Flow Maldistribution Research Articles

Numerical and Experimental Study of Microchannel Performance on Flow Maldistribution.

Miniaturized heat exchangers are well known for their superior heat transfer capabilities in comparison to macro-scale devices. While in standard microchannel systems the improved performance is provided by miniaturized distances and very small hydraulic diameters, another approach can also be followed, namely, the generation of local turbulences. Localized turbulence enhances the heat exchanger performance in any channel or tube, but also includes an increased pressure loss. Shifting the critical Reynolds number to a lower value by introducing perturbators controls pressure losses and improves thermal efficiency to a considerable extent. The objective of this paper is to investigate in detail collector performance based on reduced-order modelling and validate the numerical model based on experimental observations of flow maldistribution and pressure losses. Two different types of perturbators, Wire-net and S-shape, were analyzed. For the former, a metallic wire mesh was inserted in the flow passages (hot and cold gas flow) to ensure stiffness and enhance microchannel efficiency. The wire-net perturbators were replaced using an S-shaped perturbator model for a comparative study in the second case mentioned above. An optimum mass flow rate could be found when the thermal efficiency reaches a maximum. Investigation of collectors with different microchannel configurations (s-shaped, wire-net and plane channels) showed that mass flow rate deviation decreases with an increase in microchannel resistance. The recirculation zones in the cylindrical collectors also changed the maldistribution pattern. From experiments, it could be observed that microchannels with S-shaped perturbators shifted the onset of turbulent transition to lower Reynolds number values. Experimental studies on pressure losses showed that the pressure losses obtained from numerical studies were in good agreement with the experiments (<4%).

Improvement of Flow Distribution by New Inlet Header Configuration with Splitter Plates for Plate-Fin Heat Exchanger

The pressure drop increases along with the decrease in the flow maldistribution in the two-stage distributing inlet headers of a plate-fin heat exchanger. To solve this issue, we proposed a new inlet header configuration with splitter plates to decrease the flow maldistribution and pressure drop simultaneously. We used computational fluid dynamic technology to analyze the flow properties in the inlet header configuration and calculated the flow maldistribution degree of the outlet channels and the pressure drop in the inlet header. We performed a sensitivity analysis between the structural parameters, the flow maldistribution degree, and the pressure drop. We determined the optimum values of the structural parameters (the construction of transition duct, the number of splitter plates, the inclined angle of outermost plates, the height of splitter plates, the height of inlet header, etc.), and acquired the optimum configuration of the improved inlet header. Compared with the traditional inlet header, the flow maldistribution degree and pressure drop decreased by 91.5% and 40.9%, respectively, using the proposed improved inlet header with splitter plates, which performed better than the conventional two-stage distribution inlet header.

Numerical Simulation of Steam Flow Inside the Superheater Section of An Industrial Boiler Using a Real Gas Model

Abstract Flow mal-distribution inside manifolds hampers the overall efficiency of processes in industries. In supercritical boilers, improper flow of steam inside the superheater (SH) section is a common cause of thermal accidents. However, carrying out a numerical simulation of supercritical fluids flowing inside manifolds is challenging as the ideal gas law does not describe the behavior of these fluids properly. In the present work, numerical simulation of the flow of supercritical steam inside the superheater section of an industrial boiler has been performed using a real gas model. The proposed real gas model is first validated with experimental data associated with the steam properties. Subsequently, the effect of different inlet and outlet arrangements on the flow mal-distribution of steam in the superheater section of the boiler is investigated numerically using the real gas model. A modified inlet and outlet arrangement of the superheater header is proposed which reduces the maximum value of flow mal-distribution in the header by 19.7% and total pressure drop in the domain by 17%. The effect of the Reynolds number on flow mal-distribution in the header arrangement is found to be negligible. The absolute value of the heat absorption by the superheater tubes increases with an increase in the value of the Reynolds number.

