Effect Of Maldistribution Research Articles

Effect of liquid-vapor two-phase flow maldistribution on the thermal performance of brazed plate heat exchangers

The effect of liquid-vapor two-phase flow maldistribution on the thermal performance of brazed plate heat exchangers (BPHEs) is investigated experimentally and numerically. The two-phase refrigerant flow distributions among different channels of the BPHEs are first experimentally quantified based on the infrared radiation images of the heat exchanger sidewalls. To assess the performance degradation due to two-phase flow maldistribution, the thermal performance of the maldistributed BPHEs is measured and compared with that of the uniform-distributed BPHEs, predicted using an experimentally validated BPHE model. Several factors that affect the performance degradation are analyzed, including refrigerant inlet vapor quality and flow rate, exit superheat, and water-side flow distribution. The results show that, under the tested conditions, the two-phase refrigerant flow maldistribution causes up to a 29.8 % degradation in heat exchanger capacity and a 4 K decrease in evaporation temperature. Furthermore, the vulnerability of the BPHEs to two-phase flow maldistribution is significantly influenced by the operating conditions. This is closely related to the varying size ratios of superheated and two-phase regions within the BPHEs under different operating conditions. Reducing the refrigerant exit superheat is verified to help overcome the adverse effect brought by the flow maldistribution in the evaporators, but brings challenges in the system control. Matching the flow distributions of liquid refrigerant and water also proves beneficial to the BPHE evaporators.

Non-equilibrium overlapping grid method with two-phase porous media model for printed circuit heat exchanger based steam generator

The printed circuit heat exchanger (PCHE) is a promising alternative for the once-through steam generator (OTSG) of small modular reactors due to its compact structure and high efficiency. To study the effect of flow maldistribution on the performance of heat transfer and thermal stress, it is necessary to conduct three-dimensional full-scale simulation of two-side fluid and solid domains in the printed circuit heat exchanger based steam genarator (PCHESG). However, there is a lack of mature thermal-hydraulic models for the PCHESG. This work proposes a non-equilibrium overlapping grid method with two-phase porous media model for the PCHESG. The temperature and flow fields of the PCHESG under high-pressure conditions are evaluated. Compared to traditional methods, the new method greatly reduces the order of magnitude of the grid number from 107∼109 to 106 and the computational time from days to hours. It is found that the flow maldistribution with a threshold cold-side mass flow rate of 0.08∼0.0944 kg/s induced by the header parts of the PCHESG deteriorates the total heat transfer rate by 58.8 % and increases the spatial non-uniformity of the temperature and phase distributions. The solid domain is solved simultaneously and its temperature non-uniformity becomes significant with a threshold cold-side mass flow rate of 0.04∼0.06 kg/s. The results demonstrate that this method has great potential in achieving rapid design and optimization for the PCHESG.

Evaluation of plate heat exchangers comprising sections with different chevron angle arrangements

Gasketed Plate Heat Exchangers (GPHEs) are essential equipment for thermal control of oil facilities owing to their compactness and well-organized assembly concept. In order to satisfy heat loads of several megawatts, numerous and large plates are necessary. The application of GPHEs with plentiful plates yields flow maldistribution which affects GPHE thermal–hydraulic performance. With the purpose of matching different heat loads and processing conditions or as an alternative to reduce maldistribution effects, GPHEs with mixed arrangements, comprising sections where channels contain different geometrical features, may be selected. This works addresses the evaluation of flow and pressure distributions in GPHEs with mixed arrangements. The pressure drop in each GPHE section is initially approximated by the pressure drop in parallel pipes, neglecting the pressure drop in the ports. With this assumption, the inlet flow rate is divided into two different parts. The pressure distributions in each section are assumed to maintain flow features of m2-model for U-type arrangement. The friction factor for each GPHE segment was determined with independent experiments including only one section type and with a small number of channels as to avoid flow maldistribution. Friction factor correlations were determined for single section GPHEs where each channel can be formed with plates containing different chevron angles: 30°/30°, 60°/60°, 66°/66°, 30°/60° and 30°/66°, with channel Reynolds number ranging from 260 to 3,080. Subsequently, experiments with GPHEs containing mixed sections with high and low pressure drop channels were performed: 60°/60° and 30°/30°, 30°/30° and 66°/66°; with total number of channels up to 180. The channel pressure drop was measured in regularly spaced locations. The pressure drop in each GPHE segment has been predicted with the fractional RMS deviation equal to 2.2%. Although the mass flow rate per channel has not been measured, it is expected that the predicted flow rate distribution can reasonably stand for the real one and, therefore, assisting in proper thermal performance evaluation.

