Two-phase flow heat transfer of propane vaporization in horizontal minichannels

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1 Journal o Mechanical Science and Technology 3 (9) 599~ Journal o Mechanical Science and Technology DOI 1.17/s Two-phase low heat transer o propane vaporization in horizontal minichannels Agus Sunjarianto Pamitran 1, Kwang-Il Choi 1, Jong-Taek Oh,* and Ki-Won Park 1 Graduate School, Chonnam National University, San 9-1, Dunduk-Dong, Yeosu, Chonnam, 55-79, Korea Department o Rerigeration and Air Conditioning Engineering, Chonnam National University, San 9-1, Dunduk-Dong, Yeosu, Chonnam, 55-79, Korea (Manuscript Received August 13, 7; Revised April 3, ; Accepted September 1, ) Abstract Eperiments were perormed on the convective boiling heat transer in horizontal minichannels using propane. The test section was made o stainless steel tubes with inner diameters o 1.5 mm and 3. mm and lengths o 1 mm and mm, respectively, and it was uniormly heated by applying an electric current directly to the tubes. Local heat transer coeicients were obtained or a heat lu range o 5- kw m -, a mass lu range o 5- kg m - s -1, saturation temperatures o 1, 5, and C and quality ranges o up to 1.. The nucleate boiling heat transer contribution was predominant, particularly at the low quality region. Decreases in the heat transer coeicient occurred at a lower vapor quality with a rise o heat lu and mass lu, and with a lower saturation temperature and inner tube diameter. Laminar low appeared in the minichannel lows. A new boiling heat transer coeicient correlation that is based on the superposition model or propane was developed with.7% mean deviation. Keywords: Boiling; Heat transer; Horizontal tube; Microchannel; Propane Introduction Several studies have been devoted to natural rerigerants as an environmental protection eort. Most HCFCs chemicals break down beore reaching the ozone layer, with the chlorine produced reaching the stratosphere to deplete the ozone layer. In inding a replacement or HCFCs, there have been some studies on natural rerigerants, such as propane, as an environmental conservation eort because they do not contain chlorine. Recent awareness o the advantages o process intensiication has also led to a demand or smaller evaporators or use in the rerigeration and air conditioning and processing industries. Smaller channels o a heat echanger give a higher This paper was recommended or publication in revised orm by Associate Editor Jae Young Lee * Corresponding author. Tel.: , Fa.: address: ohjt@honnam.ac.kr KSME & Springer 9 heat transer coeicient because o its larger heat transer contact surace. However, heat transer or two-phase lows in small channels cannot be properly predicted by using eisting procedures and correlations intended or large channels. There is a small quantity o published data that relates to two-phase low and heat transer or propane in small channels compared with that in large channels. Evaporation using a smaller channel may provide a higher heat transer coeicient due to its higher contact area per unit volume o luid. In small channels, as shown in Bao et al. [1], Zhang et al. [], Tran et al. [3], Kandlikar-Steinke [] and Pamitran et al. [5] the contribution o nucleate boiling is predominant and laminar low appears. This study was undertaken to obtain eperimental data or propane and to determine the local heat transer coeicient during evaporation by using some test conditions in minichannels. The eperimental results

2 A. S. Pamitran et al. / Journal o Mechanical Science and Technology 3 (9) 599~ Preheater P, T Sight glass A Mass Flow meter Re. Pump A P, T A A V V 1 mm mm P, T 1 mm 1 mm P Receiver P, T Test Sections Condenser Subcooler A A D i : 1.5 and 3. mm Thermocouple Water Pump Fig. 1. The eperimental test acility and test section. Rerigerator Unit were compared with several eisting heat transer correlations. A new correlation or propane in minichannels that is based on the superposition model is developed in this study because the correlation or orced convective boiling in small channels is limited.. Eperimental aspects.1 Eperimental apparatus and method The eperimental acility consisted o a condenser, a subcooler, a receiver, a pump, a mass low meter, a preheater, and test sections (Fig. 1). A variable AC output motor controller was used to control the low rate o the rerigerant. A coriolis-type mass