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Brian Elmegaard - One of the best experts on this subject based on the ideXlab platform.
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Performance of heat pumps using pure and mixed refrigerants with Maldistribution effects in plate heat exchanger evaporators
International Journal of Refrigeration, 2019Co-Authors: Roberta Mancini, Benjamin Zühlsdorf, Vikrant Aute, Wiebke Brix Markussen, Brian ElmegaardAbstract:Abstract 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.
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Maldistribution in air water heat pump evaporators part 1 effects on evaporator heat pump and system level
International Journal of Refrigeration-revue Internationale Du Froid, 2015Co-Authors: Gunda Mader, Björn Palm, Brian ElmegaardAbstract:Abstract This paper presents an approach to quantify the effect of evaporator Maldistribution on operating costs of air–water heat pumps. In the proposed simulation model Maldistribution is induced by two parameters describing refrigerant phase and air flow distribution. Annual operating costs are calculated based on heat pump performance at distinct operating conditions. Results show that percentage increase of operating costs is similar for the three considered climate zones, even though the effect of Maldistribution on heat pump performance varies with operating conditions. Differences in terms of absolute cost increase for the climate zones arise mainly due to a varying number of operating hours. Absolute cost increase is considerable in the average and especially colder climate zone and can only partly be reduced by enlarging the evaporator.
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Maldistribution in air-water heat pump evaporators. Part 2: Economic analysis of counteracting technologies
International Journal of Refrigeration, 2015Co-Authors: Gunda Mader, Björn Palm, Brian ElmegaardAbstract:Abstract In this study a methodology is applied to quantify the effect of evaporator Maldistribution on operating costs of air–water heat pumps. The approach is used to investigate the cost-effectiveness of two technologies enabling to counteract Maldistribution: a flash gas bypass setup and the individual superheat control in parallel evaporator channels. In the total cost of ownership analysis, different scenarios for climatic conditions, severity of Maldistribution, and economic framework are considered. Results show that the flash gas bypass system is cost-effective only in a few conditions, namely severe Maldistribution, high electricity prices, and colder climate. Investment in the individual superheat control technology, however, can be quickly amortized in many scenarios. For the warmer climate zone with a small number of operating hours counteracting of Maldistribution does not pay off under the used economic assumptions.
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Comparison of fin-and-tube interlaced and face split evaporators with flow Maldistribution and compensation
International Journal of Refrigeration, 2013Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Lars Finn Sloth LarsenAbstract:Abstract Flow Maldistribution in fin-and tube evaporators for residential air-conditioning is investigated by numerical simulation. In particular, the interlaced and the face split evaporator are compared in flow Maldistribution conditions. The considered sources of Maldistribution are the liquid/vapor distribution and the airflow distribution. Furthermore, compensation of flow Maldistribution by control of individual channel superheat is studied for each evaporator type. It is shown that the interlaced evaporator is better at flow Maldistribution than the face split evaporator. However, if individual channel superheats are controlled, the face split evaporator achieves the best performance, i.e. an increase of 7% in overall UA-value and 1.6–2.4% in COP compared to the interlaced evaporator without compensation.
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performance of residential air conditioning systems with flow Maldistribution in fin and tube evaporators
International Journal of Refrigeration-revue Internationale Du Froid, 2011Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Wiebke Brix, Lars Finn Sloth LarsenAbstract:Refrigerant and airflow Maldistribution in fin-and-tube evaporators for residential air-conditioning was investigated with numerical modeling. Fin-and-tube heat exchangers usually have a pre-defined circuitry. However, the objective in this study was to perform a generic investigation of each individual Maldistribution source in an independent manner. Therefore, the evaporator and the condenser were simplified to be straight tubes for the purposes of this study. The numerical model of the R410A system, its verification and an investigation of individual Maldistribution sources are presented in this paper. The Maldistribution sources of interest were: inlet liquid/vapor phase distribution, feeder tube bending and airflow distribution. The results show that Maldistribution reduced the cooling capacity and the coefficient of performance of the system. In particular, different phase distribution and non-uniform airflow distribution reduced the performance significantly. Different feeder tube bendings only caused a minor decrease in performance.
Bengt Sundén - One of the best experts on this subject based on the ideXlab platform.
