CHAPTER 10 CONCLUSION AND SCOPE FOR FUTURE WORK

Transcription

1 159 CHAPTER 10 CONCLUSION AND SCOPE FOR FUTURE WORK The investigations on heat transfer and friction factor characteristics for variant twisted tapes (T-TT, S-TT, U-TT, V-TT, VW-TT, HW-TT and WN-TT) fitted in the double pipe heat exchanger with the twist ratios y = 2.0, 4.4 and 6.0 have been studied and presented. According to the past studies, it is observed that modifications on the P-TT i.e. small cuts on the tape gives assurance for enhancement of both heat transfer and thermal enhancement. The variant twisted tapes are used based on the concept of introducing small cuts on the peripheral region of the tape (T-TT, S-TT, U-TT, V-TT, VW-TT and HW-TT) and wire-nail attachment on the P-TT. The experiments are conducted to find the suitable one for the industrial applications. The conclusion arrived from the present work are presented in the following sections: 10.1 PLAIN TUBE i) The plain tube data of Nusselt number and friction factor is verified with the standard correlations in order to ensure the performance of the experimental set up for laminar and turbulent flows. The maximum deviation observed for experimental Nusselt number and friction factor is ±12% and ±15% with the standard correlations values respectively. In addition, the plain tube correlations derived from the experimental results are found to represent the experimental

2 160 data within ±5% for Nusselt number and ±7% for friction factor respectively PLAIN TWISTED TAPES (i) ( (i The validation test is conducted using the correlations available in the literature for the sake of error assessment of P-TT laminar and turbulent experimental results. The present P-TT experimental Nusslet number and friction factor values are deviated to a maximum of ±20% with standard correlations. The tape with lower twist ratio (y = 2.0) offered better thermal enhancement than that of tape with higher twist ratio (y = 4.4 and 6.0) because the residential time of flow increases with stronger swirl level which causes the flow has more time to exchange the heat between the core and the wall. The correlations are developed by fitting the P-TT laminar and turbulent experimental data within ± 9% & ±8% for Nusselt number and friction factor respectively VARIANT TWISTED TAPES i) In general, the variant twisted tapes used in the present work yield higher thermal performance than those with P-TT and plain tube. The reason for the enhancement is the small cut on the peripheral region of the tape which cause additional disturbance to the main swirl flow and there by developing secondary flow along with the main swirl flow. Similarly, the

3 161 wings and wire-nails act as additional turbulators which improves the fluid mixing nearer to the wall of the tube. i iv) In comparison made among the T-TT, S-TT and U-TT on heat transfer enhancement, the U-TT plays better than that of T-TT and S-TT due to the small radius on the corner in the U-cut which improves the fluid mixing nearer to the wall of the tube. The V-TT provides improved heat transfer enhancement than that of T-TT, S-TT and U-TT because of the effective secondary flow in addition with the main swirl flow. In addition, the increasing the depth and decreasing width of the cut yielded enhanced heat transfer characteristics because the vorticity behind the cut are further promoted resulting in increasing turbulence than that of the decrease in the depth and increase in the width of cut. v) The VW-TT gives the enhanced thermal performance than that of V-TT only at lower twist ratios (y = 2.0) for both the laminar and turbulent flows due to the higher degree of turbulence mixing. vi) The horizontal wings augment the heat transfer rate with that of vertical wings due to the fact of more disturbances to a thermal boundary layer and better mixing between the core and the fluid wall. v In the group of variant twisted tapes, HW-TT and WN-TT yields better thermal performance due to the effect of horizontal wings and wire-nails which acted as an additional turbulator which improve turbulent intensity in the flow path that leads to higher heat transfer enhancement.

4 162 vi The correlation is also developed for Nusselt number and friction factor by fitting the experimental data of the variant twisted tapes with minimum deviation CFD SIMULATION The CFD analysis is carried out to compare the experimental data for plain tube, P-TT and HW-TT using Fluent 6.3 CFD software. Among the variant twisted tapes, HW-TT offered better performance. Therefore the CFD simulations have been carried out for HW-TT among the variant twisted tapes used in the study. The numerical models are developed for plain tube, P-TT and HW-TT and conclusions arrived out of this numerical works are as follows: i) The deviation between simulation and experimentation is observed to the maximum of ±13% for both Nusselt number and friction factor. The turbulent intensity for the heat transfer enhancement of tube inserts is also verified from CFD results of P-TT and HW-TT. i The HW-TT offered increase in turbulent intensity due to presents of wings cause effective turbulence mixing which leads to the better heat transfer than that of plain tube and P-TT. Concluding remarks In a nut shell, all the variant twisted tapes provided better thermal performance than that of P-TT and plain tube both in laminar and turbulent flow conditions. In the group of variant twisted tapes used in the present work, HW-TT and WN-TT give the highest performance in laminar and

5 163 turbulent flows due to the effect of increased turbulence which improves the fluid mixing near the wall of the test tube. Therefore, HW-TT and WN-TT can be used in place of P-TT to reduce the size of heat exchanger for industrial applications SCOPE FOR FUTURE WORK i) By using the same twist set, heat transfer augmentation for various fluids can be studied. i iv) The performance of wing- cut tapes can also be studied under different combinations of depth and width ratios. The thermal enhancement of wire-nail can be studied by increasing and decreasing the number of wire nails on the P-TT. Thermal performance of T-TT, S-TT, U-TT and V-TT can be assessed by increasing and decreasing the number of cut on the P-TT. v) These variant twisted tapes can be used for heat transfer augmentation studies also in refrigeration system.