W2A-038 Particle flow-0.txt
Measurements in the separation region of a gas-particle cross flow
Introduction

Particle-laden flows occur in many engineering applications and are of particular interest in the solid fuel combustion systems associated with fluidised bed reactors. In regions beyond the splash zone, the flow environment in these systems is a dilute suspension of fine solids, particularly in circulating fluidised bed units. The presence of solid particles influences the convective heat transfer, as has been observed in pipe flows by Depew and Kramar (1972) and Sukomel et al. (1976), with increases in Nusselt number noted for fine particles. Similar findings have been reported for suspension jet flows by Kurosaki et al. (1986) and for cross flow over tubes in the first row of a staggered array by Murray and Fitzpatrick (1991). In both cases, enhancement was attributed to an increase in the thermal capacity for the suspension flows with some contribution from boundary layer changes. Murray and Fitzpatrick (1991) report different results for the second and later rows in the tube array, where decreases in the local Nusselt number were considered to be due to changes in boundary layer separation and turbulence characteristics due to the particles. The present work investigates the influence of particles on the boundary layer characteristics in the near separation zone for single cylinders and arrays of cylinders in cross flow.

Particles are known to modify the turbulence of a carrier phase, and papers by, among others, Hinze (1971) and Owen (1969) have shown which fundamental parameters are most likely to influence the interaction, with factors such as particle Reynolds number and relative (particle to fluid) time and length scales determining the nature of the change. Much of the experimental work on the effect of particles on flows has been for either jet or pipe flows. For jet flows, a detailed review of relevant work and of the techniques appropriate for measurement of two phase flow characteristics has been given by Hardalupas et al. (1989). The experimental data reported indicate that the particle phase caused a decrease in the rate of decay of centre-line gas velocity and increased the local turbulence. The decrease in velocity was attributed to particle to gas momentum transfer and the higher turbulence to an increase in the mean rate of strain. The characteristics of suspension flows in ducts or pipes have been investigated by Lee and Durst (1982), Tsuji et al. (1984) and Liljegren and Vlachos (1990). From these, depending on flow conditions and particle size, the turbulence intensity either increased or decreased and a flatter mean velocity profile was observed for the carrier phase with higher velocities close to the wall. For some cases, the velocity profile exhibited maxima at locations intermediate between the wall and the centreline. Thus, without conducting experiments, it is difficult to predict the specific influence of particles on velocity and turbulence characteristics for many flow configurations.

The possibility of boundary layer thinning or disruption by particles suspended in a flow was suggested by Boothroyd (1971). Kurosaki et al. (1986) considered this to be the dominant mechanism of heat transfer enhancement in the stagnation region of an impinging jet suspension flow. Kane et al. (1973), on the other hand, suggest that the viscous sublayer in turbulent pipe flows may increase by extending the range of near laminar flow due to turbulence suppression by the particles. The effect of particles on the characteristics of boundary layer formation on a flat plate was reported by Rogers and Eaton (1989), and their results indicated that for 20% mass loading, there was a noticeable suppression in the turbulence intensity of the flow. However, in this study, the boundary layer remained attached, unlike that of a cylinder in cross flow, and possible changes in separation or wake shedding were not considered. Boundary layer measurements have been made in the near separation region of a cylinder in single phase cross flow by Vartry and Currie (1984). The general characteristics of two phrase gas-particle cross flows over a cylinder have been reported by Lambert et al. (1988), but no specific measurements have been reported for the boundary layer near the separation region of a cylinder in cross flow.

This paper reports the results from a systematic study of the effect of particles on the boundary layer characteristics for a cylinder in cross flow. The measurements were obtained with a single channel LDA system using a pedestal amplitude discrimination technique to enable the characteristics of both the carrier phase fluid and the particles to be determined. Measurement profiles of both mean and rms velocities in the boundary layer region at 85 to 105 from the front stagnation point were obtained for two size ranges of particles (45-85 -200) at two concentrations. In addition, some results from a study of the effect of particles on the flow around the second row tube in a small tube array model are also presented.

