Queroseno

fela

zhity, Xhina

Received in revised form 8 March 2012Accepted 9 March 2012Available online 21 March 2012

Keywords:Supercritical kerosene

tribution and pressure drops in the tubes were obtained in the experiment. The coking characteristic of

temperature hydrocarbon fuels is the formation of coke. Coke gen-erally refers to the surface carbon diffusing through metals, precip-itating out at grain boundaries, and catalytically growing carbonlaments that include grains of the metal surface. The coke will re-duce the metals strength and ductility, and then reduce the heattransfer by acting as an insulator on the surface. Also, the coke willclog small passages in fuel systems and injectors [2].

Another signicant application of heat transfer enhancement isto reduce the temperature difference between the uid and thewall for a xed heat ux. For wall cooling, it is very useful. Theenhancement techniques were classied by Webb and Bergles in1983 [4] as: passive and active techniques. Passive techniques donot require direct application of external power, whereas activetechniques require an external activator/power supply to bringabout the enhancement. For example, the rough surface is a typicalpassive heat transfer enhancement technique that is used in manyactual industry situations.

Corresponding author. Tel.: +86 29 82667034; fax: +86 29 82669033.

Experimental Thermal and Fluid Science 42 (2012) 1624

Contents lists available at

a

w.eE-mail address: [email protected] (H. Wang).Hydrocarbon fuels, which usually work at supercritical pressureand high heat ux, are very important in the active cooling systemof rockets and in the regenerative cooling of scram jet engine,where they act as refrigerants. The conditions found in the regen-erative cooling passage of a modern rocket engine include intro-duction of the fuel at high initial pressures (20.748.3 MPa) anda heat ux above several MW/m2 [1]. Because of their crucialimportance in rocket engine cooling technology, the supercriticalheat transfer of hydrocarbon fuels such as kerosene demands thor-ough investigation. Another key issue in the application of high

generation heat transfer technology and is becoming widely usedin many industrial situations as it can improve the heat exchangereffectiveness. Generally, it can be implemented by increasing theow velocity, unsteadiness, and turbulence, or by limiting thegrowth of uid boundary layers close to the heat transfer surface.Actually, the goal of enhanced heat transfer can be stated as the de-sire to encourage or accommodate high heat uxes [3]. For xedtemperature difference and constant heat exchanger area the heatux can be increased by elevating the heat transfer coefcient.Alternatively, heat transfer enhancement permits the accommoda-tion of high heat uxes at a moderate temperature difference [3].Ribbed roughnessHeat transferPressure dropSquare rubeCoking

1. Introduction0894-1777/$ - see front matter 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.expthermusci.2012.03.00the square tube was also analyzed, with the method of wall temperature variation with the time. Exper-imental conditions included pressures of 522 MPa, velocities of 1560 m/s, and maximum heat ux of40,000 kW/m2. Four different ribbed structure square tubes and two ribbed circular tubes were tested.Compared with the smooth tube, the experimental results showed the ribbed roughness can improvethe heat transfer performance while the pressure drop in the tubes is increased. It was found that ofeither the square tube or the circular tube, the tube with the ratio of rib pitch to height 7.5 can getthe best integrated heat transfer and pressure drop effect. With the increase of velocity, the coking is sig-nicantly improved, and the initial coking temperatures for the square smooth and ribbed tube are about370 C and 410 C, respectively. The introduction of ribbed roughness also has the benet of the cokingweakening. The heat ux when coking occured in the ribbed roughness square tube was much higherthan that of the smooth square tube. For assessing the engineering application, and for deeper under-standing of the rib effect on the heat transfer and pressure drop, more ribbed structures need to beinvestigated.