low meter was used to measure the rerigerant low rate. The mass quality at the test section inlet was controlled by installing a preheater. For evaporation at the test section, a certain heat lu was conducted rom a variable AC voltage controller. The vapor rerigerant rom the test section was condensed in the condenser and subcooler, and then supplied to the receiver. The test sections were comprised o stainless steel smooth tubes with inner diameters o 1.5 and 3. mm and heated lengths o 1 mm and mm, respectively. The tubes were well insulated with rubber and oam. The outside tube wall temperatures at the top, both sides, and bottom were measured at 1 mm aial intervals rom the start o the heated length with thermocouples at each measured site. The test sections were heated uniormly and constantly by applying an electric current directly to their tube walls. The input electric voltage and current were adjusted in order to control the input power. The local saturation pressure, which was used to determine the saturation temperature, was measured with Bourdon tube type pressure gauges at the inlet and at the outlet o the test Table 1. Eperimental conditions. Working rerigerant Propane Test section Horizontal smooth minichannels Inner diameter (mm) 1.5, 3. Tube length (mm) 1, Mass lu (kg m - s -1 ) 5 Heat lu (kw m - ) 5 Quality. 1. Inlet T sat (ºC) 1, 5, Table. Summary o estimated uncertainty. Parameters Uncertainty Inner wall temperature ±1.5% Absolute pressure ±1.5% Mass lu ±5.5% Heat lu ±.9% Mass quality ±5.99% Heat transer coeicient ±.75% section, as shown in Fig. 1. Sight glasses with the same inner diameter as the test section were installed in order to visualize the low. Table 1 lists the eperimental test setup speciications in this study. The physical properties o the rerigerant were obtained by reerencing the REFPROP. The temperature and low rate data were recorded by using the Darwin DAQ3 Plus logger R9.1 sotware program and version.1 o the Micro Motion ProLink Sotware package, respectively. Table gives a summary o the estimated uncertainty associated with the test parameters. The uncertainties were obtained rom both random and systematic errors.. Data reduction The local heat transer coeicients at position z along the length o the test section were deined as ollows: h = T wi q T sat (1) The inside tube wall temperature, T wi was the average temperature o the top, both right and let sides, and bottom wall temperatures, and was determined by using steady-state one-dimensional radial conduction heat transer through the wall with internal heat generation. The quality,, at the measurement locations, z, was determined based on the thermodynamic properties:

3 A. S. Pamitran et al. / Journal o Mechanical Science and Technology 3 (9) 599~ 1 h tp (kw/m²k) 1 G (kg/m²s) q = 15 kw/m² D i = 3. mm T sat = 1 C h tp (kw/m²k) 1 q (kw/m²) 1 15 G = 15 kg/m²s D i = 3. mm T sat = C Fig.. The eect o mass lu on heat transer coeicient. i i = () ig The rerigerant low at the inlet o the test section was not completely saturated. Even though it was just short, it was necessary to determine the subcooled length or reduction data accuracy. The subcooled length was calculated with the ollowing equation to determine the initial point o saturation. i i i i zsc = L = L i,in,in ( Q W) The outlet mass quality was then determined with the ollowing equation: + i i i (3),in o = () ig The saturation pressure at the initial point o saturation was then determined by interpolating the measured pressure and the subcooled length. 3. Results and discussion Fig. shows the eect o mass lu on the heat transer coeicient. An insigniicant eect o mass lu on the heat transer coeicient at the low quality region o around up to. in Fig. indicates that nucleate boiling heat transer is predominant. The high nucleate boiling heat transer is supposed because o the physical properties o the rerigerants and the geometric eect o the small channels. Several studies with small tubes, including Kew and Cornwell [], Lazarek and Black [7], Wambsganss et al. [], Bao et al. [1], Zhang et al. [], Tran et al. [3] and Fig. 3. The eect o heat lu on heat transer coeicient. Pamitran et al. [5], reported that nucleate boiling is predominant in small channels, which is opposite that o the predominantly convective-dominated heat transer in conventional channels. Using a 1.95 mm tube, Bao et al. [1] showed that the heat transer