塩ビ製多孔管の水理特性

畑地散水カンガイにおける管配水に関する研究 (IV)

抄録

In this paper, the author performed experiments on the hydraulic characteristics of PVC perforated-pipe for the purpose of obtaining basic data for rationalizing the pipe distribution system for sprinkler irrigation, and investigated the applicability of the hydraulic equations of perforated pipe derived in a previous paper (III) for sloping perforated-pipe, etc.
The results obtained are summarized as follows:
(1) The actual coefficients of friction for the 40mm rigid PVC pipe agreed with the values of the Blasius equation but those for the same kind of perforated-pipe were given by f= 5.842 R_e^-0.582 (in H_f= f (l/D) (V²/2g) (1/3)), and were about 18-33% smaller than the Blasius' values in the range of Reynolds number (R_e) for practical purpose.
(2) In the following relations among the beginning pressure head H_m, the average pressure head H_a, the distal pressure head H₀ and the pressure loss of head H_f on a horizontal perforated-pipe,
H_m=H_a+αH_f, H₀=H_a-βH_f α, β: constants,
the experimental results for a pipe 16m in length were respectively α=0.711, β=0.289 and situation of H_a= 38.7%, and analytical results of the past data for the pipe of 50m, 0.740, 0.260 and 38.0%. Therefore the utility of the calculated values obtained in the third paper has been confirmed. The measured distribution curves of water pressure indicated good agreement with the equation H_x= H_f (1-x/l) ³. H_x: pressure head higher than H₀ along the pipe.
(3) For a perforated pipe which slopes downward the expression for hydraulic-grade line becomes
H_x=H_f[(1-x/l)³+kx/l]
where k=H_e/H_f, H_e: elevation difference.
By using the above equation, the values of α, βand the locations of H_a and H_min (= minimum pressure head) were computed (Table 6). The pressure distribution along the pipe is most uniform when k is 0.91. Applicability of the corrections made for sloping perforated-pipe proposed in the third paper was verified.
(4) Values of the factor F for estimating the friction loss between the first and last outlets in a pipe with multiple outlets are given by
F=1/(N-1)N^m·[(N-1)^m+(N-2)^m+...+2^m+1^m]
where N: number of outlets (equally spaced),
m: velocity exponent in friction loss formula,
and are given in Table 1 for m=1.85, 1.9 and 2.0.
(5) Relations between the mean discharge per small hole q_a (cc/ sec), mean spread W_a (cm) and beginning pressure P_m (g/cm²) in the tested 40mm PVC perforated-pipe are given by P_m= 84-84.43 q_a-100.54, P_m= 4.60 W_a-101.28. Change of the efflux direction of water in relation with the pipe axis was not recognized upstream or downstream in the pipe or by pressure.
(6) The measured diameters of the 312 holes having nominal diameter 0.8mm were 0.59-1.29mm, and 0.815mm on a average. The coefficients of discharge c for the 22 holes measured were 0.407-0.776, and 0.609 on a average. Values of c were computed by q=0.659cd²/P (l/min, mm, kg/cm²). The numerical constant of 0.659 or 0.66 is more accurate than the generally adopted 0.65.