Large Diameter Channels Research Articles

Study on coupled heat transfer of pyrolytic kerosene and supercritical CO2 in zigzag-type PCHE used for hypersonic vehicle power generation system

Supercritical CO2 closed cycle is an effective solution to the power generation challenge of hypersonic vehicles. The coupled heat transfer between aviation kerosene and supercritical CO2 within the zigzag-type printed circuit heat exchanger (PCHE) is the core thermal process of the whole cycle. Starting from the enhancement of the cycle's power generation, this study analyzes the influence mechanism of structural parameters on the heat transfer process through numerical simulation. The research indicates that a deep pyrolysis reaction of aviation kerosene, with the chemical heat absorption rate accounting for over 65 % of the total heat transfer rate. Due to the significant nonlinearity introduced by pyrolysis, traditional enhanced heat transfer design methods may even weaken its heat exchange. Unlike most studies, this type of PCEH achieves optimal heat transfer performance with a larger channel diameter and a smaller bend angle. Additionally, the increase in diameter and the decrease in bend angle enhance the power capacity of kerosene and its pyrolysis products, significantly improving the efficiency of thermal-electric conversion for the cycle. Finally, the study has determined a diameter of 2.4 mm and a bend angle of 30° constitute a set of structural parameters most suitable for the PCHE.

Dynamic Synthesis of Three−Point Circle Peripheral Docking Technology Pose

Research on space docking technology was first motivated by the application scenario of space station docking in the aviation field. Due to the maturity of this technology, its application field is expanding to other industries. To benefit the intelligent transportation field, this study uses space docking technology to realize combination and reconstruction between car bodies. Different spatial docking methods and structural characteristics are determined by different application scenarios. The docking of the space station is completed in the suspended state of the body in the outer space environment, which requires advanced technology, is high in hardware cost, and involves a small channel diameter. In this study, the docking of the reconstructed vehicle is completed on the ground; thus, it is low in hardware cost and involves a large channel diameter. Based on the above technical requirements, a three−point circular peripheral docking method is designed for the reconstructed vehicle. A visual positioning system is built, the mapping relationship between the camera coordinate system and the landmark coordinate system rotation matrix and translation vector is established, and the adaptive capture behavior is realized by path planning through the inverse pose solution model. Simulation experiments demonstrate that the active and passive vehicle bodies can achieve compliance capture with small collision force under the two conditions of forward collision and oblique collision, and the recovery coefficient after collision and docking is in the range of (0, 0.01). The acceleration decay process under two working conditions is measured in the crash test of a sample vehicle, which verifies the feasibility of the space reconstruction of the vehicle docking mechanism and the ability of this configuration to achieve tasks, providing a constructive idea for space docking technology with a large channel diameter.

CFD and Energy Loss Model Analysis of High-Speed Centrifugal Pump with Low Specific Speed

High-speed centrifugal pumps with low specific speed have the characteristics of a small flowrate, a high head, and being compact and light weight, making them promising candidates for applications in the thermal management of aerospace and electronic devices. The energy loss in the low specific speed pump is critical and complex due to the large impeller diameter, the narrow and long flow channel, and the small outlet width. In this paper, an analysis method based on an energy loss model and computational fluid dynamics simulations (ELM/CFD) is proposed to analyze the performance of the low specific speed pump with a fully sealed structure. Experiments were carried out under variable water flowrates. The results show that the empirical correlation method failed to accurately predict the performance of high-speed centrifugal pumps, because the bearing clearance leakage and motor channel leakage are ignored. Moreover, the volume loss and hydraulic loss are calculated based on the empirical parameters of commonly used pumps that are different from the high-speed pump with the low specific speed in the complex flow channel structure. The ELM/CFD method calculates various loss power based on the simulation results and can predict the head and efficiency with deviations less than 2% and 5%, respectively. ELM/CFD can accurately analyze the optimization direction of the pump. The hydraulic loss and the volume loss of the impeller are the dominant factors that restrict the pump efficiency under the lower flowrates, while the hydraulic loss of subsequent flow channels becomes important under the larger flowrates.