coeicients were independent o mass lu. At the moderate quality region, heat transer coeicients increase with increasing mass lu and vapor quality. This is similar to the results reported by Kuo-Wang [9], who used R- in a 9.5 mm tube. The eect o mass lu on the heat transer coeicient appears at moderatehigh vapor quality o around... A higher mass lu results in a greater heat transer coeicient at moderately high vapor quality due to the increasing convective boiling heat transer contribution. At the high quality region o around.7 1., a decrease in the heat transer coeicient occurs at the lower quality under the higher mass lu condition, as is shown in Fig.. It is supposed that the higher mass lu results in a lower dry-out quality. This trend agrees with Pettersen [1] and Yun et al. [11]. For the higher mass lu condition in the convective evaporation region, an increase in the heat transer coeicient appears at a lower quality, which can be eplained by the annular low becoming dominant with increasing quality. Nucleate boiling suppression appears earlier or the higher mass lu, which means that convective heat transer appears earlier under the higher mass lu condition. The lower mass lu condition results show smaller increases in the heat transer coeicient in the convective region. Fig. 3 shows that a strong dependence o the heat transer coeicients on the heat lu appears at the low quality region o around up to.3. At the low quality region, the heat transer coeicients increased with increasing heat lu. Nucleate boiling is known

4 A. S. Pamitran et al. / Journal o Mechanical Science and Technology 3 (9) 599~ Table 3. Physical properties o propane. T ( C) ρ / ρ g µ /µ g σ (1-3 N/m) P (kpa) h tp (kw/m²k) 1 1 T sat = 1 C D (mm) q (kw/m²) G (kg/m²s) Fig.. The eect o inner tube diameter on heat transer coeicient. to be dominant in the initial stage o evaporation, particularly under high heat lu conditions. The eect o heat lu on the heat transer coeicient shows the dominance o nucleate boiling heat transer. Nucleate boiling is suppressed at high quality where the eect o heat lu on the heat transer coeicient becomes lower. As the heat lu increases at high qualities, the evaporation is more active and the dryout quality becomes lower. The trend illustrated in Fig. 3 agrees with previous studies, e.g., Kew and Cornwell [], Yan et al. [1], and Kuo-Wang [9]. Fig. illustrates the eect o inner tube diameter on the heat transer coeicient. At the low quality region o around up to., smaller inner tube diameter shows higher heat transer coeicient. This is due to more active nucleate boiling in the smaller diameter tube. As the tube diameter becomes smaller, the contact surace area o heat transer increases. The more active nucleate boiling causes dry-patches to appear earlier. Thereore, the dry-out quality is relatively lower or the smaller tube. The quality or rapid increase in heat transer coeicient is lower or the smaller tube. It is supposed that the annular low appears at a lower quality in the smaller tube. The eect o saturation temperature is depicted in Fig. 5. The heat transer coeicient increases with an increase in saturation temperature, which is due to the more active nucleate boiling. Table 3 shows that the higher temperature has a lower surace tension and h tp (kw/m²k) T sat ( C) 1 5 q = 15 kw/m² G = 15 kg/m²s D i = 3. mm.... Fig. 5. The eect o saturation temperature on heat transer coeicient. higher pressure. The lower surace tension and higher pressure result in higher boiling nucleation. At the high quality region, dry-out appeared earlier or the lower saturation temperature. This result can be eplained with the propane s density ratio, ρ /ρ g, and viscosity ratio, µ /µ g. As shown in Table 3, the lower temperature has higher density ratio, ρ /ρ g, and viscosity ratio, µ /µ g. A higher density ratio, ρ /ρ g, and/or a lower viscosity ratio, µ /µ g, cause higher entrainment, which is the reason that the dry-out comes easier. The eperimental results show that the eect o density ratio, ρ /ρ g, on the dry-out is higher than that o viscosity ratio, µ /µ g. Thereore, the lower saturation temperature results in a lower dry-out quality, as shown in Fig. 5. It is concluded that the eect o density ratio, ρ /ρ g, and viscosity ratio, µ /µ g, on the heat transer coeicient is necessary to be considered in the prediction o heat transer coeicient. The heat transer coeicients