Efficient Thermal Control Research Articles

Influence of randomly distributed vacancy defects on thermal transport in two-dimensional group-III nitrides

Efficient thermal transport control is a fundamental issue for electronic device applications such as information, communication, and energy storage technologies in modern electronics in order to achieve desired thermal conditions. Structural defects in materials provide a mechanism to adjust the thermal transport properties of these materials on demand. In this context, the effect of structural defects on lattice thermal conductivities of two-dimensional hexagonal binary group-III nitride (XN, X=B, Al, and Ga) semiconductors is systematically investigated by means of classical molecular dynamics simulations performed with recently developed transferable inter-atomic potentials accurately describing defect energies. Here, two different Green–Kubo based approaches and another approach based on non-equilibrium molecular dynamics are compared in order to get an overall understanding. Our investigation clearly shows that defect concentrations of 3% decrease the thermal conductivity of systems containing these nitrites up to 95%. Results hint that structural defects can be used as effective adjustment parameters in controlling thermal transport properties in device applications associated with these materials.

Near-field radiation assisted smart skin for spacecraft thermal control

Thermal control is of critical importance for normal operation of spacecraft. Given thermal radiation is the only means of heat dissipation in space, an efficient thermal control approach for spacecraft is to coat the radiator with a tunable-emittance “skin” that can tune its heat dissipation according to various thermal conditions. The existing schemes solely relying on far-field thermal radiation, which are based on mechanical, electrochromic or thermochromic working principles, are difficult to combine the advantages of all-solid-state structure, actively and accurate tuning, and large tuning range of heat flux. In this work, we propose a near-field radiation assisted (NFRA) smart skin for thermal control which can tune the heat rejection accurately and in a large range. It contains a metal-insulator-semiconductor (MIS) structure, where the carrier distribution in the semiconductor layer can be electrically altered. In this way, the near-field heat flux, and ultimately the skin emission power expressed using effective emittance, can be controlled as a function of the applied voltage. The variation range of the effective emittance can exceed 0.7 when adjusting the applied voltage from −10 V to 100 V with our preliminary design. This work opens a new way of smart skin design for active spacecraft thermal control.

Application of Precision Thermal Control Techniques in Taiji-1 satellite

In order to detect gravitational waves in space, the Chinese Academy of Sciences has drawn up the Taiji plan. The first step of the Taiji plan is to verify the key techniques of the core payload through the Taiji-1 satellite in low Earth orbit. Because the temperature stability directly affects the measurement accuracy of the interferometer ranging and acceleration, the temperature control index of (<italic>T</italic>± 0.1) K with high precision and high stability is proposed for the Taiji-1 satellite. Based on this requirement, an efficient thermal control scheme was designed. In the scheme, the thermal control from the selection of thermal components to the application of stand-alone equipment was controlled according to high indicators. At the same time, in the process of thermal implementation, in order to ensure the efficiency of the scheme, detailed control was also carried out. The thermal control scheme adopts the constant temperature cage as the design concept, adopts the means of three-stage temperature control, and combines the principle of active and passive. Finally, the high stability index of on-orbit flight temperature control of (<italic>T</italic>±0.005) K is obtained. Through the application research of high precision and high stability technology, it is found that thermal control heating scheme, heat leakage control, single resolution of temperature controller, temperature measuring circuit, control algorithm, control precision, temperature measuring resolution of temperature measuring element are still the important factors that restrict high precision temperature control technology.

Improving thermal control efficiency of solar energy utilization based on at series single-chip microcomputer

The thesis researched and designed a solar air heating automatic control system. The system is based on the AT89C52 single-chip microcomputer. The system has automatic tracking function control of the blade heat collector, winter and summer function conversion, and active and passive function conversion functions. The advantage of this system is that it can set the operating parameters of the system as required and can typically work following the set mode under complicated weather changes. Through experimental research, it is found that the solar air heating automatic control system can effectively improve the utilization efficiency of solar energy.

Solar thermal control efficiency based on at series micro-controller

Aiming at the problems of water temperature and water level control system of conventional solar water heater and low solar thermal control efficiency, this paper developed an AT series single-chip solar water heater control system for rural areas. In this paper, the detailed design of hardware and software is presented, and the method of evaluating the collector efficiency is simplified by the temperature rise rate and solar irradiance of the collector plate. The software compensation algorithm we adopted can effectively reduce the impact of different water quality on water level measurement and improve solar thermal control efficiency. It is found that the control system designed in this paper has the characteristics of low cost, high safety, and strong practicability.

