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Fully developed laminar flows in a semicircular duct with temperature-dependent viscosity variations in the flow cross section are analyzed, where the viscosity-temperature behavior is described by the Arrhenius model. Both the T and H1 boundary conditions are considered, as they represent the most fundamental heating/cooling conditions encountered in practical compact heat exchanger applications. Numerical solutions for the flow velocity and the temperature fields have been obtained by finite difference technique. The friction factor and Nusselt number results display a strong dependence on the viscosity ratio (μw/μb), and this is correlated using the classical power-law relationship. However, results indicate that the power-law exponents are significantly different from traditional values for circular tube. They are found to be functions of the flow geometry, boundary condition, and direction of heat transfer (heating or cooling).

Charge inventory, the accounting of the distributed fluid mass in closed systems, is necessary to predict system performance at different environmental conditions for a given charge. Public domain simulation models used to predict unitary equipment performance are currently unable to accurately determine charge inventory. Sources of error in these models include incomplete internal volume accounting, neglected refrigerant-oil diffusion effects, and void fraction modeling assumptions. The results presented here suggest that the most important charge inventory issue is proper void fraction determination. The Baroczy void fraction correlation gave the best agreement with measured data for three unitary air conditioners. As the condenser holds by far the largest percentage of the total charge, accurate prediction of heat transfer and pressure drop in the condenser was found to be necessary for charge inventory modeling.

Experimental results have been obtained for single-phase forced convection in deep rectangular microchannels. The microchannels were fabricated in a 2 mm thick silicon substrate by means of chemical etching. The tested configuration has 251 μm wide channels and 119 μm thick channel walls. The channel depth is 1030 μm and the channels cover a total projected area of 2.5 cm by 2.5 cm. A thin-film heater is deposited on the back side of the silicon substrate, corresponding to the entire projected channel area. The silicon substrate measures 2.9 cm by 2.9 cm, with only a 2 mm wide edge surrounding the channel area. All tests were performed with deionized water as the working fluid, where the liquid flow rate ranged from 5.47 cc/s to 118 cc/s. A critical Reynolds number of 1500 was found for this configuration, contrary to that of larger channels. The theoretical analysis as well as previous data found in the literature agree reasonable well with the experimental findings. This channel configuration has been shown to reduce temperature non-uniformity in the substrate compared to previous studies by utilizing relatively high flow rates. In addition, the theoretical analysis shows that increasing the channel depth can significantly improve the flow and heat transfer performance.

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 liquidvapor 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.