鱼雷壳体冷凝器换热特性研究

发布时间：2018-06-23 08:06

本文选题：闭式循环 + 壳体冷凝器　；参考：《中国舰船研究院》2017年博士论文

【摘要】：闭式循环热动力系统不向雷外作任何排放,是完全无尾迹的,并且不受背压影响能适应大深度航行,是未来鱼雷动力发展的方向之一。由于系统与外界只有热量交换而无物质交换,没有尾气排放,发动机做功后的乏汽必须经壳体冷凝器冷凝成水后才能供给系统作为循环工质使用,缺之则无法构成闭式循环,其工作特性直接影响汽轮机工作性能乃至闭式循环的总效率,所以壳体冷凝器是闭式循环系统必不可少的关键组件。由于其外部结构受限、内部空间也非常有限、热流体为大流量、高能量的过热蒸汽等这些特殊条件的限制,目前对其内部流动换热特性并没有准确的认识,还有待进一步的深入研究。本文以闭式循环热动力系统核心组成部分——壳体冷凝器为研究对象,建立了壳体冷凝器内部蒸汽流动凝结过程的物理模型,设计了试验装置并搭建了蒸汽冷凝试验系统。针对复杂约束条件下壳体冷凝器单通道和壳体冷凝器整机,对其流动换热特性进行了深入的理论分析、一维仿真、三维数值模拟和试验研究,分析了各因素对蒸汽流动换热特性的影响并利用试验数据验证并修正了相关计算模型,形成了壳体冷凝器蒸汽冷凝换热预测模型。利用壳体冷凝器单通道和整机一维仿真计算模型,计算获得了冷却通道内沿轴向温度、压力、干度以及速度分布特性。分析表明:在蒸汽冷凝换热过程中,蒸汽在两相区与壁面的换热量最大,换热能力沿流向随着液膜厚度的增加而降低;由于气液两相速度差异导致强烈的剪切作用,有利于降低液膜厚度,因此提高蒸汽初速可提高换热效果。影响环状流冷凝段长度的主要因素有蒸汽入口压力、流量、温度、通道大小等工况参数。随着入口压力的升高,饱和温度上升,凝结出现位置提前,通道内流速下降,冷凝段长度减小,流动损失也下降。但压力过大时,由于蒸汽密度增大,速度减小,换热系数会随之降低,导致冷凝段的长度减小趋势慢慢变缓;当入口蒸汽流量增加时,入口蒸汽流速也随之增加,各段换热系数亦跟随增加,故虽然总体换热量增加,但对冷凝段长度影响并不大。随着蒸汽流量的继续增加,总体换热量也继续上升,此时流速对换热效果提高有限,故冷凝段长度随蒸汽流量增加迅速增大;提高入口温度主要影响进口段的换热与流动特性,增加冷凝段的长度,但对通道内的局部换热系数影响较小,当冷却通道足够长时,一定范围内的入口温差对出口参数影响较小。单位长度的蒸汽侧换热系数随通道宽度增加而减小,冷却通道轴向单位长度压损随通道宽度增加不断减小。增加通道数量能有效提高壳体冷凝器平均换热系数,增强壳体冷凝器的换热能力缩短蒸汽完全冷凝所需的冷凝器长度,且由于通流面积的增加减少流动阻力。随着通道螺旋角度的减小,通道内蒸汽流向变化梯度也越大,能有效冲刷冷凝液膜,削弱液膜在通道壁上的附着能力,进而降低液膜厚度提高冷凝器单位长度换热系数。螺旋角越小,蒸汽完全冷凝所需冷凝器长度越短,但随着通道螺旋角度的减小壳体冷凝器压损有增大趋势。通过对壳体冷凝器单通道和整机三维数值模拟分析,获得了冷凝器通道内部蒸汽冷凝过程温度场、压力场、速度场的分布情况。分析得出了不同入口温度、流量和压力等参数对通道内蒸汽冷凝过程各参数的影响规律,比较完整地阐明了鱼雷壳体冷凝器通道内蒸汽冷凝流动换热特性及其机理。基于不同参数下通道内部换热特性参数拟合,建立了单通道换热预测模型。研究结果表明:在进口段,壁面附近过热蒸汽受冷极易凝结,仅在冷却通道中心附近为过热蒸汽。由于壁面附近受冷蒸汽凝结过程中释放大量潜热,将加热其附近蒸汽,提高其局部过热度,从而推迟了其在下游的凝结,甚至使得部分壁面液膜向下游发展过程中出现二次蒸发现象。由于气液速度差异引起的强剪切作用导致界面失稳,出现较大的流动波动现象,壁面附近交替出现局部高、低速区和汽、液集中区。波动现象会加剧壁面附近低温流体与冷却通道中心高温流体间的质量和能量掺混,从而增强了换热效果,也导致较大的流动损失。随着蒸汽沿轴向发展,蒸汽温度不断下降,整个冷却通道横截面将被气液混合物充满。两相混合区内冷却通道中心附近高蒸汽体积分数区更接近下壁面,浮升力和重力对等温放热区的两相流动影响减弱,冷却通道内气液分布的发展主要由壁面温度或换热特性主导。不同进口蒸汽参数对比研究表明,随着进口压力的增大,凝结出现位置提前,管内流速下降,流动损失下降;而蒸汽温度的增加主要影响进口段的换热与流动特性,而对两相等温放热段及全液换热段的影响较小,当冷却通道长度足够长时,一定范围内的入口温差对出口参数的影响较小。在本文所研究的通道宽度下,通道换热能力随着其宽度的增加而略有下降,但其流动损失则明显减小。从管内流型演化来看,在入口段呈现间歇流状态;而后向下游演化为波状环状流状态,即冷却通道四周均被液膜覆盖,而冷却通道中心为气液混合流动状态;随着冷却通道内流体进一步冷却,最终进入全液流动状态,并以较为均匀的平行流动状态向出口发展。通过对流动与换热特性分析,建立了单通道换热经验关联式,并与试验结果进行了对比,其计算精度优于已有经验公式,对冷凝器流动换热设计与计算具有较好的工程指导意义。初步探讨了壳体冷凝器整机螺旋通道内换热与流动特性,计算结果表明,采用本文建立的单通道换热预测模型,可较好的预测螺旋通道总体换热与流动特性,但两相区出现位置存在一定的误差,这主要是由于所建立的模型没有考虑离心力影响所致。壳体冷凝器整机螺旋通道数值模拟结果还表明,螺旋通道内由于离心力影响,其冷凝水主要附着在内壁侧,而蒸汽则更易向外壁侧流动,离心力作用有助于降低冷凝液膜厚度增强蒸汽与冷却侧壁面间的换热。对壳体冷凝器单通道和冷凝器整机在不同入口蒸汽条件下的冷凝换热特性进行了试验研究。研究结果表明:通道内的蒸汽冷凝总传热量、热流密度、出口温度、蒸汽侧对流换热系数和总传热系数均随着入口蒸汽质量流量的增大而增大。试验器总传热量、热流密度以及出口温度随着入口蒸汽温度的增加略有上升,但上升幅度非常小,总传热系数和蒸汽侧对流换热系数基本保持不变。通过对壳体冷凝器整机试验数据分析得到了蒸汽冷凝传热经验关联式,可以作为壳体冷凝器的设计计算依据,预测壳体冷凝器换热效果。本文的研究不仅有利于提高对闭式循环系统壳体冷凝器通道内蒸汽冷凝换热特性及机理的理解,还有助于壳体冷凝器的开发以及优化设计。
[Abstract]:It is one of the direction of future torpedo dynamic development. Because the system and the outside world have only heat exchange and no material exchange, no exhaust emissions, the exhaust gas after the engine doing work must be subjected to a shell condenser. After condensing into water, the supply system can be used as a circulating refrigerant, and the lack of it can not form a closed cycle. Its working characteristics directly affect the performance of the steam turbine and the total efficiency of the closed cycle. So the shell condenser is a necessary key component in the closed cycle system. The thermal fluid is limited by the special conditions such as large flow, high energy superheated steam and so on. At present, there is no accurate understanding of the heat transfer characteristics of its internal flow. It still needs further study. In this paper, the shell condenser is the core component of the closed cycle thermal dynamic system, and the internal steam of the shell condenser is established. A test device was designed and a steam condensing test system was designed. The flow and heat transfer characteristics of a shell condenser with a single channel and a shell condenser under complex constraints were analyzed theoretically, one dimensional simulation, three dimensional numerical simulation and experimental study, and the analysis of the various factors on steam. The influence of the flow and heat transfer characteristics and the correlation calculation model were verified and corrected, and the prediction model of condensation heat transfer in the shell condenser was formed. The distribution characteristics of the axial temperature, pressure, dry degree and velocity in the