Forward Flow Mode Research Articles

Experimental study on convective heat transfer of an open-loop borehole heat exchanger

Open-loop borehole heat exchanger (OBHE) is a single well geothermal heat exchanger with an open-loop structure that can realize the geothermal energy extraction without mining the geothermal water. In this paper, a sandbox experiment is designed to simulate the convective heat transfer process in the reservoir area of OBHE. The mechanism of convective heat transfer in the reservoir area is studied, and the key factors that affect the convection heat transfer intensity are analyzed. The results show that the convection heat transfer of OBHE in the reservoir area is affected by both the driving effect of fluid flow inside the screen tube and the buoyancy effect. In the forward flow mode, the two effects have the opposite direction. While in the backward mode, the two effects have the same direction. The backward flow mode is more conducive to convective heat transfer. In addition, many factors influencing significantly the convective heat transfer of OBHE include inlet temperature, inlet flow rate, reservoir temperature, fluid flow direction and inner tube diameter.

Research on multiphysics coupling fields in electrochemical trepanning of lateral flow

Electrochemical trepanning (ECTr) is an effective method for machining the ruled surface parts. Generally, the forward flow mode is used in ECTr, under which the streamlines at the outlet are divergent, resulting in the obvious flow patterns at the outlet and the instability of the machining process. In ECTr of a diffuser with a special structure, the lateral flow mode is adopted to improve the uniformity of the flow field, thereby improving the surface quality at the hub. ECTr is a complicated multiphysics coupling process. To investigate the distributions of electric field, two-phase flow field and thermal field in ECTr with lateral flow, a multiphysics coupling field model was established. In this model, a coupling relationship was formed between the various physical fields through the change of the electrolyte conductivity. And a coupled multiphysics simulation was performed to show how the gas bubbles volume fraction, electrolyte temperature, electrolyte conductivity and current density change along the flow path. Compared with the inlet of the electrolyte, the gas bubbles volume fraction and the temperature at the outlet increased by 38.8% and 6.3 K, respectively. Under the combined influence, the conductivity decreased by 7.227 S/m at the outlet, resulting in a decrease of 57.81 A/cm2 in the current density. The simulation results were then verified against those from a corresponding experiment involving lateral flow ECTr. Along the flow path, the thickness of the machined blade gradually increased, varying from 2.09 mm to 2.76 mm. The surface quality gradually deteriorated along the flow path and the surface roughness varied from Ra 0.72 μm to Ra 1.05 μm. Combining the simulation and the experiment, the correctness and the effectiveness of the multiphysics coupling model and simulation were confirmed. The results can be applied to other ECM processes.

Optimization of flow field in electrochemical trepanning of integral cascades (Ti6Al4V)

Ti6Al4V is widely applied in the integral cascades of aero engines. As an effective machining method, electrochemical trepanning (ECTr) has unique advantages in processing surface parts made of hard-to-cut materials. In ECTr, the state of the flow field has a significant effect on processing stability and machining quality. To improve the uniformity of the flow field when ECTr is applied to Ti6Al4V, two different flow modes are designed, namely full-profile electrolyte supply (FPES) and edges electrolyte supply (EES). Different from the traditional forward flow mode, the flow directions of the electrolyte in the proposed modes are controlled by inlet channels. Simulations show that the flow field under EES is more uniform than that under FPES. To further enhance the uniformity of the flow field, the structure of EES is optimized by modifying the insulating sleeve. In the optimized configuration, the longitudinal distance between the center of the inlet hole and the center of the blade is 6.0 mm, the lateral distance between the centers of the inlet holes on both sides is 16.5 mm, the length to which the electrolyte enters the machining area is 1.5 mm, and the height of the insulating sleeve is 13.5 mm. A series of ECTr experiments are performed under the two flow modes. Compared with EES, the blade machined by FPES is less accurate and has poorer surface quality, with a surface roughness (Ra) of 3.346 μm. Under the optimized EES, the machining quality is effectively enhanced, with the surface quality improved from Ra = 2.621 μm to Ra = 1.815 μm, thus confirming the efficacy of the proposed methods.

Modelling foam improved oil recovery: towards a formulation of pressure-driven growth with flow reversal.

The pressure-driven growth model that describes the two-dimensional (2-D) propagation of a foam through an oil reservoir is considered as a model for surfactant-alternating-gas improved oil recovery. The model assumes a region of low mobility, finely textured foam at the foam front where injected gas meets liquid. The net pressure driving the foam is assumed to reduce suddenly at a specific time. Parts of the foam front, deep down near the bottom of the front, must then backtrack, reversing their flow direction. Equations for one-dimensional fractional flow, underlying 2-D pressure-driven growth, are solved via the method of characteristics. In a diagram of position versus time, the backtracking front has a complex double fan structure, with two distinct characteristic fans interacting. One of these characteristic fans is a reflection of a fan already present in forward flow mode. The second fan however only appears upon flow reversal. Both fans contribute to the flow's Darcy pressure drop, the balance of the pressure drop shifting over time from the first fan to the second. The implications for 2-D pressure-driven growth are that the foam front has even lower mobility in reverse flow mode than it had in the original forward flow case.

