Two-phase Operation Research Articles

Optimal planning of networks of mobile objects under uncertainty

When solving a given common task, a swarm of moving objects coordinates the state of its individual members. When planning swarm operations, there is a need to take into account the possibility of its operational regrouping, since at the time of planning the exact purpose of the swarm operation may be either not yet defined, or secret, or determined by a number of random circumstances. At the same time, the swarm resources are insufficient for one-time coverage of all possible targets in a given service area. Therefore, the execution of the operation of the swarm begins to resolve the aforementioned uncertainties. In this case, the operation time can be significantly reduced. This problem is solved by methods of the percolation theory. A concept of programmable percolation of the service area, which is implemented in two phases, is introduced. The value of the concentration of objects in a basic stochastic swarm is obtained numerically -- using the results of statistical modeling of two-phase operations -- and analytically, providing a minimum total cost of the two-phase operation. The synergy of information interaction between the swarm objects when implementing a programmable percolation path is analyzed.

Two-phase flow morphology and local wall temperatures in high-aspect-ratio manifold microchannels

Manifold microchannel heat sinks can dissipate high heat fluxes at moderate pressure drops, especially during two-phase operation. High-aspect-ratio microchannels afford a large enhancement in heat transfer area; however, the flow morphology in manifold microchannels during two-phase operation, as well as the resulting thermal performance, are not well understood. In this work, a single manifold microchannel representing a repeating unit in a heat sink is fabricated in silicon with a bonded glass viewing window. Samples of different channel lengths (750 μm and 1500 μm) and depths (125 μm, 250 μm, and 1000 μm) are considered; channel and fin widths are both maintained at 60 μm. Subcooled fluid (HFE-7100) is delivered to the channel at a constant flow rate such that the fluid velocity at the inlet is ~1.05 m/s in all cases. A high-speed camera is used to visualize the two-phase flow in the channel through the glass sidewall; an infrared camera measures the temperature distribution on the opposite channel sidewall. The flow visualizations reveal that vapor nucleation occurs at stagnation regions below the manifold near the inlet plenum and at both corners adjacent to the channel base. For deep channels (1000 μm), at sufficiently high heat fluxes, vapor completely covers the base of the channels and liquid does not re-wet the surface in this region. This newly identified vapor blanketing phenomenon causes a significant decrease in performance and an increase in the measured channel wall temperatures. This study reveals the critical role of the two-phase flow morphology in manifold microchannel heat sink design.

Fractional Frequency Reuse in Multi-tier Networks: Performance Analysis and Optimization

Frequency Reuse (FR) is an efficient approach to improve the network performance in a multi-cell cellular network. In this paper, we investigate the performance of heterogeneous networks utilising two well-known frequency reuse algorithms, called Strict FR and Soft FR, with a reuse factor of $$\varDelta$$ ($$\varDelta > 1$$). Based on a two-phase operation of the FR algorithm, we develop a new approach to analyse cellular networks based on the Poisson Point Process (PPP) model which successfully demonstrates the impact of the network parameters (such as the number of allocated Resource Blocks (RBs) and bias factor) and FR parameters (such as Base Station (BS) transmit power) on the performance of a user and overall network. Compared to related works, we propose the following novel approaches: (i) we investigate flexible FR networks in which users have connections with the BSs which deliver the highest performance; (ii) the BS observes SINR on the data channel to classify each user into either a Cell-Edge User (CEU) or Cell-Center User (CCU); (iii) the initial state of the network is considered to establish the initial network interference when new users arrive and request connections to the BSs. In the case of a single-tier network, it is proved that our analytical approach is more accurate than previous works. In the case of a two-tier network, the analytical results indicate that compared to the 3GPP model, our proposed model not only reduces up to 40.79% and 3.8% power consumption of a BS on the data channel but also achieves 16.08% and 18.63% higher data rates in the case of Strict FR and Soft FR respectively. Furthermore, the paper presents an approach to find an optimal value of SINR thresholds and bias factor to achieve the maximum network performance.

