Flow Maldistribution Research Articles

Model-based efficiency mapping and parametric analysis of low-pressure axial fans

This paper advances a methodology for axial fan efficiency mapping followed by a design optimization exercise of a low-pressure axial fan for small capacity refrigeration applications. For these purposes, a tailor-made first-principles simulation model based on the so-called blade element theory (BET) was put forward. First, the fan efficiency was deployed into two terms – namely, electric motor and blade efficiency, which in turn was split into two other terms that account not only for the irreversible losses but also for flow maldistribution – in order to identify opportunities for blade optimization. Later, a sensitivity analysis was performed to find out the optimum values of key operating and design parameters of the fan, namely motor speed, blade size and number, chord length, pitch angle, and airfoil characteristics, such as thickness and maximum camber values. Among those, the pitch angle stood out as the most influencing parameter affecting the fan characteristics, including the airflow rate, the power consumption, and the blade efficiency. Comparisons with results obtained by means of the so-called fan laws are also reported for the blade size and speed.

The importance of maldistribution matching for thermal performance of compact heat exchangers

Compact heat exchangers have gained increased attention in recent years, particularly in demanding applications where high temperatures, high pressures, and/or high power densities are required. For decades, the heat exchanger (HX) community believes that flow maldistribution is a key factor for HX effectiveness, that is, reducing the degree of flow maldistribution (MALD) can help increase the HX effectiveness. Therefore, significant efforts have been devoted in the past to optimizing the header geometry to minimize flow maldistribution. This work was initially motivated by this, and the original goal was to figure out a HX header design with the lowest maldistribution. However, by systematically constructing a comprehensive maldistribution matrix, the analysis revealed that the HX effectiveness is not actually determined by the MALD, but instead dominated by the degree of maldistribution mismatch (MISM). This conclusion was also theoretically generalized, which indicated that matching of the local heat capacity rate is key for achieving maximum performance. The MISM provides a local means of tracking this information, while the MALD only provides a global approximation of the maldistribution itself. With this new perspective, flow maldistribution needs not necessarily be avoided, but instead matched between two fluid streams, to improve the HX performance. We demonstrated that by carefully designing the header geometry to match the velocity profiles of the two fluids in a 2 MW PCHE with molten salt and supercritical carbon dioxide (sCO2) as the heat transfer fluids, the HX could achieve a higher effectiveness even when the maldistribution increased. A technoeconomic study using a CSP system as an example revealed that the use of this new HX design paradigm could result in CSP capital cost savings as large as 16.6%.

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.

A regulating method for pressure drop of supercritical cracked-kerosene in cooling tube

Flow distribution of kerosene in actively cooling panel is an important factor that constraints the cooling efficiency of the thermal protection system of scramjet. However, the flow mal-distribution is a common phenomenon in the realistic thermal protection system. In order to investigate this problem, a way to regulate the pressure drop via restriction orifice is proposed. The flow, heat transfer and thermal cracking process in cooling tube are calculated using a three-dimensional fluid-solid conjugated simulation method for supercritical cracked-kerosene. The results indicate that with an appropriate restriction orifice, the pressure drop under a lower heat flux can match with that under correspondingly higher heat flux in a same tube, which verifies the feasibility of pressure drop regulating using the restriction orifice. Therefore, this research could provides a design method of flow distribution in actively cooling panel of scramjet.

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.

Performance evaluation of fin-and-tube heat exchangers in an outdoor unit considering air flow mal-distribution

The heat transfer capacity of air conditioning outside units is inextricably related to fins and airflow distribution, and research on the combination of the two is lacking. To analyze the fins and fin-and-tube heat exchanger, three-dimensional numerical models of the fins and flow field of the outdoor unit are created in this paper. On the one hand, the heat transfer and fluid flow characteristics of the fins are quantitatively evaluated and analyzed. On the other hand, the resistance coefficients of the porous media are numerically fitted to study the uniformity of the air velocity distribution on the surface of the heat exchanger. Two indexes, the total integrated heat capacity and JF-factor, are applied to evaluate the performance of the fins and fin-and-tube heat exchanger on the overall and specific levels, respectively. The results indicate that slit fins perform better than wave fins at heat transfer, whereas plain fins perform the worst. However, improved heat transfer performance results in increased pressure drop, which has a two-sided effect. It causes both a decrease in air volume flow rate and an increase in air distribution uniformity. Comparing the comprehensive performance of different fins, JF-factors of slit fins and wave fins are on average 20.95% and 14.36% higher than plain fins. In terms of overall performance, slit fins and wave fins have higher integrated heat exchange capacities than plain fins by 18.0% and 9.33%, respectively.

