Convection Loop Research Articles

Study of the Effects of Vent Configuration on Mono-Span Greenhouse Ventilation Using Computational Fluid Dynamics

The rise in the human population, its density and scarcity of resources require cost-effective solutions for sustainable energy and water resources. Smart and sustainable agriculture is one important factor for future green cities to tackle climate change as a cost-effective solution to save energy and water. However, greenhouses (GH) require consistent ventilation due to their internal temperatures, and this can be an energy-intensive operation. Therefore, it is necessary to analyse the potential factors involved. In this study, the effect of vent configuration of a mono-span greenhouse with roof and side vents at low wind speeds was investigated using computational fluid dynamics (CFD). The validated simulations were then performed on different models to analyse the effects of the vents’ locations on the ventilation requirements. The side vents were found to contribute most to the ventilation. The position of the side vent was found to affect the convection loop in the greenhouse and the air velocity at the plant level. The humidity was shown to be highest under the windward side vent. The roof vent was found to affect the temperature and air velocity in the roof of the greenhouse but had very little effect on the distributions at the plant level.

Compatibility of FeCrAlMo with flowing PbLi at 500°-650 °C

To explore the upper temperature limit of a fusion blanket utilizing eutectic Pb-17at.%Li for tritium breeding, a series of three mono-metallic thermal convection loops have been fabricated from alloy APMT (Fe–21Cr–5Al–3Mo) and operated with commercial PbLi and increasing peak temperatures of 550°, 600° and 650 °C, flow rates of 0.4–0.7 cm/s and temperature gradients of 85–115 °C. Chains of APMT specimens including 25 mm long tensile specimens were hung in the hot and cold legs of the loops to evaluate post-exposure room temperature tensile properties after 1000 h exposures in each experiment. For specimens exposed to flowing PbLi above 500 °C, only minor changes in tensile properties were observed after exposure. Most APMT specimens were pre-oxidized to form alumina prior to exposure and exhibited small mass losses as the alumina was partially transformed to α-LiAlO2 that tended to spall after exposure. Specimens without pre-oxidation exhibited higher mass losses and formed γ-LiAlO2 but with less spallation. While promising for increasing blanket temperatures, the small mass losses suggest that the surface oxide is not completely stable. However, even at the highest temperatures, there was no indication of classic dissolution/precipitation behavior suggesting that higher temperature compatibility may be possible.

Large Eddy Simulations on a natural convection boundary layer at Pr = 0.71 and 0.025

The development of Gen IV nuclear reactors entails research oh heavy liquid metals, which are considered as appropriate coolant agents due to their high thermal conductivity and favorable nuclear safety characteristics. These fluids are characterized by a very low Prandtl number (between 0.006 and 0.025). During the design of such reactors, the scenarios of maintenance or loss-of-flow accidents consider a pure natural convection loop inside the reactor. The CFD modeling of the momentum and heat transfer must contemplate natural convection in low Prandtl number fluids, however in literature these flow configurations are not usual. This is why in the present article, Large Eddy Simulations over a natural convection boundary layer at Pr=0.025 are presented and described. Turbulence is triggered by adding a perturbation in the boundary layer. The adopted numerical approach is validated through a Large Eddy Simulation at Pr=0.71, where heat transfer, friction coefficient and mean and turbulent flow fields are compared with measurements in literature. Once turbulence is reached in the Pr=0.025 boundary layer, flow characteristics as Nusselt number, skin friction coefficient and turbulent profiles are calculated and compared with the Pr=0.71 simulation to assess the effect of Prandtl number on heat and momentum transfer.

