The fluid flows in the globe valves are driven by the pressure gradient in the established numerical model。 Pressure at the inlet boundary is higher than at the outlet boundary。 The inlet gauge pressure  is  set  as  100  bars  and  the  outlet  gauge  pressure   is 1 bar, which is close to the standard atmosphere pressure in the established numerical model。 The wall function is used to deter- mine the wall boundary condition。 In practice, the cavitation phe- nomenon in a globe valve is unsteady。 Therefore, the numerical model in this study adopts the second-order Crank-Nicolson scheme to simulate the transient  complex  flow  patterns inside the globe valve。 The time increment is set as 0。1 s in this study, which satisfies the Courant-Friedrichs-Lewy (CFL) condition。 Also, another time increment 0。05 s is used and the pressure drops predicted by those two increments are the same。 The total physical time used in each simulation is up to 10    s。

JOURNAL OF ENERGY ENGINEERING © ASCE / MARCH 2013 /  27

Cavitation occurs in the region whose pressure is less than the saturated vapor pressure inside the globe valve。 When the cavitation mode is chosen in STAR-CCM, it is necessary to specify the saturated vapor pressure of water。 It is set as 2,338 Pa at the room temperature of 20°C in this study。 In this model, Vf is set as 0 at the inlet and outlet boundaries, and it is assumed that there is no vapor bubble at both sides。

A polyhedral cell is adopted in the mesh used in the simulations。 Cells in the vicinity of the globe valve and the valve itself are refined。 Fig。 3 demonstrates the 3D computational cells utilized in the numerical simulation。 The number of total cells varies from 4 × 105 to 9 × 105 because of the different valve openings and cages。 To understand the influence of cells, a variety of computa- tional cells—405,215; 505,414; and 767,851—are used to simulate the flow in a fully open globe valve。 Table 1 shows the grid inde- pendency tests for those cells。 Because the pressure drop between

101。6 mm (2D) in front of the valve and 304。8 mm (6D) behind the valve is the most important result in characteristic coefficients; that is the reason for comparing the pressure drop predicted by various cells。 In terms of the predicted pressure drop, it is clear that results using 505,414 and 767,851 cells have close pressure drop values。 To reduce the total calculation time, 505,414 cells are adopted in the following numerical simulations。

A Windows-based PC is utilized to perform the numerical simulations in this study。 The CPU and memory of the PC are

Fig。 3。 3D computational mesh including the piping system, the globe valve, and the cage; inlet flow is in the   X-direction

Table 1。 Grid Independency Test of Various   Meshes

Intel(R) Core(TM) i7 930 2。80 GHz and 4 GB RAM, respectively。 The average total calculation time for each case is approximately 6–7 days。

Results  and Discussion

Analysis of Flow Field

Three globe valve models with or without cages are considered in this study。 All physical quantities reach stead states at the final instant。 Flow patterns are analyzed at the longitudinal X—Y plane of the globe valve in the z-direction。 The following sections provide analysis of the pressure contours and streamtraces, turbulence kinetic energy, prediction of cavitation, pressure drop along a streamtrace, and percentage of vapors as the globe valve is either fully open or half  open。

Flow Patterns and Pressure  Contours

Fig。 4 depicts flow patterns and pressure distributions in fully open globe valves without and with cages。 Streamtraces are plotted by massless particles placed in arbitrary locations in Fig。 4。 A vortex is obviously visualized by the streamtraces in Fig。 4(a) because the main flow passes around the plug, deflects downward, and im- pinges the valve bottom。 As a result, a high pressure region exists at the valve bottom。 It reveals that the impinging flow may ruin the valve bottom when no cage is installed at the globe valve。 Also, a pair of counter-rotating vortices exists at the downstream region as shown in the Y—Z plane of Fig。 4(a)。 Low pressure is observed in the counter-rotating vortices。 Another high pressure region exists below the plug because fluids flow from different directions  of the gap between the plug and the valve body and then bump into each other。 In Figs。 4(b and c), fluid flows are guided by those two cages。 Fluids exit from a number of flow channels inside the cages and flow toward the axis of the plug。 As a result, it causes a high pressure region below the plug。 In addition, the vortex located at the bottom of the valve and shown in Fig。 4(a) shrinks because the direction of the primary flow changes。 Two more vortices appear in the cage as shown in Fig。 4(b)。 Therefore, those two cages are evidently able to guide the fluids to flow through the valve cham- ber。 Moreover, the high pressure region at the bottom is clearly reduced in the globe valves with the two cages。 Especially in Fig。 4(c), the high pressure region almost vanishes owing to the one-stage step cage。 That is,  the  high  pressure  attributable  to the flow impingement at the bottom of the globe valve with the one-stage perforated cage is higher than that in the valve with the one-stage step cage。 Fig。 5 presents flow patterns and pressure contours of half-open globe valves without and with cages。 Although the vortex inside the valve chamber is still huge, the re- gion of vortex shrinks as the valve opening decreases in Fig。 5(a)。 The pressure adjacent to the plug in Fig。 5(a) is higher than that in Fig。 4(a)。 Moreover, the high-pressure region  in  the  half- open valve bottom is smaller than that in the fully open valve。 In Figs。 5(b and c), the vortices in the globe valves with the cages enlarge as a result of the half opening。 Also, the high pressure at the bottom of the globe valve with the one-stage perforated cage is still higher than that with the one-stage step   cage。论文网