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下面詳細(xì)研究壓水室形狀即相對流量系數(shù)a對水力損失的影響。為此先研究壓水室內(nèi)損失與液流速度Ur之間的關(guān)系如何。對于最佳工作狀態(tài)，壓水室內(nèi)的損失為
即損失與液流在葉輪出口處的切向分速度和壓力短管喉部速度有關(guān)。喉部液流速度是可變值;在ur=0.75cz時(shí)，壓水室內(nèi)損失為最小值。在壓水室內(nèi)保證上述選擇的最佳流量時(shí)最小損失，根據(jù)式(3-30-22)，hors=0. 44c./2g.
在壓力短管喉部，一般u-<0.75c2a，所以在Ur減小時(shí)，結(jié)果是壓水室內(nèi)的損失隨著系數(shù)a增加而增大。在保證最佳流量時(shí)的最大損失將在環(huán)形壓水室內(nèi)。
應(yīng)該考慮，研究最佳工作狀態(tài)。在其他流量時(shí)，不論大流量還是小流量，在選定尺寸的壓水室內(nèi)最佳損失將增大。
由上述情況可知，a增大，即壓水室形狀逐漸接近環(huán)形，一方面導(dǎo)致壓水室計(jì)算斷面上液流速度減小，即導(dǎo)致其壽命增加;另方面導(dǎo)致水力損失增大。
與計(jì)算斷面相比(系數(shù)k),壓力短管喉部斷面面積增大，這將導(dǎo)致在a值最小時(shí)得到環(huán)形壓水室。因此，壓水室參數(shù)A和壓水室計(jì)算斷面面積都減小。因?yàn)閗值增大，而計(jì)算斷面面積減小，所以壓力短管喉部斷面面積變化應(yīng)很小。因此，系數(shù)a對壓水室內(nèi)損失的影響不大。
在系數(shù)a為不同值時(shí)，有可能保持最住流量的恒定值。同時(shí)，隨其增大，壓水室水力損失增大。這時(shí)，理論揚(yáng)程HT將是常數(shù)。泵揚(yáng)程等于理論揚(yáng)程與葉輪內(nèi)損失hx和壓水為葉輪效率。于是，泵揚(yáng)程室內(nèi)損失hun之差。葉輪內(nèi)損失hx=Hr(1-x), 式中，加H= xHτ- hors.
在保持最佳流量時(shí)，系系數(shù)a增大，即逐漸轉(zhuǎn)向環(huán)形壓水室，這導(dǎo)致水泵揚(yáng)程H有所下降。例如，如果對于理論揚(yáng)程H-=96m的高揚(yáng)程泵選擇壓水室，那么在不同壓力短管喉部速度ur值時(shí)，揚(yáng)程變化見表3-3-5.
在確定水泵揚(yáng)程時(shí)，采用例如切向分速度等于30m/s，葉輪內(nèi)的損失為理論揚(yáng)程的7%（當(dāng)z=4時(shí)），葉輪轉(zhuǎn)速n=600r/min，葉輪直徑D=1m。
從計(jì)算結(jié)果可知，壓水室內(nèi)的損失隨 壓力短管喉部速度減小而增大。在本例中，壓力短管喉部速度變化為50%，將導(dǎo)致泵在段佳工作狀態(tài)時(shí)揚(yáng)程下降8%，同時(shí)水力效率從6%到53%，下降為與裝部液流速度變化對低揚(yáng)程系壓水室內(nèi)損失的影響。令葉輪下面列舉一例，壓力短管喉部液流速度變化對低揚(yáng)程圓周速度u2 = 19m/s,泵的理論揚(yáng)程為Hr= 31m,葉輪內(nèi)損失采用等于理論揚(yáng)程的7%，即h=2. lm,葉輪出口切向分速度C2u一16m/s.
