Heat Removal Capability Research Articles

A numerical and experimental investigation on microscale heat transfer effect in the combined entry region in macro geometries

The rising heat dissipation problem in electronic devices has led to numerous investigations on microchannel heat sink. However, literature shows that microscale heat transfer is generally not being applied to macro geometries, which is believed largely due to the fabrication and operational challenges. In the present study, experiments were conducted in a conventionally-sized circular channel which was manufactured through conventional techniques. The channel has a nominal diameter of 20 mm and length of 30 mm. An insert was placed concentrically into the channel to make the flow path small enough to behave like a microchannel in order to attain high heat removal capabilities. Under such a construction, various sizes of channel can be formed by placing different sizes of insert, one at a time, into the circular channel. The experiments and numerical simulations were conducted for nominal gap sizes of 1000 and 300 μm over a range of Reynolds numbers from 1000 to 5500 and heat fluxes from 5.3 to 37.1 W/cm2 in the combined entry region. The experimental findings showed that the design was able to achieve a maximum heat transfer coefficient of 68 kW/m2 K with single-phase water flowing through the annular channel of gap size of 300 μm at Reynolds number of 5200. Comparisons of measurements from the 300-μm case with the numerical solutions showed good agreement for pressure drop predictions with an average deviation of 4.5% but poor agreement for the Nusselt number predictions with deviation of more than 30% for cases at higher Reynolds number. Most importantly, the experiments have demonstrated the possibility of achieving microscale heat transfer effects in macro geometries using readily available conventional fabrication methods. With microscale heat transfer effects easily available, it also presents an opportunity to effectively improve the heat removal capabilities of a macroscale heat exchanger in the near future.

Investigation of self-similar heat sinks for liquid cooled electronics

Electronic systems require cooling devices that are able to deal with high heat-flux capabilities. For this purpose, microchannels are attractive for direct cooling, due to their superior performances. A study on self-similar heat sinks for liquid cooled electronics is presented in this work, where the devices are made from copper, designed for industrial application and for large scale production. Self-similarity refers to the fact that there is a certain similarity and repeatability (or pattern) of the substructures compared with the overall structure. The internal structures, the so called overflow structures (or microchannels), have been designed in order to achieve high heat transfer coefficients. To validate the design and describe the flow characteristics inside the device via analytical solutions is almost impossible, therefore a well known numerical code was employed to have an insight of the thermal-fluid distributions. As can be seen clearly from the simulation, most of the heat is removed in the overflow-structure, on the side of the device adjacent to the source of heat. This paper attempts to analyse a comprehensive list of data as well as plots in a critical manner in order to illustrate the significant characteristics of this type of device. A clear lack in a proper common definition of the heat flux may lead to a misinterpretation of the results, in fact depending on the chosen area where the heat exchange takes place (namely the internal area of the microstructures or the separation surface between the device and the heating source), we can achieve a maximum heat flux of either about 200 W/cm2 or about 700 W/cm2. A very low pressure drop together with a good heat removal capability, make this device suitable for cooling of IGBT chips in power electronic applications.

Supersonic Two-Phase Impinging Jet Heat Transfer

The experimental heat transfer rates from a supersonic two-phase impinging air jet with disperse droplets are presented. The experimental configuration consists of an expanding disperse mixture of air and water through a converging–diverging nozzle, designed for Mach 3.26 with a liquid to air mass flow ratio ranging from 1.28% to 3.83%, impinging upon a thin film heater constructed of nichrome. The spatially varying heat transfer coefficient is measured, and peak values are on the order of 200,000 W/m2K. Two distinct regions of heat transfer are identified, one dominated by the jet impingement flow and another dominated by thin film heat transfer. The heat transfer coefficient of an impinging jet with dry air and no droplets is measured during the investigation as well. The heat transfer results are compared, and it is demonstrated that the addition of disperse water droplets to the jet significantly increases the heat removal capability of the jet as well as smoothing the spatial temperature distribution of the heater surface. As much as an order of magnitude increase in heat transfer coefficient is observed near the centerline of the jet and a factor of 3–5 increase is seen at a distance of approximately 4 nozzle diameters from the jet. The fundamental heat transfer coefficient measurements should benefit applications involving supersonic two-phase jets for high heat flux thermal management.

