Exit Quality Research Articles

BWR-HD: An Optimized Boiling Water Reactor with High Power Density

Increasing the economic competitiveness of nuclear energy is vital to its future. One way to reduce the cost of the plant is by extracting more power from the same volume. A scoping study is conducted to maximize the power density in boiling water reactors (BWRs) under the constraints of using fuel with traditional materials and cylindrical geometry, and enrichments below 5% to enable its licensability with no changes to present facilities. An optimization search over all other design parameters yields a BWR with high power density (BWR-HD) at a power level of 5000 MW(thermal), equivalent to a 26% uprated Advanced BWR (ABWR), the most recently built version of BWR. The BWR-HD utilizes about the same number of wider fuel assemblies, with 16 × 16 pin arrays and 35% shorter active fuel than the 10 × 10 assemblies of the ABWR. The fuel rod diameter and pitch are also reduced to just over 70% of the ABWR values. Thus, it is possible to increase the power density and specific power by 65% while maintaining the nominal ABWR minimum critical power ratio margin. The optimum core pressure is found to be the same as the current 7.2 MPa. The core exit quality is increased to 19% from the ABWR nominal exit quality of 15%. The pin linear heat generation rate is 20% lower, and the core pressure drop and mass of uranium are 30% lower. The BWR-HD’s fuel, modeled with FRAPCON 3.4, showed similar performance to the ABWR pin design. This results in 20% reduced operations and maintenance and capital costs per unit energy, but total fuel cycle cost similar to that of the 18-month ABWR fuel cycle.

Flow Boiling Pressure Drop of R134a in Microdiameter Tubes: Experimental Results and Assessment of Correlations

The experimental results of two-phase flow boiling pressure drop of R134a in vertical microdiameter stainless steel tubes are presented in this paper. The tests were conducted using four tubes: one tube with an inner diameter of 0.52 mm and 100 mm heated length and three tubes with an inner diameter of 1.1 mm and different heated lengths (150, 300, and 450 mm). Other experimental conditions include mass flux range of 200–500 kg/m2-s, system pressure range of 6–10 bar, inlet subcooling value of about 5 K, and heat flux range of 1–140 kW/m2. The results indicated that the total measured two-phase pressure drop increases with increasing mass flux, heat flux (exit quality) and decreasing system pressure and tube inner diameter. The test section heated length was found to have a significant effect on the measured pressure drop per metre length. The total measured two-phase pressure drop results were also compared with eighteen macro- and microscale models and correlations.

Two-Phase Flow Control of Electronics Cooling With Pseudo-CPUs in Parallel Flow Circuits: Dynamic Modeling and Experimental Evaluation

On-chip two-phase cooling of parallel pseudo-CPUs integrated into a liquid pumped cooling cycle is modeled and experimentally verified versus a prototype test loop. The system's dynamic operation is studied since the heat dissipated by microprocessors is continuously changing during their operation and critical heat flux (CHF) conditions in the microevaporator must be avoided by flow control of the pump speed during heat load disturbances. The purpose here is to cool down multiple microprocessors in parallel and their auxiliary electronics (memories, dc/dc converters, etc.) to emulate datacenter servers with multiple CPUs. The dynamic simulation code was benchmarked using the test results obtained in an experimental facility consisting of a liquid pumped cooling cycle assembled in a test loop with two parallel microevaporators, which were evaluated under steady-state and transient conditions of balanced and unbalanced heat fluxes on the two pseudochips. The errors in the model's predictions of mean chip temperature and mixed exit vapor quality at steady state remained within ±10%. Transient comparisons showed that the trends and the time constants were satisfactorily respected. A case study considering four microprocessors cooled in parallel flow was then simulated for different levels of heat flux in the microprocessors (40, 30, 20, and 10 W cm−2), which showed the robustness of the predictive-corrective solver used. For a desired mixed vapor exit quality of 30%, at an inlet pressure and subcooling of 1600 kPa and 3 K, the resulting distribution of mass flow rate in the microevaporators was, respectively, 2.6, 2.9, 4.2, and 6.4 kg h−1 (mass fluxes of 47, 53, 76 and 116 kg m−2 s−1) and yielded approximately uniform chip temperatures (maximum variation of 2.6, 2, 1.7, and 0.7 K). The vapor quality and maximum chip temperature remained below the critical limits during both transient and steady-state regimes.