A new correlation for CCFL at full scale PWR excluding the Hutze effect in UPTF data

The effect of the Hutze (a secondary ECC injection pipe within the hot leg pipe of a Konvoi-type PWR) upon CCFL was experimentally investigated at COLLIDER test facility of the Technical University Munich (TUM). COLLIDER represents a PWR hot leg with a large dimeter of 190 mm and ~¼ scale. A Hutze pipe was built into COLLIDER’s hot leg according to a German Konvoi-type PWR reactor design data. The effect of the Hutze upon CCFL are shown through comparisons with previous investigations at COLLIDER without a Hutze. Visual observations using high-speed recordings are presented at different flow conditions: supercritical conditions at low air velocities, transition into subcritical flow, and the onset of CCFL with comparison against previous recording without a Hutze. Limits of: the onset of hydraulic jumps, onset of CCFL (flooding), and most importantly CCFL characteristics are also shown and compared. The Hutze was found to induce a large impact upon these limits: two hydraulic jumps are observed prior to the onset of CCFL instead of only one (in case without a Hutze), the limits of the onset of hydraulic jumps and onset of CCFL occur at much lower air velocities, and the hysteresis effect (difference between limits of flooding and deflooding) disappears. CCFL characteristics curve is also shifted toward lower values of gas velocities and less liquid flows into the reactor vessel at same air velocities in the case with a Hutze which means a less cooling potential during the reflux condensation mode. Ohnuki correction of the Hutze effect upon CCFL characteristics of 1988 is tested and it seems to work well at small diameter but it overestimates CCFL characteristics for COLLIDER data. The previous agreement between CCFL characteristics obtained from downscaled experiments with D > 50 mm without a Hutze (including COLLIDER data and other large scale facilities) with UPTF data (at full scale) is found to happen because of the Hutze effect which shifts the CCFL characteristics downwards in Wallis diagram and not because of the correct representation of occurring CCFL phenomena. The actual CCFL characteristics for the 1:1 scale without a Hutze are expected to be higher than those reported from data with Hutze at UPTF facility. This indicates that there is still a scale effect even at a large diameter channel, and that most previous downscaled experiments have underestimated CCFL characteristics at full scale. Based upon obtained results, a correlation of CCFL characteristics at full scale excluding the Hutze effect is given. This correlation is of a special interest for the design of new PWR reactors as most new designs does not include a Hutze.

Supercritical heat transfer of NOVEC 649 refrigerant in horizontal minichannels

This study investigates the influence of mass flowrates, system pressures, and channel diameters on supercritical heat transfer characteristics of NOVEC 649 refrigerant in horizontal minichannels. The experimental results demonstrate that the heat transfer coefficient significantly increases by increasing mass flux. Especially in the case of a small channel diameter, increasing mass flux can also reduce the degree of heat transfer deterioration when the fluid temperature exceeds the pseudocritical temperature. The heat transfer coefficient decreases dramatically when the fluid temperature reaches the pseudocritical temperature and then increases again when the fluid temperature exceeds and gets far from the pseudocritical temperature. Moreover, the heat transfer coefficient increases significantly with the decrease in the channel diameter. The buoyancy effect has the significant impacts on the present results, especially under experimental conditions with larger channel diameters. The heat transfer deterioration is most obvious in the results of the larger diameter. The empirical correlation of supercritical heat transfer suitable for the organic working fluid of NOVEC 649 is developed based on Jackson's correlation. Most of the errors between the values predicted by the present correlation and experimental results are within 20%, and the mean absolute error is only 11.4%.

Large eddy simulations of high-heat-flux supercritical CO2 convection in microchannels: Mixed convection and non-uniform heating

