RESULTS AND DISCUSSIONS
2.2 Effect of Number of Vortex Generators
4.3.1 Effects of Vortex generators Configuration
Velocity Vectors for Common Flow Up (CFU) and Common Flow Down(CFD) Configurations
Figure -42 Longitudinal vortices for CFU configuration at the different cross-sections (x=80, 85, 90, 95,100 in mm) at angle of attack of 15º with 7VGs
90, 95,100 in mm)
Figure -43 Longitudinal vortices for CFU configuration (Oval tubes AR=1.80) at the different cross-sections (x=80, 85, 90, 95,100 in mm) at angle of attack of 15º with 7VGs.
FLOW FLOW
49 In CFU configuration, the transverse distance between the leading edges of the winglet pair is more than the distance between the trailing edge; hence, the left winglet forms vortices rotating in a counterclockwise direction while the right winglet forms vortices rotating in a clockwise direction as shown in Figure 42 and in Figure 43. The vortices create the up wash flow between the vortices while the down wash flow on the outer region of vortices towards the lower channel wall. Also, the distance between the vortex cores decreases, hence, the interaction between the counter rotating vortex pair increases considerably. It is also observed that the core of the vortices in CFU gradually moves from the lower wall to upper wall as the flow moves downstream. The thinning of the thermal boundary layer is on the outer region of the vortices.
Figure -44 Vortices distribution for CFD configuration (Oval tubes AR=1.80) at the different cross-sections (x=80, 85, 90, 95,100 in mm) at angle of attack of 15º with 7VGs
On the contrary, The CFD configuration of a winglet pair is demonstrated when the transverse distance between the leading edges is less than that of the trailing edges. For CFD, the left winglet forms vortices rotating in a clockwise direction while the right winglet forms vortices rotating in a counterclockwise direction as shown in Figure 44.These vortices create the down wash flow towards the lower channel wall while the up wash flow is away from the wall which is found in the outside region of the vortices. Along the downstream direction, the secondary velocity vectors decreases while the distance between the vortex cores increases.
Thinning of the thermal boundary layer thus occurs in between the two vortices.
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50 Figure -45 Simulated vortices for (CFU+CFD) configuration at the different cross-sections (x=80, 85, 90, 95,100 in mm) at angle of attack of 15º with 7VGs
Figure -46 Staggered configuration creates vortices at the different cross-sections (x=80, 85, 90, 95,100 in mm) at angle of attack of 15º with 7VGs
The Effects of CFD and CFU pairs of vortices produced by LVGs with rectangular shape of winglet are studied at various Reynolds number. Results show that effect of LVGs could effectively enhance the heat transfer of the heat exchanger. The common flow up configuration shows a better overall performance than the CFU, CFU+CFD and staggered configuration for the RWP. The longitudinal vortices are generated behind the RWPs due to the pressure difference and the friction. The induced spanwise velocity of longitudinal
FLOW FLOW
51 vortices is almost 2~3 times as much as that of the frontal velocity. This strong swirling flow will transport the fluid in the tube wake region to the main flow regions .The improved bulk fluid mixing between the tube wake and main flow by the longitudinal vortices is one of the important heat transfer mechanisms. The development of the secondary flow obtained from the simulations is presented at five different locations along the stream wise direction downstream of the LVGs for Re=850.The plots show the generation of the counter rotating vortices by the LVGs, their gradual deformation and decrease in their strength as they move downstream in the channel. The effects of channel walls on vertical structures can be seen when the cross flow at any two sections of the winglets are compared. Two counter rotating main vortices behind the winglets cause the fluid to churn. This churning motion causes the fluid near the wall to flow in the central region. The circular tubes show a better heat transfer coefficient due to high velocity induced flow near the tube wall than that of oval tubes.
Variation of Heat Transfer Coefficient and Pressure Drop with Reynolds Number
Reynolds number, Re
500 600 700 800
H eat transfer coeffici ent, h(Wm
-2k
-1) 25 30 35 40 45 50
55
Circular-CFUCircular-CFD Oval-CFU Oval-CFD
Oval-(CFU+CFD) Oval-Staggered
(a)
52
Reynolds number, Re
500 600 700 800
Pressure drop (Pa)
10 20 30 40 50
60
Circular-CFUCircular-CFD Oval-CFU Oval-CFD
Oval-(CFU+CFD) Oval-Staggered
Figure 47- Area weighted average value of a) heat transfer coefficient and the b) pressure drop for a range of Re numbers at 15º angle of attack
Increasing Reynolds number increases heat transfer coefficient for all the stated cases.
Circular tube with vortex generators in CFU configuration shows a higher heat transfer capacity than that of oval tubes in CFU, but considering the pressure drops from Figure 47(b) the overall performance of oval tube is better than the circular ones.
At the same time, Oval tubes with 7 vortex generators in CFU gives the best output results considering the pressure drop penalty. Increasing Reynolds number decreases the ratio while it shows highest values at low Reynolds number due to lower pressure drop offered by the flowing fluid resistance as shown in Figure 48.
(b)
53
Reynolds number, Re
500 600 700 800
Heat transfer coefficient/pressure drop
0.5 1.0 1.5 2.0
2.5
Circular-CFUCircular-CFD Oval-CFU Oval-CFD
Oval-(CFU+CFD) Oval-Staggered
Figure 48-Performance parameter (h/p ratio) for a range of Re numbers at 15º angle of attack with 7VG