Two mounting schemes were devised for attaching heater wires to special-purpose glass tubing used in a capillary-heat-transfer experiment. Not only were the wires required to supply heat needed for the experiment; in addition, it was required that the liquid and vapor enclosed by the tubing be visible between the wires. It was a challenge to satisfy these requirements while preventing (1) delamination of wires from the tubing and (2) short-circuiting that can occur as a consequence of delamination. Finally, neither the attachment nor the operation of the heaters could be allowed to impose stresses that could break the tubing.
The tubing was made from a low-thermal-expansion glass. The heater wires were made of a Cr/Co/Al/Fe alloy and had a diameter of 0.010 in. (0.25 mm). The problem was to mount one heater wire on a cylindrical section of capillary tube and the other wire on a conical transition between the capillary tube and wider evaporator/condenser tube (see figure).
Part of the solution to the wire-mounting problem was to prepare the capillary tube separately from the evaporator/condenser tube before bending the tubes and joining them to form the flow loop needed for the experiment. The main step in the separate preparation of the capillary tube was grinding a spiral groove, in which the heater wire would later be inserted. The spiral groove was intended to prevent adjacent turns of wire from sliding together and thereby becoming short-circuited. An additional advantage of placing the wire in the groove is that it increased the effective contact area between the wire and the tube, making for greater efficiency in the transfer of heat from the wire to the glass.
Once the groove was ground, the capillary and evaporator/condenser tubes were bent and then joined, with conical transition pieces, in a glass-blowing operation. A glass rod (not shown in the figure) was installed alongside the glass tubing to provide additional support for the capillary heater wire. The glass loop was stress-relieved. The capillary heater wire was inserted in the groove, then encapsulated in the groove by epoxy and a two-piece glass cover, which aided heat transfer while affording the required transparency.
The end portions of the heater wire were made to pass through two short ceramic tubes attached to the glass rod. The tips of the capillary heater wire were welded to copper wires, which were wider than the holes in the ceramic tubes; this arrangement prevented unwrapping of the capillary heater wire. The capillary-heater-wire installation stayed intact even when the heater was operated beyond the maximum use temperature of the epoxy.
Because the conical transition pieces were formed manually, they had irregular shapes, making it impractical to grind a spiral groove to hold a heater wire at the capillary/evaporator transition. Instead, all except the end portions of the wire for this location was bent in a serpentine configuration, then wrapped around the glass, then epoxied in place on the glass at each bend in the wire. The end portions of the wire were fed through a two-hole piece of ceramic and welded to larger wires; this arrangement prevented unwrapping and short-circuiting of the wire.
No failures of glass tubing were caused by the installation and operation of the heaters. Measurements by thermocouples confirmed that heat was transferred from the heater wires to the liquid in the glass. The heaters remained in contact with the glass, operating flawlessly throughout the capillary-heat-transfer experiment.
This work was done by Greg Blank of Lewis Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 - 8
21000 Brookpark Road
Refer to LEW-16707.