2012 Elsevier Inc. All rights reserved.

Now heat transfer enhancement has indeed become a second-Article history:Received 17 March 2011

An experiment on supercritical kerosene heat transfer and ow characteristics in square and circular owchannels with ribbed roughness was carried out at Xian Jiaotong University. The wall temperature dis-Experimental investigation on heat transsupercritical pressure in square and circu

Haijun Wang a,, Yushan Luo a, Hongfang Gu a, Honga State Key Laboratory of Multiphase Flow in Power Engineering, Xian Jiaotong Universib The 11th Research Institute, Academy of Space Propulsion Technology, Xian 710001, C

a r t i c l e i n f o a b s t r a c t

Experimental Therm

journal homepage: wwll rights reserved.9r and pressure drop of kerosene atr tube with articial roughness

Li a, Tingkuan Chen a, Jianhua Chen b, Haibo Wub

ian 710049, China

SciVerse ScienceDirect

l and Fluid Science

lsevier .com/locate /et fs

al aHeat transfer enhancement technology can be used in theregenerative cooling of scram jet engines and rocket engines, inwhich it can notably improve the heat transfer. Along with the en-hanced heat transfer, another benet is the reduction of the walltemperature, which is most likely to improve the working uidcoke characteristic such as kerosene. Therefore, the introductionof enhanced heat transfer technology in rocket and jet engineregenerative cooling has important signicance.

In the 1960s and 1970s, for the purpose of developing of super-critical and ultra-supercritical pressure fossil-red power plants,there was extensive research on heat transfer to uids at supercrit-ical pressures. Recently, there has been a renewed interest in thiseld as a result of active consideration worldwide in the develop-ment of several promising new applications involving supercriticalpressure uids such as supercritical water cooled nuclear reactors,innovative air-conditioning and refrigeration systems, and regen-erative cooling systems for new generation liquid rocket engines.Comprehensive literature reviews of earlier studies on heat trans-fer to carbon dioxide and water at supercritical pressure can befound in Pitla et al. [5], Pioro et al. [6] and Cheng et al. [7]. Themajority of these studies are related to supercritical carbon dioxideand water. As for heat transfer enhancement at supercritical pres-sure, many studies focused on the ried tube, an articial rough-ness method to enhance heat transfer. Watson et al. [8] foundthat the rotational ow in the ribbed tube can greatly raise theCHF and the critical vapor quality. Nishikawa et al. [9] found thatried tubes with different geometrical structures have differentheat transfer enhancement performances. Kolher and Kastner[10] experimentally ascertained that the ried tube can effectively

Nomenclature

s rib pitch, mmh rib height, mme ratio of s to hp pressure, MPav ow velocity, m/sq heat ux, kW/m2

Re Reynolds numberNu Nusselt numberPr Prandtl numbert temperature, Cf friction coefcientDp pressure drop, MPa

H. Wang et al. / Experimental Thermdelay heat transfer deterioration and improve heat transfer in thepost dryout region. Ackerman [11] experimentally found that thetube with a rough wall surface can increase the turbulent shearstress and prevent heat transfer deterioration. Similar work wasdone by a Chinese research group, by whom heat transfer andresistance characteristics in different dimension ried tubes atsubcritical and supercritical pressure were investigated [1215].

Due to their importance in rocket and jet engine regenerativecooling, the heat transfer and ow characteristics of hydrocarbonfuels, especially kerosene, were investigated in detail. Billingsley[16] studied the thermal performance (thermal stability and heattransfer characteristics) of RP-2 fuel in a highheat ux facility aimedto provide initial RP-2 thermal performance information under con-ditions simulative of those encountered in the cooling channels of areal engine. A circular copper tube was used in the test section.Short-duration thermal stressing tests provided heat transfer infor-mation and the effects of wall temperature, bulk temperature, andow rate on the heat transfer were observed. Longer-duration testsat elevated wall temperatures provided some useful information inelucidating the conditions under which solid carbon deposits form.At the same time, some numerical research work about RP-2 fuelunder high uxes was done by the same team [17]. Zhang et al.[18] experimentally studied the heat transfer characteristics withinan air/fuel heat exchanger in an attempt to cool the cooling air fromthe compressor with RP3 kerosene as the coolant. The supercriticalfuel was injected into the combustor after the heat exchange whilethe cooled cooling air was led to the turbine for blade and disk cool-ing. A series of U-turn tubes were used in his experiment. Testresults indicated that the heat transfer coefcients were monoto-nously increasing along the tube and in the bend regions the heattransfer was enhanced due to the strong secondary ow inducedby the centrifugal force. A series of electrically heated tube experi-ments were conducted to investigate the potential of JP-7 as a cool-ant under conditions relevant to a Mach 8 propulsion system byDiane L. Linne et al. [2]. The heat transfer capabilities, carbon depo-sition, and material compatibility of JP-7 were tested in differenttubes with 0.125 in. diameter. Zhong et al. [19] experimentallyand analytically investigated the heat transfer characteristics of Chi-na No. 3 kerosene under conditions relevant to a regenerative cool-ing system for scramjet applications. The wall temperature and thebulk fuel temperatureweremeasured at the same location along theow path to analytically deduce the local heat transfer characteris-tics. Another Chinese research team also investigated the heattransfer characteristics of methane, propane, No. 21 high-densitykerosene, aero kerosene and rocket kerosene in stainless-steel andcopper circular tubes under high pressure and heat ux conditions[20], and the deposit formation rates for kerosene in stainless-steeltubes were investigated in the same test. Their results showed theheat transfer coefcient change rules and the basic coking charac-

