Biswajit Mitra

Measurement of Condensation Heat Transfer Coefficients at Near-Critical Pressures in Refrigerant Blends

This paper presents the results of an experimental study on condensation heat transfer of refrigerant blends R404A and R410A flowing through horizontal tubes of 9.4 and 6.2 mm inner diameter at nominal pressures of 80% and 90% of the critical pressure. Local heat transfer coefficients were measured for the mass flux range 200<G<800 kg/m 2 -s in small quality increments over the entire vapor-liquid region. Heat transfer coefficients increased with quality and mass flux, while the effect of reduced pressure was not very significant within this range of pressures. The heat transfer coefficients increased with a decrease in diameter.

Heat Transfer and Pressure Drop for Condensation of Refrigerant R410A at Near-Critical Pressures

This paper presents the results of an experimental study of heat transfer and pressure drop during condensation of refrigerant R410A inside a horizontal 9.4 mm I.D. tube. The experiments were conducted at nominal pressures of 80% and 90% of the critical pressure of R410A. A novel technique that addresses the conflicting requirements imposed by the need for accurate measurement of low heat transfer rates for the small quality increments, and delineation of the tube-side heat transfer coefficient from the overall resistance, was used. Local heat transfer coefficients and pressure drops were measured for the mass flux range 200 < G < 800 kg/m2 -s in small quality increments over the entire vapor-liquid region, spanning several different two-phase flow regimes. These data were compared with the heat transfer and pressure drop data obtained under similar conditions for R404A by Jiang and Garimella [1]. The data were also compared with available correlations for condensation heat transfer according to the applicable flow regime to assess the validity of these models for refrigerant blend condensation at near-critical pressures. Possible explanations for deviations between the data and the models in the literature are also provided.Copyright

Supercritical Heat Transfer and Pressure Drop in Refrigerant R410A

This paper presents the results of an experimental study on heat transfer and pressure drop at critical and supercritical pressures of refrigerant R410A inside a horizontal 9.4 mm I.D. tube. Knowledge of heat transfer and pressure drop in such refrigerants blends at elevated pressures is gaining increasing attention for the design of vapor-compression space-conditioning and water heating systems at high heat rejection temperatures. Local heat transfer coefficients and pressure drops were measured for the mass flux range 200 < G < 800 kg/m2 -s for the temperature range from 30–110°C. A technique that simultaneously allows accurate measurement of low local heat duties and deduction of the tube-side heat transfer coefficient from the measured overall resistance was used. A primary cooling loop using water at high flow rates ensures that the refrigerant side presents the governing thermal resistance. Heat exchange with a secondary cooling water stream at a much lower flow rate amplifies the coolant temperature difference, which in turn enables accurate heat duty measurements. The results show that the heat transfer coefficient exhibits a sharp peak in the vicinity of the vapor-liquid dome. These data are compared with the most relevant correlations from the literature and possible explanations for agreement and discrepancies between the data and predictions are provided.Copyright