in the present study are analyzed and compared by using seven previous heat transer coeicient correlations. Table shows the deviation percentage o the comparison and Fig. shows the selected comparisons o the eperimental heat transer coeicient with the eisting correlations. Overall, Shah s [13] correlation gave the best prediction o all seven. Shah s [13] correlation was developed by using conventional rerigerants in a conventional channel. The correlation o Tran et al. [3], which was developed or low boiling heat transer in small channels, also works well with the present eperimental data. Jung et al. s [1] correlation is a superposition model that was developed by using conventional channels. The prediction using Jung et al. s [1] correlation shows a higher deviation or the smaller diameter data. Jung et al. s [1] correlation

5 A. S. Pamitran et al. / Journal o Mechanical Science and Technology 3 (9) 599~ 3 Table. Deviatnhe heat transer coeicient comparison between the present data and the previous correlations. Deviation Shah [13] Tran et al. [3] Jung et al. Gungor- Takamatsu Kandlikar- Chen [1] (%) [1] Winterton [15] et al. [17] Steinke [] MD AD h tp, pred. /h tp, ep. h tp, pred. /h tp, ep Shah (19) MD = 15.%, AD = -.59% Tran et al. (3) MD = 1.%, AD = -9.% Fig.. The comparison o the eperimental heat transer coeicient with eisting correlations. does not consider the laminar low which appears in low with small channels. The high deviation, relatively, in the prediction using the Gungor-Winterton [15], Takamatsu et al. [1], and Chen [17] correlations was supposed because those correlations were developed by using conventional channels. The correlation reported by Kandlikar-Steinke [] was developed or low boiling heat transer in small channels but it did not work quite well with propane. The pool boiling correlations o Cooper [1] and Stephan-Abdelsalam [19] have been used to calculate the nucleate boiling heat transer contribution in several previous studies such as Jung et al. [1] and Gungor-Winterton [15]. The Cooper [1] correlation is a unction o reduced pressure, molecular weight and heat lu, whereas the Stephan-Abdelsalam [19] correlation is a unction o the thermal conductivity, surace tension, density, heat lu, saturation temperature and the Prandtl number. Thereore, the present study used the both pool boiling heat transer coeicients as a comparison in order to obtain the best result.. Development o a new correlation.1 Modiication o actor F Two important mechanisms, nucleate boiling and orced convective evaporation, mainly govern low boiling heat transer. Because o its high boiling nucleation, the appearance o convective heat transer or evaporative rerigerants in small channels is delayed compared with that in conventional channels. Thereore, the prediction o convective heat transer contribution or rerigerants in small channels will be dierent rom that in conventional channels. The new heat transer coeicient correlation in this study was developed only with the eperimental data obtained prior to dry-out. Chen [17] introduced a multiplier actor F=n(X tt ) to account or the increase in convective turbulence due to the presence o a vapor phase. Chen [17] reported the actor, F, to be a unction o X tt, which needs to be evaluated again physically or low boiling heat transer in minichannels which has laminar low condition due to the small diameter eect. The liquid-vapor low condition o this eperimental result shows 59% turbulent-turbulent, 37% laminarturbulent and % turbulent-laminar. It was obtained by calculating the Reynolds number or each liquid and vapor phase. By considering the low conditions (laminar or turbulent) in the Reynolds number actor F, Zhang et al. [] introduced a relationship between the Reynolds number actor F and the two-phase rictional multiplier based on pressure gradient or liquid alone low φ, F=n( φ ), where φ is a general orm or our conditions according to Chisholm [], φ = C X + X (5) For liquid-vapor low conditions o turbulentturbulent (tt), laminar-turbulent (vt), turbulent-laminar (tv), and laminar-laminar (vv), the values o the Chisholm parameter C are, 1, 1, and 5, respectively. The Martinelli parameter is deined as ollows:

