EXPERIMENTAL INVESTIGATION OF NONIDEALITY AND NONADIABATIC EFFECTS UNDER HIGH PRESSURE RELEASES

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1 EXPERIMENTAL INVESTIGATION OF NONIDEALITY AND NONADIABATIC EFFECTS UNDER HIGH PRESSURE RELEASES Kznetsov M.*, Pariset S., Friedrich A., Stern G., Jordan T. Karlsrhe Institte of Technology, Karlsrhe, Germany Pro-Science GmbH, Germany *Corresonding athor: ABSTRACT De to the nonideality of a high ressre hydrogen release the ossibility of a two-hase flow and its effect on the dynamics of the discharge rocess was exerimentally investigated. A small-scale facility was designed and constrcted to simlate the transient blow-down of a cryogenic flid throgh a small break. Gaseos and liqid nitrogen were lanned to were sed as a srrogate for GH and LH. The reslts will comlement the qasi-stationary safety reglation tests and will rovide time-deendent data for verification of the theoretical models. Different orifice sizes (.5,,, 4 mm) and initial N ressres (3 bar) were sed in the tests. The measred time-deendent data for vessel discharge ressre, thrst, discharge mass flow rate, and gas temeratres were comared against a theoretical model for high ressre nitrogen release. This verification for nitrogen also assres the eqation of state for hydrogen, which is based on the same methodology.. INTRODUCTION The raidly growing worldwide energy needs and the diversification of the energy mixed with an increasing share of renewable energies are challenging for existing energy infrastrctres. More flexible soltions are reqired, esecially those that integrate renewable energies more efficiently. With icefel (integrated cable energy system for fel and ower) a flexible system for energy storage, distribtion and then reconversion has been develoed []. The concet of icefel roject was based on a grid of ser-inslated cable for the transort of cryogenic fels like liqid or sercritical ressrized hydrogen or natral gas. These cryogenic fels serve as energy carrier as well as energy storage media. A damage of sch a cable with a small leak of hydrogen may lead to formation of hydrogen jet, which can be ignited being remixed with air []. Different flame roagation regimes may develo in sch hydrogen air jet deending on initial ressre and temeratre in blk volme (as icefel cable, for instance). The roblem of safety analysis in this case is non-ideal state of the released gas and the gas inside the blk vessel. This may reslt in difficlties to evalate main roerties of hydrogen jet strctre dring the release. One can see on the diagram of state of real hydrogen taken from [3] that two-hase flow of hydrogen is very robable in the case of ressrized sercritical hydrogen release starting from the conditions close to the oeration of the icefel cable (see Fig. ). Real side-view of two-hase flow of hydrogen for the test conditions #46 (- mm nozzle, 7 bar, 4K, see Fig.) is shown in Fig.. The dynamics of discharge mass flow rate may be calclated analytically from the integral form of the soltion of Bernolli eqation written as follows ρ m& = CD A max ρ d, () ρ ρ where C D is the discharge coefficient; A is the break area; and ρ is the ressre and the density of the gas stream (sb-index ) and downstream of the flow (sb-index ):

2 + > + + = + + ; ; d d d d D A C m ρ ρ &, () for critical (chocked) or sbcritical flow nder isentroic conditions. Figre. Temeratre entroy (T-S) diagram of state of real ara-hydrogen taken from [3]. Dashed lines show a range of the real gas state dring the isentroic release rocess. Initial state of the gas (ressre and temeratre) is shown as sqared oints with labelled test nmbers. Another roblem of the high ressre hydrogen release is non-adiabatic natre of the rocess. Deending on initial ressre, diameter of the orifice and thermal isolation of high ressre volme the release conditions might be more close to iso-thermic or transient. The discharge coefficient takes into accont the difference between theoretical and exerimental mass flow rate: theor D m m C & & ex =, (3) where theoretical mass flow rate can be calclated analytically as for ideal gas by Eq. () or more recisely by nmerical integration of Eq. (). Bt even with the nmerical integration, it is not clear what is the real athway of the discharge rocess with resect to the constant entroy or real state of the gas, esecially in the case of two-hase flow. Objectives The objective of crrent work is to obtain detailed exerimental data on the high-ressre releases in wide range of initial ressres and nozzle diameters to take into accont nonideality of the

