5

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Ftow Ratio M —— Pressure Ratio N ---Efficiency %

It has been shown (see References 2, 3, 4) that one-dimensional analyses successfully predict actual LJL pump performance. But theory-experiment agreement obtains only if the test-pump flow conforms with the model assumptions. Two conditions that will cause departure of measured N and h data from the theoretical curves are

• Cavitation, which occurs in the mixing throat

• Extension of the throat mixing process out into the diffuser

Operating Flow Ratio Mop In Figure 3, the flow ratio Mop is indicated on the left slope of the "parabolic" efficiency curve, which peaks at M = Mmep. The recommended location of the design operating point is to set Mop = § Mmep. Higher operating M ratios would provide slightly higher efficiencies, but at a greater risk of cavitation. (Cavitation as part of the design process is included in the following examples given.) Finding Mmep can be readily accomplished from Table 1 type data using spreadsheet successive approximations. Alternatively, Eqs. (17)-(18) can be differentiated and set equal to zero to find the peak efficiency Mmep value.

Cavitation LJL pumps may encounter cavitation, which occurs in the mixing throat. With reference to Figure 3, the LJL pump normally responds to a reduction in back pressure Pd (Q1 and Ps constant) by producing a larger Q2 secondary flow, and hence a larger M. Measured pressure ratios (N) and efficiencies (h) track along these theory-based characteristic curves as shown in Figure 3. But after the throat-inlet pressure (Po) is reduced to the vapor pressure (Pv) of the secondary liquid, any further drop in the back pressure has no effect on the flow ratio, which stabilizes at M = ML, the cavitation-limited flow ratio. Note the vertical dashed line in Figure 3: measured N and h values fall on this vertical line, under ML operating conditions. In this manner, cavitating-pump performance departs radically from predicted/normal behavior.

Published studies (see References 1 and 8) have shown that NPSH-type correlations adequately explain and predict cavitation-limited flow phenomena. Comparing the predicted Ml with the intended Mop is an essential step in designing a jet pump installation. If Mop < ML, the LJL pump can be expected to perform "on design;" that is, to follow the Eq. (17) N(M) relation, with no cavitation. ML can be predicted from the operating conditions as follows (see Reference 8):

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