Thermal Performance And Parameters Distribution Of Large-scale Horizontal-tube Falling Film Evaporators

As the main equipment of the low-temperature multi-effect evaporation desalination device,the horizontal-tube falling film evaporator has relatively high heat transfer coefficient due to the wave effect of the liquid film on the surface of the tubes as well as the phase change on both sides of the heating tube.Meanwhile,owning the advantages of allowing small temperature difference,low requirement for heat resource,excellent anti-scale and anti-corrosion performance,it has become the focus of research and application of thermal desalination technology.Many scholars have carried out research on the falling film evaporation outside a single horizontal tube or a column of tubes,detailed research on the thermal performance of the tube bundle were rarely reported in literatures and the impact of the inter-tube vapor flow on the distribution of heat transfer parameters within the evaporator is rarely reported.In addition,the difference in the variation trend and variation direction of the in-tube and out-tube heat transfer parameters causes different distributions of inter-tube heat transfer parameters.It also causes the characteristics of multiple parameters and high complexity during the analysis which lead to few reports on the coupling parameter space analysis of the heat transfer process inside and outside the tube.In this paper,comprehensive study has been experimentally and numerically carried out on the heat transfer performance of the horizontal-tube falling film evaporator and the parameters distribution characteristics within the evaporator.A test bench was established for the study of heat transfer performance in a horizontal-tube falling film evaporator.The distributions of the inner-tube and outer-tube fluid temperature as well as the tube wall temperature were studied under different seawater spray densities and salinities.In supplementary experiments,the local inner-tube steam pressure drop and the local pressure drop when vapor flowing across the falling film tube bundle were obtained under different operating conditions.Based on the database established from the extensive experimental data,together with the analysis on the heat transfer and flow process in the evaporator,the distributed parameter model was established of a large-scale falling film evaporator.The corresponding thermal calculation program was developed.The feasibility of the model was verified by comparing the numerical results with the measured value from the practical device.The thermodynamic process of the large-scale falling film evaporator was numerically simulated based on the distributed parameter model.The distributions of the heat transfer coefficient within the large-scale horizontal-tube falling film evaporator were analyzed under different top-row seawater spray densities and top-row seawater salinities which helps to provide the basic method for deeply understanding the distribution variation and mechanism of heat transfer coefficient in large-scale falling film evaporator.Based on the "self-compensation characteristic" between the steam inlet velocity and the heat transfer rate,the non-uniform distribution of the steam inlet velocity was quantitatively analyzed under different operating conditions,revealing the coupling mechanism and distribution characteristics of phase change heat transfer processes inside and outside the tube.Results show that there is remarkable influence of the non-uniform distribution of steam inlet velocity on the design accuracy of the evaporator.Based on the operating characteristics of "small temperature difference,low flow resistance,saturated state and high sensitivity" of the multi-effect evaporation desalination device,the temperature difference losses caused by inner-tube steam and inter-tube vapor pressure variance as well as the boiling point elevation were quantitatively analyzed and the detailed distribution of temperature difference was given under different operating conditions.The "effective heat transfer temperature difference area" was defined and the coverage fraction of the effective temperature difference area was proposed to characterize the utility of the apparent temperature difference.The effective heat transfer critical point and the heat transfer critical point were proposed which provide quantitative control theoretical index for the optimal design of the large-scale multi-effect evaporation desalination device.The impact of the tube length,the row-column ratio on the distribution of the heat transfer parameters and the comprehensive performance of the evaporator were discussed based on above analysis.The optimal tube length and row-column ratio were obtained with the criterion of the maximum heat flux of the evaporator.