Low-temperature Heat Pipes Research Articles

Results of an Experimental Investigation into the Use of Low-Temperature Heat Pipes for Accumulation of Winter Cold

Results of an Experimental Investigation into the Use of Low-Temperature Heat Pipes for Accumulation of Winter Cold

Design and Thermal-Hydraulic Performance Analysis of a Low-Temperature Heat Pipe Test Facility

Interest in heat pipe microreactors (HPMRs) has recently grown due to several unique advantages compared with other reactor types. These compact and mobile reactors are expected to find applications in a variety of fields to provide carbon-free power in remote or off-grid locations. Experimental work is needed to aid and expedite the design and licensing of future HPMRs, especially on the validation of heat pipe performance as key heat transfer components. A Low-Temperature Heat Pipe Test Facility (LTHPF) was designed and constructed according to previously developed scaling laws to bypass the difficulties of experimenting with liquid-metal working fluids by using surrogate fluids. The design, instrumentation, and experimental capabilities of the facility are described. The testing conditions, including various operating limits and the ranges of the nondimensional parameters used for scaling analysis, are reported. It is found that certain nondimensional parameters could yield a wide range over the operating conditions, whereas some showed minimal variation when water was used as the working fluid. Last, the performance of several types of wicks, including the annulus-screen, groove-screen, and wrapped-screen designs, were investigated for applications in the LTHPF. It is observed that the groove-screen wick structure provided slight improvement in capillary limits at higher temperatures and that the wrapped-screen wick yielded lower capillary limits due to the absence of a low-resistance flow path for the liquid.

Heat pipe integrated solar thermal systems and applications: A review

Heat pipe (HP) is a passive technique for conduction of heat from source to sink over a large distances. Being very efficient than pure metal conductor of which it is made, It has a broad spectrum of applications, from very low to very high temperatures. The discussion in this paper is limited to low and medium temperature heat pipe integrated solar thermal applications. The major focus is on construction and thermal performances of solar collectors integrated with heat pipe used for water heating (domestic and Industrial purpose), air/space heating, water desalination and indirect solar cooking system. Through literature review the observations are, heat pipe designs commonly used in thermal applications are wick, wickless (Thermosyphon), pulsating, loop and flat micro heat pipe array and feasibility prediction, performance superiority of heat pipe integrated thermal systems over systems without heat pipe are checked based on energy, exergy calculations.

О РАЗРАБОТКЕ ТЕПЛОВЫХ ТРУБ ДЛЯ ЯЭУ

The article discusses the designs of two nuclear reactors cooled by heat pipes with liquid metal coolants. Both designs were developed at IPPE JSC. The first design is a research reactor with a capacity of 1.2 MW. Evaporating sodium is used as a coolant in the core. The evaporating eutectic sodium-potassium alloy is used as a coolant in the secondary loop. The third loop contains gas as working fluid of the Stirling or Brighton cycle. The report presents the results of thermo-hydraulic experiments that confirmed the main design parameters. The second design called RIFMA is an innovative super small NPP with direct conversion of thermal energy into electricity and is supposed, in particular, to be used in the Arctic region. Thermal power is 100 kW, efficiency is not less than 10 %. Molybdenum heat pipes filled with lithium are used to transfer heat from the core. To convert energy, thermophotovoltaic converters are proposed. They are cooled by low-temperature heat pipes that remove residual heat and transfer it to air radiators. A nuclear power plant concept and three versions of the core are presented.