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On Flow Maldistribution in PEMFC Stacks
International Journal of Green Energy, 2011Co-Authors: Jinshi Wang, Junjie Yan, Jinliang Yuan, Bengt SundénAbstract:Polymer electrolyte membrane fuel cell (PEMFC) stacks are widely studied for their own advantages. The flow Maldistribution in unit cells may severely influence the fuel cell stack performance, mainly including the uniformity of current density and the voltage. Investigations of flow Maldistribution in PEMFC stacks are rarely found, and presented results are unsystematic, scattered, and even contradictory. Thus, it is necessary to review and summarize the previous concerned papers, and to get some methods or guidelines for reducing the flow Maldistribution in PEMFC stacks. In this paper, the existing literature concerning flow Maldistributions in PEMFC stacks is reviewed. The effects of the arrangement of flow configurations, design parameters, and operating conditions on the flow Maldistribution are discussed. Some suggestions are outlined to reduce the flow Maldistribution in PEMFC stacks. The need for further research is also discussed.
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Influence of Flow Maldistribution on the Pressure Drop and Water Condensation in a 100 kW PEM Fuel Cell Stack
Volume 6: Energy Systems: Analysis Thermodynamics and Sustainability, 2007Co-Authors: Takamasa Ito, Jinliang Yuan, Bengt SundénAbstract:The Proton Exchange Membrane (PEM) fuel cell has been investigated for a long time because it has a high power density, low emission, wide choice of fuel sources, etc. However, the performance of a stack of PEM cells degrades with respect to that of a single cell. This is mainly due to the introduction of the manifold in which supplies the reactants to individual cells. The manifold can induce an additional pressure drop and a flow Maldistribution. Therefore, it is critical to consider the interaction between the design of the manifold and the flow Maldistribution. In this study, the sensitivity of the friction factor in the individual cells to the flow Maldistribution and the interaction between the flow Maldistribution and water condensation are discussed. As a result, it is found that the friction factors in the individual cells strongly influences the flow Maldistribution especially when the flow in the unit cell is changed from laminar flow to turbulent. Large flow Maldistributions induce the large pressure drops. As a result, water condensation can be little. However, it also induces a degradation in stack performance. A bipolar plate with a low number of flow channels can improve the flow Maldistribution and water condensation in the unit cell although at the expense of a large pressure drop. (Less)
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thermal analysis of plate condensers in presence of flow Maldistribution
International Journal of Heat and Mass Transfer, 2006Co-Authors: Prabhakara Rao Bobbili, Bengt Sundén, S DasAbstract:Flow Maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow Maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of Maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.
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transient response of plate heat exchangers considering effect of flow Maldistribution
International Journal of Heat and Mass Transfer, 2005Co-Authors: N Srihari, Bengt SundénAbstract:Plate heat exchangers have been playing important role in the power and process industries in the recent past. Hence, it is important to develop simulation strategies for plate heat exchangers accurately. This analysis represents the dynamic behaviour of the single pass plate heat exchangers, considering flow Maldistribution from port to channel. In addition to Maldistribution the fluid axial dispersion is used to characterise the back mixing and other deviations from plug flow. Due to unequal distribution of the fluid, the velocity of the fluid varies from channel to channel and hence the heat transfer coefficient variation is also taken into consideration. Solutions to the governing equations have been obtained using the method of Laplace transform followed by numerical inversion from frequency domain. The results are presented on the effects of flow Maldistribution and conventional heat exchanger parameters on the temperature transients of both U-type and Z-type configurations. It is found that the effect of flow Maldistribution is significant and it deteriorates the thermal performance as well as the characteristic features of the dynamic response of the heat exchanger. In contrast to the previous studies, here the axial dispersion describes the inchannel back mixing alone, not Maldistribution, which is physically more appropriate. Present method is an efficient and consistent way of describing Maldistribution and back mixing effects on the transient response of plate heat exchangers using an analytical method without performing intensive computation by complete numerical simulation.
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an experimental and theoretical investigation of the effect of flow Maldistribution on the thermal performance of plate heat exchangers
Journal of Heat Transfer-transactions of The Asme, 2005Co-Authors: Bengt SundénAbstract:An experimental and theoretical study of the effect of flow Maldistribution from port to channel on the thermal performance of single and multipass plate heat exchangers is presented. In general, flow Maldistribution brings about an increase in pressure drop and decrease of the thermal performance in heat exchangers. This deterioration is found to depend on flow rate, number of channels, and port size. Experiments show that analytical predictions of pressure drop and thermal performance in presence of flow Maldistribution are quite accurate for practical purposes. The results indicate that under identical conditions, Maldistribution is more severe in Z-type plate heat exchanger compared to U type. Multipassing is found to reduce the Maldistribution effect significantly. An insight to the physical aspects of Maldistribution and its possible reduction through proper design strategy are also presented. (Less)
Lars Finn Sloth Larsen - One of the best experts on this subject based on the ideXlab platform.
