Flow visualization and pressure drop for refrigerant phase change and air-water flow in small hydraulic diameter geometries

A comprehensive study of two-phase flow mechanisms and pressure drop in horizontal small hydraulic diameter tubes was conducted. Co-current flow of air-water mixtures in four round tubes and one rectangular tube with hydraulic diameters ranging from 5.5 mm to 1.3 mm were investigated. Bubble, dispersed, elongated bubble, slug, stratified, wavy, annular-wavy, and annular flow patterns were observed. The results of this work show that diameter and surface tension effects play an important role in determining the flow patterns and transitions between them. Flow mechanisms during condensation of refngerant R134a in small diameter round, square and rectangular tubes (0.506 mm < Dh < 4.91 mm) were also investigated. Flow mechanisms were recorded and categorized into intermittent, wavy, aimular, and dispersed flow over the entire range of qualities, and for five different refrigerant mass fluxes. As the hydraulic diameter is decreased, the influence of gravity diminishes and surface tension becomes more significant, thus promoting annular and slug/plug flow, and virtually eliminating the wavy flow regime. Transition lines between the flow patterns and regimes were established based on the experimental data. Many of the significant transition lines can be represented or approximated by constant Froude number lines, both for air-water mixtures, and refngerant R134a. This common non-dimensional basis for transitions in fluids of widely different phase properties could be useful for extending the transition criteria to other fluids, geometries and operating conditions. Two-phase pressure drop measurements were taken on a set of 5 circular tubes and on 7 non-circular tubes (triangular, square, rectangular, barrel, and "N" shaped extruded tubes). Frictional components of the total measured two-phase pressure drops were determined by accounting for the small contributions due to expansion/contraction at the headers, and the deceleration component due to momentum change. Reasonable agreement was found between the pressure drops measured in this study for the larger tubes and correlations in the literature. Flow regime-based pressure drop correlations were developed for the following regimes; intermittent and discrete wave flows, annular and disperse wave flows, annular/mist flow, and mist flow. For each of these regimes, one correlation accounted for the circular geometries, while equations of the same functional form were developed for the non-circular geometries. It was found that the circular tube correlations were able to predict 90 percent of the data within 20 percent, while 92 percent of the non-circular tube data were predicted within 20 percent. I CHAPTER ONE. INTRODUCTION Conventional air-conditioning systems use round-tube, plate-fin heat exchangers as evaporators and condensers to transfer heat between the refngerant and the indoor and outdoor air. The size of these heat exchangers required to deliver a desired heat load, and conversely, the heat load delivered by a heat exchanger of a specified size, depends primarily on the heat transfer coefficients of the refngerant and the air. The refngerant undergoes a liquid-vapor phase change in the condenser and the evaporator, and consequently the tubeside heat transfer coefficients are significantly higher than the air-side heat transfer coefficients, often by a factor of 20-30. The introduction of fins on the air-side, and the resulting additional surface area of up to a factor of 10-15 over bare tubes, counteracts this to a large extent. Thus, even though the air-side often continues to have the governing thermal resistance, the tube-side resistance is not negligible. With the heat exchanger cost representing 10 to 25 percent of the cost of air-conditioning systems, any potential improvements in the overall heat transfer conductances, without commensurate increases in pressure drop, are of immense value in decreasing capital and/or operating costs. Manufacturers have achieved cost reductions in conventional heat exchangers through modification and optimization of the geometric parameters such as the number of tubes, tube dimensions, air-side fin depth, height, thickness, shape and pitch. However, further attempts in this direction will at best result in incremental improvements. Automotive heat exchanger manufacturers have developed very compact flat-tube, multilouver fin heat exchangers to replace round-tube/flat-plate fin geometries used in engine cooling and air-conditioning systems. An example of a flat-tube, multilouver-fln heat exchanger is shown in Figure I. These novel heat exchangers offer the following advantages over conventional geometries: • Flat tubes offer a smaller frontal obstruction to air flow compared to round tubes, reducing drag and fan power • Rectangular tube-side passages with typically smaller hydraulic diameters offer higher tube-side heat transfer coefficients • Interrupted multilouver fins reduce boundary-layer resistances and thus result in larger air-side heat transfer coefficients • The above two factors result in a higher overall heat transfer conductance • By using optimized header configurations, the tube-side flow may be directed in several passes and rows, with the number of tubes per pass chosen to suit the heat transfer and pressure drop requirements and constraints