Inlet Height Research Articles

Optimal profiles for one dimensional slider bearings under technological constraints

The optimal slider bearing profile for maximum bearing load is studied by using direct constrained optimal control techniques. The constraints include the Reynolds and the energy equations. The energy equation takes into account the shear strain rate in the lubricant. The dependence of lubricant viscosity on temperature is considered. Technological constraints such as the maximum lubricant pressure and temperature and the minimum lubricant film thickness are included into the model. The realistic problem considered here yields optimal bearing profiles which are much more complex than the classical Rayleigh step bearing profile. The optimal bearing profile consists of an alternation of regions of constant height and more or less abrupt height variations. The number of constant height regions depends on the type of the constraint and in many cases is larger than three. The minimum value of the bearing height is one of the most important constraints. Four levels of model approximations have been tested. The most important model improvement is to take into account the temperature dependence of the lubricant viscosity. Several bearing design and operation parameters, such as bearing length, inlet height, sliding velocity and lubricant inlet pressure and temperature, have been considered. They all have complex influence on the optimal bearing profile.

Investigation of the atmospheric boundary layer depth variability and its impact on the 222Rn concentration at a rural site in France

AbstractContinuous monitoring of the atmospheric boundary layer (ABL) depth (zi) is important for investigations of trace gases with near‐surface sources. The aim of this study is to examine the temporal variability of zi on both diurnal and seasonal time scales over a full year (2011) and relate these changes to the atmospheric 222Rn concentrations (CRn) measured near the top of a 200 m tower at a rural site (Trainou) in France. Continuous zi estimates were made using a combination of lidar and hourly four‐height carbon dioxide (CO2) profile measurements. Over the diurnal cycle, the 180 m CRn reached a maximum in the late morning as the growing ABL passed through the inlet height (180 m) transporting upward high CRn air from the nocturnal boundary layer. During late afternoon, a minimum in the CRn occurred mainly due to ABL‐mixing. We argue that ABL dilution occurs in two stages: first, during the rapid morning growth into the residual layer, and second, during afternoon with the free atmosphere when zi has reached its quasi‐stationary height (around 750 m in winter or 1700 m in summer). An anticorrelation (R2 of −0.49) was found while performing a linear regression analysis between the daily zi growth rates and the corresponding changes in the CRn illustrating the ABL‐dilution effect. We also analyzed the numerical proportions of the time within a season when zi remained lower than the inlet height and found a clear seasonal variability for the nighttime measurements with higher number of cases with shallow zi (<200 m) in winter (67.3%) than in summer (33.9%) and spring (54.5%). Thus, this pilot study helps delineate the impact of zi on CRn at the site mainly for different regimes of ABL, in particular, during the times when the zi is above the measurement height. It is suggested that when the zi is well below the inlet height, measurements are most possibly indicative of the residual layer 222Rn, an important issue that should be considered in the mass budget approach.

Numerical Simulation and Optimization of Air Distribution in Large Space

In recent years, people pay increasing attention to the issue whether indoor air conditioning system can meet the requirements of thermal comfort under the circumstance of good air quality and reduce energy consumption. FLUENT, commercial computational fluid dynamics software is used to simulate the three-dimensional distribution of temperature and velocity in the subject hotel atrium. The turbulence model modified by buoyancy was used to solve equations. The SIMPLE scheme was used for numerical discrimination. Distribution of temperature, velocity on some typical section and the atrium space can be obtained; the result of numerical simulation can be visualized by post-processing module of FLUENT and TECPLOT software. The influence to the air flow distribution is simulated and calculated by the elements of different rates, angle, and height of the air supply flow. As for the specified case of the project, an optimized solution is obtained, that is supply air speed 6 m/s, angle 15°, air inlet height 7.5m, air temperature 291K.

