The Effect of Endplates on Rectangular Jets of Different Aspect Ratios

The Effect of Endplates on Rectangular Jets of Different Aspect Ratios M. Alnahhal *, Th. Panidis Laboratory of Applied Thermodynamics, Mechanical Engineering and Aeronautics Department, University of Patras, Greece Abstract An experimental study on the influence of an endplate - a wall flush at the jet exit plane - on the flow field of turbulent free jets issuing from smoothly contracting rectangular nozzles of aspect ratio 6 and is presented. Measurements of the streamwise and lateral velocity mean and turbulent characteristics were accomplished, with x- sensor hot wire anemometry, for jets with and without endplates and identical inlet conditions, at downstream locations in the range x/d=-0. For AR=, centreline measurements were collected for Re=0,000, 0,000 and 0,000, and transverse profiles for Re=0,000. For AR=6, centreline measurements have been collected for Re=0,000 and 0,000. The results indicate that the endplate has an effect on the near and far flow field of the turbulent jet, mainly in the lower aspect ratio case. Introduction Rectangular or plane jet flows are generic flows involved in numerous natural phenomena and engineering applications. In combustion systems plane jets are of interest either as a means to enhance and control combustion as in air curtain burners or in quenching staged combustion schemes or cooling the walls of high efficiency burners []. An endplate (or front plate)) is a wall flush at the jet exit plane. It is in general expected that introducing a jet through a plate cause a decrease in velocity spread and decay rate due to the endplate s effect. Although endplates have been used in many previous investigations [,,], only few articles focusing on the comparison of the flow field in the presence or absence of endplates in jets issuing from rectangular or round nozzles are available in the literature. Papadakis and Staiano [] investigated plane jets of aspect ratio, AR=60, with and without a front plate at the exit. They observed insignificant differences between the two cases. Abdel-Rahman et al. [6] investigated the use of a front plate at the exit plane of an axisymmetric round nozzle. Their LDA measurements showed that a reduction in velocity spreading rate and reduced kinematic mass flux occurred in the case with front plate. This was attributed to reduced interaction of the jet with ambient fluid due to the front plate. Motivated by the lack of information on the effect of endplates on rectangular nozzle jets of small or moderate aspect ratios, the objective of the present experimental investigation is, to provide information on the influence of the endplates on jet development and mixing in the near field (e.g. potential core) and in the two-dimensional region (e.g. self preserving) for two rectangular jets of aspect ratios, AR=6 and at different Reynolds numbers. Experimental Set-up and Procedure The jet nozzle facility used for the present investigation (described in detail elsewhere [7]) is shown in figure. It consists of a centrifugal blower air supply system, of. kw power, used to provide the airflow to a diffuser, which decelerates the flow and converts the dynamic pressure to static in order to minimize the energy losses and enhance the uniformity of the flow field. Before reaching the nozzle, the air is passed through a settling chamber, which contains a honeycomb and five screens ( mm square mesh, 0. mm diameter wire) at cm spacing to reduce disturbance at the inlet of the nozzle. The honeycomb installation in the settling chamber (not shown in Fig. ) helps the smoothing and alignment of the flow resulting in turbulence reduction. Large scale eddies break to smaller ones and the mean velocity cross variations are minimized in this section. A D Boerger nozzle is used to accelerate the flow. The nozzle exit is rectangular of length L = 00 mm and width D = 0 or 0 mm for aspect ratios AR = L/D = or 6 respectively. The configuration facilitated the implementation of endplate when needed. The endplate, a wall flush at the jet exit plane, is made of Perspex of mm, 00mm length and 600 mm width, supported by a suitable frame to minimize vibration during operation (see figure ). Hot wire anemometry with a Dantec x-wire sensor, was used to conduct all the measurements. The x-wire sensors are µm in diameter and mm in length. The x-wire was calibrated at nozzle exit under identical inlet flow conditions with the measurements. For aspect ratio, AR=, centreline measurements for both configurations have been collected up to nozzle widths from the nozzle exit, at three Reynolds numbers, Re= U 0 D/ν =0,000, 0,000 and 0,000, where U 0 is the exit velocity from the nozzle, D is the * Corresponding author: mhnahhal@mech.upatras.gr Proceedings of the European Combustion Meeting 009