Reactor runaway due to statistically driven axial activity variations in graded catalyst beds: Loading from pre-measured single tube aliquots

Highly exothermic catalytic reactions are often carried out in fixed bed, multi-tubular reactors that use a graduated catalyst activity profile. By increasing the effective catalytic activity in zones along the axial coordinate, the decreasing driving force that occurs as reactants are consumed can be compensated by increasing catalytic activity. This not only helps balance the heat generation rate along the reactor with the heat removal capabilities but it allows the attainment of very high conversion of one or more reagents while maintaining high reactor productivity. Partial oxidation reactions often use such strategies to manage the heat of reaction while minimizing the amount of recycle required. Industrial examples of such processes include ethylene oxychlorination to 1, 2-dichloroethane, propylene oxidation to acrylic acid and ortho-xylene oxidation to phthalic anhydride.Lower activity zones of catalyst are often created by diluting active catalyst pills with similar-sized pills of inert materials. This avoids the need to develop several catalyst formulations for a single process and allows fine-tuning of catalyst zoning profiles. However, loading a mixture of active and inert pills into a reactor tube is an inherently statistical process, which produces non-uniform activity profiles. The effect of this statistical non-uniformity has been studied for the case where tubes are loaded from catalyst drums - an essentially infinite reservoir, but not for the case where the correct amounts of catalyst and diluent are pre-weighed for each tube before loading. This method ensures the total number of active pills in the diluted zone is always correct but it does not eliminate the random axial activity variability that is inherent in sampling from mixtures of active and inert pills. When loading a reactor tube from a pre-weighed single tube aliquot, the reservoir is finite; the loading process is a series of Bernoulli trials without replacement and the statistics of loading are described by the hypergeometric distribution.We have modeled the behavior of reactor tubes loaded in this way for tube to pill diameter ratios (N) between 3 and 10, which covers the typical range of commercial practice for such reactor systems. While the total number of active pills is always correct for this type of reactor tube, the statistical variability in the axial activity profile can cause significant numbers of tubes to run away, despite the fact that the tube would be stable with a flat activity profile. This effect is exacerbated by smaller tube to particle ratios (N values) and greater extents of catalyst dilution.While this loading scheme is still subject to statistical variability, our results show that tubes loaded from pre-weighed, single tube aliquots perform significantly better than those loaded from an infinite reservoir (55 gallon drum for example ) where the correct total activity is not guaranteed and the catalyst loading statistics are described by the binomial distribution.

Research Developments on Improvement of Spray Cooling Performance

Spray cooling is considered to be one of the most promising electronic cooling methods, since it has several unique advantages such as high heat dissipation, small fluid inventory, and uniformity of heat removal. However the spray cooling performance could be further improved, which can be expressed by the combination of a better heat removal capability, a higher cooling efficiency, and the more uniform surface temperature distribution. In this paper, recent research developments on spray cooling characteristics under different working conditions are reviewed, possible approaches to improve the spray cooing performance are also proposed. Especially spray cooling systems with both enhanced surface and lower system pressure would be be full of attraction.

