efficiency of evaporative cooling without adding high cost. [ABSTRACT FROM AUTHOR]

Binary non-azeotropic mixtures are increasingly used in various heat exchange devices as a heat carrier or working fluid. When using non-azeotropic mixtures, it is necessary to take into account the features of the heat transfer process determined by specific thermodynamics of these mixtures. This paper presents the results of a study of hydrodynamics and heat transfer during circulation of an alcohol-water mixture through a heated cylindrical channel. The inner diameter of the channel is 7.6 mm, the length is 4 m. The experiments were carried out using a mixture of ethyl alcohol and water at mass concentration of the volatile component of 22 %. Mass velocity range is 160–720 kg/(m2·s), the vessel pressure is 0.02 MPa. The operating parameters correspond to both single-phase and two-phase heat transfer regimes. A detailed description of the experimental setup and the results on temperature distribution along the working section are given. It is found that in a non-azeotropic mixture, the vaporization process begins significantly earlier than the equilibrium conditions determine. [ABSTRACT FROM AUTHOR]

The steady state thermodynamic investigation of vapour compression refrigeration system (VCRS) using CNT, Au and HAuCl4 nanoparticles suspended in different proportions in polyalkylene glycol lubricant is carried out and presented in this paper. The most commonly adopted R134a refrigerant is used in the system. The energetic and exergetic analysis are performed for nanoparticles with different volume fractions viz. 0.1% Au, 0.2% Au, 0.1% HAuCl4, 0.2% HAuCl4, 0.1% Au and 0.05% CNT, 0.2% Au and 0.02% of CNT. The experimental test rig is fabricated and tested with the different proportions of nanoparticles. Compressor exit temperature and pressure, condenser exit temperature, evaporator inlet and exit temperature are measured. The energy and exergy efficiency of the system components, cooling capacity, compressor power input and coefficient of performance (COP) of the system are obtained. The suspension of Au, HAuCl4 and CNT nanoparticles up to 0.2% and combination of Au(0.1%, 0.2%) and CNT(0.02%, 0.05%) increases the heat transfer characteristics and thereby system functioning. From the results it is established that the power input decreases by 16.7%, COP increases by 21.3% and the second law efficiency increases by 11.2%. [ABSTRACT FROM AUTHOR]

This paper gives an overview of several published current studies concerning heat transfer enhancement on the double pipe heat exchanger. Since double pipe heat exchanger has drawn much consideration due to simplicity and wide-range utility. The flow manipulation technique to enhance heat transfer in a double pipe heat exchanger has been frequently cited. Also, various research addressing working fluid on a double pipe heat exchanger has been discussed in detail. Another technique is known as surface modification that has an infrequent investigation comparing the other techniques. Correlation of Nusselt number and pressure drop coefficient shown in this overview. Future research opportunities are mentioned. [ABSTRACT FROM AUTHOR]

Heat pipes are promising heat removal devices widely used in a variety of fields ranging from thermal management of electronic components to terrestrial and aerospace applications. Their working principle, phase change of a working fluid, makes them superior to other conventional cooling methods. Heat pipes have been commercially available since the mid of 1960’s. Only in the past few years the electronics industries have embraced heat pipes as reliable, cost-effective solutions for high end cooling applications. This paper presents a detailed review of the materials, applications and performance of heat pipes using nanofluid as working medium. The first section reviews the heat pipes which are operating with nanofluid as working fluid. This section deals with nanofluid of different nanoparticles. The second section reviews heat pipes coated with different types of nanoparticles and third section reviews the performance of heat pipes with nanoparticle coated wick. [ABSTRACT FROM AUTHOR]

Mainly consider the heat conduction between the layers of fabric, the thermal convection between the fabric and the air to establish the model of this paper. Firstly, the heat conduction equation between multilayer fabrics is constructed by the law of conservation of energy and Fourier's law; then, the heat convection equation between the fabric and the air is expressed by Newton's cooling formula; Finally, the thermal conductivity, temperature boundary conditions and temperature load between the fabrics of each layer were determined, and a heat transfer model conforming to the external environment—fabric (I~III)—air layer—human skin system was established. Considering the wide application of finite element in heat transfer, we use ANSYS heat transfer module to calculate the temperature distribution of each layer of the system by using transient thermal analysis method. The simulation results are well fitted to the actual experimental results, and the temperature of each layer is generated. Distribution. [ABSTRACT FROM AUTHOR]