Myocardial Necrosis in ICU Patients with Severe Hemodynamic Disturbances

Abstract Background this prospective comparative observational study, to compare the incidence of myocardial injury in patients with hypovolemic shock versus septic shock, after approval of Ethical committee of critical care department on forty patients in general ICU divided into two groups of twenty patients each equal. Aim of the Work to compare the incidence of myocardial injury in patients with hypovolemic shock versus septic shock. Patients and Methods this prospective comparative observational study, carried out in Maadi Military Force Hospital, to compare the incidence of myocardial injury in patients with hypovolemic shock versus septic shock, after approval of Ethical committee of critical care department on forty patients in general ICU divided into two groups of twenty patients each equal. Results group A has higher values in SOFA score than in Group B, most probably due to toxins affected perfusion of organs and response to resuscitation, beside the poor cellular oxygen utilization, so clinical outcome among the two groups was twenty-four patients died in (sixteen patients in Group A and eight patients in Group B), so in our study there was a higher mortality in Group A patients (Septic shock) than Group B patients (Hypovolemic shock), The deceased patients had higher serum cTnI levels than survivors, this was concordant with Arlati and his colleagues, but there were no significant differences in cardiac troponin. Conclusion the incidence of myocardial infarction is increased in Group A (septic patients) more than Group B (hypovolemic patients), most probably due to the maldistribution of coronary blood flow which might produce, for a given level of hypotension, higher amounts of myocardial damage and, consequently, higher levels of troponin I than in hypovolemic shock.

Prediction of flow maldistribution in printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) is widely used in a variety of extremely high temperature and high pressure applications. Since the PCHE is a typical multi-channel heat exchanger, the flow maldistribution may happen in the PCHE channels. In this paper, a new mathematical model is established to predict the flow maldistribution in a PCHE and it demonstrates that the traditional simplified numerical model with shortened heat transfer channels has a large error in predicting the flow maldistribution of a PCHE. Inlet and outlet local hydraulic losses are essential reasons to affect the flow maldistribution. An increase in the channel length improves the flow uniformity, while the flow uniformity becomes worse when the flow rate increases. When the channel length exceeds 500 times the hydraulic diameter of channel, the flow non-uniformity can be neglected at 0.12 kg/s. When the Reynolds number is smaller than 392, the flow non-uniformity can be neglected at the channel length of 0.1 m. Finally, with the aim of helping engineers make a quick judgement whether the flow maldistribution in a PCHE needs to consider or not, the relationship between the geometrical dimensions and operating conditions of a PCHE is proposed.

Experimental and numerical study on the performance of a minichannel heat sink with different header geometries using nanofluids

Inefficient distributor and collector header design in a minichannel heat sink causes flow maldistribution which can severely degrade the thermal and hydraulic performance of the heat sink. This paper presents an experimental and numerical investigation on the thermohydraulic performance of minichannel heat sinks with two different header geometries using nanofluids. A series of tests are conducted on a minichannel heat sink with optimized and conventional header geometries by using (Al2O3-H2O) nanofluids and distilled water as coolants. The effects of header geometry, nanoparticle concentration, and coolant flow rate on the overall heat transfer coefficient, heat transfer enhancement, thermal resistance, and base temperature are investigated. The experimental and numerical results exhibited good agreement, indicating that the thermohydraulic performance of the heat sink with the optimized header geometry is superior to that with the conventional header geometry. The minichannel heat sink with the optimized header geometry exhibited 17% higher overall heat transfer coefficient and 43% reduction in pressure drop while achieving lower values for base temperature and thermal resistance for each employed flow rate and volumetric concentration of nanoparticles. Furthermore, the performance evaluation criteria (PEC) indicated a 41% improvement in hydraulic performance while using minichannel heat sink with optimized header geometry compared to that with the conventional header geometry.