2 Experimental set-up

2.1 Test rig

The tests were conducted in a closed loop rig with a 80 mm x 80 mm test section and the flow axis orientated vertically as shown in Fig. 1. For the single cylinder tests, a 38 mm diameter cylinder was installed in the centre of the section with the cylinder axis normal to the flow direction. The resulting high blockage ratio was acceptable, since the consequent accelerating mainstream flow is similar to the conditions experienced by the tubes at the front of heat exchanger arrays. Preliminary tests on the flow characteristics upstream of the cylinder indicated a uniform flow distribution with nominally isotropic turbulence. For the measurements in the second row of an array, the same cylinder was used with "half" cylinders placed upstream and downstream with a triangular pitch of 2.0, as shown in Fig. 1. (Although for such a configuration, the wake structure of the first row tubes will be somewhat different from a full scale array, the flow characteristics close to the surface of the second row near the separation region are quite similar.) The cylinder was polished to produce a mirrored surface to minimise optical noise effects encountered when measuring near surfaces. The laser, together with the transmitting the receiving optics, were mounted on a three dimensional traversing mechanism with a locational accuracy of 10 in the horizontal plane. The laser, together with the transmitting and receiving optics, were mounted on a three dimensional traversing mechanism with a locational accuracy of 10 in the horizontal plane. The laser orientation was normal to the plane shown in Fig. 1. Measurements were obtained for five positions at 85 to 105 from front stagnation. Initially, flow seeding of 1 was obtained using a DANTEC 55L18 seeding generator. After some preliminary tests, this seeding was found to cause agglomeration of the larger particles. In order to alleviate this, smoke from incense was used to seed the air. Comparative tests performed showed no difference between the results obtained for this method and those from the seeding generator. Spherical glass beads of nominal sizes 45-85 and 100-200 were used. The particles were introduced into the flow from a hopper located upstream of the test section and collected in a cyclone. Flow visualisation using a laser sheet technique showed that the particles were uniformly distributed at entry to the test section.

2.2 LDA configuration and measurement system

The laser Doppler system consisted of a 32 mW He-Ne laser operating in forward scatter. The optical arrangement consisted of a Bragg cell with 120 mm focusing and 350 mm receiving lenses and a photomultiplier which was aligned at 5-10 off-axis. The optics were aligned to monitor the velocity in the direction tangential to the cylinder surface, but the measurements were obtained in the horizontal plane rather than normal to the cylinder surface due to difficulties in aligning the traversing system. Although this leads to some small discrepancies in the data, except at 90, the emphasis of the work at present is to measure differences in the behaviour of single and two phase gas-particle flows close to the cylinder surface and these differences will not give rise to significant variations. Specific details of the set-up are given in Table 1. Signal processing of the Doppler signal was performed using a TSI 1980B counter interfaced to an IBM XT computer. A schematic of the LDA and signal processing arrangement is shown in Fig. 2.

In order to discriminate between the carrier and the particle phases, the principle of pedestal amplitude filtering as used by Lee and Durst (1982) and by Levy and Lockwood (1981) was adopted. The tests required the use of a Bragg cell, since zero and negative velocities were anticipated. A robust but simple electronic filter/discriminator system was constructed and installed between the photomultiplier and the downmixer. The photomultiplier signal was passed to both a control line and a relay line. On the control line, the signal was low pass filtered to removed the Doppler frequency and the then input to a threshold detector operating on signal rise time. When this was above a present critical value, a pulse was generated to the relay line to eliminate the signal. Thus, only Doppler signals pertinent to the carrier phase were passed to the downmixer. Conversely, when only information on the particle phrase was required, the control line was switched out and the gain and threshold levels on the photomultiplier and counter respectively set such that signals from the carrier phase were not detected. A line diagram of the discriminator system is given in Fig. 3.

3 Evaluation of measurement uncertainty

3.1 Boundary layer

The first uncertainty to be accounted for is the locational accuracy by which measurements can be obtained in a boundary layer of thickness of the order of 1 mm. Surface location was achieved at each position from the front stagnation point using a visual technique which involved setting the laser at 1 - 2 to the axis of the cylinder. The laser was traversed towards the surface until the spots intersected to form a measuring volume on the surface. The first measurement for the carrier phase was then obtained at 100 from the surface thus located. In some instances, it was not possible to obtain results for the particle phase close to the surface.
In addition to the usual errors inherent in laser Doppler measurements, two additional problems arise when measuring in the boundary layer in the near separation zone. The first is that of velocity gradient biasing and is a problem encountered in all measurements using LDA in flows where velocity differences occur across the measuring volume. It has been indicated by Vartry and Currie (1984) that biasing will be negligible if the ratio between the diameter of the measuring volume and the thickness of the boundary layer is less than 1 - 30. For the set-up used in the present work, the smallest expected value for the latter was of the order 1 mm with a measuring diameter of 80 resulting in a minimum ration of 1 to 12.5. As a consequence, gradient bias errors might be expected to influence the results. Near the surface, the effect of velocity gradient bias is to cause a positive skew characteristic to the velocity histogram whereas at the outer edge of the boundary layer, a negative skew is apparent. In both cases, the rms value of velocity will be increased. The second problem arises as a consequence of the time averaging procedures used to obtain the velocity histograms and is evident in regions where the wake fluctuations cause an unsteady velocity characteristic to be superimposed on the mean flow and turbulence characteristics. Although corrections can be applied to data to account, to some extent, for these effects, in this work the differences in the "raw" boundary layer characteristics caused by the presence of a solid phase in the flow have been studied. It will be evident from the results obtained that the effects of these errors on the observed changes in flow characteristics are not significant.
