Investigation on the boiling heat transfer characteristics of R404A under stratified flow conditions

Abstract

Regulations for the protection of the stratospheric ozone layer have led to the phasing-out of all chlorinated refrigerants including HCFC. The CFC free compounds, such as HFC (e.g. R134a) and their mixtures (e.g. R404A, R407C, R410A), have been proposed as long term substitutes for commercial low temperature refrigeration appliances. R404A a near otropic refrigerant mixture of R143a, R125 and R134a (52%:44%:4% by mass) has zero ozone depletion potential. R404A and R134a are the most widely used refrigerants in commercial appliances, such as bottle coolers, ice cream freezers, deep freezers, beer coolers and beverage dispensers. Much research work on the heat transfer characteristics of R134a has been reported, but very little research has been reported on R404A under low mass flow rate conditions. In this context, a detailed knowledge of the heat transfer characteristics of the refrigerant, R404A, boiling under low mass flow rate conditions, is necessary for an efficient design of heat exchangers. An experimental investigation on the horizontal flow boiling heat transfer of R404A under varied heat flux conditions is reported. The experimental facility consists of a coaxial counter current tube-in-tube heat exchanger to facilitate the flow boiling phenomenon in the inner tube. The tests are conducted in a commercially available smooth, horizontal, copper tube of inner diameter of 7.49 mm (3/8 inch). The necessary heat load is provided by acetone flowing in the counter current direction through the annulus of the test section. The effective length of the test section is 10 m and the entire heat exchanger is split into 20 subsections in order to estimate local heat transfer coefficient at every subsection. High accuracy instruments are connected to the test facility, to measure and log in the temperature, mass flow rate and pressure data.