Flow Maldistribution Research Articles

Expanded Microchannel Heat Exchanger: Finite Difference Modeling

A finite difference model of a heat exchanger (HX) considered maldistribution, axial conduction, heat leak, and the edge effect, all of which are needed to model a high effectiveness HX. An HX prototype was developed, and channel height data were obtained using a computerized tomography (CT) scan from previous work along with experimental results. This study used the core geometry data to model results with the finite difference model, and compared the modeled and experimental results to help improve the expanded microchannel HX (EMHX) prototype design. The root mean square (RMS) error was 3.8%. Manifold geometries were not put into the model because the data were not available, so impacts of the manifold were investigated by varying the temperature conditions at the inlet and exit of the core. Previous studies have not considered the influence of heat transfer in the manifold on the HX effectiveness when maldistribution is present. With no flow maldistribution, manifold heat transfer increases overall effectiveness roughly as would be expected by the greater heat transfer area in the manifolds. Manifold heat transfer coupled with flow maldistribution for the prototype, however, causes a decrease in the effectiveness at high flow rate, and an increase in effectiveness at low flow rate.

On the Use of Dual Cell Density Monoliths

Monolith-type substrates are extensively used in automotive catalytic converters and have gained popularity in several other industrial processes. Despite their advantages over traditional unstructured catalysts, such as large surface area and low pressure drop, novel monolith configurations have not been investigated in depth. In this paper, we use a detailed computational model at the reactor scale, which considers entrance length, turbulence dissipation and internal diffusion limitations, to investigate the impact of using a dual cell substrate on conversion efficiency, pressure drop, and flow distribution. The substrate is divided into two concentric regions, one at its core and one at its periphery, and a different cell density is given to each part. According to the results, a difference of 40% in apparent permeability is sufficient to lead to a large flow maldistribution, which impacts conversion efficiency and pressure drop. The two mentioned variables show a positive or negative correlation depending on what part of the substrate—core or ring—has the highest permeability. This and other results contribute relevant evidence for further monolith optimization.

Two-phase flow in parallel channels: Mal-distribution, hysteresis and mitigation strategies

Parallel flow channels have been widely used in industrial applications to improve reactor performance. Substantial experimental evidence in the literature suggests that two-phase flow through parallel paths can distribute non-uniformly, even when the flow paths are identical, leading to decreased system performance. Flow mal-distribution is typically related to the ill-distributed flow in the header of parallel pipes and the instability of uniform flow distribution. Firstly, we carry out a review on mal-distribution and hysteresis phenomena of gas–liquid and gas–solid two-phase flow in parallel channels. Further, flow instability and possible mal-distribution mechanisms are examined. Lastly, strategies in literature to mitigate the flow mal-distribution and hysteresis are reviewed. Given the importance of a fundamental understanding of two-phase flow instability to enhance the system performance and reliability, future research should focus on improving the instability analysis of multiphase flow systems and developing mitigation strategies for the design and operation of parallel flow systems.

Numerically investigating two-phase reactive transport in multiple gas channels of proton exchange membrane fuel cells

Understanding the coupled mechanisms between two-phase flow and reactive transport in proton exchange membrane fuel cells is important for improving cell performance. In this study, a one-dimensional + three-dimensional model is developed to simulate the air–water two-phase flow, mass transport and electrochemical reaction in the cathode side of proton exchange membrane fuel cells. Dynamic water behaviors affected by several forces in three channels are well captured by the volume of fluid method. Effects of liquid water behaviors on pressure drop, mass flow rate, oxygen concentration distribution, current density and especially the distribution characteristics among different channels are discussed. It is found that the current density increases under certain flow patterns such as droplet detachment and water film on the top wall. Compared with single-phase flow, the two-phase flow magnifies the flow maldistribution in different channels, and such maldistribution in turn leads to different water detachment sequence in the three channels. The change of the mass flow rate in the second channel is more sensitive to that in the third channel. Effects of the gas channel wall wettability and channel structures are also investigated. The results show that changing the gas channel as hydrophobic will lead to higher pressure drop, lower current density and poorer flow distribution among different channels, which thus is not desirable in application. Modified structures of rectangle channel and tapered channel are finally explored. Compared with the base case, the tapered channel results in increased flow uniformity and averaged current density by 65.9% and 1.5%, respectively.

