Thermal Control Applications Research Articles

Experimental study of the heat transfer characteristics of plasma jets in circular tubes under applied magnetic fields with different orientations

Numerical studies have demonstrated that heat transfer between magnetogasdynamic flow in a tube with the tube wall can be regulated via the application of magnetic fields. However, the development of corresponding thermal energy control applications is limited owing to a lack of effective experimental verification of these numerical findings. The present study addresses this issue by developing an experimental system for studying the effects of applied magnetic fields lying parallel and perpendicular to the direction of flow on the convective heat transfer of plasma jets generated by a dielectric barrier discharge (DBD) reactor in circular tubes, where the heat transfer parameters within the tube are calculated according to temperature measurements on the outer wall of the tube. The results demonstrate that both types of applied magnetic fields tend to suppress heat transfer between the plasma jet and the tube walls, where the effect of the perpendicular electromagnetic field on the heat transfer is anisotropic, while that of the parallel electromagnetic field is isotropic. In addition, the magnetic field intensity, the excitation voltage of the DBD reactor, and the airflow velocity at the inlet are all demonstrated to have regulatory effects on the observed heat transfer between the plasma jet and tube walls. These results lend considerable substance to the findings of numerical studies indicating that the geometrical parameters and intensities of magnetic fields must be coordinated with the specific requirements and flow parameters inherent in practical applications.

Design of Reference Junction Temperature Swing of Power Module for Thermal Management

Thermal management is a cost-effective means to improve the lifetime of power module, but it also brings negative influence to the converter. To alleviate the negative effect, this article proposes a strategy to design the reference junction temperature swing based on the distribution characteristics of consumed lifetime and the thermal control efficiency. We show that the selected reference temperature swing, including its type and amplitude, allows each application of thermal control to reduce as much consumed lifetime as possible and, at the same time, to guarantee the performance of the converter. Moreover, this approach does not need to obtain the junction temperature swing in real time, which makes the thermal management simple and efficient. The experimental results are presented to verify the merits of the proposed strategy. This article also evaluates the impact of uncertainties due to the semiconductor parameter variations and performs the error analysis of junction temperature estimation.

Design, experimental demonstration, and validation of a composite morphing space radiator

Future manned space missions will require thermal control systems that can adapt to larger fluctuations in temperature and heat flux exceeding the capabilities of current state-of-the-art technologies. Specifically, these missions will demand novel space radiators that can vary the system heat rejection rate to maintain the crew cabin at habitable temperatures throughout the entire mission. While current systems can provide a turndown ratio (defined as the ratio of maximum to minimum heat rejection rates) of 3:1 under adverse conditions, future missions are projected to demand thermal control systems that can provide a turndown ratio of more than 6:1. A novel morphing radiator concept autonomously varies the system heat rejection rate by altering the shape of the panel exposed to space, where composite materials can provide an ideal compromise between thermal conductivity, restorative stiffness and deformation capability. Shape change is accomplished through the use of shape memory alloys, a class of active materials that exhibit thermomechanically driven phase transformations and can be used as simultaneous sensors and actuators in thermal control applications. This work details progress towards testing and modeling a spaceflight-quality, high turndown ratio morphing radiator prototype in a relevant thermal environment. A prototype composite morphing radiator with shape memory alloy strip actuators and high performance thermal coatings achieved a turndown ratio of 7.2:1, while an associated multi-physical model thereof has been shown to capture all major effects and will enable future design improvements.

VO2 particle-based intelligent metasurface with perfect infrared emission for the spacecraft thermal control.

Thermochromism film can automatically adjust its emittance without additional energy consumption, which shows great prospect in the application of spacecraft thermal control. However, it is still challenging to achieve a large infrared emittance at a high temperature and emittance tunability of the thermochromism film. In this work, we propose a V O 2 particle-based intelligent metasurface for spacecraft thermal control, which consists of a square lattice array of hollow spheroidal V O 2 particles on Au substrate. The metasurface with a V O 2 particle having a large aspect ratio (∼10) displays perfect emission throughout the entire mid-infrared spectral range. The emittance tunability can exceed 0.63 with total normal emittance of 0.85. The underlying mechanisms involved in the metasurface are attributed to particle-dependent scattering, by which the infrared emittance is dramatically enhanced for the metallic state and restricted for the dielectric state. In addition, the infrared emittance at a high temperature and emittance tunability of the metasurface remain large for incident angles up to 60°. To the best of our knowledge, this work proposes the first thermochromism film structure with perfect infrared emission, which could accelerate the development and practical application of the thermochromic film in the field of spacecraft.

