abaqus热辐射的问题，求解决

The sum of 72 viewfactor matrix rows are less than one, indicating an open cavity; however, no ambient temperature has been defined. The facets with small row sums belong to elements identified in element set ErrElemVFSmallSumStep1Inc1.做辐射的时候，出现这个是为什么，东西做的是一个密闭的圆环管道，但是这提示是开环的。