The effect of the geometric scale on the critical heat flux for the top of the reactor vessel lower head

The critical heat flux (CHF) on the top part of the reactor vessel lower head external wall was measured using three test sections of a two-dimensional slice. The top region (inclination angle of 90°) of the test section, which simulates the reactor vessel lower head for the in-vessel retention through the external reactor vessel cooling (IVR-ERVC) strategy for severe accidents, was primarily considered in this study. The purpose of this study was to produce experimental data to assess the CHF limits on the top of the external vessel wall for the metal layer and investigate the geometric scaling effect on the CHF. Three test sections, with respective radius of curvatures values of 0.15m, 0.25m and 0.5m were used. The width and gap sizes were 0.03m and 0.03–0.06m, respectively. The material of the test section was type 304 stainless steel (SUS304). The CHF results were acquired under an inlet subcooling of 2 and 10K with mass fluxes between 50 and 400kg/m2s. The experimental results were compared with the results of large-scale experiments. The ULPU data were generally greater than the CHF data in this study, which is due to the surface property. Using the CHF data from Jeong et al. (2005), the CHF results were discussed based on the local condition for scales between R=0.15 and 2.5m. However, the local conditions cannot suitably explain the relationship between the R=0.25m and R=0.5m test cases. To eliminate the inconsistency in the CHF relationship of the mass flux and exit quality conditions, a flow visualization technique was used, and the formation of a two-phase boundary layer flow was observed. Applying the scaled-down gap size, the exit quality values for the experimental results were modified. CHF correlations were developed with the data from this study and from Jeong et al. (2005).

Experimental analysis of upward flow boiling heat transfer in a channel provided with copper metallic foam

This work deals with the flow boiling heat transfer inside a channel filled with metallic foam soldered to its lateral walls. The survey on this topic shows that the work in this field is at its beginning. The tested metallic foam samples are made from copper with 36 PPI (Pore per Inch Linear) and 97% porosity and the working fluid being used is n-pentane. In the present work the independent variables are the mass flux in the range from 10 to 100 kg/m2s and the heating power with values between 0 and 25 W/cm2. There are two dependent variables which are the temperature (for the heated wall and inside the channel) and the exit quality. The thermal results are compared to those given by Gungor–Winterton and Shah. The comparison with the Gungor–Winterton correlation shows that the metallic foam insert enhances the heat transfer coefficient by a factor in the range of 2–4 for low quality. Furthermore an inversion point is identified where the effectiveness of the channel filled with metallic foam is reversed compared to a plain channel. This point is given for a constant wall temperature superheat.

A Study of Critical Heat Flux During Flow Boiling in Microchannel Heat Sinks

The cooling capacity of two-phase transport in microchannels is limited by the occurrence of critical heat flux (CHF). Due to the nature of the phenomenon, it is challenging to obtain reliable CHF data without causing damage to the device under test. In this work, the critical heat fluxes for flow boiling of FC-77 in a silicon thermal test die containing 60 parallel microchannels were measured at five total flow rates through the microchannels in the range of 20–80 ml/min. CHF is caused by dryout at the wall near the exit of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The bubbles pushed back into the inlet plenum agglomerate; the resulting flow blockage is a likely cause for the occurrence of CHF which is marked by an abrupt increase in wall temperature near the exit and an abrupt decrease in pressure drop across the microchannels. A database of 49 data points obtained from five experiments in four independent studies with water, R-113, and FC-77 as coolants was compiled and analyzed. It is found that the CHF has a strong dependence on the coolant, the flow rate, and the area upon which the heat flux definition is based. However, at a given flow rate, the critical heat input (total heat transfer rate to the coolant when CHF occurs) depends only on the coolant and has minimal dependence on the details of the microchannel heat sink (channel size, number of channels, substrate material, and base area). The critical heat input for flow boiling in multiple parallel microchannels follows a well-defined trend with the product of mass flow rate and latent heat of vaporization. A power-law correlation is proposed which offers a simple, yet accurate method for predicting the CHF. The thermodynamic exit quality at CHF is also analyzed and discussed to provide insights into the CHF phenomenon in a heat sink containing multiple parallel microchannels.