A Parametric Study of Wind Catcher Model in a Typical System of Evaporative Cooling Tower Using CFD

The aims of the research work described in this paper is a part were to use computational fluid dynamics (CFD) to investigate the factors affecting the performance of a single-stage downdraught evaporative cooling device for low-energy cooling of buildings developed from a novel prototype device described by Pearlmutter et al. (1996; 2008); and to model and explore the performance of the device when integrated within a hypothetical building. This involved carrying out simulations: to select the most effective wind catcher geometry. Two types of wind catcher using curved deflector and closed cowl design were studied: In total five alternative arrangements were investigated. Arrangements 1 and 2 were bi-directional wind catchers. Arrangement 1 was modelled without a baffle and arrangement 2 was modelled with an extended baffle. Arrangements 3, 4 and 5 were uni-directional closed cowls. Arrangement 3 was modelled without a baffle, arrangement 4 was modelled with a short baffle and arrangement 5 was modelled with an extended baffle and an increased inner radius of 1 metre which had the effect of raising the mid-plane height of the cowl inlet by 1 metre. Initially, for comparison in all studies, the inlet wind speed was set at 10 m/s at a reference height of 11.5 metres which corresponded to the mid plane height of the wind catcher and wind cowl entry ducts for arrangements 1 to 4. The CFD simulations were carried out to define the optimum geometry of a wind catcher.

Numerical and experimental studies on the mixing characteristics of two jets with inclined angles in a rectangular chamber

Mixing characteristics in a rectangular chamber are investigated using experimental and numerical methods. A mixing chamber with an axial inlet (representing fuel inlet) and a side inlet (representing air inlet) is designed. Two jets with different momentum ratios through the two inlets are mixed in the chamber. Computational fluid dynamic simulation is validated by experimental data of particle image velocimetry that measures flow velocity distribution. The momentum ratios of the two jets and the height of the axial inlet significantly influence the penetration depth of axial jet into the mixing chamber and the pressure drops at recirculation zones.

THE EFFECT OF THE GAS INLET ON THE FLUID FIELD DURING FABRICATING HFCVD DIAMOND-COATED CUTTING TOOLS

In the present study, the fluid field in a process of fabricating diamond coated cutting tools using the hot filament chemical vapor deposition (HFCVD) method is investigated using the finite volume method (FVM), in which the effects of the inlet height, gas initial velocity, inlet radius and arrangement are illustrated in terms of the gas velocity magnitude and vector distribution near the filaments and the flute surface of cutting tools. In the simulations, the coupling effect of the temperature and the gas field is also considered by simultaneously calculating the temperature distribution. The simulation results suggest that either shortening the distance between the gas inlet and filaments, or increasing the gas initial velocity is helpful for the reactive gas arriving at filaments surface and being dissociated. Furthermore, increasing the inlet area is able to significantly increase the velocity of gas field around the filaments, as well as produce a much more uniform gas velocity field. Based on this conclusion, two novel multi-inlets setups are proposed to further improve the generated gas field and the simulation results show that the most superior gas field can be achieved with the one including 8 larger central inlets and 24 smaller outskirt inlets. Finally, an actual deposition experiment is carried out and its result indicates that adopting the optimized such inlet arrangement could generate a highly uniform and homogeneous growth environment on whole deposition area.

Parametric analysis of steady bifurcations in 2D incompressible viscous flow with high order algorithm

This work deals with the computation of steady bifurcation points in 2D incompressible Newtonian fluid flows. The problem is modeled with the Navier–Stokes equations with an evolving geometric parameter. The aim of the present study is to propose a reliable and efficient numerical method for parametric steady bifurcation calculations. The numerical algorithm is based on the coupling of a continuation method with a homotopy technique. The continuation method lies on the asymptotic numerical method with Padé approximants for an initial linear stability analysis with an initial geometric configuration. The homotopy technique completes the calculation with the computation of critical Reynolds numbers for different discrete values of the geometric parameter. Two classical numerical problems are approached. The first one is the flow in sudden expansion. The geometric parameter is the height of the expansion inlet. The second problem is the flow in a divergent/convergent channel. In this case, the geometric parameter is the length of the channel. Comparisons of results with those obtained from the literature are performed, showing the efficiency of the proposed algorithm. The aim of this study is to determine the critical Reynolds numbers of the flow using few computations for each geometric parameter.