The paper proposes a design of a condensing heat exchanger of thermosyphon loop heat pipe for cooling of electrotechnical box. The heat pipe appears to be a very good cooling device that does not require any mechanical device in its construction. A heat pipe is a structurally simple device capable of transmitting high heat flows over relatively long distances. It may be of circular or other cross-section. If one part of the heat pipe is heated - the evaporator and the other cooled - the condenser, there is a constant evaporation and condensation of the working fluid. The section between the evaporator and the condenser where there is no phase change is called adiabatic section. The heat transfer in the heat pipe is based on the phase change principle. The design of the condensing exchanger must ensure condensation of the vapor phase and heat the environment to the surrounding environment in an ideal state only by natural convection. The article describes the design of a condensing exchanger and the impact of the amount of working fluid on performance heat pipes. [ABSTRACT FROM AUTHOR]

This paper is concerned with design and optimization of cooling system for high-power LED luminaire. Dimension, shape and topology optimization algorithms and their program realization are developed with the use of MathCad for the maximal heat transfer rate, least temperature gradient between LED and fins, minimal weight of cooling system. Thermal design by Comsol Multiphysics showed that the system on the base of heat pipes is the most effective system for high-power LED luminaire. [ABSTRACT FROM AUTHOR]

This paper proposes an approach of calculation of permissible heat fluxes of heat pipe. Simulation algorithms and their program realization are developed with the use of MATCAD. Permissible heat fluxes in the heat pipe are determined accounting operating conditions and properties of capillary-porous materials. This approach allows us to compare heat transfer characteristics of the heat pipe by using different materials and coolant. By employing simulation data heat pipes with mesh wick and groove wick are designed. [ABSTRACT FROM AUTHOR]

The paperdeals with use of LOOP thermosyphonheat pipe to transfer heat from electrotechnical box and describe of construction individual types ofLOOP heat pipes. The LOOP heat pipe is very good cooling device which requires no mechanical parts in their design. LOOP heat pipe use only phase change during heat transfer, without a compressor, fan orpump. LOOP heat pipe is more energy saving compared to conventional cooling systems with forced convection. The main advantage of cooling by heat pipe is that electrotechnical box canbe hermetically closed (dust-free construction), because dust reduces the lifetime of electrotechnical elements in box Lifetime ofLOOP heat pipeequals to the lifetime of construction material. The paper describes mathematical model of LOOP thermosyphon heat pipe (condenser). Compares selectedtypes of working fluids which are filled with a heat pipe and construction materials of heat pipe. [ABSTRACT FROM AUTHOR]

Heat intake minimization is one of the main challenges during the design process of cryogenic storage tanks. It is widely known that connection pipes significantly contribute to this residual heat transfer from ambient temperature conditions to the cold inner vessel. A certain pipe inclination can cause a convective flow field within the fluid. This effect usually increases the total heat transfer much more dramatically than anticipated. In several previous papers we discussed the impact of pipe geometry as well as boundary conditions intensively. However, there is no suitable correlation in literature available which could be used to estimate the total heat transfer properly. The large number of experimental data we gained during our investigations allows us to propose a new correlation in order to predict the total heat transfer through an inclined pipe in function of the inclination angle. In this paper we derivate this new correlation and show its application for heat transfer estimations. Several comparisons are carried out against our own measurements as well as literature data. [ABSTRACT FROM AUTHOR]

Cryogenic loop heat pipes (CLHPs) are promising thermal links between the cryocoolers and the cooled components. This paper presents a prototype of cryogenic loop heat pipe working in the temperature range from 77 K to 100 K with nitrogen as working fluid, whose high heat transfer capacity was investigated in the earlier work. Besides a main loop, the prototype introduced a secondary loop with responsibility for the cool down and the thermal management of the CLHP during the operation. In this paper, a series of experiments were performed by changing the heat loads on the secondary evaporator during the operating process. The results were compared and analyzed to investigate the influence of the secondary evaporator to the performance of the CLHP. Beneficial conclusions were also obtained and demonstrated. [ABSTRACT FROM AUTHOR]

Heat pipes are heat transfer devices capable of transferring large quantities of heat effectively and efficiently. A vapor chamber (VC) is a flat heat pipe. A novel VC with hollow condenser tubes embedded on the top of it is proposed. This paper reports on the experimental thermal performance of three VC devices embedded with hollow tubes and employed as heat sinks. The first device consisted of a VC with a single hollow tube while the other two VCs had an array of multi-tubes with different tube lengths. All three devices were tested under natural and force air convection cooling. An electrical resistance heater was employed to provide power inputs of 10 and 40 W. Surface temperatures were measured with thermocouple probes at different locations around the devices. The results show that temperatures increased with heater input while total device thermal resistances decreased. Force convection results in lower temperatures and lower resistance. Dry-out occurs at high input power and with too much condensing area. There appears to be an optimum fill ratio which depended upon dimensions of the VC and also heating power. [ABSTRACT FROM AUTHOR]

