Pointe Aux Piments , Mauritius HEAT REJECTION IN CONDENSERS CLOSE TO CRITICAL POINT-DESUPERHEATING , CONDENSATION IN SUPERHEATED REGION AND CONDENSATION OF TWO PHASE FLUID -

Conventional modeling of condensers always assumes three zones: desuperheating, condensation and subcooling even it is clear that condensation occurs in desuperheating zone at some conditions and that subcooling occurs during condensation. This paper discusses the actual situation and provides experimental validation of the hypothesis. The experimental results show heat transfer coefficients (HTC) of CO2 and R410A at mass fluxes 100 to 240 kg ms and reduced pressures from 0.68 to 1.00 in a horizontal smooth tube of 6.1 mm inner diameter. Data are compared to correlations proposed for other working fluids or other conditions. Results show much higher values of HTC than correlation proposed for single-phase turbulent flow in superheated zone. The occurrence of condensation in superheat zone is evident when tube wall temperature is below saturation temperature. The results suggest that simplified calculations of heat rejection in superheated zone significantly oversize the condenser. The semi-empirical correlation, which is here proposed as the combination of existing correlations for single-phase turbulent and saturated condensation, satisfactorily predicts the heat transfer coefficient of the superheat zone condensation. INTRODUCTION Balekjian and Katz [1] experimentally investigated condensation from superheated vapors of R114 and steam on outer surface of a horizontal tube. Their experimental HTC suggested the lowering cooling surface temperature, below saturation point, generates condensate from super heated vapor. Altman et al. [2] provided six points of experimentally determined HTC, which are averaged from various superheated inlet to saturated outlet in a test section of 1.22 m length 8.71 mm ID. With those six data having various superheat degree at test section inlet, approximately 30 to 70 % decrease in HTC from a correlation valid for two-phase zone condensation was confirmed. Bell [3] accepted the criterion of condensation occurrence in desuperheating zone is the tube wall temperature below saturation point and cautioned that the simple use of LMTD method to calculate the overall coefficient of condensers could be invalid. Miropolskiy et al. [4] provided experimental data for the quasi-local HTC of superheated steam vapor flowing downward in a cooled vertical smooth tube. The results experimentally presented the criterion of start of condensation which is when the tube wall temperature is below saturation point. Further, their data showed the behavior of superheat zone condensation for various reduced pressure until 0.82. Fujii et al. [5] experimentally investigated condensation of R11 and R113 flow in horizontal smooth tubes. From the temperature distribution in the radial direction of the horizontal middle plane of the tube, they proved coexistence of superheated vapor and subcooled liquid in condensation flow. They varied the vapor mass quality, which indicates actual vapor and liquid mass flow rate in non-equilibrium, to analyze the mass transport process. Lee et al. [6] experimentally investigated condensation in superheated R22 vapor and proposed a physical model accounting the sensible heat on the condensation heat transfer. Their model defines HTC bulk temperature as the reference temperature. In response, Webb [7] reported that sensible heat is negligible and simplified model defines HTC with saturation temperature gives same results to Lee’s model. Various systems and application drove operating conditions closer to the critical point, very often rejected heat in superheat zone is greater than in two-phase zone. Conditions just below the critical point are increasingly common for both R410A water heaters and CO2 commercial refrigeration systems in most frequent operations from autumn to spring. The authors [8] presented experimental results for only CO2 heat rejection (cooling/condensation) flow in 6.1 mm ID smooth tube at the pressure from 5.0 to 7.5MPa. The experimental results 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics