Several microwave-cavity devices at various stages of development are designed for heating material samples or process streams with uniform temperature-versus-time histories. These devices could satisfy needs for heating according to well-regulated temperature-vs.-time schedules for batch and stream processing in diverse applications; for example, pharmaceutical processing, extrusion and molding of plastics, and processing of biological or medical samples.

The figure presents a simplified axial cross-sectional view of one such device for heating either (1) a stationary annular or cylindrical sample of material or (2) a process stream flowing axially, with azimuthal uniformity, in a narrow space between an inner and an outer encasement tube. The geometry of the device is chosen so that in the plane of the figure or any other plane along the cylindrical axis, the process stream or sample is exposed to substantially equal microwave power density at all points along its circumference.

The Sample or Process Stream Is Heated Uniformly throughout its interior because it is confined in a narrow annular region that contains an antinode of an axisymmetric TM010 electromagnetic mode of the microwave cavity, which has no axial dependence.

The microwave cavity is dimensioned to support an axisymmetric mode, and the radii of the inner and outer encasement tubes are chosen so that the annular sample or flow space between these tubes contains an antinode of either the electric or the magnetic field. This choice of dimensions minimizes the spatial variation of the electric or magnetic field within the sample, thereby minimizing spatial variations in the heating rate. The encasement tubes should be made of a low-loss dielectric material; for example, quartz.

If the sample or process stream is of a lossy dielectric material (e.g., a plastic), then for maximum and substantially uniform heating, the annular space should be dimensioned to contain an antinode of the electric field. Similarly, if the material in question is subject to heating predominantly by the magnetic field, then the annulus should be dimensioned to contain an antinode of the magnetic field.

To prevent convective cooling of the heated sample or process stream, the empty spaces within the microwave cavity are evacuated. To minimize radiative cooling, the cavity wall and end plates are coated with gold, which is highly reflective of thermal radiation. The innermost part of the cavity is occupied by a fixed dielectric tube and an axially movable dielectric rod within the tube; these components are used to tune the cavity to resonance at the frequency of a magnetron or other source that supplies the microwave power. [Tuning by use of this technique was described in "Improved Tuning of a Microwave Cavity for Heating Samples" (NPO-20409), NASA Tech Briefs, Vol. 22, No. 11 (November 1998), page 54.] In the case of a process flow, the tuning rod and tube can be tapered to compensate for the variation of the permittivity of the material with temperature and thus with axial position, to keep the desired field antinode within the annulus at all axial positions.

This work was done by Henry W. Jackson of Acro Service Corp. and Martin Barmatz of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

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Refer to NPO-20459

This Brief includes a Technical Support Package (TSP).
Microwave heating with uniform temperature history

(reference NPO20459) is currently available for download from the TSP library.

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NASA Tech Briefs Magazine

This article first appeared in the March, 1999 issue of NASA Tech Briefs Magazine.

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