Progetto ed ottimizzazione degli scambiatori di calore

Flow maldistribution in heat exchangers is a cause of reduction of thermal performances. Some maldistributions are the result of fabrication conditions, other ones are caused by the heat transfer and fluid flow process itself, fouling and/or corrosion, or the use of predisposed flows such as two phase flows. Two-phase flow distribution from a header to parallel channels is now becoming of interest in predicting the heat transfer performance of compact heat exchangers, evaporators and condensers. In case of distribution of a two-phase mixture in headers, the situation is more complex, owing to the differences in the thermophysical properties of each phase (density, viscosity and surface tension in particular). Additional problems exist for two-phase flow distribution because gravity tends to stratify the liquid and gas phases. In evaporators, a uniform distribution is essential in order to avoid dry-out phenomena with resulting poor heat exchange performance. In condensers, unequal distribution of liquid could create zones of reduced heat transfer owing to high liquid loading. The literature experiments have demonstrated important effects of many geometrical and operating factors: hydraulic diameter of the manifold (of square and circular cross-section); orientation of the manifold (horizontal and vertical) and orientation of heat exchanger tubes connected to the manifold (with upward or downward flow); intrusion depth of the channels into the header wall; length of the inlet pipe of the manifold; presence of nozzle at the inlet of the manifold. total mass flow to the manifold; inlet mass vapour fraction (inlet mass quality); heat load on the branches. The research in this field concerns the analysis of performance in phase distribution of prototypes using air-water mixtures in simulated evaporators. The experiments have been carried out by varying distributor geometrical configurations and operating conditions such as overall flow rate and inlet quality. The effects of geometric factors such as distributor length and shape, size of inlet port orifices, number of heat exchanger plates, new header fittings (protruding pipes, flute) have been analysed in order to find the critical parameters for flow distribution and to understand the improvable aspects of current distributors.

Publications

  1. A. Marchitto, F. Devia, M. Fossa, G. Guglielmini, C. Schenone, Experiments on two-phase flow distribution inside parallel channels of compact heat exchangers, Int. J. Multiphase Flow, Vol. 34, Issue 2, 2008, Pages 128-144.
  2. M. Fossa, A. Marchitto, A Simplified Approach for Predicting the Intermittent Behavior of Gas-Liquid Mixtures in Pipes, J. Fluids Eng.,Vol. 131,  Issue 1, 2009.
  3. A. Marchitto, M. Fossa, G. Guglielmini, Distribution of air–water mixtures in parallel vertical channels as an effect of the header geometry, Exp. Thermal and Fluid Science, Vol. 33, 2009, Pages 895–902.
  4. A. Marchitto, M. Fossa, G. Guglielmini, The effect of the flow direction inside the header on two-phase flow distribution in parallel vertical channels, Applied Thermal Engineering J., Vol. 36, 2012, Pages 245-251.
  5. F. Devia, A. Marchitto, M. Fossa, G. Guglielmini. CFD Simulations Devoted to the Study of Fitting Effects on the Phase Distribution in Parallel Vertical Channels, International Journal of Chemical Reactor Engineering. ISSN (Online) 1542-6580, ISSN (Print) 2194-5748, 2015.