l test section length, md hydraulic diameter, m

Greeksymbolsq density, kg/m3

s time, min

Subscriptsw walli inner surface

nd Fluid Science 42 (2012) 1624 17teristic. Bates et al. [1], utilizing a System for Thermal Decomposi-tion Studies test rig, investigated the thermal stabilityperformance of fuels from very small fuel samples, and measure-ments of heat transfer coefcients and the effect of wall tempera-ture, ow velocity, and wetted-material on deposit formation inheated test channelswere obtained from some larger test rigs. Com-prehensive numerical studies of the turbulent convective heattransfer of the cryogenic propellant, methane, owing inside a hor-izontal mini tube under supercritical pressures were conducted byWang et al. [21]. The effects of many key parameters, includingthe inlet pressure, wall heat ux, inlet velocity, and inlet tempera-ture, on the supercritical heat transfer phenomena of the cryo-genic-propellant, methane, were studied in detail.

The majority of the above studies were related to heat transferand coking characteristics in the smooth tube under supercriticalpressure and high heat ux. Very few studies of supercritical heattransfer with hydrocarbon fuels in the heat transfer enhancementtube can be found in the literature. Due to their potential practicalapplication in rockets and jet engines, it is necessary to conductsome investigation to analyze the heat transfer and ow

characteristics of hydrocarbon fuel in the heat transfer enhance-ment tube. In this study, the heat transfer and ow characteristicsof kerosene owing inside horizontal square and circular tubeswith articial roughness under supercritical pressures and highheat uxes were examined, focusing on fundamental understand-ing of the effects of the roughness parameters on the heat transferprocess. The results will provide information important to practicalengineering design.

2. Experimental apparatus

2.1. Experimental loop

The experiments were conducted on the supercritical ow withheat transfer test loop at Xian Jiaotong University, using the elec-trical heating method. The schematic diagram of the test loop isshown in Fig. 1. It consisted of an oil tank, a high-pressure pistonpump, a lter, an orice plate, a heat exchanger, a preheater, a testsection, a condenser, a rotameter and a number of valves. The heatexchanger was designed for heat recovery and the rotameter wasused to visually monitor the ow rate in the experimental system.The test section and the preheater were directly heated as resis-tance elements by alternating current. The heating power was con-tinuously regulated by transformers. The mass ux and pressure inthe experimental loop were precisely controlled by adjustingvalves. The orice plate was used to measure the mass ux inthe test section, which was calibrated with a weighing techniquewith a stopwatch, and the maximum uncertainty was 3.2%. Theuid pressure and temperature were measured at different loca-tions by pressure transmitters and NiCrNiSi K type sheathed ther-

