Cold Gas Stream Control with Liquid N2 Injection Stability
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Solution Overview
Problem
Existing MAS cold gas supply systems face challenges in maintaining stable temperature and flow at low temperatures below 130 K due to significant heat leaks and instability in two-phase flow, making it difficult to achieve consistent spinning speeds and temperatures with existing N2 gas-cooling technology.
Innovation Solution
The method involves controlled injection of liquid nitrogen into a chilled gas stream to maintain a low liquid fraction, using separate pre-cooling and condensing coils to achieve a consistent vapor fraction, and incorporating a helium-nitrogen mixture to reduce the boiling point, thereby minimizing heat leaks and stabilizing the gas flow to the MAS probe.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single cold gas stream is used to cool both the sample and bearings in MAS, then the system is simpler, but temperature stability deteriorates due to frictional heating and RF heating affecting the sample temperature
Solution Approach 1:
The patent divides the gas cooling system into separate streams: one dedicated to sample cooling and another to bearing cooling. This segmentation allows independent temperature control of each component, preventing frictional heating from affecting sample temperature and enabling stable low-temperature operation below 130 K
2Temperature
If liquid nitrogen is directly injected into the gas stream to reduce temperature, then cooling efficiency improves, but flow stability deteriorates due to two-phase flow instability
Solution Approach 1:
The patent pre-cools the nitrogen gas in heat exchangers before it reaches the injection point, reducing its temperature to near the dew point. This preliminary cooling action allows controlled condensation of a small liquid fraction (3-10%) that stabilizes the two-phase flow and maintains consistent temperature during transfer to the MAS probe
Solution Approach 2:
The patent carefully controls the liquid fraction parameter of nitrogen in the injected stream, maintaining it between 3-10%. This parameter optimization ensures sufficient cooling effect while avoiding the instability associated with high liquid fractions, achieving both low temperature and flow stability
3Temperature
If the gas flow rate is increased to compensate for heat leaks, then temperature maintenance improves, but the system requires larger gas consumption and higher pressure
Solution Approach 1:
The patent introduces a small fraction of liquid nitrogen as an intermediary cooling agent that is injected into the pre-cooled gas stream. This liquid fraction evaporates and absorbs heat efficiently, providing superior cooling per unit mass compared to gas alone, thereby reducing the total gas consumption required to maintain low temperatures
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach ensures a stable and consistent supply of cold gas with a vapor fraction of less than 20%, reducing temperature rise during transfer and achieving spinning speeds and temperatures below 120 K with improved stability and efficiency.
Implementation Method 1
Pressurized ultra-dry nitrogen gas of a controlled mass-flow rate is cooled inside fine coils bathed in liquid nitrogen to condense it to a vapor fraction less than about 20%
Implementation Method 2
controlled injection of liquid nitrogen into a chilled gas stream to maintain a low liquid fraction
Implementation Method 3
The fluid from the first (condensed) mixture is injected into the cooled gas from the second mixture
Implementation Method 4
transferred through a thermally insulated line to the input of the instrument needing a supply of cold gas
Data Source
AI summary
An improved method of supplying pressurized cold gas consistently of predominately N2 and He at low flow rate (typically under 1 g/s) with a desired N2 liquid fraction to an instrument requiring such is disclosed. Pressurized ultra-dry nitrogen gas of a controlled mass-flow rate is cooled inside fine coils bathed in liquid nitrogen to condense it to a vapor fraction less than about 20% and typically under 3%. A second gas stream consisting of predominately nitrogen plus helium, supplied from a controlled pressure, is cooled in a separate set of coils to an exit mean temperature significantly above the temperature of saturated nitrogen vapor in this mixture. The fluid from the first (condensed) mixture is injected into the cooled gas from the second mixture and transferred through a thermally insulated line to the input of the instrument needing a supply of cold gas of a target vapor fraction.