The VE/VCO2 slope: A useful tool to evaluate the physiological status of children with congenital heart disease

Cardio-pulmonary exercise test (CPET) is becoming a key examination to assess physical capacity and disease severity in paediatric cardiology. The VE/VCO2 slope has been increasingly used as a surrogate marker for morbidity and mortality in adult heart failure, pulmonary arterial hypertension and for adult patients with CHD. Nevertheless, the use of the VE/VCO2 slope in children remains limited in the absence of reference values and clearly identified clinical determinants. This study aimed to compare the VE/VCO2 slope in a paediatric cohort with CHD to that of age- and gender-adjusted healthy controls. We also intended to identify the clinical and CPET variables associated with VE/VCO2 slope in this population. This cross-sectional study was carried out between November 2010 and September 2015 in two tertiary care paediatric cardiology reference centres. A total of 700 children were enrolled (399 CHD and 301 healthy controls). The mean VE/VCO2 slope was significantly higher in CHD than in healthy subjects (31.6 ± 4.8 vs. 29.3 ± 4.8; P < 0.001). When considering CHD sub-groups, the VE/VCO2 slope was impaired in single ventricle, complex anomalies of ventriculo-arterial connections, anomalies of the atrioventricular junctions and valves. The VE/VCO2 slope according to the CHD physiological status was illustrated in the Fig. 1, it was higher in children with significant pulmonary regurgitation, tricuspid regurgitation, right ventricular hypertension and right ventricle outflow tract (RVOT) obstacle. In the CHD group, VE/VCO2 slope increase was associated with BMI, the presence of a RVOT obstacle, the number of cardiac catheter procedures, as well as low age, FVC, tidal volume, and PetCO2 (Fig. 1). Increased VE/VCO2 slope was predominantly in children with single ventricle and/or residual right heart abnormalities suggesting that maldistribution of pulmonary blood flow during exercise is an important CHD-unique determinant of VE/VCO2 slope.

A new multiscale modeling framework for investigating thermally-induced flow maldistribution in multi-stream plate-fin heat exchangers

Multi-stream Plate-fin heat exchangers (MSPFHEs) are among the most efficient heat exchangers in energy-based industries. They should be carefully designed to have maximum effectiveness and to avoid wasting energy. Flow maldistribution could significantly degrade the performance of heat exchangers. This study presents a novel modeling framework to capture the thermally-induced maldistribution in two-phase MSPFHEs. The developed model is used to show how thermally-induced flow maldistribution affects the performance of the heat exchanger, and some possible modifications are investigated to reduce its consequences. Our case study results show that thermally-induced maldistribution decreases the total heat transfer by 10.8%. Heat leakage helps to induce less flow near the leakage region, so it can be managed to overcome the initial thermally-induced flow. Moreover, thermally-induced maldistribution affects a limited region along the heat exchanger height direction. So, thermal performance deterioration decreases with increasing the number of layers for each stream. Nevertheless, increasing the heat exchanger length increases the degraded region in the heat exchanger. Thus, increasing the core length is not a good way to counteract the thermal performance deterioration caused by thermally-induced maldistribution. It is also found that an optimum layer arrangement could help to eliminate the thermally-induced maldistribution in the heat exchanger.

