In Tube Evaporation of a Low GWP Zeotropic Mixture for Navy Chiller Applications

open_access
Open Access
Author:
Hayden, Lauren
Area of Honors:
Mechanical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
Brian Matthew Fronk, Thesis Supervisor
Anne Elizabeth Martin, Thesis Honors Advisor
Keywords:
In-tube evaporation
Zeotropic refrigerant
R471A
Low GWP refrigerants
Heat transfer coefficient
Navy chillers
Naval cooling applications
Counterflow heat exchanger
Evaporative heat transfer
Two-phase flow
Experimental study
Heat transfer correlation
Abstract:
This work analyzes the evaporative heat transfer characteristics of the low global warming potential (GWP) zeotropic refrigerant mixture, R471A, in a horizontal smooth tube. R471A has recently been suggested as a replacement for conventional HFC refrigerants in Naval cooling applications due to its low GWP and comparable thermophysical properties. However, phase change of zeotropic mixtures introduces mass transfer resistance, which can reduce heat transfer performance and make predictions complicated. Currently, there is limited experimental data for zeotropic mixtures in evaporating flow, motivating the need for this study. To determine the most accurate modeling method for zeotropic mixtures, the results from this work were compared with predictions from established heat transfer correlations from literature. Correlations developed by Gungor-Winterton, Guo, Mishra, Thome-GW87, and Zhang were chosen, with Gungor-Wintertonโ€™s predicting the most accurately with a Mean Absolute Percentage Error (MAPE) of 27.30%. Experiments were conducted in a 4.62 mm inner diameter and 6.35 mm outer diameter copper tube for dew point temperatures of 22ยฐC and 33ยฐC and mass fluxes of 300 ๐‘˜๐‘”/๐‘š2๐‘  and 500 ๐‘˜๐‘”/๐‘š2๐‘ . The refrigerant entered the test section as a subcooled liquid and exited as a superheated vapor, with counterflow ethylene glycol/water in the surrounding annulus with an inner diameter of 14.73 mm and an outer diameter of 21.59 mm. Measured temperatures, pressures and flow rates were used to calculate heat transfer coefficients along the length of the tube during evaporation.