Research On Heat Transfer Enhancement In Complex Flow Channel Of Heat Exchangers

Research on heat transfer enhancement of three different kinds of heat exchangers waspresented in this paper. They were annular-finned tube heat exchanger, self-supportedrectangle converging-diverging tube bundle heat exchanger, and shell and tube heat exchangerwith regularly spaced twisted tape inserted in shell side, respectively.Experiments were performed for turbulent heat transfer and fluid flow characteristics ofannular-finned tube heat exchanger. The effects of three factors were examined: Re number,longitudinal tube spacing (Sl) and tube arrangements. Then3-D numerical simulations wereperformed to analyze the experimental results. The Reynolds number varied from4970to23620, and Slvaried from66mm to130mm. The tube arrangements were inline arrangementand staggered arrangement. It was found that Nusselt number and friction factor increasedwith the increase of Re for both inline and staggered arrangement, and the increasing trenddecreased. For inline arrangement, the heat transfer performance became better with theincrease of Sl, and it was the best when Slis110mm. However, it became worse with theincrease of Slfor staggered arrangement, and it was best when Slis66mm. Compared withinline arrangement, the heat transfer performance of staggered array was better. It was foundthat the effects of the three factors on the heat transfer performance of the annular-finned tubebanks can be well described by the field synergy principle, the area of recirculation region andthe wake vortex scale, i.e., the enhancement or deterioration of the heat transfer performancewas inherently related to the variations of the intersection angle between the velocity and thefluid temperature gradient, the area of recirculation region and the wake vortex scale.Based on the new heat exchangers, self-support of rectangle converging-diverging(SS-RCD) tube bundle heat exchangers, three-dimensional numerical investigation wasperformed for turbulent heat transfer and fluid flow characteristics of the heat exchangers withdifferent inserts. The Reynolds numbers varied from27900to41900. The baselineconfiguration (without insert) was compared with four enhanced configurations (with inserts):Torus case, regularly spaced twisted-tape elements (RSTT) case, twisted tape (TT) case, andbaffle plate (BP) case. The inserts leaded to the variation of velocity distribution anduniformity of temperature. Compared with the baseline case, the air-side heat transfercoefficient of the four enhanced cases improved by16.69~24.32%,31.07~33.08%,28.32~33.13%, and38.01~46.74%, with an associated pressure drop penalty increase of74.02~89.5%,69.32~77.42%,54.35~65.74, and68.49~87.16%, respectively. The overallperformance was conducted by thermal enhancement factor. It is found that the BP case obtained the best overall performance, followed by TT case, RSTT case and Torus case, thebaseline case was the worst. The results indicated that the key point of enhancing heat transferof shell side is to improve the heat transfer performance on converging-diverging tube. Thenumerical results were analyzed from the view point of field synergy principle. It was foundthat the reduction in the average intersection angle between the velocity vector and thetemperature gradient was one of the essential factors influencing heat transfer performance.Based on the above research,3-D numerical simulations were performed to get theoptimal structure of converging-diverging tube. The rib length (L) and height (h) wereexamined while retaining converging length ratio. The effects on heat transfer performance oftube side, shell side and whole heat exchanger were studied. The numerical results indicatedthat the smaller rib length and the larger rib height, the better the performance of heat transferand the greater the flow resistance for both shell side and tube side. Performance EvaluationCriteria (η) was adopted to evaluate the overall heat transfer performance. When L=16.5mm,η was the largest for shell side, while when L=9mm it was the largest for tube side. The riblength remained15mm was unchanged, the effect of rib height was examined. The overallheat transfer performance was best when h=0.5mm for shell side, while when h=1.25mmfor tube side it was the best. The reason is that the recirculation region increased withincreasing of rib length and height for both shell side and tube side. With the increasing ofrecirculation region, the friction in the channel increased. However, the recirculation regionreduced the average intersection angle between the velocity vector and the temperaturegradient, which was one of the essential factors influencing heat transfer performance. ThePerformance Evaluation Criteria of the total heat exchanger when the rib length was15mmand the rib height was0.75mm was η=1.136-1.155against the Reynolds number of shellside in the range of27900-41900as compared with the smooth tube bundle heat exchanger.Optimization structure research of shell and tube heat exchanger with regularly spacedtwisted tape in shell side was performed. The working fluid was air, and the tube wasconverging-diverging tube. Single-factor method was adopted in this optimization process.Two rounds of numerical simulations were performed to find the best structure parameters ofthe heat exchanger. After the initial round of research, the optimization structure parameterswas L=28mm, h=1.5mm, ratio=4.6, y=3.33, and w=180°. Then, the second round of researchwas performed. After above research, the optimization structure parameters can bedetermined. The parameters were L=28mm, h=1mm, ratio=4.6, y=3.33, and w=180°.