Pressure Drop Curve Research Articles

Comprehensive performance evaluation of HVAC systems integrated with direct air capture of CO2 in various climate zones

Direct Air Capture (DAC) is a rapidly evolving technology that extracts CO2 directly from ambient air. This study presents a comprehensive performance evaluation of integrating DAC in HVAC systems, which can reduce indoor CO2 concentration and improve energy efficiency of HVAC systems. The DAC equipment is modeled in Modelica based on isotherm and thermodynamic equations, and pressure drop curves of the CO2 sorbent described in literature. The model is validated with data from the literature, and then integrated into a typical HVAC system available in Modelica Buildings library. The HVAC system is a Variable Air Volume (VAV) with reheater system for a one-floor office building with standard ASHRAE 2006 control sequences. Demand control ventilation strategies are designed to reduce the outdoor air flowrates when indoor CO2 concentrations are lower than the threshold, which is to maximize the benefits of integrating DAC. Four cases are proposed to assess the impacts of integrating DAC and DCV in HVAC systems in 8 different ASHRAE climate zones in the USA. The results show that by integrating DAC unit into the HVAC system, the average indoor CO2 concentration can be significantly reduced by over 45 % against the baseline without a DAC unit. By integrating DCV, 0.39–21.66 % of annual energy savings and 226–9539 kg carbon emissions reduction are observed across different climate zones. The highest energy savings are found to be achieved with cold climatic conditions while the lowest energy savings occur with favorable weather.

The Structure of Left Ventricular Relaxation in Case of Ventriculography.

To study the relaxation structure of the left ventricle (LV) in patients who underwent ventriculography. LV ventriculography was performed in 37 patients. Before catheterization, echocardiography was performed in each patient. In 6 patients, the LV ejection fraction (EF) was below 40%; these patients with systolic dysfunction were not included in the study. In 31 patients, the LV EF was higher than 50%. In this group, 13 patients had NYHA functional class (FC) 2-3 chronic heart failure (CHF); the rest of the patients had FC 1 CHF. Eighteen of 31 patients had stable ischemic heart disease; 50% of these patients had a history of myocardial infarction; the rest of the patients had hypertension and atrial and ventricular arrhythmias. The dynamics of the LV pressure decrease was analyzed from the moment of the maximum rate of pressure drop, which usually coincides with the closure of the aortic valves. The pressure drop curve was logarithmized with natural logarithms and divided into 4-5 sections with different degrees of curve slope. The relaxation time constant was calculated for each section. Its inverse value characterizes the relaxation time constant (tau). In 31 patients with LV EF 52-60%, three types of the dynamics of the relaxation rate constant were identified during the pressure decrease in the isovolumic phase: in 9 patients, the isovolumic relaxation constant (IRC) steadily increased as the pressure decreased; in 13 patients, it continuously decreased; and in 9 patients, the dynamics of IRC change was intermediate, with an initial increase followed by a decrease. In diastolic dysfunction, one group of patients had an adaptation type associated with an increase in the LV wall elasticity, while the other group had a different type of adaptation associated with its decrease. Each type has advantages and disadvantages. This is probably due to changes in the structure of the sarcomeric protein connectin (titin).

Bubble Separation in a Gas–Liquid Swirling Pipe Flow

Gas–liquid swirling pipe flow plays an essential role in phase separators. Bubble motions and flow patterns transitions in the flow are crucial for further investigation of this flow. However, much attention has been paid to these in non-swirl flow instead of swirl flow. In this work, bubble separation generated by a vane-typed swirler in a vertical pipe with an inner diameter of 62 mm was experimentally studied. Superficial air velocities (0.036 − 1.41 m/s) and superficial water velocities (0.02 − 0.95 m/s). The results show that dispersed bubbles gradually transform to gas column flow with increasing superficial water velocities (0.093 − 0.640 m/s) in the swirl flow; gas column flow gradually transforms to swirling intermittent flow with increasing superficial air velocities (0.05 − 0.40 m/s), and one peak increases to two peaks in the probability density function curve of pressure drop; slug flow can transform to swirling intermittent flow. Finally, transition criteria for bubble separation were proposed: when the turbulent fluctuations are strong enough to overcome centrifugal force, then the bubbles move to the pipe center and coalescence with each other, lead to the transition of gas column flow/swirling intermittent flow.