Photocatalytic Degradation of 2-propanol Over TiO2-based Thin Films in a Simulated Pilot Microreactor

Background: Microreactor devices have attracted increasing attention over the last years due to their high surface-to-volume ratio, which ensures a high heat and mass transfer, short molecular diffusion distance and greater spatial illumination homogeneity compared to the traditional reactors. Objective: The aim of this study was to model the kinetics of photo-degradation of 2-propanol over TiO2-based thin films in a gas-phase batch-reactor and simulate their performance in a microreactor device. Methods: The reaction was carried out in a gas-phase batch-reactor, assessing the reactivity of singlelayer nitrogen (N)-doped TiO2 and a bilayer consisting of N-doped TiO2 as a bottom layer and copper (Cu)-doped TiO2 as a top layer. The kinetics of the photocatalytic process was modelled by Langmuir– Hinshelwood (LH) model. The constants obtained from the LH model were used to simulate the performance of the photocatalysts in a microreactor, operating in a continuous flow mode and investigating the effect of the volumetric flow rate (Q), initial concentration of pollutant (Co), number of microchannels (n) and microchannel length (l) on the photo-degradation of 2-propanol. Results: N-Cu-TiO2 exhibited a higher reactivity but a lower adsorption ability towards the target pollutant compared to N-TiO2. To maximize and leverage the advantages of a microreactor, optimal operating conditions for a continuous flow mode, at a steady-state, should be moderately low Q and Co, long l and n that minimizes flow maldistribution in parallel. Conclusion: The findings in this work could serve as a basis to design and fabricate efficient microreactors for the removal of VOC in air purification applications.

Two-Dimensional Flow Boiling Characteristics With Wettability Surface in Microgap Heat Sink and Heat Transfer Prediction Using Artificial Neural Network

Abstract As technology becomes increasingly miniaturized, thermal management becomes challenging to keep devices away from overheating due to extremely localized heat dissipation. Two-phase cooling or flow boiling in microspaces utilizes the highly efficient thermal energy transport of phase change from liquid to vapor. However, the excessive consumption of liquid-phase by highly localized heat source causes the two-phase flow maldistribution, leading to a significantly reduced heat transfer coefficient, high-pressure loss, and limited flow rate. In this study, flow boiling in a two-dimensional (2D) microgap heat sink with a hydrophilic coating is investigated with bubble morphology, heat transfer, and pressure drop for conventional (nonhydrophilic) and hydrophilic heat sinks. The experiments are carried out on a stainless steel (SS) plate, having a microgap depth of 170 μm using de-ionized (DI) water at room temperature. Two different hydrophilic surfaces (partial and full channel shape) are fabricated on the heated surface to compare the thermal performance with the conventional surface. Vapor films and slugs are flushed quickly on the hydrophilic surfaces, resulting in heat transfer enhancement on the hydrophilic heat sink compared to the conventional heat sink. The channel hydrophilic heat sink shows better cooling performance and pressure stability as it provides a smooth route for the incoming water to cool the hot spot. Moreover, the artificial neural network (ANN) prediction of heat transfer coefficient shows a good agreement with the experimental results as data fit within ±5% average error.

A general and rapid method to evaluate the effect of flow maldistribution on the performance of heat exchangers

The effect of non-uniform flow on thermal performance of heat exchanger was analyzed in this paper. The quantitative relationships between velocity deviation factor and heat transfer coefficient, heat transfer capacity, entransy-dissipation-based thermal resistance, were obtained, respectively. Based on these relationships, an evaluation plot was proposed to evaluate the effect of flow maldistribution on thermal performance. The plot was divided into three zones according to various degrees of deterioration. In Zone Ⅰ (stable zone), the variations of amplitude of heat transfer characteristics is negligible with the increase of the velocity deviation factor. In Zone Ⅱ (deterioration zone), the increasing velocity deviation factor results in thermal performance reducing in a modest rate. In Zone Ⅲ (rapid deterioration zone), thermal performance deteriorates rapidly with the increase of velocity deviation factor. The predicted results of the evaluation plot were compared with abundant simulations and experiments from references, which cover cases of variable velocity deviation factors, different flow distribution profiles and various types of heat exchangers. The excellent agreement of the comparisons has proved that the plot can be successfully applied to rapidly evaluate the effect of non-uniform flow on thermal performance of heat exchanger with high accuracy. Meanwhile, traditional researches on thermal performance of heat exchanger with uniform flow can be easily translated into that of non-uniform flow by this plot method.

