Refrigerant flow distribution in vertical headers of reversible microchannel heat exchangers

An outdoor reversible microchannel heat exchanger, typically having vertical headers and more than two passes, may be used in either air-conditioning or heat pump mode in a reversible system. In heat pump mode, it functions as an evaporator and refrigerant maldistribution among parallel microchannel tubes creates unwanted superheated region, which lowers the heat transfer between refrigerant and air. This thesis presents the experimental and modeling study of refrigerant distribution in the vertical headers of outdoor reversible microchannel heat exchangers. The total and liquid mass flow rate in each microchannel tube is determined based on experimental measurement. Two-phase flow in the transparent vertical header is visualized by a high speed camera, in assist to understand the distribution among microchannel tubes. The effects of inlet conditions, header geometries and fluid properties on two-phase flow in the header and refrigerant distribution among microchannel tubes are examined. Visualization reveals that refrigerant distribution is related to the size of churn flow region in the header. The best distribution is at high mass flux and low quality because the churn flow region is largest and immerses all microchannel tubes at such inlet conditions. When mass flux is lower, the churn flow region is smaller and the distribution is worse due to lack of sufficient momentum to supply liquid to the top tubes. At higher quality, besides lacking sufficient momentum to lift liquid, the churn flow region is much smaller because the semi-annular flow appears at the bottom parts of the header. The liquid film in semi-annular flow bypasses the bottom exit tubes and makes these tubes get less liquid than other tubes in churn flow region; hence, the distribution at high quality is worse. Therefore, for good distribution, the momentum in upper region of the header should be high enough to lift liquid to the top and the momentum in lower region of the header (after the last entrance tube in the middle of the header) should not be too high to create semi-annular flow in the header. Besides increasing inlet mass flow rate, the high mass flux in the header may also be achieved by increasing tube protrusion or the number of tubes. These methods usually enlarge the churn flow region in the header and improve the distribution except at xin=0.8, when the distribution may be worse by increasing tube protrusion because the semi-annular flow region is larger and more bottom tubes are bypassed by the liquid film. The size of churn flow region in the header is also affected by fluid properties. For pure refrigerants, the churn flow region of R245fa is larger than that of R134a, R410A, and R32, mainly due to its low vapor density and high liquid-to-vapor density ratio. For refrigerant-oil mixture, the distribution is better at high oil circulation rate (OCR) because enough amount of oil generates lots of small bubbles/droplets and a layer of foams at the interface between liquid and vapor, which helps the mixing between the two phases and increases the size of churn flow region, respectively. Both factors improve refrigerant distribution. Based on experimental results, a distribution function is derived to model refrigerant distribution, by relating the liquid take-off ratio (i.e. the ratio of liquid mass flow rate in the tube to that in the header immediately upstream) with the inlet quality, vapor phase Reynolds number, and vapor phase Froude number in the header immediately upstream of the microchannel tube. A microchannel heat exchanger model, incorporating the distribution function, is developed to evaluate heat exchanger capacity reduction because of refrigerant maldistribution. The capacity degradation is up to 30% for R410A and 5% for R134a in a two-pass MCHX, compared to the uniform distribution case. The capacity degradation is related to the coefficient of variation of refrigerant maldistribution for both pure refrigerant and refrigerant-oil mixture, which is a function of the maximum liquid mass flux in the header. In the last part, the pressure drop in the header is investigated. The procedures to calculate the pressure drop in the header for single-phase vapor, single-phase liquid, and two-phase refrigerant are proposed based on experimental results