Experimental Apparatus for the Study of micro Heat Exchangers with Inlet Temperatures between -200 and 200 °C and Elevated Pressures

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The current paper presents a test bench for micro-fabricated Recuperative Counter Flow Heat Exchanger (RCFHE). The bench is suitable for up to 200 K difference between inlets temperatures and operating pressures up to 32 MPa. The experimental setup allows controlling the physical state of the gas (i.e. temperature, pressure and flow rate) at the RCFHE inlets. The bench has 5 controlled parameters and 5 more that are monitored and enables studying each of the hot and cold channels separately. We demonstrate a steady supply of liquid nitrogen into the device for 10 minutes without thermal insulation of the specimen. Another run is a steady state experiment with a temperature difference of about 20-30 K between inlets. These show that the apparatus is capable of characterizing heat exchangers and serve as preliminary results

Evaluation of Heat Removal from RBMK-1500 Core Using Control Rods Cooling Circuit

The Ignalina nuclear power plant is a twin unit with two RBMK-1500, graphite moderated, boiling water, multichannel reactors. After the decision was made to decommission the Ignalina NPP, Unit 1 was shut down on December 31, 2004, and Unit 2 is to be operated until the end of 2009. Despite of this fact, severe accident management guidelines for RBMK-1500 reactor at Ignalina NPP are prepared. In case of beyond design basis accidents, it can occur that no water sources are available at the moment for heat removal from fuel channels. Specificity of RBMK reactor is such that the channels with control rods are cooled with water supplied by the system totally independent from the reactor cooling system. Therefore, the heat removal from RBMK-1500 reactor core using circuit for cooling of rods in control and protection system can be used as nonregular mean for reactor cooldown in case of BDBA. The heat from fuel channels, where heat is generated, through graphite bricks is transferred in radial direction to cooled CPS channels. This article presents the analysis of possibility to remove heat from reactor core in case of large LOCA by employing CPS channels cooling circuit. The analysis was performed for Ignalina NPP with RBMK-1500 reactor using RELAP5-3D and RELAP5 codes. Results of the analysis have shown that, in spite of high thermal inertia of graphite, this heat removal from CPS channels allows to slow down effectively the core heat-up process

Microstructure and Residual Stress Evolution of Laser Powder Bed Fused Inconel 718 under Heat Treatments

The current work aimed to study the influence of various heat treatments on the microstructure, hardness, and residual stresses of Inconel 718 processed by laser powder bed fusion process. The reduction in residual stresses is crucial to avoid the deformation of the component during its removal from the building platform. Among the different heat treatments, 800 °C kept almost unaltered the original microstructure, reducing the residual stresses. Heat treatments at 900, 980, and 1065 °C gradually triggered the melt pool and dendritic structures dissolution, drastically reducing the residual stresses. Heat treatments at 900 and 980 °C involved the formation of δ phases, whereas 1065 °C generated carbides. These heat treatments were also performed on components with narrow internal channels revealing that heat treatments up to 900 °C did not trigger sintering mechanisms allowing to remove the powder from the inner channels

Microstructure and Residual Stress Evolution of Laser Powder Bed Fused Inconel 718 under Heat Treatments

AbstractThe current work aimed to study the influence of various heat treatments on the microstructure, hardness, and residual stresses of Inconel 718 processed by laser powder bed fusion process. The reduction in residual stresses is crucial to avoid the deformation of the component during its removal from the building platform. Among the different heat treatments, 800 °C kept almost unaltered the original microstructure, reducing the residual stresses. Heat treatments at 900, 980, and 1065 °C gradually triggered the melt pool and dendritic structures dissolution, drastically reducing the residual stresses. Heat treatments at 900 and 980 °C involved the formation of δ phases, whereas 1065 °C generated carbides. These heat treatments were also performed on components with narrow internal channels revealing that heat treatments up to 900 °C did not trigger sintering mechanisms allowing to remove the powder from the inner channels