A heat pipe is a passive heat transfer device capable of transferring a large quantity of heat effectively and efficiently over a long distance and with a small temperature difference between the heat source and heat sink. A heat pipe consists of a metal pipe initially vacuumed and then filled with a small quantity of fluid inside. The pipe is separated into a heating (evaporator) section and a cooling (condenser) section by an adiabatic section. In a run-around-coil heating, ventilation and air conditioning system, a wrap-around heat pipe heat exchanger could be employed to increase dehumidification and to reduce cooling costs. The thermal performance of a thermosyphon is dependent upon type of fill liquid, fill ratio, power input, pipe inclination and pipe dimensions. The boiling and condensation processes that occur inside a thermosyphon are quite complex. During operation, dry-out, burn-out or boiling limit, entrainment or flooding limit and geysering occur. These phenomena would lead to non-uniform axial wall temperature distribution in the pipe, or worse still, ineffective operation. In order to have a better understanding of the internal heat transfer phenomena, a visual study using transparent glass tubes and high speed camera recording of the internal flow patterns would be most helpful. This paper reports on an experimental investigation conducted to visualise the flow patterns in straight and Cshape thermosyphons. The pictures recorded enabled the internal flow boiling and condensation pattern occurring inside a straight and a C-shape thermosyphon to be observed. The thermosyphons were fabricated from 10 mm O/D x 8 mm I/D x 300 mm long glass tubes and filled with water with fill ratios from 0.5 - 1.5. The evaporator sections of the thermosyphons were immersed into a hot water tank that was electrically heated from cold at ambient temperature till boiling. Cooling of the condenser section was achieved using a fan. Preliminary results showed that dry-out occurred earlier at lower evaporator temperatures with small fill ratios. Further investigations to determine saturation and thermosyphon wall temperatures with various fill liquids and at different fill ratios, inclinations and pipe sizes are necessary with a more sophisticated video recording system. [ABSTRACT FROM AUTHOR]

This paper describes an integrated thermal-mechanical architecture for responsive small satellites that combines the intrinsic thermal management capabilities of the Satellite Modular and Reconfigurable Thermal System (SMARTS) with the modular, quick assembly mechanical design offered by the Plug-and-Play Satellite-2 (PnPSat-2) structure. PnPSat-2 is the second generation Plug and Play satellite bus. SMARTS and PnPSat-2 are technology development programs funded by the Air Force Research Laboratory (AFRL) Space Vehicles Directorate at Kirtland AFB, NM. To meet the three-tiered responsive space needs of the DoD, varying levels of responsiveness from immediate (Tier 1), to days (Tier 2), to months (Tier 3) are needed. SMARTS offers an intrinsic thermal solution that combines a multi-heat pipe isothermal bus, oversized body-mounted radiators, a bus-to-radiator loop heat pipe (LHP) thermal transport/switching system, and maximum external insulation. PnPSat-2 offers an intrinsic mechanical solution that includes panels with embedded electronics, multiple component mounting locations, and features that allow the spacecraft to be transitioned to a flat-sat for easy assembly, integration, de-integration, and reassembly. To combine the two architectures, internal half-panels were redesigned to include spreader heat pipes (SHPs) for intra-panel isothermalization, side-to-top panel structural brackets were replaced by a combined bracket-header heat pipe (HHP) for inter-panel isothermalization, and an LHP evaporator was mounted to the HHP to enable variable conductance between the isothermal spacecraft bus and body-mounted (condensers) radiators. To evaluate the concept, a three-panel thermal test bed will be configured to simulate the combined SMARTS/PnPSat-2 configuration. The paper will describe the system design, key analytical trades, hardware status, and available test data. [ABSTRACT FROM AUTHOR]