Re‐establishing the paradigm for evaluating halide salt compatibility to study commercial chloride salts at 600°C–800°C

AbstractChloride salts are one candidate for a >700°C concentrating solar power (CSP) cycle, however, many reports from the literature suggest very high reaction rates between chloride salts and structural alloys. Historically, a specific methodology was established for evaluating halide salt compatibility based on solution kinetics. This study returned to that paradigm where the salts are purified and evaluated in sealed capsules before moving to a flowing experiment to determine a true corrosion rate in a temperature gradient for a commercial K–Mg–Na chloride salt. Isothermal testing focused on Ni‐based alloys 230 and 600 at 600°C–800°C. The results indicated there were promising combinations of salt chemistry, temperature, and alloy composition that reduce the extent of reaction. The results of the first monometallic thermal convection loop of alloy 600 run for 1,000 hr with a peak temperature of 700°C showed low attack with rates ≤9 µm/yr.

Control of chaos in thermal convection loop by state space linearization

Control of chaos in a thermal convection loop by state space linearization is investigated in this article. Chaotic behaviour produces temperature oscillation in thermal convection loop for certain values of the parameters, that decreases the safe operational stability of the system. It is necessary to suppress the chaos to increase the safe operational stability of thermal convection loop. In this paper, we converted a chaotic flow of the thermal convection loop into laminar by applying a technique state space linearization. This technique converts the nonlinear system into the linear system through a coordinate transformation, based on the concept of Lie brackets. The controller of a thermal convection loop is designed for arbitrary nonlinear output. Also, we have done numerical simulations for the confirmation of our analytical results. Moreover, the comparison of state space linearization with other control techniques (Active linear and nonlinear feedback) is presented.

Patterns of Natural Convection in an Irregular Arc-Shaped Enclosure

The current article deals with laminar natural convection in an irregular arc-shaped enclosure with a heated base and cooled upper top. The domain is filled with a liquid with Prandtl number of 4.0. This two-dimensional study focuses attention to understand changes in flow and temperature field development due to variation in aspect ratio from 0.1 to 0.4 for each case of Grashof number varies from 104 to 107. The paper also addresses the effect of orientation by placing the enclosure at an inclined position with angles of 45° and 90°. The flow behavior and thermal characteristics have been investigated through streamlines and isotherms, respectively. Complete domain has been chosen for analysis. Asymmetric solutions are also observed for some of the cases. These results are confirmed by the development of the corresponding convection loops and also by the variation of local Nusselt number. Finally, the paper summarizes the effects of different parameters on the rate of heat transfer.

Effect of aligned and misaligned ventilation opening affecting energy demand and air quality in buildings

This work focuses on a horizontally ventilated cavity filled with air, heated on one side wall and cooled on the floor surface. Therefore, this study has been carried out for a convective loop induced for a fixed Rayleigh number Ra = 106 and horizontal ventilation (moderate Reynolds number Re = 100) where the injection is either in cooperating or opposing to the convective loop. The study undertaken concerns different opening position in order to analyze the energy efficiency of such ventilation and the corresponding indoor air quality. The results obtained indicate that the natural convection and the forced flow (ventilation) play an important role in the flow structure and the mixing ability, the heat exchange (cooling need) and the temperature comfort. The optimum ventilating position is a compromise in order to minimize the cooling demand, keep the mixing ability and reduce the temperature heterogeneity.

A comparative study of flow boiling HFE-7100 in silicon nanowire and plainwall microchannels

Extensive experimental investigations along with high speed visualizations have been performed to assess the flow boiling characteristics in Silicon Nanowire (SiNW) microchannels. Experiments have been also performed in Plainwall microchannels to compare their performances with SiNW configurations. HFE-7100 has been used as the working fluid and experiments are conducted in a forced convection loop at mass flux range of 400–1600 kg/m2s. Arrays of microchannel consist 5 (five) parallel straight microchannels with Width, Depth and Length dimension of 220 μm, 250 μm and 10 mm respectively. Flow boiling performances including heat transfer coefficient (HTC), pressure drop, two-phase flow instabilities and critical heat flux (CHF) have been studied in both the Silicon plainwall (smooth inner surface) and Silicon Nanowire (silicon nanostructured inner surface) microchannels. High speed flow visualizations have been performed at up to 70,000 frames per s (fps) to understand the difference in boiling mechanisms between Plainwall and SiNW. SiNW performs significantly enhanced HTC (up to 400% improvement), reduces flow boiling instabilities and pressure drop (up to 70% reduction) compared to Plainwall microchannels. However, little/insignificant effect of nanostructured surface has been observed on CHF. In addition, a major difference in two-phase flow regime development has been observed between the SiNW and Plainwall microchannels during flow visualization. Specifically, SiNWs introduce explosive bubble nucleation, reduce intermittent flow regimes (slug/churn), improve rewetting, maintain thin liquid film and thus, improve system performances.

