US2959355A - Nozzle - Google Patents

Description

Nov. 8, 1960 B. c. HOUSER 2,959,355

NOZZLE Filed July 25, 1958 2 Sheets-Sheet 1 IN V EN TOR. B/F/IDFORO C. HOV-5'67? United States Patent NOZZLE Bradford C. Houser, Pasadena, Calif, assignor to Sandberg-Serrell Corporation, Pasadena, Calif., a corporation of California Filed July 25, 1958, Ser. No. 750,944

4 Claims. (Cl. 239-75) This invention relates to nozzles and more particularly to nozzles adapted to withstand high pressures and to compensate for thermal expansion at high temperatures.

Nozzles are employed for many purposes in industry. In each instance the nozzle includes an inlet endinto which fluid under relatively high pressure and temperature is admitted and an exit end from which the fluid is discharged. A region of reduced cross section, called the throat, is disposed intermediate the two ends. is actually a hollow member which converges from the inlet end to the throat and diverges from the throat to the exit end. If the inlet pressure and temperature and the dimensions of the nozzle are such that the velocity of the fluid at the throat portion is less than Mach 1, the velocity of the fluid decreases progressively below Mach 1 downstream from the throat. If the inlet temperature and pressure and the size of the nozzle are such that the velocity of the fluid at the throat is equal to or greater than Mach 1, the velocity of the fluid downstream of the throat is supersonic and progressively exceeds Mach 1 by a factor determined, among other things, by the ratio of the diam- The nozzle:

eter of the exit end of the nozzle with respect to the diameter of the throat portion. In this situation considerable heat is imparted to the nozzle, and because the combination of velocity and density of the fluid produces a higher film coeflicient at or near the throat than in other regions, more heat per unit area is imparted to the throat area than to any other single area of the nozzle. Thus the inner surface of the nozzle in the throat portion acquires a very high temperature while the outside regions in the throat area are at a relatively low temperature. Consequently a large thermal gradient exists radially across the throat region of the nozzle, and this gradient creates thermal stresses in the throat region which in turn establish forces extending longitudinally and transversely of the nozzle. The longitudinal forces tend to part or separate the nozzle at the throat because this region, being the area of smallest cross section of the nozzle, has the least structural strength. The thermal stresses established transversely of the nozzle in the throat region tend to cause expansion and thereby warp or deform the smooth inner surface of the nozzle which may lead to flow distortions. Simultaneous with the foregoing events the. high velocity fluid through the nozzles establishes :1 Iongitudinal thrust tending to cause the nozzle to fail at its weakest or throat portion.

In one earlier attempt to minimize the foregoing difficulties, an effort was made to support the thin inner nozzle wall with ribs secured to a heavy back-up shell in the hope of reducing the forces acting on the inner wall. This had the undesirable effect of drastically increasing the thermal stresses of the inner wall.

In another earlier attempt to minimize the foregoing difiiculties, resort was made to the use of a water jacket disposed about the nozzle to reduce the temperature of the nozzle, especially in the throat region. In order to minimize the effect of longitudinal forces resulting from thermal'gradients and high velocity fluid flow tending to lice part the nozzle at the throat, an apparatus has been employed to establish a force on the nozzle in an opposite direction and thereby balance out the longitudinal forces. For the purpose of securing the optimum cooling effect, it was desirable in the past to make the material of which the nozzle is constructed as thin as possible. Yet, in order to make the nozzle strong enough to withstand longitudinal forces on the other hand, it was desirable to make the material from which the nozzle is constructed as thick as possible for the purpose of securing structural strength. Consequently the problem has been one of trying to balance the thickness of the material from which the nozzle is constructed so as to give the required cooling effect consistent with obtaining the minimum structural strength. The problem of structural strength is perhaps more fully appreciated when the diificnlty is considered of handling a nozzle some 10 to 15 feet long and constructed of a material on the order of 0.01 in. thick.

The foregoing difliculties are overcome according to the present invention by providing a nozzle constructed of a relatively thin member and providing a plurality of ribs made integral therewith and disposed about the throat portion of the nozzle. The thin member has a smooth inner surface with a plurality of slots extending therethrough and into the ribs. The depth of the slots is zero at the upstream and downstream end of the ribs and increases in depth until it is a maximum at the throat portion. The ribs provide structural rigidity and eliminate or minimize the necessity of additional apparatus to compensate for longitudinal forces established by thermal gradients and fluid flow. The slots permit thermal expansion to take place whereby the slots are partially closed. This relieves most of the thermal stresses in the thin member as a result of the high temperatures in and around the throat portion.

