| Closed cycle thermal propulsion system exhausts no emission,thereby producing no torpedo wake without any wake.Besides,the exhaust backpressure has no impacts on its performance.Therefor it adapts to large running depth.Due to above advantages,most promising candidate for future torpedo power system.As no mass is exhausted from the system,the steam exhaust of the turbine must be cooled down through the shell-integrated condenser and then reutilized.The performance of the shell-integrated condenser has significant impacts on system efficiency.Due to limited space,extreme high temperature and large flow rate,its internal flow and heat transfer characteristics are not well understood at present,and requiring furture investgating.The physical model of condensation process in the shell-integrated condenser is developed and the experiment system is set up.The simplified condenser with single channel and the shell-integrated condenser with multiple channels are studied base on theoretical analysis,1D simulation,3D simulation and experiments.The impact factors of the steam flow and heat exchange characteristics are analyzed.The model is then corrected by the experiment data.On top of that,the performance prediction model for the shell-integrated condenser is established.Based on the 1D simulation model of single cooling channel and the whole condenser,temperature,pressure,dryness and velocity distribution along the axial direction are obtained.In the process of steam condensation,heat transfer amount and the heat transfer capacity of steam in the two phase region reached the maximal value.However with the liquid film is getting thicker,the heat transfer capacity decreased.In the process of steam condensation with high flow velocity,there exist huge difference between gas flow velocity and liquid flow velocity.Due to the strong shear effect,the thickness of liquid film is reduced,thereby improving heat transfer.The main factors that affect the condensation length of annular flow are the inlet pressure,temperature,flow rate,channel size,etc.The condensation length decreases with the increase of the inlet steam pressure.However,when the inlet steam pressure increases to a certain extent,the decreasing trend of the condensation length slows down.The flow velocity increases as the inlet flow rate increases,and the heat transfer coefficient increases accordingly.As a result,although the overall amount of heat transfer increases,the condensation length varies little.With further increase of steam flow rate,the overall heat transer continues to rise,while the heat transfer coefficient almost remains constant.Therefore,the condensation length increases rapidly.When inlet steam temperature increases,the overall amount of heat transfer increases accordingly which causes the increase of the condensation length.However,it is worth pointing out that the inlet steam temperature has limited impact on the local heat transfer coefficient,thereby causing little difference on outlet parameters.The heat transfer coefficient per unit length decreases with the increase of the channel width,and the pressure loss per unit length decreases with the increase of the channel width.Increase the number of channels can effectively improve the average heat transfer coefficient.In addition,due to the increase of flow area,the flow resistance reduces.As the spiral angle of flow channel decreases,the thickness of liquid film decreases,thereby enhancing the heat exchange performance.But the pressure loss of shell-integrated condenser will increase with the spiral angle decreases.Based on the analysis of the 3D numerical simulation of single channel and the whole condenser,the distribution of temperature,pressure,velocity along the axial direction are obtained.The influences of different inlet temperature,flow rate and back pressure on the condensation process are analyzed.The heat exchange characteristics of the shell-integrated condenser are illustrated.Based on the simulation of heat transfer characteristics,the prediction model of heat transfer for single channel is established.The results indicate that the superheated steam near the wall surface condenses rapidly in the inlet section.And the superheated steam only exists in the vicinity of the condensation channel.Due to the heat released in the condensation process,the steam nearby is heated to some extent,thereby delaying the downstream condensation.Under certain circumstance,the downstream liquid film is even re-evaporated.Due to the strong shear effect caused by the velocity difference between gas and liquid,the interface instability occurs,and the large fluctuating phenomenon occurs.As a result,the local high and low velocity zones,the steam and liquid zones are alternately formed near the wall.This fluctuation enhances the heat exchange performance,but also leads to certain losses.As the steam flows along the axial direction,its temperature decreases continually.The cooling channel cross section will be filled with mixde two-phase gas-liquid mixture.The region with high steam volume fraction is close to the lower wall.The effect of the gravity and buoyancy on the two-phase flow is weakened,the gas and liquid distraction is mainly decided by the heat exchange characteristics.Comparative study indicates that with the increase of inlet pressure,condensation occur earlier.Meanwhile,the flow velocity and the flow loss decrease.The increasing steam temperature mainly affects the heat transfer and flow characteristics of the inlet section,while its impact on the two-phase region and liquid phase region is limited.The heat transfer capacity decreases slightly with the increase of the width,while the flow loss reduces sharply.The flow pattern evolves in the condensation channel: intermittent flow state in the inlet,annular flow in the middle and parallel flow in the outlet.The empirical heat exchange model of single channel is obtained through analyzing flow and heat exchange characteristics.By comparing the results with experiment data,this model shows superior accuracy than previous models,making it useful for the engineering design of the condenser.This paper also investigates the heat exchange and flow characteristics in the spiral channels of the shellintegrated condenser.The calculation results indicated the heat exchange model of single channel proposed previously can predict the performance of spiral channels.However,the position of two phase region various to some extent.This is because that gravity is ignored in the single channel model.The numerical simulation of spiral channels shows that the condensed water mainly accumulates on the inner wall due to the centrifugal force;while steam tends to flow to outer wall.The centrifugal force help reduce the thickness of liquid film,thereby enhancing the heat exchange between the steam and wall.The heat transfer characteristics of the single channel and whole condenser under different inlet steam conditions are studied through experiments.The results show that the total heat transfer,the heat flux,the outlet temperature,the convective heat transfer coefficient and the total heat transfer coefficient all increase as inlet steam mass flow rate increase.While the total heat transfer and heat flux density and outlet temperature increase slightly as the inlet steam temperature increase.On the other hand,the total heat transfer coefficient and the convective heat transfer coefficient remain constant.Based on the experiment data,the empirical correlations of steam condensation heat transer are obtained,which can be used as the basis for the design and caculation of shell-integrated condenser and the prediction on the heat transfer effect of shell-integrated condenser.This research not only improves the understanding of the heat transfer characteristics and mechanism in the condenser of the closed cycle propulsion system,but also lays a solid foundation for the design and optimization of the shell-integrated condenser. |