a).0 = = U 0. c).0 0.8 = =0 0.6 U 0. 0. (All dimensions in mm, not in scale) Figure. Experimental set-up Figure Mean stream wise velocity profiles on the central xy plane nozzle width and ν is the kinematic viscosity of air at ambient temperature. For Re=0,000, transverse measurements across the jet, on the central xy plane were conducted for both configurations at different distances from the jet exit (x/d=0,,, and 0). While for aspect ratio, AR=6, centreline measurements were collected for both configurations at Re=0,000 and 0,000 up to 0 nozzle widths. Results and Discussion Mean velocity field Mean streamwise velocity, U, profiles measured along the transverse direction on the central xy plane are presented in figure. Measurements for both test cases (without endplate,, and with endplate, ) are depicted at downstream locations x/d= and. Profiles are plotted as U (U c is the streamwise mean velocity at the centreline) against (y c is the distance where the streamwise velocity is half of the centreline velocity at the same x/d location). Near the exit (x/d=) both configurations, in line with previous investigations comprising smoothly contracting nozzles, produce uniform (U ) top-hat mean velocity profiles. Measurements throughout the measured flow field have shown insignificant effect of the endplate on the mean streamwise velocity profiles as shown in figure. These findings agree with those in Papadakis and Staiano [] where no significant effect of the endplate on the mean streamwise velocity profiles for plane jet of AR=60 was observed. The corresponding plots of the non-dimensional lateral mean velocity are presented in figure. As expected zero values are observed at the centre at all stations extending in a larger area at the closest to the exit locations, depicting the presence of the potential core. Both configurations, without endplate, and with endplate,, exhibit nearly similar trends showing insignificant effect of the endplate on the mean lateralwise velocity profiles. The growth of the jet half-velocity width, y c, defined as the distance from the jet centreline to the point at which the mean streamwise velocity becomes half of the centreline value at the same x/d location for Re=0,000, is shown in figure.

6 = = AR= Re=0,000 V 0 0 0 0 0 0 - Figure Jet half-velocity widths in the stream wise Direction V 6 = =0 - Figure Mean lateralwise velocity profiles on the central xy plane Beyond a certain distance from the jet exit the half width of a jet is expected to vary linearly with the streamwise coordinate, x, the slope depending on aspect ratio and geometry of the nozzle [8]. This linear evolution is usually described with a relation of the form y c /D= A [x/d+a ] As shown in figure, experimental measurements for both test cases in the first stages of development almost collapse and then vary insignificantly suggesting no influence of the endplate on the growth of the jet half velocity width. A was found to be and 0. for the jet without and with endplate respectively. The decay behaviour of the centreline mean velocity for AR= at three Reynolds numbers, Re=0,000, 0,000 and 0,000, presented by ) versus x/d, where U 0 is the mean streamwise velocity at the exit plane, are shown in figures. An inverse square relationship of the form ) =B [x/d+b ] is demonstrated in figure for x/d greater than 8, where B is a measure of the centreline mean velocity decay rate and B is the jet s virtual origin. The decay of the centreline mean velocity of both test cases at the three Reynolds number, Re=0,000, 0,000 and 0,000 were nearly similar (see table ) and hence suggest no significant effect of the endplate. These findings agree with the results of Papadakis and Staiano [] where similar centreline decay rates for the two test cases (AR=0) were obtained. These are interesting findings since Kotsovinos [9] showed that introducing plane jet out of a wall suffer from reduction in momentum flux and slower spread rate. Later on, Abdel-Rahman et al. [6] found that, their axisymmetric round jet out of a wall suffered from reduction in momentum flux as well as slower spread and decay rates attributing this to the presence of the endplate, causing reduced interaction with the ambient fluid. Examining the initial conditions of the previous mentioned studies one may find that Kotsovinos [9] used a rectangular nozzle of an aspect ratio. while that of Papadakis and Staiano [] was 0. This difference is very significant since jets of smaller aspect ratios, less than or equal to 0, do not have a clear centreline velocity decay as planar jets. Furthermore it has been shown that the point where a rectangular jet first assumes axisymmetric behaviour moves upstream Table Jet characteristics including far field mean stream wise velocity decay, B and jet s potential core length, x p of the present investigation. AR Re Jet condition 0,000 0,000 0,000 B x p.9 0..8 0..0 0.8.7.6 0..6 6 0,000 0.8.00 0.89.89 0,000 0.7. 0..