Characterisation of a controllable loop thermosyphon for precise temperature management

Controllable loop thermosyphon in frequently start-stop operation offers an efficient heat transfer regulation. This low-cost device has a great protentional in precise temperature management of refrigerators, industrial buildings, and cutting-edge equipment. In this study, a test platform is built to investigate the thermodynamic characteristics of controllable loop thermosyphon and its effectiveness on precise temperature control of a fresh-keeping box. The comprehensive behaviours in continuous and start-stop operations are explored using R407C, R410A, R404A and R134a as working fluids. The filling ratios for above refrigerants are optimised, and the influence of thermal boundaries are studied. Given a suitable filling ratio of R407C, the average fresh-keeping temperature is reduced from 25 °C to 5 °C in 90 min and to 0 °C in 180 min during continuous operation. In start-stop operation, a minimum fresh-keeping temperature fluctuation of ± 0.3 °C is achieved at various fresh-keeping temperatures. The maximum and average heat transfer rates in the running period are 74.0–94.5 W and 48.3–80.2 W, respectively. Results indicate that the controllable loop thermosyphon with R407C provides efficient heat transfer and thermal control, and thus perform well in precise temperature management.

Thermal Performance Investigation by Infrared Analysis of Mini Pulsating Heat Pipe

A promising solution in the field of passive two-phase heat transfer devices is represented by Pulsating Heat Pipes (PHPs). They are undoubtedly appealing due to the high heat transfer capability, efficient thermal control, adaptability and low cost. In the last years they are raising concern for space applications that are characterised by extreme environmental conditions, strictly constrains in terms of compactness, reliability and the need to dissipate efficiently heat in microgravity conditions. In this study, the thermal performance of oscillating heat pipes that consists of extra-thin metallic pipes are investigated: the adoption of metallic pipes with an inner diameter less than 0.4 mm permits to couple flexibility and compactness with high heat transfer performance. HFC-134a is used as working fluid. Many authors have investigated the pulsating behaviour of this type of heat transfer devices only considering the average temperature of the evaporator and condenser. In this work, to deeply investigate the oscillating behaviour of the proposed PHP, it is adopted an approach based on the study of the local temperature distributions on the wall of the PHP, acquired with a high-speed and high-resolution infrared camera. The local analysis of the temperature trends is of fundamental importance in the understanding of the complex phenomena that govern the pulsating field.

Study of Human Thermal Comfort for Cyber-Physical Human Centric System in Smart Homes.

An environmental thermal comfort model has previously been quantified based on the predicted mean vote (PMV) and the physical sensors parameters, such as temperature, relative humidity, and air speed in the indoor environment. However, first, the relationship between environmental factors and physiology parameters of the model is not well investigated in the smart home domain. Second, the model that is not mainly for an individual human model leads to the failure of the thermal comfort system to fulfill the human’s comfort preference. In this paper, a cyber–physical human centric system (CPHCS) framework is proposed to take advantage of individual human thermal comfort to improve the human’s thermal comfort level while optimizing the energy consumption at the same time. Besides that, the physiology parameter from the heart rate is well-studied, and its correlation with the environmental factors, i.e., PMV, air speed, temperature, and relative humidity are deeply investigated to reveal the human thermal comfort level of the existing energy efficient thermal comfort control (EETCC) system in the smart home environment. Experimental results reveal that there is a tight correlation between the environmental factors and the physiology parameter (i.e., heart rate) in the aspect of system operational and human perception. Furthermore, this paper also concludes that the current EETCC system is unable to provide the precise need for thermal comfort to the human’s preference.

An efficient and innovative catalytic reactor for VOCs emission control

Efficient mixing and thermal control are important in the flow reactor for obtaining a high product yield and selectivity. Here, we report a heterogeneous chemical kinetic study of propene oxidation within a newly designed catalytic jet-stirred reactor (CJSR). To better understand the interplay between the catalytic performances and properties, the CuO thin films have been characterized and the adsorbed energies of propene on the adsorbed and lattice oxygen were calculated using density functional theory (DFT) method. Structure and morphology analyses revealed a monoclinic structure with nano-crystallite size and porous microstructure, which is responsible for holding an important quantity of adsorbed oxygen. The residence time inside the flow CJSR (1.12–7.84 s) makes it suitable for kinetic study and gives guidance for scale-up. The kinetic study revealed that using CJSR the reaction rate increases with O2 concentration that is commonly not achievable for catalytic flow tube reactor, whereas the reaction rate tends to increase slightly above 30% of O2 due to the catalyst surface saturation. Moreover, DFT calculations demonstrated that adsorbed oxygen is the most involved oxygen, and it has found that the pathway of producing propene oxide makes the reaction of C3H6 over CuO surface more likely to proceed. Accordingly, these findings revealed that CJSR combined with theoretical calculation is suitable for kinetic study, which can pave the way to investigate the kinetic study of other exhaust gases.