Performance Characteristics of an Operating Supercritical CO2 Brayton Cycle

Supercritical CO2 (S-CO2) power cycles offer the potential for better overall plant economics due to their high power conversion efficiency over a moderate range of heat source temperatures, compact size, and potential use of standard materials in construction. Sandia National Labs (Albuquerque, NM) and the U.S. Department of Energy (DOE-NE) are in the process of constructing and operating a megawatt-scale supercritical CO2 split-flow recompression Brayton cycle with contractor Barber-Nichols Inc. (Arvada, CO). This facility can be counted among the first and only S-CO2 power producing Brayton cycles anywhere in the world. The Sandia-DOE test-loop has recently concluded a phase of construction that has substantially upgraded the facility by installing additional heaters, a second recuperating printed circuit heat exchanger (PCHE), more waste heat removal capability, higher capacity load banks, higher temperature piping, and more capable scavenging pumps to reduce windage within the turbomachinery. With these additions, the loop has greatly increased its potential for electrical power generation, and its ability to reach higher temperatures. To date, the loop has been primarily operated as a simple recuperated Brayton cycle, meaning a single turbine, single compressor, and undivided flow paths. In this configuration, the test facility has begun to realize its upgraded capacity by achieving new records in turbine inlet temperature (650 °F/615 K), shaft speed (52,000 rpm), pressure ratio (1.65), flow rate (2.7 kg/s), and electrical power generated (20 kWe). Operation at higher speeds, flow rates, pressures, and temperatures has allowed a more revealing look at the performance of essential power cycle components in a supercritical CO2 working fluid, including recuperation and waste heat rejection heat exchangers (PCHEs), turbines and compressors, bearings and seals, as well as auxiliary equipment. In this report, performance of these components to date will be detailed, including a discussion of expected operational limits as higher speeds and temperatures are approached.

NUMERICAL COMPUTATION OF FLUID FLOW AND HEAT TRANSFER IN A MEMS-BASED MICRO CHANNEL HEAT SINK

Carbon nanotube (CNT) has been proven to be an effective material for the thermal management of MEMS-based devices due to its superior thermal conductivity. At the same time, micro-channel heat-sinks are widely used in electronic products as a high performance heat transfer device because of its simple construction, easy fabrication process and effective heat removal capability. A numerical study has been carried out to investigate the thermal-fluid characteristics of the aligned and staggered MWCNT (multi walled CNT) based micro pin fins having 650 µm long with hydraulic diameter of ~130 µm. Average heat transfer coefficients have been obtained for effective heat fluxes ranging from 50 to 130 W and Reynolds numbers from 14 to 160. The influence of various fluids, fluid velocity; fluid conductivity, fin material, fin conductivity, fin geometry and fin orientations on cooling effects have also been investigated and compared in this study.

Pool boiling CHF enhancement by graphene-oxide nanofluid under nuclear coolant chemical environments

External reactor vessel cooling (ERVC) for in-vessel retention (IVR) of corium as a key severe accident management strategy can be achieved by flooding the reactor cavity during a severe accident. In this accident mitigation strategy, the decay heat removal capability depends on whether the imposed heat flux exceeds critical heat flux (CHF). To provide sufficient cooling for high-power reactors such as APR1400, there have been some R&D efforts to use the reactor vessel with micro-porous coating and nanofluids boiling-induced coating. In present study, an experimental study has been conducted to investigate the viability of using graphene-oxide nanofluid under various coolant chemical environments to enhance CHF during ERVC. Pool boiling CHF experiments were carried out for the thin-wire heater with controlling the heater orientation from horizontal to vertical, or at 0<θ<90°. The dispersion stability of graphene-oxide nanofluid in the chemical conditions of flooding water that includes boric acid, lithium hydroxide (LiOH), and tri-sodium phosphate (TSP) was checked in terms of surface charge or zeta potential before the CHF experiments. Finally integral effects of graphene-oxide nanosheets and chemicals on CHF limits were investigated. Results showed that graphene-oxide nanofluids were very stable under ERVC coolant chemical environments and enhanced CHF limits up to about 40% at minimum at 90° of angle (vertical orientation) and about 200% at maximum at 0° of angle (horizontal orientation) in comparison to pure water.

Development of a porous body model for decay heat removal studies in a pool type sodium cooled fast reactor

In this paper, the primary heat transport circuit of a pool type fast breeder reactor has been analyzed to demonstrate its decay heat removal capability without exceeding design safety limits even under complete power failure condition leading to natural convection flow in the system. A three dimensional model of large sodium pool (~12 m diameter) and complex inter-wrapper flow path through narrow gaps (3.4 mm) between a large number of heat generating subassemblies is simplified to a semi-3D porous body model which is computationally less intensive. The proposed semi-3D model has been validated against the detailed three dimensional model. The analysis has been carried out using the general purpose CFD code STAR-CD. The predicted results demonstrate that the decay heat removal system is conducive for natural convection and can remove the heat, keeping all the important temperatures within design safety limits.