Utilization of sludge templated carbide slag pellets for simultaneous desulfurization and denitrification: Low and moderate temperatures and synergetic mechanism

Higher SO2 and NO removal requirements have been proposed for a low carbon, flexible power generation. In this study, a low-carbon, high-efficiency, and low-cost mixed pellet was prepared through synergistic utilization of carbide slag and sludge. Further, this work systematically explored its combustive properties and SO2 and NO removal capabilities. The results indicated that more efficient mass transfer and catalyzed reactions were integrated into the unique pellet structure. The sludge-added mixed pellets exhibited good denitrification ability, at low and moderate temperatures, and improved the desulfurization performance of carbide slag. The pellets with carbide slag to sludge mass ratio of 10:3 has a 12.3% higher desulfurization efficiency than 10:0 pellets, as sludge extended the duration of the chemical reaction control phase and reduced the diffusion resistance of the product layer during desulfurization. The sludge-added pellets had satisfactory denitrification efficiencies of up to 70% at 550 °C, primarily due to the homogeneous and heterogeneous reductions between NO, pyrolysis gas, and char, which formed in the pre-combustion phase of sludge; these NO reduction processes were significantly catalyzed through carbide slag-CaO surface reduction and reoxidation. Notably, more C(O) and C(N) groups formed on the char surfaces under trace atmospheric O2 concentration (≤1%), enhancing NO reduction. This pellet combustion process conforms to the three-dimensional diffusion reaction model. This study will provide a theoretical direction for high value waste utilization of carbide slag and sludge and facilitates the industrial application of thermal power flexible variable load pollutant control.

Flat Absorber Black PEO coatings on Ti6Al4V for spacecraft thermal control application

Black appearance and flat absorber, i.e., high solar absorptance (αs) and high IR emittance (εir) surfaces, are essential for optical benches and optical mounts in astronomical and scientific payloads/spacecraft. Generally, these optical benches and optical mounts are fabricated by Ti6Al4V alloy due to its superior mechanical and thermal properties. However, bare Ti6Al4V is restricted for spacecraft thermal control applications due to its low emittance, intermediate absorptance, and moderate corrosion resistance behaviors. Hence, plasma electrolytic oxidation (PEO) technique was utilized to develop black and flat absorber coatings on bare Ti6Al4V alloy. Sodium silicate (SS), sodium hypophosphate (SHP), and potassium fluoride (KF) were used for the preparation of the electrolytic bath. Process parameters such as current density, duty cycle, frequency, the concentration of electrolytes, and duration were judiciously altered to get uniform PEO coatings with the flat absorbing property. Six numbers of PEO coatings were finally chosen for further in-depth characterizations as they showed adequate flat absorbing property. The phase composition, microstructure, roughness, and wettability of those PEO coatings were thoroughly studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), profilometry, and water contact angle measuring techniques, respectively. Thermo-optical properties viz., εir and αs of the coatings were recorded by portable IR emissometer and solar reflectometer, respectively, and electrical sheet resistance was measured by two probe method. The Nanomechanical properties of the coatings, such as modulus (E) and nanohardness (H), were evaluated by the nanoindentation technique. The PEO coatings were also investigated with respect to potentiodynamic polarization to investigate corrosion resistance of the PEO coatings. The highest εir value of PEO coating was achieved as 0.87 for a 20% duty cycle with a combination of SS + SHP electrolytes, while the maximum αs value was achieved as 0.83 for the same 20% duty cycle with a combination of SS + SHP + KF electrolytes.

VO2-based superposed Fabry-Perot multilayer film with a highly enhanced infrared emittance and emittance tunability for spacecraft thermal control.

Thermal control coating for spacecraft based on thermochromic film attracts increasing interest due to their ability of self-adaptive emittance switch and less resource consuming compared with traditional thermal control coatings. However, practical applications of thermochromic film for spacecraft are constrained by the low infrared emittance at a high temperature and narrow emittance tunability. In this work, a thermochromic film with simple structure, nearly perfect infrared emission and large emittance tunability is proposed for the application of spacecraft thermal control. The thermochromic film is a VO2-based superposed Fabry-Perot (FP) multilayer film, which is constructed by encapsulating three thin VO2 layers in four lossless BaF2 spacer on the Al substrate. The infrared emittance and emittance tunability of the superposed FP film is dramatically enhanced by the three superposed VO2-BaF2-Al FP resonances at wavelengths of 9, 15 and 20 µm, respectively. For VO2 layers under metallic state, the spectral normal emittance of the superposed FP film is close to unity in the entire mid-infrared spectral range, while for VO2 layers under dielectric state, the film is highly reflective. For the typical growth techniques of the VO2 layers considered here, the emittance tunability of the superposed FP film can exceed 0.70 with total normal emittance larger than 0.91 at high temperature, simultaneously. The largest total normal emittance of the superposed FP film can reach 0.95 with emittance tunability of 0.78. In addition, the infrared emission and emittance tunability performances of the superposed FP film remain excellent for incident angles up to 60°. This work proposes a simple structure with highly enhanced infrared emittance and emittance tunability that outperforms the existing thermochromic films, which could accelerate the application of thermochromic films in the field of spacecraft thermal control.