Near-critical-point supercritical fluid convection is a promising solution for emerging high-flux thermal management needs because of the high fluid thermal conductivities and specific heats. Supercritical convection has been extensively studied for large diameter channels with uniform heating (4.1 < D < 22.7 mm, 0.05 < q'' < 330 W cm−2). However, limited information is available on transport processes to guide engineering of high-flux compact supercritical heat transfer equipment, which often have non-uniform heating distributions. To address this need, large eddy simulations (LES) are employed to study supercritical CO2 convection in microchannels (750μm×737μm cross-section). First, the simulation approach is validated with published experimental data. Studies are then conducted for horizontal isothermal heated channels at reduced pressure Pr=1.1, mass fluxes G=100-1000kgm-2s-1 (Re=3000-35,700), average wall heat fluxes q''=24-62Wcm-2, and bulk flow temperatures Tbulk=291-317K (inside and outside pseudocritical range). Results are used to assess the applicability of published supercritical convection correlations for microchannel heat exchangers. All available supercritical correlations are found to under-predict heat transfer coefficients at high heat fluxes (q''=58-62Wcm-2). The bottom-to-top wall heat transfer coefficient (HTC) ratios from these cases are used to determine a new criterion for the onset of significant mixed convection effects. At low mass fluxes (G=100kgm-2s-1), this HTC ratio is found to exceed 2.5×. Simulations indicate that, at these conditions, increased heat fluxes lead to reduced HTCs for low and high mass fluxes, but increased HTCs at intermediate mass flux values. Finally, an illustrative case is evaluated to assess the impact of conjugate heat transfer effects in microscale supercritical convection, and highlight limitations of conventional modeling approaches. This study quantifies the accuracy of convection correlations for high-heat-flux microchannel pseudocritical conditions, provides new criteria for predicting the onset of mixed convection in microchannel supercritical flows, and demonstrates the impact of conjugate heat transfer effects in microchannel supercritical heat exchangers.

Unusual Constriction Zones in the Major Porins OmpU and OmpT from Vibrio cholerae

The outer membranes (OM) of many Gram-negative bacteria contain general porins, which form nonspecific, large-diameter channels for the diffusional uptake of small molecules required for cell growth and function. While the porins of Enterobacteriaceae (e.g., E.coli OmpF and OmpC) have been extensively characterized structurally and biochemically, much less is known about their counterparts in Vibrionaceae. Vibrio cholerae, the causative agent of cholera, has two major porins, OmpU and OmpT, for which no structural information is available despite their importance for the bacterium. Here we report high-resolution X-ray crystal structures of V.cholerae OmpU and OmpT complemented with molecular dynamics simulations. While similar overall to other general porins, the channels of OmpU and OmpT have unusual constrictions that create narrower barriers for small-molecule permeation and change the internal electric fields of the channels. Together with electrophysiological and invitro transport data, our results illuminate small-molecule uptake within the Vibrionaceae.

Chitosan-derived three-dimensional porous carbon for fast removal of methylene blue from wastewater

Despite much progress in modifying chitosan as an absorbent for wastewater treatment, it is still difficult for current chitosan-based adsorbents to achieve the desired removal effects towards basic dyes. In this study, chitosan-derived three-dimensional porous carbon (CTC) consisting of large-diameter channels and mesopores was prepared to remove methylene blue (MB) from wastewater. The results indicate that CTC has excellent performance for MB removal, and the maximum adsorption capacity was 925.93 mg g−1 at 318 K. The adsorption isotherm and kinetics models of MB on CTC could be described well by the Langmuir isotherms and the pseudo-second-order rate model. An experiment to study the CTC removal of MB from a flowing aqueous solution was performed using a homemade device. The water treatment rate of CTC reached 250 L g−1 h−1, with high MB removal efficiency (>93.4%). Furthermore, the desorption–adsorption experiments indicate that CTC is also a reusable adsorbent that can be applied to recover MB from wastewater. The obtained CTC is a promising alternative for the current expensive absorbents and provides a concept for designing the three-dimensional (3D) structures of raw materials to improve adsorption capability.

Effect of channel geometry on flow reversal in microchannel evaporators

Boiling instability and periodic flow reversal are commonly seen in microchannel heat exchangers, but have been studied in air conditioning systems only recently. The amount and frequency of reverse flow largely depends on the geometry of the channels. This paper presents results obtained in a R134a automotive air conditioning system operated under flash gas bypass mode with venting of reversed vapor (to provide better control of the flow). Results for multiple microchannel evaporators with different channel diameters and lengths are presented. The mass flow rate and frequency of reversed vapor are measured for each geometry. Meanwhile, a mechanistic model is developed to simulate the bubble dynamics inside of a single microchannel tube. This model is validated against experimental data and employed to explain the effect of channel length and diameter on flow reversal. Both experimental and simulation results show that larger channel diameter and longer channel length result in less flow reversal with a lower frequency.