6 A. S. Pamitran et al. / Journal o Mechanical Science and Technology 3 (9) 599~ X ( ) ( d dz) 1 1 dp dz 1 ρg = = p g g ρ () In this study, the Blasius equation o riction actor was used or riction actors and g. Hence, the Martinelli parameter can be rewritten as ollows: X µ 1 ρg = µ g ρ (7) The important eects o quality, density ratio ρ /ρ g, and viscosity ratio µ /µ g on the heat transer coeicient or boiling rerigerants are represented in Eq. (7). The liquid heat transer is deined by the Dittus- Boelter correlation, ( 1 ).. k G D C p µ lo =.3 D µ k h () The actor F is a convective two-phase multiplier that accounts or enhanced convective due to the cocurrent low o liquid and vapor. A new actor F as Table 5. Summary o the new heat transer coeicient. h = Sh + Fh tp nbc lo S =. ( φ ) Bo ( ).1.55 nbc = 55 r.33ln r.5.7, where - is in Wm =.3φ k G ( 1 ) D Cp µ lo =.3 D µ k C 1 µ 1 ρg = 1 + +, X = X X µ g ρ h P P M q q F h φ C(tt) =, C(vt) = 1, C(tv) = 1, C(vv) = 5 shown in Fig. 7 was developed with a regression method rom the eperimental data. The new actor F can be epressed as ollows: F =.3φ (9) where φ is in Eq. (5).. Nucleate boiling contribution Mass lu is believed to have a signiicant eect on the suppression o nucleate boiling. A higher mass lu is corresponding to a higher suppression o nucleate boiling. For evaporation in a minichannel, the suppression is lower than that in a conventional channel. The nucleate boiling heat transer or the eperimental data was predicted by using the Cooper [1] correlation, which is a pool boiling correlation developed based on an etensive study. For a surace roughness o 1. µm, the correlation is given as ollows: ( ).55 h= 55P.33ln P M q (1) r r where the heat lu, q, is in W m - and P r is the reduced pressure (P r =P sat /P crit ). The correlation covers reduced pressure rom.1 to.9 and molecular weights rom to. Kew and Cornwell [], using R-11b in a tube with a length o 5 mm and inner diameters o 1.39 to 3.9 mm, showed that the Cooper [1] pool boiling correlation best predicted their eperimental data. Better prediction using the Cooper [1] correlation is also shown in the Jung et al. [1] study. Chen [17] deined the nucleate boiling suppression h tp /h lo 1 1 Eperimental Data Zhang et al. Factor F New Factor F h tp, pred (kw/m²k) +% -% φ h tp, ep (kw/m²k) Fig. 7. Two-phase heat transer multiplier as a unction o φ. Fig.. Diagram o the eperimental heat transer coeicient, h tp, ep., vs prediction heat transer coeicient, h tp, pred.

7 A. S. Pamitran et al. / Journal o Mechanical Science and Technology 3 (9) 599~ 5 actor, S, as a ratio o the mean superheat T e to the wall superheat T sat. Chen s actor S was developed with a conventional channel, hence urther evaluation is needed to apply it or rerigerants in a minichannel. Jung et al. [1] proposed a convective boiling heat transer multiplier actor N as a unction o the quality, heat lu, and mass low rate (represented by using X tt and Bo) to represent the strong eect o nucleate boiling in low boiling by comparing it with that in nucleate pool boiling, h nbc /h pb. To consider laminar low in minichannels, the Martinelli parameter, X tt, is replaced by the two-phase rictional multiplier, φ. Using the eperimental data rom this study, a new nucleate boiling suppression actor, a ratio o h nbc /h pb, is proposed as ollows: S ( φ ).1 =. Bo.777 (11).3 Heat transer coeicient comparison The new heat transer coeicient correlation was developed by using a regression method with 79 eperimental data points. The comparison o the eperimental heat transer coeicient, h ep,, and the prediction heat transer coeicient, h pred, is shown in Fig.. The new correlation showed good agreement with a mean deviation o.7% and an average deviation o -.1%. Table 5 gives a summary o the new correlation. The use o Stephan-Abdelsalam s [19] correlation to predict the nucleate boiling heat transer contribution in developing a new correlation showed a slightly higher deviation (mean deviation and average deviation o 9.5% and.%, respectively). 