A Two-Phase Three-Level Buck DC–DC Converter With X-Connected Flying Capacitors for Current Balancing

This letter presents a two-phase three-level (2P3L) buck converter with cross-connected flying capacitors ( $\text{X-}C_{\mathrm{ FLY}}$ ). The proposed converter suppresses the unbalanced inductor currents in two-phase operation, by alternately connecting the flying capacitors to across the branches. Meanwhile, three-level operation reduces the required inductance with lower inductor voltage, together with two-phase operation, helping to achieve fast transient response and small output ripple. Here, we design the power stage and feedback loop aiming for a bandwidth of 10 MHz. Then, the proposed 2P3L buck converter with $\text{X-}C_{\mathrm{ FLY}}$ fabricated and validated in 65-nm standard CMOS achieves a low unbalanced current of about 5 mA, and a small output ripple of 50 mV at a load current of 110 mA, using only 1-nF output capacitor. The measured peak efficiency is 86%. Besides, it can track fast 10-MHz 64QAM LTE envelope signal.

Experimental and numerical investigation of two phase ejector performance with the water injected into the induced flow

When injectors are used in petroleum industries or sea water desalination system, it is a special case that the induced gas is accompanied by water, the ejector is in two-phase operation. In the present study, the ejector performance at two phase flow is investigated experimentally and numerically. The results indicate that: the entrainment ratio would decrease when the water is injected into the induced flow, as well as the pressure ratio. The maximum decrement of entrainment ratio is around 40.5% at sub-critical mode when the motive pressure is 1.0 MPa, the induced pressure is 0.2 MPa, liquid-air ratio is 22%. For the pressure ratio, the maximum decrement is 62.5% when the motive pressure is 1.0 MPa, the induced pressure is 0.5 MPa, liquid-air ratio is 2.87%. However, the lower the induced pressure is, the more obvious the effect of mass flow rate on entrainment rate is, while the effect on pressure ratio is just opposite. Moreover, the performance becomes worse in sub-critical mode than that in critical mode. The effect of NXP on ejector performance also has been analyzed. There exists an optimum NXP whether the two phase flow ejector is in critical mode or sub-critical mode. The optimum NXP for entrainment ratio enlarges from 3.6 mm to 4.4 mm with the increase of water mass flowrate, and is bigger than that of single phase ejector (the optimum NXP is 2.8 mm). This study may provide a beneficial reference for the operation of supersonic ejectors on two phase flow.

Optimizing the Control of a Group of Mobile Objects under Uncertainty

A swarm of moving objects coordinates the position of its individual objects in order to simultaneously solve a general problem set in a distributed manner. Planning the swarm operations comes across a problem of taking into account the possibility of operational regrouping of the swarm as the exact purpose of the swarm operation is not yet determined, or is a secret, or is set by a number of random circumstances. At the same time, the swam resources are not sufficient to simultaneously cover all possible targets in the operating region. Therefore, it is advisable to carry out the swarm operation in two phases and begin the first preliminary phase before resolving the mentioned uncertainties by creating a basic network with a relatively low concentration of swarm objects therein. In this case, one can significantly reduce the operation time. In the second phase of the operation, one locally simultaneously regroups the swarm objects, which takes a minimum time, to form a programmable percolation path that provides targeted coverage of the operating region. The solution to this problem is carried out by methods of the programmed percolation theory. The value of the swarm object concentration is obtained numerically using the results of statistical modeling of two-phase operations and analytically, thereby providing a minimum of total cost of the two-phase operation. The synergetics of information interaction of the swarm of objects in the implementation of a programmable percolation path is considered.

Parametric studies on startup transients in multiple parallel channels of a natural circulation boiling system

The nature of startup transients in a natural circulation boiling system with multiple parallel channels are governed by the interplay between numerous geometric and operating parameters. Present work focuses on numerically ascertaining the relative influence of such variables, with primary objective being to identify possible options of suppressing concerned oscillations. A scaled-down version of AHWR, comprising of three parallel channels, has therefore been simulated using RELAP5. Both geysering and type-I instabilities have been observed in all three channels on the initiation of two-phase operation, commonly separated by a span of near-stable reversed flow in one of the channels. Adoption of higher initial pressure has been found to have a strong stabilizing effect, as it can completely eliminate geysering and can substantially shorten the duration of type-I instability. Increase in inlet loss coefficient and decrease in either core outlet or riser outlet loss coefficients can also subside type-I oscillations, without having any noteworthy effect on geysering. Channels with smaller diameter can also reduce both amplitude and duration of type-I fluctuations, albeit at the expense of a deteriorated steady-state flow rate. The effects of wall roughness and thermal capacitance have also been explored. The influence of initial pressure has been found to be significantly stronger than the others, particularly in removing the geysering instabilities. An augmented initial pressure is capable of neutralizing the system even when the channels are subjected to unequal power, thereby identifying itself as a very potent option for practical implementation.

