Flow Distribution Characteristics Research Articles

Investigation of two-phase flow distribution in the vertical annular distribution header of a variable refrigerant flow heat pump system

The characteristics of two-phase flow distribution from a vertically installed annular refrigerant distribution header to multiple branch ports are investigated, with a specific focus on simulating a real variable refrigerant flow heat pump (VRF HP) system. A test section is fabricated to replicate an actual distribution header for empirical investigation, utilizing an air–water mixture as the working fluid. A series of experiments are conducted to examine distribution characteristics, involving variations in the location of the main inlet port, operational mode, total mass flow rate, and void fraction. The experimental results indicate that the two-phase mixture exhibits a relatively uniform liquid phase distribution to the branch ports when the main inlet port is positioned at the bottom side. However, maldistribution to branch ports is intensified as the inlet void fraction increases and the flow rate of the two-phase mixture decreases. Furthermore, a series of computational fluid dynamics analyses is performed to investigate the distribution characteristics of R410A, the actual working fluid utilized in VRF HP systems. These analyses are conducted under the operational conditions typical of VRF HP systems. Despite identical Reynolds numbers for the two-phase R410A flow and air–water mixture flow, the former exhibits a notably uneven distribution to branch ports owing to the distinct thermophysical properties, particularly density and viscosity. The findings from this investigation enhance the comprehension of two-phase flow distribution characteristics in a vertically installed refrigerant distribution header under real VRF HP system conditions, offering valuable insights into mitigating the maldistribution of refrigerant flow to branch ports.

Research on heat transfer and flow distribution of parallel-configured microchannel heat sinks for arrayed chip heat dissipation

The heat dissipation ability and compact structure of microchannel flow boiling show great promise for high-efficiency heat dissipation in the field of chip cooling. In arrayed chip cooling scenarios, multiple microchannel heat sinks are typically arranged in parallel. Random fluctuations in the heat load of each chip can lead to non-uniform flow distribution and even thermal failure of the chip. However, there are still limited studies on the non-uniform flow distribution in the parallel use of multiple microchannel heat sinks. This study presents an experimental and theoretical study on the boiling heat transfer and flow distribution characteristics of two heat sinks in parallel using HFE-7100. The steady-state test results indicated that the flow distribution characteristics in parallel-configured heat sinks were closely correlated with the flow pattern. In the single-phase region, flow distribution in both heat sinks was primarily influenced by the physical properties of the fluid. The effect of heat flux on non-uniform flow distribution was within 7 %. When one of the heat sinks entered the two-phase region, microchannel resistance characteristics changed significantly, leading to a dramatic increase in non-uniform flow distribution, up to 26.0 %. The critical heat flux (CHF) was triggered prematurely, with a decrease of up to 31.4 %. In dynamic characteristics testing, when both heat sinks were in the two-phase region simultaneously, the increase in heat flux led to greater non-uniformity in the flow distribution of the parallel-configured heat sinks compared to the single-phase region. Furthermore, considering the transition boundary of two-phase flow patterns in microchannels, a prediction model for flow distribution in parallel-configured microchannel heat sinks was proposed, demonstrating good predictive capability. The research provides a reference for the design and optimization of two-phase cooling systems for arrayed chip configurations.

Characteristics of numbering-up and flow distribution of multi-channel microreactor with 2-D constructal inlet distributors

In this work, the flow distributions of 16-, 8-, 4-, 2-channels microreactors with 2-D constructal inlet distributors were measured. The results showed an increase in channel numbers caused a decreasing uniformity of the flow distribution. Both asymmetrical velocity profiles before a bifurcation point and asymmetrical flow resistance after a bifurcation point played an important role on the flow distribution in microchannels. The uniformity of flow distribution showed a trend of firstly increasing and then remaining unchanged with the increase of Re (4 ∼ 680). The pressure drops of microreactors were also investigated and the empirical method is in the acceptable range for estimating the energy consumption. Extending the channel length after bifurcation point from 124.46 mm to 160.95 mm and incorporating a bypass pressure equalization chamber at bifurcation point could improve the uniformity of flow distribution.

