THE INFLUENCE OF KILN GEOMETRY ON FLOW MALDISTRIBUTION ACROSS TIMBER STACKS IN KILNS

ABSTRACT

A one-dimensional mathematical model is evaluated for flow through a timber-drying kiln. This model is used to develop recommendations for the design of the plenum chambers on either side of the timber stack. The variation of the vertical air velocity was explored for a sticker spacing of 20 mm and a board thickness of 50 mm. When the width of the plenum chambers is at least equal to the sum of the thickness of the stickers, flow maldistribution is substantially reduced in single-track kilns. The width of the plenum chambers of double-track kilns may be reduced to three-quarters of the sum of the thickness of the stickers, because the increase in timber-stack resistance to airflow mitigates the effect of pressure variations down the length of the plenum chambers.     

AIRFLOW BEHAVIOUR IN TIMBER (LUMBER) KILNS

Even drying of stacked timber boards in a kiln depends, amongst other things, on having a uniform airflow through the pile. A study of possible airflow maldistribution in existing kilns has been carried out using a Perspex model of a kiln, with water as the fluid, which has been designed to be geometrically similar to an actual kiln and have dynamically similar flows. Measurements of local velocities were possible by tracing the flow of fine gas bubbles. The standard kiln arrangements result in significant recirculation zones before and after the stack, with a non-uniform incident velocity profile. Different design alternatives for the geometry of the plenum spaces on either side of the stack have been investigated.     

AIRFLOW BEHAVIOUR IN TIMBER (LUMBER) KILNS

ABSTRACT

Even drying of stacked timber boards in a kiln depends, amongst other things, on having a uniform airflow through the pile. A study of possible airflow maldistribution in existing kilns has been carried out using a Perspex model of a kiln, with water as the fluid, which has been designed to be geometrically similar to an actual kiln and have dynamically similar flows. Measurements of local velocities were possible by tracing the flow of fine gas bubbles. The standard kiln arrangements result in significant recirculation zones before and after the stack, with a non-uniform incident velocity profile. Different design alternatives for the geometry of the plenum spaces on either side of the stack have been investigated.     

USE OF COMPUTATIONAL FLUID DYNAMICS TECHNIQUES TO ASSESS DESIGN ALTERNATIVES FOR THE PLENUM CHAMBER OF A SMALL SPRAY DRYER

The inlet region of a pilot-scale, co-current spray dryer was simulated using the proprietary Computational Fluid Dynamics (CFD) codes, CFX4 and CFX5. Several design alternatives were considered for correcting uneven inlet air distribution, which is known to influence spray dryer performance and airflow patterns. The simulations were used to assess each alternative prior to construction, assuming isothermal and incompressible flow conditions. Experimental measurements were compared with the simulation results for the original and one modified design.

Drying air is supplied to this dryer via an overhead pipe feeding an annular plenum chamber, of diameter 400 mm, surrounding the atomiser. A distributor plate with two concentric rings of 50 holes, each of 5 mm diameter, forms the base of the plenum chamber. A three-dimensional grid was required to model each of the 100 holes separately and to consider the asymmetric flow behaviour. The resulting grid consisted of about 532,000 cells. The CFD simulations proved useful in predicting the trends in flow distributions in each of the designs.     

USE OF COMPUTATIONAL FLUID DYNAMICS TECHNIQUES TO ASSESS DESIGN ALTERNATIVES FOR THE PLENUM CHAMBER OF A SMALL SPRAY DRYER

The inlet region of a pilot-scale, co-current spray dryer was simulated using the proprietary Computational Fluid Dynamics (CFD) codes, CFX4 and CFX5. Several design alternatives were considered for correcting uneven inlet air distribution, which is known to influence spray dryer performance and airflow patterns. The simulations were used to assess each alternative prior to construction, assuming isothermal and incompressible flow conditions. Experimental measurements were compared with the simulation results for the original and one modified design.