Thermal performance of U-shaped and L-shaped heat pipes

A comprehensive three-dimensional numerical model is developed to evaluate the effects of bending on the performance of both low temperature and high temperature heat pipes. The model solves for wall, wick, and vapor regions and includes the compressibility effects of the vapor. The proposed model can calculate the operating pressure and temperature of the heat pipe based on the operational and physical conditions. The effects of one 90-degree bend (L-Shaped) and two 90-degree bends (U-Shaped) are evaluated based on the hydrodynamics and thermal performance of the cylindrical heat pipes. The effects of bending on the heat pipe wall temperature are more significant for a high temperature heat pipe at a lower operating temperature range compared to a low-temperature heat pipe. The effects of bending on heat pipe is not significant if the wick of the heat pipe is not damaged due to bending and the blockage of liquid flow does not occur. In addition, if the heat pipe is operating in a horizontal plane where the evaporator and condenser are at the same level, the liquid pressure drop in the wick is not affected significantly by bending. Furthermore, the effects of gravity can be important when bending heat pipes and consideration should be given for this factor.

Experimental research on heat transfer performance of CO2 low temperature heat pipe

With the development of applications in the sub-zero temperature region, the high performance heat transfer element, heat pipe is expected to play an important role in this field. However, suitable working fluid for low temperature heat pipe is limited, the two commonly used substances, Freon and ammonia are environmentally harmful. In contrast, carbon dioxide (CO2) is an environmentally friendly gas and can be a promising candidate working fluid for low temperature heat pipe because of its suitable thermal properties. At present, CO2 heat pipes are seldom investigated and used in the sub-zero temperature region. So, the heat transfer performance of CO2 heat pipe is experimentally investigated, and influences of heat loads, filling ratios, condenser conditions (temperature, length, and position) are investigated. The results show that the performance of CO2 heat pipe is satisfactory, its thermal resistance can be 0.2–0.4K/W and its equivalent thermal conductivity can be 3.7 × 104 W/(m•K). Besides, the performance of R134a heat pipe is investigated as a control group, it is found that its thermal resistance is 2–5 times that of CO2 heat pipe in sub-zero temperature region. The results illustrate that the CO2 heat pipe has a good application prospect in the field of low temperature range from -50 °C to 0 °C.

Resonant vibration heat transfer coefficient increase of short low-temperature heat pipes

The results of heat transfer coefficient studies of the short linear heat pipes (HP's) with a formed by a capillary-porous insert vapour channel in the Laval nozzle-liked form at high heat power loads and external vibrations are presented. At high heat power loads, vaporization process in the capillary-porous evaporator and vapour propagation in the channel acquires a pulsating character with a frequency of 400–500 Hz and amplitude up to 103 Pa. It is shown that under external longitudinal vibration attached to the HP's body with an limited amplitude of 0.15 mm and frequency equal to the frequency of arising internal vapour pulsations, the HP's heat transfer coefficient shows the resonance nature and increases by to 20%. The increase in the heat transfer coefficient is due to the boiling process intensification in the grid HP's capillary-porous evaporator.

Computer simulation of vortex flows near the condensation surface in the Laval-liked nozzle form vapour channel of short low temperature heat pipes at high heat loads

The results of the computer simulation of vortex formation near the condensation surface in the Laval-liked vapour channel of short low temperature heat pipes (HP’s) at high heat loads are presented. For the first time it was found that the vapour vortex of moist compressible vapour flow near the condensation surface inside the vapour channel of short HP’s changes its rotational motion direction, depending on the thermal load magnitude on the HP’s evaporator. With low thermal loads, the rotational motion direction of the vapour vortex due to the Coanda effect and sticking moving vapour jets to the walls of the channel occurs from the periphery to the center of the vapour channel. While the thermal load increasing, the direction of rotation of the vapour vortex changes to the opposite, from the center to the periphery. The thickness of the liquid condensate film located under the vortex also depends on the magnitude of the thermal load and the direction of the vortex rotation. The numerical calculation of the condensate film thickness of working fluid were compared with experimental values depending on the thermal load on the HP’s evaporator and obtained by capacitive sensors.