Process Intensification of Electrodialysis through the Investigation and Elimination of Maldistribution

Often, advancements in electrochemical engineering processes stem from improvements at the microscale and in fundamental understanding. However, there is often much insight to be gained from identifying engineering challenges associated with process operation. Electrodialysis (ED) is an emerging electro separation technology which utilises a stack of ion selective membranes and an electric field to drive salt transfer between streams. Industrial implementation and intensification of ED is currently hindered by operational efficiencies being far from ideal. In this work, we explore a system-wide approach to investigate operational inefficiencies resulting from maldistribution in the flow between ED channels.Through computational fluid dynamics (CFD) simulations, it was shown that maldistribution is prevalent in ED. Further, a simplified analytical model demonstrated that the degree of maldistribution will be far more severe upon scale-up. In order to examine the effects of maldistribution on ED performance, transport models were built. These demonstrated that the limiting current density, and thus the maximum salt flux, was reduced by as much as 23% for a standard lab-scale stack. This will be much more significant upon scaleup, and therefore, overcoming maldistribution in ED can have significant benefits for process intensification. Validation of this was conducted through particle image velocimetry (PIV) and desalination experiments.A glass cell was constructed for PIV experiments with an identical geometry to that used for CFD simulations. A camera and telecentric lens was focussed on the channel centreline, and red and blue LEDs were sequentially pulsed. This light was scattered off particles in water flowing through the cell and collected by a single exposure image. The particle velocities were calculated from the superimposed red and blue images and a flow field was constructed. The experimentally determined flow field had excellent agreement with that found from CFD simulations, validating the existence of maldistribution in ED.Desalination experiments were conducted on a PC64004 ED stack operating in a steady state mode. The degree of maldistribution was independently altered by changing the flow rate while ensuring that the overall salt convective flux was constant. The LCD was subsequently measured by collecting current-voltage curves. results showed excellent consistency with the transport model, thereby validating the relationship between increased maldistribution and a lower LCD.In summary, in this work we have demonstrated and validated the presence of maldistribution in ED, as well as the significant detrimental effect it has on the upper limits of process intensification. Further, we have demonstrated the benefits of investigating system-wide inefficiencies in addition to fundamental phenomena. Future research will focus on developing methods to overcome maldistribution in ED. Figure 1

Effect of flow maldistribution on heat transfer performance and temperature field of plate-fin heat exchangers

Plate-fin heat exchangers (PFHEs) are widely utilized in aviation precoolers, emphasizing not only heat transfer performance but also the outlet temperature distribution, with flow maldistribution impacting both parameters. This study developed a numerical model of crossflow PFHE and a two-dimensional flow distribution model under the condition of flow maldistribution. Experimental validation shows a standard deviation of 6.9%. This investigation delves into the effects of flow distribution offset (ξ) and non-uniformity (ε) on heat transfer performance and outlet temperature distribution. With large temperature differences, variations in fluid properties due to temperature lead to changes in the surface heat transfer coefficient (SHTC). The combined effects of SHTC and temperature difference determine the heat transfer performance. The results indicate optimal heat transfer performance and outlet cross-sectional temperature distribution at ξ= − 0.67 and 1, respectively. For a given ξ, increased non-uniformity deteriorates heat transfer efficiency and outlet cross-sectional temperature distribution. At ε=0.66, relative to the uniform flow, the heat transfer performance and standard deviation of outlet temperature distribution decrease by 10.3% and 45.1%, respectively.

