Convection Loop Research Articles

Vertically heated natural convection loop model

Abstract We present a comprehensive analytical model for laminar flow in a vertically heated rectangular convection loop, with applications to oil cooling in transformer systems. Starting from an integral equation, we apply a quasi-one-dimensional approach to derive equations for both loop velocities and temperature distributions. In this model, we account for temperature-dependent dynamic viscosity and introduce cooling at the loop top and in the fin. The analytical predictions are rigorously validated against finite volume method (FVM) simulations across different geometries and heat flux conditions, demonstrating strong agreement within a 2% discrepancy range for most parameters. This model provides a viable analytical alternative to computationally intensive simulations, offering insights into natural convection dynamics within closed-loop configurations.

ANALYSIS OF THE INNER SURFACE OF A SUPERCRITICAL WATER CONVECTION LOOP AFTER AN ELECTRON IRRADIATION SESSION

Supercritical water (SCW) can be an effective cooler for nuclear reactors. All sorts of research on the subject of SCW are very important, especially bearing in mind that SCW is a non-polar solvent, while subcritical water is a polar solvent. Irradiation leads to an increase in corrosion and migration of some metals into the volume of the SCW. The transition from the supercritical state to the normal state and inversely contributes to the synthesis of substances in the SCW. On the inner surface of the convection loop, a film of corrosion products is formed, which are minerals that can protect the metal from corrosion.

SPECIFICS OF DESIGNING AN INFRARED PYROMETER-REFLECTOMETER FOR SEMICONDUCTOR HETEROSTRUCTURE FABRICATION

There are general technical requirements for all types of reactors for chemical vapour deposition technology using AIII- BV metalorganic compounds. Among them, it is worth highlighting the large temperature gradients that cause the origin of convection loops, which in turn, taking into account the high speed of the gas flow, lead to turbulence in the reactor instead of the expected laminar flow. It is also important to take into account the change in parameters of the wafer surface during the growth process and the need for signal separation between the useful signal from the wafer surface and the background signal from the wafer carrier, which rotates at fixed speed for uniform deposition of compounds. To obtain high-quality heterostructures with reproducible parameters, it is important to have a system of precise temperature control on the wafer surface directly in the deposition area, since the deposition process for many complex semiconductor devices (for example, laser diodes, LEDs, photodiodes, transistors on heterojunctions) is very sensitive to temperature changes. The method of optical pyrometry is a non-contact method that allows to precisely determine the temperature of the wafer surface and meets the technical requirements of CVD epitaxy growth reactors. This article is devoted to the analysis of the features of the development of optoelectronic systems for precise temperature measurement during epitaxial growth in order to determine the criteria for the selection or development of components of the optoelectronic system of the pyrometer-reflectometer. The main physical processes, electro-optical characteristics of Si photodiode, AlGaAs/GaAs LED and parameters of bandpass interference filters were investigated. Based on the analysis of the obtained research and measurement results, scientific recommendations have been developed. The recommendations aimed at the selection and optimization of the parameters of the components of the pyrometer-reflectometer (photodetectors, light emitting diodes, optical filters) in order to improve the accuracy and temperature stability of measurements in the pyrometer’s operation conditions, which take into account the compensation of emissivity change from the surface of the wafer.

Corrosion Behavior of Candidate Structural Materials for Molten Salt Reactors in Flowing NaCl-MgCl2

Abstracts. A high-temperature molten salt natural convection loop was designed and manufactured for chloride-based salts. NaCl-MgCl2 salt was prepared and injected into the loop. Corrosion coupons composed of SS304, SS316L, and high-Ni alloys were also prepared as candidate structural materials for molten chloride salt reactors. After a corrosion experiment was conducted for 500 h, the salt was drained from the loop to the drain tank; inductively coupled plasma optical emission spectroscopy was used to investigate metal dissolution from the materials into the salt. The corroded materials were analyzed using scanning electron microscopy and energy-dispersive X-ray spectroscopy to calculate the corrosion rate. Materials exposed to NaCl-MgCl2 molten salt showed varying levels of corrosion resistance. The high-Ni alloy demonstrated the highest resistance, followed by SS316L and SS304. Furthermore, corrosion products were observed to migrate along the molten salt through natural convection, eventually depositing onto the surface of the high-Ni alloy in the cold leg of the loop.