A novel semi-analytical pseudo-steady state finite-conductivity model for optimum horizontal-well completion design

This paper introduces a new technique to determine the optimum horizontal-well completion design in terms of the optimum horizontal-well length, the minimum wellbore/reservoir pressure drop, and the maximum wellbore/reservoir productivity index (PI) at pseudo-steady state (PSS). Frictional-wellbore pressure losses (FWPL) are accounted for based on a new general semi-analytical PSS finite-conductivity model (FCM) for a horizontal well under a rectangular bounded reservoir producing at a constant rate. The PSS FCM is coupling a PSS infinite-conductivity model (ICM) with a frictional-wellbore model (FWM) by using their correct-discretization resolution of the wellbore (CDRW), which has never been achieved before. The new FWM is developed in a general form to be coupled easily with any general ICM and can be applied at any time of the well production life. Two field examples are presented to show the effect of different reservoir, well, and fluid properties on horizontal-well performance.The new technique is based on the optimization of dimensionless well length (LD) between the two systems (i.e., ICM and FWM) of the FCM because LD is directly proportional to the PI of the first system and inversely proportional to the PI of the second system. Therefore, there is always a minimum point on the wellbore/reservoir pressure drop curve corresponds to a maximum point on the wellbore/reservoir PI curve.Results show that: 1. LD has the key role in the wellbore/reservoir coupling model.2. FWPL is crucial if both well production rate (qw) and horizontal permeability (kh) are high, where a notable drop in FCM PSS PI occurs, recommending shortening to the horizontal-well length.3. FWPL is insignificant if qw is low and oil viscosity (μo) is high, where no decrease in FCM PSS PI occurs, recommending lengthening to the horizontal-well length.4. Wellbore segment length (ΔL) of the FWM must be small, ΔL = 0.25 ft in this study. If it is not enough small, a critical overestimation may be occurred in the optimum horizontal-well length and the maximum PSS PI of the horizontal-well completion design.

Production Dynamic Characteristic of Fractured Wells in Multilayer Reservoirs Considering the Effect of Non-Uniform Flux Distribution

Abstract Hydraulic fracturing stimulation, which improves matrix permeability and reduces production costs, has been extensively used in the exploitation of multilayer reservoirs. However, little research on the production dynamic characteristics of vertically fractured wells in stratified reservoirs has been done in the literature. The influence of flux variation along the fracture on the pressure transient behavior has been ignored in these previous works. Therefore, this paper introduces a novel semi-analytical model for fractured wells in multilayer reservoirs, in which the finite difference method is used to characterize fluid flow in the fracture and the Green’s function method is used to characterize fluid flow in the matrix. With the aid of the model, the production dynamic characteristics of fractured wells in multilayer reservoirs can be readily investigated. In addition, based on the assumption of nonuniform flux distribution along the fracture, we successfully recognize four flow regimes occurring in the pressure drop and pressure derivative curves. Following that, the influences of several parameters on the pressure dynamics and layered flux contribution are studied. The calculation results indicate that a larger storability ratio, as well as a larger permeability ratio, can increase the values of the pressure drop and the pressure derivative; the greater the fracture height, the greater the fluid flow into each layer of the fracture. During the production of this model, increasing the fracture conductivity can reduce the pressure drop and pressure derivative, which means lower flow resistance in the fracture.

Model and Analysis of Pump-Stopping Pressure Drop with Consideration of Hydraulic Fracture Network in Tight Oil Reservoirs

The existing pump-stopping pressure drop models for the hydraulic fracturing operation of tight oil reservoirs only consider the main hydraulic fracture and the single-phase flow of fracturing fluid. In this paper, a new pump-stopping pressure drop model for fracturing operation based on coupling calculation of the secondary fracture and oil-water two-phase flow is proposed. The physical model includes the horizontal wellbore, the fracture network and the tight oil reservoir. Through the numerical simulation and calculation, the wellbore afterflow performance, the crossflow performance between the main hydraulic fracture and the secondary fracture, the fracturing fluid leakoff and the oil-water replacement after termination of pumping are obtained. The pressure drop characteristic curve is drawn out by the bottom-hole flow pressure calculated through the numerical simulation, and a series of analyses are carried out on the calculated pressure drop curve, which is helpful to diagnose the -oil-water two-phase flow state and the fracture closure performance under the control of the fracture network after hydraulic fracturing pumping. Finally, taking a multi-stage fractured horizontal well in a tight oil reservoir in the Junggar basin, China as an example, the pump-stopping pressure drop data of each stage after hydraulic fracturing are analyzed. Through the history fitting of the pressure drop characteristic curve, the key parameters such as fracture network parameters, which include the half-length of main hydraulic fracture, the conductivity of main hydraulic fracture and the density of secondary fracture, the fracture closure pressure are obtained by inversion, thus, the hydraulic fracturing effect of fractured horizontal well in tight oil reservoirs is further quantified.