Experimental study on flow boiling-based micro-channel configurations for the PEMFC cooling

Proton Exchange Membrane Fuel Cell (PEMFC) is one of the most promising energy conversion devices. Efficient thermal management is crucial for maintaining the PEMFC's efficiency and durability. However, cooling the PEMFC can be challenging due to the small temperature differences between the stack's operating temperature and the surrounding environment. In the present study, flow boiling is applied as a cooling technique for the PEMFC. A flow boiling experimental test bench with fluid flow visualization is built for PEMFC cooling. To relieve the negative effect of mal-distribution of working fluid in the micro-channels, the cooling configurations with orifice baffles are also tested. Flow boiling performance of HFE-7100 in the micro-channel-based cooling configuration for PEMFC is experimentally investigated. The flow boiling characteristics of HFE-7100 in the micro-channel are revealed, which is beneficial for the appropriate design of flow boiling-based cooling configuration for the PEMFC. Excellent temperature distribution uniformity of flow boiling-based cooling PEMFC is achieved and is further enhanced by adding orifice baffles.

Numerical investigation on the thermal performance of cross‐cocurrent and countercurrent three fluid heat exchanger with flow maldistribution at the inlet

AbstractFlow maldistribution at the inlet of a heat exchanger (HX) is a significant parameter that needs to be considered to judge the performance of the same. In this paper, four arrangements of three‐fluid cross‐flow heat exchanger (3FCFHX) with different flow conditions are considered as shown in Figure 1 in which is the performance of cross‐countercurrent and cross‐cocurrent arrangements for uniform and flow maldistribution at the inlet. In addition, the effect of different inlet fluid flow models on the HX performance are investigated numerically. Among the three fluids in these arrangements, the central fluid is considered to be the hot fluid. From the principles of conservation of energy, the governing equations for three fluids are generated and solved using the finite element method. Four different inlet fluid‐flow models are considered for the analysis. Performance is judged using hot fluid effectiveness and the number of transfer units for a different range of governing parameters. The effects of inlet flow maldistribution (IFM) are measured using degradation factors. The results show that the performance of cross‐cocurrent arrangement is found to be superior to the cross‐countercurrent arrangement. In addition, the IFM enhances thermal performance. Further, it is determined that the flow maldistribution at the inlet will enhance the hot fluid effectiveness by 4%–4.5% and 1.8%–2% in cross‐cocurrent and countercurrent arrangements, respectively. The results give a thorough insight into the significant concerns involved in the design of such HXs. Application of the finite element method proves the ease of determining the exit temperatures of the fluids in the HX. This approach is indeed time‐saving and gives insights when compared to the CFD approach.

Effect of single-phase flow maldistribution on the thermal performance of brazed plate heat exchangers

This paper presents an experimental and numerical investigation of the effect of single-phase flow maldistribution on the thermal performance of brazed plate heat exchangers (BPHEs). A thermal performance model of the BPHEs is developed with the consideration of flow maldistribution among the channels. The proposed model is validated against the experimental results. The model simulations reveal that the flow maldistribution has a trivial impact on the overall thermal capacity of the BPHEs if two fluid streams are supplied from the same side of the heat exchangers; if two fluid streams are supplied from the opposite side, the overall thermal performance is significantly deteriorated due to mismatched flow distributions of two fluid streams. Moreover, other factors are also proven to act in this issue, including the number of plates, plate length, header/port size, sudden expansion flow at the heat exchanger entrance, and heat capacity rate ratio of two streams. They either directly change the flow distribution or potentially affect the thermal effectiveness of the BPHEs. Based on the simulation results, correlations are generated, which can evaluate the thermal capacity degradation due to flow maldistribution without the necessity of knowing the detailed flow distribution and building heat exchanger models.

A coupled combustion and hydrodynamic model for the prediction of waterwall tube overheating of supercritical boiler