Liquid metal compatibility of pre-oxidized FeCrAl in flowing Sn

A flowing Sn thermal convection loop (TCL) experiment was conducted in FeCrAlMo (Kanthal alloy APMT) tubing for 1000 h with a peak temperature of 400 °C. Oxide dispersion strengthened (ODS) Fe-(10–12)Cr-6Al and APMT specimens were exposed and all were pre-oxidized to form an external alumina scale. Unlike static exposures at 400° and 500 °C where pre-oxidation prevented dissolution, large mass losses were observed in the TCL hot leg after the specimens were cleaned in Li. The observed pitting suggests that localized failure of the pre-formed alumina surface oxide caused significant metal loss. Specimen mass gains were not observed on the cold leg implying that mass transfer did not occur and the small levels of Fe and Cr in the Sn after the experiment implies that Sn-rich phases may have formed. Post exposure room temperature tensile testing showed limited impact of the Sn exposure on the 12Cr ODS FeCrAl alloy, comparable to a 1000 h anneal in Ar at 400 °C. These results suggest that pre-oxidation is not sufficient to protect FeCrAl alloys in contact with flowing liquid Sn at 350°-400 °C.

Assessing passive thermosyphon solar water heater as low-cost, sustainable water disinfection technology: Leveraging the Germicidal effect of copper and thermal convection loops

This study investigated the passive thermosyphon solar water heater (TSWH) as a domestic size point-of-use water disinfection treatment system for inactivating bacteria by utilizing both the germicidal properties of copper in heat exchanger and the heat generated in thermal convection loops. Additionally, life cycle analysis and manufacturing cost analysis were conducted to reveal its environmental and economic benefits. Initially, the germicidal capacity of copper tube against E. coli and total coliform in rapidly flowing polluted water was tested in laboratory settings with contact times ranging from 1.5 to 94 min. Without any additional heat, the copper tubes achieved 99.9% (log 3.01) removal of E. coli, and 99.24% (log 2.12) removal of total coliform after 94 min of contact. In the presence of heat (50–80 ºC), complete removal of the bacteria was achieved after only 1.5 mins of contact time at 50ºC. A domestic-sized passive TSWH with a copper tube heat exchanger was evaluated for disinfecting 20 L/day of microbiologically polluted tap water and synthetic groundwater in a field study. The average contact time of polluted water in copper tube heat exchanger was 5.58±1.2 min in the morning (with an outlet water temperature of 87.3 ºC), and 11.83±1.41 min in the afternoon (with an outlet water temperature of 78±5.1 ºC). These contact times and temperatures, achieved throughout the year, were sufficient to meet the required minimum for complete bacterial removal in the synthetic groundwater. Importantly, the system achieved complete inactivation of pathogens at pasteurization temperatures below 75 ºC, relying solely on the combined effect of copper and thermal convection loops, without pumps or electricity. Compared to an active TSWH using circulating pumps and controls, the passive system generated 62.2% less green house gas, had 55.5% lower cumulative energy demand and cost 323.3 $ compared to 544.64 $. Highlighting significant economic and environmental benefits, the passive TSWH offers a promising solution for disinfecting water in low-income areas with limited access to safe water.

Hydrodynamic design of the Separate Effect test facility for Flow-Accelerated Corrosion and Erosion (SEFACE) studies in liquid lead

Flow-accelerated corrosion and erosion (FACE) phenomena can be crucial for performance of structural elements in heavy liquid metal (HLM) cooled reactor systems. Existing experimental observations indicate that turbulent flow characteristic can affect FACE, but there is no quantitative data that can be used for model development and validation. Main recirculation pump impellers, which operate at high relative velocities and rotational flow conditions can be especially vulnerable to FACE. For comparison, the core internals operate at lower velocities and in axial flow conditions, but at higher temperatures and neutron fluence. Hence, systematic experimental data is needed to improve our knowledge on FACE phenomena. The Separate Effect Test Facility for Flow-Accelerated Corrosion and Erosion (SEFACE) is designed to obtain such experimental data including high relative velocities (up 20 ms−1) and high temperatures (400 to 550 °C) of liquid lead. This article focuses on the hydrodynamic design of SEFACE. The aim of the design is to achieve well defined flow conditions for experiments and ensure safe operation of the facility. First, we examine three design concepts (i.e., forced convection loop, rotating cylinder, and rotating disk) and motivate the choice of the rotating disk approach for SEFACE. Second, we discuss different design options, i.e., a confined rotor–stator test chamber and the unconfined rotating disk configuration. We used Reynolds-Averaged Navier Stokes (RANS) calculations to identify and solve the issues stemming from the high rotational speed. These include, for instance, lead free surface deformation, radial pressure buildup, and axial bending forces due to asymmetric test chamber. The CFD-derived torque and power predictions in rotor–stator and rotating disk systems are verified with selected empirical turbulent friction factor correlations or/and DNS calculations. We demonstrate that the developed hydrodynamic design of SEFACE solves identified issues and enables obtaining experimental data under well-defined flow conditions. The findings are deemed to also be applicable to the design of rotating disk-type FACE installations for other liquid mediums.