A combinatorial optimization design method applied to S-shaped compressor transition duct design

This paper presents a combinatorial optimization method based on uniform design in combination with response surface methodology and genetic algorithm. Uniform design is used to obtain experimental points and response surface methodology to establish a mathematical regression model. Subsequently, genetic algorithm is employed to acquire optimal solution of the objective function. The optimization method has been applied to a two-dimensional S-shaped transition duct design. The process is performed with two design variables. One defines the drop height ratio which describes wall profile, and the other depicts the length ratio between the axial length of the S-shaped transition duct and the duct inlet height. Total pressure loss coefficient as an aerodynamic performance parameter is selected as the objective function for optimization. The objective function is numerically assessed at design points sampled by uniform design in the experimental domain. The initial transition duct was designed with a radius-change to length ratio 11.6% larger than current engine design limits, and the optimization yields a decrease of 36.9% in total pressure loss and more uniform distributions of parameters at the outlet. The paper shows that the described optimization method can be applied to turbofan engines to increase the radial offset and decrease the axial design space between the fans and cores without jeopardizing performance.

Simulation of two-phase fluid mixture flow in rectangular two-inlet cavity using lattice Boltzmann method

In this paper, two-phase fluid mixture flow in rectangular two-inlet cavity is studied using lattice Boltzmann method (LBM). To simulate two-phase fluids with large viscosity difference, the pseudo-potential model is improved. The improved model is verified for surface tension through Laplace's law and shown much better performance in simulating fluids with large viscosity difference than pseudo-potential model. The multiple-relaxation-time (MRT) scheme is used to enhance numerical stability. Then the two-phase fluid mixture flow with same and different viscosity in two-inlet cavity is simulated by present lattice Boltzmann (LB) model, pseudo-potential LB model and volume-of-fluid (VOF) method, respectively. The comparison of these numerical results shows that LB model is more suitable for such kind of flow than VOF method, since it can reflect repulsive forces and transitional region of two-phase fluids in dynamic process. Moreover, it also shows that present LB model has better dynamic stability than pseudo-potential model. Furthermore, simulations of the two-phase fluid mixture flow with different fluid viscosities, inlet velocities, inlet heights and outlet positions using present LB model are presented, exhibiting their effect to contact area of fluids.

Evaluation of Pressure Drop Performance During Enhanced Flow Boiling in Open Microchannels With Tapered Manifolds

Boiling can provide several orders of magnitude higher performance than a traditional air cooled system in electronics cooling applications. It can dissipate large quantities of heat while maintaining a low surface temperature difference. Flow boiling with microchannels has shown a great potential with its high surface area to volume ratio and latent heat removal. However, flow instabilities and low critical heat flux (CHF) have prevented its successful implementation. A novel flow boiling design is experimentally investigated to overcome the above-mentioned disadvantages while presenting a very low pressure drop. The design uses open microchannels with a tapered manifold (OMM) to provide stable and efficient operation. The effect of tapered manifold block with varied dimension is investigated with distilled, degassed water at atmospheric pressure. Heat transfer coefficient and pressure drop results for uniform and tapered manifolds with plain and microchannel chips are presented. The OMM configuration yielded a CHF of over 500 W/cm2 in our earlier work. In the current work, a heat transfer coefficient of 277.8 kW/m2 °C was obtained using an OMM design with an inlet gap of 127 μm and an exit gap of 727 μm over a 10 mm flow length. The OMM geometry also resulted in a dramatic reduction in pressure drop from 158.4 kPa for a plain chip and 62.1 kPa for a microchannel chip with a uniform manifold, to less than 10 kPa with the tapered OMM design. A tapered manifold (inlet and exit manifold heights of 127 and 727 μm, respectively) with microchannel provided the lowest pressure drop of 3.3 kPa.