Numerical Simulation of Pulsation Flow in the Vapour Channel of Short Low Temperature Heat Pipes at High Heat Loads

The results of the numerical simulation of pulsations in the Laval-liked vapour channel of short low-temperature range heat pipes (HPs) are presented. The numerical results confirmed the experimentally obtained increase of the frequency of pulsations in the vapour channel of short HPs with increasing overheat of the porous evaporator relative to the boiling point of the working fluid. The occurrence of pressure pulsations inside the vapour channel in a short HPs is a complex phenomenon associated with the boiling beginning in the capillary-porous evaporator at high heat loads, and appearance the excess amount of vapour above it, leading to the increase in pressure P to a value at which the boiling point TB of the working fluid becomes higher than the evaporator temperature Tev. Vapour clot spreads through the vapour channel and condense, and then a rarefaction wave return from condenser in the evaporator, the boiling in which is resumed and the next cycle of the pulsations is repeated. Numerical simulation was performed using finite element method implemented in the commercial program ANSYS Multiphisics 14.5 in the two-dimensional setting of axis symmetric moist vapour flow with third kind boundary conditions.

The Droplets Condensate Centering in the Vapour Channel of Short Low Temperature Heat Pipes at High Heat Loads

The results of experimental studies of the process of condensate microdroplets centering contained in the moving moist vapour in the vapour channel of short heat pipes (HPs) for large thermal loads are presented. A vapour channel formed by capillary-porous insert in the form of the inner Laval-liked nozzle along the entire length of the HP. In the upper cover forming a condensation surface in the HP, on the diametrical line are installed capacitive sensors, forming three capacitors located at different distances from the longitudinal axis of the vapour channel. With increasing heat load and the boil beginning in the evaporator a large amount of moist vapour in the vapour channel of HP occur the pressure pulsation with frequency of 400-500 Hz and amplitude up to 1·104Pa. These pulsations affect the moving of the inertial droplets subsystem of the vapour and due to the heterogeneity of the velocity profile around the particle flow in the vapour channel at the diameter of microdroplets occurs transverse force, called the Saffman force and shear microdroplets to the center of vapour channel. Using installed in the top cover capacitors we can record the radial displacement of the condensable microdroplets.

Supercritical CO 2 Rankine Cycle System with Low-Temperature Geothermal Heat Pipe

Abstract Extracting thermal energy from the geothermal reservoir, a new design of heat pipe (thermosyphon) is introduced in this paper in details. The analytical result indicated that substantial amount of heat can be transferred into the methanol (the working fluid in the heat pipe) through the outer wall in the region of high-temperature geothermal reservoir, and the heat can be transferred upward by vapour phase effectively to a CO2/methanol heat exchanger at the surface of the ground. In application of the heat pipe, a high-performance energy generation system Rankine cycle with methanol (heat pipe) using the geothermal reservoir, carbon dioxide (CO2) was selected as a model working fluid to run power cycle. The system is feasible for low temperature (low-grade) geothermal reservoir, and should be sustainable in a sense that no hot water (spa) can be wasted, in comparison with the ordinary geothermal power plant.

The Study of Condensation Processes in the Low-Temperature Short Heat Pipes with a Nozzle-Shaped Vapour Channel

The results of researches of condensation processes in the vapour channel similar to the Laval nozzle of short linear heat pipes are presented. Capacitive sensors are additionally installed in cooled top covers of the heat pipes, and electromagnetic pulses were supplied to them from the external generator. At heating the heat pipe evaporator, starting from a certain thermal power threshold value, electromagnetic pulses became modulated. It is related with the formations of the boiling process in the capillary-porous evaporator and large amount of vapour over it. Boiling process results in rapid increase of the pressure under which the average temperature of the evaporator occurs to be less than the boiling temperature of the working fluid under increased pressure. Considering condensation of excess vapour, this leads to repeated initiation and extinction of the boiling process in the evaporator, which reflects in pressure pulsations in the vapour channel. Pressure pulsations cause modulating effect on electromagnetic impulses. Pulsations frequencies are measured as well as their dependence from overheating of the evaporator. Using the capacitive sensors and a special electronic equipment we measured the local thickness of the working fluid at the condensing surface inside the heat pipes. Time-averaged values of the condensate film thickness are measured, depending on the heat load on the capillary-porous evaporator. The measurement error does not exceed 2 × 10–3 mm. It is demonstrated that the condensate film thickness lessens sharply with the increase of the heat load on the evaporator of a Laval-like low-temperature heat pipe, while the heat resistance of the film on the condensing surface reaches 60% of the total heat resistance of heat pipe with the capillary-porous evaporator.

