The Study Of Critical Technology Of Heat Ransfer Enhancement Of Water-Cooled Ondenser Used In Large Thermal Power Eneration

Shell-and-tube heat exchangers are key equipments of manyindustrial processes and are widely used in metallurgy, petrochemical,energy and nuclear power, aviation, etc. Heat transfer enhancement is thekey technology of improving heat transfer efficiency, minimizing thevolume and reducting the cost of the heat exchangers. Based on National"Twelfth Five-Year" Plan for Science&Technology Support and aimingat developing the tube side assembled rotors (TSAR) with better heattransfer enhancement performance and lower resistance, the papersystematically studied heat transfer enhancement performance of TSARthrough theoretical analysis, numerical simulation and experiment;meanwhile, TSAR were applied in condensers of guodian, sinopec powerplant and achieved good application effect. The main research studieswere listed as follows:1. Analysis of heat transfer mechanismIn laminar flow and low Reynolds number turbulent flow, there wastemperature gradient existing in the entire cross section of the tube andthe temperature distributes as a parabola. Therefore, in laminar flow andlow turbulent Reynolds number, heat transfer enhancement should focuson the strengthening of the fluid radial mixing in the whole cross sectionof the tube. That was to say, by changing the distribution of velocity fieldin the tube, velocity component (horizontal speed) in the direction of heattransfer was enhanced and the field synergy degree increased greatly, which made the heat transfer enhanced. The horizontal velocitycomponent in the cross section of the tube made many kinds of vortexessuch as longitudinal vortex and transverse vortex. In turbulent flow withlow Reynolds numbers, compared with transverse vortex, longitudinalvortex could get better heat transfer results and lower resistance increase.TSAR made many beneficial longitudinal vortexes with some transversevortexes.With the increase of Reynolds number, the fluid changed fromlaminar flow to turbulence. In case of turbulence, thinning the flowboundary layer and the thermal boundary layer, whose thicknesses areproportional to each other, was relevant. TSAR could enhance the heattransfer effect in three points summarized as follows:(1) the blade ofrotor disturbed the boundary layer of the fluid;(2) the centrifugal fluidstruck the boundary layer of the fluid;(3) the tangential fluid cut theboundary layer of the fluid.2. Experimental study on heat transfer and friction characteristicsBased on the mechanism that heat transfer enhancement wasachieved by destroying the viscous sublayer of fluid boundary layer inturbulent region and the structure styles of helical three-blade rotors andblade-discreted rotors, three new structure styles of rotors were presented:the helical blade rotor, the helical ladder rotor, and the helical blade rotorwith ladders. An experimental apparatus to test heat transfer enhancementand pressure drop characteristics of TSAR was designed and built. Fivedifferent structural forms of TSAR presented above were experimentallystudied. The experiment results showed that in the range of experimentalReynolds number, different structures forms and structure parameters ofTSAR could increase the heat transfer rate, the tube convective heattransfer coefficient and heat transfer Nusselt number, and also inevitablyincreased the pressure drop; the increase of the rotor diameter could makea significant contribution to heat transfer enhancement, but the frictionfactor correspondingly increased due to the rotation of the rotors; Thehelical blade rotor with ladders had the best heat transfer performanceduring the different structure forms of the rotors. The increase of the rotor diameter had little influence on friction factor, but enhanced heat transferof the tube side fluid; Comprehensive performance evaluation indicatedthat the helical blade rotor with ladders of22-150had the bestperformance during the experimental rotors.3. Numerical study on heat transfer and friction characteristicsThe helical blade rotor whose diameter was22mm and lead was200mm was numerically studied by the software of FLUENT. Thedistribution of the fluid velocity field and temperature field in the tubeinserted with helical blade rotors was gained. In addition, the helicalblade rotors whose outer diameters were22mm and leads were100mm,150mm and200mm respectively were also numerically studied. And theeffect of rotor lead on Nusselt number and friction factor was analysed bynumerical analysis method. The simulation results revealed that theNusselt number and friction factor of the enhanced tube increasedfollowing with the decrease of rotor lead under the same conditions.4. Fouling experimentThe anti-scale and de-scale experiments results of TSAR showedthat compared with the ordinary organic glass tube, the cycle that theorganic glass tube with rotors reached the state of fouling dynamicbalance was longer, but the dirt deposit degree is only half of that in theordinary organic glass tube after reaching the state of dynamic balance.The online cleaning performance of TSAR was listed as follows:(1) Itcould destroy the velocity conditions of fouling deposition and extend theinduction period of fouling deposition.(2) It could enhance the shearingeffect of the near wall fluid and speed up the peeling of the generatedfouling.(3) It could make thermal boundary layer thinner, increase thetemperature gradient near the wall, and make the generated dirt peelbecause of the thermal stress.(4) It could enhance the impact of the fluidin the center of the tube on the fouling of the wall.(5) It could removefouling by physical abrasion function of the rotor.Meanwhile, based on the mechanism analysis of turbulence heattransfer enhancement, according to the results of experiments andnumerical simulation, the structures of the rotors were changed with aiming at improving the performance of anti-scale and de-scale,enhancing the turbulence of boundary layer, thinning boundary layer andimproving overall heat transfer effect. Two new structures were novellypresented: the boundary layer shear rotor and helical two-blade rotor withthermal conducting and de-scaling cilia.5. Industrial applications and structure innovationAfter reformed by energy-saving technology of TSAR, the heattransfer performance of condensers in Bengbu power plant and maomingpetrochemical plant were greatly improved; the vacuum was improved by0.28~2.01KPa, and the usage of circulating water was also reduced. Thereforming projects of the condensors in maoming petrochemical plant andbengbu power plant fully displayed the wide application of TSAR, whichlaid a good foundation for the subsequent optimization and industrialapplication.