Two-phase Thermal Control Research Articles

Numerical and Experimental Investigation of Heat Transfer in the Porous Media of an Additively Manufactured Evaporator of a Two-Phase Mechanically Pumped Loop for Space Applications

Two-phase pumped cooling systems are applied when it is required to maintain a very stable temperature for heat dissipation in a system. A novel additively manufactured evaporator for two-phase thermal control was developed at NASA Jet Propulsion Laboratory (JPL). The Two-Phase Mechanically Pumped Loop (2PMPL) allows to manage the heat transfer with much wider breadth of control authority compared to capillary-based systems, while alleviating the system's sensitivity to pressure drops. The focus of this work is the understanding and capturing the micro-scale evaporation occurring in the porous structure of the evaporator. The Boiling and Phase Change Heat Transfer Laboratory at the University of California, Los Angeles (UCLA) developed an all-encompassing numerical simulation tool to predict the operational thermal behavior of the evaporator considering the effect of the liquid-vapor interface at the wick-to-vapor boundary. The numerical model incorporated the behaviour of the liquid-vapor meniscus at particle level located along the evaporative boundary between the wick structure and the vapor chamber. The numerical model allowed to study the effect of different parameters, such as boundary conditions, geometry, wick and fluid properties. An experimental setup was built at UCLA in order to characterize the heat transfer within an additively manufactured porous sample fabricated at JPL and in particular its evaporative heat load under certain heat inputs. The experimental efforts served as validation for the numerical results and aided in the characterization of the transient phenomena, such as dry-out.

Determination of the Heat-Controlled Accumulator Volume for the Two-Phase Thermal Control Systems of Spacecraft

<div>For spacecraft with high power consumption, it is reasonable to build the thermal control system based on a two-phase mechanically pumped loop. The heat-controlled accumulator is a key element of the two-phase mechanically pumped loop, which allows for the control of pressure in the loop and maintains the required level of coolant boiling temperature or cavitation margin at the pump inlet. There can be two critical modes of loop operation where the ability to control pressure will be lost. The first critical mode occurs when the accumulator fills with liquid at high heat loads. The second critical mode occurs when the accumulator is at low heat loads and partial loss of coolant, for example, due to the leak caused by micrometeorite breakdown. Both modes are caused by insufficient accumulator volume or working fluid charge. To analyze the loop characteristics in critical modes, experiments were conducted on a test bench with ammonia coolant, and a mathematical simulation of a two-phase mechanically pumped loop was performed. The results show that the loop can operate in critical modes in a certain range of heat loads. The conducted studies allow for the design of a heat-controlled accumulator with the minimum required volume, expand the performance range of a two-phase mechanically pumped loop, and increase the reliability of its operation in orbit during long-term missions.</div>

Ground Test and on-Orbit Verification of a Mechanically Pumped Two-Phase Loop Using for Thermal Control of Spacecraft

Mechanically pumped two-phase loop (MPTL) is an advanced two-phase heat transfer device through the release of latent heat of phase change during flow boiling process. In order to investigate the performance of MPTL system under different gravity conditions, one MPTL setup using a novel two-phase thermal-controlled accumulator coupled with self-cooling function was designed and constructed. Its characteristics of startup, phase change, and temperature control were tested on the ground and on orbit. Test results showed that MPTL system had good working performance and stable working characteristics both on the ground and on a satellite. The functions of fluid management, temperature control and self-cooling for accumulator were verified under different gravity conditions. It was found that the superheat time and superheat temperature for on-orbit results were longer and higher than the ground’s results respectively. The heat transfer efficient of two-phase flow of MPTL system could reach 12,000 − 60,000 W/(m2·K). Affected by the buoyancy, the heat transfer coefficient on-orbit was 10%−29% lower than the ground result. The research would contribute to the theoretical basis of two-phase flow and heat transfer and promote the development of active two-phase thermal control technology for space.

