Flow Maldistribution Research Articles

Numerical and experimental investigation on the air flow characteristics of heating, ventilation, and air-conditioning module for a small electric vehicle

In electrical vehicles, replacing positive temperature coefficient heater as heat source with an air source heat pump could improve the driving range and decrease energy consumption in cold climate. Design of the heating, ventilation, and air-conditioning module for heat pump system has a significant influence on its performance in each working mode. A newly designed heat pump heating, ventilation, and air-conditioning module was introduced in this paper. The air flow characteristics of the heat pump heating, ventilation, and air-conditioning module in four working modes were analyzed, and the air flow rate and wind resistance were obtained by numerical simulation. Experiments were also conducted for validating its airflow rate in each working mode. Results of these experiments show that some unfavorable phenomena such as flow maldistribution and vortex inside the heat pump heating, ventilation, and air-conditioning module exist, which could lead to insufficient utilization of the heat exchange area of heat exchangers and the generation of aerodynamic noise. Furthermore, the air flow rate of the original heating, ventilation, and air-conditioning module was also measured for comparison, and the designed heat pump heating, ventilation, and air-conditioning module shows nearly 15–20% decrease in each working mode.

Optimization of heat transfer effects in radial fin microchannel heat sink

ABSTRACT In the quest of saving energy, heat must be extracted from the electronic devices effectively, for which microchannel heat sinks are employed. The temperature variation in heat sinks exists due to flow mal-distribution. With increase in complexity of heat sink designs, the concerns for its reliability arises, as the large temperature variation across the substrate causes development of thermal stresses. Therefore, to tackle both the problems simultaneously the authors presents a study of heat sink with radial fins. The height of channel, number of channels on substrate, and flow rate are considered as the design parameters. The response of the design variables is obtained in terms of Nusselt number, which is put into Taguchi’s L9 orthogonal array for obtaining the optimum combination of design parameters for maximum Nusselt number. The signal-to-noise ratio and analysis of variance were used to examine which parameter significantly affects Nusselt number. The results indicate that the flow rate is the most influential parameter that affects Nusselt number.

Hybrid FEM and FDM Approach for Monolithic Catalytic Converters with Pore Diffusion

A transient, three-dimensional model is developed for monolith catalytic converters with washcoat pore diffusion. Due to the unique configuration of monolith converters, the flow within the channel can be considered fully developed laminar flow, and one-dimensional transport equations can be formulated for the flow and species. On the other hand, a three-dimensional equation is necessary for the monolith solid temperature to capture the effects of significant external heat loss or flow maldistribution. The proposed model employs a three-dimensional finite-element method (FEM) for the solid temperature equation and a one-dimensional finite-difference method (FDM) for the flow and species equations. A recently developed asymptotic approach (Bissett, Emiss. Control Sci. Technol. 1(1), 3–16, 2015, Bissett, Emiss. Control Sci. Technol. 5(1), 45–54, 2019) is utilized to solve the washcoat pore diffusion and surface coverages equations. The three-dimensional FEM meshes are chosen so the nodes also serve for the one-dimensional transport equations. All equations are solved fully coupled, without lagging or reliance upon external tools. The developed solver can efficiently simulate general cross-section geometries (circle, oval, rectangle, etc.) for a catalytic monolith under nonuniform inlet distribution and detailed washcoat pore diffusion. The solver is verified by comparing the simulation results to the one-dimensional simulation results for a simple geometry under uniform inlet conditions. The model can be applied to flow through general monolith catalytic reactors with single or dual washcoat layers and a wide range of kinetics.