2.2. Test section

In this study, two types of heat transfer enhancement tubeswere used as the test section. One was the square tube and theother was the circular tube. The tubes were made of copper.Fig. 2 shows a schematic diagram of the square and circular testsections. The hydraulic diameters for the square tube and the cir-cular tube were 2.4 mm and 1.8 mm, respectively. Both the squareand circular tubes have the same total length of 350 mm. Only100 mm and 150 mm lengths of the tubes were electrically heatedto get the heat transfer and ow data for the square and circulartubes; the rest of the 350 mm long tubes were treated as the stableow segment of upstream and downstream. The outer wall tem-perature was measured by 0.2 mm NiCrNiSi K-type thermocou-ples, which were jointed on the outer tube wall. The maximumuncertainty was 0.7%. The outer surface of the heating section ofthe test tubes was divided into 14 measurement cross-sectionsto get the outer wall temperature. The corresponding inner walltemperature was determined from a one-dimension heat conduc-tion model with a known heat ux and outer wall temperature.The heat ux was estimated from the electric power imposed onthe test section, and the thermal efciency and the maximumuncertainty was 3.8%. The pressure drops in the test section weremeasured by a differential pressure transmitter and the maximumuncertainty was 0.59%. The heat insulator was put on the outertube wall to minimize the heat loss.

An articial surface roughness rib was mounted on the wall ofeach of the test tubes. For the square tube, the rib was mountedonly on the bottom side wall, while a circular rib was mountedin the circular test tube. The different rib types were distinguishedby the combination of the rib height and pitch as shown in Fig. 2.

rim

18 H. Wang et al. / Experimental Thermal and Fluid Science 42 (2012) 1624mocouples, respectively. The maximum uncertainties were 1.4%and 2.1%, respectively.

Fig. 1. ExpeFig. 2. Schematic diagram of test sections:The e = s/h is dened as the ratio of the rib pitch to the rib height. In

ental loop.(a) circular tube and (b) square tube.

this study, four square tubes with different e were tested, and theywere 6.25, 7.5, 10 and 14.3. For the circular tube, there were onlytwo different e tubes, 7.5 and 10. The width of the rib was0.7 mm. The height of the rib was 0.35 mm for the tubes withe = 6.25, 7.5 and 10, and the height of the rib was 0.6 mm for thetube with e = 14.3.

2.3. Data acquisition

The data acquisition system consisted of an industry computer,a 24 V power supply and some IMP (Isolated Measurement Pods)35951C data acquisition plates made by British Solartron UK Co.All measurement signals were connected with the computer inthe control room through the data lines on the acquisition plate.The computer carried out the monitoring of the test working con-

points. In general, all the experimental conditions were involved

depends mostly on the mass ow rate, which enhances the heattransfer signicantly. Fig. 6 shows the heat transfer characteristics

wall temperature, the heat ux is much higher in the tube withhigher uid velocity, moreover, the higher the velocity, the steeperthe curve slope. The same phenomena can be found in other ribbedtested tubes.

H. Wang et al. / Experimental Thermal and Fluid Science 42 (2012) 1624 19in the comparisons. It can be seen that the experimental datapoints match the correlation very well, especially for the low Renumber conditions. Even for the high Re number, the deviationsbetween them do not exceed 10%.

Fig. 4 shows the heat transfer data points of water in the smoothcircular and the square tubes, separately. It can be seen that thedimensionless heat transfer data points in the smooth square tubeare in agreement with those of the smooth circular tube. From theresults of Figs. 3 and 4, it can be concluded that this test loop,dition and the acquisition and processing of data.

2.4. Experimental parameters

The main parameter range of the working condition for the testwas as follows:

Test section inlet pressure p: 5, 10, 15, 22 MPa.Inlet uid velocity v: 15 m/s, 30 m/s, 60 m/s.Heat ux q: 80040,000 kW/m2.

3. Experimental loop checking

To evaluate its reliability, the experimental loop was checkedwith smooth circular and square tubes before the formal test,and de-ionized water was used as the working uid. Except forthe tubes and working uid, all the equipment and the methodwere the same as in the kerosene articial roughness tube test.In the checking test, the water owed into the smooth tubes witha pressure of 10 MPa and a velocity range from 10 m/s to 40 m/s.

Fig. 3 shows the results of the Gnielinski Nusselt correlation[22] compared with the smooth circular tube experimental dataFig. 3. Comparison of experiment data with computed data in the smooth tube.of the e = 7.5 ribbed square tube under different velocities with apressure of 22 MPa and a uid inlet temperature of 20 C. As ex-pected, the heat transfer increases with velocity. For a given innerincluding the measurement instruments and the method, is reli-able enough to carry out the kerosene heat transfer and owexperiment.