Интенсификация теплопередачи в двухфазных системах с капиллярными насосами

Heat transfer in capillary pumped loops (CPL) is carried out by transferring the mass of the circulating coolant in the form of liquid and vapor. Therefore, the hydrodynamics of the phases in the CPL determines their heat transfer capacity (heat flow or the product of the heat flow by the heat transfer length). The influence of structural, hydraulic and thermo-physical properties of capillary structures used as capillary pumps in two-phase thermal control systems (Loop Heat Pipes - LHP) on their heat transfer capacity has been analyzed. Methods of increasing the heat transfer capacity of LHP, due to the use of anisotropic capillary structures with a decrease in pore sizes in the direction of the vaporization zone, have been determined. The conditions of LHP operability and the method of analytical calculation of the temperature field in anisotropic capillary structures for a model with pseudo-convection have been considered. The calculated and experimental data have been compared.

ПРОЕКТИРОВАНИЕ СИСТЕМЫ ТЕРМОРЕГУЛИРОВАНИЯ С ДВУХФАЗНЫМ КОНТУРОМ ДЛЯ КОСМИЧЕСКОГО АППАРАТА ПРОИЗВОДИТЕЛЬНОСТЬЮ ДО 15 кВт

This paper presents a methodological approach to the design of a spacecraft thermal control system with coolant pumping with a cooling capacity of up to 15 kW. Two variants of constructions are considered. A thermal and mass-energy analysis of the thermal control system for an automatic spacecraft with active coolant pumping using the heat of the phase transition is carried out. Structurally, the thermal control system consists of a two-phase circuit and axial heat pipes. Sequential laying of a circuit for heat collection is impractical, as it leads to excessively large diameters of steam paths. It is required to consider either a scheme with parallel laying of steam mains, or a variant with a central two-phase bus and autonomous circuits for heat collection / removal. The proposed layout of the thermal control system with two-phase circuit and active pumping of the coolant with a capacity of up to 15 kW provides for the necessary area of the radiation panels. Two variants are considered: with a central heat bus and with a parallel connection of the evaporation line. The results of the analysis show a mass acceptance of the two-phase thermal control system with a central reserved bus. The reliability parameter for the first option is also higher, since it has full redundancy of the central heat bus. In the second option, it is difficult to reserve the contour. The advantage of the second option is a lower thermal resistance during heat transfer from the equipment, therefore, large estimated reserves for the temperature of the equipment seats.

Combined Two-Phase Thermal Control System with Parallel Capillary Pumps

Combined Two-Phase Thermal Control System with Parallel Capillary Pumps

РАЗРАБОТКА КОНЦЕПЦИИ ДВУХФАЗНОЙ СИСТЕМЫ ТЕПЛООТВОДА СПУТНИКА

For spacecraft (SC) with power unit capacity more than 4 ... 6 kW promising construction of thermal control system (TCS) based on two-phase mechanically pumped loops (2PMPL). The development of 2PMPL has been carried out quite intensively since the early '80s. However, so far there are no examples of practical implementation of such high-power systems. One of the main reasons mentioned is the novelty of the system, and insufficient study of its operation in space conditions, which adds risks. The most important component of such systems is a heat rejection subsystem (HRS), whose task is to reject heat from the coolant and radiate it into space. In its turn, HRS is also a system, the design of which requires using a system approach, considering various aspects of its operation. HRS includes a heat-hydraulic network and a radiation heat exchanger (RHE). The key elements of the HRS are condensers (CC), quite new devices for space technology. This paper presents an algorithm for the design and optimization of the heat rejection subsystem (HRS) of a satellite two-phase thermal control system. The methodology of engineering synthesis of complex technical systems and informal procedures for multi-criteria optimization of elements and subsystems at various stages of HRS design is repeatedly used. t is shown that optimization should be carried out both at the level of elements and subsystems, and at the level of the whole thermal control system. As a result of the study, the HRS design is proposed, which uses condensers in the form of smooth steel tubes of constant cross-section and their series-parallel connection scheme in the hydraulic network. Main advantages of the design: traditional for single-phase loops elements are used; operation of elements and subsystems in zero gravity conditions is predictable and allows complete testing on the ground without mandatory flight experiment; the system is operable at high saturation pressures (temperatures) (on ammonia - up to 85℃).