Local flow boiling heat transfer characteristics in silicon microchannel heat sinks using liquid crystal thermography

The next generation of microelectronic devices will require better heat management through new technologies such as microchannel heat sinks. The present study focuses on the experimental investigation of onset of boiling and flow boiling heat transfer characteristics in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 276 μm, width of 225 μm, and a length of 16 mm. Experiments are carried out for mass fluxes ranging from 341 to 531 kg/m 2 s and heat fluxes from 55.5 to 154.2 kW/m 2 using FC-72 as the working fluid. The present study focuses on the characteristics of boiling incipience in microchannels including the effect of mass and heat fluxes. It provides a first qualitative and quantitative local experimental data on the position of the incipient boiling. The boiling incipience and temperature thermal maps for the heat sink surface are provided from a localized measured data using Un-encapsulated thermochromic liquid crystals (TLC). This measuring technique is also used to determine the local heat transfer coefficient for saturated flow boiling. The heat transfer coefficient decreases sharply at low exit quality and then it remains almost constant as the exit quality increases. Two-phase heat transfer coefficient asymptotic models and correlations in microchannels underpredict the data in the nucleate boiling regime.

A study of discrepancies in flow boiling results in small to microdiameter metallic tubes

There is a disagreement in the reports on flow boiling heat transfer on the dependence of the local heat transfer coefficient on local vapour quality, mass and heat flux and system pressure. As a consequence, various conclusions were reported about the dominant heat transfer mechanism(s) in small to microdiameter tubes. Yet, the reasons behind this large disagreement are not clear. The current study investigated experimentally two important parameters that may contribute in explaining the scatter in the published heat transfer results. The first parameter is the tube inner surface characteristics and the second is the length of the heated section. The surface effect was experimentally investigated through examining two stainless steel tubes manufactured by two different methods. The first tube is a seamless cold drawn tube whilst the second is a welded tube. The two tubes have similar design and dimensions and were investigated at 8 bar system pressure and 300 kg/m 2 s mass flux. The inner surface of the two tubes was examined using a scanning electron microscope (SEM) and was found to be completely different. The heat transfer results demonstrated that the trend of the local heat transfer coefficient versus local vapour quality in the seamless cold drawn tube is completely different from that in the welded tube. Three heated lengths were investigated for a seamless cold drawn tube with an inner diameter of 1.1 mm over a wide range of experimental conditions; mass flux range of 200–500 kg/m 2 s, system pressure of 6–10 bar, inlet sub-cooling value of about 5 K and exit quality up to about 0.95. The results indicated that the heated length influences strongly the magnitude as well as the local behaviour of the heat transfer coefficient. There is a progression from nucleate boiling to convective boiling as the heated length increases. The variation in the heat transfer coefficient due to differences in the heated length may also influence the performance of the existing microscale heat transfer correlations. The flow patterns observed at the exit of each test section are also presented.

