Master's Thesis Thomas Horst


Experimental analysis of the spatial drop distribution in humidifiers of air-conditioning systems

In the present thesis the spatial drop distribution within the absorption section as part of the steam humidification of room air is analyzed in an experimental duct. Particle rate and velocity of the drops are measured by laser Doppler velocimetry at 81 specific positions with defined distances to the steam injector. Influencing factors and their impacts on the system are identified by a design of experiments approach. Therefore, both linear and square effects plus their interactions are considered. The operating parameters of the humidification system are defined by temperature T , volume flow V˙ , relative humidity change ∆φ, and the final relative humidity φ out at the end of the process, which are being varied stepwise.

The evaluation of the conducted series of experiments considers several properties of the vapor plume. Those are the spatial drop distribution and the maximum measured particle rate as well as the characteristics of condensation and evaporation.

Based on a graphical representation of the particle rates, the build-up of the vapor plume is analyzed qualitatively. Over the series of experiments repeating local maxima of the particle rate in the middle and front area at a short distance to the steam injector can be observed as well as a delay in the development of drops close to the top of the duct. The overall spreading of the vapour plume is primarily defined by the variation of φ out.

The evaluation of the design of experiments regarding the maximum measured particle rate shows a linear impact for all parameters. Nonlinear effects are of no significance. Interactions are also insignificant except for interactions between T and ∆φ. A simultaneous raise of both parameters reduces the number of detected drops. A major impact results from the final relative humidity. Thus, it is a main influence on the the particle rate.

The condensation process could not be fully explained by the analysis of the collected data. At first there is a spontaneous development of drops in the close range of the steam injector. In some of the experiments a temporal delay of the following evaporation can be observed. However, no direct correlation to the experimental parameters could be found.

When the process of condensation is finished an exponential reduction of the particle rate is observed. The exponential coefficient, determined by regression analysis, was therefore defined as an indicator of the decrement of detected particles. Raising φ out or ∆φ leads to a major decrease of the particle rate drop, while raising T causes a minor increase. Neither nonlinear nor interaction effects could be verified. Also the volume flow had no significant influence on the reduction of the particle rate.