Flow Boiling Experiments of R-134A Refrigerant in a Pressurized System

This paper reports an experimental investigation of the effect of heat flux and system pressure on bubble nucleation of a refrigerant in a flow boiling environment. A high pressure test loop was built with easy physical and optical access to the test section. The test section is vertical with thermochromic liquid crystal (TLC) applied to the back of the heater for high resolution and accurate surface temperature measurement. Refrigerant R-134 is used as the test fluid at different pressures ranging from 690 to 827 kPa and different heat fluxes to quantify the nucleation process. Two synchronized high resolution and high speed cameras are used to simultaneously capture TLC images as well as bubbling events such as bubble nucleation, growth and departure at high frame rates (up to 1200 fps). In order to use TLC for temperature measurement, hue versus temperature relationship was first established through in-situ calibration. By varying flow rate and inlet temperature, TLC and bubble images were captured and analyzed. Results show that the bubble generation frequency and size increase with heat flux. An increase in pressure of 137 kPa (from 690 to 827 kPa) increased the bubble frequency and size by about 32 Hz and 20 μm respectively.