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An experimental study of airflow in lumber kilns

J. J. Nijdam, R. B. Keey

Abstract Stacked lumber in a box-shaped kiln is prone to non-uniform flows dueto the appearance of re-circulation zones before and after the stack. Flow visualisations in a hydraulic model of a kiln were conducted to test the

effectiveness of different kiln configurations for eliminating the largerre-circulation zone at the inlet to the stack. When the sharp right-angled bendsare streamlined with sufficient curvature and the geometry of the kiln is such thatthe flow converges through these transitions, then the re-circulation zones vanishand the flow distribution over the height of the stack becomes less peaky andmore even. The experiments show that progressively narrowing plenum chambersin a symmetrical kiln cause more severe flow non-uniformity over the height of the lumber stack than plenum chambers with constant width.

IntroductionAn important prerequisite for producing evenly dried lumber in a kiln is that theairflow is uniformly distributed throughout the height of the stack. Good bafflingsystems and regular stacking arrangements can improve the uniformity of flow toa certain extent. However, previous work by Nijdam and Keey (1999, 2000) haveshown that the geometry of a box-shaped kiln also influences the distribution of flow over the lumber stack, ensuring that some degree of flow maldistribution isunavoidable. Improvements to current box-kiln designs can be facilitated by, first,understanding the factors that influence flow maldistribution within a stack of lumber, and then modifying the geometry of the kiln to mitigate their effect.

There are two major contributions to the flow maldistribution that results fromthe geometry of the kiln. The first contribution is the frictional and inertial forceswhich cause pressure variations down the height of the plenum chambers. Theinertial forces in the plenum chambers on either side of the stack counterbalanceeach other when the plenum chamber widths are equal, which is the case in mostindustrial lumber kilns. However, the frictional force causes a reduction inpressure in the airflow direction, allowing more fluid to flow through the fillet

Wood Science and Technology 36 (2002) 19–26 Springer-Verlag 2002

DOI 10.1007/s002260100121

Received 1 November 1999

J. J. NijdamWood Technology Research Centre, University of Canterbury,Private Bag 4800, Christchurch, New Zealande-mail: [email protected]

R. B. Keey Wood Technology Research Centre, University of Canterbury,Private Bag 4800, Christchurch, New Zealand

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spaces near the top of the stack. Nijdam and Keey (2000) have shown that thefrictional pressure drop down the height of the plenum chamber decreases as thewidth of the plenum chambers increases, with a subsequent improvement in theuniformity of flow over the height of the lumber stack. They show that, in single-track kilns, the width of the plenum chambers should be at least equal to the sumof the width of the fillet spaces to achieve uniformity in flow over the height of thelumber stack. In double-track kilns, the plenum chamber can be narrower due tothe added resistance of the second lumber stack in series with the first.

The second major contribution to flow maldistribution is the sharpright-angled turn from the ceiling space, where the fan is located, to the inletplenum chamber. A vortex zone is generated near the lumber stack following thesharp right-angled turn, and the blocking action of this vortex can severely retardthe flow of air into the uppermost fillet spaces of the lumber stack (Nijdam andKeey 1999). Sturany (1952) has shown that contouring the corner of the sharpright-angled turn smoothes this transition, thereby eliminating the separationzone. Idelchik (1993) has demonstrated the benefits to be gained fromstreamlining the sharp right-angled bend of a duct, which significantly reducesthe extent of flow separation, improves the uniformity of flow across the ductwidth downstream of the bend and reduces the pressure drop through thetransition.

Contouring the sharp right-angled turn could be an expensive and dif ficult taskgiven the shear size of most industrial-scale kilns. In this paper, a simple andpractical design is investigated using semicircular sections to smooth thetransition of the right-angled bend, thereby improving the uniformity of flow over

the height of the lumber stack. Various geometric configurations are tested in ahydraulic model of a kiln to determine the optimum design. Flow-visualisationtechniques are used to gain a qualitative picture of the motion of the fluid in thehydraulic model, and to study the quantitative properties of the flow field.