Coupled simulation and validation of a utility-scale pulverized coal-fired boiler radiant final-stage superheater

The paper presents a coupled modelling methodology that employs a three-dimensional finite volume method computational fluid dynamics model to solve the boiler flue gas side, and a 1D FVM network approach to model the steam side flow and heat transfer of a final-stage radiant superheater. The CFD model includes the necessary upstream components that affect the thermal performance of the superheater, including combustion and heat transfer in the furnace and platen superheater. The methodology is applied to a 620MWe drum-type boiler of which the superheater was instrumented with thermocouples measuring the outlet steam temperatures on the different elements. This was augmented with boiler process data that was extracted from the distributed control system (DCS) for steady-state operation at 99% and 65% boiler loads.The results show that the approach can capture the details of the temperature maldistribution between legs in the same heat exchanger, between elements in the same leg and between the inner and outer loops of the same element for different load cases. It also captures the steam flow maldistribution between the inner and outer loops. On the flue gas side the results show that for the part-load case, the high gas temperature and velocity zones are less uniformly distributed compared to the full load case and also less symmetric than in the full load case.The calculated total heat transfer rates for the evaporator, platen superheater and final superheater compare well with the experimental values for both the 99% and 65% maximum continuous rating load cases. This shows that the coupled simulation models can capture the overall thermal performance of the three heat exchangers modelled here with good accuracy.

Analysis Exploring the Uniformity of Flow Distribution in Multi-Channels for the Application of Printed Circuit Heat Exchangers

The maldistribution of fluid flow through multi-channels is a critical issue encountered in many areas, such as multi-channel heat exchangers, electronic device cooling, refrigeration and cryogenic devices, air separation and the petrochemical industry. In this paper, the uniformity of flow distribution in a printed circuit heat exchanger (PCHE) is investigated. The flow distribution and resistance characteristics of a PCHE plate are studied with numerical models under different flow distribution cases. The results show that the sudden change in the angle of the fluid at the inlet of the channel can be greatly reduced by using a spreader plate with an equal inner and outer radius. The flow separation of the fluid at the inlet of the channel can also be weakened and the imbalance of flow distribution in the channel can be reduced. Therefore, the flow uniformity can be improved and the pressure loss between the inlet and outlet of PCHEs can be reduced. The flow maldistribution in each PCHE channel can be reduced to ± 0.2%, and the average flow maldistribution in all PCHE channels can be reduced to less than 5% when the number of manifolds reaches nine. The numerical simulation of fluid flow distribution can provide guidance for the subsequent research and the design and development of multi-channel heat exchangers. In summary, the symmetry of the fluid flow in multi-channels for PCHE was analyzed in this work. This work presents the frequently encountered problem of maldistribution of fluid flow in engineering, and the performance promotion leads to symmetrical aspects in both the structure and the physical process.

Analysis of a horizontal flow closed loop thermal energy storage system in pilot scale for high temperature applications – Part II: Numerical investigation

The storage system investigated in this work, namely the CellFlux system, consists of a regenerator type thermal energy storage volume which is coupled to a heat exchanger by a circulating intermediate working fluid. The numerical simulations in this work are based on experiments conducted with a large scale pilot plant having a bed length of more than 10 m. The storage volume is of a novel design with horizontal flow direction and is filled with hollow bricks as sensible heat storage material. So far, most publications focus on packed bed storage systems, often with molten salt or oil, only few consider regularly shaped channels with gaseous flow. For the investigation in this part of the publication, a one-dimensional dispersion concentric model for channel flow is implemented in MATLAB/Simulink.The analysis in part I of this publication has revealed a radial flow maldistribution, which can be predicted by a correction function correlated beforehand. The commonly used assumption of plug flow, however, will be afflicted with an error. Thus, the aim of this work is to determine the accuracy of the model both under the plug flow assumption and the application of aforementioned correction function. Three single blow experiments, where the storage is charged or discharged from a uniform initial temperature, are compared to the numerical model. The average temperature deviation between experiment and simulation reaches up to 10% without the correction function, but improves significantly through its application and then ranges between 0.4% and 4.8%.Moreover, studies based on larger experiments have either not observed or generally not addressed the effect of a flow maldistribution. This appears to be an issue for small scale experiments where typically more sophisticated models are applied. If it was possible to model the entire storage volume with the present 1D model, the flow maldistribution effects could average out. Also, computing effort would be low and allow simulations on system level. This approach was taken only by few authors so far, particularly for packed bed systems and was also not experimentally verified. In order to correctly depict the experimental set up, the model considers the different composition of the interior, such as flow distributors and inlet/outlet cones as well as the thermal heat capacity of the walls and losses to the environment. As a result the model shows good agreement with the experiments: the mean temperature difference of the exit temperature between experiment and simulation during cyclic operation remained below 5%.