a) AR= Re=0,000 AR=6 Re=0,000 ) ) 0 0 0 0 0 0 8 6 6 AR= Re=0,000 AR=6 Re=0,000 ) ) 0 0 0 0 0 0 0 0 c) Figure 6 Axial mean velocity decay for AR=6 ) 6 AR= Re=0,000 0 0 0 0 0 Figure Axial mean velocity decay for AR= as the aspect ratio decreases [8]. Based on these findings the influence of the endplate seems to be aspect ratio dependent. Motivated by these considerations, the effect of the endplate was tested using a smaller aspect ratio, AR=6. Both of configurations (with and without endplate) were tested at two Reynolds numbers, Re=0,000 and 0,000. The results indicate that the endplate has a significant effect on the development of the jet at both Reynolds numbers tested. The decay rates of mean velocity at Re= 0,000 and 0,000 are shown in figure 6-a and 6-b respectively. The endplate jet () is found to decay at a slower rate (see table ) than the free jet () probably due to hindered interaction of the jet with the ambient fluid. Focusing on the very near field characteristics, the potential core length, x p, for all the test cases were derived from figures and 6 to illustrate the influence of the endplate on the near field mixing for both aspect ratios at all Reynolds numbers tested. Generally the length of the jet potential core, x p may be considered as an indication of the speed of mixing in the near field, but it is also affected by the way the flow field is convecting its structures. In most of the cases (except for AR=, Re=0,000) the presence of the endplate resulted in a longer potential core length, as result which can be associated with slower near field mixing due to reduced interaction with the ambient fluid.

a) 0. = = 0.6 = = 0. 0. u' v' 8 0. = =0 0.6 = =0 0. 0. u' v' 8 Figure 7 RMS profiles of the streamwise velocity component Figure 8 RMS profiles of the lateralwise velocity component Turbulent velocity field The distributions of the root-mean square (rms) of the streamwise velocity component fluctuations for both configurations for AR= at Re=0,000, on the central xy-plane are shown in figure 7 for x/d= and. Values are non-dimensionalised by the mean streamwise velocity component at the centreline of each location and presented versus. As seen in figure 7, both test cases exhibit nearly similar trends through out the measured flow field. Similar observations hold for the lateral velocity component rms distributions presented for the same locations in figure 8. The shape and the values of the distributions are quite close to those of the streamwise velocity component. The Reynolds shear stress profiles on the central xy plane for AR= at Re=0,000 at x/d= and and x/d= and 0 are shown in figures 9-a and b respectively, presented by <uv> against. The presence of the lateral velocity fluctuations in this term makes it antisymmetric, forcing zero values on the central axis. Following the trends of the velocity components rms distributions, peak values of the shear stress are observed close to = in the early stages of the development moving towards the central axis downstream. Again the distributions of Reynolds shear stresses at =,, and 0 show insignificant effect of the endplate on the near and far turbulent flow fields. Conclusions HWA has been used in an experimental investigation of a turbulent air jet issuing from rectangular nozzles of two aspect ratio, AR=6 and at several Reynolds numbers, to study the relative effect of an endplate positioned at the nozzle exit plane on the jet flow characteristics. Based on the results of the present study, the influence of an endplate seems to be ARdependent having a stronger effect when used with rectangular nozzles of smaller aspect ratio, in accordance with previous investigations. Longitudinal and transverse velocity mean and statistical characteristics measured along the centreline or across the jet for AR=, show a minimum effect of the endplate, of the order of the uncertainty. On the

(<uv> )X00. 0.8 0. = = contrary similar measurements along the centreline of a jet with AR=6 reveal significant influence of the endplate resulting in longer potential core lengths as well as slower decay rate of the centreline velocity at the two Reynolds numbers tested, probably implying restricted mixing with ambient air. Further research focusing on the low aspect ratio jet, to verify these observations and shed light on the underlining physical mechanisms, is currently in progress. Acknowledgment The support of the State Scholarships Foundation of Greece (IKY) to Mohammed Alnahhal is gratefully acknowledged (<uv> )X00..0 0. = =0 Figure 9 Reynolds shear stress profiles at different downstream locations References [] M. Stephane,S. Camille and P. Michel, ASME FEDSM 00 000 ASME/JSME, Fluids Engineering Davison meeting, Vol. F-7 (000) [] L. W. Browne,. R.A Antonia and A.J. Chambers, IUTAM Symposium, Marseille (98), pp. -9 [] G Heskestad, Trans. ASME, J. Appl. Mech., (96) pp. 7-7 [] L.J.S Bradbury. J. Fluid Mech. Vol., (96), pp. 6. [] M. Papadakis and M.W. Staiano, AIAA, 9-06, (99), pp. - [6] A.A. Abdel-Rahman, W. Chakrroun and F. Alfahed, Mechanics Research Comunications, Vol, No., (997), pp. 77-88 [7] A. Cavo, G. Lemonis, Th. Panidis, D. D. Papailiou, Exp. Fluids Vol, (007), pp. 7 0. [8] A. Krothapalli, D. Baganoff, K. Karamcheti. J. Fluid Mech. Vol. 07,(98),, pp. 0 0 [9] N. E. Kotsovinos, J. Fluid Mech. Vol. 77, part, (97), pp. 0-. 6