INFLUENCE OF THE NOZZLE OUTLET FACE
ON NEAR FLOW FIELD MIXING
Ijaz M. Khan*,Tony Gilbert† and Mostafa Barigou‡ *‡The University of Birmingham, Department of Chemical Engineering,
Edgbaston, Birmingham, B15 2TT, UK e-mail: m.i.khan@bham.ac.uk
†
Jet Environmental Techniques, Pine Wood Business Park, Coleshill Road, Solihull, Birmingham, B37 7HG, UK
e-mail: tgilbert@jetenvironmental.com
Key words: Nozzle, Turbulence, Diffusion, Near flow field
Abstract. The shape of the nozzle geometry is increasingly attractive in heating, ventilation
and air conditioning applications. However an important consideration in the design of nozzle geometry is its effect on the dynamics of near flow field jet and to minimise the manufacturing cost for practical applications. In this investigation the effect of an identical 3d contraction of the nozzle geometry is investigated numerically with circular, square and rectangular outlets of similar effective area. Comparisons of the axial mean streamwise velocity decay, turbulence, entrainment and the temperature distribution were reported for a circular, square and rectangular outlet sections of the nozzle to evaluate its performance in the space. From the analysis of data, it was found that enhanced mixing between the jet flow and the still surrounding fluid was noticed for the JETs existing nozzle geometry with circular outlet section which generated relatively higher turbulence kinetic energy in the near flow field, which implies better diffusion of temperature for air conditioning and ventilation applications.
1 INTRODUCTION
Lc 0.035m 0.09m D i Flow direction Ln D e
Figure 1: Section of the JETs existing nozzle geometry with circular outlet face
Test Nozzle outlet face Di (m) o U (msec-1) L (m) o T (oC) De (m) a (m) b (m) Remarks 1 2 3 Circular Square Rectangular 0.16 0.16 0.16 3.75 3.75 3.75 5 5 5 60 60 60 0.07 0.062 0.070 0.062 0.055 Lc =0.055m, Ln=0.18m
Table 1: Test conditions for injection nozzle geometry. For details see figures 1 and 2
the near flow field. Khan et al.9 looked in more detail to assess the influence of the nozzle side wall geometry and to investigate the differences in mean flow, turbulence and mixing behaviour of turbulent flow jet in an enclosed numerical space. Four geometries were considered; the existing JETs nozzle, cone nozzle, a convergent divergent nozzle profile and an adjustable eyeball nozzle. The research investigation clearly demonstrated; the maximisation of turbulence kinetic energy is linked with the maximisation of entrainment of the ambient air, minimisation of potential core length, maximisation of throw and the maximisation of diffusion and mixing in the near field of turbulent jet flow.
a) Circular outlet face, Test 1
b) Square outlet face, Test 2
c) Rectangular outlet face, Test 3
2 DESCRIPTION OF THE PROBLEM
Three different outlet section of the nozzle producing a flow jet into an enclosed room are investigated numerically. These conditions are produced via existing JETs nozzle with circular outlet face, and its performance is compared with a similar collar and contraction geometry with a square and a rectangular outlet face, for details see Table 1. The cross section geometry of the JETs existing nozzle and the isometric view of various outlet faces of the nozzles can be seen in figures 1 and 2 respectively. The flow field was considered within the domain identified by cartesian-coordinate x (streamwise), and y, z (cross-stream); for details see figure 3 and 4 for the orientation of the coordinates from the jet axis. To obtain a fully developed velocity profile, we chose straight, 5m pipe having similar diameter to that of the nozzle collar. The minimum length of the flow establishment was estimated from
reported by White Di
L=4.4Re1/6 10.
3 MEASUREMENTS AND NUMERICAL VALIDATION
The prototype measurements and the model validation for the existing JETs nozzle geometry can be seen in more detail at Khan8 and Khan et al.9. For validation; the flow parameters and the geometry of the nozzle and numerical room is chosen the same as that in the experiment carried out by JETs. Boundary conditions of the numerical room can be seen in figure 3. The nozzle outlet face is fitted flush with pressure outlet boundary.