Effect of flow maldistribution on the optimal design of plate heat exchanger using constrained multi objective genetic algorithm

In this study, thermal and hydraulic optimization of plate heat exchanger is presented. In the thermal modeling of the plate heat exchanger, the effects of flow maldistribution for each side are also considered. A multi objective optimization is implemented based on the NSGA-II (Non-dominated Sorting Genetic Algorithm-II) algorithm to obtain the optimal trade-off (Pareto front) between effectiveness and pressure drop of hot side. The optimization variables in this study are considered as chevron angle, the number of plates, corrugation depth, corrugation pitch, horizontal distance of ports, vertical distance of ports, and port diameter and the goal is to achieve maximum effectiveness and minimum pressure drop. In addition, operational and geometrical constraints are also considered in the optimization procedure in order to take the advantage of the standard design and achieve feasible solutions. Next, a case study is presented to identify conflicting thermal-hydraulic behavior of plate heat exchanger. The results are reported as a set of multiple optimum solutions called ʺPareto optimal solutionsʺ. Moreover, the effect of changing these parameters on the optimal design is also presented and discussed in detail.

Optimization of Flexible Hose for Reducing Flow Maldistribution in Manifolds

Coolant Distribution Systems (CDS) are required to be optimally designed to ensure predefined mass flow rate in all parallel channels between exit and collecting manifolds, while maintaining low-pressure drop across them. Even small change in the pressure drop at component level will result in maldistribution of flow rate and increase in overall pressure drop of CDS. Numerical and experimental study had been carried out in the work proposed for change in pressure drop due to deformation of end connector’s bend cross section, in a flexible hose. The methodology for optimum modelling of the problem on CFD tool using sub-structuring method is also suggested. In sub-structuring method a part of complete hose (henceforth referred as sub-structured model), has been used instead of complete model of flexible hose. Results of sub-structuring model were compared with that of a complete model of a flexible hose. Numerical values obtained from simulation were validated with experimental results. Optimum bend cross section for avoiding maldistribution in parallel channels and increase in pressure drop of CDS were calculated. Substantial reduction in computation cost was achieved with negligible loss of accuracy in pressure drop values.

Experimental characterization of thermal-hydraulic performance of a microchannel heat exchanger for waste heat recovery

Given size and performance advantages, microchannel heat exchangers are becoming increasingly important for various energy recovery and conversion processes. In this study, detailed experimental measurements were conducted to characterize flow and heat transfer performance of a microchannel heat recovery unit (HRU) manufactured using standard photochemical etching and diffusion bonding processes. According to the global flow and temperature measurement, the HRU has delivered the predicted thermal performance under various oil and air flow rates. As expected, the heat transfer effectiveness varies between 88% and 98% for a given air and oil flow rates while it increases with air inlet temperature due to the improved thermal conductivity. However, significant flow mal distribution is identified among the air channels according to the in-depth flow distribution measurement using hot wire. The flow measurement also indicates visible misalignment of the air channels caused by the manufacturing processes. In addition, the excessive pressure drops occurred for both air and oil channels indicating reduced flow areas due to the photochemical etching process. The results of this experimental study can hopefully provide insights in improving designs of microchannel heat exchangers using the same manufacturing processes.

Performance evaluation of commercial-size proton exchange membrane fuel cell stacks considering air flow distribution in the manifold

This study offers an efficient method for commercial-size proton exchange membrane fuel cell (PEMFC) stack performance evaluation to improve designing of fuel cell and maximize power density in PEMFC stack. The flow distribution in the manifold of the stack is critical to the energy conversion of the assembled unit cells in series due to the typical short-board effect. Most existing works studying on the flow distribution focus on small fuel cell stack and does not establish a clear relationship to performance, which are not able to thoroughly guide commercial-size PEMFC stack design and development. In the present study, an effective method combining computational fluid dynamics (CFD) model and empirical model is proposed to evaluate the performance of commercial-size stack considering air flow distribution in the manifold. Firstly, the air flow distribution in the manifold is predicted by a CFD model. A performance evaluation empirical model is developed by a series of experiments to evaluate the effects of flow maldistribution on the performance of PEMFC stack. Then, the predicted flow distribution and performance are respectively validated by a novel experimental setup. Finally, the effects of stack configuration, cell number, and current density on flow distribution and performance of PEMFC stacks are discussed. The results show that U-type configuration promotes more uniform voltage among unit cells than Z-type. The voltage unevenness of unit cells caused by flow maldistribution climbs dramatically as the cell number and current density increase. The methodology developed is beneficial to the energy management and the efficiency improvement of commercial-size PEMFC stack.