Two-phase flow instability characteristics of HTGR Once Through Steam Generators

High Temperature Gas-cooled Reactors (HTGR) have the advantage of inherent safety compared with most Gen III reactors. They also have very high temperature, which make the plant have high electricity generation efficiency and have has the potential for hydrogen production. Once Through Steam Generator (OTSG) is responsible for transferring high temperature heat from primary loop helium to secondary loop composed of water and steam. The evaporation tubes of HTGR OTSG are very long due to the low heat transfer coefficient of helium convection over tube bundles. A Two-phase flow instability may occur in HTGR OTSG, which can cause pulsation of the mass flow rate, system pressure and temperature. This will not only interfere with the control system, but also cause thermal fatigue damage in the structures. Thus, it is important to predict and avoid two-phase flow instability in HTGR OTSG. Three types of flow instabilities are investigated for HTGR OTSG using both a linear analytical model and a nonlinear numerical model. They are flow excursion (or Ledinegg instability), density wave oscillation and pressure drop oscillation. For the nonlinear numerical model, RELAP5 software is adopted. Flow excursion and pressure drop oscillation are discussed based on the predicted hydraulic characteristic curve of HTGR OTSG (pressure drop vs. mass flow rate curve). Density wave oscillation is investigated using the transient variation of the outlet mass flow rate. For the linear analytical model, a set of ordinary differential equations are established using integration and small perturbation methods. The variables of differential equations are the length of the subcooling water region, the length of both the subcooling water and saturated two-phase regions, the average density of the saturated two-phase region and the inlet mass flux. The stability of OTSG is investigated using the eigenvalues of the coefficient matrix of the ordinary differential equations. Based on the results calculated using the linear analytical model and nonlinear numerical model, there is no flow excursion, pressure drop oscillation or density wave oscillation at the designed partial and full power levels (30%, 50%, 75% and 100%). Thus, HTGR OTSG can operate stably and have a sufficient safety margin at the designed power levels. Density wave oscillation will happen when decreasing system pressure. However, increasing inlet resistance coefficient can avoid density wave oscillation effectively. The tube lengths’ influences on the stability boundary of OTSG are also investigated and it shows no strong dependency.

A machine-learning method to accurately recognize the leakage pressure-drop signals in trunk natural gas pipelines

A block valve installed along a trunk natural gas pipeline can automatically shut down when it detects a pressure-drop rate that is over a threshold value for a specified duration, indicating that leakage accidents have occurred. However, current methods have difficulties on identifying the differences among the pressure-drop rate signals caused by small-hole leakage, compressor startup, and block valve emergency shut-down conditions. This usually leads to improperly shutting down the block valve. Based on an optimized support vector machine, a machine-learning method is proposed to recognize the pressure-drop rate signals through the eigenvalue of the pressure-drop curve instead of only using the pressure-drop signal and its duration. Firstly, the singular decomposition method and the support vector machine method are applied to initially classify the pressure-drop rate signals of three operating conditions: the gas leakage, compressor startup, and block valve emergency shut-down conditions. Secondly, a teaching-learning-based optimization method is applied to optimize the penalty factor and kernel function parameter in the support vector machine model. Particularly, the tent chaotic maps and adaptive inertia weight methods are applied to improve the teaching-learning-based optimization method to balance its local and global search performance. Thirdly, an improved teaching-learning-based support vector machine method is established. Finally, 960 sets of simulated pressure-drop signals taken from three trunk natural gas pipelines and pressure-drop rate signals collected from an actual pipeline are applied to verify the accuracy of the proposed model. The results show that the improved teaching-learning-based support vector machine model achieved high classification and recognition accuracy. Specifically, it achieved accuracy rates of 99.4%, 100%, 98.3%, and 100% for the pressure-drop rate signals generated by pipelines A, B, C, and Cangzhou branch in the presence of pipeline leakage (the leak hole diameter is from 25 to 125 mm). Additionally, it demonstrated an average recognition accuracy of 97.42% for the pressure-drop rate signals generated by pipelines under other operating conditions. Through cooperating with Supervisory Control And Data Acquisition system, this method provides a more relevant approach to determine the shutdown conditions of a block valve while preventing mistaken actions.

