Wispy annular flow was first identified by Bennett et al. (1965). Their description of the regime was as follows: “There is, in this regime, a continuous, relatively slow-moving, liquid film on the tube walls, and a more rapidly moving entrained phase in the gas core. This description, of course, fits annular flow also, but the ‘wispy annular’ regime was characterized by the nature of the entrained phase. This phase appeared to flow in large agglomerates, somewhat resembling ectoplasm”. These observations have been confirmed by later workers (thought the ghostly allusion to “ectoplasm” is perhaps inappropriate!), but the regime has received surprisingly little attention, particularly since it covers a very wide range of conditions of practical interest. The regime is characterized by a number of features which include:
Large pressure fluctuations (Baker, 1966).
The existence, under some circumstances, of a maximum in pressure gradient near the onset of the regime and a pressure drop minimum near the close of the regime as the gas velocity is increased with a constant liquid velocity. This is illustrated by the results of Owen and Hewitt (1986) as shown in Figure 1.
The droplet mass transfer coefficient decreases with increasing concentration, which affects the deposition rate (Govan, 1990).
There is extensive gas entrainment in the liquid film (Bennett et al., 1965).
Figure 1. Pressure gradient maxima and minima associated with the occurrence of wispy annular flow (Owen and Hewitt, 1986).
The regime was included in the flow pattern map developed by Hewitt and Roberts (1969) (see Gas-Liquid Flow).
Recent work on wispy annular flow is reported by Hawkes and Hewitt (1995). They measured the power spectral density of pressure gradient fluctuations, and their results are typified by Figure 2. Two peaks appear in the power spectrum, one at a frequency of around 16 Hz (which corresponds to the presence of the normal “disturbance waves” in the annular flow (see article on Annular Flow). Another peak occurs at around 5 Hz, and this corresponds to the frequency of the wispy zones (characterized by highly absorbing dark zones in video pictures). Hawkes and Hewitt hypothesize that these zones may be related to “hold-up waves”, which are the inverse of the void waves formed in bubbly flow (see article on Bubbly Flow). Such hold-up waves are also found in gas fluidized systems at lower particle concentrations.
Figure 2. Power spectral density of pressure drop fluctuation in wispy annular flow (Hawkes and Hewitt, 1995).
Baker, J. L. L. (1966) Flow regime transitions at elevated pressures in vertical two-phase flow, Argonne National Laboratory Report No. ANL-7093.
Bennett, A. W., Hewitt, G. P., Kearsey, H. A., Keeys, R. K. F. and Lacey, M. P. C. (1965) Flow visualisation studies of boiling at high pressure, Proc. Inst. Mech. Eng., 180 (Part 3C) (1965–66), 1–11.
Govan, A. (1990) Modelling of vertical annular and dispersed two-phase flows, PhD thesis, Imperial College, University of London.
Hawkes, N. J. and Hewitt, G. F. (1995) Experimental studies of wispy annular flow. International Symposium on Two-Phase Flow Modelling and Experimentation, 9–11 October 1995, Rome.
Hewitt, G. F. and Roberts, D. N. (1969) Studies of two-phase flow patterns by simultaneous X-ray and flash photography, UKAEA Report AERE-M 2159.
Owen, D. G. and Hewitt, G. F. (1986) an improved annular two-phase flow model, Proc. Third International Conference on Multiphase Flow, The Hague, Netherlands, paper C1.