ExperimentalA detailed account of the apparatus and techniques used has been publishedelsewhere (Nijdam and Keey 1999), but a few of the salient features are describedhere for clarity. The water-test facility has a closed-circuit arrangement. The maincomponents are the header tank, the hydraulic kiln, and the reservoir tank. Water

is pumped from the reservoir tank up to the header tank. The header tank suppliesthe hydraulic kiln with water at a flowrate which is controlled by a throttling valveon the outlet pipe of the hydraulic kiln. An overflow from the header tank ensuresthat the water level inside the header tank, and therefore the flowrate into thehydraulic kiln, remains constant throughout the experiment. Water from thehydraulic kiln and the header-tank overflow drains back into the reservoir tank.

The hydraulic kiln is made of Perspex to enable visualisation of the flow fields,and is geometrically similar to typical box-type lumber kilns found in industry.The hydraulic kiln consists of 13 equivalent fillet spaces, each having a width of 5 mm, and 13 equivalent boards, each having a thickness of 10 mm. The ‘‘ceiling

space’’ is 65-mm high and the ‘‘plenum-chamber’’ width is varied between experi-ments. Uniformity in the inlet flow is promoted by the use of flow straightenersand wire meshes. All experiments are conducted at Reynolds numbers based onthe hydraulic diameter of the fillet space of between 3000 and 12000. These Rey-nolds numbers cover the range of values found in many kilns in industry.

Cavitation-induced bubbles were used to visualise the flow fields at the higherflowrates, while the hydrogen-bubble method was employed to measure the fillet-

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space velocities at lower flowrates, when very little cavitation occurs. Thehydrogen-bubble method involves the use of a thin metallic wire to act as thecathode of an electronic circuit to generate small hydrogen bubbles. A platinumwire of diameter 0.07-mm is tautly stretched across the height of the stack so thatit passes through every fillet space transverse to the direction of flow. The wire islocated 50-mm downstream from the leading edge of the stack of Perspex boardsto remove any entrance effects. The anode is a thin, stainless steel plate placed atthe bottom of the hydraulic kiln, 35-mm from the platinum wire. When a DCvoltage is applied across the electrodes, hydrogen gas bubbles are evolved at thecathode and swept off it by the flow. The sizes of the bubbles generated by thewire are small enough so that buoyancy effects are negligible over the region thevelocity measurements are made and over the range of water velocities tested. Thehydrogen-bubble method is explained in detail by Shraub et al. (1965).

The bubbles are made visible by lighting at right angles to the line of sightusing three 150-W incandescent bulbs. The speeds of the hydrogen bubbles areextracted by visually recording the flow using a video camera, and subsequently measuring the distances travelled by individual bubbles over a frame.

ResultsSemicircular sections of different radii were placed in the inlet ceiling space of thehydraulic kiln to remove the vortex zone generated by the sharp right-angledturn. In these experiments, the plenum-chamber width was set equal to theceiling-space height at the apex of the semicircular section to ensure that the flow did not diverge through the right-angled bend. The flow visualisations and

relative velocity profiles for these configurations are shown in Figs. 1 and 2,respectively. Figure 3 shows the effect of flowrate on the relative velocity distribution over the height of the stack. The ratio c of the radius of thesemicircular section to the maximum height of the ceiling space was equal to 0.3in these particular tests.

In Figs. 4 and 5, the effect of a diverging flow is illustrated. In the first test, thesharp right-angled bend was not contoured, whereas in the second test asemicircular section with the ratio c equal to 0.3 was placed in the ceiling space of the hydraulic kiln. The ratio r of plenum-space width to the sum of the fillet-spacewidths was equal to 1.385, ensuring a diverging flow through the right-angled

bend in both these tests.In the final experiment, a straight sloping wall was placed in both plenumchambers so that r at the top of the stack was equal to 0.7, and tapered down to1=13 (or equivalently 5 mm) at the bottom of the stack. Again, a semicircularsection was placed in the ceiling space of the hydraulic kiln. The semicircularsection had a value for c of 0.3 which was found to be amply suf ficient to eliminatethe vortex zone in the inlet plenum chamber. The relative velocity distribution forthis configuration is compared against the relative velocity distribution of a kilnconfiguration with a constant plenum-chamber width (r ¼ 0.7) and a semicircularsection (c ¼ 0.3) in the ceiling space (Fig. 6).