It is desirable in many instances to secure smooth or non-turbulent flow through the nozzle. sure that the slots do not create fiow distortion in the throat region, the width of the slots must be maintained; Thewithin defined limits at the operating temperature. width of the slot is critical and must be maintained at some value equal to or less than one-half the width of the boundary layer. The boundary layer may be defined generally as a thin slow-moving film which may be considered as thin multiple layers of fluid immediately adjacent to the inside surface of the nozzle. If W equals the width of a slot and W equals the width of the boundary layer, then their relationship may be expressed as W Z2W These and other features of this invention may be more fully appreciated when considered in the light of the following specification and drawings in which:

Fig. 1 is a cross-sectional view of a nozzle constructed according to the present invention and taken on the line 11 of Fig. 2;

Fig. 2 is a cross-sectional view taken on the line 2-2 in Fig. l;

Fig. 3 is a cross-sectional view of a mandrel, used in fabricating the nozzle of Figs. 1 and 2, taken on the line 33 of Fig. 4;

Fig. 4 is an expanded cross-sectional view taken on the line 44 of Fig. 3;

Figs. 5, 6 and 7 are expanded views illustrating a nozzle in various steps of construction as might be seen along the line 44 of Fig. 3;

Figs. 8 and 9 are expanded fragmentary views illusttrating the relation of a slot in Fig. 2 and its associated boundary layer;

Fig. 10 is a curve showing the fluid velocity with re- Referring first to Figs. 1 and 2, a nozzle 10 is adapted In order to in-- to receive fluid' under pressure at the end 12 and discharge this fluid to the atmosphere at the end 14. The nozzle is formed by a relatively thin member 16 having a smooth inner surface 18 and shaped as indicated in Figs. 1 and 2. A plurality of slots 20 through 27 are disposed in the surface of the thin member 16 and run upstream and downstream from a throat portion 28. The throat portion of the nozzle is reinforced by a plurality of ribs 30 through 37 which are made integral with the thin member 16. A jacket 49 is disposed about the ribs 30 through 37 and extends longitudinally of the nozzle from a point 42 upstream of the throat to a point 44 downstream of the throat. The jacket is made integral with the ribs 30 through 37 and the member 16 at the junctions 42 and 44. Cooling fluid is supplied through an inlet 50 to a chamber 48 and is discharged from this chamber through an outlet 46. In a similar fashion cooling fluid is supplied through an inlet 56 to a chamber 54, and it is discharged from this chamber through an outlet 52. In like fashion the inlets 60 through 65 permit the entry of cooling fluid into corresponding chambers 70 through 75, and the cooling fluid is discharged through associated outlets not shown. The cooling fluid serves to reduce the temperature of that portion of the thin member 16 between the points 42 and 44. Because the thin member 16 is not a perfect thermal conductor, a temperature gradient is established thereacross because the water on the outside of this member is relatively cool and the high pressure gases on the inside are relatively hot. Consequently thermal stresses areestablished in the thin member 18, and these stresses are greatest at or near the throat portion 28 of the nozzle 10 because the gases give up more of their heat in this region. The thermal gradient establishes stresses having a component of force in the longitudinal direction. The thin member 16 is constrained from longitudinal deformation under these stresses by making the ribs 30 through 37 and the outer jacket 40 sufficiently thick so that the forces tending to separate or part the nozzle at the throat portion 28 are less than the longitudinal strength. Temperature rise in the thin member 16 in the region of the throat 28 produces lateral thermal expansion which tends to close the slots 20 through 27. The slots 20 through 27 permit this lateral thermal expansion thus preventing lateral thermal stresses from developing and the inner surface 18 of the thin member 16 remains smooth and undistorted in the region of the throat 28. The regions of the thin member 16 upstream of the point 42 and downstream of the point 44 may be cooled by the addition of conventional cooling devices where necessary to eliminate deformation as the result of hot gases flowing therethrough. The heating effect of the gases in these regions is relatively lower and does not present the problem which exists at the throat region.

Reference is made to Figs. 3 through 7 for a description of the various steps involved in constructing the nozzle 10. A mandrel 80 in Fig. 3 is formed of aluminum or any other material which later may be etched away. The mandrel 80 is circular in cross section and contoured throughout its length as illustrated by the sectional view of Fig. 3. A plurality of fins 81 through 88 are formed integral with or otherwise attached to the mandrel 89. In practice the fins may be thin metallic strips on the order of several thousandths of an inch thick tightly fitted into slots cut in the throat portion of the mandrel. Figs. 5, 6 and 7 illustrate the nozzle 10 in the various stages of electroforming operations as may be viewed in cross section along the line 44 in Fig. 3. The first step involved is to immerse the mandrel of Fig. 3 in a bath and electroform a material such as nickel or copper, for example, throughout the length of the nozzle. The depositions on the projecting fins 81 through 88 build up more rapidly than the smooth areas of the mandrel 80. Once the depositions on the fins 81 through 88 build up to a desired thickness, the mandrel is properly machined to make the electroformed ribs 91 through 98 in Fig. 4 a precise length and thickness. The regions 99 between the ribs 91 through 98 .may be machined to give a proper thickness to the thin member 16.