Airflow Distributions in a Z Type Centripetal Radial Flow Reactor: Effects of Opening Strategy and Opening Rate

Computational fluid dynamics (CFD) was adopted to investigate the influence of the three-section opening strategy in the Z type centripetal radial flow reactor on the uniformity of the gas flow, which aimed to optimize the opening rate of the reactor. The simulation results showed that as the pore-opening ratio of are 10%, 16% and 29% for three sections from top to bottom of the central channel, the opening rate of the circular channel perforated plate is 10–12%, 21–25% and 30–40% from top to bottom, respectively; the uniformity of the reactor was then achieved. Through the simulation results, it was also found that the change in the opening rate at the center pipe perforated plate had a greater contribution to the gas flow mal-distribution inside the reactor. The contribution of the change in the opening rate of the annular channel perforated plate to the uniformity of the gas flow inside the reactor was smaller than that of the center pipe. Annular channel width should not be smaller, such that gas flow malfunction inside the reactor could be avoided, although high-speed velocity cases were encountered.

Flow models of perforated manifolds and plates for the design of a large thermal storage tank for district heating with minimal maldistribution and thermocline growth

Large water tanks are used as thermal energy storage components in district heating systems to store sensible heat produced by intermittent energy sources and to decouple the production of thermal energy from its demand. Good thermal stratification is crucial for energy storage efficiency, thus flow maldistribution and mixing of water layers at different temperatures should be minimized. This paper proposes an innovative internal flow distribution configuration for a large-size thermal energy storage, and develops new simplified analytical models for the choice of its design parameters. In the novel configuration, water is injected into (and collected from) the cap volumes of the tank by flowing radially inward (outward) through several small orifices of a peripheral toroidal manifold. Two horizontal perforated plates cover the full cross sections downstream of the manifolds and rectify the vertical flow, thus reducing mixing. Uniform perforation pitch was analytically demonstrated to be the most reasonable solution both for the toroidal distributors and for the rectifying plates. A 1D model was developed to predict the time evolution of the vertical temperature distribution in the tank. The turbulence-related parameters that could not be inferred from the existing fluid-mechanics literature were initially estimated with CFD simulations. The results of CFD-calibrated model were then compared to experimental data obtained from a full-scale large water-tank facility recently built in Brescia according to the proposed design. After a re-calibration of the exponent defining the decay of homogeneous turbulence downstream of the perforated plates, good agreement was found between measured and predicted vertical temperatures. With the novel inlet design, a thermocline of about 0.5 m is established immediately downstream of the perforated plate, and remains practically constant along time. The model is important to minimize and control the thermocline thickness so as to maximize the recoverable thermal energy, not only at the tank design stage but also to identify optimal loading and unloading protocols.

Digital image analysis of gas-liquid flow in a cross-corrugated plate heat exchanger channel: A feature-based approach on various two-phase flow patterns

High-resolution photographs of the air-water flow in a transparent cross-corrugated channel are analyzed using digital image processing. Gas-liquid distribution is controlled via several inlet nozzles. Flow direction is varied between upward, downward, and horizontal flow.The image analysis algorithm is applied to a wide range of void fractions and flow patterns including bubbly, intermediate and film flow. Large bubble clusters are separated using edge detection. Local film and bubbly flow are identified with texture analysis. The method is suitable for locally inhomogeneous two-phase flow which is difficult to analyze visually.Objective and reproducible results are obtained by measuring features in the processed images: Mean and maximum bubble diameter, and the ratio of local film flow are determined. For non-uniform gas injection, expansion of two-phase flow across channel width is measured. Impact of flow direction and maldistribution on two-phase flow is demonstrated by calculating differences of the features. Uncertainties of image processing results are quantified. Bias is discussed based on visual analysis of the photographs in a previous study.Models for maximum bubble diameter and local film flow ratio, used to model flow pattern transitions in a previous study, are verified: For uniform two-phase distribution, models generally agree with measured values.

Durability Study of 3D‐Printed Catalytic Static Mixers for Hydrogenations in Chemical Manufacturing

AbstractStructured catalysts made by additive manufacturing offer the capability to seamlessly scale heterogeneous catalysis from lab to production with minimal effort to reengineer the process at intermediate steps. They can be tailored to meet certain process demands for mixing, heat transfer and flow resistance, in order to overcome flow maldistribution, hotspot or pressure drop issues, generally associated with traditional packed bed systems. Herein we investigate the long‐term durability of 3D‐printed Pd catalytic static mixers for their use in the hydrogenation of 4‐nitrobenzoic acid.