For numerical analysis, a hexahedral grid arrangement is used for modelling which can be seen in figure 5. Very fine grids have been used near the walls and near the inlet and outlet boundary. The time-averaged Navier-Stokes equations are solved using the computational fluid dynamics code solver Fluent 6.2.16, based on the finite volume method. We assume the gas flow is incompressible and the fluid properties are assumed ideal gas. The velocity profile is assumed uniform at mass flow inlet boundary condition. The values of the turbulent kinetic energy and dissipation rate are computed from the inlet velocity , the hydraulic diameter and the turbulence intensity which has been selected to be 4%. The nozzle outlet plane Reynolds number, based on the equivalent diameter (
o U o U Di m De=0.07 ), was about 98900. Equivalent diameter is the diameter of a circular outlet of the nozzle with the same outlet face area as the square and rectangular outlet face. Air flow in the working area of the numerical room is considered at , where as at the mass flow inlet boundary temperature was set to
. C o 20 C o
60 k−ε turbulence model required 7000 iterations to obtain a stationary solution; and 13000 iterations for the RNG k−ε model. Convergence of the steady state simulation was considered to be reached when the flow residuals was less than 10-4 for flow equations. Temperature was considered to be converged when residuals were less than 10-7. Calculations were performed on a PC with Pentium 4 processor with 3.2 GHz speed and 2GB RAM. 4 RESULTS
4.1 Mean velocity profiles
Equivalent diameter De at nozzle outlet face
x y z Floor as wall Ceiling as wall Wa ll Wall Wall Pre ssu re o utle t 11m 5.5 m 2 .5 m Pipe Mass flow inlet
Flow direction
Figure 3: Boundary conditions of the numerical test room, for detail see Figures 1-2 and Table 1
1m 1.5m
Floor of the room Ceiling of the room
Wall Nozzle geometry
Primary air flow X
Induced flow Nozzle collar 0.055m Induced flow y L
Figure 4: Nozzle arrangement shown in x-y plane in a numerical room
0 2 4 6 8 10 12 14 16 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Ux /Ue X /D e Circular outlet Square outlet Rectangular outlet
Figure 6: Near flow field velocity decay on the jet centre line for tests 1, 2 and 3
0 2 4 6 8 10 12 14 16 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 TKE/Ue 2 X /D e Circular outlet Square outlet Rectangular outlet
0 2 4 6 8 10 12 0 5 10 15 20 25 30 X /De E n tr ai n m en t ra ti o Circular outlet Square outlet Rectangular outlet
Figure 8: Mass entrainment for tests 1, 2 and 3
0 2 4 6 8 10 12 14 16 20 25 30 35 40 45 50 55 60 Temperature oC X /D e Circular outlet Square outlet Rectangular outlet
Figure 9: Temperature decay on the jet centre line for tests 1, 2 and 3 4.2 Turbulence kinetic energy profiles
4.3 Mass entrainment profiles
The comparison of mass entrainments for various outlet conditions of the nozzle can be seen in figure 8. Mean streamwise velocity and the gas density are acquired for the nozzle with circular, square and a rectangular outlet face in the y-z planes at various
locations. Mass entrainment was computed numerically by integrating area under the curve using Simpsons rule. Analysis of the results demonstrated that nozzles with different outlet section have no significant effect on the entrainment of the jet flow.
De X /
4.4 Temperature profiles
A similar comparison of axial temperature profiles for various nozzle outlet conditions can be seen in figure 9. The analysis of the results demonstrated that there is a negligible influence on the temperature distribution inside the numerical room. From the importance of turbulence in mixing process, a nozzle with circular outlet is associated with relatively higher TKE and therefore offers better diffusion of temperature in the space.
5 CONCLUSIONS
The near flow field of a jet is investigated under steady state conditions using RNG k−ε
turbulence model. The flow conditions into the numerical room were produced by a circular, square and rectangular outlet section of the nozzle. A circular outlet face of the nozzle is associated with relatively higher turbulence kinetic energy and therefore offers better mixing of temperature in the near flow field.
NOMENCLATURE
Di Hydraulic diameter at inlet of the nozzle
De Equivalent diameter at outlet of the nozzle L Length of the pipe
Lc Collar length
Ln Nozzle length
o
T Gas temperature at mass flow inlet boundary
TKE Turbulence kinetic energy
Ue Axial velocity at nozzle outlet face
o
U Uniform inflow velocity at the inlet boundary
X
U Axial jet velocity from the nozzle outlet face in the direction of flow X Axial distance from the nozzle outlet face in the direction of flow
a Width of the nozzle outlet face
b Height of the nozzle outlet face
x Longitudinal direction
y Upward direction
REFERENCES
[1] S.K. Robinson, S.J. Kline, and P.R. Spalart, “A review of quasi-coherent structures in a numerically simulated turbulent boundary layer”, NASA Technical Memorandum 102191 (1989).
[2] P.J. Holmes, G. Berkooz, and J.L. Lumley, Turbulence, coherent structures,
dynamical systems and symmetry, Cambridge University Press, Cambridge (1996).
[3] B. Wegner, Y. Huai, and A. Sadiki, “Comparative study of turbulent mixing in jet in cross-flow configurations using LES”, International journal of Heat and Fluid Flow, 25, 767-775 (2004).
[4] M.I. Khan, R.R. Simons, and A.J. Grass, “Effect on turbulence production due to sudden change in flow regimes”, Journal of Hydraulic Research, 43(5), 549-555 (2005).
[5] M.I. Khan, R.R. Simons, and A.J. Grass, “Upstream turbulence effect on pollution dispersion”, Journal of Environmental Fluid Mechanics, 5(5), 393-413 (2005).
[6] M.I. Khan, R.R. Simons, and A.J. Grass, “Influence of cavity flow regimes on turbulence diffusion coefficient”, Journal of Visualisation, 9(1), 57-68 (2006).
[7] M.I. Khan, R.R. Simons, and A.J. Grass, “Vertical diffusion of pollution from line source near a wall”, Journal of Hydraulic Research, Approved for publication in 2006. [8] M.I. Khan, “A performance evaluation of the HVAC-based nozzles”, Proc. The 3rd
Asian Conference on Refrigeration and Air-conditioning, Seoul National University,
Gyeongju, Korea, 609-615 (2006)
[9] M.I. Khan, M. Barigou, and T. Gilbert, “Nozzle side wall geometry effect on the near flow field mixing”, Proc. International Heat Transfer conference-13, Australia (2006).
