Sizing of an Aircraft Fuel Pump

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, ^ = £ i ^ + £ - A ^ i (p„ _ ^„) log ( £ i ^ )
2 pt Pi \Pi-pJ
(4)
The centrifugal or mixed flow impeller is sized to generate the remaining head for the above case. However, the total pump when sized for one set of conditions should be verified for all the other specified conditions. Inducer Sizing. Inducer is an axial flow pump with gradual blade loading. It is designed to handle values of V/L of up to 0.45 which corresponds to void fraction of 0.30 at the inlets. The void fraction of 0.30, is the limit of bubbly flow regime (Ishii et al., 1980). As the critical Weber number is generally of order of 10, the bubble sizes are expected to be less than 0.3 cm. Therefore, it is assumed that the two phases are well mixed at the inlet and behave as pseudo single phase. The inlet of the inducer is sized on the basis of the following conditions; {a) compute specific speed for the inducer and use a table (Stepanoff, 1957) for estimating the coefficient ka„ kcJ^gHit, (b) (5)
^m
Hale Waihona Puke tan'(0[/2)
(7)
The hub size is based on strength consideration. The lowest inlet velocity is selected for inlet sizing from these two approaches. The other inducer inlet parameters are as follows: Al = QiBJc„.,i = n(rl, where fi, = 1/(1 ni,t/2TrriSm P'l) where (8) (9)
VPi Pv.i,
The value of K varies for inducer inlet conditions and blade design and a minimum value of K which can be safely used has been suggested by many researchers (Jacobsen and Keller, 1971; Stripling and Acosta, 1962; and Wade and Acosta, 1966).
Introduction
Two-phase flow appears naturally in many situations and pumps are needed to pump the mixture. One example of this situation is the boost pump for aircraft fuel systems. The fuel has many types of dissolved gases and as the pressure reduces, the gases come out of the fuel creating two-phase conditions at the pump inlet. The aircraft has to operate at various altitudes and speeds. This will lead to different pressures resulting in different V/L (vapor to liquid volume ratio) at the pump inlet. The pump is required to handle all these situations. The other requirement on the pump is that the fluid temperature rise due to friction, and secondary losses should be small. Furthermore, the weights of aircraft components are critical and so the pumps are designed for very high speed, which may result in severe cavitation problems. In general, a combination of inducer and centrifugal pumps (as shown in Fig. 1) are used. The inducer is designed to raise the pressure sufficiently to maintain proper inlet conditions for the centrifugal pump where most of the energy transfer to fluid takes place. A volute (Fig. 2) is attached at the exit of the pump to recover the pressure. Selection for Design Conditions. The aircraft engine manufacturer supplies a set of requirements which specify the inlet fluid conditions, pump speed, flow rate, and expected pressure rise. These conditions vary with altitude. The inducer should be able to increase the pressure in such a way that the following conditions (Stepanoff, 1957; Dixon, 1978; Sabersky et al., 1971) are met at the exit of inducer or inlet of centrifugal impeller for all conditions: V/L = 0.0, and i < 3.0 rad. (1)
(6)
First eliminate the conditions for which the inducer suction specific speed is less than 3.0 rad as no inducer will be needed for these. For the remaining specified conditions, the head rise required by the inducer is estimated for supplying proper impeller inlet conditions. The inducer is sized for the worst conditions. In situations where it is not straightforward to select the worst situation, the inducer is sized for one and checked for the other. The inducer inlet pressure will depend upon V/L ( v / L ) i = e Pa - Pi
(2) (3)
p, = {Op, + (V/L),p„,,)/(# + (V/L),)
n.,
Q 27r'(/)„p,n(l - 2D
X, = ^
n,A
The pressure rise required to dissolve all the gases in this case is(po-J3i)The two phase mixture is compressible and the head rise corresponding to the pressure rise oi {p^ — Pi) will be
Sizing of an Aircraft Fuel Pump
U. S. Rohatgi
Brookhaven National Laboratory, Department of Advanced Technology, Building 475B, Upton, NY 11973
A need to pump a mixture of two-phase fluid appears naturally in many situations. One example of this situation is aircraft fuel systems, where the pump inlet may have two-phase mixture due to the desorption of the dissolved gases at low pressures at higher altitudes. A simple procedure of selecting proper design conditions for the inlet inducer and a method of sizing the inducer, impeller and volute to meet all the design requirements has been described. This procedure has also been applied to a typical fighter plane boost pump design.