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Superheating and Boiling of Water in Hydrocarbons at High Pressures*

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Superheating and Boiling of Water in Hydrocarbons at High Pressures* hr. J. Hear Mass Tran$&. Vol. 24. No. 4, pp. 695.-706. I981 ~l7-9310/81/0~695 I2 %OZ.W,‘O Printed ,n Great Britam @ 1981 Pergamcn Press Ltd SUPERHEATING AND BOILING OF WATER IN HYDROCARBONS AT HIGH PRESSURES* c. T. AVEDISIAN Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, U.S.A. and 1. GLASSMAN Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, U.S.A. (Receioed 15 April 1980 and in revisedform 18 September 1980) Abstract-The boiling of water drops superheated in some nonvolatile liquid n-alkanes was experimentally investigated at ambient pressures up to 4 MPa. It was found that boiling appeared to be initiated at the water-hydr~rbon interface by either growth of a single bubble or by streams of bubbles being released from the interface. The temperature at which boiling was first observed was found to be relatively insensitive to pressure, increasing only about 40 K over a 4 MPa change in pressure. A qualitative theory based on the homogeneous nucleation ofbubbles within superheated liquids is used to explain the effect of boiling pressure on temperature. The two observed boiling modes are justified in terms of consideration of the surface and interfacial free energies of the water and hydrocarbon, and qualitative agreement between the measured and predicted variation of nucleation pressures with temperature is demonstrated. The information obtained was used to provide insight into the mechanism by which the disruptive combustion or ‘micr~xplosion’ of burning water-in-fuel emulsifi~ droplets would be initiated in high pressure combustion applications. NOMENCLATURE 0, surface tension ; a, Peng-Robinson constant, equation (11); dl2I in terfacial tension ; A, constant defined in equation (13); ffO* pro~rtion~ity constant, equation (8); b, Peng-Robinson constant, equation (12); I** critical exponent, equation (8); B, constant defined in equation (14); 49 fugacity coefficient. W*b symbol for a vapor nucleus containing n* molecules; Subscripts f, nucleation rate; 4 species i ; k, Boltzmann constant; ie, saturation conditions for species i; % molecular mass of species i; 1, water ; Noi- molecular number density of species i; 2, hydrocarbon. p, total vapor pressure; INTRODUCTION pot liquid pressure; 1. pie* equilibrium vapor pressure of species i ; DIRECT contact heat transfer between a volatile and R gas constant ; nonvolatile liquid can often result in si~ifi~ant T, temperature; superheating, followed by explosive boiling, of the T cir critical temperature of species i; volatile liquid. Important practical instances in which T CW water-n-alkane critical solution such explosive boiling has been observed include the temperature; spilling of liquid natural gases on water [l], the 0, liquid molar volume; reaction between water and liquid metals [2] and the VOis molecular volume (vJ6.02 x 10z3); burning of water-in-oil emulsions [3]. In these situ- Y, vapor phase mole fraction; ations the rapid growth of bubbles either within the 6 vapor phase compressibility factor volatile liquid or at the interface between the volatile (Pv/RT). and nonvolatile liquid is responsible for the explosive boiling effect. Greek symbols In the absence of extraneous nucleation aids such as 6ijs interaction parameter, equation (11); particles, the initial microscopic bubble whose growth leads to the observed boiling effect forms by the natural processes of homogeneous nucleation. The *Work performed at the Department of Mechanical and rate of formation of vapor bubbles which are of a size Aerospace Engineering, Princeton University, Princeton, such that they are in both chemical and mechanical NJ 08544, U.S.A. equilibrium with the surrounding liquid (i.e. critical 695 size nuclei) depends on the physical properties of the A schematic diagram of the expernnental mstal- particular liquid in which the bubbles form. However, lation is shown in Fig. I. The apparatus consisted of a if nucleation occurs at the interface between the double chamber configuration in which an inner te\! volatile and nonvolatile liquid, the properties of both section containing the hydrocarbon P~;LXmvunted liquids may be expected to affect the nucleation rate. within an outer pressure vessel. Slotted window:. on The homogeneous nucleation of bubbles within the the pressure vessel with direct hack, !ighting providsd bulk of superheated liquids has been extensively visual access to the interior of the ICS! SecTion. The tee: studied as has been reviewed by Blander and Katz [4]. section essentially consisted of ;L glans :nhc (45 mm The experimental study of bubble nucleation at a (I.D., 41 mm I.D. and 450 mm iong) which was closed liquid-liquid