Experiments in Fluids 12, 329-341 (1992) Experiments in Fluids 9 Springer-Verlag 1992 Measurements in the separation region of a gas-particle cross flow X A. Fitzpatrick, B. Lambert and D. B. Murray Mechanical Engineering Dept., Trinity College, Dublin, Ireland Abstract. This paper reports the results from a study of the effects of particles on the boundary layer characteristics for cross flow over a single circular cylinder in the near separation region and on a cylin- der located in the second row of a small mock-up heat exchanger array. Two size ranges of particles (45-85 Ixm and 100 200 ~tm) with two concentrations were used for the single cylinder tests. For the array, the larger particles were used at a single concentration. For the single cylinder, the results with the larger particles at mass loading ratio of 1.0 show an obvious effect on the velocity profile shape and on the rms levels. For reduced concentrations, and for the smaller particles, the effects are less noticeable. For the array, similar trends are observed and a delay in the separation point is reported. 1 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 sys- tems 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 Kramer (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 attribut- ed 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 char- acteristics due to the particles. The present work investigates the influence of particles on the boundary layer characteris- tics 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 parti- cle 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 pre- dict the specific influence of particles on velocity and turbu- lence 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 dom- inant mechanism of heat transfer enhancement in the stagna- tion 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