Total Pressure Difference Research Articles

Lattice Boltzmann Simulation of Steady Flow in a Semi-Elliptical Cavity

AbstractThe lattice Boltzmann method is employed to simulate the steady flow in a two-dimensional lid-driven semi-elliptical cavity. Reynolds number (Re) and vertical-to-horizontal semi-axis ratio (D) are in the range of 500-5000 and 0.1-4, respectively. The effects of Re and D on the vortex structure and pressure field are investigated, and the evolutionary features of the vortex structure with Re and D are analyzed in detail. Simulation results show that the vortex structure and its evolutionary features significantly depend on Re and D. The steady flow is characterized by one vortex in the semi-elliptical cavity when both Re and D are small. As Re increases, the appearance of the vortex structure becomes more complex. When D is less than 1, increasing D makes the large vortexes more round, and the evolution of the vortexes with D becomes more complex with increasing Re. When D is greater than 1, the steady flow consists of a series of large vortexes which superimpose on each other. As Re and D increase, the number of the large vortexes increases. Additionally, a small vortex in the upper-left corner of the semi-elliptical cavity appears at a large Re and its size increases slowly as Re increases. The highest pressures appear in the upper-right corner and the pressure changes drastically in the upper-right region of the cavity. The total pressure differences in the semi-elliptical cavity with a fixed D decrease with increasing Re. In the region of themain vortex, the pressure contours nearly coincide with the streamlines, especially for the cavity flow with a large Re.

Influence of the total static gas pressure difference on blast-furnace performance

Further evidence is found to confirm that blast-furnace performance is not always improved when the gas pressure in the furnace is increased as a result of artificial drag in the gas-supply network. What is important is not simple increase in the gas pressure but optimization of this pressure and also the total pressure difference in the furnace in specific operating conditions.

Vacuum Drying of Wood—State of the Art

In this paper, we review the literature published on vacuum drying of wood. Vacuum drying is not a new technology, and its use for drying wood has been suggested since the early 1900s. Technologies for vacuum drying of wood can be classified by the heating method used. In this paper, we define vacuum-drying methods in four groups: conductive heating vacuum, cyclic vacuum, superheated steam vacuum, and dielectric vacuum. Advantages of drying wood below atmospheric pressure are the ability to dry at lower temperatures (and thus lower the probability of developing some drying defects), greatly reduced drying times, color preservation, greater energy efficiency, better control of volatile organic compound emissions, and the ability to dry very large cross sections. Some characteristics that differentiate vacuum from conventional drying are that in vacuum the primary driving force is the total pressure difference, the prevailing moisture transfer mechanism is water vapor bulk flow, and there is greater water migration in the longitudinal direction. While past research has focused on increasing the understanding of the fundamental mechanisms for vacuum drying and applications to specific industries and species, more recent efforts have concentrated on improving existing methods, for example, by improving moisture control and the use of pretreatments to improve drying quality.

Synthesis of zeolitic imidazolate frameworks (ZIF)‐8 membrane and its process optimization study in separation of CO2 from natural gas

ABSTRACTBACKGROUNDIn this work, well‐intergrown zeolitic imidazolate frameworks‐8 (ZIF‐8) membrane was synthesized through a microwave‐assisted secondary solvothermal growth method. A solvent evaporation seeding method was applied prior to membrane growth. Subsequently, process optimization for the separation of CO2 from natural gas through the ZIF‐8 membrane was studied using response surface methodology. The process variables studied included CO2 feed composition (36–83%), total pressure difference (100–700 kPa) and temperature (303–343 K). Three empirical models were generated using a central composite design in order to study the interaction between the process variables and the responses, including CO2 permeance, CH4 permeance and CO2/CH4 separation factor. The optimum condition was then determined for maximum CO2 permeance and CO2/CH4 separation factor and minimum CH4 permeance.RESULTSUsing this method, ZIF‐8 seeds of nano‐size were well distributed on the support due to high nucleation rate at the supersaturation condition. The presence of heavy hydrocarbons affected CH4 permeance, attributed to the molar mass and viscosity. An optimum CO2 permeance of 2.76 × 10−8 mol m−2 s−1 Pa−1 and CO2/CH4 separation factor of 3.72 was obtained at 312 K, 100 kPa and 36% CO2 feed composition. The experimental results were in agreement with the predicted values with an average error lower than ± 7%.CONCLUSIONSelective ZIF‐8 membrane was successfully synthesized using a feasible and reproducible solvent evaporation seeding method. The experimental data for the optimum condition were in good agreement with the predicted data, showing the accuracy of the empirical models. © 2016 Society of Chemical Industry

