Characteristics And Effectiveness Of Destratification Of Aeration-induced Internal Waves In A Stratified Reservoir

Lakes and reservoirs had become our main source of water supply in most cities,but now almost every long-running reservoir exists a certain degree of pollutionproblems. Low dissolved oxygen concentration in bottom water is the root cause ofinternal pollution and eutrophication, and thermal stratification is the main inducementof low dissolved oxygen concentrations. Therefore, destratification is crucial to controlinternal water quality pollution in deep reservoirs.Conventional destratification technologies exist a common problem of highenergy consumption. Based on the principles of fluid mechanics that internal waveswill be produced by moderate disturbance in stratified water, conditions andcharacteristics of internal waves induced by aeration were studied, and process andeffect of destratification of aeration-induced internal waves were preliminary analyzedin this paper. The main conclusions are as follows:(1) According to the actual stratified temperature structure of Jinpen reservoir,copper pipes of air conditioning evaporator were emplaced annularly at the bottom ofthe model reservoir with length of2.0m, width of0.6m and height of1.2m, andbottom water would be refrigerated to make water temperature constant. So stratifiedtemperature structure based on natural convection was simulated in the laboratory forthe first time. The thickness of isothermal, thermocline and epilimnion layer was about40cm,20cm,40cm, respectively. Temperature gradient of thermocline was related toair temperature and the given water temperature of isothermal layer. Buoyancyfrequency reached its maximum in the middle of thermocline.(2) Under the pilot conditions in this paper, disturbance period of water-lifting aerator decreased with the increasing air flow rates and decreased from89.2s to20.8swhen the flow rate increased from50L/h to200L/h. Temperature profiles of differentdistances from axis of the water-lifting aerator were measured in real time to obtain thecorresponding density profile. Calculation methods of characteristic parameters whichinclude wave periods, wave velocities, wave lengths and wave amplitude wereestablished. When top outlet of the aerator was at the top of the isothermal layer, theperiodic outflow under air-water two-phase conditions was used as a disturbing sourceto induce the internal waves in stratified reservoir. The wave amplitude reached a peakwhere the buoyancy frequency was the lowest in the thermocline. The wave amplitudesdecreased gradually with time due to the dissipation of energy, and the breaking ofinternal waves seemed to happen at the end of wave propagation.(3) Under the conditions of temperature gradients of0.39,0.56,0.78℃/cm in thethermocline and air flow rates of50～200L/h, characteristic parameters of internalwaves were calculated according to water temperature profiles. Wave periods, wavevelocities and wave lengths increased linearly with the disturbance period whendisturbance period T0was less than or equal to35.1s, and the parameters decreasedrapidly first and then changed slowly when T0was greater than35.1s at thetemperature gradient of0.78℃/cm. The variation of wave amplitude was morecomplex. The regularity of the characteristic parameters changed with disturbanceperiod was weaker at low temperature gradient. Wave periods and wave amplitude hada decreasing trend, while wave velocities and wave lengths had the opposite trend withthe increasing temperature gradient of thermocline.(4) Under the pilot conditions in this paper, the velocity of periodic outflow froma water-lifting aerator varied between0m/s and0.5m/s and the corresponding Froudenumber was from0to32.15. Structural characteristics of internal waves ofrepeatability test were very similar, but repeatability was less satisfactory than internalwaves generated by fixed disturbance source. The possible reason was thatbody-generated internal waves and wake-generated internal waves dominatedalternately in the fluid.(5) Thermocline moved down gradually which was as the main feature of theprocess of destratification of aeration-induced internal waves. The time of destratification decreased with the increasing air flow rates. When air flow ratesincreased from50L/h to200L/h, the time of destratification was90～270min and60～300min at temperature gradient0.39℃/cm and0.56℃/cm respectively.Compared with the conventional mixing by circulated flow, the mixing by internalwaves could be enhanced by25%～60%under the pilot experimental conditionswhich had obvious advantages of low energy consumption and high efficiency, and hadbroad application prospects.