Testing of CO2 on-orbit fill/refill for the upgraded tracker thermal pump system in the Alpha Magnetic Spectrometer

The ability of resupplying fluid is vital to many aerospace applications for the sake of lifetime prolongation and performance maintenance, especially for deep-space explorations. Compared to conventional on-orbit filling technologies with the emphasis on efficiency maximization, fluid replenishment for two-phase cooling systems in space requires an accurate control of the transferred quantity constrained by its working principle. Under microgravity, weighing is impractical, and due to the dependent relationship between the saturated temperature and the saturated pressure, mass cannot be deduced from measurements of the two thermal properties, making the filled quantity determination challenging in space. For this reason, this article presents a novel quantitative on-orbit filling method for two-phase systems, through which the transferred mass is obtained from the density variation of the supercritical fluid in the supply vessel. Via heating the fluid to supercritical, a homogeneous state independent from gravity is achieved, and therefore the fluid density can be deducted from the temperature and the pressure. The filling process is driven by pressure difference and precisely regulated by solenoid valves and a flow restrictor situated in the transfer line, constituting two fundamental components. To direct the filling, an empirical model aiming to predict the transferred mass is established in terms of the pressure difference and transfer time. Based on that, a fill/refill architecture is deployed to transfer a quantitative amount of coolant to a delicate pumped CO2 two-phase thermal control system, the Upgraded Tracker Thermal Pump System in the Alpha Magnetic Spectrometer-02 on the International Space Station, which was designed and constructed to replace the current Tracker cooling system in 2019. Results of the test conducted in a thermal-vacuum chamber showed that the density-variation approach in the upgraded two-phase system achieved a filling accuracy better than 7%, and the empirical model showed consistency with the approach within 5%. Moreover, a criterion to assess when the two-phase cooling system needs a coolant refill was proposed according to the investigation of a two-phase fluid accumulator of the system under thermal regulation, in which the heat transfer is strongly affected by liquid-vapor distribution. Given the above, the proposed method contributes to the on-orbit filling for two-phase systems with more precise control over the transferred quantity. It also delivers valuable insights to the operational status evaluation of two-phase systems for future space missions.

МАЛОИНЕРЦИОННЫЙ ТЕПЛООБМЕННИК АММИАК-АНТИФРИЗ ДЛЯ ИССЛЕДОВАНИЯ ПЕРЕХОДНЫХ ПРОЦЕССОВ В ЭЛЕМЕНТАХ ДВУХФАЗНОЙ СИСТЕМЫ ТЕРМОРЕГУЛИРОВАНИЯ СПУТНИКА

Technical progress entails the use of more powerful equipment on satellites. In connection with the growth of heat generation onboard the spacecraft, the task is to develop thermal control systems based on two-phase mechanically pumped fluid loop (2PMPFL). The advantage of such systems is the ability to transport a greater amount of heat, reduced to a unit of flow, than when using circuits with a single-phase coolant. The study of two-phase thermal control systems in terrestrial conditions is difficult because gravity affects the hydraulics and heat transfer of two-phase flows. Particularly difficult is the study of transients. This article presents the results of tests of a recuperative heat exchanger, which allows to study transient processes in 2PMPFL with high accuracy.It was designed and manufactured the heat exchanger of simple “tube in tube” type design. The thermal characteristics of the heat exchanger were determined on the experimental stand, which is a prototype of a closed-type 2PMPFL with ammonia coolant. Single-phase “liquid” modes, two-phase modes with low mass vapor content (up to 0.04), and single-phase transient modes were investigated. It has been experimentally determined that a heat exchanger under given conditions is capable of removing up to 1323 W of heat in a single-phase mode and up to 1641 W of heat - when operating in a two-phase mode. The data obtained in the course of the experiments allowed us to select the most appropriate known correlation for calculating the stationary characteristics of the heat exchanger with an error not exceeding 5%, which is a high indicator of accuracy for engineering calculations.The heat exchanger has low thermal inertia. The conclusion is relevant for the range of parameters: the ammonia temperature at the inlet is 24...60 ⁰C; antifreeze inlet temperature 5… 16 ⁰C; ammonia mass flow rate 8...17 g / s; mass flow rate of antifreeze 1...4 kg/min.Due to the low thermal inertia of the heat exchanger, it can be used to study transients with the rate of change of the coolant temperature at the inlet up to 1.85 K / min. You can use the stationary method of thermal calculation, i.e. calculate the transient process in the quasi-stationary approximation.

