# Experimental Method for Flow Calibration of the Aircraft Liquid Cooling System
## 1 Introduction  
With generous applications of the avionics and the continuous improvement of the radar power, the air cooling system cannot fully satisfy the cooling requirements, and the liquid cooling system emerges as the times requires. Compared with the air-cooling system, the liquid cooling system has several advantages, such as the larger refrigeration capacity, the simpler system design, the secondary cooling with fuel or punching air, and no influence on the normal work of the aircraft. At the same time, the heat transfer coefficient [1; 2] and specific heat of the liquid are much greater than the air, and the liquid cooling system has higher cooling efficiency and stable working ability. Therefore, the liquid cooling system for high-performance aircraft electronic equipment has become an inevitable trend.  
Under normal conditions, the main characteristic parameters of the liquid cooling system are the temperature and pressure. In order to evaluate the cooling performance of the liquid cooling system more accurately, the flow of the coolant becomes another important parameter, which can be obtained through the experimental measurement. Around the 21 st Century, many liquid flow measurement methods had been proposed [3; 4; 5; 6; 7], but there are several disadvantages among these methods proposed, such as the leakage, restriction of the flight condition, the toxic of the refrigerant and so on, which were very difficult to be applied to the liquid cooling system of the aircraft. Therefore, it is necessary to find a safe and feasible flow measurement method for the aircraft liquid cooling system.  
In recent years, the aircraft liquid cooling system has become an important development direction of the aircraft refrigeration system, and researches on domestic aircraft liquid cooling system have just started. [PERSON] et al. [8] used the experimental method to calculate the flow correction coefficient. [PERSON] et al. [9] introduced the usage, the characteristics and the application scope of common flow counters in industry briefly. [PERSON] et al. [10] used a telecentric CCD imaging system to track a moving liquid meniscus insidea glass capillary, and developed a new experimental setup for measuring ultra-low flows. [PERSON] et al. [11] developed a calibration rig, which consisted of a syringe pump and a weighing tank system, to conduct the flow calibration in the flow range of [PERSON] et al. [12] considered the diversion damper situation and the jet flow velocity profile, and developed a measurement error model of diverter to overcome the error calculation difficulties of the diverter in the liquid flow calibration facilities. [PERSON] et al. [13] constructed a flow measurement system consisting of an acoustic emission sensor and a signal processing circuit, and transformed the acoustic emission signals into the corresponding flow. Restricted by the limited space of the aircraft cabin, some liquid flow measurement equipment commonly used are difficult to implement modification on the aircraft, which can cause many safety problems, such as the leakage of the coolant and the performance of the system. Therefore, these flow measurement methods above are difficult to carry out on the aircraft.  
To overcome this problem, a new flow measuring method of the aircraft liquid cooling system is proposed in this paper. This method does not need to modify the liquid cooling system, so it will not affect the normal work of the aircraft liquid cooling system. According to the principle of the differential pressure method, the experimental platform of the aircraft liquid cooling system is established, and the flow calibration is carried out based on the pressure difference of the reducer and the square bend. Experimental results indicate: the flow calibration method proposed is correct and effective, which can be applied to the flow measurement of the aircraft liquid cooling system.