Browsing by Author "Stokes, Sheldon"
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Item Compact Oxygen Heat Exchanger for the Exploration Portable Life Support System(2020 International Conference on Environmental Systems, 2020-07-31) Izenson, Michael; Servi, Amelia; Stokes, Sheldon; Rundle, TessaThe ventilation loop in NASA’s Exploration Portable Life Support System (xPLSS) includes a compact heat exchanger that cools the ventilation gas before it enters the suit helmet. This heat exchanger must provide efficient gas cooling with low pressure losses in a very compact package. It must operate across a wide range of suit pressures and be built from materials that are compatible with oxygen and the thermal control loop. Finally, the heat exchanger must serve as a pressure drop element that can be used to measure the ventilation flow rate with high accuracy. We have developed an innovative heat exchanger that meets these requirements. The miniature shell-and-tube heat exchanger uses a core built from an array of Inconel microtubes. The microtubes provide a very high surface area and high heat transfer coefficients for efficient gas cooling with minimal pressure losses in the ventilation and thermal control loops. We have designed and built several generations of the heat exchanger and measured their heat transfer and pressure drop performance in an extensive series of tests that simulate operation in the xPLSS ventilation loop. We have found that the compact heat exchanger meets all xPLSS requirements for heat transfer and pressure loss. The heat exchanger is designed for close integration with a high-accuracy flow meter, and produces a highly linear flow vs. pressure drop characteristic that is suitable for high-accuracy flow measurement. Two units have been delivered to NASA in 2019 and three additional development, verification, and test (DVT) units are scheduled for delivery in 2020. This paper will describe the design, manufacturing, and performance of the DVT heat exchangers.Item Development of an Oxygen Heat Exchanger and Flow Meter for the Exploration Portable Life Support System(50th International Conference on Environmental Systems, 7/12/2021) Izenson, Michael; Servi, Amelia; Stokes, Sheldon; Beach, Theodore; Rundle, Tessa; Hinckley, DavidThe ventilation loop in NASA�s Exploration Portable Life Support System (xPLSS) includes a compact heat exchanger that cools the ventilation gas before it enters the suit helmet. In addition to cooling the ventilation gas, the heat exchanger also serves as a pressure drop element for measuring the flow rate of ventilation gas. We have developed an innovative heat exchanger that meets the challenging requirements for service in the xPLSS. The miniature shell-and-tube heat exchanger is built from all oxygen-safe materials around a core comprising an array of Inconel microtubes. The microtubes provide a very high surface area and high heat transfer coefficients for efficient gas cooling with minimal pressure losses. The microtube design also provides a highly linear flow vs pressure drop characteristic that enables accurate measurement of gas flow. The first design validation test (DVT) heat exchanger is installed and undergoing integrated system testing in the first xPLSS prototype. This paper reports on the status of the heat exchanger development program and reviews some of the key lessons learned from the development and testing effort: (1) Maintaining stability of test conditions for a small heat exchanger across wide range of pressures and flow rates; (2) Measurement of pressure losses in a component designed for a compact, highly integrated system; (3) Challenges obtaining flight-qualified electronics for the flow sensor. The techniques that were developed to measure performance and qualify the current set of DVT heat exchangers will be used to support flight qualification tests for future units.Item High-Accuracy Oxygen Flow Meter for the Exploration Portable Life Support System(2020 International Conference on Environmental Systems, 2020-07-31) Izenson, Michael; Servi, Amelia; Stokes, Sheldon; Beach, Theodore; Kirkconnell, Carl; Huynh, Leon; Rundle, Tessa; Lee, Steven"The xEMU’s portable life support system (xPLSS) requires a high accuracy instrument to measure the rate of oxygen flow in the ventilation system. The sensor must produce accurate readings across a wide range of flow conditions while consuming very little volume or power and introducing very little pressure loss to the ventilation loop. We have developed an innovative flow sensor built around a commercial, off-the-shelf MEMS flow-sensing chip that is designed for oxygen service. We have developed a custom flow sensor housing to channel gas flow over the MEMS sensing elements and custom electronics to control the sensor and generate signals compatible with xEMU requirements. The flow sensor operates in parallel with the ventilation loop heat exchanger, so introduces no additional pressure loss to the ventilation system. The unit meets size and shape requirements for service in the xPLSS and is replaceable in space if necessary. Data from separate-effects tests and tests of the integrated xPLSS heat exchanger / flow meter system show that the flow meter achieves high accuracy requirements across the range of specified operating conditions. We built and tested a proof-of-feasibility prototype in mid-2019, followed by a “rapid turn” demonstration sensor that meets form, fit and function requirements in late 2019. Fully-qualified DVT units are scheduled for delivery in mid-2020."Item Integrated Oxygen Flow Meter / Heat Exchanger for Portable Life Support Systems(48th International Conference on Environmental Systems, 2018-07-08) Izenson, Michael; Servi, Amelia; Phillips, Scott; Stokes, Sheldon; Campbell, ColinSpace suits for future exploration missions will have multi-mission goals with new and challenging requirements for the portable life support system (PLSS). In particular, the space suit ventilation loop requires cooling and flow measurement components that must meet specifications that go well beyond the capabilities of the components used for the existing Extravehicular Mobility Unit. The flow meter must provide high measurement accuracy over a wide flow range, compatibility with pure oxygen, low pressure losses, and very compact size. The heat exchanger that cools the ventilation loop must be built from materials that are compatible with the liquid cooling loop, and it must provide efficient gas cooling in a small package across conditions ranging from normal suit pressure to elevated pressure. This paper describes the development of a novel device that combines the flow measurement and cooling functions in a single, compact flow meter / heat exchanger (FMHX). We have developed design methods that enable us to assess trade-offs, optimize performance, and specify the design of an FMHX that meets the requirements and constraints for operation in future PLSSs. We used computational fluid dynamics analysis to validate the pressure drop and heat transfer characteristics of the FMHX design. Data from tests of a proof-of-concept FMHX show that the system meets all design requirements. We used the results from these tests to refine the design parameters and predict performance of an optimized, prototype FMHX.Item Ventilation Heat Exchanger/Flow Meter for xPLSS(2023 International Conference on Environmental Systems, 2023) Izenson, Michael; Niblick, Adam; Stokes, Sheldon; Rundle, TessaThe flow meter / heat exchanger (FMHX) in the ventilation loop of the exploration EMU cools the ventilation gas and measures the ventilation flow rate. The heat exchanger transfers heat from the ventilation gas to the thermal control loop via a miniature shell-and-tube heat exchanger. The flow meters calculate the flow rate of gas through the ventilation loop based on the pressure drop across the heat exchanger core. Creare has delivered four design validation test (DVT) heat exchangers and five DVT flow meters to NASA JSC to support development of the exploration portable life support system (xPLSS). This paper describes the design and performance of the DVT units. The heat exchangers are designed to cool the ventilation gas to a specified temperature with low pressure losses under the most challenging operating conditions. The measured performance of the DVT heat exchangers agrees well with design models and meets all performance requirements. The flow meters use a MEMS thermal flow sensor to produce a signal that is proportional to a small bypass flow around the heat exchanger core. They are designed to achieve high measurement accuracy across the full range of xPLSS operating conditions. We calibrated the flow meters in a special-purpose flow facility that simulates operation in the xPLSS ventilation loop. Calibration testing shows that DVT flow meters produce digital output for vent loop mass flow that meets NASA’s accuracy requirements across the range of xPLSS operating conditions. This paper reviews the design of the heat exchangers and flow meters and presents data from the final flow meter calibration testing, heat exchanger performance validation, and initial ground testing in NASA’s xPLSS.