INCEPT · A new strategy for chilldown enhancement in cryogenic propulsion systems

A new strategy for chilldown enhancement in cryogenic propulsion systems

Ambitious goals of total greenhouse gas (GHG) emission reduction and decarbonisation have been set by the recent policies European Green Deal, Energy Union (2030 energy and climate targets) and European Union’s 2050 long-term decarbonisation strategy, aiming for a successful green energy transition. The project’s overall objective is to minimise liquid hydrogen consumption in future applications as fuel for terrestrial, maritime and aviation transportation. Indeed, combined with partial vehicle electrification, the use of cryogenic fuels (first and foremost liquid hydrogen) in terrestrial, maritime and aviation transports has gained an increasingly prominent role thanks to their environmentally friendly nature and ability to store the energy and control its release. Cryogenic fuels can be stored as gas or liquid. Even though cryogenic liquefaction requires energy due to typical low temperatures (< 120 K), it is advantageous since it produces high fuel densities. Liquefied cryogenic fuels can also be used as coolants in integrated systems for energy regeneration and recovery. They can preserve the optimal temperature conditions of power sub-systems like electric batteries and combustion chambers. All these qualities make liquified cryogenic fuels particularly suitable for the next hybrid transport systems. However, they have some drawbacks. The chilldown (or quenching) in pipelines of fuel storage and handling systems is one of the most critical. Defined as the process of keeping the system adjusted to the low temperature (see Figure 1), the cryogenic chilldown cannot be avoided as it is the initial transient stage in any cryogenic liquid transfer operation. The chilldown time and the quenching efficiency define the performance of the cryogenic quenching process: the lower the chilldown time and the higher the quenching efficiency are, the more efficient the chilldown process becomes. In the past, the cryogenic chilldown has been treated as a typical engineering problem in transfer lines of liquid-fueled rocket engines; therefore, much scientific research has been performed directly by governmental aerospace agencies. The pioneering works date back to the end of the ’60s, but they increased exponentially in the last decade, driven by the growing interest in cryogenic fuels as an alternative to traditional fossil fuels. Today, the cryogenic chilldown is highly inefficient (average quenching efficiency < 39%), outlining the need to make the chilldown process faster and more efficient in order to minimise the energy and fuel wastage. This outlines the first knowledge gap tackled by the project: you cannot mitigate a problem if you do not understand it properly. In fact, despite the research effort, the complete mechanistic understanding of the cryogenic chilldown heat transfer dynamics is still missing, and its relation to the inner surface properties is largely unexplored. The last one requires the development of a more advanced experimental methodology and the integration of innovative optical techniques to measure the temperature field of the inner pipe wall. The low-temperature measurement range makes the use of any infrared imaging impossible, while the use of thermosensitive coatings represents a promising solution. Moreover, a crucial step towards efficient, flexible and robust cryogenic transfer lines is the control of the boiling modes occurring during chilldown, for example, using low-thermally conductive coatings with and without intermittent pulsed flow. However, the durability of these coatings is uncertain. Surface structuring at the micro or nanoscale has steeply increased in recent years, allowed by the rapid advancements in micro-and nano-manufacturing technologies (both subtractive and additive) and enables a controlled modification of the solid wettability and wickability. Experiments in flow boiling have proved heat transfer and evaporation enhancement in the presence of annular flow or thin evaporating film using surface structuring, thanks to the increased nucleation-site density and anticipated bubble departures, which help to wash out slug bubbles, maintain rewetting properties and delay the onset of flow boiling oscillation. However, the role of the structure length scale is still unknown, especially when looking at the cryogenic chilldown enhancement using surface engineering: in this regard, only a few studies exist and focus on cryogenic chilldown in pools and vessels via nanoporous structures and nanofiber coating. Despite their promising results, no chilldown enhancement strategy using surface texturing has been applied to pipes yet. The context drafted outlines that the complex transient nature of the chilldown process in pipelines is barely studied, and a full mechanistic understanding of its heat transfer process is missing (knowledge gap KG1). Furthermore, the development of new enhancement strategies, ideally passive with no energy cost, is strongly required. In this regard, the most promising ones are based on functionalizing the solid surface wettability (knowledge gap KG2) to enhance the boiling heat transfer mode and reduce the chilldown time. Following the identified knowledge gaps, this project sets two innovation objectives (IOs). (IO1) Develop a novel experimental methodology to reveal the local and global mechanisms governing the heat transfer into the pipeline undergoing cryogenic chilldown: A large part of the energy and fuel waste comes from an initial explosive two-phase flow dynamics and the establishment of the film boiling regime. Thin film evaporation phenomena drive its duration at the liquid-vapor-solid interfaces at the micro and nanoscale. The chilldown analysis must be based on advanced experimental methodologies, including novel thermographic techniques using thermosensitive coatings. Successful realisation of this objective will shed light on a series of fundamental complex physical phenomena that are unclear to the research community (KG1). (IO2) Investigate cryogenic chilldown enhancement by tuning surface wettability using femtosecond (fs) laser technology. Surfaces with textures at micrometric and nanometric scales have shown a certain potential in improving nucleation and thin film evaporation. These surfaces can be used to mitigate flow boiling instability following a quasi-periodic design (KG2). Reaching this objective will demonstrate the fs laser texturing as an emerging technique for flow boiling control and the role of surface wettability in the cryogenic chilldown transient region.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз