COMPARISON OF HEAT TRANSFER CHARACTERISTICS IN SURFACE COOLING WITH BOILING MICROJETS OF WATER, ETHANOL AND HFE7100

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1 4th Micro and Nano Fows Conference UCL, London, UK, 7-0 September 04 COMPARISON OF HEAT TRANSFER CHARACTERISTICS IN SURFACE COOLING WITH BOILING MICROJETS OF WATER, ETHANOL AND HFE700 Da0riusz Mikieewicz, Tomasz Muszynski : Gdansk Uniersity of Technoogy, Facuty of Mechanica Engineering, Department of Energy and Industria Apparatus Narutowicza /, Gdansk, Poand E-mai: Dariusz.Mikieewicz@pg.gda.p, Tomasz.Muszynski@pg.gda.p Abstract The basis of microjet technoogy is to produce aminar jets which when impinging the surface hae a ery high kinetic energy at the stagnation point. Boundary ayer is not formed in those conditions, whie the area of fim cooing has a ery high turbuence resuting from a ery high heat transfer coefficient. Appied technoogy of jet production can resut with the size of jets ranging from 0 to 500μm in breadth and 0 to 00μm in width. Presented data are used in order to aidate authors own semi-empirica mode of surface cooing by eaporating microjet impingement in the stagnation point. Main objectie of this paper was to inestigate the physica phenomena occurring on soid surfaces upon impingement of the singe microjet in case of three fuids. Intense heat transfer in the impact zone of microjet has been examined and described with precise measurements of therma and fow conditions of microjets. Reported tests were conducted under steady state conditions for surface cooing by singe microjet producing an eaporating fim. Obtained database of experimenta data with anaytica soutions and numerica computer simuation aows the rationa design and cacuation of microjet modues and optimum performance of these modues for arious industria appications. Keywords: microjets, heat transfer intensification, boiing,. Introduction Accurate contro of cooing parameters is required in eer wider range of technica appications. It is known that reducing the dimensions of the size of nozze eads to an increase in the economy of cooing and improes its quaity. Present study describes research reated to the design and construction of the nozzes and microjet study, which may be appied in many technica appications such as in metaurgy, eectronics, etc. Using iquids such as water, boiing is ikey to occur when the surface temperature exceeds the cooant saturation temperature. Boiing is associated with arge rates of heat transfer because of the atent heat of eaporation and because of the enhancement of the ee of turbuence between the iquid and the soid surface, Garimea and Rice (995). This enhancement is due to the mixing action associated with the cycic nuceation, growth, and departure or coapse of apour bubbes on the surface. In the case of fow boiing, such as boiing under impinging jets, the interaction between the bubbe dynamics and the jet hydrodynamics has significant effect on the rate of heat transfer. The common approach used to determine the rate of boiing heat transfer is by using a set of empirica equations that correate the aue of the surface heat fux or the heat transfer coefficient with the fuid properties, surface conditions, and fow conditions. These correations do not proide much insight into the underying physica mechanisms inoed in the boiing heat transfer probem, Liu and Zhu (00). The aternatie approach is to use mechanistic modes. There hae been a number of mechanistic modes deeoped for the case of poo boiing and for the case of parae fow boiing. In the atter case, the boiing heat transfer phenomenon is more compicated due - -

2 4th Micro and Nano Fows Conference UCL, London, UK, 7-0 September 04 to the strong couping between the fow, the therma fied, and bubbe dynamics. The basis of microjet technoogy is to produce aminar jets which when impinging the surface hae a ery high kinetic energy at the stagnation point, Mikieewicz and Muszynski (009), Mikieewicz et a (03). Boundary ayer is not formed in those conditions, whie the area of fim cooing has a ery high turbuence resuting from a ery high heat transfer coefficient. Appied technoogy of jet production can resut with the size of jets ranging from 0 to 500μm in breadth and 0 to 00μm in width. Main objectie of this paper was to inestigate the physica phenomena occurring on soid surfaces upon impingement of the singe microjet in case of three fuids. Intense heat transfer in the impact zone of microjet has been examined and described with precise measurements of therma and fow conditions of microjets. Reported tests were conducted under steady state conditions for surface cooing by singe microjet producing an eaporating fim. Obtained database of experimenta data with anaytica soutions and numerica computer simuation aows the rationa design and cacuation of microjet modues and optimum performance of these modues for arious industria appications. Presented data are used to aidate authors own semi-empirica mode of surface cooing by eaporating microjet impingement in the stagnation point..experimenta setup Present study shows resuts of steady state heat transfer experiments, conducted for singe phase cooing in order to obtain wa temperature and heat fuxes. Fig. shows the schematic diagram of the test section. It consisted of the probe, fuid suppying system, the measuring deices and DC power suppy. Working fuid was fed to the nozze from a suppy tank, which aso seres as the pressure accumuator. The water pressure in the test section was raised by an air compressor. Desired fuid fow rate was obtained by sustaining the constant pressure of fuid with a proper use of fow contro ae. In order to reduce pressure drop necessary to create a steady aminar jet, nozze was mm ong. Due to ow oumetric fow rate of cooant it was measured at inet and outet from the cooing chamber with a graduated fask. Fig. The schematic diagram of test section Fig. The cross-section of the probe The cooed surface was the copper truncated cone with top diameter 0mm and 0mm height. Water impingement surface was sierpated, in order to preent high temperature erosion. The radia distribution of surface temperature was determined with the aid of fie T-type thermocoupes, created from embedding 50μm thick constantan wire to the copper core. Heat is suppied by a kantha heater, eectricay isoated with sapphire gass and mounted at the bottom of the core. The whoe set is thermay insuated by gass woo and paced in the casing. Heater is powered by a DC power suppy and the tota power input - -

