Experimental And Numerical Studies Of Compound Heat Transfer Enhancement Technology By Rough Tubes With Tube Inserts

Shell and tube heat exchangers are widely used in many fields, such as chemicalengineering, power engineering, petroleum and metallurgy industries. Traditional plain tubeused in many heat exchangers has the shortage of low heat transfer which leads to poorthermal performance. However, the techniques of heat transfer enhancement can overcomethis shortcoming. According to whether external power is needed, the techniques for heattransfer enhancement can be divided into two categories. One is the active strengtheningtechnique and the other is the passive one. Due to its less energy consumption, easieroperation and higher reliability, the passive strengthening techniques are more commonlyused in the engineering. Among these passive strengthening techniques, rough tubes and tubeinserts are practical and popular. Rough tubes can improve heat transfer in the near-wall zoneby employing the rough rib structure, but it can’t further improve heat transfer in themainstream zone. Tube inserts can improve heat transfer by mixing fluid between the nearwall zone and mainstream zone. The coordination of these two heat transfer enhancementtechniques can make full use of their advantage, improving heat transfer in both near wallzone and mainstream zone, which has a great significance to acquire a higher efficient thermalperformance.The local temperature and velocity field in the converging-diverging tube (CD) fitted withthe evenly spaced twisted tapes (ESTT) is discussed firstly via numerical simulation. It isfound that the self-sustaining flow tend to decay swift, which indicates that a heat transfermechanism different from that in turbulent flow. Based on the characteristic of effective heattransfer temperature difference (EHTT), the characteristics of temperature, effective heattransfer coefficient (EHTC), heat flux and temperature utilization ratio in the plain tube, plaintube fitted with the ESTT, CD and CD fitted with the ESTT are investigated. The obtainedresults show that the ESTT can enhance both EHTT and EHTC in the section with ESTT andcan maintain a higher heat transfer rate by using the slowly varying characteristic of EHTTdownstream of ESTT. For bare CD, it can improve EHTT and EHTC by rough rib withdestroying the development of thermal boundary layer and microspur replacement. When theESTT is used simultaneously with the CD, it creates a synergy of compound heat transfer enhancement in the near wall zone and the mainstream zone.Laminar heat transfer and flow characteristics by using90%glycerol as working fluid areexperimentally performed in the plain tube and CD fitted with ESTT, respectively. It is foundthat under investigated experimental conditions the ESTTs with smaller twisted ratio, biggerrotation angle and short gap distance are in favor of improving thermal performance.Compared with the plain tube, the thermal performances of CD, plain tube fitted with ESTTand CD fitted with ESTT are improved.Turbulent heat transfer and flow characteristics by using water as working fluid in the CDfitted with twin twisted tapes are investigated. First, the model is verified with the publishedliteratures. Second, a series of geometric parameters of the bare CD and CD fitted with twintwisted tapes are performed. The results show that the CD can improve heat transfer byemploying a pair of transverse vortices in the converging section. The cooperation of CD withtwin twisted tapes can exploit the superimposed advantage from transverse vortices andlongitudinal vortices to improve heat transfer. Although the CDs fitted with twin twisted tapeshave a lower thermal performance than their counterparts of bare CDs, their overall thermalperformances are greater than unity.A numerical study is implemented to analyze the heat transfer and fluid flowcharacteristic of98%sulfuric acid based on an industrial background of absorption process insulfuric acid industry. The results indicate that CD fitted with ESTT has a better overallthermal performance than bare CD under laminar flow, but has a lower overall thermalperformance than bare CD under turbulent flow. Small twist ratio of ESTT is more efficient toimprove overall thermal performance under laminar flow but large twist ratio is more efficientunder turbulent flow.Heat transfer and flow characteristics of transverse groove tube (TG) and CD by usingwater as working fluid are experimentally investigated. Effects of different rib height and ribtype on the heat transfer and flow behaviors of TG are numerically analyzed. Moreover, underconstant temperature condition, the geometric parameters of CD are optimized. It is foundthat the heat transfer coefficient of TG is higher than that of CD, but its flow resistance is alsohigher than its counterpart. Hence, the overall thermal performance of TG is lower than thatof CD. The heat transfer of TG is more sensitive to the cases with lower rib height; however, the flow characteristic tends to be opposite. It is discovered that by mounting outwarddirection rib on the inward rib the flow separation of ribs can be effectively suppressed andthe local heat transfer on ribs can be improved, resulting in a better overall thermalperformance. Pitch length, rib height and pitch ratio have great influence on the overallthermal performance of CD. With the optimization the thermal performance of CD can beenhanced under available geometric parameters.