An Experimental Study on Performance Improvement of the Stairmand Cyclone Design

The cyclone is widely used as air cleaning device. Many studies have been made on how to enhance its particle collection efficiency or reduce the airflow resistance, but usually the alteration of only one dimension is discussed, and only the collection efficiency or the airflow resistance is considered. Some studies have compared the 50% cut-off size under the same airflow resistance, or compared the airflow resistance under the same 50% cut-off size to obtain the optimal configuration. Yet steps like this take long time to complete, which makes it difficult to compare the cyclone efficiency comprehensively. This work adopts the cyclone quality factor as the performance index, and the cyclone designed by Stairmand is adopted as the basic configuration in the present study. With the cyclone body diameter (D) fixed, we vary the cyclone length (H), inlet height (a) and width (b), cyclone length without the cone (hb), cone height (hc), cone bottom diameter (B), and vortex finder length (S). The results show that each of these design parameters has a different impact on the cyclone performance. Based on the cyclone quality factor, equivalent to the filter quality factor, it is found that the H and S of the Stairmand cyclone are already the optimal conformations. The cyclone performance could be further improved, if further changes could be made to the other conformations, for example, narrowing down the inlet (adjusting a/b from 13/5 to 20/3.2), reducing the cone bottom diameter (adjusting B from 9 mm to 4 mm) and decreasing the vortex finder diameter (adjusting De from 13 mm to 8 mm), The cyclone quality factor appears to be a function of the volumetric flow rate. Results demonstrate that while the air flow is fixed at 15 L/min, the newly improved cyclone could perform 12 times better than the Stairmand cyclone, according to the cyclone quality factor. Keyword: Cyclone quality factor; Cyclone design; Cyclone performance.

Influence of Rib Height on Heat Transfer Augmentation – Application to Aircraft Turbines

This paper presents the influence of rib height on heat transfer and friction factor augmentation in convergent rectangular channel with bottom rib-roughened walls has been conducted in experimentally. In this experiment the square sized ribbs at three different height were (3, 6 and 9 mm) used. A specified constant heat-flux is applied to the bottom surface of the test section and the ribs are arranged in the form of a staggered array. Measurements were taken of a convergent rectangular channel with an inlet aspect ratio of 1.25 and an exit aspect ratio of 1.35. This yields a rib height (e) to mean hydraulic diameter of the duct (Dm) ratio of 0.0348, 0.0697 and 0.1046 respectively, with a fixed rib pitch (p) to test the section inlet height (hin) ratio of 0.6 along the sloped surface of the test section. The Reynolds number based on the mean hydraulic diameter (Dm) of the channel was kept in a range of 20,000 to 60,000. The results from the ribbed channel are compared with that of the smooth convergent rectangular channel, to estimate the heat transfer enhancements from the ribbed channel. The comparison shows that among the tested convergent smooth channel, 3, 6 and 9 mm ribbed channel; the heat transfer from the channel with 6 mm rib height is better and showed comparatively lower friction loss than the other ribbed channels.

Aircraft and tower measurements of CO2 concentration in the planetary boundary layer and the lower free troposphere over southern taiga in West Siberia: Long‐term records from 2002 to 2011

AbstractIn situ measurements of the vertical distribution of carbon dioxide (CO2) carried out with a light aircraft over a tower site (Berezorechka; 56°08′45″N, 84°19′49″E) in the taiga region of West Siberia from October 2001 to March 2012 document the detailed seasonal and vertical variation of CO2 concentrations during daytime. The variation appears to be controlled mainly by the CO2 flux from taiga ecosystems and the height of the planetary boundary layer (PBL). We calculated average CO2 concentrations in the PBL and the lower free troposphere (LFT), both of which show clear seasonal cycles and an increasing long‐term trend. Seasonal amplitude in the PBL had a larger value (29 ppm) than that in the LFT (14 ppm), demonstrating strong CO2 source‐sink forcing by the taiga ecosystems. Mean CO2 concentrations during 13:00–17:00 local standard time observed at the four levels of the tower (5, 20, 40, and 80 m) showed lower CO2 concentrations than that observed in the PBL by aircraft during June–August (growing season). This negative bias decreased with increasing inlet height such that the minimum difference appeared at the 80‐m inlet (−2.4 ± 0.8 ppm). No such bias was observed during other months (dormant season). The daytime CO2 flux, based on multiple vertical profiles obtained on a single day, ranged from −36.4 to 3.8 µmol m−2 s−1 during July–September. There was a clear difference in the fluxes between the morning and afternoon, suggesting that these data should be considered examples of fluxes during several daytime hours from the West Siberian taiga.

