氯化铵水溶液定向凝固实验的传热分析

Heat transfer analysis of experimental unidirectional solidification of aqueous ammonium chloride solution

选用类合金NH4Cl-H2O二元溶液进行垂直定向凝固实验研究,再现过共晶合金结晶过程,测量记录凝固过程中的温度场和固、液相界面位置;重点分析了两相区的传热特性,包括局部热流和释放潜热,并尝试用实验数据与数值计算相结合的方法确定两相区局部固相分数与温度的关系曲线。研究表明:在结晶过程中,各点温度呈线性下降,局部热流在进入两相状态后达到峰值;各相区内温度梯度恒定,但相界面附近温度梯度变化显著。两相区凝固过程中,先期潜热释放总量大,总凝固分数大,两相区厚度迅速增长;随后总凝固分数随相界面迅速上移而急剧下降,经历准稳态过程后再缓慢上升。溶液沿凝固方向分层,NH4Cl质量分数逐渐增大,相应结晶温度逐渐升高。

An experimental study of unidirectional solidification was carried out for a binary NH4Cl-H2O solution cooled from below in a rectangular cavity. The temperatures were measured and the positions of phase interfaces were traced timely. Experimental data were widely employed in series calculation to describe heat transfer properties in the mushy layer incluiding local heat flux and released latent heat flux. The relationship of local solid fraction with temperature in the mushy layer was tentatively obtained by carrying out numerical computation with experimental data. It turns out that during the solidification process, local temperature descends linearly and local heat flux reaches its peak value after local solution changing into mushy state; the temperature gradient almost keeps constant in each phase layer, but the value differs from each other due to latent heat releasing. With the total solid fraction of mushy layer decreasing, the thickness of mushy layer increases rapidly at first due to high heat flux and large latent heat releasing; then it experiences qusi-stable state and finally grow slowly. Furthermore, with the phenomenon of stratification, solute （NH4Cl） concentration increases along the direction of solidification, so the crystalization temperature rises along the direction.