REMOTE MONITORING OF RADIATION TEST TEMPERATURE REGIME AT A HIGH-CURRENT ELECTRON ACCELERATOR

Methods for evaluating the radiation resistance of structural materials of nuclear reactors have been developed and research is being carried out in NSC KIPT. As a part of these studies, a supercritical water convection loop with a chamber for irradiation of the tested samples by electron beam of the accelerator LU-10 (beam energy of 8…10 MeV, beam power up to 10 kW) was developed. A pyrometric system based on a DSLR camera (Digital Single-Lens Reflex Camera) has been created for the remote continuous non-intrusive control of the surface temperature of the irradiation cell in the range of 350…550°С. The work presents the results of calibration and use of the developed system in the conditions of a 500-hour run of sample irradiation in the loop.

Natural convective loops heat transfer scaling analysis

Heat transfer arising in natural convective loops is analyzed numerically and theoretically. Combining 3D direct numerical simulations, unidirectional heat-transfer semi-analytical computations, scaling arguments and asymptotic analysis new universal scaling laws are proposed to account for dimensionless heat transfer behavior of natural convective loops. It is found that the Reynolds number Re resulting from the buoyancy driven rotating convection (as well as the Péclet number) scales as Grashof number Gr as Re∼Gr3/4 for imposed temperature boundary conditions and Re∼Gr3/5 for mixed imposed temperature/flux heating conditions. These scaling laws are successfully confronted with experimental measurements in both heating configurations. It is shown that these scaling result from boundary layers dominated transfers without noticeable influence of possibly complex 3D flow patterns found in the fluid flow. These simple scaling laws constitute a progress over previously existing correlations which have disregarded the effect of boundary layers, heating configurations as well as fluid to solid variable diffusivity/conductivity.

Rayleigh–Bénard type PCM melting and solid drops

In this paper we document both theoretically and numerically the melting of PCM in an enclosure heated from the bottom, in the presence of vertical metallic fins. We start with the analysis of the PCM melting in the absence of fins to discover the main scales of the problem, stressing the occurrence of Rayleigh-Bénard cells generated during the convection regime of melting. We continue with predicting the impact of fins on the development of the melting interface. The numerical experiments allow to understand the impact of the aspect ratio between the height of the enclosure and the distance between fins on the melting dynamics. They highlight the existence of solid drops for a certain range of aspect ratios when the horizontal flow channels in the convective loops along the fins become in contact, and they show how the solid drop is dragged downward to finish melting on the heated bottom of the enclosure.

DEPENDENCE OF TEMPERATURE DETERMINATION ACCURACY ON THE BANDWIDTH OF THE INTERFERENCE FILTER IN OPTICAL PYROMETRY SYSTEMS

Some general issues common to all reactors for gas-phase epitaxy of A3B5 metal-organic compounds include large temperature gradients in the reactor, leading to the formation of convection loops, high gas flow rates that can result in turbulence instead of the expected laminar flow, and the need for good uniformity and precision in temperature control of the semiconductor substrate surface. Since the deposition process for many complex semiconductor devices (such as infrared laser diodes or heterojunction bipolar transistors) is highly temperature-dependent, a system for precise temperature control of the process at the substrate surface directly in the deposition zone is necessary to obtain high-quality heterostructures with reproducible parameters.
 This article describes the features of the epitaxial growth process in MOCVD reactors, as well as highlights the tasks facing the temperature control system. The feasibility of using a radiation compensation system is explored in comparison to measurement methods using a thermocouple and classical pyrometry. Transformation of the input optical signal into a proportional output current in this system is performed by a sensitive photodetector, namely a silicon photodiode. The selection of the wavelength range of the radiation is determined by the optical parameters of the semiconductor substrates and compounds used in the system. The measurement spectrum is determined by the parameters of the interference filter in the system.
 The research was conducted to determine the effect of the interference filter width on the accuracy of determining the real temperature of a pyrometric control system with radiation compensation. The dependence of the interference filter characteristic width and shape on the correction value of the temperature of pyrometric systems after calibration is analyzed. Based on the results obtained, recommendations are made for selecting the optimal working wavelength and interference filter bandwidth of the optical system as a compromise between the determination error and the magnitude of the optical signal. The results can be used for the design of modern precision pyrometric systems.