壓力短管喉部液流速度變化，導(dǎo)致泵揚(yáng)程如表3-3-6所示變化
對于AF泡沫泵低揚(yáng)程泵，壓力短管喉部液流速度變化一倍時(shí)，泵揚(yáng)程下降為4%左右。
上述例子指出，壓力短管喉部斷面面積變化對壓水室損失的影響不大，即在給定流量時(shí)，泵的揚(yáng)程變化很小。這就可以這樣來選擇系數(shù)a,即在盡可能提高壓水室壽命時(shí)，泵效率值要相當(dāng)高。同時(shí)對于給定值，泵揚(yáng)程變化將處在允許的范圍內(nèi)。
Next, the influence of the shape of the pressure chamber, i.e. the relative flow coefficient a, on the hydraulic loss is studied in detail. For this reason, the relationship between the loss in the pressurized water chamber and the flow velocity Ur is studied. For the best working condition, the loss in the pressurized water chamber is as follows
That is to say, the loss is related to the tangential velocity of liquid flow at the outlet of impeller and the throat velocity of pressure short pipe. The throat fluid velocity is variable, and the pressure chamber loss is the minimum when ur equals 0.75 cz. In the pressurized water chamber, the minimum loss is guaranteed when the optimum flow rate is selected. According to formula (3-30-22), hors = 0.44c. /2g.
In the throat of pressure short pipe, generally u-<0.75c2a, so when Ur decreases, the loss in the pressure chamber increases with the increase of coefficient a. The maximum loss in ensuring the optimal flow rate will be in the annular water pressure chamber.
It should be considered to study the best working condition. When other flow rates are large or small, the optimal loss in the pressurized water chamber of the selected size will increase.
From the above situation, it can be seen that the increase of a, that is, the shape of the water chamber gradually approaches the annular shape, on the one hand leads to the decrease of the liquid flow velocity on the calculated section of the water chamber, that is, the increase of its life; on the other hand, leads to the increase of hydraulic loss.
Compared with the calculated section (coefficient k), the section area of the throat of the pressure short pipe increases, which will result in the annular pressure chamber when the value of a is the smallest. Therefore, both the parameters A and the calculated cross-section area of the water chamber are reduced. Because the K value increases and the calculated cross-section area decreases, the change of throat cross-section area of pressure short pipe should be very small. Therefore, coefficient a has little effect on the loss of the pressurized water chamber.
When the coefficient a is different, it is possible to keep the constant value of the maximum flow rate. At the same time, the hydraulic loss of the pressure chamber increases with the increase of the pressure chamber. At this time, the theoretical head HT will be constant. The pump head is equal to the theoretical head and the loss of HX in the impeller, and the pressure water is the impeller efficiency. Hence, the difference of Hun loss in pump head chamber. The loss in impeller is HX = Hr (1-x), in which H = xH_ - hors is added.
When the optimum flow rate is maintained, the system coefficient a increases, i.e. gradually turns to the annular pressure chamber, which results in the decrease of pump head H. For example, if a pressurized chamber is selected for a high-lift pump with theoretical head H-=96m, the variation of the head can be seen in Table 3-3-5 at different throat velocity ur values of the short-pipe under different pressures.
When determining the pump head, such as the tangential velocity equal to 30m/s, the loss in the impeller is 7% of the theoretical head (when z = 4), the impeller speed n = 600r/min, and the impeller diameter D = 1m.
From the calculation results, it can be seen that the loss in the pressurized water chamber increases with the decrease of the throat velocity of the pressure short pipe. In this case, the change of throat velocity of pressure short pipe is 50%, which will cause the head of pump to decrease 8% in good working condition and the hydraulic efficiency to decrease from 6% to 53%, which is the influence of the change of liquid flow velocity on the loss of pressure chamber in low-lift system. For example, the change of liquid flow velocity in the throat of pressure short pipe is U2 = 19m/s, the theoretical head of pump is Hr = 31m, and the loss in impeller is 7% of the theoretical head, i.e. H = 2. lm, and the tangential velocity of impeller outlet is C2u 16m/s.
The change of liquid flow velocity in throat of pressure short pipe leads to the change of pump head as shown in Table 3-3-6.
For low-lift pumps, when the throat fluid velocity of pressure short pipe doubles, the pump head drops to about 4%.
The example above shows that the change of throat section area of pressure short pipe has little effect on the loss of pressure chamber, that is, the change of pump head is very small when the flow rate is given. This allows the selection of coefficient a, i.e. the pump efficiency value is quite high when the life of the pressurized water chamber is increased as much as possible. At the same time, for a given value, the change of pump head will be within the allowable range.
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