5. Concluding remarks Convective boiling heat transer eperiments were perormed in horizontal minichannels with propane. Mass lu has an insigniicant eect on the heat transer coeicient at the low quality region. At the moderate quality region, the heat transer coeicients increase with mass lu and vapor quality. At the high quality region, a decrease in the heat transer coeicient occurs at a lower quality or a higher mass lu condition. A strong dependence o the heat transer coeicients on the heat lu appears at the low quality region. Jung et al. s [1] correlation gave the best prediction among the si reported correlations. The physical properties o the rerigerant and geometric eect o the small tube must be considered when developing a new heat transer coeicient correlation. Laminar low appears during low boiling in small channels. Thereore, in this study, a modiied correlation o the multiplier actor on the convective boiling contribution, F, and the nucleate boiling suppression actor, S, was developed by using a laminar low consideration. The new boiling heat transer coeicient correlation based on the superposition model or propane in minichannels showed a mean deviation and an average deviation o.7% and -.1%, respectively, which highlights the good agreement between the measured data and the calculated heat transer coeicients. Reerences [1] Z. Y. Bao, D. F. Fletcher and B. S. Haynes, Flow boiling heat transer o reon R11 and HCFC13 in narrow passages, Int J Heat and Mass Transer, 3 () [] W. Zhang, T. Hibiki and K. Mishima, Correlation or low boiling heat transer in mini-channels, Int J Heat and Mass Transer, 7 () [3] T. N. Tran, M. W. Wambsganss and D. M. France, Small circular- and rectangular-channel boiling with two rerigerants, Int J Multiphase Flow, (3) (199) 5-9. [] S. G. Kandlikar and M. E. Steinke, Predicting heat transer during low boiling in minichannels and microchannels, ASHRAE Trans, CH (3) 7-7. [5] A. S. Pamitran, K. I. Choi, J. T. Oh and H. K. Oh, Forced convective boiling heat transer o R-1A in horizontal minichannels, Int J Rerigeration, 3 (7) [] P. A. Kew and K. Cornwell, Correlations or the prediction o boiling heat transer in small-diameter channels, Applied Thermal Engineering, 17 (-1) (1997) [7] G. M. Lazarek and S. H. Black, Evaporative heat transer, pressure drop and critical heat lu in a small diameter vertical tube with R-113, Int J Heat Mass Transer, 5 (19) [] M. W. Wambsganss, D. M. France, J. A. Jendrzejczyk and T. N. Tran, Boiling heat transer in a horizontal small-diameter tube, Journal o Heat Transer, 115 (1993) [9] C. S. Kuo and C. C. Wang, In-tube evaporation o HCFC- in a 9.5 mm micro-in/smooth tube, Int J Heat Mass Transer, 39 (199)

8 A. S. Pamitran et al. / Journal o Mechanical Science and Technology 3 (9) 599~ [1] J. Pettersen, Flow vaporization o CO in microchannels tubes, Eperimental Thermal and Fluid Science, () [11] R. Yun, Y. Kim and M. S. Kim, Convective boiling heat transer characteristics o CO in microchannels, Int J Heat and Mass Transer, (5) 35-. [1] Y. Y. Yan and T. F. Lin, Evaporation heat transer and pressure drop o rerigerant R-13a in a small pipe, Int J Heat and Mass Transer, 1 (199) [13] M. M. Shah, Chart correlation or saturated boiling heat transer: equations and urther study, ASHRAE Trans, 73 (19) [1] D. S. Jung, M. McLinden, R. Radermacher and D. Didion, A study o low boiling heat transer with rerigerant mitures, Int J Heat Mass Transer, 3 (9) (199) [15] K. E. Gungor, H. S. Winterton, Simpliied general correlation or saturated low boiling and comparisons o correlations with data, Chem Eng Res, 5 (197) [1] J. C. Chen, A correlation or boiling heat transer to saturated luids in convective low, Industrial and Engineering Chemistry, Process Design and Development, 5 (19) [17] H. Takamatsu, S. Momoki and T. Fujii, A correlation or orced convective boiling heat transer o a nonazeotropic rerigerant miture o HCFC/ CFC11 in a horizontal smooth tube, Int. J. Heat and Mass Transer, 3 (1) (1993) [1] M. G. Cooper, Heat low rates in saturated nucleate pool boiling a wide-ranging eamination using reduced properties, In: Advances in Heat Transer, Academic Press, 1 (19) [19] K. Stephan and M. Abdelsalam, Heat-transer correlations or natural convection boiling, Int J Heat Mass Transer, 3 (19) [] D. Chisholm, A theoretical basis or the Lockhart- Martinelli correlation or two-phase low, Int J Heat Mass Transer, 1 (197) [1] D. Jung, Y. Kim, Y. Ko and K. Song, Nucleate boiling heat transer coeicients o pure halogenated rerigerants, Int J Rerigeration, (3) -. Jong-Taek Oh received his B.S., M.S. and Ph.D. degrees in Rerigeration Engineering rom Pukyong National University, Korea. Dr. Oh is currently a Proessor at Department o Rerigeration and Air Conditioning Engineering, Chonnam National University at Yeosu, Korea. Dr. Oh s research interests are in the area o boiling and condensation heat transer and pressure drop o rerigerants with small tubes, heat pump and transportation rerigeration.