4. Results and discussion

4.1. Heat transfer

The working uid used here was kerosene. Its critical pressureis 2.495 MPa, and its critical temperature is 404.3 C. Fig. 5 showsthe density and specic heat of kerosene varying with temperatureat a pressure of 10 MPa. Pseudo-critical temperature is dened asthe temperature, for a given pressure, at which the specic heatexhibits a maximum. Generally, the uid pseudo-critical tempera-ture increases with the pressure. When the wall temperature islower than the pseudo-critical value, the heat transfer of keroseneunder supercritical pressure can be regarded as a single phase con-vective heat transfer, i.e., the heat transfer coefcient is not inten-sied explicitly with the increase of the heat ux, rather, it

Fig. 4. Comparison of heat transfer data between the smooth circular and squaretubes.Fig. 5. Kerosene properties change with temperature.

2000

1600

a

20 H. Wang et al. / Experimental Thermal and Fluid Science 42 (2012) 1624Fig. 6. Comparison of the heat transfer characteristics of kerosene at differentvelocities.The uid inlet pressure has little effect on the heat transfer, asshown in Fig. 7. It was found that the heat transfer characteristicsexhibit almost the same trend at different inlet pressures for thee = 7.5 roughness tube. The main reason for this trend is becausethe experimental pressure is far above the critical pressure of thekerosene and the uid is in a state of supercritical pressure. There-fore, the kerosene has similar behavior as the single phase uid.The uid properties will not change dramatically with the increaseor decrease of pressure.

The heat transfer data of the ribbed roughness tubes with differ-ent e can be tted according to the following model:

Nu m Ren Pr0:4 1

The tting error is less than 7.8%. The values of m and n for the dif-ferent tubes are shown in Table 1.

Fig. 8 presents a comparison of the heat transfer characteristicsbetween the smooth square tube and the ribbed roughness squaretubes with different e. It is known that the articial roughness can

Fig. 7. Comparison of the heat transfer characteristics of kerosene at differentpressures.

Table 1Heat transfer and pressure drop characteristic of the ribbed roughness square tube.

Tube structure m in Eq. (1) n in Eq. (1)

e = 6.25 0.0186 1.04e = 7.5 0.0156 1.09e = 10 0.0037 1.216e = 14.3 0.009 1.141200

800

400

0

28000

24000

20000

16000

12000

8000

0 40000 80000 120000 160000

bproduce appreciable heat transfer enhancement. At the same walltemperature, the ribbed roughness tubes can accommodate muchhigher heat uxes, or at the same heat ux, the wall temperatureof the ribbed tube is much lower than that of the smooth tube.For example, as is shown in Fig. 8b, when the heat ux is9000 kW/m2 the wall temperature for all the ribbed tubes is about100 C lower than that in the smooth tube.

In this study, the ribbed tubes all have relatively high e valuesthat are classied as a K-type roughness element [23]. In this typeof roughness tube, the separated ow over the initial rib becomespartially reattached before encountering the upstream face of thenext rib. Although the turbulent structure is still unclear so far, itis well known that the existence of a rib can enhance the turbu-lence and reduce the boundary layer thickness. This will result inaugmentation of the heat transfer. Some research has indicatedthat reasonable articial roughness can enhance the heat transferup to 22.5 times, while the pressure drop only increases 1.31.5 times [24]. The effect of the vortex structure resulting fromthe rib on the ow and heat transfer characteristic has been dis-cussed in detail in Ref. [24].

Heat transfer times Pressure drop times

1.56 2.141.88 1.981.44 1.321.83 3.8

4000

0

0 50 100 150 200 250 300 350 400

Fig. 8. Comparison of heat transfer characteristics between the smooth and ribbedsquare tubes.

of ribbed roughness tubes can signicantly enhance the heattransfer.

4.2. Pressure drop

The pressure drop characteristic for the ribbed roughness tubeswas also tested and analyzed in this research. The friction coef-cient was calculated by the following equation:

Dp f l 1 q v2 2

a

Fig. 10. Comparison of heat transfer characteristics between the smooth and ribbedcircular tubes.

