Scction zyx 9-25 zyx S569 M.E.H. VAN DONGEN, G. LAMANNA, B. PRAST Condensing nozzle flows: Ludwieg tube experiments and numerical/ theo- retical modelling. zyxwvu The present paper deals with homogeneously condensing flows of humid nitrogen in a Lava1 nozzle. The modelling of nonequilibrium condensation phenomena can be separated in two distinct processes: homogeneous nucleation and droplet growth. Our objective is to investigate the quality of a condensation model characterised by the following combination: the (corrected) InternaEly Consistent Classical Theory for the nucleation process and a generalised transitional growth model, with the droplet temperature calculated explicitly via the wet-bulb equation. Our theoretical predictions have been then compared with our experimental results on droplet sizing showing a good agreement. 1. Introduction Supersonic nozzle flows of a condensable gas mixture are characterised by the spontaneous generation of a liquid droplet cloud, whose properties strongly depend on the coupling between the flow and the condensation process itself. Further, depending on the initial conditions, different flow regimes may occur ranging from steady to periodic oscillating motions. Along the years, many different models have been proposed and verified thoroughly with respect to the onset of condensation, shock position, frequencies, and modes of oscillations [1,2,3]. However no conclusive answer could be drawn due the lack of reliable experimental data on droplet sizes. This latter, in fact, constitutes a very sensitive parameter for assessing the quality of the proposed condensation models. The scarcity of reliable data is ascribable to the difficulties of retrieving the size information from the spectral data of a nanometre-size cloud. At the gas dynamics laboratory of Eindhoven University of Technology, a facility has been developed to determine the time dependent variation of the size distribution by means of a white light extinction method. Objective of this paper is to use these experimental results to validate condensation models and to corroborate the validity of the underlying theoretical assumptions. The modelling of nonequilibrium condensation can be separated in two distinct processes, namely homogeneous nucleation and droplet growth. Therefore, its correctness relies simultaneously on the quality of the nucleation and growth model employed. The present paper focuses essentially on this latter. By comparing two different growth laws, the importance of simulating correctly the energy flux between the droplet and its environment is ascertained. The quality of nucleation models is, instead, more difficult to evaluate and is, therefore, only marginally addressed here [4]. It is important to realise, in fact, that significant uncertainty exist with regard to the surface tension of subcooled liquid water. This uncertainty makes the assessment of nucleation models extremely critical, since it strongly depends on the extrapolation of surface data to low temperatures. Here the analysis is limited to a temperature range of [245 + 2701 K, where accurate experimental data are available from literature [5]. 2. Modelling On the basis of Luijten's experiments [6] on water-nitrogen systems, the Internally Consistent Classical Theory (ICCT) was chosen to model the nucleation process. This latter differs from the Classical Nucleation Theory (CNT) for the presence of correction factors, as indicated below: 1 zyxwvu S J = -expO JCNT; where S = p,/p"(T) is the supersaturation, zyxwvu cr is the surface tension, a0 is a molecular surface area, Icg is the Boltzmann constant, and 0 is a dimensionless surface tension. For an exhaustive review on nucleation models, the reader is referred to Luijten et al. [7]. Upon implementing the (ICCT) model to condensing nozzle flows, it resulted that a satisfactory agreement was found with a correction factor of about 0.01. Thus the implemented nucleation rate is JICCT = [ . J, where < = 0 The process of droplet growth involves the net transfer of mass (vapour molecules) towards the droplet, and the