This paper describes potential heat rejection design concepts for Brayton power conversion systems. Brayton conversion systems are currently under study by NASA for Nuclear Electric Propulsion (NEP) and surface power applications. The Brayton Heat Rejection Subsystem (HRS) must dissipate waste heat generated by the power conversion system due to inefficiencies in the thermal-to-electric conversion process. Sodium potassium (NaK) and H2O are two coolant working fluids that have been investigated in the design of a pumped loop and heat pipe space HRS. In general NaK systems are high temperature (300 to 1000 K) low pressure systems, and H2O systems are low temperature (300 to 600 K) high pressure systems. NaK is an alkali metal with health and safety hazards that require special handling procedures. On the other hand, H2O is a common fluid, with no health hazards and no special handling procedures. This paper compares NaK and H2O for the HRS pumped loop coolant working fluid. A detailed excel analytical model, HRS_Opt, was developed to evaluate the various HRS design parameters. It is capable of analyzing NaK or H2O coolant, parallel or series flow configurations, and numerous combinations of other key parameters (heat pipe spacing, diameter and radial flux, radiator facesheet thickness, fluid duct system pressure drop, system rejected power, etc.) of the HRS. This paper compares NaK against water for the HRS coolant working fluid with respect to the relative mass, performance, design and implementation issues between the two fluids. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper describes four advanced devices (or tools) that were developed to help solve problems in cryogenic integration. The four devices are: (1) an across-gimbal nitrogen cryogenic loop heat pipe (CLHP); (2) a miniaturized neon CLHP; (3) a differential thermal expansion (DTE) cryogenic thermal switch (CTSW); and (4) a dual-volume nitrogen cryogenic thermal storage unit (CTSU). The across-gimbal CLHP provides a low torque, high conductance solution for gimbaled cryogenic systems wishing to position their cryocoolers off-gimbal. The miniaturized CLHP combines thermal transport, flexibility, and thermal switching (at 35 K) into one device that can be directly mounted to both the cooler cold head and the cooled component. The DTE-CTSW, designed and successfully tested in a previous program using a stainless steel tube and beryllium (Be) end-pieces, was redesigned with a polymer rod and high-purity aluminum (Al) end-pieces to improve performance and manufacturability while still providing a miniaturized design. Lastly, the CTSU was designed with a 6063 Al heat exchanger and integrally welded, segmented, high purity Al thermal straps for direct attachment to both a cooler cold head and a Be component whose peak heat load exceeds its average load by 2.5 times. For each device, the paper will describe its development objective, operating principles, heritage, requirements, design, test data and lessons learned. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Pulsating Heat Pipes, the new two-phase heat transfer devices, with no counter current flow between liquid and vapor have become a modern topic for research in the field of thermal management. This paper focuses on the performance of methanol and distilled water as working fluid in a closed loop pulsating heat pipe (CLPHP). This performances are compared in terms of thermal resistance, heat transfer co-efficient, and evaporator and condenser wall temperature with variable heat inputs. Methanol and Distilled water are selected for their lower surface tension, dynamic viscosity and sensible heat. A closed loop PHP made of copper with 2mm ID and 2.5mm OD having total 8 loops are supplied with power input varied from 10W to 60W. During the experiment the PHP is kept vertical, while the filling ratio (FR) is increased gradually from 40% to 70% with 10% increment. The optimum filling ratio for a minimum thermal resistance is found to be 60% and 40% for distilled water and methanol respectively and methanol is found to be the better working fluid compared to distilled water in terms of its lower thermal resistance and higher heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Pulsating heat pipe (PHP) is a new, promising yet ambiguous technology for effective heat transfer of microelectronic devices where heat is carried by the vapor plugs and liquid slugs of the working fluid. The aim of this research paper is to better understand the operation of PHP through experimental investigations and obtain comparative results for different parameters. A series of experiments are conducted on a closed loop PHP (CLPHP) with 8 loops made of copper capillary tube of 2 mm inner diameter. Ethanol is taken as the working fluid. The operating characteristics are studied for the variation of heat input, filling ratio (FR) and orientation. The filling ratios are 40%, 50%, 60% and 70% based on its total volume. The orientations are 0° (vertical), 30°, 45° and 60°. The results clearly demonstrate the effect of filling ratio and inclination angle on the performance, operational stability and heat transfer capability of ethanol as working fluid of CLPHP. Important insight of the operational characteristics of CLPHP is obtained and optimum performance of CLPHP using ethanol is thus identified. Ethanol works best at 50-60%FR at wide range of heat inputs. At very low heat inputs, 40%FR can be used for attaining a good performance. Filling ratio below 40%FR is not suitable for using in CLPHP as it gives a low performance. The optimum performance of the device can be obtained at vertical position. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of a closed loop Pulsating Heat Pipe (CLPHP) as the demand of smaller and effective heat transfer devices is increasing day by day. PHP is a two phase heat transfer device suited for heat transfer applications, especially suited for handling moderate to high heat fluxes in different applications. A copper made Pulsating Heat Pipe (PHP) of 250 mm length is used in this experimental work with 2 mm ID and 3 mm OD, closed end-to-end in 8 looped, evacuated and then partially filled with working fluids. The evaporation section is 50 mm, adiabatic section is 120 mm and condensation section is 80 mm. The performance characterization is done for two working fluids at Vertical (0°) orientations. The working fluids are Methanol and Ethanol and the filling ratios are 40%, 50%, 60% & 70% based on total volume, respectively. The results show that the influence of various parameters, the heat input flux, and different filling ratios on a heat transfer performance of CLPHP. Methanol shows better performance as working fluid in PHP than ethanol at present orientation for a wide range of heat inputs and can be used at high heat input conditions. Ethanol is better choice to be used in low heat input conditions. [ABSTRACT FROM AUTHOR]

