Experimental and numerical studies on an automobile air conditioning system with refrigerants R134a, R1234yf and R1234ze(E)

Includes bibliographical references and Index.

PhD; 2023; Mechanical engineering

The HydroFluoroCarbons (HFCs) synthesized as alternatives to ChloroFluoroCarbons (CFCs), though friendly to tratospheric ozone, have high Global Warming Potential (GWP). Despite this, numerous applications currently employ HFCs for refrigeration and air conditioning. The Kyoto protocol, negotiated in 1997 and came into force in 2005, put the HFCs in the green house basket and stated that the emissions of these gases need to be checked and controlled. The 2015 EU regulation and the 2016 Kigali amendment to the Montreal protocol suggested phase-out of the HFCs and this process will go on until 2036 in industrialized nations and until 2047 in non-industrial nations to accomplish a condition of 85% decrease of HFCs. The HFC134a refrigerant used in vehicle air conditioning has a 1300 Global Warming Potential (GWP), which prompted researchers to look for new low-GWP refrigerants. Recent research has revealed that the HydroFluoroOlefin (HFO) refrigerants HFO1234yf and HFO1234ze(E), with a GWP of 4 or less, show promise for Application in Automobile Air Conditioning (AAC) field. The AAC requires special attention due to frequent leakages of HFC caused by pipe failures due to vibration. In this research, the low-GWP refrigerants R1234yf and R1234ze(E) are used to explore AAC system performance, and comparisons with the currently used refrigerant HFC134a is made. An experimental setup is developed to simulate an AAC system containing evaporator, condenser, swash plate reciprocating compressor and expansion valve (thermostatic type) as the main components with interconnecting copper pipelines and necessary controls and instrumentation. The setup also accommodates an Internal (liquid-to-suction) Heat Exchanger as an optional component. The complete experimental setup was mounted on a mild steel frame in the laboratory. The experiments were carried out to find out how various quantities of interest, such as the condenser heat rejection rate, Coefficient of Performance (COP), pressure ratio, cooling capacity & mass flow rate of the refrigerant, were affected by the evaporator face velocity, temperature at condenser inlet, compressor speed, temperature at evaporator inlet and air temperature, condenser face velocity. For all the three refrigerants the transport and thermodynamic properties are numerically generated using Helmholtz type equations of state or standard correlations and are validated against the Refprop 9.0 version, Consequently, the option to calculate the transport and thermodynamic properties using either customised algorithms or the Refprop software is provided. From the different models, an integrated model for the entire system is created using the formulations for various components. The component and system models are validated against the results of published literature. For the 3 refrigerants considered, the integrated model can simulate numerically the AAC system performance with and without the IHX. The results show that the higher evaporator inlet air temperature, higher compressor speed, higher condenser inlet air velocity, higher velocity of air at the inlet of evaporator and lower temperature of air at the inlet of condenser better performance. The deviation in results between the numerical and experimental investigations are less than 8% for a system without IHX and less than 15% for a system with IHX. The performance of R134a is better than the alternatives considered. The difference between R134a and R1234yf results are less than 15% without IHX and less than 10% with IHX. The difference between R134a and R1234ze(E) results are less than 33% without IHX and 25% with IHX. A noticeable decrease in both the power required for compression and the cooling capacity is observed in case of R1234ze(E). This indicates that in order increase the refrigerating capacity of R1234ze(E) a compressor with enhanced volumetric displacement should be used. According to the results of the current study, use of IHX is advantageous for both R1234ze(E) & R1234yf, and R1234yf performs better than R1234ze (E). The COP of R1234yf with and without IHX is on an average 9% and 5% lower respectively compared to R134a without IHX, and the COP of R1234ze(E) with and without IHX is on an average 5% and 3% lower respectively compared to R134a without IHX. The cooling capacity of R1234yf is on an average 8% less than the cooling capacity of R134a without IHX. When an IHX is interposed in the circuit, this difference is reduced to an average of 4%. The cooling capacity of R1234ze(E) is on an average 28% less than the cooling capacity of R134a without IHX. This difference is reduced to an average of 23% with the use of IHX for R1234ze(E). Even though R134a performed better, R1234yf with IHX is a better alternative in the current AAC system working with R134a without IHX, with only a slight compromise in the system's performance. Thus, if the AAC systems change to R1234yf with an IHX, the directives set out in the Kigali amendment of 2016 to Montreal Protocol (namely the discontinuation of HFCs for refrigeration) will be satisfied without any significant loss in the performance.