Convective dropwise condensation out of humid air inside a horizontal channel – Experimental investigation of the condensate heat transfer resistance

A new facility feasible for the investigation of dropwise condensation at a vertical polymer surface (9 × 6 mm, L × H) is described in detail. The droplets are liquefied out of humid air, which flows through a rectangular channel (12 × 32 mm, W × H) and passes the sub-cooled surface of a sensor. The latter one is flush mounted into one of the vertical channel walls beyond a hydrodynamic entry length of 550 mm determining perpendicular heat fluxes by measuring temperature differences. Furthermore the droplet covered surface is observed through the opposite channel wall by an infrared camera. The variable experimental parameters include temperature, water content and flow velocity of the humid air, as well as the surface temperature of the sensor. The scope of the experimental study is to investigate the mean heat transfer resistance of the condensate droplets which form on the substrate surface. The procedure is based on the determination of heat fluxes and the evaluation of the temperature differences between the substrate surface and the liquid/gas interface by means of thermography, both of them carried out synchronously. The experimental droplet heat transfer coefficient, h d , is evaluated as an average value and it considers both heat conduction through the droplets and the augmentation of heat transfer due to inner convection. The parameter variation covers Reynolds numbers between 7900 and 21400, relative humidities between 56% and 95% and air temperatures between 30 °C and 46 °C for a broad range of cooling temperatures. The latter one influences the droplet surface temperature and together with the water content of the bulk, it determines the driving force for mass transfer. h d was found to increase with the Reynolds number and the driving force for mass transfer. However, increasing the latter one beyond a certain value causes massive accumulation of noncondensable gases at the liquid/gas interface inhibiting the vapour mass flux and leading to invariant droplet heat transfer coefficients. The experimental data is correlated providing the basis for the experimental investigation of the heat transfer at the liquid/gas interface which will be done in future.