Periodic Flow Reversal Research Articles

New approach to improve performance by venting periodic reverse vapor flow in microchannel evaporator

This paper presents a proposal for a venting reverse vapor in flash gas removal A/C system in order to improve refrigerant distribution and reduce pressure drop in microchannel evaporator and thus increase system efficiency. Introduction to the reverse vapor flow observed in parallel flow microchannel evaporator was presented in earlier IJR paper by the authors. An experimental comparison of the A/C system with new approach to an FGB system revealed that vapor venting provided a 5% increase of cooling capacity and 3% of COP when operated at identical test conditions, while the maximum COP improvement was approximately 10%–12% when capacity is matched by reduction of compressor speed. The improvement compared to direct expansion system was significantly higher.

Periodic Reverse Flow and Boiling Fluctuations in a Microchannel Evaporator of an R134a Mobile Air-Conditioning System

Periodic Reverse Flow and Boiling Fluctuations in a Microchannel Evaporator of an R134a Mobile Air-Conditioning System

Periodical reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system

This paper presents the phenomenon of periodic reverse flow and associated boiling fluctuation found in experiments with a parallel microchannel evaporator used in an R134a air conditioning system. A simultaneous flow visualizations and measurements confirmed the periodic flow reversal. It caused synchronized oscillations of the evaporator inlet pressure and the pressure drop. The magnitude and frequency of oscillations increased with heat flux. Three potential impacts of flow reversal on evaporator performance are identified: 1) moderate liquid maldistribution; 2) reduced heat transfer coefficient; 3) increased refrigerant side pressure drop. Finally, to mitigate impacts of periodic reverse flow, a solution is proposed: to vent and bypass backflow vapor accumulated in the inlet header.

Overcoming the drawbacks of microsieves with micromeshes for in situ product recovery

During bioconversion processes in situ product recovery using membranes has been proven to be very effective for continuous product recovery from bioconversion processes. In particular, submerged membrane filtration offers an advantage over the conventional processes. Here it is crucial to provide effective fouling control to avoid loss in filtration performance. We recently developed an operation concept called reverse-flow diafiltration. It is characterized by a periodic flow reversal by substrate feeding, so cake-layer formation is prevented and substrate is supplied over the same membrane. In the presented research work, we focus on the use of high-flux microfiltration membranes, namely microsieves and micromeshes, using model yeast suspensions. These membranes provide of factor 5 higher permeabilities than conventional microfiltration membranes. Both membranes were tested in conventional submerged operation with backflushing and with reverse-flow diafiltration. For process evaluation transmembrane pressure profile and cell retention were analysed. The results show that (1) reverse-flow diafiltration allows easier process control at factor 2 lower transmembrane pressures than in submerged filtration and (2) micromeshes are advantageous in terms of fouling control and operability. TMP increase rates for micromeshes were about half as high as for microsieves in submerged filtration. Cell retention of the micromeshes could be improved by pore size reduction with layer-by-layer polyelectrolyte coating.

On the oceanic communication between the Western Subarctic Gyre and the deep Bering Sea

Sparse information is available on the communication between the northern North Pacific and the southern Bering Sea. We present results from a multi-decadal simulation of a high-resolution, pan-Arctic ice-ocean model to address the long-term mean and variability and synthesize limited observations in the Alaskan Stream, Western Subarctic Gyre, and southern Bering Sea. While the mean circulation in the Bering Sea basin is cyclonic, during the 26-year simulation meanders and eddies are continuously present throughout the region, which is consistent with observations from Cokelet and Stabeno (1997). Prediction (instead of prescription) of the Alaskan Stream and Aleutian throughflow allows reproduction of meanders and eddies in the Alaskan Stream and Kamchatka Current similar to those that have been observed previously (e.g. Crawford et al., 2000; Rogachev and Carmack, 2002; Rogachev and Gorin, 2004). Interannual variability in mass transport and property fluxes is particularly strong across the western Aleutian Island Passes, including Buldir Pass, Near Strait, and Kamchatka Strait. Much of this variability can be attributed to the presence of meanders and eddies found both north and south of the passes, which are found to directly cause periodic flow reversals and maxima in the western passes. Given that modeled flow reversals and maxima last for time periods ranging between three months and two years, short-term observations (months to few years) may not be representative of the actual mean flow. These extremes in the communication across the Aleutian Island Passes have a large impact on the oceanic environmental conditions in the southern Bering Sea and could directly impact biological species there and further downstream. Therefore, we identify a need for continuous monitoring of the flow through Buldir Pass, Near, and Kamchatka straits.