3 rocess. In order to simlify the conditions for two-hase flow, nitrogen will be sed instead of the ressrized hydrogen. With these work, a caability of nmerical and theoretical models for high ressre hydrogen releases will be validated against time-deendent exerimental data. Figre. Side view of cryogenic hydrogen jet [3]. Initial conditions corresond to the test #46 (- mm nozzle, 7 bar, 4K).. DIAGRAM OF STATE OF REAL GASES Figre shows the temeratre entroy (T-S) diagram of state of real ara-hydrogen. A T-S diagram is the most freqently sed to analyze adiabatic high ressre gas releases. The thick solid line corresonds to satration rocess of gas or liqid to two-hase gas-liqid state. This diagram allows to visalize the work done by or on the system and the heat added to or removed from the system. The heat transferred to or from a system eqals to the area nder the T-S crve of the rocess. Another advantage of T-S diagram is the ossibility to evalate the ratio gas/liqid within two-hase area. The more left final state of the system aears within two-hase area, the more liqid hase is resent in the system. As one can see from Fig., ractically in all exerimental cases the two-hase state of the released sbstance might be reached in the case of isentroic discharge rocess (shown by ble arrow). The mist articles of liqid hydrogen cold be clearly seen in Fig. in case of ressrized cryogenic hydrogen release in air. The roblem for the exeriments is that there is some difficlty to rovide detailed measrements of ressre, temeratre and mass flow rate for sch ressrized cryogenic hydrogen systems becase, for instance, initial blk temeratre hydrogen vessel mst be below the ambient one to reach two-hase flow. This is the reason to sbstitte hydrogen by nitrogen at ambient temeratre. Another reason to se inert nitrogen instead of hydrogen is to eliminate the ossibility for hydrogen combstion as in the case of hydrogen release in air. A T-S diagram of state for real nitrogen was calclated sing real gas eqation of state from NIST database [4]. The T-S diagram of nitrogen (Fig. 3) shows that starting from the ressre of bar at ambient temeratre the two-hase state cold be reached dring the isentroic rocess of high-ressre discharge to atmosheric ressre (vertical arrows). The ratio of liqid to vaor 3

4 will increase with an initial ressre increase above bar. NIST Nitrogen Eqation of State 3 Temeratre (K) 5 5 Satration bar 5 bar bar bar 3 bar 5 bar 75 bar bar 5 bar bar Entroy (kj/kg*k) Figre 3. Temeratre entroy (T-S) diagram of state of real nitrogen. Vertical arrows corresonds to the cases of isentroic nitrogen releases from high ressre to atmosheric ressre of bar. The area nder thick black bell-shaed line corresonds to the two-hase (gas-liqid) state of nitrogen. 3. EXPERIMENTAL DETAILS A small-scale DISCHA facility was designed and fabricated to simlate the transient blow-down of a cryogenic gas throgh a small break. Gaseos and liqid nitrogen were sed to sbstitte gaseos and liqid hydrogen (GH and LH ). The reslts will comlement the qasi-stationary tests and will rovide time-deendent data for verification of the theoretical and nmerical models develoed in [5]. The schematic of the facility is resented in Fig. 4 The test vessel (ble) is located on a low-friction sledge which is monted itself to a highresoltion scale (caacity 5 kg, resoltion 5 g). The vessel has two discharge oenings at different heights to test hase searation dring the two-hase blow-down, however only one art is sed in a given test. The test is initiated by comter controlled oening of the fast vale V5, or V4, resectively. The discharging flid will create a thrst F which can be measred by the force transdcer F. The scale rovides an indeendent measrement of the mass flow m& leaving the vessel. Both qantities are connected by the thrst eqation: F = mv & + ) A, (4) e ( e + e where F is the thrst [N]; m& mass flow rate [kg/s]; V e is the exit velocity [m/s]; e is the exit ressre [Pa]; is the ambient ressre [Pa]; A e nozzle area [m ]. When the exit flow conditions are determined theoretically, Eq. (4) allows to check a consistency of measred and calclated data. The same nozzle geometry is as in the ICESAFE facility [3] is sed in these tests. 4