Characteristics of two-phase flows in large diameter channels

Two-phase flows in large diameter channels have a great deal of importance in a wide variety of industrial applications. Nuclear systems, petroleum refineries, and chemical processes make extensive use of larger systems. Flows in such channels have very different properties from flows in smaller channels which are typically used in experimental research. In this paper, the various differences between flows in large and small channels are highlighted using the results of previous experimental and analytical research. This review is followed by a review of recent experiments in and model development for flows in large diameter channels performed by the authors. The topics of these research efforts range from void fraction and interfacial area concentration measurement to flow regime identification and modeling, drift-flux modeling for high void fraction conditions, and evaluation of interfacial area transport models for large diameter channels.

3D simulation and analytical model of chemical heating during silicon wet etching in microchannels

We investigate chemical heating of a Silicon-on-Glass (SOG) chip during a highly exothermic reaction of silicon etching in potassium hydroxide (KOH) solution in a microchannel of 100-micron width inside a 1x1 cm SOG chip. Two modeling approaches have been developed, implemented and compared. (1) A detailed 3D model is based on unsteady Navier-Stokes equations, heat and mass transfer equations of a laminar flow of viscous incompressible fluid in microchannel, coupled to the heat transfer equation in the solid chip. 3D simulation results predicted temperature distributions for different KOH flow rates and silicon etching areas. Microchannels of a small diameter do not heat the chip due to the insufficient chemical heating of the cold fluid, whereas large-area etching (large channel diameter and/or length) leads to local overheating that may have negative effects on the device performance and durability. (2) A simplified analytical model solves a thermal balance equation describing the heating by chemical reactions inside the microchannel and energy loss by free convection of air around the chip. Analytical results compare well with the 3D simulations of a single straight microchannel, therefore the analytical model is suitable for quick estimation of process parameters. For complex microstructures, this simplified approach may be used as the first approximation.

Two-group modeling of interfacial area transport in large diameter channels

A comparison of the existing two-group interfacial area transport equation source and sink terms for large diameter channels with recently collected interfacial area concentration measurements (Schlegel et al., 2012, 2014. Int. J. Heat Fluid Flow 47, 42) has indicated that the model does not perform well in predicting interfacial area transport outside of the range of flow conditions used in the original benchmarking effort. In order to reduce the error in the prediction of interfacial area concentration by the interfacial area transport equation, several constitutive relations have been updated including the turbulence model and relative velocity correlation. The transport equation utilizing these updated models has been modified by updating the inter-group transfer and Group 2 coalescence and disintegration kernels using an expanded range of experimental conditions extending to pipe sizes of 0.304m [12in.], gas velocities of up to nearly 11m/s [36.1ft/s] and liquid velocities of up to 2m/s [6.56ft/s], as well as conditions with both bubbly flow and cap-bubbly flow injection (Schlegel et al., 2012, 2014). The modifications to the transport equation have resulted in a decrease in the RMS error for void fraction and interfacial area concentration from 17.32% to 12.3% and 21.26% to 19.6%. The combined RMS error, for both void fraction and interfacial area concentration, is below 15% for most of the experiments used in the comparison, a distinct improvement over the previous version of the model.

A correlation for interfacial area concentration in high void fraction flows in large diameter channels