Experimental and Simulated Performances of a Batch-Type Longan Dryer with Air Flow Reversal Using Biomass Burner as a Heat Source

This article presents experimental performance of a batch-type longan dryer using a biomass burner with air flow reversal and also presents modeling of the longan dryer for drying of whole longan. The dryer essentially consists of a biomass burner and a drying bin with an arrangement for periodic air flow reversal. Three drying runs with loading capacity of 2,000, 1,500, and 1,000 kg of whole longan were carried out. There was no significant difference in temperatures in different positions (except inlet and outlet) inside the dryer (p < 0.05) or moisture content inside the dryer (p < 0.05). Whole longan was dried from an initial moisture content of 74% (wb) to a final moisture content of 14% (wb). The drying time of whole longan in the longan dryer was 60, 54, and 48 h for 2,000, 1,500, and 1,000 kg loading, respectively. The quality of dried product was also good in comparison to the high-quality product in markets. To simulate the performance of the longan dryer for drying of whole longan, a set of partial differential equations was developed and the equations were solved using the finite difference technique. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5 (Compaq Computer Corp., TX). The simulated moisture contents agreed well with the experimental data. This model can be used to provide the design data and it is also essential for optimal dryer design.

Reciprocating air flow for Li-ion battery thermal management to improve temperature uniformity

The thermal management of traction battery systems for electrical-drive vehicles directly affects vehicle dynamic performance, long-term durability and cost of the battery systems. In this paper, a new battery thermal management method using a reciprocating air flow for cylindrical Li-ion (LiMn 2O 4/C) cells was numerically analyzed using (i) a two-dimensional computational fluid dynamics (CFD) model and (ii) a lumped-capacitance thermal model for battery cells and a flow network model. The battery heat generation was approximated by uniform volumetric joule and reversible (entropic) losses. The results of the CFD model were validated with the experimental results of in-line tube-bank systems which approximates the battery cell arrangement considered for this study. The numerical results showed that the reciprocating flow can reduce the cell temperature difference of the battery system by about 4 °C (72% reduction) and the maximum cell temperature by 1.5 °C for a reciprocation period of τ = 120 s as compared with the uni-directional flow case ( τ = ∞). Such temperature improvement attributes to the heat redistribution and disturbance of the boundary layers on the formed on the cells due to the periodic flow reversal.

Mixing Process of Two Different Gases by a Pulsed Jet with a Periodic Reverse Flow (A Case Issued Carbon Dioxide Gas into Still Air)

A strongly pulsed jet is considered as a method for rapid mixing of two different gases. Carbon dioxide gas with a pulsation was discharged from a convergent and round nozzle into still air. An oscillatory and reversed flow at the nozzle exit was produced by the pulsation with high intensity. The spatial and temporal variations of concentration and velocity were measured in the mixing region of the pulsed jet. The mixing rate of the gases is dramatically enhanced near the nozzle exit. Neither the mean concentration nor the mean velocity monotonically decays in the stream-wise direction, because the ambient air is drawn inside the nozzle periodically by the reversed flow. The direction of the velocity changes periodically at a certain boundary, which is the position where the mean concentration becomes the minimal value. When the mixing gas is aspirated inside the nozzle, the fluid lump of any concentration is separated to two fluid lumps because of the direction change in the velocity. The issuing mean velocity becomes larger than the velocity determined by flow rate of supplied carbon dioxide gas. The axial distribution of the mean velocity increases near the nozzle exit. There is also a phase difference between the concentration and velocity oscillations.