The numerical study of multi-tube passive convective heat switches

Heat switches are widely used in cryogenic systems, playing an important role in controlling thermodynamic cycles and accelerating the cooling of cryogenic components. Among them, the convective heat switch, normally made from two identical tubes, realizes the on/off state by the presence/absence of a natural convective gas loop, which can be used to shorten the cooling time of cryogenic systems. In order to further understand the heat transfer mechanism of the passive convective heat switch, three-dimensional steady-state models of twin-tube and triple-tube configurations are established in this paper. The simulations of the twin-tube heat switch with different hot-end heat loads and different charge pressures are conducted and compared with the experimental results. The influence of the asymmetry of two tubes on thermal conductance is analyzed. Meanwhile, the simulation and comparison of twin-tube and triple-tube convective heat switches with equal void volume are carried out. It is found that the convective heat transfer performance of the twin-tube heat switch is slightly better than the triple-tube configuration, and when the cold-end temperature is 4 K and the hot end is applied 0.3 W heat load, the hot-end temperature is stable at 11.57 K, leading to a thermal conductance of 39.4 mW/K.

Characterization of Fe and Cr Dissolution and Reaction Product Formation in Molten Chloride Salts With and Without Impurities

ABSTRACT There is considerable interest in molten chloride salts for several applications including thermal storage and next-generation molten salt reactors (MSRs). Several studies have concluded that Cl salts are highly corrosive to structural materials. Using Oak Ridge National Laboratory’s established methodology, Cl salt compatibility was assessed for candidate Ni-based alloys 230, 600 and 740 H at 600°–800°C in static welded capsules and in flowing thermal convection loop (TCL) salt experiments. Simply drying commercial Cl salt at 550°–650°C and adding~0.05 wt.%Mg was able to produce small specimen mass changes and limited surface attack after 100–1000 h exposures. Intentional additions of H2O, NiCl2 and undried salt were used to better understand the role of impurities and achieve the>50 µm levels of attack reported in other studies. Characterisation of Cr depletion and oxide formation in conjunction with pre- and post-test salt chemistry measurements are being used to understand salt compatibility of structural materials.

Evaluation of the interaction between SiC, pre-oxidized FeCrAlMo with aluminized and pre-oxidized Fe-8Cr-2W in flowing PbLi

Dissimilar materials interactions are a major concern when liquid metals are used as a working fluid. To simulate a dual-coolant PbLi blanket, chemical vapor deposited (CVD) SiC, pre-oxidized FeCrAlMo (APMT) and pre-oxidized aluminized coating on Fe-8Cr-2 W (F82H) specimens were exposed to flowing commercial Pb-17at.%Li with a peak temperature of 650 °C in a pre-oxidized APMT thermal convection loop (TCL) for 1000 h. Similar to prior results in static PbLi, the coated F82H showed small mass changes and limited degradation of room-temperature tensile properties. After exposure, the α-Al2O3 on the surface coated F82H reacted with the PbLi to form LiAlO2. However, no significant Al coating loss or interdiffusion was observed. Mass transfer was observed between the APMT, TCL and the SiC specimens. Fe, Cr and Ni were dissolved into the molten flowing PbLi and transported through the TCL to react with the SiC specimens. Compared to a prior experiment with a majority of bare FeCrAl specimens that were exposed in a TCL with a peak temperature of 700 °C for 1000 h, the SiC reaction was reduced to only a few microns of carbides and silicides. Oxygen dissolved in the molten PbLi sustained the formation of an Al2O3 scale on the surface of the coating and induced the formation of a thin surface SiO2 layer on the surface of the SiC.