On the accuracy of CFD modeling of cyclic high heat flux divertor experiment

To investigate the heat removal capability of conceptual divertor cooling fingers, accurate computational fluid dynamics (CFD) analyses are indispensable. Although the cooling performance of the divertor finger has been successfully high-heat-flux tested under real DEMO conditions in a combined helium loop and electron beam facility at the Efremov Institute, Russia, an accompanying numerical simulation of the experiments is of great importance. This might help to better understand the complex thermo-hydraulic conditions with the aim of predicting other different load cases. To accurately reproduce the experimental boundary conditions, the Gaussian-like shape of the absorbed power was taken into account and the heat losses were estimated. Modeling of the structure thermal conductivity was also found to be an important source of modeling uncertainty. In the context of accurate modeling of experimental conditions, the effect of some modeling assumptions was evaluated. Transient simulations of the cyclic heat flux experiment were performed only for the solid part of the cooling finger to avoid excessively long computation times. The helium cooling was taken into account by the heat transfer coefficient (HTC) on the fluid–structure interface, obtained from the steady-state simulations of the full solid–fluid model. The HTC distribution did not vary with time throughout the entire transient simulation. The modeling error associated with such HTC approximation was estimated for the particular cyclic experiment. It is shown that the simulated temperature cycles on the top of experimental mock-up agree well with the measured data.

Reactor runaway due to statistically driven axial activity variations in graded catalyst beds

In some multi-tubular fixed bed catalytic reactors, a graduated activity profile in the axial direction is created by loading several “zones” of catalyst, each with a different target catalytic activity. This is often achieved by diluting active catalyst pellets with inert materials and can help balance the heat generation and heat removal capabilities of the reactor. Partial oxidation reactions where one or more of the reagents undergoes very high conversion often use such strategies and some examples are ethylene oxychlorination to 1,2-dichloroethane, propylene oxidation to acrylic acid and ortho-xylene oxidation to phthalic anhydride.Loading a mixture of active and inactive particles into a reactor tube is an inherently statistical process, which produces non-uniform activity profiles within the reactor tubes. In the present work, we have evaluated the impact of this purely statistical axial activity variation on the performance of a reactor for the conversion of ortho-xylene to phthalic anhydride. The simulations include tube to particle diameter ratios (N) between 3 and 10, which covers the typical range of commercial practice for such reactor systems. We have found that even when the intended zoning profile is well within the stable operating regime, significant numbers of reactor tubes are likely to run away for systems with 50% catalyst and N values of 4–6. These runaways are caused by axial activity variations rooted in the purely statistical sampling process of adding a mixture of active and inert particles into the reactor tubes. Higher dilutions and smaller values of N make this situation worse.The work shows that the frequency of runaway reactor tubes correlates reasonably well with the relative standard deviation of the binomial distribution that describes the active catalyst sampling statistics divided by N. A strategy is proposed for establishing particle size and dilution ratios that minimize the likelihood of runaway tubes.In commercial practice, imperfect catalyst-diluent blending, particle segregation during storage and handling and careless catalyst loading will all increase the likelihood of runaway tubes, so the present analysis represents an optimistic view of the axial activity profile issue.

Structured surfaces for enhanced pool boiling heat transfer

We experimentally investigated surface roughness-augmented wettability on critical heat flux (CHF) during pool boiling with horizontally oriented surfaces. Microstructured surfaces with a wide range of well-defined surface roughness were fabricated, and a maximum CHF of ∼208 W/cm2 was achieved with a surface roughness of ∼6. An analytical force-balance model was extended to explain the CHF enhancement. The excellent agreement found between the model and experimental data supports the idea that roughness-amplified capillary forces are responsible for the CHF enhancement on structured surfaces. The insights gained from this work suggest design guidelines for new surface technologies with high heat removal capability.

