Heat Transfer Characteristics and Efficient Utilization on Ice-Water Cold Storage

利用冰水的潜热蓄冷系统的蓄冷能力是不结冰的水的显热蓄冷的数倍。为了保证冰水蓄冷系统的有效运行，必须了解冰水蓄冷系统在冻结和融化过程中的传热特性。本文针对相变蓄冷器的传热特性，以简单几何形状冷却表面上冰的冻结和融化为基础，采用边界拟合法，对自然对流条件下竖直壁面上冰的冻结和融化传热特性进行了数值分析。对不同温度条件下的冻结和融化过程进行了数值模拟，得到了以下结论：在自然对流条件下，水的逆向流动对传热量的贡献很大;由于冰水界面的传热速率较低，冻结速率可以通过求解常解的Stefan问题来粗略估算。在熔化过程中，加热表面的传热系数完全取决于表面温度。当表面温度保持在8 ℃时，出现两股强度相近、流向相反的环流，换热系数最低。在强制对流条件下，对水平冰柱的融化换热特性进行了实验研究。分别对淡水冰和低共熔碳酸钠水溶液冰进行了实验。给出了碳酸钠水溶液在自由流速度为0.008 ~ 0.056 m/s、流体温度为1.4 ~ 8 ℃和-0.7 ~4.6 ℃时的熔融速率结果。还给出了自由流速度与传热量之间的简单关系式。

The latent heat cold storage utilizing ice-water system has heat storage capacity of several times as high as sensible heat cold storage using water without freezing. For effective operating ice-water cold storage, it is necessary to clear the properties of heat transfer with freezing and melting. In this report, the freezing and melting of ice on the simple geometrical shaped cooling surfaces which are the fundamentals to give better understanding the heat input and output characteristics of the latent heat cold storage.Under the free convection, heat transfer characteristics on freezing and melting of ice on vertical wall were investigated by numerical analysis with applying the boundary fitting method. The numerical simulations were made on both freezing and melting under several temperature conditions, and following matters were clarified.Under free convection, reversing flow of water make badly contributions to heat transfer rate. Because of lower heat transfer rate on ice water interface, freezing rate can roughly be estimated by the solution of usual Stefan Problem. In the melting process, heat transfer coefficient on the heating surface thoroughly depends on the surface temperature. When the surface temperature is kept at 8 degree Celsius, two circulation flows which are the nearly same strength and have mutually opposite flow directions occur and the lowest heat transfer coefficient is observed.Under forced convection, heat transfer characteristics on melting of a horizontal ice cylinder were studied experimentally. The experiences were done on both ice of fresh water and ice of sodium carbonate aqueous solution of eutectic concentration. The results of melting rate are presented for free stream velocity range 0.008 to 0.056 m/s and fluid temperatures of 1.4 to 8 degree Celsius and -0.7 to 4.6 degree Celsius for aqueous solution of sodium carbonate. The simple relation between the heat transfer rates and free stream velocities are also given.