Modified experimental method to investigate micro transport in the shale matrix for shale gas production

Matrix permeability is the primary transport property for shale gas recovery, due to the special production process of gas from matrix to wellbore. With the micro pore size of shale matrix, it is necessary to consider micro transport mechanisms for the obtained permeability from macro experimental techniques. In this study, the macro experimental permeability was investigated in micro way, by the analyzation of pressure drop curves of shale matrix at high- and low-pressure experimental conditions, with modified numerical model containing micro transport mechanisms. As selected parameters, porosity, pore radius and tortuosity were regressed to obtain apparent permeabilities of three samples. The analysis shows that, tortuosity is much higher than macro one, and has been proved to be necessary and reasonable. The Knudsen diffusion and slippage effect control the micro transport of shale matrix at low and high pressure, separately, which is the main difference of macro and micro transports. Slippage flow weight factor helps to increase of slippage flow proportion with pressure. Higher pore radius and tortuosity both could weaken the effect of Knudsen diffusion, and slippage flow appears positive relationship with pore size at high pressure. This research provides an inspiration to shale transport study and a micro perspective theoretical guidance for the macro recovery of in-site shale gas reservoir.

Diffusion model of gas hydrate formation from ice considering the gas pressure drop

A kinetic model of the process of gas hydrate formation from ice that considers the gas pressure drop caused by this process is presented. This model is based on the diffusion equation. The issues during the formation of gas hydrate from ice powder and that from a flat ice layer are considered separately. For each of these problems, a solution in the framework of a quasi-stationary approximation is obtained, allowing for the calculation of the gas pressure drop in the gas phase. The calculated data are compared with available experimental data on the kinetics of CO2 hydrate formation from pure ice powder and ice powder with the addition of sodium dodecyl sulfate (SDS) at a mass concentration of 240 ppm: the calculated and experimental curves of CO2 pressure drop are compared with each other. From this comparison, the values of the diffusion coefficient of CO2 through a layer of CO2 hydrate formed from pure ice and ice with the addition of SDS at a mass concentration of 240 ppm were determined. Based on these values, it was found that SDS molecules present in the gas hydrate affect the rate of gas diffusion through the gas hydrate layer.

A unified approach to simulate thermal hydraulic instabilities and the mutual interactions using transient distributed model

In the present work, a 1-D thermal-hydraulic model is developed to simulate two phase flow boiling for a working coolant R134a circulating inside a single horizontal channel which includes a surge tank. The TH model is the first of its kind which solves the transient conservation equations based on distributive formulations without the incorporation of quasi-steady state approximation and still can simulate pure pressure drop oscillations (PDOs), Ledinegg excursion and density wave oscillations (DWOs), and the co-existence of those instabilities as well using a unified approach by employing appropriate external characteristic curves. The model is validated with the available experimental data and then, further used for intended purpose as mentioned above. Finally, the notable conclusions are drawn based on the simulations conducted. Results indicate that the initial equilibrium position in the negative slope region of the pressure drop vs. mass flux curve is unstable, leading to Ledinegg excursion or PDOs based on the steepness of the external pump characteristic curve. Increased surge tank volume extends PDOs. For Ledinegg excursion, the unstable position shifts to positive slope regions, and gets stabilized or destabilized depending on the occurrence of DWOs. DWOs are more likely at low mass flux and positive slope region. With a constant mass flow rate from an external pump, DWOs occur with a surge tank upstream of the heated section but not without it. Simulations demonstrate coexistence of PDOs, Ledinegg excursion, and DWOs in certain situations.