Numerical investigation of the thermohydraulic characteristics of microchannel heat sinks using supercritical CO2 as a coolant

For compact electronic devices, high-pressure drops that characterize microchannel heat sinks (MCHS) is an issue that needs to be resolved to reduce the size of heat-removing systems. In this regard, supercritical carbon dioxide (sCO2) can be of great value due to its favorable thermophysical properties near the critical point. Thus, the potential of the proposed coolant (sCO2) for microchannel heat sinks (MCHS) is numerically investigated and compared with the conventional liquid coolant. Moreover, a header geometry with minimum flow maldistribution is designed for both coolants. Finally, the impact of various operating conditions on the thermal and hydraulic performance of the proposed sCO2-cooled MCHS is investigated to evaluate its optimal operating conditions. To accomplish this, a 3D Reynolds Averaged Navier–Stokes (RANS) model is developed to analyze the thermal and hydraulic performance of both coolants. Thermophysical properties of sCO2 are implemented through a high-resolution real gas property (RGP) file to capture abrupt variations in its properties and ensure the model's precision. Results suggest that replacing water with sCO2 can enhance the thermal performance of microchannel heat sinks by up to 32% at higher flow rates of the coolant, while pressure losses can be reduced by up to 7 times compared to water-cooled MCHS. Moreover, it is found that sCO2-cooled MCHS can maintain a steady base temperature, even during overload conditions. The proposed coolant can be used to design highly efficient and compact cooling systems with the capability of enhancing the MCHS overall performance by up to 2.2 times.

Effects of vortexes in feed header on air flow distribution of PEMFC stack: CFD simulation and optimization for better uniformity

Distribution uniformity is critical to achieve high performance for proton exchange membrane fuel cell (PEMFC) stacks. In this work, air flow distribution of a 30 kW “U-type” stack of 100 unit cells is investigated by computational fluid dynamics (CFD) simulation, Burgers vortex model is introduced to describe the mechanism of flow maldistribution. Three cases are presented to evaluate the effects of vortexes on the distribution uniformity under various average current densities, and obtain optimized design for better uniformity. The results show that the vortexes usually generate in the feed header, the flow vortex structures are governed by the feed header’s configuration. The distribution uniformity reduces dramatically with the increase of current density and the generation of vortexes. The cells that locate in the positions of vortexes get less flow allocation than other cells, distribution uniformity can improve by optimizing the feed header’s configuration to change the vortex structure of flow field.

Sub-Channel Analysis of a Hexagonal Sub-Assembly: Influence of Blockage and Axial Power Distribution on Critical Heat Flux

Abstract The study of thermal hydraulics of a hexagonal sub-assembly is essential to ensure the safe operation of liquid metal cooled fast reactors. Identifying the dryout location in fuel sub-assembly (FSA) is a precursor to the determination of safe Critical Heat Flux (CHF) margins. In this study, a sub-channel analysis code coupled with a film thickness model is employed to predict the CHF location in a hexagonal sub-assembly. A simple post-CHF heat transfer model is proposed and validated against the experimental data. The nature of flow resistance changes and operating conditions would significantly influence the occurrence of CHF. To this end, the effect of blockage (0.0 ≤ b ≤ 0.3) and axial power distribution (APD) on CHF is systematically investigated in a hexagonal sub-assembly. It was observed that the presence of blockage causes coolant flow maldistribution which results in an early occurrence of CHF for higher mass flux (G &amp;gt; 1500 kgm−2s−1) and lower inlet subcooling (ΔTsub ≤ 30 K) conditions for b = 0.3. Furthermore, a comparative study of uniform and sinusoidal heat flux distributions is performed. It was noticed that sinusoidal APD causes the early occurrence of CHF compared to uniform APD.