5 The iing system shown in Fig. 4 allows to fill the test vessel with gaseos or liqid nitrogen. The vessel is designed for bar, it has a wall thickness of 3 mm and a volme of.8 dm 3. For the initial blow-down tests gaseos nitrogen with to bar was sed. For the tests at ambient temeratres the ressre vessel was not thermally inslated. Figre 4. DISCHA facility for transient two-hase blow-down tests with gaseos nitrogen. The instrmentation is smmarized in Fig. 4. It consists of a force transdcer (F), the scale (M), three thermocoles inside the vessel at different heights (T-T3), and two fast ressre transdcers inside the vessel at different heights (P, P).The test erformance, data acqisition, and storage is controlled with an extensive Labview rogram. The tests with nitrogen gas were erformed with orifice sizes of.5,, and 4 mm, initial ressres between 3 and bar, initial temeratres between 9 and 36 K, sing the er and lower discharge arts of the vessel D and D (Fig. 3.75). A calclated by [4] sond seed in nitrogen will be sed for scaling measred blow-down ressres. Some exeriments were reeated to check the rerodcibility of the reslts. First, the 4 mm reslts will be resented as an examle for the measrements erformed in this roject, and then a scaling of measred transient ressres will be erformed. 4. RESULTS AND DISCUSSION Fig. 5 (to) dislays measred thrst forces for six different initial N ressres. The signals show an exonential decay which is serimosed by an oscillation dring the first second. The amlitde of the oscillation increases with increasing initial ressre. In ftre exeriments a stronger sort of the force transdcer will be tested to remove these oscillations. The bottom art of Fig. 5 dislays the calclated thrst for the bar discharge exeriment sing Eq. (4). The mass flow m& is nmerically calclated sing the relation Eq. (). Also the exit ressre e and the exit velocity V e are calclated with the adiabatic aroach same way as in [5]. The measred thrst can be well rerodced with a discharge coefficient of C D =.9. For the tye of conical nozzle as sed in the resent exeriments, C D -vales arond.97 are often qoted. The smaller C D -vale fond here may be attribted to the fact, that the calclation assmed an adiabatic discharge of N, whereas in the exeriment heat transfer from the vessel wall increased temeratre 5

6 and ressre dring the blow-down rocess. thrst [N] N(93K), d=4mm bar 5 bar bar 75 bar 5 bar 3 bar time [s] N(93K, bar), d=4mm Force Calclations thrst [N] C D =.9 ρ m& = CD A max ρ d ρ ρ time [s] Figre 5. Measred thrst in transient nitrogen gas discharge exeriments with different initial ressres (to) and calclated time deendent thrst for the bar exeriment (bottom). The orifice size was 4 mm. Fig. 6 dislays the corresonding reslts for measred and calclated ressres. The measred ressres show the exected smooth decay withot any oscillations. This decay has been analyzed in detail assming ideal gas behavior and constant vales for = c /c v [6]. Using the nitrogen real gas eqation-of-state and the methodology described in [5] for hydrogen it gives very good agreement between exeriment and theory, if the same discharge coefficient of.9 as in the thrst evalation is alied. 6

7 ressre Drck [bar] 5 3 bar 5 bar 75 bar bar 5 bar bar time Zeit [s] N(93K, bar), d=4mm Pressre(Kistler) Pressre(PCB) Calclations ressre [bar] 5 5 C D =.9 ρ m& = CD A max ρ d ρ ρ time [s] Figre 6. Measred vessel ressre in transient nitrogen gas discharge exeriments with different initial ressres (to) and calclated time-deendent ressre for the bar exeriment (bottom). The orifice size was 4 mm. Figre 7 comares measred and calclated transient gas temeratres for a nitrogen discharge exeriment starting from bar and 93 K. The nozzle diameter was 4 mm. The calclated adiabatic gas temeratre agrees very well with the thermocole data dring the first -3 seconds. Thereafter heat transfer from the walls revents frther gas cooling and later even cases warming of the nitrogen gas. Again the discharge coefficient C D =.9 was sed in the calclations. Sch a C D vale is consistent with the theoretical reslts of [6], in which an adiabatic and isothermal discharge of air is comared. The first limiting case assmes no heat transfer from walls, the second limiting case assmes sfficient time for heat transfer to maintain the temeratre of the gas in the vessel constant. The redicted mass flow for the isothermal discharge is abot 3% smaller than that of the adiabatic case. Since the N -exeriments described here, resent an 7