Heat transfer enhancement in a loop thermosyphon using nanoparticles/water nanofluid

Abstract This research focuses on two-phase thermal control systems, namely loop thermosyphons (LTS) filled with nanofluids and their use as LED cooling devices. The behavior of the fluid in the thermosyphons and the mechanisms explaining the possible impact of nanoparticles on the thermal properties of the working fluid, as well as the processes in the LTS, are addressed. Nanoparticle distribution in the nanofluid, methods of preparing nanofluids and the nanofluid degradation processes (aging) are studied. The results are obtained from a set of experiments on thermosyphon characteristics depending on the thermophysical properties of the working fluid, filling volume, geometry and nanoparticle mass concentrations. The impact of nanofluids on the heat-transfer process occurring inside the thermosyphon is also studied. The results indicate the strong influence of nanoparticles on the thermal properties of the thermosyphons, with up to a 20–25% increase in the heat transfer coefficient. It is shown that this effect is due to the aggregation of nanoparticles and the formation of a micro/nano relief on the vaporization surface. Additionally, a method of calculating the hydrodynamic limit of the LTS is proposed, which allows for estimation of the maximum heat that can be transferred by means of the LTS. The nanofluids are shown to be effective means for enhancing heat transfer in two-phase thermal management systems.

The first ammonia loop heat pipe: Long-life operation test

An extensive development of two-phase thermal control systems based on two- and multi-phase devices (standard heat pipes, loop heat pipes, heat switches, etc.) offers a potential to assume an increasingly important position in the instrumental thermal control for a number of space and earth applications. However, experimental data regarding long-life testing for such systems is insufficient. The first ammonia loop heat pipe (LHP) was developed in 1976 in the Ural Polytechnic Institute under supervision of Prof. Gerasimov, Iu. F. (1921–2001) (Kiseev, 1977; Gerasimov et al., 1981; Maydanik, 2005) [1–3]. This paper features its long-life test, covering 41years with varying operation times. The life test consists of two parts: permanent and periodic LHP operation (approx. 5000h) and LHP stand-by storage (approx. 152,600h). Experimental investigations were performed using the developed loop heat pipe based on composite nickel wick with effective pumping pores of 0.5–10µm in radius and 1×10−14m2 to 4×10−12m2 in permeability. We have provided experimental data demonstrating the impact of orientation, varying heat load and temperature on the LHP operation. In addition, a qualitative and quantitative chemical element analysis of the distilled water in a loop heat pipe without capillary structure (loop thermosyphon) was performed before and after its long-term operation (one year) to identify any possible interaction between the working fluid and the pipe materials.

Two-phase nanofluid-based thermal management systems for LED cooling

This research focuses on two-phase thermal control systems, namely loop thermosyphons (LTS) filled with nanofluids, and their use as LED cooling devices. The behavior of the fluid in the thermosyphons and the mechanisms explaining the possible impact of nanoparticles on thermal properties of the working fluid as well as the processes in the LTS are addressed. Nanoparticle distribution in the nanofluid, methods of preparation of nanofluids and nanofluid degradation processes (aging) are studied. The results are obtained from a set of experiments on thermosyphon characteristics depending on the thermophysical properties of the working fluid, filling volume, geometry and materials of radiators. The impact of nanofluids on heat-transfer process occurring inside thermosyphon is also studied. Results indicate strong influence of nanoparticles on the thermal properties of the thermosyphons, with up to 20% increase of the heat transfer coefficient. Additionally, a method of calculating the hydrodynamic limit of the LTS is proposed, which allows for estimation of the maximum heat flux that can be transferred by means of the LTS. Possible ways for further improvement of the model are proposed. The nanofluids are shown to be effective means of enhancing two-phase systems of thermal management.

