ABSTRACT
An analytical study on the heat transfer performance of a compact, microchannel water-carbon dioxide gas cooler was conducted and validated with experimental data. The gas cooler design under investigation used an array of serpentine microchannel tubes to carry refrigerant. The serpentine tubes were wrapped around water passages containing offset strip fins. The geometry led to a generally counterflow configuration between the two fluids. Unlike the isothermal heat rejection in subcritical two-phase heat transfer, or the constant property heat rejection for a single-phase fluid, cooling of supercritical CO2 results in a large temperature glide and rapidly changing fluid properties. The analytical model utilized a segmented approach to account for the steep gradients in the thermodynamic and transport properties of the supercritical CO2 refrigerant. The model predicted gas cooler heat duty with an average absolute error of 7.5% for data consisting of 126 points with varying refrigerant and water inlet conditions.
The segmented approach used in the model allowed a more detailed investigation of the heat transfer mechanisms within the gas cooler as the refrigerant properties varied. An investigation of multiple combinations of inlet conditions and gas cooler size reveal that the refrigerant-side thermal resistance is the overall limiting factor. However, the segmented model reveals that the steep change in supercritical CO2 properties near the pseudo-critical point result in a significant local decrease in refrigerant-side thermal resistance, which yields a sharp increase in local heat duty. The impact of this local property spike on gas cooler effectiveness, approach temperature difference and overall heat duty are analyzed using the segmented model for each case.
The analysis of this study can be used to better understand the heat transfer mechanisms resulting from this behavior, leading to optimized designs for CO2 gas coolers in heat pump water heater applications.