Characteristics of flow distribution in central-type compact parallel flow heat exchangers with modified inlet and header

This study numerically investigated the effect of the application of modified methods in a central-type parallel central-type heat exchanger on the flow distribution. For the flow maldistribution in the heat exchanger, it can greatly reduce the device performance. Therefore, It is significant to apply methods to the modification of the heat exchanger geometry for a better flow distribution. Except for the reducing the flow maldistribution, the reduction of the pressure drop in the heat exchanger should also be taken into account especially in the passive system such as passive containment cooling system and passive core cooling system etc., which are driven by natural circulation flow. Taken both flow maldistribution and pressure drop into consideration, four different methods have been applied in this study for a better flow distribution and less pressure drop. The methods include modifications of double-inlet and header. Besides, the inlet diameter and the insert angle and inlet have been investigated. The simulation results show that the modified double-inlet can reduce the flow maldistribution by 66.36% at most and pressure drop by 29.62% at the mean time. The modified header is able to reduce flow maldistribution by 53.16% at most and pressure drop by 36.03% meanwhile.

Stability analysis of parallel channels in flow boiling system: Flow maldistribution and density wave oscillations

In a flow boiling thermal-hydraulic system, Density wave oscillations (DWO) and Flow maldistribution (FMD) are two important instability phenomena. These instabilities can lead to undesirable consequences of system operation. Thermal-hydraulic system with an odd number of channels behave differently as compared to an even number of channels. Following FMD for a two channel system, it leads to a new steady state operating condition. Here, one channel receives higher flowrate and another channel receives lower flowrate for a forced flow system. It may be possible that the new operating point is in a DWO zone. Post FMD, the individual flowrate of the channel starts to oscillate. It shall be noted that under constant mass flowrate condition, such local behaviour in the channels will be difficult to detect. For a twin-channel forced flow system, DWO which may be observed with/without FMD is related to out-of-phase oscillations only. It is thus interesting to study the interaction of DWO and FMD phenomena for a tri-channel flow boiling system. Symmetry breaking transition in the context of excursion instability (i.e., Ledinegg instability) is analyzed in this work. Here, a two and three channel system is used to analyze the two phenomena, FMD and DWO. The difference between the two and three channel flow boiling system is brought out in this work. Unlike two channel system, it is observed that a region in three channel system exists where both symmetrical and asymmetrical equilibrium solutions are unstable. Further, post FMD the solution starts to oscillate but the total system inflow and outflow remains constant.

A novel approach to manage temperature non-uniformity in minichannel heat sink by using intentional flow maldistribution

Flow maldistribution is an uncalled phenomenon in water-cooled heat sink and accountable for the depreciation in thermal performance of the same. However, this study is proposed to manage temperature non-uniformity at the base of the mini-channel heat sink by taking advantage of flow maldistribution under non-uniform heating. Four different types of flow arrangements such as I-Type, Z-Type, C-Type and N-Type and five different types of heating arrangements such as uniform, center streamline, left streamline, right streamline and dual streamline heating are considered. A comprehensible numerical study has been performed using ANSYS-Fluent to explicate the combined effect of different flow arrangements and different heating arrangements on the thermal performance of the heat sink. The results show that for uniform heating, I-Type flow arrangement has better thermal performance with the lowest value of thermal resistance. For non-uniform heating, better thermal performance can be achieved for cases such as center streamline heating, right streamline heating, left streamline heating and dual streamline heating with I-Type, Z-Type, C-Type, and N-Type flow arrangements respectively. Hence, a suitable flow arrangement should be considered for a given heating arrangement to acquire the optimal thermal performance of the heat sink.