Analysis and Optimization of Flow Field in Electrochemical Machining of the staggered groove on Gear Tooth Surface

Self-lubricating gears can better adapt to the bad lubrication conditions without oil or oil scarcity. Staggered groove can be produced by Electrochemical machining (ECM) on the gear tooth surface to fill solid lubricating materials. The cathode with and without flow slot has been designed. Three basin models have been established which are forward flow mode, reverse flow mode and side flow mode. The gap flow field of ECM has been analyzed. It is determined that positive flow is a suitable flow mode. By optimizing the structure of the flow slot in the cathode, the deficiency of the forward flow field is improved.

Comparison of faradaic reactions in flow-through and flow-by capacitive deionization (CDI) systems

Use of flow-through electrodes, in which water flows through rather than past porous electrodes, has been shown to be a promising approach to overcoming mass transfer limitations in various electrochemical technologies (including microbial fuel cells, electrochemical disinfection, electrochemical advanced oxidation processes and capacitive deionization). In this work, we fabricated a flow-through CDI cell and investigated the saline water desalination performance and the possible Faradaic reactions occurring in the systems under different flow modes, i.e., forward mode (water flows from anode to cathode) and reverse flow mode (water flows from cathode to anode). Upon applying a charging voltage of 1.2 V, use of flow-through CDI operating under both flow conditions exhibited better desalination performance compared to that of conventional flow-by CDI. Larger pH fluctuations were found in flow-through CDI systems with effluent pH increasing to about 9.3 in forward flow mode and decreasing to 3.8 under reverse flow mode while in flow-by CDI, the treated water pH exhibited only a slight increase to 8.0. In addition, much higher H2O2 production and dissolved oxygen (DO) decay rates were observed in flow-through CDI operated in forward flow mode compared to other scenarios, possibly due to the different ion adsorption and reaction sequences. These findings provide insight into both the desalination capacity of flow-through CDI and the Faradaic reactions occurring in such a system and should be of value in developing a treatment technology capable of simultaneous water desalination, disinfection and/or micro-pollutant removal.

Flow field research on electrochemical machining with gas film insulation

Electrochemical machining (ECM) is an efficient method for fabricating crucial components of aircraft engines, such as integral blisks and diffusers. This paper proposes a new method for decreasing the stray corrosion in trepanning ECM, in which air is supplied around the machined workpiece to remove stray electrolyte. Specifically, compressed air was blown into the cathode and through the workpiece in a forward flow mode, thereby forming a gas film on the machined surface. Simulations were performed using the ANSYS software to determine the distributions of compressed air and electrolyte in the machined area. The simulation results demonstrated a uniform air–electrolyte distribution and a stable flow field under the optimal parameters. Experiments were conducted to verify these results, and the gas film was found to exert an obvious influence on reducing the taper angle and improving the surface quality of the workpiece. Severe corrosion of the workpiece occurred in the absence of air flow, whereas the stray corrosion was significantly reduced when air was supplied through the cathode. Upon increasing the feed rate from 0.5 to 1.5 mm/min, the taper angle decreased from −0.012° to −0.003° and the surface roughness decreased from Ra 0.841 μm to Ra 0.266 μm, whereas the optimal values for the samples obtained without the gas film were 0.096° and Ra 0.993 μm at the feed rate limit of 1.0 mm/min.

Analysis of single stage steam generating absorption heat transformer

Absorption heat transformer (AHT) has played a role in producing available heat from the low temperature of waste heat sources. This study proposes two operating modes regarding the way to supply heat source to the AHT system, and aims to investigate the operating characteristics of the steam generating AHT system concerning heating source temperature. First, this study describes the thermodynamic cycle of the single stage AHT operating with water-lithium bromide pair, and two operating modes are illuminated: a forward flow mode for heat source input to the evaporator and then to the generator, and a reverse flow mode for heat source input to the generator and then to the evaporator. Second, the first law of theoretical simulation characterizes the effects of the different operating modes on the heat transfer rates of each AHT component, the coefficient of performance (COP), and the steam generation rate. The experimental studies validate the theoretical simulation models; consequently, the results obtained herein lay a foundation of further investigation for the thermodynamic characteristics of the steam generating AHT system to maximize the operating performance.