Evaluation of exhaust performance of cooling towers in a super high-rise building: A case study

The exhaust performance of a cooling tower placed in the interior space of a building is crucial due to limited space and stochastic ambient wind conditions. Improper design of the cooling tower could lead to a reduction in thermal efficiency and could also deteriorate the operational performance of the chillers. In this paper, the exhaust performance of cooling towers in a super high-rise building considering both side exhaust and interlayer exhaust methods is investigated using CFD simulations. The results show that the exhaust performance of cooling towers under interlayer exhaust is better than that under side exhaust. However, the exhaust recirculation phenomenon of the cooling towers on the windward side caused by outdoor wind is still obvious because the outdoor wind speed is low. The total pressure differences between the inlet and outlet of the tower units under interlay exhaust become larger with increases in wind speed in each district. The fan total head should be carefully determined to overcome the surplus pressure drop caused by the wind. This study helps to guide other similar cases utilizing the interior space of buildings for the cooling towers.

Influence of Geometrical Parameters of Open-Cell Aluminum Foam on Thermohydraulic Performance

The influence of the geometry of open-cell aluminum foam on the thermohydraulic behavior in channel flow is investigated. The mean cell diameter and the strut cross-sectional surface area are chosen as geometrical parameters, ranging respectively between 1.2 and 5.2 mm and between 0.0125 and 0.17 mm2. The flow arrangement and the operating conditions are fixed. A numerical model is implemented in a commercial solver, based on volume averaging theory. The model is validated against experimental data. The porous properties, which take the sub-REV scaled physics into account, are written as a function of both geometrical parameters. The thermohydraulic characteristics of 16 well-chosen foams are used to build a surrogate model. An ordinary Kriging model is used for this, indicating that the root mean square error of interpolated results is lower than 0.6 and 3% for, respectively, heat transfer and total pressure. The resulting heat transfer and total pressure difference are nondimensionalized by dividing them by the results obtained from an empty channel. The relative increment of the pressure drop is an order of magnitude higher than the increment observed for heat transfer. Consequently, the applied performance evaluation criterion (defined as the ratio of dimensionless heat transfer versus total pressure) is mainly influenced by the hydraulic performance. For the given application, a clear optimum is found. The proposed method allows performing the parameter study with acceptable computational cost with a sufficient level of detail from an engineering perspective.

Total Pressure Gradient Incidence on Hygrothermal Transfer in Highly Porous Building Materials

A new experimental investigation is conducted to extract conclusions about the magnitude of moisture transfer due to the total air pressure difference, generally, caused by wind or pressurization by ventilation, on hygroscopic porous wall. A special constructed setup tolerating the creation of low total pressure gradient at different relative humidity conditions is performed. The resulting moisture flux and the hygrothermal state around and within the material are monitored. Then, a characterization of the intrinsic moisture infiltration coefficient is defined. This coefficient is used as an input parameter in simulation models. The experimental procedure points out diverging conclusions between different testing materials (wood fibrous insulation, OSB and aerated concrete). In fact, the amount of moisture infiltration is completely dependent on both combination of total pressure gradient value and hygrometry of the specimen. It has an important contribution on the moisture transfer, in walls, relatively to that induced by vapor diffusion phenomenon.

