An Experimental Investigation of Flow Patterns and Liquid Entrainment in a Horizontal-Tube Evaporator (1992)

An experimental study of two-phase flow patterns and droplet entrainment in a&#13;\nhorizontal-tube evaporator was conducted. The study is an element of the worldwide change&#13;\nfrom the current generation of refrigerants to chemicals with reduced ozone-depletion and&#13;\nglobal-warming potential. Measurements were made with serpentine aluminum and glass&#13;\nevaporators with geometries typical of those used for domestic refrigeration. The refrigerant in&#13;\nthe majority of tests was R134a (tetrafluoroethane), which will replace R12&#13;\n(dichlorodifluoromethane) for domestic refrigeration and automotive air-conditioning&#13;\napplications in 1995. Baseline data with R12 and R22 (chlorodifluoromethane, currently used in&#13;\nresidential air conditioners) were also recorded.&#13;\nThe phenomenon of primary interest was the non-equilibrium transport of liquid droplets&#13;\nwithin slightly superheated vapor at the evaporator exit. In sufficient quantity, such liquid would&#13;\nprove detrimental to evaporator performance by increasing the vapor superheat level at a given&#13;\nheat duty and reducing the effective temperature difference between refrigerant and air. Also of&#13;\nconcern were substantial variations in carry over rate with time, and corresponding fluctuations&#13;\nin exit temperature. These variations are due to the formation of slugs far upstream in the&#13;\nevaporator which rapidly transport a surplus of liquid toward the exit.&#13;\nA flow loop was constructed to circulate oil-free refrigerant through the evaporator under&#13;\nconditions spanning those of domestic refrigerator operation. Liquid carry over (expressed as a&#13;\ndimensionless entrained mass fraction, or EMF) was measured as a function of inlet quality, heat&#13;\nflux, mass flux and exit superheat level. A laser-based phase/Doppler particle analyzer was used&#13;\nto measure droplet diameters and velocities within an optical probe volume at the evaporator&#13;\nexit. Videotapes and still photographs of flow patterns within the glass evaporator were made.&#13;\nTwo types of experiments -- time-averaged and time-resolved -- were conducted. The former&#13;\nrevealed the quantitative dependence of EMF on the independent variables, while the latter&#13;\ndocumented the relationship between EMF and flow-regime transitions within the evaporator.&#13;\nTests with three refrigerants over wide ranges of operating conditions revealed time averaged&#13;\nEMFs of no more than 0.1 percent Thus, liquid carry over does not significantly affect&#13;\nthe performance of the evaporator as a heat exchanger. Analysis of variance (ANDY A) revealed&#13;\nexit superheat to have the strongest effect on entrained mass fraction, followed by mass flux,&#13;\ninlet quality and heat flux. Time-averaged EMFs varied with operating conditions by several&#13;\norders of magnitude, decreasing with increasing superheat level (due to lower entrainment rates&#13;\nnear the exit and more rapid droplet vaporization) and mass flux and inlet quality (due to a lower&#13;\nliquid inventory in the first few evaporator passes). An expression relating EMF to&#13;\ndimensionless forms of the independent variables was developed via nonlinear multiple&#13;\nregression.The incidence of slug flow within the evaporator and its effect on evaporator exit&#13;\nconditions were documented in the time-resolved experiments, using techniques such as autoand&#13;\ncross-correlation and Fourier transform. Time-resolved EMFs as high as one percent were&#13;\nobserved, as well as sharp reductions in exit superheat of as much as 8??C. These coincided with&#13;\nthe arrival at the exit of slug remnants in the form of entrained droplet clusters. The distribution&#13;\nof superheat readings with time was found to follow a Rayleigh probability density function.&#13;\nThis finding will facilitate the use of the superheat reading in control strategies, should it become&#13;\nnecessary to actively control refrigerant flow rate.