Two phase flows exist as a part of many industrial processes, including chemical processes, nuclear reactor systems, and heat exchangers. In all of these applications the interfacial area concentration is an important parameter for evaluating the interactions between the phases, including drag forces, heat transfer or chemical reaction rates. Many models for interfacial area concentration exist for dispersed bubbly flows; however this type of flow only exists at relatively low void fractions. Very few correlations exist for the prediction of cap-turbulent, slug, or churn-turbulent flows. In this paper a new correlation for predicting the interfacial area concentration beyond bubbly flows in large diameter pipes is derived using a two-bubble-group method (spherical and distorted bubbles as Group-1 bubbles and cap and churn-turbulent bubbles as Group-2 bubbles) and the two-group interfacial area transport equation. The derivation assumes steady state and fully developed flow, and is based on interfacial area transport source and sink terms for large diameter pipes developed by Smith et al., 2012a. Int. J. Heat Fluid Flow 33, 156–167. The resulting equations can be used to predict the void fraction for each group of bubbles and the Sauter mean diameter for each group of bubbles in addition to the total interfacial area concentration. The model is then benchmarked based on the data collected by Schlegel et al., 2012. Exp. Therm. Fluid Sci. 41, 12–22; Schlegel et al., 2014. Int. J. Heat Fluid Flow 47, 42–56. It is found that the correlation predicts the data for Sauter mean diameter of Group 1 bubbles with RMS error of 23.3% and bias of +1.83%. For Group 2 bubbles the RMS error is 24.0% and the bias is +5.35%. This indicates that the correlation somewhat over-predicts the bubble sizes. In spite of this the prediction error remains reasonable compared to the accuracy of previous correlations, and given that the experimental uncertainty can be as high as 15% for some flow conditions. The RMS error and bias in the total interfacial area concentration are 22.6% and −4.29%, respectively. This is consistent with the over-prediction of the Sauter mean diameters, but again is reasonable considering the experimental uncertainty and the prediction error of previous correlations. The model is also able to predict the trends found in the experimental data with varied liquid and gas velocities, representing a large improvement over previous modeling efforts. An expanded database of accurate interfacial area concentration measurements at higher pressures would allow further improvement of the model benchmark and expansion of the range of applicability of the model.

Influence of stiffness on CHF for horizontal tubes under LPLF conditions

Studies reported in the past on critical heat flux (CHF) are mostly limited to vertical flow, large channel diameter, high pressure and high mass flux. Since horizontal flow is commonly encountered in boiler tubes, refrigerating equipments and nuclear reactor fuel channels (PHWR), there is a need to understand horizontal flow CHF, generate sufficient experimental database and to develop reliable predictive method. Few studies are reported on the effect of upstream flow restrictions on flow instabilities and CHF. The present work investigates the effect of upstream flow restriction on CHF in horizontal flow at near atmospheric pressure conditions. In the present study, stiffness is defined as the ratio of upstream flow restriction pressure drop to the test section pressure drop. The classification of a flow boiling system as soft or stiff on the basis of quantification of the stiffness is attempted. Experimental data shows an increase in the CHF with the increase in the stiffness for a given initial mass flux. A correlation for the prediction of CHF under various stiffness conditions is developed. A correlation is suggested to predict the mass flux at CHF as a function of stiffness and initial mass flux. Modeling and transient analysis of the stiffness effect on CHF is carried out using the thermal hydraulic system code RELAP5. The predicted phenomena are in agreement with the experimental observations.

Local flow structure beyond bubbly flow in large diameter channels

A thorough review of the available literature has revealed a significant lack of usable data regarding the transport of interfacial area in large diameter channels. This represents a concern for various industrial systems, but especially for predicting the performance of safety systems in nuclear reactor systems. In order to remedy this gap in the current experimental database a series of experiments has been performed. These experiments included the measurement of the local interfacial area concentration and other parameters using local electrical conductivity probes in pipes with diameters of 0.152m [6in.], 0.203m [8in.] and 0.304m [12in.]. Volumetric fluxes ranged up to 2m/s [6.56ft/s] for the liquid phase and 10 m/s [32.8ft/s] for the gas phase, and two nominal pressure conditions of 180kPa [26.1psia] and 280kPa [40.6psia] were included. Gas was injected as large cap bubbles in order to provide a basis for evaluating models for cap-bubbly flow at low void fractions. Measurements were performed simultaneously at three axial locations to allow the evaluation of interfacial area transport. The resulting data provides valuable insight into the flow structure and behavior in all flow regimes other than annular flow and will serve as a valuable database for the evaluation of models for predicting the transport of interfacial area across a wide variety of flow conditions and pipe sizes.

Effect of the alkali insertion ion on the electrochemical properties of nickel hexacyanoferrate electrodes.

Nickel hexacyanoferrate (NiHCFe) is an attractive cathode material in both aqueous and organic electrolytes due to a low-cost synthesis using earth-abundant precursors and also due to its open framework, Prussian blue-like crystal structure that enables ultra-long cycle life, high energy efficiency, and high power capability. Herein, we explored the effect of different alkali ions on the insertion electrochemistry of NiHCFe in aqueous and propylene carbonate-based electrolytes. The large channel diameter of the structure offers fast solid-state diffusion of Li(+), Na(+), and K(+) ions in aqueous electrolytes. However, all alkali ions in organic electrolytes and Rb(+) and Cs(+) in aqueous electrolytes show a quasi-reversible electrochemical behavior that results in poor galvanostatic cycling performance. Kinetic regimes in aqueous electrolyte were also determined, highlighting the effect of the size of the alkali ion on the electrochemical properties.