Simulation of the OECD/NEA Sandia Fuel Project Phases I & II ignition tests with DRACCAR

This paper describes simulations of two ignition tests performed at full power in the frame of the Sandia Fuel Project (SFP) with the thermo-mechanical code DRACCAR v2.3.The OECD/NEA Sandia Fuel Project was built on an agreement between 12 countries from OECD, the Nuclear Energy Agency (NEA) and the US-NRC for the characterization of thermal-hydraulic and zirconium fire phenomena in pressurized-water reactor (PWR). The experimental program was split in two phases to focus at first on axial heating and burn propagation in one prototypic fuel assembly (Phase I), and then on axial and radial heating and burn propagation in 1 × 4 fuel assemblies (Phase II).DRACCAR is a simulation tool, developed at IRSN, for fuel assembly mechanical behavior and coolability assessment during a LOCA transient. The flexibility of DRACCAR allows the modeling of many kinds of geometries. Because the code is based on a 3D non-structured meshing, it can be used to model any non-axisymmetric geometry, like the 1 × 4 fuel assemblies geometry of the Phase II of the SFP program.In order to check the consistency of the modeling, we have optimized the code options to get best results for the Phase I, and applied the same options to the Phase II. Most of the DRACCAR results have been successfully checked against experimental ones, using additional code improvements. Air oxidation and breakaway modeling of the zircaloy claddings were successfully tested against the experimental results. Nevertheless parts of the experimental results of Phase II have been difficult to reproduce. As many causes could be involved in these difficulties, such as detailed evolution of the air convective loop, radiative heat transfers in the bundles, and the modeling of additional reactions of zirconium with nitrogen in places where oxygen is lacking, there is still room for improvement in the work of interpretation and modeling of the SFP tests.

Frequencies of Flare Occurrence: Interaction between Convection and Coronal Loops

Abstract Observations of solar and stellar flares have revealed the presence of power-law dependences between the flare energy and the time interval between flares. Various models have been proposed to explain these dependences and the numerical value of the power-law indices. Here, we propose a model in which convective flows in granules force the footpoints of coronal magnetic loops, which are frozen-in to photospheric gas, to undergo a random walk. In certain conditions, this can lead to a twist in the loop, which drives the loop unstable if the twist exceeds a critical value. The possibility that a solar flare is caused by such a twist-induced instability in a loop has been in the literature for decades. Here, we quantify the process in an approximate way with a view to replicating the power-law index. We find that, for relatively small flares, the random walk twisting model leads to a rather steep power-law slope that agrees very well with the index derived from a sample of 56,000+ solar X-ray flares reported by the GOES satellites. For relatively large flares, we find that the slope of the power law is shallower. The empirical power-law slopes reported for flare stars also have a range that overlaps with the slopes obtained here. We suggest that in the coolest stars, a significant change in slope should occur when the frozen-flux assumption breaks down due to low electrical conductivity.