Two-Phase Expander Approach for Next Generation of Heat Recovery Systems

This study presents the numerical adaptations to the semi-empirical expander model in order to examine the feasibility of piston expanders under off-design and two-phase scenarios. This expander model considers supply valve pressure drop, condensation phenomena, heat losses, leakage losses and friction losses. Using Aspen HYSYS©, the expander model is utilised in simulating the next generation of integrated engine cooling and exhaust heat recovery system for future heavy-duty engines. The heat recovery system utilises water-propanol working fluid mixture and consists of independent high pressure (HP) and low pressure (LP) expander. The results of off‑design and two-phase operation are presented in terms of expander efficiency and the different sources of loss, under two distinctive engine speed-load conditions. The heat recovery system, operating with the LP expander at two-phase and the HP expander at superheated condition, represented the design point condition. At the design point, the system provided 15.9 kW of net power, with an overall conversion efficiency of 11.4%, representing 10% of additional engine crankshaft power. At the extreme off-design condition, the two-phase expander operation improved the system performance as a result of the nullification of leakage losses due to the much denser working fluid. The optimised two-phase operation of the LP expander (x=0.55) and the HP expander (x=0.9) at the extreme-off design condition improved the system power by nearly 50% (17.4 vs. 11.7 kW) compared to the reference state. Finally, adapting piston air motors as two-phase expanders for experimental evaluation and reduction in frictional losses was a recommended research direction. ©2019. CBIORE-IJRED. All rights reserved

Two-phase operation of a Terry steam turbine using air and water mixtures as working fluids

Terry steam turbines are employed in the safety systems of many nuclear Boiling Water Reactors to drive pumps and provide cooling water to the nuclear reactor core. While the turbine efficiency is low, the more important feature is high reliability under off-normal conditions. An important aspect of reliability is the ability to function with two-phase steam-water injection into the turbine, as most likely occurred in the Fukushima Dai-ichi nuclear accidents. This study investigates the characteristics of a Terry turbine during air-water injection with gas mass fractions ranging from 1 (dry gas) to 0.05 (wet gas), to better understand the Terry turbine’s true operational capabilities and provide justification for extended Terry turbine use for reactor safety. Other parameters investigated are the inlet pressure, the exhaust backpressure and the turbine’s rotational speed. The turbine performance is presented in terms of dynamometer loading and pump performance change as functions of the gas mass fraction.

Physical layer security based on NOMA and AJ for MISOSE channels with an untrusted relay

Physical layer security based on non-orthogonal multiple access (NOMA) and artificial jamming (AJ) is considered in a one-way amplify-and-forward (AF) relay network with an untrusted relay. In the first phase of the two-phase operation of the untrusted AF relay network, a source node transmits the first information and AJ symbols, and the destination decodes the AJ symbol first and waits for the relayed replica of the first information symbol, which is forwarded by the untrusted relay. In the second phase, the source sends the second information symbol and the relay amplifies and forwards the received signal in the first phase with a fixed gain. In such an operation scenario, an untrusted relay tries to eavesdrop on the information signal while relaying the received signal properly. To decode the information symbol, the destination cancels out the AJ signal and then decodes the first information symbol by combining the signals received during the two phases. Finally, the second information symbol is decoded with the second-phase signal after cancelling out all the other symbols, i.e., the first information and AJ symbols. The eavesdropper and relay try to eavesdrop on the information signal with all possible orders of interference cancellation, i.e., NOMA receive processing. In this study, an ergodic secrecy rate is derived according to the operation scenario during the two phases. The behaviours of the ergodic secrecy rate with respect to power allocation to two information symbols as well as an AJ symbol are investigated. It is shown that the expected secrecy rate can be maximized by selecting the transmit power and rates for the three symbols properly. It is also demonstrated that the proposed physical layer security scheme achieves a higher secrecy rate than a secure beamformed transmission based on NOMA and artificial noise.