Tube overheating is the leading cause of boiler tube failures that had led to tremendous replacement and maintenance costs. Boiler tube temperature is simultaneously affected by the heating effect of hot combustion gas in the furnace and the cooling effect of steam flow in boiler tubes. Thus, prediction of tube temperature needs to include the simulations of both the gas and steam flows of the boiler in the model. To compromise the significant scale difference between the boiler furnace and tubes, a coupled model that integrates a three-dimensional (3D) CFD model describing the gas flow and combustion processes in the furnace with a one-dimensional (1D) hydrodynamic model describing the steam flow in the boiler waterwall tubes was developed in this paper. The 3D CFD and 1D hydrodynamic models are coupled by iterative data exchange of boiler heat flux and steam temperature between the two models. In this way, this coupled model can incorporate all the key parameters that strongly affect the waterwall tube temperature, including the distributions of boiler heat flux, steam temperature and the convective heat transfer coefficient of steam flow in boiler tubes. In particular, the critical effects of steam maldistribution caused by both the structure of tube network system and uneven heat absorption of parallel tubes on tube temperature can be resolved by this model. This coupled model provides the capability to predict the detailed distribution of waterwall tube temperature and locate the areas of tube overheating under different boiler operating parameters. This coupled model was employed to predict the tube temperature distribution of the spiral waterwall of a 350 MW supercritical boiler. The predicted results show that the areas having higher risk of tube overheating are located near the outlet of spiral waterwall tubes where the steam temperature is the highest and on the wall areas where the local wall heat flux is the highest. The results strengthen the necessity of incorporating both the gas and steam sides of boiler into the model for accurate prediction of waterwall tube temperature.

Transient Flow Uniformity Evolution in Realistic Exhaust Gas Aftertreatment Systems Using 3D-CFD

To precisely control a vehicle powertrain to minimize emissions, accurate and detailed models are needed to capture the spatio-temporal variability of the variables of interest. The aim of this work is to analyze flow and temperature fields in a geometrically realistic — and thus complex — exhaust gas aftertreatment system under transient conditions. The spatio-temporal response of these fields to upstream step changes is predicted using three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) kappa - omega simulations where the catalytic converter is described as a porous medium. A catalytic converter geometry with a 90^{circ }-bend and a partially dead volume is used to demonstrate the effects of time-resolved flow maldistribution on the profiles of velocity and temperature. Two sets of transient simulations in terms of step changes in velocity and temperature are performed. Uniformity indices are used to characterize the distribution and variability of the different catalyst channels under transient conditions. The evolution of the uniformity indices as functions of time and axial distance into the catalyst are calculated at different cross-sectional planes. The results show that the evolution of the temperature uniformity is rate controlling, continuously modulating the otherwise much faster flow uniformity response via the fluid properties. The temperature uniformity time scale is determined by the balance of flow, thermal inertia, and the heat losses from the system. The interplay between pressure drop and heat losses governs the transition to the new steady state in uniformity. These types of transient simulations and analyses can contribute essential information when developing reduced-order engineering models to represent the spatio-temporal variability in exhaust aftertreatment systems, in particular during rapid events such as cold start.

Flow maldistribution and heat transfer characteristics in plate and shell heat exchangers

• The pressure drop, flow maldistribution, and heat transfer rate are measured in a plate and shell heat exchanger. • Friction factor and Nusselt number correlations are provided for water and oil flows. • The plate side showed a better thermal performance than the shell side but a higher friction factor. • Flow maldistribution was more severe on the shell side than on the plate side. • The effectiveness deterioration due to the flow maldistribution was less than 4%. • An analytical comparison indicates that PHEs have a better thermo-hydraulic performance than the PSHE studied. The plate and shell heat exchanger (PSHE) was developed to overcome the traditional gasketed plate heat exchangers (PHE) operation limits. Its constructive characteristics allow higher-pressure and thermal applications, suitable for processes found in power plants and the oil and gas industry. However, studies on the PSHE thermo-hydraulic performance are still scarce. This study presents an experimental and theoretical analysis of the flow and heat transfer characteristics of a PSHE. A test rig operates with water and viscous oil to produce turbulent and laminar flow regimes, typical of the oil and gas industry. The heat transfer rate, pressure drop, and flow distribution are measured. The m 2 -model, developed to predict flow maldistribution on the PHE, is suitable for the plate side of the PSHE. An analytical model is used to correct the effects of maldistribution on the overall heat transfer coefficient and provide Nusselt number correlations. Experiments show that the flow maldistribution increases the heat exchanger pressure drop and deteriorates the heat transfer performance. The maximum and average channel flow rate ratio reaches 2.30 on the shell side and 1.75 on the plate side. Friction factor correlations are created based on the channel pressure drop data. The shell side has an inferior overall performance than the plate side, with a higher maldistribution and a lower Nusselt number. The PSHE effectiveness deterioration due to the maldistribution is 4% in the worst scenario within the experimental range. Results indicate that the PHE thermo-hydraulic performance is superior to the PSHE. Nevertheless, the structural advantages of the PSHE make it appropriate for applications involving high pressures and elevated temperatures.

