Water Nucleation Measurements in a Pulse-Expansion Wave Tube

Abstract

Wave experiments have been carried out in a pulse-expansion wave tube [1] to study water condensation, which plays an important role in a variety of industrial processes and in cloud formation models. We focused on homogeneous condensation, in which foreign bodies are absent and stable clusters (aggregates of molecules) are formed due to thermal fluctuations. Homogeneous condensation is a stochastic process of molecular collisions with a probability that colliding molecules stick together to form a condensation nucleus. Some of these nuclei will grow by catching other molecules to reach a so-called critical cluster size (nucleation) beyond which they grow to macroscopic sizes (droplet growth); other nuclei will simply evaporate into individual molecules again. The two underlying physical processes of condensation that can be distinguished are nucleation and droplet growth. Our pulse-expansion wave tube is specifically designed to produce a nucleation pulse [2] allowing to separate the processes of nucleation and droplet growth in time. We can therefore accurately measure the nucleation rate, which describes the number of water droplets that is formed per unit of time and space. Droplet growth is studied by combining two different optical techniques based on light scattering of a laser beam by the droplet cloud [3]. Once the droplet growth curve (droplet radius vs. time) is determined, the droplet number density follows from the measured extinction. The nucleation rate is finally obtained as the ratio of the droplet number density and the nucleation pulse duration. The homogeneous water nucleation rate is strongly dependent on the water vapor fraction of the gas–vapor mixture [4]. In previous work, the water vapor fraction was determined by the apparatus [5] that prepares these mixtures. In this work, we implemented two additional techniques to investigate the produced water vapor fractions. We shall present new experimental homogeneous nucleation rates of droplets of supercooled water (liquid water at temperatures lower than the equilibrium freezing temperature) in nitrogen at an elevated pressure of 1.0 MPa and a temperature of 240 K.