interface has received much attention at one end. The open end was Sealed hy ;t stainlers ,,reel [5-83 and no work has been reported at preasures cap with O-rings. The lube ~214 heated h! IM’I~ above atmospheric. The experimental evidence seems aluminum blocks with bariac reguiarcd electrical vtrlp to indicate that, at atmospheric pressure, nucleation heaters attached along its length. The ie\t ~ctL>r: and growth of bubbles at a liquid-liquid interface is assembly was hung from the tsndrrstdc of the top C‘U~C typically characterized by a string of bubbles emerging plate of the pressure cell, anii ali wires were pas& from the interface between test drop and field Liquid through Conax ‘feedthrough’ fittings screwed intc 11:~ [5,7-91, instead of the characteristic explosive boiling top cover plate. Filtered nitrogen gas was u\eii ii.3 attendant to homogeneous nucleation of bubbles pressurize the fuel and was in contact with it ani;, ~11;UI within the interior of a liquid [4, 9, IO]. external reservoir. This reservoir was placed appror;- In the present work we report the results of an imatelv 1.14m from the pressure: ccl1 in orde:- i:s experimental program in which we attempted to minimize the amount of nitrogen gas dissolved in ihe measure the incipient boiling pressure at various liquid in the test section. temperatures of water drops superheated within non- The water injection system convicted uf an externai volatile hydrocarbon field liquids. The properties of reservoir connected to a stainless steel capillar! tube the hydrocarbons were selected such that nucleation of (0.13 mm I.D. and 1.6 mm O.D.), one end ofwhieh C\S bubbles at the water- hydrocarbon interface would be passed through a heat exchanger T-h?ting motrnted OI> more likely to occur than within the bulk of the water the top cover pkife and into ihe tt)p ,>f rhc Se\i >r’ct!lin. drops. The information obtained was used to provide and the other end ofwhich wa5 cot~n~~red t(o ;I \&noiri insight into the mechanism by which disruptive burn- valve. The water was pressurized !J, nrtrogen pa\ in ing or ‘microexplosion’ of water-in-oil emulsified contact with the free liquid surface m tht: reser\olr, and drops would be initiated during droplet combustion. the reservoir was connected to the solenoid I a1t.c b! :i The objectives of this work were to investigate (I) length of standard 6.3 mm stain&\ steel I ubinp i. i4 I:> the manner in which water drops boiled in some long. A high pressure Millipore Ctter holder ~~~llt~~~nl~l~ nonvolatile tt-alkanes; (2) the variation of boiling a 0.45 i-1”’ filter was attached to the outlet of [he watch temperature with pressure; and (3) the relationship reservoir Ry pressurizing the water rcser\oir fro~r~ between nucleation theory and the experimental 30 kPa to 70 h-Pa higher than thl- pressure in !hc it’\{ observations. section. anJ then quicki! opening am.i cl~\mp thi: solenoid v&e. a group of drops iypically under 3 mm in diameter could be forced through the capillar> tubi: and into the fuel. 2.1. Description of the apparatus As the water drops fell. the pressure \%a< released b;, Water drops were injected into a column filled with manually opening a ball valve. When a watt!; drop a lighter nonvolatile liquid hydrocarbon under press- which had been selected for obsel ~;lii~~n Erorn ;rmong ure and at a uniform temperature. The initial the group of falling drop\ was,. &served io emif pressure was set equal to the saturation pressure of bubbles or change its direction tst motion. an evcn~ water at the hydrocarbon temperature to insure marker on a chart recorder was r~:rnoicl~ azti\a& h> intimate liquid-liquid contact between water and pushing a button, thus giving a record of the pres?urc hydrocarbon at the start of an experiment. As the and temperature of boiling. Temperature was record& heavier water drops fell, they were superheated by b)’ two thermocouples located ;II lixed position< releasing the pressure on the hydrocarbon. When a 70 nltn and I X0 mm from the lop <)f lhe glasq lube iC\i water drop which had been arbitrarily selected for section. Whenever possible, the decomprescion ra!c observation by the naked eye began to boil. the was regulated in a manner such that the waler drops pressure and temperature of the fuel were recorded on boiled between the two ~her~n~~~~)u~l~~whusi: lcm- a chart recorder by remotely activating an event peratures difl’ered by less than 0.4 K. The pressure was marker button. A variation of this isothermal decom- recorded by a transducer connected IO one c)!’ IWO pression method was used by Moore [6] to measure channel< on the chart recorder: the other chatmci w;is he limit of superheat of pure freon- 12 in water as a connected to the lower ofthe two th~rtnocouple~ tri the function of pressure, and by Forest and Ward [ 11, 121 test section.
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