The working fluid in solar receivers, utilized for effecting chemical reactions, is usually flown through a sealed enclosure provided with a quartz window. When one of the reactants or products of reaction is a powder care must be taken to prevent contact of the incandescent powder particles with the window, in order to obviate its destruction by overheating. Attempts made in the past to screen the window against particle deposition by a "curtain" of an auxiliary gas stream showed that very substantial flow rates of auxiliary gas (30-80% of the main stream flow rate) were necessary for perfect window screening. The heat absorbed by the auxiliary gas stream represented a major loss of energy. In an effort to reduce the auxiliary stream flow rate to a minimum, a certain flow pattern akin to the natural tornado phenomenon has recently been developed in our laboratory. It enabled effective reactor window screening by an auxiliary gas flow rate less than 5% of the main gas flow rate. The tornado effect is discussed and demonstrated by a smoke flow visualization technique.