DiscussionNijdam and Keey (1999) have shown that a vortex zone appears in the inletplenum chamber of a box-shaped kiln just after the abrupt 90 bend. Figure 1ashows that the right-angled bend continues to generate a vortex zone in theplenum chamber even when a small semicircular section, with ratio c of the radiusof the semicircular section to the maximum height of the ceiling space equal to

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Fig. 2. The effect of the ratio c of the radius of the semicircular section to the maximumheight of the ceiling space on the flow distribution across the stack: equivalent averagebetween-board air speed is 4.2 m/s at 400 K

Fig. 1. Flow patterns in the hydraulic kiln for various ratios c of the radius of thesemicircular section to the maximum height of the ceiling space: equivalent averagebetween-board air speed is 8 m/s at 400 K

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0.1, is placed in the ceiling space. Thus, an across-stack velocity distributionsimilar to the case in which no semicircular section is present can be expected.The vortex zone disappears when c is increased to 0.2 (Fig. 1b). However, flow separation persists and the separation point is located at an angle of about 130

Fig. 3. The effect of flowrate on the flow distribution across the stack (equivalent averagebetween-board air speed calculated at 400 K): ratio c of the radius of the semicircularsection to the maximum height of the ceiling space is 0.3

Fig. 4. Flow patterns in the hydraulic kiln with the ratio r of plenum space width to thesum of the fillet-space widths equal to 1.385: equivalent average between-board air speed

is 8 m/s at 400 K

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of water into the uppermost fillet space, although the restriction in flow is not sosevere.

Fig. 2 shows that increasing the radius of the semicircular section has the effectof moving the peak velocity towards the uppermost fillet space and narrowing theflow distribution in the top quarter of the stack. The vortex zone is eliminatedonce c is at least equal to 0.2, although a separation zone persists which reducesthe velocity in the uppermost fillet space. The flow restriction becomes lesspronounced as the radius of the semicircular section is increased further with c equal to 0.3. A distribution of velocities may still arise over the height of the stackeven when the radius of the semicircular section is increased suf ficiently tocompletely remove the separation zone. The frictional effect down the height of the plenum chamber will ensure that the highest velocity appears in the upper-most fillet space and the lowest velocity appears in the lowermost fillet space.Uniform velocity profiles can only be achieved when the width of the plenumchambers – and correspondingly the height of the ceiling space – as well as theradius of the semicircular section are increased suf ficiently.

In order to eliminate the diffuser effect in the right-angled bend, the ratio p of the plenum chamber width to the ceiling-space height at the apex of thesemicircular section remained constant at unity as the radius of the semicircularsection was increased. However, this has resulted in the reduction of the ratio r of the plenum-chamber width to the sum of the fillet space widths. Thedimensionless plenum chamber width r decreased from 0.9 to 0.7 as the di-mensionless radius c of the semicircular section increased from 0.1 to 0.3. Nijdamand Keey (2000) have shown that r must be at least equal to unity for the frictional

effect to become negligible. The reduction of r below unity may explain why thereappears to be no improvement in the uniformity of the relative velocity profilefrom the fifth fillet space downwards, even though the semicircular sectionincreases in radius, thus mitigating the influence of the separation zone. Clearly,the separation zone has the greatest effect on the flow distribution in the topquarter of the stack, while the frictional effect influences the flow distribution inthe remaining portion of the stack.