Characterization of Manifold Microchannel Heat Sinks During Two-Phase Operation

High-heat-flux removal is necessary for next-generation microelectronic systems to operate more reliably and efficiently. The direct embedding of microchannel heat sinks into the heated substrate serves to reduce the parasitic thermal resistances due to contact and conduction resistances typically associated with the attachment of a separate heat sink. Manifold microchannel (MMC) heat sinks can dissipate high heat fluxes at moderate pressure drops, especially during two-phase operation. High-aspect-ratio microchannels allow for a large enhancement in heat transfer area. This work focuses on designing intrachip MMC heat sinks for high-heat-flux dissipation and to characterize the flow morphology present in the MMCs during two-phase operation. A 3 × 3 array of heat sinks is fabricated into a heated silicon substrate for direct intrachip cooling. The heat sinks are fed in parallel using a hierarchical manifold distributor that is designed to deliver equal flow to each of the heat sinks. Each heat sink contains a bank of high-aspect-ratio microchannels; channels with nominal widths of 15 μm and 33 μm and nominal depths between 150 μm and 470 μm are tested. Discretizing the chip footprint area into multiple smaller heat sink elements with high-aspect-ratio microchannels ensures shortened effective fluid flow lengths. High two-phase fluid mass fluxes can thus be accommodated in micron-scale channels while keeping pressure drops low compared to traditional, microchannel heat sinks. The thermal and hydraulic performance of each heat sink array geometry is evaluated using the engineered dielectric liquid HFE-7100 as the working fluid and for mass fluxes ranging from 600 kg/m²s to 2100 kg/m²s at a constant inlet temperature of 59 °C. To simulate heat generation from electronics devices, a uniform background heat flux is generated with thin-film serpentine heaters fabricated on the silicon substrate opposite the channels; temperature sensors placed across the substrate provide spatially resolved surface temperature measurements. Experiments are also conducted with simultaneous background and hotspot heat generation; the hotspot heat flux is produced by an individual 200 μm × 200 μm hotspot heater. During uniform heating conditions, heat fluxes up to 1020 W/cm² are dissipated at chip temperatures less than 69 °C above the fluid inlet and at pressure drops less than 120 kPa. Heat sinks with wider channels yield higher wetted-area heat transfer coefficients, but not necessarily the lowest thermal resistance; for a fixed channel depth, samples with thinner channels can have increased total wetted areas owing to the smaller fin pitches. During simultaneous background and hotspot heating conditions, background heat fluxes up to 900 W/cm² and hotspot fluxes up to 2,700 W/cm² are dissipated. The hotspot temperature increases linearly with hotspot heat flux and is independent of background heat flux and mass flux. At hotspot heat fluxes of 2,700 W/cm², the hotspot experiences a temperature rise of 16 °C above the average chip temperature. The ability to fabricate and assemble a chip-integrated, compact hierarchical manifold used to deliver fluid to a 9 × 9 array of heat sinks has been demonstrated, with feature sizes significantly reduced compared to the 3 × 3 array of heat sinks. The integrated manifold provides ports to measure the pressure drop across the channel; combining these data with the overall pressure drop, the contributions of both components to the hydraulic performance is determined. The hierarchical manifold consists of eight feature layers that have a minimum feature size of 50 μm. The manifold is fabricated by etching one feature layer into each side of four silicon wafers and then thermocompression bonding the wafers together. The resulting manifold is a compact, leak-free device that is used to deliver fluid to the array of heat sinks and recollect the outlet flow from the heat sinks. A sample manifold was diced, revealing a manifold that was aligned with the channels within 5 μm. Heat fluxes up to 630 W/cm² are tested with temperatures and pressures reaching 110 °C and 135 kPa, respectively. An experiment is designed to provide simultaneous high-speed flow visualization and spatially-resolved wall temperature measurements on a single manifold microchannel. Visualizing the flow morphology inside the channel during two-phase operation is critical to being able to understand the performance MMCs. This work provides an understanding of the two-phase flow structure and wall temperature profiles in high-aspect-ratio microchannels, which cannot be extracted from the area- and time-averaged data obtained using the heat sinks containing many parallel channels. In high-aspect-ratio channels, vapor blanketing at the bottom of the channel is observed, which leads to significantly diminished thermal performance. The vapor formation characteristics in high-aspect-ratio microchannels also lead to time-periodic fluctuations that are not observed in low and intermediate aspect ratios. Opportunities for future experimental and model work to further understand flow boiling in MMCs are identified based on the work completed in this dissertation and the open literature

Characterization of Hierarchical Manifold Microchannel Heat Sink Arrays under Simultaneous Background and Hotspot Heating Conditions

A hierarchical manifold microchannel heat sink array is fabricated and experimentally characterized for uniform heat flux dissipation over a footprint area of 5 mm x 5 mm. A 3 x 3 array of heat sinks is fabricated into the silicon substrate containing the heaters for direct intrachip cooling, eliminating the thermal resistances typically associated with the attachment of a separate heat sink. The heat sinks are fed in parallel using a hierarchical manifold distributor that delivers flow to each of the heat sinks. Each heat sink contains a bank of high-aspect-ratio microchannels; five different channel geometries with nominal widths of 15 lm and 33 micrometers and nominal depths between 150 micrometers and 470 micrometers are tested. The thermal and hydraulic performance of each heat sink array geometry is evaluated using HFE-7100 as the working fluid, for mass fluxes ranging from 600 kg/m2 s to 2100 kg/m2 s at a constant inlet temperature of 59 degree C. To simulate heat generation from electronics devices, a uniform background heat flux is generated with thin-film serpentine heaters fabricated on the silicon substrate opposite the channels; temperature sensors placed across the substrate provide spatially resolved surface temperature measurements. Experiments are also conducted with simultaneous background and hotspot heat generation; the hotspot heat flux is produced by a discrete 200 micrometers x 200 micrometers hotspot heater. Heat fluxes up to 1020 W/cm2 are dissipated under uniform heating conditions at chip temperatures less than 69 degree C above the fluid inlet and at pressure drops less than 120 kPa. Heat sinks with wider channels yield higher wetted-area heat transfer coefficients, but not necessarily the lowest thermal resistance; for a fixed channel depth, samples with narrower channels have increased total wetted areas owing to the smaller fin pitches. During simultaneous background and hotspot heating conditions, background heat fluxes up to 900 W/cm2 and hotspot fluxes up to 2700 W/cm2 are dissipated. The hotspot temperature increases linearly with hotspot heat flux; at hotspot heat fluxes of 2700 W/cm2, the hotspot experiences a temperature rise of 16 degree C above the average chip temperature

