Study On Performance Of Multirow Helically Coiled Tube Heat Exchangers Used In Surface Water Heat Pump Systems

Surface water heat pumps（SWHPs）use water sources such as reservoirs,rivers,lakes,and seas as heat sources or heat sinks.It is a way of utilizing renewable energy and has been applied in many countries around the world.At present,compared with ground source heat pumps,the SWHP system projects make slow progress.In addition to the constraints of climate and surface water conditions,the main reason of sluggish development is the immaturity of system-related design methods and theories.This thesis focuses on the multi-row helically coiled tube（MHCT）heat exchanger in the SWHP system.A series of experimental and simulation studies were carried out,to comprehensively describe and analyze the heat transfer and the pressure drop performance and the characteristics of the icing on the outer wall at low temperature.The key parameters affecting the performance thus were revealed.For the convective heat transfer on the surface water side and the icing on the outer wall at low temperature,a mathematical model of heat transfer was proposed.Moreover,the correlations,design methods and tools were given for the further engineering application.Firstly,an experimental scheme for heat transfer and pressure drop performance of MHCT heat exchanger was proposed.In particular,a system for the low-velocity surface water flow was built under controllable boundary conditions.The research was conducted in terms of different operating and geometric parameters.The results showed that the mixed-convection heat transfer happened in the surface-water side at the experimental velocities.When changing surface water temperature or the temperature and velocity of internal medium at a constant surface water velocity,the convective heat transfer intensity depends on the effect of natural convection.With the circumstance of surface water velocity changing,convection intensity varies with the velocity.Regarding the influence of geometric parameters,when the distance between the outer walls of the upper and lower tubes is smaller than the outer diameter of the tube,the（?）increases by more than30%with the increase of vertical spacing.But when it is greater than the outer diameter of the tube,the increased percentage decreases to about 6%.The convective intensity on the surface water side increases with the decrease in the number of rows,however this effect is weak with a large number of rows.The increase percentage of（?）is only about5%when the number of rows decreases from 4 to 3,but this value is more than 20%when the number of rows decreases from 2 to 1.The horizontal spacing and tube length have little effect on（?）,the change percentage is only about 5%.For the calculation of（?）and friction factor on the internal medium,it is feasible to substitute the average coil diameter into the correlation of single-row helically coiled tube heat exchangers proposed by El-Genk.The produced errors are no more than±15%and±10%respectively.For the surface water side,the purely forced convection correlation could underestimate the convective intensity at a low Re_o,and the variation characteristics of convective intensity cannot be described accurately.The both effects of natural convection and forced convection should be considered in the development of（?）correlation.In addition,in the entire heat transfer process,the resistances of the thermal conduction of the tube wall and the thermal convection of the surface water side constitute the largest proportions,about 65%and 30%,respectively.Secondly,based on the experimental system,more heat transfer data of surface-water velocity and stagnant water bodies were obtained through numerical simulation and experimental methods,respectively.The results demonstrated that in the surface water side geometric parameters affecting on the convective intensity of forced and natural convections is consistent with the mixed-convection experiment.Thus,the（?）correlations of forced and natural convections were proposed respectively.According to the heat transfer flow theory of Churchill et al.,the（?）correlation was proposed for a wide Re_o range,including the mixed-convection region.A method using the dimensionless number Re_o/Gr_o¹ was also put forward to divide the convection region.For the engineering design of MHCT heat exchangers,a series of charts for the designed tube lengths were given.Thirdly,aiming at the issues of icing on the outer wall in the application of MHCT heat exchanger in cold regions,the mathematical models of icing and non-icing conditions were established.In this model,the local convective intensity along the tube length direction in the surface water side was modeled,important factors such as the convective heat transfer difference between the positive and the negative areas under the flowing water body as well as the influence of geometric parameters were considered.Through the calculations of some cases,it is found that the outer wall temperature calculated by the model is consistent with the experimental and numerical simulation results,and the results can accurately describe the variation characteristics of ice thickness,that is,the thickness of ice on the positive area is smaller than that on the negative area,and the thickness of ice on the inner row is also higher than that on the outer row.When the maximum thickness of the ice layer exceeds the vertical spacing or horizontal spacing,the ice could fill the gap between the tubes.Finally,based on the proposed icing model,the effects of operating parameters and geometric parameters on icing characteristics were further studied.The results indicated that the increase of the surface water temperature or the inlet temperature and velocity of internal medium,or the decrease of the surface water velocity significantly increase the average thickness and distribution length of the ice layer.For geometric parameters,an increase of 1cm in the vertical spacing lowers the maximum thickness of the ice layer by about 21%,while the effect of the horizontal spacing is very small.The thickness and distribution length of the ice are smaller on a heat exchanger with a smaller number of rows.The heat transfer rate of a single-row heat exchanger is more than 20%higher than that of a 4-row heat exchanger.For an increase of 1 mm in the average thickness of the ice layer,the heat exchange rate decreases by 0.62%.If the ice layer fills the gap,the heat exchange rate may decrease more.To avoid ice filling the gap or predict the icing situation for a known operating condition,a series of reference charts of recommended minimum vertical spacing were proposed for the applications in cold regions.In conclusions,according to the convective heat transfer characteristics in the surface water side as well as the heat transfer and pressure drop performance of the internal medium,this thesis proposed the calculation correlations of the corresponding important parameters.The heat transfer model of icing on the outer wall was also established,which provides an important theoretical basis for the research and analysis of MHCT heat exchangers,and greatly facilitates their engineering calculation.This work has great significance for the popularization of surface water source heat pump system.