A Review of Experimental and Numerical Analyses of Solar Thermal Walls

Nowadays, almost 30% of total energy consumption (130 EJ) is consumed for the operation of buildings, mainly by space heating/cooling and ventilation systems, hot water preparation systems, lighting, and other domestic appliances. To improve the energy efficiency of buildings, several countries are promoting the use of renewable energy. The most promising systems include active and passive solar installations. In passive systems, the solar energy is collected, stored, reflected, or distributed by the roof ponds, natural convective loops, and the most popular direct gain walls and thermal storage walls (known as Trombe walls). This paper reviews the experimental and numerical studies devoted to the different solutions of Trombe walls, including solar chimneys integrated on the vertical walls, classic Trombe walls, Trombe walls with incorporated phase change materials, and photovoltaic Trombe walls. The actual state of the art is presented in the context of reducing energy consumption and enhancing thermal comfort. Most of the analyzed studies showed that the application of thermal storage walls allowed achieving these goals, led to lower emissions of greenhouse gases, and improved living standards. Nevertheless, there is a need for more detailed feasibility studies, including cost and environmental indicators.

Термомагнитная конвекция в гидродинамическом контуре: роль капельных агрегатов

In order to clarify the effect of drop-like aggregates on the thermomagnetic convection of a fer-rofluid in a vertical hydrodynamic circuit (convective loop), two series of experiments were carried out, which differ in the dispersed composition of single-domain particles in the fluid. This particular geometry of the cavity with the ferrofluid was chosen because in a closed hydrodynamic circuit the best conditions are created for thermomagnetic convection and the heat transfer en-hancement, provided that a magnetic field is applied to the heated section of the circuit in one of its vertical sections. In this case, the field nonuniformity in the cross section of the channel form-ing the loop is sufficiently small, and the ponderomotive force acting on the liquid is directed along the tube. The heated section of the circuit, which is affected by a nonuniform magnetic field, ‘works’ as a kind of thermomagnetic pump and a circulation flow occurs in the circuit. In the first series of experiments, we used a ferrofluid containing a coarse fraction of particles capable of causing thermodynamic instability of the ferrofluid in a magnetic field (spinodal decomposition) and the formation of drop-like aggregates. The second series of experiments was carried out with a ferrofluid that had undergone magnetic separation and was free from coarse particles. It is shown that the results of convective experiments differ qualitatively depending on the disperse composi-tion of particles and the presence of drop-like aggregates. A ferrofluid with a coarse fraction com-pletely blocks the global circulation flow, including gravitational convection, while a separated ferrofluid, purified from large particles, increases the intensity of convection in a strong magnetic field and the integral heat flux by 6–7 times.

Electrodeposited NiMo Alloy Coatings for Improved Molten Salt Compatibility

The use of molten halide salts as a primary coolant within next generation molten salt nuclear reactors (MSRs) has the potential to improve efficiency and nuclear reactor safety by operating at low pressures and high temperatures without boiling. However, these coolants will require the development of new corrosion resistant material systems that will have to meet or supersede existing standard codes for these systems. Therefore, new MSRs will require validation and testing of new material systems that can produce robust component structures and enable them to withstand these corrosive environments. Furthermore, any next generation process to apply protective coatings must allow for a conformal, thick, dense, and bonded material to be formed onto an ASME code approved material (e.g. type 316H stainless steel) that will comprise the structure of heat exchanger and containment vessel components.Within this context, Faraday Technology Inc. in collaboration with Oak Ridge National Lab and Quintus Technologies is working to develop and demonstrate the effectiveness of a low cost and high value corrosion-resistant alloy coating for next generation molten salt reactors. The manufacturing process involves the application of a functionally graded NiMo coating by electrodeposition onto 316H alloy substrates and their subsequent high temperature corrosion evaluation in molten fluoride salt at 700°C. After application the components were hot isostatically pressed (HIPped; at Quintus) to create a diffusion bond between the coating and alloy substrate. Initially, 316H alloy coupons were coated for evaluation (Figure 1 Left) in Mo capsules with static FLiNaK salt known to be corrosive due to metallic impurities. The next stage of testing involved evaluating coatings on the inner diameter of 316H tubes (25mm outer diameter; Figure 1 Middle). Coated tubes (75mm long) had coated endcaps welded to either end and were filled with the same FLiNaK salt and tested for 500 h at 700°C and 1000 h at 750°C. In addition, the microstructure and salt performance of butt welds were evaluated for joints between coated 316H tubes and joints between coated and uncoated 316H tubes. Results from the static capsule testing will be presented including the effect of coating composition and heat treating conditions on the depth of attack in the molten salt as feedback for the coating development process. The final phase of testing will include 200 mm long coated 316H tube sections in the hot and cold legs of a flowing FLiNaK thermal convection loop experiment which is currently being fabricated (Figure 1 R). Acknowledgements: The financial support of DOE Contract No. DE-SC0019602 is acknowledged. Figure 1: Corrosion characterization test plan. (Left) static coupon studies, (Middle) static pipe studies, and (Right) continuous flow loop pipe studies. Figure 1