Oscillating Gadolinium Thermal Diode Using Temperature‐Dependent Magnetic Forces

AbstractHigh‐performance thermal diodes would enable improved waste heat scavenging and thermal management systems. Prior study has indicated that the temperature (T)‐dependent magnetic response of ferromagnets near the Curie temperature provides a potential mechanism for thermal rectification via thermally induced mechanical oscillations between hot and cold surfaces, but the rectification was not investigated in a macroscopic device. Here, a centimeter‐scale oscillating gadolinium thermal diode (OGTD) is constructed with steady‐state thermal rectification ratios (γ) as large as γ = 23 in air and γ = 16 in vacuum. In the forward mode when the top surface is warmer than 26 °C and the bottom surface is colder than 20 °C, an unstable balance between gravitational forces and T‐dependent magnetic forces causes a shuttle containing gadolinium to oscillate and transfer thermal energy. In the reverse mode, the shuttle does not oscillate and energy is transferred via parasitic conduction. The diode is durable over >103 oscillation cycles and can be used in thermal circuits for rapid thermal regulation in time‐varying environments or half‐wave thermal rectification with up to 50% of the ideal‐diode performance. The experiments show that the OGTD can achieve large γ in a convenient geometry and T range for thermal control applications.

Development of flat absorber black anodic coating on 3D printed Al–10Si–Mg alloy for spacecraft thermal control application

Black anodic coating (BAC) is developed on 3D printed or additively manufactured (AM) Al–10Si–Mg alloy by sulphuric acid based anodization technique at low (∼9 °C) temperature. The microstructural and surface properties of the coating are investigated by optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and water contact angle (WCA) measurement techniques. The solar absorptance and infrared (IR) emittance of the coating have been evaluated in the range 200–2500 nm and 3–30 μm, respectively. Nanoindentation technique is employed with low load to evaluate the hardness and modulus of the coating at microstructural length scale. The anodic coating shows superior hardness (∼3.95 GPa) and modulus (∼96 GPa) as compared to reported literature values, possibly due to dense microstructure and the presence of harder ceramic phases, e.g., alumina and kyanite in anodic coating. Further, due to dense microstructure, corrosion resistance improves significantly after anodization of AM alloy. Finally, thermo-optical properties are not altered after the space environmental tests which ensures the usage of the present anodic coating on AM Al alloy for spacecraft thermal control application. The solar absorptance (0.87) and IR emittance (0.78) obtained for the present coating as flat absorber characteristics are higher than the coatings processed on conventional wrought alloys. The present anodic coating shows substantial improvement in mechanical, thermo-optical and corrosion properties due to higher silicon content unlike conventional wrought alloy and refinement in microstructure during additive manufacturing aiding continuous and uniform anodic oxide layer.

Preparation of antistatic AZO/Al2O3-ZnO-Y2O3 composite thermal control coating and its study on electron-resistance radiation performance

As space exploration moves further and further into deep space, it is a big challenge to develop antistatic inorganic thermal control coating with lightweight and strong adhesion to substrates. Herein, this work reported an inorganic antistatic AZO/Al2O3-ZnO-Y2O3 composite coating fabricated by PEO combined with ALD methods. The results show that when the Zn/Al ratio of deposited AZO film is 24:1, its overall performance is the optimal, and the emissivity, absorptivity and resistivity are 0.892, 0.409 and 1.15 × 10–3 Ω·cm, respectively, which can meet thermal control application requirements of spacecraft. Furthermore, the AZO/Al2O3-ZnO-Y2O3 composite coating shows a good electron-resistance irradiation performance, which is because AZO conductive film is easy to generate free carriers, speed up some electron transport and make electrons better disperse and conduct, thus reducing the penetration depth of electron irradiation. This work provides a material basis for thermal control system of spacecraft with long life and high reliability.