A simple, rapid mathematical model to calculate the non-steady-state startup process of the jacket-type radial heat pipe is presented in this paper. The model is based on the special structure and using conditions of the jacket-type radial heat pipe, the vapor temperature in heat pipe only changes over time. The startup performance of the heat pipe with variation input heat flux and the filling rate is analyzed. The results manifest that the filling rate increased will reduce the maximum operating temperature of the heat pipe and shorten the startup time of the heat pipe. With the increase of input heat flux, the operating temperature increases and the time to reach the steady state of the heat pipe is added. The total thermal resistance of heat pipe decreases with the increase of the input heat flux and filling rate. The variation of the local convective heat transfer coefficient and the maximum temperature of the water are investigated with different cooling water inlet conditions. [ABSTRACT FROM AUTHOR]

Modeling of transient thermal-fluid phenomena, temperature control, and gravity effects for conventional and upgraded loop heat pipe (LHP) systems with complex radiators, multiple components, and varying environmental conditions, can be effectively accomplished using Thermal Desktop™. This paper outlines some recent advances in transient modeling of LHP systems with multiple components, such as a) one LHP with two parallel condensers on two orthogonal radiators with varying environmental heating loads, b) two LHPs sharing one radiator under heat loading and sink conditions changing in time, and c) a hybrid LHP with two main and one secondary evaporators undergoing transients due to the heat loads variation. Cleared for open publication, Oct. 29, 2009, OSR, DOD, 10-S-0170. [ABSTRACT FROM AUTHOR]

This paper describes a novel thermal control system for the Warm Electronics Box (WEB) on board a small lunar surface lander intended to support science activities anywhere on the lunar surface for an extended duration of up to 6 years. Virtually all lander electronics, which collectively dissipate about 60 W in the reference mission, are contained within the WEB. These devices must be maintained below 323 K (with a goal of 303 K) during the nearly 15-earth-day lunar day, when surface temperatures can reach 390K, and above 263 K during the nearly 15-earth-day lunar night, when surface temperatures can reach 100 K. Because of the large temperature swing from lunar day-to-night, a novel thermal switching system was required that would be able to provide high conductance from the WEB to the radiator(s) during the hot lunar day and low (or negligible) conductance during the cold lunar night. The concept that was developed consists of ammonia variable conductance heat pipes (VCHPs) to collect heat from WEB components and a polymer wick propylene loop heat pipe (LHP) to transport the collected heat to the radiator(s). The VCHPs autonomously maximize transport when the WEB is warm and autonomously shut down when the WEB gets cold. The LHP autonomously shuts down when the VCHPs shut down. When the environment transitions from lunar night to day, the VCHPs and LHP autonomously turn back on. Out of 26 analyzed systems, this novel arrangement was able to best achieve the combined goals of zero control power, autonomous operation, long life, low complexity, low ΔT, and landed tilt tolerance. [ABSTRACT FROM AUTHOR]

The use of heat pipes and loop heat pipes (LHPs) as thermal control devices in satellites and other spacecrafts have increased considerably during the last decades. Due to the need of a crescent demand on heat dissipation, more of these devices have been used in low orbit and geo-stationary satellites to promote the adequate thermal control for the instrumentation and equipments aboard. Even though each design is dedicated to a specific application, heat pipes and LHPs have to go through acceptance tests prior to be integrated in the satellite’s honeycomb panels (in case of most heat pipes applications) and instrumentation (in case of LHPs thermal control). In order to give a guideline for the required acceptance tests that heat pipes and LHPs have to be submitted, this paper presents the constraints that need to be considered while performing those tests. In all cases, the acceptance tests for both heat pipes and LHPs are related to the most important conditions to verify their proper operation before integrating them on their designed application in a satellite. Such acceptance tests are important to correctly evaluate these thermal control devices and avoid malfunctions while qualifying the entire satellite. [ABSTRACT FROM AUTHOR]

Radiators operating in lunar or Martian environments must be designed to reject the maximum heat load at the maximum sink temperature, while maintaining acceptable temperatures at lower powers or sink temperatures. Variable Conductance Heat Pipe (VCHP) radiators can passively adjust to these changing conditions. Due to the presence of non-condensable gas (NCG) within each VCHP, the active condensing section adjusts with changes in either thermal load or sink temperature. In a Constant Conductance Heat Pipe (CCHP) without NCG, it is possible for all of the water to freeze in the condenser, by either sublimation or vaporization. With a dry evaporator, startup is difficult or impossible. Several previous studies have shown that adding NCG suppresses evaporator dryout when the condenser is frozen. These tests have been for relatively short durations, with relatively short condensers. This paper describes freeze/thaw experiments involving a VCHP with similar dimensions to the current reactor and cavity cooling radiator heat pipe designs. [ABSTRACT FROM AUTHOR]

