9. VAMPDAP TURBOPUMP TESTS

(1)

167

Chapter 9

9. VAMPDAP TURBOPUMP TESTS

9.1. RESULTS AND DISCUSSION FOR COLD TESTS

In Figure 9.1 to Figure 9.18 the results obtained for VAMPDAP pump from the experimental campaign performed in cold flow condition are illustrated. Hence, as previously done for DAPROT3 in chapter 8, the phase and modulus of the normal and tangential components of the fluid-induced rotordynamic force are shown, along with the hydraulic efficiency. Also in this case the experiments have been carried out in water at 20 °C, where thermal effects can be neglected. The results of hot flow condition will be shown in next chapter.

Experimental curves reported in Figure 9.1 to Figure 9.9 and in Figure 9.10 to Figure 9.18 present the comparisons of the results respectively obtained at different flow coefficients with fixed cavitation number, and at different cavitation numbers with fixed flow coefficient. Experiments with both discrete and continuously varying whirl speed have been conducted, and the agreement between the two approaches is verified also for VAMPDAP pump.

9.1.1. INFLUENCE OF FLOW RATE AT FIXED

CAVITATION NUMBER

The diagrams in Figure 9.1 and Figure 9.2 show the influence of the flow coefficient at noncavitating conditions. It can be observed that as the flow coefficient decreases, a shift at higher whirl frequency ratios of the rotordynamic force minima and maxima is verified. This behavior can be observed at moderate and highly cavitating conditions too, as illustrated in Figure 9.4, Figure 9.5, Figure 9.7, and Figure 9.8. On the other hand, a decrease in flow rate corresponds to a significant higher rotordynamic force at negative whirl frequency ratios.

It is also possible to observe a different behavior for normal and tangential forces between positive and negative whirl ratios. The “classic” results, where the spectrum of FN is parabolic whereas that of

FT is quasi-linear, can be observed only for ω/Ω < 0.

The deviation from these trends can be observed at any flow coefficient for positive whirl ratio. Maxima and minima are manifest for ω/Ω of about 0.3 both for normal and tangential forces. For the nondimensional normal force, a flat experimental curve can be recognized for high positive whirl

(2)

Chapter 9

168

ratios which is placed at 𝐹𝑁∗ ≅ 0. This flat region starts from ω/Ω = 0.3 for design flow coefficient and

from ω/Ω = 0.5 both for lower and higher flow rates.

In Figure 9.1 the results for nondimensional normal and tangential forces in noncavitating cold tests at various flow rates are presented. In these operating conditions two different assessments can be done depending on whether the whirl ratio is negative or positive:

• If ω/Ω < 0 the normal force is destabilizing (thus tending to increase the impeller eccentricity) for any ω/Ω independently from the flow rate. Nevertheless, a stabilizing operational region is present at very low whirl speeds (ω/Ω > -0.06). The effect of an increase in the flow rate is mainly to decrease 𝐹𝑁∗. On the other hand the tangential force is

always stabilizing and decreases as the flow rate increases.

• If ω/Ω > 0 the stability of the normal and tangential forces depends on the flow rate. Firstly it can be observed that the normal and tangential forces are substantially lower with respect to negative whirl ratios. Moreover, at Φ = 0.111 the normal force component is stabilizing for ω/Ω < 0.27 whereas at Φ = ΦD = 0.093 for 0.08 < ω/Ω < 0.35. At low flow rate, Φ =

0.074, the stability condition is verified for ω/Ω > 0.24. On the other hand, for 0.5 < ω/Ω < 0.7 the tangential force is always stabilizing whatever the flow rate. For increasing flow coefficient the stable region becomes wider. Indeed for Φ = 0.093 it becomes destabilizing when ω/Ω < 0.4 whereas for Φ = 0.111 occurs for ω/Ω < 0.37.

The stabilizing/destabilizing behavior of rotordynamic force can be obtained from the phase diagrams where the radial and azimuthal components are analyzed. It is clear that at low flow coefficient a wide region in which both FT and FN are destabilizing occurs for 0 < ω/Ω < 0.23, whereas

in the other flow rate operating conditions, this region becomes narrower. Moreover, as previously noted, when a local minima of the rotordynamic force modulus is reached a rapid variation of its phase occurs.

Similar considerations can be made for Figure 9.4, Figure 9.5, Figure 9.7, and Figure 9.8, where the results for moderate and highly cavitation conditions are presented. In Figure 9.4 and Figure 9.7 it can be observed that for high flow rate (Φ = 0.111) the stabilizing normal force region is wider for negative whirl frequency ratios with respect to noncavitating condition. Indeed in both cavitating conditions, 𝐹𝑁∗ is negative for -0.14 < ω/Ω < 0.