Design of highly reflective film for smart radiation device

Smart radiation device (SRD) based on the asymmetrical Fabry-Perot cavity automatically tune its IR emittance depending on the ambient temperature, making it an ideal choice for thermal control system of spacecrafts. The low solar absorption is desirable for SRD to prevent the spacecraft from overheating by sun light. In this paper, a multilayer highly reflective film with LHk stacking layers is designed to reduce the solar absorptance (As) of the Ag/Al2O3/VO2 structured SRD. The reflective film achieves a high reflection band in ~220 nm bandwidth from 460 nm to 680 nm, which results in a reduction of the solar absorptance by 25.56 % for SRD working at low temperature and 24.27 % at high temperature as the stacking factor k= 5. The simulation results indicate that an economic reflective film with k= 3 can achieve effective suppression of As of SRD, demonstrating the promising potential of the proposed reflective film in thermal control application of spacecrafts.

Enhanced Electrochromic Performance of All-Solid-State Electrochromic Device Based on W-Doped NiO Films

Electrochromic materials have attracted much attention due to their promising applications in smart windows and thermal control. However, NiO is a weak point for a complementary ECD and needs to be improved due to its low optical modulation and charge density. In this work, the W-doped NiO films are designed and prepared by RF magnetron co-sputtering to improve the performance of the NiO. The results shows that the optical modulation of the W-NiO (52.7%) is significantly improved compared with pure NiO (33.8%), which can be assigned to the increase in lattice boundaries due to the W doping. The response time of W-NiO is 8.8 s for coloring and 7.2 s for bleaching, which is similar to that of NiO film. The all-solid-state electrochromic devices (ECDs) that employed W-NiO as a complementary layer are prepared and exhibit a high-transmittance modulation of 48.5% in wavelengths of 450–850 nm and an emittance modulation of 0.28 in 2.5–25 μm, showing great application potential in the field of smart windows and spacecraft thermal control devices. The strategy of preparing NiO doped by W indicates an innovative direction to obtain ECDs with high performance.

Recent progress and advances in electrochromic devices exhibiting infrared modulation

This review provides a route for constructing advanced IR-ECDs towards real-world applications in smart windows, IR sensors, thermal control and military camouflage.

Analytic characterization and operational limits of a hybrid two-phase mechanically pumped fluid loop based on the capillary pumped loop

Two-phase mechanically pumped fluid loops have the potential to provide numerous benefits for spacecraft based applications. Here a hybrid two-phase capillary and mechanically pumped fluid loop that shows promise for spacecraft thermal control applications is analyzed and its operational limits are characterized. Data from an experimental system that incorporates a 3D printed evaporator is correlated to several models that together provide a comprehensive characterization of the system behavior. The loop performance is analyzed using pointwise as well volume-averaged governing equations and resistance network models, to predict flooding and dryout limits. Also, system-level numerical (including 3-D CFD simulations) transient and linear-response analyses are performed predicting the changing loop phase distributions and the observed time-dependent behavior. With the 3D-printed evaporator, a notably high evaporator conductance of 30 W/cm2-K and heat flux over 10 W/cm2 over large area are achieved with negligible temperature non-uniformity.

Self-Driven Infrared Electrochromic Device with Tunable Optical and Thermal Management

Electrochromic devices (ECDs) exhibiting tunable optical and thermal modulation in the infrared (IR) region have attracted extensive attention in recent years due to their attractive application prospects in both military and civilian settings. However, considering the continuous energy supply needed for driving the device operation, it is desired to develop advanced IR-ECDs with low energy consumption. Herein, a flexible self-driven IR-ECD is constructed for achieving variable optical and thermal management in a low-energy mode. In this device, a built-in potential difference of 1.36 V exists between the EC polyaniline cathode and the aluminum foil anode. Consequently, there is a rapid and obvious increase in the IR reflectance of the device after connecting the two electrodes. Such a self-driven reflectance contrast is over 20% at the wavelength of 1500 nm, and the coloration efficiency of the device reaches up to 93.6 cm2 C-1. Meanwhile, the maximum apparent temperature modulation on the surface of the device reaches up to 5.6 °C. Then, the self-driven IR-ECD could recover to its original state driven by a solar cell, indicating good reversibility and stability. We anticipate that this work may provide a new insight into developing advanced self-driven IR-ECDs for applications in dynamic military camouflage and commercial thermal control.

