APPLICATION-BASED METHODOLOGY FOR ASSESSMENT OF FLOW BOILING CORRELATIONS IN MICROFIN TUBES

The high flow boiling heat transfer coefficients (HTCs) and comparatively low pressure drop of microfin tubes make them suited to meet the ever-increasing energy efficiencies and burgeoning raw material costs of HVAC & R systems. Since the late 1980s, many empirical or semi empirical correlations have been proposed to predict the in-tube boiling HTC for microfin tubes. A vast number of experimental studies have also been published for microfin tubes under different operating conditions with different refrigerants. However, due to the complex behavior of two-phase flows, it has been observed that no single correlation can predict the flow boiling HTC reasonably for the entire spectrum of geometric and operating parameters. The objective of the current study is to develop and provide researchers and practicing engineers with a predictive tool that will help them select the most accurate flow boiling HTC correlation for any practical HVAC & R application. Six popular and widely used correlations have been selected for investigation, which include Yu et al. [1], Thome et al. (as cited in [2]), Cavallini et al. [3], Yun et al. [4], and Chamra and Mago [5] and Wu et al. [6]. Each correlation has been carefully and critically assessed to discover its strengths, weaknesses and applicability to practical situations. In general, for tubes with root diameters greater than 5 mm, Thome et al. [2] and Cavallini et al. [3] predict the flow boiling HTC very well for CO2 and halogenated refrigerants respectively. For tubes with root diameters less than 5 mm, Wu et al. [6] is recommended. However, these recommendations can and do change with an alteration in flow or geometric parameters. Hence a tool is needed to measure the applicability of these correlations. The extensive experimental database collected for this research includes 1636 data points taken from 22 published articles. This wide-ranging database contains experimental points that incorporate all of the geometric and operating conditions that are used in the air-conditioning industry today. Traditionally, experimentation has been the principal means of ascertaining the HTC of microfin tubes. However, such testing is very costly and time consuming The innovative numerical methodology developed in this work provides the practitioner with a tool to quantify the performance of a correlation for a specific application. This approach has been tested by the authors for several experimental points that span the entire range of real world geometric and operating parameters. The use of this novel numerical procedure will save the HVAC & R industry significant experimental time and cost.