3 4th Micro and Nano Fows Conference UCL, London, UK, 7-0 September 04 is determined by measuring current intensity and otage. During test s heater was capabe of dissipating up to 40W. Additiona four K-type thermocoupes are attached in the copper rod axis. These thermocoupes measure axia temperature gradient at the core of a heating bock and contro temperature of the heater. They are connected to the Nationa Instruments data acquisition set. The signa from thermocoupes was processed with the aid of the LabVIEW appication. Heater operating power aues are precisey controed and measured. The appied power osses through conduction into the insuation and radiation to the surroundings are accuratey cacuated and accounted for in a tests. Data are taken from a steady state measuring points in order to excude heat capacity of the instaation. The nozze construction aows modification of its dimensions. In case of these studies nozze with hydrauic diameter of 75 µm was used. Experiments were conducted for the spacing of 50mm between the nozze exit and impinging surface. Because of imited power suppy, ow water mass fux were used in order to obtain wide range of surface temperature. 3. Theoretica mode In the anaysis of carried out experiments the simpe theoretica mode of impinging iquid microjet with phase change is presented. Proposed is the mode of heat transfer in the stagnation zone, where iquid rapidy eaporates due to contact with the hot surface. The mode is deeoped on the basis of known pressure difference between the nozze exit and the stagnation point. As a resut of eaporation of impinging jet, there is formed a apour banket on the surface. The dynamic pressure is the way in which the nozze interacts with the surface, p d =u d /. In the anaysis considered aso coud be the capiary effect p kap =/D (with D being the nozze diameter) and the hydrostatic pressure drop p g gh (where H denotes the jet suspension oer the surface). The atter two hae howeer been omitted in the present anaysis as they hae been regarded as of secondary importance. The tota pressure acting on the surface yieds: p p p p () c d The schematic of rapidy eaporating microjet is presented in fig. 3. The radius on which the spreading of iquid fim is taking pace can be determined from the energy baance on the eement of the cooed pate, which reads: q kap R m h c p TSAT T0 () From () the impingement radius reads: R m ( h c T ) q The appicabiity for further cacuations of the radius obtained from equation (3) has aso been confirmed experimentay in the course of authors own experiment (Mikieewicz et a. 009). In that study aues or the range of cooing of the surface, obtained from (3) showed a ery good consistency with experimenta findings. Substituting into that equation the aues of properties at atmospheric pressure and ambient temperature of 0 o C (c p =484.3 J/kg K, h =56.4 kj/kg, and T=80K, C=908.) the obtained radius of cooing was equa to.87mm, which agreed ery we with experimenta finding for that case when mass fow rate was kg/s and heat fux q=3.0mw/m. In the anaysis of the eaporating jet the foowing assumptions were made: ony a dynamic part of pressure difference in () is acting on the jet, iquid temperature on the radius of eaporation R is higher than saturation temperature. The mass baance on eaporation surface yieds: p g (3) dm R d dm R V d (4) Equation (4) enabes to determine the expression for estimation of the fim thickness dm d m R V (5) - 3 -

4 4th Micro and Nano Fows Conference UCL, London, UK, 7-0 September 04 Standard soution is obtained assuming that u =0 and eocity at outet is found from: Fig. 3. Scheme of rapidy eaporating microjet In order to soe equation () we must proide the means for cacuation of apour eocity V. That can be done by the reference to the concept by Kutateadze who soed a simiar probem for the outfow from a arge tank through a sma hoe. In our approach we assume that the radia apour motion from the stagnation point can be modeed in a simiar way. It is assumed in our approach that apour which is formed as a resut of impingement forms a cyinder with the base corresponding to the nozze diameter, as seen from fig. 3. Simiary to Kutateadze s approach, such motion can be modeed as fow of apour from the side wa of the cyinder of the size D, by anaogy to the outfow from the tank through a sma hoe. Such probem, as sketched in fig. can be described by Bernoui equation: u p gh p u gh Fig. 4. Outfow from the arge tank through a sma hoe (6) p u (7) In our case we are deaing with the fow of apour coming out of the nozze. Hence we assume that =. We are aware that athough in the mode due to Kutateadze eocity at state is assumed zero, in our case it has a finite aue which can be determined from: q u (8) h In subsequent works on that topic eqn (8) wi be attempted to be incuded into modeing but for the time being the apour eocity is: V pc In (9) pc ud is the microjet dynamic pressure and - the contraction number. Substituting apour eocity to (5) yieds: d (9) m R u (0) After describing the heat transfer on cooed surface as heat conduction in apour ayer, the fim thickness can be found: ( Tw TSAT ) D () 4 ud h cp( TSAT T 0 ) Knowedge of fim thickness aows to determine the heat transfer coefficient: D Nu Ja Pr ( Ja ) where T W TSAT c p Pr Ja a h c p, and ( TSAT T0 cp Ja ) h Some attention shoud be deoted to the cacuation of the contraction factor present in equation (9). A good consistency of resuts is partiay deoted to the fact that the () - 4 -