Cooling of High Heat Flux Flat Surface with Nanofluid Assisted Convective Loop: Experimental Assessment

Abstract Experimental investigation was conducted on the thermal performance and pressure drop of a convective cooling loop working with ZnO aqueous nanofluids. The loop was used to cool a flat heater connected to an AC autotransformer. Influence of different operating parameters, such as fluid flow rate and mass concentration of nanofluid on surface temperature of heater, pressure drop, friction factor and overall heat transfer coefficient was investigated and briefly discussed. Results of this study showed that, despite a penalty for pressure drop, ZnO/water nanofluid was a promising coolant for cooling the micro-electronic devices and chipsets. It was also found that there is an optimum for concentration of nanofluid so that the heat transfer coefficient is maximum, which was wt. % = 0.3 for ZnO/water used in this research. In addition, presence of nanoparticles enhanced the friction factor and pressure drop as well; however, it is not very significant in comparison with those of registered for the base fluid.

Active Control of Thermal Convection in a Rectangular Loop by Changing its Spatial Orientation

The problem of the automatic control of the fluid flow in a rectangular convective loop heated from below is studied theoretically and experimentally. The control is performed by using a feedback subsystem which changes the convection regimes by introducing small discrete changes in the spatial orientation of the loop with respect to gravity. We focus on effects that arise when the feedback controller operates with an unavoidable time delay, which is cause by the thermal inertia of the medium. The mathematical model of the phenomenon is developed. The dynamic regimes of the convection in the thermosyphon loop under control are studied. It is shown that the proposed control method can successfully stabilize not only a no-motion state of the fluid, but also time-dependent modes of convection including the irregular fluid flow at high values of the Rayleigh number. It is shown that the excessive gain of the proportional feedback can result in oscillations in the loop orientation exciting the unsteady convection modes. The comparison of the experimental data obtained for dielectric oil and dodecane with theory is given, and their good agreement is demonstrated.

Thermal-hydraulic analysis of the LORELEI test design by means of CATHARE2 V2.5

LORELEI (Light Water One-Rod Equipment for LOCA Experimental Investigation) test device is currently under design in the framework of the planned experimental facilities of the Jules Horowitz Reactor. The objective of this devise is to analyze the thermal-mechanical behavior of the fuel rod and the radiological consequences during a Loss of Coolant Accident (LOCA). This paper deals with an accurate modeling of the LORELEI test device by means of the system code CATHARE2 V2.5. The experimental device and its geometry is described together with the experimental parameters and the different phases of the test planned accident. The modeling and testing are dedicated to the study of the re-irradiation and the dry phase. The re-irradiation phase is not a part of the LOCA accidental sequence but it is modeled to provide a study concerning the nominal and abnormal conditions by finding out the device thermal-hydraulic limits. In this first phase, aiming at creating a detectable fission product inventory, the device is filled with water and the fuel rod is cooled by a convection loop. During the dry-phase, the device simulates the first part of a LOCA accidental scenario. The fuel cladding temperature is increased up to 1200°C and a small amount of water is set in the bottom of the device to produce steam needed to investigate the exothermic reaction with the fuel cladding. The object of this last part is to plan the operating test sequences and determine the best thermal conditions to reach an acceptable fuel cladding temperature profile.

Numerical prediction of thermal environment and energy consumption of three different greenhouses under hot and semi-arid climate

The evaluation of the thermal performance of a greenhouse requires an understanding of the energy exchanges between its components. In recent years, Computational Fluid Dynamics (CFD) proved to be efficient to predict the microclimate inside greenhouses, but few CFD works analyzed hot and arid climatic conditions. The aim of the present study is to investigate the impact of such specific conditions prevailing in the southern Mediterranean basin considering three different unheated greenhouses equipped with rows of canopy (tunnel, Venlo and plastic vertical wall greenhouse). 2D simulations were conducted in order to estimate the influence of the greenhouse geometry and cover material on the energy balance, focusing in particular on the heat losses. To establish the boundary conditions and validate the model, experiments were launched inside the greenhouses. Simulations were conducted for two contrasted conditions: the nocturnal period when the energy performance of each type of greenhouse was investigated, and the diurnal period for which a solar day was simulated taking account of natural ventilation. A good agreement was found with experiments concerning the temperature profiles and the energy losses. The predicted stable flow patterns slightly differed from one greenhouse to another for each condition: during the night they all disclosed two convective loops sweeping the floor, rising along the middle of the greenhouse, cooling down along the roof and moving down along the sidewalls of the greenhouse, while during the day flow patterns were strongly impacted by the vent openings. The obtained values (temperature and velocity) were coherent with results found in the literature for different greenhouse types and sizes. The developed tool offers potential applications in research on greenhouse climate to assess different cover material performances. It could also be used to investigate the energy consumption and partition for different greenhouse shapes, different crop densities different growth stages.