Effects of two-phase inlet quality, mass velocity, flow orientation, and heating perimeter on flow boiling in a rectangular channel: Part 1 – Two-phase flow and heat transfer results

Lack of understanding of flow boiling behavior in reduced gravity poses a major challenge to the development of future space vehicles utilizing two-phase thermal control systems (TCSs). A cost effective method to investigating the influence of reduced gravity on flow boiling is to perform ground experiments at different orientations relative to Earth gravity. This paper is the first part of a two-part study aimed at exploring flow boiling mechanisms of FC-72 in a rectangular channel heated along one wall or two opposite walls. Experiments are performed in vertical upflow, vertical downflow and horizontal flow, subject to large variations in mass velocity, inlet quality and wall heat flux. Detailed measurements are used to investigate the influences of orientation, and therefore gravity, on boiling curve, local and average heat transfer coefficients, and pressure drop, and their relationship with interfacial behavior is captured with high-speed video. For horizontal flow, the effects of gravity are reflected in appreciable stratification across the channel at low mass velocities, with gravity aiding vapor removal from, and liquid return to the bottom heated wall, while accumulating vapor along the top heated wall. For vertical upflow and vertical downflow, with both single-sided and double-sided heating, there is far better symmetry in vapor formation along the channel. The heat transfer coefficient shows significant variations among the different orientations and heating configurations at low mass velocities, but becomes insensitive to orientation above 800kg/m2s, proving inertia around this mass velocity is effective at negating any gravity effects. For low mass velocities, pressure drops are fairly equal for vertical upflow and vertical downflow, but greater than for horizontal flows. However, fairly equal pressure drops are achieved at high mass velocities for all orientations. Overall, this study proves that gravity effects on two-phase pressure drop and two-phase heat transfer are dictated mostly by mass velocity and, to a lesser extent, by inlet quality.

A review of recent experimental investigations and theoretical analyses for pulsating heat pipes

Pulsating heat pipe (PHP), or oscillating heat pipe (OHP), a novel type of highly efficient heat transfer component, has been widely applied in many fields, such as in space-borne two-phase thermal control systems, in the cooling of electronic devices and in energy-saving technology, etc. In the present paper, the characteristics and working principles of the PHPs are introduced and the current researches in the field are described from the viewpoint of experimental tests, theoretical analyses as well as practical applications. Besides, it is found that the state-of-the-art experimental investigations on the PHPs are mainly focused on the flow visualization and the applications of nanofluids and other functional fluids, aiming at enhancing the heat transfer performance of the PHPs. In addition, it is also pointed out that the present theoretical analyses of the PHP are restricted by further development of two-phase flow theories, and are concentrated in the non-linear analyses. Numerical simulations are expected to be another research focus, in particular of the combination of the nanofluids and functional fluids.

Miniature heat pipes as compressor cooling devices

The purpose of this paper is an analytical and experimental study on heat transfer and temperature distribution for hermetic refrigeration compressor using miniature heat pipes as two-phase thermal control system. Heat pipe based coolers, such as miniature and micro heat pipe spreaders, loop heat pipes and loop thermosyphons ensure the temperature decrease of the most important parts of the compressor – cylinder head, cylinder, oil and compressor shell down to 10–15 °C. The experimental validation of the analytical study was performed for four different designs of miniature heat pipes.

Transient analysis of boiling heat transfer in periodic drying out miniature pools

The evolution of the two-phase thermal control technique is moving in the direction of the use of devices which operate in stable periodic or chaotic unsteady regimes. In these devices the heat transfer coefficient relative to the evaporator walls therefore changes over time and it is hardly predictable, especially in the case of boiling regime. This paper deals with the analysis of the boiling coefficient evolutions over time during the operations of a thermal control devices which periodically operates, the Periodic Two-Phase Thermosyphon (PTPT). An experimental technique for measuring the transient boiling heat transfer coefficient in a thick flat evaporator is shown. This technique has been used to measure boiling heat transfer coefficients over time in the case of evaporators with small pools of liquid (lower than 64 ml) which periodically dries out. The FC72 fluid has been used in the experiments. The results show that no nucleate boiling regime has been observed in the evaporator, which works in an unstable transitional boiling regime with relative heat transfer coefficients lower than those typical of nucleate boiling regimes (lower than 50%).