Low-concentration and multi-component NMHCs capture from oil field exhaust using porous ZIF-8/iso-hexadecane slurry

Non-methane hydrocarbons (NMHCs) are a common type of volatile organic compounds (VOCs) pollutant in the petrochemical industry and have attracted widespread attention because of their adverse health effects and environmental impacts. In this paper, we report a new porous slurry formed with zeolitic imidazolate framework-8 (ZIF-8) and iso-hexadecane to capture the low-concentration and multi-component NMHCs (mainly ethane (C2H6), propane (C3H8), and n-butane (n-C4H10)) from the oil field exhaust. The sorption capacity of C2H6 in the slurry is significantly higher than that of nitrogen (N2) and methane (CH4). Moreover, the slurry demonstrated a clear advantage for C2H6 over N2 and CH4 in competitive adsorption through the pressure-drop curves. In the NMHCs capture experiments, the C3H8 and n-C4H10 concentrations after purification can be reduced to below 100 ppm, while the C2H6 concentration can reach approximately 180 ppm. More encouragingly, in the breakthrough tests, the slurry exhibits a perfect kinetic separation selectivity for multi-component NMHCs. Furthermore, to avoid structural collapse of ZIF-8 material during long-term use in acidic and wet environments, a certain amount of 2-methylimidazole was retained in the slurry as a protective agent in the material synthesis process. In this way, the ZIF-8 materials in the slurry can retain the stable characteristic structure in an aqueous and acidic environment and keep the capture capacity for NMHCs without degradation. We believe the porous ZIF-8/iso-hexadecane slurry is a promising capture agent for low-concentration and multi-component NMHCs with strong purification capacity and stability.

BIO16: Computation Fluid Dynamics Analysis of Dual Lumen VV ECMO Cannulas

Background: Venovenous extracorporeal membrane oxygenation (VV ECMO) treatment requires blood to be drained from the patient’s venous system, oxygenated in an artificial membrane, and returned. Traditionally, this has been achieved with two, single lumen cannulas (SLCs). More recently, dual lumen cannulas (DLCs) combining both drainage and return functions into a single device have become increasingly popular. Recirculation fraction, the proportion of oxygenated return blood which is directly drained out before passing through the tricuspid valve, should be low. Another key design objective for these cannulas is to minimize shear stress, as high levels increase the risk of coagulation activation (hemolysis). Whilst DLCs are thought to have superior recirculation performance compared to SLCs, this has not yet been demonstrated in a direct comparison. Furthermore, the shear stress produced by DLCs, their impact on native hemodynamics and response to changes in positioning have not been reported in adults. In this study, we aim to describe the recirculation and hemodynamic performance of DLCs. Methods: A patient-averaged model of the right atrium (RA) and the venae cavae was used to model venous flow. Two common DLCs, the Maquet Avalon Elite (Getinge) and MC3 Crescent (Medtronic) were converted into 3D models (Figure 1A.). Both were downscaled to 27Fr for our patient model. To delineate these from the off-the-shelf products we use the prefix “ds” for downscaled. Both DLCs were first positioned optimally and simulated at 2,4 and 6 L/min. Noting a high degree of similarity, insertion depth (±4cm) and rotation (±30° and ±60°) was only assessed for the dsAvalon.Figure 1. A. Downscaled (ds)Avalon and dsCrescent cannula features. B. Drainage fractions from the superior (SVC) and inferior vena cava (IVC) C. Time-averaged velocity streamlines of the high-velocity jet exiting the reinfusion port of both DLCs at 4L/min. Results: Pressure drop curves were nearly identical for the two devices. dsAvalon pressure drops agreed well with experimental data. Recirculation fraction was low for both DLCs compared to SLCs. SVC and IVC drainage adapted with ECMO flow rate to draw a greater proportion of flow from the IVC at high ECMO flow rates (Figure 1B). The reinfusion port design created a high-velocity jet of blood (Figure 1C). Consequently, time-averaged wall shear stress (TAWSS) was very high (>475 Pa) at the reinfusion port for both designs. Discussion: DLCs exhibit superior recirculation performance compared to SLCs under ideal conditions. This appears to be due to DLCs ability to drain flow from both the IVC and SVC, favoring greater IVC drainage at higher ECMO flow rates. Both designs produce a highly focused reinfusion jet which may increase risk of hemolysis and coagulation activation.

