针对低压密闭腔体内水膜的加热蒸发现象，利用斐克扩散定律构建一维蒸发模型。基于饱和水温度和压力的对应关系，发展一种确定扩散边界层厚度的方法。搭建水膜低压加热蒸发的实验系统，实验研究不同压力条件下水膜的蒸发质量和温度随时间的变化。通过对比蒸发质量和温度变化的模型预测结果和实验结果，发现二者数据的吻合度较高，表明该模型能够较为准确地预测低压加热条件下的水膜蒸发特性。基于当前的数学模型，进一步研究真空压力、加热热流密度和蒸发容积对水膜蒸发速率、蒸发质量和蒸汽压力的影响。研究发现，压力越低、加热热流密度越高和蒸发容积越大，水膜的蒸发速率越大，达到饱和状态的时间越短。
A one-dimensional evaporation model for simulating water evaporation under vacuum condition is proposed by using Fick's diffusion law. Based on the correspondence between saturated water temperature and pressure, a method for determining the thickness of diffusion boundary layer is developed. An experimental system of water film low pressure heating and evaporation is set up. The vaporization and temperature variations of water film under different conditions are studied. By comparing the predicted results and experimental data for evaporation mass and temperature changes, it is found that the model accurately predicts the evaporation characteristics of water film under low pressure and heating conditions. Based on the current mathematical model, the effects of vacuum pressure, heating heat flux, and chamber volume on water evaporation rate and vapor pressure are further studied. The results indicate that the lower the pressure, the higher the heat flux, and the larger the chamber volume are, the higher evaporation rate is and the shorter time is needed to reach saturation.
[1] 陈学, 刘晓华, 沈胜强, 等. 水平管外降膜蒸发传热实验研究[J]. 太阳能学报. 2015, 36(8):1996-2001.
[2] Liu L, Wang Y, Fan D, et al. Using phenolphthalein as a promising indicator to monitor the vacuum freeze-drying process[J]. Materials Letters, 2015, 139:245-248.
[3] Bai Y, Pang C, Hu W, et al. High capacity cobalt boride prepared via vacuum freeze-drying method and used as anode material for alkaline secondary battery[J]. Journal of Renewable and Sustainable Energy, 2013, 5(2):021401.
[4] Wheeler R R, Hadley N M, Dahl R W, et al. Microwave enhanced freeze drying of solid waste[R]. SAE Technical Paper, 2007.
[5] Cheng W, Chen H, Hu L, et al. Effect of droplet flash evaporation on vacuum flash evaporation cooling:modeling[J]. International Journal of Heat and Mass Transfer, 2015, 84:149-157.
[6] Wang C, Xu R, Song Y, et al. Study on water droplet flash evaporation in vacuum spray cooling[J]. International Journal of Heat and Mass Transfer, 2017, 112:279-288.
[7] Cammenga H K. Evaporation mechanisms of liquids[J]. Current topics in materials science, 1980, 5:335-446.
[8] 王遵敬. 蒸发与凝结现象的分子动力学研究及实验[D]. 北京:清华大学, 2002.
[9] 肖宁, 赵小明, 陈民, 等. 液体蒸发率及表面温度测量[J]. 工程热物理学报, 2007, 28(5):748-750.
[10] Lienhard J H. A heat transfer textbook[M]. 4th ed. New York:Courier Corporation, 2013.