cooling channel were obtained by using the single channel of the shell condenser and the one dimensional simulation calculation model of the whole machine. During the steam condensation heat transfer process, the heat transfer of steam in the two phase and the wall is the largest, and the heat transfer ability decreases along with the increase of the thickness of the liquid film. Because the difference of the velocity of gas and liquid leads to the strong shear effect, it is beneficial to the reduction of the thickness of the liquid film. Therefore, the increase of the initial steam velocity can improve the heat transfer effect. The main factors of the degree are steam inlet pressure, flow, temperature, and channel size. With the increase of inlet pressure, the saturation temperature rises, the position of condensation appears in advance, the flow velocity in the channel decreases, the length of condensing section decreases, and the flow loss also decreases. When the inlet steam flow rate increases, the inlet steam flow increases and the inlet steam flow velocity increases and the heat transfer coefficient increases. The increase of speed to heat transfer is limited, so the length of the condensing section increases rapidly with the increase of steam flow, and the increase of the inlet temperature mainly affects the heat transfer and flow characteristics of the inlet section, and increases the length of the condensing section, but it has little effect on the local heat transfer coefficient in the channel. When the cooling channel is long enough, the inlet temperature difference within a certain range is affected by the outlet parameters. The heat transfer coefficient of the unit length decreases with the increase of the channel width, and the axial unit length pressure loss decreases with the width of the channel. Increasing the number of channels can effectively increase the average heat transfer coefficient of the shell condenser, and enhance the heat transfer capacity of the shell condenser to shorten the condenser's length required for the complete condensation of steam. With the increase of flow area, the flow resistance is reduced. With the decrease of the channel spiral angle, the change gradient of the flow direction in the channel is also greater. It can effectively scour the condensing liquid film, weaken the adhesion ability of the liquid film on the channel wall, and then reduce the thickness of the liquid film to increase the unit length heat transfer coefficient of the condenser. The smaller the helix angle, the steam is completely condensed. The shorter the length of the condenser, the pressure loss of the condenser is increased with the decrease of the spiral angle of the channel. The distribution of the temperature field, pressure field and velocity field in the condensation process of the condenser channel is obtained by the three-dimensional numerical simulation analysis of the single channel and the whole machine. The different inlet temperature and flow rate are obtained. The influence of parameters such as pressure and other parameters on the parameters of steam condensation in the channel is described. The heat transfer characteristics and its mechanism in the condensers channel of torpedo condenser are completely clarified. A single channel heat transfer prediction model is established based on the parameters fitting of the internal heat transfer characteristics under different parameters. The results show that the inlet section is in the inlet section. The superheated steam near the wall is very easy to condense and only is superheated in the vicinity of the center of the cooling channel. Due to the release of latent heat in the process of cold steam condensation near the wall, the steam near the wall will be heated to increase its local overheat, thus postponing its condensation at the lower reaches, so that the liquid film of some wall surface will come down to the downstream process. There are two phenomena of evaporation at present. Due to the strong shear action caused by the difference of gas and liquid velocity, the interfacial instability is caused by the strong shear effect, and there is a large flow wave phenomenon. The local high, low velocity zone and steam and liquid concentration zone are alternately appeared near the wall. The wave phenomenon will