Anisotropic Evolution of Effective Stress and Pore Pressure during Coalbed Methane Drainage

Summary The anisotropy and dynamic variation in permeability of gas-adsorbing coals have a significant influence on fluid flow behavior in the cleat system. The assumption of a constant anisotropy coefficient (the ratio between permeability components in orthogonal directions) has been traditionally made to simplify the seepage-stress coupling analytical model. In this approach, the pressure drop of the coalbed is separated into desorption and nondesorption areas. To evaluate the effective stress, pore pressure, permeability distribution, and variable anisotropy coefficient more accurately, analytical formulas were developed that consider elastic mechanics and methane sorption. The results show that the anisotropy coefficient can be dynamic when cleat compressibility anisotropy exists. Pressure contours are a set of ellipses that increase in eccentricity from the near-wellbore area to the pressure drop boundary, leading to corresponding anisotropy changes in effective stress and permeability. The gas desorption-related matrix shrinkage effect causes a discontinuous pressure drop gradient at the boundary between desorption and nondesorption areas, resulting in nonsmooth pressure drop curves. The pressure gradient difference changes with the radius of the desorption area and is nonisotropic, with the high-permeability direction showing a greater difference than the low-permeability direction. These results indicate that the dynamic anisotropy coefficient has a significant impact on coalbed drainage and extraction. Compared to previous mathematical models, which assumed permeability isotropy or constant anisotropy coefficient in cleat systems, the proposed model provides a more accurate method to evaluate pressure and permeability distribution.

Experimental Study of the Effect of Humidity on Air Filter Material Performance

A large number of application cases show that air filter media are easy to fail under extreme conditions of high humidity, haze, rain and snow, thus seriously affecting the energy efficiency of gas turbines. To study the impact of humidity on the dust-holding performance of filter media, three typical filter media applied for gas turbines with a similar efficiency are selected, and their dust-holding performance is investigated at a humidity of 30%, 60% and 75%, respectively. The results showed that the dust-holding pressure drop curves of three filter media were divided into two phases. With the increase of humidity, the increasing pressure drop rate of three filter media decreased. The pressure drop synthetic fiber, glass fiber composite filter media and the synthetic fiber composite filter media with a sandwich structure were more significantly affected by humidity during the filter cake filtration phase. Under the same conditions, filter media with a sandwich structure had the highest dust-holding capacity, while the electrospun fiber composite filter material had the lowest one. The dustcake formed on the surface of the filter media that consist of a pure synthetic fiber or a small amount of plant fiber is significantly affected by humidity.

N‐shape pressure drop curve of gas–liquid microchannel reactor and modeling

AbstractAccurate prediction of gas–liquid pressure drop is essential to microreactors design; however, the understanding of general rules of pressure drop in a wide gas–liquid flow ratio, especially smaller than 1.0, remains insufficient, Accordingly, this work systematically studies the pressure drop rule within the gas–liquid flow ratio of 0.2–2.0. The results show that, under a given gas velocity, the pressure drop first increases and then decreases, and finally back increases with the liquid flow velocity, and the named N‐shape pressure drop curve could be clearly observed. Besides, the rules of gas–liquid unit length and gas‐phase holdup are explored to reveal the mechanism behind the N‐shape pressure drop curve. Finally, three semi‐empirical correlations based on the gas–liquid unit length and separated flow model are successfully proposed for the N‐shape pressure drop. This work provides some important fundamental information for the reliable design of gas–liquid microreactors.

A Study of the Non-Linear Seepage Problem in Porous Media via the Homotopy Analysis Method

A non-Darcy flow with moving boundary conditions in a low-permeability reservoir was solved using the homotopy analysis method (HAM), which was converted into a fixed-boundary mathematical model via similarity transformation. Approximate analytical solutions based on the HAM are guaranteed to be more accurate than exact analytical solutions, with relative errors between 0.0089% and 2.64%. When λ = 0, the pressure drop of the Darcy seepage model could be instantaneously transmitted to infinity. When λ > 0, the pressure drop curve of the non-Darcy seepage model exhibited the characteristics of tight support, which was clearly different from the Darcy seepage model’s formation pressure distribution curve. According to the results of the HAM, a moving boundary is more influenced by threshold pressure gradients with a longer time. When the threshold pressure gradients were smaller, the moving boundaries move more quickly and are more sensitive to external influences. One-dimensional, low-permeability porous media with a non-Darcy flow with moving boundary conditions can be reduced to a Darcy seepage model if the threshold pressure gradient values tend to zero.

