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

Abstract

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