Supercritical startup strategy of cryogenic loop heat pipe with different working fluids

Space infrared detectors of the next generation such as James Webb Space Telescope (JWST) have pressed requirement for cryogenic heat transport technology below 40 K. Cryogenic loop heat pipes (CLHP) could possess massive potential in an infrared detection system because it is developed from a loop heat pipe (LHP) as a highly efficient thermal control device for space application. This paper discusses the design procedure of CLHP and how to produce a CLHP prototype following design principles. The CLHP prototype had various working characters while adopting different working fluids such as nitrogen, neon, and hydrogen, with which the CLHP system could operate at 20–120 K. Both common and unique operating characteristics in its supercritical startup process were investigated. All three CLHPs could achieve a successful supercritical startup just with various periods. The effect of heat sink controlling measure is significant for CLHP especially for that with a narrower temperature range such as neon and hydrogen. Due to lower dynamic viscosity and more considerable latent heat of hydrogen, the automatic cooling characteristics of the primary evaporator was found in H2-CLHP, which did not appear in the nitrogen or neon experiments.

Coupled fluid-thermal investigation on non-ablative thermal protection system with spiked body and opposing jet combined configuration

In this paper, a Non-Ablative Thermal Protection System (NATPS) with the spiked body and the opposing jet combined configuration is proposed to reduce the aerodynamic heating of the hypersonic vehicle, and the coupled fluid-thermal numerical analysis is performed to study the thermal control performance of the NATPS. The results show that the spiked body pushes the bow shock away from the protected structure and thus reduces the shock intensity and the wall heat flux. In addition, the low temperature gas of the opposing jet separates the high temperature gas behind the shock from the nose cone of the spiked body, ensuring the non-ablative property of the spiked body. Therefore, the NATPS reduces the aerodynamic heating by the reconfiguration of the flow field, and the thermal control efficiency of the system is better than the Thermal Protection System (TPS) with the single spiked body and the single opposing jet. The influencing factors of the NATPS are analyzed. Both increasing the length of the spiked body and reducing the total temperature of the opposing jet can improve the thermal control performance of the NATPS and the non-ablative property of the spiked body. However, increasing the heat conductivity coefficient of the spiked body can enhance benefit the non-ablative property of the spiked body, but has little influence on the thermal control performance of the NATPS.

Penetration mode effect on thermal protection system by opposing jet

The effect of the penetration mode on the thermal protection system by opposing jet is studied in this paper. The results show that the opposing jet not only pushes the bow shock away from the blunt body and reduces the shock intensity, but also directly cools the blunt body by the low-temperature jet gas. Therefore, the opposing jet can significantly reduce the aerodynamic heating of the blunt body. When the total pressure ratio of the opposing jet is less than critical value, the opposing jet presents the long penetration mode. When the total pressure ratio is greater than critical value, the opposing jet presents the short penetration mode. The critical total pressure ratio gradually decreases with the increase of diameter of the opposing jet. Increasing the total pressure ratio and diameter of the opposing jet can improve the thermal control efficiency of the opposing jet, and increasing diameter of the opposing jet can also reduce the drag coefficient. However, the drag coefficient increases abruptly when the penetration mode changes.

An original look into pulsating heat pipes: Inverse heat conduction approach for assessing the thermal behaviour

A promising solution in the field of passive two-phase heat transfer devices is represented by Pulsating Heat Pipes (PHPs). These relatively new devices, which achieve resounding interest in terms of high heat transfer capability, efficient thermal control, adaptability and low cost, have been extensively studied in the last years by many researchers. Many authors have investigated the heat fluxes at the evaporator and the condenser area only in terms of the mean values. In this work a novel approach to investigate the local heat flux in PHPs is presented and tested: the temperature distributions on the external wall of the PHP acquired with a high-speed and high-resolution infrared camera were used as input data for the inverse heat conduction problem in the wall under a solution approach based on the Tikhonov regularization method. Infrared imaging is performed on a single loop PHP designed with sapphire inserts partially coated with a highly emissive paint, allowing to determine at the same time the external wall temperature and the fluid temperature. Results show that the technique is able to show when heat is transfer from the fluid to the sapphire wall, when the hot fluid is pushed from the evaporator towards the condenser; increasing the wall tube temperature. On the contrary, when a cold fluid flows back from the condenser, the tube releases the heat previously accumulated, thereby decreasing its temperature. This approach allows to analyze the thermal behavior of the device by investigating the direct interconnection between the thermo-fluid dynamic phenomena within the PHP and the local heat flux measurements. The results proposed in this work are a breakthrough for the improvement and validation of both VOF-based DNS simulations, for local physical phenomena, and 1D simulations of the global PHP behaviour.