Fluid–structure interaction analysis on the effect of chip stacking in a 3D integrated circuit package with through-silicon vias during plastic encapsulation

This paper presents the effect of chip stacking in a 3D integrated circuit package during plastic encapsulation. An experiment was conducted on four stacked chips with bumps in a perimeter array. The flow front advancement and chip displacement in the experiment were validated by using FLUENT 6.3 and ABAQUS 6.9, respectively. A total of four models, which consist of two, three, four, and five stacked chips with through-silicon vias, were studied numerically. A simultaneous or direct solution procedure was employed to solve the variables of the fluid/structural domain. This approach provides better visualization of the actual plastic encapsulation process by considering the fluid–structure interaction phenomenon during the process. A constant ratio of inlet and outlet gate heights was applied to create a more uniform flow front advancement among the models. Results indicate that the highest displacement occurred in Model 4, which contains the most stacked chips. The highest von Mises stress was also detected in Model 4. Therefore, unfavorable deformation is anticipated when more stacked chips are employed. The experimental and numerical studies provide useful information in understanding the fluid flow of epoxy resin and subsequent structural deformation under the effect of chip stacking.

Numerical Investigation of Thermal Characteristics in a Solar Chimney Project

In order to create interior multiclimate zones suitable for plant cultivation in a solar chimney project proposed for the World Horticultural Exposition 2016 in China, the numerical investigation is performed to a solar chimney with 2 km radius and 1 km height. The simulation methodology is validated by experimental data. The impacts of inlet height and radius to interior thermal characteristics are evaluated. It is found that the temperature field is much similar when the inlet height decreases from 3.8 m to 0.5 m. With further decrease of the inlet height to 0.2 m, its impact to temperature field is significant. The temperature field is similar though the radius of solar chimney decreases from 2.0 km to 1.725 km. Further decrease to 1.13 km results in much lower temperature field as less radiate heat is obtained. The procedure of soil surface temperature calculation is also proposed to obtain the supplementary heat amount maintaining the required temperature for plant cultivation.

An automated system for selective and continuous measurements of vertical thoron profiles for the determination of transport times near the ground

Abstract. The quantification of in-canopy transport times is of major importance for the investigation of sources, sinks and net fluxes of reactive trace gases within plant canopies. The Damköhler number, which compares timescales of chemical reactions with transport times, is a widely applied measure to evaluate flux divergences. In this study we present and evaluate a novel automated measurement system for selective vertical thoron (Tn) profiles near the Earth's surface and demonstrate its suitability for the direct and reliable determination of transport times within a natural grassland canopy. For the first time, we perform a rigorous determination of systematic and random uncertainties of Tn (and Rn) concentrations under field conditions for this type of measurement system. The obtained median precisions for three concentration classes (> 100 Bq m−3, 100–15 Bq m−3, < 15 Bq m−3) were 8.8%, 23.2% and 132.1% for Tn (and 16.6%, 25.0%, 99.2% for Rn). We calculate in-canopy transport times (τ) and propagate their uncertainty from the individual errors of the Tn concentration measurements. A quality assessment of τ for the field experiment during a period of 51 days revealed good data quality with 44% of the relative uncertainties below 50%. The occurrence of transport time uncertainties higher than 100% was caused by absolute Tn gradients lower than 70 Bq m−3 m−1, which was found for 22% of all determined transport times. In addition, the method was found to be highly sensitive to the Tn concentrations at the upper of the two inlet heights (zu). Low values of CTnzu result in high absolute uncertainties of the transport time. A comparison with empirical parameterizations revealed a much lower scatter for the τ values determined from our measurements. We found an excellent agreement with τ values obtained by the in-canopy resistance approach used, e.g., in the SURFATM model during daytime, while the SURFATM model significantly overestimated transport times during nighttime.

