| The thermal-hydraulic behavior of folded-louvered-fin, microchannel heat exchangers is explored under conditions of air-side frosting, defrosting, and refrosting. The temperature distribution within a two-dimensional composite fin is analyzed. A parametric analysis shows that for some conditions, such as those typical to frost-coated fins, the problem can be approximated as a two-dimensional slab on a one-dimensional fin. Under this approximation, an exact solution to the heat diffusion equation is obtained through an eigenfunction expansion. The analytical solution and a one-term approximation to the full solution have broad applicability in addition to their use for calculating fin efficiency for frost-coated fins.; Valid HA-LMED and UA-LMTD methods for wet- and frosted-surface heat transfer are formulated. The UA-LMTD method is shown to provide the best results for dry, partially-wet/frosting, and fully-wet/frosting conditions. Without area partitioning, the HA-LMED method is only applicable to fully-wet/frosting conditions. For all the conditions considered in a parametric study to mimic the experimental range of this work, the UA-LMTD method provides the value of the air-side convective heat transfer coefficient within 3% and is more accurate than the HA-LMED method.; Heat transfer and pressure drop data for nine different fin geometries are presented, and a decrease in the overall heat transfer coefficient and an increase in the pressure drop are observed as frost accumulates on the surfaces. A reduction in air-side flow rate and bridging of louver gaps by frost are identified as the factors most important to the reduced heat transfer performance. Correlations are presented for predicting the thermal performance of these heat exchangers under frosting conditions.; A numerical model is developed to predict the time-varying performance of folded-louvered-fin, microchannel heat exchangers. The model utilizes the correlations developed from the experimental data and incorporates a sub-model for frost properties. The model successfully predicts the heat transfer performance of the heat exchangers studied, but its ability to predict the pressure-drop behavior needs further improvement. The model can be used to evaluate geometry effects on the frosting behavior of the louvered-fin, microchannel heat exchangers, and can be easily generalized to other applications with simultaneous heat and mass transfer. |