The TacSat-4 micro-satellite uses an aluminum/ammonia loop heat pipe (LHP) to transport 700 W of heat from the electronics to two radiator sections. In addition to the thermal requirements, there were additional specifications for the primary and secondary wicks, and the flow balancer between the two LHP condensers. This paper discusses the experimental test rigs designed to verify the LHP performance against these requirements. The measured LHP performance at various operating conditions including start-up, un-balanced condenser heat removal, transient power, high power, and shut-down is discussed. [ABSTRACT FROM AUTHOR]

The paper discusses an advanced hybrid two-phase loop technology for spacecraft thermal management. The hybrid loop integrates active mechanical pumping with passive capillary pumping promising a reliable yet high performance cooling system. The advanced evaporator design using porous wick structures was developed for the hybrid loop to enhance boiling heat transfer by passive phase separation. The prototype testing using various hybrid loops and components demonstrated that the hybrid loop was capable of removing high heat fluxes from multiple heat sources with large surface areas up to 135 cm2. Because of the passive capillary phase separation, the hybrid loop operation doesn’t require any active flow control of excess liquid in the evaporator, even at highly transient and asymmetrical heat inputs. These performance results represent significant improvements over state-of-the-art heat pipes, loop heat pipes and evaporative spray cooling devices in terms of performance, robustness and simplicity. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper describes the gravity effect on heat transport characteristics in a miniature loop heat pipe with multiple evaporators and multiple condensers. Tests were conducted in three different orientations: horizontal, 45deg tilt, and vertical. The gravity affected the loop’s natural operating temperature, the maximum heat transport capability, and the thermal conductance. In the case that temperatures of compensation chambers were actively controlled, the required control heater power was also dependent on the test configuration. In the vertical configuration, the secondary wick was not able to pump the liquid from the CC to the evaporator against the gravity. Thus, the loop could operate stably or display some peculiar behaviors depending on the initial liquid distribution between the evaporator and the CC. Because such an initial condition was not known prior to the test, the subsequent loop performance was unpredictable. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

During the last several years, high temperature heat pipes have been identified as enabling thermal control devices for several emerging applications. NASA’s Prometheus initiative serves as a primary focus for much of the interest to expand thermal management capabilities to higher temperatures and increase the technology readiness level. Recently Swales and other investigators were tasked by NASA’s Glenn Research Center (GRC) to design and fabricate several titanium-water heat pipes to satisfy the very challenging requirements associated with the Prometheus initiative. This program was successfully finished in March of 2006 and three high performance titanium-water development heat pipes were delivered to GRC. This paper describes the basic heat pipe design and its predicted performance capabilities. The manufacture and test of the heat pipes are also briefly discussed. Test results are discussed and compared to the existing hardware performance. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

NASA is interested in Brayton cycle converters for both nuclear powered spacecraft, and lunar/Mars missions. The radiator to dissipate the waste heat will operate at temperatures in the 400–550 K range. To date, nearly all space radiator systems have used aluminum heat pipes with ammonia working fluid, but these heat pipes are not suitable in the higher temperature range. A Graphite Fiber Reinforced Composites (GFRC) radiator with high temperature water heat pipes is currently under development. Previous short-term life tests indicated that water would be compatible with titanium and Monel heat pipes. This paper presents the results to date of heat pipe life tests with commercially pure titanium, titanium alloys, as well as Monel K500 and Monel 400. To date, the life test pipes have operated successfully at 500 and 550 K. The Monel and CP-Titanium pipes have operated for 11,800 hours, while the titanium alloy pipes have operated for 3,300 hours. The life tests are ongoing. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Space nuclear systems require large area radiators to reject the unconverted heat to space. System optimizations with Brayton cycles lead to radiators with radiator temperatures in the 400 to 550 K range. To date, nearly all space radiator systems have used aluminum/ammonia heat pipes but these components cannot function at the required temperatures. A Graphite Fiber Reinforced Composites (GFRC) radiator with high temperature water heat pipes is currently under development. High temperature GFRC materials have been selected, and will be tested for thermal conductivity and structural properties. Titanium/water and Titanium/Monel heat pipes have been successfully operated at temperatures up to 550 K. Titanium was selected as the baseline envelope material, due to its lower mass and previous experience with bonding titanium into honeycomb panels. Heat pipes were fabricated with a number of different wick designs, including slab and grooved wicks. Since titanium cannot be extruded, the grooves are being fabricated in sintered titanium powder. The paper reports on the radiator design, materials selection, heat pipe to fin bonding, heat pipe design, and experimental results. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

