CHAPTER 4 TECHNICAL DETAILS OF VARIANT TWISTED TAPES

Transcription

1 37 CHAPTER 4 TECHNICAL DETAILS OF VARIANT TWISTED TAPES 4.1 TECHNICAL DETAILS OF TWISTED TAPES This section deals with the technical terms, design, fabrication part associated with twisted tape and details of the variant twisted tapes. The technical details related to P-TT and variant twisted tapes are discussed below: i) Pitch (H): It is distance of one length of twist in the twisted tapes for the angle of rotation 180 º shown in the Figure 4.1. ii) Twist ratio (y): The twist ratio is defined as the value of one length of twist (or) pitch length to inner diameter of tube (Agarwal and Raja Rao 1996, Wang and Sunden 2002, Dewan et al and Eiamsa-ard et al. 2006). Pitch length (H) Twist ratio (y) = Inner diameter of tube (d ) i iii) Width ratio (WR): It is the ratio between width of the cut (w) to the width of the twisted tape (W) (Eiamsa-ard et al. 2010b). Width of cut (w) Width ratio (WR) = Width of twisted tape (W)

2 38 iv) Depth ratio (DR): It is the ratio between depth of the cut (d e ) to the width of the twisted tape (W) (Eiamsa-ard et al. 2010a and 2010b). Depth of cut (d e) Depth ratio (DR) = Width of twisted tape (W) 4.2 DESIGN AND FABRICATION OF TWISTED TAPES In the test run, tapes are used with three different twist ratios, 4.4 and 6.0. Twisted tapes are made from aluminium strips of thickness 1.5mm and width 23.5mm (Agarwal and Raja Rao 1996). By referring the literature, most of the authors (Agarwal and Raja Rao 1996, Al-Fahed and Chakroun 1996, Ray and Date 2001, Saha et al. 2001, Hasim et al. 2003, Naphon 2006, Chang et al. 2007a & 2007b, Thianpong et al. 2009, Eiamsa-ard et al. 2009a & 2009b and 2010a & 2010b and Saha 2010) used the twist ratio in the range between 2 and 8 for better heat transfer enhancement. So, the twist ratios, 4.4 and 6.0 are used in the present work to study the effect of heat transfer enhancement. Figure 4.1 shows how a twisted tape integrated to a heat exchanger. Figure 4.1 P-TT fitted in a heat exchanger

3 Plain-Twisted Tape (P-TT) Figure 4.2 shows the photographic view of the P-TT of twist ratios, 4.4 and 6.0 with its details of pitch and width. Plain-Twisted Tapes W = 23.5 mm W H = 50, 110 and 150 mm H Figure 4.2 Plain-Twisted Tapes (P-TT) Trapezoidal cut-twisted Tape (T-TT) Figure 4.3 shows the photographic view of the T-TT of twist ratios, 4.4 and 6.0 with trapezoidal-cut (8mm depth, 8mm &12mm width at bottom and top respectively) alternately in the peripheral region of the tape to increase the disturbance to the flow near the walls of the test section. Trapezoidal Cut-Twisted Tapes w t w b d e w t = 12 mm and w b = 8 mm Figure 4.3 Trapezoidal Cut-Twisted Tapes (T-TT)

4 Square cut-twisted Tape (S-TT) The photographic view of the S-TT of twist ratios, 4.4 and 6.0 with square-cut dimensions of 8mm depth and 8mm width on both top and bottom alternately in the peripheral region of the tape to increase the disturbance to the flow near the walls of the test section shown in the Figure 4.4. Square Cut-Twisted Tapes w y= 2.0 d e w = 8 mm Figure 4.4 Square Cut-Twisted Tapes (S-TT) U cut-twisted Tape (U-TT) The Figure 4.5 demonstrates the U-cut twisted tape with twist ratios of 2.0, 4.4 and 6.0 for the depth and width of U-cut is 8mm respectively. U Cut- Twisted Tapes w w = 8 mm d e Figure 4.5 U Cut-Twisted Tapes (U-TT)

5 V cut-twisted Tape (V-TT) V-cut twisted tapes of twist ratios, 4.4 and 6.0 with different DR and WR are shown in Figures 4.6a, 4.6b, and 4.6c respectively for the DR = 0.34 and WR = 0.43, DR = 0.34 and WR = 0.34 and DR = 0.43 and WR = V-cut is introduced in the P-TT on both top and bottom alternately in the tape with different dimensions of depth and width to study the heat transfer enhancement by increasing and decreasing the depth and width of the V-cut. V-TT (DR=0.34, WR=0.43) w d e w = 10 mm, (a) V-TT (DR=0.34, WR=0.34) w d e w = 8 mm and (b) V-TT (DR=0.43, WR=0.34) w d e (c) Figure 4.6 V cut-twisted Tapes (V-TT) w = 8 mm and d e = 10 mm

6 Wing cut-twisted Tape (W-TT) In the present work, two types of wing-cut twisted tape are used, namely vertical wing cut-twisted tape (VW-TT) and horizontal wing cut-twisted tape (HW-TT). The modified twisted tapes comprise the vertical wing -cut and horizontal wing-cut with dimensions of 5mm width and 8mm depth alternately in the peripheral region of the tape to promote the turbulence near the walls of the test section. The geometrical configuration of VW-TT and HW-TT are shown in Figures 4.7a and 4.7b respectively. Vertical Wing Cut (VW-TT) d e (a) Horizontal Wing Cut (HW-TT) w w = 5 mm (b) Figure 4.7 Wing Cut-Twisted Tapes (a) VW-TT and (b) HW-TT Wire Nail-Twisted Tape (WN-TT) The wire nail-twisted tape used in the present work is shown in Figure 4.8. The present tape contains modification on tape geometry by inserting small wire nails on the P-TT and it is believed that the small nails

7 43 promotes the turbulence in addition with the swirl flow generated by the P-TT and also details of wire nails are given in Table 4.1 Table 4.1 Details of wire nail - twisted tape Sl. No. Twist ratio (y) Pitch length (H) (mm) Number of pitch Wire-nail/ Pitch Number of wire-nail Wire Nail-Twisted Tapes Length of wire nail (L w ) = 14 mm Diameter of wire nail (d w ) = 2 mm (a) Head diameter of wire nail (d hw )= 4mm Lw d hw d w (b) (c) (d) Figure 4.8 (a) WN-TT (b) and (c) views of WN-TT, and (d) Wire -nail in the next Chapter. The data reduction steps for the aforementioned cases are presented