Prevention of convection currents to reduce heat losses - in narrow space between two parallel surfaces

摘要

A means of preventing the formation of convection currents in a narrow space between two parallel surfaces where there is a temp. gradient across a fluid in the space is to put in a porous material which is elastic in the direction of its thickness. The material may be in the form of threads, fibres, knitted material or leaves, the distance between them being less than the critical stagnation thickness at the temp. of the space. Partic. the leaves are thin and flexible, extending across the spaced, joined at one endge and free at the other, giving a spiral profile in transverse section, covering one another, and fitted in the annular space between two coaxial cylinders. There are means of holding the leaves in place while the cylinders are fitted one inside the other, and freeing them afterwards. Partic, applicable to the space between the plug closing the top of a nuclear reactor and the wall of the reactor. The invention prevents the heat losses that can arise and be significant even when the space is very narrow and partic, in annular spaces. The critical thickness for DELTAt = 250 degrees C height 1 m, press. 80 bars is 1.2 mm for steam, 1.4 mm for N2 and 3.5 mm for helium. Above these values, convection becomes important.