Heat transfer and flow characteristics of intermittent oscillating flow in tube

Motions of piston and displacer in ideal Stirling cycle is non-sinusoidal and rise-dwell-fall-dwell. However, heat transfer and flow characteristics of present oscillating flow are based on sinusoidal motions. In this work, heat transfer and flow characteristics of intermittent oscillating flow with intermittent ratio = 1 in tube were numerically studied. The model was firstly validated based on literature's empirical equations for sinusoidal oscillating flow. Then, heat transfer and flow characteristics of intermittent and sinusoidal oscillating flows were simulated and compared. The simulation results showed that the total heat transfer coefficient and pressure drop in intermittent oscillating flow were 14.6% lower and 0.83% higher respectively compared to those in sinusoidal oscillating flow. The curves of pressure drop were nearly the same in sinusoidal and intermittent oscillating flows with removal of stagnation periods. Besides, velocity distributions in two oscillating flows were also investigated. The simulation results showed that the “velocity loop” in two oscillating flows occurred at the same time and the changing amplitudes of wall's velocity over central axis's velocity in “velocity loop” were nearly the same.

Study on fluidization characteristics and drying characteristics of fine coal in thermal vibration separation fluidized bed

By studying the fluidization characteristics of the granular bed in the separation process of fine coal, this paper explores the effect of coal particles on the distribution and evolution of the fluidization behavior of the bed, and analyzes the separation and drying characteristics of fine coal. The results indicated that the stable value of pressure drop curve of bed with medium particles is larger than that of mixed particle bed of medium and coal. Meanwhile, with the increase of the coal particle size, the stable value of pressure drop gradually decreases, as well as the drying rate of lignite increases first and then decreases with the increase of gas velocity, vibration frequency and amplitude. Contrarily, the drying time of fine-grained coal decreases and then increase with the increase of vibration amplitude. Under the conditions of N = 1.4, f = 20 Hz, A = 2 mm, the contact efficiency between the gas–solid phases is good, and the bed pressure fluctuation is stable. After the separation of −6 + 3 mm and −3 + 1 mm fine coal, the ash content of clean coal is 13.4 % and 9.1 % respectively, and the gangue ash reached 43.7 % and 63.5 % respectively. With the increase of gas velocity, vibration frequency and amplitude, the activation energy Ea of fine coal drying process decreases first and then increases, and then the activation energy Ea in the separation process of fine coal is the lowest under the conditions of N = 1.4, f = 20 Hz, A = 2 mm.

Study on non-monotonic pressure-drop of supercritical n-decane with pyrolysis in heated channels

The non-monotonic relation between pressure-drop and flow-rate leading to the flow instability of fuel with pyrolysis was investigated. A characteristic pressure-drop model of n-decane coupled pyrolysis reactions under supercritical pressure was established. The model includes one-dimensional flow control equation, species equation, RK-PR equation of state, high-pressure viscosity model and a reaction mechanism consisting of 10 species and 15 reactions. The non-isothermal friction factor formula with wall correction coefficient fitted by experimental data was used in the model. To verify the model, the pressure-drops at different flow-rates were measured in a horizontal stainless steel tube with the inner diameter of 1.93 mm under different back pressures (3, 4 and 5 MPa). The results show that the model can accurately capture the non-monotonicity of characteristic pressure-drop. Under 3 MPa condition, the characteristic pressure-drop curve of n-decane has a pseudo-critical negative slope (PCNS) region and a reaction negative slope (RNS) region caused by the pyrolysis. With the increase of pressure, the PCNS region disappears but the RNS region still exists. The high pressure makes the onset of RNS region appear earlier due to the accelerated initial reaction rate. The total pressure-drop is separated and analyzed based on the model mathematically. In the PCNS region, the derivative of acceleration pressure-drop to mass flux is negative, whereas the derivative of friction pressure-drop to mass flux is positive. In the RNS region, the two derivatives are both negative because of the sharp decrease in density and the increase in friction factor caused by the pyrolysis reaction. The two parallel channels’ instability with constant total mass flow rate is analyzed based on the characteristic pressure-drop curves for reacting and non-reactive flow. The results indicate that the fuel pyrolysis reaction not only expands the range where the flow excursion may occur but also makes it difficult for the flow-rate to reach stable state again after the flow excursion caused by pseudo-critical instability.