Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Abstract
Often in air conditioning and refrigeration vapor compression systems the working fluid is mixed with
lubricating oil, which flows through the system as it operates. The effects of this oil in the system include the
possible degradation of heat transfer, increased pressure drop, insufficient lubrication for the compressor and the
possibility of flooding the compressor suction ports with oil should the compressor valve fail. In order to
quantitatively evaluate these effects on the system, a better understanding of the flow of oil through various
operating conditions is required. The purpose of this investigation is the measurement of oil retention in small
diameter round copper tubes with various internal geometries. Test sections include an internally smooth, an axially
microfinned and a helically microfinned tube, all with an outer diameter of 9.53 mm (3/8???). Mass fluxes of 75 to
300 kg/m2s are tested, with an emphasis on 75 and 150 kg/m2s, and qualities of 0% to 100% are reached. The
refrigerant/oil mixtures examined include R134a with a polyol ester, R134a with a polyalkylene glycol, R134a with
an alkylbenzene, R22 with an alkylbenzene and R410A with a polyol ester. Additionally, it is important to examine
the void fraction and flow visualization of the mixtures to determine if the oil has an effect in these areas. Finally,
two models have been developed for oil holdup prediction. The first is based on the test section???s liquid volume
fraction and is used at mid to low-range qualities, while the second, with the Blasius turbulent flow formula as a
basis, is used to predict holdup at high qualities.Air Conditioning and Refrigeration Project 15
