Two-Phase Heat Transfer Mechanisms within Plate Heat Exchangers: Experiments, Modeling and Simulations

Summary :

Plate heat exchangers are adopted in many domestic and industrial applications, such as ventilation, air conditioning, evaporation or condensation process, heat pumps and cooling of hydrodynamic circuits in engines. In the present thesis, an extensive experimental study to characterize thermal and hydraulic performance of a compact plate heat exchanger has been carried out and then follow up with proposition of new prediction methods for single-phase and evaporating flows in the plate¿s channels, in the form of general correlations that include large additional databases culled from the literature.

Specifically, upward single-phase flow and flow boiling heat transfer of low pressure liquid refrigerants were investigated within a plate heat exchanger prototype fabricated with 1 mm pressing depth and chevron angle of 65°. High spatial and temporal resolution infrared measurements were implemented to obtain local (pixel-by-pixel) heat transfer coefficients and frictional pressure drops. Single-phase experiments were carried out for Reynolds numbers ranging from 34 to 1615 and Prandtl numbers from 4.9 until 6.5, and the associated effect of the main involved parameters on the thermal and hydraulic performance was analyzed in detail. Several of the most quoted prediction methods available in the literature were statistically evaluated against the present heat transfer and pressure drop database and a new models were proposed to predict mean and local thermal and hydraulic performance of the present compact plate heat exchanger. Two-phase experiments were carried out for mass fluxes ranging from 10 kgm-2s-1 to 85 kgm-2s-1, imposed heat fluxes from 225 Wm-2 to 4100 Wm-2, saturation temperatures from 19°C to 35°C and vapor qualities from 0.05 to 0.90. The heat transfer coefficient increased with mass flux, heat flux and saturation temperature (system pressure) whilst the frictional pressure drop rose with mass flux and vapor quality but decreased with saturation temperature. Based on local infrared measurements, a new correlation for predicting local frictional pressure gradient through the test section was proposed.

Next, a wide experimental databank was culled from thirteen literature research studies, which investigated flow boiling heat transfer and two-phase frictional pressure drop within chevron plate heat exchangers. The database was first adopted to validate the predicted capabilities of 29 literature models, and then utilized to provide the new prediction methods to evaluate local heat transfer coefficients and frictional pressure gradient. These new models were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total) and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The general flow boiling prediction methods have been programmed into a simulation code to analyze local thermal and hydraulic performance of the plate heat exchangers over a large range of test conditions. The simulation results were then validated against independent experimental database from literature, resulting in very good agreement between each other. The present simulation code represents a powerful tool to be used for practical applications by the thermal engineers in order to design and rate commercial plate heat exchangers.