Drying air is supplied to this dryer via an overhead pipe feeding an annular plenum chamber, of diameter 400 mm, surrounding the atomiser. A distributor plate with two concentric rings of 50 holes, each of 5 mm diameter, forms the base of the plenum chamber. A three-dimensional grid was required to model each of the 100 holes separately and to consider the asymmetric flow behaviour. The resulting grid consisted of about 532,000 cells. The CFD simulations proved useful in predicting the trends in flow distributions in each of the designs.     

新型多晶硅还原炉底盘流动与传热的数值模拟

多晶硅还原炉的底盘取热结构对降低能耗有很大影响,提出了一种新型的多晶硅还原炉底盘均匀取热结构,并就其温度均匀性、冷却效果与传统底盘结构进行比较。该新型底盘结构由中间隔板分为2层,并在隔板上安装电极位置的孔周围焊接竖直环隙。冷却水由隔板下层的冷却水进口进入底盘并流经各电极周围的环隙进入隔板上层的还原炉底盘上底板实现均匀取热。与传统多晶硅还原炉底盘结构相比,该结构克服了传统结构下底盘取热不均匀的问题。就新型底盘取热结构中的单棒环隙结构进行模拟优化。重点考察环隙上焊接挡板的厚度、宽度及数量对冷却效果的影响。300 K的冷却水做工作介质,底盘材料用不锈钢。模拟后得到的单棒环隙结构的最适宜结果为竖直环隙挡板厚度1 mm,挡板宽度1 mm,挡板间距10 mm;水平环面挡板厚度1 mm,挡板宽度1 mm,挡板间距10 mm;换热效果较传统底盘提高32%,温度均匀性提高54%。     

Parametric Influences on the Sensitivity of Exploding Foil Initiators

Analyses of firings by the up‐and‐down method were employed to gain a better understanding of the influences of foil bridge width, foil thickness, flyer thickness, and barrel length on the sensitivity of exploding foil initiators. Characterization was performed via the mean threshold voltage. The results showed that the mean threshold voltage of exploding foil initiators was reduced by 16 % when the foil bridge width was reduced from 0.4 mm to 0.3 mm, which means that a reduced foil bridge width could decrease the mean threshold voltage of exploding foil initiators as suggested by theoretical analysis. The mean threshold voltage of exploding foil initiators was reduced by 20 % as the flyer thickness was decreased from 50 μm to 25 μm, which could be an efficient way to improve the sensitivity of exploding foil initiators. The flyer accelerated to over 90 % maximum velocity after flying 0.15 mm in a fine barrel, which revealed that exceeding a barrel length of 0.15 mm has little effect on the performance of exploding foil initiators.     

垂直矩形窄缝通道内气液两相流型的研究

本文研究了矩形窄缝通道内气液两相流的流型.用目测和摄影方法得到气液两相向上并流的流型.用x-y函数记录仪测得各种流型下的压差频谱图.作出了以1/X_(tt)～G_t和j_(?)～Re为坐标的流型图.作者还用Lockbart-Martinelli方法计算了矩形通道内气液两相流的压降,并与实测值进行了比较.     

洗涤冷却管内垂直降膜流动特性

采用双平行电导探针对多喷嘴煤气化过程中洗涤冷却管内垂直降膜的液膜厚度分布进行了实验研究,研究发现,经洗涤冷却水分布环分配后在洗涤冷却管内形成的液膜在进口段200～400 mm处呈明显的不均匀分布,液膜厚度差值高达3 mm。流动方向上的液膜波动变化与平均液膜厚度变化相似,但具有滞后性。研究还表明,在2.4～14 m³·h^-1的进水流量范围内,整体液膜厚度随着流量的增大而增大,由此对洗涤冷却管内液膜充分发展区的平均液膜厚度建立了经验关联式。实验还发现,洗涤冷却水分布环的槽缝宽度对液膜分布影响显著,在本文研究条件下,最佳槽缝宽度为3 mm,槽缝宽度继续增大,液膜的均匀性下降。另外,随着气相表观速度的增大,整体液膜厚度减小,但一定程度上改善了液膜分布的均匀性。     

Identification of Flow Regimes in Slot‐Rectangular Spouted Beds using Pressure Fluctuations

Pressure fluctuation data recorded in slot‐rectangular spouted beds of 1.44 mm diameter glass beads were analyzed with the aid of statistical and chaotic tools. The column width was maintained at 150 mm, while the thickness and slot width were both varied. It is shown that there are significant shifts in both statistical and chaos measures as the gas flow rate is increased. The results suggest that pressure fluctuations can be used to provide diagnostics of flow regime transitions in spouted beds when viewing is impossible.     