Modified distribution parameter for churn-turbulent flows in large diameter channels

Two phase flows in large diameter channels are important in a wide range of industrial applications, but especially in analysis of nuclear reactor safety for the prediction of BWR behavior and safety analysis in PWRs. To remedy an inability of current drift-flux models to accurately predict the void fraction in churn-turbulent flows in large diameter pipes, extensive experiments have been performed in pipes with diameters of 0.152m, 0.203m and 0.304m to collect area-averaged void fraction data using electrical impedance void meters. The standard deviation and skewness of the impedance meter signal have been used to characterize the flow regime and confirm previous flow regime transition results. By treating churn-turbulent flow as a transition between cap-bubbly dispersed flow and annular separated flow and using a linear ramp, the distribution parameter has been modified for churn-turbulent flow. The modified distribution parameter has been evaluated through comparison of the void fraction predicted by the drift-flux model and the measured void fraction.

An analysis of steady/unsteady electroosmotic flows through charged cylindrical nano-channels

The steady/unsteady electroosmotic flow in an infinitely extended cylindrical channel with diameters ranging from 10 to 100 nm has been investigated. A mixture of (NaCl + H2O) is considered for the numerical calculation of the mass, potential, velocity, and mixing efficiency. Results are obtained for the channel diameters are small, equal, or greater than the electric double layer (EDL) both for steady and unsteady cases. In the present discussion, a symmetrical distribution of mole fractions is considered at the wall interface. Hence, the velocity and potential are symmetrical in nature toward the centerline of the channel, and also identical in nature at maximum and minimum time levels (i.e., at π/2 and 3π/2 for a periodic function) in the transient case. In case of steady flows, the velocity and potential satisfy the chemical equilibrium condition at the centerline. It is observed that the electric double layer reaches a local equilibrium in the presence of electroosmosis when the channel length is long compared to the characteristic hydraulic diameter and the flow is essentially one-dimensional, which depends only on channel diameter. Comparisons of NP (Nernst Plank) model with PB (Poisson–Boltzmann) model are achieved out for different published results at larger channel diameters.

마이크로채널 기판에서 비정상 복합 열전달의 수치적 연구

A numerical study of transient conjugate heat transfer on micro heater in a micro-channel substrate under pulsed heating was conducted. It was found that the time constant is not affected by the pulse heating magnitude at same operating condition. Furthermore, the time constant increases with low substrate thermal diffusivity, low Reynolds number, and large channel diameter. Since the time constant is a dominant parameter to characterize transient heat transfer, it should be considered for transient convective heat transfer coefficient.

A Microfluidic Device for High Throughput Measurements of Thermotaxis

E. coli bacteria have the ability to sense thermal gradients and congregate in locations with a preferred temperature. Studies of this phenomenon, called thermotaxis, have been complicated by the lack of tools to enable the generation of well-defined thermal gradients and the collection of data on motile E. coli in these gradients. Here we report a microfluidic experimental system which allows the collection of data on the migration of bacteria in thermal gradients at a high throughput. In our microfluidic device, a bacterial suspension slowly flows through a 10 mm long, 0.5 mm wide microchannel that has a linear temperature gradient across it. It takes ∼100 seconds for cells to reach the end of the microchannel, thus they have sufficient time to sample the temperature gradient and find their preferred position in the gradient. By analyzing the distribution of cells across the microchannel near its end, we collect data on the thermotaxis of >10,000 individual E. coli cells per experiment. The temperature gradient is generated by the continuous circulation of temperature-controlled water through two large-diameter channels flanking the microchannel. The temperature gradient is evaluated using a temperature-sensitive fluorescent dye. The microfluidic chip is made from a single cast of PDMS, making it easy to fabricate and replicate with high fidelity. We have used the experimental system to perform preliminary experiments on wild type E. coli in a variety of linear thermal gradients and have observed the congregation of cells centered about a temperature of 35–37° C.