Two-Phase Performance of a Hybrid Jet Plus Multipass Microchannel Heat Sink

Abstract The heat transfer and fluid flow performance of a hybrid jet plus multipass microchannel heat sink in two-phase operation is evaluated for the cooling of a single large area, 3.61 cm2, heat source. The two-layer branching microchannel heat sink is evaluated using HFE-7100 as the coolant at three inlet volumetric flow rates of 150, 300, and 450 ml/min. The boiling performance is highest for the flow rate of 450 ml/min with the maximum heat flux value of 174 W/cm2. Critical heat flux (CHF) was observed at two of the tested flow rates, 150 and 300 ml/min, before reaching the maximum operating temperature for the serpentine heater. At 450 ml/min, the heater reached the maximum allowable temperature prior to observing CHF. The maximum pressure drop for the heat sink is 34.1 kPa at a heat flux of 164 W/cm2. Further, the peak heat transfer coefficient value of the heat sink is 28,700 W/m2 K at a heat flux value of 174 W/cm2 and a flow rate of 450 ml/min. Finally, a validated correlation of the single device cooler is presented that predicts heat transfer performance and can be utilized in the design of multidevice coolers.

Energy Crowdsourcing and Peer-to-Peer Energy Trading in Blockchain-Enabled Smart Grids

The power grid is rapidly transforming, and while recent grid innovations increased the utilization of advanced control methods, the next-generation grid demands technologies that enable the integration of distributed energy resources (DERs)---and consumers that both seamlessly buy and sell electricity. This paper develops an optimization model and blockchain-based architecture to manage the operation of crowdsourced energy systems (CES), with peer-to-peer (P2P) energy trading transactions. An operational model of CESs in distribution networks is presented considering various types of energy trading transactions and crowdsourcees. Then, a two-phase operation algorithm is presented: Phase I focuses on the day-ahead scheduling of generation and controllable DERs, whereas Phase II is developed for hour-ahead or real-time operation of distribution networks. The developed approach supports seamless P2P energy trading between individual prosumers and/or the utility. The presented operational model can also be used to operate islanded microgrids. The CES framework and the operation algorithm are then prototyped through an efficient blockchain implementation, namely the IBM Hyperledger Fabric. This implementation allows the system operator to manage the network users to seamlessly trade energy. Case studies and prototype illustration are provided.

Control of Smart Transformer Under Single-Phase to Ground Fault Condition

In low voltage (LV) electric distribution grid, the continuity of power supply service is degraded by various grid faults. Even under single phase to ground faults, the continuous operation capability of healthy phases is also limited. The smart transformer (ST) can establish each phase voltage independently and thus can switch the operation mode from three-phase to two-phase operation during single phase faults, ensuring continuous power supply in two healthy phases and minimizing outage area. However, different from the three-phase operation mode, both high 2nd-order oscillation in dc-link and high current in neutral conductor challenge system performances and reliability under the two-phase operation. Thus, an improved two-phase voltage control strategy is needed either to limit the neutral current amplitude avoiding over ampacity (reducing the power losses and suppressing neutral potential variations), or to suppress dc power oscillation (preventing the lifetime reduction of dc-link capacitors). This paper focuses on the control system design of ST voltage control for the two-phase operation to improve the grid voltage quality. The simulation and experimental results clearly verify the effectiveness and correctness of the proposed strategy.

Design, Fabrication, and Characterization of a Compact Hierarchical Manifold Microchannel Heat Sink Array for Two-Phase Cooling

