HiETA Tests Our Highest Effectiveness Heat Exchanger
The Ultra-compact air to liquid heat exchanger, which is more than 4x smaller than a high performing conventionally manufactured system, is made from Inconel 625, a nickel superalloy, with an operating temperature of up to 800°C.
The design objective of the iBRANCH (Inverted Brayton Cycle for Waste Heat Recovery) project was to cool exhaust gas between a turbine and a compressor with minimal pressure drop. We managed to achieve an air temperature drop of 650°C to 40°C in just 45mm of core with a pressure drop of less than 1kPa.
This single-phase heat exchanger has been designed as part of the Inverted Brayton Cycle for automotive exhaust heat recovery systems. In order to achieve the design objective, the unit has been designed to employ an engine hot exhaust gas as the primary working fluid, the coolant of which is a 20/80 water/glycol mixture.
Further to this, a parametric analysis and design study was conducted to establish the most favourable heat exchanger configurations with respect to its weight and size. Considerations were taken into account for the design of the heat exchanger core geometries and manifolds so that they were able to be manufactured within the HiETA Additive Manufacturing (AM) facility located in Bristol & Bath Science Park. The AM fabrications of HX includes flanges that are attached to the ducts in order to form connections that could be welded with the mating equipment. The turbine diffuser design extends to the half section of the turbine housing and forms an integrated diffuser part turbine-housing. This design has enabled the heat exchanger to be integrated into the turbomachinery (turbine-compressor), thus reducing the system packaging. This made the unit highly compact up to 3400 m2/m3 compared to 400-800 m2/m3 of conventional heat exchangers. Further to the design, the unit also underwent thermal shock testing.
The heat exchanger is a pure crossflow plate-fin unit, with curved sections which provide coolant entry and exit to the core, which has optimised and tailored manifolding for better distribution. The low-pressure hot gas enters the heat exchanger unit from the turbine exhaust and passes through the main heat exchanger core. It is then ducted from the bottom to the compressor inlet. The liquid coolant enters from the from the curved side and passes through the horizontal ducts in a crossflow U fashion.
The compact heat exchanger unit, which is part of HiETA’s novel waste heat recovery system, was optimised for a low-pressure drop, volume and weight while achieving an effectiveness of 96%.