Chondrogenic differentiation of human adipose-derived stem cells in polyglycolic acid mesh scaffolds under dynamic culture conditions

Chondrogenic differentiation of human adult adipose-derived stem cells was studied in vitro for the development of engineered cartilage tissue. Cells cultured under dynamic conditions in polyglycolic acid (PGA) scaffolds produced substantially higher glycosaminoglycan (GAG) and total collagen levels than cells in pellet cultures. This result reflects the importance of cell attachment and cell–scaffold interactions in stem cell differentiation and chondrogenesis. Although gene expression levels for both aggrecan and collagen type II were up-regulated significantly in PGA cultures treated with transforming growth factor β1 (TGF-β1), synthesis of GAG but not collagen type II was enhanced in tissue constructs when TGF-β1 was added to the medium. Bone morphogenetic protein-6 (BMP-6) in the presence of TGF-β1 was effective in improving GAG and total collagen production when the cells were pre-treated with fibroblast growth factor-2 (FGF-2) prior to scaffold seeding. Extending the culture duration from 2 to 5 weeks did not improve cartilage development in PGA scaffolds; loss of cells from the constructs suggested that the rate of scaffold degradation exceeded the rate of replacement by ECM during the 5-week period. Stem cells in PGA scaffolds were cultured in perfusion-type recirculation bioreactors operated with periodic medium flow reversal. The highest levels of GAG and collagen type II accumulation were achieved in the bioreactor cultures after the seeding cell density was increased from 2 × 10 7 to 4 × 10 7 cells per scaffold.

Titanium-Water Loop Heat Pipe Operating Characteristics Under Standard and Elevated Acceleration Fields

An experiment has been developed to examine the behavior of a titanium-water loop heat pipe under standard and elevated acceleration fields. The loop heat pipe was mounted on a 2.44 meter-diameter centrifuge table on edge with heat applied to the evaporator via a mica heater and heat rejected using a high-temperature polyalphaolefin oil coolant loop. The loop heat pipe was tested under the following parametric ranges: heat load at the evaporator 100 ≤ Q in ≤ 600 W, heat load at the compensation chamber 0 ≤ Q cc ≤ 50 W, radial acceleration 0 ≤ a r ≤ 10 g. For stationary operation, the evaporative heat transfer coefficient decreased monotonically with heat load whereas the thermal resistance decreased to a minimum then increased. Heat input to the compensation chamber was found to increase the evaporative heat transfer coefficient and decrease the thermal resistance for Q in = 500 W. Transient periodic flow reversal in the loop heat pipe was found for some cases, which was likely due to vapor bubble formation in the primary wick. Operation in an elevated acceleration environment revealed that dry out was dependent on both Q in and a r , and the ability for the loop heat pipe to reprime after an acceleration event that induced dry out was influenced by the evaporator temperature. The evaporative heat transfer coefficient and thermal resistance were found not to be significantly dependent on radial acceleration. However, the evaporator wall superheat was found to increase slightly with radial acceleration at high heat loads.

Repetitive model predictive control of a reverse flow reactor

This paper deals with the control of a catalytic reverse flow reactor (RFR) used for methane combustion. The periodic flow reversals effected on the system makes it both continuous and discrete in nature (i.e., a hybrid system). Control of this system is challenging due to the unsteady state behavior of the process along with its mixed discrete and continuous behavior. Although model predictive control (MPC) is proven to be a powerful technique for several processes it becomes less effective in systems such as the RFR where the model prediction errors and the effect of disturbances on the plant output repeat from time to time. In such cases, control can be improved if the repetitive error pattern is exploited. A novel repetitive model predictive control (RMPC) strategy, that combines the basic concepts of iterative learning control (ILC) and repetitive control (RC) along with the concepts of MPC, is proposed for such systems. In the proposed strategy, the state variables of the model are reset periodically along with predictive control action such that the process follows the reference trajectory as closely as possible. The results obtained prove that the RMPC approach provides an excellent performance for the control of the RFR.

Efficient reheating of a reverse-flow reformer—An experimental study

Energy supply for strongly endothermic reactions like steam reforming can be provided efficiently by internal combustion in an adiabatic fixed-bed operated under periodic flow reversal. The key to an optimal process with high productivity is a periodic reheating of the fixed-bed to a favourable temperature profile avoiding hotspots by homogeneous ignition. Therefore, combustion experiments have been performed under realistic conditions. Homogeneous combustion can be suppressed by addition of an inhibitor like steam or CO 2 even if hydrogen is burned; however, in reverse-flow operation, a time-variant fuel addition needs to be installed in order to limit the maximum temperature. Autothermal methane steam reforming can then be conducted with high productivity using a synthetic PSA-offgas for combustion.

