TWO PHASE CFD FLOW ANALYSIS OF R134a AND R1234yf REFRIGERANTS AND ITS EFFECT ON COOLING PERFORMANCE IN AN AUTOMOBILE HVAC EVAPORATOR CORE

The purpose of this research is to conduct a two-phase fluid analysis and simulation for examining the influence of refrigerant (R134a/R1234yf) properties on cooling performance of an automobile HVAC (Heating, Ventilation and Air Conditioning) evaporator core and compare their performances. In this research, streamline deviation, static pressure, velocity magnitude, pressure co-efficiency, Reynolds number and turbulent kinetic energy play a key role in the comparison of the refrigerants R1234yf and R134a two-phase fluids. Subsequently, it was found that R134a properties were better towards the cooling performance of an automobile HVAC evaporator core.

As stated by Hrnjak et al (2008), an evaporator is incorporated for heat exchange between the refrigerant and ambient air in the air conditioning system. The orifice tube or expansion valve inserts the refrigerant which is a highly pressurized liquid, into the evaporator, extending the refrigerant. The resultant evaporator core is left to the circumstance through the large evaporator surface and routed to the interiors of the vehicle through the airflow of ventilation. A defective evaporator indicates symptoms such as poor performance of cooling and ventilation and air conditioning system failure. Causes for evaporator failure are leakage in the evaporator at the connection that occurs due to damage, blockage of pipes in the evaporator and evaporator contamination disturbing the passage of air. Refrigerant R134a is mostly an adopted refrigerant in vapour compression cycles particularly in automotive systems for air conditioning. R1234yf was developed as a substitute for R134a in mobile systems for air conditioning (Sanaye et al, 2012).

Recognised Problems:
Condenser sub-cooling is considered as a practical problem in daily air conditioning and refrigeration systems. A temperature difference is needed between the refrigerant in the heat exchanger and surrounding air for allowing the heat exchange to occur. In standard systems, the temperature differs around 5°C between the humidity at the evaporator and evaporation and 20°C between the outdoor temperatures and condensing at the condenser. Outcomes in a lift of temperature around 40°C among the condensing temperatures and evaporating which is subsequent to about 9 bar of difference in the pressure that the compressor has to give in both the R1234yf and R134a systems (Incropera et al, 2006). Therefore this research intends to focus on influence of refrigerant (R134a/R1234yf) properties on cooling performance of an automobile HVAC evaporator core.

Aim:
Aim of this research is to investigate in detail the influence of refrigerant (R134a/R1234yf) properties on cooling performance of an automobile HVAC evaporator core.

Research Study:
Following are the course of investigations to be carried for the research: I S S N 2 3 2 1 -807X V o l u m e 1 3 N u m b e r 1 0 J o u r n a l o f A d v a n c e s i n c h e m i s t r y 5915 | P a g e F e b r u a r y 2 0 1 7 w w w . Pottker and Hrnjak (2012) empirically and numerically investigated the condenser sub-cooling effect on the performance of vapour compression systems. It demonstrated that, as sub-cooling of the condenser maximizes, COP (coefficient of performance) reaches as highest as an outcome of trade-off between maximizing effects of refrigerating and compression. Thermodynamic properties linked with maximum effect of refrigerating that is latent vaporization heat and liquid specific heat are dominant for determining the maximum COP enhancement with sub-cooling of the condenser. Refrigerants with large latent vaporization heat would gain less from sub-cooling of the condenser. For a typical air conditioning system, the numerical results demonstrate that R1234yf would gain the most because of their minimal latent vaporization heat from sub-cooling of condensers when compared to R134a, R410A and R717. In contrast, COP value maximizes the subcooling which does not prove to be a strong thermodynamic property. During the comparison of R134a and R1234yf, the numerical research for the operation of a condition, the system COP maximized up to 18% for R1234yf and 9% for R134a.
Lee and Jung (2012) conducted a brief comparison of R134a and R1234yf performance in a test bench for automobile applications. R134a and R1234yf performance comparison is carried out in a heat pump tester under the mobile airconditioning conditions. Outcome of the test demonstrates that the co-efficient of capacity and performance of R1234yf are up to 2.7% and 4.0% lower than that of R134a. Temperature of compressor discharge and charge amount of R1234yf is 6.5 o C and 10% lower than R134a. It was summarized from the outcomes that R1234yf is adopted as a long term solution which is eco-friendly in MACs because of its excellent properties for environment with acceptable performance.
Gomma (2015) theoretically and experimentally conducted a thermal performance of R134a automotive air conditioning. The performance of the R134a alternatives(R1234ze, R1234yf and R152a) is characterized by low global warming potential which is less than 150. From the findings of the research, it was noted that with speed maximization in the compressor, it gives a lower COP value for all the condensing temperature values. When the temperature for condensing is maximized by 5 o C, the capacity for cooling is minimized by 9%, and COP is minimized by 27%. The performance of R1234yf refrigerant is most similar to R134a refrigerant in all parameters. For all evaporating and condensing values, performance of highest co-efficient is acquired for R1234e among all examined refrigerants. From thermal and environmental performance perspective, R1234yf refrigerant has the best performance among all examined refrigerants. It was also noticed that for all examined refrigerants, maximizing the condensing temperature in order to elevate power consumption in the compressor, further maximizes the load of condenser at the same rate of refrigerant flow. Gaurav and Kumar (2015) studied about environmental sustainability of automobile system for air conditioning with Refrigerant R1234yf. This research summarizes that eco-friendly R1234yf refrigerant is the replacement of R134a in mobile applications for air-conditioning with temperature set for the indoors is between 20 o C and 24 o C . This is beneficial in minimizing R1234yf flammability since it minimizes the ratio of compression, load reduction and system temperature further minimizing the refrigerant leakage and oil from the environmental system. It was noticed that cooling time minimizes with speed maximization of the blower for both refrigerants. Similar to that, maximized rate of air flow at higher speed of blower minimizes the relative humidity when the temperature for cabin modifies from 320 o C to 240 o C. It was identified that cooling time is lesser when R1234yf refrigerant is used, in comparison with R134a and minimizes the range between 4% and 6%. There is no relative modification for the blower speed effecting relative humidity of R1234yf than R134a.

Research Design:
Two-phase fluid Simulations setting for two-phase fluid analysis are as follows: Types of the simulation are three dimensional and unsteady. Solvers in the two-phase simulation are implicit, pressure based and double precision. Multi-phase models in the two-phase simulation are 2-phase, implicit and mixture. Coupling of pressure velocity in two-phase fluid analysis is simply phase coupled. Turbulence model is k-e and mixture model.

Conclusion and Future Work:
Purpose of the research is to conduct two-phase fluid analysis on examining influence of refrigerant (R134a/R1234yf) properties on cooling performance of an automobile heating, ventilation and air conditioning evaporator core and compare their performance.
It was noticed that saturated suction temp (k), saturated discharge temp (k), cooling capacity (W), volumetric efficiency (%) was same as input. Output values of the refrigerant are as follows: The simulations and two-phase fluid analysis determined thatR134a was a feasible idea than R1234yf towards the cooling performance of an automobile HVAC evaporator core.
Extensive study can be carried out on other refrigerants i.e., R410a and R22. Comparison of their performances in the cooling of the automobile HVAC evaporator core.