Investigation on flow distribution of supercritical natural gas in a printed circuit heat exchanger

Printed circuit heat exchanger is considered as an promising candidate for floating storage and regasification unit of liquefied natural gas due to its high compactness and efficiency. The flow maldistribution is an essential issue in practical applications of the printed circuit heat exchanger, which leads to the performance degradation. In this study, the flow distribution of supercritical natural gas in a printed circuit heat exchanger plate is three-dimensionally simulated, and the thermal-hydraulic performance of the channels is investigated. To save the computational cost, a simplified two-dimensional model is proposed. The comparisons of results between the two- and three-dimensional models show that the effects of backflows, vortex, interaction between properties and flow distribution are the reasons that result in the differences between the two models. The findings suggest that the simplified two-dimensional model is capable for engineering demands with the satisfaction of computational accuracy and efficiency. It helps to overcome numerical difficulties and save computational resources in engineering applications such as the PCHE design.

CFD Assessment of Flow Maldistribution in Plate Fin and Tube Heat Exchangers

The primary goal of this work is to use CFD assessment simulating framework is to anticipate the impact of injecting air flowing mal-distribution on heat exchanger architecture and thermal flow characteristics. Using CFD (computational fluid dynamics) simulation-studies, determine the impact of air intake flow unequal distribution or mal-distribution on the complete redesign as well as high - temperature hydraulic performing ability of HEX (heat exchangers). The findings demonstrate that a plate as well as tube fin heating element consisting of an elongated tubular setup slanted by 30 degrees can achieve the highest temperature transmission, having a heat transfer ability of 23.22 percent greater than a (SPHEX) spherical pipe heat exchanger.

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.

Parameter Study of Geometrically Induced Flow Maldistribution in Shell and Tube Heat Exchangers

Abstract Shell and tube heat exchangers (STHEs) are the most common type of heat exchanger in preheat trains (PHT) of oil refineries and in chemical process plants. Most commercial design software tools for STHE assume uniform distribution over all tubes of a tube bundle. This leads to various challenges in the operation of the affected devices. Flow maldistribution reduces heat duty of STHE in many applications and supports fouling buildup in fluids that tend to particle, bio, and crystallization fouling (Verein Deutscher Ingenieure, ed., 2010, Heat Atlas, 2nd ed., VDI-Buch., Springer-Verlag). In this article, a fluid mechanics study about tube side flow distribution of crude oil and related hydrocarbons in two-pass PHT heat exchangers is described. It is shown that the amount of flow maldistribution varies significantly between the different STHE designs. Therefore, a parameter study was conducted to investigate reasons for maldistribution. For instance, the nozzles diameter, type, and orientation were identified as crucial parameters. In consequence, simple design suggestions for reducing tube side flow maldistribution are proposed.

Oscillatory valve effect on temperature synchronization in microchannel cooling systems

• Flow maldistribution with oscillation can result in large temperature differences. • Periodic opening and closing of expansion valve can mitigate flow maldistribution. • Only some combinations of valve opening amplitude and frequency can balance flow. • Large amplitude periodic valve opening can balance flow over wider frequency range. • Very small valve opening amplitudes do not synchronize flow for any frequency. Microchannel evaporator cooling systems are widely studied for their ability to remove high heat fluxes. However, flow instabilities during microchannel boiling, like pressure drop oscillation and flow maldistribution, can cause an uneven temperature variation and deteriorate the cooling performance. This study analyzes the effect of using an oscillatory valve opening on the flow distribution and channel temperatures across a two-channel evaporator. Experiments and prediction models indicate that a specific range of valve oscillation amplitude and frequency may cause synchronization resulting in balancing the flow distribution and temperature across the channels, even when they are thermally isolated from each other. Analysis of system performance at different conditions indicates that larger oscillation amplitudes in the valve opening may allow a wider frequency range for temperature synchronization. For a fixed valve opening amplitude, using either significantly small or large frequencies may not be helpful. Moreover, for small amplitudes in valve opening, synchronization may never occur for any frequency. These observations lead us to believe that deliberate oscillations in the valve opening can serve as an active control strategy to mitigate flow maldistribution in parallel channels, suppress temperature oscillation, and improve the overall cooling performance.