Experimental investigation on a novel method for set point temperature controlling of the active two-phase cooling loop

How to cool the accumulator and realize a stable set point temperature at the lowest cost is the key for the practical spacecraft application of two-phase thermal control system. The paper points out a novel method to combine thermoelectric coolers (TECs) with thermal bus of an active two-phase cooling loop. The novel method is realized by using the cold plate of a TEC to control the accumulator temperature while the hot plate is cooled down by subcooled liquid in the main thermal bus. Experiments have proved the feasibility and stability of the new design, which not only improves the accuracy from ±0.4 °C to ±0.1 °C but also increases the cooling speed to 0.31 °C/min when the input power of the TEC is 10 W. Some important performance characteristics demonstrated in this paper included: (1) operation of an active two-phase thermal system driven by mechanical pump; (2) a special evaporator with 15 m length and 2.6 mm diameter; (3) robustness and reliability of the system under various test conditions; (4) effectiveness of TECs and a rapid dropping speed in controlling the MPCL operating temperature; and (5) a higher controlling accuracy of the evaporator.

Performance Characteristics of Electrohydrodynamic Conduction Pump in Two-Phase Loops

The electric field applied in dielectric fluids causes an imbalance in the dissociation-recombination reaction generating free space charges. The generated charges are redistributed by the applied electric field resulting in the heterocharge layers in the vicinity of the electrodes. Proper design of the electrodes generates net axial flow motion pumping the fluid. The electrohydrodynamic (EHD) conduction pump is a new device that pumps dielectric fluids utilizing heterocharge layers formed by imposition of electrostatic fields. This paper investigates the performance characteristics of EHD conduction pump in two-phase thermal control loops. The EHD twophase loop consists of an EHD conduction pump, condenser, evaporator, transport lines, and reservoir (accumulator). The tests are performed with two different loops alternating two different EHD pumps to measure the generated pressure head and the mass flowrate at various applied voltages and sink temperatures. The power consumption of EHD conduction pumps is also determined by measuring the electric current value. The de-aerated R134a and unprocessed R134a are used as the working fluid. The dependence of EHD pump performance on the fluid temperature, effects of de-aeration of working fluid, effects of electrode design, pressure loss within EHD pump, influence of the loop size (tube diameter and length) are investigated.

Design Optimization of Condensers in a CO2 Two-Phase Thermal Control System for Space Detector Application

Design Optimization of Condensers in a CO2 Two-Phase Thermal Control System for Space Detector Application

Thermal Control Utilizing an Electrohydrodynamic Conduction Pump in a Two-Phase Loop With High Heat Flux Source

The electric field applied in dielectric fluids causes an imbalance in the dissociation-recombination reaction generating free space charges. The generated charges are redistributed by the applied electric field, resulting in the heterocharge layers in the vicinity of the electrodes. Proper design of the electrodes generates net axial flow motion pumping the fluid. The electrohydrodynamic (EHD) conduction pump is a new device that pumps dielectric fluids utilizing heterocharge layers formed by imposition of electrostatic fields. This paper experimentally evaluates the performance of a two-phase (liquid-vapor) breadboard thermal control loop consisting of an EHD conduction pump, condenser, preheater, evaporator, transport lines, and reservoir (accumulator). This study is performed to address the feasibility of the EHD two-phase loop for thermal control of a laser equipment with high heat flux source. The generated pressure head and the maximum applicable heat flux are experimentally determined at various applied voltages and sink temperatures. Recovery from the evaporator dryout condition by increasing the applied voltage to the pump is also demonstrated. The performance of the EHD conduction pump in this study confirms that the EHD conduction pump can be used as a stand-alone system for high heat flux thermal control.