Next a material such as a lead-tin alloy or other suitable alloy having a low melting point is poured in between the ribs 91 through 98. This material may be referred to as a filler and is indicated at 100 in Fig. 6. The outer surface may be machined again to give smoothness thereto after which the mandrel is againplaced in a bath for electroforming a relatively thick outer layer of material such as nickel or copper, for example, this material preferably being the same material used in electroforming the ribs 91 through 98. This outer layer forms the jacket 40 illustrated in Fig. 7. Then the nozzle is inserted in a bath of basic solution such as sodium hydroxide or other suitable etching bath, and the mandrel 88 is etched away without disturbing the material of which the nozzle is formed. Next apertures are formed in the outerjacket 40 for-receiving the inlets andoutlets shown in Figs. 1 and 2 The filler material 100 is heated above the melting point. and allowed to run out of these apertures. At this point the chambers 48, 54 and 70 through 75 of Fig. 2 are formed. After this operation is completed, inlets and outlets are secured to the jacket 40 as by welding, brazing or otherwise, and. the. nozzle of Figs. 1 and 2 results.

While copper and nickel have been set. forth as suit able examplesof materials from which a nozzle may. be electroformed by the foregoing technique, it is pointed out that numerous other materials are capable of being used. The characteristics of the various materials in: cluding such factors as thermal conductivity, thermal expansivity, modulus of elasticity and the strength of the material at relatively high temperatures determine, which material is suitable for given combinations of temperature and pressure at the end 12 of the nozzle in Fig. 1'. If the nozzle 10 in Fig. 1 is to receive combinations of pressure and temperature varying between 2200 psi. and; 1000" F.

to 1500 p.s.i. and 1500 F. the nozzle 10 in Fig. 1 may have the following dimensions if. made with nickel:

inch

Width of ribs 91 through 98 0.1 Length of ribs 91 through 98 between thin member 16 and the jacket 40 0.25 Thickness of the thin member 16 0.04 Width of the slots 20 through 27 Q. 0.002 Inside diameter of the thin member 16 at the throat I 0 Another and perhaps more suitable material is beryllium-copper for such a nozzle. Beryllium-copper can be applied by flame spraying techniques using a sequence of steps similar to that used to electroform nickel, As previously described a mandrel with projecting fins may first be prepared and berylliurncopper then formed thereon by flame spraying until the nozzle is built up.

