Search

Abstract: Recent reviews of flow boiling heat transfer in small tubes and channels have highlighted the need for predictive correlations that are applicable over a wide range of parameters and across different studies. A composite correlation is developed in the present work which includes nucleate boiling and convective heat transfer terms while accounting for the effect of bubble confinement in small channels. The correlation is developed from a database of 3899 data points from 14 studies in the literature covering 12 different wetting and non-wetting fluids, hydraulic diameters ranging from 0.16 to 2.92mm, and confinement numbers from 0.3 to 4.0. The mass fluxes included in the database range from 20 to 3000kgm−2 s−1, the heat fluxes from 0.4 to 115Wcm−2, the vapor qualities from 0 to 1, and the saturation temperatures from −194 to 97°C. While some of the data sets show opposing trends with respect to some parameters, a mean absolute error of less than 30% is achieved with the proposed correlation. [Copyright &y& Elsevier]

Abstract: Flow boiling of refrigerant HFC-134a in a multi-microchannel copper cold plate evaporator is investigated. The heat transfer coefficient is measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. The test piece contains 17 parallel, rectangular microchannels (0.762mm wide) of hydraulic diameter 1.09mm and aspect ratio 2.5. The design of the test facility is validated by a robust energy balance as well as a comparison of single-phase heat transfer coefficients with results from the literature. Results are presented for four different mass fluxes of 20.3, 40.5, 60.8, and 81.0kgm−2 s−1, which correspond to refrigerant mass flow rates of 0.5–2.0gs−1, and at three different pressures 400, 550 and 750kPa corresponding to saturation temperatures of 8.9, 18.7, and 29°C. The wall heat flux varies from 0 to 20W/cm2 in the experiments. The heat transfer coefficient is found to vary significantly with refrigerant inlet quality and mass flow rate, but only slightly with saturation pressure for the range of values investigated. The peak heat transfer coefficient is observed for a vapor quality of approximately 20%. [Copyright &y& Elsevier]

An important feature of detailed system simulation models for unitary air conditioners is the calculation of charge inventory. Void fraction determination in the two-phase regions of the heat exchangers is the primary challenge associated with charge inventory calculations. Annular flow is one of the predominant flow regimes encountered in horizontal heat exchangers. Analytical annular flow models typically fail to accurately represent void fraction. Thus, many of the available void fraction models are empirically based. To improve the prediction capabilities of void fraction models, a mechanistic void fraction model has been developed for annular flow in horizontal tubes. The present model considers the effect of momentum eddy diffusivity damping at the liquid–vapor interface. Two approaches are presented for determining the wall shear stress. The modeling results are compared to predictions from various void fraction models found in the literature. The present model is found to work well at moderate mass fluxes. [Copyright &y& Elsevier]