5 4th Micro and Nano Fows Conference UCL, London, UK, 7-0 September 04 contraction factor present in (9) is cacuated from the empiricay adjusted formua: Bo (3) 4 Experimenta aidation Fig. 7 shows the experimenta resuts of fuy deeoped nuceate boiing heat transfer at the stagnation zone for impinging saturated water jet using the nozze dimensions of 0.x0.05mm. Heat transfer data are potted in the form of boiing cures for different iquid mass fux. The Kutatieadze s empirica correation is aso presented as a soid ine, and Mostinski s correation as dashed ine to point out the simiarity of the considered jet cooing to the poo boiing case of iquid. For nuceate poo boiing of water at atmospheric pressure, correation due to Kutateadze (95) can be simpified to: 0W 3 qcr 5.58 ( TW TS ) m (4) The Mostinski mode (963) takes into account ony the critica parameters of working fuid: P T T P* q (5) crit where: p p p P* Pcrit Pcrit Pcrit A cear trend can be obsered that increasing mass fux of iquid eads to the higher aue of critica heat fux. The cures foow in a quaitatie way the distribution of the boiing cure predicted by the equation due to Kutateadze. Higher mass fuxes aso resut in smaer wa superheats, howeer are shifted eft. Figures 5 to 7 show the heat fux on cooed surfaces with water, ethano and HFE700, for different iquid eocities. In eery case increased fuid inet eocity resuted in higher heat fuxes remoa. Fig. 8 presents the stagnation point heat transfer coefficients in function of predicted aues of heat transfer coefficient resuting from equation (), in form of dimensioness Nusset number. w sat Appication of (3) enabes cacuation of heat transfer coefficients with a ery reasonabe accuracy. In Fig. 9 presented are distributions of the ratio of experimenta to theoretica Nusset number in function of wa superheating, which shows that the mode () is capabe of capturing the trends in boiing heat transfer in impinging in the stagnation point, within reasonabe error imits. A trend to predict ower HTC than experimenta aues, in higher wa superheat can be diminished by improement of eq.(3). Fig. 5. Heat fux density in function of jet Reynods number for boiing HFE700 jet Fig. 6. Heat fux density in function of jet Reynods number for boiing jet of ethano - 5 -

6 Nu th [-] Nu exp /Nu th [-] 4th Micro and Nano Fows Conference UCL, London, UK, 7-0 September 04 aue aries with iquid subcooing. Future actiities shoud proceed in the direction of better determination of that parameter..5.5 ETHANOL HFE700 WATER +30% -30% Fig. 7. Heat fux density in function of jet Reynods number for boiing water jet ETHANOL HFE700 WATER +30% -30% Nu exp [-] Fig. 8. Experimenta and theoretica heat transfer coefficients cacuated using () CONCLUSIONS In the paper presented hae been theoretica and experimenta studies of eaporating microjet impingement. Anaytica mode of stagnation Nusset number was presented. Modifications and further deeopment of this mode wi take pace in course of further work. Very important issue in cacuation of heat transfer in microjet cooing is cacuation of friction factors. It has been found that its Tw[ o C] Fig. 9. Reation between experimenta and theoretica heat transfer coefficients in function of wa temperature. References Garimea S.V., Rice R.A. (995). Confined and submerged iquid jet impingement heattransfer, J. of Heat Transfer, 7, pp Kutateadze S.S., (95) Heat Transfer in Condensation and Boiing, United States Atomic Commission. Liu Z-H, Zhu Q-Z, (00). Prediction of critica heat fux for conectie boiing of saturated water jet impinging on the stagnation zone, J. of Heat Transfer, 4, pp Mikieewicz D., Muszynski T. (009). Experimenta study of heat transfer intensification using microjets, Int. Symp. on Conectie Heat and Mass Transfer in Sustainabe Energy, Hammamet, Tunisia. Mikieewicz D., Muszyński T., Mikieewicz J. (03).Mode of heat transfer in the stagnation point of rapidy eaporating microjet, Archies of Thermodynamics. 33, No., Mostinski I.L. (963), Cacuation of heat transfer and critica heat fux in boiing iquids based on the aw of corresponding states, Tepoenergetika, 0,