In the second part of this study a sensitivity analysis on the prediction methods is performed to consider the effect of plate geometry on thermal-hydraulic performance and an extensive comparison of all the two-phase pressure drop and flow boiling heat transfer prediction methods available in the open literature is also performed versus the large diversified database presented in Part 1. The experimental databank, from numerous independent research studies, is then utilized to develop the new prediction methods to evaluate local heat transfer coefficients and pressure drops. These new methods were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total), respectively, and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The prediction for flow boiling heat transfer coefficients was broken down into separate macro-and microscale methods. Keywords Dimensional analysis Flow Boiling General heat transfer prediction method General pressure drop prediction method Multiple regression technique Plate heat exchangers Contents Nomenclature Introduction Sensitivity analysis Comparison of prediction methods Statistical comparison of prediction methods to data Dimension analysis and new flow boiling heat transfer and pressure drop prediction methods Conclusions Conflict of interests References
Infrared (IR) thermography was used to measure the local heat transfer coefficients within two plate heat exchanger geometries. The chevron patterns were machined into polycarbonate and IR transparent calcium fluoride plates, both of which were electrically heated using flexible film heaters at heat fluxes up to 0.8 W cm-2. The test fluid was a refrigerant (HFE7100) at mass fluxes between 25 and 100 kg m-2 s-1 , and qualities from 0 to 0.9. The apparatus and data reduction technique were validated by comparing the single-phase heat transfer and pressure drop data against the prediction methods from the literature. Adiabatic flow visualizations were conducted to link the flow patterns with the observed heat transfer. The frictional pressure gradient and heat transfer coefficient were compared with available correlations. It was shown that the heat transfer coefficient and the frictional pressure gradient increased with mass flux and quality. The comparison indicated the need for new prediction methods for predicting the local thermalhydraulic performance over a wide range of operating conditions.
This article is the second in a two-part study on two-phase flow of R245fa in a new promising and more compact plate heat exchanger (PHE). Two-phase adiabatic frictional pressure drops were detailed in Part I, while the flow boiling heat transfer coefficients were obtained and the results presented here in Part II. Upward flow boiling heat transfer was investigated locally within the PHE prototype using an IR camera to measure the local PHE wall temperatures with high resolution. A new experimental approach for PHEs was introduced in order to reduce the data and obtain the local pixelby-pixel flow boiling heat transfer coefficients throughout the PHE. The influences of heat flux, mass flux, saturation temperature, vapor quality, and inlet condition on flow boiling heat transfer coefficients were obtainable. During the tests, the vapor qualities changed from 0 to 0.78 for mass fluxes from 7.5 to 40 kg m −2 s −1 , heat fluxes from 250 to 3700 W m −2 , and saturation temperatures from 19 • C to 35 • C. In addition, comparative flow boiling experiments were carried out with a small subcooling and then repeated with liquid-vapor (xin ≈ 0.1-0.2) flow at the PHE inlet. Several of the most widely used prediction methods in the PHE literature were evaluated with respect to the current mean and quasi-local (obtained in six windows) experimental databases. The prediction methods mostly showed a better agreement with the mean flow boiling heat transfer coefficients rather than the local database.KEY WORDS: two-phase flow, plate heat exchanger, flow boiling heat transfer, pressure drop, R245fa
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