Analysis of Flow Maldistribution in Compact Heat Exchangers

Abstract Most theoretical models related to thermal performance and the evaluation of pressure drop on compact heat exchangers, consider that the mass flow rate is constant and equal in all core channels. However, this hypothesis in some situations is erroneous, the effect of fluid maldistribution affects heat exchangers performance. Therefore, in the present work, an analysis of the phenomenon of fluid maldistribution in headers was performed. Experimental tests were carried out on polymeric headers, using Polyamide 2200, in order to evaluate the fluid nonuniformity. The tests were carried out with compressed air at room temperature in a wide range of mass flow rate totaling 300 experimental data. A theoretical model was used to predict the fluid maldistribution level inside the header. Based on the coefficient of variation (CoV), the presence of fluid maldistribution for low flows was verified. Besides, a new theoretical model, the standard deviation of fluid nonuniformity (σ), based on the shape factor (SF), was presented to measure maldistribution level without experimental tests. The proposed model proved to be efficient to predict the fluid nonuniformity inside the header, presenting an average error of 18% in comparison to the CoV.

Effect of asymmetric fluid flow distribution on flow boiling in a microchannel heat sink – An experimental investigation

Flow boiling in microchannels is emerging as an exclusive cooling solution for miniaturized high-power electronic devices alongside having other high heat flux applications. Size miniaturization at microscale strangely increases heat transfer performance as well as flow boiling instabilities. Many flow boiling instabilities are interrelated and result from imperfect hydrodynamic conditions. One of such problems is flow maldistribution among parallel channels of a microchannel heat sink. Very limited studies have dedicatedly investigated the negative effects of flow maldistribution on the boiling process in microchannels. A microchannel heat sink with twenty-five rectangular microchannels (width × height × length = 0.45 × 0.725 × 25 mm) made on a copper block base of 25 × 25 × 85 mm using wire electrical discharge machining under an I-type flow configuration is investigated for that purpose. Flow boiling patterns of central and side microchannels as well as the temperature profile of central and side microchannels are recorded. The boiling process always incepts in side microchannels and rapidly converts into a periodic flow reversal up to the inlet manifold, whereas weak, stable, bubbly flow and single-phase liquid flow are observed in the neighboring and central microchannels, respectively. Furthermore, with the increase of heat flux, flow boiling intensity increases and more parallel microchannels start experiencing rapid bubble growth; consequently, the intensity of flow reversal in side channels also increases. At high heat fluxes, the vapor backflow of side microchannels reaches the central microchannel and blocks the flow through it, named mirage flow confinement. Boiling in the central channel aggravates under the influence of the mirage flow reversal processes. Temperature non-uniformity across the microchannel heat sink increases with the increase of heat flux and mass flux caused by the early appearance of a partial dryout of side channels and the escalation of the flow distribution asymmetry. Whereas, better temperature distribution is observed at higher inlet fluid temperatures.

Barriers to electrodialysis implementation: Maldistribution and its impact on resistance and limiting current density

Electrodialysis is an emerging low-energy membrane-based water-treatment technology with strong potential for industrial use. However, various engineering challenges have prevented widespread utilisation. The flow maldistribution in electrodialysis has been evaluated through computational fluid dynamics simulations in Ansys Fluent. A typical lab-scale stack geometry with ten cell pairs was simulated, and significant flow non-uniformity was found. An analytical model showed good agreement with simulations and was used to quantify the maldistribution through a single dimensionless number. The inlet flow rate and stack geometry were varied to investigate the distribution of flow. Maldistribution was exacerbated when the inlet flow rate, channel width, distributor angle, or number of cell pairs was increased. Further, the effect of maldistribution on the limiting current density (LCD) and stack resistance was evaluated using a one-dimensional model. Maldistribution was found to significantly impact the LCD, with a reduction of 23% found for the typical geometry relative to uniformly distributed flow. The resistance was impacted much less, showing an increase in resistance of only 2%. This highlights that maldistribution is a localised issue rather than a global one and goes some way to explaining why there has been a distinct lack of research into flow maldistribution in electrodialysis.