Numerical analysis of the optimal turbine pressure drop ratio in a solar chimney power plant

In a solar chimney power plant, only a fraction of the available total pressure difference can be used to run the turbine to generate electrical power. The optimal ratio of the turbine pressure drop to the available total pressure difference in a solar chimney system is investigated using theoretical analysis and 3D numerical simulations. The values found in the literature for the optimal ratio vary between 2/3 and 0.97. In this study, however, the optimal ratio was found to vary with the intensity of solar radiation, and to be around 0.9 for the Spanish prototype. In addition, the optimal ratios obtained from the analytical approach are close to those from the numerical simulation and their differences are mainly caused by the neglect of aerodynamic losses associated with skin friction, flow separation, and secondary flow in the theoretical analysis. This study may be useful for the preliminary estimation of power plant performance and the power-regulating strategy option for solar chimney turbines.

Numerical shape optimization of a centrifugal pump impeller using artificial bee colony algorithm

Centrifugal pumps consume huge amounts of energy in various industrial applications. Therefore for these pumps, the improvement of machine efficiency has become a major challenge. Since the hydraulic performance of a centrifugal pump strictly depends on its impeller shape, in the present work, an efficient and original approach has been developed and applied to the design of centrifugal pump impellers in order to achieve a higher efficiency. A global optimization method based on the Artificial Neural Networks (ANNs) and Artificial Bee Colony (ABC) algorithm has been used along with a validated 3D Navier–Stokes flow solver to redesign the impeller geometry and improve the performance of a Berkeh 32-160 pump as a case study. In the next step, to verify the optimization results, all the domains within the centrifugal pump were simulated using the CFD method. The complete numerical characteristic curves of the pump with the optimized impeller were compared to the validated (using the available experimental data) numerical characteristic curves of the initial pump. The numerical results show an efficiency improvement of 3.59% at only 6.89m increase of total pressure difference for the Berkeh 32-160 centrifugal pump. The new impeller geometry presents much more changes in the meridional channel and blade profile. The results indicate a reasonable improvement in the optimal design of pump impeller and a higher performance using the ABC algorithm.

A numerical investigation of moment coefficient and flow structure in a rotor–stator cavity with rotor-mounted bolts

The torque associated with overcoming the losses on a rotating disc is of particular importance to the designers of gas turbine engines. Not only does this represent a reduction in useful work, but it also gives rise to unwanted heating of metal surfaces and the adjacent fluid. This article presents a numerical study on the effect of rotor-mounted bolts on the moment coefficient and velocity distributions within a rotor–stator cavity under conditions representative of modern gas turbine engine design. Steady-state, two-dimensional and three-dimensional, computational fluid dynamics simulations are obtained using the FLUENT commercial code with a standard k–ɛ turbulence model. The model is validated against experimental data and then used to investigate the effects of varying the number of bolts and also a continuous ring. Two test cases are investigated: one corresponds to where the flow structure is dominated by the superimposed flow ( λT = 0.35); and the other, where rotation is expected to govern the flow structure ( λT = 0.35). The principal flow phenomena in the vicinity of the bolts were described using the simulation results. Increasing the number of bolts will decrease the relative total pressure difference around bolts and increase the tangential velocity of the core of fluid between the rotor and stator. It was also found out that the free-stream Reynolds number of the flow approaching the bolts decreases and angle of attack increases with increasing number of bolts. As a consequence, the bolts’ wake character changes leading to a situation where the wake of one bolt is not fully collapsed in advance of the following bolt. The occurrence of Taylor columns were also investigated in the rotor–stator system with rotor-mounted bolts. It was found that while increasing the number of bolts can decrease the Rossby number to produce an intermediate rotation in the system, the Taylor columns will not be produced even for 60 bolts under the both flow conditions. The contribution of skin friction to the overall moment coefficient reduces as the number of bolts increases and the pressure-related losses increase. Increasing the number of bolts will decrease the moment produced by each individual bolt. However, since there are more bolts in the system the total moment of all bolts will increase by increasing their number. There also appears to be a point where increasing the number of bolts does not bring about an increase in the overall moment coefficient. It is also interesting to report that the moment coefficient associated with a continuous ring is similar to that for a plain disc.