Analysis of a horizontal flow closed loop thermal energy storage system in pilot scale for high temperature applications – Part I: Experimental investigation of the plant

A unique large scale pilot plant of the CellFlux thermal energy storage concept is experimentally investigated. This storage concept consists of a regenerator type thermal energy storage volume, which is coupled to a finned tube heat exchanger by a circulating intermediate working fluid. The system investigated in this work operates at a temperature of 390 °C and uses air as intermediate working fluid which is conveyed by a centrifugal fan. The storage volume has a bed length of over ten meters and is of a novel design, where the air flows in horizontal direction. Since this approach could cause a flow maldistribution, a thorough analysis is of major interest for the accuracy of subsequent numerical simulations. The experiments reveal that the mass flow along the centerline can be up to 20% higher than the mean bulk flow. A significant maldistribution between top and bottom area, however, is not observed. As an alternative to the typically used rock filling, the storage volume is equipped with standard hollow bricks. These bricks are cost effective but do not have a well-defined shape. Thus, the predictability of the pressure drop by correlations found in the literature is unclear. It turns out that the measured pressure drop is evenly distributed in axial flow direction but generally higher than expected from the assumption of pure channel flow. Further experiments are conducted to validate the heat capacity of the bricks and to derive a correlation for the inner heat transfer between bricks and storage walls. Eventually, the aim of the experimental investigation is a general proof of concept as basis for the numerical investigation. Thus, all specifications of the plant and the storage material are provided. The plant is analyzed towards plausibility of heat losses, showing that heat losses can be predicted well within the given uncertainties.

Effect of water vapor condensation on the flow distribution in a PEM fuel cell stack

In this study, the effect of water vapor condensation on the flow distribution in the manifold of a PEMFC stack is investigated. A solver is developed in OpenFOAM software and is validated against experimental and numerical data. Due to the condensation, the water vapor concentration in the mixture decreases near the manifold walls. Thus, beginning and last cells receive more oxygen than the middle cells. The condensed water forms small droplets on the manifold walls. The small droplets coalesce into larger droplets. Descending large droplets into the last cell of the 26-cell stack leads to the formation of the slug. This process results in blockage of flow through the last cell, causing a significant increase in the flow non-uniformity. The non-uniformity index increases significantly from 0.046 to 0.94 in the water removal process. When the number of cells increases from 26 to 39, the large droplet detaches from the manifold and sinks into seven middle cells. The flow passing through these cells significantly reduces, resulting in a severe flow maldistribution. A comparison between flow distribution parameters indicates that the root mean square value of the deviation from the mean flow rate is a better criterion than the non-uniformity index to determine the severity of maldistribution.

3D CFD simulation of turbulent flow distribution and pressure drop in a dividing manifold system using openfoam

The flow distribution in a manifold system is a critical design parameter which affects the performance of majority of the chemical equipment. The flow mal-distribution may lead to serious issues such as (i) reduced heat or mass transfer (ii) enhanced pressure drop (iii) high energy dissipation and (iv) creation of dead zones or hot spots. The objective of the present investigation is to identify potential design strategies for attaining uniform fluid flow, reduced pressure drops and minimum energy dissipation inside dividing manifold system. 3D CFD simulations using OpenFoam have been performed to study the effect of eight different design strategies on the extent of non-uniformity (ENU) and pressure drop inside manifold system. The results reveal the dominance of momentum effect near the inlet which results in reverse flow through the branched tubes near the inlet whereas maximum discharge occurs through the tubes near the closed end. The turbulent kinetic energy (k) and dissipation rate (ε) are very high near the T junction, decreases as the flow precedes in the downstream direction and vanishes completely near the closed end of manifold. This study for the very first time reveals the roles of turbulent parameters (k and ε) in controlling the flow mal-distribution and pressure drop inside manifold systems. The most effective design strategies for achieving maximum flow uniformity and minimum energy dissipation are (i) inclusion of perforated baffle which reduces the vortex formation and results in 95% reduction in ENU¯ and (ii) converging header which results in 66% reduction in ENU¯.