Measurement of flow maldistribution induced by the Ledinegg instability during boiling in thermally isolated parallel microchannels

Flow boiling in a network of heated parallel channels is prone to instabilities that can cause uneven flow distribution, thereby degrading the heat transfer performance of the system and limiting predictability. This study experimentally investigates flow maldistribution between two parallel microchannels that arises due to the Ledinegg instability. The channels are heated uniformly and are thermally isolated from each other, such that both channels are subjected to the same input power regardless of the flow distribution. The channels are hydrodynamically connected in parallel and deionized water is delivered at a constant total flow rate shared by both channels. Direct measurements of the flow rate, wall temperature, and pressure drop in individual channels are performed simultaneously with flow visualization. At low power levels, when both channels remain in the single-phase liquid regime, the flow is evenly distributed between the channels and they attain the same wall temperature. As the power is increased, boiling incipience in one of the channels triggers the Ledinegg instability, which causes the flow to become maldistributed and induces a temperature difference between the channels. The severity of flow maldistribution, as well as the temperature difference between the channels, grows with increasing power. In the most extreme condition measured in this study, 96.5% of the total flow rate is directed to the channel operating in the single-phase liquid regime, while the boiling channel is starved and receives just 3.5% of the flow. The quantitative account of the flow maldistribution and temperature non-uniformity presented here provides a mechanistic understanding of the effects of Ledinegg instability-induced flow maldistribution on the heat transfer characteristics of thermally isolated parallel microchannels.

Full-Field Comparison of MRV and CFD of Gas Flow through Regular Catalytic Monolithic Structures

Understanding the influence of gas flow maldistribution in honeycombs can be beneficial for the process design in various technical applications. Although recent studies have investigated the effect of maldistribution by comparing the results of numerical simulations with experimental measurements, an exhaustive 3D full-field comparison is still lacking. Such full-field comparisons are required to identify and eliminate possible limitations of numerical and experimental tools. For that purpose, spatially resolved flow patterns were simulated by computational fluid dynamics (CFD) and measured experimentally by non-invasive NMR velocimetry (MRV). While the latter might suffer from a misinterpretation of artefacts, the reliability of CFD is linked to correctly chosen boundary conditions. Here, a full-field numerical and experimental analysis of the gas flow within catalytic honeycombs is presented. The velocity field of thermally polarized methane gas was measured in a regular 3D-printed honeycomb and a commercial monolith using an optimized MRV pulse sequence to enhance the obtained signal-to-noise ratio. A second pulse sequence was used to show local flow propagators along the axial and radial direction of the honeycomb to quantify the contribution of diffusion to mass transport. A quantitative comparison of the axially averaged convective flow as determined by MRV and CFD shows a very good matching with an agreement of ±5% and 10% for printed and commercial samples, respectively. The impact of maldistribution on the gas flow pattern can be observed in both simulation and experiments, confirming the existence of an entrance effect. Gas displacement measurements, however, revealed that diffusive interchannel transport can also contribute to maldistribution, as was shown for the commercial sample. The good agreement between the simulation and experiments underpins the reliability of both methods for studying gas hydrodynamics within opaque monolith structures.

Analysis and improvement of flow distribution in manifold for proton exchange membrane fuel cell stacks

The flow distribution in manifold has significant effect on the output performance of the stack according to the Cannikin Law, especially for multi-cell proton exchange membrane fuel cell (PEMFC) stacks. This study presents an analytical model which simultaneously considered local pressure losses, pressure recovery phenomenon, electrochemical reactions, and liquid water to calculate the flow distribution using the flow network method. Both U-shape and Z-shape flow configuration stack are investigated. The analytical model is simultaneously and quantitatively validated with three distinct variables (the dimensionless mass flow rates, the stack pressure drop and the pressure distribution in manifold) to ensure the accurateness of modelling results. Effects of reactant consumption and two-phase flow in unit cell on the flow distribution are quantitatively investigated. Results show that the flow maldistribution is overestimated when ignoring gas consumption and two-phase flow for both 200-cell U-shape and Z-shape stack. Balance the pressure drop of inlet and outlet manifold significantly improves the flow distribution for U-shape stack. Finally, the differentiated assembly strategy is first proposed to improve the flow distribution in manifold for multi-cell PEMFC stacks. Compared to the original design, the ratios of the improvements for 200-cell U-shape and Z-shape stack are 85% and 25%, respectively.

