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Some knowledge of flow regime is necessary to assess flow‐induced vibration in two‐phase flow. Flow regimes are governed by a number of parameters, such as surface tension, density of each phase, viscosity of each phase, geometry of flow path, mass flux, void fraction and gravity forces. Flow regime conditions are usually presented in terms of dimensionless parameters in the form of a flow regime map. Grant (1975) and Grant and Chisholm (1979) used available data to develop the flow regime boundaries shown in Fig. 2-5. The axes on the Grant flow regime map are defined in terms of a Martinelli parameter, X, and a dimensionless gas velocity, U_g. The Martinelli parameter is formulated as follows:

(2‐7)

where μ_ℓ and μ_g are the dynamic viscosity of the liquid and gas phases, respectively. The dimensionless gas velocity is defined as follows:

(2‐8)

where is the pitch mass flux of the gas phase, d_e ≅ 2(P − D) is the equivalent hydraulic diameter and g is acceleration due to gravity.

The Grant map of Fig. 2-5 shows three flow regimes: spray, bubbly and intermittent. The terms spray and bubbly are used loosely here. Perhaps they would be more appropriately defined as “continuous flow” covering the whole range from true bubbly flow to wall‐type flow to spray flow. Intermittent flow is characterized by periods of flooding (mostly liquid) followed by bursts of mostly gas flow. As discussed by Pettigrew et al (1989a) and Pettigrew and Taylor (1994), this is an undesirable flow regime from a vibration point‐of‐view. Thus, intermittent flow should be avoided in two‐phase heat exchange components and, particularly, in the U‐bend region of steam generators.

Flow regime is discussed in more detail in Chapter 3.