Surface effects in flow boiling of R134a in microtubes

The inner surfaces of microtubes may be influenced strongly by the process of making them due to manufacturing difficulties at these scales compared to larger ones, e.g. the surface characteristics of a seamless cold drawn tube may differ from those of a welded tube. Accordingly, flow boiling heat transfer characteristics may vary. In addition, there is no common agreement between researchers on the criteria of selecting tubes for flow boiling experiments. Instead, tubes are usually ordered from commercial suppliers, in many cases without taking into consideration the manufacturing method and its effect on the heat transfer process. This may explain some of the discrepancies in heat transfer characteristics which are found in the open literature. This paper presents a comparison between experimental flow boiling heat transfer results obtained using two different metallic tubes. The first one is a seamless cold drawn stainless steel tube of 1.1 mm inner diameter while the second is a welded stainless steel tube of 1.16 mm inner diameter. Both tubes have a heated length of 150 mm and the flow direction is vertically upwards. The tubes were heated using DC current. Other experimental conditions include: 8 bar system pressure, 300 kg/m 2 s mass flux, about 5 K inlet sub-cooling and up to 0.9 exit quality. The results are presented in the form of local heat transfer coefficient versus local quality and axial distance. Also, the boiling curves of the two tubes are discussed. The results show a significant effect of tube inner surface morphology on the heat transfer characteristics.

사다리꼴 미세유로의 대류비등 2상유동 : 1부-압력강하 특성

Characteristics of two-phase pressure drop in microchannels were investigated experimentally. The microchannels consisted of 9 parallel trapezoidal channels with each channel having of bottom width, of depth, of sidewall angle, and 7 cm of length. Pressure drops in convective boiling of Refrigerant 113 were measured in the range of inlet pressure 105~195 kPa, mass velocity , and heat flux . The total pressure drop generally increased with increasing mass velocity and/or heat flux. Two-phase frictional pressure drop across the microchannels increased rapidly with exit quality and showed bigger gradient at higher mass velocity. A critical review of correlations in the literature suggested that existing correlations were not able to match the experimental results obtained for two-phase pressure drop associated with convective boiling in microchannels. A new correlation suitable for predicting two-phase friction multiplier was developed based on the separated flow model and showed good agreement with the experimental data.

Experimental investigation of flow boiling characteristics in a cross-linked microchannel heat sink

This paper experimentally investigates flow boiling characteristics in a cross-linked microchannel heat sink at low mass fluxes and high heat fluxes. The heat sink consists of 45 straight microchannels each with a hydraulic diameter of 248 μm and heated length of 16 mm. Three cross-links, of width 500 μm, are introduced in the present microchannel heat sink to achieve better temperature uniformity and to avoid flow mal-distribution. Flow visualization, flow instability, two-phase pressure drop, and two-phase heat transfer measurements are conducted using the dielectric coolant FC-72 over a range of heat flux from 7.2 to 104.2 kW/m 2, mass flux from 99 to 290 kg/m 2 s, and exit quality from 0.01 to 0.71. Thermochromic liquid crystals are used in the present study as full-field surface temperature sensors to map the temperature distribution on the heat sink surface. Flow visualization studies indicate that the observed flow regime is primarily slug. Visual observations of flow patterns in the cross-links demonstrate that bubbles nucleate and grow rapidly on the surface of the cross-links and in the tangential direction at the microchannels’ entrance due to the effect of circulations generated in those regions. The two-phase pressure drop strongly increases with the exit quality, at x e,o < 0.3, and the two-phase frictional pressure drop increases by a factor of 1.6–2 compared to the straight microchannel heat sink. The flow boiling heat transfer coefficient increases with increasing exit quality at a constant mass flux, which is caused by the dominance of the nucleation boiling mechanism in the cross-link region.

Analysis of power and cooling cogeneration using ammonia-water mixture

Abstract Development of innovative thermodynamic cycles is important for the efficient utilization of low-temperature heat sources such as solar, geothermal and waste heat sources. This paper presents a parametric analysis of a combined power/cooling cycle, which combines the Rankine and absorption refrigeration cycles, uses ammonia-water mixture as the working fluid and produces power and cooling simultaneously. This cycle, also known as the Goswami Cycle, can be used as a bottoming cycle using waste heat from a conventional power cycle or as an independent cycle using solar or geothermal energy. A thermodynamic study of power and cooling cogeneration is presented. The performance of the cycle for a range of boiler pressures, ammonia concentrations and isentropic turbine efficiencies are studied to find out the sensitivities of net work, amount of cooling and effective efficiencies. The roles of rectifier and superheater on the cycle performance are investigated. The cycle heat source temperature is varied between 90-170 °C and the maximum effective first law and exergy efficiencies for an absorber temperature of 30 °C are calculated as 20% and 72%, respectively. The turbine exit quality of the cycle for different boiler exit scenarios shows that turbine exit quality decreases when the absorber temperature decreases.