Finally, an important consideration from previous diagrams, is that the rotordynamic forces are much more higher at negative whirl ratios with respect to the values obtained at positive whirl speeds.

9.1.2. INFLUENCE OF CAVITATION AT FIXED FLOW

RATE

From Figure 9.10 to Figure 9.18 the effects of the cavitation number at fixed flow rates (Φ = 0.065, 0.078, and 0.052, respectively) for cold tests (T = 20 °C) are presented.

From the experimental diagrams it is clear that the effect of cavitation on fluid-induced rotordynamic forces is not significant for any flow rate. Nevertheless a slightly increase in tangential force for decreasing inlet pressure is manifest for ω/Ω < 0.

On the other hand, the normal force tends to increase slightly with cavitation number only at negative whirl ratios whereas no effects can be observed at ω/Ω > 0. The main consequence of this phenomena is found for high flow rate (Φ = 0.111) where the presence of cavitation yields to a lower normal force at negative whirl frequency ratios.

(3)

VAMPDAP TURBOPUMP TESTS

169

9.1.3. INFLUENCE OF ROTORDYNAMIC FORCES ON

THE HYDRAULIC EFFICIENCY

The experimental campaign has included the assessment of the rotordynamic effects on the hydraulic efficiency also for the VAMPDAP pump. With the same procedure exploited for the evaluation of the hydraulic efficiency of the DAPROT3 different tests have been performed at various operating conditions.

The total pressure rise across the pump has been evaluated exploiting equations 5.10 and 5.11 since the flow outlet from the main impeller presents a spiral volute which is part of the VAMPIRE pump.

The hydraulic efficiency of the VAMPDAP pump at T = 20 °C and Φ = ΦD = 0.092 at zero

eccentricity conditions (2 mm blade tip clearance for both impellers) is equal to 61% (chapter 6.1.1). The noncavitating results of the hydraulic efficiency for different flow rates as a function of the whirl frequency ratio are presented in Figure 9.3 and show that the presence of rotordynamic forces and moments may affect the hydraulic efficiency by less than 1% both at design and off-design conditions. Nevertheless a slightly increase can be observed at positive whirl frequency ratios.

At moderate cavitation condition (Figure 9.6) the same trends are verified whereas for highly cavitating flow (Figure 9.9), the effects of rotordynamic forces are even less, especially at low flow rate, where they can be assumed as negligible.

The effect of the cavitation on the hydraulic efficiency is presented in Figure 9.12, Figure 9.15, and Figure 9.18, where the results for Φ = ΦD, Φ = 1.2ΦD, and Φ = 0.8ΦD are respectively reported. From

the experimental curves it is clear that this phenomena tends to slightly increase the efficiency of the turbopump up to 0.9% at design. On the other hand, at off-design conditions, the influence of the cavitation can be considered as negligible since no particular differences are found (variations in less than 0.5%).

(4)

Chapter 9

170

9.2. ROTORDYNAMIC FORCE DIAGRAMS

Figure 9.1 Influence of the flow coefficient on the nondimensional normal force (top) and nondimensional tangential force (bottom) of the rotordynamic force with σN = 0.604 and T = 20 °C.

(5)

171

Figure 9.2 Influence of the flow coefficient on the modulus (top) and phase(bottom) of the rotordynamic force with σN = 0.604 and T

(6)

Chapter 9

172

Figure 9.3 Influence of the rotordynamic effects on the hydraulic efficiency (top) with close-up view (bottom) at different flow coefficients when σN = 0.604 and T = 20 °C.

(7)

173

(8)

Chapter 9

174

(9)

175

(10)

Chapter 9

176

(11)

177

(12)

Chapter 9

178

(13)

179

Figure 9.10 Influence of the cavitation number on the nondimensional normal force (top) and nondimensional tangential force (bottom) of the rotordynamic force with Φ = ΦD = 0.092 and T = 20 °C.

(14)

Chapter 9

180

Figure 9.11 Influence of the cavitation number on the modulus (top) and phase(bottom) of the rotordynamic force with Φ = ΦD =

(15)

181

Figure 9.12 Influence of the rotordynamic effects on the hydraulic efficiency (top) with close-up view (bottom) at different cavitating conditions when Φ = 0.092 and T = 20 °C.

(16)

Chapter 9

182

Figure 9.13 Influence of the cavitation number on the nondimensional normal force (top) and nondimensional tangential force (bottom) of the rotordynamic force with Φ = 0.111 and T = 20 °C.

(17)

183

Figure 9.14 Influence of the cavitation number on the modulus (top) and phase(bottom) of the rotordynamic force with Φ = 0.111 and T = 20 °C.