High-heat-flux removal is critical for the next-generation electronic devices to reliably operate within their temperature limits. A large portion of the thermal resistance in a traditional chip package is caused by thermal resistances at interfaces between the device, heat spreaders, and the heat sink; embedding the heat sink directly into the heat-generating device can eliminate these interface resistances and drastically reduce the overall thermal resistance. Microfluidic cooling within the embedded heat sink improves the heat dissipation, with two-phase operation offering the potential for dissipation of very high heat fluxes while maintaining moderate chip temperatures. To enable multichip stacking and other heterogeneous packaging approaches, it is important to densely integrate all fluid flow paths into the device; volumetric heat dissipation emerges as a performance metric in this new heat sinking paradigm. In this paper, a compact hierarchical manifold microchannel design is presented that utilizes an integrated multilevel manifold distributor to feed coolant to an array of microchannel heat sinks. The flow features in the manifold layers and microchannels are fabricated in silicon wafers using deep reactive-ion etching. The heat source is simulated via Joule heating using thin-film platinum heaters. The on-chip spatial temperature measurements are made using four-wire resistance temperature detectors. The individual manifold layers and the microchannel-bearing wafers are diced and bonded into a sealed stack via thermocompression bonding using gold layers at the mating surfaces. Thermal and hydrodynamic testing is performed by pumping the dielectric fluid HFE-7100 through the device at a known flow rate, temperature, and pressure at different levels of chip heat input. A volumetric heat density of up to 2870 W/cm3 is dissipated at a chip temperature less than 112 °C and microchannel pressure drop less than 27 kPa. The overall pressure drop is governed by flowing through the manifold, rather than the microchannels, in this compact heat sink that occupies envelope of 5 mm $\times 5$ mm $\times2.3$ mm including all the functional flow features.

Smart Transformer-Based Single Phase-To-Neutral Fault Management

The smart transformer (ST), with its high control capability, offers new features for optimizing the management of the distribution grid. The ST, by controlling the voltage in each phase independently, is able to manage the operations during grid faults, in particular in the single-phase ones. The ST, when a single-phase-to-ground fault occurs, can rapidly reduce the voltage in the phase under fault. Thus, it guarantees system safety and operates with the remaining two healthy phases to avoid unnecessary interruption in the healthy lines. However, the two-phase asymmetrical operation challenges the system performances due to the high power 2nd harmonic ripple in the dc voltage, that reduces the capacitors, lifespan and the high current flowing in the neutral conductor. These issues would force the grid operator to increase the ST maintenance, in order to avoid unplanned faults in the ST hardware, due to failure of aged capacitors. This paper presents a flexible-control strategy, based on the phase-shift angle control between two healthy-phase voltages, that reduces the impact of the power 2nd harmonic oscillation and thus can delay the maintenance intervention. The proposed strategy aims to improve system performances by avoiding neutral-line current overload and attenuating ac side active power (low voltage dc-link voltage) oscillation. Depending on the grid conditions (small dc link capacitance or low ampacity of neutral cable), the voltage angle can be adapted, minimizing the impact of the two-phase operation system in the ST-fed grid. The effectiveness and feasibility of the proposed approach have been validated experimentally with a simplified microgrid setup.

A comparative study of continuous operation between a dynamic baffle crystallizer and a stirred tank crystallizer

Crystallization, often as the final isolation and purification step in drug substance manufacturing, has substantial impact on downstream efficiency and final drug product quality. It is a critical but challenging step in developing end-to-end continuous manufacturing, which has been identified as an emerging technology in the pharmaceutical manufacturing sector by the U.S. Food and Drug Administration (FDA). A traditional stirred tank crystallizer (STC) operated as a mixed-suspension-mixed-product-removal (MSMPR) system is a popular choice for continuous crystallization for its utilization of existing knowledge and equipment. However, there are disadvantages associated with agitational systems like the STC, such as poor local mixing and high shear rate. A commercial dynamic baffle crystallizer (DBC) was studied here as an alternative unit operation. The DBC in this study consists of a jacketed glass vessel with dynamic ‘donut’ shaped baffles to provide oscillatory mixing to improve heat and mass transfer while exerting less shear. Our goal is to compare continuous operation performances in the DBC with the STC as MSMPR systems. First, residence time distribution (RTD) studies of both systems were performed considering single phase (liquid only) and two-phase (solid-liquid) operation. The RTD gives good insights into the mixing dynamics without computationally heavy fluid dynamic simulations. Continuous cooling crystallization of paracetamol was performed in the DBC and the STC. The DBC showed good potential to be used as an MSMPR system producing more uniform RTDs as well as higher quality crystallization products compared to a traditional STC.