Investigation on uneven flow distribution in triply periodic minimal surface heat exchangers

The development in manufacturing engineering has made it possible to adopt the unique porous structure known as the triply periodic minimal surface to increase energy efficiency in a variety of energy-related sectors. With the unique morphological characteristics, triply periodic minimal surface exhibits remarkable mechanical strength and thermal performance, which makes it a potential candidate for compact, high-performance heat exchanger constructions. In this study, a theoretical model based on triply periodic minimal surface porous media is developed. Based on the hypothesis that the triply periodic minimal surface is isotropic and the heat transfer intensity of fluid and solid is uniform throughout the core of the heat exchanger, the simulation of the full-size heat exchanger is simplified by using a porous media model. The flow distribution characteristics of triply periodic minimal surface heat exchangers, as well as the effect mechanism of key factors, including the channel porosity, header configuration, the type of triply periodic minimal surface, the core length and Reynolds number on the flow distribution performance, are investigated. A dual-header configuration is proposed to improve the flow uniformity of triply periodic minimal surface heat exchangers. The results indicate that the flow distribution performance of triply periodic minimal surface based heat exchangers can be enhanced by the reduction of channel porosity. When the cold-side porosity is 0.7, the flow rate deviates from the mean value by 220% at its maximum for a Primitive heat exchanger. A dual-header configuration eliminates the columnar extreme high velocity space in the core and greatly reduces the “weak zone” of flow distribution, which improves the flow non-uniformity by 45–90% over the single-inlet headers. This paper provides data and theoretical support for the optimal design and practical application of the complex, high-performance triply periodic minimal surface based heat exchangers.

Flow distribution characteristics of parallel narrow rectangular channels under deformation conditions -Part I: An experimental study

Plate-type fuel assembly has been a promising option for core miniaturization applications. The parallel coolant channels, characterized by narrow rectangular with a large aspect ratio, may generate flow distribution deviation due to structural deformation. This study is dedicated to a comprehensive investigation of flow distribution characteristics accompanied by fluid boiling at high-pressure conditions in parallel twin narrow rectangular channels. The study will be presented in two parts: Part I of this paper comprises experimental methodology and flow distribution results. The experimental setup includes an integrated parallel channels geometry designed to improve pressure-bearing capabilities. Additionally, experiments are conducted on severe channel deformation modes, such as swelling and bending, by modifying the shape and dimensions of the ceramics within the test apparatus.Based on the experimental data, including pressure difference, wall and main fluid temperatures, the flow and heat transfer performance between twin channels is analyzed. Results shows that a significant deviation in flow distribution occurs in the swelling channel. Then methods involving thermal balance and pressure difference are employed to ascertain the mass flow rate of each distinct channel. The research reveals that the impact range of flow distribution is categorized into single-phase and two-phase control regions. The corresponding flow deviation of swelling channel surges from 5% to approximately 30%, significantly exceeding the design threshold specified for plate-type fuel assembly. Regarding the bending channel, the overall deviation consistently remains within 3%. Even when the fluid transitions into two-phase region, the flow deviation does not enlarge further. Moreover, a lower mass flux and higher heat flux aggravate the uneven flow distribution, while an increase in system pressure alleviates the flow deviation. Furthermore, Part II of this investigation introduces a numerical model derived from the experimental data, and the flow distribution characteristics under various channel deformations are examined through model calculations.

Effect of microchannel cross-section geometry on the flow resistance with stainless steel flat-plate solar collectors

The stainless steel flat plate solar collector is a new type of collector that has a longer lifespan in high-temperature and high-humidity environments. When using stainless steel as the base material of the collector, the microchannel structure is typically employed to increase the heat transfer area and compensate for the lack of heat transfer coefficient of stainless steel. However, the shape of the channel section in microchannel structures affects the flow resistance of the heat transfer medium. This article presented the equation for the optimal feature size of the channel, obtained through fluid theory analysis of the stainless steel flat plate collector. The study analyzed the impact of different section parameters on the flow distribution and pressure drop characteristics of the medium in the channel through numerical simulation. The results showed that for a certain inlet flow rate, the cross-sectional geometry of the microchannel heat-absorbing plate determined the parameters such as the type of core cross-section, the degree of wall urgency, and the geometrical length of the microchannel, which in turn affected the distribution of the fluid in the flow channel and the energy loss. The optimal thermal performance of the stainless steel collector was observed when the width of the micro-channel was 8.7 mm and the corrugation height of the tube group was 3.00 mm.