Heat transfer characteristics of triple-stage impingement designs and their application for industrial gas turbine combustor liner cooling

This paper reports a triple-stage impingement concept with two optimized designs to tackle the common design challenges of the adverse effect of cross flow and coolant maldistribution on impingement cooling for modern gas turbine components. Flow structure and heat transfer characteristics were investigated at two different constant uniform wall temperatures with lower wall temperature related to lower wall to jet temperature ratio and the higher wall temperature related to higher wall to jet temperature ratio. The feasibility of application the triple-stage impingement cooling designs to a gas turbine combustor liner was conducted through Conjugate Heat Transfer analysis by applying a wall with realistic material thermal properties at two different constant wall heat fluxes. All studies were carried out at engine operation conditions with high air pressure and temperature. The results show that the triple-stage impingement cooling designs can reinitiate impingement jets at each stage, which greatly reduces the cross flow impact and local thermal gradient. The staging of cooling air for combustor cooling also offers better utilization of the cooling capacity. Even with 50% reduction in the cooling air consumption compared to conventional single-stage impingement design, both triple-stage impingement designs can still sufficiently cool the combustor liner. Both triple-stage impingement designs can be practically applied to gas turbine combustor liner cooling.

MOF-808 and its hollow fibre adsorbents for efficient diclofenac removal

• MOF-808 is studied as a highly effective adsorbent for diclofenac removal. • The maximum adsorption capacity of MOF-808 is found to be at 833 mg g −1 . • MOF-808 can remove almost 95% diclofenac within the first hour. • A MOF-808 supported adsorbent is fabricated with its high removal efficiency. • The supported adsorbent can be easily regenerated and reuse for at least 4 times. The popularity of diclofenac, one of the highly effective non-steroidal anti-inflammatory drugs (NSAIDs), has led to its excessive consumptions. Consequently, its presence in water bodies is continually increasing and poses serious threats to ecosystems. Adsorption processes have been employed to address this issue. However, most of the commercially available adsorbents such as activated carbon, zeolite, etc. suffer from relatively low diclofenac adsorption capacity (100–200 mg g −1 ) and slow kinetics. Herein, we explore the potential of MOF-808 as a superior adsorbent for diclofenac removal. Benefitting from its high specific surface area and large pore aperture, the maximum diclofenac adsorption capacity achieved in this study reaches 833 mg g −1 with almost 95% removal within first hour. This performance is one of the highest reported adsorbents for diclofenac removal so far and has led us to have further commercial exploitation by employing a yttria-stabilised zirconia (YSZ) hollow fibre support where the MOF-808 adsorbent is deposited on its outer surface. Such a configuration is beneficial from a practical point-of-view, as it eliminates the requirement of separation of powdered adsorbents from treated water once the adsorption process finishes. Also, the hollow fibre configuration not only substantially increases the surface-area-to-volume ratio of adsorption systems, but also reduces the maldistribution of flows commonly unavoidable in conventional packed bed adsorption columns. The high-efficiency of the MOF-808 hollow fibre adsorbent is exhibited by retainment of its equilibrium adsorption capacity of 796 mg g −1 . More importantly, the MOF-808 hollow fibre adsorbent can withstand at least 4 adsorption-regeneration cycles without any significant reduction in the adsorption performances, indicating the potential for possible commercialisation of the MOF-808 adsorbent for diclofenac removal from water.