A low-power relaxation oscillator with high frequency stability for RFID

A low-power relaxation oscillator with a high frequency stability is presented for radio frequency identification (RFID). This oscillator implements an improved self-biased and a comparator multiplexing techniques to reduce the power consumption and chip area. A capacitor resetting delay cancellation technique based on a two-phase operation and a proportional to absolute temperature (PTAT) current with an upward curvature versus the temperature are adopted to maintain the frequency stability. The oscillator is designed in a 65 nm standard CMOS process and occupies a small area of 0.0216 mm2. The post-layout simulation results show that a frequency drift of 0.95% from 0.7 to 1.7 V and a temperature stability of 27 ppm/°C as the temperature varies from − 40 to 85 °C at a typical working frequency of 1.92 MHz. Under a supply voltage of 0.7 V, the maximum power consumption is only 15.4 μW at − 40 °C. At room temperature, the figure of merit (FOM1 and FOM2) are 8 nW/kHz and 89.5 dB, respectively, which makes it more efficient than relaxation oscillators reported to date.

Fukushima Unit 2 Accident Simulation With MELCOR 2.1

A VTT Fukushima Daiichi Unit 3 (1F3) method for estimation of liquids and consequences of releases (MELCOR) model was modified to simulate the Fukushima Daiichi Unit 2 (1F2) accident. Five simulations were performed using different modeling approaches. The model 1F2 v1 includes only the basic modifications to reproduce the 1F2 accident. The model 1F2 v2 includes the same modifications used in 1F2 v1 plus the wet well (WW) improvement. In the 1F2 v3 model, the reactor core isolation cooling (RCIC) system logic was modified to avoid the use of tabular functions for the mass flow inlet and outlet. Because of this analysis, it is concluded that there is a strong dependency on parameters that still have many uncertainties, such as the RCIC two-phase flow operation, the alternative water injection, the suppression pool (SP) behavior, the rupture disk behavior and the containment failure modes, which affect the final state of the reactor core.

Prediction of Helium Vapor Quality in Steady-State Two-Phase Operation of SST-1 Superconducting Toroidal Field Magnets

Steady-state superconducting tokamak (SST-1) at the Institute for Plasma Research is the first superconducting tokamak in India and is an “operational device.” Superconducting magnets system (SCMS) in SST-1 comprises sixteen toroidal field (TF) magnets and nine poloidal field magnets employing cable-in-conduit conductor of multifilamentary high-current-carrying high-field-compatible-multiply-stabilized NbTi/Cu superconducting strands. SST-1 superconducting TF magnets are successfully and regularly operated in a cryostable manner being cooled with two-phase (TP) flow helium. The typical operating pressure of the TP helium is 1.6 bar (a) and the operating temperature is the corresponding saturation temperature. The SCMS cold mass is nearly 32 t and has a typical cool-down time of about 14 days from 300 K down to 4.5 K using the helium refrigerator/liquefier (HRL) system of an equivalent cooling capacity of 1350 W at 4.5 K. Using the available experimental data from the HRL, we have estimated the vapor quality during the cryostable operation of the TF magnets using the well-known correlation of TP flow. In this paper, we report the detailed characteristics of TP flow for given thermohydraulic conditions during long steady-state operation of the SST-1 TF magnets as observed in the SST-1 experimental campaigns.

Thermodynamic Modelling of Supersonic Gas Ejector with Droplets

This study presents a thermodynamic model for determining the entrainment ratio and double choke limiting pressure of supersonic ejectors within the context of heat driven refrigeration cycles, with and without droplet injection, at the constant area section of the device. Input data include the inlet operating conditions and key geometry parameters (primary throat, mixing section and diffuser outlet diameter), whereas output information includes the ejector entrainment ratio, maximum double choke compression ratio, ejector efficiency, exergy efficiency and exergy destruction index. In single-phase operation, the ejector entrainment ratio and double choke limiting pressure are determined with a mean accuracy of 18 % and 2.5 % , respectively. In two-phase operation, the choked mass flow rate across convergent-divergent nozzles is estimated with a deviation of 10 % . An analysis on the effect of droplet injection confirms the hypothesis that droplet injection reduces by 8 % the pressure and Mach number jumps associated with shock waves occuring at the end of the constant area section. Nonetheless, other factors such as the mixing of the droplets with the main flow are introduced, resulting in an overall reduction by 11 % of the ejector efficiency and by 15 % of the exergy efficiency.