Study On Refrigerant Flow Characteristics In Adiabatic Capillary Tubes And Match With System

In this dissertation refrigerant flow characteristics in adiabatic capillary tubes and the component matching with the system are investigated by both simulation and experiments. The main work and results are as follows: 1. Numerical modeling of refrigerant flow characteristics in straight adiabatic capillary tubes: A homogenous model is developed which includes subcooled liquid region, metastable liquid region, metastable two-phase region and equilibrium two-phase region. Effects of various combinations of friction factor equations and two-phase viscosity correlations on the model precision are compared. Also, the effect of superheated liquid viscosity in metastable two-phase region is considered. Carl's correlations are introduced to calculate refrigerant thermophysical properties in the model. Numerical results show quite good agreement with the experimental data of many researchers. Flow characteristics of R22 and R290 in adiabatic capillary tubes are compared with the model developed and sensitivity analysis show that mass flow rate of R290 is less effected by geometric parameters and operating conditions when compared with that of R22. 2. Numerical modeling of refrigerant flow characteristics in coiled adiabatic capillary tubes: It is verified whether the friction factor equations for coiled pipes in literature are applicable to adiabatic capillary tubes and the results show that Mori&Nakayama or Ito friction factor equation can only be used for small-roughness capillary tubes and not for large-roughness capillaries. A new method is presented to calculate the friction factor for coiled capillaries, which is that Churchill equation is fitted into the form of Blasius equation and the parameters are applied into the MN type friction factor correlations. Numerical results show good agreement with many experimental data. 3. Experimental investigation of refrigerant flow characteristics in adiabatic capillary tubes: With the one-pass-through experimental system designed and set up, amounts of experimental work are done on the performance of R22 in straight, coiled and parallel adiabatic capillary tubes. The results show that mass flow rate of coiled capillary decreases when the coiled diameter reduces. And the phenomenon of hysteresis in coiled capillary tubes is more prominent than in straight tubes and the data are more disperse. With path of increasing subcooling, data points tend to be consistent, while for path of decreasing subcooling, it shows that mass flow rate data points are more disperse. The character of hysteresis in coiled capillaries tends to be weaker with the coiled diameter increasing. Also, it is found and analysed that there exists inlet pressure fluctuation of coiled capillary tubes when the degree of subcooling is changed from one to another. Mass flow rate of various parallel capillary tubes is almost the same with the sum of the mass flow rate of each single tube under the same operating condition. Further, hysteresis also exists for parallel capillary tubes. However, the fluctuation of inlet pressure is weaker than that in a single capillary. 4. Simulation and experimental study on the match of capillary tube with the system: A system simulation program is developed to match the capillary tube length and refrigerant charge on the condition that the other components of air conditioner have been chosen. Further, a sensitivity analysis is made with the developed model. System experiment results show that mass flow rate tends to increase when the coiled diameter increases. The experiment also shows that system mass flow rate with refrigerant of R290 is less than half of that with R22, and the input power and cooling capacity with R290 also decrease compared to that with R22.However, the energy efficiency ratio is higher than R22 and the system performance with R290 is less effected than with R22 when the operating condition changes. Capillary tubes with different inner diameters can all meet the designed performance. No other effects of jumping pressure difference on the system performance are found except that the vibration of capillary is a little more distinct for the capillary with the higher jumping pressure difference.