In operation fluid under high temperature and pressure enters the nozzle 10. in Fig. l at the, end 12 and con-. verges until it reaches the throat 28 and diverges down stream of the throat 28. In the process a large quantity of heat is conveyed to the thin member 16. The velocity of the gases at the throat 28 is Mach 1, and the temperature is greatest at or near the throat. Although a large quantity of heat is conveyed to. the nozzle 10 throughout the length of the thin member 16, the largest quantity of heat is conveyed to the thin member 16 at or near the throat 28. Consequently thermal stresses in the throat region are quite high. Because the crosssectional size of the nozzle is smallest at the throat, this is normally the region of least mechanical strength, and the nozzle tends to break or separate in the throat region under forces established longitudinally either by thermal stresses or thrust from the high velocity fluid. The structural strength of the nozzle according to the present invention is increased by the reinforcing effect of the ribs 30 through 37 disposed about the thin member 16 between the jacket 40 and the throat region. The material used is one having a relatively high strength under the extreme combination of operating temperature and pressure. In order to minimize the thermal stresses the slots 20 through 27 are disposed in respective ribs 30 through 37. The slots are deepest at the throat and taper to zero depth at their upstream and downstream ends. Note, for example, this construction in slot 20 of rib 30 in Figs. 1 and 2. It is essential for many purposes, however, to secure smooth or nonturbulent flow through the nozzle 10. To insure that the slots 20 through 27 do not create flow distortion in the throat region 28, the width of the slot must be maintained within defined limits. The width of the slots is critical and must be maintained at some value equal to or less than one-half the width of the boundary layer where the boundary layer is defined as mutiple layers of fluid immediately adjacent to the inside surface of the thin member 16. If W equals the Width of a slot and W equals the width of the boundary layer, then the relationship between them may be expressed as W ;2W Fig. 8 shows the slot 22 in Fig. 2 broken out and expanded in size for ease of viewing. The width of the slot 22 is indicated by W and the width of a boundary layer of fluid flowing adjacent to the inner surface of the thin member 16 is indicated as W The velocity of fluid flow with respect to distance taken transversely of the nozzle at the throat 28 is indicated by the curve 110 in Fig. 10. This curve represents the velocity distribution for air flow which is aerodynamically perfect. The distance W at either end of the curve 110 in Fig. 9 represents the thickness of the boundary layer adjacent the inner surface 18 of the nozzle and shows the velocity of the multiple layers which form the boundary layer. Note that that portion of the boundary layer adjacent to the inner surface of the thin member 16 at either side of the cross section is zero as indicated by the points 112 and 114. The velocity of each succeeding adjacent layer, moving inwardly from the points 112 and 114, increases until the points 116 and 118 are reached. Between the points 116 and 118 the flowing gases theoretically have a uniform velocity. As long as the width of the boundary layer W remains at some value equal to or greater than twice the width of W a velocity distribution of this type may be obtained which represents substantially perfect aerodynamic flow as illustrated in Fig. 8, an expanded view of the slot 22 in Fig. 2. If on the other hand the width of the boundary layer W becomes less than twice the slot width W turbulence or imperfect aerodynamic flow results, and this is illustrated in Fig. 9 where the boundary layer presumably extends into the slot 22 substantially as shown. Since the width of the boundary layer is difficult if not impossible to change, it therefore is necessary to control the width of the slot. Since the boundary layer W is normally in the neighborhood of 0.005 in. thick at the throat 28 for the temperature and pressure conditions assumed, the slots 20 through 27 in Fig. 2 must be not greater than half of this amount or approximately some value less than 0.0025 in. The slots 20 through 27 close partially when the thin member 16 heats up during operation. For this reason slots may be about 0.003 in. or even slightly greater when the nozzle is at room temperature. After the thin member 16 is heated up during Operation, the slots close to some value less than 0.0025 in. It is seen therefore that the slots may be slightly larger than one-half the width of the boundary layer W when the nozzle 10 is at room temperature. During operation the slots partially close to a point where their width is less than one-half that of the boundary layer W and at the same time relieve thermal stresses. Since various materials have different coefiicients of thermal expansion, this must be taken into account when determining the width of the slots 20 through 27 for various materials. For this reason the use of nickel in constructing the nozzle along with the dimensions indicated for the various parts are to be taken as illustrative, for many other types of materials having different dimensions may be suitably employed. Where smooth flow is not required, reinforcing ribs with slots nevertheless may be used to give structural rigidity and to relieve thermal stresses.

What is claimed is:

l. A nozzle having one end adapted to receive fluid under pressure and another end adapted to discharge the fluid, a throat portion disposed intermediate the two ends, a plurality of ribs disposed about the nozzle at the throat and running upstream and downstream from the throat, said nozzle including a smooth inner surface having slots extending through the smooth inner surface into said ribs whereby thermal expansion of the smooth inner surface may take place and partially close said slots, said slots having a maximum width during operation of said nozzle which is equal to or less than one half the thickness of the boundary layer of a fluid flowing by said smooth inner surface whereby the fluid flows smoothly past said slots.

2. A nozzle having first and second ends with a throat disposed therebetween, said nozzle having a smooth inner surface which converges from said first end to said throat and diverges between said throat and said second end, a plurality of ribs disposed about the throat portion of said nozzle, said smooth inner surface having a plurality of slots which extend into said ribs.

3. A nozzle comprising a hollow member having a smooth inner surface, said member having a cross-sectional configuration which converges from the first end to said throat and diverges from said throat to the second end, said nozzle including a reinforced portion disposed about the throat and extending upstream and downstream of the throat, said reinforced portion having a plurality of slots which extend from the inner smooth surface into said reinforced portion.

4. A nozzle comprising a hollow member having a smooth inner surface, said member having a cross-sectional configuration which converges from the first end to said throat and diverges from said throat to the second end, said nozzle including a reinforced portion disposed about the throat and extending upstream and downstream of the throat to provide structural rigidity, said reinforced portion having a plurality of slots which extend from the inner smooth surface into said reinforced portion, said slots having a width which is equal to or less than one-half the width of the boundary layer of fluids passing through said nozzle whereby thermal expansion may take place in the throat region of said hollow member without disturbing the smooth flow of fluids therethrough.

References Cited in the file of this patent FOREIGN PATENTS Y

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