aggravate the mass and energy mixing between the low temperature fluid near the wall and the high temperature fluid at the center of the cooling channel. With the development of the steam along the axial direction, the steam temperature decreases and the cross section of the whole cooling channel will be filled with gas-liquid mixture. The high steam volume near the center of the cooling channel near the center of the two phase mixing area is closer to the lower wall, the floating lift and the two phase flow in the gravity heating zone. The development of the gas and liquid distribution in the cooling channel is mainly dominated by the wall temperature or heat transfer characteristics. The comparison of the parameters of the imported steam shows that with the increase of the inlet pressure, the position of the condensation appears in advance, the flow velocity in the tube decreases and the flow loss decreases, while the increase of the steam temperature mainly affects the heat exchange and flow characteristics of the inlet section. The influence of the two equal temperature exothermic section and the full liquid heat transfer section is smaller. When the length of the cooling channel is long enough, the inlet temperature difference in a certain range has little effect on the outlet parameters. The heat transfer capacity of the channel decreases slightly with the increase of the width of the channel, but the flow loss decreases obviously. In the evolution, there is a state of intermittent flow in the entrance section, and then into the state of the wave like flow in the downstream, that is, the cooling channel is covered by the liquid film for 4 weeks and the center of the cooling channel is a gas-liquid mixed flow state. With the further cooling of the fluid in the cooling channel, the flow state eventually enters the full liquid state, and the flow state is more uniform parallel to the flow state. Through the analysis of the flow and heat transfer characteristics, a single channel heat transfer empirical correlation is established and compared with the test results. The calculation accuracy is better than the existing empirical formula. It has a better engineering guiding significance for the flow and heat transfer design and calculation of the condenser. The calculation results show that the single channel heat transfer prediction model established in this paper can better predict the overall heat transfer and flow characteristics of the spiral channel, but there is a certain error in the position of the two phase region. This is mainly because the model does not consider the influence of the centrifugal force shadow. The numerical model of the spiral channel of the shell condenser is the whole model. The results also show that the condensate is mainly attached to the inner side of the inner wall because of the influence of centrifugal force in the spiral channel, while the steam is easier to flow outside the wall. The centrifugal force helps to reduce the thickness of the condensate film to enhance the heat transfer between the steam and the cooling side wall. The results show that the total heat transfer, heat flux, outlet temperature, the convection heat transfer coefficient and the total heat transfer coefficient in the steam side increase with the increase of the inlet steam mass flow. The total heat transfer, heat flow density and outlet temperature of the tester are along with the inlet steam temperature. The increase is slightly increased, but the increase is very small. The total heat transfer coefficient and the convection heat transfer coefficient of the steam side are basically unchanged. Through the analysis of the experimental data of the shell condenser, the empirical correlation of the steam condensation heat transfer is obtained. It can be used as the calculation basis for the design and calculation of the shell condenser. The study of the heat transfer effect of the shell condenser is predicted. It is not only helpful to improve the understanding of the characteristics and mechanism of condensing heat transfer in the channel of the closed loop system, but also to the development of the shell condenser and the optimization design.
【学位授予单位】：中国舰船研究院
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TJ630.3

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