H. Wang et al. / Experimental Thermal and Fluid Science 42 (2012) 1624 21Table 1 lists the values of m and n for different ribbed tubes inEq. (1) and the average heat transfer enhancement times comparedto the smooth square tube, separately. It was found that differentribbed roughness structures resulted in different heat transferenhancement effects. For the ribbed roughness tubes with e = 7.5and e = 14.3, the heat transfer Nu numbers were around two timesthose of the smooth square tube. This was followed by the ribbedroughness tubes of e = 6.25 and e = 10, for which the Nu numberswere about 1.5 times those of the smooth square tube. So, it wasconcluded that e = 7.5 and e = 14.3 give the best heat transferenhancement effect.

For the circular tube, two ribbed roughness types were tested,e = 7.5 and e = 10. Fig. 9 shows the heat transfer results of theribbed circular tube e = 7.5. For the circular tube, the increase ofkerosene velocity can notably improve the heat transfer effective-ness. As shown in Fig. 9, when the wall temperature was 300 C,the heat ux transferred in the tube was about 9076 kW/m2 witha velocity of 15 m/s. When the uid velocity increased to 30 m/sand 60 m/s, the corresponding heat uxes were 17,345 kW/m2

and 33,981 kW/m2, which were 1.9 and 3.7 times that of 15 m/s.For the two tested types of ribbed roughness circular tubes, the

heat transfer data also can be tted according to the model of Eq.(1). The coefcients of m and n are listed in Table 2.

Fig. 10 shows the comparison of the heat transfer characteris-tics between the smooth circular tube and the ribbed roughnesscircular tubes with different e. It is consistent with the square tuberesult that the rib roughness produces a remarkable heat transfer

Fig. 9. Heat transfer characteristics of ribbed circular tube with e = 7.5.enhancement effect. As seen in the comparison between the tworibbed tubes, the tube with e = 7.5 has a better effect, whose Nunumber is about 2.5 times that of the smooth circular tube in theexperimental range. For the e = 10 tube, this value is about 2.2.From the perspective of heat ux, it was found that when the walltemperature reached the upper limit, the heat ux of the smoothtube was about 20,000 kW/m2, while the value of the two ribbedroughness circular tubes was about 40,000 kW/m2. From the pointof view of wall temperature, it can be seen that when the uidvelocity was 60 m/s and the heat ux was 20,000 kW/m2 the walltemperature of the smooth tube was about 375 C, while the walltemperatures of the ribbed tubes at the same conditions were230 C (e = 10) and 215 C (e = 7.5) separately. There was adecrease of about 145 C. So it is undoubtedly that the two types

Table 2Heat transfer and pressure drop characteristic of ribbed roughness circular tube.

Tube structure m in Eq. 1 n in Eq. 1

e = 7.5 0.022 1.094e = 10 0.025 1.067bd 2

Here Dp is the measured pressure drop in the test section, l is thelength of the test section, d is the hydraulic diameter, q is the den-sity and v is the average velocity.

Fig. 11 shows the results of f for the tests of the ribbed rough-ness square tubes and the smooth square tube. It was found thatthe friction factor of the e = 14.3 tube was much higher than thatof the other ribbed roughness square tubes. Although it had anobvious heat transfer enhancement effect, the increase in the pres-sure drop was remarkable. Its friction factor was about 3.5 timesthat of the smooth square tube. On the contrary, the e = 7.5 tubethat had the best increase in heat transfer showed a moderatepressure drop increase. Its friction factor was about 1.9 times thatof the smooth square tube, while the f of the tube with e = 10 just

Heat transfer times Pressure drop times

2.5 2.032.2 2.07

22 H. Wang et al. / Experimental Thermal ahad a slight rise compared to the smooth tube. All the friction fac-tor times of the ribbed roughness square tubes to smooth tube arelisted in Table 1.