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Comparison of fin-and-tube interlaced and face split evaporators with flow Maldistribution and compensation
International Journal of Refrigeration, 2013Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Lars Finn Sloth LarsenAbstract:Abstract Flow Maldistribution in fin-and tube evaporators for residential air-conditioning is investigated by numerical simulation. In particular, the interlaced and the face split evaporator are compared in flow Maldistribution conditions. The considered sources of Maldistribution are the liquid/vapor distribution and the airflow distribution. Furthermore, compensation of flow Maldistribution by control of individual channel superheat is studied for each evaporator type. It is shown that the interlaced evaporator is better at flow Maldistribution than the face split evaporator. However, if individual channel superheats are controlled, the face split evaporator achieves the best performance, i.e. an increase of 7% in overall UA-value and 1.6–2.4% in COP compared to the interlaced evaporator without compensation.
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performance of residential air conditioning systems with flow Maldistribution in fin and tube evaporators
International Journal of Refrigeration-revue Internationale Du Froid, 2011Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Wiebke Brix, Lars Finn Sloth LarsenAbstract:Refrigerant and airflow Maldistribution in fin-and-tube evaporators for residential air-conditioning was investigated with numerical modeling. Fin-and-tube heat exchangers usually have a pre-defined circuitry. However, the objective in this study was to perform a generic investigation of each individual Maldistribution source in an independent manner. Therefore, the evaporator and the condenser were simplified to be straight tubes for the purposes of this study. The numerical model of the R410A system, its verification and an investigation of individual Maldistribution sources are presented in this paper. The Maldistribution sources of interest were: inlet liquid/vapor phase distribution, feeder tube bending and airflow distribution. The results show that Maldistribution reduced the cooling capacity and the coefficient of performance of the system. In particular, different phase distribution and non-uniform airflow distribution reduced the performance significantly. Different feeder tube bendings only caused a minor decrease in performance.
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Compensation of flow Maldistribution in fin-and-tube evaporators for residential air-conditioning
International Journal of Refrigeration, 2011Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Wiebke Brix, Lars Finn Sloth LarsenAbstract:Compensation of flow Maldistribution in multi-channel fin-and-tube evaporators for residential air-conditioning is investigated by numerical modeling. The considered sources of Maldistribution are distribution of the liquid and vapor phases in the distributor and non-uniform airflow distribution. Fin-and-tube heat exchangers usually have a predefined circuitry, however, the evaporator model is simplified to have straight tubes, in order to perform a generic investigation. The compensation of flow Maldistribution is performed by control of the superheat in the individual channels. Furthermore, the effect of combinations of individual Maldistribution sources is investigated for different evaporator sizes and outdoor temperatures. It is shown that a decrease in cooling capacity and coefficient of performance by flow Maldistribution can be compensated by the control of individual channel superheat. Alternatively, a larger evaporator may be used.
Ken S Chen - One of the best experts on this subject based on the ideXlab platform.
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Analytical model of flow Maldistribution in polymer electrolyte fuel cell channels
Chemical Engineering Science, 2010Co-Authors: Suman Basu, Chaoyang Wang, Ken S ChenAbstract:Gas–liquid, two-phase flow through channels of a polymer electrolyte fuel cell (PEFC) is of great interest as reactant oxygen is supplied and liquid product water is removed via these PEFC channels. Gas diffusion layer (GDL) intrusion in the channels, which is inherent to the process of PEFC cell and stack assembling, increases the local flow resistance in the intruded channels and consequently lowers their flowrates. This flow Maldistribution renders the intruded channels more susceptible to liquid water accumulation or flooding. A one-dimensional analytical model is developed in this work to elucidate the two-phase flow Maldistribution in PEFC channels resulting from GDL intrusion. Relative humidity (RH) and the stoichiometric flow ratio of inlet gases are found to be the two key parameters controlling the flow Maldistribution in PEFC channels. Interestingly, our analysis shows that decreasing the inlet RH worsens flow Maldistribution. As GDL intrusion in channels is inevitable, a good flow-field design must be inherently tolerable to flow Maldistribution. Using the analytical model presented herein, the number of flow channels and their U-turns are optimized to minimize the detrimental effect of GDL intrusion.