8 intermediate case between the two limiting discharge cases, a slight decrease of the discharge coefficient C D de to wall-gas heat transfer can be exected. 5 N(93K, bar), d=4mm temeratre [ o C] T (5 mm) T (75 mm) T3 (5 mm) Calclations C D = time [s] Figre 7. Measred and calclated gas temeratres in a N discharge exeriment starting from bar and 93 K. The exerimental data indicate heat transfer from vessel walls and thermal stratification. Nozzle diameter 4 mm. The thermocole measrements indicate a thermal stratification in the vessel dring the blowdown rocess. The coldest temeratres are recorded at the bottom (thermocole T, 5 mm below the vessel cover). Thermocole T is also the closest one to the discharge nozzle D sed in this test (see Fig. 4). Heat transfer dring the blow-down rocess was also analyzed by calclating the time-deendent N gas entroy S (, T) from measred ressres and gas temeratres dring the discharge, sing the NIST real gas eqation-of-state [4]. Figre 8 dislays calclated gas entroies for a 5- bar, 5- bar, -bar, and -bar discharge exeriment, sing the 4 mm nozzle and the data of thermocole T. The initial gas temeratre in these tests was near 3 K. The gas entroies are initially constant, following the isentroic ath very closely, bt later heat transfer cases temeratre and entroy increases, reventing the formation of two-hase states, which wold occr an ideally isentroic exansion from bar. In the resent set- heat transfer becomes significant after deressrization to abot % of the initial ressre, excet in the 5-bar test which was nearly adiabatic de to the small discharge time and smaller temeratre gradient. The inflence of the discharge time becomes very obvios if the exeriments from bar initial ressre bt with different nozzle diameters (Fig. 9). The blow-down times reqired from bar to bar were 7 s, 59 s, 4 s, and 3.4 s for the.5 mm, mm, mm, and 4 mm nozzles, resectively. The longer the discharge time, the smaller the temeratre decrease in the gas de heat transfer from the walls. The resonse time of the thermocole, which consisted of bare (nsheathed). mm Tye K wires, was less than ms. 8

9 Temeratre (K) Satration bar 5 bar 5 bar(4 mm Ex) bar bar 3 bar 5 bar 5 bar(4 mm Ex) 75 bar bar bar(4 mm Ex) 5 bar bar bar(4 mm Ex) Entroy (kj/kg*k) Figre 8. Calclated N gas entroies in N-discharge exeriments with 5, 5,, and bar initial ressre and 4 mm nozzle diameter. The high ressre tests are inflenced by heat transfer from the wall to the gas, which revents two-hase release for the and bar tests. Temeratre (K) Satration bar 5 bar bar bar 3 bar 5 bar 75 bar bar 5 bar bar bar(.5 mm Ex) bar( mm Ex) bar( mm Ex) bar(4 mm Ex).5 mm nozzle mm nozzle mm nozzle 4 mm nozzle Entroy (kj/kg*k) Figre 9. Calclated nitrogen gas entroies in nitrogen discharge exeriments from bar initial ressre with different nozzle diameters. 9

10 The GN exeriments have rovided imortant information on how heat transfer inflences the blow-down of a high-ressre gas system. It shows that heat transfer from ambient atmoshere may revent two hase flow for relative small leaks. This means that it needs larger nozzle diameter, higher ressre, lower initial temeratre and better thermal isolation to really reach the two-hase state. 5. SCALING OF TRANSIENT DISCHARGE PRESSURES The transfer of exerimental data from any small scale facility to ractical alications on larger scale reqires a dimensional analysis of the governing rocesses. For the discharge of ideal gases from higher ressre vessels non-dimensional analytical soltions have been resented for the time-deendent ressre in the reservoir [6]: ( + ) + ( + ) + = + t, (5) where t is the time; is the ressre; dimensionless ressre, + = (t)/ ; is the initial blk ressre; adiabatic exonent, = c /c v ; dimensionless time, t + = t/t char ; characteristic release time, t char = V/(A c ); V is the vessel volme; A is the nozzle area; c is the initial sond seed of gas in vessel. Measred exerimental ressres (t) are dislayed in Fig. in terms of + vs. t + for the different nozzle diameters of., and 4. mm. Scaling with + and t + reslts in very good agreement of the tests with different initial ressres for a given nozzle. There is a small systematic sreading of the crves for + <., which is robably de to gas-wall heat transfer. The measres ressres (t) were evalated from the initial ressre down to end = 3 bar to remain in the choked flow regime nozzle. mm bar 5 bar bar 5 bar bar t+ nozzle 4. mm 3 bar 5 bar bar 5 bar bar t+ Figre. Scaling of measred transient ressres for different initial ressres (3- bar) and nozzle diameters (. and 4. mm) sing the er discharge ort D. Note that the sed characteristic time incldes the sond seed of the gas c in its initial state /T, which varies significantly with the initial ressre. Althogh Eq. (5) was originally derived for ideal gases with constant and constant sond seed dring the blow-down. It also allows a very good scaling of the resent non-ideal high-ressre discharge exeriments with nitrogen. Figre comares the scaled ressre histories for the er discharge ort and for the five different nozzle diameters. For each given nozzle diameter only the largest initial ressre is