Fig. 12 presents the friction factors of all the tested circulartubes. It is reasonable that with the increase of heat transfer thepressure drop will increase synchronously. It can be seen fromFig. 12 that the friction factors for the two types of ribbed rough-ness tubes were all much larger than that of the smooth circulartube, and they were almost two times that of the smooth tube.By comparing Figs. 11 and 12, it can be found that the friction fac-tor of the ribbed circular tube at the same condition is slightly lar-ger than that of the ribbed square tube.

To integrate the heat transfer and friction pressure drop factorcharacteristics of all the tested tubes, it can be concluded that ofeither the ribbed circular tubes or the ribbed square tubes, the tubewith e = 7.5 has the best effect. It has an excellent increase in heattransfer, with only a moderate rise in pressure drop. For the squaretube with e = 14.3, although it had the best heat transfer increaseeffect, its pressure friction factor was extremely high comparedwith other ribbed tubes. In addition, the Nu number of the e = 10square tube was only about 1.44 times the smooth square tube,while its pressure friction factor was the lowest among the ribbedroughness square tubes. For the ribbed roughness e = 10 circulartube, its heat transfer and pressure drop all had a good effect. Sothe rib structure of e = 10 is also an acceptable option. In this study,only four types of e ribbed roughness tubes were tested. To get a

Fig. 11. Comparison of friction factors of all the tested square tubes.deeper understanding, more tubes with different rib structureneed to be investigated in the future and then more choices canbe supplied for the engineering design.

Fig. 12. Comparison of friction factors of all the tested circular tubes.4.3. Coking

In the actual industrial application of kerosene, due to heating,carbon deposit, i.e. coking, will happen on the metal wall surface.This can reduce the metals strength and ductility, and then reducethe heat transfer by acting as an insulator on the surface. Some-times it will lead to equipment burning. So it is necessary to under-stand the characteristic of coking to avoid it in practicalengineering.

In this study, the coking was not measured quantitatively. Anindirect method was used, monitoring the tube wall temperaturechanging with the time, to qualitatively analyze the coking charac-teristic. It is well known that if there is coking on the wall surface,the deposited carbon will give additional heat transfer resistance.It will make the wall temperature increase abnormally. In theexperiment, at rst the tube wall temperature was heated to250 C, then this temperature was maintained for 20 min. Duringthis period, the wall temperature was monitored and recorded. Ifthere was no abnormal change in temperature, the heat ux wasincreased to make the wall temperature rise up to 300 C. It wasthen kept at that temperature for 20 min to monitor and recordthe wall temperature change. If there was no obvious temperaturechange observed, the heat ux was increased again. The settingtemperature points were 330 C, 350 C, 380 C and 400 C, whichtemperatures were each maintained for 20 min to observe. If thewall temperature at any temperature point exhibited an abnormalchange, the wall temperature was monitored and recorded until itreached 500 C. Then the experiment was stopped for the sake ofsafety. In the coking experiment, only the square tube was tested.

Fig. 13 shows the wall temperature varying with the time forthe smooth square tube in the coking experiment. Here, two mea-surement sections in the test tube were selected to reect the cok-ing situation of the tube. The two sections were close to the outletof the tube. This was mainly because, in the electrical heating mod-el, the wall temperature will be higher near the tube outlet. And itis known coking tends to occur at high temperatures. Between thetwo sections, Section 2 was closer to the outlet. The test parame-ters for Fig. 13 are: uid velocity 30 m/s, pressure 15 MPa. It canbe seen that the measuring temperature at a lower heat ux,5582 kW/m2, almost does not change with time for a long experi-mental time. This means that the tube is in a stable condition andthere is no carbon deposit (coking) on the inner wall surface. Whenthe heat ux rose to 8116 kW/m2, the wall temperature exhibitedan abnormal increase. From the initial 370 C to the test end570 C, the wall temperature in the section 1 top side rose 200 Cduring a period of 90 min. And the growing rate of the wall tem-perature with time became more and more quick. This indicatedthere was a carbon deposit or coking occurring on the wall surface.Also the test result showed that the initial coking temperature forthe smooth square tube was about 370 C.