Effect of heat exchanger design on seasonal performance of heat pump systems

This paper presents the effect of heat exchanger design on heat pump performance based on partial load operating conditions. 3-D numerical analysis was conducted to calculate face velocity profiles for each outdoor heat exchanger design (rectangular, cylindrical, and trapezoidal) in 10 different operating conditions. Heat exchanger circuits were modified considering heat exchanger face velocity distributions, and seasonal heat pump performances were calculated with modified heat exchanger design. The maximum seasonal performance enhancement of 7.07% was achieved with a modified heat exchanger design. Air-side flow maldistribution could affect significantly refrigerant path design and heat exchanger performance as well as system performance. The analysis results also revealed that smaller refrigerant circuits at the upper part of the heat exchanger interacting with higher air velocity could further enhance the annual system performance.

Combining the Nonuniform Structure and Flow Maldistribution for the Accurate Prediction of the Process Performance of Monolithic Adsorbent Systems

Monolithic adsorbent systems offer the potential to achieve high throughput in fast-cycle adsorption processes. Modeling these systems is typically based on the “ideal monolith” assumption, neglecting channel nonuniformities. A new model that accounts for both structural nonuniformity and inlet flow maldistributions is presented and validated using literature data of breakthrough experiments and full-cycle process performance for carbon capture separation. A detailed comparison between predictions using an apparent mass transfer coefficient (MTC) obtained by fitting the breakthrough curve and the intrinsic MTC is presented. The process calculations based on the apparent MTC grossly underpredict the carbon dioxide recovery. A procedure is established to obtain the structural nonuniformity parameters and flow maldistribution effects from the breakthrough curve. With this model, the full-cycle predictions are within 1.20 and 1.25% of the experimental observations in terms of heavy product purity and recovery, respectively. The evidence reported in this article makes a strong case for independent measurement of the intrinsic MTCs in monolithic systems.

Assessment of flow pattern and temperature profiles by residence time distribution in typical structured packed beds

The flow and temperature fields in packed beds are rather complex but are of great importance since they will influence macroscopic parameters and the overall performance of the packed bed system. Regions with either too high or too low velocity are not preferred since they will decrease the heat and mass transfer characteristics of packed beds. The residence time distribution (RTD) is a way to assess the characteristics of the flow maldistribution, where a narrower distribution is closer to the ideal plug flow with less dispersion. In this article, the RTD in three typical structured packed beds, namely, the simple cubic (SC) packing, the body-centered cubic (BCC) packing and the face-centered cubic (FCC) packing, are studied by using ANSYS FLUENT. Results show that, firstly, the RTD curve can be used to diagnose the flow pattern in packed beds where the skewed peak indicates the channeling effect and a long tail relates to recirculation. It is revealed secondly that, FCC packing has the narrowest RTD curve in the studied structures, BCC packing comes second and SC packing has the flattest RTD curve, which demonstrates that the flow uniformity in FCC packing is the best. For the same packing form, the flow is less maldistributed at higher velocity. Finally, it is shown that the packing form with less dispersion, which behaves more like the ideal plug flow, has a more uniform distribution of the particle-to-fluid heat transfer coefficient near the particle surface.

Heat-triggered two-phase flow maldistribution in a micromachined cryogenic cooler

Counter flow heat exchangers (CFHXs) are essential elements of micromachined Joule–Thomson (JT) cryogenic coolers. The performance of CFHXs has a crucial influence on the overall behavior of JT microcoolers. Flow maldistribution, especially those with phase change, can adversely affect the performance of JT microcoolers. However, the underlying mechanisms of flow maldistribution for two-phase flow in JT microcoolers remain unexplored. Here we report the heat triggered switching of two-phase flow maldistribution in the heat exchanger of JT microcoolers by both microscopic observations and temperature measurements. A measure against flow maldistribution is proposed.