Fabrication of Graphene‐Covered Micro‐Tubes for Process Intensification

Graphene is known for its high surface‐area‐to‐mass ratio. However, for using graphene in engineering processes such as catalytic reactors or heat exchangers, a high surface‐area‐to‐volume ratio is essential. Currently, graphene is only prepared in sheet form, which limits its surface‐area‐to‐volume ratio to around 200 m2 m−3. Herein, a technique based on chemical vapor deposition (CVD) to realize graphene on a micro‐tubular substrate to not only substantially increase its surface‐area‐to‐volume ratio to a value over 2000 m2 m−3 but also to eliminate the maldistribution of flows commonly unavoidable in flat‐sheet configurations is proposed and demonstrated. This approach uses a dual‐layer micro‐tubular substrate fabricated by a phase‐inversion technique. In the substrate, a thin copper outer layer is used to enable the CVD growth of graphene, and an inner Cu–Fe layer is adopted to provide a strong mechanical support. This approach is feasible to produce graphene with a very high surface‐area‐to‐volume ratio for possible applications in catalytic reactors or heat exchangers, although problems such as inter‐diffusion between the two metal layers and defects in graphene need to be addressed. To the best of our knowledge, this study is the first attempt to prepare graphene with high surface‐area‐to‐volume ratios by a CVD route.

Improvements in Performance of a Self-Similarity Heat Sink Through Structure Modification

A self-similarity heat sink (SSHS) exhibits better overall performance than a conventional heat sink whereas although serious flow maldistribution still occurs. A modification to a conventional SSHS is proposed to overcome this disadvantage and obtain a more uniform heat transfer. Straight fins on the cover plate were replaced by zigzag fins to form a smaller incidence angle of fluid from the inlet manifold to the overflow channels, and the inlet manifold with constant cross-section was changed to a tapered inlet manifold to restrict excess flow to the downstream end of the inlet manifold. Numerical analysis was subsequently conducted to verify the modification. The results showed that the modification scheme successfully addressed the disadvantage of flow maldistribution of the original SSHS. Effects of the incidence angle on flow and heat transfer inside a single unit were analyzed at flow rates of 0.58–1.73 kg/h. The optimal incidence angle was found to be in the range of 20°–25°. For an entire SSHS with this optimal design, uniformity in flow distribution and heat transfer was significantly improved compared with original design, although the pressure drop increased slightly.

A numerical evaluation of next generation additive layer manufactured inter-layer channel heat exchanger

A Concept Heat Exchanger (HE) design manufactured using the Additive Layer Manufacturing (ALM) technique Selective Laser Melting (SLM) is proposed and numerically evaluated. It is composed of a HE corrugation which introduces inter-layer flow conduits between the parallel HE layers of the same fluid. These pathways are provided by hollow elliptical tubes which serve several functions: to disturb the flow to promote heat transfer, to provide additional heat transfer area and to minimise flow maldistribution inside the HE core. The corrugation is incorporated into a counter-flow prototype HE unit model meaning to exploit the installation volume and design freedom made possible via ALM. Three Computational Fluid Dynamics (CFD) models are utilised to evaluate the performance of the proposed HE unit. Firstly, a traditional two step HE design methodology is utilised which works by initially evaluating a fully symmetric channel of the proposed HE corrugation (termed single channel). Then the results this model are incorporated into a simplified HE unit model. The second approach evaluates the HE unit performance based on a fully detailed CFD analysis that fully resolves flow and heat transfer inside the HE core. The third modelling approach involves splitting the inter-layer HE unit model into parts, which results in HE header models and allows simplification of the HE core into a single corrugation period width HE core model (termed superchannel). The results of these models are then compared to a conventional pin–fin HE unit model, formed by blocking the elliptical inter-layer conduits. It was found that in all the HE unit models the pressure drop is similar whilst the heat transfer was enhanced by between 7% and 13% in terms of the overall ΔT by the inter-layer channels (increasing with the Reynolds number). All simulations were completed using a CFD package OpenFOAM.

