State of the art – Background
In two phase capillary pumped cooling systems (CPL), the selection of a suitable fluid is a critical issue to ensure operation and allow the design of efficient, safe and economic CPL systems. Many properties have to be considered : heat transfer is related to the vaporization enthalpy and the heat capacity. Viscosity and density are related to the fluid flowing through the pipes. Surface tension is related to the ability to wet the porous wick where heat transfer occurs and to pump the fluid. Temperature limits must be set because the CPL apparatus may undergo severe conditions during aircraft operation, from very cold weather situations to very hot conditions. Typically the fluid cannot freeze nor fully evaporate during operation. Besides some existing fluids display inappropriate effects in terms of toxicity and safety, which must be improved and adds new products requirements. Hence, the problem of finding a fluid is a multi-objective problem, considering that numerous properties need to be matched. Although many CPL apparatus are in operation, none of the existing fluids (acetone, methanol, ethanol, ammonia, water for instance) are satisfactory with regards to the specifications targeted.
With such an extended set of specifications, it was initially forecasted that suitable pure compound fluids might not be found and that mixture candidates should be looked at as well. This defines a scientific challenge because mixtures are reputedly exhibiting non ideal behavior which can be described only by nonlinear property predictions models, which is a difficult task to perform during a large search.
The DIPHASICFLUID project has set the objective of finding new fluids for CPL systems under severe conditions. As an all-around search in databases and suppliers portfolio could be very time consuming and unable to cover all properties at the same time, the project has been implementing a strategy for finding solutions based on the use of property prediction tools and first principle thermodynamic models, to orient the search towards suitable chemical families that would eventually contain pure fluids and later to search for mixtures. Besides, this computer aided in silico search was deemed the most efficient to handle the multicriteria search that we performed as all relevant properties were considered together.
Description of work
Six work packages were defined :
• WP1 : performance function building
• WP2 : in silico search with top-down and bottom-up approach
• WP3 : accurate models prediction
• WP4 : fluid choice
• WP5 : property measurements
• WP6 : project management
Initially, the strategy was designed to run the tasks sequentially from WP1 to WP5, but after a year as WP2 results were mitigated for mixtures, it became evident to imagine a more efficient strategy. The final search strategy kept all six work packages with small adaptation of the tasks objectives. It ended up in combining in parallel WP3 (property estimation refining) WP4 (fluid choice) and WP5 (property measurements) :
The task (WP1) consisted in building a mathematical performance function encompassing all the property specifications and screen potentially interesting chemical families in order to define an electronic signature of the ideal fluid. Second, a systematic computer-based search was run to result in a list of candidate fluids (WP2). It combined two existing computer tools from the partners : a bottom-up approach that constructs molecules with feasible chemical synthesis pathways starting from a core molecule, using GRASS software, and a top-down search based on group contribution property estimation methods to explore new pure compounds and mixtures built from a pool of chemical fragments, using IBSS software. Third, the candidate list was narrowed by updating the performance function of each candidate through the refining of property values either extracting them from databases or predicting them with more accurate first principle methods, including molecular simulation tools COSMO-RS, Gibbs Medea and Material Studio (WP3). The fourth task objective was to choose the fluid (WP4). The fifth task was dedicated to experimental measurements so as to verify the real performance of selected fluids (WP5). A coordination tasks (WP6) oversaw the whole project and handled dissemination issues.
The project has been successful. With the help of the efficient strategy combining computer-based calculation, several tens of thousands of molecules covering all chemical families were screened. The in silico search was notably fit to handle the multicriterion search as we considered both environment, health and safety related properties in addition to physico-chemical properties related to CPL apparatus functioning. Not a single molecule achieved the maximum performance regarding all properties, but suitable chemical families were identified and drawbacks of the most promising single molecules were pointed out. Hence, mixtures were screened with the idea of benefiting from synergies in the mixture where one compound property could compensate the far-from-target property value of another compound in the mixture. At that time the aforementioned non-ideal behavior in mixture was precisely evaluated with the help of models based on first principle thermodynamics and measurements. With that in mind, several hundred mixtures were imagined, their property predicted and verified with experiments. Ultimately, the project found several fluid mixture candidates which performance was been validated. Additionally, new cooling fluids not fitted for the CPL case study have been found to be used in other heat transfer processes. Both solutions are undergoing patenting at the time of the project end.
a) Timeline & main milestones
The timeline spanned over 36 months.
• WP1 : performance function building lasted 6 months [M1-M6]
• WP2 : in silico search with top-down and bottom-up approach lasted 9 months [M7-M15]
• WP3 : accurate models prediction lasted 23 months [M7-M29]
• WP4 : fluid choice lasted 21 months [M9-M29]
• WP5 : property measurements ran over the whole project duration.
• WP6 : project management ran over the whole project duration.
The main milestones were :
• M1 : The new property calculation models are available and allow building a performance function that incorporates all the project fluid specifications. (reached at the end of WP1 duration)
• M2 : candidate fluids (pure and mixtures) are proposed by the in silico approach. (reached at the end of WP2 duration)
• M3 : stability of candidate fluids is evaluated. (reached at the end of WP3 duration)
• M4 : fluids (pure and mixtures) are selected.
(reached at the end of WP4 duration)
b) Environmental benefits
A key feature of the new fluids found is that they exhibit reduced impacts in terms of environment, health and safety (EHS). This was achieved by considering systematically these issues during the search, in addition to properties related to the CPL apparatus functioning. Indeed, during the search many fluids suitable for CPL were discarded based on these EHS impacts.
c) Maturity of works performed
In line with the project objectives, new fluids have been found and two patents are pending, premises of a future test in operation of a CPL with these fluids.
Several partial results have been released in conferences, and two articles are in preparation.