Characteristics of heat transfer and homogeneous nucleation process during quenching high temperature surface with an impinging water jet

对冲击水射流作用下材料的传热特性和均匀成核进行了实验研究。实验条件为射流速度3 ~ 15m /s，液体温度20 ~ 95℃，射流直径2 ~ 3mm，初始阻塞温度250 ~ 600℃。采用铜、黄铜和碳钢三种不同的材料作为测试块。当用液体冷却高于莱顿弗罗斯特温度的块体时，是否出现润湿高温表面的条件，在很大程度上取决于在表面温度保持高于液体过热度的热力学极限的情况下，冷却能力是否足够。对于冷却表面高于液体过热度的热力学极限且液体有足够的冷却能力的情况，在很短的时间内交替重复湿、干状态，在此期间表面温度逐渐下降。当表面温度低于液体过热的热力学极限时，在有限的表面区域，即撞击射流区中心附近，确定了稳定的润湿，但润湿区域不会在受热表面上扩展。这种情况持续到表面温度达到表面温度。我们将这个持续时间定义为停留时间t^*，然后可以确定停留时间的表面温度Tw^*。停留时间和表面温度可由下式预测，(1)(2)停留时间过后，湿润区域开始扩大，覆盖整个表面。我们可以清楚地表温度和地表热通量是如何随着湿区移动而变化的。例如，我们可以指出最大热流密度q_<max>出现在湿区内的位置，并可以用下式估计最大热流密度；(3)式中p、c和λ分别为密度、热容量和导热系数，q_c为冲击射流稳定冷却下的临界热流密度。少

An experimental study has been made of heat transfer characteristics and homogeneous nucleation with an impinging water jet. The experimental condition is varied from jet velocity of 3 to 15 m/s, liquid temperature of 20 to 95 ℃, jet diameter of 2 and 3 mm, and initial block temperature of 250 to 600 ℃. Three different materials of copper, brass and carbon steel are employed as tested block.When the block temperature higher than Leidenfrost temperature is cooled by liquid, condition whether an occurrence of wetting high temperature surface appears or not, strongly depends on whether in the case that the surface temperature is kept higher than the thermodynamic limit of liquid superheat the cooling capacity is enough or not. For the case of cooling the surface higher than the thermodynamic limit of liquid superheat and enough cooling capacity of liquid, the wet and dry conditions are alternatively repeated at very short time during which the surface temperature gradually goes down. When … More the surface temperature becomes lower than the thermodynamic limit of liquid superheat, a stable wetting is confirmed on a limited area of the surface, namely near the center of impinging jet zone, but the wetted area never expand over the heated surface. This condition is lasting until the surface temperature reaches a surface temperature. We define this lasting time as a resident time, t^*, and then can determine the surface temperature, Tw^*, at the resident time. The resident time and the surface temperature can be predicted by the following equations,(1)(2)After the resident time, the wetted area starts expanding to cover the entire surface. We can clarify how the surface temperature and surface heat flux change with moving the wetted area. For example, we can point out the position at which the maximum heat flux, q_<max>, appears within the wetted zone and can estimate the maximum heat flux by the following equation ;(3)where p, c and λ are density, heat capacity and thermal conductivity, respectively and q_c is critical heat flux under the steady cooling with impinging jet. Less