Experimental Investigation on Heat Spreader Integrated Microchannel Using Graphene Oxide Nanofluid

Thermal design consideration is highly essential for efficient heat dissipation in advanced microprocessors which are subjected to conjugate heat transfer under high heat flux with a minimal area for cooling. Generally, these multicore processors develop a localized high density heat flux referred to as hotspot. The effective use of microchannel in order to mitigate the hotspot is found in literature; however, the flow induced hotspot still exist due to maldistribution of flow inside the microchannel. Henceforth, the present study provides an experimental insight on laminar forced convection in a parallel microchannel heat sink accompanied with 1.2 mm thin copper heat spreader with a surface area of 30 mm2 to effectively migrate the maldistribution flow induced hot spot. The present experimental study provides a profound insight about the hotspot and migration of hotspot to safe zones; as a result, not only the performance of the multi core microprocessor is highly improved but also the reliability of neighboring components is well secured.

Interaction of density wave oscillations and flow maldistribution for two-phase flow boiling parallel channels

Density wave oscillations (DWO) and flow maldistribution (FMD) are two important instabilities among others in two-phase flow boiling systems. As both the phenomena can lead to undesirable consequences such as channel burnout, the design intent is to avoid the same. DWO is caused by the interaction between single-phase and two-phase region as perturbation propagates slowly in the later region. Whereas in case of FMD, it leads to one channel receiving higher flowrate and other channel receiving lower flowrate for a twin parallel channels system under forced flow condition. Thus, channels subjected to similar thermal hydraulic and geometric condition receive non-identical flowrate. Bifurcation analysis is used to identify DWO and FMD instabilities in this work. DWO is identified by Hopf bifurcation, thus Hopf curve which constitutes Hopf bifurcations represents the DWO boundary. FMD has been observed earlier through numerical simulation but the region which depicts the same in a parametric space has not been demarcated in previous studies. In this work, FMD is identified by a pitchfork bifurcation. This leads to straight-forward identification of FMD boundary as a pitchfork curve. A combined analysis of multiple instabilities using a single stability map can demarcate regions with the dominance of particular instability. It can thus provide a better perspective for system design and system instabilities in terms of suitable operating region selection. A stability map which can capture and demarcate both the phenomena simultaneously i.e, DWO and FMD in a parametric space is the interest of present work. The boundary of DWO and FMD splits the parametric space into three distinct regions. These are DWO region, a region of symmetrical stable solutions and FMD region which contains non-identical (asymmetric) stable solutions. This breakdown of symmetry of the system is observed by non-identical flowrate in the channels. In some cases, boundary depicting DWO and FMD intersects. Hence, a fourth region is also observed which possess the characteristics of both the instabilities. The stability map shows the transition between these regions.

Effect of geometry parameters on the hydrocarbon fuel flow rate distribution in pyrolysis zone of SCRamjet cooling channels

Hypersonic air-breathing propulsion (>Mach 5) based on SCRamjet promotes the revolutions of both military and civilian applications. However, the engine thermal protection is seriously challenged due to high Mach number. One of the key problems is mal-distribution of hydrocarbon fuel in regenerative cooling channels under the complex thermal boundaries and channel geometry, which may cause structural failure. To improve the cooling channel design and achieve better fuel flow distribution, the effect of channel geometry parameters on flow distribution in pyrolysis zone is numerically studied. Both geometry induced and heat flux induced flow mal-distribution are concerned. The study indicates that the flow and heat transfer features in pyrolysis zone of parallel channels differs with those in non-pyrolysis zone. The flow distribution deviations in pyrolysis zone are lower. When the channel aspect ratio varies, thermal stratification dominates flow distribution more than the variation of heat flux on heated surface in pyrolysis zone, which is opposite of that in non-pyrolysis zone. Besides, thinner rib and smaller total flow area achieve better cooling performance while consuming less fuel chemical heat sink. The results help to conclude possible references to the design of cooling channels with fuel pyrolysis considered.

