A study on the characteristics of the flow inside a thermostatic expansion valve

Thermostatic expansion valve;spray;atomization;Phase Doppler Anemometry (PDA)

Huo, Ming ; Lee ; Chia-Fon

关键词: Thermostatic expansion valve;    spray;    atomization;    Phase Doppler Anemometry (PDA);   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/17072/1_Huo_Ming.pdf?sequence=2&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Thermostatic expansion valves (TEV) and electronic expansion valves (EEV) are expansion devices commonly used in air-conditioning system which involves the spray and atomization process of the refrigerant. Most previous studies focused on the macro-features of the expansion valve, such as the refrigerant pressure drop through the valve, the response time of the valve to the change of superheat in the evaporator, or the relationship between the location of the valve and the system efficiency, etc. However, few studies have looked into the valve and fundamentally studied the physical process of the refrigerant taking place near the orifice, where the refrigerant goes through the major pressure drop. The atomization and spray process are crucial to the performance of the expansion valve since the downstream refrigerant are mainly determined by these processes in terms of mass flow rate, quality and homogeneity. Non-homogenous flow can cause poor refrigerant distribution among the circuit inside the evaporator, and essentially decrease the efficiency of the entire air-conditioning system. The other issue involved with the TEV and EEV is the mass flow control, which is namely called as “valve hunting”. One of the greatest advantages such expansion valves can provide compared with traditional expansion valves is their capability to control the refrigerant mass flow rate according to the superheat degree of the evaporator, so that the system can always run in the most efficient mode and achieve the desired cooling capacity at the same time. Yet TEV or EEV can either starve or over feed the evaporator, due to the response time to the change of superheat degree, and inaccurate control of the valve opening and closing. Either starving or over feeding the evaporator can hurt the system performance: the former can decrease the cooling capacity while the later can result in liquid refrigerant going into the compressor, and cause damage to the compressor, in the worst scenario. Although the “valve hunting” has been addressed for a long time, few papers have tried to explain and solve this problem regarding to the refrigerant spray and atomization process. In this study, such techniques were applied to the study of the TEV and EEV. The expansion process was studied by introducing a valve with optical access. The break-up and atomization of the refrigerant were visualized near the outlet of the orifice under different feeding conditions on micro-second scale applying backlit illumination technology. A new image processing method is proposed for cone angle and film thickness determination. A Phase Doppler Anemometry (PDA) system was used later to measure the size and velocity of individual droplets passing the location at the outlet of the orifice. It is found that the increase of the feeding pressure tends to expand the spray cone angle while its impact on the film thickness is not quite obvious. The expansion of the cone angle resulted in more drops splashed from the edge of the needle base and the presence of the drops becomes more random. To further evaluate the impact of the feeding pressure, the drops size distribution under different pressure difference along the radial directions is measured. The drops size dependency on radial distance and pressure difference are acquired based on curve fit of the results.