Regular Transient Loading Response in a Vapor-Phase Flow-Direction-Switching Biofilter

The principal objective of this study was determination of the response of a laboratory-scale vapor-phase flow-direction-switching biofilter to loading changes associated with normal operations such as lunch breaks, overnight shutdowns, and single-shift operation of commercial and industrial facilities. Three regular transient loading cases were considered: (a) variable flow-reversal interval lenghts, (b) variable feed-on/off interval lengths, and (c) variable inlet concentration during a repeating feed-on/off cycle. Toluene was used as the model contaminant compound. The most significant findings of the study were: (1) Relative to unidirectional mode of operation, periodic flow reversal produced a more uniform distribution of reaction capacity along the length of the packed bed; (2) a 12 h flow reversal interval was sufficiently short to maintain the toluene-degrading microbial community in a near-fully active state throughout the unit whereas a 2 day flow reversal interval resulted in diminished removal rates in the first half of the bed and (3) Increasing off-period length resulted in greater penetration of contaminant into the bed and more uniform removal rates along the length of the bed. Information developed in this study should provide a more complete basis for establishing operating protocols and monitoring regulations for vapor-phase biofiltration systems.

Transient Response of Flow-Direction-Switching Vapor-Phase Biofilters

Transient loading of vapor-phase biofilters may result in exceedence of the local reaction or mass transfer capacity of the inlet region. In such cases, higher concentrations of contaminants are carried deeper into the bed where there is less active biomass and, in some cases, breakthrough of contaminants may occur. Previous studies have demonstrated that periodic reversal of the flow direction results in improved transient-loading response. However, quantitative information on the extent of the benefit is lacking. Step function increases in toluene concentration were applied to unidirectional-flow and flow-direction-switching laboratory reactors operated in parallel. Contaminant concentration was monitored at several points along the packed beds. Relative to unidirectional mode of operation, periodic flow reversal produced a more uniform distribution of microbial reaction capacity along the length of the packed bed. Directional switching at a 12-h interval did not result in a loss of activity or removal capacity. Mass-removal rates under transient-loading conditions were similar in the first-half of both biofilters but, in the second-half of the units, significant removals were observed only in the flow-direction-switching biofilter. As a result, maximum mass-removal rates under transient-loading conditions were approximately twice as great for the flow-direction-switching biofilter relative to the conventional unidirectional-flow biofilter receiving similar mass loading.

Time-dependent, three-dimensional flow and mass transport during solution growth of potassium titanyl phosphate

A finite-element, numerical model is used to compute time-dependent, three-dimensional fluid flow, mass transfer, and continuum growth kinetics in the potassium titanyl phosphate (KTP) solution crystal growth system of Bordui et al. The effects of a periodically-reversing crystal rotation schedule are analyzed for two different crystal-mounting geometries. Results suggest a lower probability of the occurrence of defects when the mounting geometry is designed to take advantage of periodic flow reversal effects on the supersaturation field.

Upscaling and reversibility of Taylor dispersion in heterogeneous porous media

Tracer flow in stratified porous media is dominated by the interaction between convective transport and transverse diffusive mixing. By averaging the tracer concentration in the transverse direction, a one-dimensional non-Fickian dispersion model is derived. The model accounts for the relaxation process that reduces the convective transport to dispersive mixing. This process is (short-) time correlated and partially reversible upon reversal of flow direction. For multiscale velocity fields, the relaxation is a multiscale process. To date only single scale processes have been successfully upscaled. Our procedure extends this to multiscale processes, using scale separation. The model parameters can be calculated a priori based on the velocity profile. For periodic flow reversal, the results are essentially the same. Despite the non-Fickian behavior during a cycle, the net contribution of each cycle to the spreading relaxes to a Fickian process in a similar way as for unidirectional flow. The cycle time averaged dispersion coefficient is a monotonically increasing function of the reversal time. It asymptotically converges towards the effective dispersion coefficient in the absence of flow reversal.

A MATLAB implementation of upwind finite differences and adaptive grids in the method of lines

In this paper, we report on the development of a MATLAB library for the solution of partial differential equation systems following the method of lines. In particular, we focus attention on upwind finite difference schemes and grid adaptivity, i.e., grid movement or grid refinement. Several algorithms are presented and their performance is demonstrated with illustrative examples including a fixed-bed reactor with periodic flow reversal, a model of flame propagation, and the Korteweg–de Vries equation.

