| Under the background of industrial transformation towards green and low-carbon energy,generating electricity through waste incineration plays a significant role in economy,environment and society.Consequently,it receives financial support and technical guide from the government.During the long-term process of industrialization and urbanization,industrial solid waste(ISW)has the characteristics of large production,various kinds and difficult disposal.Therefore,the development of waste-to-energy plants dealing with multi-source combustible solid waste is imperative under the circumstance.In-depth studies of measurements to increase the flexibility towards waste of different heating values,improve the transformation efficiency between incineration and power generation,and prolong the service life of waste-to-energy devices are of great necessity.The application of heat transfer enhancement elements is beneficial to make more use of available energy and optimize steam parameters.In addition,it is conductive to exploit potentialities of energy conservation during process of incineration and power generation.Supported by the National Key Research and Development Program of China ‘Key Technologies for Efficient Co-disposal of Industrial Organic Solid Waste in Waste Incineration Facilities’(Grant No.2020YFC190093),the present work studied and explored the application of heat transfer enhancement elements with spiral structure in grate furnace,aiming at improving heat utilization efficiency.Apart from this,this dissertation evaluated the exergy efficiency of the entire plant in terms of exergy loss,after utilization of heat transfer augmentation elements with spiral structure.The main contents and achievements are as follows:(1)The effect of structural parameters,including start number,corrugated depth and pitch,on flow field characteristics and temperature distribution of multi-start spirally corrugated tubes(SCT)were deeply investigated.The heat transfer enhancement characteristics including thermal performance and irreversible loss were evaluated.The results show that from the perspective of the First Law of Thermodynamics,eight-start SCT possesses the highest ability of heat transfer augmentation under the same pumping power.From the point of view of exergy loss,with the increase of Re,the irreversible energy loss of six-start SCT has a tendency of stability.The spiral structure can generate secondary flow and enhance convective heat transfer,accordingly,a hybrid smooth and spirally corrugated tube(HSSCT)was designed taking advantage of this feature.The length of each part,the structural parameter and rotation direction of corrugated part were adjusted for structure optimization.Compared with SCT,the irreversible available energy loss of HSSCT decreases about 8.13% to 14.93% during the convective heat transfer process.(2)The specific expression of exergy loss was derived for convective heat transfer process inside the pipe.After application of heat transfer enhancement elements with spiral structure,the variation of exergy can be obtained.In combination with heat transfer capacity,key structural parameters such as pitch ratio can be optimized.An experimental apparatus was set up for measuring thermal performance and loss property of spirally twisted tapes,which were slightly higher than numerical results.Apart from this,parametric analysis of multi-channel twisted tape inserts and staggered twisted tape inserts was made,respectively,both of which contain spiral structure.For multi-channel twisted tape inserts,the heat transfer enhancement can be maximized with a channel number of two and a unit pitch ratio of 1.5,under the premise of minimal irreversible loss.With regard to staggered twisted tape inserts,increase of pitch ratio under high Re conditions is beneficial for available energy loss reduction.However,the conclusion is opposite for low Reynolds number.Decreasing the deflection angle between the front and rear unit twisted tape inserts helps to reduce exergy loss.(3)The SCT was applied in the first stage low temperature superheater of the grate furnace.Through the Surface Response Method,the effect of structural parameters on heat transfer performance at the steam-side was studied.The relationship between target variables,including Nusselt number,friction factor and so on,and independent variables was achieved.Results indicate that the field synergy angle varies within a small range with the increase of Reynolds number.At the same time,the proportion of available energy loss at the steam side keeps rising.The exergy loss resulting from temperature difference is minimized with a pitch ratio of 0.4 and a corrugation depth ratio of 0.125.On the basis of available potential theory,the specific expression of thermal exergy loss number in heat exchanger is derived,which can make quantitative analysis of available energy loss due to temperature differences.During use,the phenomena of entropy production paradox is exempted effectively.(4)An attempt was made for installation of twisted tape inserts in the second flue and the third flue inside the grate furnace.Based on orthogonal experiment method,the effects of row spacing,column spacing and mounting angle on both velocity uniformity index and temperature uniformity index were investigated.When it comes to the uniformity of the flow field,exclusive use of spirally twisted tape inserts in the second flue is better than the third flue.In accordance with the optimal combination scheme of spirally twisted tape inserts in both the second flue and the third flue,the utilization efficiency of the available energy reaches the maximum value.Based on this,the proportion of utilized available energy to the total input available energy can be further improved with an application of SCT inside the low temperature superheater.Accordingly,the concept of the enhanced exergy efficiency was proposed.It realizes the rapid energy efficiency evaluation of systems utilizing heat transfer enhancement techniques from a macro perspective.Apart from this,it overcomes disadvantages of the traditional exergy analysis method,which requires too many parameters. |