Time-periodic thermal rectification in heterojunction thermal diodes

Thermal diodes that rectify time-periodic temperature (T) fields have been proposed for applications in waste heat scavenging and thermal control. Although these applications involve transient temperatures, prior analytical solutions for heat conduction in thermal diodes have focused on the steady-state response. We use analytical perturbation methods supported by finite-element method (FEM) calculations to study time-periodic rectification in a one-dimensional heterojunction diode consisting of two materials with thermal conductivity k and heat capacity c that each scale linearly with T. One boundary of the diode experiences a sinusoidal boundary condition with an average temperature T0, an angular frequency ω, and a temperature amplitude ΔT, while the other boundary is maintained at a constant T0. The analytical perturbation solution shows that the nonlinear response has frequency components at direct current (dc) and at the second harmonic of ω for small ΔT/T0. We introduce a rectification asymmetry metric φ as the difference in the forward and reverse diode orientation dc heat fluxes and find that φ can be enhanced at large ω by a factor of two compared to the quasi-steady φ at small ω. We identify the optimal dimensionless parameters to maximize φ and find that φ is independent of the heat capacity temperature coefficient for all ω. We use FEM calculations to validate the perturbation solution and to study the effects of larger ΔT oscillations, interface thermal contact resistances, and heat losses. Our modeling results can be used to design heterojunction thermal diodes for applications and experiments involving time-periodic thermal rectification.

Passive Smart Thermal Control Coatings Incorporating CaF2/VO2 Core-Shell Microsphere Structures.

Existing smart radiation devices suffer from numerous disadvantages such as large thicknesses, limited dimensions, or requirements for sustained electrical power. The present study addresses these issues by proposing a smart thermal control coating based on CaF2/VO2 core-shell (CaF2@VO2) structured microspheres prepared by a solvent/hydrothermal-calcination method and distributed within an easily applied polymer matrix. Here, the dielectric-to-metallic transition property of the VO2 shell material with increasing temperature is used to regulate the optical scattering and absorption characteristics of the CaF2@VO2 core-shell microspheres to realize a positive and reversible increase in the emissivity of the coating from 0.47 at 30 °C to 0.83 at 90 °C. The mechanisms behind this effect are investigated by theoretical analyses and numerical simulations. The present work can expect to promote the further research and development of new coating materials for smart thermal control applications.

A comprehensive review of micro/nano enhanced phase change materials

Enhancement in properties of thermal storage materials improves their performance and contributes to reducing the greenhouse gas emissions. The enhancement can be made in a passive way, which is cost-effective and hardly requires management. For decades, phase change materials (PCMs) have been used in many applications for thermal storage, thermal control and thermal insulation purposes. PCM can store a huge amount of energy with low or no temperature swing. However, the major drawback of PCM is the low thermal conductivity. Techniques of different scales (macroscopic, microscopic and nanoscopic) have been adopted in thermal engineering to enhance the thermal conductivity of PCM. This paper presents a comprehensive review of the literature dealing with micro/nano enhancement techniques of PCMs. Enhancement effects as well as limitations of each technique are discussed in detail. Moreover, direction for future research and possible challenges are pointed out along with conclusions drawn from the studies.

Ultra-Broadband Infrared Metamaterial Absorber for Passive Radiative CoolingSupported by the National Natural Science Foundation of China (Grant Nos. 52022018 and 52021001), and the Program for Changjiang Scholars and Innovative Research Team in University.

Infrared metamaterial absorber (MMA) based on metal-insulator-metal (MIM) configuration with flexible design, perfect and selective absorption, has attracted much attention recently for passive radiative cooling applications. To cool objects passively, broadband infrared absorption (i.e. 8–14 μm) is desirable to emit thermal energy through atmosphere window. We present a novel MMA composed of multilayer MIM resonators periodically arranged on a PbTe/MgF2 bilayer substrate. Verified by the rigorous coupled-wave analysis method, the proposed MMA shows a relative bandwidth of about 45% (from 8.3 to 13.1 μm with the absorption intensity over 0.8). The broadband absorption performs stably over a wide incident angle range (below 50°) and predicts 12 K cooling below ambient temperature at nighttime. Compared with the previous passive radiative coolers, our design gets rid of the continuous metal substrate and provides an almost ideal transparency window (close to 100%) for millimeter waves over 1 mm. The structure is expected to have potential applications in thermal control of integrated devices, where millimeter wave signal compatibility is also required.

A Simple Method to Reversibly Switch the Reflectance Spectrum of a Layered Structure Consists of an Ultra-Thin Film Phase-Change Material GST

By applying pulsed laser and CW laser to irradiate the top layer phase-change material GST, we realized reversible optical switching of reflectance of a layered structure. The FTIR measured reflectance spectrum proved that the reflectivity at 8 μm could be changed reversibly from 0.19 to 0.90. Our research may have some inspiration for active thermal control and other applications.