Thermal-hydraulic performance and entropy generation of supercritical carbon dioxide in heat exchanger channels with teardrop dimple/protrusion

Supercritical carbon dioxide (SCO2) is an excellent candidate for refrigeration system, energy utilization and new power cycle due to its easy-to–reach pseudo-critical point and the advantages of thermo-physical properties. As an efficient and passive thermal control technology, teardrop dimple/protrusion has great potential in heat transfer enhancement and resistance reduction. In this paper, three types of teardrop dimple and protrusions are novelly introduced into the SCO2 rectangular channels. Numerical calculations have been carried out to obtain the detailed flow structures, friction and heat transfer characteristics, overall thermal performance and amount of entropy generation under full turbulence regions. Moreover, the effects of geometrical structure, eccentricity of teardrop dimple/protrusion, Re, temperature and pressure of SCO2 are discussed with in-depth analysis. According to the results, complex and different evolution and development patterns of vortices are presented for three different structures. Positive eccentricity dimple (PED) produces the best overall thermal performance, owing to significant heat transfer enhancement and less friction, and the maximum TP reaches 1.17, indicating a 17% improvement compared with the smooth case. The variation of eccentricity has little effect on overall thermal performance and entropy generation for PED structure, while plays a major role in positive eccentricity protrusion (PEP) and negative eccentricity protrusion (NEP) cases. However, the increase of Re is unfavorable to the overall thermal performance. In addition, the temperature and pressure of CO2 have significant effects on the overall thermal performance for PED cases, and corresponding high-TP and thermal performance deterioration regions have been captured, which is advantageous to fast positioning the specific ranges of temperature and pressure, and whose coupled mode. Specifically, the maximum TP greater than 6 can be obtained near the temperature 305 K and pressure 8 MPa.

A tuning methodology of Model Predictive Control design for energy efficient building thermal control

The Model Predictive Control (MPC) approach is based on the prediction of indoor and outdoor thermal loads in order to counter the deviation of the indoor temperature from the occupants' preference in advance. The minimization of temperature alteration allows for efficient energy use and improvement of indoor thermal environment. Unlike the classical reactive control, MPC is able to act in advance and to explicitly handle constraints on building variables when selecting the best scenario. This peculiarity makes MPC particularly suitable for the optimal energy management of buildings but, despite the increasing research activity in the field, the commercial technologies are lagging behind. Among the reasons are the lack of a well established design approach easily applicable to the buildings domain and the lack of a design optimization. In this paper a tuning methodology of MPC design for energy efficient building thermal control is presented. The tuning is performed on the controller parameters, and aims at identifying the best parameter set in terms of energy saving and temperature deviation from the chosen setpoint. To demonstrate the effectiveness of the methodology, a simulation based analysis using a model estimated on a real case study is presented. The methodology shows that by improving the control parameters it is possible to reduce energy consumption and improve thermal comfort for the final user.

Experimental validation of additively manufactured optimized shapes for passive cooling

This article confirms the superior performance of topology optimized heat sinks compared to lattice designs and suggests simpler manufacturable pin-fin design interpretations. The development is driven by the wide adoption of light-emitting-diode (LED) lamps for industrial and residential lighting. Even for advanced lighting technology as LEDs, a large fraction of the input power is still converted to heat. Thus, efficient thermal control lowers energy waste, increases lifetime and reduces maintenance costs of this rapidly growing, expectedly soon to be governing, illumination technology. The presented heat sink solutions are generated by topology optimization, a computational morphogenesis approach with ultimate design freedom, relying on high-performance computing and simulation. Optimized devices exhibit complex and organic-looking topologies which are realized with the help of additive manufacturing. To reduce manufacturing cost, a simplified interpretation of the optimized design is produced and validated as well. Numerical and experimental results agree well and indicate that the obtained designs outperform lattice geometries by more than 21%, resulting in a doubling of life expectancy and 50% decrease in operational cost.