Limit cycle analysis and maldistribution mitigation for multi-channel cooling system

Flow boiling in microchannel can be applied for large heat load dissipation due to its high heat transfer coefficient, compact structure, and low refrigerant flow. However, instabilities in microchannel cooling systems, like pressure drop oscillation and flow maldistribution, can deteriorate cooling performance, cause component damage, and result in unbalanced temperatures. In this study, we explore the effects of heat load on limit cycles and temperature distributions for microchannel cooling systems with single, two, and multiple channels. The objective is to analyze the pressure drop oscillation characteristics and understand the changing patterns of limit cycles at different operating conditions, based on which control strategies can be developed for flow instability suppression. Numerical results show that the limit cycles are identical for channels with equal heat loads and the same pressure drop characteristics. Thus, mass fluxes and temperatures are evenly distributed. For unequally heated channels, the one with the smallest heat load would have a limit cycle crossing both positive and negative regions of the pressure drop characteristic curve, while the limit cycles of the channels with larger heat loads would oscillate only at the positive slope region (the high vapor quality side). Based on the understanding of limit cycles for multiple channels, we investigated the temperature distributions and pressure drop oscillations under the individual and combined effects of thermal and mass flow connections among channels. Results show that the thermal and mass flow connections would reduce the temperature difference for channels experiencing unequal heat loads, and a strong enough connection would result in the same limit cycles and synchronized temperatures for all channels.

Performance Degradation of a Shell-and-Tube Heat Exchanger Due to Tar Deposition

Biomass represents a programmable renewable energy source that is useful for reducing issues related to the transfer from fossil fuels to the renewable energy era. The exploitation of biomass is strongly related to the development of power technologies that are designed to improve efficiency; however, at the same time, they have to be designed to improve the life cycle of the entire installation—especially in relation to maintenance operations. In this paper, a numerical analysis is proposed to assess the performance of a heat exchanger used for separating condensing tar from syngas generated by the gasification of lignocellulosic wood chips and pellets. The analysis included clean, fouled, and clogged conditions. Flow maldistribution characterized the inlet section of shell-and-tube configurations and was responsible for clogging phenomena. Starting from field detection, analyses of fouled and clogged conditions showed a reduction in the effectiveness of the heat exchanger, causing dangerous conditions for the internal combustion engine used to exploit the syngas flow.

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.

Thermal Management of Multi Core Processor Using U Configured Parallel Microchannel Cooling System

Thriving technologies in the electronics industry demands effective cooling systems for proper thermal management of the devices. To propose an effective cooling system, real-time challenges need to be taken into examination. One of the challenges is the non-uniform heat load emitted by the device. In this paper, a multicore processor has mimicked for the heat load and examined with U configured parallel microchannel cooling system (PMCS) for its capacity to cool the processor effectively. A detailed numerical examination has been conceded out by mimicking the AMD Ryzen-7 octa-core processor for its size, shape and heat load. To make it convenient for applying non-uniform heat load, the shape of the PMCS is divided into 4*4 array of a total of 16 heaters. The present has witnessed that, there is an uneven distribution of fluid flow across the channels. The initial channels do get more quantity of fluid whereas the end channels do get a very less quantity of fluid. Why because of the flow maldistribution, it formed the hot spots (high-temperature zones), which will be further intensified due to the non-uniform heat load emitted by the processor. It perceived from the present numerical analysis, there is a formation of hotspots not only due to the random location of active cores but also due to the flow maldistribution across the channels. The flow arrangement of U configuration having worse flow distribution among other existing configuration. The present work conclusively demonstrates that high maldistribution leads towards dropping the uniformity of cooling. But the existing maldistribution can be effectively exploited to tackle the non-uniform heat load released by the processor when the active core location falls in the place of initial channels.

Experimental Investigation of Two-Phase Flow Maldistribution in Plate Heat Exchangers

After conducting a critical survey of the different categories of existing heat exchangers, the results of several experiments about the behaviour of a two-phase current in an open channel are reported. The results confirm the complexity of the problems induced in heat exchangers by flow maldistribution, especially when two-phase flows are considered in multi-channel systems. It is shown that severe misalignment of heat exchangers can lead to a loss of economic performance of more than 25%. Improper distribution of fluid flow causes longer fluid coils to form, and the liquid cochlea can eventually occupy a large space, thereby reducing heat transfer and disrupting the considered biphasic system. The use of a small diameter distribution pipe with properly spaced outlet holes seems to be a promising approach to fix many of these issues. It is found that the current distribution in the channels, in addition to the header pressure distribution, also depends on factors such as the geometry and the initial flow regime in the header.