A relatively simple thermal technique has been proposed to enable detection of bubbles of gas or bulk gas in a tube that is meant to contain a flowing liquid only. In a system in which gas cannot be tolerated, the technique could be used to trigger a shutoff valve downstream of a location where gas is detected.
The technique would involve the placement of a circumferential electrical heating element around the tube and a circumferential thermometer around the tube a suitable short distance downstream of the heating element. The thermometer reading would be indicative of the rate of transfer of heat from the heating element to the thermometer; this rate would depend on the temperature of the heater, the rate of convective transfer of heat between the tube and the fluid at the heater and thermometer locations, and the thermal capacity of the fluid in the tube. These variables and thus the thermometer reading would be affected by any gas that might be present.
A device to implement the technique could be fabricated by placing a thin layer of a dielectric material around the tube, sputtering a thin platinum coat over the dielectric, and patterning the platinum into narrow rings around the tube at two (or more for redundancy) positions along the tube. The upstream platinum ring would be operated as a heater and resistance thermometer, and the downstream platinum ring would be operated as a resistance thermometer. For a specific combination of heater temperature and rate of flow, the temperature sensed by the downstream resistance thermometer would include a component proportional to the transport of heat by thermal mass flow in the tube. Intrusion of gas into the tube would reduce the effective thermal mass density of the fluid in the tube, and the temperature read by the downstream resistance thermometer would decrease accordingly.
This work was done by Frank T. Hartley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.
NPO-20996
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Thermal Detection of Gas in a Tube Containing Flowing Liquid
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Overview
The document discusses a novel thermal detection technique developed by Frank T. Hartley at NASA's Jet Propulsion Laboratory (JPL) for identifying the presence of gas in tubes intended to carry only flowing liquids. This method is particularly significant in systems where gas intrusion is unacceptable, as it can trigger a shutoff valve downstream when gas is detected.
The technique involves the placement of a circumferential electrical heating element and a thermometer around the tube at specific locations. The thermometer measures the temperature downstream of the heating element, which reflects the rate of heat transfer from the heater to the fluid in the tube. This heat transfer rate is influenced by several factors, including the heater's temperature, the convective heat transfer between the tube and the fluid, and the thermal capacity of the fluid. The presence of gas in the tube alters these variables, leading to a decrease in the temperature reading of the downstream thermometer.
To implement this technique, a thin layer of dielectric material is applied around the tube, followed by a sputtered thin platinum coating. The platinum is then patterned into narrow rings, with one ring functioning as both a heater and a resistance thermometer (upstream) and the other as a resistance thermometer (downstream). The temperature sensed by the downstream thermometer includes a component that is proportional to the thermal mass flow in the tube. If gas intrudes into the tube, it reduces the effective thermal mass density of the fluid, resulting in a lower temperature reading at the downstream thermometer.
This thermal detection method offers a relatively simple and effective solution for monitoring gas intrusion in liquid-carrying systems, which is critical for maintaining system integrity and preventing potential failures. The work is documented under NASA Contract No. NAS 7−918 and is part of the JPL New Technology Report NPO−20996. The document emphasizes that the information provided does not imply any endorsement by the U.S. Government or JPL and is intended for identification purposes only.
Overall, this innovative approach represents a significant advancement in the field of fluid dynamics and thermal detection, with potential applications in various industries where liquid purity is essential.