Transient Behavior and Maldistribution of Two-Phase Flow in Parallel Channels

Compared with a single channel of equivalent volume, parallel channels provide a larger surface area for heat transfer and are, therefore, a common design feature in two-phase heat exchangers. However, flows through parallel channels often encounter uneven flow distribution, resulting in the deterioration of thermal performance, leading to permanent device failure. Therefore, there is a need to understand the process and nature of flow maldistribution in parallel channels. This study investigates factors that influence the process of flow maldistribution in two parallel channels with the same inlet and outlet manifolds. Both model predictions and experimental characterization show significant flow maldistribution (flow ratio >10) when geometrically similar parallel channels (diameter ~1 mm, and length ~0.1 m) encounter disturbances such as sudden variations in heat load (e.g., 50% increase) and total flow rate (e.g., 50% decrease). Due to flow maldistribution, the resulting temperature distribution in the parallel channels can be drastically different. Besides changes in operating conditions, other factors influencing flow distribution include channel geometry and thermophysical properties of channel wall material. Although the wall properties do not affect the individual channel pressure drop curve at steady state, they influence the transient process that leads to flow maldistribution in parallel channels. Furthermore, we found that the predicted flow distribution assuming a radial lumped analysis can differ from models accounting for heat diffusion in the radial direction. We believe these observations are crucial in understanding the effect of flow maldistribution on parallel channel evaporators’ performance and can be used as a guideline to develop better heat sinks for electronics thermal management.

Heat transfer enhancement using non-equally structure in a plate-fin heat exchanger with offset fins

In this study, a cross-flow plate-fin heat exchanger with offset fins is optimized by considering the effects of flow maldistribution for air side. For this purpose, the study is focused on an increase in the rate of heat transfer, which can be achieved by using non-equally fin structure. Numerical simulations have been carried out to investigate the thermodynamic characteristics of the non-equally full-size plate-fin heat exchanger by using the porous media approach. Based on the numerical model, flow distribution, total heat rate and pressure drop of the plate-fin heat exchanger are studied. A asymmetric structure with heat transfer enhancement is presented in this study. After comparing numerical predictions of the total heat transfer rate and the pressure drop under various Reynolds number. It is observed that, the percentages of increase in effectiveness for the final non-equally structure are in the range of 2.5-6.2% and the pressure drop remains almost constant in the cases of air inlet velocity fixed at 9.5627 m/s. The increasing asymmetric structure is numerically verified to improve the flow distribution of the plate-fin heat exchanger.

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.

CFD investigation of heat and moisture recovery from air with membrane heat exchanger

• Proposing a newly designed channelized plate module for air-to-air enthalpy exchanger. • Independent behavior of co-current system due to the small zone involved in transfer. • Investigation of the simultaneous effects of maldistribution, flow rate and residence time. • Superiority of multi-channel to single channel exchanger at flow rates lower than 10 m 3 /h. • Lower effectiveness and higher energy recovery and water emission at larger flow rates. Enthalpy exchangers using hydrophile membrane cores, are attractive devices to replace regular heat exchangers in ventilating systems. Among the various experimental and numerical works carried out in air-to-air exchangers, the present study is carried out to numerically investigate a plate module enthalpy exchanger channeled with separator blades. The simulation is conducted by the development of a user-defined function (UDF) using a CFD tool. Sensible and latent effectiveness, water emission rate, energy recovery, and pressure drop are the performance parameters investigated depending on different system characteristics and operating conditions, including the number of channels on each side, air flow rate, and flow configuration. In the co-current design, due to the small zone involved in mass and heat transfer, channeling and the change in flow rate do not affect the effectiveness. In the counterflow design, 10 m 3 /h was the highest flow rate, which showed the multi-channel system's superiority. Channeling was not beneficial at higher flow rates due to a large number of stagnation points and maldistribution caused by the separator blades. However, the comparison at the same velocity and Re number leads to the multi-channel system's superiority due to the longer residence time.

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.