Influence of Operating Parameters on Raceway Properties in a Model Blast Furnace Using a Two-Fluid Model

In the lower part of an ironmaking blast furnace, a hot air jet is injected horizontally into a bed of coke particles, forming a void zone, called a “raceway”. In this paper, two-fluid gas–solid modeling is used to study the influence of various operational parameters on the size and shape of the raceway (void zone). The model employs state-of-the-art closure relations to describe solid-phase stress, which then incorporates frictional forces based on soil mechanics, and k-epsilon equations to describe gas-phase stress. The results show that raceway size and shape depend on jet velocity, outlet pressure, particle size, bed height, height of the jet inlet and the particle downward extraction rate. The influence of jet injection angle, domain geometry, and initial porosity on the raceway properties are found to be insignificant for the investigated range, although changing initial porosity suggests a small degree of hysteresis.

Conveying Characteristics of Pulverized Coal in a Top‐Discharge Blow Tank System

AbstractBased on the experiments of pulverized coal pneumatic conveying using nitrogen as carrier, the influences of riser inlet height above the gas distribution plate, riser diameter, pulverized coal external moisture content, and supplemental gas flow rate on the conveying characteristics such as pulverized coal mass flow rate and solid‐gas ratio were investigated in a laboratory‐scale experimental setup of a top‐discharge blow tank under atmospheric pressure. There exists an optimal riser inlet height for a given top‐discharge blow tank. The supplemental gas is one of the critical factors affecting the conveying stability and continuity. A model for material mass flow rate prediction with errors ranging from –25 % to +15 % is presented.

CFD modeling and multi-objective optimization of cyclone geometry using desirability function, artificial neural networks and genetic algorithms

The low-mass loading gas cyclone separator has two performance parameters, the pressure drop and the collection efficiency (cut-off diameter). In this paper, a multi-objective optimization study of a gas cyclone separator has been performed using the response surface methodology (RSM) and CFD data. The effects of the inlet height, the inlet width, the vortex finder diameter and the cyclone total height on the cyclone performance have been investigated. The analysis of design of experiment shows a strong interaction between the inlet dimensions and the vortex finder diameter. No interaction between the cyclone height and the other three factors was observed. The desirability function approach has been used for the multi-objective optimization. A new set of geometrical ratios (design) has been obtained to achieve the best performance. A numerical comparison between the new design and the Stairmand design confirms the superior performance of the new design. As an alternative approach for applying RSM as a meta-model, two radial basis function neural networks (RBFNNs) have been used. Furthermore, the genetic algorithms technique has been used instead of the desirability function approach. A multi-objective optimization study using NSGA-II technique has been performed to obtain the Pareto front for the best performance cyclone separator.

Numerical simulation of airflow in a solar chimney for cooling buildings in the city of Yazd

This paper investigates numerically a two-dimensional turbulent flow in a solar chimney due to free convection heat transfer. For evaluation of the fluid flow and heat transfer in the system, the basic equations including conservation of mass and momentum in the x, y directions and energy in the Cartesian coordinate are converted to the algebraic form using the finite volume method. This simulation (use the commercial CFD code) provides the profile of temperature, velocity, and mass flow rate variations in the solar chimney in different conditions. These results can be used to evaluate the effective parameters and optimize the performance of the system for natural evaporative cooling and ventilating the buildings. Based on climate conditions in Yazd, the optimum dimension of the width of the chimney is 0.3–0.4 m, inlet height is 0.3 m, and the flow rate created in the system and at the outlet is at 0.18 m3/s. The mean cooling rate resulted from the solar chimney is about 800 W/m2. Considering the long hot and sunny days of summer (about 12 h in Yazd), the use of this system in warm and dry region is recommended. We can save a considerable amount of energy sources such as natural gas or oil and lower CO2 emissions.