Study On Flow Characteristics Of R-22 Through Stepped Short Tube Orifices For Domestic Air-conditioners

Nowadays, an assembly of two capillary tubes and a by-pass check valve is widely used as throttling valve to achieve different mass flow rates between cooling mode and heating mode in domestic heat pump air-conditioning systems. However, the by-pass check valve increases the manufacturing cost and the leakage probability of refrigerant, meanwhile, the movement of valve could cause noises. Short tube orifice has been popularly used as expansion device in refrigeration and air conditioner because of its simplicity, low initial cost, high reliability and ease of maintenance. In order to reduce the manufacturing cost, a stepped short tube orifice consisting of two serial- connected short tube orifices with different diameters is employed as an alternative in this research. In the same working condition, because the flashing point locations are different between forward flow and reverse flow of refrigerant inside stepped short tube orifice, the mass flow rate is different between forward flow and reverse flow.In this work, the flow characteristics between forward flow and reverse flow of Refrigerant-22 inside stepped short tube orifice was investigated via experimental study and calculated model research. The effect of working conditions, including upstream pressure, downstream pressure and upstream degree of subcooling, and geometry size of stepped short tube orifice were tested. Meanwhile, the cycle performance of the heat pump air-conditioner with stepped short tube orifice used as expansion device was investigated in an air enthalpy laboratory. Finally, a calculated model for predicting the mass flow rate was proposed based on the experimental data of mass flow rate between forward flow and reverse flow of R-22 inside stepped short tube orifice. And the geometry optimization is possible for practical application of stepped short tube orifice.The main conclusions in this study are listed as follow:(1) The mass flow rate and its difference between forward flow and reverse flow of R-22 inside stepped short tube orifice is mainly determined by the upstream working condition and the geometry of the stepped short tube orifice.(2) With variation of the upstream pressure and degree of subcooling, the mass flow rate of forward flow is always greater than the mass flow rate of reverse flow when the R-22 flow through No.11 stepped short tube orifice(D1=0.8mm, D2=1.0mm, L1=3mm, L2=6.5mm) in opposite flow directions. What’s more, the variation of mass flow rate difference is small, and the mass flow rate difference is larger than 10kg/h.(3) Compared to the assembly of two capillary tubes and a by-pass check valve, the manufacturing cost saving of stepped short tube orifice is nearly 59%. Moreover, the cycle performance of air-conditioner with No.18 stepped short tube orifice(D1=0.9mm, D2=1.05 mm, L1=3mm, L2=6.5mm) can be comparable to the system adopting conventional expansion device in nominal working condition and non- nominal working condition.(4) A calculated model for predicting the mass flow rate of forward flow and reverse flow of R-22 inside stepped short tube orifice is established. The relative deviation of prediction is ±15% for 98% of the measured result.