Calculation of radiant heat exchange in tunnel kilns

Conclusions A new program for calculating the error when a two-dimensional model of heat exchange in tunnel kilns of various sizes and with different temperature gradients along the axis of the channel is used, has been developed. It is established that the use of a two-dimensional model is valid in the calculations of large tunnel kilns but may lead to significant errors when calculating small kilns where it is necessary to allow for the connection between the positions and the radiant component of the heat flow.Translated from Ogneupory, No. 4, pp. 23–27, April, 1982.     

250 t钢包底吹氩卷渣和钢液裸露的模拟

针对某钢厂250 t钢包底吹氩气过程进行水模型实验和数值模拟,考察了吹气量和渣层厚度对卷渣行为和吹气量、渣层厚度及透气砖透气性能对钢液裸露面的影响,分析了卷渣形成机理. 结果表明,吹气量对卷渣形成具有决定性作用,吹气量控制在0.96 m3/h(对应实际流量69 m3/h)以下可避免卷渣;随着吹气量的增加,钢液裸露面积逐渐增大,当吹气量达到0.70 m3/h时,钢液裸露面积百分比约达14%,继续增大吹气量,其增加幅度变缓;随着渣层厚度的增加,临界卷渣吹气量和钢液裸露面积逐渐减小,以37 mm(对应实际渣厚150 mm)厚渣层覆盖,可有效防止钢液二次氧化;透气砖堵塞对钢液裸露面积影响较大,顶部钢液形成两不同大小的裸露亮圈,并加重对包壁耐材的冲刷与侵蚀,降低钢的洁净度. 工艺优化后,钢包水口结瘤率降低至0.1%以下,且可降低生产成本.     

Voidage Profiles in a Slot‐Rectangular Spouted Bed

Local voidages were determined using optical fibre probes for 1.4 and 2.4 mm glass beads in slot‐rectangular spouted beds of width 150 mm. The effects of air inlet flow rate, slot width and bed thickness were investigated. Spout widths were determined from standard deviations of local voidage fluctuations. Both spout and annulus voidages increased with increasing air flow. Annulus voidages were usually higher than corresponding loose‐packed voidages and decreased with increasing height. Annulus and spout voidages were very sensitive to the air entry slot width and particle diameter. With increasing height, spouts became increasingly circular (three‐dimensional), despite the initial rectangular geometry.     

Effect of the shape of hydrogel samples on their stretch-fracture properties

Hydrogels have broad application prospects, but the measurement of their mechanical properties often lacks stability. This study investigates the mechanical properties of hydrogels, with a specific focus on the influence of sample geometry on the tensile-fracture testing results. In the process of stretching the hydrogel along its length, increasing the width and thickness will result in uneven stress distribution. When the width of PAM hydrogel is three times that of initial sample (5 mm of width), the elastic modulus, maximum stress, and maximum strain of PAM hydrogel are reduced by about 16.8%, 69.2%, and 26.5%, respectively. Similarly, compared to the initial sample (1 mm of thickness), the elastic modulus of the triple thickness sample was reduced by about 6.5%, the maximum stress was reduced by 31%, and the maximum strain was reduced by 18.3%. In contrast, increasing the length of the hydrogel can improve the tensile properties of the hydrogel. Finite element calculations support these findings that the size increase in the loading direction improves the stress dispersion uniformity. These results indicate that the shape (length, width and thickness) of the hydrogel sample affects the tensile properties of the hydrogel and should be paid attention in related studies.     

