Speaker
Description
Microchannel heat exchangers (MCHXs) have been the focus of intense study in the academic sector for around 20 years, with an escalation in recent times due to the severe heat management issues associated with current and emerging applications in technology areas such as energy production and space. The space sector, in particular, is implementing thermodynamic cycles in what can be considered very high temperature and high pressure environments. One such example is the thermodynamic cycle of the Synergistic Air Breathing Rocket Engine (SABRE), a hypersonic precooled aero-engine developed by Reaction Engines Ltd. SABRE is unique among rockets since it can operate both as a turbojet engine (air breathing) and a conventional rocket, and in such applications, extreme pressures and temperature differentials are present.
In a project supported and funded by the European Space Agency (ESA), work was carried out where the overarching technical objective was to develop and validate a compact, light-weight and high performance helium-hydrogen heat exchanger model, based on computational fluid dynamics (CFD) and analytic theoretical models. The validated model was then used to provide the basis for an HX concept used in the power management system of a reusable launch system. The main technical objective was to increase the thermal performance of the MCHX to achieve a Nusselt number of over 10 (Nu>10). This is primarily achieved through the addition/modification of geometrical features and channel arrangement, and through adoption of a cross-flow H2-He design as part of a wider Brayton heat cycle concept.
A practical design case scenario is considered, whereby 35K Hydrogen at 20MPa is used as the coolant in a MCHX that must transfer close to 90MW of heat whist ensuring a 42K outlet temperature for the initially 700K Helium. The model results indicate that substantial size and weight reduction will be achieved provided that specifically engineered microchannels can be manufactured at the sub-micron scale. The resulting MCHX was designed such that it could be manufactured using photochemical etching to form the micron-scale channels on individual shims, which are then stacked and diffusion bonded to produce 316L stainless steel MCHX breadboards for the on-going validation programme itself. This method of manufacture produced what are commonly known as a printed circuit heat exchangers (PCHEs). With high strength, leak-tight bonds, good chemical compatibility whilst being compact and lightweight nature, lends themselves well to the extreme service conditions as outlined, whilst achieving the desired heat transfer goals.
