Characteristics of Refrigerant Film Thickness, Pressure Drop, and Condensation Heat Transfer in Annular Flow

Abstract

Results are presented for void fraction, pressure drop, and condensation heat transfer for refrigerants R-11, R-12, R-22, and R-134a over a range of mass fluxes, qualities, and tube sizes. General characteristics and trends in void fraction, pressure drop, and heat transfer for different refrigerants are discussed. Annular flow modeling relations are used to develop ratiometric equations for liquid fraction, pressure drop, and heat transfer that allow general trends to be predicted when the operating parameters of a refrigeration system are changed. INTRODUCTIONThe recent conversion to alternative refrigerants and the continued search for new refrigerants have resulted in a significant number of recent investigations exploring characteristics of refrigerants in condensation and evaporation (Schlager et al. these investigations are resulting in better models (e.g., Jung and Radermacher 1991, Dobson and Chato 1998, Kattan et al. 1998c) that are being built directly on an extensive refrigerant database rather than the models of the past that often relied on data from fluids other than "refrigerant-like" fluids.The purpose of this paper is to examine three primary characteristics of refrigerants during condensation: void fraction, pressure drop, and heat transfer. When a refrigeration system is designed, these are the characteristics that determine how much tubing is required (heat transfer), how much refrigerant will be required (void fraction), and how much power will be required to move the refrigerant through the tubing (pressure drop). Additionally, comparisons between refrigerants of the past and present are explored. These results are used to provide guidance for estimating how design changes (refrigerant, tube size, mass flow rate) may affect a system.A mechanistic model based on uniform annular flow in a tube is the basis for the modeling. Although refrigerants at condensation conditions tend to pass through a variety of flow regimes as they move from high-to low-quality conditions, the annular flow region tends to be the central region that dominates most in-tube condensation conditions and therefore provides a useful reference to explore the significance of deviations between predicted and experimental conditions as one moves into other flow regions.This paper is organized into three sections. First, a background section describes the uniform annular flow model. The model allows a consistent basis for predicting void fraction, pressure drop, and heat transfer. Next, results from model predictions are compared to experimental data from a past study of common older refrigerants (R-11, R-12, and R-22) and from a more recent 230 HVAC&R RESEARCH study of R-134a and R-22. Finally, some simple relations are developed to provide guidance in determining how a change of system parameters affects void fraction, pressure drop, and heat transfer. These relations are set up in a manner similar to the fan laws.

Refrigerant Condensation Flow Regimes in Enhanced Tubes and Their Effect on Heat Transfer Coefficients and Pressure Drops

“…These data suggest a liquid redistribution with the thickest and thinnest regions rotated in the direction of the (anti-clockwise) groove rotation. In their extensive and systematic study of liquid films in horizontal tube flow, Hurlburt and Newell [11] came to similar conclusions as the other researchers. Cho and Kim [10] performed circumferential temperature measurements in test sections with U-bends.…”

“…with the two-phase multiplier, , being that of Souza and Pimenta [56], given by (11) The liquid-only pressure drop is the normal Darcy-Weisbach equation, modified by Friedel [57] for two-phase flow:…”

Condensation in Minichannels and Microchannels

“…C Hurlburt and Newell (1999) developed scaling equations for condensing flows that would enable the prediction of void fraction, pressure drop, and heat transfer for a refrigerant at a given condition and tube diameter from the available results for another similar fluid (R-11, R-12, R-22, and R-134a) operating at a different condition in the diameter range 3À10 mm. To achieve this, they assumed that the bulk of the condensation process occurs under annular flow conditions, and that even though in reality the condensing film is nonuniform around the circumference, an equivalent average film thickness would suffice for the prediction of shear stress.…”

“…It was mentioned in the previous section that Hurlburt and Newell (1999) developed scaling equations for condensing flows that would enable the prediction of void fraction, pressure drop, and heat transfer for a refrigerant at a given condition and tube diameter from the available results for another similar fluid (R-11, R-12, R-22, and R-134a) operating at a different condition in the diameter range 3À10 mm. They developed an equivalent average film thickness model assuming annular flow, which enabled the prediction of shear stress.…”

An Investigation of Refrigerant Void Fraction in Horizontal, Microfin Tubes

“…Pressure drop, however, is primarily caused by the vapor core's interaction with the liquid film's turbulent layer above the viscous sublayer and buffer region. Hurlburt and Newell (1999) theoretically predict the thickness of these layers and the relative thennal resistances in the liquid film region.…”