A Compressible Transient Model of the Solar Chimney and Heat Collector

The compressible transient model of solar chimney power plant system was proposed. It was added to the pressure equation and the ideal gas state equation basis on the heat balance equation for the solar collector model. The air flow station can be easily calculated with the improved model. The results of dynamic changes of the total pressure difference calculated in the model were in good agreement with the given actual measured values in references. The solar chimney model was considered the influence of fluid pressure on the density. The influence of the structural chimney on the chimney efficiency was analyzed with the established model. It was shown that the chimney efficiency changes significantly with the chimney height and its diameter. The chimney efficiency was decreased with a convergent chimney shape while increased with the divergent one. When the tilt angle of chimney reached a peak and then further increased, the chimney efficiency was the constant. These results will provide the important reference to improving the system efficiency.

Separation of pentane isomers using MFI zeolite membrane

A BZSM-5 zeolite membrane layers supported on a porous tubular alpha alumina support were prepared by hydrothermal synthesis. Permeation of single and binary mixture of normal pentane (nC5) and 2-methylbutane (2MB) through BZSM-5 zeolite membrane was studied as a function of total pressure, temperature and feed composition using total pressure difference and concentration gradient methods. The nC5/2MB selectivity decreased with increasing of temperature. The rapid decrease in separation selectivities with increase in temperature indicated that preferential adsorption was responsible for the selectivity. The selectivity decreased as the feed pressure increased in total pressure difference method. The performance of membrane in concentration gradient method is higher than that in total pressure drop method. It can be addressed by decreasing of non-zeolite effect on membrane permeation in the concentration gradient method. In addition, in contrast of pressure drop method, the nC5/2MB selectivity of membrane increased with increase in feed pressure for concentration gradient method.

Separation of CO2 from the CO2/N2 mixed gas through ionic liquid membranes at the high feed concentration

Separation of CO2 from the CO2/N2 mixtures through ionic liquid membrane was carried out in this study. The CO2 volume fractions (XCO2) at feed side were 0.3–0.5. The microporous polyvinylidine fluoride (PVDF) membrane and two species of ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]), were used as the material for supported membrane and room temperature ionic liquids, respectively. The PVDF membranes were impregnated with the ionic liquids. The total pressures at the feed side were varied from 1.05 to 1.21atm. The experiment was performed under constant temperature at 40°C. The gas flow rates of CO2/N2 mixed gas at feed side were varied in range of 100–300mlmin−1. The effects of total pressure difference, the total flow rate at the feed side and the CO2 concentration were investigated. It was found that the higher permeability of CO2 was obtained from the lower total pressure differential conditions. The feed flow rates do not have the significant effect on permeability of N2. The [bmim][PF6] and [bmim][Tf2N] liquid membrane are given the equivalent CO2/N2 selectivity for XCO2=0.5 even though the feed gas flow rate was adjusted. The carrier saturation inside the ionic liquid membrane at high CO2 concentration and high pressure difference constrain the permeation of CO2. The highest selectivity was obtained in the case of XCO2=0.3 at constant total pressure difference and feed gas flow rate.

Electrokinetic flow in connected channels: a comparison of two circuit models

We apply the recently developed ‘ion current model’ (ICM) to investigate mixed pressure-driven/electro-osmotic flow in electrokinetic series circuits. Under the ICM, the local geometric mean ion concentration varies throughout a microfluidic circuit as a consequence of ion conservation. ICM predictions are used to assess the accuracy of the earlier ‘total current model’ (TCM), which is instead based upon the simplifying assumption that the geometric mean ion concentration is uniform in all channels within a circuit. The model comparison is performed over an experimentally relevant range of physical parameters and channel dimensions. When the ratio of total current to flow rate is small, or when the inlet concentration is very low or very high, the ICM and TCM give similar predictions of the total pressure and potential differences across simple three-channel contraction–expansion networks. Otherwise, the errors introduced using the TCM may be very large. Our results demonstrate that the common assumption that the electrolyte concentration throughout a device is equal to the concentration of fluid supplied to the device is not always valid, even if there is little or no electric double layer overlap.

