(83l) Visualisation of Flow in Microchannel Array Water Evaporators by High-Speed Videography

In thermal and chemical process engineering evaporation is an important and well-known unit operation. It is done by conventional heat exchangers in most instances. Transition from water to vapour with these conventional devices is well known and understood, as well as flow patterns like bubbly flow, liquid ring flow, slug flow or annual flow in conventional channels. Thermal and chemical process engineering in an industrial environment requires heat exchangers or evaporators with exactly controlled phase transitions which generate strictly defined vapour quality. In many cases this will be obtained with conventional devices. In some applications the use of conventional devices is dangerous or may create some major process difficulties like runaway of the process or decomposition of liquid mixtures by exceed of a certain temperature limit. Since a couple of years microstructure heat exchangers became more and more important in thermal and chemical process engineering. The channels integrated into these devices are in the size range of a few hundred micrometers with related hydraulic diameters. The main advantage of microstructure devices compared to conventional heat exchangers is the very high surface-to-volume-ratio of up to several 10,000 m²∙m^-3, which is about one or two orders of magnitude higher than that of conventional devices [1, 2]. Therefore the heat transfer capability of microstructure heat exchangers is respectively much higher than the heat transfer capability of conventional heat exchangers. Due to the small hold-up and the overpressure resistance, which depends on material and manufacturing technique, microstructure heat exchangers may be well suited for high-pressure applications or dangerous fluids [3]. The evaporation process of liquids, e.g. water, in microstructure heat exchangers is not well-investigated yet. The transition from water to vapour in micro channels is accompanied by several phenomena like pulsation at sub audio frequencies, vapour clogging or the occurance of vapour slugs in micro channels [4]. These phenomena are widely observed but not investigated in detail. A technique to explore phase transition in micro channels (especially micro channel evaporators) is the investigation by high-speed videography [4, 5]. This method is used to study phase transition at the Institute for Micro Process Engineering (IMVT) of the Forschungszentrum Karlsruhe. A special test device which allows to look inside the micro channels was developed for this purpose. The device is heated electrically by conventional resistor cartridges, and the temperature inside the device is monitored by Type K thermocouples. A detailed description can be found in [6]. A photo of the device is shown in Fig. 1. In combination with a microscope, this test device allows a detailed view inside the micro channels. Therefore, it is possible to observe the flow pattern and the phase transition directly inside the micro channels applying visual methods. Optical analysis of flow pattern and phase transition inside micro channels is supported by digital high-speed videography. This method enables to slow down the recorded phase transition process for a staggering observation of phase transition in microstructure channels. It is also used to visualize the non-uniformity of flow in parallel micro channels. Information about the void fraction is approachable by extraction of pixel intensity from single pictures or picture strings by digital image processing. Exploration of vapour quality is assisted by a special laser photometer which was assembled at the IMVT. The functional principle of this laser photometer is based on measuring of total reflection by the outlet vapour steam. The work presented here focuses on exploration of phase transition in micro channels. Fundamental issues and dependency of phase transition to, e.g., geometric parameters in micro channels obtained by investigation with high-speed videography will be given. This method of investigation is also suited to provide information about formation and constituents of multiphase flow in micro channels, which will be shown in examples. This information is required to describe several processes in micro heat exchangers, e.g. phenomena during evaporation. Theory of known fluid dynamics will be used and, if necessary, modified for description of flow arrangement and regime. Another point of interest is exploration of vapour quality depending on microstructures, their dimensions and arrangement. It is required to investigate the composition of vapour at the outlet of the microstructure devices. Especially the content of liquid droplets and their size define the character of the outlet steam, e.g. saturated steam or superheated steam. A method of measuring the steam quality will be described, and first results will be given. These fundamental issues will lead to the development of special design rules for micro heat exchangers to generate well-defined vapour quality with stable and controllable phase transition. The design rules will also allow to control or to minimize phenomena like pulsation at sub audio frequencies, vapour clogging and the occurance of vapour slugs in microchannels. References [1] Brandner, J. J., Anurjew, E., Bohn, L., Hansjosten, E., Henning, T., Schygulla, U., Wenka, A., Schubert, K., Concepts and realization of microstructure heat exchangers for enhanced heat transfer. Experimental Thermal and Fluid Science 30, pp. 801?809, 2006. [2] Wörz, O., Jäckel, K. P., Richter, Th., and Wolf, A., Microreactors, A New Efficient Tool for Optimum Reactor Design, Proc. 2nd Int. Conf. on Microreaction Technology, pp. 183-185, 1998. [3] Schubert, K., Brandner, J. J., Fichtner, M., Lindner, G.,Schygulla, U., and Wenka, A., Microstructure Devices for Applications in Thermal and Chemical Process Engineering, Microscale Thermophysical Engineering, Vol. 5, No. 1, pp. 17-39, 2001. [4] Henning, T., Brandner, J. J., and Schubert, K., Comparison of microchannel array evaporators by high-speed videography. S. G. Kandlikar, ed., Proc. 3rd Int. Conf. on Microchannels and Minichannels (ICMM2005), Toronto/ON, Canada, June 13-15, pp. 128-136, 2005. [5] Henning, T., Brandner, J. J., and Schubert, K., 2004. High-speed imaging of flow in microchannel array water evaporators. S. G. Kandlikar, ed., Proc. 2nd Int. Conf. on Microchannel and Minichannels (ICMM2004), Rochester/NY, June 17-19, pp. 483-490, 2004. [6] Henning, T., Brandner, J. J., and Schubert, K., 2004. Characterisation of electrically powered micro heat exchangers. Chem. Eng. J., 101, pp. 339-345, 2004. Fig.1: Photo of a test device to observe flow pattern, phase transition and appearing phenomena during phase transition in micro channels. The wires in front of the device are thermocouples to measure temperature inside the microstructure device.