Reducing thermal imbalances and flow nonuniformity in solar collectors through the selection of free flow area ratio

The effect of the riser to header diameter ratio and total cross-sectional area ratio on the flow distribution and thermal performance of flat-plate solar collectors is investigated. An analytical model based on the mass and momentum conservation equations is developed to determine the pressure drop and flow distribution in flat plate collectors. A numerical model is also developed to the assess the thermal effects of flow maldistribution. The accuracy of both models is verified against comparison with experimental measurements taken from the open literature. The use of the models is then extended to cover the flow distribution in solar collector networks. It is found that for laminar and turbulent conditions, cross-sectional area ratios and diameter ratios equal or lower than 0.75 and 0.25 respectively, minimize flow nonuniformity and reduce thermal imbalances. The results can be used for design purposes.

Numerical study of flow inhomogeneity and heat transfer enhancement in structured packed beds

Packed beds are widely used in engineering applications due to their high specific surface area and good heat transfer characteristics. A grille-sphere composite packed bed is proposed previously and has been proved to have higher overall heat transfer coefficient than the simple cubic packing structure. In the present paper, the flow inhomogeneities in both the grille-sphere composite packed bed and the simple cubic packing are studied and the relationship between the flow inhomogeneity and the heat transfer characteristics is revealed by numerical simulations. The simulations are performed on ANSYS FLUENT software. The turbulence flow is modelled by the renormalization group k- model. Both dispersion of the velocity distribution and the residence time distribution are employed to assess the flow maldistribution. When the inlet velocity equals 2.17 m/s, the variance of the residence time distribution of the composite packed bed is 5.91% smaller than that of the simple cubic packing while the Nusselt number is 10.64% higher. The results indicate that less flow maldistribution can lead to heat transfer enhancement.

Simulating the Effects of Poisoning on the Rate of the Oxidation of Ammonia over a V&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt;/TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Monolithic Diesel SCR Catalyst Using a Multichannel Model

The background to this study is the need to find out if some reactions of O₂ oxidation of ammonia oxidation are important in the Selective Catalytic Reduction (SCR) of NO by NH₃. The objective of the study was to shed light on the influence of poisoning on these reactions over a diesel SCR catalyst by compounds in the exhaust gases. The method used was to experimentally determine the amounts of products formed at several temperatures and compared them to simulated values. About 700 ppm NH₃ was oxidized by 2% O₂ in helium yielding N₂, N₂O, and NO at increasing temperatures. Comparisons are given for a 4.56% vanadia on titania fresh catalyst and the ones used for 890 and 2299 h. The kinetics was simulated using a multichannel model of the monolithic catalyst. The experimental values of the products were nicely fitted by the kinetic model where all three ammonia oxidation reaction rates were of the first order in the concentration of ammonia. The fit was somewhat better for the non-isothermal case than the isothermal one. The deactivation reduces the activation energies for the formation of all products. Effects of flow and concentration maldistribution are shown to be present but are quite small. The temperature increase is 1.30 K for the most active catalyst at the highest temperature (733 K). The use of the multichannel model shows that quite considerable deviations in inlet ammonia concentrations are obtained over the catalyst cross section. This means that the catalyst is not used to its full potential.

A Numerical Study of Response Surface Based Shape Optimization Using Neural Networks

Industrial pipes that are used for fluid transport generally have to undergo many changes of shape to accommodate interfacing equipment related to plant operation, which results in flow maldistribution zones and higher pressure drops, and in turn leads to higher power consumption. In an attempt to redress this problem, ANSYS, a commercial Computational Fluid Dynamics (CFD) software, is used to perform numerical simulations based on a deterministic computational model of the internal fluid flow using the Reynolds Averaged Navier Stokes equations (RANS), a multi objective optimization study employing Response surface methodology and artificial neural networks. This numerical analysis has been performed on a galvanized steel duct for water recirculation. The focus of the paper is the study of the effect of a chosen set of several geometrical dimensions on the pressure drop and flow distribution inside the duct. Subsequently, a new set of designs with different geometrical parameters has been obtained to minimize the pressure drop and achieve a more uniform flow distribution by using artificial neural networks to generate a response surface and further employing Screening (Shifted-Hammersley sampling) as the optimization method that was used to select the best designs from amongst those that have been generated from the response surface.