Infrared long nanosecond laser pulse ablation of silicon: Integrated two-dimensional modeling and time-resolved experimental study

Nanosecond (ns) laser ablation may provide good solutions to many industrial silicon micromachining applications. However, most of the previous work is on lasers in ultraviolet (UV) or visible spectral ranges, and ns laser ablation of silicon at the infrared (IR) wavelength has not been well understood, particularly for long ns pulses with durations on the order of ∼100ns. IR ns lasers often have lower costs and less external energy consumption for the same laser energy output than UV or visible lasers, which is desirable for many practical applications. This paper aims to understand the mechanism of IR ns laser ablation of silicon, by combining time-resolved experimental observations with physics-based modeling study. The observation is through a ns-gated intensified charged-coupled devices (ICCD) camera coupled with a microscope tube, while the model is based on two-dimensional (2D) gas dynamic equations for the gaseous phases coupled with the condensed phase heat transfer equation through the Knudsen layer relations. The research shows that the material removal mechanism under the studied laser ablation conditions is surface vaporization in the early stage (yielding a plasma plume above the target), followed by subsequent liquid ejection. The measured plasma front propagation matches reasonably well with the model prediction. The experimentally observed spatial distribution of the plasma radiation intensity is consistent with and has been understood through the model. The study also shows that the observed liquid ejection is induced by the total surface pressure difference between the near-boundary region of the target melt pool and the other remaining region of the pool. The pressure difference is mainly due to the surface vaporization flux drop after laser pulse ends.

Experimental investigation of pressure distribution between the piston rings and its formation in reciprocating compressors

The severe non-uniformity of pressure distribution has been suggested as the essential reason for the premature failure of piston rings in reciprocating compressors. A test rig was built to investigate the dynamic pressure distribution and its formation process, so that the root cause of the non-uniform pressure distribution could be revealed. The experimental results showed that the pressure distribution between the rings was always significantly non-uniform under various test conditions and the first ring bore more than 75% of the total pressure difference. Further analysis of the experimental data indicated that when the suction pressure was not higher than that in the crank case, the first piston ring switched its contact position with the piston ring groove twice in one cycle, at the angles of around 63° and 170°, respectively, while the others switched contact positions at about 90° and 270°. If the suction pressure was higher than the pressure in the crank case, the first ring still switched its contact position twice in a cycle, at the crank angle of about 47° and 195°, respectively, but the other rings no longer changed their positions. The experimental results also demonstrated that the formation of pressure difference on different rings was not synchronous, which indicated that the rings could not work until their pressure difference reached a critical value.

Optimal Design Study on the Radian and Length of the Guide Plates for the Pipeline Elbow

Pipeline elbows are extensively applied in the liquid-conveying piping system, which vibration and noise control are always difficult and critical. The structural style of the guide plates for the liquid-conveying pipe noise eliminating elbow is numerical calculated in this paper, then we find that the fluid liquidity in the liquid-conveying pipe elbow is improved very well by the cross-sectional bending radian and export length of the guide plates, and the two parameters are also optimal designed. The calculated results show that the total pressure difference in the liquid-conveying pipeline elbow decreases first and then increases with the cross-sectional bending radius of the guide plate increases; The total pressure difference decreases first and then increases with the export length of the guide plate increases; When the guide plates according to the gold ratio arrangement, the number, the cross-sectional bending radius and the length are 2, 200 mm and 40 mm respectively, we obtain the total pressure difference is 10707.87 Pa in the liquid-conveying pipeline elbow, comparing it to the result of the original program, it decreases 38.58%, so the internal flow field is improved better, the optimal design has a very obvious effect.

Methane steam reforming in a Pd–Ag membrane reformer: An experimental study on reaction pressure influence at middle temperature

In this experimental work, methane steam reforming (MSR) reaction is performed in a dense Pd–Ag membrane reactor and the influence of pressure on methane conversion, CO x-free hydrogen recovery and CO x-free hydrogen production is investigated. The reaction is conducted at 450 °C by supplying nitrogen as a sweep gas in co-current flow configuration with respect to the reactants. Three experimental campaigns are realized in the MR packed with Ni–ZrO catalyst, which showed better performances than Ni–Al 2O 3 used in a previous paper dealing with the same MR system. The first one is directed to keep constant the total pressure in both retentate and permeate sides of the membrane reactor. In the second case study, the total retentate pressure is kept constant at 9.0 bar, while the total permeate pressure is varied between 5.0 and 9.0 bar. As the best result of this work, at 450 °C and 4.0 bar of total pressure difference between retentate and permeate sides, around 65% methane conversion and 1.2 l/h of CO x-free hydrogen are reached, further recovering 80% CO x-free hydrogen over the total hydrogen produced during the reaction. Moreover, a study on the influence of hydrogen-rich gas mixtures on the hydrogen permeation through the Pd–Ag membrane is also performed and discussed.

