Direct Contact Heat Exchanger for Compact Brayton Cycle Propulsion
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Solution Overview
Problem
Current underwater propulsion systems face issues such as violent reactions due to heat exchanger leaks, noise generation, severe stress on oxidant injectors, and slow start-up times in closed Rankine cycle systems, while conventional Brayton cycle systems are too large for underwater use and inefficient due to high compressor power requirements.
Innovation Solution
A closed Brayton cycle power system utilizing a liquid metal fuel with a compact heat exchanger and direct heat transfer, where a compressor compresses the working gas, which is preheated in a regenerator and bubbled through a liquid metal fuel/oxidant mixture in a reactor/storage tank to generate heat, with a turbine expanding the gas to produce power and a cooler recirculating the gas.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional Brayton cycle heat exchangers are used in underwater systems, then the system can operate with gas phase working fluid, but the heat exchanger becomes too large for the restricted space available
Solution Approach 1:
The heat transfer tubes are nested within the liquid metal fuel container, with the tubes arranged in a cylindrical annulus around the central fuel column. This nested configuration allows the heat exchanger to utilize the existing volume of the reactor/storage tank, eliminating the need for separate external heat exchanger components and reducing overall system volume.
Solution Approach 2:
The patent transitions from conventional external heat exchanger configurations to an internal heat transfer tube arrangement that utilizes the vertical and radial dimensions within the reactor/storage tank. The tubes are arranged in a cylindrical annulus, maximizing heat transfer surface area within the restricted volume available in underwater vehicles.
2Temperature
If gas velocity is increased to achieve higher heat transfer coefficients in compact heat exchangers, then heat transfer efficiency improves, but heat exchanger pressure drop increases
Solution Approach 1:
The working fluid flows continuously through the heat transfer tubes in direct contact with the liquid metal fuel, maintaining continuous heat transfer without interruption. This continuous flow pattern ensures consistent heat transfer efficiency while managing pressure drop through optimized flow paths and tube arrangements.
Solution Approach 2:
The patent optimizes various parameters including tube diameter, tube spacing, gas velocity, and fluid properties to achieve the desired balance between heat transfer efficiency and pressure drop. By adjusting these parameters, the system achieves high heat transfer coefficients while keeping pressure drop within acceptable limits for the Brayton cycle operation.
3Power
If liquid metal fuel is used with water as working fluid in Rankine cycle, then chemical reaction heat is generated, but leaks cause violent reactions and toxic gas release
Solution Approach 1:
The patent extracts the water working fluid from the chemical reaction zone and uses it only as a heat transfer medium in the Brayton cycle. The liquid metal fuel reacts with oxygen in a separate zone, and the resulting heat is transferred to the working fluid through heat transfer tubes, eliminating direct contact between water and liquid metal fuel and preventing violent reactions and toxic gas release.
Solution Approach 2:
The heat transfer tubes serve as an intermediary between the liquid metal fuel combustion zone and the water working fluid. This intermediary structure allows heat transfer without direct contact between the reactive liquid metal fuel and the water, eliminating the risk of violent chemical reactions and toxic gas generation while maintaining efficient heat transfer.
4Power
If compressor power is increased to meet Brayton cycle requirements, then gas compression is achieved, but the system becomes less efficient compared to Rankine cycle
Solution Approach 1:
The patent merges the functions of the compressor and turbine into a unified system where the compressor compresses the working gas and the turbine expands it to produce power. This integration allows for better matching of pressure ratios and improved overall efficiency, reducing the net power loss associated with separate compression and expansion processes.
Solution Approach 2:
The patent optimizes compressor and turbine parameters including pressure ratios, temperature differentials, and flow rates to minimize energy loss. By carefully selecting and adjusting these parameters, the system achieves efficient operation despite the inherent energy requirements of gas compression, reducing the gap between Brayton and Rankine cycle efficiencies.
Data Source
AI summary
A liquid metal fueled Brayton cycle power system with a direct contact heat exchanger. In this invention, a compressor compresses the working gas. A regenerator preheats the compressed working gas and passes the working gas to a reactor/storage tank with liquid metal fuel stored therein. An oxidant is injected into the reactor/storage tank to react with the liquid metal fuel. The compressed working gas bubbles through the liquid metal fuel in the reactor/storage tank and is heated by direct contact with the fuel-oxidant mixture. A turbine expands the heated working gas and thereby withdraws power from the system. The spent working gas exits to the regenerator where it warms the compressed gas. A cooler reduces the working gas temperature and recirculates the gas to the compressor.