Heat transfer , condensation and fog formation in crossflow plastic heat exchangers
-In this paper heat transfer of air-water-vapour mixtures in plastic crossflow heat exchangers is studied theoretically and experimentally. First, a model for heat transfer without condensation is derived, resulting in a set of classical differential equations. Subsequently, heat transfer with wall condensation and fog formation are considered in some detail. Separate attention is paid to the heat transfer and condensation of pure steam in the heat exchanger. Finally, the experiments performed are reported and the results compared with the models presented. From this comparison it can be learnt that the models are well able to predict the rate of heat transfer and phenomena such as condensation and fog formation.
Condensation of a vapour with incondensables: An improved gas phase film model accounting for the effect of mass transfer on film thickness
Abstract An improved gas phase film model is presented for calculation of local skin friction, heat and mass transfer coefficients in condenser design. The model accounts for the effect of the local mass transfer intensity on film thickness in turbulent flow. Predictions from the proposed model and from the classical ‘film theory’ and ‘condensation curve’ methods are compared with experimental data for a saturated steam-air mixture inside a 2 cm diameter tube under the following conditions: pressure, 1–1.5 bar; air mass fraction, 2–30%; inlet velocity, 5–50 m s −1 . Results indicate that for low values of the heat flux density (and for low values of the condensation rate) the three methods above are in excellent agreement with data. For high values of the heat flux density, the proposed model gives excellent agreement with the data but the classical methods do not.
Experimental Study for a Double-Glazed Forced-Flow Solar Collector/Regenerator
Experimental study of a double-glazed forced-convection solar collector/regenerator for absorption solar cooling is presented. The south facing experimental solar collector/regenerator with 10 deg slope is located at Kaohsiung, Taiwan at 120{degree}19minutesE longitude and 22{degree}34minutesN latitude. The size of the collector is 1 m wide and 7 m long with an effective regeneration area of 0.9 m by 6 m. Previous study for single-glazed forced-convection solar collector/regenerator operated at the same location has shown to have a best day-average efficiency of 17%. In order to raise the system performance, a double-glazed collector/regenerator is constructed such that air can be preheated in the upper channel flow. The preheated air is then conducted into the lower channel where it contacts with the film flow of solar heated lithium-chloride solution and regenerates the solution by carrying out the evaporated water vapor. The preheated air has lower relative humidity but the same humidity ratio since it is sensibly heated. Therefore, the regeneration driving potential is increased. The present study shows that the best day-average efficiency can reach 20% which increases the feasibility of the open-cycle absorption solar cooling system. Effects of controlling parameters on the collector/regenerator performance are studied, and heat and mass transfer correlationsmore » are also presented for design purposes.« less