辐射屏数及密度对超导腔垂直测试杜瓦漏热的影响

超导腔垂直测试杜瓦为广口液氦容器，对绝热性能有较高要求，其内筒体采用高真空多层绝热，顶部盖板侧为多屏绝热。为减少杜瓦顶部盖板侧漏热，提出了变密度辐射屏方法，以盖板、辐射屏、液面和氦气为对象建立了导热、对流与辐射耦合换热的传热模型，并通过实验进行了验证。通过数值计算得到了辐射屏数对漏热的影响规律及最优辐射屏密度。结果表明：1） 实验测得的辐射屏温度与传热模型计算结果较为一致，平均相对偏差为8.37%，认为传热模型是合理的；2） 靠近液面处第1层辐射屏（屏1）与液面间氦气的格拉晓夫数Gr随屏1温度T₁的升高存在极大值（T₁=9.14 K，Gr=1.12×10¹⁴），T₁超过35 K后Gr基本保持不变；3） 等间距分布时，辐射屏数大于11层后总漏热变化不明显，一定辐射屏数下，相邻两屏之间气体导热占主导地位，低温区域（靠近液面侧）至高温区域（靠近盖板侧）导热热流比重减小，辐射热流比重升高；4） 11层辐射屏数下，高温区域布置7层、低温区域布置4层的变密度辐射屏方式漏热最小，与等间距分布相比漏热可减少4%。

Influence Analysis of Number and Density for Radiation Shields on Leakage Heat of Superconducting Cavity Vertical Test Dewar

The superconducting cavity vertical test Dewar is a liquid helium open vessel, which has high requirement for thermal insulation performance. The inner vessel of the Dewar is insulated with high-vacuum multilayer insulation, and the top cover side is multi-screen insulation. In order to reduce the leakage heat on the top cover side of the Dewar, a variable density radiation shield method was proposed. The coupling heat transfer model of heat conduction, convection and radiation was established for the cover, radiation shield, liquid surface and helium gas, and was verified by experiment. The influence of the number of radiation shield on the leakage heat and the optimal radiation shield density were obtained by numerical calculation. The result shows that the calculated result of the radiation shield temperature is consistent with the experimental result, with an average relative deviation of 8.37%, and the heat transfer model is considered to be reasonable. The Grashof number (Gr) of the helium gas between the first radiation shield (shield 1) near the liquid surface and the liquid surface has a maximum value (T₁=9.14 K, Gr=1.12×10¹⁴) with the increase of temperature T₁ of shield 1, and after T₁ exceeds 35 K, Gr remains basically unchanged. When the radiation shield is arranged at equal space, the total leakage heat change is not obvious after the number of radiation shield is greater than 11. Under certain number of radiation shield, the gas heat conduction between two adjacent shields is dominant, and the heat conduction transfer decreases and the radiation heat flux increases from the low temperature area close to the liquid surface to the high temperature area near the cover. As the number of radiation shield is 11, the method of the variable density radiation shield, which is to be arranged for 7 radiation shields in the high temperature area and 4 radiation shields in the low temperature area, has minimal leakage heat. The method can reduce the heat leakage by 4% compared with the equal space method.