Side-Coupling Scheme for a High-Power Laser Diode Array With Grating Couplers: Thermal and Geometrical Issues

Grating couplers for side coupling of the light from a high-power laser diode array (LDA) into the inner cladding of a double-clad fiber were studied theoretically and implemented experimentally. In the experiments, two types of grating couplers were designed and fabricated: a gold-coated surface relief grating coupler and a gold-embedded silica grating coupler. A suitable design for heat dissipation for each grating coupler was employed to minimize thermal expansion due to the heat accumulated from the light absorption by the metal part of the grating coupler. The experimental results show that the gold-embedded silica grating coupler is superior to the surface relief gold grating coupler, because of its higher resistance to thermal expansion and better heat removal capability. In addition, the grating pitch and groove width were optimized for the highest overall coupling efficiency by taking account the backward diffraction loss and the groove wall nonverticality due to fabrication distortion. The experimental results show that the gold-embedded silica grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave LDA into the inner cladding of a 400- m-diameter double-clad fiber with an overall coupling efficiency of 50%.

Evaluation of energy and particle impact on the plasma facing components in DEMO

We analyze the first wall blanket W/EUROFER configuration for DEMO under steady-state normal operation and off-normal conditions, such as vertical displacement events (VDE) and runaway electrons (RE). The main issue is to find the optimal thickness of the W armor which will prevent tungsten surface from evaporation and melting and, on the other hand, will keep EUROFER below the critical thermal stresses. Under steady-state operation heat transfer into the coolant must remain below the critical heat flux (CHF) to avoid the possible severe degradation of the coolant heat removal capability. From the plasma side it is particularly demanding to keep the bulk plasma contamination during the reactor long operational discharges below the fatal level. The possible damage of the FW materials due to the plasma sputtering erosion is estimated. The minimum thickness of the tungsten amour about 3mm for W/EUROFER sandwich structure will keep the maximum EUROFER temperature below the critical limit for EUROFER steel under steady-state operation and ITER like cooling conditions.

Analytical studies of the heat removal capability of a passive auxiliary feedwater system (PAFS)

As passive safety features for nuclear power plants receive increasing attention, various studies have been conducted to develop safety systems for third-generation (GEN-III) nuclear power plants that are driven by passive systems, such as natural circulation, gravity, and resistance to high temperatures. Thus, South Korea has designed the Advanced Power Reactor Plus (APR+) with a two-loop PWR and 1500 MWe by adding passive safety features to the Advanced Power Reactor 1400 MWe (APR1400). The Passive Auxiliary Feedwater System (PAFS) is one of several passive safety systems being designed for the APR+, and extensive studies are being conducted to complete its design and to verify its feasibility. Because the PAFS removes decay heat from the reactor core under transient and accident conditions, it is necessary to evaluate the heat removal capability of the PAFS under hypothetical accident conditions. Therefore, in this paper, after introducing the characteristics of the PAFS and its design requirements, a performance analysis of the PAFS is performed for two accident cases: Loss of Condenser Vacuum (LOCV) and Feedwater Line Break (FLB). For the analysis, the Multi-dimensional Analysis of Reactor Safety (MARS) code is used, and a MARS model is developed by adding the PAFS model to the existing APR1400 model. The analysis results show that the PAFS has enough capacity to remove decay heat under the postulated accident conditions. In addition, the adequacy of modified control logic for main steam isolation valve (MSIV) is validated by comparing the traditional control logic.

Research on the designed emergency passive residual heat removal system during the station blackout scenario for CPR1000

The concept of passive safety is put forward to improve the safety and reliability of the nuclear power plant. The secondary emergency passive residual heat removal system (EPRHRS) is an innovative design for China Pressurized Reactor (CPR1000), which can remove the core residual heat effectively and improve the inherent safety by passive methods when the station blackout (SBO) occurs. In this work, the EPRHRS, which is connected to the steam generator (SG) and consists of a heat exchanger (HX), a water tank and corresponding pipes and valves, was designed for CPR1000. The primary loop and EPRHRS were modeled using RELAP5/MOD3.4 to investigate the residual heat removal capability of the EPRHRS and the transient characteristics of the primary loop system in the event of the SBO accident. In addition, sensitivity studies were also performed to investigate effects of the main parameters of the EPRHRS on the transient behavior of the primary loop and the EPRHRS. The effect of isolation valve opening time on the primary loop and the EPRHRS was also studied in the paper. The results of the study indicate that the optimized HX area is 200m2 when the EPRHRS water tank height is 10m and the optimized isolation valve opening time is 30s. The EPRHRS can remove the core decay heat from the primary loop effectively through the stable natural circulation established in the primary loop and the EPRHRS loop based on the calculation results. Therefore, it illustrates that the EPRHRS designed in the paper is effective and meaningful to CPR1000.