Total pressure losses minimization in turbomachinery cascades using the exact Hessian

A method for the design of turbomachinery cascades with minimum total pressure losses, subject to constraints on the minimum blade thickness and flow turning, is presented. It is based on the Newton–Lagrange method which requires the computation of first- and second-order sensitivities of the objective function and the constraints, with respect to the design variables. The computation of the exact Hessian of the function which expresses the difference in total pressure between the inlet to and the outlet from the cascade, is new in the literature. To compute the Hessian, the direct differentiation of the viscous flow equations is used for the first-order sensitivities of the functional and the flow-related constraints, followed by the discrete adjoint method. Since the objective function is defined along boundaries other than those controlled by the design variables, it is challenging to investigate the significance of all terms comprising the exact second-order sensitivity expressions. All these terms were temporarily computed using automatic differentiation and those which proved to be significant are hand-differentiated to minimize CPU cost and memory requirements. Insignificant terms are eliminated, giving rise to the so-called “exact” Hessian matrix. An “exactly” initialized quasi-Newton method was also programmed and tested. In the latter, at the first cycle, the exact gradients and Hessians are computed and used; during the subsequent optimization cycles, the discrete adjoint method provides the exact gradient whereas the Hessian is updated as in quasi-Newton methods. The comparison of the efficiency of the aforementioned methods depends on the number of design variables used; the “exactly” initialized quasi-Newton method constantly outperforms its conventional variant in terms of CPU cost, particularly in non-convex and/or constrained optimization problems.

A note on a bronchial tree with 23 levels of bifurcation

Our recent article ''Why do we have a bronchial tree with 23 levels of bifurcation?'' (1) came out in Heat and Mass Transfer in 2009, in which we proposed an analytical model for the lung architecture, namely, quasi-steady one- dimensional mass transfer model for the lung. In its introduction, we referred to the paper of Prof. Reis' group, namely, ''Constructal theory of flow architectures of the lungs'' (2) as one of the most important papers in this particular topic, explicitly stating, ''Others (i.e. Prof. Reis' group) were certainly successful to some extent in relating the number of the bifurcation levels to the best oxygen access to the alveolar sacs.'' We believe that Prof. Reis' group certainly deserves credit for explaining for the first time the reason why we have a bronchial tree with 23 levels of bifurcation on the basis of Bejan's Constructal law. In our paper, we mentioned the originality of our paper, stating, ''Here, we shall present a simple and yet rational model for the respiratory system, namely, a quasi-steady one-dimensional mass transfer model. The model describes the mass transfer between the airway inlet and the red blood cell interior. Using this mass transfer model, we shall show that there exists the optimal number of the bifurcation levels, namely, 23, that yields the minimum overall mass transfer resistance for the mass transport from the external air to the red blood cells.'' Although the conclusions drawn in the two papers, namely, the optimal number of the bifurcation levels being 23, are the same, the difference in the approaches taken in the two papers is quite clear. We proposed a quasi-steady one-dimensional mass transfer model to explain the reason why we have a bronchial tree with 23 levels of bifurcation, whereas they elegantly resorted to Bejan's Constructal law with some assumptions such as isothermal and adiabatic flows with all species having the same chemical potential. Naturally, the resulting equations giving the total num- ber of bifurcations differ substantially from each other. Our equation requires the total pressure difference for breathing and the alveolar sac effective diffusivity, whereas theirs requires quantities such as the gas constant, absolute tem- perature, the oxygen relative concentration at the entrance of the trachea and that in the external air. Despite of the substantial difference in appearance of the two equations, both equations give the same optimal number of the bifurcation levels. This, however, is not surprising, since all rational scientific approaches should lead to the same fact in nature.