Electric-thermal collaborative control and multimode energy flow analysis of fuel cell hybrid electric vehicles in low-temperature regions

The energy flow distribution characteristics of electric vehicles operating in various propulsion modes and all climatic scenarios have not been thoroughly explored. To achieve effective electric-thermal collaborative energy management, intelligent control methods must be applied considering various climatic conditions to alleviate mileage anxiety. In this study, we developed a novel electric–thermal collaborative energy management strategy based on an improved deep neural network and energy quantification model to increase the global energy conversion efficiency. The complete energy consumption distribution characteristics are summarized under various strategies and propulsion modes based on an experiment data collected by the vehicle control unit that involves battery self-heating, cabin heating, acceleration consumption, and fuel consumption in the temperature range of −10°C-35 °C. Our findings indicate that, for a fuel cell hybrid bus in the cycle including the initial cabin heating process, the heating consumption in the pure electric mode was 9.9 kWh/cycle and 13 kWh/cycle when the ambient temperature is −2 °C and −10 °C, respectively, accounting for 33 % and 42 % of the total consumption, respectively. After using the waste heat from the fuel cell, the consumption of electric heating under the same conditions is only 3.7 kWh/cycle. In the high-temperature scenario, the cabin cooling consumption is 3.26 kWh/cycle, accounting for only 18 % of the total energy consumption. Finally, in low-temperature scenarios, the electric–thermal collaborative strategy reduced the cost by 14.7 % and 9.2 % in the pure electric and hybrid modes, respectively. Thus, our approach significantly improves energy utilization and conversion efficiency, especially at low temperatures.

Multi-Physics Coupled Acoustic-Mechanics Analysis and Synergetic Optimization for a Twin-Fluid Atomization Nozzle

Fine particulate matter produced during the rapid industrialization over the past decades can cause significant harm to human health. Twin-fluid atomization technology is an effective means of controlling fine particulate matter pollution. In this paper, the influences of the main parameters on the droplet size, effective atomization range and sound pressure level (SPL) of a twin-fluid nozzle (TFN) are investigated, and in order to improve the atomization performance, a multi-objective synergetic optimization algorithm is presented. A multi-physics coupled acoustic-mechanics model based on the discrete phase model (DPM), large eddy simulation (LES) model, and Ffowcs Williams-Hawkings (FW-H) model is established, and the numerical simulation results of the multi-physics coupled acoustic-mechanics method are verified via experimental comparison. Based on the analysis of the multi-physics coupled acoustic-mechanics numerical simulation results, the effects of the water flow on the characteristics of the atomization flow distribution were obtained. A multi-physics coupled acoustic-mechanics numerical simulation result was employed to establish an orthogonal test database, and a multi-objective synergetic optimization algorithm was adopted to optimize the key parameters of the TFN. The optimal parameters are as follows: A gas flow of 0.94 m3/h, water flow of 0.0237 m3/h, orifice diameter of the self-excited vibrating cavity (SVC) of 1.19 mm, SVC orifice depth of 0.53 mm, distance between SVC and the outlet of nozzle of 5.11 mm, and a nozzle outlet diameter of 3.15 mm. The droplet particle size in the atomization flow field was significantly reduced, the spray distance improved by 71.56%, and the SPL data at each corresponding measurement point decreased by an average of 38.96%. The conclusions of this study offer a references for future TFN research.

Control and Protection of Medium Voltage Distribution Network Based on Unified Power Flow Controller

The integration of diverse load types, such as large-scale distributed generators and charging stations, presents considerable challenges to distribution networks. The Unified Power Flow Controller (UPFC) provides multiple benefits, including its compact dimensions, cost efficiency, seamless regulation, and swift response. When deployed in distribution networks, the UPFC facilitates the creation of a Distribution Unified Power Flow Controller (DUPFC) system. The DUPFC system enhances medium voltage distribution network regulation but also affects power quality, fault characteristics, and relay protection accuracy. To address these issues, a power flow equivalent circuit model for the medium voltage distribution loop network was developed. The operating principle and steady-state model of the UPFC were investigated, based on distribution loop network parameters and power flow distribution characteristics. A UPFC control strategy was designed to create a suitable DUPFC system for the medium voltage distribution loop network, ensuring coordination between UPFC protection and the distribution system’s protection mechanisms. This system accomplishes rapid fault isolation and automatic load transfer. Finally, a simulation model based on a typical 20 kV distribution loop network structure was selected to validate the effectiveness of the UPFC control strategy and the DUPFC protection system in MATLAB/Simulink.

