Pure Refrigerants R134a Research Articles

FILM CONDENSATION OF SATURATED AND SUPERHEATED VAPORS ALONG ISOTHERMAL VERTICAL SURFACES IN MIXED CONVECTION

An analysis for condensation from an isothermal vertical flat plate in mixed convection is reported. The entire mixed convection regime is divided into two regions. One region covers the forced-convection-dominated regime, and the other covers the free-convection-dominated regime. The governing system of equations is first transformed into a dimensionless form by the nonsimilar transformation, separately for each regime, and then solved using the local nonsimilarity method along with a finite difference scheme. Two nonsimilarity parameters are introduced. The parameter xi f = Grx Re2 x characterizes the effect of buoyancy force on forced convection, while the parameter xi n = Rex Gr1 x /2 characterizes the effect of forced flow on free convection. Numerical results for pure steam and refrigerant R-134a are presented for both saturated and superheated cases. It is found that the buoyancy force significantly increases the wall shear stress and condensate mass flux. To a lesser degree, the buoyancy force also increases the wall heat flux. Superheating is found to have an insignificant effect on wall heat flux for a pure vapor.

Transport properties of refrigerants R32, R125, R134a, and R125+R32 mixtures in and beyond the critical region

A practical representation for the transport coefficients of pure refrigerants R32, R125, R134a, and R125+R32 mixtures is presented which is valid in the vapor–liquid critical region. The crossover expressions for the transport coefficients incorporate scaling laws near the critical point and are transformed to regular background values far away from the critical point. The regular background parts of the transport coefficients of pure refrigerants are obtained from independently fitting pure fluid data. For the calculation of the background contributions of the transport coefficients in binary mixtures, corresponding-states correlations are used. The transport property model is compared with thermal conductivity and thermal diffusivity data for pure refrigerants, and with thermal conductivity data for R125+R32 mixtures. The average relative deviations between the calculated values of the thermal conductivity and experimental data are less than 4–5% at densities ρ⩾0.1 ρ c and temperatures up to T=2 T c.

Transfert thermique pour la condensation du R123, du R134a et de leurs mélanges, en écoulement forcé entre deux plaques planes horizontales. Etude numérique.

Heat transfer for forced convection condensation of R123, R134a and their mixtures flowing between two horizontal parallel plates. Numerical study. Film condensation of pure and binary mixtures flowing between parallel plates is treated numerically. The coupled equations of mass, momentum, species and energy conservations for the two phases are solved with an implicite scheme. In this study, we retained the pressure forces, the liquid and vapor interfacial shear stress, the Dufour effect, the inertia and enthalpy convection terms, the turbulence in the two phases and the variation of the physical properties with the temperature and concentration. The results obtained for the condensation of refrigerants R123 and R134a, show strong influence of the composition of mixture on the mean heat transfer coefficient and the total pressure loss. The calculated mean Nusselt number is in good agreement with the experimental correlations of Mochizuki and Inoué [6] and Akers and Rosson [24]. A new correlation for the mean heat transfer for forced convection condensation of pure refrigerants R123 and R134a and their mixtures between horizontal flate plates is proposed.