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.
The mean condensate heat resistance of dropwise condensation with flowing, inert gases
The quantification of the condensate heat resistance is studied for dropwise condensation from flowing air-steam mixtures. Flows are essentially laminar and stable with gas Reynolds numbers around 900 and 2000. The condensate shaping up as hemispheres on a plastic plane wall and the presence of inert gases make it possible that thermocapillary convection occurs making the resistance less than the mean condensate thickness (ca. 0.185 mm) divided by the heat conduction coefficient. The analysis of experiments shows that the effective mean condensate resistance might indeed be less, by a factor of 0.8±0.2. The analysis takes account of the sensible heat transfer which may be as large as 35% of the total heat transfer if inlet vapor concentration,cin, is low (ca. 0.07). A method is presented to determine the gas-condensate interface temperature,ti, that is needed in the analysis of the heat resistance. The highest temperature differences(ti−tw),tw being the mean temperature of the condenser plate at the gas side, have been found to occur for relatively high values ofcin (ca. 0.3).ZusammenfassungEs wird der thermische Kondensationswiderstand bei Tropfenkondensation aus einem strömenden Luft-Dampf-Gemisch untersucht. Die Strömung ist laminar und stationär mit Gas Reynolds-Zahlen zwischen 900 und 2000. In Anwesenheit von Intertgas kann thermokapillare Konvektion am halbkugelförmigen Kondensattropfen auftreten, wobei der thermische Widerstand kleiner ist, als der Quotient aus der mittleren Kondensatdicke (ca. 0,185 mm) und dem Wärmeübergangskoeffizienten. Die Analyse der Experimente zeigt, daß der effektive mittlere Kondensatwiderstand tatsächlich um einen Faktor 0,8±0,2 kleiner sein kann. Der Energieanteil infolge Kondensatunterkühlung wird berücksichtigt, er kann bis zu 35% der gesamten Wärmeübertragung ausmachen, falls die Dampfkonzentrationcin am Eintritt klein ist (ca. 0,07). Zur Bestimmung der in die Berechnung des thermischen Widerstandes eingehende Temperatur an der Grenzfläche zwischen Gas und Kondensatti, wird eine spezielle Methode entwickelt. Die größte Temperaturdifferenz (ti−tw) ergibt sich bei relativ hohencin-Werten (ca. 0,3), wobeitw die mittlere Temperatur der Kondensatorplatte auf der Gasseite ist.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.