Numerical simulation of flow distribution characteristics in carbon dioxide microchannel evaporator

Microchannel heat exchangers find widespread use in household and automotive air conditioning due to their compact structure and high efficiency. However, two-phase refrigerant flow mal-distribution in microchannels limits their application. For the CO2 heat pump, the performance reduction by flow mal-distribution could be as large as 20%. This study proposed an approach to mitigate flow mal-distribution by inserting a perforated tube into the header of a microchannel evaporator in CO2 heat pump. Numerical simulations were carried out to investigate the impact of perforated tube parameters and CO2 inlet conditions on flow distribution. Results show a 24.4% increase in total non-uniformity (S) of flow distribution with a perforated tube. A higher number of openings and smaller diameters enhance uniformity, with the 10-hole, 2 mm diameter perforated tube proving most effective. Additionally, the most uniform flow distribution was achieved at an inlet CO2 mass flow rate of 30 g/s and a vapor quality of 0.2. And the flat tubes with an insertion depth of 5 mm and a cross-section of 16 × 2 mm is the most beneficial for uniform flow distribution.

Numerical simulation on gas-solid separation characteristics of in-situ pyrolysis products of tar-rich coal

ABSTRACT Tar-rich coal holds the potential to substitute the supply of oil-gas resources. The in-situ pyrolysis contributes to efficient resource utilization and can potentially achieve net-zero CO2 emissions. However, it is challenging to efficiently separate oil-gas resources, which are predominated by tar, from multiphase products of in-situ pyrolysis of tar-rich coal. Additionally, the difference in fluid properties yields significant influences on separation performance, while the understanding of the separation characteristics of high-viscosity fluid remains insufficient. In this study, the separation process of pyrolysis products from tar-rich coal was investigated using the computational fluid dynamics simulation approach. The flow field, pressure drop, separation efficiency and distribution characteristics were analyzed. The simulation results indicate that the volute cyclone separator exhibits commendable separation efficiency, exceeding 98%. Meanwhile, in handling tars containing a high concentration of light oil, the separator demonstrates remarkable adaptability. The separation efficiency of solid particles decreases from 89% to 83% when the system contains a high content of pitch. The particle diameter exerts great influence on the flow field. The separation efficiencies are 99% and 0.37%, respectively, when the particle sizes are selected as 0.1 mm and 0.001 mm. In the scope of this study, the optimal working conditions of the cyclone separator were determined, which provides data support for improving the separation efficiency and recovery rate of the oil-gas resources.

Numerical simulation of spray atomization in crossflow using an empirical primary atomization model

The hybrid Eulerian-Lagrangian approach is used to simulate the atomization process of spray jet in subsonic crossflow. A coupled Volume of Fluid (VOF) - Lagrangian Particle Tracking (LPT) solver is implemented in the open-source platform OpenFOAM to solve the governing equations for the continuous and discrete phase respectively. An empirical model for primary atomization is proposed, in which the aerodynamic effect is taken into account, along with the turbulence effect. Thus, it is suitable for the flow at a low liquid-gas density ratio, i.e., less than 500. The performance of the presented model is evaluated and verified by comparisons of the simulated results with the experimental data. The flow characteristics of flow fields and distribution of droplet diameters are analyzed in detail. It is revealed that the vortex structures have a direct impact on the distribution of the droplets. The effects of the empirical constants in the atomization model are also examined. Finally, it is shown that the model performs better for the flow at a higher Weber number, in which the aerodynamic effect is more significant.