Single-phase and Two-phase Flow and Heat Transfer in Microchannel Heat Sink with Various Manifold Arrangements

The manifold microchannel (MMC) heat sink, which possesses better heat transfer performance and less pressure drop than the traditional microchannel heat sink, has attracted much attention from researchers in recent years. But not much of them, either single-phase or two-phase ones, focused on the problems about flow maldistribution and wall temperature nonuniformity that might ultimately lead to system breakdown. To investigate the flow distribution in the MMC heat sink, six types of manifold, namely Z-type, C-type, H-type, U-type, ZU-type, and HU-type are considered in the current study. All six manifolds are investigated in single-phase cases with water, while the latter three are adopted in two-phase cases with HFE7100. The cases are solved with the self-developed codes based on the OpenFOAM package. The manifold arrangement has a strong impact on the overall heat transfer performance and pumping power required. The HU-type and ZU-type MMC heat sinks are recommended to be fabricated in the single-phase heat transfer application due to their small thermal resistance lower than 2×10−5 m2·K/W and pressure drop less than 500 Pa. Besides, if the pumping power is not a restriction factor in the system, Z-type MMC is also recommended. In the two-phase cases, the flow distribution is naturally not uniform within a parallel microchannel system, and the microchannels with large flow velocity appear to have higher wall temperatures. The authors recommend using HU-type MMC heat sink in the flow boiling application due to its bottom wall temperature distribution is relatively uniform with a difference of less than 2 K.

Two-phase flow distribution in a refrigerant distributor having four indoor-unit connections of a variable refrigerant flow system

• Multiple indoor units are connected to a refrigerant distributor of a VRF system. • Distribution of two-phase flow was experimentally and numerically investigated. • Maldistribution of two-phase flow is significantly influenced by void fraction. • CFD simulation was performed to estimate distribution of R410A. • Maldistribution of R410A appeared to be less than maldistribution of air-water. In a variable refrigerant flow (VRF) system, the refrigerant is supplied from one outdoor unit to multiple indoor units through a header or refrigerant distributor. When the two-phase refrigerant flows into the distributor, uneven distribution occurs and deteriorate system performance. An experimental apparatus simulating the refrigerant distributor of a VRF system was constructed and air-water two-phase flow distribution characteristics were investigated. The air-water two-phase flow was more mal-distributed as the void fraction increased. A change in void fraction affected the distribution more than a change in mass flow rate in the experimental range. The flow distribution in the header was little affected by variation of the downstream flow resistance. Previous empirical correlations were investigated to predict the air-water two-phase flow distribution measured in this experiment, but performance was poor. To estimate the refrigerant two-phase flow distribution, computational fluid dynamics (CFD) simulations were performed using R410A as a working fluid. The mal-distribution of R410A appeared to be mitigated compared with air-water flow due to the large vapor phase density of R410A.

Pressure Drop Oscillations With Symmetry Breakdown in Two-Phase Flow Parallel Channels

Abstract Two-phase flow in parallel heated channels is prone to symmetry breakdown resulting in mass flow maldistribution. Moreover, in the presence of compressible volume (CV), such systems also undergo pressure drop oscillations (PDOs). The performances of such systems depend on the effect of these flow instabilities. However, the simultaneous occurrence of these two-phenomena has been rarely reported in the literature. In this work, an approach is applied in a two-channel system to demarcate the parameter space of mass flow rate and inlet temperature into several areas, where these two phenomena take place. The loss in the symmetry in the flow rate is observed as the mass flow rate is varied, which leads to flow maldistribution. The PDO are also observed for specific values of mass flow rate in the system. One unique feature of the parallel channel system is the existence of the oscillatory and stable (albeit asymmetric) states at the same parameter values. For these parameter values, the final state of the system is dependent on the type of initial disturbance. The flow maldistribution due to symmetry breakdown is identified by the pitchfork bifurcation, and oscillations of mass flow rate are identified by the presence of Hopf bifurcation. Moreover, the physical interpretation of the different phenomena in the system is carried out using internal and external pressure drop characteristics curves.