A CFD Energetic study of the influence of the panel orientation in Open Joint Ventilated Façades

The arrangement of the panels in Open Joint Ventilated Façades (OJVF) is a potential factor in improving the energy efficiency of this building system. The distribution of joints in the façade influences the behaviour of the air flow in the channel which in turn could affect the overall heat exchanges with the envelope and thus the internal conditions of the building. Tiling panels can be installed on ventilated façades with different arrangement patterns according to the layout of the joints: lined up, staggered, stepped, diagonal or random, although manufacturers recommend a façade layout with in-line gaps to avoid costly façade maintenance. Thus, landscape and portrait layout with continuous joints are the most frequent arrangement in ventilated façades. This research assesses the benefit of the installation of OJVF panels in both layouts in order to reduce the cooling loads. Two real OJVFs with different panel arrangements, landscape and portrait, are analysed. Also, they are compared with a conventional façade with a sealed air cavity. All solutions are modelled and simulated using the commercial computational fluid dynamics software ANSYS FLUENT to evaluate the fluid-dynamic and thermal behaviour of the façades in summer and winter conditions. The energy performance of these solutions is evaluated, analysing different parameters such as panel’s temperature, mean air velocity inside the cavity, fluid pathlines through the open joints and thermal flux in the air cavity and to the room. The airflow inside the cavity is mainly driven by thermal buoyancy in all façades but differs from bi-dimensional convective loops in conventional façades to three-dimensional complex and asymmetrical airflows in OJVFs. The results obtained show that both OJVF configurations perform much better than the conventional sealed façade, reducing the heat transfer into the room by 30% in the summer period. In any case, the landscape OJVF façade reduces the transfer in the same period to a minimum value of 7.3 W/m2, which is 3% less than the flux transferred through the vertical one. This small difference in the energy performance of OJVFs makes the choice of panel orientation more based on other criteria such as aesthetics.

Micro heat and mass transfer by bubble-trains from the gas discharging at nucleation sites

In this study consideration is given to the possibility of heat and mass transfer from the characteristic columnar bubble-trains observed when a diluted gas precipitates upon contact with nucleation sites at the surface of the container. Last impressive advances in micro/nanoscale surface modification start to allow the control -in number, roughness and spacing, of such nucleation sites and with this opening interesting and so far unexplored possibilities for heat and mass transfer. One of them is the kind of convection induced by bubble-trains when a supersaturated gas is discharging at nucleation sites clusters. Because the bubble generation from the nucleation sites is continuous, the bubble-trains are moving in a confined container, and because surfaces may be deliberately coated by design with such nucleation sites, then the constant upward ascent of the fluid entrained by the bubble train eventually set a convective loop driven by the bubble trains. This kind of convection -although expected to be rather small, nevertheless could be of potential interest for microelectronic applications and then it is worthy a first theoretical assessment in the mass and heat transfer enhancement attainable. Utilizing a simplified idealized two-dimensional geometrical model an expression for the upper limit in the Nusselt number was derived and comparison with natural convection performed.

A preliminary study of 4He convective heat switches

Heat switches are key components in sub-kelvin refrigeration. It is widely used in cryogenic systems, such as controlling thermodynamic cycle, coupling/decoupling redundant cryocoolers, accelerating the cooling of cryogenic components and so on. Convective heat switch features simple structure, no moving parts and great thermal conductance, which is mainly used to precool cryogenic components. In order to accelerate the pre-cooling of a test platform to 4K, the passive convective heat switch is investigated in this paper, which consists of a circuit comprising two identical stainless steel tubes and two copper cavities respectively connected to the cold and warm stages. The ON/OFF state of the switch is realized by the presence/absence of a gravity-driven convective helium-4 gas loop. Several convective heat switches with different cavity structures and charge pressures are designed and assembled. The preliminary experimental results on thermal conductance are presented. With a typical dimension of a cylindrical cavity with a diameter of 28mm and a height of 16.8mm, a stainless steel tube with a diameter of 9.6mm and a length of 72mm, thermal conductance of 40-50mW/K in ON state has been obtained.