Improvement of two-phase refrigerant distribution for upward flow of a parallel flow minichannel heat exchanger using insertion devices

In a parallel flow heat exchanger, significant mal-distribution of flow occurs due to phase separation. In this study, various insert devices (perforated tube, perforated tube with perforated plate, orifice and perforated tube, concentric perforated tube) were investigated to obtain an improved flow distribution in a 36 channel parallel flow heat exchanger. The test section was made to closely simulate an actual heat exchanger. Tests were conducted for upward flow for the mass flux from 57 to 241 kg m−2 s−1 and quality from 0.2 to 0.4 using R-410A. Of the investigated insert devices, concentric perforated tube yielded the best flow distribution. Insertion of the concentric perforated tube reduced the thermal degradation from 61% to 14%. Furthermore, the preferred number of holes of the concentric perforated tube was dependent on the mass flux. At a low mass flux, an insert having small number of holes was preferred, whereas the reverse was true at a high mass flux. At a low mass flux, the effect of inlet vapor quality on flow distribution was significant. At a high mass flux, however, the effect of vapor quality on flow distribution was minimal. Possible explanations on the flow distribution behavior were provided through flow visualization in the header.

Channel Blockage and Flow Maldistribution during Unsteady Flow in a Model Microchannel Plate heat Exchanger

This paper describes the problem of channel blockage as a result of flow maldistribution between the channels of a model mini channel plate heat exchanger consisting of one pass on each leg. Each leg of the heat exchanger contains 51 parallel and rectangular minichannels of four hydraulic diameters namely 461 µm, 571 µm, 750 µm and 823 µm. In addition, a more complex geometry has been investigated where for the sake of breaking the development length the inclined transverse cuts have been incorporated. The moment of liquid phase transition through the exchanger (the working medium: water) was recorded for the mass fluxes ranging from 18.67 to 277.76 kg/m2s in 51 parallel channels with the use of a fast speed camera. The Reynolds numbers Re in the individual channels were from 10.76 to 90.04. The relationship between the mass flux and the size of the minichannels in the presence of the maldistribution is discussed here. The existence of the threshold in the mass flux below which the phenomenon occurs has been shown. Two mechanisms of channel blocking have been recorded and described in detail. A miniscale variation of one of them containing the extended geometry was created as well.

Performance of heat pumps using pure and mixed refrigerants with maldistribution effects in plate heat exchanger evaporators

This paper presents a combined plate heat exchanger (PHE) - heat pump simulation framework for the evaluation of flow maldistribution in PHE evaporators and its effect on the cycle thermodynamic and economic performance. A case study of heat pump integration for waste heat recovery purposes in data centres was chosen to demonstrate the utilization of the simulation tool. The analyses were made for the pure fluids butane and propane, and for the zeotropic mixtures propylene/butane at (0.5,0.5) mass composition and CO2/dimethyl ether (DME) (0.2,0.8) as refrigerants. Both liquid/vapour maldistribution and the effect of end plates were considered in the heat exchanger models. Results show that butane is most sensitive to maldistribution, with a maximum Coefficient of Performance (COP) reduction of 5.9%, while propane experiences the lowest reduction of 2.5%. The different sensitivity of the working fluids to maldistribution was found to be related to the evaporator design, refrigerant pressure drop, and fluid properties. Last, the results of the economic analysis show that a higher specific cost of heat is obtained when considering maldistribution effects.

Modeling framework to predict two-phase flow distribution in heat exchanger headers

Mini- and microchannel heat exchangers have the potential to drastically decrease the size and cost of thermal systems, but they often underperform because of flow maldistribution. Maldistribution can be particularly acute when a two-phase mixture enters a heat exchanger header, which is difficult to address because the flow phenomena in these situations are poorly understood. Experimental studies in recent years have attempted to gain a better understanding of the flow regimes and distribution in manifolds, but methods to predict distribution in multi-channel heat exchangers have been lagging. A new modeling approach, which predicts the liquid and vapor flow rates in each channel of a microchannel heat exchanger based on header flow patterns and component pressure drop, is presented here. The proposed model shows good agreement with previously reported experimental results, predicting most of the data within the experimental error.