Analysis of coolant flow distribution characteristics at core inlet of small pressurized water reactor under rolling condition

The flow distribution characteristics at core inlet directly determines the thermal and hydraulic characteristics in the core, and it is closely related to the safety of nuclear reactor. The primary coolant is subjected to additional inertial force in ocean motion, which influence the flow distribution characteristics at core inlet. In the present paper, numerical simulation method is adopted to analyze the coolant flow distribution characteristics at core inlet of the Reactor Pressure Vessel (RPV) of a Small Reactor under rolling condition. Then the results of the flow distribution characteristics were compared with those under steady state. The results show the variation range of flow distribution factors in each channel under rolling condition is within 5%. And the uneven distribution of coolant at the core inlet increases with the increase of rolling amplitude, and decrease with the rolling period in short period. Standard deviation of flow distribution factor at the core inlet changes little under rolling condition, which indicates the flow distribution at the core inlet keeps good uniformity.

Numerical investigation the flow distribution characteristics of the evaporative cooling plate in a pump-driven two-phase flow system

Numerical investigation the flow distribution characteristics of the evaporative cooling plate in a pump-driven two-phase flow system

A study on flow distribution characteristics of heat exchanger module integration scheme for aircraft environmental control system based on CFD

With the rapid development of the civil aviation industry, the performance requirements of the environmental control system for domestic commercial large aircraft are gradually improved, and the ram air inlet heat exchanger system is its main component. Generally speaking, a ram air heat exchanger system mainly includes an air conditioning system heat exchanger, air preparation system heat exchanger, and auxiliary cooling system heat exchanger. The integration mode of the heat exchanger will directly affect the flow and flow distribution characteristics of the ram air heat exchanger system. This paper develops three integration schemes referring to the integration mode of heat exchangers in Boeing 787 and Airbus 350 aircraft. Based on Fluent software, the calculation and simulation of three integrated scheme models under the seven working conditions of taking off, climbing, cruising, descent, hovering, emergency, and landing during flight were carried out, and the flow distribution and flow resistance values of different schemes were obtained, which were compared to meet the heat transfer requirements.

Optimal design of silver-loaded zeolite filter structure based on porous media model

Silver zeolite filters have potential applications in nuclear power plant filtration and emission systems due to their high-efficiency adsorption properties for radioactive methyl iodide. The adsorption performance of the filter is closely related to the flow distribution characteristics. To analyze the flow distribution characteristics of the filter element, we established a reasonable numerical calculation model and verified the model through experiments. Then based on the model, the advantages and disadvantages of different filter unit structures are analyzed. We proposed two new structural forms of filters and evaluated the performance of the filters based on the pressure distribution and flow increment curve, considering the difficulty of processing and space utilization. Comparative analysis and characteristic research have been carried out under different sizes, from the cylindrical filter to the round table filter and, finally, to the square filter. Our results show that round table and square filters have significantly improved flow distribution compared to traditional cylindrical filters. However, the structure of the round table filter is irregular, and the space utilization rate is low. Finally, through optimization design, the square filter with channel spacing L of 250 mm has the best performance. Our research results have a certain guiding significance for the design of the silver zeolite filter prototype.

Experimental Study on Gas-Solid Two-Phase Flow Characteristics of a Vertical Cyclone Combustor System.

According to the design and operational parameters of the cyclone liquid slag-discharging boiler, an experimental platform for the cyclone burner was designed and constructed in a cold state based on the principle of similarity. The experimental study investigated the effects of parameters, such as swirl-vane angles, coal concentration, operating parameters, and particle size, on the flow distribution and vertical riser resistance characteristics of the vertical cyclone burner. The results showed that there were differences in flow distribution among the cyclone burners, and the most uniform flow distribution was achieved when the swirl-vane angle of the primary air was 30°. The concentration of pulverized coal significantly influenced the pressure drop in the vertical ascending section, which increased with higher concentrations of pulverized coal. When the concentration of pulverized coal remains constant, the pipeline pressure drop is minimized at a primary air velocity of 7.5 m/s. As the secondary wind speed increased, the pressure drop consistently rose; when the secondary wind speed is 22 m/s, the pressure drop of the pipeline is the maximum; however, excessively high secondary wind speeds were found to be detrimental to the formation of an optimal aerodynamic field in the burner. Furthermore, when the pulverized coal concentration was held constant, materials with larger particle sizes exhibited the highest pressure drop. When the particle size increases from 50 to 150 μm, the pressure drop of the vertical riser segment also increases. Finally, based on the Barth additional pressure drop theory, the pressure drop formula of the vertical riser is fitted by a dimensional analysis method, and the correlation formula of the pressure drop test of gas-solid two-phase flow in the vertical riser is obtained.