Ultrasonic Doppler Velocimetry Experiment of Lead-Lithium Flow with Oroshhi-2 Loop

The lead-lithium eutectic alloy (PbLi) is a promising coolant for nuclear fusion reactors. Although PbLi flow and heat transfer in a nuclear fusion reactor condition needs to be comprehended, no velocity profile measurement techniques have been developed yet. Since PbLi is a high-temperature liquid metal, ultrasonic Doppler velocimetry (UDV) is a promising method. In the present study, we investigated a UDV measurement in a high-purity PbLi forced convection loop environment. In summary, naturally contained oxide particles as flow tracers could differ by PbLi purity, and this difference could influence UDV measurability.

Compatibility of an FeCrAl alloy with flowing Pb-Li in a thermal convection loop

A mono-metallic thermal convection loop (TCL) fabricated from alloy APMT (Fe21Cr5Al3Mo) tubing and filled with 0.025 m long tensile specimens of the same alloy was operated continuously for 1000 h with commercially pure Pb-17 at.%Li (Pb-Li) at a peak temperature of 550 ± 1.5 °C and a temperature gradient of ∼116 °C. The resulting Pb-Li flow rate was ∼0.0067 m/s. A 1050 °C pre-oxidation treatment (to form an external alumina scale) given to most specimens exposed within the TCL decreased total mass loss by a factor of 3–30 compared to adjacent specimens that were not pre-oxidized. However, all specimens exposed above 500 °C lost mass suggesting that the alumina scale was not entirely stable in flowing Pb-Li at these temperatures. Post-exposure room temperature tensile tests indicated that the mechanical properties of APMT were substantially influenced by extended exposures in the range of 435–490 °C, which caused an increase in yield strength (∼65%) and a corresponding decrease in ductility associated with α′ embrittlement. Specimens annealed in argon at the same temperature exhibited identical changes without exposure to Pb-Li. Scanning transmission electron microscopy revealed Cr-clusters within the microstructure in specimens exposed in the low temperature regions (<490 °C) of the TCL, indicating the formation of α′ consistent with the mechanism of α′ embrittlement.

Cooling capacity of high porosity open-cell metal foams as passive cryogenic radiators

This work presents an innovative avenue for employment of high porosity open-cell metal foams as extended heat transfer surfaces in passive cryogenic radiators. Metal foams are known for being light in weight and possess high surface area density. In contrast to a solid surface, porosity of metal foams makes it feasible for penetration of radiation thereby resulting in higher radiatively interactive surface area. Two 20 PPI metal foams made of copper and aluminum with 94.9% and 90.3% porosity respectively have been chosen for this study. A laboratory-scale test rig measures the radiative cooling capacity of metal foams in vacuum (10−6mbar) subjected to liquid nitrogen environment. Heat load to the foam has been provided by means of convective fluid loop. Simultaneously, a theoretical model based on radiation-conduction fin analysis has been developed to predict the foam cooling capacity at a specified temperature. The required radiation heat transfer coefficient has been obtained from a previous experiment wherein the foam samples are freely suspended in similar conditions but with no heat load. Lastly, performance of the foams under study has been expressed in terms of a commonly used performance parameter (surface area/cooling capacity) for passive cryogenic radiators.

