Boiling Heat Transfer of R-134a and M09 Refrigerant Mixture in a Stratified Flow – A Comparison Under Varied Heat Flux Condition


B. Raja1, D. Mohan Lal2,a and R. Saravanan2,b

1IIITDM-Kancheepuram, IIT Madras Campus, Chennai, India.

rajab@iiitdm.ac.in

2R&AC Division, College of Engineering, Anna University, Guindy, Chennai, India.

amohanlal@annauniv.edu
brsaravanan@annauniv.edu

ABSTRACT

The boiling heat transfer coefficients of R-134a and M09 zeotropic refrigerant (R-134a/R-290/R-600a: 91% / 4.068% / 4.932% by mass) were evaluated in a smooth horizontal tube of 9.52 mm diameter. The experiments were conducted for similar working pressure, reduced pressure and liquid entry temperature of the test fluids. The conditions were selected to mimic the evaporators of refrigerators and deep freezers, which operate under varied heat flux (2 to 8 kW m-2) and low mass flow rate (3 to 5 g s-1) with stratified flow patterns. The varied heat flux to the test fluids was provided using a coaxial countercurrent heat exchanger test section with the test fluids flowing in the inner tube and the acetone, the heating fluid, flowing in the annulus. The experiments revealed that in the aforementioned evaporators, between the two mechanisms of flow boiling, which are the nucleate boiling and convective vaporization, the former played an important in the magnitude of the heat transfer coefficient. Further, unlike high mass flow rate conditions, the presence of nucleation was witnessed in both the test fluids even at higher vapor qualities. Under similar working pressure and reduced pressure, the heat transfer coefficients of R-134a and M09 at low vapor qualities were found to be almost equal. However, at higher vapor qualities the values of M09 were found to be 15 to 40% higher than that of R-134a. Similarly, under the same liquid entry temperature, the heat transfer coefficients of M09 were lower than R-134a at lower vapor qualities by 20 to 40% and higher than R-134a at higher vapor qualities by 15 to 35%. The presence of low-density hydrocarbons in M09 and thus, the increased vapor velocity was reasoned for the higher values of M09 at higher vapor qualities.

Keywords: Flow boiling, Refrigerant mixture, Heat transfer coefficient, Stratified flow, Varied heat flux.


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