One of the primary goals in designing a fission power system is to ensure that the system can be developed at a low cost and on an acceptable schedule without compromising reliability. The Heatpipe Power System (HPS) is one possible approach for producing near-term, low-cost, space fission power. The Heatpipe Operated Moon Exploration Reactor (HOMER-25) is a HPS designed to produce 25-kWe on the lunar surface for 5 full-power years. The HOMER-25 core is made up of 93% enriched UO2 fuel pins and stainless-steel (SS)/potassium (K) heatpipes in a SS monolith. The heatpipes transport heat generated in the core through the water shield to a potassium boiler, which drives six Stirling engines. The operating heatpipe temperature is 880 K and the peak fast fluence is 1.6e21 n/cm2, which is well within an established database for the selected materials. The HOMER-25 is designed to be buried in 1.5 m of lunar regolith during operation. By using technology and materials which do not require extensive technology development programs, the HOMER-25 could be developed at a relatively low cost. This paper describes the attributes, specifications, and performance of the HOMER-25 reactor system. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The S∧4 reactor has a sectored, Mo-14%Re solid core for avoidance of single point failures in reactor cooling and Closed Brayton Cycle (CBC) energy conversion. The reactor is loaded with UN fuel, cooled with a He-Xe gas mixture at ∼1200 K and operates at steady thermal power of 550 kW. Following a launch abort accident, the axial and radial BeO reflectors easily disassemble upon impact so that the bare reactor is subcriticial when submerged in wet sand or seawater and the core voids are filled with seawater. Spectral Shift Absorber (SSA) additives have been shown to increase the UN fuel enrichment and significantly reduce the total mass of the reactor. This paper investigates the effects of SSA additions on the temperature and burnup reactivity coefficients and the operational lifetime of the S∧4 reactor. SSAs slightly decrease the temperature reactivity feedback coefficient, but significantly increase the operating lifetime by decreasing the burnup reactivity coefficient. With no SSAs, fuel enrichment is only 58.5 wt% and the estimated operating lifetime is the shortest (7.6 years) with the highest temperature and burnup reactivity feedback coefficients (-0.2709 ¢/K and -1.3470 $/atom%). With europium-151 and gadolinium-155 additions, the enrichment (91.5 and 94 wt%) and operating lifetime (9.9 and 9.8 years) of the S∧4 reactor are the highest while the temperature and burnup reactivity coefficients (-0.2382 and -0.2447 ¢/K; -0.9073 and 0.8502 $/atom%) are the lowest. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A nuclear space power system that consists of a solid metal nuclear reactor core with heat pipes carrying energy to a cylindrical thermoelectric converter surrounding each of the heat pipes with a heat pipe radiator surrounding the thermoelectric converter is the most simple and reliable space power system. This means no single point of failure since each heat pipe and cylindrical converter is a separate power system and if one fails it will not affect the others. The heat pipe array in the solid core is designed so that if an isolated heat pipe or even two adjacent heat pipes fail, the remaining heat pipes will still transport the core heat without undue overheating of the uranium nitride fuel. The primary emphasis in this paper is on simplicity, reliability and fabricability of such a space nuclear power source. The core and heat pipes are made of Niobium 1% Zirconium alloy (Nb1Zr), with rhenium lined fuel tubes, bonded together by hot isostatic pressure, (HIPing) and with sodium as the heat pipe working fluid, can be operated up to 1250K. The cylindrical thermoelectric converter is made by depositing the constituents of the converter around a Nb1%Zr tube and encasing it in a Nb 1% Zr alloy tube and HIPing the structure to get final bonding and to produce residual compressive stresses in all brittle materials in the converter. A radiator heat pipe filled with potassium that operates at 850K is bonded to the outside of the cylindrical converter for cooling. The solid core heat pipe and cylindrical converter are mated by welding during the final assembly. A solid core reactor with 150 heat pipes with a 0.650-inch (1.65 cm) ID and a 30-inch (76.2 cm) length with an output of 8 Watts per square inch as demonstrated by the SP100 PD2 cell tests will produce about 80 KW of electrical power. An advanced solid core reactor made with molybdenum 47% rhenium alloy, with lithium heat pipes and the PD2 theoretical output of 11 watts per square inch or advanced higher temperature converter to operate at 1350K could produce a greater output of approximately 100KW. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper discusses the development of an advanced hybrid loop technology that incorporates elements from both passive and active loop technologies. The result is a simple yet high performance cooling technology that can be used to remove high heat fluxes from large heat input areas. Operating principles and test results of prototype hybrid loops are discussed. Prototype hybrid loops have been demonstrated to remove heat fluxes in excess of 350W/cm2 from heat input areas over 4cm2 with evaporator thermal resistances between 0.008 and 0.065°C/W/cm2. Also importantly, this performance was achieved without the need to actively adjust or control the flows in the loops, even when the heat inputs varied between 0 and 350W/cm2. These performance characteristics represent substantial improvements over state of the art heat pipes, loop heat pipes and spray cooling devices. The hybrid loop technology was demonstrated to operate effectively at all orientations. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