Correlations for saturated critical heat flux in microchannels

Experimental results of the saturated-flow boiling critical heat flux (CHF) in microchannels for both multi- and single-channel configurations were obtained from the literature. The collected database contains 629 data points, covering five halogenated refrigerants, nitrogen, and water, for a wide range of operational conditions, and different microchannel dimensions. The whole database was analyzed by using five empirical correlations to verify their respective accuracies. However, none of the existing correlations could predict the entire database precisely. A saturated CHF correlation was proposed by using boiling number, length-to-diameter ratio, and exit quality. Combining with the energy balance equation, the new correlation can predict the overall microchannel database accurately on the whole. It predicts almost 97.0% of the non-aqueous data (except R12 data points located in the macro-scale region) and 94.0% of the water data within the ±30% error band.

A semi-empirical approach for predicting two-phase flow discharge through branches of various orientations connected to a horizontal main pipe

The subdivision of two-phase flow in branching conduits consisting of a large horizontal main pipe with upward, downward, or lateral branches of reduced diameter is of great interest in various technological fields. For example, these conduits are important in light-water nuclear reactors (LWRs) in the case of a small break loss-of-coolant accident (SBLOCA) in a leg of the reactor's primary coolant loops, as well as for breaks or valve malfunctions in a large pipeline. In these kinds of circumstances, the relevant phenomenology often involves phase stratification coupled with possible liquid entrainment or gas pool-through phenomena. Therefore, these phenomena were studied in depth to evaluate the pressure drop across conduit elements resulting from the redistribution of flow phases and the discharged mass among them. In the past, several experiments have been performed along with studies in flow modelling. As a result, several formulae and models for branch exit quality and consequent discharge mass flow rate predictions have been proposed. In recent years, we have been engaged in extensive research on this subject, resulting in a new semi-empirical formulation to express branch exit quality in terms of the physical processes taking place in the conduits, the operating parameters and the branch geometry. In this paper, we applied these correlations to representative sets of experiments previously reported in the literature, comprising a wide range of branch-to-main-pipe diameter ratios and operating pressures, which proved our formulation to be very accurate.

Analytical Modeling of Annular Flow Boiling Heat Transfer in Mini- and Microchannel Heat Sinks

An analytical model is proposed to predict the flow boiling heat transfer coefficient in the annular flow regime in mini- and microchannel heat sinks based on the separated model. The modeling procedure includes a formulation for determining the heat transfer coefficient based on the wall shear stress and the local thermophysical characteristics of the fluid based on the Reynolds’ analogy. The frictional and acceleration pressure gradients within the channel are incorporated into the present model to provide a better representation of the flow conditions. The model is validated against collected data sets from the literature produced by different authors under different experimental conditions, different fluids, and with mini- and microchannels of hydraulic diameters falling within the range of 92–1440 μm. The accuracy between the experimental and predicted results is achieved with a mean absolute error of 10%. The present analytical model can correctly predict the different trends of the heat transfer coefficient reported in the literature as a function of the exit quality. The predicted two-phase heat transfer coefficient is found to be very sensitive to changes in mass flux and saturation temperature. However, it is found to be mildly sensitive to the change in heat flux.