Fig. 14 shows the coking result of the smooth square tube at ahigher velocity. Comparison with Fig. 13 shows that the increasein uid velocity can greatly increase the heat ux of the cokingoccurrence. The heat ux was 17,129 kW/m2 when the wall tem-peratures abnormal rise was monitored. This heat ux was almosttwice that of the low uid velocity in Fig. 13. In other words, withdouble the uid ow velocity, the corresponding heat ux whencoking occurs is also approximately doubled. And again the test re-sult showed that the initial coking temperature for the smoothsquare tube at higher velocity was about 370 C.

In addition, it was found that the coking wall temperature var-iation characteristic under high velocity was not exactly the sameas under low velocity. It indicates an unstable coking phenomenon.

nd Fluid Science 42 (2012) 1624From Fig. 13, it can be seen that when coking occurs, the wall tem-perature goes up monotonically. Fig. 14 shows that under highervelocity the coking wall temperature will increase in a zigzag path,

al a340

320

300

a

H. Wang et al. / Experimental Thermwhich is up down up again down again and so on. This maybe due to the uid scouring capacity enhancement with the in-crease of velocity, with the result that the deposited carbon cannotbe stable on the wall surface. Sometimes some little depositedcarbon will be ushed out by the high velocity uid. But the cokingtrend is inevitable, and then the wall temperature will rise and fallrepeatedly.

Fig. 15 shows the coking characteristic of the ribbed roughnesssquare tube with e = 7.5. The test parameters are: velocity 60 m/sand pressure 15 MPa. Due to the ribbed roughness, the heat uxwhen coking occurs will greatly increase. Comparison of Figs. 15and 14 shows that the critical heat ux will rise from the

the heat transfer characteristic of kerosene at supercritical

Fig. 14. Variation in wall temperature over time for the smooth square tube at highvelocity.

280

260

600

550

500

450

400

350

0 20 40 60 80 100

0 10 20 30 40 50 60

b

Fig. 13. Variation in wall temperature over time for the smooth square tube.pressure in the square or circular tube. At the same time,the pressure drop of kerosene will also increasesignicantly.

(2) In the combined view of heat transfer and pressure drop,either the square or the circular tube with the ribbed struc-ture of e = 7.5 will give the best comprehensive effect17,129 kW/m2 of the smooth square tube to the 27,813 kW/m2 ofthe ribbed tube. The initial coking temperature for the ribbed squaretube is also raised to 410 C. So either the initial coking temperatureor the heat ux is improved by the ribbed roughness. The tempera-ture variation curve also exhibits a zigzag pattern in the ribbedsquare tube. Theunstable cokingphenomenon is veriedagainhere.

5. Conclusions

In this study, an experiment on the heat transfer and ow char-acteristics of kerosene owing inside horizontal square and circu-lar tubes with articial roughness under supercritical pressuresand high heat uxes was conducted. The wall temperatures andpressure drops in the tubes were obtained in the experiment.The following conclusions are the ndings of this investigationand the results will provide important information for practicalengineering design.

(1) The introduction of ribbed roughness can greatly improve

Fig. 15. Variation in wall temperature over time for the ribbed squarer tube withe = 7.5.nd Fluid Science 42 (2012) 1624 23among the experimental tube structures.(3) For the smooth square tube, the initial coking temperature

is about 370 C, and at relatively high uid velocity, the cok-ing presents a non stable phenomenon.

(4) Compared with the smooth tube, the heat ux, when cokingoccurs, in the ribbed roughness square tube is much higher,and the corresponding initial coking temperature will alsoincrease.

(5) Not only the heat transfer, but also the coking characteristicof the tubes benets from the ribbed roughness. For engi-neering applications, more ribbed roughness structuresneed to be investigated to nd the best parameters.

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24 H. Wang et al. / Experimental Thermal and Fluid Science 42 (2012) 1624

Experimental investigation on heat transfer and pressure drop of kerosene at supercritical pressure in square and circular tube with artificial roughness1 Introduction2 Experimental apparatus2.1 Experimental loop2.2 Test section2.3 Data acquisition2.4 Experimental parameters

3 Experimental loop checking4 Results and discussion4.1 Heat transfer4.2 Pressure drop4.3 Coking

5 ConclusionsReferences