An Experimental Investigation on Flow Boiling of Ethylene-glycol/water Mixture

Mixtures of ethylene glycol and water are used in cooling the engines in automotive applications. To avoid the two-phase flow in the engine, the mixture is subcooled in the radiator before entering the engine block. Heat transfer is therefore essentially under subcooled flow boiling conditions. Very little information is available in the literature on the subcooled flow boiling characteristics of this mixture, and there is no predictive method established in this region. The present work focuses on obtaining experimental heat transfer data for mixtures of ethylene-glycol (0 to 40 percent mass fraction, limited by the maximum allowable temperature in the present setup) and water in subcooled flow boiling region. The experimental setup is designed to obtain local heat transfer coefficient values over a small circular aluminum heater surface, 9.5-mm in diameter, placed at the bottom wall of a rectangular channel 3mm x 40-mm in cross-section. The applicability of the available model for subcooled flow boiling of pure liquids to the mixtures is examined. NOMENCLATURE ” Bo Boiling number = q I G i,fg cP,r specific heat of liquid, J/kgK D12 diffusion coefficient of 1 (ethylene glycol) in 2 (water) Dh hydraulic diameter of the flow channel, m Fn Mass diffusion induced suppression factor, eq. (4) Fo fluid surface parameter in Kandlikar (1990) correlation G mass flux, kg/m’s i, latent heat of vaporization, J/kg 4” heat flux, W/m2 Re Reynolds number, pVD&t T temperature, K 1 V flow velocity, m/s Vt Volatility parameter, defined by Kandlikar (1998c), eq. (3) xt liquid mass fraction of ethylene glycol in aqueous solution yt vapor mass fraction of ethylene glycol in aqueous solution Greek Letters (r heat transfer coefficient, W/m2K a* a based on (T,-T,,,), eq. (1) K thermal diffusivity, =W(pc,r), m2/s h thermal conductivity, W/mK p viscosity, N-s/m2 p density, kg/m3 Subscripts conv = convective component 6 bulk fluid lo = liquid only nb = nucleate boiling component snf = saturation value tp = two phase w = wall INTRODUCTION A major application of flow boiling of ethylene-glycol mixtures is in automotive engine cooling. Although this mixture has been used for over several decades, there is very little information available in the open literature on its heat transfer characteristics under flow boiling conditions. The present work is aimed toward obtaining experimental data and characterizing the mixture effects on the heat transfer performance. Subcooled flow boiling of binary mixtures involves the combination of two phenomena that are quite extensively studied: subcooled flow boiling of pure components, and flow boiling of binary mixtures. A brief overview of these topics is presented here. Subcooled Flow Boiling of Pure Components Figure 1 shows Ute lteat transfer cltamcteristics during subcooled flow boiling of a pure liquid. Heat flux is plotted as a function of local wall temperature in this plot. The plot represents conditions existing at a certain location in a uniformly heated tube with const<anult liquid subcooling. As the heat flux is increased, path A-B-C-E-G is followed. Initially the lteat transfer is by single-phase tnode in Ute region A-C wltere an increase in the flow velocity or a decrease in local fluid temperature increases Ute heat transfer rate. At location C on the plot, the local wall superheat is sufficient to cause nucleation (Onset of Nucleate Boiling) under a given set of flow conditions (hysteresis delays nucleation to D in some cases). Following Ute ONE3 condition, heat transfer is by combined nucleation and convection modes in Ute region C-E (P,artial Boiling). Finally beyond E, Fully Developed Boiling (FDB) is established in the region E-G, and heat transfer is entirely by nucleate boiling mode. In Ute single-phase region A-B, the single-phase correlations apply, while specific correlations are developed for the FDB and p<artial boiling regions. Kandlikar (1998c) provides further details of various regions. The FDB correlation plotted in Fig. 1 represents a line that is approached as an asymptote front Ute partial-boiling region. Kandlikar (1998c) used tlte nucleate boiling component in the Kandlikar (1990) flow boiling correlation to predict the fully developed flow boiling heat transfer with pure liquids. He contpared experimental data from several sources with predictive ntodels by McAdams et al. (1919), Jens and Lottes (195 I), Tltom et al. (1965), Mikic and Roltsenow (1969), and Sltalt (1987) and found good agreement between ltis model and tlte data as well as Ute trends in their p~arametric relationships. Flow Boiling of Binarv Mixtures Flow boiling of binary mixtures consists of two components, convective and nucleate boiling. The convective component is sitttilar to that for pure liquids, and can be predicted from the existing pure contponent correlations using ntixhre properties. The nucleate boiling component, however, depends on Ute mlcleation of bubbles and their growtlt in a binary liquid. The difference in Ute liquid and vapor phase compositions during evaporation sets off a mass diffusion process presenting an additional resistance to lteat transfer. Tlte resulting suppression in the nucleate boiling component depends on the thermodynamic properties and Ute nahue of Ute vapor-liquid equilibrium curves (dew point artd bubble point) for Ute mixture. Kandlikar (1998a, b) present a review of Ute previous work done in pool as well as flow boiling of mixtures, and offer a contpreltensive treatntent of the phenomena. He classifies the level of heat transfer suppression into three regions: near azeotropic region (heat transfer clt‘aracteristics <are sintil<ar to tltat for a pure liquid), mild diffusion-induced suppression o5 LL 5 4 I I G I I I I FDB I I 1 sat Wall Temperature, Tw Fig. 1 Heat flux dependence on wall superheat at constant local subcooling during subcooled flow boiling region, and severe diffusion-induced suppression region. The correlations presented by Kandlikar (1998b) for these regions <are presented in this paper while discussing the results. Subcooled Flow Boiling of Etltvlene-Glvcol/Water and Propylene-Glvcol/Water Mixtures Finlay et al. (1987) conducted experiments with eUtyleneglycol/water mishtres covering an operating range appropriate to the automotive engine cooling conditions using copper and aluntinum tubing and cast-iron sections. Their results indicated a need for better predictive methods for correlating the heat tr~ansfer data, especially in the high heat-flux region (corresponding to FDB). Recently Utere ltas been an increased thrust to shift to propylene-glycol/water mixhtres. Propylene glycol is less toxic Utan etltylene glycol, possesses very similar heat transfer clt,aracteristics, ‘and appears to be an ideal replacement. Antbrogi et al. (1997) presented a comp‘arison of the two coolants under a wide range of engine operating conditions. McAssey et al. (1993, Bltowmick et al. (1996, 1997), and McAssey and Kandlik‘ar (1999) conducted experiments to compare Ute performance of propylene-glycoYwater and ethylene-glycol/water mixtures in engine cooling application. Tlte entire range from single-phase to saturated boiling was covered. Under Utese conditions, the performance wiUt the two mixhtres was very similar. A need for more reliable predictive methods for these mixtures was noted in these studies. OBJECTIVES OF THE PERESENT WORK In the present work, the subcooled flow boiling heat transfer performance for ethylene-glycol/water mixtures is obtained experimentally using a rectangular channel and a localized spot heater. The heat transfer characteristics in the fully developed boiling region is studied, and the FDB correlation for pure liquids is extended to binary mixtures by incorporating the mass-diffusion suppression effects.