A study of organic working fluids on system efficiency of an ORC using low-grade energy sources

Abstract Rankine cycles using organic fluids (as categorized into three groups: wet, dry, and isentropic fluids) as working fluids in converting low-grade energy are investigated in this study. The main purpose is to identify suitable working fluids which may yield high system efficiencies in an organic Rankine cycle (ORC) system. Efficiencies of ORC systems are calculated based on an assumption that the inlet condition of the working fluid entering turbine is in saturated vapor phase. Parameters under investigation are turbine inlet temperature, turbine inlet pressure, condenser exit temperature, turbine exit quality, overall irrversibility, and system efficiency. The low-grade energy source can be obtained from a solar pond or/and an ocean thermal energy conversion (OTEC) system. Results indicate that wet fluids with very steep saturated vapor curves in T-s diagram have a better overall performance in energy conversion efficiencies than that of dry fluids. It can also be shown that all the working fluids have a similar behavior of the efficiency-condenser exit temperature relationship. Furthermore, an appropriate combination of solar energy and an ORC system with a higher turbine inlet temperature and a lower condenser temperature (as operated deeply under sea level) would provide an economically feasible and environment-friendly renewable energy conversion system.

Two-phase pressure drop and flow visualization of FC-72 in a silicon microchannel heat sink

The rapid development of two-phase microfluidic devices has triggered the demand for a detailed understanding of the flow characteristics inside microchannel heat sinks to advance the cooling process of micro-electronics. The present study focuses on the experimental investigation of pressure drop characteristics and flow visualization of a two-phase flow in a silicon microchannel heat sink. The microchannel heat sink consists of a rectangular silicon chip in which 45 rectangular microchannels were chemically etched with a depth of 276 μm, width of 225 μm, and a length of 16 mm. Experiments are carried out for mass fluxes ranging from 341 to 531 kg/m 2 s and heat fluxes from 60.4 to 130.6 kW/m 2 using FC-72 as the working fluid. Bubble growth and flow regimes are observed using high speed visualization. Three major flow regimes are identified: bubbly, slug, and annular. The frictional two-phase pressure drop increases with exit quality for a constant mass flux. An assessment of various pressure drop correlations reported in the literature is conducted for validation. A new general correlation is developed to predict the two-phase pressure drop in microchannel heat sinks for five different refrigerants. The experimental pressure drops for laminar-liquid laminar-vapor and laminar-liquid turbulent-vapor flow conditions are predicted by the new correlation with mean absolute errors of 10.4% and 14.5%, respectively.

Study of the critical heat flux condition with water and R-123 during flow boiling in microtubes. Part II – Comparison of data with correlations and establishment of a new subcooled CHF correlation

This study’s objective was to better understand the CHF condition in microchannels. The effect of different operating parameters – mass flux, inlet subcooling, exit quality, heated length and diameter – were assessed in detail in Part I of the study and compared to the behavior in conventional sized channels. Part II of the study compares the water and R-123 data with existing micro/macrochannel correlations. Existing correlations for predicting CHF in large-sized channels do not seem to be applicable to microchannels. This study has provided new subcooled CHF data for low mass fluxes and the earlier available subcooled boiling CHF correlation for microchannels (based on the data available for very high mass fluxes) is not suitable to predict such data. Based on the new subcooled CHF data, a correlation to predict CHF in low-flow subcooled boiling has been developed.

Study of the critical heat flux condition with water and R-123 during flow boiling in microtubes. Part I: Experimental results and discussion of parametric effects

Extensive experimentation was performed to obtain flow boiling critical heat flux data in single stainless steel microtubes with diameters from 0.286 to 0.700 mm over a wide range of mass fluxes, inlet subcoolings, and exit pressures for two different working fluids (water and R-123). The effect of different operating parameters – mass flux, inlet subcooling, exit quality, heated length and diameter – were assessed in detail (Part I of the paper). The conventional DNB-type behavior is observed in the high subcooled region, and the typical dryout type behavior is seen in the high-quality saturated region when the flow is completely annular. The flow in transitional flow patterns (churn–annular or slug–annular) causes a peculiar increase of CHF with exit quality. Also, the increased void fraction near the saturated region in subcooled boiling results in increased subcooled CHF values. Part II of the paper deals with comparison of data with existing correlations and development of a new correlation to predict the CHF condition in the subcooled liquid region.

E211 高出口乾き度条件下のマイクロ管内限界熱流束特性(熱伝達II)

E211 高出口乾き度条件下のマイクロ管内限界熱流束特性(熱伝達II)