Probabilistic Flow Regime Map Modeling of Two-Phase Flow

The purpose of this investigation is to develop models for two-phase heat transfer, void fraction, and pressure drop, three key design parameters, in single, smooth, horizontal tubes using a common probabilistic two-phase flow regime basis. Probabilistic two-phase flow maps are experimentally developed for R134a at 25 ??C, 35 ??C, and 50 ??C, R410A at 25 ??C, mass fluxes from 100 to 600 kg/m2-s, qualities from 0 to 1 in 8.00 mm, 5.43 mm, 3.90 mm, and 1.74 mm I.D. horizontal, smooth, adiabatic tubes in order to extend probabilistic two-phase flow map modeling to single tubes. An automated flow visualization technique, utilizing image recognition software and a new optical method, is developed to classify the flow regimes present in approximately one million captured images. The probabilistic two-phase flow maps developed are represented as continuous functions and generalized based on physical parameters. Condensation heat transfer, void fraction, and pressure drop models are developed for single tubes utilizing the generalized flow regime map developed. The condensation heat transfer model is compared to experimentally obtained condensation data of R134a at 25 ??C in 8.915 mm diameter smooth copper tube with mass fluxes ranging from 100 to 300 kg/m2-s and a full quality range. The condensation heat transfer, void fraction, and pressure drop models developed are also compared to data found in the literature for a wide range of tube sizes, refrigerants, and flow conditions.Air Conditioning and Refrigeration Project 18

Investigation of Adiabatic Refrigerant Pressure Drop and Flow Visualization in Flat Plate Evaporators

Adiabatic pressure drop and flow visualization in chevron plate, 1:1 aspect ratio bumpy plate, and 2:1 aspect ratio bumpy plate heat exchangers were investigated for vertical upward flow with R134a. Qualities ranging from subcooled liquid to superheated vapor were investigated. Mass fluxes ranged from 16 kg/m2-s (for superheated vapor) to approximately 300 kg/m2-s (for sub-cooled liquid). The pressure drop experiments were conducted for 10o C and 20o C inlet temperatures. The flow visualization experiments were conducted at a 10o C inlet temperature. The following is the order of highest to lowest pressure drop geometries on both a mass flux and mass flow bases: chevron plate, 1:1 aspect ratio bumpy plate, and 2:1 aspect ratio bumpy plate. These trends are more pronounced on a mass flow basis. Four flow regimes were observed for the flat plate geometries investigated and are mapped out on a mass flux versus quality basis for each geometry. The chevron geometry was seen to undergo flow transitions at lower qualities and mass fluxes than the bumpy plate geometries. The kinetic energy per unit volume of the flow was found to have a strong linear relationship with pressure drop for both single-phase and two-phase flow, suggesting that inertial effects are the dominant mode of pressure drop in flat plate heat exchangers. Vapor pressure drop prediction models based on the kinetic energy of the flow are presented, which predict pressure drop within 20%. A two-phase pressure drop model is developed, also based on kinetic energy per unit volume of the flow. A pseudo void fraction is defined in order to correlate the two-phase pressure drop to the single-phase pressure drop. The two-phase pressure drop model predicts two-phase pressure drop to within 15% of experimental measurements. A description of and modifications to the experimental test facilities are provided. In addition, the geometries and construction of the plates are provided.Air Conditioning and Refrigeration Project 12

Investigation of an R134A Refrigerant/Iso 32 Polyol Ester Oil Mixture in Condensation

Air Conditioning and Refrigeration Project 12

An Investigation of Void Fraction in the Stratified/Annular Flow Regions in Smooth, Horizontal Tubes

Refrigerants R134a and R410A have been used for void fraction measurements in smooth horizontal tubes with diameters between 4mm and 7mm. Quality and mass flux were varied from 5% to 90% and 75 kglm2-s to 700 kglm2-s, respectively. Two test loops, one for condensing flows at 35C and the other for evaporating flows at 5C, were used in the investigation. Results show that near the transition from annular to stratified flow void fraction changed from viscousinertial dependence to gravitational-inertial dominated dependence. An important feature observed is the annular flow region's relative insensitivity to mass flux while the border region between annular and stratified flows is characterized by strong mass flux dependence.Air Conditioning and Refrigeration Project 7

The Behavior of a Near-Azeotropic Refrigerant Mixture of R-321R-125 in an Enhanced Tube

Condensation of a near-azeotropic mixture of 45% R-32 and 55% R-125 has been studied in a microfinned tube. The local behavior of heat transfer coefficient and the pressure drop was found to be a function of both quality and mass flux. The overall enhancement for a microfinned tube was determined to be a result of increased heat transfer combined with an increase in surface area due to the addition of the fins. Another important t result of this study was that the flow regime was found to be an important factor in characterizing the enhancement of the microfinned tube. The effect of the fins on the pressure drop was not readily apparent, but needs further testing in order to be determined.Air Conditioning and Refrigeration Center Project 3

Optimized Design of Refrigerant Condensers

A Microsoft Excel spreadsheet program developed previously at ACRC to optimize condenser size with respect to surface area has been modified. A new method of modeling pipe bends has been developed incorporating a new pressure drop correlation. Estimation of total liquid and vapor masses has been added. The results using this new, more accurate method of modeling pipe bends proved to vary almost negligibly from those obtained from the previous, simpler method, confmning the usefulness of the older version of this program with the addition of the new pressure drop correlation and the estimation of the refrigerant inventory. The optimization method is also applicable to evaporator design with appropriate modifications.Air Conditioning and Refrigeration Project 3

The Heat Transfer and Pressure Drop Behavior of a Zeotropic Refrigerant Mixture in a Microfinned Tube

Air Conditioning and Refrigeration Center Project 3

Review of Recent Research on Boiling and Condensation Heat Transfer With Mixtures

The fundamental physics and application of boiling and condensation of mixtures have attracted more and more attention. In this paper, the recent research in this field is reviewed. More than one hundred papers concerned with pool boiling, flow boiling, falling film evaporation, free and forced convective condensation of mixtures inside and outside tubes are summarized.Air Conditioning and Refrigeration Center Project 0

Optimal Sizing of Two-Phase Heat Exchangers

Air Conditioning and Refrigeration Center Project 0

Experimental Measurement and Modeling of Oil Holdup

Often in air conditioning and refrigeration vapor compression systems the working fluid is mixed with lubricating oil, which flows through the system as it operates. The effects of this oil in the system include the possible degradation of heat transfer, increased pressure drop, insufficient lubrication for the compressor and the possibility of flooding the compressor suction ports with oil should the compressor valve fail. In order to quantitatively evaluate these effects on the system, a better understanding of the flow of oil through various operating conditions is required. The purpose of this investigation is the measurement of oil retention in small diameter round copper tubes with various internal geometries. Test sections include an internally smooth, an axially microfinned and a helically microfinned tube, all with an outer diameter of 9.53 mm (3/8???). Mass fluxes of 75 to 300 kg/m2s are tested, with an emphasis on 75 and 150 kg/m2s, and qualities of 0% to 100% are reached. The refrigerant/oil mixtures examined include R134a with a polyol ester, R134a with a polyalkylene glycol, R134a with an alkylbenzene, R22 with an alkylbenzene and R410A with a polyol ester. Additionally, it is important to examine the void fraction and flow visualization of the mixtures to determine if the oil has an effect in these areas. Finally, two models have been developed for oil holdup prediction. The first is based on the test section???s liquid volume fraction and is used at mid to low-range qualities, while the second, with the Blasius turbulent flow formula as a basis, is used to predict holdup at high qualities.Air Conditioning and Refrigeration Project 15