Study on structure optimization and distribution characteristics of centrifugal refrigerant distributor for air conditioner

The refrigerant distributor helps resolve non-uniform flow distribution issues in multi-path evaporators. Using the CFD method, this paper investigates the distribution mechanism of R410A in a centrifugal distributor under air-conditioning conditions and explores the impact of structural factors on its distribution performance. The Taguchi method is employed to identify the optimal combination of structural parameters and optimization direction for achieving distribution uniformity. Furthermore, the flow distribution characteristics of the optimized distributor are investigated under different conditions. The results show that, under the working conditions of an inlet mass flow rate of 50 kg·h−1 and an inlet quality of 0.1, the non-uniformity of the mass flow rate in the outlet branches of the optimized distributor is only 1.07 %, which represents a decrease of 7.64 % compared to the original distributor. Additionally, the non-uniformity of the outlet branch quality also decreased by 9.41 %. These findings indicate that the optimized distributor exhibits superior distribution performance. Nonetheless, the degradation of the optimized centrifugal distributor's uniform distribution performance with increasing refrigerant inlet mass flow rate and quality suggests that it is more suitable for low mass flow rate and quality conditions.

The Seepage Evolution Characteristics in Undisturbed Loess under Dynamic Preferential Flow: New Insights from X-ray Computed Tomography

Preferential flow is widely developed in varieties of voids (such as macropores and fissures) in loess areas, affecting slope hydrology and stability and even leading to geological disasters. However, the model of seepage evolution with dynamic preferential flow is not clear, which obstructs the disclosure of the mechanism of landslides induced by the preferential flow. This study aimed to capture the seepage and occurrence status of water in loess voids, explain the variability characteristics of the loess pore structure, and reveal the seepage evolution model of dynamic preferential flow. Preferential infiltration experiments were conducted by combining X-ray computed tomography (X-ray CT) nondestructive detection with contrast techniques under dynamic seepage conditions. Three-dimensional (3D) visualized reconstruction, digital image correlation (DIC), image processing, and quantitative analyses were performed in AVIZO 2019.1, including two-dimensional (2D) and 3D characteristics of preferential flow distribution and macropore changing, dynamic variation of the porosity, pore number, volume, dip angle, and connectivity. Results showed that (1) preferential flow exists under saturated and unsaturated conditions in loess with strong uniformity and anisotropy; (2) preferential flow not only migrates into existing connected macropores, but also connects the original isolated pores into channels and forms larger percolation groups of contrast medium under the gradually increased high pressure; (3) the seepage develops with the evolution model of ‘preferential flow–piston flow–preferential piston mixture flow–piston flow’ in the dynamic process. The new insights into the characteristics of the seepage evolution in undisturbed loess under dynamic preferential flow will enrich the understanding of loess seepage and provided an important reference for future research on the slope instability of the loess induced by preferential flow.

Numerical study on flow distribution of supercritical CO2 in multiple channels of printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) is a promising candidate in the application of the supercritical CO2 Brayton cycle. The flow maldistribution of PCHE is a practical issue due to the multi-channels in the core region, which may affect the performance of PCHE in the operation. Therefore, in this paper, the flow distribution and thermohydraulic characteristics of PCHE with three different channel configurations including the straight channel, wavy channel, and airfoil channel are investigated. The flow non-uniformity with two types of headers (free manifold and straight rib manifold) is compared among the three configurations under different mass flow rates. Based on the results, it is found that the core region of PCHE with larger flow resistance shows better flow uniformity and heat transfer performance. And the header structure is of great significance to the flow distribution based on the analysis of flow vector contours. Therefore, a modified manifold, with bent fins and airfoil fins, is proposed to improve the flow maldistribution of PCHE. Compared to the free manifold and straight rib manifold, the flow non-uniformity of the modified manifold is reduced by 39.4% and 61.8%, respectively; and the overall performance is averagely increased by 5% and 8.5%, respectively. Hence, it is recommended that the designers adopt the modified manifold for the performance improvement of integrated PCHE.