Figure 3 shows the effect of increasing the equivalent average air velocity overthe range from 2.5 to 4.2 m/s through the stack on the flow distribution over theheight of the stack when c is 0.3. There does not appear to be a significant

difference between the relative velocity profiles at these different flowrates, whichsuggests that flow maldistribution is not strongly affected by the flowrate over thelimited range investigated, when the vortex zone is eliminated. By contrast, in aprevious investigation, Nijdam and Keey (1999) have shown that the velocity distribution becomes significantly more peaky as the flowrate is increased, when avortex zone exists in the plenum chamber. The blocking action of the vortex zoneappears to become more severe as the flowrate rises.

The vortex zone is not eliminated when the ratio p of the width of the plenumchamber to the height of the ceiling space at the apex is greater than unity, asshown in Fig. 4. The higher the value of p, the greater the diffuser effect and

consequently the larger the separation zone. Thus, the velocity distribution acrossthe width of the plenum chamber downstream of the 90 turn is a function of thedegree of expansion of the right-angled bend. Streamlining the right-angled bendhas very little benefit when the width of the duct expands through the transition.Indeed, the flow maldistribution appears to worsen, as shown in Fig. 5. Thesemicircular section reduces the height of the ceiling space, which increases theflowrate at the constriction. The fluid is able to resist a change in direction for

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longer as it travels along the ceiling space due to the increase in momentuminduced at the constriction. Thus, the size of the vortex zone is increased and theflow through the uppermost fillet spaces is reduced even further.

Figure 6 shows that sloping walls in the plenum chambers do not improve theuniformity of flow over the height of the stack. Previous, Nijdam and Keey (1999,2000) have shown that the uniformity in flow worsens when the plenum chamberwidth in a box-shaped kiln is reduced, due to an increased average velocity in theplenum chamber which exacerbates the frictional effect. A straight sloping wallessentially equalises the velocity down the height of the plenum chamber, becausethe width of the plenum chamber is reduced proportionally with a loss involumetric flowrate through the fillet spaces. Thus, straight sloping walls increasethe average velocity in the plenum chamber, which intensifies the frictional effect,thus accentuating the velocity profile over the height of the stack. This differsfrom single-pipe manifolds, such as those used in pipe burners (Dow andShreveport 1950). Sloping walls are effective for improving flow uniformity inthese cases, because the inertial and frictional forces can be counterbalanced tobring about a zero pressure gradient along the length of the manifold. However,in a symmetrical lumber kiln, the inertial forces in the plenum chambers on eitherside of the stack already counterbalance each other, leaving only the frictionalforce.

ConclusionsThe effectiveness of various kiln configurations for eliminating the inlet vortexzone and improving the uniformity of flow over the height of the lumber stack is

investigated. One kiln configuration employed a semicircular section in theceiling-space of a box-shaped kiln to streamline the sharp right-angled bend. Twoconditions must be met in order to eliminate the vortex zone using thesemicircular section. First, the radius of a semicircular section must be at leastequal to one-fifth of the height of the ceiling space. Second, the width of theplenum chamber must be less than or equal to the height of the ceiling-space atthe apex of the semicircular section.

We have found that the magnitude of the flowrate over the range from 2.5 to4.2 m/s has very little effect on the uniformity of flow over the height of the stack,provided the vortex zone has been eliminated. Finally, progressively narrowing

plenum chambers do not improve the uniformity of flow over the height of thestack since inertial forces in the plenum chambers counterbalance.

ReferencesIdelchik IE (1993) Fluid dynamics of industrial equipment: flow distribution designmethods. Hemisphere Publishing Corporation. New YorkNijdam JJ, Keey RB (1999) Airflow behaviour in timber (lumber) kilns. Drying Technology.17: 1511–1522Nijdam JJ, Keey RB (2000) The influence of kiln geometry on flow maldistribution acrosstimber stacks in kilns. Drying Technology (in press)Shraub FA, Kline SJ, Henry J, Runstadler PW, Littell A (1965) Use of hydrogen bubbles for

quantitative determination of time-dependant velocity fields in low speed water flows.ASME J. Basic Eng. 87: 429–444Sturany H (1952) Questions of flow in the drying of wood. Holz Roh- Werkstoff 10: 201–207

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