Surveillance of countries' borders is of greatest interest to monitor to detect enemies. Saab's radar aircraft has air, ground and sea surveillance capabilities and is characterized by the large radar that attaches to the fuselage of the aircraft. When designing future aircraft and unmanned vehicles, weight and energy efficiency are sensitive parameters to consider. The weight of aircraft and its equipment has an important impact on, among other things, fuel consumption and thus how long they can stay in the air. In addition, all mission equipment must be provided with cooling to avoid overheating. The aim of this study has been to analyze and evaluate existing refrigeration systems in some of Saab's radar aircraft during various operational scenarios and also review alternative designs for future vehicles and carbon dioxide as future cooling media.
The refrigeration systems Environmental Control System and Mission Air Cooling System ensure that cabin and cockpit areas are tempered and pressurized and that all heat generated in the equipment is transported away and out to the surrounding atmosphere as a heat sink. The Environmental Control System uses ambient air as media. Compressed air is drained from engines or an auxiliary power unit and supplies the refrigeration system with air. The Mission Air Cooling System is a conventional refrigeration machine with R134a as the refrigerant. The existing cooling circuit absorbs heat in the evaporator which is placed in the air distribution where the air circulates and cools the equipment. The refrigerant then transports the heat to the surrounding atmosphere. Carbon dioxide as a refrigerant has been used since the 19th century. With the ongoing phasing out of conventional refrigerants, R134a included, carbon dioxide is starting to become relevant again. Despite carbon dioxide's good heat transfer properties, there is a major challenge regarding the high-pressure conditions, which places demands on the components of the refrigeration system. Carbon dioxide can reduce pressure losses and dimensions of components and pipelines as well as reduce installation weight.
The performance of the refrigeration systems has been evaluated based on their coefficient of performance and how much energy from the engine corresponds to the amount of fuel that the refrigeration systems require. Pressure, temperature and enthalpy conditions were developed in a simulation program, alternatively previous calculation templates were reused to calculate heat transfers and work in each refrigeration system. In addition, the installation weights of the refrigeration systems in relation to each other were provided in order to be able to analyze these against other parameters, such as performance and complexity, for future aircraft and unmanned vehicles. An alternative construction in the Mission Air Cooling System was investigated where the air distribution is excluded and instead the cooling circuit is led all the way to the devices. The evaporator thus functions as a cooling plate. The calculations for a carbon dioxide machine were carried out using research articles.
For the Environmental Control System, it turned out that the design of the air intake together with the air velocity profile in some cases generates low mass flows, which causes abnormal heat exchanges and temperature conditions in the cabin and cockpit. The Mission Air Cooling System had better performance in terms of coefficient of performance but has a long chain of energy conversions required for the electricity supply which contributes to energy losses. Calculations carried out for a carbon dioxide machine resulted in the compressor's displacement being able to be reduced by 89\%.
Finally, based on the analyses and calculations carried out, a section is presented that explains which parameters should be considered for future designs for unmanned vehicles, as well as a figure that can be seen as an example of a system structure. The system structure is a conventional refrigeration machine with carbon dioxide as the refrigerant. Based on the analyses made regarding the installation weight and the performance of the refrigeration systems, it is likely that the presented system structure also contributes to the lowest weight and is an example of a refrigeration system in future aircraft and vehicles.
2024. , s. 79