Применение емкостных датчиков для изучения процессов конденсации в коротких низкотемпературных тепловых трубах. Калибровка и измерения

Применение емкостных датчиков для изучения процессов конденсации в коротких низкотемпературных тепловых трубах. Калибровка и измерения

Characteristics research of low-temperature heat pipes for energy-saving equipment

The efficiency of the heat pipes (HP) is largely dependent on the characteristics of capillary structures (CS) used therein. Due to the composite materials developed in Institute for Problems in Materials Science (IPMS) of NAS of Ukraine, which were used as the CS, Ukrainian heat pipes with high hydrodynamic and thermal characteristics were developed. The aim of the experimental work of the authors was the comparison of thermal characteristics of heat pipes that are based on composite porous structures with similar HP characteristics with fiber porous structures (which are considered to be the most effective view of insert HP). The results showed that HP with composite CS provides better thermal characteristics, especially when the heating of pipes was from the top, and cooling – from the bottom. In this position, a composite HP with CS steadily functioned under heat load up to 25 watts, whereas HP with fiber CS stopped stable operation at a load of 15 watts. As with the horizontal and vertical arrangement of the pipe (at «below» heat input), the value of thermal resistance (R [K/W]) for pipes with composite CS did not exceed values R that are typical for HP with fiber porous structures. Thus, thanks to the authors, it was found that the new composite structure is more appropriate to use when placing the HP at an angle to the upper heat supply than fiber HP.

Characteristics of low-temperature short heat pipes with a nozzle-shaped vapor channel

This paper presents the results of experimental and numerical studies of heat transfer and swirling pulsating flows in short low-temperature heat pipes whose vapor channels have the form of a conical nozzle. It has been found that as the evaporator of the heat pipe is heated, pressure pulsations occur in the vapor channel starting at a certain threshold value of the heat power, which is due to the start of boiling in the evaporator. The frequency of the pulsations has been measured, and their dependence on the superheat of the evaporator has been determined. It has been found that in heat pipes with a conical vapor channel, pulsations occur at lower evaporator superheats and the pulsation frequency is greater than in heat pipes of the same size with a standard cylindrical vapor channel. It has been shown that the curve of the heat-transfer coefficient versus thermal load on the evaporator has an inflection corresponding to the start of boiling in the capillary porous evaporator of the heat pipe.

Исследование характеристик коротких низкотемпературных тепловых труб с паровым каналом в виде сопла

Исследование характеристик коротких низкотемпературных тепловых труб с паровым каналом в виде сопла

Heat exchangers based on low temperature heat pipes for autonomous emergency WWER cooldown systems

The new schemes of NPP independent passive emergency reactor facility cooldown systems are considered. Nuclear power plant is considered with water-moderated energy reactor WWER (NPP with WWER). There are low-temperature heat pipes heat exchangers for decay heat rejection under conditions of complete unit long-term loss of station power as used in the emergency system. Schemes of independent systems of emergency cooldown of “hot” reactor facility based on two-phase thermosiphons or heat pipes are presented. The new circuits of WWER NPP independent passive emergency reactor cooldown systems with low-temperature heat pipes heat exchangers under conditions of complete unit long-term de-energization are considered. The circuits of emergency reactor cooldown systems and emergency repair cooldown are presented.