(18)

Chapter 9

184

(19)

185

(20)

Chapter 9

186

(21)

187

(22)

Chapter 9

188

9.3. RESULTS AND DISCUSSION FOR HOT TESTS

In Figure 9.19-Figure 9.30, the results of the tests performed in water at 70 °C are presented, corresponding to moderate thermal cavitation conditions. The influence of the flow rate can be observed in Figure 9.19-Figure 9.24 whereas the effect of cavitation is shown in Figure 9.25-Figure 9.30. It is clear that the behaviors observed for cold tests are verified also for hot flow condition, since no significant differences are found.

Indeed maxima and minima of fluid-induced rotordynamic force tends to shift towards higher whirl ratios for decreasing flow rate whereas an increase in its intensity at negative whirl ratios occurs. Also in hot tests the deviation from the classic trend is verified for any flow rate at ω/Ω > 0. On the other hand, the parabolic and quasi-linear trend, for normal and tangential forces respectively, are respected for ω/Ω < 0 in any cavitation or flow rate operating conditions.

The points of stability transition are not affected by the presence of a hot flow in place of a cold flow. Hence the transition regions remains the same of the discussion presented in chapter 9.2.

Figure 9.25-Figure 9.30 show the influence of the cavitation number at fixed flow rate (Φ = ΦD =

0.065, Φ = 0.078, and Φ = 0.052, respectively) for hot flow condition. As observed for cold tests, the main influence of cavitation can be detected at high flow rate and negative whirl ratios, where the stability transition shifts from ω/Ω = -0.05 to -0.14. For the lowest flow rate the effect of cavitation is clearly negligible.

A direct comparison between cold and hot tests, as reported in Figure 9.31-Figure 9.34 for two different flow rates at two different cavitating conditions, show that the higher fluid temperature is associated with a slightly increase in the normal force only at higher values of |ω/Ω|. Conversely, the effect on the tangential force can be considered totally negligible. The same effects can be observed in any other operating condition and thus the results are here omitted.

Finally, the hydraulic efficiency presents similar trends with respect to cold tests. From a comparison between T = 20 °C and T = 70 °C, it has been observed that the temperature influence the hydraulic efficiency only for the conditions reported in Figure 9.35 to Figure 9.37. In the other cases the effect of a hot flow can be considered negligible and the results obtained for cold flow are verified. From these figures it is clear that the temperatures may increase or decrease the hydraulic efficiency depending on the operating condition:

• For Φ = 0.111 and σN = 0.053. The efficiency obtained for T = 70 °C increases up to 1% at

positive whirl ratios whereas at ω/Ω < 0 it increases less (up to 0.5%).

• For Φ = 0.092 and σN = 0.081. The increase in efficiency is verified also at these condition

where a maximum of 1% can be obtained for any whirl ratio.

• For Φ = 0.074 and σN = 0.604. In this third case the efficiency tends to decrease with

increasing temperature and it can be in the order of 1% at any ω/Ω.

From previous results it is clear that the presence of a high temperature inhibits the cavitation phenomena due to the thermal boundary layer at the liquid/vapor interface as shown in chapter 3.5. Hence the pump hydraulic efficiency increases when the cavitation is inhibited and the large scale behaviors of the flow are not predominant, in which the thermal benefit is limited by stronger secondary flows.

On the other hand a high temperature leads to a cavitating flow that would not cavitate at lower temperature due to the increase in vapor pressure of the fluid.

(23)

189

(24)

Chapter 9

190

(25)

191

(26)

Chapter 9

192

(27)

193

(28)

Chapter 9

194

(29)

195

Figure 9.25 Influence of the cavitation number on the nondimensional normal force (top) and nondimensional tangential force (bottom) of the rotordynamic force with Φ = ΦD = 0.092 and T = 70 °C.

(30)

Chapter 9

196

Figure 9.26 Influence of the cavitation number on the modulus (top) and phase(bottom) of the rotordynamic force with Φ = ΦD =

(31)

197

(32)

Chapter 9

198

(33)

199

(34)

Chapter 9

200

(35)

201

Figure 9.31 Influence of the temperature on the nondimensional normal force (top) and nondimensional tangential force (bottom) of the rotordynamic force with Φ = 0.093 andσN = 0.604.

(36)

Chapter 9

202

Figure 9.32 Influence of the temperature on the modulus (top) and phase(bottom) of the rotordynamic force with Φ = 0.093 andσN =

(37)

203

Figure 9.33 Influence of the temperature on the nondimensional normal force (top) and nondimensional tangential force (bottom) of the rotordynamic force with Φ = 0.111 andσN = 0.053.

(38)

Chapter 9

204

Figure 9.34 Influence of the temperature on the modulus (top) and phase(bottom) of the rotordynamic force with Φ = 0.111 andσN =

(39)

205

Figure 9.35 Influence of the rotordynamic effects on the hydraulic efficiency for different flow temperatures at Φ = 0.111 and σN =

0.053.

(40)

Chapter 9

206