BACKUP AND ULTIMATE HEAT SINKS IN CANDU REACTORS FOR PROLONGED SBO ACCIDENTS

In a pressurized heavy water reactor, following loss of the primary coolant, severe core damage would begin with the depletion of the liquid moderator, exposing the top row of internally-voided fuel channels to steam cooling conditions on the inside and outside. The uncovered fuel channels would heat up, deform and disassemble into core debris. Large inventories of water passively reduce the rate of progression of the accident, prolonging the time for complete loss of engineered heat sinks.The efficacy of available backup and ultimate heat sinks, available in a CANDU 6 reactor, in mitigating the consequences of a prolonged station blackout scenario was analysed using the MAAP4-CANDU code. The analysis indicated that the steam generator secondary side water inventory is the most effective heat sink during the accident. Additional heat sinks such as the primary coolant, moderator, calandria vault water and end shield water are also able to remove decay heat; however, a gradually increasing mismatch between heat generation and heat removal occurs over the course of the postulated event. This mismatch is equivalent to an additional water inventory estimated to be 350,000 kg at the time of calandria vessel failure. In the Enhanced CANDU 6 reactor ∼2,040,000 kg of water in the reserve water tank is available for prolonged emergencies requiring heat sinks

Molecular mechanisms of acquired thermotolerance in plants

21 Molecular mechanisms of acquired thermotolerance in plants Andrija Finka Department of Ecology, Agriculture and Aquaculture, University of Zadar, Trg Kneza Višeslava 1, 23000 Zadar [email protected] Sessile organisms such as plants experience daily heat-shocks, with temperature variations rising up to 30°C between 2 am and 2 pm. Diurnal temperature variations may be particularly extreme for organisms that evolved far from large temperate water bodies, such as oceans, and populated intemperate inland habitats exposed to abrupt environmental stresses, such as freezing, heating, excess light, excess oxygen radicals and to more gradual seasonal stresses, such as dehydration, nutriment starvation and salt stress. To cope with a noxious heat shock, exposed organisms need to timely develop appropriate molecular defenses that can prevent and repair heat damages, mostly in labile lipids and proteins. A mild rise of ambient temperature transiently increases the fluidity of the plasma membrane, which in turn, activates the transient opening and depolarization of specific heat-sensitive Ca2+ channels. Members of the cyclic nucleotide gates channels (CNGCs) were recently identified as being the primary heat-sensors of land plants. Depending on the rate of temperature increase, the duration and intensity of the heat priming preconditions, terrestrial plants may thus acquire an array of heat shock protein-based thermotolerance mechanisms against upcoming, otherwise lethal, extreme heat waves. Quantitative proteomics reveals absolute changes of cellular protein concentrations during heat-shock treatments

Refrigerant two-phase flow behaviour and pressure drop up- and downstream of a sharp return bend

The European Union’s goals for climate and energy aim for a reduction of Europe's greenhouse gas emissions by 80-95% compared to 1990 levels by the year 2050. One of the technologies that can help to attain such a low-carbon society is a heat pump. It is estimated that the use of heat pumps could reduce the CO2 emissions of the building sector by 50%. An important component of the heat pump system is the heat exchanger, which typically consists of tubes and fins. This type of heat exchanger is also common in other applications such as air-conditioning. For domestic applications it is important that this heat exchanger is constructed in a compact way. To attain a heat exchanger with a limited size, the tubes are folded up into a sequence of short straight channels interconnected by 180° bends. The fluid flowing through these tubes extracts heat from the ambient air and the addition of this heat causes the fluid to evaporate. Research confirms that the presence of the bends do affect the evaporating flow in these channels. However, the underlying mechanisms are still largely unknown. In this work, the effect of the return bend on the flow is investigated and linked to the occurring pressure drop in the channel. The results yield more insight in how the bend geometry affect the flow and what the consequences are for the occurring pressure drop