Interaction of rarefaction waves with a finite-thickness layer near a rigid boundary. Equilibrium approximation

Interaction of a rarefaction wave with a layer of solid particles near the end face of a shock tube is considered. A one-dimensional unsteady approximation of mechanics of heterogeneous media with identical pressures of the phases and with allowance for a finite volume concentration of particles in the layer is used as a mathematical model. The wave pattern of the flow and the mechanism of wave interaction with the layer, including the dynamics of the layer boundary depending on the layer thickness and rarefaction-wave width, are determined. The mathematical model proposed is verified against the dependence of the layer-boundary coordinate on time and also the dependence of the velocity of upward motion of the layer on the difference in pressures between the high-pressure and low-pressure chambers. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 126–135, September–October, 2007.     

Inlet plenum pressure drop calculation for a cross-flow module

Outlined here is a method for applying a model of discharge flow through a series of sharp-edged orifices from an inlet plenum, to accurately predict the total pressure drop across the flow distribution geometry. This particular case relates to a cross-flow filtration membrane module, but is applicable for any flow distribution system of a similar format.     

Stack-Wide Behavior for Hardwood Drying

The assessment and development of drying schedules for hardwood timber that account for stack-wide behavior is required for practical implementation in kilns of finite width. The concept of a characteristic drying curve was not found to be applicable for the hardwood timber considered here (Australian ironbark), so a full (Fickian) diffusion model was used. Air flow reversals every 4 h gave less variation in the average moisture contents across the stack than greater time periods between each reversal. An incorrectly timed air flow reversal was found to do more damage than no air flow reversals at all. Even with optimization of the drying schedules to account for stack widths, a reconditioning period is still needed to reduce the moisture content variation within the limits for a quality class of A (as defined by the Australian standards). The frequency of air flow reversals is recommended to be no greater than 4 h between each reversal in order to reduce the moisture content variation across the kiln within specified limits and limit the maximum strain to 1.8%.     

并行流道风冷式电池热管理系统的导流板形状优化

针对并行流道风冷式动力电池热管理系统,开发了一种导流板形状优化方法。采用控制点描述导流板的形状,结合数值模拟方法,以电池组温差极小化为优化目标,通过逐步调整控制点高度优化导流板形状。典型算例结果表明,采用提出的优化方法优化Z形风冷系统的进口导流板形状,可显著提高系统的散热性能。在不同冷却空气流量下,与原始系统相比,优化后系统在压降增加20%的情况下,电池组最高温度下降了3.7 K以上,最大温差减小了85%以上;与文献中的Z形流道优化系统相比,本研究的优化系统在保证系统压降基本不变的情况下,电池组温差减小了48%以上。     

一种新型具有均匀流动分布的多流道片材机头的几何设计

提出了一种新型的全部流道由平直板条组成的片材机头结构。全部流道中在熔体特征黏度相等的假定条件下，给出了确定构成流道的栅条尺寸的算法及相关计算公式。在幅宽为0．87m、口模间隙为1mm的实验机头上进行了PP片材的挤出实验以证实机头结构的可行性和设计方法的正确性。结果表明，用这种形式的机头可以获得厚度均匀的挤出片材(沿幅宽最小厚度与最大厚度的比值为0．97)。熔体沿幅宽的平均停留时间分布分析结果表明，平均停留时间分布在幅宽上是不均匀的，但其不均匀性远低于T形机头。     

Investigation of natural convection heat transfer in a unique scaled‐down dual‐channel facility

Multiphase Reactors Engineering and Applications Laboratory (mReal) has designed and constructed a scaled‐down dual‐channel facility to investigate plenum‐to‐plenum natural circulation heat transfer through two channels for coolant flow that would be encountered during a loss of flow accident in the prismatic modular reactor (PMR). Heat transfer characterization of the current facility has been investigated under different upper plenum and cooled channel outer surface temperatures using sophisticated flush mounted heat transfer sensors. Results show a reduction in the values of local heat‐transfer coefficient and Nusselt number along the heated channel with increasing outer surface temperatures. One significant observation was the heat transfer reversal close to heated channel exit, where the heat flows from gas to the channel wall. This flow reversal is attributed to recirculation at the heated channel exit to the upper plenum. The average heat transfer results, when compared with previous literature, showed a similar qualitative trend. © 2016 American Institute of Chemical Engineers AIChE J, 63: 387–396, 2017