11 lotted (3 and bar). A good overall correlation is observed. The differences are de to the above discssed heat transfer effects and the discharge time. The slowest exeriments (.5 and mm nozzles) show the highest vales for + (t + ). For instance the scaled time t + to reach a scaled ressre + =. is abot t + = 3.5 for the.5 mm nozzle. This nmber shifts to smaller vales for increasing nozzle diameter and decreasing discharge time, reaching abot t + =.5 for the 4 mm nozzle. 5 N(93K, bar) d=.5 mm d= mm d= mm d=3 mm d=4 mm ressre [bar] time [s] N(93K, bar).8 d=.5 mm d= mm d= mm d=3 mm d=4 mm t+ Figre. Scaling of measred transient ressres for different nozzle diameters and initial ressre of bar sing the er discharge ort D. The comarison of exerimental reslts and calclations by Eq. (5) shows that the exerimental reslts with heat transfer tend to converge against the theoretical soltion withot heat transfer with decreasing discharge time. The above discssed discharge ressres were all measred sing the er discharge nozzle D in Fig. 4. The same test series was erformed with the lower discharge nozzle D giving identical (t) within the ressre transdcer recision (.5%). As follows from crrent exeriments, in case of single-hase discharge of nitrogen from the test vessel no effect of the release location on the

12 discharge ressre was observed, and the scaling discssed above leads to ractical identical reslts. 6. SUMMARY AND CONCLUSIONS A small-scale facility for transient discharge of cryogenic nitrogen was designed, constrcted and tested with gaseos nitrogen. Different orifice sizes (.5,,, 3, 4 mm) and initial N ressres (3 bar) were investigated. The measred time-deendent data for vessel discharge ressre, thrst, discharge mass flow, and gas temeratres cold be well rerodced sing the NIST database for the real gas eqation-ofstate of nitrogen [4]. This verification for nitrogen also assres the EOS for hydrogen, which is based on the same methodology [5]. Another imortant finding is, that the newly develoed critical discharge analysis method for a re sbstance [5] redicts correctly the transient blow-down of a high-ressre gas system. Becase a nmber of hysical model assmtions and nmerical aroximations are involved in the methodology, correctness and recision for transient simlations had to be demonstrated. The dynamic nitrogen tests have rovided an additional verification which is comlementary to the qasi-static DISCHA exeriments. The measred ressre histories cold be scaled very well sing initial ressre and sond seed of the gas, vessel volme and nozzle area as characteristic qantities. New reslts abot heat transfer effects in blow-down of gaseos high-ressre systems have been obtained. For relatively small nozzle diameter, lower initial ressre and absence of thermal isolation the heat from srronding may comletely eliminate the two-hase scenario of high ressre release. The facility is ready for extension of the exeriments to liqid nitrogen and investigation of cryogenic two-hase discharge in a ftre roject hase. ACKNOWLEDGEMENTS This work was done within the Icefel roject ( fnded by the German Federal Ministry of Edcation and Research. REFERENCES. A. Dylla, H-transort drch Kabel. Energieversorgng "HZwei Das Magazin für Wasserstoff nd Brennstoffzellen", (Aril 7) 7. Jahrgang. A. Veser, M. Kznetsov, G. Fast, A. Friedrich, N. Kotchorko, G. Stern et al. The strctre and flame roagation regimes in trblent hydrogen jets, International Jornal of Hydrogen Energy, 36,, A. Friedrich, W. Breitng, G. Stern, A. Veser, M. Kznetsov, et. al, Ignition and heat radiation of cryogenic hydrogen jets, International Jornal of Hydrogen Energy, 37,, P.J. Linstrom and W.G. Mallard, Eds., NIST Chemistry WebBook, National Institte of Standards and Technology, Gaithersbrg MD, 899, htt://webbook.nist.gov, (retrieved May 3, 3) 5. J. Xiao, J.R. Travis, W. Breitng, Hydrogen release from a high ressre gaseos hydrogen reservoir in case of a small leak, International Jornal of Hydrogen Energy, 36,, J. Dtton, R. Coverdill, Exeriments to stdy the gaseos discharge and filling of vessels, Int. J. Eng. Ed., 3, 997,