The use of nanomodified heat storage materials for thermal stabilization in the engineering and aerospace industry as a solution for economy

The paper presents studies on the use of nanomodified materials with phase transitions. The rmostable heat exchange devices based on materials modified with carbon nanotubes allowing to control thermal contact have been developed. It is established that through a controlled thermal contact it is possible to provide a heat flux of up to 100 [kW·m-2 ]. The application of heat-resistance storage material in technical devices such as self-rescuers is justified. The efficiency of thermal control is established, since the temperature after the heat exchanger is reduced to 40 ° C at an incoming temperature of about 100 °C.

Passive cooling concept for onboard heat sources in aircrafts

A passive cooling system based on heat pipe technology was tested in-flight in an Embraer test aircraft. Avionics thermal behavior was simulated by employing electrical resistances with input power ranging from 40 to 850W. Heat is transported from the resistances to the evaporator of a recently patented heat exchanger system (HES) by intermediary heat transfer elements (IHTEs), consisting of one heat pipe and four thermosyphons. The HES consists of loop-thermosyphons with a shared evaporator connected to two parallel condensers, one at the fuselage and another within the air-conditioning system. The couplings between HES evaporator and IHTE condensers were designed to assure practical fitting and low contact resistance. Experiments were conducted with Mach numbers of 0.55 up to 0.78 at altitudes of 4.5, 9.1 and 10.6km, corresponding to air static temperatures of 0, −30 and −43°C, respectively. IHTEs and HES behaviors were also investigated during roller coaster and G-load turn maneuvers. Heat sink changes surrounding the HES condensers owing to different altitudes hardly affect IHTEs. Heat pipe and thermosyphons with 0.7m length can dissipate 120W and 500W, respectively. Convection can be an alternative where heat conduction between avionics and IHTE evaporators is not possible. Two thermosyphons with evaporator fins dissipated 586W with the air temperature within the convective system of about 80°C. Aircraft maneuvers do not affect the thermal behavior of heat pipe technologies. Efficient thermal control of avionics is at hand.

Opportunities of synergistically adjusting voltage-frequency levels of cores and DRAMs in CMPs with 3d-stacked DRAMs for efficient thermal control

Chip-Multiprocessors (CMPs) with 3D-stacked DRAMs is promising for solving the memory wall problem, but the high power density makes 3D ICs frequently operate at or near the thermal limit. System hot spot of a CMP with 3Dstacked DRAMs is usually in DRAMs that are in the layers farthest from the heat sink. Heat from DRAMs and cores are all accumulated in DRAMs. Therefore, existing thermal managements for 3D ICs all perform thermal control on cores only because lowering the power-level of cores can also lower DRAM access frequency. However, as the power consumption of single DRAM access increases with the number of DRAM stacks and the width of the vertical links, the instantaneous DRAM accesses may easily overheat the system. So, in addition to lowering the access frequency of DRAMs, reducing the power consumption per DRAM access is also crucial. In this paper, we characterize the thermal and performance behavior of the target architecture when the voltage and frequency levels of cores and DRAMs are synergistically controlled. We also evaluate the thermal and performance behavior of existing thermal control methods that can be applied to the target architecture. The insights provided by the characterizations presented in this paper are important for developing an effective thermal management policy for CMPs with 3D-stacked DRAMs. Our results show that, synergistically controlling the voltage-frequency levels of cores and DRAMs does achieve higher thermal efficiency than controlling cores only.

Non equilibrium lumped parameter model for Pulsating Heat Pipes: validation in normal and hyper-gravity conditions

As relatively new and promising members of the wickless heat pipes family, Pulsating Heat Pipes, with their high effective thermal conductivity, construction simplicity, low weight, and potential high power loads may answer the present industry demand of high heat transfer capability, efficient thermal control, flexibility and low cost. Numerous are the attempts to simulate their complex thermal behavior, but none of the existing models is validated for transient operative conditions and under various gravity levels. Thus, a novel lumped parameters model able to compute the steady state as well as the transient performance of PHPs has been developed. It consists of a two-phase separated flow model applicable to a confined operating regime (slug–plug flow). A complete set of balance differential equations accounts for thermal and fluid-dynamic phenomena. The main originalities of this tool lay in the suppression of the standard assumption of saturated vapor plugs as well as in the consequent embedding of heterogeneous and homogeneous phase changes. In addition, an experimental work has been carried out with a 16 turns copper capillary PHP filled with FC-72. Therefore, the model has been validated in several operative conditions and under various gravity levels by comparison with these experimental data both in normal and hyper-gravity conditions showing very good prediction capability.