The effect of heating area orientation on flow boiling performance in microchannels heat sink under subcooled condition

Subcooled flow boiling heat transfer experiments were conducted in this work in order to investigate the effect of the heating area orientation of microchannels heat sink on flow boiling heat transfer and pressure drop characteristics. The flow boiling heat transfer experiments were conducted in a 31 parallel “U” shaped microchannels (width 305µm and depth 290µm) heat sink with deionized water as the working fluid. The tests were conducted for different orientations such as, Horizontal upward facing (HU), Horizontal with heating area vertically aligned (HV), Vertical with up flow (VUF), Vertical with downflow (VDF) and Horizontal downward facing (HD) in a forced convection loop with volume flow rate of 50ml/min, 100ml/min and 150ml/min. From the experimental results, it was observed that the performances of the heat sink under all orientation conditions were found to be almost identical except the vertical downflow orientation (VDF). The critical heat flux values are found to be less in the case of vertical downflow orientation. In the case of vertical downflow orientation, the critical heat flux values corresponding to 50, 100 and 150ml/min flow rate were 44.1W/cm2, 74W/cm2 and 99.3W/cm2 respectively. For VDF orientation the buoyancy force acts on bubbles against the flow direction. Consequently, the bubbles were built up and merge each other due to the difficulty in draining and reversed flow was created with less heat flux input. The total pressure drop observed to be more for vertical downflow orientation compared to other orientations. Significant pressure fluctuations were observed during flow boiling in microchannels with VDF and HD orientations at low flow rates. The percentage reduction in effective heat flux value at the incipience of critical heat flux (CHF) in VDF orientation for flow rate of 50, 100 and 150ml/min were 13%, 10.30% and 7.40%, and the corresponding percentage reduction in maximum outlet heat transfer coefficient was 30%, 23% and 19% respectively.

ОБ ЭФФЕКТИВНОСТИ АКТИВНОГО УПРАВЛЕНИЯ ТЕПЛОВОЙ КОНВЕКЦИЕЙ В ТЕРМОСИФОНЕ ПРИ ЗАПАЗДЫВАЮЩЕМ КОНТРОЛЛЕРЕ

The paper deals with the problem of automatic control of the motion of an inhomogeneously heated liquid in long vertical channels connected at the ends. Such a rectangular loop (a non-simply connected thermosyphon) makes it easy to maintain the mechanical equilibrium of a liquid with a vertical temperature gradient. The fluid flow is controlled by small discrete channel orientation changes with respect to the static component of the inertial field, for example, gravity. Such changes are made by the computer according to the law of proportional negative feedback. The effect of a time delay, which naturally arises in the controller's commands, is considered. To study the effect in detail, an artificial time delay has been also added. A mathematical model of the phenomenon is developed. Dynamic modes of the work of a controlled convective loop are numerically studied. It was found that the excessive feedback gain can cause non-stationary convection modes in the system. Methods for weakening the delay effect are discussed.

Delay-induced oscillations in a thermal convection loop under negative feedback control with noise

We study both experimentally and theoretically the problem of active control of the mechanical equilibrium of a fluid in a convection loop heated from below and cooled from above. In order to easily obtain and maintain the mechanical equilibrium of fluid we have designed a rectangular-shaped loop with long vertical channels and short crosspieces between them. The control is performed by using a negative feedback subsystem which inhibits the convection by introducing small discrete changes in the spatial orientation of the loop with respect to gravity. In this paper, we focus on effects that arise when the feedback controller operates with time delays and/or is subjected to random fluctuations. Both these intrinsic features of the controller could be tuned in experiments to explore their effects together and separately. When the noise is absent, the excess feedback was found to lead to the excitation of delay-related oscillations. In addition, we show that time delay coupled with noise can cause a system to be oscillatory even when its deterministic counterpart exhibits no oscillations. So, we give an example of a hydrodynamic system having, generally, a large number degrees of freedom, which behaves like a small-sized stochastic system heavily dependent on fluctuations, even far from the point of bifurcation. The experimental data and theory is shown to be in good agreement.