Hydrogen generation in a reverse-flow microreactor: 2. Simulation and analysis

The second part in this series presents parametric simulation and sensitivity analysis of methane partial oxidation in a microreactor operated with periodic flow reversal. The reverse-flow (RF) operation provides up to a 5% increase in the hydrogen yield compared to that of the unidirectional (UD) operation. The effect of varying the inlet CH4:O2 feed ratio, the inlet velocity, and the inlet gas temperature is studied. The optimal feed ratio is found to be CH4:O2 = 1.15:1 with the feed at ambient room temperature. The effects of varying the reactor length and heat losses to the ambient are also investigated. In both these cases, the performance of the UD operation degrades significantly, whereas the performance of the RF operation is more robust to the variations in reactor size and heat losses. The issue of choosing the optimal switching time is also addressed. Switching the input and output ports near the natural timescale of reaction heat release is shown to provide the optimum yield. Through these results, design and operation guidelines based on simple timescale analysis have emerged. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Tissue engineering of human cartilage in bioreactors using single and composite cell-seeded scaffolds

Chondrocytes isolated from human fetal epiphyseal cartilage were seeded under mixed conditions into 15-mm-diameter polyglycolic acid (PGA) scaffolds and cultured in recirculation column bioreactors to generate cartilage constructs. After seeding, the cell distributions in thick (4.75 mm) and thin (2.15 mm) PGA disks were nonuniform, with higher cell densities accumulating near the top surfaces. Composite scaffolds were developed by suturing together two thin PGA disks after seeding to manipulate the initial cell distribution before bioreactor culture. The effect of medium flow direction in the bioreactors, including periodic reversal of medium flow, was also investigated. The quality of the tissue-engineered cartilage was assessed after 5 weeks of culture in terms of the tissue wet weight, glycosaminoglycan (GAG), total collagen and collagen type II contents, histological analysis of cell, GAG and collagen distributions, and immunohistochemical analysis of collagen types I and II. Significant enhancement in construct quality was achieved using composite scaffolds compared with single PGA disks. Operation of the bioreactors with periodic medium flow reversal instead of unidirectional flow yielded further improvements in tissue weight and GAG and collagen contents with the composite scaffolds. At harvest, the constructs contained GAG concentrations similar to those measured in ex vivo human adult articular cartilage; however, total collagen and collagen type II levels were substantially lower than those in adult tissue. This study demonstrates that the location of regions of high cell density in the scaffold coupled with application of dynamic bioreactor operating conditions has a significant influence on the quality of tissue-engineered cartilage.

Comparing flow‐reversal and inner recirculation reactors: Experiments and simulations

AbstractThe operation of reactors with flow reversal operate similar to a reactor with internal recirculation, which the feed enters through one (say, inner) reactor and then turns around and flows out through (the outer) another, when the heat‐transfer coefficient between the tubes is large. In this study, we compare the behavior of a packed‐bed reactor operating in flow‐reversal or internal‐recirculation modes, using ethylene oxidation on Pt/Al2O3 as a model reaction. The reactor was built from two concentric tubes (with 28.5 and 42.5 mm in diameter), both packed with a 20 cm catalytic bed and 10 cm inert beds (of alumina‐pellets) on each side. An adjustable opening between the tubes allowed for an internal recycle mode and the whole system could be operated with periodic flow reversal. The reactor can be employed then either as a simple once‐through bed or as a bed with flow reversal in the inner tube or as bed with internal recirculation flowing from the inner to outer tube, or in the opposite direction, as well as an internal‐recirculation reactor with flow reversal. Due to heat losses, the latter two modes were inferior to the others. The experiments, backed by simulations using a homogeneous model with independently determined parameters, showed that the technically‐simpler inner‐outer internal‐recycle reactor operated better at low flow rates, than that with flow reversal, but the conclusion is reversed at high flow rates. The domain where the internal‐recirculation reactor is superior depends on the heat‐transfer coefficient between the streams. By lowering the feed concentration, the extinction point was determined for each mode highlighting again the conclusions drawn above that inner‐recirculation operation may be superior to flow reversal at low flow rates. Simulations revealed also the existence of solutions with stationary fronts or oscillatory fronts.