Effects of an upstream tetrahedron on the circular cylinder–flat plate juncture flow

A technique of installing a tetrahedron at the upstream corner of the circular cylinder–flat plate juncture is developed to control the characteristic horseshoe vortices appearing in the natural juncture flow. The Reynolds numbers based on the cylinder diameter are within the range of 500–2900. The flow patterns and time-averaged velocity fields in the vertical symmetry plane and a horizontal plane near the flat plate of the natural and tetrahedron-controlled juncture flows are examined by using the laser-assisted particle flow visualization method and particle image velocimetry in a towing water tank. The flow approaching the circular cylinder–flat plate juncture can induce a characteristic horseshoe vortical flow consisting of a single vortex, dual vortex, or triple vortex. These horseshoe vortices appearing in the natural case may be changed to a characteristic mode of vortical flow, reverse flow, or forward flow when a tetrahedron is installed at the upstream corner of the juncture. The appearance of the vortical flow, reverse flow, or forward flow mode depends on the geometric parameters of normalized axial length, expansion angle, and tilt angle as well as the flow parameter of the Reynolds number. The vortical flow mode appears at small axial length of tetrahedron. The forward flow mode appears at the large axial length of tetrahedron. When the forward flow mode appears, the boundary-layer upstream of the circular cylinder does not separate. Therefore, the horseshoe vortices induced in the natural juncture flow disappear. The data bank consists of the design parameters of axial length, tilt angle, and expansion angle of the tetrahedron, which is provided as a figure.

Math-based torque converter modelling to evaluate damping characteristics and reverse flow mode operation

This paper presents math-based torque converter modelling and simulation in both forward flow mode and reverse flow mode operations. Since a torque converter plays an important role in transferring power from an engine shaft to the transmission shaft and vice versa and affects the fuel consumption and vehicle longitudinal dynamics, simulating the dynamic behaviour of this component in different operating modes is of great importance. Our torque converter model is validated with the experimental results of the Honda CRV during the forward flow mode operation. The main focus of this research is on reverse flow mode simulation, and the application of the proposed math-based torque converter model to evaluate damping characteristics of a torque converter due to undesired disturbances generated either from engine pulsations or from road bumps and potholes. The simulation results show that a torque converter efficiently damps high frequency disturbances introduced from the engine shaft to the transmission side and vice versa.

CFD simulation of flow in vortex diodes

AbstractVortex diodes are used as leaky nonreturn valves in applications, where it is desirable to avoid valves with moving parts. Despite their use in practice for several decades, no detailed analysis of the flow inside the vortex diodes is available. A strategy was derived for the CFD simulations of the vortical flow in diodes. A good agreement was seen between pressure drop (ΔP) across the inlet‐outlet ports from CFD simulations, and the experimental data for five diode sizes. The simulations showed that in the reverse flow situation tangential velocity was dominant and resulted in conservation of angular momentum in the chamber until it reaches the axial exit port. This vortical motion induced a significant pressure drop (ΔPr). The axial velocity gradient over the chamber cross‐section helps in inducing relaminarization of the flow. In the forward flow mode, the fluid gets distributed radially over the chamber and exits through the tangential port, yielding low ΔPf. The analysis showed that the performance of a diode is strongly affected by diode geometry, size, aspect ratio, nozzle configuration and Reynolds number. Among different configurations, the nozzles with entry port size equal to diode yielded higher diodicity. Simulations showed that using angle of divergence for diffuser sections of nozzles of the order of 7° exhibited higher diodicity than smaller angles. It was also observed that at higher flow rates significantly higher diodicity was obtained using axial nozzles with larger radius of curvature for expander section. The modeling methodology and results presented will be useful for evolving better designs of vortex diodes. © 2008 American Institute of Chemical Engineers AIChE J, 2008

In-line membrane separation method for sulfide monitoring in wastewaters exploiting multisyringe flow injection analysis

A fully automated multisyringe flow injection analysis (MSFIA) system coupled to gas-diffusion (GD) separation is proposed for the spectrophotometric determination of sulfide in urban wastewaters containing suspended solids. No preliminary batch sample treatment is required. The method carried out in a multicommutated system is based on analyte release as hydrogen sulfide from the donor channel of the GD module into a stagnant recipient solution composed of N, N-dimethyl- p-phenylene diamine (DMPD) and Fe(III). The methylene blue dye generated in-line via oxidative coupling is subsequently delivered downstream to a miniaturized flow-through light-emitting diode (LED)-based optical fiber plug-in spectrophotometer. The effect of various experimental variables on gas transfer across the membrane and concentration enhancement in the receiver solution via iterative flow reversals are discussed. Under the optimized chemical and hydrodynamic variables, the GD–MSFIA method features coefficients of variation better than 0.8% ( n=10) at the 10 mg l −1 level, a linear working range of 0.5–20 mg l −1 sulfide, a 3 σ blank-detection limit of 0.03 mg l −1 and an injection throughput of 13 h −1 for a sample volume of 700 μl and a forward flow mode. The analytical performance of the proposed GD–MSFIA system is also critically compared with that of in-line dialysis separation schemes using passive semi-permeable hydrophilic membranes. Accuracy was assessed by using the batch APHA–AWWA–WPFC standard method as an external reference.