R&D activities on manufacturing plasma-facing unit for prototype of ITER divertor outer target in JADA

Japan Domestic Agency (JADA) carried out R&Ds activities to improve joining CFC monoblocks onto a CuCrZr cooling tube in PFUs to boost the success rate of joint and to confirm load carrying capability of the monoblock attachments to Steel Support Structure (SSS) against tensile force simulating electromagnetic load to pull PFUs from SSS. In joining the CFC monoblocks to the cooling tube, JADA has adopted brazing by using noble-metal-free filler with the following improvements; (1) metalizing joint surface of CFC using Ti-coating with accurate thickness controlling, (2) Changing buffer layer material from soft pure copper to Cu–W alloy. By using the present improved joint, JADA has manufactured three mock-ups with 5 CFC monoblocks and tested against repetitive high heat loads more than 20MW/m2. All of CFC monoblocks of each mockup can survive the high heat loads throughout 1000 cycles with no degradation of heat removal capability. Regarding the load carrying capability of monoblock attachments to SSS, tensile experiments were carried out using the same geometries of CFC and tungsten monoblocks in PFUs and the results show that both geometries and joints meet the ITER requirements, that is, 3kN and 8kN, respectively.

Multi dimensional analysis of Design Basis Events using MARS-LMR

Abstract KALIMER-600 (Korea Advanced Liquid Metal Reactor), which is a pool type SFR (Sodium-cooled Fast Reactor), was developed by KAERI (Korea Atomic Energy Research Institute). DBE (Design Basis Events) for KALIMER-600 has been analyzed in the one dimension. In this study, the one dimensional analyzed sodium hot pool is modified to a three dimensional node system, because the one dimensional analysis cannot represent the phenomena of the inside pool of a big size pool with many compositions, such as UIS (Upper Internal Structure), IHX (Intermediate Heat eXchanger), DHX (Decay Heat eXchanger), and pump. The results of the multi-dimensional analysis compared with the one dimensional analysis results in normal operation, TOP (Transient of Over Power), LOF (Loss of Flow), and LOHS (Loss of Heat Sink) conditions. First, the results in normal operation condition show the good agreement between the one and multi-dimensional analysis. However, according to the sodium temperatures of the core inlet, outlet, the fuel central line, cladding and PDRC (Passive Decay heat Removal Circuit), the temperatures of the one dimensional analysis are generally higher than the multi-dimensional analysis in conditions except the normal operation state, and the PDRC operation time in the one dimensional analysis is generally longer than the multi-dimensional analysis. In particular, we can see the reverse and mixing flow phenomena in LOHS better than TOP and LOF, which affect the peak temperature in the one dimensional analysis at 330 s but is not observed in the multi-dimensional analysis. In addition, the different temperatures at the central line of fuel and cladding are shown due to different mass flow in the core. The two PDRCs show the almost same results in one and multi-dimensional analysis, respectively. However, in the comparison between the one and multi-dimension, the heat removal capability of PDRC in the multi-dimensional analysis is better than the one dimensional analysis at the beginning of calculation. The aspects change at the ending of calculation due to the different mass flow in PDRC and over flow sodium temperature. In wrap up, the sodium temperatures of the core in the one dimensional analysis are generally higher than the multidimensional analysis. The temperature peak point in the one dimensional analysis under LOHS does not show in the multi-dimensional analysis, because the reverse and mixing flow pattern between the hot pool and the core outlet region. Even thought there are some different results between the two different dimensional analyses, we can say that the KALIMER-600 system has the safety margins.

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