There is a need for heat pipes and loop heat pipes in the temperature range of 450–700 K for spacecraft radiator and high temperature electronics cooling. Currently, the only heat pipe fluid that has been used extensively is water in the lower portion of the range (and alkali metals at higher temperatures). In this paper, heat pipe designs are compared with different fluids in this working range. Water is the optimum fluid for the lower temperatures in this range, and cesium for the higher temperatures. Iodine and antimony tribromide have good predicted performance in the middle of the range. Life test should be conducted with these fluids, as well as antimony trichloride. In addition, more physical property data should be collected on the halides. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Heat pipes are among the most promising technologies for space radiator systems. The paper reports further evaluation of potential heat pipe fluids in the intermediate temperature range of 400 to 700 K in continuation of two recent reports. More thermo-physical property data are examined. Organic, inorganic and elemental substances are considered. The evaluation of surface tension and other fluid properties are examined. Halides are evaluated as potential heat pipe fluids. Reliable data are not available for all fluids and further database development is necessary. Many of the fluids considered are promising candidates as heat pipe fluids. Water is promising as a heat pipe fluid up to 500–550 K. Life test data for thermo-chemical compatibility are almost non-existent. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The NASA Marshall Space Flight Center’s Early Flight Fission Test Facility (EFF-TF), provides a facility to experimentally evaluate nuclear reactor related thermal hydraulic issues through the use of non-nuclear testing. This facility provides a cost effective method to evaluate concepts/designs and support mitigation of developmental risk. Electrical resistance thermal simulators can be used to closely mimic the heat deposition of the fission process, providing axial and radial profiles. A number of experimental and design programs were underway in 2004 which include the following. Initial evaluation of the Department of Energy Los Alamos National Laboratory 19 module stainless steel/sodium heat pipe reactor with integral gas heat exchanger was operated at up to 17.5 kW of input power at core temperatures of 1000 K. A stainless steel sodium heat pipe module was placed through repeated freeze/thaw cyclic testing accumulating over 200 restarts to a temperature of 1000 K. Additionally, the design of a 37- pin stainless steel pumped sodium/potassium (NaK) loop was finalized and components procured. Ongoing testing at the EFF-TF is geared towards facilitating both research and development necessary to support future decisions regarding potential use of space nuclear systems for space exploration. All efforts are coordinated with DOE laboratories, industry, universities, and other NASA centers. This paper describes some of the 2004 efforts. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Technologies for Space Reactor Power Systems are being developed to enable future NASA’s missions early next decade to explore the farthest planets in the solar system. The choices of the energy conversion technology for these power systems require radiator temperatures that span a wide range, from 350 K to 800 K. Heat pipes with carbon-carbon fins and armor are the preferred choice for these radiators because of inherent redundancy and efficient spreading and rejection of waste heat into space at a relatively small mass penalty. The performance results and specific masses of radiator heat pipes with cesium, rubidium, and potassium working fluids are presented and compared in this paper. The heat pipes operate at 40% of the prevailing operation limit (a design margin of 60%), typically the sonic and/or capillary limit. The thickness of the carbon-carbon fins is 0.5 mm but the width is varied, and the evaporator and condenser sections are 0.15 and 1.35 m long, respectively. The 400-mesh wick and the heat pipe thin metal wall are titanium, and the carbon-carbon armor (∼ 2 mm-thick) provides both structural strength and protection against meteoroids impacts. The cross-section area of the D-shaped radiator heat pipes is optimized for minimum mass. Because of the low vapor pressure of potassium and its very high Figure-Of-Merit (FOM), radiator potassium heat pipes are the best performers at temperatures above 800 K, where the sonic limit is no longer an issue. On the other hand, rubidium heat pipes are limited by the sonic limit below 762 K and by the capillary limit at higher temperature. The transition temperature between these two limits for the cesium heat pipes occurs at a lower temperature of 724 K, since cesium has lower FOM than rubidium. The present results show that with a design margin of 60%, the cesium heat pipes radiator is best at 680–720 K, the rubidium heat pipes radiator is best at 720–800 K, while the potassium heat pipes radiator is the best performer and lightest at higher temperatures >= 800 K. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Loop heat pipes (LHPs) are versatile two-phase heat transfer devices that have gained increasing acceptance for space and terrestrial applications. The operating temperature of an LHP is a function of its operating conditions. The LHP usually reaches a steady operating temperature for a given heat load and sink temperature. The operating temperature will change when the heat load and/or the sink temperature changes, but eventually reaches another steady state in most cases. Under certain conditions, however, the loop operating temperature never really reaches a true steady state, but instead becomes oscillatory. This paper discusses the temperature oscillation phenomenon using test data from a miniature LHP. [ABSTRACT FROM AUTHOR]