A device and technique to measure the heat transfer limit of a planar heat pipe wick

Heat pipes are increasingly being utilized in applications for cooling electronics such as micro-processors. Heat pipes afford a very high heat transfer rate with only a small temperature difference between the heat source and heat sink. However, heat pipes have several operating limits, known as heat transfer limits, that can cause a severe reduction in the heat transfer rate and lead to catastrophic failure of the micro-processor. Two of these limits, known and the capillary limit and boiling limit are primarily a function of the heat pipe wick. In the present work, we developed a test apparatus and technique to determine the heat transfer limit of a planar heat pipe wick. The advantage of this apparatus and technique is a quantifiable measurement of the wick’s heat transfer limit without being confounded by other operating limit mechanisms such as the viscous or sonic limits. The apparatus and technique also yields the wick’s effective thermal conductivity. The apparatus is compatible with many of the low temperature heat pipe working fluids such as water or methanol, can be operated in any orientation with respect to the gravity vector, accounts for parasitic heat losses, provides for flow visualization studies, and is adaptable to a wide range of wick thicknesses.

Fluid Flow in Axial Reentrant Grooves with Application to Heat Pipes

The fully developed laminar flow within a reentrant groove has been analyzed using a finite element model. A parametric analysis was carried out to determine the Poiseuille number Po = fRe, the dimensionless mean velocity v ∗ , and the dimensionless volumetric flow rate ˙ V ∗ as functions of the geometry of the reentrant groove (groove height 1.0 < ‐ H ∗ < 4.0, slot half-width 0.05 < W ∗ /2 < 0.9, and fillet radius 0.0 < R ∗ < ‐ 1.0), and the liquid‐vapor shear stress (0.0 < −τ ∗ lv < 2.5). The case in which the meniscus recedes into the reentrant groove was examined and could be a result of evaporator dryout or insufficient liquid fill amount. The cross-sectional area of the liquid in the groove, A ∗ , the meniscus radius R ∗, and the aforementioned flow variables were calculated as functions of the meniscus contact angle (0 < ‐ φ < ‐ 40 deg) and the meniscus attachment point (0.0 < H∗ l < ‐ 2.75). Finally, the results of the numerical model were used to determine the capillary limit of a low-temperature heat pipe with two different working fluids, water and ethanol, for a range of meniscus contact angles.

Elaboration of thermal control systems on heat pipes for microsatellites Magion 4, 5 and BIRD

The strong restricted requirements to mass and power budget of microsatellite subsystems lead to choice of mainly passive thermocontrol system, specifically with heat pipe (HP) integration. An objective of this paper is to present the experience collected by authors in the field of elaboration and flight exploitation of thermocontrol systems (TCS) with heat pipes for microsatellites Magion 4 (1995), Magion 5 (1996) and BIRD (2001). Microsatellites Magion 4, 5 (Project INTERBALL) were completed with TCS including four low-temperature U-shaped heat pipes (fabricated by Kyiv Polytechnic Institute) installed in pairs on symmetrical facets of microsatellites’ bodies. Each pair of heat pipes was placed on two general flanges: one combined HP heating zones, the second combined their condensation zones. Heat pipes were fabricated from thin-wall copper shells with capillary structure of metal felt. Flanges were of aluminium, and acetone was used as heat carrier. Rated transferred heat power was not less than 60 W in the range of working temperatures from −20 to +50 °C. TCS for microsatellite BIRD included two U-shaped aluminium ammonia heat pipes (design and manufacture––Kiev Polytechnic Institute) symmetrically placed on opposite faces of the satellite. Heat pipe capillary structure was formed from 30 grooves of rectangular section. Rated transferred heat power reached 90 W in the range of working temperatures from −50 to +60 °C. Symmetrical layout of elements of TCS on microsatellite is the most acceptable due to stability of spatial orientation and geometrical symmetry of microsatellites and to enlarge the reliability of system operation. Microsatellites Magion 4, 5 and their parts have passed the full cycle of tests stipulated by Project INTERBALL. Currently Magion 5 active functioning exceeds 4.5 years and heat pipes’ operational time reaches 40 000 h. The microsatellite BIRD has passed the qualification and acceptance test programs and has been launched on 22.10.2001 by Indian Polar Satellite Launch Vehicle. Heat pipes’ technical characteristics, operating modes and their adaptation into TCS design, test and flight operation results are presented.