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Two-Phase Flow Maldistribution and Mitigation in Polymer Electrolyte Fuel Cells
Journal of Fuel Cell Science and Technology, 2009Co-Authors: Suman Basu, Chaoyang Wang, Ken S ChenAbstract:Flow Maldistribution among polymer electrolyte fuel-cell (PEFC) channels is of concern because this leads to nonuniform distributions of fuel and oxidizer, which in turn result in nonuniform reaction rates in the catalyst layers and thus detrimentally affect PEFC performance and durability. Channels with low flow rates risk flooding by liquid water. This can cause catalyst support corrosion and hence the undesirably accelerated aging of PEFCs. Multiphase flow computations are performed to examine the effects of gas diffusion layer (GDL) intrusion and manifold design on reducing flow Maldistribution. Velocity field, hydrodynamic pressure, and liquid saturations are computed in the parallel gas channels using the multiphase-mixture formulation in order to quantify the flow nonuniformity or Maldistribution among PEFC channels. It is shown that, when channel flow is in single phase, employing two splitter plates in the header manifold can bring down the flow Maldistribution to less than half of that for the case with 20% area Maldistribution due to the GDL intrusion. When channel flow occurs in the two-phase regime, the liquid-water front can be pushed downstream and the effect of GDL intrusion on the maximum liquid saturation can be decreased by more than one-third by using flow splitters.
Martin Ryhl Kærn - One of the best experts on this subject based on the ideXlab platform.
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Comparison of fin-and-tube interlaced and face split evaporators with flow Maldistribution and compensation
International Journal of Refrigeration, 2013Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Lars Finn Sloth LarsenAbstract:Abstract Flow Maldistribution in fin-and tube evaporators for residential air-conditioning is investigated by numerical simulation. In particular, the interlaced and the face split evaporator are compared in flow Maldistribution conditions. The considered sources of Maldistribution are the liquid/vapor distribution and the airflow distribution. Furthermore, compensation of flow Maldistribution by control of individual channel superheat is studied for each evaporator type. It is shown that the interlaced evaporator is better at flow Maldistribution than the face split evaporator. However, if individual channel superheats are controlled, the face split evaporator achieves the best performance, i.e. an increase of 7% in overall UA-value and 1.6–2.4% in COP compared to the interlaced evaporator without compensation.
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performance of residential air conditioning systems with flow Maldistribution in fin and tube evaporators
International Journal of Refrigeration-revue Internationale Du Froid, 2011Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Wiebke Brix, Lars Finn Sloth LarsenAbstract:Refrigerant and airflow Maldistribution in fin-and-tube evaporators for residential air-conditioning was investigated with numerical modeling. Fin-and-tube heat exchangers usually have a pre-defined circuitry. However, the objective in this study was to perform a generic investigation of each individual Maldistribution source in an independent manner. Therefore, the evaporator and the condenser were simplified to be straight tubes for the purposes of this study. The numerical model of the R410A system, its verification and an investigation of individual Maldistribution sources are presented in this paper. The Maldistribution sources of interest were: inlet liquid/vapor phase distribution, feeder tube bending and airflow distribution. The results show that Maldistribution reduced the cooling capacity and the coefficient of performance of the system. In particular, different phase distribution and non-uniform airflow distribution reduced the performance significantly. Different feeder tube bendings only caused a minor decrease in performance.
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Compensation of flow Maldistribution in fin-and-tube evaporators for residential air-conditioning
International Journal of Refrigeration, 2011Co-Authors: Martin Ryhl Kærn, Brian Elmegaard, Wiebke Brix, Lars Finn Sloth LarsenAbstract:Compensation of flow Maldistribution in multi-channel fin-and-tube evaporators for residential air-conditioning is investigated by numerical modeling. The considered sources of Maldistribution are distribution of the liquid and vapor phases in the distributor and non-uniform airflow distribution. Fin-and-tube heat exchangers usually have a predefined circuitry, however, the evaporator model is simplified to have straight tubes, in order to perform a generic investigation. The compensation of flow Maldistribution is performed by control of the superheat in the individual channels. Furthermore, the effect of combinations of individual Maldistribution sources is investigated for different evaporator sizes and outdoor temperatures. It is shown that a decrease in cooling capacity and coefficient of performance by flow Maldistribution can be compensated by the control of individual channel superheat. Alternatively, a larger evaporator may be used.