| Heat transfer enhancement of heat exchanger has been a hot issue in engineering and scientific research. As a kind of technology of passive heat transfer enhancement, twisted tape insert relies on its advantages in the aspects of heat transfer and online cleaning is gradually being studied and applied. The purpose of this paper is to explore the heat transfer and friction characteristics in laminar flow of a new kind of twisted tape insert, which combined with reverse twist and incision. In this paper, the means of numerical simulation and experimental research were used to study the various types of twisted tape.Three types of twisted tape are adopted in the numerical simulation, which are traditional twisted, left-right twisted tape and left-right twisted tape with notch, respectively. In addition, use plain tube to do a comparison. For each type of twisted tape, three different pitches are set. The pitches are 42 mm,63 mm and 84 mm, corresponding pitch ratios are 3.0,4.5 and 6.0. Through the analysis of velocity field and temperature at the outlet, we found that the velocity and temperature distribution of the cross section were getting more uniform by inserted twisted tape. The longitudinal motion of the fluid in the tube was effectively enhanced. Therefore, strengthen the mixing of the fluid. At the same time, the laminar boundary layer was destroyed and then reduced the thermal resistance. These effects made the outlet temperature rise and Nusselt number enlarge. However, due to the strong blocking effect, the friction factor of the tube also increased accordingly by inserted twisted tape. Compared to the traditional twisted tape, inserted the left-right twisted tape got higher Nusselt number, friction factor and performance evaluation criteria (PEC). The left-right twisted tape with a pitch ratio of 3.0 has the best effect on heat transfer enhancement. Calculation results revealed that Nusselt number caused by this type of twisted tape was 2.72-3.75 times of that for the plain tube, while the value for friction factor was 5.87-6.42. The corresponding PEC was 1.51-2.03. And for left-right twisted tape with notch, twisted tape with pitch ratio of 3.0 has the best effect on heat transfer enhancement, similarly. The Nusselt number reduced little compared to left-right twisted tape, which was 2.14-3.61times of plain tube. And the value for friction factor reduced a lot, which was 5.05-5.70. The corresponding PEC was 1.25-2.02.In the experiment, only one type of twisted tape was used in order to save cost. Left-right twisted tape with notch whose pitch ratio is 3.0 was chosen due to its superior performance in numerical simulation. Four different lengths of this type twisted tape were arranged at the entrance of the tube, which are 1/4 length, half length,3/4 length and full length. Experimental results showed that the full length twisted tape has the best performance. The Nusselt numbers was 1.92-3.80 times of that for the plain tube, while the value for friction factor was 3.69-6.30. The highest PEC was 2.06.At the end of this paper, the experimental data were polynomial fitting and obtain the experimental correlations of Nusselt number and friction factor. This will have a certain reference value in the future research. |
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